Direct band gap carbon superlattices with efficient optical transition

At a Glance

Metadata	Details
Publication Date	2016-02-04
Journal	Physical review. B./Physical review. B
Authors	Young Jun Oh, Sunghyun Kim, In‐Ho Lee, Jooyoung Lee, K. J. Chang
Institutions	Korea Research Institute of Standards and Science, Korea Institute for Advanced Study
Citations	15
Analysis	Full AI Review Included

Technical Analysis and Documentation for Direct Band Gap Carbon Superlattices

Executive Summary

This documentation analyzes the computational research predicting a family of pure carbon superlattices (C(100)_n/C(5-7)) that exhibit intrinsic direct wide band gaps, positioning them as highly competitive materials for deep ultraviolet (UV) optoelectronics.

Novel Direct Band Gap Carbon Allotrope: Computational models confirm C(100)_n/C(5-7) superlattices possess direct band gaps ranging from 5.6 eV to 5.9 eV, overcoming the indirect gap limitation of standard cubic diamond (SCD).
Deep UV Emitter Potential: The band gap range corresponds to deep UV emission wavelengths of 210-221 nm, addressing the critical need for efficient emitters below 250 nm, where current AlGaN alloys struggle.
High Optical Efficiency: Calculated dipole matrix elements (|M|²) for optical transition are comparable to or greater than those of Gallium Nitride (GaN), suggesting high external quantum efficiencies are achievable.
Extreme Thermal and Dynamical Stability: Molecular dynamics simulations confirm thermal stability up to 2000 K, making these materials ideal candidates for high-power, high-temperature optoelectronic applications.
Practical Synthesis Route: The proposed synthesis method—wafer bonding of atomically smooth, homoepitaxially grown C(100) diamond surfaces—directly aligns with 6CCVD’s core MPCVD growth and polishing capabilities.
Low Formation Energy: The excess energy required to form superlattices with n ≥ 3 is less than 90 meV/atom relative to cubic diamond, indicating high thermodynamic feasibility.

Technical Specifications

A summary of the critical physical and electronic properties extracted from the density functional theory (DFT) and G₀W₀ calculations.

Parameter	Value	Unit	Context
Material Family	C(100)_n/C(5-7) Superlattices	N/A	Stacked layers of diamond (100) with defective 5-/7-membered rings.
Direct Band Gap (E_g, G₀W₀)	5.57 - 5.94	eV	For structures with n = 2 to n = 7.
Peak Emission Wavelength	210 - 221	nm	Deep UV range, competitive with specialized AlGaN alloys.
Thermal Stability Limit	2000	K	Confirmed stable for 100 ps simulations, suitable for high power.
Optical Transition Efficiency	≥ 1.0 - 1.4	Ratio to	M
Excess Energy (n ≥ 3)	< 90	meV/atom	Low relative formation energy compared to cubic diamond.
Hole Effective Mass (Average, m_h)	0.34 - 0.39	m_o	Mass comparable to cubic diamond (0.43 m_o).
Electron Effective Mass (Average, m_e)	0.53 - 1.42	m_o	Increases with n, suggesting short-period SLs (low n) offer higher electron mobility.
Required Wafer Orientation	C(100) 2x1	N/A	Surface reconstruction required for successful low-barrier wafer bonding.

Key Methodologies

The theoretical framework and the proposed practical route for synthesis highlight specific engineering requirements critical for material production.

Computational Modeling (Prediction)

Global Optimization: Conformational Space Annealing (CSA) was employed for global optimization, minimizing enthalpy and specifically designing the objective function to promote direct band gap formation.
DFT Calculations: Electronic properties were analyzed using Density Functional Theory (DFT) with the Perdew, Burke, and Ernzerhof (PBE) functional.
Quasiparticle Correction: Accurate band gaps were determined using G₀W₀ quasiparticle calculations, yielding values (5.57-5.94 eV) significantly higher than PBE (4.00-4.31 eV).
Optical Transition Analysis: Dipole matrix elements (|M|²) were calculated by solving the Bethe-Salpeter equation (BSE) together with the G₀W₀ approximation to assess deep UV light source efficiency.
Stability Testing: Dynamical stability was confirmed via full phonon spectra calculation (no imaginary modes), and thermal stability was confirmed using ab initio molecular dynamics simulations at 2000 K.

