Conversion of an ultra-wide bandgap amorphous oxide insulator to a semiconductor

The variety of semiconductor materials has been extended in various directions, for example, to very wide bandgap materials such as oxide semiconductors as well as to amorphous semiconductors. Crystalline β-Ga2O3 is known as a transparent conducting oxide with an ultra-wide bandgap of ~4.9 eV, but amorphous (a-) Ga2Ox is just an electrical insulator because the combination of an ultra-wide bandgap and an amorphous structure has serious difficulties in attaining electronic conduction. This paper reports semiconducting a-Ga2Ox thin films deposited on glass at room temperature and their applications to thin-film transistors and Schottky diodes, accomplished by suppressing the formation of charge compensation defects. The film density is the most important parameter, and the film density is increased by enhancing the film growth rate by an order of magnitude. Additionally, as opposed to the cases of conventional oxide semiconductors, an appropriately high oxygen partial pressure must be chosen for a-Ga2Ox to reduce electron traps. These considerations produce semiconducting a-Ga2Ox thin films with an electron Hall mobility of ~8 cm2V−1 s−1, a carrier density Ne of ~2 × 1014 cm−3 and an ultra-wide bandgap of ~4.12 eV. An a-Ga2Ox thin-film transistor exhibited reasonable performance such as a saturation mobility of ~1.5 cm2 V−1 s−1 and an on/off ratio >107. Reversing the usual role of oxygen during deposition of semiconducting thin films makes it easier to produce bendable electronics. Crystalline gallium oxide is a large-bandgap semiconductor that can handle the strong electric fields used to supply power components. However, it usually turns into an insulator when produced as amorphous, flexible thin films. Now, Junghwan Kim from the Tokyo Institute of Technology in Japan and colleagues have found that high partial pressures of oxygen gas during rapid laser deposition transforms amorphous gallium oxide films into semiconductors. While most oxide films are manufactured under low-oxygen conditions, the researchers discovered their procedure suppresses the formation of electron-trapping defects that diminish conductivity. Prototype thin-film transistors and diodes demonstrated the potential for using amorphous gallium oxide for high-efficiency devices. Crystalline Ga2O3 is an ultra-wide bandgap oxide semiconductor, but electron conduction in amorphous Ga2Ox has never been attained to date. Here we succeeded in converting amorphous Ga2Ox to a semiconductor with the bandgap of ~4.12 eV and the electron mobility ~8 cm2 V−1 s−1. The key is to suppress charge compensation defects by increasing the film density and suppress formation of oxygen-poor and oxygen-excess defects. It produces thin-film transistors and Schottky diodes with high on currents and on/off ratios.