Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

Abstract

Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase-change material Ge 2 Sb 2 Te 5 as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless permits the detection of the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable Ge 2 Sb 2 Te 5 . First, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of 10 −4 nm −2 are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the buildup of vacancy layers.

Electrical and optical properties of epitaxial binary and ternary GeTe-Sb2Te3 alloys

“…The value for Sb 2 Te 3 is in good agreement with the energy gap determined by scanning tunneling microscopy (STM) 30 , whereas the optical gap of α-GeTe is in a good agreement with results of DFT calculations 31 . For GST225, the optical gap is also in good agreement with recent STM data 32 . Lately, GeTe/Sb 2 Te 3 superlattices were studied using FTIR 33 .…”

Strong and Weak 3D Topological Insulators Probed by Surface Science Methods

“…The particular sample grown by MBE exhibits a defect density of ∼ 1.5 · 10 12 /cm 2 . This implies a potential disorder due to the positive charging of most of the defects, in particular, vacancies [45,114,122]. The dI/dV (V ) curves ( Fig.…”

“…This gives direct access, e.g., to spatial variations of the band gap for a semiconductor or insulator. Figure 6(a) shows the (111) surface of the strong 3DTI Ge 2 Sb 2 Te 5 exhibiting Te as the top layer with hexagonal atomic structure [45]. Several, largely triangular bright protrusions appear on top of the atomic lattice ( Fig.…”

“…6(b)−(d)). They have been identified as subsurface defects by comparison with DFT data [45]. The lateral size of the triangle increases with the depth of the defect below the surface.…”

“…The potential disorder can be mapped on small length scales by scanning tunneling spectroscopy (STS). [ 46,47 ] Therefore, one either uses the spatial variation of features in the local density of states (LDOS) related to E D or the band edges [ 48–50 ] or, more precisely in energy, by spatially tracking Landau level energies in magnetic field B . [ 39,51,52,53,54 ]…”