Encapsulation of Zinc Tin Oxide Based Thin Film Transistors - The

Jun 1, 2009 - In this paper, we study the effect of thin film encapsulation on transparent thin film transistors (TFTs) with zinc tin oxide channel la...
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Encapsulation of Zinc Tin Oxide Based Thin Film Transistors Patrick Go¨rrn,*,† Thomas Riedl,‡ and Wolfgang Kowalsky‡ Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey 08544, and Institute of High-Frequency Technology, Technical UniVersity of Braunschweig, Schleinitzstrasse 22, 38106 Braunschweig, Germany ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: May 6, 2009

In this paper, we study the effect of thin film encapsulation on transparent thin film transistors (TFTs) with zinc tin oxide channel layers deposited by oxygen plasma assisted pulsed laser deposition (PAPLD). Chemisorption and desorption of oxygen are identified as the most important physical processes governing the impact of surface passivation on the device characteristics. For an in-depth study, we apply three different methods to remove chemisorbed oxygen from the channel surface: thermally activated desorption in vacuum; light induced desorption; and chemical reaction with the highly reactive agent trimethyl aluminum (TMA) during atomic layer deposition (ALD) thin film encapsulation. In each case, the removal of chemisorbed oxygen significantly shifts the threshold voltage of the TFTs on the order of 1-2 V. On the basis of the understanding gained in these experiments, a two-step encapsulation process is proposed that enables the application of high-performance ALD grown permeation barriers layers without alteration of the device characteristics. This finding is an essential step toward active matrix OLED displays with metal oxide driving electronics. 1. Introduction Thin film transistors (TFTs) with a transparent amorphous oxide semiconductor (TAOS) channel combine the excellent uniformity of amorphous devices with a high saturation field effect mobility, 1 order of magnitude larger than that of TFTs based on amorphous silicon (a-Si:H).1 Moreover, they are transparent in the visible part of the spectrum. A significant amount of work has been devoted to the most promising application for TAOS-TFTs: TFT backplanes for active matrix organic light emitting diode (OLED) displays.2-4 Since the sensitivity of OLEDs to moisture and oxygen causes device degradation, they must be encapsulated.5 In active matrix displays, this encapsulation will also cover the TFT backplane. It is well-known that the electrical properties of oxide semiconductors are significantly affected by interaction with the outside atmosphere.6 Hence, for industrial application of oxide based TFT as driver electronics for OLEDs it is essential to understand how their performance depends on the interaction of the channel surface with the atmosphere and to study the impact of encapsulation on the TFT characteristics. Various deposition techniques and materials have been considered for thin film encapsulation of TFTs. It is well-known that for example polymer thin films are poor gas permeation barriers.7 It is thus very surprising that passivation with only the polymeric resin SU8, of bottom gate TFTs based on sputtered channel layers of indium gallium zinc oxide (IGZO) or zinc tin oxide (ZTO), already causes pronounced changes in the characteristics of the transistors.8 The thermal evaporation of insulating oxides or fluorides for the passivation of ZTOTFTs shifts the transfer characteristics to lower voltages or even leads to a complete absence of channel saturation.9 The dominant general response of oxide TFTs to encapsulation is a decrease * To whom correspondence should be addressed. Phone: (609) 258-2509. Fax: (609) 258-6279. E-mail: [email protected]. † Princeton University. ‡ Technical University of Braunschweig.

