Investigation Of GeSi Low Dimensional Structure Fabrication

GeSi are widely investigated in opto-electronics and material science, because of its special advantage like low cost and compatibility with current mature Si technology. But the application in opto-electronic devices was hindered by the inherent indirect band gap of GeSi. This could be solved by the low dimensional structure like quantum dots and film.Due to the 4.2% lattice mismatch between Ge and Si, strain relaxation leads to self-assembled Ge quantum dots as we grow Ge epitaxially on the Si substrate. It's widely accepted that uniform Ge quantum dots with small size and large density are essential to its device performance. However, due to the existence of wetting layer, the density of Ge quantum dots can not reach further than 1010cm-2 in traditional epitaxial growth. The diameter can not be smaller than 30nm either, with positions randomly distributed. Besides, high quality Ge film with low density misfit dislocation is essential to its device performance.Molecule Beam Epitaxy (MBE) system was exploited to investigate Ge quantum dots and Ge film growth. This thesis mainly covers the topics as below:1. Firstly we introduced the MBE equipment, as well as several characterization methods that frequently used in GeSi epitaxial growth, such as Etch pit Density(EPD), Scanning Electron Microscopy(SEM), Raman Spectroscopy and Photoluminescence. Special attention was paid to the investigation of Reflected High Energy Electron Diffraction (RHEED). As an in-situ monitoring system, it would help to know more about the growing mechanism by the explanation of the diffraction pattern.2. Chemically induced SiO2 film was proved to be an efficient buffer layer for uniform and high density (～1011cm-2) Ge quantum dots growth, including direct deposition and post annealing treatment after room temperature growth. Based on the results from various characterization, we investigate the growing mechanism of Ge nucleation SiO2 layer. Because of the surface energy difference, Ge atoms tends to form smaller islands on the SiO2 surface, rather than wetting layer in S-K mode. Besides, the temperature and thickness effect for the nanostructure formation was also evaluated.3. To improve the ordering of Ge dots nucleation, Porous Alumina Membrane(PAM), with inherent hexagonal periodicity, was utilized as pattern tied on Si substrate. After Ge deposition in MBE system, Ge quantum dots selectively nucleate in these holes. Then highly ordered Ge dots were achieved after the membrane was peeled off. At the same time, the dots uniformity could also be improved by optimizing the growth parameter(substrate temperature 400℃-500℃,deposition thickness 25nm).4. Based on the S-K growth mode, we take use of various methods to reduce the dislocation density in Ge film, like low temperature seed layer and post growth cyclic annealing. The dislocation density decreased from 108cm-2 to 106cm-2. Besides, MSM (Metal- Semiconductor-Metal) devices was fabricated based on the high quality Ge film. We introduced device fabrication process in detail. Testing system was set up to evaluate the dark current and responsivity of the devices.