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Thermal stability of amorphous InGaZnO thin film transistors passivated by AlOx layers
Solid-State Electronics 104 (2015) 39–43
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Thermal stability of amorphous InGaZnO thin film transistors passivated by AlOx layers Zhe Hu, Daxiang Zhou, Ling Xu, Qi Wu, Haiting Xie, Chengyuan Dong ⇑ National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 12 July 2014 Received in revised form 12 October 2014 Accepted 28 October 2014
The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: Amorphous indium gallium zinc oxide (a-IGZO) Thin film transistor (TFT) Thermal stability Passivation-layer Intrinsic excitation
a b s t r a c t Thermal stability of amorphous InGaZnO thin film transistors (a-IGZO TFTs) passivated by AlOx layers was investigated in this paper. The passivation-layer thickness (0–60 nm) and measurement temperature (298–573 K) were intentionally controlled to study the temperature dependent performance of a-IGZO TFTs with sputtered AlOx passivation-layers. Generally, there was a negative shift in threshold voltage under higher temperatures, which was due to thermally excited carriers through intrinsic excitation and oxygen vacancy formation. A qualitative model was proposed to effectively ascertain the aforementioned two physical mechanisms. With passivation-layer thickness decreasing oxygen vacancy formation became more and more evident while intrinsic excitation could apparently worsen the characteristics of a-IGZO TFTs under the temperature higher than 473 K. In addition, the ‘‘passivation-layer thickness effect’’ for thermal stability of a-IGZO TFTs was theoretically explained by the variation of defect formation energy with the device passivation-layer thickness. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Transparent amorphous oxide thin film transistors (TFTs) have been attracting considerable attention for applications in mobile displays, televisions, and other consumer electronics. In particular, amorphous In–Ga–Zn–O (a-IGZO) TFTs have been regarded as one of the most promising candidates for the addressing devices in next-generation flat-panel displays including active-matrix liquid crystal display (AMLCD) and active-matrix organic light-emitting diode (AMOLED) due to their outstanding features such as high field-effect mobility, low-temperature fabrication ability and good large-area uniformity [1–5]. However, some critical issues such as environmental instabilities of a-IGZO TFTs still remain to be solved [6–10]. Especially, the characteristic variations of a-IGZO TFTs, such as the threshold voltage (Vth) variation, often occur under temperature stresses and hence seriously limit the actual applications of this novel technology [6]. So far several reports about the temperature influence on transistor characteristics of a-IGZO TFTs have been published [7–9]. For the n-type oxide semiconductors, it is well known that oxygen vacancy (VO) plays an important role in the electrical performances of the thin films and the corresponding ⇑ Corresponding author at: 800 DongChuan Rd., Shanghai 200240, China. Tel.: +86 21 34207894. E-mail address: [email protected] (C. Dong). http://dx.doi.org/10.1016/j.sse.2014.10.012 0038-1101/Ó 2014 Elsevier Ltd. All rights reserved.
TFT devices. In fact, VOs in oxide semiconductors create additional states near the conduction band and hence generate carriers, as may induce a negative shift in the device threshold voltage [8]. Therefore, any factors influencing the concentration of VOs in oxide semiconductors might lead to instabilities of the oxide thin film transistors. Naturally the free carriers in a-IGZO mainly originate from point defects (oxygen vacancies) which closely relate the temperature-dependent characteristics of oxide thin film materials and devices [10–12]. Therefore it is important to investigate how the ambient temperature affects the concentration of point defects in a-IGZO as well as the performance of the corresponding TFT devices. On the other side, passivation layers are often used to improve the stability of oxide TFTs by isolating the ambience and the device back-channels [13–16]. However, how passivation layers exactly affect the thermal stability of a-IGZO TFTs has not yet been covered, although it is apparently the essential knowledge for the mass production of this novel technology. In this article, the temperature dependence of the electrical characteristics of a-IGZO TFTs with variously thick AlOx passivation-layers was investigated. A qualitative model was proposed to ascertain the two physical mechanisms relating thermal stability of a-IGZO TFTs with passivation layers. We believe the experimental and theoretical results in this paper could benefit application of a-IGZO TFTs, especially for choice of passivation-layers and working temperatures in mass production.
