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Direct-bandgap emission from hexagonal Ge and SiGe alloys

Nature volume 580, pages 205–209 (2020)Cite this article

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Abstract

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal1 of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades2,3,4,5,6. Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III–V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.

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Fig. 1: Calculated band structure of hex-Si1 − xGex.
Fig. 2: Overview of the hex-Si1 − xGex material system.
Fig. 3: Emission from hex-Ge and hex-Si0.20Ge0.80.
Fig. 4: Time-resolved photoluminescence measurements of single hex-Si0.20Ge0.80 nanowires.
Fig. 5: Tunability of the direct bandgap of hex-Si1 − xGex alloys.

Data availability

All data underlying this study are available from the 4TU ResearchData repository at https://doi.org/10.4121/uuid:68e75799-0378-4130-9764-b80cb1f2319b.

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Acknowledgements

We thank D. Buca, A. Polimeni, J. de Boeck and C. Palmstrøm for discussions. We thank R. van Veldhoven for the technical support of the MOVPE reactor and providing the MQW samples. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III and we thank F. Bertram for assistance in using beamline P08. This project received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement number 735008 (SiLAS), the Dutch Organization for Scientific Research (NWO) and from a Marie Skłodowska-Curie Action fellowship (GA number 751823). We acknowledge Solliance, a solar energy R&D initiative of ECN, TNO, Holst, TU/e, imec, Forschungszentrum Jülich, and the Dutch province of Noord-Brabant for funding the TEM facility. We thank the Leibniz Supercomputing Centre for providing computational resources on SuperMUC (project number pr62ja).

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Author notes

  1. Sebastian Kölling

    Present address: Department of Engineering Physics, École Polytechnique de Montréal, Montreal, Québec, Canada

  2. These authors contributed equally: Elham M. T. Fadaly, Alain Dijkstra, Jens Renè Suckert

Authors and Affiliations

  1. Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands

    Elham M. T. Fadaly, Alain Dijkstra, Marvin A. J. van Tilburg, Chenyang Mao, Yizhen Ren, Victor T. van Lange, Ksenia Korzun, Sebastian Kölling, Marcel A. Verheijen, Jos E. M. Haverkort & Erik P. A. M. Bakkers

  2. Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Jena, Germany

    Jens Renè Suckert, Claudia Rödl, Jürgen Furthmüller, Friedhelm Bechstedt & Silvana Botti

  3. Institute of Semiconductor and Solid-State Physics, Johannes Kepler University, Linz, Austria

    Dorian Ziss & Julian Stangl

  4. Eurofins Materials Science Netherlands, Eindhoven, The Netherlands

    Marcel A. Verheijen

  5. Physik Department & Walter-Schottky-Institut, Technische Universität München, Garching, Germany

    David Busse & Jonathan J. Finley

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Contributions

E.M.T.F, C.M. and Y.R. carried out the growth of wurtzite nanowire cores. E.M.T.F. carried out the growth of hex-SiGe shells and analysed the data. A.D. and D.B. carried out the photoluminescence spectroscopy. A.D. analysed the optical data. M.A.J.v.T., A.D. and V.T.v.L. performed time-resolved spectroscopy on single nanowires; K.K. performed optical finite difference time domain simulations. J.R.S., C.R., J.F. and S.B. performed the DFT calculations. D.Z. and J.S. performed the XRD measurements. S.K. performed the APT characterization. M.A.V. performed the TEM analysis. J.R.S., J.F., S.B., J.E.M.H. and E.P.A.M.B. supervised the project. F.B. contributed to the interpretation of data and E.M.T.F., A.D., D.Z., S.B., J.J.F., J.E.M.H. and E.P.A.M.B. contributed to the writing of the manuscript. All authors discussed the results and commented on the manuscript

Corresponding author

Correspondence to Erik P. A. M. Bakkers.

