Intra- and inter-conduction band optical absorption processes in β -Ga2O3

Abstract

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/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). 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.

Ultrafast Dynamics of Gallium Vacancy Charge States in $β$-Ga$_2$O$_3$

“…The probe wavelength is 800 nm. When the probe is polarized along the a*-axis (perpendicular to the b-and c-axis), the change in transmission is due to optical absorption from photoexcited free electrons (intra-conduction band absorption [30]). As the polarization of the probe is changed away from the a*-axis toward the c-axis, additional absorption due to defects is observed that makes ∆T /T more negative in the first few hundred picoseconds.…”

“…Very notably, the shape of the ∆T /T transient is also polarization dependent suggesting that different loss mechanisms are contributing to the probe absorption when the probe is polarized along the c-axis and a*-axis. In recently reported work [30], we have examined the ∆T /T transient for probe polarization along the a*-axis in detail and shown that, for this polarization, the probe experiences optical loss only due to intra-conduction band transitions (a form of free-carrier absorption) characterized by a 1/ω 3 frequency dependence, where ω is the center frequency of the probe pulse. Optical absorption related to defects is not observed for probe polarized along the a*-axis.…”

“…where, L i is the pump-probe interaction length determined by the spatial overlap of the pump and probe beams in the sample (≈ 260 µm in our measurement scheme), n o is the equilibrium electron density (∼ 5×10 18 cm −3 ), n(τ ) is the total electron density in the conduction band at time τ , σ f c (ω) is the absorption crosssection associated with free-carrier intra-band absorption [30]. As shown recently by the authors [30,31], σ f c (ω) is proportional to ω −3 , where ω is the frequency of the probe. n d is the defect density.…”

Ultrafast Dynamics of Optical Nonlinearities in β-Ga2O3

“…This is a typical defect state absorption. In Singh et al (2020), they did the absorption spectra of different probe wavelengths (515, 600, 660, 800, 940 nm) under 450 nm excitation, and found that they were related to the transition in the conduction band (intraband free-carrier absorption) through α∝ω −3 fitting (Drude model, as shown by the red dotted line in Figure 4), while the extra absorption due to defect state between 500 and 650 nm is ignored. The difference between our experimental results and the Drude line proves the existence of defect state absorption.…”

“…The study of gallium oxide nonlinear absorption/refraction dynamics is of great significance to the design of all-optical switches and ultrafast optoelectronic devices. In 2020, Okan Koksal et al verified the intra-and inter-conduction band optical absorption processes of β-Ga 2 O 3 through steady state and ultrafast optical spectroscopy measurements (Singh et al, 2020), but they ignored the influence of defect state at 500-650 nm on absorption, which we will be explained explicitly in this paper. Gallium oxide has many impurities or intrinsic defects, such as Si, Sn, oxygen vacancy and gallium vacancy, which promote the radiative and nonradiative recombination of carriers (Varley et al, 2010;Dong et al, 2017;Neal et al, 2018).…”