Band alignment amorphous interface

The interfaces prepared by sputter deposition of ZnO (filled square) or (Zn,Mg)0 (filled triangles) exhibit a valence band offset of AEyb = 1.2 eV. The ZnO and (Zn,Mg)0 films were prepared at room temperature in pure Ar and therefore exhibit a large disorder and a large BEve(Zn 2p3/2)- Compared with the interface with reverse deposition sequence, the offset is 0.35 eV larger. This indicates a rather strong influence of the deposition sequence on the band alignment at the CdS/ZnO interface, which is most likely related to the amorphous nucleation layer when ZnO is deposited onto CdS. 

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

To give an individual value for the band alignment is not possible. Structurally well-ordered interfaces, which are obtained e.g., by deposition of CdS onto ZnO layers deposited at higher temperatures and/or with the addition of oxygen to the sputter gas, show a valence band offset of A TV is = 1.2 eV in good agreement with theoretical calculations [103]. Sputter deposition of undoped ZnO at room temperature in pure Ar onto CdS also leads to a valence band offset of 1.2 eV. In view of the observed dependencies of the band offsets this agreement is fortuitous, as the influence of the local disorder and of the amorphous nucleation layer most likely cancel each other. 

Fig. 4.25. Influence of the amorphous nucleation layer of the ZnO film on the band alignment at a hypothetical CdS/ZnO interface (a) CdS and ZnO before contact (b) in contact with charge equilibrium established by space charge layers (c) in contact with equilibrium established by charges localized in an amorphous ZnO nucleation layer...

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