Defect-creation effects on abnormal on-current under drain bias illumination stress in a-IGZO thin-film transistors

Journal Pre-proofs Defect-creation effects on abnormal on-current under drain bias illumination stress in a-IGZO thin-film transistors Tae-Kyoung Ha, Yongjo Kim, Yong-Jung Cho, Yun-Seong Kang, SangHee Yu, GwangTae Kim, Hoon Jeong, Jeong Ki Park, Ohyun Kim PII: DOI: Reference:

S0038-1101(19)30596-9 https://doi.org/10.1016/j.sse.2019.107752 SSE 107752

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Solid-State Electronics

Received Date: Revised Date: Accepted Date:

10 October 2019 11 December 2019 16 December 2019

Please cite this article as: Ha, T-K., Kim, Y., Cho, Y-J., Kang, Y-S., Yu, S., Kim, G., Jeong, H., Ki Park, J., Kim, O., Defect-creation effects on abnormal on-current under drain bias illumination stress in a-IGZO thin-film transistors, Solid-State Electronics (2019), doi: https://doi.org/10.1016/j.sse.2019.107752

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Defect-creation effects on abnormal on-current under drain bias illumination stress in a-IGZO thin-film transistors Tae-Kyoung Ha*, Yongjo Kim, Yong-Jung Cho, Yun-Seong Kang, SangHee Yu, GwangTae Kim, Hoon Jeong, Jeong Ki Park and Ohyun Kim Tae-Kyoung Ha, Yongjo Kim and Prof. Ohyun Kim Department of Electrical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 37673, Korea E-mail: [email protected] Yong-Jung Cho, Yun-Seong Kang Oxide Development Group, LG Display, 245, LG-ro, Wollong-myeon, Pasu-si, Gyeonggi-do, 10845, Korea SangHee Yu, GwangTae Kim, Hoon Jeong, Jeong Ki Park IT Development Group, LG Display, 245, LG-ro, Wollong-myeon, Pasu-si, Gyeonggi-do, 10845, Korea

Keyword: a-InGaZnO, thin film transistors, drain bias stress, illumination, donor-like state, acceptor-like state

On-current ION and field effect mobility FE changed abnormally by drain bias illumination stress (DBIS) in amorphous InGaZnO thin-film transistors. First, ION dropped to 35% of its initial value and FE decreased from 11.8 cm2/(V∙s) to 3.0 cm2/(V∙s) because of acceptor-like tail states ATAIL, which were generated by rupture of weak oxygen bonds near the drain region. The ATAIL trap electrons, and therefore have negative charge, which causes scattering of charged carriers and decrease in FE. As DBIS time elapsed, ionized oxygen vacancies (VO2+) were generated near the drain region, so ION was re-elevated to 1.57 times as high as its dropped value (i.e., to 55% of its initial value); FE increased from 3.0 cm2/(V∙s) to 6.9 cm2/(V∙s) (i.e., to 59% of its origin value);. These compensation effects happened when ATAIL and VO2+ occurred in the same place, so generation of VO2+ near drain region was the main cause of the re-elevation.

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1. Introduction Amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) are promising candidates to replace hydrogenated amorphous silicon (a-Si:H) TFTs in wide and thin displays.[1] Compared to a-Si:H TFTs, a-IGZO TFTs have good carrier mobility , low off-current IOFF, good optical transparency, high ratio of on-current ION to IOFF, good uniformity and low fabrication temperature.[1,2] Therefore, a-IGZO is a promising material for the channel of TFTs in liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs), which are main components of wide and thin displays. However, the reliability of a-IGZO can be insufficient because the TFTs in LCDs and OLEDs in displays are exposed to various stresses such as bias, temperature, humidity and illumination

[3-6].

Under these conditions, a-IGZO TFTs can be afflicted by these stresses,

then undergo threshold-voltage shift VT, decrease in , increase in subthreshold slope (SS), the hump phenomenon, the scaling effect, and drop in ION.[7-11] Usually, these problems are caused by charge trapping and/or by generation of sub-gap defect states in the a-IGZO channel or in the interface between gate insulator (GI) and a-IGZO channel.[7-11] Charge trapping and defect-state generation can occur concurrently. In practical applications, the drain voltage remains fixed even during the ‘off’ state (gate voltage = 0), so a-IGZO TFTs are continuously subjected to drain-bias stress (DBS). Furthermore, since a-IGZO TFTs are mainly used in displays, so TFTs are also continuously exposed to illumination. Therefore, to achieve a-IGZO TFTs that have good stability, the influence of DBS under illumination should be understood. Thus, the combined effects of DBS and illumination stress have widely been investigated.[12-14] Previous reports have hypothesized that when drain-bias illumination stress (DBIS) is applied to TFTs, the hump phenomenon and SS degradation occur because ionized oxygen vacancies (VO2+) are generated then migrate or that negative VT happens under DBS or DBIS because of hole trapping or because of generation of donor-like states.[12-14] 2

