Investigation of disorder effects in amorphous GaAs and GaP by EXAFS

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Abstract
Disorder effects in amorphous GaAs and GaP are investigated by EXAFS measurements at the K edge of Ga and As for GaAs and of Ga only for GaP, performed with the DCI synchrotron radiation at LURE. The samples are unannealed films with controlled stoichiometry prepared by flash-evaporation under ultrahigh-vacuum conditions. In the case of GaAs, the EXAFS data are analysed by a classical Fourier filtering technique, using crystalline powders as a reference material. The results are compared with those obtained in the same experiments on evaporated amorphous Ge films. No contribution from second neighbours can be detected in either set of amorphous data. The mean first-neighbour distances around Ga and As absorbing atoms are found to be identical, in contrast to previous observations, and larger than the crystalline distance by 0.01 AA only. The spread of these distances around their mean value, deduced from the EXAFS amplitudes, supports the conclusion that the amorphous GaAs samples are chemically ordered. In the case of GaP, the EXAFS data are compared with computed model curves constructed within different assumptions about chemical disorder characteristics. From the study of the amplitude, there is some indication of the presence of Ga-Ga wrong bonds, with a length slightly larger than the Ga-P normal bonds. The difficulties in EXAFS analysis related to disorder are discussed.