The present study investigates the NBIS degradation of a-IGZO TFTs with heterojunctions as buried active channels. Drain current and gate–source voltage (ID–VGS) measurements were used to determine electrical properties and transfer characteristics. In addition, capacitance–voltage (C–V) measurements were used to verify the degradation mechanism.
Cross-sectional diagrams of the single-channel (control group) and dual-channel a-IGZO TFTs used in the present study are shown in Fig. 1(a). These top-gate a-IGZO TFTs are fabricated on a glass substrate with a 10 μm channel length and 15 μm channel width. Production of the dual-channel a-IGZO TFTs commenced with the radio frequency magnetron sputter deposition of 20 and 40 nm a-IGZO (In2O3:Ga2O3:ZnO = 1:1:1 mol. %) channel layers with high oxygen flow (HOF, Ar: O2 = 50:14) and low oxygen flow (LOF, Ar: O2 = 50:6). In contrast, production of the single-channel a-IGZO TFTs commenced with the sputter deposition of a 60 nm thick a-IGZO channel layer with LOF. A plasma-enhanced chemical vapor deposition (PECVD) method was then used to deposit a 140 nm thick SiOX as a gate insulator (GI) layer on the a-IGZO. A patterned metal gate consisting of 30 nm thick Ti and 250 nm thick MoN was subsequently deposited on the GI layer by sputtering. Thereafter, a 300 nm thick interlayer dielectric (ILD) of SiOX was deposited over the metal gate by PECVD. A contact-type MoN source and drain electrode (S/D metal) was subsequently deposited on the ILD layer by sputtering. Finally, a 200-nm-thick SiOX passivation layer was formed on the surface of the ILD SiOX layer.The transfer characteristics of the dual-channel a-IGZO TFTs were examined using an Agilent B1500 semiconductor parameter analyzer with a Cascade M150 microprobe station. These transfer characteristics were obtained under illumination stress using a 375 nm ultraviolet light emitting diode (UV LED). Transfer characteristic curves were obtained in a dark environment under light stress of VGS = −15–15 V and VDS = 0.1 V. In this instance, threshold voltage (Vth) was defined as the drain current (ID) at 1 nA.
Figure 1(b) presents the energy band diagram for the dual-channel a-IGZO TFTs. At the interface between the HOF and LOF layers in the a-IGZO TFTs, an oxygen concentration difference produced an offset bandgap. This is due to the HOF layer having higher oxygen content, it has a wider bandgap. In contrast with the HOF layer, the LOF layer has lower oxygen content, so it has a narrower bandgap. This bandgap yielded conduction band discontinuities, which formed energy barriers that confined carriers in the LOF layer. These carriers were kept away from interfacial defects, and as a result, Coulomb scattering was avoided.Figure 2 shows the transfer characteristic curves for the single-channel and dual-channel a-IGZO TFTs with equal dimensions. In Fig. 2, the solid circles represent ID, and the open circles of the same color represent μFE. For the single-channel a-IGZO TFTs, μFE = 7.91 cm2/V s at VGS = Vth+15 V and VDS = 0.1 V. For the dual-channel ones, μFE is 12.29 cm2/V s at VGS = Vth+15 V and VDS = 0.1 V. The dual-channel a-IGZO TFTs exhibited a 1.5× enhancement in μFE. This enhancement can be attributed to the carriers confined to the buried LOF channel layer with a discontinuous conduction band.Figures 3(a) and 3(b), respectively, show the transfer characteristic curves of the dual-channel and single-channel a-IGZO TFTs under NBIS with 375-nm UV light for 1000 s (at VG = −30 V + Vth, VD = 0 V, and VS = 0 V). In Fig. 3(a), the solid circles represent forward sweeps, and the open circles of the same color represent reverse sweeps. The transfer characteristic curves illustrate significant NBIS degradation of the dual-channel a-IGZO TFTs. An abnormal hump was detected after short periods of stress. On the contrary, a clockwise hysteresis loop was observed after long periods of stress. This hysteresis loop indicates degradation through the NBIS-induced parallel negative shift in Vth. In comparison, only a parallel negative shift in Vth is detected in the transfer characteristics in Fig. 3(b), which is consistent with the transfer characteristics of a typical a-IGZO TFT under NBIS. Through comparison of Figs. 3(a) and 3(b), it becomes apparent that the dual-channel and single-channel a-IGZO TFTs exhibit significantly different degradation characteristics under