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Synthesis of Germanium-Tin Alloys by Ion Implantation and Pulsed Laser Melting: Towards a Group IV Direct Band Gap Semiconductor

机译：离子注入和脉冲激光熔融合成锗锡合金：面向IV族直接带隙半导体

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摘要

The germanium-tin (Ge1-xSnx) material system is expected to be a direct bandgap group IV semiconductor at a Sn content of 6.5--11 at.%. Hence there has been much interest in preparing such alloys since they are compatible with silicon and they raise the possibility of integrating photonics functionality into silicon circuitry. However, the maximum solid solubility of Sn in Ge is around 0.5 at.% and non-equilibrium deposition techniques such as molecular beam epitaxy or chemical vapour deposition have been used to achieve the desired high Sn concentrations.;In this PhD work, the combination of ion implantation and pulsed laser melting (PLM) is demonstrated to be an alternative promising method to produce a highly Sn concentrated alloy with good crystal quality. In initial studies, it was shown that 100 keV Sn implants followed by PLM produced high quality alloys with up to 6.2 at.%Sn but above these Sn concentrations the crystal quality was poor. The structural properties of the ≤6.2 at.% alloys such as soluble Sn concentration, strain distribution and crystal quality have been characterised by Rutherford backscattering spectrometry (RBS), Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The optical properties and electronic band structure have been studied by spectroscopic ellipsometry. The introduction of substitutional Sn into Ge is shown to either induce a splitting between light and heavy hole subbands or lower the conduction band at the Gamma valley.;However, at higher implant doses needed to achieve >6.2 at.% Sn, ion-beam-induced porosity in Ge starts to occur, which drastically reduces the retained amount of the implanted Sn and such microstructure also hinders good crystallisation of the material during PLM. To solve this problem, it was shown that a nanometer thick SiO2 layer deposited on the Ge substrate prior to the implantation can largely eliminate the formation of porosity. This capping SiO2 layer also helps to increase the retained Sn concentration up to 15 at.% after implantation, as well as significantly improving the crystal quality of the Ge-Sn layer after PLM. With the use of the capping layer, a good quality Ge-Sn layer with ~9 at.% Sn has been achieved using Sn implants at an energy of ~120 keV. However, the thin film alloys produced by 100 keV or 120 keV Sn implantation and PLM are shown to contain compressive strain as a result of the large lattice mismatch between Ge and high Sn content alloys. Such strain compromises the tendency towards a direct bandgap material and hence strain relaxation is highly desirable. A thermal stability study showed that the thin film strained material is metastable up to ~400°C, but thereafter Sn comes out of solution and diffuses to the material surface.;To investigate a possible pathway to the synthesis of strain-relaxed material, a higher Sn implant energy of 350 keV was used to produce thicker alloy layers. XRD/reciprocal space mapping showed that this thicker alloy material is largely relaxed after PLM, which is beneficial for the direct band gap transition and solves the trade-off between higher Sn concentration and compressive strain. However, RBS indicates a sub-surface band of disorder which suggested a possible mechanism for the strain relaxation. Indeed, TEM examination of such material showed the material relaxed via the generation of non-equilibrium threading defects. Despite such defects, a PL study of this relaxed material found photon emission at a wavelength of ~2150 nm for 6--9 at.% Sn alloys. However, the intensity of the emission was variable across different Sn content alloys, presumably as a result of the threading defects. A possible pathway to removing such defects is given that may enable both photodetectors and lasers to be fabricated at wavelengths above 2mum.
机译：锗-锡（Ge1-xSnx）材料系统有望成为Sn带含量为6.5--11 at。％的直接带隙IV型半导体。因此，制备这样的合金引起了很多兴趣，因为它们与硅相容并且它们增加了将光子功能集成到硅电路中的可能性。然而，Sn在Ge中的最大固溶度约为0.5 at。％，并且已采用非平衡沉积技术（例如分子束外延或化学气相沉积）来实现所需的高Sn浓度。离子注入和脉冲激光熔化（PLM）技术被证明是生产具有良好晶体质量的高锡浓缩合金的另一种有前途的方法。在最初的研究中，表明100 keV Sn植入物，然后进行PLM可以生产出高质量的合金，其锡含量高达6.2 at。％，但高于这些Sn浓度，晶体质量就很差。 ≤6.2at。％合金的结构特性，例如可溶性锡浓度，应变分布和晶体质量，已通过卢瑟福背散射光谱（RBS），拉曼光谱，X射线衍射（XRD）和透射电子显微镜（TEM）进行了表征。 。光学性质和电子能带结构已通过椭圆偏振光谱法研究。已显示将替代性Sn引入Ge会引起轻空穴子带和重空穴子带分裂或降低Gamma谷处的导带;但是，在达到高于6.2 at。％Sn的情况下，需要更高的注入剂量，离子束锗中产生的多孔性开始出现，这大大减少了注入的锡的保留量，并且这种微结构也阻碍了PLM过程中材料的良好结晶。为了解决该问题，已表明在注入之前沉积在Ge衬底上的纳米厚的SiO 2层可以大大消除孔隙的形成。该覆盖的SiO2层还有助于在注入后将保留的Sn浓度提高到15 at。％，并显着提高PLM之后的Ge-Sn层的晶体质量。通过使用覆盖层，使用锡植入物以约120 keV的能量实现了锡含量约为9 at。％的高质量Ge-Sn层。但是，由于Ge和高Sn含量合金之间的晶格失配较大，结果表明，通过100 keV或120 keV Sn注入和PLM制备的薄膜合金含有压缩应变。这种应变损害了直接带隙材料的趋势，因此非常需要应变松弛。热稳定性研究表明，薄膜应变材料在高达约400°C的温度下是亚稳态的，但随后Sn从溶液中逸出并扩散到材料表面。为研究可能的应变松弛材料的合成途径， 350 keV的较高Sn注入能量用于生产较厚的合金层。 XRD /倒数空间映射表明，这种较厚的合金材料在PLM之后会大量松弛，这有利于直接的带隙跃迁，并解决了较高的Sn浓度与压缩应变之间的折衷问题。然而，RBS指示了表面下的无序带，这提示了应变松弛的可能机制。确实，这种材料的TEM检查显示该材料通过产生非平衡螺纹缺陷而松弛。尽管存在此类缺陷，但对这种松弛材料的PL研究发现，对于6--9 at。％的Sn合金，在〜2150 nm的波长处有光子发射。但是，发射强度在不同的锡含量合金之间是可变的，这大概是由于螺纹缺陷造成的。给出了消除此类缺陷的可能途径，该途径可以使光检测器和激光器都能够以大于2μm的波长制造。

著录项

  • 作者

    Tran, Tuan Thien.;

  • 作者单位

    The Australian National University (Australia).;

  • 授予单位 The Australian National University (Australia).;
  • 学科 Electrical engineering.;Materials science.
  • 学位 Ph.D.
  • 年度 2017
  • 页码 151 p.
  • 总页数 151
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 生理学;
  • 关键词

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