Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

At a Glance

Metadata	Details
Publication Date	2021-12-29
Journal	Photonics
Authors	V. V. Kononenko, М. С. Комленок, P. A. Chizhov, V. V. Bukin, V. V. Bulgakova
Institutions	Institute of Radio-Engineering and Electronics, Prokhorov General Physics Institute
Citations	7
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Efficiency Terahertz Generation in Nitrogen-Doped Diamond PCAs

Executive Summary

This research validates nitrogen-doped diamond as a superior material for high-power, large-aperture Photoconductive Antennas (PCAs) operating in the Terahertz (THz) regime. 6CCVD is uniquely positioned to supply the custom materials required to replicate and advance this technology.

Material Validation: Synthetic Monocrystalline (SCD) and Polycrystalline (PCD) CVD diamonds, supplied by 6CCVD, are confirmed as highly effective substrates for 400 nm pumped THz PCAs.
Doping Control is Critical: The efficiency of THz generation is tightly correlated with the substitutional nitrogen doping level ($N_s$, 0.1-100 ppm), which governs the optical absorption and subsequent saturation fluence ($F_{sat}$).
Performance Metrics: Achieved saturation fluence as low as 40 µJ/cm², demonstrating excellent photoconductive response under deep UV excitation (400 nm).
High Power Potential: Diamond’s exceptional dielectric strength (10 MV/cm) projects potential optical-to-THz conversion efficiencies exceeding 3%, significantly higher than current reported yields.
Customization Requirement: The study highlights the need for precise control over material origin (CVD vs. HPHT), doping uniformity, and surface quality (Rayleigh scattering minimization in PCD).
6CCVD Advantage: We provide custom-doped SCD and large-area PCD wafers with integrated metalization and ultra-low roughness polishing, essential for high-field PCA device fabrication.

Technical Specifications

The following hard data points were extracted from the analysis of nitrogen-doped diamond substrates for THz PCA applications:

Parameter	Value	Unit	Context
Nitrogen Doping Range ($N_s$)	0.1 - 100	ppm	Range of substitutional nitrogen investigated.
Diamond Bandgap	5.46	eV	Wide bandgap semiconductor requiring UV excitation.
Breakdown Electric Field	10	MV/cm	Maximum dielectric strength of diamond substrate.
Carrier Mobility (SCD)	~4500	cm² V^-1 s^-1	Record mobility at room temperature.
Pump Wavelength	400	nm	Second harmonic excitation wavelength.
Laser Pulse Duration	150	fs	Femtosecond pump source used.
Maximum Bias Voltage Applied	3	kV	Pulsed bias voltage used in the experiment.
Minimum Saturation Fluence ($F_{sat}$)	~40	µJ/cm²	Achieved with high N-doped HPHT diamond.
Maximum THz Pulse Energy	~200	pJ	Measured at 25 kV/cm bias field.
Projected Conversion Efficiency	>3	%	Expected efficiency at high bias fields (1 MV/cm).
Sample Thickness Range	250 - 1570	µm	Range of substrate thicknesses tested.
THz Emission Peak Frequency	0.2 - 0.3	THz	Low-frequency maximum observed in the spectrum.

Key Methodologies

The experimental procedure focused on correlating nitrogen doping levels with optical absorption and subsequent THz emission performance across various diamond types.