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Extended data figures and tables

Extended Data Fig. 1 Schematic illustration of the nanowire growth process.

a, The core nanowire growth starts with a GaAs (111)B substrate patterned with Au catalyst seeds, which is introduced in the MOVPE reactor and annealed at a temperature higher than the eutectic temperature forming an alloy between the catalyst seed and the substrate. b, Next, the GaAs gas precursors (TMGa and AsH3) are introduced, and Au-catalysed GaAs core nanowires are grown. To proceed with the SiGe shell growth, Au seeds are chemically etched away from the GaAs cores and the surface of the cores is repaired (c), and the sample is reintroduced to the MOVPE reactor (d). A hex-Si1 − xGex shell is epitaxially grown around the GaAs cores from precursors (Si2H6 and GeH4). (The molecules are drawn with the freely available MolView Software (http://www.molview.org/)). The 30° tilted scanning electron microscopy images in the bottom panels of a–d are representative of the results of the growth steps in the top panels, with insets in b and c displaying a magnified image of the nanowire. e, A 30° tilted-overview representative scanning electron microscopy image of hex-Ge/GaAs Core shells corresponding to what is shown in Fig. 2, confirming the uniformity of the growth across the sample.

Extended Data Fig. 2 Crystal quality of the wurtzite GaAs nanowire cores.

a, Bright-field TEM images recorded in the [11\(\bar{2}\)0] zone axis of five representative GaAs core nanowires of a pure wurtzite crystal (stacking faults indicated with red lines), with a stacking-fault density of 0–6 stacking faults per micrometre. b, A zoomed-in bright-field TEM image of the top part of one of the nanowires in a (blue circle), indicating the purity of the crystal structure. c, HAADF-STEM image of the red box in b, displaying the ABAB stacking of the GaAs atomic columns, which is the hallmark of the hexagonal crystal structure. The red shading highlights a stacking fault forming one cubic layer in the hexagonal structure.

Extended Data Fig. 3 Ge content calibration curve.

A calibration curve for the incorporated atomic fraction of Ge in the as-grown SiGe shells discussed in Fig. 2e, f and Fig. 5. Owing to growth kinetics, the input percentage of gas precursors in the MOVPE reactor does not always match the actual incorporated atomic percentage in the grown Si1 − xGex shell structures. To map the compositional output onto the input Ge fraction, the real content of four Si1 − xGex shells was measured by EDX-STEM and plotted as solid black circles. The compositions of additional samples were determined based on their input Ge fraction by interpolating the EDX-STEM data points and are indicated by blue triangles. The error bars for the black data points represent the standard deviation in the composition across three different analysed nanowires per sample. The accuracy of EDX in STEM is confirmed by determining the composition of a single sample, corresponding to MOVPE input Si0.10Ge0.90, with both EDX-STEM and APT. The APT data is shown in Extended Data Fig. 6 and was performed on three different nanowires. Both techniques yielded almost the same Ge composition—0.74 and 0.75 respectively—which is within the standard deviation across the whole sample. The reproducibility of this calibration method is also confirmed by photoluminescence, where almost identical spectra are observed for two different samples grown with the same input.

Extended Data Fig. 4 Full series of symmetric (0008) reflections of hex-Si1 − xGex.

a, An RSM of as-grown wurtzite GaAs nanowires on a cub-GaAs substrate, containing the wurtzite-GaAs (0008) reflection and the cub-GaAs (444) reflection. b, An RSM for a sample similar to that in a but with a thick Ge shell, including the cub-GaAs (444) substrate reflection and the hex-Ge (0008) reflection. c–f, Additional RSMs are shown for samples with Si1 − xGex shells, for x = 0.92 (c), for x = 0.86 (d), for x = 0.74 (e) and for x = 0.61 (f). The intensities (Int) of the reflections are colour-coded on a logarithmic scale, in units of counts per second (cps). A clearly increasing shift of Qout-of-plane can be observed for increasing Si content, corresponding to a decreasing lattice constant. For the RSMs in d and e a reflection from a parasitic, epitaxial cub-SiGe layer is also found. g, Tabulated hexagonal lattice parameters of all measured hex-Si1 − xGex samples with corresponding error values extracted from XRD measurements. The errors given take into account the accuracy of defining the peak position with a two-dimensional fit as described, as well as the scattering of the individual lattice parameter values extracted from the evaluation of multiple peaks. EDX-STEM was used to determine the composition of some samples and the rest were interpolated on the basis of the curve in Extended Data Fig. 3.

