mlod

slea

e, OUn

formine

ide/tin oxide, and tin oxide/indium oxide.

Amorphous binary oxides composed of heavy-metal cations channel layers compare favorably with other types of TTFTs

Thin Solid Films 515 (2006(HMCs) with (n1)d10ns0 (n4) electronic configurationsconstitute an interesting class of materials since they are oftentransparent in the visible portion of the electromagneticspectrum and they also possess relatively high electronmobilities in spite of their amorphous character [13].Examples of such materials include indium oxide doped withtin (ITO) [4] and zinc tin oxide [5] for which free carrieramorphous-state mobilities as large as 40 and 30 cm2 V1 s1,respectively, have been reported.

Our current interest in HMC ternary oxides, compoundscontaining two HMC elements and oxygen, stems from theiruse as channel layers in transparent thin-film transistors(TTFTs). Specifically, highly transparent TTFTs exhibitingrelatively high mobilities have been reported using indium

reported to date, involving channel layers composed ofpolycrystalline ZnO [916], single-crystal InGaO3(ZnO)5[17], or polycrystalline SnO2 [18].

A fundamental challenge inherent in utilizing HMC ternaryoxides involves basic materials selection. Since 15 elementshave been identified as candidate HMCs [2], i.e., Cu, Zn, Ga,Ge, As, Ag, Cd, In, Sn, Sb, Au, Hg, Tl, Pb, and Bi, there are 105possible ternary oxide combinations. Moreover, the composi-tion of a ternary oxide, in terms of the relative amount of eachsingle cation oxide constituent, is variable. For example, thefraction of ZnO to SnO2 in zinc tin oxide may be specified as(ZnO)x(SnO2)1x, where 0x1, underscoring that the relativesingle cation oxide composition is continuously variable fromzero to one. Finally, there is no reason to restrict attention 2006 Elsevier B.V. All rights reserved.

PACS: 81.15.Cd; 85.30.Tv; 73.61.Jc; 81.05.GcKeywords: Indium oxide; Zinc oxide; Tin oxide; Zinc indium oxide; Zinc tin oxide; Sequential layer deposition

1. Introduction [8] as the TTFT channel layer. The performance and potentialmanufacturability of TTFTs using these HMC ternary oxideof high-performance TTFTs with channel layers composed of zinc oxA novel deposition methodology is employed for exploratory development of a class of high-performance transparent thin-film transistor(TTFT) channel materials involving oxides composed of heavy-metal cations with (n1)d10ns0 (n4) electronic configurations. The methodinvolves sequential radio-frequency sputter deposition of thin, single cation oxide layers and subsequent post-deposition annealing in order toobtain a multi-component oxide thin film. The viability of this rapid materials development methodology is demonstrated through the realizationShort com

Transparent thin-film transistor explayer deposition an

David Hong a,, Hai Q. Chiang a, Rick E. PreCheol-Hee Park b, John F. W

a School of Electrical Engineering and Computer Sciencb Department of Chemistry, Oregon State

Received 18 July 2005; received in revisedAvailable onl

Abstractgallium zinc oxide [6], zinc tin oxide [7] and zinc indium oxide

Corresponding author.E-mail address: hongda@engr.orst.edu (D. Hong).

0040-6090/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.03.050unication

ratory development via sequentialthermal annealing

y a, Nicole L. Dehuff a, Jeffrey P. Bender a,ger a, Douglas A. Keszler b

regon State University, Corvallis, OR 97331-5501, USAiversity, Corvallis, OR 97331-4003, USA

15 March 2006; accepted 27 March 20065 May 2006

) 27172721www.elsevier.com/locate/tsfexclusively to ternary oxide combinations; it is possible thatoptimal HMC materials will involve multiple combinations ofsingle cation constituent oxides. These considerations suggestthat HMC multi-component oxide materials selection andoptimization may be complicated and time-consuming.

