Laser vibrational excitation of radicals to prevent crystallinity degradation caused by boron doping in diamond

At a Glance

Metadata	Details
Publication Date	2021-01-20
Journal	Science Advances
Authors	Lisha Fan, Loïc Constantin, Zhipeng Wu, Kayleigh A. McElveen, X. G. Chen
Institutions	Institut de Chimie de la Matière Condensée de Bordeaux, Centre National de la Recherche Scientifique
Citations	15
Analysis	Full AI Review Included

Executive Summary

This research demonstrates a novel laser-assisted chemical vapor deposition (CVD) route to overcome the critical trade-off between high doping concentration and crystal quality in boron-doped diamond (BDD).

Core Achievement: Highly conductive, metallic-type BDD films were grown at a fast rate (35 µm/hour) while maintaining excellent crystallinity, a feat previously prohibitively difficult in high-level doping regimes.
Mechanism of Control: The process uses laser vibrational excitation (10.494 µm CO₂ laser) tuned to the fundamental bending mode (v₂) of the growth-critical radical, boron dihydride (BH₂).
Thermal Nonequilibrium: Laser excitation induces a thermal nonequilibrium state in the flame chemistry, which preferentially suppresses the relative abundance of detrimental boron hydrides (BH) compared to carbon-containing radicals (C₂, CH).
Performance Metrics: The resulting BDDs achieved a high boron concentration (4.3 x 10²¹ cm^-3), ultra-low resistivity (28.1 milliohm-cm at 300 K), and record-high Hall hole mobility (55.6 cm² V^-1 s^-1).
Crystallinity Confirmation: High-resolution TEM confirmed sharp grain boundaries free of amorphous or graphitic carbon, explaining the superior hole mobility.
Application Validation: The highly crystalline and conductive BDD electrodes showed enhanced efficiency in glucose sensing, confirming their potential for high-performance electrochemical devices.

Technical Specifications

Parameter	Value	Unit	Context
Doping Concentration ([B])	4.3 x 10²¹	cm^-3	Measured by SIMS/Hall effect (21 W absorbed laser power)
Film Resistivity (ρ)	28.1	milliohm-cm	At 300 K (21 W absorbed laser power)
Hole Mobility (µ_h)	55.6	cm² V^-1 s^-1	At 300 K (21 W absorbed laser power); 200x higher than nanocrystalline BDD
Growth Rate	35	µm/hour	Laser-assisted combustion CVD
Laser Wavelength (λ)	10.494	µm	Matches BH₂ bending mode (v₂)
Substrate Temperature (T_sub)	780 ± 10	°C	Maintained during deposition
Undoped Diamond d₁₁₁	2.06	Angstrom	Reference lattice spacing
Doped Diamond d₁₁₁	2.12	Angstrom	Lattice spacing showing expansion due to B incorporation
Working Potential Window	Up to 2.9	V	In 0.1 M H₂SO₄ (21 W absorbed laser power)
Commercial BDD Resistivity	4616	ohm-cm	At 300 K (for comparison)
Commercial BDD [B]	6.8 x 10²⁰	cm^-3	For comparison
B Flow Rate (Max Tested)	100	sccm	Acetylene line carrying B source

Key Methodologies

The BDD films were prepared using a laser-assisted combustion diamond CVD method, actively controlling the gas-phase chemistry.

Substrate Preparation: p-type silicon (Si) wafers were used as substrates, pre-seeded ultrasonically in a 5-nm diamond slurry.
CVD Environment: An open-air combustion flame was generated using standard gas flows: Acetylene (1685 sccm) and Oxygen (1795 sccm).
Doping Introduction: Boron was introduced by bubbling a third acetylene line through a boric oxide methanol solution (10 g/liter). The B flow rate (0 to 100 sccm) controlled the doping density.
Temperature Control: Substrates were placed on a water-cooled, three-axis moving stage, maintaining the deposition temperature at 780° ± 10°C.
Laser System: A wavelength-tunable, continuous-wave Carbon Dioxide (CO₂) laser (9.2 to 10.9 µm range) was used as the irradiation source.
Resonant Excitation: The laser was tuned precisely to 10.494 µm to match and resonantly excite the BH bending mode (v₂) of the BH₂ radical.
Laser Coupling: The laser beam was focused to a ~2 mm diameter to cover the inner core of the oxyacetylene flame, parallel to the substrate. Absorbed power was varied up to 21 W.
Diagnostics:
- Flame Chemistry: Optical Emission Spectroscopy (OES) and Laser-Induced Fluorescence (LIF) were used to analyze excited-state and ground-state radicals (C₂, CH, BH) in the flame.
- Film Quality: FESEM, micro-Raman spectroscopy, HRTEM, and SIMS were used for morphological, structural, and compositional analysis.
- Electrical Properties: Resistivity and Hall effect measurements were performed using a homemade system and a Quantum Design PPMS.

Commercial Applications

The ability to produce highly crystalline, highly conductive BDDs at fast growth rates removes a major limitation for diamond technology adoption in several high-value sectors.

Advanced Electrochemistry:
- High-Performance Sensors: Direct application in biosensing (e.g., glucose sensors, as demonstrated) due to enhanced electrochemical activity, low background current, and wide potential window.
- Water Treatment: Highly stable electrodes for advanced oxidation processes (AOPs) and wastewater remediation.
Semiconductor Devices:
- High-Power/High-Frequency Electronics: Utilizing diamond’s superior thermal conductivity and wide bandgap (5.5 eV) for devices requiring high current density and stability.
- Doping Flexibility: The technique offers a pathway to achieve high-level doping in other rigid-lattice semiconductors (e.g., GaN, SiC) without sacrificing crystal integrity.
Materials Science:
- Fundamental Research: Provides a tool for “microscopic” control over chemical reactions, enabling the design and execution of previously unidentified doping schemes.
Energy Storage:
- Capacitors and Batteries: Use as stable, highly conductive electrode materials in extreme environments or high-performance energy storage systems.

View Original Abstract

An active, “microscopic,” laser-enabled control of energy coupling channel produces highly conductive and crystalline diamond.

Tech Support

Original Source

DOI: https://doi.org/10.1126/sciadv.abc7547