Structure and electronic properties of amorphous Ge-Pd alloys manifesting the metal-insulator transition. II. Photoemission spectroscopy and electronic structure calculations

Photoemission valence band spectra of amorphous Ge-Pd alloys produced hy Ar-ion-beam sputtering are measured with XPS and UPS techniques over a wide range of Pd concentration. A Pd-derived hand composed of Pd4d-Ge4p bonding states develops with Pd concentration and governs the upper valence band region in high concentrations. The UPS profile at the Fermi edge shows that a metallic continuous hand is formed in alloys above 1.3% Pd. To interpret the photoemission spectra and reveal the electronic structure of these amorphous alloys: DV-X alpha molecular orbital calculations are performed for Ge17-nPdn (n = 0, 1, 2, 4, 5) clusters with diamond structure which may simulate some local structures of amorphous alloys of low Pd concentrations. Calculated partial and total density of states well reproduce the valence band features revealed by XPS and UPS. A bandstructure change from an open band to a continuous band at the Fermi level caused by alloying with Pd is also confirmed by the calculation. The localization-delocalization behavior of the molecular orbital at the Fermi level in the cluster appears to be related with the metal-insulator transition in amorphous Ge-Pd alloys.