NBIS.Mechanistically, the NBIS degradation of a-IGZO TFTs could have occurred due to the entrapment of photo-induced holes at the GI/channel layer interface.33,35,3633. K. H. Lee, J. S. Jung, K. S. Son, J. S. Park, T. S. Kim, S. Lee et al., Appl. Phys. Lett. 95, 232106 (2009). https://doi.org/10.1063/1.327201535. M. P. Hung, D. Wang, J. Jiang, and M. Furuta, ECS Solid State Lett. 3, Q13 (2014). https://doi.org/10.1149/2.010403ssl36. J. Gwang Um, M. Mativenga, P. Migliorato, and J. Jang, Appl. Phys. Lett. 101, 113504 (2012). https://doi.org/10.1063/1.4751849 Alternatively, NBIS degradation could have occurred due to the accumulation of ionized oxygen vacancies (VO+ or VO2+) at the insulator/channel interface.37–4037. J. W. Jo, Y. H. Kim, and S. K. Park, Appl. Phys. Lett. 105, 043503 (2014). https://doi.org/10.1063/1.489154138. P. Migliorato, M. D. H. Chowdhury, J. G. Um, M. Seok, and J. Jang, Appl. Phys. Lett. 101, 123502 (2012). https://doi.org/10.1063/1.475223839. H. Oh, S. M. Yoon, M. K. Ryu, C. S. Hwang, S. Yang, and S. H. K. Park, Appl. Phys. Lett. 97, 183502 (2010). https://doi.org/10.1063/1.351047140. B. Ryu, H. K. Noh, E. A. Choi, and K. J. Chang, Appl. Phys. Lett. 97, 022108 (2010). https://doi.org/10.1063/1.3464964 The accumulation of oxygen vacancies and the entrapment of holes result in positive charges that generate an electric field that causes a-IGZO TFTs to turn on at an applied gate voltage less than Vth. To determine the predominant mechanism of NBIS degradation, transfer characteristics of the dual-channel a-IGZO TFTs were obtained under UV illumination stress (IS), as shown in Fig. 3(b). Based on these results, it was confirmed that the negative shift in Vth is due to electron–hole entrapment and not oxygen vacancy generation.To confirm the hump phenomenon observed in Fig. 3(a), transfer characteristics of the dual-channel a-IGZO TFTs were measured with different channel widths. Shown in Fig. 4(a) are the transfer characteristic curves obtained under NBIS for 150 s (with adjusted gate voltage = Vth). In Fig. 4(a), the current of the sub-channel is related to the channel width of the device. This result indicates that the sub-channel is associated with the surface and the buried channels instead of the side channels.Based on the electrical and transfer characteristics obtained in this study, a physical model describing the mechanism for NBIS degradation of the dual-channel a-IGZO TFTs was proposed. Before NBIS, dual-channel a-IGZO TFTs have active heterojunctions, which form an energy barrier that keeps carriers in the buried channel (LOF) layer. During NBIS, electron–hole pairs are induced by UV LED light, as shown in Fig. 4(b). Owing to the negative gate bias, these photo-induced holes become entrapped in the defects located at the GI/HOF interface in the a-IGZO TFTs. This entrapment leads to the downward bending of the energy band. Simultaneously, the repulsion of electrons under the negative gate bias voltage results in the entrapment of electrons in the defects located at the HOF/LOF interface. This entrapment leads to the upward bending of the energy band.Figure 4(c) shows the evolution of the band diagram for the dual-channel a-IGZO TFTs under stress. Under the initial stress, carriers were transported in the LOF layer. During stress for 300 s, some of the holes and electrons generated by UV light were trapped in the defects at the GI/HOF and HOF/LOF interfaces. At this moment, the slightly curved energy band results in conduction in both the surface and buried channels. Hence, a hump is detected. After the stress has been applied, the energy band bends severely because more electron–hole pairs become entrapped in the interface defects. The severe energy band bending results in preferred conduction in the surface channel over that in the buried channel. Therefore, the conduction path migrates from the LOF layer to the HOF layer, where the main current is dominated by the surface channel, and a parallel shift is subsequently observed, as shown in Fig. 4(c).Figure 5(a) shows the relationship between hysteresis and stress time. Clockwise hysteresis is generally ascribed to the trapping and de-trapping of photo-induced electron–hole pairs35,4135. M. P. Hung, D. Wang, J. Jiang, and M. Furuta, ECS Solid State Lett. 3, Q13 (2014). https://doi.org/10.1149/2.010403ssl41. M. P. Hung, D. Wang, T. Toda, J. Jiang, and M. Furuta, ECS J. Solid State Sci. Technol. 