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2. Experiment The TFTs with bottom-gate staggered structure, as shown in Fig. 1, were fabricated on the substrates of n-type silicon wafers with 300-nm-thick thermal oxide layers (SiO2). Here, the n-type silicon and thermal oxide were served as the gate electrode and gate insulator, respectively. First, 40-nm-thick a-IGZO films were deposited as active-layers on the wafers by RF magnetron sputtering technique using a target of polycrystalline InGaZnO (In:Ga:Zn:O = 1:1:1:4, mol%) at room temperature (RT). The sputtering deposition was performed using a gas mixture of Ar/O2 (30 sccm/0 sccm) at a total pressure of 3 m Torr and the RF power density of 2.19 W/cm2. Then 100-nm-thick indium–tin oxide (ITO) source/drain (S/D) electrodes were prepared in pure Argon at RT by DC magnetron sputtering. AlOx passivation layer was then sputtered with ambience of pure Argon, working pressure of 2 m Torr, deposition temperature of RT and RF power of 100 W. Four samples (A, B, C and D) with passivation-layer thicknesses of 0 nm, 15 nm, 30 nm and 60 nm were prepared to study the influence of passivation-layer thickness on the thermal stability of a-IGZO TFTs. Finally, the devices were annealed at 400 °C in air for 1 h to improve the properties of thin films and interfaces in a-IGZO TFTs. Shadow masks were used to pattern the films including active-layer, S/D electrodes and passivation layer, resulting in channel length of 275 lm and channel width of 1000 lm for the a-IGZO TFTs. The transfer characteristics of the a-IGZO TFTs were measured with Keithley 4200 SCS parameter analyzer for the gate voltages Vgs ranging from 20 to 40 V with a fixed drain voltage Vds = 10 V. A heating stage was used to modulate the measurement temperature (298–573 K) in characterizing thermal stability of a-IGZO TFTs. 3. Results and discussion 3.1. Temperature dependent transfer characteristics Typical transfer characteristics of the four a-IGZO TFTs (samples A, B, C and D) at various temperatures are depicted in Fig. 2. First, one may notice that the four samples showed fairly good electrical performance at RT (298 K) in spite of their different thicknesses in passivation layers. With the same methods used in Ref. [16], some device parameters could be extracted from the corresponding curves presented in Fig. 2. For instance, the field effect mobility (lFE) approximated 10 cm2/V s, the on–off current ratio (Ion/Ioff) was greater than 107, and the subthreshold swing (SS) was less than 0.3 V/decade. However, with measurement temperature increasing the transfer characteristics of the four samples more or less degraded, especially for the device without passivation (sample A). As shown in Fig. 2(a), the Ids–Vgs curve exhibited evidently negative shifts when measurement temperature rose. Even worse, sample A failed to show transfer characteristics with measurement temperature being greater than 473 K. The a-IGZO TFTs with AlOx passivation-layers (samples B, C and D) exhibited much better thermal stabilities at low-temperature range (298–473 K). When the
Fig. 1. Schematic cross section of the a-IGZO TFT structure used in this study.
measurement temperature increased to high-temperature range (>473 K), however, the samples B, C and D also showed serious threshold voltage (Vth) shifts in the negative direction. Another interesting result in Fig. 2 is that the on-current (Ion) gradually degraded with measurement temperature increasing, which also seemed to somewhat relate the device passivation-layer thickness. Being contrary to the percolation theory which was believed to fit a-IGZO films [3], the related physical essence of this fact is still unclear. In order to quantitatively study thermal stability of a-IGZO TFTs, one useful term, DVth, was defined as,
DV th ¼ V th ðTÞ V th ð298 KÞ
ð1Þ
where threshold voltage (Vth) was extracted from the transfer curves as the corresponding gate voltage (Vgs) to Ids = 108 A. Fig. 3 shows the temperature dependence of DVth for the four a-IGZO TFTs with variously thick passivation-layers. DVth value increased with measurement temperature increasing, implying that a-IGZO TFTs became more unstable at higher ambience temperature. Additionally, when passivation thickness for a-IGZO TFTs increased, DVth value tended to decrease evidently. Sample A, the device without passivation, negatively shifted much more than the other three samples, which was due to the exposure of its back channel to the environments. Therefore, one can draw a natural but important conclusion that thicker passivation layer for a-IGZO TFTs could lead to more thermally stable a-IGZO TFTs. We should emphasize here that the thermal stability of a-IGZO TFTs with passivation layers was quite different from that of the unpassivated device. As shown in Fig. 3, the a-IGZO TFTs passivated by even very thin AlOx film (e.g. 15 nm) could significantly improve the device thermal stabilities in low-temperature range (298–473 K). With further increase in passivation-layer thickness, DVth at temperature less than 473 K gradually decreased and approached zero if 60-nmthick AlOx was employed. However, thermal stability of a-IGZO TFTs in high-temperature range (>473 K) seemed to be hardly dependent on device passivation-layer thickness (as shown in Fig. 3), implying another mechanism might dominate here. 3.2. Analysis and discussion According to the aforementioned experimental results one may notice that thermally induced Vth shift in a-IGZO TFTs is quite complicated since it concerns not only measurement temperature but also device passivation-layer thickness. From internal essence point of view, two physical mechanisms are involved here. Generally, if the valence electrons in the semiconductor get enough energy from the outside such as light, temperature, electromagnetic field excitation, etc., some of the valence electrons can be free of covalent bond and become approximately free electrons. And holes will be generated at the same time. This is the intrinsic excitation for semiconductor [17], naturally followed by a-IGZO materials. But, as one of the wide band-gap semiconductors, a-IGZO thin film shows weak intrinsic excitation at low temperatures, where the intrinsic carrier concentration can be ignored compared with the carrier concentration induced by impurity ionization. However, with the increase in ambient temperature, electrons produced by intrinsic excitation will increase quickly. So, intrinsic excitation should not be completely ignored in oxide semiconductors, especially at high temperatures. On the other side, Takechi et al. pointed out that oxygen vacancy (VO) formation played an important role in thermal stability of a-IGZO TFTs without passivation layers [18]. Unfortunately how exactly these two aforementioned physical mechanisms, i.e. intrinsic excitation and VO formation, worked in thermal stability of a-IGZO TFTs was still unclear. We believe this is quite essential knowledge for the actual applications of this novel technology. In order to ascertain the related physical
Z. Hu et al. / Solid-State Electronics 104 (2015) 39–43
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Fig. 2. Transfer characteristics at different temperatures for a-IGZO TFTs: (a) sample A (passivation-layer thickness: 0 nm), (b) sample B (passivation-layer thickness: 15 nm), (c) sample C (passivation-layer thickness: 30 nm) and (d) sample D (passivation-layer thickness: 60 nm).