of the threshold voltage Vth, which can be related to an increase of the density of free electrons within the channel. These additional electrons originate in the passivated surface of the channel that has been exposed to atmosphere before encapsulation. Recently, Kang et al. reported that the channel of IGZOTFTs is very sensitive to oxygen adsorption.10 A decrease in the oxygen pressure from 760 to 8.5 × 10-6 Torr was found to shift the turn-on voltage (Vto) from -7 to -54 V. The underlying desorption process was characterized as very sluggish. Upon subsequent exposure to air, Vto returned to the initial value before evacuation. In view of these results, it can be expected that a dense inorganic encapsulation of IGZO TFTs, for example, by using atomic layer deposition (ALD), will result in very low threshold voltages, comparable to those found in vacuum.11 The magnitude of that shift of Vto upon encapsulation renders the sensitivity to oxygen a very important issue of oxide based electronics in general. Moreover, the effects of bias stress and illumination on oxide based TFTs also depend essentially on oxygen desorption. The underlying processes are field induced adsorption of oxygen by trapping of electrons and light induced desorption of oxygen.12,13 In some of our experiments, we will use the latter mechanism to deliberately remove adsorbed oxygen from the channel of our TFTs. No systematic studies of species adsorbed on a TAOS surface have been published. However, in the case of crystalline ZnO surfaces thermal desorption spectroscopy (TDS) experiments were conducted some time ago.14,15 The results distinguished between weakly bound physisorbed oxygen molecules and chemisorbed negatively charged oxygen. For chemisorbed O2at the 10-10 surface, an activation energy of about 1.1 eV has been found.15 Consequently, in a temperature range between 40 and 80 °C the concentration of chemisorbed oxygen remains nearly constant while the density of weakly bound physisorbed oxygen decreases by the factor of 2.5 as the temperature is increased from 150 to 200 K.14 When the oxygen background pressure is reduced from 10-4 to 10-8 mbar surface coverage

10.1021/jp9018487 CCC: $40.75  2009 American Chemical Society Published on Web 06/01/2009

Encapsulation of Oxide Based Thin Film Transistors Θ of chemisorbed oxygen drops by the order of 1 × 10-4. Θ ) 1 corresponds to one O2- for every ZnO molecule at the surface. Studies of the resistance change of sintered ZnO pellets upon heating in vacuum at 510 °C and subsequent exposure to oxygen allowed the determination of the amount of ultimately chemisorbed oxygen in ZnO, corresponding to a coverage of Θ ) 3 × 10-4.16 We assume that O2- chemisorption from ZnO dominates the Vth shift in PAPLD-ZTO TFTs in vacuum and that this process is furthermore comparable for (poly-)crystalline and amorphous ZnO. The density of ZnO units (binding sites for oxygen) at the ZTO channel is approximately 1015 cm-2. Thus, reducing Θ by 3 × 10-4 reduces the density of O2- adsorbed to the channel surface by approximately 3 × 1011 cm-2. This translates to a change in the surface charge density of 5 × 10-8 As/cm2. The gate capacitance of the ZTO TFT in our study is 60 nF/ cm2. Therefore we estimate that this change causes a gate voltage shift of about 1 V. 2. Experimental Details 2.1. Sample Preparation. As substrates we used glass coated with the transparent conductive oxide (TCO) indium tin oxide (ITO) and a 220 nm thick Al2O3/TiO2 laminate, which served as the gate contact and dielectric, respectively. The substrates have been provided by Planar Systems Inc. (Finland). The 60-80 nm thick ZTO channels have been deposited by oxygen plasma assisted pulsed laser deposition (PAPLD). As source and drain contacts we used the PLD deposited TCO aluminum doped zinc oxide, structured by standard photolithography, on top of the channel. All investigated TFTs are fabricated in the described staggered bottom gate structure with a width to length ratio of 5 (1 mm/200 µm). Further details about the TFT fabrication can be found elsewhere.2 Al2O3 and ZrO2 thin films were grown in an 8 in. ALD reactor (Savannah 200 from Cambridge Nanotech). For the preparation of the Al2O3 layers H2O and trimethyl aluminum (TMA) were used as precursors, and for the preparation of the ZrO2 layers tetrakis(dimethylamido)zirconium(IV) (TDMA(Zr)) and H2O. TMA and water precursors were kept at room temperature while the TDMA(Zr) was heated to 75 °C. Al2O3 layers were prepared by repeated dosing of TMA and H2O for 15 ms with 20 to 30 s of N2 purging intervals (20 sccm). The reactor temperature was kept at 60 °C. ZrO2 layers were prepared by repeated dosing of TDMA(Zr) and H2O for 0.3 s and 30 ms, respectively. Nanolaminates were prepared by repeated deposition of 20 cycles of Al2O3 and 20 cycles of ZrO2.17 2.2. Instrumental Methods. Illumination or bias stress has been found to shift the threshold voltage of an oxide TFT.18,19 We took particular care to limit these parasitic effects. To study the influence of the background atmosphere, our TFTs were placed in a completely dark test chamber that was still open to the atmosphere. To reduce the impact of bias stress, the transfer characteristics of the TFT were measured only every 300 s. The devices were kept under these conditions until the threshold voltage has become nearly constant in time before the chamber was evacuated. Saturation field effect mobility µsat and Vth were determined in the saturation regime by extracting the axis intercept and the slope above Vth, respectively. Using the same test chamber without an obscured window, we exposed the ZTO channel to violet light (λ ) 425 nm, 1.5 W/cm2) to deliberately remove adsorbed oxygen from the TFT surface. For this purpose LEDs of 425 nm with a spectral width of approximately 20 nm were used. Light intensities were measured by using a Coherent Laser Mate-Q with a Coherent RW80 measuring head.

