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Effect of SiO2 and SiO2/SiNx Passivation on the Stability of Amorphous Indium-Gallium Zinc-Oxide Thin-Film Transistors Under High Humidity Md Delwar Hossain Chowdhury, Student Member, IEEE, Mallory Mativenga, Member, IEEE, Jae Gwang Um, Ravi K. Mruthyunjaya, Gregory N. Heiler, Timothy John Tredwell, Senior Member, IEEE and Jin Jang, Member, IEEE Abstract— We studied the environmental stability of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with single-layer (SiO2 ) and bilayer (SiO2 /SiN x ) passivation under high-humidity (80%) storage. During the 30 days of investigation, all single-layer passivated TFTs showed negative turn-ON voltage shifts (VON ), the size of which increased with storing time. The negative VON is attributed to donor generation inside the active a-IGZO caused by the diffusion of ambient hydrogen/water molecules passing through the SiO2 passivation layer. The X-ray photoelectron spectroscopy depth profile for the SiO2 passivated structures confirms that the concentration of oxygen vacancies, which is initially larger at the a-IGZO/SiO2 interface, compared with the bulk a-IGZO, decreases after 30 days of storage under high humidity. This can be explained as the passivation of oxygen vacancies by diffused hydrogen. On the other hand, all bilayer passivated TFTs showed good air stability at room temperature and high humidity (80%). Index Terms— Amorphous-indium-gallium zinc oxide (a-IGZO), oxide thin-film transistors (TFT) reliability, SiO2 and SiN x passivation layer, TFT.
I. I NTRODUCTION HE trend in display technology has always been toward large area, optical transparency, and flexibility. As a substitute for hydrogenated amorphous-silicon (a-Si:H) thin-film transistors (TFTs), amorphous indium-gallium zinc-oxide (a-IGZO) TFTs seem to be promising candidates, as they meet most of the requirements needed to achieve these goals. a-IGZO TFTs exhibit 10 times higher field-effect mobility (μEFF ) compared with a-Si:H TFTs [1], [2], even though a similar manufacturing process is used and both materials are amorphous. Recently, many results for the
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Manuscript received May 19, 2014; accepted January 10, 2015. Date of publication February 2, 2015; date of current version February 20, 2015. This work was supported in part by the Industrial Strategic Technology Development Program (10045269, Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV) funded by MOTIE/KEIT. The review of this paper was arranged by Editor J. Huang. M. D. H. Chowdhury, M. Mativenga, J. G. Um, and J. Jang are with the Advanced Display Research Center and Department of Information Display, Kyung Hee University, Seoul 130-701, Korea (e-mail:
[email protected]). R. K. Mruthyunjaya, G. N. Heiler, and T. J. Tredwell are with Carestream Health Inc., Rochester, NY 14615 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2392763
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electrical stability of a-IGZO TFTs have been published. Most focused on various gate insulator/passivation materials [2]–[7], hydrogen and nitrogen treatments [8], [9] of the a-IGZO back surface, and also time and/or temperaturedependent postfabrication anneals in vacuum, hydrogen, nitrogen, and ambient oxygen [7], [10]–[13]. Amorphous-oxide semiconductors (AOSs) show high sensitivity to hydrogen and oxygen, where the former is commonly considered to be a donor and the latter is an oxygen-vacancy compensator [14]–[16]. These two species are, therefore, used to control the carrier concentration and turn-ON voltage (VON ) of AOS TFTs. To overcome the diffusion of ambient hydrogen/oxygen into the active AOS of a TFT, a good passivation layer is required to protect the devices from external contamination, thus extending the lifetime of the device. Many materials have been used to passivate AOS TFTs [3], [7], and only short-term light and bias stabilities were investigated. For instance, it is a common practice to use SiO2 rather than SiNx as a gate insulator or passivation material for a-IGZO TFTs because SiO2 forms less interface trap densities with a-IGZO compared with SiNx [5], [17]. The TFTs with SiO2 gate insulators and/or passivation have thus been reported to show good stability under positive bias stress (PBS) [18] or negative bias stress [19]. However, the investigation of long-term environmental stability of the SiO2 passivated TFTs under harsh conditions, such as high humidity, are required to understand the lifetime of the devices. In the present work, we investigated the influence of single-layer (SiO2 ) and bilayer (SiO2 /SiNx ) protection layers on the stability of a-IGZO TFTs under high humidity (80%) and room temperature (RT) conditions. It was found that the TFT with the SiO2 /SiNx bilayer is very stable, but the TFT with the SiO2 passivation degrades significantly during 30 days of storage. The degradation is related to the diffusion of hydrogen (H) into the a-IGZO layer and the consequential generation of oxygen vacancies, which act as donors. II. E XPERIMENT Bottom-gate, inverted-staggered TFTs with an etch stopper were fabricated on glass substrates. Two sets of samples were prepared. The first set of samples, Sample-A, had a
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Fig. 2. Evaluation of environment stability of a-IGZO TFTs, stored at 25 °C and 80% humidity conditions in dark for one month. Transfer characteristics were measured at RT for TFTs with channel length (L) = 50 μm and channel width (W ) = 20 μm, from (a) Sample-A and (b) Sample-B.
