Low-temperature processed Schottky-gated field-effect transistors based onamorphous gallium-indium-zinc-oxide thin filmsM. Lorenz, A. Lajn, H. Frenzel, H. v. Wenckstern, M. Grundmann, P. Barquinha, R. Martins, and E. Fortunato Citation: Applied Physics Letters 97, 243506 (2010); doi: 10.1063/1.3525932 View online: http://dx.doi.org/10.1063/1.3525932 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/97/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improvement in gate bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors usingmicrowave irradiation Appl. Phys. Lett. 105, 213505 (2014); 10.1063/1.4902867 Temperature dependence of negative bias under illumination stress and recovery in amorphous indiumgallium zinc oxide thin film transistors Appl. Phys. Lett. 102, 143506 (2013); 10.1063/1.4801762 Thermal stability of metal Ohmic contacts in indium gallium zinc oxide transistors using a graphene barrierlayer Appl. Phys. Lett. 102, 113112 (2013); 10.1063/1.4796174 Room-temperature-operated sensitive hybrid gas sensor based on amorphous indium gallium zinc oxide thin-film transistors Appl. Phys. Lett. 98, 253503 (2011); 10.1063/1.3601488 Thin-film transistors with amorphous indium gallium oxide channel layers J. Vac. Sci. Technol. B 24, 2702 (2006); 10.1116/1.2366569

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

69.166.47.134 On: Sat, 20 Dec 2014 07:09:36

Low-temperature processed Schottky-gated field-effect transistors basedon amorphous gallium-indium-zinc-oxide thin films

M. Lorenz,1,a� A. Lajn,1 H. Frenzel,1 H. v. Wenckstern,1 M. Grundmann,1 P. Barquinha,2

R. Martins,2 and E. Fortunato2

1Institut für Experimentelle Physik II, Fakultät für Physik und Geowissenschaften, Universität Leipzig,Linnéstrasse 5, 04103 Leipzig, Germany2Departamento de Ciência dos Materiais, CENIMAT/I3N, Faculdade de Ciências e Tecnologia, FCT,Universidade Nova de Lisboa and CEMOP-UNINOVA, 2829-516 Caparica, Portugal

�Received 14 September 2010; accepted 19 November 2010; published online 15 December 2010�

We have investigated the electrical properties of metal-semiconductor field-effect transistors�MESFET� based on amorphous oxide semiconductor channels. All functional parts of the deviceswere sputter-deposited at room temperature. The influence on the electrical properties of a 150 °Cannealing step of the gallium-indium-zinc-oxide channel is investigated. The MESFET technologyoffers a simple route for processing of the transistors with excellent electrical properties such as lowsubthreshold swing of 112 mV/decade, gate sweep voltages of 2.5 V, and channel mobilities up to15 cm2 /V s. © 2010 American Institute of Physics. �doi:10.1063/1.3525932�

Thin-film-transistor �TFT� technology is nowadaysmostly based on hydrogenated amorphous silicon �a-Si:H�.1

The rather low mobility below 1 cm2 /V s �Refs. 2 and 3� islimiting the device performance. The amorphous structureof a-Si:H leads to localized tail states in the top of the va-lence band and the bottom of the conduction band.4 In amor-phous oxides such as gallium-indium-zinc-oxide �GIZO� theeffect of disorder on the carrier mobility is rather weak5–7

and thus Hall-effect mobilities �Hall of up to 60 cm2 /V swere reported.8

The current research of TFTs based on amorphous oxidechannel materials is entirely focused on metal-insulator-semiconductor field-effect transistors �MISFETs�.7,9 Thechannel mobility �ch reaches several 10 cm2 /V s;10,11 how-ever, MISFETs often require the deposition of a thick insu-lator and hence generally exhibit high operating voltages�V�10 V�. Our approach is based on metal-semiconductorfield-effect transistors �MESFETs�. MESFETs exhibit lowoperating voltages and reach channel mobilities that equalthe Hall-effect mobilities. Due to the missing insulator-semiconductor interface, scattering of charge carriers by in-terface defects can be neglected. We have fabricated MES-FETs based on pulsed-laser deposition-grown polycrystallineZnO on sapphire12 and on low cost glass substrates.13 Also,fully transparent MESFETs based on ultrathin gate contactshave been realized.14 In this work we demonstrate the com-patibility of the MESFET device concept with room tem-perature �RT� deposited amorphous oxide channel materialson Corning 1737 glass substrates using radio-frequency �rf�magnetron sputtering. Schottky contacts �SCs� on amorphous

silicon have been reported,15,16 but there are no reports onMESFETs based on amorphous silicon or oxide channel ma-terials. In this work the compositions of the sputter target andfilm thickness were chosen, such that the resulting net dop-ing density and the related extension width of the Schottkycontact space charge region permit a low switching voltageoperation.12

