Defect Characterization and Control in Metastable GeSn Optoelectronic Alloy Nanostructures

非技术性描述半导体材料的一个重要应用领域是光电器件;例如，在使用电子激发光发射的激光器或发光二极管（LED）等器件中。在科学和技术上，人们对能够发射波长比可见光稍长的光（即光谱的中红外部分）的半导体有很大的兴趣。这种光源在硅芯片上制造时，可能成为未来无处不在的化学传感器网络的关键组件，加速硅芯片之间和硅芯片上的数据传输，以及自动驾驶汽车所需的运动传感器。本研究项目的重点是一种半导体材料系统，锗-锡，它在硅芯片上的中红外光发射方面具有很大的前景。锗锡的发光效率受到合成时生长到材料中的原子级缺陷的限制。该项目描述了这些缺陷的性质和数量，并研究了消除或改变它们的方法，以尽量减少它们对锗锡的影响。本科生参与这些研究活动，并特别努力从美国科学和工程劳动力中代表性不足的群体中招募具有高度竞争力的本科研究人员。该项目包括与斯坦福大学的RISE外展计划合作，以激励高中生考虑STEM领域的进一步教育和职业生涯。技术说明在足够大的情况下显示直接带隙（x ~ 10原子%）锡组成，Ge（1-x）Sn（x）合金对于中红外（IR）光发射器和吸收器具有很大的希望，同时还与硅电子和光子技术单片兼容。先前对生长在硅上的锗-锡外延膜的研究已经证明了中红外光泵浦激光，并且已经有逐渐增加Sn含量以获得更长波长操作的趋势。Ge（1-x）Sn（x）的发光特性仍然远未达到最佳。用于促进高Sn含量合金的低生长温度（300°C）导致形成大浓度的受主型空位缺陷。 理论预测Sn原子与这些空位的强配对将导致增强的非辐射载流子复合，降低光发射和吸收效率。该项目使用应变工程的核-壳纳米线结构作为平台，研究生长后退火以解离Sn-空位对并消除Ge（1-x）Sn（x）壳层中的空位。厚度高达500 nm的壳是特别感兴趣的，以实现能够有效引导中红外光的线结构。同步辐射漫x射线散射是用来表征的相对浓度的空位绑定到锡原子，双空位，集群和单空位的合金与退火时间和温度的趋势。一个关键的目标是了解这些合金中空位平衡的速率和机制。扩展的X射线吸收精细结构分析提供了一个额外的探针周围的Sn原子和Sn空位对的稳定性的本地键合。该项目还研究了原子氟作为化学空位钝化剂，建立在先前的经验与F钝化的Si表面状态和空缺的Ge。核-壳线的X射线和光电表征之间的耦合可以揭示点缺陷和器件相关特性之间的连接的基本见解。温度相关的光致发光、光电导和超快泵浦探测测量用于探测Ge（1-x）Sn（x）能带结构和不同空位缺陷数对载流子复合动力学的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响评审标准进行评估，被认为值得支持。

Nontechnical DescriptionAn important area of application for semiconductor materials is in optoelectronic devices; for example, in devices such as lasers or light-emitting-diodes (LED’s) that use electrons to stimulate the emission of light. There is great interest, both scientifically and technologically, in semiconductors that can emit light with wavelengths somewhat longer than visible light, i.e. in the mid-infrared part of the spectrum. Such light sources, when fabricated on silicon chips, could become key components in future ubiquitous chemical sensor networks, in speeding up data transfer between and on silicon chips, and in motion sensors required by autonomous vehicles. This research project focuses on a semiconductor material system, germanium-tin, that holds great promise for mid-infrared light emission on silicon chips. The efficiency of light emission by germanium-tin is limited by the presence of atomic scale defects that grow into the material when it is synthesized. This project characterizes the nature and number of such defects, and investigates methods for annihilating or altering them to minimize their effects on germanium-tin. Undergraduates are involved in these research activities, with special efforts made to recruit highly competitive undergraduate researchers from groups that are under-represented in the US science and engineering workforce. The project includes a partnership with Stanford’s RISE outreach program, to inspire high school students to consider further education and careers in STEM fields.Technical DescriptionExhibiting a direct bandgap at sufficiently large (x ~ 10 atomic %) tin composition, Ge(1-x)Sn(x) alloys hold great promise for mid-infrared (IR) light emitters and absorbers, while also being monolithically compatible with silicon electronic and photonic technologies. Previous research on germanium-tin epitaxial films grown on silicon has demonstrated mid-IR optically-pumped lasing, and there has been a gradual trend of increasing Sn content to access longer wavelength operation. The light emission characteristics of Ge(1-x)Sn(x) are still far from optimal. Low growth temperatures ( 300°C) used to promote high Sn content alloys cause large concentrations of acceptor-type vacancy defects to form. Strong pairing of Sn atoms with these vacancies is predicted theoretically and will result in enhanced non-radiative carrier recombination, reducing the efficiency of light emission and absorption. This project uses strain-engineered core-shell nanowire structures as a platform to study post-growth annealing to dissociate Sn-vacancy pairs and to annihilate vacancies incorporated in the Ge(1-x)Sn(x) shells during their growth. Shells of thickness up to 500 nm are of particular interest, to achieve wire structures capable of efficiently guiding mid-IR light. Synchrotron diffuse x-ray scattering is used to characterize trends in the relative concentration of vacancies bound to Sn atoms, divacancies, clusters and monovacancies in the alloys versus annealing time and temperature. A key goal is to understand the rates and mechanisms governing the approach to vacancy equilibrium in these alloys. Extended x-ray absorption fine structure analysis provides an additional probe of local bonding around Sn atoms and the stability of Sn-vacancy pairs. The project also examines atomic fluorine as a chemical vacancy passivant, building on prior experience with F passivation of Si surface states and vacancies in Ge. Coupling between x-ray and optoelectronic characterization of the core-shell wires can reveal fundamental insights into the connection between point defects and device-relevant properties. Temperature-dependent photoluminescence, photoconductivity and ultra-fast pump-probe measurements are used to probe Ge(1-x)Sn(x) band structure and the effects of different vacancy defect populations on carrier recombination dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.