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, it is found here 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 1 / ω 3 dependence in the visible to near-IR wavelength range. This frequency dependence markedly differs from the 1 / ω 2 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). Steady state and ultrafast optical spectroscopy measurements unambiguously identify both these absorption processes and enable quantitative measurements of the inter-conduction band energy and the frequency dependence of absorption. Whereas the intra-conduction band absorption does not depend on light polarization, inter-conduction band absorption is found to be strongly polarization dependent. 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 and are also of importance for high electric field transport effects in this emerging semiconductor.