Free-running Sn precipitates: an efficient phase separation mechanism for metastable Ge1\u2212xSnx epilayers

Heiko Groiss,Dagmar Gerthsen,Martin Glaser,Dietmar Roth,Peter Bauer,Magdalena Schatzl,Moritz Brehm,Friedrich Schäffler

doi:10.1038/s41598-017-16356-8

Abstract

The revival of interest in Ge1−xSnx alloys with x ≥ 10% is mainly owed to the recent demonstration of optical gain in this group-IV heterosystem. Yet, Ge and Sn are immiscible over about 98% of the composition range, which renders epilayers based on this material system inherently metastable. Here, we address the temperature stability of pseudomorphic Ge1−xSnx films grown by molecular beam epitaxy. Both the growth temperature dependence and the influence of post-growth annealing steps were investigated. In either case we observe that the decomposition of epilayers with Sn concentrations of around 10% sets in above ≈230 °C, the eutectic temperature of the Ge/Sn system. Time-resolved in-situ annealing experiments in a scanning electron microscope reveal the crucial role of liquid Sn precipitates in this phase separation process. Driven by a gradient of the chemical potential, the Sn droplets move on the surface along preferential crystallographic directions, thereby taking up Sn and Ge from the strained Ge1−xSnx layer. While Sn-uptake increases the volume of the melt, single-crystalline Ge becomes re-deposited by a liquid-phase epitaxial process at the trailing edge of the droplet. This process makes phase separation of metastable GeSn layers particularly efficient at rather low temperatures.

Highlights

This process differs substantially from phase separation of immiscible solids based on solid-state diffusion, such as, the precipitation of Si in the Si/Al system[41] or topological transitions and surface-mediated growth of nanostructures formed by immiscible phases in the PbTe/CdTe42 or the ErSb/GaSb43 heterosystems, respectively
Contact to hydrophilic and hydrophobic regions can be distinguished by the contact angle of the liquid, which is shallower in regions where the droplet wets the surface
A related behavior can be qualitatively observed in our in-situ video sequences (Supplementary Materials Section S3) in which the Sn droplets are in the liquid state and moving: Under this condition the contact angle of the Sn droplet is shallower in the GeSn region in front of the droplet, and steeper on the side of the trail, indicating a higher degree of wetting in the former region

Summary

Experiments

Sample series A was grown as a reference for an assessment of the accessible growth conditions in our MBE system. The homogeneity of the GeSn layers gets lost during film growth above TEC To further assess this finding, we recorded AFM and SEM images of the degraded samples of series B. Investigations of sample B6 by TEM (Fig. S2 in the Supplementary Materials) and EDXS revealed that the film between the solidified droplets is single crystalline Ge with a small fraction of dissolved Sn, whereas the droplets consist of β-Sn. above TEC the Ge0.9Sn0.1 films become phase-separated already during MBE growth, with the Sn phase segregating at the film surface in liquid form, as inferred from the shape of the precipitates. The volume of the Sn precipitates increases gradually as a function of time until the droplets come to a halt as they run into trail regions of other droplets This can be seen in the sequence of still images in Fig. 4 that are extracted from video sequence V1. For the FFTs the entire images of (c) and (e) were used

Findings

Discussion

Methods