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Figure 1. Threshold voltage shift of a ZTO-TFT in vacuum (5 × 10-7 mbar) as a function of time. The arrow indicates the time the sample chamber is evacuated.

For in situ monitoring of the TFT performance during ALD encapsulation the staggered bottom gate devices have been contacted with use of gold bond wires. The electrical feedthrough has been established on the ISO-KF pumping line of the ALD reactor. For a further investigation in the test chamber described above the encapsulated devices have been transferred through the air. 3. Results and Discussion For experimental verification of the estimated threshold voltage shift upon oxygen desorption, we applied three different methods to desorb oxygen from the channel surface: activated thermally at room temperature in vacuum, by illumination of visible light, and by chemical reaction with TMA during ALD thin film encapsulation. In all of these experiments the transfer characteristics of the TFTs are measured in situ during the desorption processes. In none of these studies was a significant change in the saturation field effect mobility, subthreshold regime, the off-current, or hysteresis observed. However, all studied desorption processes result in a decrease of the threshold voltage. The initial threshold voltage of PAPLD-ZTO TFTs without encapsulation that have been stored in atmosphere in the dark for several days is typically between -1 and 3 V. 3.1. Thermally Activated Desorption at Room Temperature. We investigated the influence of thermally activated desorption of oxygen at room temperature on the performance of PAPLD-ZTO TFTs. For this purpose the devices were placed in a vacuum chamber. Then the chamber was evacuated to a pressure of 5 × 10-7 mbar. Figure 1 shows the resulting Vth shift for a typical PAPLD-ZTO-TFT. The initial threshold voltage is around 2.5 V. The overall change of Vth was less than 1 V within 500 h. The time constant of this process was on the order of days. This Vth shift agrees with the value estimated from measurements on crystalline ZnO. The model of O2- chemisorption from ZnO might be sufficient to describe not only the order of the absolute change of the threshold Vth, but also the extremely long time constants found as a result of the activation energy of around 1.1 eV for chemisorbed O2- on ZnO. The sensitivity of amorphous PAPLD-ZTO channels to oxygen is comparable to the sensitivity that is expected for a single crystalline ZnO surface. In contrast, Kang et al. observed a shift of about 40 V in their R-IGZO TFTs upon evacuation to 8.5 × 10-6 Torr. Normalized to the properties of the gate dielectric the impact of oxygen desorption on the threshold voltage is about an order of magnitude larger than that for PAPLD-ZTO TFTs. This observation can be explained by a higher activation energy for the electron transfer from chemi-

