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COMPOUNDSEMICONDUCTOR

November 2006 Volume 12 Number 10

C O N N E C T I N G T H E C O M P O U N D S E M I C O N D U C T O R C O M M U N I T Y

UK defense chiefs feel the need for GaN p5

HEADLINE NEWS HB-LEDS INTERVIEW

SixtiesthrowbackIf you’re going to SanFrancisco, you might see this LED-packed VW Transporter. p8

Heart of glassDavid Jordan explains why making GaN LEDs on a glass substrate makes sense. p12

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Compound Semiconductor November 2006 compoundsemiconductor.net 1

TECHNOLOGY

14 Extended VCSEL wavelengths offer convenient source for gas sensing: Markus Ortsiefer from Vertilas explains why long-wavelength tunable VCSELs are a better option for gas sensing than the lead salt, quantum cascade and diode-pumped solid-state lasers that are used today.

17 Application Focus: Big step forward in silicon integration The hybrid III-V/silicon laser manufactured by Intel and itscollaborators may not represent true silicon photonics, but it is a significant development in optical interconnects for chip and board communications, discovers Michael Hatcher.

20 Crossed swords over SEMI standards : Competition among SiC substrate manufacturers has boosted wafer diameters and eliminated micropipe defects in commercial SiC wafers. However, there is still some division over the effectiveness of the standards used to define the quality of the material, as Richard Stevenson discovers.

24 ECSCRM: applications steal the show: Commercial applications and market penetration of devices dominatedthe recent ECSCRM conference in Gateshead, UK, which was held on the centenary of the first SiC paper. Alton Horsfall reports.

26 Equipment Update: Steam generator purifies oxidation A Californian start-up claims that its ultrapure steam generator can boost the production yield and speed the oxide aperture process used widely in VCSELmanufacturing. Richard Stevenson talks to the tool’s inventor, Jeff Spiegelman.

27 Product Showcase

28 Research Review: Chinese Academy of Science researchers boost GaAs photodetector efficiency by factor of 10...Osram investigates nonradiative defects... Reflector increases InGaN LED emission

Compound Semiconductor’s circulation figures are audited by BPA International

INDUSTRY

5 Headline News: UK defense chiefs feel the need for GaN...AlGaAs standard to make industry ‘more efficient’.

6 The Month in RFICs: Silicon process update prompts Nitronex ramp...Skyworks’switch increases focus on GaAs products...AXT to lobby against China tax changes...Hittite enjoys huge gain in profit...Epiwafer foundry IQE wins new deal.

8 The Month in HB-LEDs: BridgeLux retaliates with Cree lawsuit...SemiLEDs scales up GaN production...Osram markets latest chip designs...Luminus samples super-bright chipset.

10 The Month in Optoelectronics: Sumitomo Chemical sets up joint venture to exploit split-band material for solar energy...50 million and counting: Finisar honors VCSELmilestone in Texas...Hydride VPE suitable for quantum wells...Shell turns to cascade lasers for oil prospecting.

12 Interview: Industry veteran aims to steer GaN from start-up to mass production BluGlass’s low-cost GaN deposition method has already won favor among Australian investors. Jon Cartwright catches up with CEO David Jordan to find out what direction the company is headed for next.

Bright lightsLuminus has optimized its new RGB

LED chipset for use with Texas

Instruments’ digital light processors,

which feature millions of tiny mirrors. p9

Integration breakthroughIntel’s new hybrid chip can integrate up

to 36 lasers on a single die. p17

Main cover image: Observed using cross-polarized light to highlight defects, the quality of these two SiC substrates might look completely different, butactually they both conform to equivalent SEMI standards for the material. As a result, some believe that the standards should be reviewed. Images courtesy of Northrop Grumman/Trans Tech Publications.

Small is beautifulNIST has made available tiny pieces

of AlGaAs that should help III-V

manufacturers become more efficient. p5

http://compoundsemiconductor.net

compoundsemiconductor.net November 2006 Compound Semiconductor2

E D I T O R I A L

Global milestonesThe daily churn of stories on our news servicecompoundsemiconductor.net last month included a couple ofnotable milestones in the world of commercial compoundsemiconductor device manufacturing. One of these came fromSpectrolab, which specializes in multi-junction photovoltaicdevices that are mostly used to power satellites, revealing that it

had now made two million GaAs-based solar cells.It’s an impressive achievement. Having overcome the incredibly stringent

technical demands of applications in space, Spectrolab’s technology cannow be found in more than half of all the satellites orbiting Earth, includingthe International Space Station. Its solar cells also feature in six of the sevencraft still to be found either on the Martian surface or in orbit around the redplanet, and have helped the Mars Global Surveyor and the Spirit andOpportunity rovers to operate way beyond their predicted lifespans.

Acouple of weeks before Spectrolab, and a step up in terms ofmanufacturing volume, Finisar shipped its 50 millionth VCSEL. For that we

must also doff our hats to Honeywell, whichbegan making the surface-emitting laserscommercially back in 1996. Finisar hasemerged as a force to be reckoned with withinthe optical data communications sector, andhas even commissioned a new epitaxymachine to meet the expected demand.

But do these admittedly impressivemilestones really matter, or are they just nice, round numbers? Well, maybehere’s one that does matter. The continued global craving for cell phonesand, in turn, the GaAs transistors that sit at the heart of pretty much all ofthem these days, has prompted a number of analysts, including ABIResearch and IDC, to predict another big milestone for 2006 – that a billionmobile phones will be sold.

One billion? It’s the sort of number that is barely possible to conceive,and one that inevitably results in metaphors involving things being stacked end-to-end and stretching all the way around the world. Come to think of it, with the triple-junction solar cells in orbit, the surface-emitting lasers sending daily news updates around the world viaoptical networks, and the transistors deployed in phones from Africa to the Antarctic, it would be fair to say that the world has already beenwrapped in compound semiconductors.

Michael Hatcher Editor

“The continued globalcraving for cell phoneshas prompted analyststo predict that a billionwill be sold in 2006.”

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Production Ruth Harwood, Ruth LeopoldAd production Joanne Derrick, Mark TrimnellArt director Andrew GiaquintoTechnical illustrator Alison Tovey

SubscriptionsAvailable free of charge to qualifying individualsworking at compound semiconductor fabs andfoundries. For further information visitcompoundsemiconductor.net/subscribe. Subscriptionsfor individuals not meeting qualifying criteria:individual £86/$155 US/7125; library £193/$348US/7280. Orders to Compound Semiconductor, WDIS, Units 12 & 13, Cranleigh Gardens IndustrialEstate, Southall, Middlesex UB1 2DB, UK. Tel: +44 208 606 7518; Fax: +44 208 606 7303. General enquiries: [email protected].

9314 average total qualified circulation**June 2006 BPA audit statement

Editorial boardMayank Bulsara Atlas Technology (USA); Andrew Carter Bookham Technology (UK); Jacob TarnOCP/Gigacomm (Taiwan); Ian Ferguson GeorgiaInstitute of Technology (USA); Toby Strite JDSU(USA); Mark Wilson Motorola (USA); Dwight StreitNorthrop Grumman (USA); Joseph Smart Crystal IS(USA); Colombo Bolognesi Swiss Federal Institute ofTechnology (Switzerland); Shuji Nakamura Universityof California at Santa Barbara (USA)

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I NDUSTRY H E A D L I N E N E W S

GaN electronics has been specified as a pri-ority technology by UK defense chiefs. In itsDefence Technology Strategy (DTS), launchedon October 17 by procurement minister LordGrayson, the Ministry of Defence (MOD)highlighted GaN circuit technology as a keyarea of science and technology and one thatdemanded a domestic supplier.

According to the DTS document, the UKcannot currently guarantee on-shore access toGaN components, which the MOD believeswill be crucial to the country’s future militarycapability. “There is a need for the UK toensure continued access to some criticaladvanced components and circuits,” reportsthe MOD’s strategy document. “The mostimportant of these are the solid-state RFtransmit/receive modules that are based onadvanced semiconducting materials. It isessential that the UK maintains on-shoreaccess to these technologies.”

“[The] MOD will work with defensemanufacturers and European procurementagencies to identify what UK investment is needed to ensure that UK companies will be able to access GaN technology within

Europe,” claims the document.Currently, the UK has a very limited GaN

development program, although it is part of aEuropean research effort focused on GaNHEMTs. The US and Japan are well ahead ofEurope in terms of GaN electronics, thankslargely to central government investment inthe technology, as well as being home to theincumbent GaN device manufacturers foroptoelectronic applications such as blue LEDsand violet-blue lasers.

Part of the problem is that the UK currentlyrelies on Filtronic as its only major III-Vfoundry for RFICs. This has been the case eversince Bookham decided to shut its CaswellGaAs line in May 2004 and concentrate solelyon photonics components.

The MOD acknowledges that the veryexistence of the Filtronic foundry depends onthe viability of that company’s overall busi-ness, and the presence of a large non-defensemarket for its products.

With the UK’s GaN electronics expertisecurrently limited to some leading universityresearch groups and Qinetiq, the research anddevelopment company that was previously

part of the MOD, defense chiefs will need toinvest some of the ministry’s £2.6 billion($4.9 billion) annual research budget in the wide-bandgap material.

“The development of GaN can be consid-ered as part of an ongoing systematic devel-opment of wide-bandgap materials, includingdiamond,” the MOD said. “This is likely tolead to a range of new components.”

Although GaN expertise is relatively scarcein the UK, one domestic company that couldprovide some of the required material is IQE.Its new Somerset, NJ, division has plenty ofexperience in GaN-based epitaxy for RF appli-cations. The US location is not expected toprove any barrier to the MOD, just as IQE hasbeen involved in US Department of Defenseapplications for many years.

“GaN HEMTs feature as key products onIQE’s roadmap, and while the major driver is currently dominated by defense-relatedapplications, we believe this has the potentialto help refine GaN processes for RF compo-nents and drive down costs to open upcommercial opportunities in the longer term,”the company said.

UK defense chiefs feel the need for GaNN I T R I D E S

AlGaAs standard to make industry ‘more efficient’S T A N D A R D S

Compound semiconductor manufacturers willnow be able to measure the composition ofAlGaAs films more accurately than everbefore, thanks to a new material standardissued by the National Institute of Standardsand Technology (NIST).

The Boulder, CO, authority says that the1 cm2 AlGaAs/GaAs pieces (denoted SRM2841) will improve the accuracy of AlGaAsfilm composition analysis by 10 times com-pared with current methods.

AlGaAs layers are used widely in bothmicroelectronic and optoelectronic deviceapplications, in particular for the red lasers thatare deployed in optical disc drives. KrisBertness, the NIST scientist who led thestandards development, told CompoundSemiconductor: “There really have not beenany absolute standards in the past.”

“People are much more used to thinking interms of how reproducible their measurementsare, but that does not address the absoluteaccuracy of the measurement.”

As a result, Bertness says, the III-V indus-try has been operating largely on knowing howthe material they grow one day compares withwhat they grew the month before. “Most lab-

oratories have an internal reference system thatworks as a bootstrap standard,” she said.

Comparisons between the way that differ-

ent laboratories measure AlGaAs composi-tions have shown that not only is there avariation in the calibration equations used, butalso a tendency by the experimentalists toignore absolute calibration of their ownmeasurement equipment.

In response to an initial request frommembers of the Optoelectronics IndustryDevelopment Association, and with NISTfunding, Bertness and colleagues set about the task of developing the standard back in1998. The new standard is certified with ref-lection high-energy electron diffraction(RHEED) growth-rate measurements andphotoluminescence.

