Microscopic Properties of Thin Films: Learning About Point Defects
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rized as follows, (a) Thin films of the highest quality are bounded by interfaces that are microscopically rough. Moreover, thinfilm interfaces contain roughness on many length scales, each affecting a subset of the physical properties of interest.67 Tailoring this roughness to optimize specific properties of thin films is an important experimental task, (b) Thin films relax during typical processing steps, such as annealing or implantation, in ways not often encountered in bulk solids.a9This is because point defects injected during processing are affected by the multilayer nature of many thin films, which introduces inhomogeneities in at least one dimension, (c) Thin films can be used as microscopic laboratories to investigate the fundamental properties of intrinsic point defects.10 In this way, one can measure parameters such as the formation and migration energies of point defects, not easily separable in standard measurements in bulk materials. However, the analysis of these experiments is, in general, not unique, and must be based on results from more than one technique, and guided by theory.
Interfacial Roughness With a few important exceptions, such as the Si/SiO2 system, thin-film interfaces of the highest perfection are those between isostructural, crystalline materials with closely similar lattice parameters, differing only in composition. Traditional microscopic techniques, developed for structural examination of solids, are un-
able to reveal the configuration of such "chemical interfaces." However, chemical lattice imaging reveals compositional variations in the sample by encoding them into pattern changes in the image.11 Pattern recognition methods can then be used to quantify the local pattern changes, and hence compositional variations in the sample.12 Figure la is a chemical lattice image of a GaAs/AlQ4Ga()6As multilayer, and Figure lb is the quantitative chemical map deduced from it by pattern recognition. In this figure, the height of the surface represents the local composition of the sample. Chemical mapping, reviewed in detail elsewhere,11 is able to measure the composition of individual atomic columns in a range of materials, often with near-atomic sensitivity. Chemical maps such as Figure lb show that interfaces of the highest quality are microscopically rough.4 This was initially surprising, because many properties of semiconductor thin films (such as the luminescence properties of quantum wells) had been interpreted to indicate a total absence of microscopic roughness. Real interfaces contain roughness over many length scales ranging from microscopic to macroscopic. Each experimental technique probes only that part of the roughness lying within its characteristic "acceptance window".6'713 This window is delimited by the field of view and the spatial resolution of the technique in question. To describe an interface adequately, it is necessary to compile data from a variety of experimental techniques, each of which reveals only a part of the "roughness spectrum".6 Here, an important point
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