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sorbed oxygen to the channel in PAPLD-ZTO compared to sputtered IGZO channels. Aside from the material itself, the deposition technology has to be considered as well, because the sensitivity of the investigated channels deposited by PAPLD appears very low compared to that of sputtered channels of the same material.8,9 While the threshold in atmosphere is influenced by chemisorbed oxygen, the threshold in vacuum is correlated with the free density of electrons in the channel volume. Chemisorption of oxygen relies on the transfer of electrons from the channel to the adsorbate. With a higher electron density the extent of the charge transfer will increase. It is well-known that oxygen vacancies form shallow donors in ZnO and increase the carrier density in the film.20 Consequently, a higher degree of oxidation in the volume of the channel layer will decrease the density of free electrons and thus the rate of oxygen chemisorption at the surface. Therefore, the surface coverage of chemisorbed oxygen in equilibrium will be reduced. In this context, the low sensitivity of PAPLD grown ZTO TFTs is attributed to an optimized oxidation during the deposition enabled by the highly energetic PLD plasma and the additional application of oxygen radicals supplied by an additional oxygen plasma source. 3.2. Chemisorption in Controlled Atmosphere. While the low sensitivity of the PAPLD ZTO channels to thermally activated desorption of oxygen is expected to be beneficial for the application of ZTO TFTs in electronic backplanes, the large time constant complicates the study of this phenomenon. Therefore, to deliberately remove adsorbed oxygen from the TFT surface, we expose the ZTO channel to violet light (λ ) 425 nm, 1.5 W/cm2). The photon energy (2.9 eV) clearly exceeds the mentioned activation energy for desorption of 1.1 eV. In ZnO nanowires, sub-bandgap irradiation excites electrons trapped to adsorbed oxygen into the conduction band.13 Thereby, weakly bound neutral oxygen is produced that may leave the surface upon thermal activation at room temperature. The investigated TFTs were illuminated for about 15 min until the threshold voltage decreased by 4 V. This shift is not exclusively due to desorption of chemisorbed oxygen but also due to the excitation of other defects in amorphous ZTO. We plausibly assume that a significant part of the shift of the threshold originates in oxygen desorption. Hence, after illumination, the temporal dependence of the recovery of the threshold voltage Vth(t) in the dark was analyzed at various oxygen background pressures. Figure 2a shows these Vth(t) transients for a bottomgate ZTO-TFT with an exposed (nonencapsulated) channel. At high vacuum (∼ 10-7 mbar), the recovery of Vth(t) is slow and it becomes significantly faster at moderate pressure of pure oxygen (5 × 10-4 mbar). This clearly demonstrates that oxygen adsorption governs the recovery of Vth(t) after illumination. Furthermore, the strong dependence on the oxygen background pressure makes this experiment a useful technique for studying the thin-film encapsulation of oxide TFTs. 3.3. Effect of ALD Encapsulation. In view of the previous discussion, we decided to study the effect of an ALD grown thin film encapsulation on ZTO-TFTs. Atomic layer deposition (ALD) is a promising technique that allows the deposition of very dense films at low substrate temperature. Therefore ALDgrownthinfilmshavebeenconsideredforOLEDencapsulation.17,21-23 Al2O3 films grown at 120 °C reduce permeation rates for water to 6.5 × 10-5 g/(m2 day).23 With Al2O3/ZrO2 nanolaminates deposited at lower deposition temperatures (80 °C), even less permeable gas diffusion barriers are possible.17 Even though a deposition temperature up to 80 °C might be tolerable to encapsulate some OLEDs with optimized thermal stability, even lower process temperatures would be desirable. On the other

Go¨rrn et al.

Figure 2. (a) Threshold recovery of a ZTO-TFTs after illumination at different background pressures before (exposed) and after encapsulation (encapsulated) and (b) the corresponding transfer characteristics in darkness of the TFT before and after encapsulation.

hand, toward lower substrate temperatures one typically encounters a compromised quality of the encapsulation layers and dramatically increased processing times due to the requirement for significantly longer purge times between precursor doses. As a trade-off, we chose a deposition temperature of 60 °C in our ALD processes. Figure 2a also shows the recovery of a ZTO-TFT encapsulated with 130 nm Al2O3/ZrO2 nanolaminate. The channel is identical with that of the nonencapsulated TFT. The recovery of Vth is much slower than that of the TFT without encapsulation. Within 25 h, the overall recovery of Vth is less than 0.5 V. No significant influence of varying oxygen pressure between 5 × 10-7 and 5 × 10-4 mbar is observed, which proves that the barrier layer is effective. Our study of the relatiVe threshold shifts after illumination clearly showed that the interaction of oxygen with the TFT channel significantly affects the threshold voltage. It is therefore quite reasonable to assume that the encapsulation of a TFT changes the absolute value of the threshold voltage of the device. Figure 2b shows the transfer characteristics of the same TFT before and after ALD encapsulation with 130 nm of an Al2O3/ZrO2 nanolaminate. The threshold voltage of the encapsulated device is decreased by about 2 V. The deposition of an Al2O3 layer by ALD is expected to lead to a nearly complete removal of chemisorbed oxygen from the ZTO surface, as the applied precursor, trimethyl aluminum (TMA), is a highly reactive agent that will react with oxygen at the surface.24 Very interestingly, the use of thinner encapsulation layers on the order of about 20 nm also leads to a similar shift of Vth, but this shift reversibly recovers to the initial values within days or weeks for devices stored at ambient atmosphere. This behavior can be attributed to the elevated permeation rates for oxygen found in thinner barrier layers. Consequently, oxygen or water may penetrate the thin barrier and restore the former threshold Vth by adsorption at the critical channel surface. Consequently, a pre-encapsulated TFT with essentially unaffected electrical characteristics can be obtained. Very surpris-