Fig. 3. Temperature dependence of DVth for the four samples with variously thick passivation-layers (0, 15, 30 and 60 nm).
mechanisms, we proposed a qualitative model to describe the thermal stability of oxide thin film transistors with different passivation-layer thickness. The basic concept of our model was exhibited in Fig. 4(a), where we assumed that VO formation was dominated by both measurement temperature and passivationlayer thickness while intrinsic excitation was only related to measurement temperature. Since VO formation directly relate to the oxygen/water exchange between device back-channel and ambience, we believe passivation-layer thickness effectively influence this mechanism. As for the other physical mechanism, intrinsic excitation, we believe it is a simply thermal process, so only ambient temperature dominates here. Accordingly four regions might be divided for the dependence of DVth on measurement
temperature and passivation-layer thickness, as shown in Fig. 4(a). In region I (low temperature/thick passivation) it would bring very stable devices because neither of these physical mechanisms (VO formation and intrinsic excitation) existed here. On the contrary, region IV (high temperature/thin passivation) where both VO formation and intrinsic excitation efficiently worked could make a-IGZO TFTs become extremely unstable with the largest DVth. As for the other two regions II and III, however, either VO formation or intrinsic excitation could be apparently observed, so aIGZO TFTs would exhibit moderately stable properties for thermal stability measurements. In order to have a more intuitive vision to the shift of Vth and meanwhile confirm the validity of our model, the experimental dependence of DVth on both device passivation-layer thickness and measurement temperature was depicted, as shown in Fig. 4(b). Here, the different color areas represented various DVth values. The dark blue part in the bottom-right corner where DVth < 0.4 V on the conditions of 60-nm-thick passivation layer and 298–373 K testing temperature might correspond to the region I as shown in Fig. 4(a). On the contrary, the red part in the top-left corner where DVth > 18.0 V for the devices without passivation under temperature higher than 473 K showed the case when both VO formation and intrinsic excitation efficiently worked (region IV as shown Fig. 4(a)). In other areas, however, the colors indicated that DVth values were between those in the aforementioned two areas, implying only one physical mechanism dominated here (regions II and III as shown in Fig. 4(a)). More specifically, intrinsic dominated in region II while VO formation dominated in region III. All in all, the correspondence between the experimental data as shown in Fig. 4(b) and the qualitative model as shown in Fig. 4(a) could prove that our theoretical model was effective to describe the thermal stability of a-IGZO TFTs passivated by sputtered AlOx thin films. Moreover, we could understand the passivation-layer thickness effect on the thermal stability of a-IGZO TFTs more clearly based on
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Fig. 4. (a) A qualitative model for thermal stability of oxide thin film transistors. (b) The corresponding experimental results of Vth shifts for a-IGZO TFTs passivated by variously thick AlOx films under different measurement temperatures.
this model. By comparing region I with III or region II with IV in Fig. 4(a), a conclusion might be drawn like this: under the same measurement temperature, the thicker the passivation layer, the smaller the negatively shift of Vth, and the better the thermal stability of the a-IGZO TFT devices. Even very thin passivation layer of AlOx could well protect the device back-channel, and therefore efficiently improved the thermal stability of the a-IGZO TFTs. As for our devices in this paper, 60-nm-thick AlOx seemed strong enough to isolate the device back-channel and ambience, and therefore nearly eliminate VO formation in temperature range of 298–573 K. On the other side, intrinsic excitation was assumed to only relate to measurement temperature in our model. Since this physical mechanism is strongly dependent on the assistance of thermal energy, it is reasonable to deduce that intrinsic excitation may only take effect at high temperatures. However, the ‘‘threshold temperature’’ above which intrinsic excitation could work apparently is difficult to decide. As for the a-IGZO TFTs in this paper, we could roughly estimate their ‘‘threshold temperature’’ as 473 K because the four samples showed different performance above this temperature (as shown in Fig. 2). Contrary to intrinsic excitation, VO formation is a unique physical mechanism for oxide semiconductors. According to the aforementioned experimental data and theoretical analysis this mechanism relates to not only measurement temperature but also passivation-layer thickness. Therefore, investigating the exact physical essence of VO formation in thermal stability of a-IGZO TFTs should be meaningful. Here, a theoretical analysis was employed based on the model proposed by Takechi et al. [18], where temperaturedependence of threshold voltage shift can be expressed as:
lnðDV th Þ ¼
W cg ln 3K B T 2 e tIGZO C 1
ð2Þ
where W stands for the defect formation energy, kB the Boltzmann constant, Cg the capacitance of the gate insulator per unit area, e the electronic charge, tIGZO the thickness of the a-IGZO semiconductor layer, and C1 the constant related to the entropy for the formation of one vacancy and two free electrons. By using the experimental data shown in Fig. 3 and the least square method to fit these points, W and C1 can be extracted from the slope and the intercept of the straight line expressed using Eq. (2), as shown in Fig. 5. Here, defect formation energy W other than C1 was calculated since the later one was beyond the scope of this article. It is worth noting that the fitting points for Fig. 5 vary between 323 K and 473 K because extrinsic excitation would take effect when the measurement temperature exceeded 473 K. The experimental plots leaded to
Fig. 5. Experimental plots and the proximate straight lines for ln(DVth) e/kBT in the temperature range of 323–473 K for the four a-IGZO TFT samples with passivation-layer thickness of 0, 15, 30, and 60 nm respectively. Inset: the dependence of defect formation energy W on the passivation-layer thickness of aIGZO TFTs.
the proximate straight lines with W of 0.59, 0.74, 0.79 and 0.84 eV for sample A, B, C and D respectively. The defect formation energy values were of the same order as those for typical oxide semiconductor crystals ranging from approximately 0.5–2.0 eV [19–21]. Importantly, defect formation energy apparently increased with the rise of passivation layer thickness, as shown in the inset of Fig. 5. It is well known that electron concentration in a-IGZO thin films is closely related to oxygen vacancies determined by defect formation energy W. Generally speaking, the higher the W value, the smaller the variation of carrier concentration. Therefore, a conclusion might be made that thicker passivation for a-IGZO TFT could result in higher defect formation energy and thus lead to smaller variation of carrier concentration and Vth shift. If there is no passivation-layer, device back channel will directly expose to the ambience, resulting in the easily oxygen exchange between the active layer and the outside. Passivation-layer tends to prevent this kind of exchange happening and the isolation effect will be more obvious with the increase in passivation-layer thickness. In this case, defect formation energy W reasonably becomes bigger. Therefore, oxygen vacancy generation will become more difficult for the a-IGZO TFTs with thicker passivation layers.
