Boron-doping effects on local structures of semiconducting ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films fabricated via coaxial arc plasma - an x-ray absorption spectroscopic study

At a Glance

Metadata	Details
Publication Date	2021-06-09
Journal	Semiconductor Science and Technology
Authors	Naofumi Nishikawa
Institutions	Japan Advanced Institute of Science and Technology, Kyushu University
Analysis	Full AI Review Included

Executive Summary

This study investigates the structural and electrical implications of boron doping in Ultrananocrystalline Diamond/hydrogenated Amorphous Carbon (UNCD/a-C:H) composite thin films fabricated using Coaxial Arc Plasma Deposition (CAPD).

P-Type Conduction Mechanism: The successful incorporation of boron preferentially forms σ* C-B bonds localized at the UNCD grain surfaces (Grain Boundaries, GBs), which is the primary structural origin for enhanced p-type electrical conductivity.
Doping Location: Boron atoms substitute hydrogen atoms terminating the UNCD surfaces at GBs, rather than substitutionally replacing carbon atoms within the diamond lattice, a mechanism distinct from conventional CVD diamond doping.
Electrical Enhancement: The formation of σ* C-B bonds creates acceptor-like localized levels above the valence band, leading to a substantial increase in electrical conductivity (up to 2 x 10^-1 Ω^-1cm^-1) via hopping conduction.
Structural Distortion: Low-level boron doping (less than 1.0 at. %) causes initial structural distortion, resulting in the formation of metallic boron (B-B) at GB interfaces and an increase in unstable π* C=C bonds.
Device Potential: The films exhibit a high light absorption coefficient (> 10⁵ cm^-1) across the UV range, making them highly promising candidates for deep-UV photodetector applications.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Method	Coaxial Arc Plasma Deposition (CAPD)	N/A	Used for low-temperature, high-absorption film growth.
Substrate Temperature	550	°C	Deposition temperature.
Hydrogen Pressure	53.3	Pa	Operating pressure during deposition.
Base Pressure (Evacuation)	10^-5	Pa order	Chamber base pressure prior to deposition.
Boron Target Content (Range)	0 to 10	at. %	Boron concentration in graphite targets.
Indirect Optical Bandgap	1.7	eV	Associated with the amorphous carbon (a-C:H) matrix.
Direct Optical Bandgap	2.9	eV	Associated with Grain Boundaries (GBs).
Light Absorption Coefficient	> 10⁵	cm^-1	High absorption across 3 to 6 eV photon energy range (Deep-UV).
Boron Activation Energy	0.37	eV	Relatively low energy for p-type conduction in diamond.
Undoped Conductivity (RT)	2 x 10^-7	Ω^-1cm^-1	Room temperature conductivity of intrinsic films.
Doped Conductivity (Max. RT)	2 x 10^-1	Ω^-1cm^-1	Achieved with sufficient boron doping to form σ* C-B bonds.
σ* C-B NEXAFS Resonance	286.9	eV	Photon energy corresponding to the key p-type bonding state.

Key Methodologies

The UNCD/a-C:H films were fabricated and characterized using the following procedures:

Film Deposition (CAPD):
- Technique: Coaxial Arc Plasma Deposition (CAPD) was employed, utilizing a coaxial arc plasma gun equipped with graphite targets.
- Doping: Targets were blended with Boron contents ranging from 0.1 to 10 at. % to control the doping level.
- Substrate: Films were deposited onto insulating Si substrates (resistivity > 10 kΩ·cm).
- Conditions: Deposition occurred at a substrate temperature of 550 °C under a hydrogen pressure of 53.3 Pa.
Structural Characterization (NEXAFS):
- Spectroscopy: Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was performed at the Kyushu Synchrotron Research Center.
- Measurements: Both Boron K-edge and Carbon K-edge spectra were obtained.
- Detection: Total Electron Yield (TEY) technique was used to record the data.
- Analysis: C K-edge spectra were deconvoluted using Gaussian and arctangent functions to quantitatively determine the relative amounts of carbon hybridization states (e.g., π* C=C, σ* C-H, σ* C-B, σ* C-C).
Supporting Characterization:
- X-ray Photoelectron Spectroscopy (XPS) was used to confirm boron incorporation (B 1s spectra) and the presence of oxygen impurities (O 1s spectra).

Commercial Applications

The unique structural and electrical properties induced by localized boron doping make these UNCD/a-C:H composite films suitable for advanced semiconductor and sensing technologies:

Deep-UV Photodetectors: The high light absorption coefficient (> 10⁵ cm^-1) and the ability to generate photo-induced carriers efficiently under UV irradiation make these films excellent candidates for high-sensitivity UV sensors.
P-Type Diamond Electronics: The creation of stable, highly conductive p-type layers is crucial for fabricating high-performance p-n heterojunction diodes and other active diamond-based semiconductor devices.
High-Frequency/High-Power Devices: Diamond’s inherent wide bandgap and thermal stability, combined with tunable p-type conductivity, are essential for next-generation power electronics and high-frequency components.
Electrochemical Sensors: The conductive nature of the grain boundaries, enhanced by boron doping, is beneficial for creating robust electrodes for fuel cells, biosensors, and other electrochemical applications.
Radiation Hard Electronics: Diamond-based materials are inherently radiation-hard, and the composite structure may offer advantages in devices intended for harsh environments (e.g., space or nuclear applications).

View Original Abstract

Abstract Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma possess a marked structural feature of diamond grains embedded in an amorphous carbon and a hydrogenated amorphous carbon matrix which are the largest constituents of the films. Since the amorphous nature yields much larger light absorption coefficients as well as a generation source of photo-induced carriers with UV rays, these films can be potential candidates for deep-UV photodetector applications. From some previous studies p-type conduction of the films has been realized by doping boron in experimental conditions. In addition, their optical and electrical characteristics were investigated previously. However, the bonding structures which largely affect the physical properties of the devices have not been investigated. In this work, near-edge x-ray absorption fine structure spectroscopy characterizations are carried out. The result reveals that a bonding state σ * C-B of diamond surfaces is formed preferentially and structural distortion is caused at an early stage of boron-doping. Further doping into the films lessens the amount of unsaturated bonds such as π * C≡C, which may be a cause of the device performance degradations. Our work suggests a fundamental case model of boron-doping effects on a local structure of the film.

Tech Support

Original Source

DOI: https://doi.org/10.1088/1361-6641/ac09d0

References

1982 - Vapor deposition of diamond particles from methane [Crossref]
1982 - Growth of diamond particles from methane-hydrogen gas [Crossref]
1983 - Diamond synthesis from gas phase in microwave plasma [Crossref]
1985 - Chemical vapour deposition of diamond in RF glow discharge [Crossref]
1987 - Synthesis of diamond films in a rf induction thermal plasma [Crossref]
1990 - Thermal diffusivity of isotopically enriched 12C diamond [Crossref]
1993 - Thermal conductivity of isotopically modified single crystal diamond [Crossref]
1964 - Intrinsic edge absorption in diamond [Crossref]
2001 - Synthesis and characterization of highly-conducting nitrogen-doped ultrananocrystalline diamond films [Crossref]
1997 - Growth and characterization of phosphorous doped {111} homoepitaxial diamond thin films [Crossref]