Hexagonal SiGe Phase Revealed by Nano-XRF and XLD XAS

Highly crystalline hexagonal Si_1-xGe_x nanowires (NWs) are tunable light emitting semiconductors that opens the silicon technology to photoelectronic applications. Under perpendicular crystallographic orientations, by using a hard X-ray nanobeam it is possible to probe axial vs radial coordination environment and detect any preferential order modulation that impacts the anisotropic optical response. This proposal aims to record XEOL-detected X-ray linear dichroism (XLD) microscopy around Ge atoms in Si_1-xGe_x alloyed NWs to illustrate 2D projections of the optical anisotropies that relies on different short-range structural orders.<br/>Although silicon is a critical material in semiconductor industry, its indirect electronic band gap structure makes silicon an inefficient light emitter, limiting its applications. Cubic Si has the lowest conduction-band minimum close to the high-symmetry X-point and a second-lowest minimum at the L-point. On the other hand, cubic Ge also has an indirect bandgap, but its lowest conduction-band minimum is located at the L-point. Thus, by modifying the crystal structure from cubic to hexagonal, the symmetry along the &lt;111&gt; crystal direction can be changed. As a consequence, for hexagonal Ge the band-folding effect results in a direct bandgap at the Γ-point with a magnitude close to 0.3 eV. However, the goal of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades. Recently, Fadaly et al. demonstrated efficient light emission from direct bandgap hexagonal Ge and SiGe NWs¹. These findings were possible thanks to the growth of Ge-rich Si_1−xGe_x alloys around a thin (about 35 nm in diameter) wurtzite gold-catalysed GaAs NW that is lattice-matched to Ge. These NWs have shown a sub-nanosecond, temperature-insensitive radiative recombination lifetime and an emission yield similar to that of direct-bandgap III–V semiconductors. Moreover, by controlling the composition of the hexagonal Si_1−xGe_x alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap structure. Therefore, such results represent a huge step in the silicon technology and a thorough investigation could provide further insights into their singular physical properties. However, it is hard to filter out the SiGe phase from GaAs phase by XRD technique. They are expected to present the same hexagonal structure and close lattice parameters due to the close element number of Ga (31) and As (33) compared with Ge (32). So far , nanoXRF and nanoXAS have proven to be a valuable tool to image different types of semi-conductor alloy NWs^2,3. Here in we propose to combine the power of these two approaches with elemental and polarization selectivity via X-ray linear dichroism (XLD) at the Ge K-edge to explore in details the property-function relationships in these novel SiGex alloyed NWs.<br/>1. Fadaly, E. M. T. <i>et al.</i> Direct-bandgap emission from hexagonal Ge and SiGe alloys. <i>Nature</i> <b>580</b>, 205–209 (2020).<br/>2. Troian, A. <i>et al.</i> Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires. <i>Nano Lett.</i> <b>18</b>, 6461–6468 (2018).<br/>3. Martínez-Criado, G. <i>et al.</i> Exploring single semiconductor nanowires with a multimodal hard X-ray nanoprobe. <i>Adv. Mater.</i> <b>26</b>, 7873–7879 (2014).