Photo stability of solution-processed low-voltage high mobility zinc-tin-oxide/ZrO2thin-film transistors for transparent display applicationsTae-Jun Ha and Ananth Dodabalapur

Citation: Applied Physics Letters 102, 123506 (2013); doi: 10.1063/1.4795302 View online: http://dx.doi.org/10.1063/1.4795302 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Solution-processed zinc-indium-tin oxide thin-film transistors for flat-panel displays Appl. Phys. Lett. 103, 072110 (2013); 10.1063/1.4818724 Low temperature processing of indium-tin-zinc oxide channel layers in fabricating thin-film transistors J. Vac. Sci. Technol. B 29, 021008 (2011); 10.1116/1.3553205 Band transport and mobility edge in amorphous solution-processed zinc tin oxide thin-film transistors Appl. Phys. Lett. 97, 203505 (2010); 10.1063/1.3517502 Solvent-mediated threshold voltage shift in solution-processed transparent oxide thin-film transistors Appl. Phys. Lett. 97, 092105 (2010); 10.1063/1.3485056 Solution-processed zinc–tin oxide thin-film transistors with low interfacial trap density and improved performance Appl. Phys. Lett. 96, 243501 (2010); 10.1063/1.3454241

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Photo stability of solution-processed low-voltage high mobilityzinc-tin-oxide/ZrO2 thin-film transistors for transparent displayapplications

Tae-Jun Ha and Ananth DodabalapurMicroelectronics Research Center, The University of Texas at Austin, Austin, Texas 78758, USA

(Received 25 January 2013; accepted 28 February 2013; published online 28 March 2013)

We report solution-processed low-voltage zinc-tin-oxide (ZTO)/zirconium-oxide thin-film transistors

(TFTs) possessing a field-effect mobility of �10 cm2/Vs, a threshold voltage of 0.1 V, and an on-off

current ratio of �1� 109. These TFTs exhibit very small hysteresis windows in both dark and

illuminated conditions. We also investigate the photo stability combined with prolong negative bias

in these devices. Large threshold voltage shifts and sub-threshold swing degradation typically

observed in ZTO TFTs are not present in our devices. We believe that these device characteristics,

which stem from the electronically clean semiconductor-dielectric interface, satisfy the requirement

for high quality and low power-consuming transparent displays. VC 2013 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4795302]

Metal-oxide semiconductors such as zinc-oxide (ZnO),

zinc-tin-oxide (ZTO), and indium-gallium-zinc-oxide (IGZO)

have attracted noticeable interest for transistor applications

due to their visible light transparency compared to amorphous

and poly silicon.1–4 This transparency potentially enables the

fabrication of stacked structures for various applications.

However, there have been reports of hysteresis and instability

caused by illumination on metal-oxide field-effect transistors

(FETs) despite the wide band-gap in these semiconductors.5,6

Furthermore, photo-induced instability combined with bias-

stress lead to more serious failure of switching and driving

transistors in pixels for transparent displays.7–9 In this work,

we present solution-processed low-voltage ZTO thin-film

transistors (TFTs) employing a solution-processed zirconium

dioxide (ZrO2) dielectric. These devices possess good mobil-

ity and very small hysteresis windows under both dark and

illuminated measurement conditions. We also investigate the

extent of photo-induced instability caused by the negative

bias illumination stress on ZTO TFTs. Very little change in

device characteristics is observed even without passivation.

These excellent characteristics stem from the electronically

clean interface between ZTO and ZrO2. We believe that this

work is very promising in helping understand the causes of

electric- and photo-induced stability in transparent devices.