Fig. 1. Cross-sectional view of a-IGZO TFT with (a) single-layer SiO2 as the passivation layer (Sample-A) and (b) bilayer SiN x /SiO2 as the passivation layer (Sample-B).
200-nm-thick SiO2 passivation layer. The second set, Sample-B, had a double passivation layer consisting of SiO2 and SiNx . The thickness of the SiO2 in SampleB was 150 nm and that of the SiNx was 100 nm. The schematic cross sections of Sample-A and Sample-B are shown in Fig. 1(a) and (b), respectively. For both samples, a 200-nm-thick SiO2 layer is used as the gate insulator. The fabrication process begins with deposition by the sputtering of a 150-nm-thick Mo layer that is patterned by wet etch to form the gate electrode. This is followed by the deposition of the gate dielectrics by plasma-enhanced chemical vapor deposition (PECVD) at 380 °C. The active layer is a 20-nm-thick a-IGZO layer, which is deposited on top of the gate insulator, by sputtering, at 200 °C, using a polycrystalline IGZO target (InO3:Ga2 O3 :ZnO = 1:1:1 mol %) in an Ar and O2 gas environment (with an Ar:O2 ratio of 4:8). After the deposition of the active layer, a 100-nm-thick PECVD SiO2 layer is deposited and patterned to form an etch stopper, which is used to protect the a-IGZO during the formation of the source/drain electrodes. The gate insulator, active layer, and etch stopper are deposited without breaking the vacuum in a cluster-deposition tool. This ensures clean interfaces at the back and front channel, as the a-IGZO is protected immediately after deposition without exposure to air. Note that breaking the vacuum before depositing a protective layer on the a-IGZO may lead to significant environmental contamination because the a-IGZO is exposed to air when the sample is moved from one place to another. After the formation of an active island, 150-nm-thick Mo is deposited by sputtering at 200 °C and patterned by a wet-etch process, which uses an HNO3 -based Mo etchant to form the source/drain electrodes. The fabrication process is completed with the deposition of the passivation layers by PECVD at 200 °C. The X-ray photoelectron spectroscopy (XPS) depth-profile spectra of a stack of thin films exactly matching the deposition conditions of the TFT stack in Sample-A (glass/SiO2 (200 nm)/a-IGZO(80 nm)/SiO2 (300 nm)) were measured using a PHI 5000 Versa Probe with a base pressure of 6.7 × 10−8 Pa and monochromated Al Kα radiation
