Tellurium doping of InGaP for tunnel junction applications in triple junction solar cells

Abstract

Tellurium doped InGaP is an ideal material for the n side of a tunnel junction for triple junction solar cell structures grown by MOCVD. In this paper, we discuss the growth process for abrupt turn-on and turn-off of tellurium in InGaP when InGaP must be highly doped and uniformly doped with tellurium to provide for epitaxial films suitable for tunnel junctions. Results show that tellurium pre-doping of the layer before InGaP growth provides for a sharp turn-on and that using a growth pause at elevated growth temperatures after InGaP provides for a sharp turn-off in the doping profile. Results from a series of experiments in which partial pressure of phosphine and growth temperature were varied show that doping levels of >1×10 19 cm −3 can be achieved uniformly over 4 in. diameter germanium wafers by lowering growth temperature and V/III ratio. Results also showed that high tellurium doping levels strain InGaP and adjustment of indium mole fraction in InGaP is required to produce smooth epitaxial layers. These MOCVD growth processes can be incorporated in the growth of tunnel junction layers used in the production of triple junction solar cells.