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 an epitaxial GaAs and an 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 blue-shift 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 Finite-Elements Method calculations confirms that this dependence on 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.