3, Q3023 (2014). https://doi.org/10.1149/2.005409jss under illumination. Hysteresis widens with increasing stress time, thus resulting in greater charge migration through channels (trapping and de-trapping). This can be attributed to the HOF/LOF interface having fewer defects then the GI/HOF interface. During stress, the conduction path migrates from the LOF layer to the HOF layer with a poorer interface quality [inset in Fig. 5(a)]. In other words, the number of trapped charges increases with rising interface defects.To verify the mechanism for NBIS degradation of the dual-channel a-IGZO TFTs, C–V measurements were performed. Figure 5(b) shows the C–V measurements of the dual-channel and single-channel (inset) a-IGZO TFTs before and after 1000-s stress, and the equivalent circuit in the dual-channel transistor is shown in Fig. 5(c), where CSiOx is the equivalent capacitance of the GI layer. Cit1 and Rit1 are the equivalent capacitance and equivalent resistance induced by interface defects between the SiOX and HOF layers. Similarly, Cit2 and Rit2 are induced by interface defects between the HOF and LOF layers. CHOF and CLOF are the capacitances of the HOF and LOF layers.Before the dual-channel device is under stress, the equivalent capacitance is that of CSiOx and CHOF in series since the carrier can conduct in the LOF layer. After stress, the conduction path migrates from the LOF layer to the HOF layer. Hence, the equivalent capacitance is that of CSiOx. In addition, when capacitors are connected in series, the total capacitance value must be less than the capacitance value of any individual capacitor in the series combination. Therefore, the capacitance value before the stress was less than that after the stress, as shown in Fig. 5(b). In comparison, the capacitance value of the single-channel a-IGZO TFTs remains the same before and after the stress [inset in Fig. 5(b)].In conclusion, a channel migration phenomenon was observed in dual-channel a-IGZO TFTs after NBIS stress. Under NBIS conditions, the separation of electron–hole pairs under an electrical field resulted in the entrapment of holes at the oxide/HOF interface and the entrapment of electrons at the HOF/LOF interface. After long-term stress, μFE was reduced, and degradation hysteresis occurred due to the accumulation of electron–hole pairs in interface defects at the oxide/HOF interface. The deterioration of electrical characteristics indicates that a severe energy band shift results in an invalid barrier height for the buried channels. The phenomenon of channel migration between the oxide/HOF interface and the HOF/LOF interface was confirmed by utilizing different device dimensions and taking capacitance–voltage measurements. As a recommendation, future work should focus on the development of heterojunction a-IGZO TFTs that can mitigate the effects of channel migration during operation. Such TFTs would be able to withstand NBIS degradation when exposed to light.
This work was performed at National Sun Yat-sen University Joint Center for high value Instruments and Taiwan Semiconductor Research Institute. The authors are grateful for the technical support from the Center for Nanoscience and Nanotechnology, National Sun Yat-sen University. The whole project was supported by the National Science and Technology Council in Taiwan.
This work was supported by the National Science and Technology Council, Taiwan, under Contract No. MOST-109-2112-M-110-015-MY3.
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Han-Yu Chang: Conceptualization (equal); Data curation (lead); Formal analysis (equal); Investigation (lead); Validation (lead); Writing – original draft (lead). Ting-Chang Chang: Conceptualization (equal); Formal analysis (equal); Resources (equal); Supervision (lead); Validation (equal). Mao-Chou Tai: Conceptualization (lead); Formal analysis (equal); Investigation (supporting); Resources (supporting). Bo-Shen Huang: Conceptualization (supporting); Investigation (supporting); Resources (supporting). Kuan-Ju Zhou: Resources (supporting); Writing – original draft (supporting). Yu-Bo Wang: Investigation (supporting); Resources (supporting). Hung-Ming Kuo: Investigation (supporting); Resources (supporting). Jen-Wei Huang: Formal analysis (equal); Resources (supporting).
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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