NANOELECTRONICS

Nanoengineering of materials for field emissiondisplay technologies

S.R.P. Silva, J.D. Carey, G.Y. Chen, D.C. Cox, R.D. Forrest, C.H.P. Poa, R.C. Smith, Y.F. Tang and J.M. Shannon

Abstract: The holy grail in terms of flat panel displays has been an inexpensive process for theproduction of large area ‘hang on the wall’ television that is based on an emissive technology.Electron field emission displays, in principle, should be able to give high quality pictures with goodcolour saturation, and, if suitable technologies for the production of cathodes over large areas wereto be made available, at low cost. This requires a process technology where temperatures must bemaintained below 4501C throughout the entire production cycle to be consistent with the softeningtemperature of display glass. In this paper we propose three possible routes for nanoscaleengineering of large area cathodes using low temperature processing that can be integrated into adisplay technology. The first process is based on carbon nanotube–polymer composites that can bescreen printed over large areas and show electron field emission properties comparable with someof the best aligned nanotube arrays. The second process is based on the large area growth of carbonnanofibres directly onto substrates held at temperatures ranging from room temperature to 3001C,thereby making it possible to use inexpensive substrates. The third process is based on the use ofexcimer laser processing of amorphous silicon for the production of lithography-free large areathree terminal nanocrystalline silicon substrates. Each route has its own advantages and flexibilityin terms of incorporation into an existing display technology. The harnessing of these synergies willbe highlighted together with the properties of the cathodes developed for the differing technologies.

1 Introduction

The flat panel display (FPD) market is one of the largestconsumer electronic sectors, with sales within the US aloneexceeding one billion dollars per annum. There are manycompeting FPD technologies, as shown in Fig. 1, withactive matrix liquid crystal displays (AMLCDs) leading theway. For the larger flat displays, plasma display panels(PDPs) dominate, however, recent developments by Sam-sung have seen the emergence of AMLCDs with 52 inchdiagonal screens. Samsung have also produced a prototype38 inch full colour video rate carbon nanotube (CNT)display which shows all the positive attributes associatedwith field emission such as high brightness, high contrast,excellent viewing angles, low power consumption and largearea. Other field emission display (FED) technologies basedon metal ‘Spindt’ tips favoured by companies such asCandescent, Pixtech and Motorola have all delivered highquality displays. The UK based company, Printable FieldEmitters, has opted for a screen printed graphite–silicondioxide binder cathode to make their large area displays.Canon, Toshiba, Noritake, MEW and Sony all have theirown field emission (FE) based technologies currently beingdeveloped for different segments of the market in this fastmoving sector. All these companies see the merits associatedwith having a fully scalable FED technology, but need thecost of production to be lowered in order to enter theconsumer market. Other emerging display technologies

vying for honours in this sector include polymer andorganic light emitting diodes (OLEDs), with no onetechnology being able to show all the attributes neededfor a high quality large flat display that can be produced ata suitable cost and scale (for example [1]).

FEDs operate in a manner which is a hybrid of theAMLCD and the PDP. The addressing of the pictureelements is based on the matrix address system developedfor AMLCDs, with the emissive display componentshowing similarities to the PDP output. Hundreds ofmultiple gated matrix addressed field emission cathodesemit electrons that hit a single pixel, whose brightness to afirst order is controlled by the acceleration voltage appliedbetween the cathode and the phosphor anode. The keyphysical parameters of importance in selecting a suitablecathode material for such an application is, in addition to itslongevity, robustness and an ability to readily integrate intoa production process, the requirement of being able tosource high current densities at relatively low electric fields.In addition, an ability to produce uniform electron emissioncurrent–voltage characteristics with little or no hysteresis isalso required. This tightness of the electron emission curveswith applied field is important in being able to design matrixdriver strategies with the required precision, where suitableoffset voltages can be used for turning on gated cathodes. Interms of phosphors, standard high and medium voltagephosphors are at present preferred over the low voltagevariety due to the reliability and testing that has beenperformed in both the CRT arena and plasma displays.

The results presented in this paper are based on threecompeting strands, all working towards a common goal ofaffordable large area FED cathodes. The first technology isassociated with the production of inexpensive cathodesbased on mixing CNT–polystyrene (PS) composites that arecastable or screen printable over large areas [2]. Their

The authors are with the Advanced Technology Institute, School of Electronicsand Physical Sciences, University of Surrey, Guildford GU2 7XH, UK

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doi:10.1049/ip-cds:20040996

Paper first received 31st October 2003 and in revised form 25th May 2004

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performance as an electron source is tested against othercompeting nanotube (NT) emitters, as well as Spindt tips.The key marketable features of CNT–composite emittersare reliability and costs, which compare favourably with theperformance of other more expensive and complicatedcathodes. We show that the current density and thresholdfields afforded by these cathodes are some of the bestavailable in the published literature.

