Effect of n-type doping level on direct band gap electroluminescence intensity for asymmetric metal/Ge/metal diodes

The direct band gap electroluminescence (EL) intensity was investigated for asymmetric metal/Ge/metal diodes fabricated on n-type Ge with doping levels in the range of 4.0 x 10(13) -3.1 x 10(18) cm(-3). Up to a doping level of 10(16) cm(-3) order, commercially available (100) n-Ge substrates were used. To obtain a doping level higher than 10(17) cm(-3) order, which is commercially unavailable, n(+)-Ge/p-Ge structures were fabricated by Sb doping on p-type (100) Ge substrates with an in-diffusion at 600 degrees C followed by a push-diffusion at 700 degrees C-850 degrees C. The EL intensity was increased with increasing doping level up to 1.0 x 10(18) cm(-3). After that, it was decreased with a further increase in n-type doping level. This EL intensity decrease is explained by the decreased number of holes in the active region. One reason is the difficulty in hole injection through the PtGe/n-Ge contact due to the occurring of tunneling electron current. Another reason is the loss of holes caused by both the small thickness of n(+)-Ge layer and the existence of n(+)p junction.