High-performance flexible zinc tin oxide field-effect transistorsW. B. Jackson, R. L. Hoffman, and G. S. Herman

Citation: Applied Physics Letters 87, 193503 (2005); doi: 10.1063/1.2120895 View online: http://dx.doi.org/10.1063/1.2120895 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of deposition power on the electrical properties of Al-doped zinc oxide thin films Appl. Phys. Lett. 97, 082109 (2010); 10.1063/1.3483232 Resistivity characteristics of transparent conducting impurity-doped ZnO films for use in oxidizing environmentsat high temperatures J. Vac. Sci. Technol. A 28, 861 (2010); 10.1116/1.3455814 Improved electrical transport in Al-doped zinc oxide by thermal treatment J. Appl. Phys. 107, 013708 (2010); 10.1063/1.3269721 Influence of interface roughness on the performance of nanoparticulate zinc oxide field-effect transistors Appl. Phys. Lett. 93, 083105 (2008); 10.1063/1.2972121 High-performance ZnO nanowire field effect transistors Appl. Phys. Lett. 89, 133113 (2006); 10.1063/1.2357013

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APPLIED PHYSICS LETTERS 87, 193503 �2005�

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High-performance flexible zinc tin oxide field-effect transistorsW. B. Jacksona�

Hewlett-Packard Labs, 1501 Page Mill Road, Palo Alto, California 94304

R. L. Hoffman and G. S. HermanHewlett-Packard Company, 1000 NE Circle Boulevard, Corvallis, Oregon 97330

�Received 11 July 2005; accepted 8 September 2005; published online 1 November 2005�

Flexible transistors were fabricated by sputter deposition of zinc tin oxide �ZTO� ontoplasma-enhanced chemical vapor deposition gate dielectrics formed on flexible polyimide substrateswith a blanket aluminum gate electrode. The flexible transistors exhibited high on-currents of 1 mA,on/off ratios of 106, subthreshold voltage slopes of 1.6 V/decade, turn-on voltages of −17 V, andmobilities of 14 cm2 V−1 s−1. Capacitance measurements indicate that the threshold voltage andsubthreshold slope are primarily influenced by residual doping in the ZTO rather than by defects atthe semiconductor/dielectric interface, and are useful for assessing contact resistance. © 2005American Institute of Physics. �DOI: 10.1063/1.2120895�

In many mobile electronics applications, the mechanicalcharacteristics of the electronics are becoming important;particularly characteristics such as weight, flexibility, and du-rability. Inexpensive, light-weight, flexible, durable displaysare desirable for laptops, PDAs, and cell phones. In order toproduce such displays, low-cost transistors with reasonableperformance must be fabricated on flexible, inexpensive,light-weight, plastic substrates, at low temperatures. Organicthin-film transistor �TFT� channel materials are flexible andinexpensive, but have limited performance with typical mo-bilities at or below �0.5 cm2 V−1 s−1, short lifetimes, andlimited on-currents �less than �10 �A�.1 Moreover, the lay-ers necessary to eliminate water and oxygen permeation,which limit organic electronic lifetimes, are quite expensive.Flexible hydrogenated amorphous silicon devices have alsobeen produced, but their mobility is also rather low �less than�1 cm2 V−1 s−1�, with on-currents similarly limited to�10 �A or smaller, for typical transistor dimensions.2,3

While being marginally acceptable for active-matrix pixeldrive transistors, neither of these aforementioned solutionshas sufficient speed or on-current to power drive-electronicsaround the display edge.

