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

Abstract

The growth of Sn-rich group-IV semiconductors at the nanoscale provides new paths for understanding the fundamental properties of metastable GeSn alloys. Here, we demonstrate the effect of the growth conditions on the morphology and composition of Ge/GeSn core/shell nanowires by correlating the experimental observations with a theoretically developed multi-scale approach. We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires changes from hexagonal (bounded by {112} facets) to dodecagonal (bounded by both {112} and {110} facets) upon increasing the supply of the Sn precursor. This transformation strongly influences the Sn distribution as a higher Sn content is measured under {112} facets. Ab-initio DFT calculations provide an atomic-scale explanation by showing that Sn incorporation is favored at the {112} surfaces, where the Ge bonds are tensile-strained. A phase-field continuum model was developed to reproduce the morphological transformation and the Sn distribution within the wire, shedding light on the complex growth mechanism and unveiling the relation between segregation and faceting.pulling during the shell growth. 14 However, at larger core diameters the shell will experience higher strain, eventually inducing plastic deformation with the nucleation of defects and surface roughening. 18,19 In this work, we show how the morphology and composition of the GeSn shell on a Ge core NW are strongly dependent on the growth conditions. At a higher supply of the Tintetrachloride (SnCl4) precursor, the symmetry of the NW cross-section changes from 6-fold to 12-fold by increasing the size of the six {110} (corner) facets in the GeSn shell with respect to the (main) six {112} facets. At the same time, enhanced segregation is observed, with an increasing difference in composition between Sn-poor <110>-oriented stripes and Sn-rich {112}oriented facets. In addition, at the highest supply of the Sn precursor, phase separation occurs and multiple Sn droplets are visible on the NW sidewall. The experimental observations are then rationalized theoretically by a multi-scale approach. First, the shape transition will be interpreted by a continuum kinetic growth model, including surface diffusion. Then, first principle calculations will be exploited to assess the origin of the different composition within the facets and to extend the growth model in order to simultaneously trace the evolution of shape and composition. The agreement between experiments and theory highlights the strong correlation between faceting and segregation dynamics in the Ge/GeSn core/shell NW system. RESULTS AND DISCUSSIONExperimental analysis. The effect of the SnCl4 precursor flow on the morphology of the GeSn shell grown around 100 nm Ge cores is shown in Fig. 1. A fixed growth time of 2 h was used in combination with a Ge/Sn ratio in gas phase ranging from 1285 to 300. An increase in the diameter of the core/shell NWs is visible with increasing (decreasing) supply of SnCl4

Thermodynamic driving force in the formation of hexagonal-diamond Si and Ge nanowires

“…The calculated surface energy value is reported in Table 1, both for 3C-Si and Ge, and it is compared to the value calculated for the (110) surface. In fact, this is another crystal orientation that has been identified in the delimiting facets of 3C-Si or Ge NWs grown in the [111] direction, particularly in case of the dodecagonal morphology of the NWs [37,43]. Indeed, the surface energy of the Ge(110) is very close to that calculated for the Ge(112), and an equilibrium shape of the [111] NWs showing 12 facets with altering orientations is thermodynamically consistent.…”

Strain engineering in Ge/GeSn core/shell nanowires

“…A detailed understanding of this complex behavior, mostly influenced by the Sn incorporation dynamics, is beyond the scope of the present Letter, and will be addressed in a separate work. 30 We now focus on the characterization of the residual strain in the GeSn shell. A quantitative determination of the strain distribution in cross-sectional TEM samples using electron diffraction is challenging.…”

Rational Control of GeSn Nanowires

“…[ 108,109 ] By increasing the supply of Sn atoms in the gas phase during growth, the cross‐section morphology of Ge/GeSn core/shell NWs changes from hexagonal to dodecagonal and Sn segregation is enhanced. [ 110 ] This transition strongly affects the Sn distribution, resulting in a higher Sn content along the [112] direction compared to the one in the [110] direction as shown in Figure 10b. [ 110 ] By establishing a multi‐scale analysis model reproducing the shape and composition profiles observed in the experiments, the authors [ 110 ] propose that the interaction between surface dependent incorporation and surface adsorption atomic dynamics is the key process that determines Sn segregation and GeSn shell morphology.…”