Abstract. We investigated AC bias stress instability of indium-gallium-zinc- oxide (IGZO) Thin-film transistors (TFTs). AC bias frequency dependence showed ...
P-28 / S.-J. Kim
P-28: The Effect of AC Bias Frequency on Threshold Voltage Shift of the Amorphous Oxide TFTs Sun-Jae Kim1, Young-Wook Lee1,2, Soo-Yeon Lee1, Jong-Suk Woo1, Jang-Yeon Kwon1, Min-Koo Han1 1
Seoul National University, Seoul, 151-742, Korea
Woo-Geun Lee2, Kap-Soo Yoon2, 2
Samsung Electronics, Yongin-Si, 449-711, Korea
Abstract We investigated AC bias stress instability of indium-gallium-zincoxide (IGZO) Thin-film transistors (TFTs). AC bias frequency dependence showed different aspect in IGZO TFTs and a-Si:H TFTs. Influencing factors to AC bias frequency dependence of instability is charge accumulation characteristic under negative bias stress, and detrapping characteristics of shallow trapped charges under positive bias stress. Fig.1 The structure of fabricated IGZO TFT
1. Introduction Recently, amorphous oxide-based thin film transistors (oxide TFTs) have gained considerable attention for their application in active-matrix displays. Oxide TFTs exhibit superior electrical properties, such as field-effect mobility or off-current level, compared with conventional amorphous silicon TFTs. [1-3] The high electrical performance of oxide TFTs makes them one of the promising candidates for the backplane elements of ultra highdefinition (UD) liquid crystal displays (LCDs) or active-matrix organic light emitting diode (AMOLED) displays. [4-7] However, the stability of the oxide TFT, such as VTH shift, still needs to be verified. [8] Although considerable amount of efforts analyzed the origin of instability of oxide TFT, dominant mechanism of degradation is still not well-established. The oxide TFT is reported to be vulnerable due to various factors, such as hydrogen, water, and oxygen. [9] The instability investigation of oxide TFTs has mainly concentrated on applying DC bias stress. Since the TFTs goes through dynamic bias during active matrix display operation, instability under AC bias stress should be studied in detail. The purpose of our work is to investigate the AC bias instability of the oxide TFTs. For the investigation of mechanism of instability, we compare it with the amorphous silicon TFTs case.
2. Experiments The amorphous indium-gallium-zinc-oxide (IGZO) TFTs with an etch stop layer were fabricated on glass substrates by a widely used a-Si:H TFT-compatible process. Fig 1 shows the structure of fabricated IGZO TFT. Molybdenum was used as the gate material. The gate insulator was fabricated as SiO2 employing plasma enhanced chemical vapor deposition (PECVD) (2000 Å). The amorphous IGZO layers with thicknesses of 400 was deposited by DC sputtering at room temperature The SiO2 etch-stop layer was fabricated by PECVD. Molybdenum was used as the source and drain material. A PECVD SiNX layer was employed for passivation (2000 Å).
For the comparison of instability with the oxide TFTs, the aSi:H TFTs with an inverted staggered bottom gate type was also fabricated employing a standard commercial process. Triple layer of SiNx (4500 Å), a-Si:H (2000 Å), n+ a-Si:H (500 Å) was deposited by plasma-enhanced chemical vapor deposition (PECVD) after gate patterning. Active island, source and drain electrode were patterned on the deposited triple layer. After patterning the source and drain electrode by a wet etching, the n+ a-Si layer between the source and drain electrode was removed by a dry etching to make an etch back type channel structure. 300nm thick SiNx was deposited for a passivation. After contact holes are formed, an indium tin oxide (ITO) electrode was deposited and patterned. The transfer characteristics and reliability tests of devices were measured by HP B1500A semiconductor analyzer in air and in dark.
3. Results and discussion Figure 2 (a) shows the I-V characteristics of fabricated IGZO TFT. Overall field effect mobility of IGZO TFTs is about 9-10 cm2·V-1·s-1. For conventional a-Si:H TFTs, overall field effect mobility is in the range of 0.3~0.7 cm2·V-1·s-1 and the off current level is 1~2 orders higher than that of IGZO TFTs. As shown in Fig. 2 (b), in IGZO TFTs, we can observe the recovery characteristic from degradation after bias stress of 30V at 60 oC, which is remarkable compared with a-Si:H TFTs. This characteristic was also reported by several efforts and is related with detrapping of trapped charges. [10,11]
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Although, in early stage of effective stress time, the tendency is indistinct, a smaller shift of VTH corresponds a larger AC bias frequency after 1000 sec. This result agrees well with early works of a-Si:H TFT reliability. [13] It may attributed to low conductivity of holes and high contact resistance between the n+ a-Si:H/a-Si:H layers for hole accumulation under negative e-field. As AC bias frequency increases, there is not enough time for the accumulation of positive charges, which is supposed to be trapped into the interface between aSi:H/gate insulator layers, so that VTH degradation is less prominent as compared to high frequency bias stress. Consequently, it can be inferred that the main parameter affecting to AC bias frequency dependence is related with hole, especially channel accumulation time. Although degradation of IGZO TFTs under negative electric field is not apparent, reliability under not only bias stress, but also under illumination or high temperature, for the generation of carriers, should be investigated more.
