Evaluation of graphene as a transparent electrode in GaN ...

U M E Å U N I V E R S I T Y

linus johansson

[email protected]

Evaluation of graphene as a transparent

electrode in GaN-based LEDs by PECVD

synthesis of graphene directly on GaN

Examiner :Thomas Wågberg, University Lector

Supervisors:Jie Sun, Assistant Professor

Åsa Haglund, Associate Professor

May 4, 2016

Linus Johansson: Evaluation of graphene as a transparent electrode inGaN-based LEDs by PECVD synthesis of graphene directly on GaN, ©2016

A B S T R AC T

evaluation of graphene as a transparent electrode

in gan-based leds by pecvd synthesis of graphene di-

rectly on gan

A transparent conductive electrode (TCE) is an important component inmany of our modern optoelectronic devices like photovoltaics, light emit-ting diodes and touch screens. These devices require good current injectionand spreading as well as a high transparency. In this thesis we explore theuse of graphene as an alternative to the current widely used indium tinoxide (ITO) as TCE in gallium nitride (GaN) based light emitting diodes(LEDs). Monolayer crystalline graphene can be produced on copper foilsusing chemical vapor deposition (CVD), where metals (especially copper)has a catalysing eect on the formation of graphene. However, transferof graphene from copper foils is not suitable for an industrial scale andit results in a poor contact with the target substrate. We investigate thepossibility of directly integrating graphene on GaN-based LEDs by usingplasma-enhanced chemical vapor deposition (PECVD). We try to obtainthe optimal conditions under these catalyst-free circumstances and pro-pose a recipe adapted for the setup that we used. We will also study ideasof using a metal (we tried copper and nickel) to assist the direct growththat could help to increase the fraction of sp2 carbon bonds and reducethe sheet resistance. The metals are evaporated onto our samples eitherbefore or after we grow a carbon film to either assist the growth or rear-range the carbon respectively. The focus was not on trying to optimizethe conditions for one metal treatment but rather to briefly explore multi-ple methods to find a suitable path for further studies. The direct grownpristine carbon films shows indications from Raman measurements of be-ing nanocrystalline graphene with a sheet resistance ranging from about20–50 k/ having a transmittance of approximately 96 % at 550 nm. Atransmittance at this level is closely related to the value of an ideal mono-layer graphene, which indicates that our carbon films could be close toone atom in thickness while being visually homogeneous and complete incoverage. Due to the use of a temperature close to the melting point ofcopper we struggled to keep the assisting copper from evaporating too fastor staying homogeneous after the treatment. Nickel has a higher meltingtemperature, but it appears as if this metal might be diusing into theGaN substrate which changes the properties of both the GaN and carbonfilm. Even though the metal treatments that we tested did not provideany noticeable improvements, there is need for further investigations toobtain suitable treatment conditions. We suggest that the treatments in-volving copper are a more promising path to pursue as nickel seem tocause unavoidable intermixing problems.

iii

S A M M A N FAT T N I N G

utvärdering av grafen som transparent elektrod i gan-

baserade leds genom pecvd-syntes av grafen direkt på

gan

En transparenta ledande elektrod (TCE) är en viktiga komponent i mångaav våra moderna optroniska enheter som solceller, lysdioder och pekskär-mar. Dessa enheter kräver en god ströminjektion och -spridning samten hög transparens. I detta examensarbete undersöker vi användning avgrafen som ett alternativ till det nuvarande ofta använda indiumtennoxid(ITO) som TCE i galliumnitrid-(GaN)-baserade lysdioder (LEDs). Mono-lager kristallint grafen kan framställas på kopparfolie genom kemisk ångde-ponering (CVD), där metaller (särskilt koppar) har en katalyserande ef-fekt vid bildandet av grafen. Dock är överföringen av grafen från koppar-folier inte lämpligt för en industriell skala och det resulterar i en dåligkontakt med målsubstratet. Vi undersöker möjligheten att direkt integr-era grafen på GaN-baserade LEDs med hjälp av plasma-förstärkt kemiskångdeponering (PECVD). Vi försöker uppnå optimala förhållanden underdessa katalysatorfria omständigheter och föreslår ett recept anpassat föruppställningen som vi använde. Vi kommer också att studera idéer om attanvända en metall (vi testade koppar och nickel) för att underlätta dendirekta tillväxten som kan bidra till att öka andelen sp2-kolbindningaroch minska ytmotståndet. Metallerna förångas på våra prover antingenföre eller efter vi växer en kolfilm för att antingen hjälpa tillväxten re-spektive arrangera om kolet. Fokus var inte på att försöka optimera förut-sättningarna för en metallbehandling utan att kortfattat undersöka flerametoder för att hitta en lämplig väg för fortsatta studier. De direktod-lade ursprungliga kolfilmerna visar antydningar från Ramanmätningar avatt vara nanokristallint grafen med en ytresistans omkring 20–50 k/och en transmittans av cirka 96 % vid 550 nm. En transmittans på dennanivån är nära relaterad till värdet av ett idealt monolager grafen, vilkettyder på att våra kolfilmer kan vara nära en atom i tjocklek samtidigt somdet är visuellt homogent och med en fullständig täckning. På grund av an-vändningen av en temperatur nära smältpunkten för koppar fick vi kämpaför att hålla den assisterande kopparen från att avdunsta för snabbt ellerförbli homogen efter behandlingen. Nickel har en högre smälttemperatur,men det verkar som om denna metall kan diundera in i GaN-substratetvilket ändrar egenskaperna hos både GaN-substratet och kolfilmen. Ävenom metallbehandlingarna som vi testade inte gav några märkbara för-bättringar, finns det behov av ytterligare undersökningar för att uppnålämpliga behandlingsförhållanden. Vi föreslår att de behandlingarna medkoppar är en mer lovande väg att fullfölja eftersom nickel verkar orsakaoundvikliga sammanblandningsproblem.

iv

C O N T E N T S

1 introduction 11.1 Previous studies . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 theory 32.1 Gallium nitride . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 GaN light emitting diode . . . . . . . . . . . . . . . 32.2 Chemical vapor deposition . . . . . . . . . . . . . . . . . . . 4

2.2.1 Plasma-enhanced chemical vapor deposition . . . . . 52.3 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3.1 Electrical properties . . . . . . . . . . . . . . . . . . 72.3.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Characterization techniques . . . . . . . . . . . . . . . . . . 102.4.1 Raman spectroscopy . . . . . . . . . . . . . . . . . . 102.4.2 Transmittance . . . . . . . . . . . . . . . . . . . . . 122.4.3 Electrical characterization . . . . . . . . . . . . . . . 12

3 experiments and results 143.1 CVD setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2 PECVD growth and equipment handling . . . . . . . . . . . 143.3 Metal treatments . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3.1 Metal evaporation prior to growth . . . . . . . . . . 233.3.2 Metal evaporation subsequent to growth . . . . . . . 23

4 discussion and conclusions 264.1 Direct growth on gallium nitride . . . . . . . . . . . . . . . 264.2 Metal treatments . . . . . . . . . . . . . . . . . . . . . . . . 284.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.4 Suggested future work . . . . . . . . . . . . . . . . . . . . . 29

references 30

a plasma waveform analysis 33

b supplementary experimental details 34b.1 Dicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34b.2 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34b.3 Growth optimization . . . . . . . . . . . . . . . . . . . . . . 34b.4 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . 36

b.4.1 TLM fabrication attempts . . . . . . . . . . . . . . . 38

v

L I S T O F F I G U R E S

Figure 1 Simplified process schemes of the two metal treat-ments tested in this work. . . . . . . . . . . . . . . 2

Figure 2 Simplified schematic drawing of a GaN LED struc-ture. . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3 Principle of a plasma-enhanced chemical vapor de-position system. . . . . . . . . . . . . . . . . . . . . 5

Figure 4 The hexagonal lattice formation of a graphene crys-tal with lattice vectors a

1

and a

2

indicated. . . . . 6Figure 5 Electron configuration of a carbon atom and the

sp2-hybridization. . . . . . . . . . . . . . . . . . . . 7Figure 6 Dispersion relation of graphene. . . . . . . . . . . . 8Figure 7 The dierent types of scatterings. . . . . . . . . . . 11Figure 8 Illustration of the two-terminal and four-terminal

resistance measurement setup. . . . . . . . . . . . . 13Figure 9 Illustration of the reaction chamber of the CVD

system used. . . . . . . . . . . . . . . . . . . . . . . 15Figure 10 Snapshot of the user interface for the CVD equip-

ment. . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 11 Temperature progression during a typical experi-

ment. . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 12 Raman spectrum obtained from a sample grown

with the recipe in Table 1. . . . . . . . . . . . . . . 18Figure 13 Optical microscope pictures showing the metal con-

tacts connected to the Hall Bar (left) and the maskof the Hall Bar (right). . . . . . . . . . . . . . . . . 19

Figure 14 Raman spectrum on the carbon Hall Bar as well asbeside it. . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 15 I-V characteristics the Hall Bar devices from oneof the samples. . . . . . . . . . . . . . . . . . . . . . 21

Figure 16 Sheet resistance of similar transmittance measuredon Hall Bar devices. . . . . . . . . . . . . . . . . . . 21

Figure 17 Transmittance measurement for 10, 20 and 40 mingrowth. . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 18 Sheet resistance and Raman D/G intensity ratiobefore and after 400 nm Cu treatment. . . . . . . . 24

Figure 19 Sheet resistance before and after 100 nm Cu treat-ment. . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 20 The plasma voltage waveform and its frequencyspectra. . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 21 Tilted SEM image of GaN exposed in high temper-ature growth of 940 ¶C. . . . . . . . . . . . . . . . . 36

Figure 22 The process steps in the fabrication of Hall Bardevices with photolithography. . . . . . . . . . . . . 37

vi

Figure 23 Optical microscope images of TLM photoresist pat-terns. . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 24 Optical microscope images of TLM pattern aftermetal lift-o attempt. . . . . . . . . . . . . . . . . . 39

L I S T O F TA B L E S

Table 1 Growth parameters acquired from testing numer-ous recipes. . . . . . . . . . . . . . . . . . . . . . . 18

Table 2 Growth parameters acquired for the semi insulat-ing GaN wafers. . . . . . . . . . . . . . . . . . . . . 19

Table 3 First recipe. . . . . . . . . . . . . . . . . . . . . . . 35Table 4 Recipe introducing hydrogen. . . . . . . . . . . . . 35Table 5 Recipe combining hydrogen and methane. . . . . . 36

vii

1I N T RO D U C T I O N

Gallium nitride (GaN) based optoelectronic devices has gained an in-creased attention in recent years as it has opened up a gate to high bright-ness blue and white light in the area of solid state lighting. A transparentconductive electrode (TCE) is a necessary component in a GaN-basedlight emitting diode (LED) to enable uniform current injection and e-cient light extraction. Indium tin oxide (ITO) is today widely used for thispurpose but as indium is a very scarce material with increasing prices aswell as its restricted potential in flexible devices, there is a demand of anew material that can replace it. Not many materials known today pos-sesses both high transparency and conductivity simultaneously, but somematerials used for this are for instance metal grids, carbon nanotubes andnanowires. Graphene, which is completely made of carbon, is an idealcandidate with almost all the right qualities, that could also have the po-tential to outperform ITO [1]. However, the conductivity of syntheticallyproduced graphene is much lower than that of perfect graphene, hencethe conductivity dierence between fabricated graphene and ITO is stilllarge.

Today, graphene is most successfully synthesized by chemical vapor de-position (CVD) on copper foils, where the metal acts as a catalyst. Thismethod can generate samples of rather homogeneous monolayer grapheneover quite large areas [2]. To be able to use such graphene films on LEDs,a transfer process is required. This is however not a an ideal method as itresults in a poor contact graphene and GaN, as well as it requires a compli-cated transfer process which is not optimal to incorporate in an industrialscale utilization. Therefore, a viable way to directly grow graphene on GaNis desirable. The current techniques are still to be improved and there areseveral challenges to overcome as GaN does not provide any catalytic eectlike copper does. Furthermore, GaN does not cope well with temperaturescommonly used for growing graphene, which is typically 1000 ¶C or morewhen using purely thermal CVD. On the other hand, a plasma-enhancedCVD (PECVD) system uses the assistance of plasma to add an extrasource of energy that allows the system to operate at a lower temperatureand is therefore a promising technique to use with temperature sensitivematerials.

1.1 previous studies

Synthetically produced graphene using CVD on copper and other metalshave been thoroughly studied. To fabricate graphene on any semiconduct-

1

1.2 objective

ing substrate would be a great accomplishment for graphene applications.A first attempt to produce graphene as a transparent electrode on GaNby direct growth was made by Sun et al. [3]. They used thermal CVD andachieved graphitic carbon films of controllable thicknesses. More investi-gations have been made since then [4, 5]. Other low temperature growthusing PECVD have also been studied on for instance various dielectricsubstrates [6] and copper [7, 8]. The use of copper as an assisting materialin direct growth on dielectric materials have also been tested [9].

1.2 objective

This work aims to pursue a method of direct integration of a graphene-likecarbon film as TCE on GaN without the need of a transferring process.Such a method might also be applicable to other semiconducting sub-strates.

We want to continue the work by Sun et al., where the first step will beto develop a recipe using PECVD, similar to the work made by Kim et al.[5]. We will need to learn how to handle the PECVD equipment and thenlook at the eect of dierent process parameters and try to achieve thebest possible growth conditions. After that we will continue by exploringadditional treatments that can provide improvements to the carbon film.One of the ideas is to evaporate a thin layer of metal (on the order of afew 100 nm) onto the GaN prior to growth in order to trigger a catalyticeect. The carbon could then ideally diuse through the metal and forma graphene layer in between the metal and GaN as well as on top of themetal. The principle of this treatment is illustrated in Figure 1 as the firstscheme at the top. A second idea is to evaporate a metal layer after thegraphene has been grown and reheat it in the CVD chamber. The desiredeect is that this would rearrange and repair the pre-grown carbon filmto increase the grain sizes and fraction of sp2 bonds. The metal layercould then be etched by a chemical metal etchant and hopefully leave thecomplete structure undamaged and intact. This treatment is illustratedin the second scheme at the bottom of Figure 1.

1.

2.

Figure 1: Simplified process schemes of the two metal treatments tested in thiswork. The dierent materials represent GaN (as the bottom layer),metal and graphene. The metals are evaporated onto our samples eitherbefore (1) or after (2) we grow a carbon film to either assist the growthor rearrange the carbon respectively.

2

2T H E O RY

In this chapter we will present the theory that is necessary to be able toconduct the experiments and interpret the findings as well as understandthe benefits and limitations of graphene as TCE. The key to interpret-ing the results will be based on the comprehensive understanding of allprocess steps, including both sample fabrication and characterization tech-niques. Sample fabrication here refers to chemical vapor deposition, metalevaporation, etching and photolithography. Photolithography is neededto construct devices on samples which makes some characterization tech-niques more accurate. The main characterizations used in this work areRaman spectroscopy, optical transmittance and sheet resistance includingfour-terminal measurement.

2.1 gallium nitride

Gallium nitride (GaN) is a wide band gap semiconductor where the bandgap energy is E

g

= 3.4 eV. Furthermore, it has a direct band gap, meaningthat the maximum energy of the valence band and the minimum energy ofthe conduction band appear at the same k-values in momentum space. Asemiconductor with these qualities is well suited for optoelectronic appli-cations to emit light from the blue region of the electromagnetic spectra.In fact, the discovery of blue LEDs using GaN by Nakamura, Akasaki andAmano was a major breakthrough for bright and ecient lighting whichled to the Nobel prize in Physics 2014 [10].

Gallium nitride has a Wurtzite crystal structure, meaning that it has ahexagonal symmetry. The unit cell is tetrahedral with a lattice constantof 3.19 Å [11].

2.1.1 GaN light emitting diode

Very simplified, a LED consists of a p-n junction, i. e. a semiconductorwhich is n-type doped on one side and p-type doped on the other side.This way of doping causes a depletion region to form due to the exchangeof loosely bound charges on each side, and a built-in potential arise inthe depletion region. When a device is connected with the p-type side toa positive electrode and the n-type side to a negative electrode, calledforward bias, it will emit light at a certain threshold voltage caused bythe recombination of the injected electrons and holes.

P-type GaN has a higher conductivity than undoped GaN (u-GaN) andn-GaN. This causes the p-type material to mainly conduct electricity in

3

2.2 chemical vapor deposition

the longitudinal direction. Therefore one typically need a layer that canspread the current laterally from the positive terminal but at the sametime have a good transparency to the emitting light. Without a transpar-ent conductive electrode (TCE) the injected carriers will gather near thecontact and mainly just transport in the longitudinal direction which lim-its the device usage significantly. A simplified GaN LED with grapheneas a current spreading layer can be seen in Figure 2. Very few materials

p-GaN

MQW

n-GaN

Graphene

Electrode

Electrode

Figure 2: Simplified schematic drawing of a GaN LED structure with grapheneas a transparent current spreading layer.

known today have both high transparency and good electrical conductiv-ity. Most transparent materials are either insulators or semiconductorssince they have a band gap that is wider than the energy of visible light.However, there are a few materials that possesses both these propertiesand one of them is graphene. Another material which is widely used asTCE at the moment is indium tin oxide (ITO). Although this materialmight have a low sheet resistance (often < 100 /), it transmits light inthe blue and ultra-violate range poorly. Moreover, it is costly and it is alsoa brittle material which limits its usage in flexible optoelectronic devices[12]. Graphene can compete on all these aforementioned aspects and onsome points even outperform ITO in theory (the properties of graphenewill be considered further in Section 2.3). However, the flexibility of theTCE in GaN-based devices is of no significance since GaN is not a flexiblematerial in the first place. Therefore this particular property has no valuein this project.


Chemical vapor deposition (CVD) is a bottom-up approach in nanofab-rication. Bottom-up means that one adds material to a target substrateopposed to a top-down approach which by implication removes material.In a general sense, CVD can be described as a process where a solid isdeposited onto a target surface (referred to as substrate) by a chemicalreaction with a substance in its gas phase [13]. CVD is a widely used tech-nique in nano- and microfabrication to produce various structured mate-rials, mostly thin films. It relies on self-assembly of the supplied buildingblocks into the desired structure, obtained by well-controlled conditions,such as temperature, pressure, gas flows, etcetera. The building blocks areprovided as a gas, often called precursor or deposition gas, which has tocontain the desired building element.

To fully understand the principles of CVD one needs good knowledgein several areas, such as thermodynamics, fluid mechanics, plasma physicsand chemical reactions [14]. But fortunately, we do not have to dig too

4


deep into these theoretical aspects as we are more interested in the workingprinciple of the system.

2.2.1 Plasma-enhanced chemical vapor deposition

There are numerous variants of CVD, but the one we are using and there-fore will consider is plasma-enhanced CVD (PECVD). The general princi-ple of a PECVD system (and also a standard CVD system when excludingthe plasma) is illustrated in Figure 3. The conventional CVD uses thermal

SubstrateHeater

Reaction

Energy

PlasmaResidual gasesEnergy

Thermal

Supplied gases

Figure 3: Principle of a plasma-enhanced chemical vapor deposition system.

energy to initiate the reaction. In PECVD the addition of plasma con-tributes to another source of energy which allows the system to operateunder lower temperatures. A plasma can be generated by subjecting a gasto an external electric field under low pressure. This can be done by apply-ing an electric potential dierence between two electrodes where the gasis located in between the electrodes. As the electric field becomes strongenough, electrons are able to separate from the gas molecules, which isthen called a plasma. The plasma will then glow as some electrons relaxesto a lower energy level, which causes a release of photons. The generatedradicals are then more susceptible to form chemical bondings with thesurface because of their unpaired valence electrons. Since the fundamen-tal principles and reactions in PECVD are much more complex comparedto conventional CVD, it is dicult to fully understand the connectionsbetween process parameters and the properties of the deposited material[14]. For this reason, one have to experimentally investigate the influenceof dierent parameters to come up with the right conditions for dierentsetups.

In this work, the PECVD system is investigate as a viable method togrow graphene on GaN with hopes that it will keep the damaged of GaNat a minimum and at the same time yield as high quality as possible for adirect growth approach. The GaN can take damaged from either too hightemperature or if the plasma power is too high. A temperature of morethan ≥1000 ¶C can cause roughness in the GaN surface and in the case ofa too high plasma power it can cause severe bombardment of ions ontothe substrate, which can result in a dissociation of the nitrogen elementfrom the surface and a modification in the band structure. Therefore, itis important to take this into consideration and not use too high temper-

5

2.3 graphene

ature or plasma power to ensure minimal damage. The plasma induceddamage have been investigated by Choi et al. who found that several Ra-man modes was related to these defects. They discovered that the peaks at300 cm≠1 and 360 cm≠1 are related to disorder-activated scattering and thepeaks at 453 cm≠1 and 639 cm≠1 are due to vacancy scattering [15]. Thefundamentals of Raman spectroscopy will be considered in Section 2.4.1.