Quasistatic capacitance-voltage �QSCV� measurementswere carried out to derive the net doping density. The QSCVspectroscopy was performed on the actual MESFET devicesusing the source and the gate contact. In this measurementmode a voltage sweep around each measurement point isapplied.17 The maximum frequencies are less than 40 Hz,which is below the typical frequencies used in CV-spectroscopy �40 Hz–10 GHz�. 1 /C2 directly calculated froma QSCV measurement, as well as the derived net dopingdensity Nd�5�1016 cm−3 being obtained in the bulk regionof the channel for annealed GIZO channel material, areshown in Fig. 2�a�. Apparently there is an increase in thecarrier concentration that is related to excessive leakage cur-rents of the forwardly biased diode. Please note that the in-crease in reverse direction—for a larger extension of thespace charge region—is an artifact, due to the depletion layerreaching down to the substrate.

Depending on the deposition conditions, the rf magne-tron sputtering at RT of the thin films causes low mobilitiesof electrons in the thin films �Table I�. For the specific depo-sition conditions used herein, annealing at 150 °C for60 min has strong effects on the electrical properties of thesamples. The remarkable change in the Hall-effect mobility,

a�Electronic mail: lorenz.ml@physik.uni-leipzig.de.

TABLE I. Channel thickness d, width-to-length ratio W /L of the Schottky gate contact, Hall-effect data �derived from 120 nm thick films�, and measuredelectrical parameters of the MESFETs.

d�nm�

W /L��m /�m�

�Hall

�cm2 /V s�NHall

�1016 cm−3�Nd

�1016 cm−3��ch

�cm2 /V s��ch,med

�cm2 /V s�Von

�V�S

�mV/decade�On/off�106�

GIZO RT 160 430/20 2.56 1.3 1 7.3 0.01 �0.5 123 4.4GIZO 150 °C 160 430/10 20.8 4.5 5 14.1 14.7 �1.9 112 25

APPLIED PHYSICS LETTERS 97, 243506 �2010�

0003-6951/2010/97�24�/243506/3/$30.00 © 2010 American Institute of Physics97, 243506-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

69.166.47.134 On: Sat, 20 Dec 2014 07:09:36

which increases by two orders of magnitude, is probablycaused by a reduction in the trap state density, local atomicrearrangement, and improvement of film compactness.10

Nevertheless we were able to keep the maximum processingtemperature below 150 °C limited by an annealing step forthe amorphous channel material. The low-temperature fabri-cation reduces production cost, and furthermore the wholeprocess is compatible with temperature-sensitive polymersubstrates required for flexible electronics.

All films were processed by means of standard photoli-thography using lift-off technique. Details on the processingsteps and the device structure �Fig. 1� can be foundelsewhere.12 We used AgxO for the SC, which was depositedat RT by reactive dc-sputtering of a Ag target in an argon/oxygen atmosphere. Deposition under oxygen atmosphereleads to a partial oxidization of Ag and results in a higherSchottky barrier height �B compared to the pure metal.18,19

Barrier heights and ideality factors for SCs of various metalson a-Si:H were reported to be in the range 0.65–1.1 eV and1.1, respectively.20–22 Reports of rectifying metallic-contactson amorphous oxides are sparse besides23 almost no rectifi-cation was reported. Figure 2�c� depicts the Schottky gate

characteristics of the respective contacts. The diodes showexcellent IV-characteristics with an on/off current ratioISG�2 V� / ISG�−2 V� of eight orders of magnitude. By con-sidering thermionic emission only, a value of �B�0.95 eVhas been derived by fitting the IV-curves �Fig. 2�c��. Theideality factors of the contacts were determined for the an-nealed GIZO thin films to be ��1.9 and for the as-grownthin film ��2. The data are similar to our recently publishedvalues of the effective barrier height and ideality factors forSC on ZnO grown on sapphire and glass substrates withvalues ��1.7 and �B�0.89–0.95 eV,13 revealing the sta-bility of the barrier formation toward alloying the zinc oxidewith Ga and In.

Figure 2�a� depicts the output characteristics of a tran-sistor based on an annealed GIZO thin film. The drain cur-rent can be controlled over eight orders of magnitudefrom several 10−5 A for a gate voltage VG=0.6 V toID�10−13 A for VG=−1.9 V; hence a voltage sweep of�VG=2.5 V is necessary to switch the transistor from theon- into the off-state. MESFETs from as-grown GIZO chan-nel material show a similar behavior, but due to the lower netdoping density of the thin films the gate voltage sweep nec-essary is �VG=2 V. These low switching voltages areamong the best reported so far for oxide based channel tran-sistors and are a huge advantage of the metal-semiconductorbased transistors making them especially favorable for lowvoltage mobile applications.