Z. Hu et al. / Solid-State Electronics 104 (2015) 39–43
So far, passivation-layer thickness effect in thermal stability of a-IGZO TFTs was well analyzed by connecting it to VO formation mechanism. With passivation-layer thickness for a-IGZO TFTs decreasing, defect formation energy of oxygen vacancy dropped, resulting in larger variation of VO concentration with temperature increasing and hence more thermally unstable TFT devices. From the above, enough thick passivation-layers are inevitable for actual applications of a-IGZO TFTs. At high temperatures, however, intrinsic excitation tends to take effect. Fortunately, the a-IGZO TFTs seldom work under such extreme temperatures as active addressing devices for flat panel displays. 4. Summary The transfer characteristics of a-IGZO TFTs with variously thick AlOx passivation-layers were measured at temperatures ranging from 298 K to 573 K in this work. Generally, there was a negative shift in threshold voltage under higher temperatures, which was probably due to thermally excited carriers through intrinsic excitation and oxygen vacancy formation. A qualitative model was proposed to effectively ascertain the above two physical mechanisms. With passivation-layer thickness decreasing, VO formation became more evident for a-IGZO TFTs because the defect formation energy dropped. Meanwhile, intrinsic excitation for a-IGZO thin film could apparently worsen the performance of a-IGZO TFTs under the temperature higher than 473 K. The above experimental and theoretical results in the study were believed to benefit both actual applications and basic understanding of a-IGZO TFTs. Acknowledgments This work was supported by National 973 project of China (Grant No. 2013CB328803) and National Natural Science Foundation of China (Grant Nos. 61136004 and 61474075). References [1] Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H. Roomtemperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004;432:488. [2] Kamiya T, Nomura K, Hosono H. Present status of amorphous In–Ga–Zn–O thin film transistors. Sci Technol Adv Mater 2010;11:044305.
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[3] Takagi K, Nomura K, Ohta H, Yanagi H, Kamiya T, Horano M, et al. Carrier transport and electronic structure in amorphous oxide semiconductor aInGaZnO4. Thin Solid Films 2005;486:38–41. [4] Kumomi H, Nomura K, Kamiya K, Hosono H. Amorphous oxide channel TFTs. Thin Solid Film 2008;516:1516–22. [5] Yabuta H, Sano M, Abe K, Aiba T, Den T, Kumomi H, et al. High-mobility thinfilm transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering. Appl Phys Lett 2006;89:112123. [6] Chen C, Abe K, Kumomi H, Kanicki J. Current temperature stress study of RF sputter a-InGaZnO TFTs. SID DIGEST 2008:104–7. [7] Chowdhury MDH, Migliorato P, Jang J. Time-temperature dependence of positive gate bias stress and recovery in amorphous indium–gallium-zincoxide thin-film-transistors. Appl Phys Lett 2011;98:153511. [8] Godo H, Kawae D, Yoshitomi S. Temperature dependence of transistor characteristics and electronic structure for amorphous In–Ga–Zn-Oxide thin film transistor. Jpn J Appl Phys 2010;49:03CB04. [9] Lee JM, Cho IT, Lee JH. Bias-stress-induced stretched-exponential time dependence of threshold voltage shift in InGaZnO thin film transistors. Appl Phys Lett 2008;93:093504. [10] Yao JK, Xu NS, Deng SZ, Chen J, She JC, Shieh HPD, et al. Electrical and photosensitive characteristics of a-IGZO TFTs related to oxygen vacancy. IEEE Trans Elec Dev 2011;58:1121. [11] Kim S, Jeon YW, Kim Y, Kong D, Jung HK, Bae MK, et al. Impact of oxygen flow rate on the instability under positive bias stresses in DC sputtered amorphous InGaZnO thin film transistors. IEEE Electron Dev Lett 2012;33:62. [12] Kang D, Lim H, Kim C, Song I, Park J, Park Y, et al. Amorphous gallium indium zinc oxide thin film transistors: sensitive to oxygen molecules. Appl Phys Lett 2007;90:192101. [13] Hong D, Wager JF. Passivation of zinc–tin-oxide thin film transistors. J Vac Sci Technol B 2005;23:L25. [14] Cho DH, Yang S, Shin J, Ryu MK, Cheong WS, Park SHK, et al. Post annealing and passivations of transparent bottom gate IGZO thin film transistors. Proc SID 2008;1243. [15] Park SHK, Ryu MK, Yoon SM, Yang S, Hwang CS, Jeon JH. Device reliability under electrical stress and photo response of oxide TFTs. J Soc Inf Display 2010;18:779. [16] Wu J, Chen YT, Zhou DX, Hu Z, Xie HT, Dong CY, et al. Sputtered oxides used for passivation layers of amorphous InGaZnO thin film transistors. Mater Sci Semicond Process 2014.. [17] Pierret Robert F. Semiconductor device fundamentals. Addison Wesley Longman; 1996. p. 65–67. [18] Takechi K, Nakata M, Eguchi T. Temperature-dependent transfer characteristics of amorphous InGaZnO4 thin-film transistors. Jpn J Appl Phys 2009;48:011301. [19] Kofstad P. Studies of electrical conductivity of Nb2O5 as a function of oxygen pressure at 600–1200 °C. J Phys Chem Solids 1962;23:1571. [20] Bonasewicz P, Hirschwald W, Neumann G. The investigation of the pressure and temperature dependence of the electrical conductivity of thin zinc oxide films with high resistances. Phys Status Solidi 1986;A97:593. [21] Gavryushin V, Raciukaitis G, Juodzbalis D, AKazlauskas, Kubertavicius V. Characterization of intrinsic and impurity deep levels in ZnSe and ZnO crystals by nonlinear spectroscopy. J Cryst Growth 1994;138:924.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Thermal stability of amorphous InGaZnO thin film transistors passivated by AlOx layers Zhe Hu, Daxiang Zhou, Ling Xu, Qi Wu, Haiting Xie, Chengyuan Dong ⇑ National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 12 July 2014 Received in revised form 12 October 2014 Accepted 28 October 2014
