Direct-bandgap emission from hexagonal Ge and SiGe alloys

At a Glance

Metadata	Details
Publication Date	2020-04-08
Journal	Nature
Authors	Elham Fadaly, Alain Dijkstra, Jens Renè Suckert, Dorian Ziss, Marvin A. J. van Tilburg
Institutions	Eindhoven University of Technology, Friedrich Schiller University Jena
Citations	351
Analysis	Full AI Review Included

Technical Documentation and Analysis: Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys

Analyzed Paper: Elham M.T. Fadaly et al. “Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys” (arXiv:1911.00726v1)

Executive Summary

This paper presents a fundamental breakthrough: the demonstration of efficient, direct bandgap light emission from Hexagonal (WZ) Germanium and Silicon-Germanium (Hex-Si_1-xGe_x) alloys, paving the way for monolithic integration of optical functionality onto silicon platforms.

Key Achievement: Hexagonal SiGe alloys (x > 0.65) were confirmed, via ab initio theory and experiment, to be direct bandgap semiconductors.
Performance: Achieved sub-nanosecond (ns) radiative lifetimes ($\sim 1 \text{ ns}$), demonstrating efficiency comparable to established direct bandgap III-V semiconductors (e.g., GaAs).
Wavelength Tunability: Emission wavelength is continuously tunable in a broad spectral range (0.3 eV to 0.7 eV, or 4.4 µm to 1.8 µm) by controlling the Si:Ge composition.
Radiative Efficiency: The material exhibits a high radiative recombination coefficient ($B_{\text{rad}}$), spanning $0.7 \times 10^{-10} \text{ cm}^{3}/\text{s}$ to $11 \times 10^{-10} \text{ cm}^{3}/\text{s}$ at 300 K.
Thermal Stability: High-quality samples maintain temperature-independent lifetime and photoluminescence (PL) intensity up to 220 K, crucial for practical device application.
Future Impact: Hexagonal SiGe provides a pathway for tightly integrated, high-performance light sources compatible with existing cubic Si-photonics circuitry, significantly enhancing energy efficiency and data throughput.

Technical Specifications

The table below summarizes critical performance and material parameters extracted from the Hex-Si_1-xGe_x experiments:

Parameter	Value	Unit	Context
Material Structure	Wurtzite (Hexagonal)	N/A	Hex-Si_1-xGe_x core/shell Nanowires (NWs)
Direct Bandgap Range (Γ-point)	0.3 to 0.7	eV	Tunable via Ge content (x > 0.65)
Wavelength Range	4.4 to 1.8	µm	Mid-IR/Near-IR region, critical for optical interconnects
Radiative Lifetime (τ_rad)	0.4 to 1.6	ns	Measured at 300 K (Sample I extrapolation)
B-Coefficient (B_rad)	$0.7 \times 10^{-10}$ to $11 \times 10^{-10}$	cm³/s	Comparable to GaAs and InP (III-V semiconductors)
N-Doping Concentration (n₀)	$9 \times 10^{18}$	cm^-3	Unintentional As-doping, leading to degenerate semiconductor behavior
Crystal Quality (Stacking Faults)	2-4	SFs/µm	Along the [0001] crystalline direction
PL Intensity Stability	Constant	N/A	Integrated PL intensity stable up to 220 K (Sample I)
NW Shell Thickness	200-400	nm	Epitaxial growth on WZ GaAs core
Growth Temperature	650-700	°C	MOVPE growth of SiGe shell

Key Methodologies

The experimental approach relies on template-assisted selective area growth of nanowires (NWs) using Metal Organic Vapor Phase Epitaxy (MOVPE).

