Fabrication and characterization of low temperature (<450 °C) grown p-Ge/n-Si photodetectors for silicon based photonics

P–n hetero-junctions were fabricated by depositing p-Ge thin films on n-Si substrates using molecular beam epitaxy and electron-beam evaporation, with processing temperatures less than 450 °C, to be compatible with back-end silicon processing. The surface preparation of the Si substrate prior to Ge deposition was found to significantly affect the crystallinity of the deposited Ge layers and, hence, the p–n photodetector diode characteristics. The quality of the deposited Ge layers was inferred both through electrical and optical measurements as well as through structural characterization, i.e. X-ray diffraction (XRD). Surface desorption treatments to remove adsorbed hydrogen, oxygen and hydrocarbons were attempted to improve the Si surface quality to increase the minority carrier diffusion lengths and minimize dark current densities. Hydrogen desorption treatment at 450 °C prior to Ge deposition gave the best performance with diffusion lengths greater than 25 nm and dark currents of 0.3 mA/cm2. The observed performance from the p–n diodes is expected to be sufficient for fabricating waveguide integrated photodetectors with high responsivities.