【International Papers】Defect Analysis of the β– to γ–Ga₂O₃ Phase Transition

      Researchers from the  GFZ Helmholtz Centre for Geosciences have published a dissertation titled "Defect Analysis of the β– to γ–Ga₂O₃ Phase Transition" in Advanced Functional Materials.

Background

      Throughout the history of science and engineering, there has always been a drive to find new and superior materials to advance technology. This is also true for power electronics, which needs new materials and related research activities to ensure future developments. Gallium oxide (Ga₂O₃) is a promising candidate for future ultrawide-bandgap (UWB) semiconductor materials for power electronic devices. It has two main advantages over the traditional UWB semiconductor materials, such as silicon carbide and gallium nitride. Ga₂O₃ does not only have a superior breakdown voltage, which allows to increase the device performance, but cost-effective large wafer production by melt growth methods is also possible with Ga₂O₃. Furthermore, the existence of different polymorphs of Ga₂O₃ makes it a compelling material for crystal structure engineering. This might be achieved via fabrication of different polymorphic layers on a single wafer as well as engineering of nanostructures with different polymorphs. However, most research has focused on the β phase, which is the most chemically and thermally stable polymorph of Ga₂O₃.

Abstract

      This study investigates the ion irradiation induced phase transition in gallium oxide (Ga₂O₃) from the β to the γ phase, the role of defects during the transformation, and the quality of the resulting crystal structure. Using a multi-method analysis approach including X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry in channeling mode (RBS/c), Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS), and variable energy positron annihilation lifetime spectroscopy (VEPALS) supported by density functional theory (DFT) calculations, defects at all the relevant stages of the phase transition are characterized. A reduction in backscattering yield is observed in RBS/c spectra after the transition to the γ phase. This goes hand in hand with a significant decrease in the positron trapping center density due to generation of embedded vacancies intrinsic for the γ–Ga₂O₃ but too shallow in order to trap positrons. A comparison of the observed positron lifetime of γ–Ga₂O₃ with different theoretical models shows good agreement with the three-site γ phase approach. A characteristic increase in the effective positron diffusion length and the positron lifetime at the transition point from β–Ga₂O₃ to γ–Ga₂O₃ enables visualization of the phase transition with positrons for the first time. Moreover, a subsequent reduction of these quantities with increasing irradiation fluence is observed, which attributes to further evolution of the γ–Ga₂O₃ and changes in the gallium vacancy density as well as relative occupation in the crystal lattice.

Conclusion

      In conclusion, we performed a multi-method analysis approach to investigate the defect structure of the β-Ga₂O₃ to γ-Ga₂O₃ phase transition. XRD analysis of (-201)-oriented β-Ga₂O₃ following 140 keV Ne⁺ irradiation indicates a phase transition occuring above a certain fluence threshold. TEM imaging confirms the transition from the monoclinic to a defective spinel cubic crystal structure and reveals a transformed layer approximately 260 nm thick. Enhanced crystal quality was observed in the RBS/c spectra following the completion of the γ-Ga₂O₃ phase transition. DB-VEPAS and VEPALS were employed to utilize the sensitivity of positrons as atomic-scale probes for non-destructive measurement and characterization of defects, such as single vacancies and their agglomerates. VEPfit results show a reduced defect concentration and an increased positron diffusion length after irradiation with a fluence of 3.5 × 10¹⁶ ion cm⁻², which serve as a clear signature of the ion-induced phase transition. This reduced defect concentration is also in accordance with the enhanced crystal quality observed by RBS/c, after the phase transformation. Positron lifetimes calculated for the three-site γ-Ga₂O₃ phase model show an excellent agreement with the experimental results. Notably, the reduction in positron lifetime with increasing irradiation fluence in γ-Ga₂O₃ is an unexpected behavior for many other semiconductor materials and is a unique characteristic of γ-Ga₂O₃. We attribute this to the increased formation of different gallium vacancy positions in the 3-site γ-Ga₂O₃ model at high damage levels, which have a shorter positron lifetime. Our results show that the relatively good crystalline quality as observed by XRD and RBS/c in heavily irradiated Ga₂O₃ is related to a) the change in polymorph at around 3.5 × 10¹⁶ ion cm⁻² (in the case of 140 keV Ne⁺ irradiation) and b) the dominant formation of further defects in the already defective spinel structure of γ-Ga₂O₃ at specific tetrahedral sites in the cubic lattice. At high irradiation fluences, saturation of gallium vacancies at T_{Ga2} sites and their aggregation into larger defect clusters are observed. Continued irradiation leads to the formation of single vacancies at T_{Ga1} sites. This shift in vacancy formation within the γ phase may explain and contribute to the high radiation tolerance of the β/γ dual-phase polymorphic structure.