Intra- and Inter-Conduction Band Optical Absorption Processes in β-Ga₂O₃.

β-Ga₂O₃ is an ultra-wide bandgap semiconductor and is thus expected to be optically transparent to light of sub-bandgap wavelengths well into the ultraviolet. Contrary to this expectation, we find that free electrons in n-doped β-Ga₂O₃ absorb light from the IR to the UV wavelength range via intra- and inter-conduction band optical transitions. Intra-conduction band absorption occurs via an indirect optical phonon mediated process with a 1/ω³ dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the 1/ω² dependence predicted by the Drude model of free-carrier absorption. The inter-conduction band absorption between the lowest conduction band and a higher conduction band occurs via a direct optical process at λ ~ 349 nm (3.55 eV). We use steady state and ultrafast spectroscopy measurements to quantify the frequency and polarization dependence, and absorption coefficients of both these absorption processes. The experimental observations, in excellent agreement with recent theoretical predictions for β-Ga2O3, provide important limits of sub-bandgap transparency for optoelectronics in the deep-UV to visible wavelength range.