The photoluminescence measurement is calibrated against both RHEED and twodirect chemical analyses – one made using anelectron microprobe, and another based oninductively-coupled plasma optical emissionspectroscopy.

Bertness believes that the SRM 2841 stan-dard will reduce wasteful duplication of ref-erence wafers and increase the free exchangeof thin-film materials between vendors andtheir customers. “We hope to make the indus-try more efficient,” she concluded.

Tiny pieces of AlGaAs/GaAs like this one are now

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I NDUSTRY T H E M O N T H I N R F I C S

Silicon process updateprompts Nitronex ramp

W I D E - B A N D G A P T R A N S I S T O R S

RF power transistor maker Nitronex says thatits GaN-on-silicon manufacturing process isnow fully qualified for volume production. TheRaleigh, NC, developer has released severalproducts manufactured with the “NRF1”process for applications in WiMAX connectiv-ity and general wireless infrastructure.

While most GaN transistor makers, includ-ing RF Micro Devices and Cree, use SiC sub-strates, Nitronex believes that its use of siliconprovides economic benefits without sacrific-ing transistor performance. Just over a year ago,Nitronex revealed that it had signed a deal tosupply the Korean company RFHIC with GaN-on-silicon transistors for WiMAX applications,but since then it has refined its manufacturingprocess. Chris Rauh, the vice-president of salesand marketing at Nitronex, explained: “Wemade the decision to improve upon the processthat was used to make the early samples.”

“It was clear to us that our product lineswould be improved by moving from 0.7 μmgates to 0.5 μm gates and by adding a sourcefield plate to the process.”

Those changes required a complete relia-bility study and qualification, which pushedback the release of the new Nitronex transis-tors until now. The new products, which areaimed at WiMAX infrastructure applications,include a 50 W/28 V device with an averagepower of 6 W over 3.3–3.8 GHz.

Two transistors producing 15 W and oper-ating at 28 V have also been released, one ofwhich covers the lower frequency band of2.3–2.7 GHz. According to some commenta-tors, these lower frequencies will remain thepreserve of silicon LDMOS power transistorsfor the foreseeable future. Rauh’s response isthat while LDMOS is now viable for frequen-cies up to 3.5GHz, this is only part of the story:“The real test is when you look at the carriersdemanding 200 MHz of bandwidth, as well asvery tough linearity requirements, and 25 Waverage OFDM power amplifier output withefficiencies greater than 10%,” he remarked.

“Higher-frequency markets (more than3.8 GHz) are clearly in our sights, so I am notin disagreement that there is a strong play forGaN at these frequencies, but I don’t believethat the game is completely settled at 2.5 GHzand 3.5 GHz for WiMAX.”

Rauh is expecting the process qualificationto have an immediate impact on Nitronex sales.He believes that although the notoriouslyconservative wireless infrastructure markethas had its fingers burned in the past by adopt-ing new power transistor technology too early,the reliability data that Nitronex can nowpresent meets or exceeds customer needs.“They can now fully embrace GaN technol-ogy because they can review the completedreliability and qualification report.”

Fremont, CA, substrate vendor AXT reported$12.5million sales for the financial quarter thatended on September 30 – a sequential rise of21% and more than double its revenue duringthe same period last year. And despite an oper-ating loss of $1 million, AXT made a small netprofit of $0.6million thanks to the sale of sharesin Finisar and an income tax benefit.

With demand for semi-insulating GaAsmaterial rising strongly for use in HBT andPHEMT devices for cell phones, and sales ofgermanium substrates growing in line with theemerging market for multi-junction solar cells,the outlook for AXT is looking very positive.

However, the firm’s management also war-ned that changes to tax law in China could have

a major impact on its finances. AXT says thatChinese authorities are planning to impose cus-tom duties on purchases of certain raw materi-als, as well as eliminate refunds of value-addedtax for these transactions. Currently, the list ofaffected materials includes gallium and arsenic.

“The combination of these actions couldsignificantly increase our costs,” warned AXTin its financial outlook. “Lobbying efforts arebeing made to remove gallium and arsenicfrom the list of materials, but there is no wayof knowing whether these efforts will have anyimpact on the final form of the regulations.”

AXT may decide to restructure its Chineseoperations, which includes five joint-venturefirms that produce raw materials and key man-ufacturing equipment, in light of the new taxlaws. On the positive side, CEO Phil Yin stillbelieves the supply of both 4 and 6 inch semi-insulating GaAs substrates to be “severely”constrained, and that AXT’s decision to expandits capacity will reap rewards in the near future.

AXT to lobby againstChina tax changes

S U B S T R A T E S

Skyworks’ switchincreases focuson GaAs products

C O M P A N Y S T R A T E G Y

GaAs chip manufacturer Skyworks Solutions,of Woburn, MA, has ceased the operation ofits baseband business unit. The move, whichdoes not affect the firm’s compound semi-conductor operations directly, will result in a10% drop in total headcount, equivalent to425 employees.

The switch will, however, increaseSkyworks’ focus on advanced GaAs-basedproducts for current cell-phone handset appli-cations and emerging markets such as broad-band wireless access. Skyworks claims to haveincreased its worldwide market share forpower amplifiers used in cell phones from only30% in 2002 to 42% currently.

Encouraged by this rapid pace of growthand increasing dollar content in phonesthrough higher levels of integration, coupledwith the development of BiFET components,Skyworks is focusing squarely on this as itscore business. “We will partner with, ratherthan compete against, leading basebandsuppliers such as Texas Instruments, Qual-comm, Freescale and Infineon,” explainedSkyworks’CEO David Aldrich.

“We are implementing a strategic restruc-turing to…realize our vision of becoming theglobal leader in semiconductors enablingmobile connectivity,” Aldrich explained.Ultimately, the switch will provide a massiveboost to Skyworks’ bottom line. Pro formaprofit for fiscal 2007, in which the restructuringcharges are ignored, should now be at least$0.55 per share, or double the estimate that wasmade before the restructuring.

How exactly that profit will be spent is notyet known, but it is possible that Skyworks isconsidering ways to increase its GaAs capac-ity to meet the expected ramp in demand forbroadband wireless connectivity.l Skyworks accounted for the restructuring inits latest financial quarter, which ended onSeptember 29. Including the various severanceand inventory charges, bad debts and licensewrite-downs associated with the companyrestructuring, Skyworks posted a net loss of justover $96 million on total sales of $193 million.Excluding those one-off charges, the companymade a profit of $11.6million. According to thecompany, shipments of power amplifiers forwideband-CDMA handset applications morethan doubled in the recent quarter, reflectingthe strong demand currently being felt foradvanced handsets with higher GaAs content.


Compound Semiconductor November 2006 compoundsemiconductor.net

Hittite enjoys huge gain in profitC O M P A N Y F I N A N C E S

Hittite Microwave, the fabless Chelmsford,MA, company that sells a range of GaAs, SiGeand silicon-based ICs, has reported a huge gainin profit.

The firm posted sales of $34.6 million dur-ing the three-month period that ended onSeptember 30 – an increase of more than 60%compared with the same quarter in 2005.Thanks to its lean cost structure, Hittite trans-lated those sales into a profit of $11.6 million.That figure is more than double the $5 millionprofit that it posted one year ago.

“Favorable market conditions and productmix allowed us to deliver strong results,” said

Hittite CEO Stephen Daly, who pointed outthat the company had launched no fewer than20 new products during the past three months.Those include a range of nine new GaAsMMIC receivers operating at up to 28 GHz.These receivers, which are available as baredie and manufactured at one of Hittite’s manyfoundry partners, are typically used in radioarchitectures for military and back-haul links,as well as satellite-based communication.

Hittite is ranked among the top 10 GaAsdevice vendors by market researchers StrategyAnalytics. It is also the highest-ranked firmwithout its own chip fab.

INDUSTRY T H E M O N T H I N R F I C S

7

Epiwafer foundry IQE has won a contract tobecome the preferred external supplier ofepitaxial material to an unspecified Europeanradio-frequency wireless chip foundry. Thefoundry will use MBE-grown material fromIQE to make GaAs PHEMT switches.

IQE says that it has signed a memorandumof understanding with the customer, to theeffect that its material will be sourced if thecustomer’s demand for epiwafers increases.IQE told Compound Semiconductor that it isalready supplying the customer in smallquantities, with volume production expectedto begin in the second half of 2007.

“It is anticipated that any increase in epi-wafer requirements beyond the customer’s

internal capacity will flow directly to IQE,”said the Cardiff, UK, firm.

IQE’s US-based MBE operation already hasa substantial contract in place with a US man-ufacturer of wireless chips based on GaAs,while the recent acquisition of Emcore’s RFepiwafer division, which employs MOCVDreactors, has significantly expanded IQE’s cus-tomer and technology base.

The latest deal is another sign that many III-V chip makers are taking a new approachto epiwafer production and sourcing strategies,believes IQE’s CEO Drew Nelson. “Thisdemonstrates the strength of the outsourcemodel, which is now becoming a key part ofcustomer’s strategic considerations,” he said.

Epiwafer foundry IQE wins new dealO U T S O U R C I N G

From our Web pages...visit compoundsemiconductor.net for daily news updates

...TriQuint and Anadigics on the upOregon-based TriQuint Semiconductor postedsales of $103.3 million in its latest financialquarter, up 37% on last year and marking thefirm’s best operating results since 2001. TriQuintand rival GaAs chip maker Anadigics, NJ, bothreported their sixth consecutive quarters of salesgains, with Anadigics posting $44.8 million.

...NBC uses Cree chips for football showGaN HEMTs made by Cree are deployed in blimpsand aircraft to transmit broadcasts of NBC’sSunday Night Football television show in the US.According to Cree’s Jim Milligan, the Powerlineardevices have reduced the weight and size ofamplifiers and doubled their energy efficiency.

...new chief at UK foundryFiltronic, the UK-based GaAs chip foundry, haspromoted its finance director Charles Hindsonto CEO. The company also completed the saleof its filter-based transmit receive and poweramplifier business unit to Powerwave.

...RFMD powers to record salesLeading GaAs chip manufacturer RF MicroDevices posted record sales of $246.9 millionin the financial quarter that ended onSeptember 30. The Greensboro, NC, companyadded that the current quarter will likely deliversales of up to $280 million. If that is anaccurate forecast, RFMD will be on course toreach $1 billion revenue for the full fiscal year.




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I NDUSTRY T H E M O N T H I N H B - L E D S

Shortly after being accused of patent infringe-ment by the major LED manufacturer Cree,BridgeLux has responded with a similar law-suit of its own.

The Californian company, which special-izes in elongated chip designs for high-powerlight emission, has filed a motion to dismissthe action brought by Cree and BostonUniversity, and alleged that its North Carolinanrival has infringed US patent 6,869,812.

Cree sued BridgeLux first, citing infringe-ment of a number of US patents, including6,657,236 and 5,686,738. The ’738 patent,filed back in 1997 and assigned to BostonUniversity, details an MBE-based method formaking GaN films. GaN LEDs are generallyproduced using MOCVD. The more recent’236 patent lists Stephen DenBaars from theUniversity of California, Santa Barbara, as oneof three inventors, and is concerned withimproving LED light extraction through theuse of “optical elements”.

BridgeLux’s patent was filed more recentlystill, in May 2003. Entitled “High powerAlInGaN based multi-chip light emittingdiode”, the patent is attributed to Heng Liu ofSunnyvale, CA, but does not mention either

BridgeLux or eLite Optoelectronics, the firm’sformer incarnation.

Meanwhile, BridgeLux has released a prod-uct line based on smaller die in a bid to pene-trate the mobile handset market in which Creehas become such a dominant player.

