Surface Reconstructions of Heusler Compounds in the Ni-Ti-Sn (001) System

TL;DR

This study investigates surface reconstructions of NiTiSn(001) and Ni1+xTiSn(001) Heusler compounds using experimental techniques and DFT calculations to optimize growth conditions for thin-film synthesis.

Contribution

It provides a detailed analysis of surface reconstructions in Ni-Ti-Sn Heusler compounds, combining experimental observations with theoretical modeling.

Findings

Sn-rich samples show c(2x2) reconstruction

Ti-rich samples exhibit (1x1) surface

Stoichiometry influences surface reconstruction type

Abstract

As progress is made on thin-film synthesis of Heusler compounds, a more complete understanding of the surface will be required to control their properties, especially as functional heterostructures are explored. Here, the surface reconstructions of semiconducting half-Heusler NiTiSn(001), and Ni1+xTiSn(001) (x=0.0-1.0) are explored as a way to optimize growth conditions during molecular beam epitaxy. Density functional theory (DFT) calculations were carried out to guide the interpretation of the experimental results. For NiTiSn(001) a c(2x2) surface reconstruction was observed for Sn rich samples, while a (1x1) unreconstructed surface was observed for Ti-rich samples. A narrow range around 1:1:1 stoichiometry exhibited a (2x1) surface reconstruction. Electrical transport is used to relate the observed reflection high energy electron diffraction (RHEED) pattern during and after growth with carrier concentration and stoichiometry. Scanning tunneling microscopy and RHEED were used to examine surface reconstructions, the results of which are in good agreement with density functional calculations. X-ray photoelectron spectroscopy was used to determine surface termination and stoichiometry. Atomic surface models are proposed, which suggest Sn-dimers form in reconstructed Ni1+xTiSn(001) half-Heusler surfaces (x<0.25) with a transition to Ni terminated surfaces for x > 0.25.