Figure 1(a) shows the schematic cross-section of a

ZTO TFT possessing a ZrO2 dielectric. A 2.5 nm titanium

adhesion layer and a 47.5 nm gold-palladium bottom-gate

electrode were first deposited on glass substrates by e-beam

evaporation under a base pressure less than 10�6 Torr. A

ZrO2 precursor solution was synthesized by dissolving zir-

conium chloride and zirconium isopropoxide isopropanol

powers (1.158:1.927) in 2-methoxyethanol (0.5M concen-

tration) with magnetic stirring at room temperature for 6 h

in an inert environment. A 90 nm thick high-k dielectric

was formed by spin-coating, followed by storing in a nitro-

gen ambient for 1 h to enable gradual evaporation of the re-

sidual solvent followed by a annealing step at 500 �C for

1 h in air. This process was repeated and a second layer of

ZrO2 was deposited. The capacitance value, extracted by

CV measurements, is 240 nF/cm2. The ZTO precursor solu-

tion was formed by dissolving zinc chloride (ZnCl2) and tin

chloride (SnCl2) powders in acetonitrile (CH3CN). The

concentration of the ZTO precursor solution is 0.24 M with

the ZnCl2/SnCl2 molar ratio being unity. A 30 nm thick

ZTO layer was formed by first spin-coating the precursor

on the ZrO2-coated substrate in a nitrogen atmosphere. The

samples were pre-bake processed at 100 �C at 30 min under

an inert atmosphere and then converted to ZTO by anneal-

ing at 500 �C for 1 h in air. 40 nm thick aluminum source

and drain electrodes were deposited by thermal evaporation

after defining the source/drain electrodes via a lift-off pro-

cess. ZTO TFT devices possess a channel width of 80 lm

and a channel length of 4 lm. All samples were character-

ized by a semiconductor parameter analyzer and measured

in air. The magnitude of voltage sweep is 5 V and the sweep

steps are from 0.01 to 0.2 V. The transfer characteristics

were measured in the saturation and linear regions with the

application of source-drain biases of 5 V and 0.1 V, respec-

tively. The light source provided an optical power density

of 6.7 mW/cm2 from a halogen lamp source.

FIG. 1. Schematic cross-section of a solution-processed ZTO TFT possess-

ing solution-processed ZrO2 dielectrics.

0003-6951/2013/102(12)/123506/3/$30.00 VC 2013 American Institute of Physics102, 123506-1

APPLIED PHYSICS LETTERS 102, 123506 (2013)

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Figure 2 shows the output and transfer characteristics of

solution-processed ZTO FETs with a ZrO2 gate dielectric.

The calculated linear field-effect mobility is 6.1 cm2/Vs and

the saturation field-effect mobility is 9.5 cm2/Vs. The thresh-

old voltage (Vth) is 0.1 V, the on-off current ratio is up to

1� 109, and the sub-threshold swing (S.S.) is 0.1 V/decade at

low gate-voltage operation (<5 V). These device characteris-

tics satisfy the requirement for use in high quality displays

(FHD or UHD, 240 Hz) with low power consumption. Very

little hysteresis is observed during the sweep of gate voltage

in both forward and reverse directions. Since the hysteresis

typically results from trapped charges in shallow trap states at

the interface between the ZTO semiconductor and the gate in-

sulator, this result indicates a good quality interface. We cal-

culated the density of trap states in ZTO TFTs and the value

is �1� 1012 /cm2, which is significantly less than that previ-

ously reported solution-processed metal-oxide TFTs and com-

parable to values obtained with sputtered metal-oxide

TFTs.10,11 Key to realizing such high mobilities and little hys-

teresis in ZTO TFTs are the high channel carrier concentra-

tions made possible by using the high-k dielectric and the

electronically clean interface between ZTO and ZrO2.12,13 We

believe that this clean interface is especially important in real-

izing a number of favorable properties that we report below.

Figure 3 shows the transfer characteristics of ZTO TFTs

with different sweep magnitudes. The hysteresis window

increases with the sweep magnitude of gate voltage as more

carriers are initially trapped at the interface; therefore, the

carrier trapping and de-trapping rates during the gate voltage

sweep will be different. In the reverse sweep direction, ini-

tially trapped electrons assist channel depletion at a higher

gate voltage, which results in a positive shift of the transfer

characteristics. Since different rates of charge carrier trap-

ping and de-trapping influence the size of the hysteresis win-

dow, the hysteresis window increases with gate voltage.