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In this paper, we applied DBIS to a-IGZO TFTs and observed abnormal current behavior that cannot be explained by the mechanisms proposed in refs. 12, 13 and 14. We divided this new behavior into two stages: ION drop and re-elevation. To identify the cause of the abnormal behavior, we conducted current-voltage (I-V) analysis of DBIS and DBS, leakage current test, simulation of electric field (E-field). Also, to prove our mechanisms, we conducted capacitance-voltage (C-V) analysis and 2D TCAD simulation.

2. Experimental methods

Figure 1. Cross-sectional schematic diagram of back-channel-etched a-IGZO TFT along the length direction.

a-IGZO TFTs with bottom-gate, back-channel-etched structure were fabricated on glass substrate (Figure 1). All electrodes were Mo-Ti, and channels with width W = 250 m and length L = 4 m was formed by sputtering. During bias stress, positive drain-to-source voltage VDS = 40 V was applied for 100,000 s at 90 °C with gate-to-source voltage VGS and source voltage VS grounded. Illumination stress was applied to the top of TFTs by a

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5,000-lx white LED lamp using ML903 with 5-V DC bias; this lamp has equal power over all wavelengths that it emits. To measure transfer characteristics, VGS was swept from -20 V to +20 V with VDS = 0.1 V at 90 °C. Threshold voltage VT was defined as VGS when drain current IDS = 1 nA ∗ (W/L). SS was chosen by curve fitting of points that were extracted using SS = 𝑑𝑉GS/𝑑log10(𝐼DS) for 0.1 nA < IDS < 10 nA from normalized value. Field effect mobility  FE was calculated from transconductance 𝑔M of the I-V curve as 𝜇 FE =

𝐿𝑔𝑀 𝑊𝐶𝑂𝑋𝑉𝐷𝑆

,

where COX is the gate capacitance per unit area. I-V measurements were conducted using an HP 4156A parameter analyzer. C-V measurements were obtained using an Agilent 4284A LCR meter, by sweeping VGS from -10 V to +10 V with 100 kHz at 30 °C before and after DBIS. 2D TCAD simulation was conducted using Silvaco ATLAS TCAD simulation tool.

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3. Results and discussion

Figure 2. (Color) (a) The I-V characteristics of ION drop and (b) that of re-elevation and (c) VT, SS and (d) FE vs. stress time in a-IGZO TFT measured with VDS = 0.1 V under DBIS (insets: log-scale curves for transfer characteristics) Transfer characteristics of a-IGZO TFTs were measured under DBIS with VDS = 40 V at 90 °C for 100,000 s with illumination by 5,000-lx white light (Figure 2). When DBIS was applied to a-IGZO TFTs for 100,000 s, ION first dropped and then underwent re-elevation, that could be divided into two stages: during stage 1, from 0 s to 1,000 s, ION decreased to 35% of the initial state (Figure 2a); during stage 2, after ION drop was terminated at 1,000 s, ION increased to 1.57 times the dropped value of ION at 1000 s (i.e., to 55 % of the initial value) (Figure 2b). To investigate the phenomenon, VT, SS and FE were extracted (Figure 2c-d). VT denoted inconsistent change with no apparent change in the SS during DBIS (Figure 2c). Under

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DBIS for 100,000 s, the shifts of VT and SS were regarded as minor because the range of VT shift was within 1 V range, and SS did not change during DBIS. However, despite this maintenance of VT and SS, FE initially decreased from 11.8 cm2/(V∙s) to 3.0 cm2/(V∙s), then increased by 6.9 cm2/(V∙s) (Figure 2d) (i.e., to 59 % of its initial value); this variation is analogous to the change of ION. Some papers have tried to explain ION drop; two cases have been observed. 1) ION drops with positive VT shift as a result of electron trapping or generation of deep acceptor-like states; in this case, ION seems to decrease when operation voltage is kept constant.[15] 2) ION drops with little VT shift and SS degradation by acceptor-like tail states.[16-18] In this case, as VGS increases, electrons are captured in acceptor-like tail states at VGS > VT; therefore, ION drops as a result of electron capture.[16] In our devices, VT shift and SS did not change much when ION decreased, so our results were more similar to case 2 than to case 1. However, we considered that only electron capture is insufficient to explain ION decrease, so we believe that  degradation, due to Coulomb scattering, is the main mechanism of ION drop. Furthermore, we observed re-elevation, which was new phenomenon; therefore, we conducted further study of our results. Drain bias stress is asymmetrical, so we conducted an asymmetric saturation-mode test. Furthermore, high drain bias stress could cause high leakage current and extreme E-fields, and thereby generate hot-carriers, which can induce defect states; therefore we quantified source-drain (S-D) leakage current and conducted simulation of the E-field. In addition, DBS (i.e., drain bias stress without illumination), was applied to a-IGZO TFTs to isolate the effect of illumination.

