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

The in-situ Ga doping technique was used to form heavily p-type doped germanium-tin (Ge_1−xSn_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 × 10²⁰cm⁻³ can be realized for Ge_1−xSn_x. The impacts of varying the Ga concentration on the crystalline quality and the mobility of p-type Ge_1−xSn_x were investigated. High crystalline quality Ge_0.915Sn_0.085 can be realized with an active Ga concentration of up to ∼1.2 × 10²⁰cm⁻³. More than 98% of the Sn atoms are located on substitutional lattice sites, although the substitutionality of Sn in p-type Ge_1−xSn_x decreases with an increasing Ga concentration. When the Ga concentration introduced is higher than 3.2 × 10²⁰cm⁻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_0.915Sn_0.085 epitaxy was integrated in a Ge_0.915Sn_0.085-on-Si p-i-n (PIN) photodiode fabrication process, and well-behaved Ge_0.915Sn_0.085/Si PIN junction characteristics were obtained. A large forward-bias current to reverse bias current ratio of 6 × 10⁴ and a low reverse current (dark current) of 0.24 μA were achieved at V_bias = −1 V.