The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: Amorphous indium gallium zinc oxide (a-IGZO) Thin film transistor (TFT) Thermal stability Passivation-layer Intrinsic excitation
a b s t r a c t Thermal stability of amorphous InGaZnO thin film transistors (a-IGZO TFTs) passivated by AlOx layers was investigated in this paper. The passivation-layer thickness (0–60 nm) and measurement temperature (298–573 K) were intentionally controlled to study the temperature dependent performance of a-IGZO TFTs with sputtered AlOx passivation-layers. Generally, there was a negative shift in threshold voltage under higher temperatures, which was due to thermally excited carriers through intrinsic excitation and oxygen vacancy formation. A qualitative model was proposed to effectively ascertain the aforementioned two physical mechanisms. With passivation-layer thickness decreasing oxygen vacancy formation became more and more evident while intrinsic excitation could apparently worsen the characteristics of a-IGZO TFTs under the temperature higher than 473 K. In addition, the ‘‘passivation-layer thickness effect’’ for thermal stability of a-IGZO TFTs was theoretically explained by the variation of defect formation energy with the device passivation-layer thickness. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Transparent amorphous oxide thin film transistors (TFTs) have been attracting considerable attention for applications in mobile displays, televisions, and other consumer electronics. In particular, amorphous In–Ga–Zn–O (a-IGZO) TFTs have been regarded as one of the most promising candidates for the addressing devices in next-generation flat-panel displays including active-matrix liquid crystal display (AMLCD) and active-matrix organic light-emitting diode (AMOLED) due to their outstanding features such as high field-effect mobility, low-temperature fabrication ability and good large-area uniformity [1–5]. However, some critical issues such as environmental instabilities of a-IGZO TFTs still remain to be solved [6–10]. Especially, the characteristic variations of a-IGZO TFTs, such as the threshold voltage (Vth) variation, often occur under temperature stresses and hence seriously limit the actual applications of this novel technology [6]. So far several reports about the temperature influence on transistor characteristics of a-IGZO TFTs have been published [7–9]. For the n-type oxide semiconductors, it is well known that oxygen vacancy (VO) plays an important role in the electrical performances of the thin films and the corresponding ⇑ Corresponding author at: 800 DongChuan Rd., Shanghai 200240, China. Tel.: +86 21 34207894. E-mail address: [email protected] (C. Dong). http://dx.doi.org/10.1016/j.sse.2014.10.012 0038-1101/Ó 2014 Elsevier Ltd. All rights reserved.
TFT devices. In fact, VOs in oxide semiconductors create additional states near the conduction band and hence generate carriers, as may induce a negative shift in the device threshold voltage [8]. Therefore, any factors influencing the concentration of VOs in oxide semiconductors might lead to instabilities of the oxide thin film transistors. Naturally the free carriers in a-IGZO mainly originate from point defects (oxygen vacancies) which closely relate the temperature-dependent characteristics of oxide thin film materials and devices [10–12]. Therefore it is important to investigate how the ambient temperature affects the concentration of point defects in a-IGZO as well as the performance of the corresponding TFT devices. On the other side, passivation layers are often used to improve the stability of oxide TFTs by isolating the ambience and the device back-channels [13–16]. However, how passivation layers exactly affect the thermal stability of a-IGZO TFTs has not yet been covered, although it is apparently the essential knowledge for the mass production of this novel technology. In this article, the temperature dependence of the electrical characteristics of a-IGZO TFTs with variously thick AlOx passivation-layers was investigated. A qualitative model was proposed to ascertain the two physical mechanisms relating thermal stability of a-IGZO TFTs with passivation layers. We believe the experimental and theoretical results in this paper could benefit application of a-IGZO TFTs, especially for choice of passivation-layers and working temperatures in mass production.
