Chemical Interfaces: Structure, Properties, and Relaxation
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oncepts a n d e x p é r i m e n t a l r e s u l t s , however, are more generally valid.
Définition of a Chemical Interface The (chemical) interface between two lattice-matched, isostructural materials can be uniquely defined on ail length scales, provided each atom type occupies an ordered set of lattice sites. As an example, consider the GaAs/AlAs System. The interface is simply the plane across which the occupants of the Group III sublattice change from Ga to
Al. The interfacial plane thus defined can in principle hâve a complex shape or "waveform," with undulations ranging from atomic to macroscopic length scales. Therefore, it is convenient to describe an interface in terms of its Fourier spectrum, by specifying the amplitude of the undulations as a function of their spatial frequency. 1 A "white noise" interface, for example, would be characterized by a constant roughness amplitude over ail possible length scales. When a given expérimental technique is used to investigate an interface, it provides information about the interfacial configuration within a certain frequency window — delimited on the high frequency side by the spatial resolution of the technique and on the low frequency side by its field-of-view. For any single technique, this window spans only a small portion of the spatial frequencies needed for a realistic description of the interface. It is thus necessary to collate the data from a large variety of techniques to obtain a realistic picture of the interfacial configuration. This is a major challenge, because information from the atomic to the centimeter range (i.e., over eight orders of magnitude) is required to provide such a description. Often, however, only spécifie characteristics, such as the optical or electronic properties of an interface are of concern, and knowledge of a limited range of frequencies is adéquate.
Figure 1. Schematic représentation of interface formed by depositing random alloy Al04Ga06As on an atomically smooth GaAs surface (cross-sectional view). Only Group 111 atoms are shown. White represents pure Al, black pure Ga, and other shades of gray represent intermediate compositions. In each panel, the composition of each atomic column is averaged over the "thickness" of the sample. When the sample is only one monolayer thick (i.e. when there is no averaging), no continuous Une can be drawn to contain only the Ga (or only the Al) atoms, illustrating that an interface cannot be defined on an atom-by-atom basis. Only as the "thickness" increases, does the interface become well-defined.
MRS BULLETIN/SEPTEMBER1990
Chemical Interfaces: Structure, Properties, and Relaxation
The simple définition of an interface in terms of the location of the chemical constituents becomes inadéquate when either of the parent materials is not chemically ordered (i.e., when some of the atom types are distributed randomly one a set of sites).1"3 In the GaAs/
AlxGaj.x As System, for example, the second material is a random alloy. Thus, the Ga and Al atoms are distributed randomly on the Group I
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