Optical cavities for Si/SiGe tetrahertz quantum cascade emitters

The use of buried tungsten silicide layers for confinement of terahertz optical modes is described. Silicon-on-silicide substrates are prepared using a bond and etch-back technique, and the successful growth of extremely long (600 period) strain-balanced p-Si/SiGe quantum cascade heterostructures on these substrates is demonstrated. THz electroluminescence is observed from these structures at low temperature, when the structure is biased so as to obtain interwell ('diagonal') transitions between heavy and light hole subbands. The emission shows a strong polarisation dependence, indicating the efficacy of the silicide layer in confining long wavelength TM modes.