The firm claims that its “BSV” series ofside-view products will allow designers of cell-phones, digital cameras and other portableelectronic devices to develop thinner displays.The new chips are based on elongated die mea-suring 10 × 18 mm and 10 × 23 mm that aremanufactured at BridgeLux’s volume pro-duction facility in Taiwan.

BridgeLux says that these new products arealready being sampled, and the chips are set tobe ramped into volume production by the endof the year. CEO Bob Walker says that the aimof the launch is to apply high-quality, high-power LED technology to mobile applications.

Mobile devices are still the key applicationmarket for high-brightness LEDs, accountingfor around half of the total value of the sector.However, it has become a somewhat com-moditized business.With the declining sellingprice of HB-LEDs used in mobile phone back-lights now outweighing the market’s growth,

chip manufacturers are pushing to penetratehigher-value applications such as large-areabacklighting for high-definition television.

This trend was highlighted in Cree’s latestfinancial quarter. The Durham, NC, companyposted revenue of $103.9 million during thethree months ending September 24, equiva-lent to only a 1 per cent increase on the sameperiod last year.

That was because an increase in unit ship-ments of 32 per cent was heavily off-set by a26 per cent decline in the average selling priceof Cree’s emitters. This has had a massiveimpact on the company’s bottom line, withCree delivering a net profit of only $13.3 mil-lion in the recent quarter compared with$21.7 million one year ago.

While revenue from LEDs has remainedlargely flat for the past year, Cree has had moresuccess with its SiC wafers and high-powerproduct lines.

Sales of high-power devices such as SiCSchottky diodes rose to $4.5 million in thelatest quarter, up more than 35 per cent sincelast year. Meanwhile, Cree’s wafer revenue hit$6.6 million – representing a yearly rise ofnearly 30 per cent.

BridgeLux retaliates with Cree lawsuit

US-headquartered SemiLEDs has ordered ahigh-volume MOCVD reactor with which itintends to scale up the manufacture of itsvertical-design GaN LEDs. The 24 × 2-inchAixtron machine will be installed in thecompany’s facility at the Hsinchu Science Parkin Taiwan, alongside its existing range ofMOCVD equipment.

SemiLEDs has pioneered a unique chipdesign that is based on the use of copper-alloy substrates. The novel substrate materialaids heat dissipation and enables the chips to be driven at a very high current to increaselight output without degrading efficacy (see Compound Semiconductor September2006, p16).

SemiLEDS initially grows its blue, greenand ultraviolet emitters on sapphire substrates,before transferring the epitaxial layers onto themetal alloy. The devices deliver an opticaloutput of 75 lm/Wat 350mAdrive current, andcan withstand a drive current of more than 3 Awithout any noticeable light output powersaturation, claims the company.

SemiLEDs scales upGaN production

Automotive lighting and high-definitiontelevisions are two critical applications that theGerman LED maker Osram Opto Semiconductorsis trying to penetrate with its latest chip designs.

The company is focusing its marketing efforton the benefits of its O-Star, TopLED and newPlatinum Dragon HB-LEDs. For example, Osramoffers an optical module that features one red,one blue and one green O-Star HB-LED.Together, the three chips output 300 lumens,enough to backlight a 56-inch television screen.

O-Star LEDs have also featured in apartnership between Osram and Volkswagenthat has updated the German motor company’sType 2 Transporter – the 1960s favorite alsoknown as the “hippy bus” – for the 21st century.

The concept vehicle’s headlamps are poweredby O-Star LEDs, while its daytime running lampsfeature Osram’s Golden Dragon chips. TopLEDswere used in the front and rear turn signals,making this vehicle the first to be built with LEDsin all the major exterior lighting functions.

P A T E N T D I S P U T E

C H I P M A N U F A C T U R I N G OS

RA

M


INDUSTRY T H E M O N T H I N H B - L E D S


Luminus Devices, the Woburn, MA, LEDmanufacturer that has made a big splash withits “PhlatLight” chipsets for television back-lighting, has now introduced a 1500 lm white-light (RGB) source.

The Massachusetts Institute of Technologyspin-out’s new PT120 product has been opti-mized for use with Texas Instruments’hugelysuccessful digital light processor (DLP).

Texas Instruments has already sold morethan 10 million DLP subsystems for applica-tions such as high-definition projectors. Eachfeatures up to 2.2 million tiny mirrors thatswitch thousands of times per second to delivercrisp, full-color images on a large screen.

The PT120 is currently being sampled tomakers of microdisplay projection televisions,and Luminus says that it has already beendesigned into a number of high-definition tele-visions based on the DLP. “Several modelsfrom various manufacturers will be commer-cially available in 2007,” said the company.

One of the critical advantages of usingLEDs in place of fluorescent sources in tele-vision applications is the speed with which thesolid-state emitters can be switched. “In a DLPhigh-definition television, the red, green andblue PhlatLight LEDs can cycle at 2.9 kHz,which is 48 times faster than traditional tele-vision frame rates – providing superior motionquality,” explained Christian Hoepfner, VPofproducts at Luminus.

Recently, Luminus has been issued with sixUS patents covering its photonic lattice tech-nology, an approach that results in its LEDsoperating with high light extraction efficiency.One of those patents dates from late 2004, butthe other five were all awarded since August2006. Before the end of this year, the companyalso expects to receive more of the 100 or soadditional patents that it has applied for.

Claiming to have invested more than

$10 million in its intellectual property port-folio, Luminus will soon be looking topenetrate beyond the relatively limited marketfor high-specification televisions. Luminusfounder and CTO Alexei Erchak said, “There

are unlimited opportunities for LEDs usingphotonic lattice technology today and in thefuture. Luminus...will also support other LEDapplications through collaborations andlicensing agreements.”

Luminus samples super-bright chipset

Long-lasting, reliable lighting. Accessible in themost inaccessible locations imaginable. That’s thepromise of LEDs. And thanks to NuSil, high-poweredversions will soon be available from Kaohsiung toCopenhagen to Kodiak, Alaska.

While our advanced packaging materials are helpinghigh-brightness LEDs fulfill their potential, yourneeds might be very different. From LEDs to fiberoptics, large batches to small, our Lightspan brandof products deliver precise, custom formulationsand the most complete line of high-refractive indexmatching adhesives, encapsulants and thermosetsavailable. All backed by more than 25 years ofengineering materials expertise.

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The digital light processors made by Texas Instruments

feature millions of tiny mirrors no larger than an ant’s leg

– as this microscopic image shows.

A P P L I C A T I O N S

TI

http://www.nusil.com



I NDUSTRY T H E M O N T H I N O P T O E L E C T R O N I C S

As it celebrated the official opening of itsnewly renovated manufacturing facility inAllen, TX, optical component and modulemanufacturer Finisar revealed that it hasshipped 50 million VCSELs.

The Californian company has been res-toring the Allen fab for more than a year at anestimated cost of $13 million. With an add-itional MOCVD reactor now refurbished anda second wafer fabrication line in place, thefacility is ready to support the production of4 Gbit/s products to upgrade storage areanetworks (SANs) and 10 Gb/s VCSELs forlocal area network applications.

“This state-of-the-art facility will enable usto power more of our customers’applications,including video-on-demand, high-definitionTV, and mobile computing,” said Finisar chiefexecutive Jerry Rawls. Over 300 people nowwork at Finisar’s Allen site, which is expectedto easily meet the firm’s production needs.

The 50 millionth VCSEL was officially sold to high-profile customer HP. At theribbon-cutting ceremony to celebrate the

renovated chip fab, Edward Flynn, the direc-tor of the SAN infrastructure unit within HP’sStorageWorks division, described the mile-stone as an extraordinary achievement.

Commercial production of VCSELs beganat Honeywell Labs’Richardson, TX, facilityin 1996. Finisar acquired Honeywell’s VCSELbusiness, including the Richardson facility, inMarch 2004 and turned it into the autonomousAdvanced Optical Component (AOC) division.

Unlike many of its rivals in the same sector,Finisar has enjoyed very strong growth indemand for its data communications productsover the past couple of years, culminating inthe expansion of AOC’s production capacity.“We worked our way back from one of themost difficult market downturns in history andsucceeded in achieving our plan for returningto profitability,” explained Rawls.l In a deal that it says will free up its operationalcash flow between now and 2010, Finisar hasagreed to exchange $100 million worth ofexisting “outstanding notes” for a new seriesof convertible notes.

Ajoint venture company is aiming to becomethe first to commercialize ZnMnTe and InGaNsemiconductors for solar energy applications.

Japanese conglomerate Sumitomo Chemicaland US start-up RoseStreet Labs have set up anew firm called RSL Energy in Phoenix, AZ,to develop, manufacture and sell what theydescribe as full-spectrum solar cells.

Having signed exclusive technology lic-enses from Cornell University and LawrenceBerkeley National Laboratory (LBNL), RSLEnergy believes that the novel materials couldbe commercialized for solar energy produc-tion within the next three years.

Wladyslaw Walukiewicz and Kin Man Yuhave developed the ZnMnTe material systemat LBNL. They found that the conduction bandof ZnMnTe can be split by adding oxygen tothe semiconductor alloy. The bands split so thatthe normally single-band material becomesmulti-band, with the energy levels widelyspaced so that the three energy transitions fallwithin the range of the solar spectrum.

This means that by tailoring the exact pro-portion of oxygen impurities in the alloy, the

energy levels can, in theory, be tuned to pro-duce triple-band material capable of more than50% conversion efficiency. According to theresearch team, the energy bands could be fur-ther split to yield quadruple-band semicon-ductors with up to 73% calculated efficiency.

RSL Energy also has a license to commer-cialize a different approach that was developedthrough a collaboration between LBNL andCornell. This has focused on InGaN as a mate-rial that can be used in more conventionalmulti-junction solar cells. The joint venturecompany believes that the InGaN approachcould produce solar cells with practical effi-ciencies of more than 48%.

The US Defense Advanced Research ProjectsAgency has also identified InGaN as a promis-ing photovoltaic material, with a quantum effi-ciency of up to 60% in the ultraviolet range.

With a start-up capital of $6.6 million, RSLEnergy is 50% owned by each of SumitomoChemical and RoseStreet Labs. Sumitomo’ssubsidiary Sumika Electronic Materials hasthe first right of refusal to produce the com-pound semiconductor materials required.

50 million and counting: Finisarhonors VCSEL milestone in Texas

M A N U F A C T U R I N G

Sumitomo Chemical sets up joint venture toexploit split-band material for solar energy

P H O T O V O LT A I C SFrom our Web pages...visit compoundsemiconductor.net

for daily news updates

...Spectrolab passes two million markBoeing subsidiary Spectrolab hasmanufactured its two millionth multi-junctionGaAs-based solar cell. The company beganworking on the high-efficiency converters in1983 and estimates that its cells power 60%of all the satellites currently orbiting the Earth –including the International Space Station. Intotal, Spectrolab cells have powered525 satellites or interplanetary missions suchas the Mars Global Surveyor.

...brighter UV-LEDsThe latest AlGaN-based ultraviolet LEDs madeby Seoul Semiconductor are twice as brightand twice as reliable as the previousgeneration, according to the Korean company.At a drive current of 20 mA, the latestcomponents have a maximum optical outputpower of 1.5 mW at 280 nm. Just four monthsago, volume device maker Seoul and AlGaNepitaxy specialist Sensor ElectronicTechnology, US, signed an equity-basedpartnership to develop and sell UV-LEDs. Thedevices have a number of potentially very high-volume applications, chiefly surfacedecontamination and water purification, wherethey could replace mercury discharge lamps.