Encapsulation of Oxide Based Thin Film Transistors

Figure 3. (a) TFT threshold voltage during encapsulation in an ALD process; (b, c) magnified view of Vth(t) upon consecutive ALD cycles; and (d) transfer characteristics of the TFT before the ALD deposition, after the first ALD cycle, and at the end of the experiment.

ingly, this device does not show a further significant change in Vth if a subsequent encapsulation layer is deposited until the desired overall encapsulation layer thickness in the range of 130 nm is reached. Thereby a highly efficient, dense encapsulation layer can be formed on top of the TFT channel without relevant changes to the TFT characteristics. For a better understanding of these phenomena, we studied the TFTs in situ during the ALD encapsulation process. First of all, the saturation field effect mobility of the device remains in the range of (8.36 ( 0.09) cm2/(V s) throughout the entire experiment (not shown here). Again, no significant change in the subthreshold regime, the off-current, or hysteresis in the transfer characteristics is observed throughout the study. We will, therefore, focus on the threshold voltage Vth as the parameter that is found to be most sensitive to the surrounding atmosphere. Figure 3a shows the overall history of Vth during this experiment. The substrate temperature within the ALD chamber is set to 60 °C, the background pressure is 5 × 10-1 mbar of pure N2 (6 N). Upon transfer of the TFT into the ALD

J. Phys. Chem. C, Vol. 113, No. 25, 2009 11129 chamber with its higher temperature and lower pressure compared to the outside atmosphere, adsorbates at the TFT surface are forced to desorb from the surface. As a consequence, a slowly decreasing threshold is observed, similar to the observation discussed above. We again wait until the device has reached a quasi-stationary state, where the variation of Vth has become sufficiently slow to reduce the influence on the following studies. Before the encapsulation is started with the first ALD cycle, the influence of water on the threshold Vth is tested. For this purpose, a 15 ms long H2O dosage is applied. This dosing is identical with the water pulses used in the subsequent ALD process. As a consequence of the H2O pulse, the pressure in the ALD reactor is increased by about 1 order of magnitude. The increased pressure recovers within seconds owing to the continuous N2 purge. There is no detectable effect of this exposure to water on the threshold of the device (Figure 3a). The H2O pulse generates a monolayer of adsorbed water on the surface. When followed by the pulse of a metalorganic precursor the first cycle of an ALD process is completed.25 In our study we use the precursor trimethyl aluminum (TMA). A pulse of water followed by a TMA pulse will be called an ALD cycle. Through a hydrolysis reaction a monolayer of Al2O3 is formed. The growth rate of Al2O3 by the TMA/water ALD process at temperatures below 100 °C is 1.3 Å/cycle.17 Figure 3b shows the fast decrease of Vth by about 360 mV after the first ALD cycle. TMA reacts with oxygen and water at the TFT channel surface. A subsequent ALD cycle (2.5 h after the first one) further reduces the threshold by 74 mV. The relative shift of Vth upon additional consecutive TMA/water cycles (with a 30 min time interval between each) decreases from cycle to cycle. After cycle 7 no further shift of Vth is observed. The total shift of Vth after the 7 cycles is 530 mV. This voltage corresponds to the density of chemisorbed oxygen (2.0 × 1011 cm-2) left at the channel surface after prolonged storage at 60 °C at a background pressure of 5 × 10-1 mbar. Apparently, 7 ALD cycles are required to complete the reaction of TMA at the surface. An explanation is found by normalizing the shift after cycle one to the overall decrease of the threshold (360 mV/530 mV). The first cycle immediately removes 68% of the chemisorbed oxygen. This number is explained by the ligand coverage of 70-80% for TMA.24 Because of the steric hindrance of its methyl groups the TMA covers only 70% of the surface. After 7 cycles no further reaction with oxygen adsorbed to the semiconductor is seen and consequently no further shift of Vth occurs. Closer inspection reveals even a slight increase of Vth after the first 1 + 9 cycles (Figure 3a). We assume that the origin of this behavior is readsorption of oxygen or water, which hints to the assumption that the thin ALD barrier still allows the smaller O2 and H2O species to penetrate this layer and then interact with the surface of the TFT channel. To verify the impact of water, we again apply a single water pulse (15 ms duration) and later a further water pulse with an increased duration (300 ms). As is evident from Figure 3a, these two exposures to water cause no shift of Vth. We conclude that a potential readsorption of water molecules only has a minor influence on the threshold Vth. Most strikingly, upon venting the ALD chamber and exposure of the pre-encapsulated TFT to ambient air (still in the dark) Vth rises quickly by about 950 mV. After reaching a threshold voltage even higher than the initial value before the ALD process, the chamber was evacuated again to 5 × 10-1 mbar. Vth immediately starts to decrease in a similar manner as in the beginning of the experiment. Figure 3c shows this decrease in Vth, and the impact of further ALD

