Sputter Deposition of ZnO onto CdS

A determination of the band alignment at the CdS/ZnO interface where ZnO has been stepwise deposited by magnetron sputtering has been published by Venkata Rao et al. [71]. A more extended series of spectra recorded during ZnO deposition by dc magnetron sputtering onto CdS are presented in Fig. 4.19. During ZnO deposition the sample was held at room temperature. 

The bottom spectra are taken from a freshly evaporated CdS film. The only emissions observed are from Cd and S core levels and Auger levels and from the valence band region. With increasing deposition time of ZnO, the Cd and S levels are attenuated, while the emissions from the growing ZnO film 

8 Chemical shifts are not very pronounced for the Cd3d core level [114] and might therefore be hardly identified. 

Following these studies, a microstructure of sputter-deposited ZnO films on polycrystalline CdS substrates is outlined in Fig. 4.21. The different evolution of the Zn 2p and O Is binding energies can consequently be attributed to the amorphous ZnO nucleation layer with a different chemical bonding between Zn and O. The model is also valid for polycrystalline In2S3 and Cu(In,Ga)Se2 substrates and for deposition of (Zn,Mg)0 films, as these show the same behavior (see Figs. 4.20 and 4.24). It is not clear whether an amorphous nucleation layer occurs also when the ZnO is deposited by other techniques as MBE, CVD, or PLD, as no data are available for such interfaces. In addition, the influence of the polycrystallinity of the substrates is not clear so far. 

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No interface reaction is observed at the CdS/ZnO interfaces. Even the sputter deposition of ZnO onto CdS with or without oxygen in the sputter gas does not lead to an increased reactivity. This is most likely related to the poor ability of the CdS and ZnO surfaces to dissociate oxygen atoms (compare Sect. 4.2.2.2 and discussion of the reactivity in Sects. 4.4 and 4.5). 

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