In-situ gallium-doping for forming p~+ germanium-tin and application in germanium-tin p-i-n photodetector

摘要

The in-situ Ga doping technique was used to form heavily p-type doped germanium-tin (Ge_(1-x)Sn_x) layers by molecular beam epitaxy, avoiding issues such as Sn precipitation and surface segregation at high annealing temperatures that are associated with the alternative implant and anneal approach. In this way, an electrically active Ga concentration of up to ～3.2 × 10~(20)cm~(-3) can be realized for Ge_(1-x)Sn_x. The impacts of varying the Ga concentration on the crystalline quality and the mobility of p-type Ge_(1-x)Sn_x were investigated. High crystalline quality Ge_(0.915)Sn_(0.085) can be realized with an active Ga concentration of up to ～1.2 × 10~(20)cm~(-3). More than 98% of the Sn atoms are located on substitutional lattice sites, although the substitutionality of Sn in p-type Ge_(1-x)Sn_x decreases with an increasing Ga concentration. When the Ga concentration introduced is higher than 3.2 × 10~(20)cm~(-3), excess Ga atoms cannot be substitutionally incorporated, and segregation of Ga and Sn towards the surface during growth is observed. The in-situ Ga-doped Ge_(0.915)Sn_(0.085) epitaxy was integrated in a Ge_(0.915)Sn_(0.085)-on-Si p-i-n (PIN) photodiode fabrication process, and well-behaved Ge_(0.915)Sn_(0.085)/Si PIN junction characteristics were obtained. A large forward-bias current to reverse bias current ratio of 6 x 104 and a low reverse current (dark current) of 0.24μA were achieved at V_(bias) = -1 V.