Free Carrier Concentration Gradient along the c-Axis of a Freestanding Si-doped GaN Single Crystal

The charge carrier concentration variance in a freestanding Si-doped GaN single crystal grown by hydride vapor phase epitaxy (HVPE) has been investigated. A transverse micro-Raman scattering measurement along the [0001] c -axis on the cross section of the n-type GaN was performed to obtain the profile of the free carrier concentration. The analysis of the coupled modes of plasmons and longitudinal optical (LO) phonons revealed the presence of a free carrier concentration gradient along the c -axis, which gradually varies from 2.3×10 17 (N face) to 9.3×10 17 cm -3 (Ga face). The possible physical origins of the charge carrier concentration gradient in the GaN crystal were discussed on the basis of current theories and experimental results of the growth mechanism of an n -doped GaN single crystal.