Effects of Base Pressure on Growth and Optoelectronic Properties of Amorphous In-Ga-Zn-O: Ultralow Optimum Oxygen Supply and Bandgap Widening

摘要

It is generally thought that deposition of devicequality oxide semiconductors requires somewhat high oxygen supply in order to reduce oxygen deficiency and to suppress generation of free electrons. This paper reports that such high oxygen supply is not an essential requirement for a representative amorphous oxide semiconductor, amorphous InGaZnO (aIGZO). That is, the optimum oxygen flow rate ratio (RO2) is 3% for standard (STD) sputtering with the base pressure of 104Pa, while it is reduced dramatically to 103105% for ultrahigh vacuum (UHV) sputtering with the base pressure of 107Pa. We consider that the ultralow optimum oxygen supply originates from residual hydrogen, because the STD and the UHV films have different hydrogen contents at the orders of 1020 and 1019cm3, respectively. This comparison also suggests that the inclusion of impurity hydrogen widens the optical bandgap by 0.1eV, which causes a bandgap minimum by post annealing at 500°C for the STD aIGZO films accompanying the removal of incorporated hydrogen as H2O. Furthermore, crystallization onset temperature is reduced remarkably by 100K for the UHV aIGZO films.