2.3 graphene

Graphene is a quasi-two-dimensional nanomaterial and it is an allotropeof carbon, formed as a honeycomb hexagonal lattice only one atom inthickness, equivalent to one layer in a graphite crystal. An illustration ofthe graphene lattice is presented in Figure 4. Graphene is the final piece

a

1

a

2

Figure 4: The hexagonal lattice formation of a graphene crystal with lattice vec-tors a

1

and a

2

indicated.

that completes the gap in carbon materials, with 0D fullerenes, 1D carbonnanotubes to 3D bulk graphite crystal. It has been attracting attention inboth applications as well as in more fundamental research [16]. It displaysremarkable optical, electrical, thermal and mechanical properties. Eventhough every individual property might not be particularly extraordinaryby them self, but rather the fact that this material combines all of themis very rare and noteworthy. Graphene behaves almost like a metal interms of its electrical properties. In principal it has a very high mobility,estimated to be limited at 2 ◊ 105 cm2/(V s) [17]. Meaning that electronsmove with very little scattering and can be accelerated easily in an ex-ternal electric field. With a realistic carrier density of 1012 cm≠3 [17] itsconductivity would be ≥108 S/m, which in turn would be higher than thatof any metal. But unlike metals, graphene is highly transparent and theoptical transmittance can be predicted with electrodynamics theory to97.7 % per layer. Interestingly this has also been observed by measuringthe transmittance of incident white light [18] through single and bilayergraphene. Although the transmittance is decreasing somewhat in the UVregion with a minimum at about ≥270 nm, the transmission spectrum israther flat in a wide range of the visible light. It is worth noting thoughthat merely a single graphene layer absorbs 2.3 %, which is a rather highfigure if we consider the amount of absorption per thickness. However, asingle layer graphene is far better than the conventionally used ITO interms of transmittance for a transparent electrode used in the UV region[19]. The reason for the optical behaviour of graphene is its peculiar elec-tronic structure which will be considered later on. Furthermore, grapheneis mechanically flexible and has a tensile strength of 130 GPa which makes

6

2.3 graphene

it the strongest material known so far [20]. Bear in mind that this is givenas a pressure which scales it to the thickness of graphene. With this in con-sideration it is believed that graphene will be a major component for futureflexible electronics. But as previously mentioned this cannot be utilized inGaN-based devices. Graphene also have a supreme thermal conductance,where values up to ≥5000 W/(m K) have been reported [21].

The many dierent properties all combined by one material motivatesthat graphene could have potential applications in numerous areas. Amongthese are optoelectronics, transistors, sensors, fuel cells, batteries, superca-pacitors, composites, bioapplications and more [22]. However, every areahave certain requirements on the graphene properties and quality whichis in some cases limited by the current technology. For example, to e-ciently be able to use graphene in transistor applications a bang gap isrequired and researchers are currently working on solving this issue byvarious methods. But the task is dicult and the progress is slow, hencethe application in this area is predicted several years ahead [22]. However,implementation in other fields are closer to being reality because the spe-cific graphene requirements are more easily achieved as the technologicalbarrier is lower.

As mentioned earlier, graphene is composed of carbon atoms forming ahexagonal crystal lattice in a two-dimensional plane. In Figure 4 we cansee the graphene lattice and its unit-cell vectors a

1

and a

2

. It has a carbon-carbon distance of 1.42 Å and a lattice constant of a =

Ô3 · 1.42 = 2.46 Å,

i. e. the size of a unit-cell vector.

2.3.1 Electrical properties

Carbon has atomic number six, meaning that it has six electrons andits electron configuration is therefore 1s22s22p2. A sketch of the electronconfiguration of carbon can be seen in Figure 5 to the left with rough en-ergy levels for each orbital. The hexagonal Bravais lattice of the graphene

Energy

1s

2s2p

x

2py

2pz

1s

sp2

2p

Figure 5: Electron configuration of a carbon atom (left) and the sp2-hybridization(right). The energy levels are just set for an illustrative purpose.

crystal originates from the electron configuration of the carbon atom.The 2s and 2p electrons can form hybrid states called sp-, sp2- or sp3-hybridization where the 2s electron form intermediate states with eitherone, two or three 2p electrons respectively. For this to happen one 2s elec-tron first need to be excited to the 2p subshell. The hybridizations can oc-cur under the right conditions since they are energetically more favourable.The sp2-hybridization is illustrated in Figure 5 to the right. There are twotypes of bonds that can occur: ‡- and fi-bondings. The ‡-bond is strong

7

2.3 graphene

and stable while the fi-bond is weak and reactive. In graphene every car-bon atom forms three sp2-hybridized ‡-bonds (covalent bonds) with threeother carbon atoms. The remaining 2p electron can form fi-bondings to an-other graphene layer called van der Waals binding which is what happensin graphite. However, in a single isolated graphene layer this 2p electron,also called fi-electron, becomes delocalized in the crystal structure. Weknow now that graphene is made of completely sp2 bonded carbon. Inreality however, when graphene is produced synthetically, we will havefractions of other bonding types as well. This can be due to, for exampleedges, wrinkles, grain boundaries or interstitial defects.

Because graphene is a 2D material down to an atomic scale, the freeelectrons are constrained in one direction but moves freely in the graphenecrystal plane. This is the reason for the appearance of its electronic bandstructure, which can be referred to as a zero band gap semiconductor [23].Because it has a zero width band gap it will however display metallicelectrical properties. To derive the band structure a common approachis to use the Hamiltonian of a tight-binding approximation as well asmolecular dynamics calculations such as ab initio [23, 24]. This will resultin an expression for the dispersion relation as

E(k) = ±t

Ò3 + f(k) ≠ t

Õf(k),

f(k) = 2 cos1Ô

3ak

y

2+ 4 cos

AÔ3ak

y

2

B

cos33ak

x

2

4,

(1)

where a is the lattice constant, t and t

Õ represents fitting parameters. Theplus and minus signs in E(k) corresponds to the conduction and valanceband respectively. Furthermore, the second term of E(k) (i. e. t

Õf(k)) has

been introduced to compensate for the asymmetry of the conduction andvalence band which is an eect of the next nearest neighbour interaction[23]. ab initio calculations have shown that t

Õ is in the range 0.02t < t

Õ<

0.2t [24]. In Figure 6 we can observe the dispersion relation of graphenein the first Brillouin zone expressed by Equation 1 with t = ≠2.7 eV andt

Õ = 0.15t. There are six points where the conduction and valence band

Figure 6: Dispersion relation of graphene using Equation 1 with t = 2.7 eV andt

Õ = ≠0.15t.

meat, called Dirac points. The Fermi level in graphene is located where

8

2.3 graphene

the valance band and conduction band meet, i. e. at the Dirac points. Inthe vicinity of these points the number of states approach to zero andthe dispersion relation can be approximated as a linear relation. This issimilar to the dispersion relation of photons which also displays a linearrelation between energy E and wave vector k. While in many cases thedispersion relation for a particle is quadratic. For this reason the electronsin graphene behaves as massless Dirac fermions [25]. This means that thefree electrons in graphene can easily be accelerated by an external force,hence the high mobility.

We know from our solid state physics that the conductivity can beexpressed as

‡ = neµ

e

, (2)

where n is the concentration of electrons, e is the elementary charge andµ

e

is the electron mobility. If holes are present this extends to ‡ =e (nµ

e

+ pµ

h

), where p is the concentration of holes and µ

h

is the holemobility. The reason for graphene’s high conductivity is mainly due to itshigh mobility. We can interpret from this relation that the conductivitycan also be increased by increasing the carrier density which can be doneby doping. Although this will also decrease the mobility as a result of theionized impurity scatterings that will occur with the dopants.

2.3.2 Synthesis

The synthesis of graphene can be done in several dierent ways. Mechan-ical exfoliation on a graphite crystal using ordinary scotch tape was thefirst method of isolating graphene by Novoselov et al. in 2004 [26]. How-ever, this method is neither scalable nor cost-eective but it can generategraphene flakes of very good quality which can be used for more researchedoriented purposes. Since the discovery in 2004 numerous methods haveemerged which generate graphene with dierent properties and potentialapplications [22].

Chemical vapor deposition generated graphene is likely the most promis-ing method for applications in optoelectronic devices as of right now due toits balance between quality and cost-eectiveness. The best quality CVDgraphene has been obtained when grown on polycrystalline copper foilsby thermal CVD at ≥1000 ¶C using methane and hydrogen as precursorand reduction gas respectively [2]. The resulting graphene is dominantlysingle layer and polycrystalline with typical crystal grains of micrometersize. The copper, working as a catalyst, enables graphitization even underthese conditions when normally this formation takes place at >≥2500 ¶C[27]. The copper catalyst is believed to improve on at least two things.The first being that it helps to decompose the carbon precursor. Secondly,it is believed that the metal, somehow plays a key role in the formationof a graphene structure. Furthermore, the low solubility of carbon in cop-per is believed to be the reason for the self limiting eect, i. e. limitingthe graphene to grow to mainly monolayer. A major advantage of CVDfor LED applications is that it has the ability to deposit the graphene-like structure with variable quality directly on the target substrate. Eventhough the graphene quality becomes much worse on a semiconducting

9

2.4 characterization techniques

or insulating substrate than on copper foils there are other advantages ofdirect growth which cannot be met by a transferred graphene film froma copper substrate. The first and most important issue is that the trans-fer process is complicated and limits the industrial utilization. Secondly,the transfer process introduces vacancies between the GaN and graphenewhich makes a poor electrical contact in the interface, i. e. a Schottky bar-rier. Graphitization by a CVD process without a metal catalyst is moredicult to obtain and the process parameters are less forgiving on thefinal result. Graphite is the most stable carbon allotrope but to obtaingraphitization one needs to overcome a large energy barrier. Since the en-ergy barrier is more dicult to overcome in the metal catalyst-free systemthe received quality will be worse. Polycrystalline multi-layer films can beobtained but the grain sizes are expected to be on the nanometer scalerather than micrometer scale.

It is known that hydrogen has the eect of removing amorphous carbonin CVD-grown graphene [28]. This might be because the amorphous car-bon is less stable than graphene which makes it possible for hydrogen toform hydrocarbons that can easily be transported away from the surfacein a gaseous phase. In literature this is referred to as an etching process[28] which could be a more general description. A second property of hy-drogen is that it works as a surface activator that changes the properties ofthe surface phase, e. g. adhesivity. Vlassiouk et al. found that the growthrate on copper substrates is correlated to the hydrogen partial pressurewith a maximum rate at about 200–400 times higher partial pressure forhydrogen compared to methane.


In this section we go through the basic theory of the characterization tech-niques used in this work which is necessary to be able to handle equip-ments, prepare measurements and interpret results.

2.4.1 Raman spectroscopy

Raman spectroscopy utilizes inelastic scattering of a monochromatic lightsource hitting a certain material. Inelastic scattering refers to a scatteringevent where the light interacts with the substrate and changes its en-ergy and hence its wavelength. The interactions can be with for examplephonons, i. e. vibrations in a crystal. This is then used to identify dierentexcitations in the substrate giving information about the phonon modesin the material. Every material will have its own spectrum which can beused as an identification or to get information about its chemical bond-ings. Raman spectroscopy is relatively easy to use and has a wide areaof application where it can be used in numerous fields. The number ofscattering events is dependent on the wavelength of the light. Therefore,the wavelength of the laser being used will impact the Raman spectrum.Most scattering events are elastic, meaning that the light is just absorbedand then emitted in a random direction without losing any energy. Theinelastically scattered light are more probable to receive a lower energy,

10


but in rare cases it can also gain energy. These dierent types of eventsare called Stokes and Anti-Stokes scattering respectively. A common illus-tration of dierent types of scattering can be seen in Figure 7. In Ramanspectroscopy one commonly just observes the light with a higher wave-length than the original light source, i. e. the lower energy light.

