pubs.acs.org/NanoLett Letter Unveiling Planar Defects in Hexagonal Group IV Materials Elham M. T. Fadaly,△ Anna Marzegalli,△ Yizhen Ren, Lin Sun, Alain Dijkstra, Diego de Matteis, Emilio Scalise, Andrey Sarikov, Marta De Luca, Riccardo Rurali, Ilaria Zardo, Jos E. M. Haverkort, Silvana Botti, Leo Miglio, Erik P. A. M. Bakkers, and Marcel A. Verheijen* Cite This: Nano Lett. 2021, 21, 3619−3625 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present. KEYWORDS: Nanowires, hexagonal group IV, hexagonal Ge, hexagonal Si, I3 basal stacking fault, defects G roup IV materials, especially silicon (Si) and germanium (Ge), are considered cornerstone materials in the semiconductor industry. Their unique physical and electronic conventional material synthesis techniques in the planar geometry. Capitalizing on this unique attribute, the hexagonal crystal phase has been achieved in group IV materials via the properties in addition to their well-established technology “crystal transfer” technique reported by some of us.1,9−11 The qualify them for a broad range of applications in the fields of hexagonal crystal structure of group IV materials was adopted electronics and optoelectronics. However, the fundamental from a hexagonal nanowire core material template. This limitation is that bulk crystalline cubic Si and Ge are incapable technique allows the fabrication of single-crystalline materials of efficiently emitting light due to the indirect nature of their in large volumes.1 band gaps. This has so far restrained them from being fully Hexagonal (hex) Ge and Si crystals have been epitaxially utilized in active optical circuitry. Recently, Fadaly et al.1 grown around almost-lattice-matched wurtzite (WZ) gallium experimentally realized direct band gap Si1−xGex alloys by arsenide (GaAs) and gallium phosphide (GaP) nanowires. In exploiting the metastable hexagonal crystal structure. This this paper, we focus on studying the GaAs/Ge system in detail. transformative result can open new frontiers toward uniting the The GaP/Si system, which displays a similar defect geometry, electronic and optoelectronic functionalities on a single chip, is discussed in the Supporting Information. The material paving the way toward the long-held goal of integrated group systems studied in this work were fabricated using metal− IV photonics. For this to happen, it is pivotal to master the organic vapor phase epitaxy (MOVPE) with trimethylgallium structural, electronic, and optical properties of this new class of (TMGa), arsine (AsH3), germane (GeH4), phosphine (PH3), hexagonal group IV materials (i.e., Si and Ge), which are and tetrasilane (Si4H10) precursors; for the growth details, see mostly unexplored. A high crystalline quality is required to the Methods section in the Supporting Information. The fully understand the intrinsic electronic and optoelectronic growth of GaAs/Ge (and GaP/Si) was performed in three properties of the materials, since structural defects can be main steps. First, gold (Au)-catalyzed untapered WZ III−V detrimental to the material properties. In group IV materials, the hexagonal crystal phase is a metastable phase, which is not readily achievable via Received: February 17, 2021 conventional thin-film growth techniques and can only be Revised: April 1, 2021 realized under extreme conditions in small volumes, as Published: April 12, 2021 demonstrated in the literature.2−8 Here, a unique feature of bottom-up-grown nanowires is employedtheir intrinsic capability to create metastable crystal phases inaccessible by © 2021 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.nanolett.1c00683 3619 Nano Lett. 2021, 21, 3619−3625 Nano Letters pubs.acs.org/NanoLett Letter nanowire arrays were grown with the vapor−liquid−solid formation seems to be intrinsic to the hexagonal group IV (VLS) method. Then, the Au catalysts were chemically system. The atomic arrangement of hex-Ge and the defect, in removed from the nanowire cores, and hex-Ge and hex-Si particular, was studied in the high-resolution STEM images shells were eventually epitaxially grown around the Au-free shown in Figure 1f and Supporting Figure 1c. The defect was nanowire cores.1 A representative scanning electron micros- revealed to be composed of two stacking