Investigation of the blue color center in β-Ga2O3 crystals by the EFG method

Abstract

Beta-type gallium oxide (β-Ga₂O₃) has attracted enormous attention for application as a transparent conductive oxide (TCO) semiconductor due to its wide bandgap at around 4.9 eV. However, unintentionally doped β-Ga₂O₃ crystals by the edge-defined film-fed growth (EFG) method have blue coloration in some cases, making their application as TCO a challenge. Herein, we focus on the investigation of the blue color center in β-Ga₂O₃ crystals grown by EFG. Glow discharge mass spectrometry (GDMS) was used to measure the impurity concentrations. X-ray diffraction revealed contraction of the crystal lattice in the blue area. The wet etching method was used to demonstrate the density of etch pits. The blue emission with high intensity from cathodoluminescence (CL) proved the existence of a sufficient number of vacancies in the blue area. X-ray photoelectron spectroscopy (XPS) analysis showed that Ga^I3d (Ga⁺) abounded in the blue area. Transmission spectroscopy combined with capacitance–voltage (C–V) measurement provided evidence that the blue area with lower transmittance from UV to NIR was more conductive. Ga^I3d (Ga⁺) induced by vacancies was speculated to be the blue color center. The blue color center can be removed by high-temperature annealing in an oxygen atmosphere.