基于锡组分和双轴张应力调控的临界带隙应变Ge1−xSnx能带特性与迁移率计算

摘要

Optoelectronic integration technology which utilizes CMOS process to achieve the integration of photonic devices has the advantages of high integration,high speed and low power consumption. The Ge1?xSnxalloys have been widely used in photodetectors,light-emitting diodes,lasers and other optoelectronic integration areas because they can be converted into direct bandgap semiconductors as the Sn component increases. However,the solid solubility of Sn in Ge as well as the large lattice mismatch between Ge and Sn resulting from the Sn composition cannot be increased arbitrarily: it is limited,thereby bringing a lot of challenges to the preparation and application of direct bandgap Ge1?xSnx. Strain engineering can also modulate the band structure to convert Ge from an indirect bandgap into a direct bandgap, where the required stress is minimal under biaxial tensile strain on the (001) plane. Moreover, the carrier mobility, especially the hole mobility, is significantly enhanced. Therefore, considering the combined effect of alloying and biaxial strain on Ge, it is possible not only to reduce the required Sn composition or stress for direct bandgap transition,but also to further enhance the optical and electrical properties of Ge1?xSnxalloys. The energy band structure is the theoretical basis for studying the optical and electrical properties of strained Ge1?xSnxalloys. In this paper,according to the theory of deformation potential,the relationship between Sn component and stress at the critical point of bandgap transition is given by analyzing the bandgap transition condition of biaxial tensile strained Ge1?xSnxon the (001) plane. The energy band structure of strained Ge1?xSnxwith direct bandgap at the critical state is obtained through diagonalizing an 8-level k·p Hamiltonian matrix which includes the spin-orbit coupling interaction and strain effect. According to the energy band structure and scattering model, the effective mass and mobility of carriers are quantitatively calculated. The calculation results indicate that the combination of lower Sn component and stress can also obtain the direct bandgap Ge1?xSnx,and its bandgap width decreases with the increase of stress. The strained Ge1?xSnxwith direct bandgap has a very high electron mobility due to the small electron effective mass,and the hole mobility is significantly improved under the effect of stress. Considering both the process realization and the material properties, a combination of 4% Sn component and 1.2 GPa stress or 3% Sn component and 1.5 GPa stress can be selected for designing the high speed devices and optoelectronic devices.%能带工程通过改变材料的能带结构可以显著提升其电学和光学性质,已广泛应用于半导体材料的改性研究.双轴张应力和Sn组分共同作用下的Ge1?xSnx合金,不仅可以解决直接带隙转变所需高Sn组分带来的工艺难题,而且载流子迁移率会显著提升,在单片光电集成领域有很好的应用前景.根据形变势理论,分析了(001)面双轴张应变Ge1?xSnx的带隙转变条件,并给出了在带隙转变临界点Sn组分和双轴张应力的关系;采用8k·p 方法,得到了临界带隙双轴张应变Ge1?xSnx在布里渊区中心点附近的能带结构,进而计算得到电子与空穴有效质量;基于载流子散射模型,计算了电子与空穴迁移率.计算结果表明:较低Sn组分和双轴张应力的组合即可得到直接带隙Ge1?xSnx合金,且直接带隙宽度随着应力的增大而减小;临界带隙双轴张应变Ge1?xSnx具有极高的电子迁移率,空穴迁移率在较小应力作用下即可显著提升.考虑工艺实现难度和材料性能两个方面,可以选择4% Sn组分与1.2 GPa双轴张应力或3% Sn组分与1.5 GPa双轴张应力的组合用于高速器件和光电器件的设计.