fault planes and the copy (SEM) image in Figure 1a shows an array of nanowires related terminating dislocations. In the defect-free part of the hex-Ge crystal, the atomic layers were found to follow perfect −ABAB− stacking, verifying the hexagonal phase of the material. The stacking-fault terminating dislocations are highlighted by a yellow circle, and the altered stacking sequence is highlighted in red. It is worth pointing out that there is exactly the same number of atomic planes on either side of the terminating dislocations. The change in the dumbbell orientation occurs at the dislocation cores (see Figure 1f). The stacking fault extends to the edge of the core/ shell structure, as clarified in detail in Supporting Figure 3. The result is a planar structure consisting of four-layer stacking that resembles the 4H-polytype stacking sequence.12 The altered stacking sequence is −ABCBA−, which is identical to the I3 basal stacking fault (BSF) type.13 The I3 BSF was theoretically predicted in 1998 in WZ III−V semiconductors such as AlN, GaN, and InN.14 It was first observed experimentally in WZ- Figure 1. Epitaxial hexagonal GaAs/Ge core/shell nanowires. (a) A GaN in only 2005, showing the faulty stacked planes when representative SEM image of an array of hex-GaAs/Ge core/shell viewed from the WZ ⟨112̅0⟩ zone axis.13 Neither of these nanowires grown in the ⟨0001⟩ growth direction on a GaAs(111)B studies considered the possibility of the stacking fault substrate. (b) EDX compositional map for the axial direction of one termination being in the bulk of the material instead of at representative GaAs/Ge core/shell nanowire. (c) Overlapped the surface. HAADF-STEM and EDX images of a cross-sectional thin lamella of To fully characterize the in-plane boundaries of the defect, a similar nanowire. (d) BF-TEM image of a GaAs/Ge core/shell an extensive BF-TEM study was performed on a thin GaAs/Ge structure, with white dashed lines as a guide for the eye indicating the core/shell nanowire. The same section of the core/shell interfaces between the two material systems. The dark partial lines across the structure represent the stacking faults in the Ge shell. (e) structure was imaged along two different ⟨112̅0⟩ zone axes Aberration-corrected HAADF-STEM zoomed-in image for a region of orthogonal to the ⟨0001⟩ long axis of the nanowire, with a the GaAs/Ge structure obtained in the ⟨112̅0⟩ zone axis, which mutual difference in viewing angle of 60°. The ⟨112̅0⟩ zone indicates the starting position of the partial stacking faults. (f) A axis is equivalent to the cubic ⟨110⟩ axis, and both allow for a magnified image of the framed region in panel e that displays the discrimination between the cubic and hexagonal stacking ABAB stacking of the hexagonal crystal structure of the Ge shell. The sequence and the identification of stacking faults in either of red highlighted region designates the double-layer partial-stacking the lattices. An ensemble of partial dislocations is visible in the fault, which leads to a 4H-polytype-like stacking that originates from BF-TEM images, as shown in Figure 2a and b, appearing as the terminating partial dislocations encircled in yellow and indicated dark lines that extend radially. The positions of the dark lines by a white arrow. in the top TEM images are illustrated by colored lines in the bottom panels. Seven defects of different lengths can be clearly with thin hex-Ge shells around thin WZ-GaAs cores. The distinguished along the two-zone axes. Imaging the defects elemental composition of the nanowires was studied by along the two different zone axes allows the reconstruction of energy-dispersive X-ray spectroscopy (EDX) using trans- the in-plane shape of the seven defects. In Figure 2c, the mission electron microscopy (TEM). The elemental maps defects are projected on the hexagonal cross-sectional core/ across the axial and radial directions of the wires, as depicted in shell shape. From this analysis, we can conclude that two Figure 1b and c, respectively, confirm that the GaAs/Ge core/ dislocation pairs terminate this planar defect with a mutual shell structure shows no intermixing between GaAs and Ge. A angle of 60° and the layers between the dislocations form the I3 detailed analysis of the