Figure 4(a) shows the transfer characteristics in ZTO

TFT measured under dark as well as illuminated condi-

tions.14,15 Little hysteresis was also observed in our devices

under the illuminated state, which results from low density of

trap states.16–18 Rim et al. reported that the incorporation of

zirconium (Zr) in ZTO TFTs lowers the off-current and

improves sub-threshold characteristics.10 This was attributed

to Zr being readily oxidized than tin (Sn) or zinc (Zn), which

can reduce the concentration of oxygen vacancies in ZTO

close to the interface with ZrO2.10 It has been reported that

zinc interstitials and oxygen vacancies play a critical role in

metal-oxide semiconductors as defect states.19,20 We hypothe-

size that in our ZTO/ZrO2 TFTs, the presence of Zr results in

a very low interface trap density arising from oxygen vacan-

cies. Very likely, this also results in a lower density of trap

states in the ZTO.21,22 Transport studies of ZTO/ZrO2 TFTs

reveal very low activation energies implying a small density

of trap states in the forbidden gap.23 For the above reasons,

ZTO TFTs employing a ZrO2 gate dielectric exhibit improved

optical response and stability, resulting in reduced photo-

induced hysteresis. This enhances electrical device perform-

ance as well.

When ZTO TFTs are used as a switching device in the

backplane, the instability caused by the negative gate bias

stress can lead to failure in the operation because switching

TFTs will be in their off-state during the majority of display

time. Furthermore, when ZTO TFTs are employed for com-

ponents of transparent displays, the instability caused by

bias-stress can be enhanced by generated charge carriers

under light illumination. The use of ZrO2 gate dielectrics

greatly reduced the extent of such bias stress. Figure 4(b)

shows the results of negative bias-stress under illumination

for the duration of 5000 s. Large threshold voltage shifts and

hump formation with S.S. degradation as reported previously

FIG. 2. (a) The output and (b) the transfer characteristics and field-effect

mobility in ZTO TFTs employing ZrO2 dielectrics.

FIG. 3. The transfer characteristics of ZTO TFTs at different sweep

magnitudes.

123506-2 T.-J. Ha and A. Dodabalapur Appl. Phys. Lett. 102, 123506 (2013)

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by other groups are not observed in our devices.7,24 It must

be noted that we achieve such stability without any passiva-

tion.25,26 Our results are in conformity with a model in which

the presence of Zr at the interface suppresses charge trapping

and the formation of oxygen vacancies. This is reflected in

a number of favorable properties: higher mobility, better

S.S., and less sensitivity of the Vth to light and negative bias,

as shown in Figure 4(c).

Solution-processed low-hysteresis ZTO TFTs employ-

ing solution-processed ZrO2 gate dielectric have been

described. Such TFTs operate at low voltages (<5 V) and ex-

hibit good device characteristics with very small hysteresis

windows under both dark and illuminated conditions. These

favorable properties result both from the high channel carrier

concentration made possible by using the high-k dielectric

and also, importantly, the electronically clean interface

between ZTO and ZrO2. Photo stability combined with pro-

long negative gate-bias stress in ZTO TFTs has been also

investigated. We believe that the excellent stability we

observe in our unpassivated devices make them promising

candidates for use in transparent displays with low power

consumption.

This work is supported by the NSF-NASCENT ERC.

1J. F. Wager, Science 300, 1245 (2003).2K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono,

Nature 432, 488 (2004).3E. M. C. Fortunato, P. M. C. Barquinha, A. C. M. B. G. Pimentel, A. M. F.

Goncalves, A. J. S. Marques, L. M. N. Pereira, and R. F. P. Matins, Adv.

