Optical Pump–Terahertz Probe Diagnostics of the Carrier Dynamics in Diamonds

At a Glance

Metadata	Details
Publication Date	2023-12-26
Journal	Materials
Authors	V. V. Bulgakova, P. A. Chizhov, A. А. Ushakov, P. V. Ratnikov, Yuri Goncharov
Institutions	Lomonosov Moscow State University, MIREA - Russian Technological University
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study utilizes the Optical Pump-Terahertz Probe (OPTP) method to analyze photoinduced carrier dynamics in doped diamond and a novel diamond-silicon composite, aiming for ultrafast THz modulation applications.

Novel Material: A polycrystalline Chemical Vapor Deposition (CVD) diamond film (19 µm thick) was fabricated with embedded silicon microparticles (100 nm-2 µm).
Key Finding (Dual Relaxation): The Si-diamond composite exhibited dual photocarrier relaxation dynamics, significantly different from standard doped diamonds.
Ultrafast Component: A fast “hot”-carrier relaxation time (t_sifast) of 4 ± 0.5 ps was observed, corresponding to over one-third of the total transmission change.
Delayed Component: A subsequent slow relaxation time (t_sislow) of 220 ± 100 ps was measured, attributed to charge separation at the Si-diamond interface.
Mechanism: Photoexcited holes transfer from Si to diamond, while electrons are retained in Si by high potential barriers, creating a doubly charged layer that delays recombination.
Doping Comparison: Nitrogen-doped (100 ppm) and Boron-doped (1 ppm) HPHT diamonds showed single, slower relaxation times (10-17 ps), confirming the unique dynamics of the composite.
Application Potential: The observed dual relaxation dynamics can be leveraged for ultrafast, flexible THz wave modulation.

Technical Specifications

Parameter	Value	Unit	Context
Si-Diamond Fast Lifetime (t_sifast)	4 ± 0.5	ps	Composite, 800 nm excitation
Si-Diamond Slow Lifetime (t_sislow)	220 ± 100	ps	Composite, 800 nm excitation
N-Doped Lifetime (100 ppm)	10 ± 1	ps	HPHT Diamond, 400 nm excitation
B-Doped Lifetime (1 ppm)	17 ± 1	ps	HPHT Diamond, 800 nm excitation
CVD Growth Temperature	840 ± 10	°C	Measured by METIS M322 pyrometer
CVD Microwave Power	4.5	kW	MPCVD system (ARDIS-100, 2.45 GHz)
CVD Gas Pressure	75	Torr	H₂-CH₄ mixture
CVD Methane Content	4	%	CH₄ in H₂
Composite Film Thickness	19	µm	Total polycrystalline diamond film
Si Particle Size (Embedded)	100 nm-2	µm	Milled silicon suspension
Laser Central Wavelength	800	nm	Ti-sapphire system
Laser Pulse Duration	40-150	fs	Used for pump and probe
THz Frequency Range	0.3-3	THz	Probing range
Diamond Raman Peak	1333.7	cm^-1	FWHM = 4.3 cm^-1
Silicon Raman Peak	520.6	cm^-1	Embedded Si particles

Key Methodologies

The study employed a two-step Microwave Plasma Chemical Vapor Deposition (MPCVD) process for composite fabrication, followed by Optical Pump-Terahertz Probe (OPTP) spectroscopy for carrier dynamics analysis.

I. Si-Diamond Composite Fabrication (MPCVD)

Substrate Preparation: Silicon <100> substrate (10 × 10 mm) was seeded with nanodiamond particles (3-7 nm, zeta potential greater than +50 mV).
First Diamond Layer: A 15 µm thick polycrystalline diamond layer was grown using MPCVD.
Si Particle Embedding: The surface was spin-coated (3000 rpm, 0.5 min) with a suspension of milled silicon particles (10 mg/mL in Dimethylsulfoxide, DMSO).
Second Diamond Layer: A 4 µm thick diamond layer was grown via MPCVD to fully encapsulate the Si microparticles.
Growth Parameters: Fixed total gas flow (500 sccm), 4% methane content, 75 Torr pressure, 4.5 kW microwave power, and 840 °C temperature.
Substrate Removal: The silicon substrate was partially removed via chemical etching (HF:HNO₃, 3:1, 40 °C) to form a diamond membrane.