...JDSU’s Kwong joins BinopticsIthaca, NY, laser chip developer Binoptics hasappointed Norman Kwong as its new executivevice-president of business development.Kwong previously worked at JDSU, where hewas director of advanced technology, and hasalso enjoyed stints at Ortel (now owned byEmcore) and Archcom Technology. Binoptics’lasers are made using a proprietary etched-facet process – the kind of innovation thatKwong believes will be important for meetingcustomer expectations for low-cost, high-reliability, high-performance lasers.

...Avanex shells another CFOAfter less than seven months in his post,Avanex CFO Cal Hoagland has left thestruggling optical component and modulemanufacturer. Giving no reasons for thesudden departure, the Fremont, CA, companyhas appointed Marla Sanchez as its new CFO.Sanchez previously worked as corporatecontroller at the laser diode manufacturer SDL.Yves Le Maitre, formerly VP of Avanex’s opticalcomponents division, has been promoted tothe unusual role of chief marketing officer.Avanex has also posted another loss-makingquarter, recording sales of $50.9 million for thethree months that ended on September 30.



INDUSTRY T H E M O N T H I N O P T O E L E C T R O N I C S

Compound Semiconductor November 2006 compoundsemiconductor.net

Across-disciplinary UK team is working on a£2.4 million ($4.5 million) project to developan optical system for oil and gas prospectingthat is based on quantum cascade lasers (QCLs).

Comprising the III-V foundry CompoundSemiconductor Technologies (CST), theuniversities of Sheffield and Glasgow, ShellGlobal Solutions and laser system specialistCascade Technologies, the consortium hasreceived just over £1 million from the UKgovernment’s Department of Trade andIndustry. The industrial partners are providingthe other £1.4 million. Shell will fund fieldtesting of the instrument once the developmentis completed.

The detection system will be based on pho-toacoustic spectroscopy, a method that can beused to measure gas concentrations withextreme sensitivity. CST commercial directorWyn Meredith explained that ethane (C2H6) isthe key hydrocarbon that gives a clue to thelocation of an undiscovered field of oil or gas.

Ethane and methane (CH4) are producedwhen larger hydrocarbons – typically found inoil and natural gas reserves – “crack” intosmaller molecules. Unlike methane, however,ethane is not produced by biological decay andso it is a much more reliable indicator.

Shell already uses a system that is based ona mid-infrared lead-salt laser to probe for ethanein its “LightTouch” prospecting equipment.

However, because they require cryogeniccooling, these are relatively bulky systemswhose mobility can be a problem. By using III-V-based QCLs instead, their size could begreatly reduced.

“Employing the photoacoustic spectrosc-opy approach, we hope to produce an instrum-ent that has an ethane sensitivity of around100 parts per trillion,” said Meredith.

Researchers at the University of Sheffieldwill grow QCL structures using novel anti-monide-based epitaxy to reach the crucial3.35 μm absorption band of ethane. CurrentQCLs operate at longer wavelengths than this.

CST will fabricate laser devices fromSheffield’s epiwafers, while Cascade willprovide module-level packaging and controlelectronics.

Glasgow’s physics department will thendevelop the photoacoustic detection systemwith Shell until it is ready for field trials.

“The long-term aim is to set up a UK sup-ply chain – from design to packaged device tosystem implementation,” explained Meredith.“This is strengthened by Cascade securing therights to source QCLs from foundry manu-facturers,” he added, in reference to a recentintellectual property relating to QCLs dealsigned by Cascade and Lucent Technologies.QCLs were invented at the US company’s BellLabs research facility.

While QCLs are only in the very early stagesof commercial deployment, they could end upbeing as useful to cancer specialists as theymay be to oil prospectors. That is becauseethane gas and other hydrocarbons are releasedwhen cancer causes free radicals in the humanbody to break down cell membranes.

CST is now looking at a number of III-Vdevice technologies that could turn into a valu-able business opportunity for the foundry overthe next five years.

“Once the baseline technology is estab-lished, the foundry model will allow rapidprototyping of novel designs for new productdevelopment and embed this technologyfirmly in the UK,” Meredith concluded.

Shell turns to cascadelasers for oil prospecting

11

Hydride VPE suitable for quantum wellsHydride vapor phase epitaxy (HVPE) can beused to produce very thin layers of GaN andAlGaN quantum wells, according to US-basedmaterials specialist Technologies and DevicesInternational (TDI).

Until now, this method, which can depositmaterial much more quickly than either MBEor MOCVD, had only been suitable for grow-ing very thick semiconductor layers.

Engineers at the Silver Spring, MD, com-pany say that a new HVPE machine is able toslow down from its normal deposition rate ofaround one micron per minute by more thantwo orders of magnitude.

This means that the same machine can becontrolled to grow sharp heterostructure lay-ers just 1nm thick, as well as thicker layers like

that of the GaN material in GaN-on-sapphiretemplates. One application of the approachcould be in nitride laser manufacturing.

TDI president Vladimir Dmitriev said:“This opens up a completely new path for theformation of low-defect substrate materialsand device structures.”

Scientists at Virginia CommonwealthUniversity and Arizona State University(ASU) have already characterized materialgrown in the novel HVPE machine. SubhashMahajan from ASU was surprised by the suc-cess: “We never expected this technology toproduce nanometer-thick GaN layers andmulti-layer structures. However, our trans-mission electron microscopy measurementsproved that it [has been] achieved.”

A P P L I C A T I O N S

E P I T A X Y




I NDUSTRY I N T E R V I E W

The shrewd investors who snapped up the shares ofAustralian GaN epitaxy start-up BluGlass must be bask-ing in their blue-tinted glory. After the company wasfloated on the Australian Stock Exchange in mid-September, its share value jumped 67% from AUS$0.30to AUS$0.50 (US$0.39). And flicking through theircorporate presentation it’s easy to see why. Over the past10 years the GaN industry has grown more than ahundred fold and by 2009 forecasters predict it will beworth more than US$7 billion (Strategies Unlimited).

BluGlass is still in its infancy. The firm was conceivedin July via the acquisition of Gallium Enterprises, acompany originally set up by the commercial arm of

Macquarie University in Sydney after a decade-longresearch project to develop low-cost GaN epitaxy.Immediately afterwards, BluGlass launched an initialpublic offering (IPO) of shares to raise AUS$6 million.However, as the deadline drew closer, unrelentingdemand pushed BluGlass to release a supplementaryprospectus to attract even more cash. All told, BluGlassraised AUS$10 million through the IPO.

Pioneering technologyThis capital now rests in the hands of the new CEO,David Jordan, for whom the semiconductor industry issecond nature. “Most of my time before BluGlass hasbeen almost exclusively spent in the photovoltaics andsilicon semiconductor field,” he says.

“In my previous role at BP Solar I managed to inter-face research and development programs and high-volume manufacturing worldwide. I very much see thatas the basis of what we’re doing here at BluGlass. Weare taking a very promising, university-based technologyand examining its potential as a business case, with thehope of taking it on to full commercialization and high-volume production.”

Most GaN material produced today is manufacturedthrough MOCVD. Though an effective, industry-provenmethod, MOCVD requires high temperatures in theregion of 1000 ºC and the use of highly toxic ammoniagas. The technology that BluGlass is pioneering, calledremote plasma chemical vapor deposition (RPCVP),requires no ammonia and proceeds at the slightly morebenign temperature of 700 ºC. This means that hardy butrelatively expensive wafer materials such as sapphire orSiC can be substituted by glass (hence the companyname) at a fraction of the cost. French firm Saint-GobinRecherche, which supplies both glass and sapphire forLED production, has just signed an 18 month deal to co-develop specially engineered substrates for BluGlass.

RPCVPis the subject of two international patents andone Australian patent held by BluGlass. These describepassing nitrogen plasma over the heated gallium. “Thegallium is transported into the reaction chamber in anitrogen carrier gas stream through a trimethyl galliumbuffer,” explains Jordan. The BluGlass team has alreadyreduced the surface roughness of its thin films from9–13 nm down to 1 nm, thus improving crystallinity andsuggesting that a high-quality GaN layer is produced.

“We are also attacking the scalability,” Jordan says.“BluGlass can already deposit at 4 inch diameters,whereas most of the MOCVD industry deposits on adiameter of 2 inches. And there’s no reason why we can’textend that to 8 inches, 16 inches or beyond.”

If this scale-up can be achieved, it points to a high

Industry veteran aims to steer GaNfrom start-up to mass productionBluGlass’s low-cost GaN deposition method has already won favoramong Australian investors. Jon Cartwright catches up with CEO David Jordan to find out what direction the company is headed for next.

B L U G L A S S

CEO David Jordan will be drawing on many years of experience in both the silicon and solar power

industries as he looks to revolutionize the GaN epitaxy sector with Australian start-up firm BluGlass.

BLU

GLA

SS



INDUSTRY I N T E R V I E W

throughput of cheaper GaN devices, which have becomeubiquitous in consumer technology. For blue LEDs, themainstay of the GaN market, 68% of applications are inLCD backlighting such as mobile phones and digitalcameras. Just 4% are in general lighting, but this couldall change if BluGlass can make an impact. Every year,a fifth of the global electricity supply is used in lightingapplications, most of which is still provided by ineffi-cient bulbs. When it is possible for white LEDs to bemass-manufactured at a much lower cost, this hugemarket beckons.

“There is no doubt that in terms of efficiency whiteLEDs are currently on a par with compact fluorescenttubes and are significantly better than incandescentbulbs,” says Jordan. “Compared with the light from flu-orescent tubes, which is fairly harsh, white LEDs can betuned to give a much softer light. In five years or so LEDsare set to take a very significant proportion of the gen-eral lighting market,” he predicts. “All we have to do isto address the cost, which is still too high. Once the costcomes down, the market will open up.”

There is another emerging market that could benefitfrom economic GaN optoelectronics: high-definition(HD) video. Found in Sony’s new Playstation 3 gamesconsole and Blu-ray Disc players, HD discs can store10 times more data than a regular DVD. The technologyhas been slow to take off because of problems makingblue laser diodes and their resulting high cost. BluGlass’sepitaxy could offer an attractive, lower-cost alternativeto MOCVD-based laser diode production and help to

ensure that HD discs fulfil their potential as the naturalsuccessor to DVDs.

Despite the three patents under its belt and ampleinvestment, Jordan is revealing no definite indication ofhow exactly BluGlass is going to generate sales revenue.“We will be working with strategic partners, eitherlicensing the technology once it’s proven, or formingjoint ventures with different groups,” says the CEO. “Butthat’s down the track, and we need to prove that we areable to hit the targets of cost-structures and device per-formance that we believe this technology offers us.”

Having already demonstrated a functional single p-njunction LED using the RPCVD method, the next stepfor BluGlass will be to complete a pilot production plantin collaboration with cleanroom specialist M+WZander.“This is an 18 month to two-year program,” says Jordan.“We envisage moving into the pilot production plant atthe beginning of next year, and then the commercial toolsthat we will use to implement the technology will be putin place in the first and second quarter. All of those thingsare required and they’re coming together now.”

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“We are taking a very promising, university-basedtechnology and examining its potential as a business case, with the hope of taking it on to fullcommercialization and high-volume production.”David Jordan

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T ECHNOLOGY O P T O E L E C T R O N I C S

Extended VCSEL wavelengths offer

Gas sensors are employed for numerous applications,including detecting moisture, monitoring gas leaks andgreenhouse gas emissions, searching for toxic and haz-ardous species, and optimizing combustion processes.Although many of these tasks can be carried out withchemical detectors, there is now a growing interest inoptical systems that have several inherent advantages.These include rapid data collection and a non-contactapproach that is more suited to the awkward measure-ment conditions in power stations and chemically reac-tive environments. And in contrast to chemical sensors,which only detect gases at a fixed location, optical tech-niques can monitor target molecules anywhere withinthe detection path of the measurement system.