crystal structure of hex-Ge reveals the BSF. It is worth mentioning that BF-TEM imaging is more presence of one predominant type of defect. This type of suited to determine the defect density and the in-plane defect appears in bright field (BF) TEM images as dark lines, extension than aberration-corrected HAADF-STEM imaging starting from random spots in the hexagonal shell and due to the limited depth-of-focus of the latter despite its better continuing toward the edge of the shell as presented in Figure visualization of the atomic arrangement at the dislocations (see 1d and, for hex-Si, Supporting Figure, 1a. In the high-angle Supporting Figure 4 and Supporting Information S1 for more annular dark-field scanning TEM (HAADF-STEM) images, details).15 To further confirm that the structure of the bright lines are visible, indicating the contribution of the dominant defect, which is intrinsic to hexagonal group IV diffraction contrast to the atomic number contrast (see Figure materials, is the I3 BSF and not another type of planar defect, 1e and Supporting Figure 1b). The formation of the defect is we have compared it to similar defects in the literature, as intriguing for two main reasons: (1) there is no significant elucidated in Supporting Information S2.16,17 lattice strain present between the core and the shell of our Atomistic modeling and ab initio density functional theory nanowires to be released in the form of a defect and (2) the (DFT) calculations corroborate the structural characterization defects are absent in the core nanowires, as elucidated in inferred by the TEM analysis, as schematically illustrated in Supporting Figure 2, and nucleate in the shell, and their Figure 3a. A hexagonally shaped GaAs nanowire (blue) with 3620 https://doi.org/10.1021/acs.nanolett.1c00683 Nano Lett. 2021, 21, 3619−3625 Nano Letters pubs.acs.org/NanoLett Letter dislocation line, while the dislocations not parallel to the viewing direction cannot be clearly identified, as confirmed by TEM analysis (see Supporting Figure 6). Notice that we consider the 60° boundary, terminating the faulty stacked region in each basal plane, as two different dislocations with opposite Burgers vectors. These two dislocations have the same origin and both point toward the nanowire surface, as illustrated in Supporting Figure 7a. However, it should be highlighted that these two terminating dislocations can also be described as a single partial dislocation that forms a semiloop and is characterized by a unique Burgers vector, as seen in Supporting Figure 7b. Mathematically, the latter description means a simultaneous reversal of both the line direction and the Burgers vector of one of the dislocation arms in each basal plane with respect to the first description. Such procedure has no effect on the real atomic arrangement, so both the mentioned descriptions of the boundary of the stacking-fault regions are equivalent. Although the identification of the nucleation mechanism needs further investigation, the tendency of the system to introduce the illustrated specific defect (I3 BSFs with pairs of 30° partial dislocations terminating them) can be explained via energetic considerations. Stacking faults in the hexagonal structure introduce cubically stacked layers in the perfect hexagonal structure. Since the cubic phase is the ground state of bulk Ge and Si, these stacking faults in hex-Ge and hex-Si Figure 2. Reconstruction of the in-plane shape of the partial defects. have negative formation energies. The energy gain is (a and b) The top panels show BF-TEM images of the same section proportional to the number of cubic rows formed, which is of a 9 nm thick GaAs core and a 45 nm thick Ge shell nanowire, two in this case as shown in Supporting Figure 8c, contrary to which were imaged along two ⟨112̅0⟩ zone axes orthogonal to the the case of a single stacking fault that forms a unique row of ⟨0001⟩ long axis of the nanowire. Several partial dislocations are cubic sites. Due to the null total Burgers vector, the visible. The bottom panels show a schematic illustration of the GaAs/ deformation field induced by the partial dislocations pairs is Ge core/shell structure, and the colored horizontal lines indicate the negligible, and their core energies