Vibrationalenergy levels

Virtualenergy levels

Rayleighscattering Raman

scattering

Stokes Anti-Stokes

Figure 7: The dierent types of elastic and inelastic scatterings that can takeplace when light interacts with a material.

2.4.1.1 Raman spectra of carbon materials

All sp2 carbon materials exhibit primarily three characteristic Ramanpeaks, often called D peak (≥1350 cm≠1), G peak (≥1580 cm≠1) and 2Dpeak (≥2700 cm≠1) [29–32]. Other higher order peaks can also be observedbut they will not be considered in this report. The D peak is a disordergenerated Raman mode which means it will be triggered by any impurities,edges or finite grain sizes [33]. It originates from the breathing mode of thesixfold symmetric sp2 carbon rings and it is absent in perfect graphene orgraphite [29]. In reality there is usually some disorder, meaning that a Dpeak will be present. The G peak on the other hand is a feature of all sp2-containing carbons and it simply relates to the sp2 bonded carbon-carbonvibrations [29]. Lastly, the 2D peak is a two-phonon, second-order process.Its shape and size depends on several variables, one being the excitationlaser energy. Another factor is that it is sensitive to the number of graphenelayers resulting in dierent shapes [30–32]. Some studies have investigatedthe relation of the intensity ratio between D and G peak, I

D

/I

G

, to aver-age grain size, L

–

[29, 34]. The essence is an empirical formula saying thatI

D

/I

G

Ã 1/L

–

in the range from graphite to nanocrystalline graphite.However, in the range from nanocrystalline graphite to amorphous car-bon another relation is dominating which in this case is I

D

/I

G

Ã L

2

–

. Thetransition can also be noticed by a shift in the G peak position as well asa broadening of the peaks as the disorder gets worse. For amorphous car-bon the two peaks are dicult to distinguish and appear to have almostmerged together [32].

11


2.4.2 Transmittance

Transmittance is regarded as the ratio of transmitted to incident electro-magnetic power. The transmittance is often analysed as a function of wave-length, ⁄, in the visible electromagnetic spectra, ranging from ultravioletto near-infrared (UV-VIS-NIR), commonly referred to as spectrophotom-etry. Spectrophotometric measurements can be used for many dierentpurposes, where chemical analysis to determine content and concentra-tion is one example. But in this work it will merely be used to study thelight transmission of the visible spectra for a thin, graphene-like, carbonfilm.

When dealing with a uniformly thin absorbing layer on a much thicker,more transparent substrate, interference eects can occur. This can ob-scure the analysis of what we actually want to study. To cope with thiswe can estimate the interference free transmittance, T

–

, as the geomet-ric mean of the upper and lower extreme values in the interference, T

M

and T

m

, that encloses the raw transmission spectrum, as explained bySwanepoel [35]. That is

T

–

=

T

M

T

m

, (3)where T

M

and T

m

are considered to be continues functions that containsa discrete number of maxima and minima from the interference patternrespectively. Therefore T

M

and T

m

have a corresponding value for every⁄. It should be noted that Equation 3 is valid in the region of weak tomedium absorption.

2.4.3 Electrical characterization

This subsection is devoted to the electrical characterization techniquesused herein. We briefly cover the required theory to be able to interpretthe results.

Electrical characteristics of a typical measurement is commonly investi-gated by looking at the current-voltage behaviour (I-V characteristics). Inreal devices, containing non-metallic parts, we cannot assume that Ohm’slaw will apply where the I-V characteristics shows a linear behaviour. Bylooking at the I-V curve we can extract information about the whole orparts of a device to tell if it has an Ohmic or non-Ohmic behaviour. Insome cases the resistance in all the connections and wires, R

wires

, is com-parable or much larger than the resistance where we actually want tomeasure, R. The measured resistance is therefore

R

tot

= R

wires

+ R = U/I,

where U and I are the measured voltage and current respectively. Thismeans that we are unable to determine the required resistance by an ordi-nary two-terminal measurement, where voltage and current are measuredthrough the same wires. A viable technique to counteract this problemis to use a four-terminal measurement instead, where the voltmeter andammeter are parallel connected. The two-terminal and four-terminal se-tups are illustrated and compared in Figure 8. By measuring this way thevoltage drop from the actual device we want to measure to the voltmeter

12


V A

V

A

R R

R1 R2 R1 R3 R2R4

Figure 8: Illustration of the two-terminal (left) and four-terminal (right) resis-tance measurement setup. The wire resistances are indicated as R

i

,i = 1, 2, 3, 4.

is much less than it would be if the current and voltage are measured inthe same two-terminal circuit. Therefore, the measured resistance is cor-responding better to the actual resistance we want to measure. But thisrequires that the voltmeter in the four-terminal setup only dissipates avery small fraction of the current going through the device with resistanceR.

2.4.3.1 Sheet resistance

When dealing with thin films it is often convenient to introduce the sheetresistance. It is defined as

R

s

= fl/t,

where fl is the bulk resistivity and t is the film thickness. We know thatresistance is expressed in terms of its bulk resistivity as R = flL/A, whereL and A is the length and cross-sectional area of the resistance, respec-tively. However, the cross-sectional area for a thin film is the width of thefilm, W , multiplied by its thickness, i. e. A = Wt. The resistance in thefilm can therefore also be denoted as

R = fl

L

Wt

= R

s

L

W

. (4)

From this relation we can see that the sheet resistance is related to theresistance of the film by the aspect ratio of the rectangle. It is also worthnoting that in the case where the film consists of a perfect square, thesheet resistance is exactly equal to the resistance independent of the sizeof the square. Allthough the dimension of sheet resistance and resistanceis in fact the same, the sheet resistance is often expressed in terms of /,to avoid confusion with regular resistance.

13

3E X P E R I M E N T S A N D R E S U LT S

In this chapter we will go through the experimental work that has beendone to obtain the results. First, the CVD system will be considered in-cluding its functionality and operation. This will then be followed by anominal growth optimization with PECVD by improving the sheet re-sistance and transmittance. Finally, a few metal treatment ideas will betested that could improve the results.

3.1 cvd setup

The CVD system used in this work is a Black Magic 2-inch from AIX-TRON NanoInstruments. It is a cold-wall, vertical furnace with a resis-tive graphite heater and a DC-generated plasma with the possibility toadd a variable frequency of 1–100 kHz (the plasma waveform is describedand analysed further in Appendix A)1. The system typically operates ataround 10 mbar, up to ≥1000 ¶C and a maximum2 plasma power of 1 kW.The mass flow controllers are calibrated for 500 sccm of C

2

H2

, 1000 sccm ofNH

3

and 1000 sccm of N2

as deposition, reduction and inert gases respec-tively. But they can of course be adjusted as required below the calibratedvalues. The deposition gas can also be changed to a CH

4

/Ar mixture of5 % methane and 95 % argon, the reduction gas can be change to hydro-gen and the inert gas has the option to use argon. Changing gases fromthe calibrated, default gases, requires a rescaling in the software. It isalso equipped with a computer and a process monitoring user interfacewhere the process parameters can be manually controlled or by automat-ically executing recipe files. The temperature of the heater is measuredby a thermocouple and it is used as feedback to the power supply of theheater to match the set temperature, through a so called PID-controller.A schematic drawing of the reaction chamber and a snapshot of the userinterface can be seen in Figure 9 and Figure 10 respectively.

3.2 pecvd growth and equipment handling

To begin with the PECVD growth process of graphene on gallium nitridehad to be at least nominally optimized to use as a starting point to evaluatethe results from dierent treatments (see Section 3.3). We will try to find

1 This analysis was definitely not necessary, but it might be useful for someone working

with this equipment in the future.

2 Although, using a considerably lower plasma power is recommended to not risk any

equipment of getting damaged.

14


HeaterSubstrate

Gas inlet

Gas outlet

Plasma electrodeHeat reflector

Showerhead

Thermocouple

Quartz belljar

Figure 9: Illustration of the reaction chamber of the CVD system used.

a recipe that produces samples where the optical transmittance is as highas possible and the sheet resistance is as low as possible using the providedequipment. There are of course also other factors that are significant toevaluate the level of success when we recall that the purpose is for GaNLED applications. One such factor is for example contact resistance. Butthe focus lies on improving the transmittance and sheet resistance, as theyare easy to quickly evaluate by rough estimations. Raman spectroscopywill be another frequently used technique due to its simplicity of instantlyobtaining information. We will also propose some recommendations on theequipment handling to obtain the best possible repeatability.

Because there was previously limited experience using the plasma inthis particular CVD setup, we first and foremost made a few test runs tolearn how the plasma works and behaves, as well as complimentary readingthe instructions manual. We discovered that the plasma works only undersome specific conditions, with restrictions on plasma power and chamberpressure. When the pressure is too low, the plasma becomes more andmore diuse and widespread. However, this is not a recommended way ofusage, according to the operations manual. On the contrary, if the pressureis too high, the plasma will separate at the heater surface and not havea complete coverage, upon where it finally disappears entirely. To easierallow the plasma to strike when it is turned on, the plasma voltage is setto 800 V. It is then controlled by fixating the plasma power which willautomatically alter the plasma current and voltage to match the givenpower, where the current is restricted to an upper limit in the plasmaoptions window. All plasma options are loaded in the recipe by the line‘TUNE PLAS’ (see Figure 10) which sets the power, current limit andwaveform.

15


Figure 10: Snapshot of the user interface for the CVD equipment.

It is crucial that experiments between dierent runs are as repeatable aspossible. This is however limited by the uncertainty of the growth param-eters. In order to create the best possible conditions there are a few thingsthat can be considered. The substrates being used should be as similar toeach other as possible. That is, the GaN wafers should be uniform anddierent wafers should be similar. The second thing is the cleaning of thesubstrates prior to the experiments. We also desire that the growth pa-rameters should be under control. For this we want our growth process tobe stable and that the parameters readouts are close to their actual values.This mainly involves temperature, plasma power and gas flows, where thetemperature is the most critical and the most dicult to control. We tryto place the thermocouple at the same spot, monitor the heater powerand brightness of the heater to get as much information as possible ofthe temperature status. The biggest diculty was to maintain the sametemperature reading between dierent experiment sessions3. The plasmais sometime a bit unstable which can be due to impurities. We found thatthis could be counteracted to some extent by running the machine emptywithout any substrate the first time of each session. By doing this one canrun the recipe but without any carbon precursor and manually increasethe plasma power gradually to a maximum of 100 W. We believe that thishelps to burn away impurities which will make the plasma more stable.At the beginning of every session, the heater is also cleaned by wipingit with IPA before mounting it into the chamber. The final growth pa-rameters that we consider are the gas flows. The flow of any gas shouldnot be too low since this can give inaccurate readouts. A recommendedrule of thumb is to use gas flows above 5 % of the maximum calibratedvalue. They should also be applied by keeping in mind that they aect thechamber pressure and therefore also the plasma as mentioned earlier.

