Solute Trapping in Low-Temperature Vapor-Liquid-Solid Growth: A Route to Direct-Gap Ge-Sn Single Crystal Nanowires

非技术描述：从液体中冷冻生长块状晶体是一种成熟的方法，用于制造高度完美的半导体材料，用于许多应用，包括电子、光的探测或发射。在这个项目中研究了一种用于生产非常小（几十到几百纳米）直径的半导体线的晶体生长过程。重点是锗锡线的生长，这有可能使令人兴奋的设备成为可能，比如可以直接在硅上生长的激光器。首席研究员和他的研究生们正在研究如何在半导体线生长的过程中将异常高浓度的锡原子冻结在半导体线中。该项目还通过在所有三年的计划活动中为本科生提供研究经验（REU），将本科生纳入实验工作。与他们的研究经验并行，REU学生与首席研究员一起开发简单的实验室演示和半导体，光固相互作用和晶体生长的演示材料。最后，通过参加斯坦福大学RISE暑期实习项目，这项研究工作被用作向低收入的旧金山湾区高中生进行科学推广的工具。技术描述：该项目扩展到纳米尺度的溶质捕获研究，作为实现亚稳高半导体合金成分的一种手段。在这样做的过程中，它利用了蒸汽-液体-固体（VLS）锗纳米线生长可能产生的大液体过饱和和深亚泛温度。该项目的重点是生长独立的锗锡二元合金纳米线，这是非常有趣的，因为它们具有高载流子迁移率和实现非常有效的光吸收和发射直接带隙的可能性。外延薄膜的研究表明，相对于L谷，金刚石立方锗的Sn合金化可以降低导带γ谷的能量。在实践中，锡合金达到了预测的6at。在非应变薄膜中实现直接间隙所需的成分是困难的。锡在体积锗中的平衡溶解度极限为1at .%。此外，Ge-Sn与Ge和Si的晶格错配会导致沉积在这些最有用的衬底上的薄膜产生压缩双轴应变，从而使子带能量偏移，与Sn添加的影响相反，因此需要更大的Sn浓度。这些考虑清楚地表明，开发生产无相干应变的独立单晶锗锡纳米线的方法可能产生的科学和技术影响。采用一种综合的实验方法来探测由Au或Sn纳米级催化剂生长的Ge-Sn纳米线的溶质捕获，包括透射电子显微镜的能量色散光谱和局部电极原子探针层析成像。在研究VLS溶质捕获动力学和机制的同时，利用光致发光和瞬态吸收等光学探针研究了Sn掺入对合金能带结构和载流子复合动力学的影响。

Non-technical Description: Growing bulk crystals by freezing them from a liquid is a well-established method for making highly perfect semiconductor materials for a host of applications including electronics, and the detection or emission of light. A crystal growth process that is used to produce very small (tens to hundreds of nanometers) diameter semiconductor wires is investigated in this project. The focus is on growth of germanium-tin wires that have the potential to enable exciting devices, such as lasers that can be grown directly on silicon. The principal investigator and his graduate students are studying how unusually large concentrations of tin atoms can be frozen into these semiconductor wires as they grow. This project also integrates undergraduates in the experimental effort by involving research-experiences-for-undergraduates (REU) students in all three years of planned activities. In parallel with their research experiences, the REU students work with the principal investigator to develop simple laboratory demonstrations and presentation materials on semiconductors, light-solid interactions and crystal growth. Finally, this research effort is used as a vehicle for science outreach to low income San Francisco Bay Area high school students, through participation in Stanford RISE summer internship program.Technical Description: This project extends to the nanoscale the study of solute trapping as a means of achieving metastably high semiconductor alloy compositions. In so doing, it exploits the large liquid supersaturations and deep subeutectic temperatures possible with vapor-liquid-solid (VLS) Ge nanowire growth. The project focusses on growth of free-standing Ge-Sn binary alloy nanowires, which are of great interest for their high carrier mobilities and the possibility of achieving a direct band gap for very efficient light absorption and emission. Research on epitaxial thin films suggests that Sn alloying of diamond cubic Ge can decrease the energy of the Gamma valley of the conduction band relative to the L valley. In practice, Sn alloying up to the predicted 6 at.% composition required to achieve a direct gap in an unstrained thin film is difficult. The equilibrium solubility limit of Sn in bulk Ge is 1 at.%. Moreover, the lattice mismatch of Ge-Sn to Ge and Si induces compressive biaxial strain in films deposited on these most useful substrates that shifts the sub-band energies in opposition to the effect of Sn addition, thus requiring even larger Sn concentrations. These considerations make clear the potential scientific and technological impact of developing methods to produce free-standing, single crystal Ge-Sn nanowires without coherency strains. A comprehensive experimental approach is used to probe solute trapping in Ge-Sn nanowires grown from either Au or Sn nanoscale catalysts, including energy dispersive spectroscopy in the transmission electron microscope, and local electrode atom probe tomography. In parallel with these studies of VLS solute trapping kinetics and mechanisms, optical probes such as photoluminescence and transient absorption are used to study the effects of Sn incorporation on alloy band structure and carrier recombination dynamics.