Activation study on Ge-implanted gallium nitride and its usage in dopant-selective nanoporous electrochemical etching

Abstract

Porosification of high n-doped GaN by electrochemical etching allows us to modify its refractive index drastically. For optoelectronic applications, lateral control of the doping profile is necessary. Ion implantation would be highly desirable, due to its high scalability. However, for a high degree of electrically active donors, post-implantation annealing has to be employed. For this, germanium ions have been implanted into unintentionally doped GaN layers at energies ranging from 40 to 350keV to form a homogeneous implantation profile. The capping layers of either SiN/Al2O3 or SiO2/SiN effectively suppress the decomposition of GaN during annealing up to 1300 °C. Degradation of the capping increases with increasing temperature until the GaN surface becomes heavily pitted at 1500 °C. X-ray diffraction measurements were used to investigate the structural changes caused by implantation and annealing. Electrochemical capacitance–voltage in combination with secondary ion mass spectrometry reveals electrical activation of implanted donors with net donor concentrations of up to 3 × 1019 cm−3. Rapid thermal annealing at 1400 °C thereby achieves the highest activation and the highest reduction in implantation-induced defects. The different cappings have no major influence on the activation rate. However, the combination of SiN/Al2O3 is more stable at higher temperatures and is, therefore, preferable. Activated Ge+-implanted GaN is electrochemically etched to porosify the previously unintentionally doped layer. The comparison to an MOVPE-grown n-doped GaN layer with a matching donor concentration shows that the implanted and annealed GaN exhibits similar pore sizes and porosity.