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cycles (numbers 11-20). Compared to the first 10 ALD cycles the overall shift of Vth for cycles 11-20 remains negligible. Even the following 180 cycle ALD process (periodicity 50 s) does not significantly influence Vth any further. It is essential to note that Vth drops because of oxygen desorption in vacuum. This temporal behavior is superimposed on the impact of the encapsulation process. The thickness of the encapsulation film grown with this 200-cycle process is approximately 26 nm. The in situ study of the threshold during ALD encapsulation explains the previous observation, that a threshold change caused by the deposition of a thin “pre-encapsulation” can be reversed upon readsorption of oxygen in atmosphere. Moreover, Vth does not show any further significant change if a subsequent denser encapsulation layer is deposited. We assume that the preencapsulation produces a barrier for the large TMA molecule that protects the readsorbed oxygen from further reaction with this precursor. 4. Conclusions The sensitivity of oxide TFTs to atmosphere was studied for the case of ZTO based devices prepared by plasma assisted PLD. Their threshold voltage is found to strongly depend on the chemisorption and desorption of oxygen, respectively, while other device properties like the subthreshold regime, the offcurrent, or hysteresis remain essentially unaffected. The shift of the threshold voltage under varied oxygen partial pressures is directly correlated to the varied density of chemisorbed oxygen molecules at the channel surface. Upon passivation of the TFT channel by using dense ALD barrier layers, a shift of the threshold by typically 2 V is found. On the basis of a detailed study of the encapsulation process and the understanding of the interplay of oxygen with the TFT channel, a strategy is developed that allows for the formation of barrier layers without significant alteration of the device characteristics. Essentially, a two-step encapsulation process based on a thin “preencapsulation” is proposed. This thin layer (in the range of 1 nm) keeps the reactive precursor away from the channel while it remains permeable for oxygen. Thus, the shift of the threshold voltage upon deposition of the pre-encapsulation may be reversed by a subsequent exposure of the sample to oxygen (air). Importantly, a subsequent encapsulation process to attain a barrier layer with low permeation rates for oxygen no longer affects the threshold of the device. This will enable the application of high-performance ALD grown permeation barriers for active matrix OLED displays with use of metal oxide driving electronics.

