Single Crystalline 100% epi-Ge Layers on Engineered Oxide Heterostructures on Si

Abstract

Single crystalline Germanium (Ge) has gained a lot of attention for applications as new material in microelectronics, photovoltaics and for photodetectors. The integration on the mature and predominating Silicon (Si) technology platform is a challenging technical task, which offers many basic scientific questions to be answered. This thesis is concerned with the integration of a functional Ge layer on the Si platform via an engineered oxide heterostructure, namely cubic PrO2. The oxide is incorporated to compensate for the 4% lattice constant mismatch of Ge and Si, with its lattice constant between the two semiconductors. An in situ reflection high energy electron diffraction (RHEED) monitoring of the layer deposition by molecular beam epitaxy (MBE) indicates that the initial growth mode of Ge on PrO2 follows a Volmer-Weber growth mode due to interface reactions, surface and strain energies. By properly tuning the growth parameters of MBE a growth recipe is developed, leading to the growth of atomically smooth single crystalline Ge (111) layers on the Pr2O3 (111) / Si (111) support system. The oxide is subject to a chemical reduction process during the Ge deposition, resulting in a Pr2O3 stoichiometry. The closed layers are not achieved by a change to van der Merwe growth, but by the adjustment of the growth kinetics, resulting in a smoothing out of the Volmer-Weber growth. The development of the recipe for the Ge layer growth is monitored with RHEED, ex situ x-ray reflectivity (XRR) and x-ray diffraction (XRD) measurements as well as scanning electron microscopy (SEM). These methods confirm the closed and smooth Ge surface and the sharp interface with the underlying Pr2O3. The closed layer stacks are investigated by synchrotron radiation x-ray diffraction under bulk sensitive and surface sensitive measurement conditions. This first study unveils a single crystalline type A / B / A stacking configuration of the Ge (111) / Pr2O3 (111) / Si (111) heterostack system. Driven by the results from the structural investigation a second study reveals the main defect mechanismsat work by XRD pole-figure measurements and reciprocal space maps (RSMs), supported by real space cross section transmission electron microscopy (TEM) images along a stacking sensitive direction. The defects limiting the long range order in the Ge layer are identified as stacking twins, microtwins and stacking faults (Fig. 2). The investigation of the thickness dependent behaviour discloses a threading behaviour of microtwins and stacking faults while stacking twins are confined to the interface. First results of high temperature UHV annealing experiments show the reduction of diffuse scattering by strain fields in defective Ge is possible, indicating a reduction of stacking faults, while microtwins as well as stacking twins are not nfluenced by the annealings. Future defect engineering approaches are required to improve the long range order of the epi-Ge layer for technological applications.