Figure 2�b� shows the transfer characteristics of theMESFETs. The turn-on voltage Von of the particular as-grown GIZO-based transistor depicted in Fig. 2�b� isVon=−0.5 V. Von averaged over all 28 TFTs on the samplechip is �−0.2 V. Comparing Von of the as-grown withthe annealed samples, it is clear that annealing has a pro-nounced effect on the GIZO thin films since Von shifts byabout �1.4 V. This is related to an increased carrier concen-tration that was confirmed by Hall-effect measurements aswell as by QSCV spectroscopy �Table I�. In Fig. 2�b� the gatecurrents of the respective transfer characteristics are depicted�dashed lines�. Evidently the leakage current is similar to theoff current of the respective MESFETs �for VG0 V�. Forthe annealed GIZO MESFETs the leakage current in the on-state of the device �VG�0.6 V� is �10−13 A. Accordingly,there is low power consumption24 of the device when thetransistor is in the on-state. For the transistors based on as-grown channel material gate voltages VG1 V are not rea-sonable since the gate current will dominate the drain cur-rent. However, it should be emphasized that the leakagecurrents for forwardly biased gate contacts �VG0 V� arean inherent part of the working principle of MESFETdevices.25

From the transfer characteristics, the channel mobilitycan be derived26 as

�ch =gmax

�W/L�eNdd,

with gmax, W /L, e, and d being the maximum transconduc-tance, the width-to-length ratio of the channel, electroncharge, and the channel thickness, respectively.

Hall-effect measurements on as-grown GIZO thinfilm samples revealed a mobility of �Hall=2.6 cm2 /V s.This is lower than the calculated channel mobility��ch=7.3 cm2 /V s� obtained from the transfer characteristic

glass

GIZOAu AuAg

FIG. 1. Top and side schematics �left/right� of the MESFET device.

FIG. 2. �Color online� �a� Net doping density vs the width w of the spacecharge region below the Schottky contact and 1 /C2 vs the applied gatevoltage �channel thickness 160 nm�. �b� Output characteristic of a MESFETwith a GIZO channel annealed at 150 °C after thin film deposition. �c�Transfer characteristics and gate currents IG �dashed lines� for as-grown andannealed GIZO channel materials, VSD=2 V. �d� Schottky gate characteris-tics of the respective GIZO transistors.

243506-2 Lorenz et al. Appl. Phys. Lett. 97, 243506 �2010�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

69.166.47.134 On: Sat, 20 Dec 2014 07:09:36

depicted in Fig. 2�b�. The median of the channel mobilityis more than 2 decades lower ��ch=0.01 cm2 /V s� in-dicating poor reproducibility. The subthreshold swing isS=123 mV /decade. By means of annealing the thin filmsample the electrical parameters of the TFTs were largelyimproved. The Hall-effect mobility ��Hall=20.8 cm2 /V s�and the channel mobility ��ch=14 cm2 /V s� are similar aspredicted by MESFET theory. Comparing both values to themedian ��ch,med=14.7 cm2 /V s� it is obvious that there is anincrease in the reproducibility of the TFTs on the samplechip compared to the as-grown GIZO thin film. Also, thesubthreshold swing was reduced to S=112 mV /decade. Thevalues for S described in this letter are among the best re-ported so far for oxide TFTs. Typical values of the sub-threshold swing for MISFETs are in the range between120 and 300 mV/decade for GIZO-based transistors9,10,27,28

and 470 mV/decade down to 150 mV/decade for TFTs withIZO channels.9,11,29

In conclusion Schottky-gated TFTs with annealed amor-phous GIZO channel material exhibit the largest channel mo-bility and reproducibility of the device types investigated.

The authors gratefully acknowledge financial support byDeutsche Forschungsgemeinschaft in the framework ofSonderforschungsbereich 762 “Functionality of Oxide Inter-faces” and the Graduate School “Leipzig School of NaturalSciences-BuildMoNa.” The authors also thank the EuropeanResearch Council for the ERC 2008 Advanced Grant, alsoPTDC/CTM/73943/2006 and PTDC/EEA-ELC/64975/2006by the Portuguese Science Foundation �FCT-MCTES�. A.L.is thankful to the Studienstiftung des deutschen Volkes.

1R. A. Street, Adv. Mater. 21, 2007 �2009�.2J.-J. Huang, C.-J. Liu, H.-C. Lin, C.-J. Tsai, Y.-P. Chen, G.-R. Hu, andC.-C. Lee, J. Phys. D: Appl. Phys. 41, 245502 �2008�.

