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6CCVD Research

n-type diamond synthesized with tert-butylphosphine for long spin coherence times of perfectly aligned NV centers

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-11-02
JournalJournal of Applied Physics
AuthorsRiku Kawase, Hiroyuki Kawashima, Hiromitsu Kato, Norio Tokuda, Satoshi Yamasaki
InstitutionsKyoto University, Spintronics Research Network of Japan
Citations7
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Novel Synthesis Method: N-type diamond was successfully synthesized using tert-butylphosphine (TBP) via Chemical Vapor Deposition (CVD), offering a significantly safer alternative to highly toxic phosphine gas.
  • Record Spin Coherence Time (T2): The longest room-temperature T2 time achieved for this synthesis method was 1.62 ± 0.10 ms, observed in the sample with the lowest unintentional nitrogen (N) concentration.
  • Impurity Control: Unintentional N incorporation was effectively suppressed by incrementally increasing the H2 and CH4 gas flow rates during CVD growth.
  • Perfect NV Alignment: Optically Detected Magnetic Resonance (ODMR) confirmed that all measured Nitrogen-Vacancy (NV) centers were perfectly aligned along the desired [111] crystallographic direction.
  • High Electron Mobility: Hall measurements confirmed n-type conduction and yielded a maximum electron mobility of 422 cm2/(Vs) at room temperature (RT).
  • Quantum Device Readiness: This study establishes optimized CVD conditions for growing high-quality, phosphorus-doped n-type diamond with aligned NV centers, crucial for manufacturing robust quantum diamond devices.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Longest Spin Coherence Time (T2)1.62 ± 0.10msAchieved in Growth Type C1 (lowest N concentration) at RT.
Maximum Hall Mobility (”)422cm2/(Vs)Measured at RT for Sample B1.
Donor Ionization Energy (ED)0.53 to 0.62eVDerived from Hall measurements (close to standard 0.6 eV for P-doped diamond).
NV Center AlignmentPerfectN/AAll measured NV centers aligned along the [111] direction.
Target Phosphorus (P) Concentration (CP)1016 - 1017cm-3Target range for long T2 performance.
NV Center Concentration (CNV)~1012cm-3Estimated from confocal fluorescence imaging.
Carrier Compensation Ratio (η = NA/ND)55% to 64%%Estimated from Hall fitting (Sample B1/B2: 55%; Sample A2: 64%).
CVD Gas Pressure25kPaCommon pressure used for all growth types.
Substrate Off-Angle1-3°Along the [112] direction.

Key Methodologies

Section titled “Key Methodologies”

The n-type diamond films were synthesized using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD) on Ib-type (111) diamond substrates.

  1. Substrate Preparation:

    • Substrates were Ib-type (111) diamond (2 x 2 x 0.5 mm3 or 2 x 2 x 0.3 mm3).
    • Substrates had an off-angle of 1°-3° along the [112] direction.
    • Post-growth cleaning involved mixed acid (H2SO4 : HNO3 = 3:1) treatment at 150°C for 30 min.

  2. Gas Sources and Doping:

    • Carbon Source: 12C-enriched CH4 (purity > 8N, [13C] < 0.005 at%) or natural CH4 (purity 6N).
    • Dopant Source: tert-butylphosphine (TBP) (purity 3N3), diluted with H2 (purity > 8N). TBP was used due to its significantly lower toxicity compared to phosphine.
    • Doping Control: TBP flow rate was adjusted to target a phosphorus concentration of 1016-1017 cm-3. In some cases (Type A and B), residual P in the chamber was sufficient for doping.

  3. Growth Condition Optimization (Types O, A, B, C):

    • Growth temperature ranged from 700°C to 1200°C.
    • Total gas flow rates were varied (100 sccm or 400 sccm).
    • CH4/H2 ratio was varied (0.1% to 1.0%).
    • Nitrogen Suppression: The longest T2 times were achieved by increasing the total gas flow rates (e.g., from 100 sccm to 400 sccm) and adjusting the CH4/H2 ratio (e.g., 0.25% for Type C) to suppress unintentional N incorporation.

  4. Electrical Contact Fabrication (Hall Measurements):

    • Heavily phosphorus-doped layers (concentration ~1020 cm-3) were selectively grown under the electrode areas to reduce contact resistance.
    • Metal stack (Ti (30 nm)/Pt (30 nm)/Au (100 nm)) was deposited via electron beam evaporation.
    • Final annealing was performed in a muffle furnace at 420°C for 30 min in air to form TiC at the interface, further suppressing contact resistance.

  5. Characterization:

    • Impurity Concentration: Secondary-Ion Mass Spectrometry (SIMS).
    • Electrical Properties: Hall measurements using the van der Pauw method (300-900 K, 0.5 T magnetic field).
    • Spin Coherence (T2): Measured using the Hahn echo pulse sequence (π/2, π, π/2-pulse).
    • NV Axis Orientation: Continuous Wave (CW)-ODMR spectroscopy with a static magnetic field applied perpendicular to the (111) surface.

Commercial Applications

Section titled “Commercial Applications”

The successful synthesis of high-quality, n-type diamond with perfectly aligned NV centers and long spin coherence times directly supports the development of next-generation quantum technologies.

  • Quantum Computing: Aligned NV centers serve as robust, room-temperature qubits, essential for building scalable quantum processors and quantum memory devices.
  • Quantum Sensing and Metrology: The long T2 times enhance the sensitivity of diamond-based quantum sensors for measuring magnetic fields, electric fields, and temperature with high precision (e.g., in medical diagnostics or materials analysis).
  • Spintronics: N-type diamond, confirmed by high Hall mobility (422 cm2/(Vs)), acts as a high-performance semiconductor host material, enabling the integration of spin-based devices (spintronics) operating at room temperature.
  • High-Power Electronics: The high mobility and wide bandgap of phosphorus-doped diamond make it suitable for high-frequency, high-power electronic devices, particularly where thermal stability is critical.
  • Safer Manufacturing: The use of TBP significantly reduces the safety risks and infrastructure costs associated with handling highly toxic phosphine gas in industrial CVD processes.
View Original Abstract

The longest spin coherence times for nitrogen-vacancy (NV) centers at room temperature have been achieved in phosphorus-doped n-type diamond. However, difficulty controlling impurity incorporation and the utilization of highly toxic phosphine gas in the chemical vapor deposition (CVD) technique pose problems for the growth of n-type diamond. In the present study, n-type diamond samples were synthesized by CVD using tert-butylphosphine, which is much less toxic than phosphine. The unintentional incorporation of nitrogen was found to be suppressed by incrementally increasing the gas flow rates of H2 and CH4. It was found that the spin coherence time (T2) increased with decreasing the nitrogen concentration, which suggests that the nitrogen concentration limits the length of T2. In the sample with the lowest nitrogen concentration, T2 increased to 1.62 ± 0.10 ms. Optically detected magnetic resonance spectra indicated that all of the measured NV centers were aligned along the [111] direction. Hall measurements confirmed n-type conduction in three measured samples prepared under different growth conditions. The highest measured Hall mobility at room temperature was 422 cm2/(V s). This study provides appropriate CVD conditions for growing phosphorus-doped n-type diamond with perfectly aligned NV centers exhibiting long spin coherence times, which is important for the production of quantum diamond devices.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2010 - Quantum Computation and Quantum Information
  2. 2013 - Solid-state electronic spin coherence time approaching one second [Crossref]

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