The second technology introduced in this paper isassociated with the availability of a low temperature growthprocess for the production of carbon nanotubes and carbonnanofibres (CNFs) [3]. In this case we show that thestructure of the deposited material is ideally suited toelectron field emission, and due to the very low temperatureplasma based growth process, can easily be incorporatedinto a large area display technology. We highlight theadvantages associated with a direct in situ cold cathodegrowth environment, that gives nanometre precision in theCNT/CNF growth due to the requirement of a suitablecatalyst, with the correct dimensions [4]. Due to the lowgrowth temperature, the requirement for a barrier layerbetween the catalyst and the substrate to prevent thetransition metal from diffusing may be relaxed. The plasmabased growth process is also scalable to very large areas inan inexpensive manner, and can grow materials on non-uniform and non-conformal surfaces such as gated cathodetracks. The technology also lends itself to domestic lightingapplications [5].

The third technology is based on nanosilicon fieldemitters and was originally introduced in order to exploita holistic approach to the production of FEDs. The idea isthat as amorphous silicon and polycrystalline silicon thinfilm transistors (TFTs) were already being prepared oververy large areas in AMLCDs, if the electron emissioncomponent of the FED could also be produced using thesame material it would allow a seamless transition of the

thin film silicon fabrication facilities to a FED technology.We show how amorphous silicon excimer laser crystallisedover large areas can be used as a cathode material. Theexcimer laser treatment, at low energy densities, affects onlythe top few nanometres of the a-Si:H layer and producesnanocrystalline structures, which would give rise to theenhanced emission properties. This process is then extendedto produce lithography–free three-terminal field emissionstructures using large area processes that lends itself to fullyintegrated low cost thin film silicon based cold cathodetechnology.

2 Electron field induced emission

Field emission is the extraction of electrons from a surfaceunder the influence of an applied electric field. The frontsurface potential barrier for electron emission is reduced bythe application of voltage Va to an anode located at adistance D away. Far from the emitter surface themacroscopic field is simply Va/D. For tip based structuresthis macroscopic field is enhanced in the neighbourhood ofan emitter by a geometric field enhancement factor b. Thisresults in a local electric field which is larger than theapplied field. The most common definition of the fieldenhancement factor b, is the ratio of the local field to theapplied field. In the case of an isolated vertically alignedCNT in the electrode geometry presented in Fig. 2a, thelocal field depends on the height h, radius r and anode–substrate separation D. Shown in Fig. 2b are the results of a2D electrostatic simulation for the variation of b with D forthree metallic tubes of heights 2, 4 and 6mm capped with ahemisphere. It is apparent that the enhancement factor inthis case is only constant (i.e. independent of the electrodegeometry) when D is about (2–2.5)h. As a result care mustbe taken in analysing FE measurements on electrodegeometries similar to the one described in Fig. 2a to ensurethat the anode is sufficiently far away from the emitter forthis effect to be ignored. The discussion above is based on asingle isolated emitter. When there are a large number ofemitters nearby screening of the applied field can occur.This has been observed experimentally by Nilsson et al. whoconcluded that screening is an important factor when theintertube separation is less than twice the nanotube length[6].

In terms of experimental method, FE measurements maybe conducted in a number of ways. In this paper twocommon methods are employed. The first utilises a sphere-to-plane geometry with a 5mm stainless steel ball bearingsuspended 30mm above the cathode surface with a highpositive potential applied in a vacuum better than 4 106

mbar. Although a spherical anode is used it is assumed thatthe electric field between the anode and the surface of thefilm can be modelled as a parallel plate. The probe anode isattached to a high precision motor allowing FE measure-ments to be made for varying anode–substrate distance. Thethreshold field, Eth, is defined as the macroscopic electricfield which gives an emission current of 1nA. It hassometimes been observed that there are differences betweenthe upward and downward I–E cycles and that sometimes alarge initial field needs to be applied. This is referred to as‘conditioning’ of the emitter and tends to be present in lessconductive materials such as amorphous carbon thin films[7]. As a measure of this ‘hysteresis effect’ the difference inthe applied fields between the upward and downwardvoltage cycles at a current of 1nA is used. The second wayof performing FE measurements uses a planar anode,usually a transparent conducting oxide, onto which

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Fig. 1 Generalised classification of displaysDMD – digital micromirror display, CRT – cathode ray tube, LCD –liquid crystal display, FPD – flat panel display, TFT – thin filmtransistor, MOS – metal-oxide-semiconductor, FLC – ferroelectricliquid crystal, PDLC – polymer dispersed liquid crystal and OLED –organic light emitting diodes

490 IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004

phosphor has been coated to allow emission site densitymeasurements to be made.

