Temperature and doping concentration dependence of the energy band gap in β-Ga_2O_3 thin films grown on sapphire

Abstract

This paper presents the effects of temperature and n-type doping concentration on the energy band gap of β-Ga2O3 thin films grown on c-plane sapphire substrates by low pressure chemical vapor deposition (LPCVD). The β-Ga2O3 thin films were grown using high purity gallium (Ga) and oxygen (O2) as precursors, and Si as the n-type dopant. The transmission electron microscopy (TEM) diffraction pattern showed that the thin films are single crystals that have a monoclinic crystal structure. The dependence of the energy band gap on temperature and n-type doping concentration have been experimentally determined from photoluminescence excitation (PLE) and absorbance spectra. The PLE spectra were measured in the temperature range of 77-298 K. The results indicate that both temperature and carrier concentration play important roles in determining the energy band gap of β-Ga2O3 thin films. The optical gap increased with the electron concentration for ne ≤ 2.52x1018 cm−3, which is due to the dominant Burstein-Moss (BM) shift. The sudden decrease in the energy gap at a doping concentration of 6.23x1018 – 3.05x1019 cm−3 is consistent with the theoretical prediction of Mott criterion for Ga2O3 semiconductor-metal transition. The energy band gap shrinks with an increasing temperature from 77 to 298 K.