Comparison of Fe and Si doping of GaN: An EXAFS and Raman study

The effect of Fe and Si doping in GaN grown epitaxially on Al2O3 is studied using Extended X-ray Absorption Fine Structure (EXAFS) and Raman spectroscopies. The EXAFS analysis shows that in GaN samples grown by Molecular Beam Epitaxy (MBE) at 750 °C the Ga–Ga distance is about 0.19% longer than the corresponding distance in samples grown by Metalorganic Vapour Phase Epitaxy (MOVPE) at 1050 °C. As this distance is affected strongly by the value of the a lattice constant, its smaller value complies with the higher compressive biaxial thermal strain, present in samples grown at a higher temperature. This finding is in line with the smaller blue shift observed for the E2 Raman peak in the MBE samples. Si doping, contrary to Fe doping, reduces the mean square relative displacement of the Ga atoms in the second nearest neighboring shell of Ga. Most probably a stress relaxation mechanism, e.g. formation of dislocations, affects the c/a ratio which in turn modifies the difference of the Ga–Ga distances in- and out- of the c-plane. Contrary to Fe doping, the incorporation of Si atoms in the Ga sublattice induces compressive hydrostatic strain that blue-shifts the E2 Raman peak. Finally, the carrier concentration determined by proper fitting of the A1(LO) Raman peak, that takes into account the phonon–plasmon coupling, shows that continuous Fe doping compensates slightly the n-type conductivity of GaN.