Evidence for Covalent Bonding in Crystalline and Amorphous As, Sb, and Bi from Valence-Band Photoelectron Spectra

X-ray photoelectron spectra (XPS) of the valence bands of crystalline and amorphous As, Sb, and Bi have been measured. The results are compared with band-structure calculations for the crystalline materials. A splitting, $\Delta E$, of the "$s$-like" peak for the crystalline phase disappears in the amorphous phase and the "$p$-like" peak is shifted towards higher energies. This similarity to tetrahedrally coordinated covalent semiconductors is explained by describing the semimetals as layers of distorted covalently bonded hexagonal rings. This generalizes the "even-odd" ring effect previously observed in group-IV semiconductors to the $A7\mathrm{}$ lattice of As, Sb, and Bi. The relation of $\Delta E$ to the interatomic distance is described by a universal curve, which applies to the group-V semimetals as well as group-IV semiconductors.