are predicted to be low.18 positions of seven defects that can clearly be recognized along the two different zone axes. (c) A top view for the reconstruction of the in- Moreover, there is no interaction between the two dislocation plane shape of the seven defects indicated in the BF-TEM images, pairs, which is again due to the null total Burgers vector of each which was created by projecting the defects into a hexagonal shape. pair, allowing the double-stacking fault to be stable and evolve All defects have symmetrically equivalent in-plane shapes. All defects until the maximum possible extension. The characteristic zero have their origin in the shell, i.e., remote from the core/shell interface. total Burgers vector of the pairs also explains their stability with respect to the nanowire free surface; no net image force is {11̅00} facets and a Ge shell (red) with hexagonal crystal acting on them, so they cannot be expelled. Moreover, it is lattice positions was constructed. Analytical expressions for the confirmed that the dislocation nucleation is not due to strain displacement field of the straight dislocations were used to release because null Burgers vector defects cannot release insert two 30° partial dislocations with opposite Burgers strain, which is consistent with the negligible lattice mismatch vectors in a basal plane along two ⟨112̅0⟩ directions with an between the core and the shell materials. angle of 60° between them (labeled A in Figure 3b), starting A multiscale approach was employed to investigate the from an arbitrary point.18 The same approach was repeated in electronic properties of the I3 BSF and its terminating the subsequent basal plane (labeled B in Figure 3b). In the dislocations, considering the defect supercell model shown in upper and lower defected planes, opposite Burgers vectors for Figure 3a and b. A smaller atomistic model, as shown in the dislocation pairs were chosen. Molecular dynamics (MD) Supporting Figure 9, was used to perform the atomistic simulations using a Tersoff potential were performed, which relaxation and obtain the minimum energy geometry, as allowed the group IV atoms to rearrange in their minimum depicted in Figure 3c. Details of the DFT calculations are energy configuration (see the Methods section of the provided in the Methods section of the Supporting Supporting Information for the MD simulations details).19 Information. The band structure diagram shows the local The terminations of the I3 BSF, as seen in Figure 1f and density of states (LDOS) along the ⟨101̅0⟩ crystallographic Supporting Figure 1c, are fully reproduced and confirmed as a direction, which contains the I3 BSF and the dislocations minimum energy configuration by MD. In Figure 3, atoms embedded in the hex-Ge crystal averaged in the perpendicular belonging to the dislocation cores are colored in green and plane (the <0001> direction).20 Notably, the LDOS across the blue; the same colors indicate the same sign of the Burgers ⟨101̅0⟩ direction in Figure 3c shows no states in the band gap vector. The I3 BSF formation in the internal triangular region is associated with the I3 BSF or the dislocation cores. Intuitively, a natural consequence of the insertion of dislocations, which this can be understood by the fact that cubic layers are formed are indeed partial dislocation dipoles on each plane. Figure 3b at the stacking fault, which have a larger bandgap than the shows that the termination of the I3 BSF can be observed only hexagonal crystal and result in states in the conduction band. when looking along a ⟨112̅0⟩ viewing direction parallel to its Similar results were obtained for hex-Si crystals, as explained in 3621 https://doi.org/10.1021/acs.nanolett.1c00683 Nano Lett. 2021, 21, 3619−3625 Nano Letters pubs.acs.org/NanoLett Letter Figure 3. Atomistic model and electronic band structure calculations of I3 BSF in hex-Ge and their terminating dislocations. The WZ- GaAs nanowire core atoms are represented in blue, and the hex-Ge shell atoms are represented in red. (a) Top-view atomic model of four atomic layers of the structure containing I3 BSF and four bounding partial dislocations. There are two at each stacking fault plane, which are separated with a 60° angle. The bottom shows a side-view