Highly Asymmetric Optical Properties of β-Ga₂O₃ as Probed by Linear and Nonlinear Optical Excitation Spectroscopy

Abstract

β-Ga2O3 is a highly promising semiconductor for a deep ultraviolet emitter owing to its wide bandgap, which significantly varies in the range of 4.49 eV to 4.74 eV due to its optical trirefringence in the monoclinic crystal structure (C2h). However, dominant photoluminescence (PL) emissions in β-Ga2O3 occur far below the bandgap, where the underlying PL mechanism has been under intense debate. In order to address this issue, we monitored the most intense PL peaks at 3.0 eV and 3.5 eV at room temperature by employing both linear and nonlinear absorption spectroscopy. The PL of β-Ga2O3 was found to be highly polarized along the (102) direction regardless of input polarization, and to be essentially the same for one-photon absorption (1PA), two-photon absorption (2PA), and three-photon absorption (3PA). However, absorption polarization dependence is quite distinct for 1PA, 2PA, and 3PA, as theoretically explained by optical selection rules for C2h symmetry. Based on the input-output method, 2PA and 3PA coefficients were determined to be 𝛽 = 3.45 cm GW -1 and 𝛾 = 0.013 cm 3 GW -2 at the excitation wavelength of 460 nm and 550 nm, respectively. These values are several times higher than theoretical values predicted by two-band models, indicating that β-Ga2O3 possesses strong optical nonlinearity with high polarization contrast. The absorption power dependences for 1PA, 2PA, and 3PA indicate that optical excitation is essentially excitonic, but bound to Ga vacancies (3.0 eV) and O vacancies (3.5 eV), which is different from the so-called self-trapped exciton based on the polaron model. Our main thesis is also clearly supported by the dependence of the PL shape on the excitation depth as probed by 2PA depth scan. This implies that densities for the Ga vacancy and the O vacancy vary differently as a function of depth from the excitation surface. A broad set of highly asymmetric optical properties clarified in our work is critical for the understanding of this wide-gap semiconductor and its potential use for ultraviolet sources especially when band-edge emission is realized.

Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na1–xKxAsQ2

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Control and understanding of metal contacts to β-Ga2O3 single crystals: a review

“…Guo et al found that thermal conductivity along the [010] direction is approximately three times larger than along the [100] direction [13]. Anisotropic nonlinear optical properties in β-Ga 2 O 3 have been reported [14,15]. Liu et al grew β-Ga 2 O 3 epilayers on r-and c-plane sapphire substrates and showed that β-Ga 2 O 3 epilayers on r-plane revealed higher UV responsivity and detectivity [16].…”

Third‐Order Nonlinear Optical Response‐Driven Upconversion Phosphors

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