Compound wurtzite materials surfaces

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. 

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. 

For noncubic compounds, the bulk crystal may be optically anisotropic. This is the case not only for wurtzite or chalcopyrite crystal structures but also for ternary and quaternary III-V and II-VI compounds showing ordering effects on the sublattices. In aU these cases, the compound semiconductor is an optically anisotropic material. The RAS signal is then a superposition of two contributions, one originating from the bulk and the other from the surface. In order to analyze the surface, the two contributions have to be separated, a task for optical modeling. 

---