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We investigate the effect of strain on the morphology and composition of GeSn layers grown on Ge/Si virtual substrates. By using buffer layers with controlled thickness and Sn content, we demonstrate that the lattice parameter can be tuned to reduce the strain in the growing top layer (TL) leading to the incorporation of Sn up to 18 at. %. For a 7 at. % bottom layer (BL) and a 11-13 at. % middle layer (ML), the optimal total thickness tGeSn = 250-400 nm provides a large degree of strain relaxation without apparent nucleation of dislocations in the TL, while incorporating Sn at concentrations of 15 at. % and higher. Besides facilitating the growth of Sn-rich GeSn, the engineering of the lattice parameter also suppresses the gradient in Sn content in the TL, yielding a uniform composition. We correlate the formation of the surface cross-hatch pattern with the critical thickness hG for the nucleation and gliding of misfit dislocations at the GeSn-Ge interface that originate from gliding of pre-existing threading dislocations in the substrate. When the GeSn layer thickness raises above a second critical thickness hN, multiple interactions between dislocations take place, leading to a more extended defective ML/BL, thus promoting additional strain relaxation and reduces the compositional gradient in the ML. From these studies, we infer that the growth rate and the Ge-hydride precursors seem to have a limited influence on the growth kinetics, while lowering temperature and enhancing strain relaxation are central in controlling the composition of GeSn. These results contribute to the fundamental understanding of the growth of metastable, Sn-containing group-IV semiconductors, which is crucial to improve the fabrication and design of silicon-compatible mid-infrared photonic devices.

Understanding the nature and behavior of vacancy-like defects in epitaxial germanium-tin (GeSn) metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as a function of Sn content, depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range. The investigated samples were grown by chemical vapor deposition method at temperatures between 300 and 330 °C. Regardless of the Sn content, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, relative to that of epitaxial Ge reference layers. These observations confirmed the presence of open volume defects in as-grown layers. The measured average positron lifetimes were found to be the highest (380-395 ps) in the region near the surface and monotonically decrease across the analyzed thickness, but remain above 350 ps.All GeSn layers exhibit lifetimes that are 85 to 110 ps higher than those recorded for Ge reference layers. Surprisingly, these lifetimes were found to decrease as Sn content increases in GeSn layers.

We report on the behavior of Ge-Ge, Ge-Sn, Sn-Sn like and disorder-activated vibrational modes in GeSn semiconductors investigated using Raman scattering spectroscopy. By using an excitation wavelength close to E1 gap, all modes are clearly resolved and their evolution as a function of strain and Sn content is established. In order to decouple the individual contribution of content and strain, the analysis was conducted on series of pseudomorphic and relaxed epitaxial layers with a Sn content in the 5-17at.% range. All vibrational modes display qualitatively the same behavior as a function of content and strain, viz. a linear downshift as the Sn content increases or the compressive strain relaxes. Simultaneously, Ge-Sn and Ge-Ge peaks broaden, and the latter becomes increasingly asymmetric. This asymmetry, coupled with the peak position, is exploited to implement an empirical approach to accurately quantify the Sn composition and lattice strain from Raman spectra.Understanding the behavior of different vibrational modes in a semiconductor is of paramount importance to probe its crystal phase and symmetry, composition, lattice strain, isotopic content, electronic and phononic properties. [1][2][3] In this regard, Raman scattering spectroscopy has thus become an ubiquitous characterisation technique as information-rich spectra are acquired from straightforward and non-destructive measurements. Therefore, it is commonly used to evaluate the chemical composition and lattice properties of, for instance, group-IV semiconductors such as strained Si, 4-6 strained Ge, 7-10 SiGe, [11][12][13][14] and GeSn layers. [15][16][17][18][19][20][21][22][23][24][25] The latter are particularly of growing interest because of their relevance to Si-compatible light emission and detection applications in the short-and mid-wavelength infrared, [26][27][28][29][30][31][32][33][34][35] which can lead to the integration of optoelectronic and photonic circuits on complementary metal-oxide-semiconductor (CMOS) platforms. [36][37][38] Previous reports on the vibrational modes of GeSn mainly focused on Ge-Ge longitudinal optical (LO) mode as the analyses relied on the use of 488 nm 15,16 or 532 nm 18-24 excitation lines. Under these conditions, the signal-to-noise ratio is too low to clearly distinguish Sn-related vibrational modes in the vicinity of the more prominent Ge-Ge LO peak. This also applies to the study of ternary SiGeSn semiconductors. [39][40][41] When using a 633 nm excitation laser, the signal-tonoise ratio is significantly enhanced, thus allowing a clear distinction of Ge-Ge and Ge-Sn modes, in addition to other features such as disorder-activated (DA) and Sn-Sn like modes. This higher sensitivity is attributed to the increase in Raman scattering cross section when the excitation wavelength becomes close to the material's E1 gap. 39,42 Oehme et al. 25 and D'Costa et al. 17 provided a quantitative description of the evolution of peak positions as a function of the composition. However, in these studies, the investigated sampl...

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.