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 × 10¹³-3.1 × 10¹⁸ cm^-3. Up to a doping level of 10¹⁶ cm^-3 order, commercially available (100) n-Ge substrates were used. To obtain a doping level higher than 10¹⁷ 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 °C followed by a push-diffusion at 700 °C-850 °C. The EL intensity was increased with increasing doping level up to 1.0 × 10¹⁸ 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.