Electronic effects of Cd on the formation of the CdS/CuInS2 heterojunction
He I UPS valence band spectra of CdS on CIS substrates from different chemical bath depositions. The nonreproducibility of the positions of the band edges can be clarified through differing amounts of band bending on the underlying CIS surface.
XPS measurements of the Cd 3d 5/2 core level from an ∼35 nm CdS layer deposited on CIS at four different x-ray fluxes denoted by the photoelectron count rates. The total shift of ∼250 meV caused by an SPV in the sample is evident between the measurements with the highest and lowest incident fluxes.
XPS measurements of the (a) Cd 3d 5/2 and (b) In 3d 5/2 core levels from an ∼1 nm CdS layer deposited on CIS at four different x-ray fluxes denoted by the photoelectron count rates. There is a minimal shift of ∼0.10 eV between the sample measured with the lowest incident flux and the other three.
He I UPS valence band spectra of the same CIS absorbers found in Fig. 1, without the 7:00 min sample, after the removal of CdS with 8% HCl. Reference measurements are also shown from a KCN-etched as well as a KCN- and then HCl-etched (no CdS) absorber. The prominent Cu 3d peak at 3 eV is evidence for a Cu-richer surface after HCl etching.
Cu L 3 absorption edges on unetched, KCN-etched and KCN–HCl-etched CIS samples. The large feature at 930 eV in the unetched spectrum (black arrow) is due to the CuS and CuO phases on the surface of the sample with a Cu 3d 9 4s 0 valence structure. The HCl etching, although it produces a Cu-richer CIS surface than found on KCN-etched CIS, produces no phase with a Cu 3d 9 4s 0 valence structure. The spectra are normalized to the peak at 932 eV following the pre-edge feature and are shifted vertically for clarity.
UPS valence band measurements of unetched, KCN-etched, KCN–HCl-etched and KCN–HCl–KCN-etched CIS absorbers. The metallic nature of the unetched sample can be clearly seen (Fermi level, inset) as well as the effect of the KCN etching when it follows HCl etching. The measurement of the KCN–HCl–KCN sample lies between that of the KCN and KCN–HCl samples.
Measured values of the CdS and CIS valence band edges, E vb,CdS, and E vb,CIS, respectively, as well as the band bending on the CIS surface induced by the CdS deposition, E bb. The valence band offsets, ΔE vb, calculated using Eq. (3) are also presented.
Core level binding energies in eV from the KCN-etched sample and the samples after CdS deposition.
Values for the differences between electronic positions of the sample with 35 nm CdS deposition time and the other three samples in Fig. 1 as noted in the column on the left. The binding energies of the 35 nm sample were always smaller than those of the other samples. The differences in valence band positions are denoted by , the differences in core level positions with with x = 0.7, 1, and 2.5 nm.
Core level binding energies in eV from the KCN, KCN–HCl–KCN, and KCN–HCl-etched sample surfaces.
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