At Vertilas, in Garching, Germany, we have beendeveloping a unique VCSELstructure that can providethe source for a modern sensing technique – tunablediode laser spectroscopy (TDLS). Our lasers cover thewavelength range 1300–2300 nm and have been usedto determine the presence of numerous gases.

With the tunable laser as a source, individual gasesare identified by their unique optical absorption finger-print. By focusing on a specific wavelength where agas has a characteristic absorption, the amount of lightabsorbed reveals the concentration of the gas.

TDLS has frequently been applied to wavelengthsoutside the range covered by our lasers. By analyzingthe strong absorption lines in the mid-infrared region(2.5–50 μm) the technique can determine the concen-tration of common gases such as water vapor, carbondioxide and ammonia. These lines are associated withso-called “rovibronic” ground energy transitions, aterm that describes the combination of molecular rota-tions and oscillations. The absorption strength of thesetransitions enables detection of gas concentrations evenin the parts per billion range.

However, the various lasers that are used to probethese gases at mid-infrared wavelengths, includinglead salt, quantum cascade and diode-pumped solid-state lasers, tend to be either expensive or inconvenientto use. This can be avoided by focusing on alternativetransitions in the near-infrared (800–2500 nm). Theabsorption strength of these transitions is one to twoorders of magnitude weaker than that of the rovibronictransitions. However, more reliable, cost-effective andeasy-to-use sources are available that draw on the tech-nology associated with lasers used for telecommuni-cation or data storage. Sensing in this spectral range is

also an advantage because uncooled high-sensitivitydetectors are available that can partly offset the weakerabsorption strength of the higher order transitions.

VCSELs versus edge emittersThe first TDLS systems operating in the near-infraredused conventional edge-emitting distributed feedback(DFB) lasers, but improvements to the performance ofVCSELs operating at 1.3 μm and above have made abetter source for gas sensing (see table on p16 for thebenefits of VCSELs over DFB lasers for TDLS appli-cations). These VCSELs have a tuning range that canextend to several nanometers, which is much wider thanthat of a DFB laser. This is because VCSELs have a muchsmaller active region, so that their internal temperatureis much more susceptible to increases in drive current

Vertilas is currently developing single-mode versions of its 2.3 μm VCSELs. These

Markus Ortsiefer from Vertilas explains why long-wavelength tunable VCSELs are a better option for gas sensing than the lead salt, quantum cascade anddiode-pumped solid-state lasers that are used today.

light

n-side contac

epitaxial top m

active region

isolation

contact

dielectric bott

0.25 mm

InP

n InAlAs

p InAlAs

n+p+

nBTJ

n+ n+

p-side contactgold coating (integrated he

Fig. 1. Vertilas’ VCSELs feature an InP-based active region, a buried tunnel juncti

dielectric mirror. The emission of these devices can be varied from 1.3–2.3 μm b

the composition and thickness of both the active region and epitaxial and dielect



TECHNOLOGY O P T O E L E C T R O N I C S

than a DFB laser. The tunability yields a wider range ofmeasurements. For example, pressure-broadened ab-sorption lines can be examined, several lines relating toone species can be observed simultaneously and multi-ple species can be detected in a gas mixture. The VCSELwavelength can also be tuned through the absorptionline several million times per second, enabling real-timemonitoring of rapid combustion processes.

Although GaAs-based VCSELs for optical com-munication applications are made in their millions,their use in TDLS systems is restricted to the detectionof oxygen, which has an absorption line at 760 nm.Emission can be extended to 1300 nm while main-taining an adequate level of performance, but this isstill short of the absorption lines for important speciessuch as methane (1651nm), carbon dioxide (2004nm),

water vapor (1854 nm) and ammonia (1512 nm).Unfortunately, the performance of the long-wave-

length VCSELs has lagged behind that of their short-wavelength counterparts, due to technological difficultieslinked with the different material system required. Theternary and quaternary InP-based compounds that areneeded to reach the longer wavelengths suffer from arelatively small index contrast, which means that a verylarge number of mirror pairs are needed to produce therequired reflectivity for the laser cavity. Thermal con-ductivity is also more than an order of magnitude belowthat of the AlGaAs/GaAs compounds, making thermalmanagement a major problem. Despite efforts to improvethermal dissipation by attaching a heat sink to one of themirrors, most of the long-wavelength VCSELs fabri-cated so far have poor performance characteristics.

r convenient source for gas sensing

e lasers could be used to detect the carbon monoxide produced by car engines.

t

mirror

tom mirrort +

eatsink)

on and a

y adjusting

tric mirrors.

Fig. 2. Using a thermistor and thermo-electric cooler stops drift in

the emission wavelength of the buried tunnel junction VCSEL.

Optical gain in InP-based lasers is very sensitive to temperature variation, so internalheating has to be suppressed to produce sufficient performance. Vertilas’ buried tunneljunction (BTJ) design addresses this issue by reducing the thickness of the p-dopedlayers. Low-resistance, n-doped layers are used because of their negligible absorption.Thermal characteristics are also improved by attaching a hybrid gold-dielectric mirror toone side of the VCSEL. This decreases heat resistance by almost an order of magnitudecompared with the ternary and quaternary InP-based semiconductor Bragg mirrors, andhas no detrimental effect on reflectivity.

The dielectric mirror on the bottom of the BTJ VCSEL can be made with electricallyinsulating materials, which opens up the design space to pairs of materials with greaterrefractive index contrast. The upper mirror carries no heat flow, which means that thispart of the VCSEL can still be made from normal semiconductor alloys.

The dielectric mirrors typically have a total reflectivity of 99.5–99.9% and areconstructed from either 2.5 or 3.5 periods of CaF2/a-silicon (refractive indexdifference,Δn of 2.1) or CaF2/ZnS (Δn of 0.9) that are coated with gold. The VCSELs alsofeature an electroplated metal layer on the bottom of the structure that providesmechanical stability and serves as an excellent heatsink. Finally, the InP substrate iscompletely removed from the device.

Coping with heat

0.6

0.4

0.2

0

abso

rban

ce

1539 1538 1537 1536 1535

wavelength (nm)

–3.6 °C15.2 °C

Fig. 3. By setting the operating temperature of the laser to 15.2 °C and –3.6 °C, Vertilas’ VCSEL can

be used to investigate the absorption spectrum of ammonia. The measurements were made on

ammonia gas contained in a 44 cm quartz tube and held at a pressure of 9.6 mbar. The absorption

measurements are in excellent agreement with the simulated data taken from the database standard.



TECHNOLOGY O P T O E L E C T R O N I C S

Success is possible, however, if one turns to noveldesigns, such as our InP-based VCSELs that feature aburied tunnel junction (BTJ) and dielectric mirrors (seefigure 1, p14). We demonstrated this approach in 1999and since then have improved the performance of ourdevices and increased their spectral coverage. Our BTJVCSELs now operate at over 100 ºC, have good ther-mal management properties (see “Coping with heat”box, p15) and deliver singlemode output powers of morethan 1 mW at 85 ºC. They also have a wavelength tun-ing range of several nanometers, modulation speeds inexcess of 10Gbit/s and are compatible with high-volumemanufacturing – the full-wafer processes we use todayon 2 inch material can be scaled to larger sizes.

Our BTJ VCSELs are mounted on a TO-header withan integrated thermo-electric cooler and thermistor tomonitor and control the laser stage temperature (figure2, p15). Temperature stability is demanded by gas sens-ing applications because the temperature-dependentwavelength is crucial to accurate detection. In a finalproduction step, the header is completed with a cap and

a wavelength-specific antireflection coating. The lasers’gas sensing capability has been demonstrated by absorp-tion measurements of ammonia, which have shownmany fine details (figure 3, p15). We acquired thesespectra by operating the laser at –3.4 ºC and then 15.2 ºCto provide a coarse control of the emission wavelengthand using current tuning for fine adjustments.

By adjusting the thickness and composition of theactive region and epitaxial and dielectric mirror layers,we produced singlemode BTJ VCSELs operating from1.3 to 2.05 μm. We also demonstrated the first electri-cally pumped room-temperature continuous-wave2.3μm BTJ VCSEL. This produces 1.47mWat 0ºC and0.74mW at 20 ºC, and a tuning range of 4.2nm at roomtemperature. It only operates in a multimode fashion thatis unsuitable for spectroscopy, but we are developing asinglemode version that could be used to detect carbonmonoxide, which has a 2332 nm absorption line. Thiswork will extend the range of our BTJ VCSELs, whichcan provide a cost-effective, tunable source and offer agood choice for a variety of gas sensing applications.

DOWA ELECTRONICS MATERIALS COMPANY, LTD.Semiconductor Business Unit,

14-1, Sotokanda 4-Chome, Chiyoda-ku, Tokyo, 101-8617, JapanPhone: +81-3-6847-1253 Fax: +81-3-6847-1260

About the authorMarkus Ortsiefer is a co-founder of Vertilas GmbH and is responsible for thecompany’s production andresearch and developmentdivisions. He would like to thankM Lackner from ProcessEng fortechnical discussions.

VCSEL 0.5–2.0 0.4–3.0 0.3–0.8 3.0–5.0 ~0.1 >1.0

DFB 20 10 <0.05 <1.0 ~0.1 <0.1

Laser type Threshold Output Current tuning Current tuning Temperature tuning Maximum frequencycurrent (mA) power (mW) rate (nm/mA) range (nm) rate (nm/°C) modulation (MHz)

The characteristics of VCSELs and distributed feedback lasers


http://www.dowa.co.jp/semicon

http://www.shivatec.com




T ECHNOLOGY A P P L I C A T I O N F O C U S

Until the last couple of years, research into silicon pho-tonics had been moving at snail’s pace. Now things seemto be happening at breakneck speed. “Silicon photonicsis on fire!” is how Graham Reed, head of electronic engi-neering at the University of Surrey, UK, puts it.

“Just look at the number of Nature papers in the pasttwo years,” he said. “There is a daily increase in the num-ber of groups and companies in this field.”

One of those companies is Intel, the biggest player insilicon electronics bar none. But it is also at the forefrontof the emerging silicon photonics research field. InSeptember, it revealed details of an electrically pumpedhybrid III-V/silicon laser, developed in collaborationwith researchers at the University of California, SantaBarbara (UCSB). While many key optical functions havebeen demonstrated on a silicon platform, finding asuitable light-emitting source has posed the biggestproblem – until now, it appears.

According to Intel, the breakthrough addresses oneof the last major barriers to producing silicon photonicsfor use inside and around future computers and data cen-ter architectures. In essence, that means using compo-nents compatible with volume silicon manufacturingmethods to generate, amplify, route, modulate and detectlight in very high speed terabit optical interconnects ofa suitable size for a high-performance computer.

Reed, whose credentials include initiating the fieldof silicon optical integrated circuits in the UK, concurs:“This device has a lot of potential” he said. “There’s nodoubt that it’s certainly a very significant piece of thepuzzle. While it’s not truly a silicon laser, it’s probablythe next best thing”.

The flurry of silicon photonics papers appearing inthe leading science journals over the past couple of years

suggests that with the fundamental building blocks inplace, integration of these various components in an opti-cal system has now become the major challenge.

For example, electro-optic modulators are one ofthese key building blocks. Earlier this year a Danish teampublished details in Nature of just such a device. Usinga silicon nitride layer to compressively strain the activelayer of silicon, Rune Jacobsen from the TechnicalUniversity of Denmark and colleagues discovered thatthis strain induces a linear electro-optic effect that canmodulate light at high speeds. In theory, it could be usedinstead of an electronic “bus” to transport data incomputer architectures.