Template Preparation: Au catalyst seeds were patterned via electron beam lithography on a GaAs (111)B substrate.
Core Growth (WZ GaAs): Wurtzite (WZ) GaAs core NW templates were grown using VLS mechanism at $650 \text{ °C}$ using trimethylgallium (TMGa) and Arsine (AsH₃) precursors. Core diameter was approximately 35 nm.
Catalyst Removal: The Au catalyst particles were removed using wet chemical etching to avoid gold contamination in the subsequent SiGe shell.
Shell Epitaxy (Hex-SiGe): The sample was reintroduced into the MOVPE reactor. The Hex-Si_1-xGe_x shell (200-400 nm thickness) was grown epitaxially around the WZ GaAs core template using Silane (Si₂H₆) and Germane (GeH₄) precursors.
Growth Parameters:
- Reactor Flow: 8.2 slm.
- Reactor Pressure: 50 mbar.
- Shell Growth Temperature: $650 \text{ °C}$ to $700 \text{ °C}$.
- SiGe Molar Fraction ($X_{\text{SiGe}}$): $1.55 \times 10^{-4}$.
Characterization: Techniques included High-Resolution XRD (synchrotron radiation), TEM/HAADF-STEM (confirming hexagonal ABAB stacking), EDX (core/shell geometry), Atom Probe Tomography (APT, confirming n-doping concentration), and Time-correlated Single Photon Counting (TCSPC) for lifetime measurement.

6CCVD Solutions & Capabilities

This breakthrough research, which targets the integration of high-performance light sources onto Si platforms, directly aligns with 6CCVD’s expertise in providing custom, high-quality material solutions necessary for advanced optoelectronic and quantum applications.

The challenge of Si-based optoelectronics is often managing defects, thermal load, and lattice mismatch. While this paper uses strain-management via NW templates, a robust, large-area substrate platform is critical for scale. 6CCVD offers unique materials that can serve as superior substrates or buffer layers to replicate or extend this Hex-SiGe research.

Applicable Materials

The high thermal conductivity and lattice matching potential of diamond offer significant benefits for scaling Hex-SiGe devices, especially for high-power integrated lasers and modulators where heat management is paramount.

Material Solution	Specification & Benefit	Application Relevance to Hex-SiGe Research
Optical Grade Single Crystal Diamond (SCD)	Low nitrogen (Optical Grade), Polished (Ra < 1nm), Thick layers (up to 500 µm).	Ideal platform for passive optical circuitry, leveraging diamond’s extreme thermal conductivity ($\gt 2000 \text{ W}/\text{mK}$) to manage heat generated by high-power integrated Hex-SiGe emitters.
Heavy Boron Doped Diamond (BDD)	Highly conductive, tunable doping concentrations (PCD or SCD), custom thickness (0.1 µm to 500 µm).	Potential strain engineering buffer layer or highly conductive electrical contact layer for Hex-SiGe devices, replacing or supplementing the role of the heavily doped Si template.
Polycrystalline Diamond (PCD) Wafers	Large dimensions (up to 125 mm), Excellent thermal sink (PCD thermal conductivity significantly exceeds Si).	Scaling platform for manufacturing large arrays of Si-compatible optoelectronic components, offering superior mechanical and thermal management compared to standard Si wafers.

Customization Potential for Optoelectronics

The success of Hex-SiGe devices depends heavily on precise geometry, clean interfaces, and custom electrodes—all areas where 6CCVD provides specialized services:

Custom Dimensions and Shapes: While the paper used NWs, future planar integration requires highly specific substrate sizes and shapes. 6CCVD offers custom plates and wafers up to 125mm (PCD) and precise laser cutting services for highly specialized designs.
Interface Control and Polishing: Epitaxial growth of the Hex-SiGe shell requires an atomically smooth surface. 6CCVD provides ultra-low roughness polishing (Ra < 1 nm for SCD and < 5 nm for inch-size PCD), essential for high-fidelity material transfer or direct heteroepitaxy experiments.
Integrated Metalization: The paper references the need for custom optical components (mirrors) and electrical contacts. 6CCVD offers in-house metalization services (Au, Pt, Pd, Ti, W, Cu) to deposit contacts, mirrors, or strain-modulating layers directly onto diamond substrates.

Engineering Support

This research demonstrates a clear path toward high-efficiency Group IV photonics. 6CCVD’s in-house PhD-level engineering team is available to assist researchers and engineers in selecting the optimal diamond material platform—whether SCD or BDD—for similar integrated Si-compatible laser and detector projects. We offer consultation on crystal orientation, doping concentration, and surface preparation to maximize the fidelity of material integration (e.g., template-assisted growth or strain-induced phase transformation).

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41586-020-2150-y