Interfaces atomically abrupt

The fifth contribution by M. Putkonen and L. NiinistO presents an overview of Organometallic Precursors for Atomic Layer Deposition (ALD). The key principle of ALD in contrast to CVD is the exclusion of any gas-phase prereaction allowing the thin film growth to be fully controlled by surface reactions and adsorption/desorption kinetics. ALD is thus ideally suited for the growth of ultra-thin layers and atomically abrupt interfaces needed in future nanoelectronic devices. While CVD and ALD have many aspects in common, precursors suitable for ALD generally need to be much more reactive than those used for CVD. Another challenge is to combine low steric demand with very high selectivity of the surface reactions. 

Superconducting BSCCO films were produced in situ on 3" diameter sapphire substrates by plasma-enhanced halide LPCVD [266, 267]. Films consisting mainly of the Bi-2212 phase were deposited with metal halide precursors at 580°C under 0.1 Torr system pressure in the presence of a rf plasma. These films became superconducting at 70 K with = 2.5 x 10 Acm at 10 K. Plasma-enhanced halide LPCVD was also used to grow Bi-Sr-Ca-Cu-O/Bi-Sr-Cu-O superconductor-normal metal (S-N) heterostructures [259], HRTEM images showed the S/N interface to be atomically abrupt while variable temperature resistivity measurements gave T. = 75 K for the S/N heterostructure. 

Figure 2-41. Cross-sectional HRTEM image of a YBCO/PrGaOi/LaAIOi trilayer showing atomically abrupt interfaces as well as the presence of (fK)l) and (110) growth domains in the PrGaOi layer. The PrGaOi layer was grown by MOCVD and the YBCO layer by PLD.

In Chapter 12 Abeles and Tiedje describe a novel layered structure alternating between a-Si H and a-SiN , H with nearly atomically abrupt interfaces. Although the material is amorphous, a periodic superlattice is formed... 

In compositional analysis of very small precipitates, or in interface segregation studies, using a probe-hole type atom-probe, one is always faced with the fact that the probe-hole may cover both the matrix and the precipitate phases, or the interface as well as the matrix. Thus any abrupt compositional changes will be smeared out by the size of the probe-hole and also by the effect of ion trajectories. A similar uncertainty seems to exist in the compositional analysis of nitride platelets formed in nitrided Fe-3 at.% Mo alloy, aged between 450 and 600°C, where Wagner ... 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

The interface obtained by sputtering is not abrupt but on the contrary, it is a region of strong mixing of Al with Si, O and C atoms over a depth of several tens of A, as observed from the chemical shifts. This finding is also evidenced by another type of measurements related to the intensity measurements. Figure 8 shows for instance, the comparison of the variation of Al/O and C/O atomic ratios determined for the two types of interfaces as a function of Al thickness. The Al/O ratio increases with a smaller slope for sputtered interfaces when compared to evaporated interfaces. Furthermore, the O/C ratio is constant (= 0.55) for evaporated film indicating that the attenuation of... 

In all of these systems, certain aspects of the reactions can be uniquely related to the properties of a surface. Surface properties may include those representative of the bulk material, ones unique to the interface because of the abrupt change in density of the material, or properties arising from the two-dimensional nature of the surface. In this article, the structural, thermodynamic, electrical, optical, and dynamic properties of solid surfaces are discussed in instances where properties are different from those of the bulk material. Predominantly, this discussion focuses on metal surfaces and their interaction with gas-phase atoms and molecules. The majority of fundamental knowledge of molecular-level surface properties has been derived from such low surface area systems. The solid-gas interface of high surface area materials has received much attention in the context of separation science, however, will not be discussed in detail here. The solid-liquid interface has primarily been treated from an electrochemical perspective and is discussed elsewhere see Electrochemistry Applications in Inorganic Chemistry). The surface properties of liquids (liquid-gas interface) are largely unexplored on the molecular level experimental techniques for their study have begun only recently to be developed. The information presented here is a summary of concepts a more complete description can be found in one of several texts which discuss surface properties in more detail. ... 

As mentioned previously, CTRs arise as a result of the abrupt termination of a crystal lattice, and the diffuse diffracted intensity connects Bragg points in reciprocal space. In this case, the scattering vector is normal to the surface, and as a result, this technique is very sensitive to surface and interface roughness but not to in-plane atomic correlations. Thus, it yields information that is complementary to that obtained by grazing incidence diffraction. The most important feature of CTR is the characteristic decay of the scattered intensity described by Eq. (38). For surfaces that are not perfectly terminated (i.e., rough) the intensity will decay faster than predicted by this equation, and this can be used as a measure of root-mean-square surface roughness. 

Layered nanostructures can be deposited from the electrochemical environment by applying a time dependent voltage program to the working electrode (5) or by using a sequential deposition scheme such as electrochemical atomic layer epitaxy (EC-ALE) (6-10). In EC-ALE, a surface-limited electrochemical reaction, such as underpotential deposition (upd), is used to synthesize a binary compound by successive deposition of each element from its respective solution precursor. EC-ALE is an attractive electrosynthetic alternative to conventional deposition methods that is inexpensive, operates at ambient temperature and pressure and provides precise film thickness control. This technique promises to overcome many problems associated with other electrosynthetic approaches, such as the formation of highly polycrystalline deposits and interfacial interdiffusion. For example, we have recently used EC-ALE to fabricate stable semiconductor heterojunctions with extremely abrupt interfaces (11). 

There have been only a few major attempts to apply interface tracking to primary atomization [9, 10]. However, both of these codes treated all variables, excepting density, as continuous across the interface, which could have implications for their accuracy. For example, the discontinuity in viscosity causes the first derivative of velocity to change abruptly at an interface. 

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