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6CCVD Research

Three-dimensional diamond planar spiral detectors

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-03-12
JournalScientific Reports
AuthorsRebecca J. Watkins, Patrick S. Salter, Ralph J. Moors, Richard B. Jackman
InstitutionsUniversity of Oxford, University College London
Citations2
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research details the fabrication and characterization of novel three-dimensional (3D) diamond planar spiral detectors designed for high-radiation environments, achieving “thin” detector performance in structurally “thick” substrates.

  • Core Innovation: A 3D network of laser-written Nano-Carbon Network (NCN) spiral “wall” electrodes was embedded 20 ”m deep into a thick (300 ”m) single-crystal diamond substrate, defining a small active detection volume (50 ”m separation).
  • Radiation Resilience: Reducing the effective electrode spacing (L) to 50 ”m makes the detector highly resilient to radiation damage, as it tolerates a reduction in carrier mean free path caused by high radiation doses.
  • Performance Enhancement: The incorporation of internal NCN wall electrodes significantly improved detector performance, boosting the Charge Collection Efficiency (CCE) by 25-30% compared to control planar spiral detectors lacking the NCN walls.
  • High Efficiency and Speed: The NCN-spiral detectors demonstrated CCE close to 100% (54.4 ± 0.9 fC collected charge, exceeding the CIVIDEC reference detector at optimal bias) and fast response times (1.22 ± 0.01 ns rise time).
  • Low Operating Bias: Full charge collection was achieved at a relatively low operating field of 2 V ”m-1 (100 V), compared to the 0.8 V ”m-1 (400 V) required by the reference Metal-Insulator-Metal (MIM) detector.
  • Fabrication Technique: An optimized femto-second pulsed laser writing process, utilizing adaptive optics to correct for diamond lattice aberrations, enabled the precise 3D fabrication of conductive NCN tracks.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Substrate Thickness300”mSingle-crystal CVD diamond
NCN Wall Depth20”mDefines active detection thickness
NCN Wall Separation (L)50”mInternal electrode spacing
Surface Contact Separation35”mTi/Pt/Au surface contacts
Laser Wavelength790nmTi:Sapphire pulsed fs laser
Laser Pulse Energy70nJUsed for NCN writing
Laser Repetition Rate1kHzUsed for NCN writing
Dark Current (Max)< 2nAMeasured at 0 to ±100 V
Max Collected Charge (NCN-spiral)54.4 ± 0.9fCTCT measurement at 100 V
Max Collected Charge (CIVIDEC MIM)50.3 ± 0.3fCTCT measurement at 400 V
CCE (NCN-spiral, Spectroscopy)94 ± 6%Compared to CIVIDEC reference
CCE Improvement (NCN vs Control)25-30%Based on TCT measurements
Rise Time (NCN-spiral)1.22 ± 0.01nsFast response time
Operating Field (NCN-spiral, Max)2.4V ”m-1Corresponds to 120 V bias
Priming Time (Required)> 13hoursRequired with 241Am source to reach max CCE
Radiation Tolerance (Target)> 1016particles cm-2Required for future HEP/Fusion applications

Key Methodologies

Section titled “Key Methodologies”

The fabrication relies on precise 3D laser writing of conductive nano-carbon networks within the diamond bulk, followed by standard microfabrication techniques for surface contacts.

  1. Substrate Selection and Characterization:

    • Used four 4 x 4 x 0.3 mm SC diamond substrates (CMO13, CMO14, CME05, CME04).
    • Raman spectroscopy (532 nm laser) confirmed the presence of Nitrogen-Vacancy (NV) defects and significant nitrogen content, classifying the material as optical grade.

  2. 3D Nano-Carbon Network (NCN) Writing:

    • Internal spiral NCN “walls” were fabricated using a Ti:Sapphire pulsed femto-second laser (790 nm, 70 nJ, 1 kHz, 0.2 mm/s).
    • A Liquid Crystal Spatial Light Modulator (SLM) was employed to correct for spherical aberrations caused by refraction at the diamond interface, enabling deep, precise focusing.
    • The writing process started 20 ”m below the surface and built up layer-by-layer (2 ”m focus raise per layer) to create 20 ”m high walls, defining the active volume.

  3. Surface Preparation:

    • Samples were immersed in an acid etching solution (H2SO4:(NH4)2S2O8 at 200 °C) followed by an alkaline solution (H2O2:NH4OH) to achieve an oxygen-terminated surface, ensuring a clean interface between the NCNs and the metal contacts.

  4. Metal Contact Deposition:

    • Surface Ti(50 nm)/Pt(20 nm)/Au(150 nm) spiral contacts were deposited using photolithography and electron beam evaporation, resulting in a 35 ”m separation between the surface contacts.

  5. Detector Priming:

    • All fabricated detectors required > 13 hours of “priming” (irradiation) with a sealed 241Am alpha source to fill deep traps and reach maximum CCE.

  6. Characterization:

    • IV Measurement: Dark current was measured (0 to ±100 V) and found to be less than 2 nA.
    • Alpha Spectroscopy: Performed using a 241Am source and a CIVIDEC Cx-L Spectroscopic Amplifier to determine peak amplitude and CCE.
    • Transient Current Measurements (TCT): Performed using a C2-TCT Broadband Amplifier (current sensitive, no shaping) to extract raw charge signals, rise times, and total collected charge as a function of applied bias.

Commercial Applications

Section titled “Commercial Applications”

This technology is critical for applications requiring robust, high-resolution radiation detection in extreme environments where conventional semiconductors fail due to radiation damage.

  • Nuclear Power and Decommissioning:
    • In-situ monitoring of alpha and neutron radiation in corrosive liquid environments (e.g., primary cooling loops of pressurized water reactors).
    • Robust detectors for long-term use in high-dose environments, leveraging the small electrode spacing for radiation hardness.
  • High Energy Physics (HEP):
    • Particle tracking and detection in future experiments demanding radiation fluences exceeding 1016 particles cm-2.
  • Fusion Energy Research (e.g., ITER):
    • Real-time neutron and alpha particle diagnostics, requiring detectors that maintain performance under high thermal and radiation stress.
  • Dosimetry:
    • High-resolution, thin-film alpha detectors for precise dose measurement in medical or industrial settings.
  • X-ray Imaging:
    • Potential use in high-speed X-ray pixel detectors, leveraging the fast response time and high charge collection efficiency demonstrated by the NCN architecture.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2004 - Semiconductors and Semimetals

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