Longitudinal optical Raman mode A1 to calculate the indium molar fraction of epitaxial InGaN layers grown by LP-MOCVD on polar and non-polar planes

Abstract

This work discusses the frequency shift of Raman mode A1(LO) for InGaN epitaxial layers grown on polar (0002) and non-polar (11–20) planes concerning strain state, indium composition, and the probe excitation energy. Furthermore, it proposes MOCVD growth conditions to grow fully relaxed polar and non-polar InGaN layers, without indium droplets formation. Then, using the probe excitation energies of 2.33 eV and 2.68 eV, Raman measurements exhibited phonon mode A1(LO) frequency shift in polar and non-polar InGaN. Besides, theoretical calculations showed that for x < 0.20, and changing the excitation energy from 2.33 to 2.68 eV, the expected frequency shift is less or equal to 15 cm−1. Also, the XRD spectra exhibited two different indium compositions for non-polar a-plane InGaN and one composition for polar c-plane InGaN. According to XRD, for non-polar a-plane InGaN, the frequency shift measured corresponds to an indium composition gradient along the growth direction and the probe excitation energy change. On the other hand, for polar c-plane InGaN, the frequency shift measured is attributed to the probe excitation energy change. Also, AFM showed that there is an excellent island coalescence in GaN and InGaN epitaxial growth. The average surface roughness for polar InGaN was 75.678 nm and was 61.216 nm for non-polar InGaN. Moreover, these values are ten times smaller than their peak-to-peak distance, and the growth follows the Stranski–Krastanov model. FWHM for all samples corresponds to a total dislocations density of ~ 109/cm2 near the surface.