Record performance in intrinsic, impurity-free lateral diamond photoconductive semiconductor switches

At a Glance

Metadata	Details
Publication Date	2025-04-01
Journal	Applied Physics Letters
Authors	Zhuoran Han, J. Lee, Anik Mazumder, Hubert N. Elly, Stephen Messing
Institutions	University of Illinois Urbana-Champaign
Analysis	Full AI Review Included

Technical Documentation and Analysis: Ultra-Pure Diamond for High-Power Photoconductive Switches

This document analyzes the requirements and achievements detailed in the research paper on intrinsic lateral diamond photoconductive semiconductor switches (PCSS). It highlights how 6CCVD’s advanced MPCVD capabilities directly support the replication and extension of this high-performance research.

Executive Summary

The research successfully demonstrates record-breaking performance in diamond PCSS by utilizing ultra-low impurity, intrinsic Type IIa substrates. This material choice is critical for achieving high-power, high-speed switching capabilities that surpass traditional wide bandgap (WBG) semiconductors.

Record Performance: Achieved a peak photocurrent of 8.0 A and an unprecedented on/off ratio of 2.3x10¹¹ at 1.2 kV DC bias.
Material Purity is Key: Performance is directly correlated with impurity concentration; the best device (PCSS A) used CVD IIa diamond with boron [B] < 1.0x10¹⁴ cm^-3 and nitrogen [N] < 3.0x10¹⁵ cm^-3.
High-Speed Operation: Demonstrated fast switching transients with a rise time of 2.2 ns and a fall time of 6.5 ns, suitable for high-frequency applications.
Intrinsic Triggering: Superior results were achieved using above-bandgap UV excitation (218 nm), maximizing photoelectric conversion efficiency by avoiding sub-bandgap impurity states.
High-Power Potential: The linear current-voltage (I-V) relationship up to 1.2 kV suggests the current can be further scaled at higher operating voltages.
Market Advantage: Diamond PCSS is positioned as a competitive, superior alternative to SiC and GaN solutions for high-power, high-speed switching.

Technical Specifications

The following table summarizes the critical performance metrics and material properties achieved by the best-performing device (PCSS A).

Parameter	Value	Unit	Context
Substrate Type	CVD IIa	N/A	Lowest impurity concentration
Boron Impurity [B]	< 1.0x10¹⁴	cm^-3	Below SIMS detection limit
Nitrogen Impurity [N]	< 3.0x10¹⁵	cm^-3	Below SIMS detection limit
Peak Photocurrent	8.0	A	At +1.2 kV DC bias, 65 µJ pulse
On/Off Ratio (R_OFF/R_ON)	2.3x10¹¹	N/A	Record high for diamond PCSS
Minimum On-Resistance (R_ON)	115.07	Ω	At +1.2 kV DC bias
Normalized Responsivity	9.1x10^-8	A-cm/W-V	Highest reported for intrinsic triggering
Rise Time (10%-90%)	2.2	ns	High-speed switching performance
Fall Time (90%-10%)	6.5	ns	Limited by intrinsic carrier lifetime
DC Bias Voltage Range	±1.2	kV	Tested range for high-power assessment
Trigger Wavelength	218	nm	Above-bandgap UV excitation
Laser Pulse Energy	65	µJ	Maximum energy used for high-current test
Surface Roughness (RMS)	0.4 to 1.2	nm	Comparable across all substrates
Diamond Bandgap	5.47	eV	Ultra-wide bandgap (UWBG)

Key Methodologies

The PCSS devices were fabricated using precise diamond processing techniques, emphasizing surface preparation and high-quality metal contacts.

Substrate Selection: Use of double-side polished Type IIa diamond substrates (4.5 mm x 4.5 mm x 0.5 mm) with varying, but precisely characterized, impurity levels measured via Secondary Ion Mass Spectrometry (SIMS).
Surface Cleaning: Substrates underwent an RCA cleaning process to eliminate organic and metallic contaminants prior to metalization.
Surface Termination (Initial): Oxygen termination achieved by treating samples in a boiling H₂SO₄:HNO₃ mixture for 2 hours.
Electrode Deposition: Lateral rectangular electrodes were deposited via electron-beam evaporation.
- Metal Stack: Ti (30 nm) / Pt (30 nm) / Au (120 nm).
- Geometry: 2.1 mm spacing, 3.9 mm length, 0.9 mm width.
Contact Annealing: Samples were annealed at 450 °C under an Argon (Ar) ambient for 1 hour to optimize contact resistance.
Surface Termination (Stabilization): Post-annealing, devices were treated with ozone at room temperature for 1 hour to stabilize the oxygen-terminated surface.
Optical Triggering Setup: A tunable Optical Parametric Oscillator (OPO) laser provided UV pulses (212 nm - 240 nm) with a 4 ns FWHM and 10 Hz repetition rate.
Electrical Circuit: Transient switching was evaluated using a 95 nF capacitor (C_s) charged via a 1 MΩ resistor (R_s), discharging through the PCSS and a 47 Ω load resistor (R_L).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-purity MPCVD diamond materials and custom fabrication services required to replicate and advance this record-setting PCSS research. The core finding—that performance scales directly with material purity—is a direct validation of 6CCVD’s specialization in electronic and optical grade Single Crystal Diamond (SCD).