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Figure 3. (Color online) I-V saturation characteristics of a-IGZO TFT measured with lVDSl= 15 V before and after DBIS when reverse mode. After DBIS, ION was maintained in forward mode, whereas ION dropped in reverse mode. (inset: I-V characteristics of forward mode) Bias stress was applied asymmetrically, so forward and reverse mode, which could get I-V characteristics of saturation condition, were conducted to compare the symmetry of degradation at the source and drain of the channel. Forward/reverse mode was conducted under saturation condition, which means |𝑉𝐷𝑆| ≥ 𝑉𝐺𝑆. Forward mode provides information about the source-side condition because the channel forms except drain region, whereas reverse mode provides information except source region because the channel forms on the drain side.[19] Considering stable operation of TFTs in saturation condition, when forward/reverse mode were conducted, VDS was 15 V, and VGS was swept from -5 V to +10 V before and after DBIS for 100,000 s (Figure 3). Before DBIS, the I-V characteristic of forward mode was approximately the same as that of reverse mode. However, after DBIS, ION dropped only during reverse mode; this result means that ION drop is caused by degradation of the drain-side condition. 7

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Figure 4. (Color) (a) ISTR vs. VDS of a-IGZO TFT. (b) Simulated lateral and total E-field vs. lateral channel position of a-IGZO TFT when VDS is 40 V. We investigated stress current (ISTR) at VGS = 0 V with various VDS (Figure 4a) because the stress condition of DBIS was VGS = 0 V with VDS = 40 V under 5000 lx illumination. When VDS was 40 V for DBIS, the ISTR (~2.6 A) had the similar value as the current stress level

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of ref. 20 (~10 A) that could make hot carriers.[20] Furthermore, a strong E-field is applied near the drain side during DBIS. To clarify the effect of the E-field, 2D simulation was conducted (Figure 4b); the peak value of the simulated E-field was 0.4 MV/cm, where 0.45 MV/cm could make hot carriers in ref. 20.[20] When carriers pass through the region that has a high E-field, they acquire energy from the E-field.[21] These energetic carriers are called hot carriers, and have kinetic energy that is above the energy level of the conduction band edge; i.e., during DBIS, a strong lateral E-field can give electrons energy to become ‘hot’ while they move to the drain side.[21]

Figure 5. (Color online) (a) Simplified channel diagram of proposed mechanism during stage 1. Qualitative description of band structure of a-IGZO TFT near drain side of the TFT about acceptor-like tail states after DBIS during VGS sweep (b) before VT and (c) after VT. 9

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When a strong lateral E-field drives the hot electrons to the drain side, the electrons can break weak oxygen bonds there, and thereby generate sub-gap states, especially acceptor-like states. VT and SS did not change much during DBIS, so the energy level of these states must be higher than the Fermi level EF at the VGS of VT; therefore, the acceptor-like states would be shallow acceptor-like states, namely acceptor-like tail states ATAIL near the conduction band (Figure 5a), so the decrease in ION is a result of generation of ATAIL on the drain side.[16-18] When ATAIL have energy level < EF, the states are negatively charged because they are filled by electrons, whereas when ATAIL have energy level > EF, the states are neutral because they are empty (Figure 5b, box).[22] When VGS is swept from -20 V to +20 V, EF of the a-IGZO channel increases toward the conduction band; when VGS < VT, ATAIL have energy above EF, so they are neutral, and have no influence (Figure 5b). In contrast, when VGS > VT, EF would be within the energy level of ATAIL, so the EF pinning effect happens because ATAIL accept electrons (Figure 5c). These ATAIL become negatively charged, and disturb the movement of electrons; this disturbance can cause Coulomb scattering which reduces FE, so ION decreases.[23]

