Since the thin film transistors (TFTs) using amorphous oxide semiconductor (AOS) was firstly reported in 2004[1], amorphous In–Ga–Zn–O (a-IGZO) have been studied intensively because they have large advantageous features such as large electron mobilities (larger than 10 cm2/Vs), small subthreshold voltage swing, and low temperature fabrication process [2,3]. A-IGZO with the nominal chemical composition of In:Ga:Zn=1:1:1 is now commercialized for liquid crystal displays (LCDs) for smart phone or high-resolution TV and large-size organic light-emitting diode displays, etc..

Although the a-IGZO TFTs are already commercialized, stability is still a critical issue if we use it for next generation display such as transparent or flexible. In particular, it is recognized that the negative shift of the threshold voltage (V_th) is associated with the excitation of subgap photons from deep-energy defects just above the valence band maximum (called near-VBM defects), which was firstly found by hard X-ray photoemission spectroscopy (HAXPES) [4].

We recently reported that a-IGZO deposited at room temperature has large film-density-distribution in the depth direction [5]. In this study, we measured depth profile of near-VBM states including IGZO/substrate interface. It is important to understand the depth distribution also of defect density, because mobile electrons in field-effect device are induced at vicinity of substrate in case of bottom gate TFT structure. Although depth analysis was already reported for a-IGZO deposited by pulsed laser deposition (PLD), we report for a-IGZO deposited by sputtering method which is standardly used in mass-production process. As a result, we found that the depth profile is different between PLD and sputtering method.

We used radio-frequency (RF) magnetron sputtering to deposit a-IGZO films on Si wafer with 150 nm SiO₂ at room temperature. RF power, oxygen flow rate ratio (Ar:O₂), deposition pressure were fixed at 70 W, 97:3 and 0.55 Pa, respectively. HAXPES measurements were carried out at BL15XU undulator beamline (hv = 6 keV) in SPring-8. Film thickness dependences were observed for 5 – 100 nm a-IGZO films. For extracting buried spectra, we used total reflection X-ray to modify the penetration depth [6].

From thickness dependence of VB spectra, we found that the near-VBM defect density increase with decreasing film thickness. It may suggest that a-IGZO at the vicinity of substrate has large defect density compared to bulk region. To separate the effect of surface and buried region, we extracted the buried spectra using total reflection of X-ray in HAXPES measurement and difference method. We successfully observed the depth profile in wide range from 0 to 100 nm and found that the defect density at the vicinity of substrate is several times higher than that at thick enough region. We will discuss more details including the relationship between the microstructure and near-VBM defects at the conference.

References

[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya,M. Hirano, H. Hosono, Nature 432 (2004) 488–492.

[2] K. Nomura, A. Takagi, T. Kamiya, H. Ohta, M. Hirano, H. Hosono, Jpn. J. Appl. Phys. 45 (2006) 4303–4308.

[3] H. Yabuta, M. Sano, K. Abe, T. Aiba, T. Den, H. Kumomi, K. Nomura, T. Kamiya, H. Hosono, Appl. Phys. Lett. 89 (2006) 112123.

[4] K. Nomura, T. Kamiya, H. Yanagi, E. Ikenaga, K. Yang, K. Kobayashi, M. Hirano, H. Hosono, Appl. Phys. Lett. 92 (2008) 202117.

[5] K. Ide, M. Kikuchi, M. Ota, M. Sasase, H. Hiramatsu, H. Kumomi, H. Hosono, T. Kamiya, Jpn. J. Appl. Phys. 56, (2017) 03BB03.

[6] S. Ueda, 7th International Conference on Hard X-ray Photoelectron Spectroscopy (2017).