Strain related relaxation of the GaAs-like Raman mode selection rules in hydrogenated GaAs1−xNx layers

The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAs1-xNx (x = 0.01) layers-with different H doses and similar low-energy irradiation conditions-was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAs1-xNx reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.