GROWTH OF HETEROEPITAXIAL CdTe LAYERS ON REUSABLE Si SUBSTRATES AND A LIFT-OFF TECHNIQUE FOR THIN FILM SOLAR CELL FABRICATION

Abstract

Heteroepitaxial (111) and (100) oriented CdTe layers have been grown on Si substrates by conventional molecular beam epitaxy (MBE) and photo-assisted MBE (PAMBE) using stacked BaF₂-CaF₂ as a buffer to overcome the 19% lattice mismatch between Si and CdTe. Heteroepitaxial As doped p-type CdTe(lOO) layers have been grown on BaF₂-CaF₂/Si(100). The dopant activation is accomplished using an extra Cd source and laser illumination of the substrate during growth. The growth kinetics and surface reconstructions have been studied using RHEED during CdTe growth under different conditions, and the induced effects on Te-desorption, Cd-migration, and As-substitution on Te-vacancy site have been correlated. The resistivity of As doped CdTe layers is down to 20 ohm cm. The 8 K photoluminescence spectra of such a layer shows a dominant (A^°, X) peak at 1.590 eV and the As acceptor level corresponds to a shallow level with = 60 me V activation energy. A lift-off technique has been used to separate the single crystal CdTe thin films from the Si wafer by dissolving the fluoride buffer. CdS/CdTe solar cells have been fabricated in these layers.

KEY WORDS:

• Solar cell
• CdTe thin film