Unlike some other approaches to optical modulatorsbased on silicon, the device proposed by the Jacobsenteam is not limited by any charge mobility or carrierrecombination effects. They argue that this could be acritical advantage when it comes to commercial deploy-ment of silicon-based photonics because these effectscould limit modulation speeds.

Despite the apparently different material platformsthat the Danish team’s modulator and Intel’s hybrid sil-icon laser are based upon, Reed believes that these twodevices could, in theory, function together as part of anoptical system. “Most of the parts that are emerging arecompatible at a high level because they are almost allbased on a silicon-on-insulator (SOI) platform,” he said.

Intel’s hybrid laser is made by fusing together aAlGaInAs/InPquantum well structure and a silicon stripwaveguide. That waveguide is first formed on the sur-face of an undoped SOI substrate. The “special sauce”that allows the laser to function is the way that the sili-con waveguide and the active AlGaInAs layers bond.

Both surfaces are treated in an oxygen plasma reac-tive ion etch chamber, pressed together and annealed.The materials couple so well that the III-V and siliconelements each form a critical part of the laser structure,with the silicon waveguide fundamental to the lasingaction. The degree of alignment between the two typesof material does not require a particularly high level ofprecision. John Bowers from UCSB has been leadingthe research: “All of the optical mode confinement – inthe laser, in subsequent modulators, in output wave-guides and in a fiber holder – is determined by the sameCMOS lithography step,” he explained.

According to Graham Reed, the complexities of integ-rating lasers, modulators and detectors will always throwup issues of compatibility, particularly when it comesto processing optical and electronic parts in the samelayer of material. For high-end applications capable of40 Gbit/s communications, it will also be necessary todevelop another part – a silicon-based optical isolator –that can be easily integrated into a circuit.

These high modulation frequencies will have to be

Big step forward in silicon integrationThe hybrid III-V/silicon laser manufactured by Intel and its collaborators may not represent true silicon photonics, but it isa significant development in optical interconnects for chip and board communications, discovers Michael Hatcher.

O P T I C A L I N T E R C O N N E C T S

A slice of the hybrid silicon

laser chip. Up to 36 lasers can

be integrated into a single die

and Intel envisages being able

to build future chips with tens or

even hundreds of emitters.

INTE

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TECHNOLOGY A P P L I C A T I O N F O C U S

achieved in Intel’s vision for commercialization of“silicon” photonics, however. Although it is funding abig effort into the development of the hybrid laser, thechip giant’s overall photonics strategy is difficult to under-stand given recent events. In September, Intel sold its opti-cal networking components business (which makescontrol electronics, rather than optical communicationsmodules) to venture-backed newcomer Cortina Systems.

So why the push to develop the hybrid laser? Intelknows that its commercialization at any level is at leastfive years away, but it is already talking about using inte-grated photonic chips as part of its terascale computingresearch program. This program is based on the idea ofmulti-core processing and Intel’s plan is to house tensor even hundreds of these cores onto a single chip.

The problem arises when you try to move this amountof data around using conventional copper interconnects.While copper can support very fast interconnectionspeeds, it can only do this over a very short distance. Todeliver terabit communications, it seems more thanlikely that photonic interconnections will be needed.

But what exactly would such a system look like? Intelhas already proposed its idea of a terabit optical trans-ceiver. It consists of a row of hybrid silicon lasers, all ofwhich are tuned to operate at a slightly different wave-length. The individual wavelengths can be defined byaltering the width of each silicon waveguide and itsgrating pitch, and can be tightly controlled thanks to theprecision that state-of-the-art photolithography allows.

If 25 lasers are used and each of these individual datastreams is modulated at 40 Gbit/s, the result would beterabit per second data transmission from a single chip.Even Intel’s prototype hybrid die featured as many as36 lasers, although only 26 actually lased.

With the hybrid laser and the other building blocksrequired for silicon photonics on the road to commer-cial development, the long-awaited marriage betweenoptoelectronics and computing suddenly appears to bewithin reach. And not, as some had predicted, to theexclusion of compound semiconductors.

INTE

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In its concept of a future

integrated silicon optical

transmitter, Intel proposes using

25 hybrid silicon lasers, each

coupled with a silicon modulator

and multiplexed together into a

single output fiber.

Benefit from reprints of articlesfeaturing your company

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INTE

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The Intel-UCSB team with hybrid silicon laser device –left to right: Hyundai Park, UCSB graduate student; John Bowers,

UCSB professor; Mario Paniccia, director of Intel’s Photonics

Technology Lab; Richard Jones, Intel researcher; and Alex Fang,

UCSB graduate student and former Intel intern.



http://www.optotaiwan.com


T ECHNOLOGY S I C S U B S T R A T E S

Crossed swords over SEMI standards Competition among SiC substrate manufacturers has boosted wafer diameters and eliminatedmicropipe defects in commercial SiC wafers. However, there is still some division over the effectivenessof the standards used to define the quality of the material, as Richard Stevenson discovers.

Intense competition between rival SiC substratemanufacturers has been good news for their customers.Wafer sizes are up and the micropipe defects that killdevices and hit yields have been cut to very low levels,with “zero micropipe” material now available. So doesthis mean that very-high-quality substrates are rou-tinely available from a wide variety of suppliers?

Well, not according to several researchers speakingat the recent European Conference on SiC and RelatedMaterials (ECSCRM) in Newcastle, UK. In theiropinion, today’s substrates contain high-density thread-ing-screw dislocations, basal-plane dislocations andunwanted polytypes, and there is a huge variation inquality, not only between vendors but between manymanufacturer’s nominally identical substrates. Thesediscrepancies are not exposed by the SemiconductorEquipment and Materials Institute (SEMI) specifica-tions, which has led some researchers to claim thatthese standards need updating.

One of these researchers is Tom Ryan, from char-

acterization equipment supplier Accent OpticalTechnologies, now part of Nanometrics. He and hiscolleagues have been assessing the quality of 2 and3 inch 4H polytype substrates from seven differentvendors. The team employed photoluminescence toidentify electrically active defects and X-ray mea-surements that provide a metric for crystalline per-fection. “The results blew us away,” says Ryan. “Thesenominally identical wafers are so different.”

Photoluminescence can distinguish between 4H and6H material, and Ryan has also employed this methodto identify boron-related defects. By altering the powerdensity of the incident laser source he has producedwhole-wafer maps that reveal 6H polytypes and stack-ing faults (see figure 1).

Ryan has also mapped smaller areas using a micro-photoluminescence tool that has a typical spatialresolution of 1 μm. By recording the intensity of the4H band edge emission he has revealed lines that areassociated with crystalline defects such as low-angle

Fig. 1. Photoluminescence

intensity mapping can identify

unwanted 6H polytypes in 4H

SiC substrates (see the whole

wafer map (a) and the

expanded view of a part of this

substrate (b)). These maps were

produced by measuring the

photoluminescence intensity at

392 nm, the peak of the

emission from 4H material.

High-quality regions of the

substrate with just 4H material

produce relatively intense

emission at this wavelength (c).

Regions containing both

polytypes have weaker emission

at 392 nm, and an additional

peak at a longer wavelength due

to the 6H polytype (d).

0.144

0.115

0.087

0.058

0.029

0.000349 408 466 525 584

4H polytype

yellow-band PL

wavelength (nm)

inte

nsity

(a.

u.)

0.110

0.088

0.066

0.044

0.022

0.000349 408 466 525 584

6H polytype

yellow-band PL

wavelength (nm)

inte

nsity

(a.

u.)

4H polytype

(a)

(d)(c)

(b)

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“SEMI standardslack state-of-the-artmeasurementsand are toosimplistic.”Tom RyanNanometrics



TECHNOLOGY S I C S U B S T R A T E S

grain boundaries, threading dislocations and interfa-cial misfit locations (see figure 2).

Ryan’s X-ray rocking curve studies involved whole-wafer mapping of the full-width half-maximum(FWHM) of an X-ray peak (see table 1). This value pro-vides a measure of crystalline quality – perfect crystalshave a theoretical FWHM approaching zero arc sec-onds, and this value increases through dislocations andgrain boundaries that disrupt the lattice. The highestquality regions of the best SiC substrates have a rock-ing curve FWHM width of 14arc seconds, which is onlya little higher than that for typical silicon and GaAsmaterial. However, even these SiC substrates have largeregions with FWHM values of up to 100 arc seconds,where material is typically much more disordered.

Synchrotron measurementsSome of the defects responsible for this disorder havebeen identified by Patrick McNally from Dublin CityUniversity, along with colleagues at the University ofFreiberg, Helsinki University of Technology, GEGlobal Research and the Institute for Synchrotronstudies in Germany. Using a synchrotron source for X-ray analysis, the team investigated 2 and 3 inch sub-strates from five manufacturers and discovered whatMcNally described as “an alarmingly large variationin wafer quality” (see figure 3, p22).

Although SiC substrate and chip manufacturersmight be interested to see McNally’s results, they arehardly likely to install an X-ray synchrotron instrumentin their foundries. “It is not something that you’re goingto find in a company’s backyard,” says McNally. “It’sa scientific tool.” However, SiC manufacturers couldreplicate the technique with standard X-ray kit con-figured for Lang topography, which can map out wholesubstrates for different types of defect at a lower spatialresolution than the synchrotron.

Also at ECSCRM, James Oliver from NorthropGrumman Electronic Systems’Advanced TechnologyCenter presented the results from his survey of SiCsubstrates. He compared material from five vendorsand found that variability in crystal quality is common,even within individual vendors. Although the bestsubstrates are free of macroscopically observabledefects, this is not the norm, according to Oliver.Polytype inclusions, low-angle grains, precipitates andstrain can be seen in many substrates with the unaidedeye using a cross-polarizer technique. Finer details canbe gained using a cross-polarized light method and a

microscope (see figure 4, p22).Oliver has also looked at substrate surface quality

with an atomic force microscope. He believes that thesurface roughness, which is typically less than 0.4 nmand at best 0.2 nm, is acceptable.

In addition to the studies comparing material fromvarious vendors, Kurt Gaskill from the US NavalResearch Laboratory in Washington, DC, presentedthe results of his investigation into 2 inch 4H-SiC sub-strates from Intrinsic, which is now part of Cree. Gaskillfound large variations in the X-ray data and cross-polarization images of three substrates, which wereoff-cut at 4º towards (11–20) and had a micropipedensity of 0–5 cm–2. There was little correlationbetween the X-ray and cross-polarization images,which led him to conclude that cross-polarized lightimages may not be sensitive to crystalline imperfec-tions in high-quality, low-micropipe substrates.

Sub-standard SEMI standards?Although the substrates studied by Ryan, McNally,Oliver and Gaskill have a very wide range of charac-teristics, they all conform to SEMI’s specifications for2 and 3 inch material. This is because the standard doesnot set a value for the micropipe density, which mustbe agreed between customer and supplier, and requiresno tests based on X-ray diffraction or photolumin-escence. Included in the SEMI standards are thesubstrate’s dimensions, including warp and bow figuresand off-axis angle tolerances. SEMI also defines limitsfor the number of visible defects and pits on the sur-faces, the number of cracks in the material and the pro-portion of crystallite area.

Ryan is critical of these specifications: “SEMIstandards lack state-of-the-art measurements and aretoo simplistic.” Arnd-Dietrich Weber, the global taskforce manager for SEMI’s SiC standardization com-mittee and manager of test and metrology at substratemanufacturer SiCrystal, is not so sure: “It is known thatif you look at the market – and the feedback fromcustomers – that material from one vendor is not iden-tical to that from a second vendor. But what is not clearat the moment is the impact of such variations on deviceproduction [and performance].”

