Direct-bandgap emission from hexagonal Ge and SiGe alloys
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-04-08 |
| Journal | Nature |
| Authors | Elham Fadaly, Alain Dijkstra, Jens RenĂš Suckert, Dorian Ziss, Marvin A. J. van Tilburg |
| Institutions | Friedrich Schiller University Jena, Technical University of Munich |
| Citations | 351 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Direct Bandgap Breakthrough: The research demonstrates efficient, direct bandgap light emission from hexagonal (WZ structure) Germanium (Ge) and Silicon-Germanium (SiGe) alloys, overcoming the indirect bandgap limitation of conventional cubic Si and Ge.
- Si-Compatibility: Hexagonal Si1-xGex is presented as an ideal material system for monolithic integration, fully uniting electronic and optoelectronic functionalities on a single silicon chip.
- High Emission Efficiency: The radiative recombination lifetime is measured to be sub-nanosecond (~1 ns), comparable to conventional direct bandgap III-V semiconductors (like GaAs or InP).
- Tunability: The direct bandgap emission wavelength can be continuously tuned across the 0.3 eV to 0.7 eV range by controlling the Ge content (x > 0.65) in the Hex-Si1-xGex alloy.
- Radiative Performance: The calculated radiative coefficient (B-coefficient) for Hex-Si0.20Ge0.80 is 0.7 x 10-10 cm3/s to 11 x 10-10 cm3/s, which is up to 5 orders of magnitude larger than cubic Si.
- Thermal Stability: The radiative efficiency is nearly temperature independent up to 220 K for high-quality samples, confirming strong evidence for pure radiative recombination in the degenerate doping limit.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Direct Bandgap Condition | x > 0.65 | Ge Content (x) | Hex-Si1-xGex alloy |
| Tunable Emission Range | 0.3 - 0.7 | eV | Hex-Si1-xGex alloy |
| Hex-Ge Bandgap (4 K) | 0.353 | eV | Lowest temperature measurement |
| Hex-Ge Bandgap (300 K) | 0.28 | eV | Room temperature measurement |
| Radiative Lifetime (Hex-Si0.20Ge0.80) | ~1 | ns | Low temperature (4 K) |
| Radiative Lifetime (Hex-Si0.20Ge0.80) | 0.46 - 0.98 | ns | Range across 60 wires at 300 K |
| Radiative Coefficient (Brad) | 0.7 x 10-10 to 11 x 10-10 | cm3/s | Hex-Si0.20Ge0.80 (Comparable to GaAs) |
| Unintentional n-Doping (As) | 9 x 1018 | cm-3 | Estimated donor concentration |
| Stacking Fault Density | 2 - 4 | SFs/”m | Along the crystalline [0001] direction |
| Hex-Ge Debye Temperature | 66 | K | Fitted from bandgap shrinkage (Vina equation) |
| PL Quenching Ratio (4K/300K) | 15 - 100 | Factor | Hex-SiGe, favorable compared to III-V |
| Hex-Si0.20Ge0.80 Lattice Parameter (a) | 3.9505 ± 0.0008 | Angstrom | In-plane lattice parameter (XRD) |
| Hex-Si0.20Ge0.80 Lattice Parameter (c) | 6.5257 ± 0.0001 | Angstrom | Out-of-plane lattice parameter (XRD) |
Key Methodologies
Section titled âKey Methodologiesâ- Template Preparation: Gold (Au) catalyst seeds were deposited onto a GaAs (111)B substrate using electron beam lithography.
- Core Nanowire (NW) Growth (MOVPE VLS): Wurtzite (WZ) Gallium Arsenide (GaAs) core NWs (~35 nm diameter) were grown via Metal Organic Vapor Phase Epitaxy (MOVPE) using the Vapor-Liquid-Solid (VLS) mechanism.
- Temperature: 650 °C.
- Precursors: Trimethylgallium (TMGa) and Arsine (AsH3).
- V/III Ratio: 2.4.
- Catalyst Removal: The Au catalytic particles were removed using wet chemical etching (cyanide-based solution) to prevent gold contamination in the subsequent shell growth.
- Shell Growth (MOVPE Epitaxy): Thick (200-400 nm) Hexagonal (WZ) Si1-xGex shells were grown epitaxially around the WZ-GaAs cores.
- Temperature: 650-700 °C.
- Precursors: Germane (GeH4) and Disilane (Si2H6).
- Structural Characterization: High-resolution X-ray Diffraction (XRD) using synchrotron radiation (DESY P08 beamline) and Transmission Electron Microscopy (TEM/HAADF-STEM) confirmed the hexagonal crystal structure (ABAB stacking) and high crystal quality.
- Compositional Analysis: Electron Dispersive X-ray (EDX) spectroscopy and Atom Probe Tomography (APT) were used to confirm core/shell geometry and determine Ge content and unintentional Arsenic (As) doping levels.
- Optical Characterization: Power and temperature-dependent Photoluminescence (PL) spectroscopy and Time-Correlated Single Photon Counting (TCSPC) were performed on single nanowires to measure emission spectra, lifetimes, and efficiency. Data were fitted using the Lasher-Stern-WĂŒrfel (LSW) model to confirm Band-to-Band (BtB) recombination.
Commercial Applications
Section titled âCommercial Applicationsâ- Silicon Photonics and Optoelectronics: Enabling the monolithic integration of efficient light sources (lasers, LEDs, modulators) directly onto silicon chips, reducing stray capacitances and energy consumption.
- Optical Interconnects: High-speed, low-energy data transfer within computing systems and data centers, particularly relevant for 2 ”m communications.
- Quantum Photonic Circuits: Potential use in silicon-based quantum technologies due to the direct bandgap and strong optical matrix element.
- Optical Sensing: Applications in integrated optical sensors and detectors, leveraging the tunable emission spectrum (0.3 eV to 0.7 eV).
- Green Information Technology: Reducing the energy footprint of information and communication technologies by improving the efficiency of on-chip light generation.