N-type CdTe Thin Films via In-Situ Indium Doping

Abstract

Indium-doping of CdTe thin films to produce n-type conductivity for solar cell device applications was investigated. Films were deposited via close spaced sublimation (CSS) from a 1 at% in-situ In-doped CdTe source. Films analyzed with SIMS exhibited a difference in indium incorporation of several orders of magnitude depending on whether a reducing or oxidizing ambient was used during CSS deposition. X-ray photoemission measurements using both soft and hard x-rays further indicated that for samples rapidly cooled after deposition under a high N2 gas flow, a Cd-rich surface layer developed, which was not observed for more slowly cooled films. Photoemission measurements also allowed the conductivity type of the films to be inferred from the valence band positions in hard x-ray spectra, with n-type conductivity confirmed for In-doped films as-deposited, as well as an undoped CdTe film, with the latter thought to be due to a small deviation from stoichiometry. The Fermi levels of In-doped films varied with deposition conditions, with In-doping, a reducing ambient, and/or slow cooling tending to produce higher Fermi levels. Chloride activation treatments using MgCl2 tended to lower the Fermi level, with more significant impacts on less highly doped films, in the case of the lowest doped film causing a switch from n-type to p-type conductivity. These results offer insights expected to be of great utility to the production of n-CdTe based solar cells, especially those fabricated via CSS.