(1486.6 eV). The TFT’s VON is taken when the gate voltage (VGS ), at which the drain current (IDS ) starts, monotonically increases. The field-effect mobility (μFE ) is derived from transconductance (g M ), ∂ IDS /∂ VGS . The subthreshold swing (SS) is taken as (d log(IDS )/d VGS )−1 of the range 10 pA ≤ IDS ≤ 100 pA, with VDS = 0.1 V. The capacitance versus gate voltage was measured using Agilent E4980A Precision LCR Meter with the TFT of width (W ) = 2000 μm and length (L) = 20 μm. III. R ESULTS AND D ISCUSSION Fig. 2(a) and (b) shows, respectively, the transfer characteristics of Sample-A and Sample-B measured over a period of 30 days. The TFTs were stored at RT and in a high-humidity (80%) environment for the 30 days. The transfer characteristics of Sample-A show negative VON shift (VON ), the magnitude of which increases with increasing storage time [Fig. 2(a)]. Such degradation could not be seen for the TFT of Sample-B [Fig. 2(b)]. Regardless of size, all TFTs from Sample-B showed good stability over time [Fig. 3(a)–(c)]. It is clear that the SiO2 passivation layer alone cannot protect a-IGZO TFTs from ambient gas diffusion at 80% humidity conditions. It is important, however, to note that the TFT characteristics of Sample-A recover completely to the initial state after annealing at 250 °C in vacuum (Fig. 4). To confirm hydrogen/water diffusion in and out of the a-IGZO thin film via the SiO2 protection layer, we analyzed the XPS depth profiles of thin-film stacks (glass/SiO2 /a-IGZO/SiO2 ) stored for 0, 7, and 30 days under 80% humidity and at 25 °C. Fig. 5(a) and (b) shows the oxygen (O) 1s spectra at the middle of the SiO2 /a-IGZO interface, which can be considered the interface between the etch stopper and the a-IGZO back channel in real TFTs, and 40-nm away from interface, which can be considered the bulk of the a-IGZO active layer in real TFTs. At the interface region [Fig. 5(a)], we observed two broad peaks of O 1s at ∼532.3 and ∼530.05 eV. The energy peak at 532.3 eV is related to the oxygen vacancy and the metal-OH (hydroxyl) sites [20], [21]. From depth profiling, it was observed that the oxygen vacancy and the −OH concentration is higher at the interface compared with the bulk a-IGZO. This may be due to the effect of the postfabrication anneal, which increases
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Fig. 3. Evaluation of environment stability of a-IGZO TFTs, stored at 25 °C and 80% humidity in the dark for one month. Transfer characteristics were measured at RT for varying channel widths (W ) and a fixed channel length (L) = 20 μm. (a)–(c) Transfer characteristics for Sample-B. TABLE I PARAMETERS E XTRACTED F ROM THE C OMBINED A NALYSIS OF TFT C URRENT–V OLTAGE AND C APACITANCE C HARACTERISTICS B EFORE (0 D AY ) AND A FTER 7, 15, AND 30 D AYS S TORING THE S INGLE L AYER PASSIVATED TFT AT 25 °C AND 80% H UMIDITY
Fig. 4. Transfer characteristics of a TFT from Sample-A at day 0, day 30, and after annealing at 250 °C for 2 h in vacuum.