Recently, a new set of materials—ZnO and related metaloxides—has been used to make low-temperature transistorswith mobilities and on-currents similar to micro- and poly-crystalline silicon, but without the associated difficulties indeposition and fabrication. Devices fabricated from suchmetal oxides on rigid substrates have shown considerablepromise. Nomura et al.4 have fabricated transistors with mo-bilities as high as 80 cm2 V−1 s−1, however, these devicesrequire single-crystal yttria-stabilized zirconia substrates,pulsed-laser deposition of multilayer indium gallium oxide/zinc oxide channels, and annealing at 1400 °C, proceduresnot compatible with flexible substrates. Subsequently, usingZnO as the channel material and a gate dielectric of atomiclayer deposited �ALD� Al2O3, mobilities in the range of2cm2V−1s−1 were achieved for long channel TFTs with sig-nificantly lower process temperatures �600 °–800 °C rapidthermal annealing�.5 Further reduction in processing tem-perature for ZnO-channel devices has been reported by sev-

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Electronic mail: gs.herman@hp.com

0003-6951/2005/87�19�/193503/3/$22.50 87, 19350ticle is copyrighted as indicated in the article. Reuse of AIP content is subje

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eral groups, with comparable mobilities obtained using near-room temperature processing.6,7 In addition to ZnO, othermetal oxides have been investigated for their suitability inTFT applications. In particular, Chiang et al.8 have shownthat zinc tin oxide �ZTO� provides good transistor character-istics for relatively low processing temperatures, using ther-mal silicon oxide and ALD Al2O3 gate dielectrics; mobilitiesin the range of 15 cm2 V−1 s−1 were obtained for samplesannealed as low as 300 °C. Recent results by Nomura et al.9

have demonstrated that mobilities as high as 8 cm2 V−1 s−1

can be achieved for flexible transistors formed near roomtemperature on polyethylene terephthalate substrates usingamorphous indium gallium zinc oxide as a channel material.

In this work, we extend the work of Refs. 4–9 to dem-onstrate high-performance transistors on low-cost, flexiblesubstrates using dielectrics produced by plasma-enhancedchemical vapor deposition �PECVD�, a deposition methodcompatible with high-throughput, low-cost manufacturing.Specifically, we describe the fabrication, characterization,and performance of ZTO transistors on flexible polyimidesubstrates with low-temperature PECVD gate dielectrics.

The flexible transistors were fabricated using thefollowing process steps. Al �135 nm thick� was sputterdeposited over stainless-steel-backed polyimide sheets�50 �m thick�, followed by deposition of 375 nm of siliconoxynitride �SiON� by PECVD at 300 °C. The roll-to-rollstack was cut into 10 mm�50 mm strips and taped to astainless steel carrier for subsequent processing. ZTO �50 nmthick� was sputter deposited through a shadow mask from aZnO/SnO2 �1:1 molar ratio� target to define the active �chan-nel� layer of the transistor; the substrate was held near roomtemperature during ZTO deposition. Postdeposition anneal-ing was performed in air at 250 °C for 10 min. The sourceand drain electrodes were deposited through a secondshadow mask, using either Al or indium tin oxide �ITO� toform the direct source/drain contact, followed by Al or Au toform a source/drain contact overlayer and contact pads. Thechannel length �L�, defined by the separation between sourceand drain electrodes, was 80 �m; the channel width �W�,defined by the width of the patterned ZTO, was 1000 �m.Access to the gate metal was obtained by etching through a

portion of the SiON layer with a dilute hydrogen fluoride

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193503-2 Jackson, Hoffman, and Herman Appl. Phys. Lett. 87, 193503 �2005�

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solution and directly probing the aluminum gate. The devicegeometry is depicted in the inset of Fig. 1.

