Amorphous nucleation

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]...

A molten metal alloy would normally be expected to crystallize into one or several phases. To form an amorphous, ie, glassy metal alloy from the Hquid state means that the crystallization step must be avoided during solidification. This can be understood by considering a time—temperature—transformation (TTT) diagram (Eig. 2). Nucleating phases require an iacubation time to assemble atoms through a statistical process iato the correct crystal stmcture... 

In an amorphous material, the aUoy, when heated to a constant isothermal temperature and maintained there, shows a dsc trace as in Figure 10 (74). This trace is not a characteristic of microcrystalline growth, but rather can be well described by an isothermal nucleation and growth process based on the Johnson-Mehl-Avrami (JMA) transformation theory (75). The transformed volume fraction at time /can be written as... 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Fine-grained polycrystals Amorphous deposits Gas-phase nucleated snow... 

Nucleation. Crystal nucleation is the formation of an ordered soHd phase from a Hquid or amorphous phase. Nucleation sets the character of the crystallization process, and it is, therefore, the most critical component ia relating crystallizer design and operation to crystal size distributions. 

C fi3 diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film. 

This approach is an alternative to quantitative metallography and in the hands of a master gives even more accurate results than the rival method. A more recent development (Chen and Spaepen 1991) is the analysis of the isothermal curve when a material which may be properly amorphous or else nanocrystalline (e.g., a bismuth film vapour-deposited at low temperature) is annealed. The form of the isotherm allows one to distinguish nucleation and growth of a crystalline phase, from the growth of a preexisting nanocrystalline structure. 

In the secondary nucleation stage, the remaining amorphous portions of the molecule begin to grow in the chain direction. This is schematically shown in Fig. 16. At first, nucleation with the nucleus thickness /i takes place in the chain direction and after completion of the lateral deposition, the next nucleation with the thickness k takes place, and this process is repeated over and over. The same surface nucleation rate equation as the primary stage can be used to describe these nucleation processes. 

Using the fluxing technique, Lau and Kui [33] determined that the critical cooling rate for forming a 7-mm diameter bulk amorphous Pd4QNi4()P2o cylinder was 0.75 K/sec. From this value, they estimated that the steady-state nucleation frequency was on the order of lO" m s. On the other hand, Drehman and Greer [34] estimated that the steady state nucleation frequency at 590 K is 10 m" s, which is also the maximum... 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

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