Lonsdaleite germanium has a direct band gap, but it is not an efficient light emitter due to the vanishing oscillator strength of electronic transitions at the fundamental gap. Transitions involving the second lowest conduction band are instead at least three orders of magnitude stronger. The inversion of the two lowest conduction bands would therefore make hexagonal germanium ideal for optoelectronic applications. In this work, we investigate the possibility to achieve this band inversion by applying strain. To this end we perform ab initio calculations of the electronic band structure and optical properties of strained hexagonal germanium, using density functional theory with the modified Becke-Johnson exchange-correlation functional and including spin-orbit interaction. We consider hydrostatic pressure, uniaxial strain along the hexagonal c axis, as well as biaxial strain in planes perpendicular to and containing the hexagonal c axis to simulate the effect of a substrate. We find that the conduction-band inversion, and therefore the transition from a pseudo-direct to a direct band gap, is attainable for moderate tensile uniaxial strain parallel to the lonsdaleite c axis.
Model for the on-site matrix elements of the tight-binding hamiltonian of a strained crystal: Application to silicon, germanium and their alloys
我们讨论了应变金刚石或锌-金刚石晶体或纳米结构的sp 3d 5s * 紧束缚哈密顿量的现场矩阵元的模型。该模型具有现场，非对角耦合之间的s，p和d轨道，并能够再现任意应变的带能量和有效质量在整个布里渊区的影响。它只引入了几个额外的参数，并且没有任何含糊之处，这些含糊之处可能来自于将宏观应变定义为原子位置的函数。以硅、锗及其合金为例，说明了该模型的应用。特别是，我们做了一个详细的比较紧束缚和从头计算的数据应变硅，锗和SiGe。
Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first principles
Direct observation of strain and confinement shaping the hole subbands of Ge quantum wells
Eidgenossische Technische Hochschule Zürich（苏黎世联邦理工学院）；Paul Scherrer Institut（保罗谢尔研究所）；University of Basel（巴塞尔大学）；National Centre for Computational Design and Discovery of Novel Materials (MARVEL), École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院新材料计算设计与发现国家中心（MARVEL））；University of Fribourg（弗里堡大学）
Impact of stoichiometry and strain on Ge$_{1-x}$Sn$_{x}$ alloys from first principles calculations
用密度泛函理论（DFT）计算了锗锡（Ge $_{1-x}$ Sn $_{x}$）二元合金$0 \leq x \leq 1$的电子结构.确定了具有半导体或半金属行为的弛豫合金作为Sn组合物$x$的函数，并且通过将垂直于[001]生长方向的超晶胞晶格常数约束到Ge、碲化锌（ZnTe）或碲化镉（CdTe）衬底的晶格常数来包括外延应变的影响。已发现，与在$Γ$处具有相同带隙的弛豫Ge $_{1-x}$ Sn $_{x}$合金相比，施加1%拉伸应变将使（正）直接带隙为零所需的Sn组分减少约5%。另一方面，压缩应变对$Γ$处的合金带隙的影响相对较小。利用DFT计算的合金晶格常数和弹性常数，估算了Ge $_{1-x}$ Sn $_{x}$薄膜的临界厚度随$x$和衬底晶格常数的变化关系，并与超晶胞DFT计算结果进行了对比验证.该分析正确地预测了Sn的组成范围，在该范围内，Ge $_{1-x}$ Sn $_{x}$ /Ge变得非晶态在能量上变得有利。化学计量比和应变的影响进行检查有关的反向（“负”）$Γ_{7}^{-}$ - $Γ_{8}^{+}$带隙，这是半金属合金电子结构的特征的大小减少。基于我们的研究结果，提出了通过Ge $_{1-x}$ Sn $_{x}$纳米结构中的应变和量子限制来实现半金属到半导体转变的策略。
基于III-V族半导体材料的晶体管已经用于由III-V族材料中的高电子迁移率驱动的各种模拟和高频应用。另一方面，III-V族材料中的空穴迁移率始终落后于IV族半导体，例如硅和锗。在本文中，我们探讨了使用应变和异质结设计的带结构模型，以提高空穴迁移率的III-V族材料。使用各种实验技术定量测量对于优化空穴传输重要的参数，例如应变、价带偏移、有效质量以及轻重空穴带之间的分裂。空穴的峰值霍尔迁移率为960 cm ~ 2/Vs，并且即使在高的薄层电荷下也保持了高的空穴迁移率。
