Impact of Photo-Excitation on Leakage Current and Negative Bias Instability in InSnZnO Thickness-Varied Thin-Film Transistors

InSnZnO thin-film transistors (ITZO TFTs), having high carrier mobility, guarantee the benefits of potential applications in the next generation of super-high-definition flat-panel displays. However, the impact of photo-excitation on the leakage current and negative bias stress (NBIS) of ITZO TFTs must be further explored. In this study, the ITZO thickness (TITZO) is designed to tailor the initial performance of devices, especially for the 100 nm TITZO TFT, producing excellent electrical properties of 44.26 cm2V−1s−1 mobility, 92 mV/dec. subthreshold swing (SS), 0.04 V hysteresis, and 3.93 × 1010 ON/OFF ratio, which are superior to those of the reported ITZO TFTs. In addition, incident light coupled with tunable photon energy is introduced to monitor the leakage current of various TITZO devices. The OFF-current results demonstrate that under the identical photon energy, many more electrons are photo-excited for the thicker TITZO TFTs. NBIS-induced Vth shift and SS deterioration in all TFTs are traced and analyzed in real time. As the TITZO thickens to near Debye length, the degree of degradation is exacerbated. When the thickness further increases, the notorious instability caused by NBIS is effectively suppressed. This study provides an important research basis for the application of ITZO-based TFTs in future displays.


Introduction
In the last two decades, thin-film transistors (TFTs) based on metal oxide semiconductors are one of encouraging low-input voltage electronic devices in transparent and flexible flat panel display (FPD) applications where traditional silicon-related TFTs are hard to match [1][2][3]. Among various metal oxide active layers, amorphous InGaZnO (a-IGZO)-dependent transistors have rapidly developed because they easily achieve high mobility (µ) of~10 cm 2 V −1 s −1 and long-term stability through creative approaches [4][5][6], including interface modification, doping engineering, and device structure adjustment. Nevertheless, their mobility is insufficient to satisfy the requirement of driving integrated circuits in ultra-and super-high-definition FPDs.

Experimental Methods
To construct the ITZO TFTs with various T ITZO , the glass substrate was ultrasonically treated with acetone, isopropyl alcohol, and deionized water for 5 min, separately. The chromium film was fabricated on the treated glass and was dry etched to form the bottom electrode. The 150 nm thick SiO 2 gate insulator was then using utilizing plasma-enhanced chemical vapor deposition (PECVD). In the case of the active layers, DC magnetron sputtering technology was adopted. The sputtering parameters, which included a deposition power of 60 W, operation pressure of 1 Pa, working gases of Ar and O 2 with ratio of 15/15 sccm, and sputtering temperature of 30 • C, were fixed. In addition, the deposition duration was adjusted. Consequently, the ITZO films with the T ITZO of 25, 45, 75, and 100 nm were tailored. After the pattern process, the geometric factor of length-width ratio was 20:50 µm as a result of the identical design specification. The SiO 2 film (200 nm) was deposited again using PECVD to form an etch-stopper layer. The ITO source and drain electrodes were sequentially fabricated with the help of the sputtering method and etching technology. The SiO 2 passivation layer was finally deposited to obtain bottom-gate top-contact ITZO TFTs. Subsequently, the devices were thermally heated in N 2 at 350 • C for 1 h. In addition, the photo-excitation OFF-current results were investigated under illumination at wavelengths in the region of 400-530 nm. With regard to the operation conditions of NBIS evaluation, the wavelength, gate voltage, and stress duration were 460 nm, −20 V, and 10 4 s, respectively. The current-voltage (I-V) response was measured using an Agilent 4156C semiconductor parameter analyzer. Figure 1 displays the transfer properties of ITZO thickness-varied TFTs measured at V DS = 10.1 V. The corresponding electrical parameters calculated from the forward scan are listed in Table 1. With respect to the TFT with 25 nm T ITZO , outstanding electrical properties were observed: µ sat of 34.73 cm 2 V −1 s −1 , V th of 1.95 V, ON/OFF ratio of 2.38 × 10 10 , hysteresis ∆V H of 0.17 V, and SS of 206 mV/dec. When the T ITZO increased to 45 nm and continued to thicken to 100 nm, the µ sat slightly increased to 37.61 cm 2 V −1 s −1, and gradually changed to 44.26 cm 2 V −1 s −1 . Additionally, as the T ITZO increased, the I D rose. Considering the relationship between I D and µ sat [15], the improvement in the µ sat contributed to the increase in the ON current. The V th negatively changed to 1.13 and −0.25 V, which correspond to 45 and 100 nm T ITZO devices, respectively, as a result of the increase free carrier concentration in the thicker T ITZO . The ON/OFF ratio reached more than 10 orders of magnitude and the hysteresis is almost negligible irrespective of the T ITZO . The SS of 92 mV/dec. was obtained when the T ITZO thickened to 100 nm. The SS value is a standard criterion of the total defect densities in the active layer and its adjacent interfaces [16]. This value suggests that high quality ITZO layer and frontand back-interfaces were designed in this work. Furthermore, compared with the performance of reviewed ITZO TFTs [17][18][19][20][21][22][23], as tabulated in Table 2, the 100 nm T ITZO device in this study is superior. Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 8 stress duration were 460 nm, −20 V, and 10 4 s, respectively. The current-voltage (I-V) response was measured using an Agilent 4156C semiconductor parameter analyzer. Figure 1 displays the transfer properties of ITZO thickness-varied TFTs measured at VDS = 10.1 V. The corresponding electrical parameters calculated from the forward scan are listed in Table 1. With respect to the TFT with 25 nm TITZO, outstanding electrical properties were observed: μsat of 34.73 cm 2 V −1 s −1 , Vth of 1.95 V, ON/OFF ratio of 2.38 × 10 10 , hysteresis ΔVH of 0.17 V, and SS of 206 mV/dec. When the TITZO increased to 45 nm and continued to thicken to 100 nm, the μsat slightly increased to 37.61 cm 2 V −1 s −1, and gradually changed to 44.26 cm 2 V −1 s −1 . Additionally, as the TITZO increased, the ID rose. Considering the relationship between ID and μsat [15], the improvement in the μsat contributed to the increase in the ON current. The Vth negatively changed to 1.13 and −0.25 V, which correspond to 45 and 100 nm TITZO devices, respectively, as a result of the increase free carrier concentration in the thicker TITZO. The ON/OFF ratio reached more than 10 orders of magnitude and the hysteresis is almost negligible irrespective of the TITZO. The SS of 92 mV/dec. was obtained when the TITZO thickened to 100 nm. The SS value is a standard criterion of the total defect densities in the active layer and its adjacent interfaces [16]. This value suggests that high quality ITZO layer and front-and back-interfaces were designed in this work. Furthermore, compared with the performance of reviewed ITZO TFTs [17][18][19][20][21][22][23], as tabulated in Table 2, the 100 nm TITZO device in this study is superior.    The typical transfer curves of ITZO-varied devices were recorded from ON-to OFF-current direction under the external light exposure and V DS bias of 20.1 V, as described in Figure 2a-d. In general, the OFF-current is dependent on the incident photon energy and the T ITZO , as summarized in Figure 2e. For the 25 nm T ITZO TFT, when the incident light energy was greater than 2.70 eV, the photo-excited OFF-current slightly increased. This increase depends on the photon energy of incident light. As the T ITZO s increased to 45 nm, high OFF-current was obtained, even under the identical photon energy of 2.70 eV. When the T ITZO was further thickened to 75 and 100 nm, the photon energy of excited OFF-current gradually reduced to 2.48 and 2.34 eV, respectively. These phenomena can be interpreted as follows: for the channel layer with T ITZO of 25 and 45 nm, the high-density defect states occupy the valence band maximum (E V ), which is~2.70 eV away from the conduction band (E C ), similar to the results of a-IGZO devices [24]. In terms of the T ITZO at 75 and 100 nm, given the long deposition duration in the sputtering chamber, the ITZO films suffer from strong plasma bombardment. A possible correction is the generation of more defect states located in the positions of 2.48 or 2.34 eV away from the E C . With regard to a-IGZO, the high density of oxygen vacancy (V O ) defects with a width of~1.5 eV is located near the E V [25]. We therefore think that the long-term bombardment effect results in much higher densities of V O defect states with the increased energy width, and gradually broadens with the increase in T ITZO . The typical transfer curves of ITZO-varied devices were recorded from ON-to OFF-current direction under the external light exposure and VDS bias of 20.1 V, as described in Figure 2a-d. In general, the OFF-current is dependent on the incident photon energy and the TITZO, as summarized in Figure 2e. For the 25 nm TITZO TFT, when the incident light energy was greater than 2.70 eV, the photo-excited OFF-current slightly increased. This increase depends on the photon energy of incident light. As the TITZO s increased to 45 nm, high OFF-current was obtained, even under the identical photon energy of 2.70 eV. When the TITZO was further thickened to 75 and 100 nm, the photon energy of excited OFF-current gradually reduced to 2.48 and 2.34 eV, respectively. These phenomena can be interpreted as follows: for the channel layer with TITZO of 25 and 45 nm, the high-density defect states occupy the valence band maximum (EV), which is ~2.70 eV away from the conduction band (EC), similar to the results of a-IGZO devices [24]. In terms of the TITZO at 75 and 100 nm, given the long deposition duration in the sputtering chamber, the ITZO films suffer from strong plasma bombardment. A possible correction is the generation of more defect states located in the positions of 2.48 or 2.34 eV away from the EC. With regard to a-IGZO, the high density of oxygen vacancy (VO) defects with a width of ~1.5 eV is located near the EV [25]. We therefore think that the long-term bombardment effect results in much higher densities of VO defect states with the increased energy width, and gradually broadens with the increase in TITZO. To further explore the collaborative effect of photo-excitation and negative bias on the stability of TITZO-varied TFTs, we conducted a routine NBIS investigation. Regarding the forward scan (Figure 3a-d), the transfer curves of 25 nm TITZO device presented a positive Vth shift associated with SS decay. The NBIS-caused situation showed a