3 Carbon nanotube–polymer composite cathodes

Multiwalled carbon nanotubes (MWNTs) and boron-doped MWNTs (B-MWNTs) were produced by the arcdischarge method [2]. In the case of B-MWNTs boron–containing electrodes were used. The resultant tubes werethen purified using the vacuum-filtering technique and werethen oxidised at 500–6001C for 40–50min to remove theamorphous carbon and carbon particles as describedelsewhere [2]. Transmission electron microscope images ofthe purified tubes showed that about 90% of the tubes havediameters in the 10–50nm range. By mechanically bendingthe film to produce a 4mm fracture it is possible to allowsome of the tubes to appear out of the composite as shownin Fig. 3a. Electron energy loss spectroscopy indicates thatthe B concentration is about 1 at. %. Nanotube polymercomposites were prepared by mixing with polystyrene andtoluene and using vacuum casting techniques. The as-castfilms were hot pressed to remove residual solvent andprevent voids in the films as described elsewhere [2]. Thevariation in the film’s surface resistivity with nanotubeloading is shown in Fig. 3b for both doped and undopedNTs where it can be observed that the sheet resistivity of the

doped composite falls from 2.5O cm to 5 104O cm asthe nanotube loading increases from 11% to 33%. Over thesame nanotube loading range the sheet resistivity of theundoped composite film falls from 15 to 0.1 O cm. The filmresistivity decreases as the CNT mass fraction increaseswhich indicates that electrical conduction across the film ismainly controlled by the CNT network. The lower sheetresistivity of the B–containing film is attributed the presenceof B doping.

The variation of the threshold field for emissionmeasured using a probe anode with the CNT concentration

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is shown in Fig. 3c, where it is apparent that the thresholdfield increases with the CNT content for both MWNT- andB-MWNT-PS films. Despite the B-MWNT being moreconductive (as inferred from the sheet resistivity measure-ments) there is no significant difference of Eth comparedwith the undoped MWNT. Indeed, it can also be observedthat the threshold field of the doped film is higher than thatof the undoped film at low nanotube concentrations. Theincrease in threshold field with CNT content is probablydue to the field screening effects as described previously atthe composite surface.

The FE characteristics measured using the planar set upare shown for the undoped and doped composites in Fig. 4aand b. The threshold fields measured are similar to thoseobtained from the single probe method. The current densityis determined across a fixed test area of 5mm2 and theemission area is controlled by the shape of polytetrafluor-oethylene spacer during the experiment. All of the filmsshow current saturation at the high current density of about4 10–4A/cm2. The origin of the current saturation effect isunclear and has been attributed to several sources such asconduction limited transport in the cathode [8], contactresistance [9], vacuum space-charge effects, [10] and to theeffect of absorbates atoms on the CNT surface [11].However, for a typical field emitter with a work functionof 5 eV and an estimated vacuum space-charge emission

current density of 108 A/cm2, then the saturation current fora single CNT with an emission area of 100nm2 would occurat a current of 10–4 A. Based on the current saturation levelsshown in Fig. 4, which are the limiting values discussed fora single emitter, vacuum space-charge effects appear to beunlikely in this case where multiple emitters are present. Therole of energy balance between the apex and body of theemitter as well as dynamic thermal effects such as theNottingham cooling effect may also need to be taken intoconsideration [12].

The emission site density maps for a 3mm2 surface areaof B doped MWNT with 20% nanotube loading in PS isshown in Fig. 5. It can be seen that the emission across the

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Fig. 5 Emission site density maps made over a 3 mm2 area from a20% B-doped MWNT-PS composite at different applied fieldsThe maps are taken over the same areaa 5V/mmb 6 V/mmc 7 V/mmd 8 V/mm

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phosphor plate increases as the applied field increases from5V/mm to 8V/mm. The estimated emitter site density is1.3 107 cm-2. Figure 6 shows the variation of emissioncurrent over 50h for a 20% MWNT-PS film. The currentdensity was measured before and after current saturationeffect at 0.8mA/cm2 and 1.2mA/cm2 respectively. Despiteslight variations initially for a current density of 1.2mA/cm2, stable emission has been observed over the 50hduration. Such a value of current density is suitable for aFED structure.