3. Results and discussion

A 150-nm XRD layered sample consists of 20 total layers ofzinc oxide and tin oxide, tin oxide being the bottom layer. Thesample is deposited directly on Corning 1737 glass substratesand annealed at 600 C. All other processing conditions are asdescribed previously. A corresponding 120-nm zinc tin oxidesample is deposited directly on Corning 1737 glass substrate andannealed at 600 C for comparison. Fig. 1 shows an XRD patternof layered zinc oxide/tin oxide and zinc tin oxide samples. Thelayered structure appears to be an amorphous film of zinc tinoxide with phase segregated SnO2. The zinc tin oxide sampleexhibits a rather broad peak at 29, characteristic ofamorphous films. The layered sample exhibits a very similarXRD pattern to the amorphous zinc tin oxide; however, there areadditional peaks at 2 values of 12 and 18, corresponding tocrystalline SnO2. Electron probe microanalysis of the layered

Films 515 (2006) 27172721The purpose of the work described herein is to describe asimple approach that we have found to be effective in theexploration of HMC multi-component oxides. The methodinvolves sequential radio-frequency (RF) sputter deposition ofthin binary oxide layers and subsequent interdiffusion via post-deposition annealing. The advantages of this methodologyinclude the use of fewer targets to explore a wide range ofmaterial combinations and stoichiometries. The viability of thisrapid materials development methodology as a screeningtechnique for assessing the potential of multi-component oxidecombinations is demonstrated via identification of indium tinoxide as a promising material system for TTFT channel layerapplications.

2. Experimental technique

All TTFTs are prepared on Nippon Electric Company glasssubstrates (NEG OA2) coated with a 100-nm sputtered indiumtin oxide (ITO) gate electrode film and a 220-nm atomic layerdeposited superlattice of AlOx and TiOx (ATO) [19]. The ATOhas a dielectric constant of 15. The ITO and ATO layersconstitute the gate contact and insulator, respectively, of abottom-gate TTFT. An 20-nm channel layer is then depositedvia RF magnetron sputtering through a shadow mask forchannel definition using 5.0-cm-diameter sputter targets, atarget-to-substrate distance of 7.5 cm, a power of 50 W, apressure of 5 mTorr, and an Ar/O2 ratio of 9:1. Next, an 200-nm layer of indium tin oxide is deposited by RF sputteringthrough a shadow mask for source and drain patterning using a7.5-cm target in pure Ar at a pressure of 30 mTorr.Aluminum via thermal evaporation is used as an alternativesource and drain contact material. There is no measurabledifference between aluminum source and drain TTFTs and ITOsource and drain TTFTs. The channel length and width are1520 m and 7170 m, respectively. The entire thin film stackis then furnace annealed in air using a 1-h ramp and a 10-minhold at 600 C for 10 min; the sample is then allowed to coolinside the furnace for 5 h to reach room temperature.Annealing occurs prior to aluminum source and draindeposition when it is utilized. All sputtering targets used forthis study are purchased from Cerac.

Consider zinc tin oxide as an example of the multi-component oxide channel layer deposition procedure employed.Deposition of layered zinc tin oxide is accomplished by firstpositioning the ITO/ATO substrate directly in front of a 5.0-cm SnO2 target to deposit a layer of SnO2, after which the targetis moved directly in front of a 5.0-cm ZnO target to deposit alayer of ZnO. The deposition duration is approximately 30 s foreach layer. Typically, this process is repeated for a total of fourlayers, two SnO2 layers and two ZnO layers. An 5.0-cmZn2SnO4 [(ZnO)0.67(SnO2)0.33] target and identical RF sputter-ing deposition processing parameters are used to fabricateTTFTs for comparison.

X-ray diffraction (XRD) patterns reported here are obtained

2718 D. Hong et al. / Thin Solidfrom a Rigaku Rapid image-plate system with AgK radiation(10 incident angle) provided by a rotating anode and a 0.5-mmpinhole collimator.sample shows a high zinc-to-tin ratio, 1.3:1; however, the XRDpattern shows no zinc oxide peaks, indicating that all of the zincis in the form of zinc tin oxide. Amorphous zinc oxide is unlikelydue to the high annealing temperature and sputtering conditions.

Electrical characteristics also indicate that the layers haveintermixed. The transistor characteristics of layered zinc oxide/tin oxide TTFTs are vastly improved over tin oxide transistorsand zinc oxide transistors and are very similar to zinc tin oxideTTFTs sputtered from a single target.

