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

非技术描述：通过从液体中冷冻晶体来生长大块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，包括电子学，以及光的检测或发射。本计画研究一种用来制造直径非常小（数十至数百奈米）的半导体导线的晶体成长制程。重点是锗锡线的生长，这些锗锡线有可能实现激发器件，例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这些半导体线生长时将异常大浓度的锡原子冻结到这些半导体线中。该项目还通过让本科生研究经验（REU）学生参与所有三年的计划活动，将本科生融入实验工作。在他们的研究经验的同时，REU的学生与首席研究员合作，开发简单的实验室演示和演示材料的半导体，光固体相互作用和晶体生长。最后，这项研究工作是作为一种工具，科学推广到低收入的旧金山弗朗西斯科湾区的高中生，通过参加斯坦福大学上升暑期实习program.Technical Description：该项目扩展到纳米级的溶质捕获的研究，作为一种手段，实现亚稳高半导体合金compositions。在这样做时，它利用了大的液体过饱和度和深亚临界温度可能与气-液-固（VLS）锗纳米线的生长。该项目的重点是独立的Ge-Sn二元合金纳米线的生长，这是非常感兴趣的高载流子迁移率和实现非常有效的光吸收和发射的直接带隙的可能性。对外延薄膜的研究表明，金刚石立方Ge的Sn合金化可以降低导带的γ谷相对于L谷的能量。在实践中，Sn合金化高达预测的6at.在无应变薄膜中实现直接间隙所需的组合物是困难的。Sn在体Ge中的平衡溶解度极限为1at. %。此外，Ge-Sn与Ge和Si的晶格失配在沉积在这些最有用的衬底上的膜中引起压缩双轴应变，其与Sn添加的效果相反地移动子带能量，因此需要甚至更大的Sn浓度。这些考虑清楚地表明了开发生产无相干应变的独立单晶Ge-Sn纳米线的方法的潜在科学和技术影响。一个全面的实验方法是用来探测从Au或Sn纳米催化剂生长的Ge-Sn纳米线中的溶质捕获，包括在透射电子显微镜的能量色散谱，和局部电极原子探针断层扫描。在平行于这些研究的VLS溶质捕获动力学和机制，光学探针，如光致发光和瞬态吸收的合金能带结构和载流子复合动力学的Sn掺入研究的影响。

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.