Micro-Raman characterization of homo-epitaxial n doped GaN layers for vertical device applications.

N-doped homo-epitaxial GaN samples grown on freestanding GaN substrates have been investigated by micro-Raman spectroscopy. Quantitative analysis of the E 2 h and the A₁(LO) modes' behavior has been performed while intentionally increasing the carrier density using silicon doping. We noticed that as the carrier concentration increases up to 1.8 × 10¹⁸ cm⁻³, the E 2 h mode remains unchanged. On the other hand, when the doping gets higher, the A₁(LO) position shifts to a higher frequency range, its width becomes larger, and its intensity drastically diminishes. This change in the A₁(LO) behavior is due to its interaction and its coupling with the free negative charge carriers. Furthermore, we calibrated the A₁(LO) frequency position shift as a function of the n-carrier concentration. We found out that for low n doping, the change in the A₁(LO) position can be considered as a linear variation while in the overall doping range, a sigmoid growth trend with a Boltzmann fit can be tentatively applied to describe the A₁(LO) position shift. This calibration curve can also be used to describe the coupling strength between the carriers and the A₁(LO) phonon. Eventually, this study shows that micro-Raman spectroscopy is a powerful non-destructive tool to probe the doping concentration and the crystalline quality of GaN material with a microscopic spatial resolution. [ABSTRACT FROM AUTHOR]