Material Sourcing and Preparation: Nineteen diamond substrates (Monocrystalline CVD, Polycrystalline CVD, and Monocrystalline HPHT) were used. Samples ranged from 3 mm to 7 mm in size and 250 µm to 1570 µm in thickness. All surfaces were mechanically polished to optical quality.
Nitrogen Quantification: Substitutional nitrogen concentration ($N_s$) was determined primarily by measuring the optical absorbance peak at 4.6 eV (270 nm) in the UV-visible spectrum. Secondary verification was performed using IR spectroscopy (1130 cm^-1 and 1344 cm^-1 bands) where applicable.
PCA Assembly: Aluminum foil electrodes were glued to the edges of the diamond substrates, creating electrode gaps ranging from 0.5 mm to 2 mm.
Excitation Setup: A femtosecond Ti:sapphire laser (800 nm fundamental, 150 fs pulse duration, 1 kHz repetition rate) was used. The second harmonic (400 nm) was generated using a BBO crystal and filtered by a cold mirror.
Bias Application: A pulsed bias voltage (up to 3 kV) with a duration of ~10 ns was synchronized with the 400 nm laser pulse. Static high voltage did not produce a THz yield.
Performance Measurement: The emitted THz radiation was collected by a telescope system using PTFE lenses and measured using a Golay detector (Tydex GC-1P) modulated at 15 Hz.

6CCVD Solutions & Capabilities

This research confirms that high-quality, nitrogen-doped CVD diamond is the enabling material for next-generation, high-power THz PCAs. 6CCVD offers the precise material control and customization necessary to optimize $F_{sat}$ and maximize conversion efficiency.

Applicable Materials for THz PCA Development

To replicate and extend the performance demonstrated in this paper, researchers require diamond substrates with highly controlled doping and superior crystalline quality.

Application Requirement	6CCVD Material Solution	Key Specification Match
High Mobility/Low Scattering	Optical Grade SCD (Single Crystal Diamond)	Required for the highest performance monocrystalline PCAs, minimizing Rayleigh scattering losses.
Large Aperture Devices	High Purity PCD (Polycrystalline Diamond)	Available in custom dimensions up to 125 mm diameter, enabling the large-aperture designs necessary for high THz power.
Controlled Absorption	Custom N-Doped SCD/PCD	We offer precise nitrogen doping control during MPCVD growth to tune the 4.6 eV absorption peak, optimizing $F_{sat}$ for 400 nm pumping.
High Field Operation	Substrates up to 10 mm Thick	Provides the mechanical and dielectric stability required to handle bias fields up to 1 MV/cm and beyond.

Customization Potential for Advanced PCA Fabrication

The paper utilized simple glued aluminum foil electrodes. To achieve the projected 3%+ conversion efficiency, integrated, high-quality electrodes capable of handling high pulsed fields are essential.

Integrated Metalization Services: 6CCVD offers in-house metalization capabilities, including deposition of Ti/Pt/Au, W, Cu, or Pd electrode structures directly onto the diamond surface. This eliminates the performance limitations associated with glued foil electrodes and allows for precise lithographic patterning of antenna gaps (0.5 mm to 2 mm range used in the study).
Custom Dimensions and Thickness: We provide SCD and PCD wafers in the required thickness range (0.1 µm to 500 µm for active layers, up to 10 mm for substrates). We can supply large-aperture PCD wafers up to 125 mm in diameter, far exceeding the 3-7 mm samples used in the study.
Ultra-Low Roughness Polishing: The research noted that Rayleigh scattering significantly degrades performance, especially in PCD. 6CCVD guarantees ultra-smooth surfaces: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, minimizing optical losses and maximizing pump coupling efficiency.

Engineering Support

The observed data dispersion and efficiency spikes (e.g., the two monocrystalline CVD samples performing twice as well as predicted) underscore the complexity of optimizing diamond PCA performance.

6CCVD’s in-house PhD team specializes in MPCVD growth kinetics and defect engineering. We can assist researchers and engineers with:

Material Selection Optimization: Consulting on the ideal nitrogen doping concentration and uniformity required to achieve the lowest possible saturation fluence ($F_{sat}$) for specific pump wavelengths (e.g., 400 nm).
Defect Control: Analyzing the trade-offs between substitutional nitrogen concentration ($N_s$) and carrier mobility to mitigate the emission weakening observed in high-nitrogen crystals.
Custom Electrode Design: Providing guidance on metal stack selection and geometry for high-voltage pulsed bias applications, ensuring robust operation at fields approaching 10 MV/cm.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available) to support your cutting-edge THz research.

View Original Abstract

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1-100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics9010018

References

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