Structural studies of GaN1‐x Asx and GaN1‐x Bix alloys for solar cell applications

Abstract

AbstractWe have discovered materials with a wide tunability of the band gap that opens the possibility to achieve a desirable material for multi‐junction solar cells. Two different alloy systems have been studied. GaN1‐xAsx alloys over the entire composition range were grown by plasma‐assisted MBE at low temperature on crystalline and amorphous (glass) substrates. For growth on sapphire the alloys are amorphous in the composition range of 0.17<x<0.75, but for growth on glass the composition range for amorphous alloys extends to x∼0.1. The bandgap of the GaN1‐xAsx alloys ranges from 3.4‐0.7 eV, covering a large part of the solar spectrum, therefore this material is potentially useful for solar energy conversion.The substitution of N by Bi in GaN forming GaN1‐x Bix is extremely difficult since Bi and N have even larger differences in size and electronegativity. TEM studies of GaN1‐xBix with x up to 0.2 show that the alloy is not entirely amorphous. Small crystallites embedded in an amorphous matrix are observed and Bi segregation was confirmed by Electron Energy Loss studies. A strong reduction in band gap is observed in samples with increasing Bi, suggesting the formation of a random alloy but low energy absorption tail may be related to the presence of the crystallites observed by TEM (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)