Figure 6. (Color) (a) The I-V characteristics and (b) VT, SS and (c) FE vs. stress time in a-IGZO TFT measured with VDS = 0.1 V under DBS. To clarify the effect of illumination related to re-elevation mechanism, DBS was applied to TFTs in darkness for 100,000 s (Figure 6). When DBS was applied, ION dropped (Figure 6a) and FE decreased (Figure 6b) without re-elevation in the TFTs; this result means that DBS causes decrease of ION and FE, and that illumination contributes to 10

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re-elevation because removal of illumination does not lead to re-elevation. During DBS, VT did not change consistently, and SS changed negligibly (Figure 6b); these reactions are similar to those during DBIS. Therefore, the ION drop mechanism of DBS is the same as the ION drop mechanism of DBIS. FE also decreased from 11.5 cm2/(V∙s) to 3.2 cm2/(V∙s) continuously, like the trend in ION (Figure 6c). In general, bias stress and illumination stress are related to generation of shallow donor-like states such as ionized oxygen vacancies (VO2+).[24] When bias stress and light stress are applied to the a-IGZO channel, neutral oxygen vacancies (VO) receive energy and release two electrons to become VO2+ [12, 25]; these vacancies have energy level below and near the conduction band minimum, and VO are deep donor-like states with the energy level above and near the valance band maximum.[26, 27] Generation of VO2+ is the difference between DBIS and DBS, and is the core of re-elevation mechanism.

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Figure 7. (Color online) (a) Simplified channel diagram of proposed mechanism during stage 2. (b) Qualitative description of band structure of a-IGZO TFT near drain side of the TFT about ATAIL and VO2+after DBIS. During stage 1, VO are also ionized to VO2+ consistently due to DBIS; but, this process seems to have little effect on I-V characteristics, possibly because the rate at which ATAIL are generated is higher than the rate at which VO are ionized. As the number of generated states increases, the rate of generation decreases, then becomes saturated.[28] Similarly, as the amount of ATAIL increases, the generation rate of ATAIL decreases; consequently, during stage 2, the ionization rate of VO becomes higher than the generation rate of ATAIL. Therefore, during stage 2, the VO2+ could affect the I-V characteristics. During continuous 12

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application of DBIS, the amount of VO2+ increases near the drain because drain region is the place which have high E-field by drain bias and illumination, so positively-charged and negatively-charged defects coexist in the same place in the drain region (Figure 7a). As a result, each compensates for the other’s effects, in a manner similar to the doping compensation (Figure 7b).[29] In conclusion, the decreases in ION and FE, which were caused by scattering of acceptor defects, could be re-elevated by compensation that is induced by mutual cancelling of positive and negative defects.

Figure 8. (Color online) (a) CGD-VGS and (b) CGS-VGS curves (frequency = 100,000 Hz) measured for initial state and after DBIS. 13

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To identify the mechanism of ION drop and re-elevation, C-V measurements were conducted. ‘Gate-drain’ capacitance CGD-V and ‘gate-source’ capacitance CGS-V data were extracted before and after application of DBIS for 100,000 s. CGD was stretched out in the region of ‘off’ capacitance COFF, then after DBIS, ‘on’ capacitance CON was slightly lower than before DBIS (Figure 8a), whereas DBIS had almost no effect of CGS (Figure 8b). We hypothesize that the change of COFF may be a consequence of shallow donor-like states, particularly VO2+, whereas CON is related to shallow acceptor-like states (ATAIL).[16,17,29] When VO are ionized, they give electrons to the conduction band, and thereby increase its net charge. The additional net charges cause increase in COFF when voltages are less than the transition voltage.

[17,30]

In contrast, CON decreased, possibly because ATAIL were

generated in the drain side. When VGS was swept from -10 V to +10 V, EF encountered the energy level of ATAIL, which capture electrons and therefore become negatively charged

(Figure 8a).[16,30] Figure 9. (Color) (a) 2D simulation structure of a-IGZO TFT (W = 250 m, L = 4 m) with locations of defects. (b) Simulated density of state for a-IGZO TFT layer, and I-V 14

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characteristics of a-IGZO TFT with (c) various NTA when fixed NGD and (d) various NGD when fixed NTA. To test these proposed mechanisms of ION drop and re-elevation, a 2D simulation was conducted (Figure 9). The tail states in the active layer were composed of conduction band tail states NATAIL and valence band tail state NDTAIL that can be expressed as

[

Ec ― E

[

Ev ― E

𝑁ATAIL(E) = 𝑁𝑇𝐴exp ― 𝑁D𝑇𝐴𝐼𝐿(E) = 𝑁𝑇𝐷exp ―

WTA

WTD

],

(1)