Every time a new recipe is tested, we start from a previously used recipeand run it without any substrate. From there we change the parameters

3 Probably due to the heavy usage of the equipment that changes the setup slightly every

time someone uses it.

16


gradually and one at a time to match the new recipe. We use this procedureso that no substrates are wasted and to make sure that the machine isused within its limitations.

Before any samples can be treated in the PECVD furnace, the samplesneed to be prepared by dicing and cleaning as described in Appendix B.Every recipe mentioned hereon will follow the basic structure presentedby Figure 11 and in the following numbered list, unless stated otherwise.

0 200 400 600 800 1000 1200 1400 1600 1800Time (s)

Tem

pera

ture

(a. u

.)

Growth

Figure 11: Temperature progression during a typical experiment. Note that thisis just an example and the events in this graph can occur at othertimes for dierent recipes.

1. Flow 1 and 2 – ON.

2. Heat – ON.4

3. Wait – T > T

growth

≠ 5 ¶C.

4. Wait – 60 s.

5. Plasma – ON.

6. Flow 3 – ON.

7. Wait – t

growth

.

8. Plasma – OFF.

9. Flow 3 – OFF.

10. Heat – OFF.

11. Wait – T < 100 ¶C.5

12. Flow 1 and 2 – OFF.

Here temperature is denoted with T and time is denoted with t. In this listwe refer to the reduction, inert and deposition gases as flow 1, 2 and 3 re-spectively. The parameters subscripted with “growth” defines the shaded

4 Ramped at 300

¶C/min to the set temperature, T

growth

.

5 In the earliest work we use 150

¶C to save time.

17


rectangular area in Figure 11. A detailed description of the procedurewere we find the final growth conditions is presented in Appendix B, Sec-tion B.3. The GaN wafers we used first was reused low grade LED struc-tures, meaning that they would have a layer of p-type GaN at the surface.These wafers contain surface roughness at dierent extents and could givelarge dierences in a Raman spectra. From the nominal growth optimiza-tion in Section B.3 we used a recipe given by Table 1, estimated to givethe so far best samples, of high transparency and low resistance, usingthese wafers. This recipe produced samples of down to a few k/ andk by measuring with 4-point probe CMT-SR2000N and a multimeter re-spectively. In Figure 12 we can see a Raman spectrum obtained from oneof the samples using this recipe. This spectrum shows a clear presence ofthe D, G and also 2D peak. The 2D peak in this particular measurementis much more prominent than usual. Instead, most of the Raman measure-ments showed a much smaller tendency of a 2D peak. Additionally, thetwo sharp peaks at ≥1240 cm≠1 and ≥1270 cm≠1 originate from the GaNsubstrate. For proper sheet resistance measurements we require a wafer

parameter value

Plasma power 40 WTemperature 800 ¶CGrowth time 10 minH

2

flow 800 sccmAr flow 300 sccmCH

4

/Ar flow 50 sccm

Table 1: Growth parameters acquired from testing numerous recipes.

1000 1500 2000 2500 3000Raman shift [cm-1]

0

0.2

0.4

0.6

0.8

1

Inte

nsity

[a.u

.]

D @ 1334 cm-1

G @ 1608 cm-1

Figure 12: Raman spectrum obtained from a sample grown with the recipe inTable 1.

which is conducting as little current as possible and in that case the p-typeGaN is not the most suitable choice. Therefore we choose another type ofwafer to do such measurements which is a semi insulating homogeneousGaN on sapphire. However, the recipe in Table 1 did not produce thesame results with this type of wafer. We could observe that these samplesbecame much darker than the previous wafers when growing on both of

18


them at the same time. For this reason there had to be some tuning to theparameter values to produce brighter samples and this was achieve aftermany test runs later as the recipe presented in Table 2. Note that in this

parameter value


2


4

/Ar flow 20 sccm

Table 2: Growth parameters acquired for the semi insulating GaN wafers.

recipe we have reduced the plasma power as well as the methane flow toget a slower growth rate. In addition to these parameter changes we nowalso added a pretreatment of two minutes before the actual growth wherewe use the same conditions as during the growth but without any carbonsource. By doing this we allow the plasma to become more stabilized be-fore the growth and the hydrogen plasma during the pretreatment couldbe activating the surface and removing unwanted impurities. To clarify,this means that we add a line in the numbered list on page 17, betweennumber 5 and 6 which then says: Wait – 120 s. With the samples createdfrom the semi insulating wafer we can perform more thorough sheet re-sistance measurements by making Hall Bar patterns of the carbon film.A guide to the photolithography procedure for the patterning of the HallBars is presented in detail in, Appendix B, Section B.4. The final devicesappears as in the left picture in Figure 13. Every fabricated sample fits

Figure 13: Optical microscope pictures showing the photolithography-patternedmetal contacts connected to the carbon film Hall Bar in the center(left) and the photomask of the Hall Bar (right). Inset shows a mag-nification of the marked center part.

nine such devices in a three by three array. The metal contacts in Fig-ure 13 (left) connect to a Hall Bar device made by the carbon film. Thiscarbon-made Hall Bar is patterned by applying photoresist to cover thedesired area while the remaining, uncovered carbon is etched by oxygen

19


plasma. The mask we use to pattern the carbon into a Hall Bar deviceis shown to the right in Figure 13. There is no clear contrast dierencebetween the carbon Hall Bar in the middle and the etched surface whenobserving in an optical microscope due to the carbon film’s very finitethickness. However, Raman and electrical measurements support its pres-ence. By measuring with the Raman microscope we can check if there iscarbon where we expect it to be and vice versa. This Raman measurementcan be seen in Figure 14 where there is a clear dierence of carbon relatedD and G peaks as anticipated.

1000 1500 2000 2500 3000Raman shift (cm−1)

Inte

nsity

(a.u

.)

Spot 1Spot 2

1

2

Figure 14: Raman spectrum on the carbon Hall Bar as well as beside it. Insetindicates the measurement spots. Spot 1 measures on the Hall Barand spot 2 measures beside it.

We used a Keithley 4200-SCS parameter analyzer to do the electricalmeasurements on the Hall Bar devices. The pattern is made such that theaspect ratio of the length and width of the Hall Bar is 1:1, meaning thatthe measured resistance equals the sheet resistance, see Section 2.4.3.1,Equation 4. We conduct the measurements using four-terminal measure-ments according to Section 2.4.3 by applying four probes to 4/6 of themetal contacts in a device. This allows us to measure the resistance ofonly the carbon square in the Hall Bar without the resistance in the wiresand contacts. We leave either the bottom two or top two contacts uncon-nected but the choice makes no dierence since it will be two symmetriccases. We then measure the current voltage characteristics in the rangeof ≠5 V to 5 V. As an example we show a typical measurement from oneof the samples in Figure 15. There is a possibility that some of the de-vices gets damaged during the fabrication. Therefore, the number of linesin each plot correspond to the number of working Hall Bar devices on asample with a maximum of nine. We can see that we have a small recti-fying tendency for the two-terminal measurement while the four-terminalmeasurement appears linear at the displayed range. By making a linear fitto each four-terminal measurement we can estimate the slope which is theinverse of the resistance in the ideal Ohmic case. The estimated resistancewill also be equal to the sheet resistance in this case since the Hall Barhas a 1:1 size ratio. In Figure 16 we can see the sheet resistance at dier-ent locations, as we measure on dierent devices, which is done for all six

20


−6 −4 −2 0 2 4 6−15

−10

−5

0

5

10

15

Voltage (V)

Cur

rent

(µA)

−0.4 −0.2 0 0.2 0.4−15

−10

−5

0

5

10

15

Voltage (V)

Cur

rent

(µA)

Figure 15: I-V characteristics the Hall Bar devices from one of the samples usingtwo-terminal (left) and four-terminal (right) measurements.

samples grown using the recipe in Table 2. We estimate the uncertaintyof the slop with a 95 % confidence interval to plot error bars at each mea-surement in Figure 16. The error bars can hardly be spotted because theseerrors are relatively small compared to the scale of the graph. The sheet

0 2 4 6 8 101.5

2

2.5

3

3.5

4

4.5

5x 104

Sample location (Hall Bar #)

Shee

t res

ista

nce

(Ω/s

q)

S1S2S3S4S5S6

Figure 16: Sheet resistance for six samples of seemingly similar transmittancemeasured on Hall Bar devices.

resistance for these samples range from about 20 k/ to about 50 k/with dierences within one sample being < 10 k/, except for sample 1having a much larger variation.

Transmittance measurements seen in Figure 17, show ≥96 %, ≥93 %and ≥87 % transmittance at 550 nm for 10, 20 and 40 min growth respec-tively. These values are estimated from the interference free transmittanceT

–

calculated by the geometric mean of the enclosing functions T

M

andT

m

as explained in Section 2.4.2, Equation 3, see Figure 17 (b) for clar-ification. We then divided by the interference free transmittance of bareGaN to get the transmittance of the carbon films alone. The calculatedvalue are also in good agreement with triangular moving averages of theraw data, see Figure 17 (c). Comparing to the work by Swanepoel [35] wecan safely assume that the above calculated values are within the weak tomedium absorption region. The measurement equipment is set such thatfull transmittance in the raw data correspond to no sample in the beam.Judging from the low level of transmittance of the raw data we can as-sume that the samples must have a high reflectance considering that GaN

21

3.3 metal treatments

400 600 800 1000Wavelength (nm)

0

1

2

3

4

5

6

Tran

smitt

ance

(%)

(a)Eg GaN

Bare GaN10 min growth20 min growth40 min growth

400 500 600 700 800 900 1000 1100Wavelength (nm)

3

3.5

4

4.5

5

5.5

6

6.5

Tran

smitt

ance

(%)

(b)

Bare GaNT,

TMTm

400 500 600 700 800 900 1000 1100Wavelength (nm)

82

84

86

88

90

92

94

96

98

Rel

ativ

e tra

nsm

ittan

ce (%

)

(c)T', 10 min

T', 20 min

T', 40 min

TMA 10 minTMA 20 minTMA 40 min

Figure 17: (a) Raw data of the transmittance measurement for 10, 20 and 40 mingrowth, where the band gap of GaN is indicated with a dashed red line.(b) Raw data for bare GaN enclosed by linear interpolations betweenall interference maxima and minima, T

M

and T

m

, used to calculatethe interference free transmittance, T

–

, by geometric mean (valid inthe region of weak to medium absorption). (c) Relative interferencefree transmittance, T

Õ–

, and triangular moving average for 10, 20 and40 min growth with indicators of T

Õ–

at 550 nm.

and sapphire are high transparency materials. The abrupt absorption at365 nm is due to the band gap of GaN which is 3.4 eV.