Proposed Practical Synthesis (Realization)

The research proposes a scalable path to create these superlattices, utilizing high-quality diamond surface engineering:

Homoepitaxial Diamond Growth: Prepare atomically smooth, high-purity C(100) wafers via Chemical Vapor Deposition (CVD).
Surface Reconstruction: Use thermal annealing to induce hydrogen desorption, resulting in clean C(100) 2x1 surface reconstruction.
Wafer Bonding: Bond two clean C(100) 2x1 surfaces. The simulation shows this process occurs without an energy barrier as the distance between the two surfaces decreases, leading directly to the formation of the C(5-7) defective layer.

6CCVD Solutions & Capabilities

6CCVD provides the necessary material science foundation—ultra-pure, highly engineered MPCVD diamond wafers—required to replicate and advance this cutting-edge research into functional Deep UV devices.

Applicable Materials

To realize the direct band gap superlattice via the proposed wafer bonding route, researchers require precision-engineered diamond substrates:

Optical Grade SCD Wafers (C(100) Orientation): High purity (low nitrogen/defects) Single Crystal Diamond (SCD) is essential to maximize optical transparency and thermal performance, as the superlattice structure is largely derived from the bulk diamond layers.
Precision (100) Surfaces: The core synthesis route relies entirely on atomically smooth, homoepitaxial C(100) growth. 6CCVD offers SCD wafers with custom orientations and guaranteed ultra-low defect densities necessary for large-area surface reconstruction.
PCD Substrates for Cost/Scale: For potential up-scaling or device integration after the initial proof-of-concept, 6CCVD’s large-area Polycrystalline Diamond (PCD) plates (up to 125mm) offer high thermal management capability for high-power devices operating at high temperatures (up to 2000 K stability).

Customization Potential for Direct Gap SL Replication

The success of the wafer bonding technique hinges on perfect alignment and surface quality, leveraging 6CCVD’s unique capabilities:

Service	6CCVD Capability	Application Relevance
Custom Dimensions	Plates/wafers up to 125mm (PCD)	Enables scaling experiments beyond small coupons, critical for industrial viability.
Extreme Polishing	Ra < 1 nm (SCD), Ra < 5 nm (PCD)	Atomically smooth surfaces are non-negotiable for low-barrier wafer bonding and subsequent formation of the epitaxial C(5-7) interface.
Thickness Control	SCD layers from 0.1 µm up to 500 µm	Provides flexible starting materials—either ultra-thin layers for complex layer transfer or robust substrates up to 10 mm.
Custom Metalization	Au, Pt, Pd, Ti, W, Cu (In-house)	While not explicitly detailed for this material, subsequent Deep UV LED fabrication will require custom electrode deposition, a standard internal 6CCVD service.

Engineering Support

This research validates the pursuit of carbon-based optoelectronics operating in extreme environments. 6CCVD’s in-house PhD team can assist researchers and engineers with material selection and optimization for:

Deep UV Optoelectronics: Selecting the ideal SCD or PCD grade based on required purity, transmission cutoff, and thermal load for UV LED/LD projects.
High-Power/High-Temperature Devices: Utilizing the exceptional thermal properties of diamond (validated up to 2000 K in this study) for advanced thermal management components.
Advanced Wafer Processing: Consultation on achieving and maintaining the atomically smooth C(100) surface quality required for state-of-the-art bonding and integration processes.

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

View Original Abstract

We report pure carbon-based superlattices that exhibit direct band gaps and\nexcellent optical absorption and emission properties at the threshold energy.\nThe structures are nearly identical to that of cubic diamond except that\ndefective layers characterized by five- and seven-membered rings are\nintercalated in the diamond lattice. The direct band gaps lie in the range of\n5.6~~5.9 eV, corresponding to wavelengths of 210~~221 nm. The dipole matrix\nelements of direct optical transition are comparable to that of GaN, suggesting\nthat the superlattices are promising materials as an efficient deep ultraviolet\nlight emitter. Molecular dynamics simulations show that the superlattices are\nthermally stable even at a high temperature of 2000 K. We provide a possible\nroute to the synthesis of superlattices through wafer bonding of diamond (100)\nsurfaces.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.93.085201