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Fig. 3 (b) shows the VTH shift of the IGZO TFTs and the a-Si:H TFTs along with the effective stress time under the negative bias stress. First of all, as shown in Fig. 3 (b), we can observe that the IGZO TFTs are very stable under the negative bias field, and there is no apparent relationship between VTH shift and AC bias frequency. It may be attributed to the characteristic of n-type IGZO semiconductors that supply of hole is not efficient and charge trapping is not occurred under negative e-field due to lack of mobile charges. [12] On the other hand, the inset of Fig. 3 (b), which is the a-Si:H TFTs case, shows that the VTH shift of a-Si:H TFT is AC bias frequency dependent.
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For investigation of reliability after the AC bias stress, we performed several pulsed bias on the IGZO TFTs and a-Si:H TFTs. The pulsed bias was applied on the gate of TFTs, and the inset of Fig.3 (a) describes the waveform of the pulsed bias. The duty ratio, defined as the ratio between pulse width and pulse period, in all experiments was 0.5. Frequency of pulsed bias, which is 1/pulse period, was varied from 10 to 1000 Hz. All negative pulsed bias has a high level of 0 V and a high level of -30 V. Also, all positive pulsed bias has a low level of 0V and a high level of 30V. The effective stress time was 3,600 sec. The W/L of IGZO TFTs used in the experiments was 100/15 and that of a-Si:H TFTs was 120/12.
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P-28 / S.-J. Kim Fig. 4 shows the VTH shift of the IGZO TFTs and the a-Si:H TFTs along with the effective stress time under the positive bias stress. The results are quite different from the negative bias stress case. As shown in Fig. 4, we can observe the apparent AC bias frequency dependence of VTH shift of the IGZO TFTs. Nevertheless, in the inset of Fig. 4 showing the VTH shift of the aSi:H TFT, AC bias stability doesn’t show the AC bias frequency dependence, and the VTH shift under AC bias stress is smaller than under DC bias stress. This result is consistent with early works of a-Si:H TFT reliability, assuming fast channel accumulation of electrons. [13] 10
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detrapping process of shallow trapped charge near interface between IGZO/gate insulator, compared to a-Si:H TFTs. As pulse width shortened, deep trapping of charge less occurred. On the contrary, detrapping of shallow trapped charges might results less VTH shift of IGZO TFTs. On the other hand, Consequently, we might consider that the factor causing different AC bias frequency of IGZO TFTs and a-Si:H TFTs is different detrapping characteristics of shallow trapped charge.
4. Conclusion We fabricated the IGZO TFTs with a bottom gate structure and investigated the AC bias stress instability of the TFTs. The AC bias frequency shows a different dependence in the IGZO TFTs and a-Si:H TFTs. Under negative bias stress, a-Si:H TFTs shows strong AC bias frequency as compared to IGZO TFTs. On the contrary, under positive bias stress, IGZO TFTs exhibits AC bias frequency dependence as compared to a-Si:H TFT. The important factors to the AC bias frequency dependence of VTH shift under bias stress is charge accumulation characteristic for negative bias stress, and detrapping characteristics of shallow trapped charges for positive bias stress.
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Fig. 4. Delta threshold voltage of IGZO TFTs under the positive AC bias stress (30 V / 0 V) and the positive DC bias stress (30 V) along with effective stress time : Dot : original data / Line : fitting curve ( inset: a-Si:H TFT case) Interestingly, in positive bias stress, the AC bias frequency dependence shows different from negative bias stress. The main factor, influencing the frequency dependence was the hole accumulation time in the negative bias stress. It was considered main VTH degradation mechanism that a negative charge trapping, since there is not apparent change of field effect mobility and subthreshold slope. [10,11] However, in positive bias stress, main carrier, accumulated in channel, is negative charges, especially electron, which is very short accumulation characteristic time. Thus, we cannot explain the different AC bias frequency dependence by only charge trapping. As discussed above, in oxide TFT, such as IGZO TFTs and IZO TFTs, the recovery characteristic is remarkable. [11,14] Although positive AC bias frequency dependence was observed by several works, main responsibility is still unclear. [10,15] Recovery characteristic of IGZO TFTs can be considered as fast
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