Recent progress in Amorphous Oxide Semiconductors
Toshio Kamiya
*1,2)
and Hideo Hosono
1,2)
1)
Laboratory for Materials and Structures, Tokyo Institute of Technology, Mailbox R3-4,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
2)
Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
* E-Mail:
TEL: +45-924-5357 FAX: +45-924-5350
Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) are employed in
state-of-the-art flat-panel displays including very high-resolution (460 ppi) LCDs and jumbo
size (77”) OLED TVs [1,2]. In this paper, we will first introduce the present status of display
technology mainly focusing on AOS-based displays.
Then, we will review the present understanding about electronic structure and defect states
in AOSs. In particular, we have clarified that three major factors, oxygen-related defects,
hydrogen-related impurities, and packing density of AOS films, are important to understand
and control the device performances and stability of AOS TFTs [3,4]. Fabricating amorphous
In-Ga-Zn-O (a-IGZO) thin films under an oxygen-poor condition, some oxygen deficiencies
produced shallow donors, but other ones form deep electron traps, leading to strong charge
compensation and insulating a-IGZO films if the film density is low. On the other hand, an
oxygen-rich condition introduces excess/weakly-bonded oxygen and those also work as
electron traps [5] and recombination centers. Some hydrogen passivates defects and improve
TFT performances, but some other degrade the TFT stability. As described above, low film
density generates electron traps, and also such flexible structure is an origin of TFT instability
in particular for photon excitation.
These knowledges about defects are very useful to develop new AOS materials. To date,
semiconducting pure a-GaO
x
were not reported, which is considered due to its large bandgap
(4.9 eV for crystalline Ga
2
O
3
) but ultra-wide bandgap AOS would be useful as it provides
more flexibility in device design. We have clarified that this is due to charge compensation by
electron traps originating from low film density, oxygen deficiency, and excess oxygen.
Therefore, formation of high-density films and fine tuning of oxygen pressure during pulsed
laser deposition produces ultra-wide bandgap (4.12 eV) semiconducting a-GaO
x
films [6]. As
AOS films have low defect densities, AOSs are expected also as hosts of thin-film phosphors.
Actually, we have succeeded in producing light-emitting AOS thin films at room temperature
on glass by doping rare-earth active centers [7,8]. At the conference, we will explain their
fabrication conditions are also controlled by the above defects.
[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature
432
(2004) 488.
[2] T. Kamiya, K. Nomura, H. Hosono, Sci. Technol. Adv. Mater.
11
(2010) 044305.
[3] T. Kamiya, H. Hosono, ECS Transactions
54
(2013) 103.
[4] T. Kamiya, H. Kumomi, H. Hosono, Digest of IDW’14, AMD2-1
[5] K. Ide, Y. Kikuchi, K. Nomura, M. Kimura, T. Kamiya, H. Hosono, Appl. Phys. Lett.
99
(2011)
093507.
[6] J. Kim, N. Miyokawa, T. Sekiya, K. Ide, Y. Toda, H. Hiramatsu, H. Hosono, T. Kamiya, Thin Solid
Films
614
(2016) 84.
[7] J. Kim, N. Miyokawa, K. Ide, Y. Toda, H. Hiramatsu, H. Hosono, T. Kamiya, AIP Advances
6
(2016) 015106.
[8] J. Kim, N. Miyokawa, K. Ide, H. Hiramatsu, H. Hosono, T. Kamiya, J. Cerm. Soc. Jpn.
124
(2016)
532.
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