Conductive Nanorods in DLC Films Caused by Carbon Transformation

At a Glance

Metadata	Details
Publication Date	2017-07-01
Journal	Ukrainian Journal of Physics
Authors	А.А. Еvtukh, В. Г. Литовченко, М. В. Стріха, Anatolii I. Kurchak, Oktay Yilmazoglu
Institutions	V.E. Lashkaryov Institute of Semiconductor Physics
Citations	3
Analysis	Full AI Review Included

Technical Analysis & Documentation: Conductive Nanorods in DLC Films

6CCVD Ref No.: NANO-2017-0526 Application Focus: High-Efficiency Field Emission Cathodes

Executive Summary

The investigated research details a highly effective methodology for improving Electron Field Emission (EFE) efficiency using doped Diamond-Like Carbon (DLC) films. 6CCVD, as an expert in high-purity MPCVD diamond, offers materials that can replicate or significantly advance this research by providing superior $sp^{3}$ control and stable conductivity.

Ultra-Low Work Function: Achieved minimum effective work function of 0.92 eV by precisely controlling Nitrogen ($\text{N}_{2}$) concentration (25%) during PECVD deposition.
EFE Enhancement Mechanism: Enhanced EFE efficiency driven by high-field “pre-breakdown conditioning” which locally transforms insulating diamond-like $sp^{3}$ bonds into conductive graphite-like $sp^{2}$ channels.
Conductive Nanorods: The formation of these stable, highly conductive nanochannels (estimated diameter 5 nm to 25 nm) embedded within the insulating $sp^{3}$ matrix facilitates local electric field enhancement.
Material Instability Addressed: The methodology relies on structural transformation and localized heating of amorphous carbon, which can be unstable; 6CCVD offers stable, intrinsically conductive materials (BDD) to achieve similar or better results reliably.
Scalability Proof: Results indicate that forming conductive channel arrays in carbon films allows for significantly enhanced EFE performance even on large, flat surfaces.

Technical Specifications

The following hard parameters define the material synthesis and performance metrics achieved in the research paper.

Parameter	Value	Unit	Context
Minimum Effective Work Function ($\Phi_{eff}$)	0.92	eV	Measured for DLC film at 25% $\text{N}_{2}$ content
DLC Film Thickness (d)	60 - 80	nm	Deposited by PECVD on Si tips
$\text{N}_{2}$ Gas Content (Maximum)	45	%	Maximum concentration used in gas mixture ($\text{CH}{4}:\text{H}{2}:\text{N}_{2}$)
RF Bias Voltage (PECVD)	~1900	V	Applied bias during film deposition
Minimum Band Gap ($\text{E}_{g}$)	2	eV	Corresponds to DLC film optimized for EFE (25% $\text{N}_{2}$)
Estimated Conductive Nanochannel Diameter	5 - 25	nm	Estimated from reduced threshold voltage after conditioning
Si Tip Radius Curvature (r)	10 - 20	nm	Geometry prior to DLC deposition
Emitter-Anode Spacing (L)	20	µm	Diode configuration measurement
Vacuum Operating Pressure	10^-6	Torr	EFE measurement environment

Key Methodologies

The experiment centered on controlling the ratio of $sp^{3}$ (diamond-like) to $sp^{2}$ (graphite-like) carbon bonds via plasma deposition parameters and subsequent electrical conditioning.

Si Tip Preparation: Formation of high-density arrays of Si emitter tips on (100) n-type wafers via chemical etching and thermal oxidation sharpening (900 °C in wet oxygen).
PECVD System: Utilization of a 13.56 MHz RF-powered Plasma Enhanced-Chemical Vapor Deposition system.
Gas Mixture Control: Deposition utilized a $\text{CH}{4}:\text{H}{2}:\text{N}_{2}$ gas mixture. The nitrogen concentration, which serves as the doping source, was varied systematically from 0% up to 45%.
Process Environment: Substrates were placed on a water-cooled cathode (200 mm diameter) and deposited under low pressures (0.2, 0.6, and 0.8 Torr).
Characterization: DLC film properties ($\text{E}_{g}$, thickness, refractive index) measured using spectroscopic ellipsometry ($\lambda$ = 632.8 nm); elemental nitrogen content confirmed by Auger Electron Spectroscopy.
EFE Measurement: Current-voltage characteristics measured in a vacuum system (10^-6 Torr) and analyzed using Fowler-Nordheim (F-N) coordinates.
Pre-Breakdown Conditioning: High-field operation was used to locally heat and induce the transformation of the insulating $sp^{3}$ phase into the conductive $sp^{2}$ phase, generating permanent nanochannels.

