Probing Metastable Space-Charge Potentials in a Wide Band Gap Semiconductor

At a Glance

Metadata	Details
Publication Date	2020-12-18
Journal	Physical Review Letters
Authors	Artur Lozovoi, Harishankar Jayakumar, Damon Daw, Ayesha Lakra, Carlos A. Meriles
Institutions	City College of New York, The Graduate Center, CUNY
Citations	19
Analysis	Full AI Review Included

Technical Documentation & Analysis: Probing Metastable Space-Charge Potentials in Wide Bandgap Semiconductors

This document analyzes the research paper detailing the visualization and control of metastable space-charge potentials in CVD diamond using Nitrogen-Vacancy (NV) centers. The findings are directly relevant to advanced quantum sensing, high-power electronics, and next-generation energy conversion systems.

Executive Summary

The research successfully demonstrates the ability to generate, visualize, and control long-lived metastable space-charge (SC) fields within a wide bandgap semiconductor (CVD diamond).

Core Achievement: Visualization of dynamic space charge patterns using Nitrogen-Vacancy (NV) color centers as local, optically-activated charge pumps and probes.
Material Requirement: The study relies on high-quality, low-defect Type 1b synthetic Single Crystal Diamond (SCD) grown via Chemical Vapor Deposition (CVD), featuring a low substitutional nitrogen concentration (0.25 ppm).
Methodology: Utilizes pulsed optical excitation (532 nm/594 nm) combined with high-speed switching of external electric fields (up to 560 V across a 100 µm gap) to tailor the interplay between carrier drift and SC field formation.
Key Finding (Fields): The resulting internal space charge fields (E_sc) were found to reach amplitudes comparable to the externally applied fields (exceeding 10⁶ V/m).
Application Potential: The engineered charge patterns serve as “blueprints” to confine and guide carrier propagation, offering a pathway for novel quantum devices, solar cells, and high-voltage electronics.
6CCVD Relevance: Replication and extension of this work require high-purity, custom-dimensioned SCD wafers with precision metalization, all of which are core 6CCVD capabilities.

Technical Specifications

Hard data points extracted from the experimental setup and material characterization (Table S1).

Parameter	Value	Unit	Context
Material Used	Type 1b Synthetic Diamond	CVD	Required for NV center studies
Substitutional Nitrogen Density (P)	0.25	ppm	Primary defect concentration
Nitrogen Vacancy Density (Q)	2.5	ppb	Concentration of the NV probe
Operating Temperature	293	K	Room Temperature
Electrode Gap	100	µm	Distance between planar surface electrodes
Maximum Applied Voltage (V)	560	V	Used to generate external electric fields
External Electric Field (E_ext)	> 10⁶	V/m	Field strength across the inter-electrode gap
Green Excitation Wavelength	532	nm	NV ionization and charge initialization
Orange Readout Wavelength	594	nm	NV fluorescence imaging
Laser Focus Depth	10	µm	Below the diamond surface (axial resolution ~3 µm)
Electron Mobility (µ_n)	2.4 * 10¹¹	µm²/(V·s)	Calculated parameter for carrier drift
Hole Mobility (µ_p)	2.1 * 10¹¹	µm²/(V·s)	Calculated parameter for carrier drift

Key Methodologies

The experiment relies on precise control over material defects, optical timing, and high-voltage switching to isolate and measure space charge dynamics.