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Z. Hu et al. / Solid-State Electronics 104 (2015) 39–43
2. Experiment The TFTs with bottom-gate staggered structure, as shown in Fig. 1, were fabricated on the substrates of n-type silicon wafers with 300-nm-thick thermal oxide layers (SiO2). Here, the n-type silicon and thermal oxide were served as the gate electrode and gate insulator, respectively. First, 40-nm-thick a-IGZO films were deposited as active-layers on the wafers by RF magnetron sputtering technique using a target of polycrystalline InGaZnO (In:Ga:Zn:O = 1:1:1:4, mol%) at room temperature (RT). The sputtering deposition was performed using a gas mixture of Ar/O2 (30 sccm/0 sccm) at a total pressure of 3 m Torr and the RF power density of 2.19 W/cm2. Then 100-nm-thick indium–tin oxide (ITO) source/drain (S/D) electrodes were prepared in pure Argon at RT by DC magnetron sputtering. AlOx passivation layer was then sputtered with ambience of pure Argon, working pressure of 2 m Torr, deposition temperature of RT and RF power of 100 W. Four samples (A, B, C and D) with passivation-layer thicknesses of 0 nm, 15 nm, 30 nm and 60 nm were prepared to study the influence of passivation-layer thickness on the thermal stability of a-IGZO TFTs. Finally, the devices were annealed at 400 °C in air for 1 h to improve the properties of thin films and interfaces in a-IGZO TFTs. Shadow masks were used to pattern the films including active-layer, S/D electrodes and passivation layer, resulting in channel length of 275 lm and channel width of 1000 lm for the a-IGZO TFTs. The transfer characteristics of the a-IGZO TFTs were measured with Keithley 4200 SCS parameter analyzer for the gate voltages Vgs ranging from 20 to 40 V with a fixed drain voltage Vds = 10 V. A heating stage was used to modulate the measurement temperature (298–573 K) in characterizing thermal stability of a-IGZO TFTs. 3. Results and discussion 3.1. Temperature dependent transfer characteristics Typical transfer characteristics of the four a-IGZO TFTs (samples A, B, C and D) at various temperatures are depicted in Fig. 2. First, one may notice that the four samples showed fairly good electrical performance at RT (298 K) in spite of their different thicknesses in passivation layers. With the same methods used in Ref. [16], some device parameters could be extracted from the corresponding curves presented in Fig. 2. For instance, the field effect mobility (lFE) approximated 10 cm2/V s, the on–off current ratio (Ion/Ioff) was greater than 107, and the subthreshold swing (SS) was less than 0.3 V/decade. However, with measurement temperature increasing the transfer characteristics of the four samples more or less degraded, especially for the device without passivation (sample A). As shown in Fig. 2(a), the Ids–Vgs curve exhibited evidently negative shifts when measurement temperature rose. Even worse, sample A failed to show transfer characteristics with measurement temperature being greater than 473 K. The a-IGZO TFTs with AlOx passivation-layers (samples B, C and D) exhibited much better thermal stabilities at low-temperature range (298–473 K). When the
Fig. 1. Schematic cross section of the a-IGZO TFT structure used in this study.
measurement temperature increased to high-temperature range (>473 K), however, the samples B, C and D also showed serious threshold voltage (Vth) shifts in the negative direction. Another interesting result in Fig. 2 is that the on-current (Ion) gradually degraded with measurement temperature increasing, which also seemed to somewhat relate the device passivation-layer thickness. Being contrary to the percolation theory which was believed to fit a-IGZO films [3], the related physical essence of this fact is still unclear. In order to quantitatively study thermal stability of a-IGZO TFTs, one useful term, DVth, was defined as,
DV th ¼ V th ðTÞ V th ð298 KÞ
ð1Þ
where threshold voltage (Vth) was extracted from the transfer curves as the corresponding gate voltage (Vgs) to Ids = 108 A. Fig. 3 shows the temperature dependence of DVth for the four a-IGZO TFTs with variously thick passivation-layers. DVth value increased with measurement temperature increasing, implying that a-IGZO TFTs became more unstable at higher ambience temperature. Additionally, when passivation thickness for a-IGZO TFTs increased, DVth value tended to decrease evidently. Sample A, the device without passivation, negatively shifted much more than the other three samples, which was due to the exposure of its back channel to the environments. Therefore, one can draw a natural but important conclusion that thicker passivation layer for a-IGZO TFTs could lead to more thermally stable a-IGZO TFTs. We should emphasize here that the thermal stability of a-IGZO TFTs with passivation layers was quite different from that of the unpassivated device. As shown in Fig. 3, the a-IGZO TFTs passivated by even very thin AlOx film (e.g. 15 nm) could significantly improve the device thermal stabilities in low-temperature range (298–473 K). With further increase in passivation-layer thickness, DVth at temperature less than 473 K gradually decreased and approached zero if 60-nmthick AlOx was employed. However, thermal stability of a-IGZO TFTs in high-temperature range (>473 K) seemed to be hardly dependent on device passivation-layer thickness (as shown in Fig. 3), implying another mechanism might dominate here. 3.2. Analysis and discussion According to the aforementioned experimental results one may notice that thermally induced Vth shift in a-IGZO TFTs is quite complicated since it concerns not only measurement temperature but also device passivation-layer thickness. From internal essence point of view, two physical mechanisms are involved here. Generally, if the valence electrons in the semiconductor get enough energy from the outside such as light, temperature, electromagnetic field excitation, etc., some of the valence electrons can be free of covalent bond and become approximately free electrons. And holes will be generated at the same time. This is the intrinsic excitation for semiconductor [17], naturally followed by a-IGZO materials. But, as one of the wide band-gap semiconductors, a-IGZO thin film shows weak intrinsic excitation at low temperatures, where the intrinsic carrier concentration can be ignored compared with the carrier concentration induced by impurity ionization. However, with the increase in ambient temperature, electrons produced by intrinsic excitation will increase quickly. So, intrinsic excitation should not be completely ignored in oxide semiconductors, especially at high temperatures. On the other side, Takechi et al. pointed out that oxygen vacancy (VO) formation played an important role in thermal stability of a-IGZO TFTs without passivation layers [18]. Unfortunately how exactly these two aforementioned physical mechanisms, i.e. intrinsic excitation and VO formation, worked in thermal stability of a-IGZO TFTs was still unclear. We believe this is quite essential knowledge for the actual applications of this novel technology. In order to ascertain the related physical
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Fig. 2. Transfer characteristics at different temperatures for a-IGZO TFTs: (a) sample A (passivation-layer thickness: 0 nm), (b) sample B (passivation-layer thickness: 15 nm), (c) sample C (passivation-layer thickness: 30 nm) and (d) sample D (passivation-layer thickness: 60 nm).
