Correlated Electrical Conductivities to Chemical Configurations of Nitrogenated Nanocrystalline Diamond Films

At a Glance

Metadata	Details
Publication Date	2022-03-03
Journal	Nanomaterials
Authors	Abdelrahman Zkria, Hiroki Gima, Eslam Abubakr, Ashraf M. Mahmoud, Ariful Haque
Institutions	Texas State University, King Khalid University
Citations	16
Analysis	Full AI Review Included

Executive Summary

This research details the synthesis and characterization of nitrogen-doped Nanocrystalline Diamond (NCD) films, establishing a critical correlation between nitrogen content, chemical bonding configuration, and electrical conductivity for advanced electronic applications.

Core Value Proposition: Successful fabrication of low-activation energy, n-type NCD films using a scalable Physical Vapor Deposition (PVD) method (Coaxial Arc Plasma Gun, CAPG).
Electrical Performance: Electrical conductivity increased by several orders of magnitude (up to ~1 S/cm) as nitrogen content was raised from 3 at.% to 8 at.%.
Low Activation Energy: The thermal activation energy (E_a) decreased from 123 meV (3 at.% N) to 108 meV (8 at.% N), confirming efficient n-type semiconducting behavior.
Doping Mechanism Confirmation: Synchrotron Near-Edge X-ray Absorption Fine-Structure Spectroscopy (NEXAFS) proved that nitrogen doping weakens the diamond σ*C-C (sp³) phase.
Structural Correlation: The enhanced conductivity is directly attributed to the strengthening of π*C=N bonds formed at the grain boundaries (GBs), which act as sources for free electron generation.
Morphology: The deposited films were uniform, 400 nm thick, and exhibited a low root mean square (RMS) surface roughness of only 8 nm.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Method	Coaxial Arc Plasma Gun (CAPG)	PVD	Used for NCD film growth
Substrate Temperature	550	°C	During deposition
Deposition Pressure	53	Pa	Final operating pressure
N₂/H₂ Inflow Ratio (I_N/H) Range	0.3 to 1.5	Dimensionless	Controls nitrogen doping level
Maximum Nitrogen Content	8	at.%	Achieved at I_N/H = 1.5
Film Thickness	400	nm	Measured via SEM cross-section
Surface Roughness (RMS)	8	nm	Measured via AFM
Highest Electrical Conductivity	~10⁰	S/cm	N 8 at.% film (at 500 K)
Lowest Activation Energy (E_a)	108	meV	N 8 at.% film
Highest Activation Energy (E_a)	123	meV	N 3 at.% film
Intrinsic Diamond Bandgap	5.45	eV	Reference property
Intrinsic Diamond Thermal Conductivity	3320	W m^-1 K^-1	Reference property

Key Methodologies

The NCD films were synthesized using a Coaxial Arc Plasma Gun (CAPG) approach, a type of Physical Vapor Deposition (PVD).

Synthesis Parameters (CAPG)

Target: Bulk graphite (99.9% purity).
Substrate: p-type mirror-polished Si (100) or quartz (for electrical measurements).
Gas Mixture: H₂ and N₂ gases (3N purity).
Doping Control: Nitrogen concentration was controlled by varying the N₂/H₂ inflow ratio (I_N/H) from 0.3 to 1.5.
Plasma Power: Applied voltage was 100 V; discharge pulse repetition rate was 5 Hz.
Pre-treatment: Substrates were cleaned using acetone, methanol, and deionized water before being fixed 15 mm from the target.

Characterization Techniques

Morphological Analysis:
- High-Resolution Scanning Electron Microscope (HRSEM) (15 kV, 10,000× magnification) for phase identification (nanograins in amorphous carbon matrix).
- Atomic Force Microscopy (AFM, close contact mode) for surface roughness measurement.
Chemical Composition and Bonding:
- X-ray Photoemission Spectroscopy (XPS): Used MgKα line (1253.6 eV) to quantify nitrogen content (N/C ratio) based on N1s and C1s peak areas.
- Near-Edge X-ray Absorption Fine-Structure Spectroscopy (NEXAFS): Performed using synchrotron radiation (350 eV photon energy). This sensitive tool probed the structural evolution, specifically tracking the conversion of σC-C (sp³) bonds to πC=N (sp²) bonds at grain boundaries.
Electrical Characterization:
- Conductivity Measurement: Van der Pauw method was used on films deposited on quartz substrates.
- Activation Energy (E_a): Calculated from the temperature-dependent electrical conductivity (Arrhenius plot), confirming semiconducting behavior.

Commercial Applications

The unique combination of wide bandgap, high stability, and tunable n-type conductivity in NCD films makes them highly relevant for next-generation electronic and optoelectronic devices.

Deep-Ultraviolet (DUV) Optoelectronics: Diamond’s wide bandgap (5.45 eV) is ideal for DUV detectors, emitters, and light sources, particularly where high sensitivity and stability are required.
High-Power/High-Frequency Devices: The material’s intrinsic high thermal conductivity and chemical stability are crucial for developing robust transistors and diodes capable of operating efficiently at high temperatures and frequencies.
Semiconductor Heterojunctions: N-doped NCD films serve as the active layer in p-n heterojunction diodes (e.g., NCD/p-Si), offering potential for high rectification ratios and integration into existing silicon technology.
Advanced Sensing Platforms: The composite structure (nanograins embedded in an amorphous matrix) allows for tailored surface chemistry and electronic properties, applicable in electrochemical or biological sensors.
Cold Cathode Emitters: Control over the sp² content and C=N bonding at grain boundaries influences electron emission characteristics, relevant for field emission displays and vacuum microelectronics.

View Original Abstract

Diamond is one of the fascinating films appropriate for optoelectronic applications due to its wide bandgap (5.45 eV), high thermal conductivity (3320 W m−1·K−1), and strong chemical stability. In this report, we synthesized a type of diamond film called nanocrystalline diamond (NCD) by employing a physical vapor deposition method. The synthesis process was performed in different ratios of nitrogen and hydrogen mixed gas atmospheres to form nitrogen-doped (n-type) NCD films. A high-resolution scanning electron microscope confirmed the nature of the deposited films to contain diamond nanograins embedded into the amorphous carbon matrix. Sensitive spectroscopic investigations, including X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS), were performed using a synchrotron beam. XPS spectra indicated that the nitrogen content in the film increased with the inflow ratio of nitrogen and hydrogen gas (IN/H). NEXAFS spectra revealed that the σC-C peak weakened, accompanied by a πC=N peak strengthened with nitrogen doping. This structural modification after nitrogen doping was found to generate unpaired electrons with the formation of C-N and C=N bonding in grain boundaries (GBs). The measured electrical conductivity increased with nitrogen content, which confirms the suggestion of structural investigations that nitrogen-doping generated free electrons at the GBs of the NCD films.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12050854

References

2020 - Laser-Induced Phosphorus-Doped Conductive Layer Formation on Single-Crystal Diamond Surfaces [Crossref]
2020 - Low temperature synthesis of transparent conductive boron doped diamond films for optoelectronic applications: Role of hydrogen on the electrical properties [Crossref]
2022 - Materials Science in Semiconductor Processing Laser-induced novel ohmic contact formation for effective charge collection in diamond detectors [Crossref]
2021 - Infrared photodetectors based on multiwalled carbon nanotubes: Insights into the effect of nitrogen doping [Crossref]
2020 - Boosting Lithium Storage in Free-Standing Black Phosphorus Anode via Multifunction of Nanocellulose [Crossref]
2021 - Flexible electronics based on 2D transition metal dichalcogenides [Crossref]
2008 - On-state behaviour of diamond Schottky diodes [Crossref]