4 Low temperature nanostructured carboncathodes

There is general agreement that the mechanism for CNTgrowth proceeds with C adsorption on to the surface of ananometre-sized Ni catalyst (usually in the form of nearspherical islands) followed by bulk diffusion of C throughthe Ni forming a supersaturated solution from whichcarbon precipitates [13]. The Ni is raised from the substrateitself which is energetically favourable especially for smalldiameter islands–this is known as the ‘tip’ growth mechan-ism. One disadvantage of using Si as a substrate, especiallyat the high temperature at which CNT growth is usuallyperformed, is that a Ni silicide can be formed and as a resultthere has been considerable effort in using alternative oxidesubstrates onto which Ni is deposited [14]. By growing CNF(which are larger diameter CNTs) at lower temperatures,the formation of the silicide is largely suppressed. The driveto lower temperatures has also resulted in the study of otherways to produce nanometre sized Ni structures. One suchway is to use plasma treatment of Ni thin films. Laserannealing of Ni has also allowed for a low temperatureroute to produce large area Ni catalyst for CNT growth[15].

Carbon nanofibres (CNFs) were grown on Ni thick filmswhich had previously been subjected to an Ar ion plasma.The Ar plasma treatment of the Ni film produces aroughening of the film surface and the appearance ofcircular features with diameters between 3mm and 500nanometre. Nanofibre growth was then achieved using CH4

as a source gas at a pressure of 1 Torr. The reverse power ofthe plasma was continually adjusted to keep it as close tozero as possible in order to prevent substantial substrateheating. The water cooled substrate table temperature wasmonitored to be about 301C throughout the deposition. ASEM image of the surface after growth (Fig. 7), shows fibre-like material arranged radially in clusters forming starshaped objects, approximately 50mm in size. The nanofibres

predominantly grew on the surface rather than verticallyaligned, and towards the centre of the cluster a higherconcentration of fibres was presented, with diameters of100–400nm, as can be seen in Fig. 7a. No evidence of metalparticles in the tips of the CNFs is observed suggesting thatgrowth does not proceed in a ‘tip-based’ growth process.The FE characteristics of the CNFs were examined using asphere-to-plane probe testing system. The film deposited atroom temperature displayed excellent emission character-istics with a threshold field of 5V/mm as shown in Fig. 7b.The first two current–voltage characteristics are presentedand no significant difference between the two can be seen.Further voltage cycles show the same behaviour, whichimplies that there is no need for a ‘conditioning’ phase. Inaddition, there is no evidence of hysteresis behaviourbetween the upward and downward cycle of either I–Echaracteristic. To exclude the possibility of emission fromthe roughened Ni film substrate, or from the surroundingamorphous carbon film, the emission characteristics fromthese two samples were also tested. In the case of the formeronly background noise was observed and from the latter athreshold field 20V/mm was required for emission. Thispreliminary study shows promising FE characteristics arepossible from low temperature grown CNFs [16].

5 Nanosilicon based material cathodes

In the realms of large area FEDs, the possibility ofexploiting silicon based materials as the electron source hasa number of significant advantages. Silicon, especially the

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amorphous and polycrystalline forms, is now well estab-lished as the preferred choice in industry for display driverelectronics as well as in other large area electronicapplications such as photovoltaic devices. In recent timesthe ability to produce micro- or polycrystalline silicon overlarge areas has negated the need to use large area andexpensive single crystalline silicon as substrates. While FEfrom tip based crystalline silicon has led to silicon fieldemitters arrays (FEAs) [17] the problems associated with thescalability of this emitter system has contributed topreventing this cathode system being exploited in FPDs.Indeed, many of the advantages of silicon that are oftenquoted to justify its use as a FEA material, such as excellentelectronic properties, ready availability coupled with a well–developed growth and fabrication process can equally beapplied to micro- and polycrystalline Si. An early study [16]of FE from PECVD grown a-Si:H showed that an initialconditioning process was required before the onset of stableand reproducible emission could be observed. Afterconditioning a threshold field of about 5V/mm wasmeasured. These films have a low defect density (1016 cm-

3) and possess about 10 at. % H and as such are similar tothose found in device applications such as the switchingelements in matrix addressed screens. Lifetime tests madeon a-Si:H cathodes demonstrated that for a matrix lineaddressed image with a frame time of 20 ms, a lifetime of25,000 hours was possible [18]. The mechanism of emissionproposed was based on the introduction of hot electronsfrom the substrate into the conduction band of theamorphous silicon film which are then accelerated towardsthe front surface due to a combination of depletion field atthe back contact and the high degree of field penetrationfound in this low defect density material [18].

