A study of n-type conduction in amorphous chalcogenide sputtered films

Abstract

A detailed study of the X-ray photoemission spectroscopy (XPS) and the transport properties of amorphous sputtered films with compositions approximated by a formula Ge25Se75-xBix where x=0-17.4 is reported. The XPS result indicates that, on a local atomic scale, the chemical states of the films are a mixture of different bonding environments of GeSe2 and Bi2Se4 for all compositions and that the Bi is introduced as a positive charged centre. The addition of 15.6 at.% Bi leads to an increase of ten orders of magnitude in the electrical conductivity as well as a decrease in the optical and electrical gaps. A negative sign of the thermoelectric power as observed for x>10, indicating that the conduction type changed from p type (low x) to n type, which is in agreement with the glass. The results are interpreted as the effect of the shift of the Fermi level closer to the conduction band due to the incorporation of the positive-U defects that unpin the Fermi energy.