Go¨rrn et al. Acknowledgment. The authors gratefully acknowledge financial support by the German Federal Ministry for Education and Research BMBF (FKZ 13N9152) and the Deutsche Forschungsgemeinschaft (DFG) through the Gottfried Wilhelm Leibniz award. P.G. thanks the Alexander von HumboldtFoundation for a Feodor Lynen Fellowship. References and Notes (1) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Nature 2004, 432, 488. (2) Go¨rrn, P.; Sander, M.; Meyer, J.; Kro¨ger, M.; Becker, E.; Johannes, H.-H.; Kowalsky, W.; Riedl, T. AdV. Mater. (Weinheim, Ger.) 2006, 18, 738. (3) Jeong, J. K.; Jeong, J. H.; Choi, J. H.; Im, J. S.; Kim, S. H.; Yang, H. W.; Kang, K. N.; Kim, K. S.; Ahn, T. K.; Chung, H.-J.; Kim, M.; Gu, B. S.; Park, J.-S.; Mo, Y.-G.; Kim, H. D.; Chung, H. K. Dig. Tech. Pap.Soc. Inf. Disp. Int. Symp. 2008, 39, 1. (4) Park, S.-H. K.; Hwang, C.-S.; Lee, J.-I.; Chung, S.-M.; Yang, Y. S.; Do, L.-M.; Chu, H. Y. Dig. Tech. Pap.-Soc. Inf. Disp. Int. Symp. 2006, 37, 25. (5) Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochack, L. S.; McCarty, D. M.; Thompson, M. E. Appl. Phys. Lett. 1994, 65, 2922. (6) Lampe, U.; Mu¨ller, J. Sens. Actuators 1989, 18, 269. (7) Charton, C.; Schiller, N.; Fahland, M.; Holla¨nder, A.; Wedel, A.; Noller, K. Thin Solid Films. 2006, 502, 99. (8) Chiang, H. Q. Ph.D. Thesis, Oregon State University, 2007. (9) Hong, D.; Wager, J. F. J. Vac. Sci. Technol. B 2005, 23, L25. (10) Kang, D.; Lim, H.; Kim, C.; Song, I.; Park, J.; Park, Y.; Chung, J. Appl. Phys. Lett. 2007, 90, 192101. (11) Cho, D.-H.; Yang, S.; Shin, J.; Ryu, M.-K.; Cheong, W.-S.; Byun, C.; Yoon, S.-M.; Park, S.-H. K.; Lee, J. I.; Hwang, C.-S.; Chu, H.-Y. Dig. Tech. Pap.-Soc. Inf. Disp. Int. Symp. 2008, 39, 1243. (12) Jeong, J. K.; Yang, H. W.; Jeong, J. H.; Mo, Y.-G.; Kim, H. D. Appl. Phys. Lett. 2008, 93, 123508. (13) Li, Q. H.; Gao, T.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2005, 86, 123117. (14) Go¨pel, W. Prog. Surf. Sci. 1985, 20 (1), 9. (15) Go¨pel, W. J. Vac. Sci. Technol. 1979, 16 (5), 1229. (16) Glemza, R.; Kokes, R. J. J. Phys. Chem. 1965, 69 (10), 3254. (17) Meyer, J.; Go¨rrn, P.; Bertram, F.; Hamwi, S.; Winkler, T.; Johannes, H.-H.; Weimann, T.; Hinze, P.; Riedl, T.; Kowalsky, W. AdV. Mater. 2009, 21, 1845. (18) Go¨rrn, P.; Lehnhardt, M.; Riedl, T.; Kowalsky, W. Appl. Phys. Lett. 2007, 91, 193504. (19) Go¨rrn, P.; Ho¨lzer, P.; Riedl, T.; Kowalsky, W.; Wang, J.; Weimann, T.; Hinze, P.; Kipp, S. Appl. Phys. Lett. 2007, 90, 063502. (20) Minami, T.; Nanto, H.; Shooji, S.; Takata, S. Thin Solid Films 1984, 111, 167. (21) Yun, S. J.; Ko, Y.-W.; Lim, J. W. Appl. Phys. Lett. 2004, 85, 21. (22) Ghosh, A. P.; Gerenser, L. J.; Jarman, C. M.; Fornalik, J. E. Appl. Phys. Lett. 2005, 86, 223503. (23) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Groner, M. D.; George, S. M. Appl. Phys. Lett. 2006, 89, 031915. (24) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. (25) Suntola, T. Appl. Surf. Sci. 1996, 100/101, 391.

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