Untersuchung von Interdiffusion, elastischer Gitterverzerrung und Relaxation in epitaktischen Si$_{1-x}$Ge$_{x}$/Si-Heterostrukturen mit Ionenstreuung

Si/Si$_{1-x}$Ge$_{x}$-heterostructures and -superiattices are favourable candidates for new high-speed electronic and optoelectronic devices based on Si-technology. Due to the large lattice mismatch of 4.2% between Si and Ge, epitaxial growth of heterostructures leads to tetragonal elastic strain of the unit cell. Above the critical layer thickness the structures start to relax by the formation of misfitand threading-dislocations. Thermal annealing, which is frequently required for device fabrication, can result in relaxation and interdiffusion between the individual layers. The dependence of elastic strain and defect densities on layer thickness and growth temperature has been studied in MBE-grown Si/Si$_{1-x}$Ge$_{x}$ - heterostructures using Rutherhord backscattering spectroscopy (RBS), ion channeling, transmission electron microscopy and X-ray diffraction. Defect densities in single layers above critical thicknesses have been obtained from planar dechanneling measurements. Rapid thermal annealing at temperatures above 1000°C was used to improve the crystalline quality and resulted in a decrease of the threading dislocation density of about 70%. Pseudomorphic, fully strained Si$_{.67}$Ge$_{.33}$-layers with thicknesses of 2000 $\mathring{A}$ have been fabricated at a low growth temperature of only 450°C. The minimum yield value of only 3.2% indicates a very low defect density. Angular scans along (100)-planes through [110]-orientations have been performed to obtain a measurement of the tetragonal lattice distortion. Thermal interdiffusion between individual layers has been studied using 5-period channeling, transmission electron microscopy and X-ray diffraction. Defect densities in single layers above critical thicknesses have been obtained from planar dechanneling measurements. Rapid thermal annealing at temperatures above 1000°C was used to improve the crystalline quality and resulted in a decrease of the threading dislocation density of about 70%. Pseudomorphic, fully strained Si$_{.67}$Ge$_{.33}$-layers with thicknesses of 2000 $\mathring{A}$ have been fabricated at a low growth temperature of only 450°C. The minimum yield value of only 3.2% indicates a very low defect density. Angular scans along (100)-planes through [110]-orientations have been performed to obtain a measurement of the tetragonal lattice distortion. Thermal interdiffusion between individual layers has been studied using 5-period Si/Si$_{1-x}$Ge$_{x}$-superlattices with periods of 200 $\mathring{A}$ on both Si (asymmetrically) buffer layers. Interdiffusion coefficients have been obtained from a Fourier analysis of the measured RBS spectra. In both cases the thermal dependence of the interdiffusion coefficients for a fixed Ge concentration, x, follows an Arrhenius law. Inderdiffusion is strongly dependent on the Ge concentration in the Si/Si$_{1-x}$Ge$_{x}$ layers. Medium energy ion scattering (MEIS) using a position sensitive, electrostatic analyser was employed to investigate very thin Si/Ge-superlattices with layer thicknesses below 40 $\mathring{A}$. The relative energy resolution of 4x10$^{-3}$ corresponds to a depth resolution of about 5 $\mathring{A}$, which is more than one order of magnitude better than conventional RBS using a Si surface barrier detector