the density of hydrogen at the interface compared with that in the bulk region as reported before [22]. In Fig. 5(a) and (b), we see a significant difference in the O 1s spectra at day 0 and that of day 30, which confirms significant changes in the chemical composition of the a-IGZO thin film as a result of the exposure to ambient hydrogen/water molecules. At the interface [Fig. 5(a)] and the bulk a-IGZO [Fig. 5(b)], the total O 1s intensities, respectively, decreased by ∼29% and 14% after 30 days in the high-humidity test. The decrease in the intensity can be explained by the passivation of oxygen vacancy sites with ambient hydrogen/water molecules, consistent with the reported in [23]. The existence of hydrogen is usually confirmed by the oxidized (−OH) subband at the binding energies of ∼532.3 eV in the O 1s spectrum [24], but our experimental evidence indicates the reduction in intensity of both the oxygen vacancy (531.55 eV) and the −OH (532.3 eV), after the incorporation of hydrogen/water molecules. In Fig. 5(c) and (d), we checked the O1s spectrum of the a-IGZO at the interface and in the bulk for bilayer (SiNx /SiO2 ) passivated thin films. The results show no significant changes over a period of 30 days. In a-IGZO TFTs, the negative VON is considered the generation of donor states and/or hole traps, the effect of which is commonly observed after the application of negative bias illumination stress. Many observations suggested that neutral oxygen vacancies (Vo ) and ionized oxygen vacancies states (Vo+ /Vo2+ ) are the origin of such degradations [3], [16], [19], [25]. Recently, a high
concentration of hydrogen was observed in a-IGZO thin films, although there was no intentional exposure of the active films to hydrogen during their deposition [22]. Hydrogen atoms are known to act as donors in a-IGZO [14], [26]. Hydrogen incorporation into the AOS films can fill oxygen vacancy sites [26] and form a stable +1 charge state as a donor. In this investigation, the negative VON undergone by the TFTs with the SiO2 passivation [Fig. 4(a) and (b)] under high humidity can be explained by the change in the chemical composition of the a-IGZO active film. Fig. 6(a) and (b) shows schematic to describe the distribution of metal (In/Ga/Zn) atoms, oxygen (O) atoms, and oxygen vacancies, before and after exposure to hydrogen, respectively. At the flat-band, the energy level diagram for the distributions in Fig. 6(a) and (b), is shown in Fig. 6(c). At RT and under high humidity, the diffused hydrogen atoms can fill oxygen vacancy sites, thereby detrapping free carriers, which are originally occupied by the vacancy sites [23]. This is consistent with the decrease in the oxygen vacancy concentration shown by the XPS results in Fig. 5. When hydrogen fills the vacancy site, it forms a positively charged center, which raises the Fermi level (E F ), as shown in Fig. 6(c), hence, the negative VON . However, the positive charges induced by the exposure to high humidity are stable until high-temperature (>200 °C) annealing is performed [23]. Note that, a 2 h anneal at 250 °C in vacuum resulted in the return of the TFT characteristics to the original state [Fig. 4(c)]. This recovery process can be explained by the out-diffusion of hydrogen/water particles at high temperature and/or the formation
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Fig. 5. Comparison of O 1s spectrum for 0 to 30 days stored thin films under 80% humidity at 25 °C. (a) and (b) Interface (PASS-SiO2/a-IGZO) and IGZO bulk (middle of a-IGZO film) for Sample-A, respectively. (c) and (d) Interface (PASS-SiNx + SiO2 /a-IGZO) and IGZO bulk (middle of a-IGZO film) for Sample-B, respectively.
Fig. 6. Schematic (a) before and (b) after hydrogen diffusion in a-IGZO. (c) Energy level diagram for SiO2 passivated TFTs before and after exposure to hydrogen.
of −OH by annealing [26]. To confirm the donor generation in a-IGZO TFTs under high humidity (80%), we extracted the Fermi-level position (E C − E F ), flat-band carrier concentration (n FB ) and bandgap density of the donor state distribution (d N gap /d E) from the combined analysis of capacitance-voltage and current-voltage characteristics [27]. The results are summarized in Table I. Overtime, the E F shifted consistently toward the conduction band (E C ) as a result of increased free-carrier concentrations. The donor state densities in all TFTs also increased, which can be assumed by the addition of hydrogen/water in the a-IGZO film under high-humidity conditions.
IV. C ONCLUSION In conclusion, we found that the SiO2 passivation layer cannot protect our devices under high-humidity storage at RT, which can be solved by bilayer (SiO2 /SiNx ) passivation. XPS data confirms that the degradation of transfer characteristics in SiO2 passivated TFTs is due to the generation of the donor states in a-IGZO. R EFERENCES [1] E. Fortunato, P. Barquinha, and R. Martins, “Oxide semiconductor thinfilm transistors: A review of recent advances,” Adv. Mater., vol. 24, no. 22, pp. 2945–2986, May 2012.