3H. Lee, G. Yoo, and J. Kanicki, J. Appl. Phys. 105, 124522 �2009�.4J. Singh, Phys. Rev. B 23, 4156 �1981�.5H. Hosono, N. Kikuchi, N. Ueda, and H. Kawazoe, J. Non-Cryst. Solids

198–200, 165 �1996�.6R. Martins, P. Barquinha, I. Ferreira, L. Pereira, G. Gonçalves, and E.Fortunato, J. Appl. Phys. 101, 044505 �2007�.

7K. Nomura, H. Ohta, A. Tagaki, T. Kamiya, M. Hirano, and H. Hosono,

Nature �London� 432, 488 �2004�.8E. Fortunato, A. Pimentel, A. Gonçalves, A. Marques, and R. Martins,Integration of Advanced Micro- and Nanoelectronic Devices-Critical Is-sues and Solutions, MRS Symposia Proceedings No. 811 �Materials Re-search Society, Pittsburgh, PA, 2004�, p. E3.9.

9M. Grundmann, H. Frenzel, A. Lajn, M. Lorenz, F. Schein, and H. vonWenckstern, Phys. Status Solidi A 207, 1437 �2010�.

10P. Barquinha, L. Pereira, G. Gonçalves, R. Martins, and E. Fortunato, J.Electrochem. Soc. 156, H161 �2009�.

11E. Fortunato, P. Barquinha, A. Pimentel, L. Pereira, G. Gonçalves, and R.Martins, Phys. Status Solidi �RRL� 1, R34 �2007�.

12H. Frenzel, A. Lajn, M. Brandt, H. von Wenckstern, G. Biehne, H. Hoch-muth, M. Lorenz, and M. Grundmann, Appl. Phys. Lett. 92, 192108�2008�.

13H. Frenzel, M. Lorenz, A. Lajn, H. von Wenckstern, G. Biehne, H. Hoch-muth, and M. Grundmann, Appl. Phys. Lett. 95, 153503 �2009�.

14H. Frenzel, A. Lajn, H. von Wenckstern, and M. Grundmann, J. Appl.Phys. 107, 114515 �2010�.

15C. R. Wronski, D. E. Carlson, and R. E. Daniel, Appl. Phys. Lett. 29, 602�1976�.

16M. Khaidar, A. Essafti, A. Bennouna, E. L. Ameziane, and M. Brunel, J.Appl. Phys. 65, 3248 �1989�.

17See Agilent 4156C PrecisionSemiconductor Parameter Analyzer, Users-guide: Measurement and Analysis http://cp.literature.agilent.com/litweb/pdf/04156-90020.pdf.

18M. W. Allen, S. M. Durbin, and J. B. Metson, Appl. Phys. Lett. 91,053512 �2007�.

19A. Lajn, H. v. Wenckstern, Z. Zhang, C. Czekalla, G. Biehne, J. Lenzner,H. Hochmuth, M. Lorenz, M. Grundmann, S. Wickert, C. Vogt, and R.Denecke, J. Vac. Sci. Technol. B 27, 1769 �2009�.

20F. Balon and J. Shannon, Solid-State Electron. 50, 378 �2006�.21M. Sahin, H. Durmus, and R. Kaplan, Appl. Surf. Sci. 252, 6269 �2006�.22I. Ay and H. Tolunay, Solid-State Electron. 51, 381 �2007�.23Y. Shimura, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, and H.

Hosono, Thin Solid Films 516, 5899 �2008�.24H. Frenzel, F. Schein, A. Lajn, H. von Wenckstern, and M. Grundmann,

Appl. Phys. Lett. 96, 113502 �2010�.25H. Frenzel, A. Lajn, H. von Wenckstern, M. Lorenz, F. Schein, Z. Zhang,

and M. Grundmann, “Recent Progress on ZnO-Based Metal-Semiconductor Field-Effect Transistors and Their Application in Transpar-ent Integrated Circuits,” Adv. Mater. �2010�.

26M. Grundmann, The Physics of Semiconductors �Springer, Leipzig, 2006�.27K. Nomura, T. Kamiya, H. Ohta, M. Hirano, and H. Hosono, Appl. Phys.

Lett. 93, 192107 �2008�.28A. Sato, K. Abe, R. Hayashi, H. Kumomi, K. Nomura, T. Kamiya, M.

Hirano, and H. Hosono, Appl. Phys. Lett. 94, 133502 �2009�.29K.-B. Park, J.-B. Seon, G. H. Kim, M. Yang, B. Koo, H. J. Kim, M.-K.

Ryu, and S.-Y. Lee, IEEE Electron Device Lett. 31, 311 �2010�.

243506-3 Lorenz et al. Appl. Phys. Lett. 97, 243506 �2010�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

69.166.47.134 On: Sat, 20 Dec 2014 07:09:36