The devices were measured using both standard andunique measurement methods. In Fig. 2, the ID-VDS outputcharacteristics �drain current versus drain-to-source voltage,with the gate voltage as a parameter�, are presented for aZTO transistor with Al source and drain contacts. The non-ideal, concave upwards behavior �current crowding� and lowon-current indicate that the Al contacts are limiting the draincurrent. This result is further confirmed using capacitance-voltage �CV� measurements. The CV characteristics of thesource contact with respect to the gate �CSG� were measuredat various frequencies ranging from 500 Hz to 500 kHz for adevice with Al source and drain contacts; for this measure-ment, a dc source-to-gate bias �VSG� was applied, while thedrain was left electrically floating. The results of this mea-surement are shown in Fig. 1. For low frequencies and nega-tive source-to-gate biases, the capacitance represents the ca-pacitance of the gate dielectric �SiON� layer only, becausethe device is biased in accumulation and the time required tocharge the dielectric capacitance through the contact resis-tance is short compared to the period of the oscillating volt-age. The insulator capacitance per unit area is Ci=15 nF cm−2. The contact resistance can be roughly esti-mated by noting that RcontactCi2�f �1 when the measuredcapacitance at negative source-to-gate biases drops to ap-proximately half of its low-frequency value. Hence, Rcontact�30 k� for devices with Al contacts; such a device will notbe able to supply on-currents much greater than �35 �Awithout significant voltage drop at the contacts. For largepositive source-to-gate biases, the measured capacitancecomprises the series capacitance of the dielectric and the

FIG. 1. The source-gate capacitance vs dc source-to-gate voltage bias�CSG-VSG� for various frequencies, for a transistor with Al source and draincontacts �output characteristics shown in Fig. 2�; measurement frequenciesare 0.5, 1, 5, 10, 50, 100, and 500 kHz �CSG decreases with increasingfrequency�. The insets indicate the device layout �upper right�, and the bandstructure for negative �left� and positive �right� dc bias conditions.

FIG. 2. Output �ID-VDS� characteristics for a transistor with Al source anddrain contacts and W /L=1000 �m/80;VGS=0 to 30 in 10 V increments �IDticle is copyrighted as indicated in the article. Reuse of AIP content is

increases with increasing VGS�.

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depleted ZTO layer, regardless of the frequency. At high fre-quencies, the oscillating field does not have enough time topenetrate the contact, so that the total series capacitance re-mains at a minimum value defined by the series capacitanceof the dielectric and the depleted ZTO layer, with little biasdependence. The rapid change from accumulation to deple-tion for low frequencies, taking place within a dc bias rangeof �3 V, indicates that the defect density at the channel/gatedielectric interface and within the bulk material is relativelylow. In fact, a few volts above the transition voltage, theindependence of the capacitance on dc bias indicates that thedepletion width extends through the bulk of the ZTO layer.Thus, the defect density of the interface and the bulk materialis relatively low, despite the amorphous nature of the ZTOactive layer.8 In order to maximize the performance of tran-sistors using ZTO channels, an improved contact is required,as described subsequently.

A flexible ZTO transistor with reduced contact resistancewas fabricated using ITO as source and drain electrodes;shadow masks were used to define the ITO source and drain,as well as the Ta/Au contact pads. The output characteristicsof a flexible transistor with ITO source and drain contacts areshown in Fig. 3. The shape of the ID-VDS curves is convex,indicating minimal current crowding. Hence, the contact re-sistance is not appreciably limiting the performance of thisdevice; the ITO provides a low-resistance, efficient n+ con-tact to the ZTO channel. Four-probe measurements �pre-sented elsewhere�10 in fact indicate that the contact resistanceis about 1 k�, whereas the channel on-resistance is about18 k� for the same device. For a transistor with W /L=1000 �m/80 �m, the on-current of �1 mA is quite high,indicating a relatively large mobility; however, we havefound that heat dissipation may become an issue if the tran-sistors are operated at these currents for extended periods ��seconds�. The ID-VGS transfer characteristics �drain currentversus gate-to-source voltage, with a fixed VDS� for this de-vice are shown in Fig. 4, indicating a drain current on-to-offratio is about 1.4�106. The subthreshold slope is1.65 V/decade and the threshold voltage �VT� and turn-onvoltage �Von� �see Ref. 11� are −8.8 V and −17 V, respec-tively. Although these voltages are somewhat more negativethan ideally desired �values near zero are preferable�, theycan be controlled, for example, by changing the O:N ratio inthe SiON dielectric and by control of stoichiometry of theZTO �an oxygen deficient stoichiometry is responsible forn-type conductivity in many oxides such as ZTO�. Reductionof the residual bulk electron concentration through control ofstoichiometry may also improve the subthreshold slope. The

FIG. 3. Output characteristics for a transistor with ITO source and draincontacts and W /L=1000 �m/80 �m;VGS=−10 to 30 in 10 Vincrements �ID

increases with increasing VGS�. The inset depicts the device cross section.

