Ge-doping in polycrystalline GaN layer through electron beam evaporator deposition with successive ammonia annealing

We studied the impact of Ge-doping on material properties of polycrystalline GaN layers with different Ge percentages of 2%, 5% and 10%. The carrier concentration for the undoped polycrystalline GaN layer is ~ 6 × 10¹⁹ cm^− 3, and the value increases up to ~ 1.1 × 10²¹ cm^− 3 by the Ge-doping with 5% of Ge. Meanwhile, the electron mobility is the lowest at 98.6 cm²/Vs with 5% of Ge. The result is comparable to some reported Ge-doped single crystal GaN layers with the carrier concentration of above 10²⁰ cm^− 3. Additionally, the surface of the polycrystalline GaN layer changes significantly with the Ge percentage above 5%. In particular, GaN grain protrusions and GaN grain-like rods are observed. It is found that Ge-N related compounds can form on the GaN grain-like rods. The grain protrusions and grain-like rods lead to the broadening of the Raman E₂ peak, indicating that the crystalline properties can be degraded by excessive Ge-doping.