Surface Structure of Zinc Blende Materials

Most III-V and II-VI compound semiconductors crystallize in the cubic zinc blende structure. This applies to the binary III arsenides. III phosphides, and III antimonides, for example, GaAs, GaP, InAs, InP, GaSb, and InSb, and also to some II-VI compounds such as ZnSe. 

One important factor that determines the surface formation is the crystalline surface orientation of the zinc-blende-type materials, that is, which of the main surface orientations is considered. In the following, we focus on the 001, 110, and 111 surfaces. The chemical composition of the surface plane is then another important parameter determining surface structure formation. Despite these crystallographic and stoichiometric variations, there exist common rules for structure formation that are derived from the chemical properties of the elemental constituents of the compound. 

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In contrast, for studying materials with an isotropic bulk structure, for example, zinc blende, RAS is a very versatile optical method for surface analysis. In this case, the optical anisotropy is surface specific, regardless of the large penetration depth of the light, which usually far exceeds the surface region (a few atomic layers). Therefore, the RAS represents a quite usefiil optical method for surface investigations of zinc blende materials. 

The wurtzite crystal structure is prominent in the class of II-VI compounds (ZnO, CdS, etc.) and, concerning the III-V materials, the III nitrides, tliat is, GaN, AlN, InN, and their multinary compounds. In the following, we will discuss surface structures of wurtzite crystals, using III nitrides as examples. The fundamental rules of surface formation are the same as for the zinc blende structures, treated in the last chapters 12.3.2 in detail. 

Characteristic of the c-plane (0001) and (0001) surfaces are N or group III adatom-terminated structures and for very group-III-rich surface conditions, the formation of metallic surface bilayers. The atomic structure of these polar surfaces ((0001) and (OOOT)) is comparable to the 111 surfaces of zinc blende III-V materials (Section 13.2.2). Nonpolar surfaces of the group III nitrides are the m-plane (1100) and the a-plane (1120), which are similar to the (110) cleavage planes. 

Most surfaces of compound semiconductors are polar, that is, the number of anions and cations per surface unit cell is not balanced. While for the zinc blende materials there is only one nonpolar exception, the (110) face, for the wurtzite structures, there are two nonpolar surfaces, the m-plane (1100) and a-plane (1120) [98]. In wurtzite materials, a (110) surface does not exist because of the different crystal structure. 

An attempt was made in this paper to sketch the behavior of elemental semiconductors (with the diamond-type structure) and of the IH-V compounds (with the zinc blende strut ture) in aqueous solutions. These covalent materials, in contrast to metals, exhibit properties which sharply reflect their crystalline structure. Although they have already contributed heavily to the understanding of surfaces in general, semiconductors with their extremely high purity, crystalline perfection, and well-defined surfaces are the most promising of materials for surface studies in liquid and in gaseous ambients. 

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