progressive deterioration with increasing stress duration. A similar observation was made in the 45 nm TITZO case. Moreover, these phenomena were further amplified. With the increase in TITZO, conditions relating to the positive movement of Vth and the degradation of SS value remarkably recovered. In For100 nm TITZO TFT, the instability that originated from NBIS was considerably suppressed. In the subsequent reverse scan (Figure 3e To further explore the collaborative effect of photo-excitation and negative bias on the stability of T ITZO -varied TFTs, we conducted a routine NBIS investigation. Regarding the forward scan (Figure 3a-d), the transfer curves of 25 nm T ITZO device presented a positive V th shift associated with SS decay. The NBIS-caused situation showed a progressive deterioration with increasing stress duration. A similar observation was made in the 45 nm T ITZO case. Moreover, these phenomena were further amplified. With the increase in T ITZO , conditions relating to the positive movement of V th and the degradation of SS value remarkably recovered. In For100 nm T ITZO TFT, the instability that originated from NBIS was considerably suppressed. In the subsequent reverse scan (Figure 3e-h), the SS decay phenomenon was then generally restored. For the thin 25 nm T ITZO particularly, the V th positively changed 3.72 V with a stable SS. When the T ITZO slightly thickened to 45 nm, the range of the V th positive shift extended to 8.00 V, and the signs of SS decline were still found after a 5000 s stress duration. In addition, as the channel continued to thicken to 75 and 100 nm, similar phenomena were still observed. The changes in V th drift reduced to 1.47 and 1.98 V, as plotted in Figure 4a, and we observed a tiny SS fluctuation.

Results and Discussion
Vth positively changed 3.72 V with a stable SS. When the TITZO slightly thickened to 45 nm, the range of the Vth positive shift extended to 8.00 V, and the signs of SS decline were still found after a 5000 s stress duration. In addition, as the channel continued to thicken to 75 and 100 nm, similar phenomena were still observed. The changes in Vth drift reduced to 1.47 and 1.98 V, as plotted in Figure 4a, and we observed a tiny SS fluctuation.  Based on the results of photo-excited OFF-current and our previous research [13], we confirmed that the photo-generated holes and electrons drifted and were captured at the front-and back-interfaces under the action of the negative VGS-induced electric field, and the new donor-like defect states, such as VO + /VO 2+ , were created at the Fermi level around the turn-on voltage. In order to visually compare the role of NBIS on the hysteresis of TITZO-varied TFTs, the transfer curves that  Vth positively changed 3.72 V with a stable SS. When the TITZO slightly thickened to 45 nm, the range of the Vth positive shift extended to 8.00 V, and the signs of SS decline were still found after a 5000 s stress duration. In addition, as the channel continued to thicken to 75 and 100 nm, similar phenomena were still observed. The changes in Vth drift reduced to 1.47 and 1.98 V, as plotted in Figure 4a, and we observed a tiny SS fluctuation.  Based on the results of photo-excited OFF-current and our previous research [13], we confirmed that the photo-generated holes and electrons drifted and were captured at the front-and back-interfaces under the action of the negative VGS-induced electric field, and the new donor-like defect states, such as VO + /VO 2+ , were created at the Fermi level around the turn-on voltage. In order to visually compare the role of NBIS on the hysteresis of TITZO-varied TFTs, the transfer curves that  Based on the results of photo-excited OFF-current and our previous research [13], we confirmed that the photo-generated holes and electrons drifted and were captured at the front-and back-interfaces under the action of the negative V GS -induced electric field, and the new donor-like defect states, such as V O + /V O 2+ , were created at the Fermi level around the turn-on voltage. In order to visually compare the role of NBIS on the hysteresis of T ITZO -varied TFTs, the transfer curves that were scanned from two different directions were combined (Figure 3i-l), and the corresponding hysteresis was quantitatively calculated (Figure 4b). For the 25 nm T ITZO TFT, the combined actions of the created defect states and the captured photo-generated holes resulted in SS decay and the negative shift in V th for the curves when V GS was scanned from −10 to 20 V. After the forward scan, the V O + /V O 2+ defect states stabilized due to a high V GS . However, the trapped holes at the front-interface were hard to desorb. Simultaneously, the electrons excited from deep level states were driven by electric field  Figure 5a. We found that the T ITZO of 45 nm is a point of inflection because it is close to the Debye length (~40 nm), which has the longest transmission path [16]. When the T ITZO thickened to 75 and 100 nm, far from the Debye length, it was difficult for the excited electrons to be trapped at the back-channel interface, thereby resulting in the slight movement of the reverse-swept transfer curves. At the same time, these free electrons stabilized the created V O + /V O 2+ defects and recombined some of the photo-generated holes, which is sketched in Figure 5b, consequently contributing to the relatively small hysteresis values of 1.99 and 2.36 eV, respectively.