Fig. 2 illustrates the drain currentdrain voltage (IDVDS)curves of a TTFT with a channel layer composed of alternatinglayers of zinc oxide and tin oxide, for a total of four layers with abottom layer of tin oxide. An interesting feature is the presenceof a negative ID slope in the saturation regime, which weattribute to slow traps, as discussed later in this paper. The peakincremental mobility, INC [15], for this TTFT is 12 cm

2 V1

s1. The threshold voltage for the TTFT shown in Fig. 2 is5 V, as estimated by linear extrapolation of an IDVGS curve(not shown) [20]. The turn-on voltage is approximately 2 Vfor this TTFT, where turn-on voltage is defined as the gate

Inte

nsity

(arb

itrar

y unit

s)

15 25 35 4552 (Degrees)

(110)

(101)

(211)

(200)

Fig. 1. Glancing-angle X-ray diffraction patterns from a layered structure of zincoxide and tin oxide (top curve) and a corresponding plot of zinc tin oxidesputtered from a single target (bottom curve). Vertical bars represent cassiteritestructure SnO2 (JCPDS 41-1445). The zinc tin oxide sample sputtered from a

single target shows very strong amorphous tendency. The layered sampleexhibits a strong amorphous tendency, superimposed on a background ofcrystallized tin oxide.

0.0 5.0 10.0 15.0 25.0 30.0 35.0 40.020.0

2.0

6.0

8.0

10.0

4.0

VDS (V)

I D (m

A)

2719Films 515 (2006) 27172721voltage corresponding to the onset of the initial sharp increase incurrent in a log(ID)VGS curve [15]. Typical TTFTs fabricatedwith a layered zinc oxide/tin oxide channel layer possess peakincremental mobilities of 515 cm2 V1 s1, thresholdvoltages of 510 V, turn-on voltages of about 5 to +5 V, anddrain current on-to-off ratios of 106.

TTFTs having a zinc oxide or a tin oxide channel layer, andwhich are processed using the same parameters as employed forlayered zinc tin oxide channels, have peak incrementalmobilities of 25 cm2 V1 s1 and 0.81 cm2 V1 s1,respectively. The transistor performance of TTFTs with alayered zinc oxide/tin oxide channel layer is found to besuperior to that of TTFTs with either zinc oxide or tin oxidechannel layers, indicating that the transistor characteristics areno longer zinc oxide or tin oxide dominated.

Zinc tin oxide TTFTs fabricated using a Zn2SnO4 sputteringtarget and identical process parameters as used to deposit thelayered zinc tin oxide channel layers have peak incrementalmobilities of 515 cm2 V1 s1, threshold voltages of 5 V,turn-on voltages of about 5 V, and drain current on-to-off

6

0.0 5.0 10.0 20.0 25.0 30.0 35.0 40.0VDS (V)

15.0

0.1

0.2

0.3

0.4

0.5

0.6I D

(m

A)

Fig. 2. Drain currentdrain voltage (IDVDS) characteristics of a representativelayered zinc oxide/tin oxide channel layer TTFT with a width-to-length ratio of4.7. VGS is decreased from 30 V (top curve, showing maximum current) to 0 Vin 5-V steps. The channel consists of alternating layers of zinc oxide and tinoxide, for a total of four layers with a bottom layer of tin oxide.

D. Hong et al. / Thin Solidratios of 10 . Thus, layered zinc tin oxide TTFTs exhibitsimilar device characteristics to those of TTFTs in which thezinc tin oxide channel layer is sputtered from a single target.Other stoichiometries of zinc tin oxide show similar character-istics to Zn2SnO4 [7].

The improved transistor performance of the layered zinc oxide/tin oxide TTFTs over TTFTs with zinc oxide or tin oxide channellayers and the similar characteristics between the layered TTFTsand single-channel layer zinc tin oxide TTFTs indicate that thepost-deposition anneal induces interdiffusion and remixing of thebinary oxide constituents, thus resulting in the formation of aternary oxide. Several additional observations support this hypo-thesis that the layers remix. First, appreciable interdiffusion ofSnO2 and ZnO is witnessed at temperatures as low as 350 C [21].In addition, very similar TTFT device performance trends areobservedwhen the layer sequence is reversed. These observationssuggest that the individual zinc oxide and tin oxide layers haveformed a single zinc tin oxide layer.