Since many dierent metals have a positive eect on the growth of grapheneby CVD, an idea was to test metal assisted growth directly on GaN orby letting a thin evaporated metal layer repair an all ready grown carbonfilm. The first technique involves a thin layer of metal to be evaporatedonto a GaN substrate which is then put into the CVD machine. By doingthis, carbon atoms can form a film in between the metal and GaN by dif-fusing through the metal, possibly in the grain boundaries. In the secondtechnique we evaporate a thin metal layer on a sample which has all readybeen grown with PECVD. After evaporation we treat it a second time inthe CVD chamber but this time mainly focusing on annealing the sam-ple along with a low dose of carbon precursor to support extra buildingblocks. By doing this we intend for the existing carbon layer to rearrange

22


and “repair” into a more favourable state (i. e. increasing the fraction ofsp2 bonds). We will study both these methods using copper and nickel ofdierent thicknesses and try to get the right conditions to achieve someimprovements compared to the pristine direct grown carbon film. We usedan electron beam evaporator for all metal evaporation in these tests.

3.3.1 Metal evaporation prior to growth

We will go through what we investigated for the pre-growth metal treat-ments and cover the necessary details. We only managed to cover onethickness per metal for these tests and the samples that we used werereused low grade GaN LED wafers with a rough surface structure, whichmeans they have a p-GaN layer at the top.

3.3.1.1 Copper prior to growth

We started from a few pieces of GaN with 400 nm copper and used therecipe in Table 1, roughly optimized for these type of wafers. After PECVDgrowth the metal changed shape and color from having a typical coppercolor to a more yellow/gold-like color. We then etched the remaining metalby immersing the samples in 1M concentration of FeCl

3

solution for about≥5 min followed by rinsing in DI water and blow drying with pressurizedN

2

gas. The result showed a dark coverage of carbon at the former metallocation and less to no coverage at the other area where the evaporatorsample holder was covering the samples.

3.3.1.2 Nickel prior to growth

For this experiment we used 600 nm nickel evaporated onto a few GaNsamples as a starting point. Now we used a similar recipe to the onein Table 1 except that no plasma was used this time and half of thesamples where exposed to twice as much carbon precursor during growth.All samples showed very similar results, specifically a matte white, possiblyporous, conducting surface with a low transmittance. An explanation forthis could be that there might be some intermixing between GaN and Niwhich could give a lower conductance as well as the white appearance.

3.3.2 Metal evaporation subsequent to growth

For every sample in these experiments we measured both the Raman spec-trum and resistance after growth but before the metal treatment to be ableto track any changes. The resistance here was always estimated using a4-point probe CMT-SR2000N measurement station. This is less accuratethan a thorough Hall Bar sheet resistance measurement but it is enoughto get a good estimate and track changes.

3.3.2.1 Copper subsequent to growth

We started from ten samples grown with our standard recipe for LEDstructured GaN wafers (Table 1). After that we applied a 400 nm copper

23


layer using the electron beam evaporator. We then took five samples at atime and put them back into the CVD machine, but this time we excludedthe plasma. Starting of with the first five samples we noticed that the cop-per layer began to deform and partially melted or dewetted. We thoughtthat this might cause possible catalysing/repairing eects to cease andtherefore we lowered the temperature about 20 ¶C for the first five sam-ples and about 40 ¶C for the next five. When we finished this treatmentwe etched the remaining copper with FeCl

3

using the same procedure asbefore (see Section 3.3.1.1). We found that the sample now had a very ir-regular and a bit white looking appearance. We performed resistance andRaman measurements after this treatment and the collected data are pre-sented in Figure 18. We can see that the measurements are scattered in a

1 2 3 4 5 6 7 8 9 10Sample #

101

102

103

104

105

106

107

Shee

t res

ista

nce

(+/s

q)

5000 +/sq

50 +/sq

BeforeAfter

1 2 3 4 5 6 7 8 9 10Sample #

1

1.2

1.4

1.6

1.8

2

2.2

D/G

BeforeAfter

Figure 18: Sheet resistance (left) and Raman D/G intensity ratio (right) beforeand after 400 nm Cu treatment. Note the logarithmic scale for thesheet resistance. We marked 50 / and 5 k/ for visual support.

broad interval. The Raman data seem more or less unchanged whereas theresistance measurements are generally higher after the metal treatment ex-cept for the first four samples which showed significantly lower values ofdown to almost 10 /. These resistance measurements where fluctuatinga lot depending on where on the sample we took our measurement, whichis not surprising considering the largely irregular appearance.

We conducted another experiment using the same method but using a100 nm copper layer instead. The samples we used this time were froma high quality semi insulating GaN wafer, hence we used recipes moresimilar to the ones given in Table 2. However, we used three of our samplesfrom our optimization procedure that we no longer needed, therefore therecipes diered slightly. But this is nevertheless of no major significance,since we are only interested in tracking possible improvements. Duringthe reheating process we did not mind if the metal were going to meltor dewett this time and we altered the treatment parameters slightly foreach sample to increase our chances of finding any improvements. Thefirst sample was treated at 750 ¶C for 10 min using 300/800/10 sccm ofH

2

/Ar/(CH4

/Ar) flows. While the second sample was treated at 650 ¶Cinstead and the third sample at 700 ¶C for 20 min using the same flows. Onall samples the copper was dewetting and the small amount of remainingcopper were etched in ammonium persulfate. This experiment showed no

24


evident change in the appearance of the samples or the estimated sheetresistance, seen in Figure 19.

1 2 3Sample #

0.4

0.6

0.8

1

1.2

1.4

1.6

Shee

t res

ista

nce

(+/s

q)

#104

BeforeAfter

Figure 19: Sheet resistance before and after 100 nm Cu treatment.

3.3.2.2 Nickel subsequent to growth

We performed an experiment using 600 nm nickel evaporated onto LEDstructured samples prepared with a carbon film. These samples, unlike theones without carbon in Section 3.3.1.2, was subjected to severe flaking ofthe nickel. This is known to be a common problem when evaporating ametal of too high thickness. We gently blowed o the worst flakes withpressurized N

2

gas. However, many samples were almost useless to treat,but we decided to give it a try with the least bad samples which hadsome areas of metal that was still adhering. After treating these samplesusing the same recipe as usual for this kind of wafer (Table 1) but withoutplasma, we received similar results as for the 600 nm nickel evaporationprior to growth. We recall that these samples had a matte white lookingsurface with reasonably good conductance and poor transparency, exceptfor the flaked areas which resembled an untreated sample.

As a final test we investigated if a thin layer of just 10 nm nickel couldhave any catalysing or repairing eects for a carbon film which was pre-grown on eight semi insulating GaN. We did six variations in the recipesfor the treatment process but all without plasma. After treating thesesamples, none of them showed any visual dierences that would indicatethe metal to melt or dewett. The etching of the metal in ammoniumpersulfate was slow and had to be left overnight. However, when finished,all eight samples tested with six recipes showed that the carbon filmsseemed to have been removed from every sample. The samples had thesame appearance as ungrown GaN substrates and resistance measurementsshowed insulating behaviours, although some Raman measurements stillshowed low intensities of sp2 related carbon peaks. This suggests that anyremaining carbon film was discontinuous and fragmental.

25

4D I S C U S S I O N A N D C O N C L U S I O N S

We had plans to make TLM patterns for contact resistance measurementsbut unfortunately it was unsuccessful and due to time constraints and alimited amount of high quality semi insulating GaN substrates we couldnot proceed to make more tries. The procedures of the attempts thatwe did are presented in Appendix B. However, we are optimistic that theelectrical contact between the carbon film and GaN is better than a typicalcontact of a transferred graphene film. The contact resistance is just asimportant as any other property, as it contributes to the overall voltagedrop of the LED and hence its power consumption and eectiveness.

The Raman measurements were not entirely reliable. As we comparedspectra taken at the same spot, on the same sample and using the samesettings, they were not matching in a desirable way. This did not improvesignificantly despite our eort of increasing the number of accumulationsup to a practical level where each complete measurement would not taketoo long. This prevented us from being able to cancel out measured spec-tra with the corresponding spectra of bare GaN, which would have beena useful method to isolate the carbon spectra. Even though Raman mea-surements are said to be a very comprehensive technique that can extracta lot of information from carbon materials, we mostly just used it as a wayof detecting the presence of carbon. If we could categorize our carbon filmusing our measured Raman spectra it would probably be characterized asnanocrystalline graphene based on the intensity ratio of D and G peaksthat typically ranged between 1 and 2 as well as the position of the G peakand relatively good distinction of the two peaks [29]. Regarding the twoGaN related peaks at ≥1240 cm≠1 and ≥1270 cm≠1, they appeared onlyto a small extent for the semi insulating GaN substrates. Whereas for theLED structured GaN substrates, they appeared very dierently for dier-ent wafers. For some wafers these peaks were too obstructive to be ableto distinguish the carbon related peaks. The exact origin of these peaks isunclear, but a guess is that they are either doping or defect related whichare both present for these wafers.

4.1 direct growth on gallium nitride

Many of the samples experienced an increase in the resistance when mea-suring at a later moment. This could be due to degradation by exposure tothe external environment. It could be, for example, that oxygen in the airreacts with our samples by occupying defects and interstitial sites whichchanges its properties. It is unclear though whether this phenomenon hap-

26

4.1 direct growth on gallium nitride

pened at the same extent to every sample. By taking measurements onsamples as early as possible after growth we are doing all we can to coun-teract this issue.

Measuring with a multimeter is not a particularly accurate method todetermine the sheet resistance. But we can argue that it will be roughlyproportional to the sheet resistance since this method measures in thelateral plane. We should also keep in mind that electrical measurementson a p-GaN substrate will not be intrinsic compared with the real sheetresistance of the carbon layer alone. For this reason we used semi insulatingGaN to characterize the intrinsic sheet resistance of the carbon layer.