atomic model of the area indicated by the black lines, which is viewed from the direction of the arrow in the top panel. It displays the atomic arrangement at both partial dislocation pairs; for more details on the imaging directions, see Supporting Figure 4. (b) Three-dimensional view for the visualization of the four modeled layers in panel a; the top-most and bottom-most layers are defect-free layers, and the middle layers contain the partial dislocations that enclose the planar stacking faults. A refers to the top defected layer, and B refers to the bottom defected layer. The dislocations, as seen along two different ⟨112̅0⟩ viewing directions, are shown in the framed boxes with dashed and solid lines, which indicate viewing directions 1 and 2, respectively. Blue and green atomic colors highlight the dislocations in the upper and lower defected planes, and the same color means an equal Burgers vector. (c) Local density of states (LDOS) of the hex-Ge supercell modeled in panels a and b averaged in the ⟨0001⟩ direction, which is the plane perpendicular to the ⟨101̅0⟩ crystallographic direction. The supercell that contains the I3 BSF and the local band structure plotted along the ⟨101̅0⟩ clearly shows no midgap states. Figure 4. Raman fingerprint of the I3 BSF. (a) Calculated atomic displacement vectors of the defect-enabled phonon mode at 241 cm−1 at the Γ- point for the smallest cell. (b) Scheme of the scattering configurations employed in the Raman measurements. (c) Raman spectra of four hex-Ge nanowires that differ in their I3 BSF defect densities. The spectra have been normalized to the intensity of the E2g mode for a direct comparison of the defect peak intensities. For each sample, several nanowires (two to six) were measured, and the average results are shown in panel d. (d) Left scale, circular symbols: intensity of the ∼244 cm−1 peak relative to E2g intensity as a function of defected shell volume percent. Right scale, triangular symbols: theoretical values. Notice that the scales of the y-axes of the experimental and theoretical intensities are different, as they are not directly comparable. Experimental error bars on the y-scale represent the half-difference between the maximum and minimum value found for that sample, and the error bars on the x-scale come from the standard deviation of the estimated defect density. The lines are guides to the eyes. Supporting Figure 9. Importantly, this means that the in the photoluminescence intensity or spectral line shape electronic properties of a hex-Ge crystal are not affected by between nanowires with different defect densities. The the I3 BSF and their terminating dislocations, which is the photoluminescence quenching rates (the decrease of the PL dominant defect in our hexagonal group IV nanocrystals. intensity with increasing temperature) in defected hex-Ge and We have analyzed the photoluminescence efficiency of hex- hex-Si0.20Ge0.80 nanowires are very comparable to those Ge and hex-SiGe nanowires containing I3 BSFs elsewhere,1 and observed in high-quality III/V semiconductor nanowires. we concluded that we have emission from band-to-band Additional data show that our hex-Si0.20Ge0.80 nanowires recombination. We have not observed a substantial difference were capable of reaching the radiative limit up to room 3622 https://doi.org/10.1021/acs.nanolett.1c00683 Nano Lett. 2021, 21, 3619−3625 Nano Letters pubs.acs.org/NanoLett Letter temperature, indicating a low density of nonradiative centers. non-destructive tool for assessing the density of I3 BSFs in The slightly lower photoluminescence quenching rate in hex- group IV semiconductor nanostructures. Ge compared to that in hex-SiGe indicates that this conclusion In summary, we have identified and characterized the should also be valid for hex-Ge. These observations atomistic microstructure of a unique planar intrinsic basal conclusively show that the I3 defects do not significantly defect, which is dominant in the hexagonal group IV family of degrade the photoluminescence. Although we cannot exclude materials. The investigation of the planar extension of such a the idea that I3 defects provide a shallow defect level with a low defect revealed that it is formed by an I3 BSF, and its atomic