Direct determination of the band offset in ALD-grown ZnO/hydrogenated amorphous silicon heterojunctions from XPS valence band spectra

The chemical composition and band alignment at the heterointerface between ALD-grown zinc oxide (ZnO) and hydrogenated amorphous silicon (a-Si:H) is investigated using monochromatized X-ray photoelectron spectroscopy. A new approach for obtaining the valence band offset EV is developed, which consists in fitting the valence band (VB) spectrum obtained for a-Si:H with a thin ZnO overlayer as the sum of experimentally obtained VB spectra of a bulk a-Si:H film and a thick ZnO film. This approach allows obtaining EV = 2.71±0.15 eV with a minimum of assumptions, and also yields information on the change in band bending of both substrate and ZnO film. The band offset results are compared to values obtained using the usual approach of comparing valence band edge-to-core level energy differences, EB,CL – EB,VB. Furthermore, a theoretical value for the VB offset is calculated from the concept of charge neutrality level line-up, using literature data for the CNLs and the experimentally determined ZnO/a-Si:H interface dipole. The thus obtained value of EV CNL = 2.65±0.3 eV agrees well with the experimental EV.