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

      Researchers from the  Linköping University have published a dissertation titled "Effect of substrate pretreatment on the epitaxial growth of κ-Ga₂O₃ layers on sapphire by halide vapor phase epitaxy" in Journal of Crystal Growth.

Background

      Gallium oxide (Ga₂O₃) is an ultra-wide bandgap semiconductor attracting significant attention due to its high breakdown electric field, making it vital for modern high-voltage, energy-efficient power electronics. Ga₂O₃ can crystallize in several polymorphs (α, β, ε, γ, κ), and even more phases are predicted from first-principles calculations. Among these, a stable monoclinic β-phase, with a bandgap of ∼ 4.8 eV and a breakdown field of ∼ 8 MV/cm, has been widely studied and shows performance potential exceeding that of SiC and GaN. However, other Ga₂O₃ phases, such as the metastable α, ε or κ polytypes, are also attracting increasing interest. The next most studied phase after the β-polymorph is the α-phase, which has a rhombohedral (corundum) structure, a lower growth temperature of ∼ 500 °C, and a wider bandgap of ∼ 5.3 eV or higher, making it highly promising for power devices.

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.

Highlights

      ● Ga₂O₃ is a promising ultra-wide band gap semiconductor for high-power and high-voltage applications.

      ● Metastable α- and κ-Ga₂O₃ layers were grown on sapphire by HVPE at 575 °C with smooth morphology.

      ● Substrate pretreatment in hydrogen or oxygen at 1000 °C is critical for achieving high-quality single-crystal κ-Ga₂O₃.