Vibrational spectra of adsorbed hydrogen on GaN(001) surfaces

The infrared spectroscopy (IR) of adsorbed hydrogen on a surface is an experimental technique used to analyze semiconductor surfaces. The hydrogen atoms are bonded to the surface dangling-bonds and the vibrational frequencies of the M–H bonds are very sensitive to the local chemical character at the surface. Usually the analysis of the results is based on the comparison with the IR spectra of molecules in the gaseous phase. This comparison is difficult when structures at the surface have no exact analogous in the gas phase. One alternative way is to base the comparison on theoretical results obtained for molecular clusters simulating the complex adsorbed at the surface. The calculated frequencies of the ‘stretching’ mode of the M–H bonds corresponding to the various ways the hydrogen binds to the surface may be associated to the experimental IR spectra. In the present work we carry out ab initio calculations of GaxNyHz clusters in order to analyze the adsorption models of hydrogen by Ga or N dimers occurring on the reconstructed GaN(001) surfaces. We use the Gaussian 94 code of the quantum chemistry in order to calculate the molecular structures; the clusters are allowed to relax in order to minimize the total energy. We obtain the vibrational spectra and the associated normal modes. q 2003 Elsevier Science Ltd. All rights reserved.