Fig. 3. Temperature dependence of DVth for the four samples with variously thick passivation-layers (0, 15, 30 and 60 nm).
mechanisms, we proposed a qualitative model to describe the thermal stability of oxide thin film transistors with different passivation-layer thickness. The basic concept of our model was exhibited in Fig. 4(a), where we assumed that VO formation was dominated by both measurement temperature and passivationlayer thickness while intrinsic excitation was only related to measurement temperature. Since VO formation directly relate to the oxygen/water exchange between device back-channel and ambience, we believe passivation-layer thickness effectively influence this mechanism. As for the other physical mechanism, intrinsic excitation, we believe it is a simply thermal process, so only ambient temperature dominates here. Accordingly four regions might be divided for the dependence of DVth on measurement
temperature and passivation-layer thickness, as shown in Fig. 4(a). In region I (low temperature/thick passivation) it would bring very stable devices because neither of these physical mechanisms (VO formation and intrinsic excitation) existed here. On the contrary, region IV (high temperature/thin passivation) where both VO formation and intrinsic excitation efficiently worked could make a-IGZO TFTs become extremely unstable with the largest DVth. As for the other two regions II and III, however, either VO formation or intrinsic excitation could be apparently observed, so aIGZO TFTs would exhibit moderately stable properties for thermal stability measurements. In order to have a more intuitive vision to the shift of Vth and meanwhile confirm the validity of our model, the experimental dependence of DVth on both device passivation-layer thickness and measurement temperature was depicted, as shown in Fig. 4(b). Here, the different color areas represented various DVth values. The dark blue part in the bottom-right corner where DVth < 0.4 V on the conditions of 60-nm-thick passivation layer and 298–373 K testing temperature might correspond to the region I as shown in Fig. 4(a). On the contrary, the red part in the top-left corner where DVth > 18.0 V for the devices without passivation under temperature higher than 473 K showed the case when both VO formation and intrinsic excitation efficiently worked (region IV as shown Fig. 4(a)). In other areas, however, the colors indicated that DVth values were between those in the aforementioned two areas, implying only one physical mechanism dominated here (regions II and III as shown in Fig. 4(a)). More specifically, intrinsic dominated in region II while VO formation dominated in region III. All in all, the correspondence between the experimental data as shown in Fig. 4(b) and the qualitative model as shown in Fig. 4(a) could prove that our theoretical model was effective to describe the thermal stability of a-IGZO TFTs passivated by sputtered AlOx thin films. Moreover, we could understand the passivation-layer thickness effect on the thermal stability of a-IGZO TFTs more clearly based on
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Fig. 4. (a) A qualitative model for thermal stability of oxide thin film transistors. (b) The corresponding experimental results of Vth shifts for a-IGZO TFTs passivated by variously thick AlOx films under different measurement temperatures.
this model. By comparing region I with III or region II with IV in Fig. 4(a), a conclusion might be drawn like this: under the same measurement temperature, the thicker the passivation layer, the smaller the negatively shift of Vth, and the better the thermal stability of the a-IGZO TFT devices. Even very thin passivation layer of AlOx could well protect the device back-channel, and therefore efficiently improved the thermal stability of the a-IGZO TFTs. As for our devices in this paper, 60-nm-thick AlOx seemed strong enough to isolate the device back-channel and ambience, and therefore nearly eliminate VO formation in temperature range of 298–573 K. On the other side, intrinsic excitation was assumed to only relate to measurement temperature in our model. Since this physical mechanism is strongly dependent on the assistance of thermal energy, it is reasonable to deduce that intrinsic excitation may only take effect at high temperatures. However, the ‘‘threshold temperature’’ above which intrinsic excitation could work apparently is difficult to decide. As for the a-IGZO TFTs in this paper, we could roughly estimate their ‘‘threshold temperature’’ as 473 K because the four samples showed different performance above this temperature (as shown in Fig. 2). Contrary to intrinsic excitation, VO formation is a unique physical mechanism for oxide semiconductors. According to the aforementioned experimental data and theoretical analysis this mechanism relates to not only measurement temperature but also passivation-layer thickness. Therefore, investigating the exact physical essence of VO formation in thermal stability of a-IGZO TFTs should be meaningful. Here, a theoretical analysis was employed based on the model proposed by Takechi et al. [18], where temperaturedependence of threshold voltage shift can be expressed as:
lnðDV th Þ ¼
W cg ln 3K B T 2 e tIGZO C 1
ð2Þ
where W stands for the defect formation energy, kB the Boltzmann constant, Cg the capacitance of the gate insulator per unit area, e the electronic charge, tIGZO the thickness of the a-IGZO semiconductor layer, and C1 the constant related to the entropy for the formation of one vacancy and two free electrons. By using the experimental data shown in Fig. 3 and the least square method to fit these points, W and C1 can be extracted from the slope and the intercept of the straight line expressed using Eq. (2), as shown in Fig. 5. Here, defect formation energy W other than C1 was calculated since the later one was beyond the scope of this article. It is worth noting that the fitting points for Fig. 5 vary between 323 K and 473 K because extrinsic excitation would take effect when the measurement temperature exceeded 473 K. The experimental plots leaded to
Fig. 5. Experimental plots and the proximate straight lines for ln(DVth) e/kBT in the temperature range of 323–473 K for the four a-IGZO TFT samples with passivation-layer thickness of 0, 15, 30, and 60 nm respectively. Inset: the dependence of defect formation energy W on the passivation-layer thickness of aIGZO TFTs.
