Silicon-Germanium Photodetectors for Optical Telecommunications
Richardson, Christopher J.K.
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This thesis investigates the design and growth of silicon-germanium p-i-n photodetectors for optical telecommunications applications. Two types of heterostructures are considered: strained silicon-germanium layers grown directly on silicon substrates, and strain-balanced silicon-germanium/silicon superlattice grown on relaxed buffer layers. The heterostructures are designed using existing band structure models and are grown using solid source molecular beam epitaxy (SS-MBE). To facilitate these growths, an atomic absorption spectroscopy- based flux monitor for the silicon source is developed and calibrated. In addition, the development of a substrate preparation procedure for relaxed buffer layers that is compatible with SS-MBE is developed and allows the growth of epitaxial films with low defect densities. P-i-n diodes processed from these films are shown to have low reverse leakage currents densities compared to other competing devices. Photocurrent spectroscopy is used to characterize these structures. A clear reduction in the bandgap of the heterostructures over that of the constituent alloys due to exploitation of the Type-II band offsets in the silicon-germanium material system is demonstrated in both, the strained and strain-balanced photodetectors. Finally, the low leakage current densities are exploited to fabricate devices with noise equivalent powers comparable to or better than competing approaches based on the growth of germanium on silicon substrates.