Mater. 17, 590 (2005).4W. B. Jackson, R. L. Hoffman, and G. S. Herman, Appl. Phys. Lett. 87,

193503 (2005).5P. G€orrn, M. Lehnhardt, T. Riedl, and W. Kowalsky, Appl. Phys. Lett. 91,

193504 (2007).6X. Huang, C. Wu, H. Lu, F. Ren, and Q. Xu, Appl. Phys. Lett. 100, 243505

(2012).7S.-Y. Lee, S.-J. Kim, Y. W. Lee, W.-G. Lee, K.-S. Yoon, J.-Y. Kwon, and

M.-K. Han, IEEE Electron Device Lett. 33, 218 (2012).8J.-Y. Kwon, J. S. Jung, K. S. Son, K.-H. Lee, J. S. Park, T. S. Kim, J.-S.

Park, R. Choi, J. K. Jeong, B. Koo, and S. Y. Lee, Appl. Phys. Lett. 97,

183503 (2010).9D. Gupta, S. Yoo, C. Lee, and Y. Hong, IEEE Trans. Electron. Devices

58, 1995 (2011).10Y. S. Rim, D. L. Kim, W. H. Jeong, and H. J. Kim, Appl. Phys. Lett. 97,

233502 (2010).11J. K. Jeong, J. H. Jeong, H. W. Yang, J.-S. Park, Y.-G. Mo, and H. D.

Kim, Appl. Phys. Lett. 91, 113505 (2007).12C.-G. Lee and A. Dodabalapur, Appl. Phys. Lett. 96, 243501 (2010).13Y.-G. Ha, S. Jeong, J. Wu, and M.-G. Kim, V. P. Dravid, A. Facchetti, and

T. J. Marks, J. Am. Chem. Soc. 132, 17426 (2010).14J. Reemts and A. Kittel, J. Appl. Phys. 101, 013709 (2007).15S. Yasuno, T. Kugimiya, S. Morita, A. Miki, F. Ojima, and S. Sumie,

Appl. Phys. Lett. 98, 102107 (2011).16B. Ryu, H.-K. Noh, E.-A. Choi, and K. J. Chang, Appl. Phys. Lett. 97,

022108 (2010).17S. Oh, B. S. Yang, Y. J. Kim, M. S. Oh, M. Jang, H. Yang, J. K. Jeong, C.

S. Hwang, and H. J. Kim, Appl. Phys. Lett. 101, 092107 (2012).18B. S. Yang, S. Park, S. Oh, Y. J. Kim, J. K. Jeong, C. S. Hwang, and H. J.

Kim, J. Mater. Chem. 22, 10994 (2012).19L. Schmidt-Mende and J. L. MacManus-Driscoll, Mater. Today 10, 40

(2007).20Y. Jeong, C. Bae, D. Kim, K. Song, K. Woo, H. Shin, G. Cao, and J. Moon,

ACS Appl. Mater. Interfaces 2, 611 (2010).21C.-G. Lee and A. Dodabalapur, J. Electronic Materials 41, 895 (2012).22S. Lee and A. Nathan, Appl. Phys. Lett. 101, 113502 (2012).23C.-G. Lee, B. Cobb, and A. Dodabalapur, Appl. Phys. Lett. 97, 203505

(2010).24D. W. Kwon, J. H. Kim, J. S. Chang, S. W. Kim, W. Kim, J. C. Park, C. J.

Kim, B.-G. Park, IEEE Trans. Electron Devices 58, 1127 (2011).25P. G€orrn, T. Riedl, and W. Kowalsky, J. Phys. Chem. C 113, 11126 (2009).26S.-J. Seo, J. H. Jeon, Y. H. Hwang, and B.-S. Bae, Appl. Phys. Lett. 99,

152102 (2011).

FIG. 4. (a) The transfer characteristics in a ZTO TFT in the dark and under

illumination. The inset shows the photo current which is the current differ-

ence between dark and illuminated conditions, (b) the results of negative

bias-stress under illumination, and (c) the shift in Vth as a function of time at

each stress condition.

123506-3 T.-J. Ha and A. Dodabalapur Appl. Phys. Lett. 102, 123506 (2013)

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