II. Optical Pump-Terahertz Probe (OPTP) Setup

Laser Source: A Ti-sapphire laser (800 nm, 40-150 fs, 1 kHz) was split into three beams: THz generation (67%), optical pump (30%), and THz detection (3%).
THz Generation: The main beam generated THz pulses via optical rectification in a nonlinear LiNbO₃ crystal using an inclined intensity front.
Excitation Wavelengths:
- 800 nm (Fundamental): Used for excitation of boron-doped diamond and the Si-diamond composite (single-photon absorption in Si).
- 400 nm (Second Harmonic): Generated using a BBO crystal in the pump path, used for excitation of nitrogen-doped diamond (due to its band gap) and the Si-diamond composite.
THz Detection: The THz pulse transmitted through the sample was detected via electro-optical gating in a Zinc Telluride (ZnTe) crystal, using the weakest laser beam for registration.
Measurement: The relative change in THz transmission (ΔE/E) was measured as a function of the pump-probe delay time to track photocarrier recombination dynamics.

Commercial Applications

The research focuses on optimizing semiconductor substrates for Terahertz (THz) technology, particularly those requiring high electric field breakdown thresholds and tunable carrier lifetimes.

Industry/Application	Relevance of Diamond/Si Composite	Technical Advantage
THz Communication (6G/7G)	Ultrafast, flexible THz wave modulation.	The 4 ps fast relaxation time allows for high-speed switching and modulation of THz signals.
Large-Aperture Photoconductive Antennas (LAPCAs)	Diamond is a prospective substrate due to its high electric field breakdown threshold (critical for high-power THz sources).	Shortened carrier lifetime (4 ps) is crucial for improving the efficiency and speed of photoconductive detectors and emitters.
High-Power THz Sources	Diamond’s superior thermal conductivity and breakdown voltage compared to traditional semiconductors (like GaAs or Si).	Enables operation at higher electric fields (up to 30 kV/cm THz field strength demonstrated in the setup) without damage.
Industrial Inspection/Medical Diagnosis	Development of compact and powerful THz sources and detectors.	The ability to engineer specific, dual-mode carrier dynamics offers new design flexibility for THz components.
Photoconductive Detectors	The introduction of defects (or Si interfaces) is a known method to shorten photocarrier lifetimes, enhancing detector speed.	The engineered Si-diamond interface provides a controlled method for achieving sub-10 ps lifetimes.

View Original Abstract

Diamond is a promising material for terahertz applications. In this work, we use a non-invasive optical pump-terahertz probe method to experimentally study the photoinduced carrier dynamics in doped diamond monocrystals and a new diamond-silicon composite. The chemical vapor deposited diamond substrate with embedded silicon microparticles showed two photoinduced carrier lifetimes (short lifetime on the order of 4 ps and long lifetime on the order of 200 ps). The short lifetime is several times less than in boron-doped diamonds and nitrogen-doped diamonds which were grown using a high temperature-high pressure technique. The observed phenomenon is explained by the transport of photoexcited carriers across the silicon-diamond interface, resulting in dual relaxation dynamics. The observed phenomenon could be used for ultrafast flexible terahertz modulation.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma17010119

References

2020 - Real-time terahertz imaging with a single-pixel detector [Crossref]
2021 - Functional THz emitters based on Pancharatnam-Berry phase nonlinear metasurfaces [Crossref]
2021 - High-power portable terahertz laser systems [Crossref]
2019 - Broadband THz to NIR up-converter for photon-type THz imaging [Crossref]
2016 - Intense THz pulses with large ponderomotive potential generated from large aperture photoconductive antennas [Crossref]
2021 - Intense terahertz generation from photoconductive antennas [Crossref]
2020 - An overview of terahertz antennas [Crossref]