A large portion of the potential drop appears in the contacts to thecarbon square sheet as we saw in Figure 15, indicating that it has a quitehigh resistance. Furthermore, the I-V characteristics in the carbon sheet(in the Hall Bar) seem very linear (at least in the voltage range used in themeasurements). Thus, the carbon film shows an Ohmic nature, i. e. similarto metal behaviour which is also the case for graphene as explained in thetheory, Section 2.3.1. The applied voltage range to the device is ≠5 V to5 V but the range over the actual carbon sheet is just about ≠0.2 V to 0.2 V.Extending the range would support a continued linearity even at highervoltages. A reasonable range to test would be one that the carbon filmwould be subjected to in a real LED, which logically should be much lessthan the voltage drop over the LED otherwise the carbon film would beimpractical to use. High quality CVD graphene grown on copper displayssignificantly lower sheet resistance (down to ≥100 /) [1] which indi-cates the large barrier in quality dierences. Scattering of charge carriersat the domain boundaries reduces the carrier mobility and therefore alsothe conductivity (Equation 2), which means that lower sheet resistance isrelated to larger average grain size. Comparing to a more similar work byKim et al. [5], our result is more than one order of magnitude higher (theymeasured 1.4 k/ whereas we measure minimum ≥20 k/). One con-tributing reason could be our dierent use of GaN substrates where theyopt to grow on LED structures, i. e. p-GaN which has a higher conduc-tance. Though it is unclear if this is enough to justify the nonconformityto our result. But judging from our dierences in Raman spectra, morespecific 2D peak intensity (in comparison to D ang G peaks), they seem tohave obtained a more graphene-like carbon film, whereas we rarely couldobserve a prominent 2D peak (Figure 12 being an exception).

The repeatability depends on the uncertainty of the growth parametersas well as the substrate regularity. The moment that any of these con-ditions change unintentionally from one experiment to another we willintroduce an uncertainty to the results. We struggled to keep a high re-peatability in this work, but we gave a few suggestions that might behelpful to narrow the distribution. The temperature measurement is with-out doubt the most uncertain and influencing process parameter for thisequipment. A way of dealing with this might be to add more sources oftemperature measurements like another thermocouple or an infrared ther-mometer. It should be noted that this could also be influenced by theradiation from the plasma. In such a case, this would be most viable as acomparison to the thermocouple before the initiation of the plasma.

27


We speculate that the plasma is largely responsible for the decomposi-tion and therefore deposition of the gases in the reaction chamber whilethe diusion on the substrate surface is mostly dependent on the substratetemperature. This would imply that temperature is indeed crucial to ob-tain high quality graphene, hence the temperature need to be at leastmoderately high. The highest possible temperature without damaging theGaN is preferred. We experienced that temperatures of 850 ¶C or morewere starting to roughen the surface which could mean that the GaN weredissociating. A simple test could be done by intentionally inducing rough-ness and comparing SEM images (or similar) before and after growth aswell as etching the carbon in oxygen plasma. Alternatively just annealingthe GaN to a critical temperature without introducing any growth.

Another reason why graphene can be grown successfully on coppermight be that copper in its (111) crystal phase has a structure with only≥3.4 % mismatch to graphene. In Section 2.1 and 2.3 we presented thelattice constants of gallium nitride (3.19 Å) and graphene (2.46 Å) respec-tively. From this aspect we have a lattice mismatch of about ≥30 % whichis another factor that hampers the ability to grow graphene on GaN epi-taxially. For this reason, along with other factors discussed later on, thedeposited carbon might be formed into many nanocrystalline grains whichis also in accordance with the observations of the Raman spectra.

A general thought is that a slower growth rate will result in a higherquality graphene. But another issue that we experienced for our directgrowth method was a lower limit of the growth rate. Other groups haveshown successful PECVD growth on semiconductors when growing for onehour or more [5, 6]. Whereas in our work our growth rate was limiting thegrowth time to a maximum of 10 min to maintain a thin and transparentcarbon cover. The major growth rate depending parameters are the carbonprecursor flow rate, temperature and plasma power. These groups usedtemperatures of 500–600 ¶C, but as we experienced a significant decrease inquality we decided to keep the temperature as high as possible. The carbonprecursor flow rate and the plasma power were limited to 20 sccm and 20 Wfor practical equipment usage. However, these groups used dierent typesof PECVD setups which creates other conditions.


A reason for the unsuccessful nickel treatments could be a tendency ofnickel to mix with carbon and GaN at elevated temperatures. As we thenimmerse the samples in a metal etchant most of the intermixed carbonwill vanish as well and the remaining nickel doped GaN has changed ap-pearance and conductivity. To successfully grow graphene using nickel onemight need to do some further research on the growth mechanism. Oneadvantage of using nickel is that it has a higher melting point than copperand hence it can be kept in a solid phase during the treatment, whichleads to a very homogeneous result.

The high value measurements in Figure 18 could have been related toareas where the samples had regions of dierent appearance. As we keptthe temperature during these experiments just at the limit of melting

28

4.3 conclusions

or dewetting, the samples had regions of dierent phases which couldbe the reason for the inhomogeneous appearance. The unrealistically lowmeasurements were probably related to copper residues or doping eects.In a similar work made by Ismach et al. they managed to produce grapheneon various dielectric substrates with the assistance of a metal layer [9].They had copper evaporated prior to their growth. However, in their workthey did not avoid the dewetting of the copper. This might have been amore eligible strategy which could have been tested for the 400 nm copperlayer both prior and subsequent to growth. We tried this for a thin copperlayer of only 100 nm were the surface was dewetted in a less than a minuteafter reaching the final growth temperature. However, by adopting thesame method for a thicker copper layer, the dewetting would take placeat a slower rate which would give the carbon more time in the presenceof copper and a higher temperature might be able to use.

4.3 conclusions

In our eort to produce a graphene based TCE for GaN LEDs we didnot manage to achieve a sucient result at this stage. We believe thatthe relatively high growth rate is a major reason for this, but that thegraphene quality is going to be limited by the lattice mismatch betweengraphene and GaN in a direct growth approach.

We believe that the possible intermixing eects of nickel makes it dif-ficult to use for these metal treatments. However, previous studies hasshown that copper assisted growth can work on various dielectric sub-strates and we managed to achieve a penetrating growth using 400 nmcopper prior to growth. By optimizing these growth parameters as well asmetal thickness using a low growth rate there can be progress to make.

4.4 suggested future work

To improve the quality and thereby resistance of the direct grown films wesuggest that a low growth rate is used. This can be further investigated bylooking deeper at the influences of temperature, plasma power, precursorflow and hydrogen partial pressure.

There is an evident potential in using metal assisted methods. The useof copper rather than nickel seems more promising. We managed to growcarbon below a copper layer deposited on GaN without much eort or priorexploration. This technique can be further analysed to get the optimalgrowth conditions and copper thickness. Also, trying to use copper withthe dewetting method using a metal thickness of more than 100 nm wouldgive the carbon more time in the presence of the copper. With this methodthe amount of residues from the copper seems to be very small and thecarbon film appears to be undamaged.

Research has kept pushing the limit of CVD graphene and it is just amatter of time before it will be able to compete with the current standardsof ITO TCEs.

29

R E F E R E N C E S

[1] Sukang Bae, Hyeongkeun Kim, Youngbin Lee, Xiangfan Xu, Jae-SungPark, Yi Zheng, Jayakumar Balakrishnan, Tian Lei, Hye Ri Kim,Young Il Song, et al. Roll-to-roll production of 30-inch graphenefilms for transparent electrodes. Nature nanotechnology, 5(8):574–578, 2010.

[2] Xuesong Li, Weiwei Cai, Jinho An, Seyoung Kim, Junghyo Nah,Dongxing Yang, Richard Piner, Aruna Velamakanni, Inhwa Jung,Emanuel Tutuc, et al. Large-area synthesis of high-quality and uni-form graphene films on copper foils. Science, 324(5932):1312–1314,2009.

[3] Jie Sun, Matthew T Cole, S Awais Ahmad, Olof Bäcke, Tommy Ive,Markus Löer, Niclas Lindvall, Eva Olsson, Kenneth BK Teo, JohanLiu, et al. Direct chemical vapor deposition of large-area carbon thinfilms on gallium nitride for transparent electrodes: a first attempt.IEEE Trans. Semicond. Man., 25(3):494–501, 2012.

[4] Zhao Yun, Wang Gang, Yang Huai-Chao, An Tie-Lei, Chen Min-Jiang, Yu Fang, Tao Li, Yang Jian-Kun, Wei Tong-Bo, Duan Rui-Fei,et al. Direct growth of graphene on gallium nitride by using chemicalvapor deposition without extra catalyst. Chinese Physics B, 23(9),2014.

[5] Yong Seung Kim, Kisu Joo, Sahng-Kyoon Jerng, Jae Hong Lee, Daey-oung Moon, Jonghak Kim, Euijoon Yoon, and Seung-Hyun Chun. Di-rect integration of polycrystalline graphene into light emitting diodesby plasma-assisted metal-catalyst-free synthesis. ACS nano, 8(3):2230–2236, 2014.

[6] Lianchang Zhang, Zhiwen Shi, Yi Wang, Rong Yang, Dongxia Shi,and Guangyu Zhang. Catalyst-free growth of nanographene films onvarious substrates. Nano Research, 4(3):315–321, 2011.

[7] Jaeho Kim, Masatou Ishihara, Yoshinori Koga, Kazuo Tsugawa,Masataka Hasegawa, and Sumio Iijima. Low-temperature synthesisof large-area graphene-based transparent conductive films using sur-face wave plasma chemical vapor deposition. Applied physics letters,98(9):091502, 2011.

[8] Golap Kalita, Koichi Wakita, and Masayoshi Umeno. Low temper-ature growth of graphene film by microwave assisted surface waveplasma cvd for transparent electrode application. RSC Advances, 2(7):2815–2820, 2012.

[9] Ariel Ismach, Clara Druzgalski, Samuel Penwell, Adam Schwartzberg,Maxwell Zheng, Ali Javey, Jerey Bokor, and Yuegang Zhang. Directchemical vapor deposition of graphene on dielectric surfaces. Nanoletters, 10(5):1542–1548, 2010.

[10] The 2014 nobel prize in physics - press release, October2014. URL http://www.nobelprize.org/nobel_prizes/physics/

laureates/2014/press.html.

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http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/press.html

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[16] Konstantin S Novoselov, Z Jiang, Y Zhang, SV Morozov, HL Stormer,U Zeitler, JC Maan, GS Boebinger, P Kim, and AK Geim. Room-temperature quantum hall eect in graphene. Science, 315(5817):1379–1379, 2007.

[17] Jian-Hao Chen, Chaun Jang, Shudong Xiao, Masa Ishigami, andMichael S Fuhrer. Intrinsic and extrinsic performance limits ofgraphene devices on sio2. Nature nanotechnology, 3(4):206–209, 2008.

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32

AP L A S M A WAV E FO R M A N A LY S I S

The plasma power supply can be controlled using either a pure DC volt-age or a user-specified waveform. A reason for using a waveform poweredplasma is to prevent the substrate from accumulating charges and even-tually lead to an electric breakdown through arc discharge, which candamage the substrate. The waveform is somewhat customizable by speci-fying the frequency (1–100 kHz) and inverse time (1–10 µs). However, theinverse time cannot exceed 1/3 of the period. Figure 20 shows two exam-ples of how the waveform can look like and their corresponding frequencyspectra. In the example to the left of Figure 20 the waveform is specifiedto have a frequency of 10 kHz and an inverse time of 10 µs. We can see inthe frequency spectra that there is a large bias component and an infinitenumber of frequency components that decays and are multiples of 10 kHz.We know that modifying the frequency of the signal will of course alsoreflect the frequency spectra to show multiples of the specified frequency.However, when we change the inverse time the corresponding change inthe frequency spectra is not as intuitive. Figure 20 also shows the signaland its spectra when we change the inverse time to 100/3 µs or 1/3 of theperiod. We clearly see in the frequency spectra that the non-zero frequencycomponents becomes more significant compared to the voltage bias. As wecan expect, the non-zero frequency components vanishes when the inversetime approaches zero.