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[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, no. 7016, pp. 488–492, Nov. 2004. [3] M. D. H. Chowdhury, P. Migliorato, and J. Jang, “Light induced instabilities in amorphous indium–gallium–zinc–oxide thin-film transistors,” Appl. Phys. Lett., vol. 97, no. 17, p. 173506, Sep. 2010. [4] K. H. Ji et al., “Comparative study on light-induced bias stress instability of IGZO transistors with SiNx and SiO2 gate dielectrics,” IEEE Electron Device Lett., vol. 31, no. 12, pp. 1404–1406, Dec. 2010. [5] J. B. Kim, C. Fuentes-Hernandez, W. J. Potscavage, Jr., X.-H. Zhang, and B. Kippelen, “Low-voltage InGaZnO thin-film transistors with Al2 O3 gate insulator grown by atomic layer deposition,” Appl. Phys. Lett., vol. 94, no. 14, p. 142107, Mar. 2009. [6] Y. S. Chun, S. Chang, and S. Y. Lee, “Effects of gate insulators on the performance of a-IGZO TFT fabricated at room-temperature,” Microelectron. Eng., vol. 88, no. 7, pp. 1590–1593, Jul. 2011. [7] K. Nomura, T. Kamiya, and H. Hosono, “Stability and high-frequency operation of amorphous In–Ga–Zn–O thin-film transistors with various passivation layers,” Thin Solid Films, vol. 520, no. 10, pp. 3778–3782, Mar. 2012. [8] J. Kim et al., “A study on H2 plasma treatment effect on a-IGZO thin film transistor,” J. Mater. Res., vol. 27, no. 17, pp. 2318–2325, Sep. 2012. [9] J.-C. Jhu et al., “N2 O plasma treatment suppressed temperature-dependent point defects formation with amorphous indium–gallium–zinc-oxide thin film transistors,” ECS Trans., vol. 45, no. 7, pp. 169–178, 2012. [10] M. D. H. Chowdhury, S. H. Ryu, P. Migliorato, and J. Jang, “Effect of annealing time on bias stress and light-induced instabilities in amorphous indium–gallium–zinc-oxide thin-film transistors,” J. Appl. Phys., vol. 110, no. 11, p. 114503, 2011. [11] M. D. H. Chowdhury, P. Migliorato, and J. Jang, “Time-temperature dependence of positive gate bias stress and recovery in amorphous indium-gallium-zinc-oxide thin-film-transistors,” Appl. Phys. Lett., vol. 98, no. 15, p. 153511, Mar. 2011. [12] S.-I. Oh, G. Choi, H. Hwang, W. Lu, and J.-H. Jang, “Hydrogenated IGZO thin-film transistors using high-pressure hydrogen annealing,” IEEE Trans. Electron Devices, vol. 60, no. 8, pp. 2537–2541, Aug. 2013. [13] K. Nomura, T. Kamiya, H. Ohta, M. Hirano, and H. Hosono, “Defect passivation and homogenization of amorphous oxide thin-film transistor by wet O2 annealing,” Appl. Phys. Lett., vol. 93, no. 19, p. 192107, Oct. 2008. [14] H. J. Kim et al., “Role of incorporated hydrogen on performance and photo-bias instability of indium gallium zinc oxide thin film transistors,” J. Phys. D, Appl. Phys., vol. 46, no. 5, p. 055104, 2013. [15] H.-K. Noh, K. J. Chang, B. Ryu, and W.-J. Lee, “Electronic structure of oxygen-vacancy defects in amorphous In-Ga-Zn-O semiconductors,” Phys. Rev. B, vol. 84, no. 11, p. 115205, Sep. 2011. [16] K. H. Ji et al., “Effect of high-pressure oxygen annealing on negative bias illumination stress-induced instability of InGaZnO thin film transistors,” Appl. Phys. Lett., vol. 98, no. 10, p. 103509, Feb. 2011. [17] C.