Skip to content
6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-12-18
JournalPhysical Review Letters
AuthorsArtur Lozovoi, Harishankar Jayakumar, Damon Daw, Ayesha Lakra, Carlos A. Meriles
InstitutionsCity College of New York, The Graduate Center, CUNY
Citations19
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research introduces a novel methodology for studying non-equilibrium charge transport in wide bandgap semiconductors, focusing on the formation and manipulation of metastable space charge potentials (SCPs) in diamond.

  • Core Achievement: Demonstrated the ability to optically inject carriers and locally probe the resulting space charge fields using Nitrogen-Vacancy (NV) centers in Type 1b CVD diamond.
  • Non-Equilibrium Dynamics: The study moves beyond traditional steady-state models, revealing that charge dynamics are highly dependent on the preparation protocol (timing of optical excitation and applied voltage).
  • Field Strength: The induced SCPs are significant, reaching amplitudes comparable to or exceeding the externally applied fields (over 106 V/m).
  • Engineering SCPs: Using pulsed protocols (alternating laser excitation and electric fields), the shape and directionality of the trapped charge patterns can be precisely engineered.
  • Carrier Guiding: The engineered SCP patterns act as “blueprints” that guide subsequent carrier propagation along specific trajectories, effectively suppressing trapping and enhancing transport directionality.
  • Material Focus: The experiments utilize Type 1b synthetic diamond (0.25 ppm N concentration), but the methodology is applicable to other wide bandgap materials like SiC and GaN.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
MaterialType 1b synthetic diamond (DDK)N/AGrown via Chemical Vapor Deposition (CVD)
Nitrogen Concentration0.25ppmBulk concentration
NV Center Concentration (Modeling)2.5ppbInitial density used in simulations
External Voltage (Max)560VApplied across electrodes
Electrode Gap100”mDistance between planar metal pads
External Electric Field (Max)5.6 x 106V/mCalculated maximum field (560 V / 100 ”m)
Induced SCP Field (Observed)> 106V/mSCPs reach values comparable to external fields
Green Laser Wavelength (Excitation)532nmNV charge initialization (NV0 ↔ NV-)
Orange Laser Wavelength (Readout)594nmNV fluorescence imaging
Continuous Green Laser Power1mWUsed during continuous park protocols
Pulsed Green Laser Power500”WUsed during pulsed protocols
Readout Laser Power100”WOrange laser scanning power
Laser Focus Depth10”mBelow the diamond surface
Axial Resolution~3”mDepth resolution of confocal microscope
Operating TemperatureRoom (293)KExperimental condition
Electron Mobility (Modeling)2.4 x 1011”m2/(V·s)Parameter used in master equations
Hole Mobility (Modeling)2.1 x 1011”m2/(V·s)Parameter used in master equations

Key Methodologies

Section titled “Key Methodologies”

The experimental approach combines high-resolution confocal microscopy with precise control over optical excitation and external electric fields.

  1. Confocal Setup and Probing:

    • A custom confocal fluorescence microscope is used, combining green (532 nm) and orange (594 nm) lasers, controlled by on/off logic pulses.
    • NV centers act as local probes, where changes in their charge state (NV- vs. NV0) are mapped via differential fluorescence (SF).

  2. Sample and Field Application:

    • A Type 1b CVD diamond sample is used with surface metal electrodes separated by a 100 ”m gap.
    • A high-voltage power supply and a high-speed switch allow for variable, time-dependent electric fields (Eext) to be applied across the gap.

  3. Charge Initialization and Continuous Drift:

    • The NV ensemble is charge-initialized to the NV- state via a preliminary green laser scan.
    • The green laser is then parked at a specific location (midpoint) while Eext is applied continuously.
    • Holes generated by NV0 recombination drift along Eext, are trapped by surrounding defects (like P1 centers), and form a dark, asymmetric NV- depletion pattern (a “tail”).

  4. Space Charge Engineering via Pulsing:

    • A novel pulsed protocol is implemented where the total park time (tp) is divided into pairs of pulses (tp1 + tp2).
    • Eext is applied only during tp1 (carrier injection and drift). Eext is turned OFF during tp2 (voltage-free period).
    • This alternating field allows the locally induced space charge field (ESC) to neutralize the effects of Eext, resulting in highly directional, jet-like charge patterns, demonstrating precise control over SCP formation.

  5. Carrier Guiding Demonstration:

    • A pre-existing SCP pattern (F1 blueprint) is created using the pulsed protocol.
    • A subsequent, voltage-free green laser park (F2) is applied.
    • The differential fluorescence (F2 - F1) shows that carriers generated during F2 propagate along the trajectory defined by the residual, long-lasting SCP field of F1.

Commercial Applications

Section titled “Commercial Applications”

The ability to locally control and map charge dynamics in wide bandgap materials has direct relevance across several high-tech engineering sectors:

  • Quantum Sensing and Computing:
    • NV centers are primary candidates for solid-state quantum devices. Understanding and mitigating charge noise (SCPs) is crucial for maintaining spin coherence and reliable readout.
  • Wide Bandgap Power Electronics (SiC, GaN):
    • The methodology provides a tool for investigating defect ionization and charge trapping in SiC and GaN, which are critical for high-power, high-frequency devices (e.g., MOSFETs, HEMTs).
  • Energy Conversion Systems:
    • Solar Cells and LEDs: The findings are directly applicable to understanding space-charge-limited currents (SCLC) and optimizing carrier collection efficiency in PIN junctions and thin-film solar cells based on low-mobility semiconductors.
  • Non-Volatile Memory and Data Storage:
    • The creation of long-lived, metastable charge states (like those in rare-earth-doped inorganic insulators) is essential for developing rewritable multilevel optical data storage devices.
  • Magnetoresistance Devices:
    • The techniques can be combined with photo-current measurements to probe the dynamics of magneto-resistance effects induced by space-charge fields in silicon and other materials.
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.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1940 - Electronic Processes in Ionic Crystals

Reads Similar to This

A Flexible Thermal Interface Material as the Heat Switch for Solid‐State Cooling Formation of Nitrogen-Vacancy Centers in Homoepitaxial Diamond Thin Films Grown via Microwave Plasma-Assisted Chemical Vapor Deposition Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications