Strain engineering in Ge/GeSn core/shell nanowires

Abstract

Strain engineering in Sn-rich group IV semiconductors is a key enabling factor to exploit the direct band gap at mid-infrared wavelengths. Here, we investigate the effect of strain on the growth of GeSn alloys in a Ge/GeSn core/shell nanowire geometry. Incorporation of Sn content in the 10-20 at.% range is achieved with Ge core diameters ranging from 50nm to 100nm. While the smaller cores lead to the formation of a regular and homogeneous GeSn shell, larger cores lead to the formation of multi-faceted sidewalls and broadened segregation domains, inducing the nucleation of defects. This behavior is rationalized in terms of the different residual strain, as obtained by realistic finite element method simulations. The extended analysis of the strain relaxation as a function of core and shell sizes, in comparison with the conventional planar geometry, provides a deeper understanding of the role of strain in the epitaxy of metastable GeSn semiconductors.Strained semiconductor heterostructures provide a rich playground for investigating the epitaxy of lattice-mismatched materials. 1 In the last decades SiGe alloys grown with a graded composition on Si were extensively studied to relieve strain by nucleating misfit dislocations in the buffer layers. [2][3][4] Recently, direct band gap and metastable GeSn alloys gained tremendous interest as a 2 platform for Si-compatible photonics operating at mid-infrared wavelengths. [5][6][7][8][9] In unstrained GeSn the direct band gap is achieved at Sn contents higher than 10at.%, hence well above the ~1at.% equilibrium solubility of Sn in Ge. The incorporation of Sn is highly sensitive to the inplane strain that the GeSn layer experiences during growth. 10,11 Due to the large lattice mismatch between Ge and α-Sn (>10%), the growth of GeSn layers has been developed on high-quality Gevirtual substrates (Ge-VS) on Si. 12,13 Partial strain relaxation can induce a compositional grading in GeSn, 8,14-16 eventually leading to segregation and precipitation of Sn, compromising material quality. [17][18][19] In GeSn layers grown on Ge-VS, the compressive strain is reduced in a multi-layer buffered heterostructure grown with different Sn contents by controlling the growth temperature 20,21 and precursors flows. 22 The high amount of strain induces nucleation of dislocations in the low (7-11at.%) Sn content buffer layers, 11,23 and the resulting uniform (plastic) strain relaxation enhances the Sn incorporation above 16at.% in the GeSn layers grown on top. 8,11,14,19 One-dimensional nanowires(NWs) provide additional degrees of freedom in tuning the effect of strain in the growth of lattice-mismatch heterostructures 24,25 when using a core/shell NW geometry. 26 The shell displays an increasing strain relaxation with thickness provided by the free surfaces at the sidewalls, while the elastic compliance of the NW core allows for enhanced strain relaxation in the shell, accommodating the lattice mismatch of the system and avoiding bending. 26,27 Recent studies on Ge/GeSn core/shell NWs 15,16,28,29...

Kinetic Control of Morphology and Composition in Ge/GeSn Core/Shell Nanowires

“…Interestingly, the model here discussed provides a nice description of the experimental findings, despite it does not cover the whole complexity of the GeSn growth dynamics, including the misfit strain and plastic relaxation, which might occur for large cores. 15 We conclude such further effects do not significantly affect the growth dynamics.…”

“…2a,c). Since the Ge cores expose six {112} and six {110} facets with similar lateral extension, 15 any change in the shell morphology occurring during the growth of the GeSn shell stems from the competition between the {112} and the {110} growth fronts. In a minimal model, we consider the GeSn growth process as resulting from the combined effect of deposition from the precursors in the gaseous phase and redistribution of adatoms by surface diffusion.…”

“…2). 15 The negative slope of � � also indicates that the incorporation of Sn becomes more favorable when considering Sn rich growth conditions (i.e.…”

Phase-field investigation of the stages in radial growth of core–shell Ge/Ge1−xSnx nanowires

“…Under this condition, significant inhomogeneity of Sn composition were observed, leading to largely altered wire geometry and surface properties. 18 As shown in Fig. 1a, starting with a stressfree [111] Ge nanowire, the Ge 1−x Sn x alloy with a Sn concentration x = 4.2% was continuously deposited under a constant chemical potential driving force.…”

“…14 To achieve better control over the quality and yield of the core-shell nanowires, a better understanding of the wire growth mechanisms is of great significance. For this purpose, recently several studies have been conducted, [15][16][17][18] where modeling and simulations were employed to explain the strain distributions across the cross section of the wire. In comparison with these static calculations, modeling the dynamics of the heterogeneous core-shell structure formation should provide new insight and description of the growth process from a different angle, which motivates the work presented in this paper.…”