-Y. Jeong et al., “Border trap characterization in amorphous indium-gallium-zinc oxide thin-film transistors with SiOX and SiNX gate dielectrics,” Appl. Phys. Lett., vol. 103, no. 14, p. 142104, Sep. 2013. [18] M. Mativenga, J. W. Choi, J. H. Hur, H. J. Kim, and J. Jang, “Highly stable amorphous indium–gallium–zinc-oxide thin-film transistor using an etch-stopper and a via-hole structure,” J. Inf. Display, vol. 12, no. 1, pp. 47–50, Mar. 2011. [19] J. G. Um, M. Mativenga, P. Migliorato, and J. Jang, “Increase of interface and bulk density of states in amorphous-indium-gallium-zincoxide thin-film transistors with negative-bias-under-illumination-stress time,” Appl. Phys. Lett., vol. 101, no. 11, pp. 113504-1–113504-4, Aug. 2012. [20] A. C. Thompson and D. Vaughan, X-Ray Data Booklet. Berkeley, CA, USA: Lawrence Berkeley National Laboratory, 2001. [21] Y. Ueoka et al., “Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy,” J. Appl. Phys., vol. 114, no. 16, p. 163713, Oct. 2013. [22] K. Nomura, T. Kamiya, and H. Hosono, “Effects of diffusion of hydrogen and oxygen on electrical properties of amorphous oxide semiconductor, In-Ga-Zn-O,” ECS J. Solid State Sci. Technol., vol. 2, no. 1, pp. P5–P8, 2013. [23] Y. Hanyu et al., “Hydrogen passivation of electron trap in amorphous In-Ga-Zn-O thin-film transistors,” Appl. Phys. Lett., vol. 103, no. 20, p. 202114, Nov. 2013.
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[24] J. H. Kang et al., “Mobility enhancement in amorphous InGaZnO thin-film transistors by Ar plasma treatment,” Appl. Phys. Lett., vol. 102, no. 22, p. 222103, Jun. 2013. [25] M. D. H. Chowdhury, P. Migliorato, and J. Jang, “Temperature dependence of negative bias under illumination stress and recovery in amorphous indium gallium zinc oxide thin film transistors,” Appl. Phys. Lett., vol. 102, no. 14, p. 143506, Mar. 2013. [26] H.-K. Noh, J.-S. Park, and K. J. Chang, “Effect of hydrogen incorporation on the negative bias illumination stress instability in amorphous In-Ga-Zn-O thin-film-transistors,” J. Appl. Phys., vol. 113, no. 6, p. 063712, Jan. 2013. [27] P. Migliorato, M. Seok, and J. Jang, “Determination of flat band voltage in thin film transistors: The case of amorphous-indium gallium zinc oxide,” Appl. Phys. Lett., vol. 100, no. 7, pp. 073506-1–073506-4, Jan. 2012.
Md Delwar Hossain Chowdhury (SM’15) received the M.S. degree in information display engineering from Kyung Hee University, Seoul, Korea, in 2011, where he is currently pursuing the Ph.D. degree in information display engineering.
Mallory Mativenga (M’14) received the Ph.D. degree in information display from Kyung Hee University, Seoul, Korea, in 2014. He is currently a Researcher of Information Display Engineering with Kyung Hee University.
Jae Gwang Um received the M.S. degree in information display engineering from Kyung Hee University, Seoul, Korea, in 2013, where he is currently pursuing the Ph.D. degree in information display.
Ravi K. Mruthyunjaya received the B.S. degree in electrical engineering from Clarkson University, Potsdam, NY, USA, and the M.S. degree in electrical engineering from the Rochester Institute of Technology, Rochester, NY, USA. He is currently with Carestream Health, Inc., Rochester.
Gregory N. Heiler received the B.S. degree from the Rochester Institute of Technology, Rochester, NY, USA, in 1985. He is currently with Carestream Health, Inc., Rochester, with a specialization on X-ray detector design.
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Timothy John Tredwell (M’79–SM’84) received the Ph.D. degree in solid-state physics from the University of Rochester, Rochester, NY, USA, in 1975. He has been a Senior Research Fellow with Carestream Health, Inc., Rochester, since 2007.
Jin Jang (M’94) received the Ph.D. degree in physics from Korea Advanced Institute of Science and Technology, Seoul, Korea. He is currently a Professor of information display and a director of Advanced Display Research Center in Kyung Hee University, Seoul and published more than 450 SCI Journal papers.