Weber’s is a point that all of the researchers agreewith. In fact, many are now in the early stages ofprojects to find out whether there is a correlationbetween substrate quality and the performance of cer-tain devices. Ryan and McNally both expect to find

Fig. 2. Micro-photoluminescence

maps of the intensity of emission

at 392 nm, the band-edge of

4H SiC, can reveal lines

associated with non-radiative

defects. These dark lines identify

low-angle grain boundaries,

threading dislocations and

interfacial misfit dislocations.

Vendor A 75 21.3 14.5 61

Vendor B 75 21.2 15 55

Vendor C 75 267 74 700

Vendor D 75 130 47 275

Vendor E 75 460 118 1300

Vendor F 50 39 16.5 140

Vendor G 50 42 19.5 120

Wafer size (mm) FWHM average (arc sec) FWHM min (arc sec) FWHM max (arc sec)

Table 1. Variations in 4H substrates revealed by X-ray measurements

NA

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“The papers atECSCRM are veryinteresting, butthe SEMIstandards arethe requirementsneeded foreconomicsuccess.”Arnd-Dietrich WeberGlobal task forcemanager, SEMI SiCstandards committee

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TECHNOLOGY S I C S U B S T R A T E S

this link. Ryan believes that the non-4H inclusions –such as basal-plane dislocations, stacking faults andthreading dislocations – increase the leakage currentin Schottky diodes and will influence MOSFETyields.McNally thinks that the threading-screw dislocationswill affect the performance of reverse-biased devices,like photodetectors. Gaskill, however, is less certainand does not want to speculate on the outcome ofinvestigations relating current substrate quality anddevice performance.

Weber points out that research findings can also leadto claims that ignore the economics of substrate pro-duction. “You have two completely different worldsdealing with the same material. The papers at ECSCRMare very interesting, [but] the SEMI standards are therequirements needed for economic success.”

The SEMI standards for SiC substrates were drawnup to reflect what is currently possible, explainedWeber, and to provide a guideline that does nothamper future developments. He asserts that the SiCtask force is not biased towards the material suppli-ers because it has representation from both thesevendors and their customers. “It is a balanced com-position that enables us to make good technicalprogress and produce a healthy document.” Withoutthis document every manufacturer could generate itsown standard and substrate, which would be bad forboth customers and the industry in general.

The current concerns for Weber’s SiC task force arenot the variations in substrate quality that are revealedby X-ray diffraction and photoluminescence, butagreeing a standard for 100 mm substrates and amethod for counting micropipes. Substrate thicknesshas proved the sticking point for the 100 mm standard.

Customers want thinner material to improve deviceperformance, but vendors say that this could lead toproblems associated with warp, bow and other geo-metrical features.

The micropipe counting issue is being addressed byanother organization, the American Society for Testingand Materials (ASTM). “Without a standard in placeit’s impossible to compare the numbers published bydifferent vendors directly,” says Weber. This has ledcustomers to request different micropipe densities fromdifferent known suppliers in order to ensure a consis-tent benchmark for incoming material. Unfortunately,good standardization is a very slow process, oftentaking five years. The ASTM committee has only beenmeeting for two years, so a universal method formicropipe counting will have to wait.

A manufacturer’s perspectiveSiCrystal’s European sales manager Thomas Kippesoffers the most pragmatic perspective. He believes thatthe key test for SiC quality is not the X-ray or thephotoluminescence results, but whether the substrateprovides a platform for high-yield manufacturing.He relates the story of a customer making Schottkydiodes. The customer focused on X-ray diffractionmaps, but observed no correlation with device yield.“Maybe it’s a different story when you make aMOSFET,” said Kippes, before pointing out that acustomer’s SiC experience and its individual processhave to also be considered.

Today’s SiC substrates are far from perfect andriddled with imperfections. But this may not matter.The pertinent question for device makers is “how gooddoes my substrate need to be?”

7000

5000

4000

0vendor A

(n-4H-SiC)vendor B

(n-4H-SiC)vendor C

(n-4H-SiC)vendor D

(n-4H-SiC)vendor E

(n-4H-SiC)vendor B

(n-6H-SiC)vendor A

(p-4H-SiC)

dis

loca

tion

den

sity

(per

cm

2)

3000

2000

6000

1000

1600

1400

1200

1000

800

600

400

0vendor A

(n-4H-SiC)vendor B

(n-4H-SiC)vendor C

(n-4H-SiC)vendor D

(n-4H-SiC)vendor E

(n-4H-SiC)vendor B

(n-6H-SiC)vendor A

(p-4H-SiC)

aver

age

BPD

are

a (p

er c

m2)

200

Fig. 3. Large-area white beam

synchrotron back reflection X-ray

topography can be used to

analyze the density of threading-

screw dislocations (a) and

basal-plane dislocations (b).

Fig. 4. Cross-polarized light

images, which have been

cropped to remove the laser

markings, can reveal various

features on SiC substrates.

Substrates a and b have

micropipe densities of 1.3 cm–2

and 32 cm–2, respectively.

Substrate c, which is of poorer

quality, has many features that

can correlate to the micropipe

map (d). Images courtesy of

Trans Tech Publications (TTP).

(a) (b)

(a) (b) (c) (d)

TTP


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T ECHNOLOGY C O N F E R E N C E R E P O R T

ECSCRM: applications steal the showCommercial applications and market penetration of devices dominated the recent ECSCRM conferencein Gateshead, UK, which was held on the centenary of the first SiC paper. Alton Horsfall reports.

There was a shift in emphasis at the sixth EuropeanConference on Silicon Carbide and Related Materials(ECSCRM). The characteristics of specific substratesand devices stopped hogging the limelight, and issuessurrounding silicon carbide (SiC) in commercial sys-tems emerged as the key talking point.

This theme was taken up by Peter Friedrichs, man-aging director of SiCED, the German joint venturebetween Siemens and Infineon. In his plenary talk heasked whether SiC was a suitable technology for invest-ment. According to Friedrichs, the first commerciallyavailable SiC products suffered from signs of over-heating, caused by circuit developers using the devicesbeyond their rated values.

This problem has been addressed by reducing theinternal electric fields within devices, and the latestproducts now deliver a stability comparable with thatof their commercial silicon counterparts. Friedrichsbelieves that this advance, allied to a doubling in thecost of silicon over the last two years driven by in-creased demand from the photovoltaics industry, haschanged the commercial landscape for SiC. “Shouldwe invest in SiC?” is probably not the pertinent ques-tion any more, argued Friedrichs, but rather “Is nowthe right time to invest?”

In a session that focused on industrial applications,Roger Bassett from energy consultant Areva outlinedthe benefits of using SiC devices in high-voltage func-tions, such as power transmission and distribution. SiC

offers low-loss switching of voltages in excess of 10kVand this provides greater flexibility in the generationand distribution of electrical power. These attributesmake the existing power network a major target for SiCdevices – it could generate $200million in product salesby 2020. SiC devices can offer an additional benefitover silicon, says Bassett, because they can reduce thearea of land needed for construction of large-scalepower conversion facilities.

The use of SiC devices in circuit applications wasdiscussed in other talks by Ty McNutt from NorthropGrumman and Leon Tolbert from the University ofTennessee. Both of these speakers said that the promisedreduction in on-state losses in circuit applications canonly be delivered by making dramatic changes to pack-aging technology that aids heat dissipation. The tworesearch groups have already demonstrated inverter cir-cuits featuring SiC switches and diodes.

At previous conferences the capabilities of SiCdevices at high temperatures has been a strong theme.This year’s meeting was no exception. NASA’s PhilNeudeck revealed that SiC FETs can run at 500 ºC formore than 2400 h, which means they can be deployedinto environments where traditional semiconductortechnology cannot function. These could includeengine exhausts, the surface of Venus and aerospaceapplications such as aircraft. NASAresearchers havealso studied behavior of this type of device in an ampli-fier circuit operating at 500 ºC.

The sixth EuropeanConference on Silicon Carbideand Related Materials,

which took place at The Sage

Gateshead on 3–7 September,

attracted 420 delegates. A total

of 293 papers were presented.

About the authorAlton Horsfall([email protected]) is alecturer in microelectronics atNewcastle University and waspart of this year’s ECSCRMorganizing team.

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TECHNOLOGY C O N F E R E N C E R E P O R T

For several years SiC has also been touted as asuitable technology for power devices operating in highradiation environments, due to the material’s widebandgap. At this year’s ECSCRM Infineon Tech-nologies’Gerald Soelkner presented the first analysisof device failure from cosmic radiation. His workdemonstrates that SiC devices can deliver greaterreliability than their silicon equivalents when themaximum electric field under blocking conditions isreduced enough. When this occurs, the effects of impactionization are not important, says Soelkner.

Another application for SiC devices is gas sensingin extreme environments. Our team at NewcastleUniversity has shown, for the first time, that gas con-centrations can be recorded at high temperatures bymonitoring the leakage current through the sensor’scapacitor. The research shows that sensor arrays canbe built that determine the concentration of variousgases in a mixture. The arrays are made by varying thecapacitor’s dielectric for each detector, which givesevery element a unique and identifiable characteristicthat can be used to trace specific gases.

Improving the oxide interfaceSiC MOSFETs have the potential to deliver a low on-state resistance but progress has been limited by the qual-ity of the SiC/SiO2 interface. Typical device mobilitiesare 40 cm2V–1s–1, which is significantly lower than thesilicon equivalents due to the high density of near-inter-face traps near to the conduction band edge.

Einar Sveinbjornsson, from Chalmers University,Sweden, said that this trap density is related to the pres-ence of sodium in the oxide, which causes many deepinterface traps (typically 1 × 1013 cm–2) that can lead toa large hysteresis in capacitance-voltage characteris-tics. The density of near interface traps can be reducedby oxidation in alumina because the sodium that is pres-ent from the processing converts these defects into deeptraps, which can then be reduced by annealing in hydro-gen gas. With this approach MOSFETs can be producedwith mobilities in excess of 100 cm2V–1s–1.

While applications took center stage this year,progress in the quality and size of SiC substrates still

arouses intense interest. Cree, which acquired IntrinsicSemiconductor this summer, picked the event to launchits zero micropipe material. Cem Basceri, from Cree’sDulles site in Virginia that previously housed Intrinsic,says that the micropipes are linked to polytype inclu-sions, which propagate in the [11–20] direction. Withcareful control of the growth conditions the formationof these inclusions is suppressed and the micropipedensity can be reduced to zero, says Basceri. The com-pany is now transferring its technology to a 100 mmplatform. Recent efforts have led to 100 mm conduct-ing substrates with just seven micropipes and high-purity semi-insulating material of the same size withmicropipe densities of 2.5 cm–2.

Substrate quality is also improving at SiCrystal, saysThomas Straubinger, the firm’s leader in the researchand development of crystal growth. The German out-fit has observed that substrate quality is dependent onthe initial seed and has improved its production processby switching to a new generation of 3 inch seed crys-tals. Straubinger revealed that these efforts have con-centrated on reducing the substrate’s basal planedislocation density because this type of defect has thegreatest impact on power devices. The basal plane dis-locations are linked to stress generated during thegrowth process and they can be substantially reducedby minimizing temperature gradients.

II-VI, a US manufacturer of SiC substrates, alsoreported improvements in material quality. AndySouzis, the technology and program manager for thecompany’s wide bandgap group, claimed that II-VI hascut the density of all types of dislocation in its 3 inchmaterial by two orders of magnitude over the last12 months. Typical values for comet counts are now31 and total dislocation densities average 7 × 103 cm–2.