],

(2)

where EC [eV] is conduction band energy, EV [eV] is valance band energy, NTA [/(cm3·eV)] is the acceptor trap density at the conduction band edge, NTD [/(cm3·eV)] is the donor trap density at the valance band edge, WTA = 0.07 eV is conduction band characteristic decay energy, and WTD = 0.01 eV is valance band characteristic decay energy. The shallow donor-like state NDSH is generally defined as the Gaussian donor trap density located near the conduction band edge, and is given as

[

𝑁DSH(E) = 𝑁𝐺𝐷exp ―

(E ― 𝐸𝐺𝐷)2 𝑊2𝐺𝐷

],

(3)

where NGD is the peak value of shallow donor-like state, EGD = 0.28 eV is the mean energy level of the states, and WGD = 0.08 eV is the half-width of Gaussian state donor trap density. In simulations, the region for ATAIL and VO 2+ was set between 3 m and 4 m at the lateral position of the channel with various densities of states (DOS) (Figure 9a-b). Simulation of the state generation rate is impossible, so we only simulated the number of states. To clarify the mechanism of ION decrease, ATAIL were added to the drain side of the a-IGZO channel with fixed NGD (Figure 9a). As NTA increased from 1 x 1018 /(cm3·eV) to 2 x 1019 /(cm3·eV), ION dropped (Figure 9c); this result means that ATAIL could induce ION drop because they reduce FE by scattering. To identify the cause of re-elevation, VO

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2+

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were added at the same place with fixed NTA (Figure 9a). As NGD increased from 1 x 1017 /(cm3·eV) to 1 x 1019 /(cm3·eV), re-elevation occurred (Figure 9d); this phenomenon means that VO2+ could compensate for the effect of ATAIL because the positive and negative defects offset each other. These simulation data support our proposed mechanisms for ION drop and re-elevation by obtaining characteristics that show the same trends as the experimental results. 4. Conclusions We investigated how drain bias and illumination affect ION in a-IGZO TFTs. During DBIS, ION dropped and then re-elevated, so we divided the abnormal current behavior into two stages. Asymmetric saturation mode test, ISTR measurements, simulations of E-field and clarification of the illumination effect were conducted to identify the mechanism of the abnormal current behavior. Our analysis indicated that the ION drop and re-elevation were caused by generation of sub-gap states in the drain-side. We hypothesized that the main mechanism of stage 1 was generation of ATAIL as a consequence of hot carriers inducing

FE degradation by Coulomb scattering, and that the main mechanism of stage 2 was ionization of VO induced by DBIS; i.e., that generated VO2+ compensated for the effect of ION drop by offset of the influence of ATAIL, so FE drop could be re-elevated with ION re-elevated. I-V saturation analysis, C-V analysis and 2D simulation data supported our hypothesized mechanism of degradation. Our results and the proposed mechanism can be useful to guide design of displays that use a-IGZO TFTs.

Acknowledgments This research was supported by the Ministry of Science, ICT and Future Planning (MSIP), Korea, under the “ICT Consilience Creative Program” (IITP-2018-2011-1-00783) supervised by the Institute for Information & communications Technology Promotion (IITP), and technically by the LG display. The authors thank LG Display for technical support.

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Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

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Author Biography TaeKyoung Ha received the B.S. degree in electrical engineering from Kyungpook National University, Daegu, Korea, in 2015, and is currently pursuing the Ph.D. degree in the Department of Electrical Engineering in Pohang University of Science and Technology (POSTECH). His research interest is amorphous oxide thin-film transistors for application in display device.

Ohyun Kim received the B.S. degree in electrical engineering from Seoul national University, in 1977, and the M.S. and Ph.D. degrees from Korea Advanced Institute of Science and Technology, Daejeon, Korea, in 1983. From 1983 to 1986, he was with Samsung Electronics, Korea, where he was involved in DRAM development. From 1989 to 1990, he was with Bell Communications Research, NJ, USA; during the period his research was focused on high speed devices. Since 1986, he has been a Professor in the Department of Electrical Engineering, Pohang University of Science and Technology (POSTECH). His research interests include EUV lithography, polymer memories, AMOLEDs, Oxide TFTs, Graphene FETs and ReRAMs, and strained high-voltage MOSFETs.

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Author Photo

Taekyoung Ha

Oh Kim

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Highlights  When drain bias illumination stress was applied, Ion dropped and re-elevated.  The main cause of Ion drop was generation of acceptor-like tail states.  The main factor of re-elevation was generation of ionized oxygen vacancies.  CV measurements and TCAD simulation were conducted to identify the mechanism.

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