photoluminescence quenching rate, our experimental results stacking exhibited a triangular shape terminated by two partial support the theoretical claim that the I3 BSF and its dislocation dipoles. The I3 BSF is the dominant stacking fault terminating dislocations do not provide a strong photo- embedded in the hexagonal crystal matrix. In addition to TEM, luminescence quenching center within the bandgap. we confirmed that I3 BSFs can also be identified by Raman A complementary method for the structural characterization spectroscopy by observing a distinct phonon mode enabled by of the I3 defects is Raman spectroscopy. We have computed the stacking fault, which is absent in the pure-phase hexagonal the Raman spectra of I3-defected hex-Ge supercells of different crystal. Eventually, the electronic band structure calculations sizes by employing density functional perturbation theory showed that the I3 BSF and its terminating dislocations do not (DFPT); for details, see the Methods section of the introduce states in the energy gap of hex-Ge and hex-Si. These Supporting Information. In addition to the expected E2g and theoretical calculations are consistent with the reported TO/LO peaks,21,22 the computed spectra of defected hex-Ge efficient light emission from hex-Ge and hex-SiGe alloys with exhibit a phonon mode at 241 cm−1, which is absent in pure I3 BSFs. The identification of the dominant defects in a newly hex-Ge and does not result from the artificial folding imposed discovered class of materials such as the hexagonal group IV by the periodic boundary conditions (for more details, see family is essential for the development of well-established technology for future optoelectronic devices. ■ Supporting Figure 12). The atomic displacements of the 241 cm−1 mode at the Γ-point were also calculated and are shown in Figure 4a. The motion is not localized at the stacking faults, ASSOCIATED CONTENT but it involves all atoms in the unit cell and is oriented along * Supporting Information sı the z-axis. Raman scattering experiments were performed on The Supporting Information is available free of charge at individual hex-GaAs/Ge core/shell nanowires with varying https://pubs.acs.org/doi/10.1021/acs.nanolett.1c00683. defect densities. The measured nanowires were also examined Materials synthesis, TEM characterization, molecular by TEM, and the defect densities were calculated in terms of dynamics simulations, stacking fault-energy calculations, the percentage of defected volume over the total shell volume; electronic band structure calculations, experimental for details see Supporting Figures 10 and 11 and Supporting Raman spectroscopy measurements, theoretical Raman Information S3. Figure 4b exemplifies the backscattering calculations, I3 BSF visualizations and comparison with geometry adopted in the Raman measurements and calcu- the literature, and quantification and volume fraction lations and the polarization of the incoming and collected calculations of the defects (PDF) photons. In Figure 4c, we compare the Raman spectra of several GaAs/Ge core/shell nanowires, which differ in the Atomic models of the simulated hex-Ge and hex-Si density of the I3 BSFs in the hexagonal crystal. The spectra of supercells containing the I3 defect (ZIP) ■ all the nanowires display the transverse optical E2g mode and the degenerate transverse and longitudinal optical A1g and E1g AUTHOR INFORMATION modes, respectively, which are typical of hex-Ge.21,22 In Corresponding Author addition to this, we observe a weak feature at 244 cm−1 with a varying intensity among the nanowire samples. To directly Marcel A. Verheijen − Department of Applied Physics, compare the spectral weight of this peak between different Eindhoven University of Technology, 5600 MB Eindhoven, samples, we normalized each spectrum to the intensity of the The Netherlands; Eurofins Materials Science Netherlands BV, E2g mode. We fitted the spectra with Lorentzian curves to 5656 AE Eindhoven, The Netherlands; orcid.org/0000- 0002-8749-7755; Email: m.a.verheijen@tue.nl extract the intensities of the 244 cm−1 modes relative to those of the E2g modes. In Figure 4d, we summarize the relative Authors intensities as a function of the percentage of the defected Elham M. T. Fadaly − Department of Applied Physics, nanowire shell volume. We observe a direct proportionality Eindhoven University of Technology, 5600 MB Eindhoven, between the intensity and the defect density for defect The Netherlands; orcid.org/0000-0001-7074-8784 densities above 1.5% of the shell volume; below this threshold, Anna Marzegalli − Dipartimento di Fisica, Politecnico di the experimental intensities are very low and similar. Milano, 22100 Como, Italy; L-NESS and Dipartimento di Considering the non-localized nature of the vibration and Scienza dei Materiali, Università di Milano-Bicocca, 20125 the fact that the phonon dispersion of bulk hex-Ge has a mode Milano, Italy at that frequency with no Raman activity, we attribute the Yizhen Ren − Department of Applied Physics, Eindhoven observation of the 244 cm−1 peak to the presence of the I3 BSF University of Technology, 5600 MB Eindhoven, The that alters the Raman selection rules, thus making a silent Netherlands mode detectable. We suggest that the observation of a Lin Sun − Institut für Festkörpertheorie und -optik, Friedrich- threshold I3 BSF density is due to the fact that these defects Schiller-Universität Jena, 07743 Jena, Germany might be too diluted below a certain density to significantly Alain Dijkstra − Department of Applied Physics, Eindhoven enhance the intensity of the 244 cm−1 phonon mode. Overall, University of Technology, 5600 MB Eindhoven, The these results indicate that Raman spectroscopy is a powerful Netherlands; orcid.org/0000-0003-2661-5460 3623 https://doi.org/10.1021/acs.nanolett.1c00683 Nano Lett. 2021, 21, 3619−3625 Nano Letters pubs.acs.org/NanoLett Letter Diego de Matteis − Departement Physik, Universität Basel, the Generalitat de Catalunya under Grant no. 2017 SGR 1506. 4056 Basel, Switzerland L.S. acknowledges financial support from the China Scholar- Emilio Scalise − L-NESS and Dipartimento di Scienza dei ship Council. S.B. acknowledges funding from the Volkswagen Materiali, Università di Milano-Bicocca, 20125 Milano, Stiftung (Momentum) through the project “Dandelion” and Italy; orcid.org/0000-0003-1465-0939 the DFG through projects SFB-1375 and BO4280/8-1. Andrey Sarikov − L-NESS and Dipartimento di Scienza dei Computational resources were also provided by the Leibniz Materiali, Università di Milano-Bicocca, 20125 Milano, Supercomputing Center through projects pr48je and pr62ja. Italy; V. Lashkarev Institute of Semiconductor Physics, Notes National Academy of Sciences of Ukraine, 03028 Kyiv, The authors declare no competing financial interest. ■ Ukraine Marta De Luca − Departement Physik, Universität Basel, ACKNOWLEDGMENTS 4056 Basel, Switzerland; orcid.org/0000-0001-6061- 3261 We thank Dr. Sebastian Kölling for performing the focused ion Riccardo Rurali − Institut de Ciència de Materials de beam experiment to prepare a lamella of the Hex-GaAs/Ge Barcelona, ICMAB-CSIC, 08193 Bellaterra, Spain; core/shell nanowire structure for TEM characterization. orcid.org/0000-0002-4086-4191 Solliance and the Dutch province of Noord Brabant are Ilaria Zardo − Departement Physik, Universität Basel, 4056 acknowledged for funding the TEM facility. E.S. and A.S. Basel, Switzerland; orcid.org/0000-0002-8685-2305 acknowledge the CINECA consortium for the availability of Jos E. M. Haverkort − Department of Applied Physics, high-performance computing resources, which were granted Eindhoven University of Technology, 5600 MB Eindhoven, under the ISCRA initiative. The Netherlands; orcid.org/0000-0003-3051-673X Silvana Botti − Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, 07743 Jena, Germany; ■ REFERENCES (1) Fadaly, E. M. T.; Dijkstra, A.; Suckert, J. R.; Ziss, D.; van Tilburg, orcid.org/0000-0002-4920-2370 M. A. J.; Mao, C.; Ren, Y.; van Lange, V. T.; Korzun, K.; Kölling, S.; Leo Miglio − L-NESS and Dipartimento di Scienza dei et al. Direct-Bandgap Emission from Hexagonal Ge and SiGe Alloys. Materiali, Università di Milano-Bicocca, 20125 Milano, Nature 2020, 580 (7802), 205−209. Italy; orcid.org/0000-0002-1329-527X (2) Barth, S.; Seifner, M. S.; Maldonado, S. Metastable Group IV Erik P. A. M. Bakkers − Department of Applied Physics, Allotropes and Solid Solutions: Nanoparticles and Nanowires. Chem. 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