6CCVD Solutions & Capabilities

6CCVD provides the specialized MPCVD materials necessary to stabilize and enhance the results demonstrated in this field emission research, replacing transient structural transformation with intrinsic material control.

Applicable Materials

To replicate and improve the performance of highly controlled carbon thin films for stable EFE, 6CCVD recommends:

Optical Grade Single Crystal Diamond (SCD): Required for achieving the ultimate benchmark $sp^{3}$ purity and stability. Ideal for fundamental research on defect generation (such as those described in the paper’s modeling section involving vacancies and $\text{H}$ defects) or for use as highly insulating dielectric layers.
Heavy Boron-Doped Diamond (BDD): The preferred material for highly conductive field emission cathodes. Unlike the $\text{N}_{2}$ doping method used in the paper, BDD provides stable, intrinsic p-type conductivity and a reliably low effective work function, eliminating the need for high-voltage, pre-breakdown structural conditioning.
Polycrystalline Diamond (PCD) Wafers: Suitable for developing the large-area, flat EFE cathodes suggested by the authors. PCD offers stable mechanical properties and controlled grain boundary structures which can be utilized as predetermined conductive pathways.

Customization Potential

6CCVD’s advanced processing capabilities directly address the technical complexity of building stable EFE devices:

Research Parameter	6CCVD Capability	Technical Advantage
Film Thickness: Paper used 60-80 nm DLC.	Precise Thickness Control: SCD and PCD available from 0.1 µm up to 500 µm (and substrates up to 10 mm).	Allows engineering of optimal electron tunneling barriers and precise junction integration.
Scalability: Paper used 8x8 cm² arrays.	Large Diameter Substrates: Custom plates/wafers up to 125 mm (PCD) for large-scale cathode fabrication.	Facilitates industrial scale-up of flat-panel electron sources.
Surface Finish: Highly polished diamond reduces field enhancement variability.	Ultra-Low Roughness Polishing: SCD polishing to $\text{Ra}$ < 1nm; Inch-size PCD polishing to $\text{Ra}$ < 5nm.	Minimizes morphological features that can cause unstable emission current density fluctuations.
Electrical Contact/Integration: Requires robust interface to Si tips/substrate.	In-House Custom Metalization: Expertise in depositing ohmic contact stacks including Au, Pt, Pd, Ti, W, and Cu directly onto diamond surfaces.	Ensures reliable, low-resistance interfaces critical for handling the high current densities required for EFE.

Engineering Support

The formation of conductive nanorods relies on the complex physics of bond rehybridization ($\text{sp}^{3}$ to $\text{sp}^{2}$) under electrical and thermal stress. 6CCVD’s in-house PhD material science team specializes in controlling the defect physics and doping of MPCVD diamond. We can assist researchers in substituting the transient DLC films with stable, intrinsically conductive BDD or highly controlled SCD to optimize performance for similar Field Emission Cathodes or Vacuum Microelectronic Devices.

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

View Original Abstract

The influence of diamond-like carbon (DLC) films deposited under various conditions on the electron field emission (EFE) of silicon (Si) tips has been investigated. During the nitrogen-doped DLC film deposition, the nitrogen content in a gas mixture is varied from 0% to 45%. In this context, the effective work function is optimized, by reaching the values less than 1 eV. A sharp increase in the emission current at high electric fields and a decrease of the threshold voltage after the pre-breakdown conditioning of a DLC film on Si tips have been measured. At high current densities and the resulting local heating, the diamond-like sp3 phase transforms into a conducting graphite-like sp2 phase. As a result, an electrical conducting nanostructured channel is formed in the DLC film. The diameter of the conducting nanochannel is estimated from the reduced threshold voltage after the pre-breakdown conditioning to be in the interval 5-25 nm. The presence of this nanochannel in the insulating matrix leads to a local enhancement of the electric field and a reduced threshold voltage for EFE. The obtained results can be used for the development of highly efficient field emission cathodes. To explain the experimental EFE results based on a transformation of DLC films and the generation of conduction nanochannels, the changes of the electron affinity (x0) for various carbon structures and impurity point defects have been calculated. The influence of the rehybridization of bonds in various carbon crystal structures on x0 is shown. The formation of conducting channel arrays in DLC films will allow us to significantly enhance EFE even on flat surfaces without tips.

Tech Support

Original Source

DOI: https://doi.org/10.15407/ujpe62.06.0526

References

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