Activation of Ge in Ga₂O₃—Role of Capping Layer, Ambient and Defect Formation

β-Ga₂O₃ has extensive potential as a UWBG semiconductor for power electronics. A key advantage of β-Ga₂O₃ is the availability of several shallow n-type dopants that can be ion implanted to achieve low resistance ohmic contacts. While Si implants have been extensively studied, there has been far less exploration of Ge and Sn as alternatives. In this work, we report on thermal annealing of ion-implanted Ge in β-Ga₂O₃ as a function of the annealing ambient, temperature, time, and capping layers. Under optimized conditions, we demonstrate over 40% activation of 50 nm box implants to 5x10¹⁹ cm^-3 (sheet carrier 1.6x10¹⁴ cm^-2) with a mobility of 64 cm²/V-s. This is the highest reported activation for Ge-doped β-Ga₂O_3.
Homoepitaxial UID films of β-Ga₂O₃ (~500 nm) were grown by MOCVD on Fe-doped (010) native substrates. Prior to ion implantation, samples were capped with a 20 nm SiO₂ by ALD to ensure uniform doping to the surface. Four implant energies were used to yield 100 nm box profile concentrations of 5x10¹⁹ and 3x10¹⁹ cm^-3, and a 50 nm box profile of 5x10¹⁹ cm^-3. Samples were annealed under high purity nitrogen at temperatures from 100 to 1050 ^oC, times from 5 to 20 min, and with and without the SiO₂ capping layer. Carrier concentrations and mobility were determined by van der Pauw Hall measurements, diffusion profiles were determined by SIMS, and lattice damage was characterized by XRD.
As-implanted samples showed clear evidence of γ-phase formation by XRD. Implant damage recovery was observed to begin at temperatures of 100 ^oC with loss of the γ-phase beginning at 600 ^oC. Complete lattice recovery was observed after 950 ^oC for 10 minutes for all but the 5x10¹⁹/100 nm. Annealing in the absence of the SiO2 cap resulted in reduced carrier activation of 15% with mobilities of 74 cm²/V-s. However, when annealed under identical conditions with the cap in place, activation increased to 40% with mobilities above 60 cm²/V-s, which is consistent with the significantly enhanced ionized donors. A subsequent 20 min anneal at 950 ^oC, after the removal of the cap, showed carrier activation loss to 20% with an increase in mobility. Based on the behavior of Si implant annealing, we believe the presence of the cap reduces the formation of compensating Ga vacancies, resulting in the enhanced activation.
SIMS diffusion measurements show substantial redistribution of Ge at these concentrations to depths >220 nm. The diffusion profiles indicate that Ge diffusivity is dramatically enhanced at concentrations above 1x10¹⁹ cm^-3 resulting in nearly constant concentration profiles. In addition, near the implant and damage peak, approximately 30% of the Ge is immobile and likely trapped in defect complexes. These data provide a framework for understanding the dopant diffusion and activation of Ge in β-Ga₂O_3.