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field-effect mobility ��� of the device is about4 Sep 2014 21:26:02

193503-3 Jackson, Hoffman, and Herman Appl. Phys. Lett. 87, 193503 �2005�

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14 cm2 V−1 s−1, as obtained from the transfer characteristicsin the linear and saturation regimes. Figure 4 �inset� presentsthe incremental field-effect mobility ��inc� obtained from therelation11

�FEincr = �dGCH

dVGS��W

LCi�−1

, VDS � VGS − Von. �1�

In this equation, GCH is the channel conductance �i.e., theslope of an ID-VDS curve for a given value of VGS�. Thismeasure of mobility describes the mobility of the carriersadded by the last increment of gate voltage.11 This quantity isuseful in noncrystalline materials wherein much of the chan-nel charge is immobilized in deep traps and interface statesand therefore does not contribute strongly to the channelcurrent. As the gate voltage is increased, the free chargecomprises an increasing fraction of total induced charge, andthe incremental mobility increases toward that of the freecarriers.

The performance of these flexible ZTO transistorsmatches the performance obtained using high-quality �andhigh-temperature� dielectrics and rigid substrates,7,8 and eas-ily exceed the performance of hydrogenated amorphous sili-con transistors used in existing applications.2,3 This level ofperformance shows that these devices may be suitable forcurrent-demanding applications such as pixel-drive transis-tors in organic light-emitting diode �OLED� displays, andperhaps for more demanding applications such as displayedge-drive electronics. Further improvements in current-drive capabilities are possible. In fact, one limitation mayarise from thermal power dissipation in a thin ZTO layer onan insulating substrate; it is likely that power dissipation con-trol such as enhanced cooling could reduce such heating andyield further improvement in electrical characteristics. Themeasured contact resistance of the ITO source/drain contactis �1 k�, which also begins to limit the on-current above

FIG. 4. Transfer �ID−VGS� characteristics for the device with ITO sourceand drain contacts �output characteristics shown in Fig. 3�; VDS

=1,4 , and 16 V �ID increases with increasing VDS�. Inset: the incrementalfield-effect mobility, derived from the �ID−VGS� curve with VDS=1 V.

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�1 mA. Thus, the ZTO material itself appears to be able tosupport even higher-performance devices.

The characteristics requiring the greatest improvementsare threshold voltage �turn-on voltage� and subthresholdslopes. The threshold voltage can be made more positive, forexample, by increasing the nitrogen content in the SiON gatedielectric or through the use of a composite dielectric. Inaddition, both the threshold voltage and the subthresholdslope can be improved by minimizing the oxygen deficiencyin the ZTO that induces n-type conductivity; the carriers thusintroduced must be depleted in order to turn the device off.The rapid change from depletion to accumulation with smallchanges in gate voltage �Fig. 1� indicates that excess carriers,rather than interface states, are responsible for the degrada-tion of the subthreshold slope. Interface states would causeboth a decreased subthreshold slope and a stretchout of theCV curve, while residual doping only affects the former.

In summary, ZTO TFTs are fabricated on polyimide sub-strates, using a low-temperature, manufacturable PECVDgate dielectric process and a maximum channel anneal tem-perature of 250 °C. Excellent performance is obtained withmobilities as high as �14 cm2V−1s-1. The importance of theappropriate selection of source and drain electrode material,in order to minimize contact resistance, is shown; ITO pro-vides a low-resistance contact to the ZTO channel layer.These results provide further evidence identifying ZTO as apromising channel material for the fabrication of high-performance TFTs on flexible substrates.

We would like to thank Frank Jeffries, Steve Braymen,and Jason Hauschildt of Iowa Thin Film Technologies forproviding the dielectric-coated thin films, and Carl Taussigfor many helpful comments.

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