β-Ga2O3中强声子诱导的Urbach带尾的量化及其对电击穿的影响
Quantification of the strong, phonon-induced Urbach tails in \b{eta}-Ga2O3 and their implications on electrical breakdown
AI总结：
本研究通过偏振光学透射光谱量化了β-Ga2O3的Urbach能量（60-140 meV），发现静态无序主导带尾，且强电子-声子耦合要求扩展传统电击穿理论以考虑动态电离阈值波动。
摘要：
在超宽禁带（UWBG）氮化物和氧化物半导体中，禁带宽度的增加与更高的离子性和强电子-声子耦合相关。这通过极性光学声子散射限制了迁移率，通过极化子和自陷使载流子局域化，通过动态无序展宽了光学跃迁，并改变了击穿场。在此，我们使用77-633 K的偏振光学透射光谱研究了Fe掺杂和Sn掺杂的β-Ga2O3块状晶体多个取向的Urbach能量（Eu）。我们发现，在293 K时，Eu值范围为60-140 meV，且静态（结构缺陷加零点声子）无序对Eu的贡献大于动态（有限温度声子诱导）无序。这通过缺乏系统的Eu各向异性以及Eu与X射线衍射摇摆曲线展宽的相关性大于与Sn掺杂的相关性而得到证明。测得的最低Eu约为传统半导体的10倍，表明在这些材料用于高场电子器件时，带尾效应需要仔细考虑。我们证明，由于厚样品的光学透射对亚带隙吸收敏感，常用的通过厚度大于1-3微米的Ga2O3透射提取带隙的Tauc方法容易产生误差。结合Fe掺杂样品的Eu(T)和椭偏法得到的Eg(T)，我们提取了有效电子-声子耦合的度量，表明加权的二阶形变势随温度增加，且E平行b的值大于E平行c。β-Ga2O3中大的电子-声子耦合表明，传统半导体的电击穿理论需要扩展，不仅要考虑更低的散射时间，还要考虑在时间和空间上波动的碰撞电离阈值。
英文摘要：
In ultrawide bandgap (UWBG) nitride and oxide semiconductors, increased bandgap (Eg) correlates with greater ionicity and strong electron-phonon coupling. This limits mobility through polar optical phonon scattering, localizes carriers via polarons and self-trapping, broadens optical transitions via dynamic disorder, and modifies the breakdown field. Herein, we use polarized optical transmission spectroscopy from 77-633 K to investigate the Urbach energy (Eu) for many orientations of Fe- and Sn-doped \b{eta}-Ga2O3 bulk crystals. We find Eu values ranging from 60-140 meV at 293 K and static (structural defects plus zero-point phonons) disorder contributes more to Eu than dynamic (finite temperature phonon-induced) disorder. This is evidenced by lack of systematic Eu anisotropy, and Eu correlating more with X-ray diffraction rocking-curve broadening than with Sn-doping. The lowest measured Eu are ~10x larger than for traditional semiconductors, pointing out that band tail effects need to be carefully considered in these materials for high field electronics. We demonstrate that, because optical transmission through thick samples is sensitive to sub-gap absorption, the commonly-used Tauc extraction of bandgap from transmission through Ga2O3 greater than 1-3 micro meter thick is subject to errors. Combining our Eu(T) from Fe-doped samples with Eg(T) from ellipsometry, we extract a measure of an effective electron-phonon coupling indicating increases in weighted 2nd order deformation potential with temperature and a larger value for E parallel b than E parallel c. The large electron-phonon coupling in beta-Ga2O3 suggests that theories of electrical breakdown for traditional semiconductors need expansion to account not just for lower scattering time but also for impact ionization thresholds fluctuating in both time and space.
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