Surfaces, interfaces, grain boundaries

This section gives a brief summary of the most critical aspects of surfaces, interfaces and grain bormdaries. Numerous books have been written on this subject. For a single source on the physics of surfaces the reader is referred to Desjonqueres or Zangwill in the recommended readings. 

The effect of large densities of defect states at an interface was alluded to in Chapter 3 in the discussion of real Schottky barriers. Here even the presence of a metal in contact with the semiconductor does not supply enough electrons to fill all of the defects and still deplete the semiconductor, as would be expected for an ideal Schottky diode. 

Typical surfactants have a difference in valence of one electron relative to the atom they replace. Thus, As atoms passivate Si surfaces, while S atoms can be used to passivate GaAs. H treatment of a Si surface reduces its reactivity significantly and passivates surface states. However, H is a small atom and relatively weakly bound to the surface. Furthermore, it reacts readily with other atoms that may adsorb. Hence, H passivation is delicate and of limited long-term value. Of more significance is that H can fit into many vacancies in solids, will passivate their dangling bonds, and so reduces the number of electrically-active defects. This is one reason why many chemical vapor deposition crystal growth processes include copious amounts of H as a dilutant gas. It not only contributes to control of the deposition reaction process, but it helps reduce the effects of growth defects. 

At this point, we will not belabor the issue of surfaces and interfaces further. To summarize the situation, their primary behavior results from large densities of dangling bonds. The effects of these dangling bonds are similar to the effects of vacancies and their influence may be reduced by the presence of surfactants if appropriate ones can be found. 

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Surfaces, interfaces, grain boundaries, and second phase precipitates may accumulate charge due to local variations in work function as in any heterojunction. 

X-Ray Emission and Fluorescence. X-ray analysis by direct emission foUowing electron excitation is of Hmited usefulness because of inconveniences in making the sample the anode of an x-ray tube. An important exception is the x-ray microphobe (275), in which an electron beam focused to - 1 fim diameter excites characteristic x-rays from a small sample area. Surface corrosion, grain boundaries, and inclusions in alloys can be studied with detectabiHty Hmits of -- 10 g (see Surface and interface analysis). 

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

The microstructure of YBa2Cu30y thin films and related heterostructures, which are the basis for several types of edge-based Josephson junction were investigated by high-resolution transmission electron microscopy. The results show that the microstructure, interfaces, grain boundaries and defect configurations in these films are strongly influenced by the substrate and the often complex structure of the film surfaces on which the subsequent layers are deposited. 

Most oxides contain impurities, which in an attempt to reduce the strain energy of the system tend to migrate to the interfaces, grain boundaries, and surfaces. It is usually the segregation of these impurities that is responsible for the surface charge. This charge is usually compensated, however, with bulk ionic defects. 

Nowotny, J., Surface and grain boundary segregation in metal oxides, in Surfaces and Interfaces of Ceramic Materials, L.-C. Dufour, C. Monty and G. Petot-Ervas (eds), Kluwer Academic Publ., Dordrecht, 205-39, 1989. 

The formation of oxide ion vacancies is expected to be facilitated at surfaces and grain boundaries of ceria due to the lower coordination number of oxide ions occupying such sites. The expected enhanced reducibility of surfaces and interfaces is apparent in nanocrystalline ceria due to the increased surface and grain-boundary density. Indeed, a number of experimental results using various measuring techniques provide unequivocal evidence for the enhanced reducibility of ceria surfaces and interfaces. Early evidence came from temperature-programmed reduction by Hg (Hg-TPR) experi-ments. " An analysis of the H2-TPR results of Zimmer et al. by... 

Another interesting consequence of the increased surface and grain-boundary density of nanocrystalline ceria is the enhanced solubility of dopants with a large size mismatch as they can be accommodated at the interfaces. The apparent solubility of copper in nanocrystalline CeOs.x was determined to be 10 mol%/ which is a significant enhancement relative to microcrystalline ceria, where a solubility limit of 1 mol% is expected (see Section 12.2.2.2). 

Because densification occurs via tire shrinkage of tliennodynamically unstable pores, densification and microstmcture development can be assessed on tire basis of tire dihedral angle, 0, fonned as a result of tire surface energy balance between tire two solid-vapour and one solid-solid interface at tire pore-grain boundary intersection [, 78, 79 and 80],... 

The performance characteristics of ceramic sensors are defined by one or more of the foUowing material properties bulk, grain boundary, interface, or surface. Sensor response arises from the nonelectrical input because the environmental variable effects charge generation and transport in the sensor material. 

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

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