Dislocation Pipe Diffusion and Solute Segregation during the Growth of Metastable GeSn

Abstract

Controlling the growth kinetics from the vapor phase has been a powerful paradigm enabling a variety of metastable epitaxial semiconductors such as Sn-containing group IV semiconductors (Si)GeSn. In addition to its importance for emerging photonic and optoelectronic applications, this class of materials is also a rich platform to highlight the interplay between kinetics and thermodynamic driving forces during growth of strained, nonequilibrium alloys. Indeed, these alloys are inherently strained and supersaturated in Sn and thus can suffer instabilities that are still to be fully elucidated. In this vein, in this work the atomic-scale microstructure of Ge0.82Sn0.18 is investigated at the onset of phase separation as the epitaxial growth aborts. In addition to the expected accumulation of Sn on the surface leading to Sn-rich droplets and sub-surface regions with the anticipated equilibrium Sn composition of 1.0at.%, the diffusion of Sn atoms also yields conspicuous Sn-decorated filaments with nonuniform Sn content in the range of ~1 to 11at.% .The latter are attributed to the formation and propagation of dislocations, facilitating the Sn transport toward the surface through pipe diffusion. Furthermore, the interface between the Sn droplet and GeSn shows a distinct, defective layer with Sn content of ~22at.%. This layer is likely formed by the expelled excess equilibrium Ge as the Sn solidifies, and its content seems to be a consequence of strain minimization between tetragonal Sn-rich and cubic Ge-rich equilibrium phases. The elucidation of these phenomena is crucial to understand the stability of GeSn semiconductors and control their epitaxial growth at a uniform composition.

Quantitative Correlation Study of Dislocation Generation, Strain Relief, and Sn Outdiffusion in Thermally Annealed GeSn Epilayers

“…The Ge 0.99 Sn 0.01 line belongs to the bright spot areas on the sample surface (inset in Figure b), which was confirmed using selective-area XRD measurements over the spot and off-spot regions. In addition, the occurrence of the rectangular-shaped spots is also consistent with recent reports on Sn segregation . Interestingly, the Ge 0.99 Sn 0.01 peak was not detected for sample S2 annealed for 8 h.…”

“…The low-temperature diffusion in relaxed layers is commonly explained by the Sn transfer along the dislocation lines that act as preferential diffusion channels. The mechanism for the so-called “pipe diffusion” of Sn atoms was recently proposed in ref , where it was argued that Sn transport is facilitated by the propagation of dislocations toward the surface, such as by threading arms of MDs. Moreover, the Sn transport toward the surface may lead to the accumulation of liquid Sn precipitates, which are known to play a major role in phase separation at temperatures above the eutectic temperature of GeSn (231 °C). , In these studies, the phase-separation process is believed to involve the free movement of Sn droplets on the sample surface that aid to decompose the GeSn layer with the subsequent redeposition of equilibrium-composition Ge 0.99 Sn 0.01 .…”

Midinfrared Emission and Absorption in Strained and Relaxed Direct-Band-Gap Ge1−xSnx Semiconductors

“…Thus, the control of both strain and composition uniformity is highly useful for engineering the emission operational range and linewidth in 024031-11 Ge 1-x Sn x optoelectronic devices [17]. Applications requiring a narrower spectral range would benefit from the use of uniform, Ge 1-x Sn x layers, where a large amount of Sn can be incorporated while avoiding phase segregation [49]. Whereas, a graded composition would enhance absorption at a larger layer thickness and cover a broader spectral range.…”

Combined Iodine- and Sulfur-Based Treatments for an Effective Passivation of GeSn Surface

“…1b. Compressive in-plane strain 𝜀𝜀 || =-0.8 % is estimated for the Ge0.9Sn0.1 layer, without showing phase separation and segregation of Sn at the surface 34 . XPS analysis was performed with a Mg Kα X-ray source at 1253.6 eV operated under an anode bias of 15 kV with an emission current of 20 mA.…”