Extended Data Fig. 5 Crystal-truncation-rod scan and polar scan.

For a representative sample with Si0.20Ge0.80 shells and a GaAs core, a scan along the asymmetric [\(\bar{1}\)01L] nanowire crystal truncation rod is shown, covering a total of three wurtzite reflections, [\(\bar{1}\)015], [\(\bar{1}\)016] and [\(\bar{1}\)017]. L refers to a crystallographic coordinate according to the HKiL Miller-Bravais (i = −(H + L)) index notation. In addition, also the cubic [224] substrate reflection and the parasitic cub-SiGe epilayer is visible (as a double-peak feature around the [224] reflection). In between the wurtzite reflections no additional reflections are visible, which indicates that only the pure wurtzite 2H phase is present in the nanowire system. The inset shows an XRD polar scan around the [\(\bar{1}\)016] reflection, confirming the six-fold symmetry of the wurtzite lattice. Each reflection, seen in the polar scan, is separated by a sample rotation of exactly 60° from the next reflection (φ rotation axis, χ tilt axis). The absence of additional Bragg reflections in between indicates a pure wurtzite phase and high crystal quality.

Extended Data Fig. 6 APT characterization of hex-Ge and Si0.25Ge0.75.

a, c, Ge (red) and Si (cyan) is shown to form a shell around the hexagonal Ga (green) and the As (blue) core. a, A three-dimensional volume reconstruction of part of a hex-GaAs/Ge core/shell nanowire with thicknesses of 35 nm/80 nm. For clarity, only a slab of 50 nm thickness of the entire 1.4-µm-long analysis is shown. c, A three-dimensional volume reconstruction of part of a hex-GaAs/Si0.25Ge0.75 core/shell nanowire with thicknesses of 35 nm/46 nm. For clarity, only a slab of 40 nm thickness of the entire 1.1-µm-long analysis is shown. b, d, A plot of the atomic species concentration in the Ge (Si0.25Ge0.75) shell in the rectangles in a (white) and c (yellow) as a function of the radial distance across the core/shell structure. Every data point in the plot represents a 2-nm slice taken along the entire length of the nanowire analyses excluding the cubic top part of the nanowire. Constant incorporation of As at a level of approximately 400 parts per million (b) (200 parts per million (d)) is observed in the entire shell while the Ga concentration quickly drops to a value close to the noise level of ~10 parts per million. e, A radial profile of the SiGe core/shell structure from the APT measurement integrated over a 1.0-µm length of the structure showing a Ge content of around 0.75 as shown in d. On the dotted rectangular volume of e, we carry out a nearest-neighbour analysis for Si atoms as previously used to evaluate random alloys of GeSn83,84. The nearest-neighbour analysis evaluates the distances between each pair of Si atoms and its first (to fourth) neighbours (NN1 to NN4). f, In b and d and f, the atomic concentration of the individual elements is plotted in the volume of the shaded regions indicated in a, c and e. A plot comparing the nearest-neighbour analysis on the measurement data to a randomized dataset. This gives us no indication of Si clustering and has been established as a reliable way to evaluate random alloys46.

Extended Data Fig. 7 LSW fit of photoluminescence-spectra on hex-Ge.

LSW fits were performed both on low-temperature (4 K), excitation-density-dependent photoluminescence spectra of hex-Ge as plotted in Fig. 3a, as well as on a temperature series of photoluminescence spectra of hex-Ge at an excitation density of 1.8 kW cm−2 as plotted in Fig. 3b. Additional fitting results are given here, with a full description of the model given in the Methods section ‘The LSW model’. a, d, The bandgap and the quasi-Fermi-level splitting as a function of excitation density and temperature, respectively. b, e, The electron temperature as function of excitation density and temperature, respectively. c, f, The Urbach-tail coefficient γ also as a function of excitation density and temperature, respectively. g, An LSW fit, plotted as a dashed green line, which was performed on a low-excitation-density (ExDens) (45 W cm−2) photoluminescence spectrum, plotted as a red line, at a temperature of 80 K. Two vertical dotted lines indicate the bandgap (BG) on the left in black and the quasi-Fermi-level splitting Δμ to the right in blue. At this lattice temperature all dopants are ionized and the Δμ equals the electron-Fermi level, so these numbers are used to determine the electron quasi-Fermi level.

