Tight-binding molecular-dynamics study of density-optimized amorphous GaAs

We present the results of a tight-binding molecular-dynamics study of the structural and electronic properties of amorphous GaAs (a-GaAs), emphasizing the relationship between density and topological and chemical disorder. We find the amorphous state to have lower density than the crystal, in agreement with experiment. The coordination number (3.94) is very close to that of the crystal; nevertheless, a significant number of atoms possess defective coordination - either threefold or fivefold. We find, as a consequence, a proportion of wrong bonds of about 12%, consistent with experiment; yet, the system remains a semiconductor, with a band gap of 1.12 eV. We have also studied the effect of chemical disorder through random exchanges of atoms in the amorphous samples; both the density and the band gap decrease upon increasing chemical disorder, suggesting that the lower density of a-GaAs is partly a consequence of chemical disorder.