【Member Papers】Edge-Dependent Stacking Fault Upward Transformation in Pseudo-Symmetric (100) β-Ga₂O₃ HVPE Epitaxy: A Common Source of Hillocks

      Researchers from the Zhejiang University have published a dissertation titled "Edge-Dependent Stacking Fault Upward Transformation in Pseudo-Symmetric (100) β-Ga₂O₃ HVPE Epitaxy: A Common Source of Hillocks" in Acta Materialia.

Background

      Beta-phase gallium oxide (β-Ga₂O₃) is a semiconductor material with ultra-wide bandgap of ∼4.8 eV, high critical breakdown field of 8 MV/cm, and optical absorption edge near 260 nm, making it highly attractive for applications in Schottky barrier diodes (SBDs), metal–oxide–semiconductor field effect transistors (MOSFETs) and solar-blind photoelectric devices. To achieve outstanding performance, the development of device-quality β-Ga₂O₃ epilayer is imperative. Halide vapor phase epitaxy (HVPE) has been investigated and successfully commercialized due to its much faster growth rate and capability to produce thick epilayers, achieving a dramatic growth rate up to 250 μm/h. To date, HVPE homoepitaxial growth had already been widely explored for (001) and (010) oriented β-Ga₂O₃ planes. However, HVPE growth on (100) plane, which is a promising plane for commercializing due to its easy size-enlarged and wafer processing, remains largely unexplored.

Abstract

      Stacking faults pose a major challenge in the epitaxial growth of β-Ga₂O₃ thick films on the (100) planes, as they degrade the surface morphology and severely deteriorate the performance of power devices. In this work, we systematically investigate the formation, evolution and electrical influence of stacking faults and related defects in (100) β-Ga₂O₃ homoepitaxial layers grown by halide vapor phase epitaxy (HVPE). Using multiple characterization techniques, we reveal that the commonly observed stacking fault SF_100-B undergoes an edge-dependent upward transformation into out-of-phase boundaries (OPBs) during epitaxial growth. The formation of OPBs directly trigger the nucleation of pyramidal hillocks on the epitaxial surface and act as killer defect, leading to premature breakdown in Schottky barrier diodes. Atomic-scale analysis demonstrates that SF_100-B originates from consecutive twinning processes on the pseudo-symmetric (100) plane of β-Ga₂O₃, whereas its mismatch with the perfect lattice at (-201) facets drives the formation of OPBs. Furthermore, two key parameters, the Ga adatom diffusion lengthand the terrace width, were demonstrated directly related to the formation of SF_100-B. Accordingly, two effective strategies, reducing the Ⅵ/Ⅲ ratio and increasing the offcut angle, were developed to suppress stacking faults. The insights gained from this work offer valuable guidance for engineering defect-controlled surfaces to enable high-performance β-Ga₂O₃-based power electronic devices.

Conclusions

      In conclusion, we systematically investigated the origin of pyramidal hillock defects on the [00-1]-offcut (100) β-Ga₂O₃ epitaxial layer growth by HVPE. It unveils that in addition to dislocations, out-of-phase boundary (OPB) constitutes a new source of hillocks formation. In β-phase homoepitaxial layers, OPBs are formed by shear slip along the translation vector of 1/6[132], leading to in-phase overlapping of octahedron Ga atoms on (100)-A and (100)-B, accompanied by a characteristic overlaid structure. Furthermore, these OPBs were demonstrated to be detrimental to the leakage current of SBD devices, emphasizing the controlling of OPBs during (100) β-Ga₂O₃ epitaxial growth. Atomic-scale investigations via HRSTEM were performed to elucidate the underlying formation mechanisms of OPBs. We found that the exposed facet at step edge dominates whether the upward transformation of SF_100-B yields OPB or SF₀₀₁, depending on the offcut direction of (100) substrate towards [00-1] or [001]. The formation of SF_100-B is driven by an atomic-scale consecutive twinning process on the pseudo-symmetric (100) plane. Subsequently, SF_100-B will simultaneously upward transformation into OPB when it impinges with the (-201) step facet of normal lattice, which ultimately gives rise to pyramidal hillocks on the epilayer surface. Therefore, the behavior of stacking faults on the [00-1]-offcut (100) β-Ga₂O₃ is fully clarified. Moreover, it is proposed that the formation of SF_100-B is directly related to the balance between two critical parameters: the Ga adatom diffusion length and the width of terrace. Based on this principle, two corresponding effective strategies, lower Ⅵ/Ⅲ ratios (e.g., VI/III=10) and larger offcut angles (up to 6°), were developed to achieve hillock-free (100) β-Ga₂O₃ epitaxy.