【Member Papers】Structural and electrical characterization of homoepitaxial (⁠-102) β-Ga₂O₃ layers grown by halide vapor phase epitaxy using synchrotron x-ray topography and emission microscopy

      Researchers from the Novel Crystal Technology, Inc. have published a dissertation titled "Structural and electrical characterization of homoepitaxial (⁠-102) β-Ga₂O₃ layers grown by halide vapor phase epitaxy using synchrotron x-ray topography and emission microscopy" in Applied Physics Letters.

Background

      Beta-gallium oxide (β-Ga₂O₃) is an emerging ultra-wide bandgap (UWBG) semiconductor characterized by a bandgap of approximately 4.8 eV and a high critical electric field strength of 8MV/cm, both of which are higher than those of SiC and GaN. An additional significant advantage of β-Ga₂O₃ is that its single crystal can be achieved by melt growth, which enables large-scale production of high-quality material at low cost. Various melt-growth methods have been employed to synthesize β-Ga₂O₃ single crystals, including edge-defined film-fed growth (EFG), floating zone (FZ), Czochralski (CZ), and vertical Bridgman (VB) techniques.7–11 Among these methods, EFG has been adopted for industrial-scale production due to its high crystal quality and high growth rate. As a result, (001)-oriented β-Ga₂O₃ EFG substrates with diameters up to 4 in. are already commercially available. These merits have accelerated the development and research of β-Ga₂O₃-based devices. To date, besides (001)-oriented β-Ga₂O₃ substrates, (-102)-oriented substrates have also attracted considerable attention as a promising alternative due to the following advantages: Large-size (-102) substrates can be achieved using the EFG method similar to that established for (001) substrates. They also demonstrate a higher polishing rate, resulting in ease of wafer processing. Moreover, (-102) β-Ga₂O₃ substrates enable smooth (100)-sidewalls, making them suitable for the fabrication of vertical fins and trench applications. However, the suitability of this orientation for practical device applications, as well as the influence of crystal defects in the (-102) β-Ga₂O₃ epitaxial layer on device performance, is yet to be thoroughly investigated. Therefore, this work provides a comprehensive study of the (-102) wafer, covering substrate and epitaxial evaluation, SBD characterization, and the identification of killer defects.

Abstract

      This study demonstrates that (⁠-102) β-Ga₂O₃ is a promising candidate for homoepitaxial growth via halide vapor phase epitaxy. Synchrotron x-ray rocking curve measurements confirmed a uniform full-width at half-maximum of approximately 16 arc sec across the wafer for both the substrate and the epitaxial layer. A donor concentration in the range from 7 × 10¹⁵ to 1 × 10¹⁶ cm⁻³ was confirmed by capacitance–voltage measurements of the Schottky barrier diodes (SBDs). Among the 35 SBDs, 32 exhibited a leakage current density lower than the detection limit under a reverse bias of −300 V, resulting in a high yield of approximately 91%. Synchrotron x-ray topography observation further identified that residual polycrystalline defects within the epitaxial film, which persist even after chemical mechanical polishing, were responsible for the reverse leakage current of the SBDs.

Conclusion

      In conclusion, we confirmed that (-102) β-Ga₂O₃ is a promising candidate for HVPE homoepitaxial growth. Synchrotron XRC measurements confirmed a uniform FWHM of approximately 16 arc sec across the wafer for both the substrate and the epitaxial layer, indicating high crystallinity and showing no significant degradation of crystal quality occurred during HVPE growth. Additionally, C-V measurements of the SBDs revealed a donor concentration ranging from  7 × 10¹⁵ to 1 × 10¹⁶ cm⁻³. Among the 35 SBDs, three exhibited high leakage current in the reverse-bias region, resulting in a high yield of approximately 91%. Furthermore, XRT measurement of the as-grown epilayer revealed that the residual polycrystalline defects remaining after the CMP process are responsible for the reverse leakage current observed in HVPE (-102) β-Ga₂O₃ SBDs. The effectiveness of CMP in fully eliminating polycrystalline defects may vary from defect to defect, depending on the timing of their formation during HVPE growth. However, to date, these polycrystalline defects are commonly confirmed to originate at the substrate/epilayer interface, which poses a significant challenge for their complete removal from the epilayer through CMP.