0 100 200 300 400 500−1000

−500

0

Time (µs)

Volta

ge (V

)

0 0.05 0.1 0.15 0.2 0.250

0.5

1

Frequency (MHz)

|Y(f)

| (a.

u.)

0 100 200 300 400 500−1000

−500

0

Time (µs)

Volta

ge (V

)

0 0.05 0.1 0.15 0.2 0.250

0.5

1

Frequency (MHz)

|Y(f)

| (a.

u.)

Figure 20: The plasma voltage waveform and its frequency spectra for a 10 kHzand 10 µs inverse time signal (left) and a 10 kHz and 100/3 µs inversetime signal (right).

33

BS U P P L E M E N TA RY E X P E R I M E N TA L D E TA I L S

In this appendix follows a detailed enough description of all the samplepreparation steps carried out before the actual experiments and measure-ments can take place.

b.1 dicing

The GaN samples are prepared by dicing ≥50 mm (2-inch) GaN wafersinto 7 mm ◊ 7 mm chips using a Loadpoint Microace 3+ dicing saw. Thisis done with a spindle speed of 20 krpm and a feed rate of 0.2 mm/s usinga 250 µm thin and 58 mm in diameter blade (K-010-325-H). The 400 µmthick wafers are cut by leaving 25 µm of the thickness uncut to avoid lossof chips during the dicing process.

b.2 cleaning

Before the CVD takes place the GaN samples are cleaned using commonsolvents used in many sample cleaning procedures. The samples are firstand foremost placed in a hot bath of acetone at 50 ¶C and ultrasonicatedfor 5 min. After that, the samples are rinsed in a beaker of methanol andlastly rinsed in a beaker of isopropyl alcohol.

b.3 growth optimization

The intention with the first few experiments were to obtain some growth.The initial test was done by using the parameter values listed in Table 3.From this PECVD treatment some carbon deposition could be observedon the substrate, but the uniformity was poor and the resistance, mea-sured with a simple multimeter, was high (in the order of M) and barelynoticeably lower than the bare substrate. NH

3

was used as a reductiongas because it was believed that this would compensate for some nitrogenloss in the GaN substrate that would lead to damage as mentioned in Sec-tion 2.2.1. These parameter values were then changed to some extent butno significant improvements could be detected. As an example 300 sccmof NH

3

flow was used keeping the other conditions at the same values asin Table 3, but with no substantial progress.

For this reason we moved on to try acetylene instead of methane, sincethis gas is know to decompose more easily than methane which makesmore carbon available. At first, something similar to the recipe in Table 3was used, but with 10 sccm C

2

H2

instead of CH4

/Ar. This gave quite uni-

34

references

parameter value

Plasma power 80 WTemperature 700 ¶CGrowth time 5 minNH

3

flow 500 sccmCH

4

/Ar flow 300 sccm

Table 3: First recipe.

form deposition and high resistance but no improvements could be seenwhen the growth time and the precursor flow were increased. Therefore, itwould be of interest to see if the use of hydrogen instead of ammonia couldmake a dierence. To make the plasma work properly when using hydro-gen we also added some argon as inert gas. This next recipe is presented inTable 4. It resulted in a significant improvement where growth could be ob-

parameter value


2

flow 600 sccmAr flow 200 sccmC

2

H2

flow 10 sccm

Table 4: Recipe introducing hydrogen.

tained consistently and where alterations in the parameter values wouldcorrespond better to the expectations. For the first run using hydrogenwe detected a resistance in the order of ≥100 k with the multimeter. Bychanging the growth time and the precursor flow the consecutive exper-iments would show expected reduction in the resistance (down to a fewk) as well as a darker coverage on the substrates, most likely as a resultof a thicker carbon layer. A recipe using methane instead of acetylene byreplacing the argon and acetylene in Table 4 with 200 sccm of CH

4

/Arwould give similar results. This recipe using methane was also tested with-out hydrogen which had a clear eect on increasing the growth rate as thiswould give a very dark sample. The developed recipe from Table 4 withmethane was then further investigated at dierent temperatures: 750 ¶C,800 ¶C, 850 ¶C, 900 ¶C and 940 ¶C (temperatures above 940 ¶C where di-cult to obtain with the used equipment and settings). This also seemed tohave a positive correlation to the growth rate. However, at temperaturesabove 900 ¶C the samples were getting a matt surface which could be in-dicating a roughening in the surface structure, i. e. damage. In Figure 21we can see the induced roughness on a GaN sample exposed to 940 ¶C.One reason for using plasma is to lower the temperature so that no tem-

35

references

200 nm

Figure 21: Tilted SEM image of GaN exposed in high temperature growth of940 ¶C.

perature related damage will occur; henceforth, we kept the temperaturebelow 850 ¶C.

Summarizing the progress so far we now have a recipe presented byTable 5. Here the methane/argon mixture has been reduced in order tohave a slower growth rate and extra argon has been introduced to keepthe pressure high enough. The growth rate is believed to influence on theoverall quality of the grown layer, where a more patient growth is prefer-able for the quality. Therefore, we are investigating this by reducing themethane flow and increasing the growth time. As we learned more aboutthe growth process and mechanism, the focus was then on managing togrow a layer that is just noticeable to the naked eye and keeping the sametransmittance through new experiments while trying to improve the sheetresistance. We tried dierent methane and hydrogen flows and changed

parameter value


2


4

/Ar flow 100 sccm

Table 5: Recipe combining hydrogen and methane.

the argon to work with the plasma. After growing more than 60 sampleswe made a final decision for a recipe that we judged resulted in the bestrelation between transparency and resistance.

b.4 photolithography

In this work we used a well tested photolithography recipe to fabricatethe Hall Bar devices. The device pattern is shown in Figure 13 and thephotomask consists of nine such patterns in a three by three array. Thisphotomask has an inverse metal pattern compared to the final metal pat-terns on the samples, therefore we used a recipe with a positive photoresist.

36

references

Figure 22 illustrates the process steps and the following list contains a de-tailed description of each step.

Mask

UV-light

2 31

5 6

9 10

4

87

CarbonGaNPhotoresistMetal

Figure 22: The process steps in the fabrication of Hall Bar devices with pho-tolithography.

1. We applied one drop of positive photoresist S1813 onto the carbon/-GaN samples, spin coated at 3000 rpm for 1 min and then baked thesamples at 110 ¶C for 1 min.

2. Next, we loaded the samples into a Karl Süss MA/BA6 mask alignerand exposed the samples for 10 s.

3. The exposed areas are now easily removed, but prior to developingwe submerged the samples in toluene for 2 min. Toluene will hardenthe surface of the photoresist which will create the preferable neg-ative profile that is advantageous for metal deposition and lift-o.Instantly after this treatment we blow dried the samples with pres-surized N

2

gas and then developed the samples in MF319 developerfor 80 s followed by rinsing in water and finally blown dry again.

4. The samples are now ready for metal evaporation. We first evapo-rated 5 nm titanium at a rate of 0.5 Å/s followed by 95 nm gold at arate of 2 Å/s using an electron beam evaporator.

5. We removed the remaining photoresist using a remover called 1165heated to 90 ¶C. We applied a gentle force using a pipette to flushthe samples with the remover. When all the desired metal appearedto have been lifted-o we rinsed the samples in IPA and blew themdry in pressurized N

2

gas.

6. Now, we repeated step 1 with the only dierence that we used arotation speed of 6000 rpm when spin coating our samples. We didthis because the thickness of the photoresist is not as crucial for theobjective which is to provide a cover for the underlying carbon.

7. We did another UV-lithography as in step 2 but exposed only for 6 sinstead.

8. For the development we used the same procedure as in step 3 butwithout the toluene treatment and we developed for only 60 s thistime.

9. To remove the uncovered carbon areas we etched the samples byoxygen plasma at 50 W for 1 min.

37

references

10. Finally, we removed the last remaining photoresist in acetone heatedto 50 ¶C for 10 min.

b.4.1 TLM fabrication attempts

The TLM photomask we used is an exact copy of the wanted metal pat-terns that we want to project on our samples. We tried two dierentmethods of which none was completely successful. In the first recipe weactually used a positive photoresist AZ5214E which has image reversalcapabilities that activates at temperatures above 110 ¶C. This recipe wasconducted with two p-GaN/carbon samples as follows.

1. We applied one drop of AZ5214E and spin coated for 30 s at 4000 rpmthen baked for 60 s at 110 ¶C.

2. We then exposed the samples with the photomask for 3.5 s.

3. Next, we reverse baked the samples for 90 s at 125 ¶C.

4. We did a flood exposure (without mask) for 90 s.

5. Finally we developed one of our samples (to check the result) for30 s in AZ351B diluted 1:5 in DI water.

Using this recipe we discovered residues of the resist everywhere on thesamples with one area possibly having less resist than the other. By thelooks of the sample from optical microscope images in Figure 23 it seemsalmost as if the thicker photoresist cover is on the opposite areas fromwhere we want the photoresist to be. The final sample with metal patternshould have metal remaining after lift-o on the rectangles and circles. Wetried to develop this sample further but with no evident progress. Due toa limited amount of time we had no possibility to investigate the optimalreverse bake temperature, which apparently is a quite critical procedurewhere too high temperature can lead to crosslinking all over the sample.In a second attempt we tried a recipe with a pure negative photoresist

Figure 23: Optical microscope images of TLM photoresist patterns .

instead. This time the recipe was as follows.

1. We applied one drop Ma-N1410 negative photoresist and spun for1 min at 4000 rpm then baked for 90 s at 100 ¶C.

38

references

2. We then exposed the samples with the photomask for 5 s.

3. Next, we developed in ma-D 533s for 40 s, rinsed in water and blewthem dry.

4. We made an identical evaporation as previously mentioned (evapo-rated them at the same time) with 5 nm/95 nm of Ti/Au (see step4 on page 37 for a more detailed description).

5. The same technique were used for the metal lift-o as before (seestep 5 on page 37 for details).

Unfortunately, the metal lift-o was not successful as the metal was di-cult to remove and was coming o everywhere. We can see the result of thelift-o process in Figure 24. The reason could be an insucient negativeprofile of the photoresist making the deposited metal continuous all overthe sample. Another possible reason could be that the photoresist layer istoo thin or that there are residues of the resist on areas where it is notsuppose to be.

Figure 24: Optical microscope images of TLM pattern after metal lift-o attempt.

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