Applicable Materials for High-Performance PCSS

To achieve the record-setting performance of PCSS A, researchers require diamond with impurity levels below the detection limits of standard SIMS analysis. 6CCVD guarantees the necessary material quality:

Optical Grade Single Crystal Diamond (SCD): This material is the direct equivalent of the ultra-low impurity CVD IIa substrate used in PCSS A. We offer SCD with guaranteed low background concentrations of Boron and Nitrogen, essential for maximizing carrier mobility (µ) and lifetime (τ), thereby ensuring superior photoconductive efficiency (α ∝ µητ).
Electronic Grade SCD: For applications requiring slightly thicker or larger wafers, our Electronic Grade SCD maintains exceptional purity suitable for high-voltage blocking layers.

Customization Potential & Fabrication Services

The successful fabrication of the lateral PCSS device relied on precise dimensions and a specific metal stack. 6CCVD offers comprehensive customization capabilities to meet these exact engineering requirements:

Requirement from Paper	6CCVD Capability	Benefit to Researcher
Substrate Dimensions	Custom plates/wafers up to 125 mm diameter.	We can supply the exact 4.5 mm x 4.5 mm plates or larger wafers for scaled production.
Thickness	SCD thickness from 0.1 µm up to 500 µm.	The 0.5 mm (500 µm) thickness used is standard, but custom thicknesses are available for specific thermal or electrical requirements.
Metalization Stack	In-house deposition of Au, Pt, Pd, Ti, W, Cu.	We can replicate the critical Ti (30 nm) / Pt (30 nm) / Au (120 nm) stack used for low-resistance ohmic contacts, ensuring device consistency.
Surface Finish	Polishing to Ra < 1 nm (SCD).	We provide the necessary double-side polishing (DSP) to achieve the ultra-smooth surface (0.4 nm to 1.2 nm RMS) required for reliable device fabrication and high-voltage operation.
Device Geometry	Precision laser cutting and shaping services.	We can pre-cut substrates to specific geometries, reducing post-processing time and material waste for high-volume device fabrication.

Engineering Support

6CCVD’s in-house PhD engineering team specializes in the material science of diamond for advanced electronic and optical applications. We offer consultation services to assist researchers in optimizing material selection for similar High-Power Photoconductive Switching projects.

Purity Optimization: Assistance in specifying the exact impurity limits required to maintain high carrier mobility and minimize Shockley-Read-Hall recombination centers.
Contact Engineering: Guidance on selecting the optimal metal stack and annealing recipe to minimize effective contact resistance (R_EC), a critical factor identified in the research.
Scaling: Support for transitioning from laboratory-scale 4.5 mm x 4.5 mm devices to larger, inch-size wafers for commercial or industrial applications.

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

View Original Abstract

Photoconductive semiconductor switches (PCSSs) are fabricated on type IIa diamond substrates with varying boron and nitrogen impurity levels (&lt;1014-1016 cm−3). The photoresponse of lateral PCSS is reported over the incident laser wavelength range (212-240 nm), energy per pulse (5-65 μJ), and DC bias (−1.2 to +1.2 kV). The PCSS device with the lowest boron and nitrogen impurity concentration achieves the highest normalized responsivity of 9.1 × 10−8 A-cm/W-V, peak photocurrent of 8.0 A, and on/off ratio of 2.3 × 1011 at a DC bias of +1.2 kV with the potential for even higher currents at increased DC bias. All PCSS display fast rise times (&lt;3 ns), limited by the laser’s rise time. However, photoresponse measurements reveal that higher impurity levels reduce the photocurrent and decrease the on/off ratio. These results highlight the performance advantages of using low background concentration type IIa diamond substrates for PCSS fabrication and present a promising route toward advanced high-power, high-speed diamond-based switches.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0266565

References

2010 - 1.55-μm GaNAsSb-based photoconductive switch for microwave switching [Crossref]
2012 - Performance and optimization of a 50 kV silicon carbide photoconductive semiconductor switch for pulsed power applications
2018 - Current state of photoconductive semiconductor switch engineering [Crossref]
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2021 - Impact ionization coefficients and critical electric field in GaN [Crossref]
2014 - High power operation of a nitrogen doped, vanadium compensated, 6H-SiC extrinsic photoconductive switch [Crossref]
2018 - High-gain persistent nonlinear conductivity in high-voltage gallium nitride photoconductive switches
2014 - Zr/oxidized diamond interface for high power Schottky diodes [Crossref]
2024 - High field transport in (ultra) wide bandgap semiconductors: Diamond versus cubic GaN [Crossref]
1992 - Unusually high thermal conductivity in diamond films [Crossref]