Low-Temperature Processed Metal-Semiconductor Field-Effect Transistor with In-Ga-Zn-O/AgOx Schottky Gate
Authors: Magari Yusaku , Hashimoto Shinsuke , Hamada Kenichiro , Furuta Mamoru 
Published: February 2020

Amorphous oxide semiconductors (AOSs), such as In-Ga-Zn-O (IGZO), are expected to be used as a channel material for thin-film transistors (TFTs) because the IGZO TFTs exhibit field-effect motilities (μFE) of over 10 cm2/Vs and good uniformity even fabricated at room temperature [1]. Metal-insulator-semiconductor field-effect transistor (MIS-FET) has been employed widely for the IGZO MIS-FET. However, processing temperature of 300-400 °C is required to guarantee the performance and reliability of the IGZO MIS-FET. In comparison with the MIS-FET, metal-semiconductor field-effect transistor (MES-FET) has several advantages such as low processing temperature since Schottky gate can be formed at low temperature. Thus, the IGZO MES-FET is a good candidate for flexible device applications.
However, there is only a few reports for AOSs based MES-FET due to difficulty of the formation of stable and good Schottky contacts with AOSs [2, 3]. Previously, we reported the top-gate coplanar MES-FETs with an IGZO channel, which was deposited by mist chemical vapor deposition, and sputtered AgOx Schottky gate [4]. A μFE of 3.2 cm2/Vs and a subthreshold swing of 356 mV/decade were achieved. However, the maximum processing temperature of reported MES-FET was 350 °C.
In this study, we fabricated MES-FET with room-temperature deposited sputtered IGZO channel. We investigated the effect of O2 gas flow ratio during the IGZO deposition and mid-annealing at 200 °C.
A 100-nm-thick IGZO channel was deposited by dc magnetron sputtering in the mixture of Ar/O2 gas at room temperature, using a ceramic target (In2O3 : Ga2O3 : ZnO = 1 : 1: 1 atm.%). The O2 gas ratio was chosen at 1 and 0.8 % at a deposition pressure of 1 Pa. After patterning of the IGZO channel, one of the IGZO deposited at 1 % of O2 was annealed in air at 200 °C for 1 hour. Then, an AgOx Shcottky gate was deposited by reactive sputtering at room temperature and Au was deposited on the AgOx. Finally, source and drain electrodes were formed by Mo.
Fig. 1 shows I-V characteristics of the Schottky diode. The rectification ratio (VG = ±2 V) of IGZO (1% O2) and IGZO (0.8 % O2) and IGZO (1 % O2+mid-annealing at 200 °C) was 2.3×106, 2.1×107, 3.3×107, and ideality factor was 1.75, 1.41, 1.39, respectively. Fig. 2 shows transfer characteristics of the IGZO MES-FETs with W/L of 100/10 µm. The electrical properties of the TFTs and Hall effect measurement results of IGZO channels were summarized in Table I. From the TFT characteristics, it was confirmed the Von shifted from 0.4 to 0.2 V when O2 ratio decrease from 1 to 0.8 %. When the IGZO was annealed at 200 °C, the on-current markedly increased accompanied by the negative Von shift. The μFE of the IGZO MES-FET, Hall carrier concentration and Hall mobility of the IGZO films were also summarized in Table I. The μFE and Von of the IGZO MES-FET were well correlated with the Hall mobility and carrier concentration of the IGZO channel, respectively. Detail of the results will be discussed in the conference.

Acknowledgement

Authors would like to thank Drs. Giang T. Dang and Martin W. Allen, University of Canterbury, New Zealand, for their helpful discussions regarding formation process and mechanism of the Schottky contact with AOS.
[1] 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. 25, pp. 488–492, (2004).
[2] M. Lorenz, A. Lajn, H. Frenzel, H-V. Wenckstern, M. Grundmann, P. Barquinha, R. Martins. And E. Fortunato, “Low-temperature processed Schottky-gated field-effect transistors based on amorphous gallium-indium-zinc-oxide thin films” Appl. Phys. Lett., vol. 97, no. 10, pp. 243506-1–243506-3, (2010)
[3] D-H. Lee, K. Nomura, T. Kamiya, and H. Hosono, “Metal-Semiconductor Field-Effect Transistor Made Using Amorphous In-Ga-Zn-O Channel and Bottom Pt Schottky Contact Structure at 200°C” ECS Solid State Lett., vol. 1, no. 1, pp. Q8–Q10, (2012).
[4] G-T. Dang, T. Kawaharamura, M. Furuta, Member, IEEE, and M-W. Allen, “Metal–Semiconductor Field-Effect Transistors With In–Ga–Zn–O Channel Grown by Nonvacuum-Processed Mist Chemical Vapor Deposition,” IEEE Electron Device Lett., vol. 36, no. 5, pp. 463-465 (2015).

Figure 1