INFLUENCE OF HIGH NITROGEN FLUX ON CRYSTAL QUALITY OF MBE GROWN GaN LAYERS USING RAMAN SPECTROSCOPY: A THEORETICAL JUSTIFICATION

Abstract

Crystal quality of plasma assisted MBE grown hexagonal GaN have been investigated at varying nitrogen and gallium fluxes using Raman spectroscopy. Theoretical calculations revealed the position of E2(low) and E2(high) in quality crystal of GaN to be 144 cm-1 and 568 cm-1 respectively. Crystal quality of the layers is discussed by comparing the experimentally measured Raman spectra with present calculations and other standard results. Room temperature Raman spectra of Ga-rich layers and stoichiometric GaN are similar as exhibiting excitation modes at 434 cm-1, 568 cm-1 and 729 cm-1 identified as residual laser line, E2H and LO mode contribution ( A1(LO) and E1(LO) ) respectively. Similarity of Ga-rich and stoichiometric GaN layers is interpreted as the indication of comparable crystal quality of both GaN layers. In Raman spectrum associated with N-rich GaN samples all the spectral features are disappeared into a broad band in the range of 250–650 cm-1. This typical observation, intensity distribution and missing of regular hexagonal modes, all are signatures of rough surface, bad crystal quality and high concentration of defects. More careful analysis of N-rich spectrum, based on atomic displacement scheme has revealed that these defects are actually Ga vacancies. Therefore, in order to obtain better crystal quality and smooth surface morphology, the optimal growth condition is stoichiometric or slight Ga-rich GaN.