A study of other material systems also indicates the remixingof binary constituents because layering of zinc oxide or tin oxidewith other binary oxides yields varying results. Zinc oxide/titanium oxide channel layer TTFTs and tin oxide/titanium oxidechannel layer TTFTs show no transistor behavior; both of thechannels containing titanium oxide result in highly resistivefilms that show no channel formation under any applied gatebias. The specific combination of zinc oxide layered with tinoxide results in good transistor characteristics. Other systemscomposed of HMC oxides also show good transistor character-istics. Successful TTFTchannel layer combinations identified todate include zinc oxide/tin oxide, zinc oxide/indium oxide,gallium oxide/indium oxide, and indium oxide/tin oxide, all ofwhich exhibit gate modulation when used as a channel material.These systems indicate that the layering methodology is a goodoption for exploring chemical systems for use as channelmaterials and indicate several HMC oxide systems are goodcandidates for use as channel materials in TTFTs.

Fig. 3 shows the IDVDS characteristics of one of thesuccessful combinations, a TTFT with a channel layercomposed of four alternating layers of indium oxide and tinoxide, tin oxide constituting the bottom layer. The magnitude of

Fig. 3. Drain currentdrain voltage (IDVDS) characteristic of a representativelayered indium oxide/tin oxide channel layer TTFT with a width-to-length ratioof 4.7. VGS is decreased from 30 V (top curve, showing maximum current) to0 V in 5-V steps. The channel consists of alternating layers of indium oxide andtin oxide, for a total of four layers with a bottom layer of tin oxide.ID is very large for this TTFT, whose width-to-length ratio is4.7. Fig. 4 shows the corresponding incremental mobility as afunction of applied gate bias for VDS of 1 V. This TTFT exhibitsa peak incremental mobility of 40 cm2 V1 s1. The relativelyhigh mobility is attributed to the larger 5s orbital of indium

-5.0 0.0 5.0 15.0 20.0 25.010.0

10

20

30

40

FE (c

m2/V

-s)

VGS (V)

Fig. 4. Incremental mobility (INC) as a function of gate voltage (VGS) for arepresentative layered indium oxidetin oxide channel layer TTFT. This curve isderived from an assessment of the IDVDS data shown in Fig. 3.

Filrelative to the 4s orbital of zinc, resulting in larger overlapbetween atomic orbitals and improved electron transport. Therelatively high current drive capability and mobility of thisTTFT suggest that indium tin oxide holds promise as a channellayer for TTFT applications. However, note that this is adepletion-mode TTFT with a threshold voltage of about 8 V;enhancement-mode behavior is preferable. Additionally, thisTTFT exhibits a very small drain current on-to-off ratio of 102.Both the depletion-mode behavior and poor on-to-off ratio areattributed to high carrier concentration within the channel layer[22]. Even though the performance of this layered channelTTFT is rather poor in terms of its depletion-mode behavior,and drain current on-to-off ratio, indium tin oxide merits furtherconsideration as a TTFT channel material since a non-optimizedprocess leads to high mobilities and large current drive.

As mentioned previously, some TTFTs exhibit a slightnegative slope in the saturation regime of a IDVDS plot. Theideal square-law model for a long-channel transistor predictsonly a flat ID in saturation, due to pinch-off, or a positive slope,due to bulk conductance. The negative slope is attributed to slowtraps near the semiconductorinsulator interface, which is nottaken into account when deriving the ideal model. Severalexperiments point to the existence of slow traps. By holding aconstant VGS and sweeping VDS multiple times from 0 V to 40 V,ID decreases for each progressive scan as more traps are filled.Also, when holding both VGS and VDS constant, the currentdiminishes as time progresses. Filled slow traps reduce thenumber of free carriers, resulting in a diminishing current. Byholding a constant VDS and sweeping VGS from 0 V to 40 V, IDdoes not decrease significantly for each subsequent sweep,indicating that the slow traps are emptied when a VGS of 0 V isapplied. It should be noted that a negative saturation slope is alsoobserved sometimes in single layer channel TTFTs, showing thatthis phenomenon is not peculiar to layered structures. Moreover,not all zinc oxide/tin oxide layered TTFTs exhibit a negativesaturation slope.

