Growth of high quality germanium films on patterned silicon substrates and applications

The principal objective of this work is to determine optimal pattern structures for highest quality (defect free) heteroepitaxial growth. High quality films of Ge on Si are of significant importance and can be used in high electron mobility devices, photodetectors for optical communications (1.3mum or 1.55mum) and integrating III-V optoelectronic devices. However, a 4% lattice mismatch and ∼ 50% thermal expansion mismatch between Ge and Si create three major challenges in growing high quality Ge films on Si, (a) high surface roughness due to a pronounced <110> crosshatch pattern, (b) high dislocation densities in Ge films and (c) high density of microcracks and wafer bending. A common way of reducing lattice and thermal expansion mismatch is to form a "virtual substrate (VS)" by growing a graded composition followed by a uniform layer of the desired epitaxial film on a defect-free Si substrate. Virtual graded layers could not decrease the dislocation densities to the numbers acceptable for most of the devices. Mathews et al. first proposed that limiting the lateral dimensions of the sample prior to growth could reduce the dislocation density. Later On Fitzgerald proposed that patterning decreases the dislocation density in the films. In this work we show high quality crosshatch-free Ge films with dislocation density ∼ 105 cm-2 on the nano-patterned Si and also high quality GaAs films on the Ge/Si virtual substrate.; The first step in this research was to perform a systematic study to identify the role of pattern width on the quality of Ge growth. We investigated micrometer and submicrometer scale patterns. We demonstrated that the quality of the heteroepitaxial layers improves as the pattern width decreases. Then we have decreased the pattern width to nanometer-scale dimensions. Significant improvement of the Ge film quality was observed. We used novel interferometric lithography techniques combined with reactive ion and wet chemical etching to fabricate Si structures. The patterning was done using standard photomask based lithography. We analyzed the quality of the Ge films using high resolution x-ray diffraction, TEM and SEM. We performed etch pit density (EPD) measurements by counting the pits formed using a Nomarski optical microscope. In order to correlate characterization with device performance, we designed an inter-digitated pattern to form Ge based metal semiconductor metal photodetector and measured the photoresponse of the Ge films. Preliminary results were very promising. We then grew 4 mum GaAs on the Ge/Si using MBE (0.5 mum/hr and 570°C) and analyzed the GaAs film quality. We also performed modeling to calculate strain energy density and wafer bending in multi-layer films grown epitaxially on planar Si substrates. We have also compared the models with experiments. (Abstract shortened by UMI.)...