Deposition sequence

As shown above, a thermodynamic analysis indicates what to expect from the reactants as they reach the deposition surface at a given temperature. The question now is, how do these reactants reach that deposition surface In other words, what is the mass-transport mechanism The answer to this question is important since the phenomena involved determines the reaction rate and the design and optimization of the CVD reactor. 

It should be first realized that any CVD process is subject to complicated fluid dynamics. The fluid, in this case a combination of gases, is forced through pipes, valves, and various chambers and, at the same time, is the object of large variations in temperature and to a lesser degree of pressure before it comes in contact with the substrate where the deposition reaction takes place. The reaction is heterogeneous which means that it involves a change of state, in this case from gaseous to solid. 

In some cases, the reaction may take place before the substrate is reached while still in the gas phase (gas-phase precipitation) as will be reviewed later. As can be expected, the mathematical modeling of these phenomena can be complicated. 

The sequence of events taking place during a CVD reaction is shown graphically in Fig. 2.3 and can be summarized as follows 1  

These steps occur in the sequence shown and the slowest step determines the deposition rate. The rules of the boundary layer apply in most CVD depositions in the viscous flow range where pressure is relatively high. In cases where very low pressure is used (i.e., in the mTorr range), the rules are no longer applicable. 

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Radiant heating, which in the past has been used mostly in experimental reactors, is gradually being introduced into production systems. The basic design is shown in Fig. 5.10, It is, of course, essential that the walls of the reactor be transparent to radiation and remain so during the deposition sequence, so that heating can proceed unhindered. 

The ECALE deposition of ternary II-VI compound semiconductors such as CdxZni xS, CdxZni xSe, and CdSjcSei c, on Ag(lll), has been reported [51-53]. The compounds were prepared by sequential deposition of the corresponding binaries in submonolayer amounts for instance, alternate deposition of CdS and ZnS was carried out to form Cd cZni cS. The stoichiometry of the ternaries was seen to depend on the deposition sequence in a well-defined and reproducible way, with the limit that only certain discrete x values were attainable, depending on the adopted sequence profile. Photoelectrochemical measurements were consistent with a linear variation of the band gap vs. the composition parameter x of the mixed compounds. 

The depositional sequences observed in evaporites do not generally follow the predictions obtained from a fixed-volume evaporation. This is largely due to natural processes that act to replenish the evaporated water with fresh seawater. The geohy-drological mechanisms by which this occurs are discussed in the next sections of this chapter. 

TiN film of approximately 300 A is typically used in the back-end interconnect process, as both the cap layer for the aluminum metal deposition sequence and an antireflective coating for the subsequent photolithography step. Since this TiN cannot be a substrate for oxide thickness measurement in ILD, the aluminum beneath the TiN must be used as the substrate. In other words, the TiN is a component of the film to be measured. Thus, its refractive index or thickness must be known to determine the unknown oxide thickness. However, the refractive index of TiN is not constant, but varies with thickness. As a result, the TiN thickness must be precisely controlled to enable the validity of the substrate modeling. 

To study the influence of the preparation conditions on the interface properties, a number of different interfaces have been prepared. Details of the preparation and the determined valence band offsets are listed in Table 4.2. The experiments include not only both deposition sequences, but also interfaces of Al-doped ZnO films, which have been conducted to elucidate the role of the undoped ZnO film as part of the Cu(In,Ga)Se2 solar cell. Details of the experimental procedures and a full set of spectra for all experiments are given in [70]. Table 4.2 includes a number of interfaces between substrates of undoped ZnO films and evaporated CdS layers (ZOCS A-D). In a recent publication [90] different values were given for the valence band offsets, as the dependence of BEvb(CL) on the deposition conditions was not taken into account in this publication. 

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. 

Interface formation between In2S3 and ZnO has also been studied for the reverse deposition sequence with Al-doped ZnO films used as substrates [70]. In this case, only degenerately doped substrates were used. Photoemission spectra indicate no chemical reactivity at the surface. 

The doped and intrinsic silicon layers (p, i, n) are packed between a TCO front contact and a highly reflective back contact. The back contact is usually either a metal like silver (Ag) or aluminum (Al), or a TCO/metal double layer structure. The latter has been shown to reduce absorption losses due to a better grain growth of Ag layers onto ZnO. Additionally, absorption losses due to surface plasmons in the metal film have to be considered [33]. Both effects result in a higher reflectivity of the TCO/Ag back reflector. In module production, magnetron sputtered ZnO is usually applied as TCO-material for the back reflector in combination with either Ag (highest reflectivity) or Al (low cost). Depending on the deposition sequence of the doped and intrinsic silicon layers, one speaks of so-called superstrate (p-i-n) or substrate (n-i-p) cell structure (see Fig. 8.4). 

Viles, H.A., Taylor, M.P., Nicoll, K. Neumann, S. (2007) Facies evidence of hydroclimatic regime shifts in tufa deposition sequences from the arid Naukluft Mountains, Namibia. Sedimentary Geology 195, 39-53. 

The depositional sequence within the two phosphatic shale members comprise sapropelic sediments that include peloidal phosphorite and subordinate phosphatic mudstone near the base, organic-carbon-rich mudstone within the central part, and peloidal phosphorite and phosphatic mudstone in the upper part. Shelfward, the bases of the shale members are in sharp contact with the underlying rocks, but the contacts seem to be sharply transitional in basinal areas. The upper contacts are transitional upward into the cherty beds of the overlying member in all areas. 

Hiranaka, K. Yoshimura, T. Yamaguchi, T. (1989). Effects of the Deposition Sequence on Amorphous Silicon Thin-Film Transistors, pn.. Appl. Phys., Vol. 28, 2197-2200, ISSN 0021-4922... 

Curtis, C.D. Coleman, M.L. (1986) Controls on the precipitation of early diagenetic calcite, dolomite and siderite concretions in complex depositional sequences. In Roles of Organic Matter in Sediment Diagenesis (Ed. Gautier, D.L.), Spec. Publ. Soc. Econ. Paleont. Miner., Tulsa, 38, 23-33. 

Stratigraphically, the sediments filling the basin can be subdivided into five major units (Matter et ai, 1980). The depositional sequence is shown in... 

Fig. 2. Schematic stratigraphy of the northern Apennines Epi-Ligurian sequences. (A) Upper Eocene-Lower Miocene succession. (B) Bismantova-Termina succession. S1-S4, depositional sequences B1-B3, arenite petrofacies. Bl, hybrid arenites B2, arkosic arenites B3, feldspathic lithic arenites 1, marls 2, resedimented arkosic and lithic arenites 3, hybrid arenites 4, silty marls. Modified from Amorosi Spadafora (1995).

The average framework compositions of the four depositional sequences are reported in Table 1. The triangular plots of Fig. 4 show the composition of the total framework (Fig. 4A), the terrigenous framework (Fig. 4B) and the fine-grained rock fragments (Fig. 4C). 

Table 1. Average and range of detrital, authigenic and textural components of the four defined depositional sequences (SI-S4) in the Bismantova-Termina succession in the Bologna area the number of samples is given in parenthesis...

Fig. 8. 6 CpDB versus 5 Opdb plot of representative diagenetic and detrital calcite in the various depositional sequences of the Bismantova-Termina succession. Arrow denotes the probable derivation of S4 cements from marine carbonate rock fragments.

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