The structural, optical, and electrical properties of HVPE-grown GaN-on-sapphire templates were studied. The c and a lattice constants of the GaN layers were measured by x-ray diffraction. It was observed that the c and a lattice constants vary non-monotonically with Si-doping. The proper selection of Si-doping level and growth conditions resulted in controllable strain relaxation, and thus, influenced defect formation in GaN-on-sapphire templates. It was also observed that HVPE homoepitaxial GaN layers grown on the templates have better crystal quality and surface morphology than the initial templates.