One of the principal disadvantages of a-Si:H thin films isthe relatively low carrier mobility. As a consequence efforthas been employed to improve the electrical properties ofamorphous silicon based materials. Thermal furnaceannealing for several hours at high temperatures has beenemployed but the temperatures usually used are often abovethe softening temperature of the typical soda glass used indisplays. An alternative is to perform rapid thermalannealing in which the glass substrate is held at a hightemperature for a short time such that softening does notbecome a significant factor. One disadvantage of RTA isthat the grain size of the resultant film is smaller than thatfound after furnace annealing. Furthermore, both types ofannealing result in modification of the whole thin film, as aresult alternative techniques which allows controlledmodification, ideally on a nanometre scale, of the regionnear the film surface are attractive. One such technique isthe use of laser crystallisation in which the large uniformbeam size coupled with a suitable choice of wavelengthallows large area modification but in which the high transferof energy takes place within a narrow nanometre-sizedregion near the surface. Laser crystallisation of amorphoussilicon has been performed using both excimer laserexcitation [19] at 248nm and a frequency doubled Nd:YAGlaser operating at 532nm. [20]. Both the grain size andsurface RMS roughness can be controlled by a propercombination of energy density and/or number of pulsesemployed. FE measurements made on excimer lasercrystallised a-Si:H films at an energy density of 204mJ/cm2 showed that a threshold field for emission of about15V/mm was required. It was reported that when theemission was imaged on a phosphor plate at a field of 30 V/mm, the site density coverage was in excess of 80%.Individual emission sites could not be imaged as theemission observed was uniformly bright throughout except

for two or three very high brightness regions. Significantly,it was observed that no conditioning stage was required forthe onset of stable emission and that the hysteresis in theupward and downward voltage cycle is less than 1V/mm. Acareful study of the microstructure of the crystallised filmusing transmission electron microscope images showedevidence of columnar Si nanocrystals around 90nm inheight and around 50nm in diameter, surrounded by grainboundaries. The observation of FE at an applied field of 15V/mm compares favourably with that of emission from Sinanowires B15V/mm [21], from poly-Si by LPCVD withoxidation sharpening [22] and poly-Si microtips [23] atB20V/mm as well as from single c-Si microtips [24] at around 20V/mm.

Laser crystallisation of a-Si:H has also been applied toproduce a self-aligned microtip array without the need forcomplex lithography or processing [25]. Using an excimerKrF laser at an energy density of 242mJ/cm2, crystallisedstrips of films 4mm wide were produced. This crystallisedarea was then capped with a 150nm thick a-SiN layerfollowed by a 25nm layer of evaporated Cr as schematicallyshown in Fig. 8a. An atomic force microscope image(Fig. 8b) of the surface shows it to be uniformly covered in adensely packed irregular array of spheroid structures about300nm in height and with an estimated emitter density of108–109 cm2. The crystallised film was then subjected to anRF/microwave reactive ion (CF4/Ar) etch treatment of200WRF power, 900Wmicrowave power. It was observedthat etching for different times up to 14min resulted insuccessive etching of the Cr and SiN layers and theemergence of the Si tip with the tip level with the Cr rim, asshown in Fig. 9. Etching for longer times (21min) resultedin the Si tip lying below the Cr rim as shown in Fig. 9d.However, while the self-aligned array structure is obvious,

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breaks in the gate insulator can be observed for this etchingtime. For this structure to be of practical use, control of theleakage current as well as a balance between etching of thegate rim material and optimum height of the tip materialneeds to be found.

6 Conclusions

In conclusion, we have described and characterised threepotential cathode systems that could be used in the next

generation of field emission flat panel displays. We haveshown that by engineering the cathodes at a nanometre levelit is possible to tailor the emission characteristics. ForCNT–polymer composite the ability to adjust the nanotubeconcentration allows us to control the threshold field andemission site density. Carbon nanofibre emitters grown atsubstrate temperatures close to room temperature open upa new avenue for FED cathodes. Finally, laser crystal-lisation of amorphous silicon has been shown to producenanometre sized self-aligned gated structures. All thesetechniques offer unique advantages over each other and aresuitable for scale up to 1m and beyond. The technologysuitable for field emission is currently available; however,the commercial viability and market impact remain to beestablished.

7 Acknowledgments

The authors would like to thank the EPSRC for fundingthrough Portfolio Partnership Award and through theCarbon Based Electronics programmes. One of the authors(JDC) also thanks the EPSRC for an Advanced ResearchFellowship. The authors are also grateful to ProfessorSir H.W. Kroto and the group at the University of Sussexfor the development of the CNT–polymer composites aspart of a joint EPSRC programme.

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Fig. 9 Scanning electron microscope image of an Si nanotip arraysa Unetchedb After 7minutesc After 14 minutesd 21minutes

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