Nucleation layer

Thus, it was established that adsorption of metal hydroxide species on the surface of the substrate provides a nucleation layer which is chemically converted to the metal chalcogenide. The forming metal chalcogenide layer acts then as a catalytic surface for subsequent anion and cation adsorption. 

The effect of a surfactant on skin depends on the type of surfactant as described earlier. Wilhelm et al. demonstrated the irritation potential of anionic surfactants.21 They evaluated the effects of sodium salts of n-alkyl sulfates with variable carbon chain length on TEWL and found that a C12 analog gave a maximum response. They suggested that the mechanisms responsible for the hydration of SC are related to the irritation properties of the surfactants. Leveque et al. also suggested22 that the hyperhydration of SC is consecutive to the inflammation process. They demonstrated that the increase of TEWL was induced by SDS without removal of SC lipids. SDS might influence not only SC barrier function, but also the nucleated layer of epidermis and dermal system associated with inflammation.23 Recently, no correlation was found between the level of epidermal hyperplasia and TEWL increase on the SDS-irritated skin.23 Further work would be needed to determine the effects of surfactants on skin. 

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. 

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. 

The amorphous nucleation layer has the consequence that the Fermi level of the growing ZnO films reaches its equilibrium value already at very low thickness ( 2nm). This is particularly important for ZnO Al films, where the Fermi level changes by more than 1 eV upon the addition of oxygen to the sputter gas. The amorphous nucleation layer, therefore, substitutes the space charge layer, which is usually necessary for charge equilibration at the interface. This important effect is illustrated in Fig. 4.25. 

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...

Fig. 4.30. Evolution of valence band maxima in dependence on ZnO deposition time as derived from core-level binding energies of the spectra shown in Fig. 4.29. The ZnO films were deposited by magnetron sputtering from an undoped ZnO target at room temperature using 15 W dc power. Core level to valence band maxima binding energy differences are comparable to those presented in Fig. 4.15 for ZnO and to those given in [36] for Cu(In,Ga)Se2. The different evolution of the Zn2p and O Is derived valence band positions for ZnO deposition times indicates the presence of an amorphous nucleation layer, as already discussed in Sect. 4.3.2...

According to our experience, it is more difficult to determine a reliable valence band offset for the Cu(In,Ga)Se2/ZnO interface than for the CdS/ZnO interface. This is related to the lower substrate core-level intensities because of the presence of multiple cations. The substrate intensity might, therefore, be already completely suppressed when the Zn 2p and the O Is derived valence band maxima (see filled circles and squares in Fig. 4.30) reach the same value, and, therefore, reflect a proper ZnO valence band maximum (end of the amorphous nucleation layer). This difficulty is not present in the data set in Fig. 4.30 and for a deposition time of 64 s a valence band offset of A/ y vis = 2.15 0.1 eV can be determined. In another experiment, we have derived a slightly smaller valence band offset of AEyb = 1.98 0.2 eV [70]. The larger uncertainty is due to the above-mentioned difficulties. 

Fig. 4.37. Evolution of valence band maxima of In2S3 and ZnO Al for the two experiments displayed in Figs. 4.35 (left) and 4.36 (right). The difference between the curves derived from the Zn 2p and O Is level at low coverage indicates the presence of an amorphous nucleation layer. Reproduced with permission from [136]...

Fig. 7.15. Peak behavior of Hall mobility at 300 K of undoped ZnO thin films on c-plane sapphire at carrier concentrations around 3 x 1016 cm-3. For these particular growth conditions with a target-to-substrate distance of 50 mm, a low-temperature nucleation layer was used. Reprinted with permission from [51]...

PLD in UHV (laser-MBE) MBE-like background pressure and in situ RHEED to ensure clean and controlled deposition of high-quality nucleation layers and films. For particular systems as SrTiC>3 and BaTiC>3, atomically smooth surface and interface were obtained [128,132]... 

A1 and O. Upon heat treatment in hydrogen atmosphere the surface flatness is much improved the RMS roughness is 0.2 - 0.3 ML (0.04 - 0.06 nm) [26,28], This heat treatment process is commonly used before growth of low temperature buffer (or nucleation) layers of AIN or GaN as shown in FIGURE 4 [24], Sapphire surfaces also become flat at a similar level by heat treatment in air [27],... 

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