the proximate straight lines with W of 0.59, 0.74, 0.79 and 0.84 eV for sample A, B, C and D respectively. The defect formation energy values were of the same order as those for typical oxide semiconductor crystals ranging from approximately 0.5–2.0 eV [19–21]. Importantly, defect formation energy apparently increased with the rise of passivation layer thickness, as shown in the inset of Fig. 5. It is well known that electron concentration in a-IGZO thin films is closely related to oxygen vacancies determined by defect formation energy W. Generally speaking, the higher the W value, the smaller the variation of carrier concentration. Therefore, a conclusion might be made that thicker passivation for a-IGZO TFT could result in higher defect formation energy and thus lead to smaller variation of carrier concentration and Vth shift. If there is no passivation-layer, device back channel will directly expose to the ambience, resulting in the easily oxygen exchange between the active layer and the outside. Passivation-layer tends to prevent this kind of exchange happening and the isolation effect will be more obvious with the increase in passivation-layer thickness. In this case, defect formation energy W reasonably becomes bigger. Therefore, oxygen vacancy generation will become more difficult for the a-IGZO TFTs with thicker passivation layers.
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So far, passivation-layer thickness effect in thermal stability of a-IGZO TFTs was well analyzed by connecting it to VO formation mechanism. With passivation-layer thickness for a-IGZO TFTs decreasing, defect formation energy of oxygen vacancy dropped, resulting in larger variation of VO concentration with temperature increasing and hence more thermally unstable TFT devices. From the above, enough thick passivation-layers are inevitable for actual applications of a-IGZO TFTs. At high temperatures, however, intrinsic excitation tends to take effect. Fortunately, the a-IGZO TFTs seldom work under such extreme temperatures as active addressing devices for flat panel displays. 4. Summary The transfer characteristics of a-IGZO TFTs with variously thick AlOx passivation-layers were measured at temperatures ranging from 298 K to 573 K in this work. Generally, there was a negative shift in threshold voltage under higher temperatures, which was probably due to thermally excited carriers through intrinsic excitation and oxygen vacancy formation. A qualitative model was proposed to effectively ascertain the above two physical mechanisms. With passivation-layer thickness decreasing, VO formation became more evident for a-IGZO TFTs because the defect formation energy dropped. Meanwhile, intrinsic excitation for a-IGZO thin film could apparently worsen the performance of a-IGZO TFTs under the temperature higher than 473 K. The above experimental and theoretical results in the study were believed to benefit both actual applications and basic understanding of a-IGZO TFTs. Acknowledgments This work was supported by National 973 project of China (Grant No. 2013CB328803) and National Natural Science Foundation of China (Grant Nos. 61136004 and 61474075). References [1] Nomura K, Ohta H, Takagi A, Kamiya T, Hirano M, Hosono H. Roomtemperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004;432:488. [2] Kamiya T, Nomura K, Hosono H. Present status of amorphous In–Ga–Zn–O thin film transistors. Sci Technol Adv Mater 2010;11:044305.
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[3] Takagi K, Nomura K, Ohta H, Yanagi H, Kamiya T, Horano M, et al. Carrier transport and electronic structure in amorphous oxide semiconductor aInGaZnO4. Thin Solid Films 2005;486:38–41. [4] Kumomi H, Nomura K, Kamiya K, Hosono H. Amorphous oxide channel TFTs. Thin Solid Film 2008;516:1516–22. [5] Yabuta H, Sano M, Abe K, Aiba T, Den T, Kumomi H, et al. High-mobility thinfilm transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering. Appl Phys Lett 2006;89:112123. [6] Chen C, Abe K, Kumomi H, Kanicki J. Current temperature stress study of RF sputter a-InGaZnO TFTs. SID DIGEST 2008:104–7. [7] Chowdhury MDH, Migliorato P, Jang J. Time-temperature dependence of positive gate bias stress and recovery in amorphous indium–gallium-zincoxide thin-film-transistors. Appl Phys Lett 2011;98:153511. [8] Godo H, Kawae D, Yoshitomi S. Temperature dependence of transistor characteristics and electronic structure for amorphous In–Ga–Zn-Oxide thin film transistor. Jpn J Appl Phys 2010;49:03CB04. [9] Lee JM, Cho IT, Lee JH. Bias-stress-induced stretched-exponential time dependence of threshold voltage shift in InGaZnO thin film transistors. Appl Phys Lett 2008;93:093504. [10] Yao JK, Xu NS, Deng SZ, Chen J, She JC, Shieh HPD, et al. Electrical and photosensitive characteristics of a-IGZO TFTs related to oxygen vacancy. IEEE Trans Elec Dev 2011;58:1121. [11] Kim S, Jeon YW, Kim Y, Kong D, Jung HK, Bae MK, et al. Impact of oxygen flow rate on the instability under positive bias stresses in DC sputtered amorphous InGaZnO thin film transistors. IEEE Electron Dev Lett 2012;33:62. [12] Kang D, Lim H, Kim C, Song I, Park J, Park Y, et al. Amorphous gallium indium zinc oxide thin film transistors: sensitive to oxygen molecules. Appl Phys Lett 2007;90:192101. [13] Hong D, Wager JF. Passivation of zinc–tin-oxide thin film transistors. J Vac Sci Technol B 2005;23:L25. [14] Cho DH, Yang S, Shin J, Ryu MK, Cheong WS, Park SHK, et al. Post annealing and passivations of transparent bottom gate IGZO thin film transistors. Proc SID 2008;1243. [15] Park SHK, Ryu MK, Yoon SM, Yang S, Hwang CS, Jeon JH. Device reliability under electrical stress and photo response of oxide TFTs. J Soc Inf Display 2010;18:779. [16] Wu J, Chen YT, Zhou DX, Hu Z, Xie HT, Dong CY, et al. Sputtered oxides used for passivation layers of amorphous InGaZnO thin film transistors. Mater Sci Semicond Process 2014.