On germanium doping of gallium nitride: Ion implantation vs in-situ doping during growth

The objective of the present work is to explore the potential of germanium-doped gallium nitride (GaN:Ge), focusing specifically on the challenges and opportunities associated with Ge ion implantation (I/I) into GaN. Thin GaN layers were grown on semi-insulating native GaN substrates using the metal-organic vapor-phase epitaxy (MOVPE) technique and subsequently implanted with germanium ions. The implantation parameters were chosen to obtain 200-nm-thick layers with target doping levels ranging from 5 × 1018 to 1020 cm-3. Following implantation, the samples were subjected to Ultra-High-Pressure Annealing (UHPA). High-resolution X-ray diffraction (HRXRD) measurements were performed to assess the structural damage recovery during UHPA. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to confirm the absence of thermal diffusion during this process. Density functional theory (DFT) calculations were carried out to explain the observed lack of diffusion in the implanted samples. Hall-effect measurements determined the free carrier concentration, mobility, and conductivity of the Ge-doped layers. A strong dependence of the electrical parameters on the annealing time was revealed. Prolonging the annealing time from 1 h to 5 h led to an increase in the free carrier concentration in all analyzed cases. While a further increase in annealing time (to 10 h) did not significantly augment the electron concentration, the carrier mobility and conductivity were enhanced. Finally, the ion-implanted samples were systematically compared with bulk HVPE-GaN:Ge crystals doped in situ and MOVPE-grown GaN:Ge layers in terms of their electrical properties.