Study of band alignment at CBD-CdS/Cu(In1-xGax)Se2 (x=0.2-1.0) interfaces

Dependence of band alignments at interfaces between CdS (grown by chemical bath deposition) and Cu(In1-xGax)Se-2(CIGS) (by conventional 3-stage co-evaporation) with 0.2<x<1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). In-rich CIGS samples had their conduction band minimum (CBM) lower than that of CdS. Their conduction band offset (CBO) was positive and approximately +0.3 similar to +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 similar to 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x, which results in the sign change of CBO around 0.4 similar to 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical compared to the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.