High Ge content photodetectors on thin SiGe buffers

PIN SiGe photodetectors (PD) are grown by MBE with Ge contents x=0, 0.10, 0.2, 0.27 and 0.5. For PDs with high Ge content, which are grown beyond the SiGe layer critical thickness, thin (sub 100 nm) strain relaxed, p+(B)-doped SiGe buffers are of special importance. The layer structure of the samples consists of a 5×1018 cm−3 p+(B)-doped buffer layer as a bottom contact, followed by a 300 nm intrinsic active zone covered with a 3×1020 cm−3 n++(Sb)-doped top contact layer. Extremely low temperatures (LT) during the first growth stage of the SiGe buffers are implemented. Process windows for high strain relaxation on different substrates are determined. The role of growth conditions in crystal structure formation is in situ monitored by time resolved reflectivity (TRR) measurements. For comparison, pseudomorphic PDs and that on conventional graded buffers were also realised. Secondary ion mass spectrometry (SIMS), μ-Raman-spectroscopy and energy dispersive X-ray (EDX) analysis are performed to measure Ge content, composition profiles and degree of relaxation. The microstructure is characterised by cross-section transmission electron microscopy (XTEM). By optical microscopy with Nomarski differential interference contrast (NIC) combined with defect etching technique, typical defects in the layers were studied. Electrical measurements are performed to determine the different current components. The DC current–voltage characteristics show a distinct diode ‘s behaviour of the detectors, which are optically characterised in terms of reverse current for different incident wavelengths.