Extended X-ray absorption fine structure investigation of Sn local environment in strained and relaxed epitaxial Ge1−xSnx films

Abstract

We present an extended X-ray absorption fine structure investigation of the local environment of Sn atoms in strained and relaxed Ge1−xSnx layers with different compositions. We show that the preferred configuration for the incorporation of Sn atoms in these Ge1−xSnx layers is that of a α-Sn defect, with each Sn atom covalently bonded to four Ge atoms in a classic tetrahedral configuration. Sn interstitials, Sn-split vacancy complexes, or Sn dimers, if present at all, are not expected to involve more than 2.5% of the total Sn atoms. This finding, along with a relative increase of Sn atoms in the second atomic shell around a central Sn atom in Ge1−xSnx layers with increasing Sn concentrations, suggests that the investigated materials are homogeneous random substitutional alloys. Within the accuracy of the measurements, the degree of strain relaxation of the Ge1−xSnx layers does not have a significant impact on the local atomic surrounding of the Sn atoms. Finally, the calculated topological rigidity parameter a** = 0.69 ± 0.29 indicates that the strain due to alloying in Ge1−xSnx is accommodated via bond stretching and bond bending, with a slight predominance of the latter, in agreement with ab initio calculations reported in literature.

The thermal stability of epitaxial GeSn layers

“…In our analysis, we have used the linear interpolation between α-Sn (6.49 Å) and Ge (5.657 Å) diamond lattice parameters. 9 The composition values of the as-grown samples are in very good agreement with those measured by Rutherford backscattering spectrometry, witnessing the accuracy of the procedure used. All the three as-grown relaxed GeSn layers still show a residual heteroepitaxial compressive strain (i.e., ε 0 < 0), corresponding to a strain relaxation in the 70% and 79% range.…”

“…[6][7][8][9] Furthermore, the strain relaxation mechanisms in GeSn remain unclear and contradictory reports can be found in the literature. 10 This is due to the fact that several experiments were performed adopting an interpretative framework similar to that developed for SiGe alloys, a material system for which the strain relaxation and dislocation dynamics are well understood and often aprioristically considered valid for all the group IV alloys.…”

“…In fact, the highly defected lattice close to the dislocation core, with its rather loose covalent bonds, enables the occupation of interstitial sites by Sn atoms, which can subsequently bind and form β-Sn defects. 9 Consequently, in the PS5 pseudomorphic sample [ Fig. 3(d)], the Sn segregation is limited to the top sample surface only.…”

Short-range atomic ordering in nonequilibrium silicon-germanium-tin semiconductors

“…Since bulk diffusion is energetically less favorable than surface diffusion, it is reasonable to conclude that the observed departure for an ideal solid solution occurs during the layer-by-layer growth. It is important to note that this phenomenon is peculiar to Sn-rich ternary alloys since extended x-ray absorption fine structure (EXAFS) investigations indicated the atomic distribution in Sn-rich strained and relaxed GeSn binary alloys to be random [28], also asserting the fact that epitaxial strain is not responsible for the observation we made in Fig 2. Interestingly, the analysis of ternary layers with lower Sn contents 4 at. % indicates that the aforementioned departure from a perfectly random alloy is either absent or too small to be detected.…”

Pushing the Composition Limit of Anisotropic Ge1–xSnx Nanostructures and Determination of Their Thermal Stability

“…This calculated value is quite accurate for structures, where no strain from a substrate has to be taken into account; 25,26,27,30 however, for highly accurate determination of the composition of surface bound epitaxial layers a small deviation is corrected by the bowing parameter which is highly dependent on the literature reports. 47,48,49 In an earlier report, we quantified the Sn concentration via EDX using the Sn(K) line, which leads to an underestimation of the actual Sn content in Ge 1−x Sn x NWs 33 . In addition, the large variation of the related Raman shift in literature on thin epitaxial films can be misleading.…”