It is a safe bet to assume that further improvementsin SiC material will be reported at the next InternationalConference on Silicon Carbide and Related Materials,which will be held in Nara, Japan, in October 2007.However, as production of this material matures andhigh-quality substrates become the norm rather thanthe exception, the increasing focus on applications issurely set to continue. ●

The Sage Gateshead, by the River Tyne in Gateshead, was the venue for this year’s ECSCRM conference. The building opened in 2004.

This year’s ECSCRMmarked the centenary ofSiC publications with areview by Nick Wright fromNewcastle University.Some of the majorbreakthroughs over thelast 100 years include:● 1906 Henry Dunwoodypatents a SiC-based waveresponsive or detectingdevice’, which couldreplace magnetic ‘coherer’devices in radio receivers.● 1907 First observationof the semiconductor LED,which was fabricatedusing SiC. Captain JosephRound, personal assistantto Marconi, noticed thatyellow light was emittedfrom certain crystals whenbiased at typically 100 V.● 1908 Marconi used thewave responsive device inhis transatlantic telecomssystem.● 1925 The waveresponsive device waslaunched commercially byCarborundum Corporation.● 1955 A method to growSiC crystals of sufficientquality for electronicdevices was developed byLely. However, thesehexagonal crystals werethe wrong shape forsemiconductor processing.● 1960s The firstcommercial SiC LEDs werereleased, just a few yearsbefore the launch of III-Vdevices.● 1978 Tairov andTsvetkov from the IoffeInstitute developed aseeded crystal growthtechnique for producingcircular SiC substrates.● 1991 Cree released thefirst commercial SiC wafers.● 2001 Infineon launchedthe first SiC Schottkydiodes.● 2006 Cree shipsmicropipe-free substrates.

100 years of SiCpublications

CH

RIS

WIL

SO

N



T ECHNOLOGY E Q U I P M E N T U P D A T E

Oxide apertures feature in most of the VCSELs deployedin optical mouse sensors and data communicationnetworks. The structures deliver optical and currentconfinement (see figure 1), and are added by oxidationin a furnace with a controlled water vapor source.

The water vapor is generated by a bubbler or vapor-izer. However, Jeff Spiegelman, president of Californianstart-up Rasirc, says that VCSEL production yield andthroughput improvements can be made if manufactur-ers switch to his company’s steam purification system.“We are able to deliver [water vapor] at a rate and com-position that gives us an advantage over everybody.”

The water vapor source provides the furnace with acontinuous stream of gaseous water at a well definedtemperature and pressure. If this uniformity is achieved,all of the VCSELs on the various processed wafers areoxidized identically and yields are very high.

Spiegelman believes that bubbler-based water vaporsources compromise this uniformity. Water is absorbedduring the process, but the carrier gas – typically nitro-gen or argon – is not consumed, which leads to changesin the composition and pressure of the gas. These vari-ations alter the oxidation rate, resulting in non-uniform

oxidation across the wafer. “The more nitrogen you havein your bubbler, the slower the growth rate and the worsethe uniformity,” claims Spiegelman.

Bubblers also suffer from contaminants in the gasstream. Any metallic or ionic contaminant in the gas canbe absorbed by the oxide layer, which reduces the film’sinsulating properties and impacts the VCSEL’s opticaland current confinement.

According to Spiegelman, vaporizers, which are alsoknown as clean steam systems, can also produce highlevels of contamination. This is because metallic partscan corrode and release ions. In addition, their perfor-mance is hindered by incomplete vapor formation. “Thewater drops onto a heated surface and you get somevapor, but you also get a clump of water molecules thathaven’t gone into the vapor state,” explains Spiegelman.“When they go into the furnace they vaporize. [Youneed] a lot of energy to go from a liquid to a gas, whichcan lead to non-uniformity.”

Spiegelman believes that Rasirc’s Intaeger ultrahighpurity (UHP) steam purification system, which waslaunched six months ago and is based around a non-porous hydrophilic membrane, can address the short-comings of vaporizers and bubblers. The machine canbe fed with de-ionized water, which is relatively cheap,and contamination is also restricted by the membrane’sselectivity. The membrane allows water molecules topass through a million times more quickly than nitrogenmolecules, thereby providing a source that is free froma carrier gas while removing contaminants such as othergases, ions, organic material, particles, viruses, bacteriaand metals. Tests have shown that the Intaeger systemcan reduce 67 different metal contaminants to levelsbelow detectable limits, and “virtually eliminate” nitro-gen and carbon dioxide.

Rasirc is yet to sell its first UHP Intaeger steam gen-erator to a VCSEL manufacturer, but it has increasedoxidation rates at three silicon fabs. All of them haveseen an improvement in growth rate of at least 14%, saysSpiegelman. One fab reported a hike of 18%, which isequivalent to a time saving of four days a month. “Thattranslates to three months or less for payback on thetool.” Oxidation uniformity has also improved: one cus-tomer went from ±5% at best to ±1.8 %, based on datafrom three lots of 25 wafers, which included wafer-to-wafer, across the wafer and across the furnace variations.

Rasirc’s steam generator is more expensive than abubbler and closer to the price of a vaporizer. However,Spiegelman points out that once the capital expense hasbeen met, running costs are as low as $20 a year for thewater, aside from the utility bill for boiling it. “And inthese high-volume applications, like VCSELs, that costdifferential is going to be critical.”

Steam generator purifies oxidationA Californian start-up claims that its ultrapure steam generator can boost the production yield and speed the oxideaperture process used widely in VCSEL manufacturing. Richard Stevenson talks to the tool’s inventor, Jeff Spiegelman.

V C S E L P R O C E S S I N G

aperture

active regionoxide extent

sacrificialAl layer

Fig. 1. Water vapor reacts with

the sacrificial aluminum-rich

layer to produce an oxide that

forms on the outside of the

device first, and moves towards

its center. The rate of this step is

key as it defines the size and

position of the aperture – the

remaining un-oxidized

material. A uniform flow of

high-purity water vapor ensures

consistency between devices

and a high yield.

Rasirc’s Intaeger ultrahigh steam purification system provides a controlled water vapor source for

VCSEL oxidation that has only a few contaminants and is free from a carrier gas.



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Optoelectronics manufacturer Osram and theUniversity of Duisburg-Essen, Germany, havemade the first detailed investigation of defectsin red-emitting AlGaInPlaser diodes generatedduring catastrophic optical damage (COD).

Understanding the formation of these def-ects, which are often seeded by enhanced non-radiative recombination due to surface statesat the laser’s facets, could aid the performanceof AlGaInPlasers in high-power applications.

COD damage is caused by an avalanche-likeprocess, which occurs at either the mirror facetor the adjacent semiconductor material. “Itsoccurrence is accelerated by local temperatureat the mirror facet, the local current density inthat region and the optical power density at the

mirror,” said Osram’s Peter Brick.The team conducted its investigation on

MOCVD-grown 650 nm lasers 1200 μm longand 100 μm wide, which were grown on (100) GaAs substrates cut at 6° towards the(111) direction.

COD was instigated by turning up thecontinuous-wave driving current until the

laser’s power output suddenly dropped. Then-doped AlGaInPcladding layer was exposedby selective wet etching and then a 488 nmlaser was employed to map the surface’s photo-luminescence with a spatial resolution of 1μm.This revealed that highly nonradiative defectsoriginate from the front mirror of the laser,propagating in a direction perpendicular to theoutput facet for more than half the resonatorlength (see figure).

The team then used focused ion beammicroscopy to examine the epitaxial layers thatwere affected by COD. Most of the defects werein the InGaP single quantum well active layerbut some veered into the AlGaInPwaveguides.The researchers also found that the defects werenoncrystalline and that they can sometimes startfrom multiple locations on the facet.


T ECHNOLOGY R E S E A R C H R E V I E W

Researchers from the Chinese Academy ofSciences’ Institute of Semiconductors haveboosted the detection efficiency of 1.55 μmGaAs photodetectors by more than 10 timesby adding a resonant cavity into the structure.

This aids the development of GaAs-basedreceivers for optical fiber communication,which are preferable to the complex hybriddevices in use today that use silicon electron-ics and germanium or InP-based photonics.

Absorption in the infrared is made possibleby the existence of midgap arsine-adatomdefects in low-temperature-grown GaAs.

Until now, the low absorption coefficient fortransitions between the midgap defect stateand the conduction band has only producedefficiencies of 0.2–0.5 mA/W. The ChineseAcademy researchers have increased thisdetection efficiency to 7.1 mA/W by addingtwo GaAs/AlAs-distributed Bragg reflectorsthat produce constructive interference andcapture more of the incident light.

The team introduced the midgap defects ina GaAs absorption layer by growing the mate-rial by MBE at 200 °C and at a growth rate of0.7 μm/h. Standard lithography was then used

to fabricate the device, which had top and bot-tom reflectors with 25 and 15 pairs of undopedAlAs/GaAs quarter-wave stacks, respectively.

According to the research group’s leader,Qin Han, the addition of a resonant cavitywould not add much to the detector cost,although as yet there are no plans for com-mercialization. “The structure of the pho-todetector is similar to a VCSEL – it can begrown all at once by MBE or MOCVD.”

Chinese Academy of Science researchers boostGaAs photodetector efficiency by factor of 10

D E T E C T O R S

Journal referenceQ Han et al. 2006 Appl. Phys. Lett. 89 131104.

Rensselaer Polytechnic Institute, NY, in part-nership with the Samsung Advanced Instituteof Technology, South Korea, has increased thelight extraction of its InGaN 400 nm LEDs by38% by adding a GaN/SiO2/Al omnidirec-tional reflector (ODR).

The team says that its latest ODRs are bet-ter suited to GaN emitters than silver mirrors,which suffer from low reflection and a highindex of refraction in the near-ultraviolet.

Aluminum-based ODRs do not suffer thesame limitations, although they do exhibit poor

conductivity. However, the researchers over-came this by using an array of NiZn/Ag micro-contacts that reduced the resistance betweenthe layers of p-type GaN and aluminum.

The InGaN LED structures were grown byMOCVD on sapphire substrates, and standardphotolithographic patterning and etching was used to form the 300 × 300 μm mesas.105 square microcontacts with dimensions of4×4μm, 6×6μm and 10×10μm, were patter-ned onto a quarter-wavelength-thick SiO2 layerand etched to expose the p-type GaN layer.NiZn/Ag microcontacts were then depositedand annealed, before the final aluminum layerwas deposited by electron-beam evaporation.

At an injection current of 20 mA, the GaN/SiO2/Al ODR LED delivered an average of16% and 38% higher light output than similar

LEDs fabricated with SiO2/Ag ODRs andsilver reflectors, respectively. At higher cur-rents the effect was less noticeable. The sizeof the microcontacts also had a marked impact– devices with 10 μm wide microcontacts hada greater light output than those with micro-contacts 6 μm or 4 μm wide.

Fred Schubert of Rensselaer thinks that theupper limit for the surface coverage of themicrocontacts would be about 75% of the totalarea: “Beyond 75% it may be better to useregular broad-area metal contacts.” he toldCompound Semiconductor.

The ODR has now been licensed to an undis-closed US start-up firm for commercialization.

Reflector increasesInGaN LED emission

D E V I C E D E S I G N

Journal referenceJ K Kim et al. 2006 Appl. Phys. Lett. 89 141123.

Journal referenceM Bou Sanayeh et al. 2006 Appl. Phys. Lett. 89

101111.

Microphotoluminescence mapping reveals that

enhanced nonradiative defects start at the output laser

facet and then divide into a number of branches.

Osram investigatesnonradiative defects

R E D L A S E R S

laser facet


http://www.riber.com


http://www.veeco.com/nitrades

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