Effect of substrate pretreatment on the epitaxial growth of κ-Ga2O3 layers on sapphire by halide vapor phase epitaxy

Abstract

Ultra-wide band gap semiconductor Ga₂O₃ emerges as a promising material for next generation high-power devices. Metastable α- and κ-phases of Ga₂O₃ are obtained by halide vapor phase epitaxy (HVPE) on sapphire (0001) substrates at a growth temperature of 575 °C. Substrate annealing at 1000 °C in a hydrogen or oxygen atmosphere prior to growth is critical for achieving single-crystal κ-Ga₂O₃ layers, as HVPE on untreated substrates results in the formation of single-crystal α-Ga₂O₃, as confirmed by X-ray diffraction measurements. The impact of substrate pretreatment on layer quality and phase transformation is further investigated across growth temperatures ranging from 670 °C to 970 °C. The results demonstrate that pretreatment of sapphire substrates in oxygen or hydrogen facilitates the synthesis of single crystal κ-Ga₂O₃, while it is unfavorable for the formation of β-phase. Cathodoluminescence measurements reveal that the κ-phase exhibits notable emission differences, with a peak maximum around ∼ 550 nm, compared to the β-phase and α-phase, which exhibit maximum emissions at ∼ 370 nm and 414 nm, respectively. Absorption measurements yield optical band gap values of ∼ 5.12 eV for the α-phase and ∼ 4.68 eV for the κ-phase. These findings provide valuable insights into controlling metastable phase synthesis in HVPE growth on sapphire.