Surface reconstruction of half-Heusler semiconductors – CoTiSb and NiTiSn

Heusler materials are an exciting category of materials and are believed to be ternary cousins to III-Vs, exhibiting wider range of electronic and magnetic properties than III-Vs. These materials can be grown on III-Vs and can potentially be integrated into novel electronic devices. Many of the electronic applications depend on electronic surface states, like spin injection in MTJs. Therefore, it is critical to understand the surface structure and electronic properties, and the driving forces of surface reconstruction in these materials. CoTiSb and NiTiSn are representative of a large class of 18 valence electron count half-Heusler materials, and have demonstrated promising applications in energy related fields. Here we explore the mechanisms of surface reconstruction of CoTiSb and NiTiSn (001) using density functional theory calculations. We present a simple electron counting model which explains the stability and driving force for Sb-Sb or Sn-Sn dimerization, similar to conventional semiconductors. We find that the (001) surface is metallic due to unsatisfied dangling bonds and that the surface energy is minimized by forming Sb-Sb or Sn-Sn dimers at low Ti surface coverage. We then compare our theoretical results with available experimental data.