Material Preparation: A Type 1b CVD diamond wafer was prepared with a pair of planar metal electrodes patterned onto the surface, defining a 100 µm gap.
Optical Excitation System: A custom confocal fluorescence microscope utilized 532 nm (Green, 1 mW) and 594 nm (Orange, 100 µW) lasers, with timing controlled by high-speed on/off logic pulses.
Charge Initialization (Standard): The 532 nm green laser was scanned across the area to preferentially initialize the NV ensemble into the negatively-charged state (NV^-).
Carrier Injection & Drift: The green laser was parked (t_p = 60 s) at the midpoint while a variable DC voltage (up to 560 V) was applied externally (E_ext), inducing hole drift and space charge formation.
Pulsed Protocol for SC Tailoring: To gauge the influence of the space charge field (E_sc), the total park time (t_p) was fractioned into a train of pulse pairs (t_p1 + t_p2). The external field was switched ON during t_p1 (e.g., 1 ms) and OFF during t_p2 (e.g., 10 µs or 1 ms) to neutralize or enhance E_sc effects.
Readout: The resulting spatial NV charge pattern was imaged by scanning the 594 nm orange laser, yielding differential fluorescence (SF) maps indicative of carrier propagation paths.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality, engineered CVD diamond substrates. 6CCVD is uniquely positioned to supply the materials and customization services required to replicate, scale, and extend these advanced charge transport studies.

Applicable Materials

The study requires a material with low intrinsic defect density but controlled nitrogen incorporation to form the NV probes.

Research Requirement	6CCVD Material Recommendation	Rationale
High-Purity Diamond Substrate	Optical Grade Single Crystal Diamond (SCD)	Our SCD material offers the high crystalline quality and low background defect concentration necessary to achieve the reported high carrier mobilities (µ_n, µ_p > 10¹¹ µm²/(V·s)) and minimize unintentional trapping.
Controlled Defect Concentration	Tailored Nitrogen-Doped SCD	We can supply SCD wafers with precise, low-level nitrogen doping (e.g., 0.25 ppm used in the paper) to ensure consistent NV center formation via post-growth irradiation/annealing, crucial for reproducible probe density (2.5 ppb).
Alternative Probes (SiV/GeV)	High-Purity SCD for Implantation	For extending the work to Silicon-Vacancy (SiV) or Germanium-Vacancy (GeV) centers, 6CCVD provides ultra-low-defect SCD substrates optimized for ion implantation and subsequent annealing.

Customization Potential

The experiment relies heavily on precise geometry and high-quality electrical contacts. 6CCVD offers full customization to meet these engineering demands.

Customization Service	Relevance to Space Charge Research	6CCVD Capability
Custom Dimensions	Required for scaling up electrode arrays and integrating into device architectures.	We offer SCD plates and PCD wafers up to 125 mm in diameter.
Precision Thickness	The experiment focused 10 µm below the surface; precise thickness control is essential for optical alignment and field modeling.	SCD and PCD wafers available from 0.1 µm up to 500 µm thickness, with substrates up to 10 mm.
Surface Metalization	High-voltage application (560 V) requires robust, low-resistance ohmic contacts.	Internal Metalization Capability: We deposit custom stacks including Ti/Pt/Au, Pd, W, or Cu tailored for specific contact requirements and high-field environments.
Surface Finish	Minimizing surface defects is critical for studying non-local space charge stemming from the metal-dielectric interface.	Superior Polishing: SCD surfaces achieve Ra < 1 nm, ensuring minimal scattering and defect trapping at the interface.

Engineering Support

The complex interplay between carrier dynamics, defect states, and electric fields requires specialized expertise.

6CCVD’s in-house PhD team specializes in CVD diamond growth and defect engineering. We can assist researchers and engineers with material selection, doping optimization, and surface preparation protocols for similar Charge Transport and Quantum Sensing projects. Our technical support ensures that the material properties (e.g., mobility, defect concentration, surface termination) are perfectly matched to the experimental requirements.

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

View Original Abstract

While the study of space-charge potentials has a long history, present models are largely based on the notion of steady state equilibrium, ill-suited to describe wide band gap semiconductors with moderate to low concentrations of defects. Here we build on color centers in diamond both to locally inject carriers into the crystal and probe their evolution as they propagate in the presence of external and internal potentials. We witness the formation of metastable charge patterns whose shape-and concomitant field-can be engineered through the timing of carrier injection and applied voltages. With the help of previously crafted charge patterns, we unveil a rich interplay between local and extended sources of space-charge field, which we then exploit to show space-charge-induced carrier guiding.