Returning to the sequential layer deposition/thermal anneal-ing methodology, a similar approach has been employedpreviously to form YBaCuO thin films for superconductorapplications [23]. Layers of three different materials, yttrium,copper and a barium/copper ceramic, interdiffused to form asingle layer when annealed at temperatures of 800930 C. Thehigh temperature required for interdiffusion is specific to the YBaCuO films under consideration. As indicated previously,interdiffusion of SnO2 and ZnO is witnessed at temperatures aslow as 350 C [21].

Other reports have shown that zinc oxide does not diffuseinto an adjacent film during a post-deposition anneal if the filmis highly crystalline [24,25]. Specifically, a zinc oxide layerdeposited at a 300 C substrate temperature, resulting in ahighly crystalline zinc oxide film, does not diffuse into ITO at apost-deposition temperatures of 350 C [25]. Remixing ofbinary oxide constituents appears to depend strongly on thecrystallinity of the individual layers; enhanced crystallinity

2720 D. Hong et al. / Thin Solidinhibits interdiffusion. Polycrystalline zinc oxide films exhibitsignificant atomic reordering at room temperature when imagedin a high-resolution transmission electron microscope with anirradiation density of 3105 A m2, the extent of whichappreciably diminishes as the film approaches single crystal[24]. Interdiffusion can also diminish with increasing crystal-linity because of the decrease of diffusion via grain boundariesand defect sites.

Our as-deposited zinc oxide thin films experience substantialatomic reordering during the annealing process, as evident fromthe fact that Scherrer crystallite size is 10 nm for the as-deposited film, increasing to 60 nm after a 700 C post-deposition anneal.

One of the benefits of employing the sequential layerdeposition/thermal annealing methodology is that the indium tinoxide system could be explored as a TTFT channel materialeven though we have not yet been successful in acquiring anindium-doped tin oxide target from commercial vendors. Otheradvantages of the sequential layer deposition/thermal annealingmethodology include the fact that no specialized equipment isrequired, film uniformity can be controlled via processparameters for the individual layers, and composition gradientsacross the sample can be engineered into the film by varying thesubstrate angle or substrate scan speed.

4. Conclusions

Sequential layer deposition/thermal annealing methodologyis demonstrated to be a viable exploratory methodology forTTFT channel layer applications. Electron imaging, XRDassessments and electrical characterization indicate that asingle layer is present after post-deposition annealing, resultingfrom the interdiffusion of multiple layers into a single channellayer.

Indium tin oxide is identified as a promising TTFT channellayer, yielding TTFTs with large current drive, high mobilityand a mobility which is relatively constant over a wide range ofapplied gate bias. Additionally, zinc oxide/tin oxide, zinc oxide/indium oxide, gallium oxide/indium oxide, and indium oxide/tin oxide have been identified as other promising TTFT channellayers. In additional to these chemical systems, a sequentialsputtering exploratory method could also be useful for the studyof materials outside the realm of HMC oxides, such as systemscontaining sulfides or nitrides when the deposition and theanneal are carried out in a suitable environment.

Acknowledgements

The authors thank Arto Pakkala for supplying ITO/ATOcoated substrates and Kim Maxwell and Paul Kingzett fromIntel for use of dual-beam and electron imaging equipment. Thiswork was funded by the U.S. National Science Foundationunder Grant No. DMR-0245386 and by the Army ResearchOffice under Contract No. MURI E-18-667-G3.

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2721D. Hong et al. / Thin Solid Films 515 (2006) 27172721

Transparent thin-film transistor exploratory development via sequential layer deposition and th.....IntroductionExperimental techniqueResults and discussionConclusionsAcknowledgementsReferences