(Invited) Recent Progress on Anisotropic Electrical and Optical Studies in Ultrawide Bandgap β-Ga2O3 Electronics and Photonics

Abstract

Ultrawide bandgap semiconductor beta-phase gallium oxide (β-Ga2O3) has recently garnered significant research interests in various electronic and photonic devices such as power devices, RF electronics, photodetectors, self-powered sensors, and high temperature and radiation-hardened devices. This is mainly driven by two factors. The first factor is β-Ga2O3’s advantageous material properties, including large bandgap (4.8 eV), high critical electric field (8 MV/cm), high device figure of merits, and excellent material stability in harsh environments. The second factor is that cost-effective bulk β-Ga2O3 substrates are commercially available, and high-quality β-Ga2O3 epilayers can be grown using various methods (e.g., MOCVD, MBE, and HVPE) for electronic and photonic device fabrication. One of the most important aspects to consider for β-Ga2O3 device design and fabrication is crystal orientation. Since β-Ga2O3 crystalizes in a highly asymmetric monoclinic structure, there are many selections available in terms of crystal orientations such as (010), (-201), (001), and (100). These different crystal orientations may lead to anisotropic electrical and optical properties in β-Ga2O3 electronics and photonics, which warrants a systematic study. This talk will discuss our recent studies of electrical and nonlinear optical properties in β-Ga2O3 with different crystal orientations and their effects on β-Ga2O3 devices. In the first part of the talk, a comparative investigation will be presented for vertical (010) and (-201) β-Ga2O3 Schottky barrier diodes, where their temperature-dependent forward and reverse I-V and C-V characteristics are compared. At forward bias, they showed different on-resistances, turn-on voltages, electron mobilities, and Schottky barrier heights. At reverse bias, different leakage currents, breakdown voltages, and leakage mechanisms were observed in the two devices. These discrepancies are ascribed to different atomic configurations and dangling bonds, and distinct surface band bending revealed by x-ray photoelectron spectroscopy (XPS) on the two surfaces of different crystal orientations. In addition, we also examined the deep-level defects in β-Ga2O3 with different crystal orientations using deep level transient spectroscopy (DLTS). The second part of the talk will focus on the nonlinear optical properties of (010) and (-201) β-Ga2O3, including two-photon absorption coefficient, Kerr nonlinear refractive index, and their polarization dependence. (-201) β-Ga2O3 has a smaller two-photon absorption coefficient, a larger Kerr refractive index, and strong in-plane nonlinear optical anisotropy. Interestingly, the two-photon absorption coefficient of β-Ga2O3 is 20x smaller than wide bandgap GaN, indicating its potential for low-loss waveguides, stable resonators, and integrated photonics. We recently demonstrated β-Ga2O3 optical waveguides with a low loss of 3.7 dB/cm in the ultraviolet to near infrared spectral region. More investigations are expected in this area as β-Ga2O3 electronics and photonics research progresses. And the choice of crystal orientations for the mass production of β-Ga2O3 devices rests on applications, device designs, and costs of substrate of a certain crystal orientation.