hysteresis was quantitatively calculated (Figure 4b). For the 25 nm TITZO TFT, the combined actions of the created defect states and the captured photo-generated holes resulted in SS decay and the negative shift in Vth for the curves when VGS was scanned from −10 to 20 V. After the forward scan, the VO + /VO 2+ defect states stabilized due to a high VGS. However, the trapped holes at the front-interface were hard to desorb. Simultaneously, the electrons excited from deep level states were driven by electric field and trapped at the back-interface. The number of captured electrons increased with the extension of NBIS duration, leading to the positive shift in curves without SS deterioration. In combination with all the factors, a 4.69 V hysteresis was obtained after a duration of 10 4 s NBIS. The above analysis is also applicable to the 45 nm TITZO device, where all the factors amplified and intensified, including electron/hole trapping and VO + /VO 2+ creation, which facilitated a severe hysteresis of 7.78 V, as described in Figure 5a. We found that the TITZO of 45 nm is a point of inflection because it is close to the Debye length (~40 nm), which has the longest transmission path [16]. When the TITZO thickened to 75 and 100 nm, far from the Debye length, it was difficult for the excited electrons to be trapped at the back-channel interface, thereby resulting in the slight movement of the reverse-swept transfer curves. At the same time, these free electrons stabilized the created VO + /VO 2+ defects and recombined some of the photo-generated holes, which is sketched in Figure 5b, consequently contributing to the relatively small hysteresis values of 1.99 and 2.36 eV, respectively. On the basis of all the results, the high-performance ITZO TFTs were produced by tailoring the TITZO. For traditional a-IGZO TFTs, their overall performance is commonly improved by introducing other elements, optimizing the post-treatment process, and adjusting the test conditions. However, these devices, especially for a TITZO of 100 nm, are successfully implemented with perfect initial electrical performance and relative long-range NBIS stability without further modification and treatment.

Conclusions
The sputtering processed ITZO films with different TITZO for TFT application were reported. By using the traditional bottom-gate top-contact device architecture, the ITZO-based TFTs exhibit outstanding electrical properties, specifically with regard to the 100 nm TITZO case, which are generally superior to the performance of reviewed ITZO TFTs. Furthermore, the roles of photo-excited OFF-current and NBIS-provoked instability in TITZO-varied TFTs were determined. The leakage current analysis revealed that many more electrons are generated for the thicker TITZO (a) T IGZO < 45 nm On the basis of all the results, the high-performance ITZO TFTs were produced by tailoring the T ITZO . For traditional a-IGZO TFTs, their overall performance is commonly improved by introducing other elements, optimizing the post-treatment process, and adjusting the test conditions. However, these devices, especially for a T ITZO of 100 nm, are successfully implemented with perfect initial electrical performance and relative long-range NBIS stability without further modification and treatment.

Conclusions
The sputtering processed ITZO films with different T ITZO for TFT application were reported. By using the traditional bottom-gate top-contact device architecture, the ITZO-based TFTs exhibit outstanding electrical properties, specifically with regard to the 100 nm T ITZO case, which are generally superior to the performance of reviewed ITZO TFTs. Furthermore, the roles of photo-excited OFF-current and NBIS-provoked instability in T ITZO -varied TFTs were determined. The leakage current analysis revealed that many more electrons are generated for the thicker T ITZO device. When the T ITZO is close to Debye length, we used NBIS-caused hysteresis to analyze the instability. NBIS-induced stability deteriorations are remarkably improved in the thicker T ITZO TFTs. This study demonstrates the value of T ITZO for tailoring photo-induced degradation mechanisms in the devices and facilitates the commercialization of high-performance ITZO TFT-based FPDs.