Extended Data Fig. 8 Arrhenius plots of hex-Si0.20Ge0.80 with varying quality and their growth parameters.

a, The plots show the same data as presented in the left panel of Fig. 4c, but here presented in an Arrhenius representation. For the lowest-quality sample III, two non-radiative processes are found with activation energies of 16 meV and 91 meV. For sample II only a single activation energy is found, of 34 meV, whereas sample I does not show any decay in intensity over the full measured temperature range. The photoluminescence intensities at 4 K are proportional to the absorption cross-sections, which scale approximately with the volume fractions of the samples that fit within the 5-µm-diameter laser spot. The volume ratios are 1:0.32:0.033 for samples I:II:III, respectively, where only 5/8 of the volume of sample I was counted (see Extended Data Fig. 8b). The measured photoluminescence intensities extrapolated to 4 K are proportional to 1:0.3:0.04 for the same samples, closely agreeing with the probed volumes. b, Tabulated growth parameters of the three different hex-SiGe samples studied, with increasing quality and the dimensions of the nanowires presented in Fig. 4b, c. The total diameter is the core diameter plus twice the shell thickness.

Extended Data Fig. 9 Relative photoluminescence intensity between compositions.

Here the relative photoluminescence intensities are given for the samples presented in Fig. 5. We note that only measurements on ensembles of as-grown standing wires are included where an equal number of wires is probed, and so the Si0.35Ge0.65 sample is not included. For this comparison all the samples were excited with an excitation density of 5 kW cm−2 at a temperature of 4 K using the same MCT detector and the same KBr beamsplitter. The samples are found to be very similar in intensity despite the change in setup efficiency, a variation in total excited volume due to thickness differences and potentially strain-induced nonradiative recombination centres for high Si-content samples. Because of these additional factors, no conclusive experimental argument can be made on whether the material becomes more efficient for higher Si contents. Additionally, we acknowledge that, despite efforts, we have not been able to measure photoluminescence spectra from wires with x < 0.65, which suggests that the direct–indirect-bandgap transition lies near this point.

Extended Data Fig. 10 External radiative efficiency.

a, Integrated photoluminescence intensities of hex-SiGe in comparison with high-quality InGaAs/InP multiple quantum well (QW) samples as measured in a micro-photoluminescence setup using 1,030-nm wavelength, 125-pJ pulses. The photoluminescence intensities of both the MQWs and the hex-SiGe wires were corrected using their respective absorption and emission efficiencies as given in the Methods section ‘External radiative efficiency of hex-SiGe’. b, A cross-sectional schematic demonstrating the layer structure of the InGaAs/InP multiple QW sample, showing a total absorption thickness of 625 nm. The bandgap wavelengths (WL) of the layers that are not lattice-matched to InP are indicated. c, A schematic illustration of the geometry of our horizontally oriented hex-SiGe nanowire, showing the emission into a NA = 0.48 cassegrain objective with a NA = 0.22 obscurity. The polarization of the emitted light is indicated as V and H for vertical and horizontal, respectively.

Extended Data Fig. 11 Comparison between different generations of hex-Ge samples.

a, Photoluminescence spectra from the first hex-Ge shells, which were grown using wurtzite-GaP cores, thus creating many defects caused by the large lattice mismatch between the core and the shell. b, The first hex-Ge shells grown on lattice-matching GaAs cores where the hex-Ge is grown at a temperature of 600 °C. c, Spectra of hex-Ge shells grown at a temperature of 650 °C, further improving the optical quality.

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Fadaly, E.M.T., Dijkstra, A., Suckert, J.R. et al. Direct-bandgap emission from hexagonal Ge and SiGe alloys. Nature 580, 205–209 (2020). https://doi.org/10.1038/s41586-020-2150-y

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