Structural studies of GaN_1-xAs_x and GaN_1-xBi_x alloys for solar cell applications

We 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. GaN_1-xAs_x 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 GaN_1-xAs_x 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 GaN_1-xBi_x is extremely difficult since Bi and N have even larger differences in size and electronegativity. TEM studies of GaN_1-xBi_x 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.