Depth-dependent optical properties of a HVPE-grown freestanding Si-doped GaN single crystal

Results of the cathodoluminescence (CL) and micro-Raman measurements in a freestanding 300 μm-thick Si-doped GaN single crystal grown by hydride vapor phase epitaxy (HVPE) are presented. The depth-dependent CL results along the c-axis on the cross-section show that the integrated CL intensities of the near-band-gap emission gradually increase with the CL linewidths broadened from the N face to the Ga face. The physical origin of increase in the CL intensity has been studied by the micro-Raman measurements along the c-axis, which revealed the carrier concentration gradually increases from 2.3×10 ¹⁷ (the N face) to 9.3×10¹⁷ cm^-3 (the Ga face). Our experimental evidence indicates that the broadening of the CL linewidth is due to the increased fluctuation of the local bandgap caused by the heavier concentration of the donor impurities.