$β$ $_2$ O $_3$的导带内和导带间光吸收过程
Intra- and Inter-Conduction Band Optical Absorption Processes in $β$-Ga$_2$O$_3$
摘要：
$β$ -Ga $_2$ O $_3$是一种超宽带隙半导体，因此预期对于亚带隙波长的光在紫外线中是光学透明的。与该预期相反，在此发现，n掺杂的$β$ $_2$ O $_3$中的自由电子经由导带内和导带间光学跃迁吸收从IR到UV波长范围的光。导带内吸收通过间接光学声子介导过程发生，在可见光到近红外波长范围内具有$1/ω^{3}$依赖性。这种频率依赖性明显不同于自由载流子吸收的Drude模型预测的$1/ω^{2}$依赖性。最低导带和较高导带之间的导带间吸收经由直接光学过程在$λ\sim 349$ nm（3.55 eV）处发生。稳态和超快光谱测量明确地识别这两个吸收过程，并使导带间能量的定量测量，以及吸收的频率依赖性。而导带内吸收不依赖于光的偏振，导带间吸收被发现是强烈的偏振依赖。实验观察，在最近的理论预测的$β$ -镓$_2$ O $_3$，提供了重要的限制的子带隙的光电子学在深紫外可见光波长范围内的透明度，也是在这个新兴的半导体的高电场传输效应的重要性。
英文摘要：
$β$-Ga$_2$O$_3$ 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 $β$-Ga$_2$O$_3$ 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 $1/ω^{3}$ 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 $λ\sim 349$ 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 $β$-Ga$_2$O$_3$, 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.
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