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

At a Glance

Metadata	Details
Publication Date	2022-11-02
Journal	Journal of Applied Physics
Authors	Riku Kawase, Hiroyuki Kawashima, Hiromitsu Kato, Norio Tokuda, Satoshi Yamasaki
Institutions	Kanazawa University, Spintronics Research Network of Japan
Citations	7
Analysis	Full AI Review Included

Technical Documentation & Analysis: n-Type Diamond for Quantum Devices

Executive Summary

This research successfully demonstrates the synthesis of high-quality, phosphorus-doped (n-type) diamond using tert-butylphosphine (TBP), a significantly safer precursor than traditional phosphine, achieving performance metrics critical for quantum technology.

Quantum Performance: Achieved a long spin coherence time ($T_2$) of 1.62 $\pm$ 0.10 ms at room temperature in n-type diamond, a key requirement for quantum sensing and memory.
Perfect Alignment: Confirmed that all measured Nitrogen-Vacancy (NV) centers were perfectly aligned along the [111] direction, essential for maximizing quantum device efficiency.
Electrical Properties: Verified n-type conduction with a high Hall mobility of 422 cm²/(Vs) at room temperature, demonstrating excellent semiconductor characteristics.
Safety & Scalability: Utilized TBP, which is orders of magnitude less toxic than phosphine, paving the way for safer and potentially more scalable MPCVD production of n-type diamond.
Impurity Control: Demonstrated effective suppression of unintentional nitrogen incorporation by optimizing H₂ and CH₄ gas flow rates, directly correlating lower nitrogen concentration with longer $T_2$.
Device Integration: The methodology included selective heavy P-doping and Ti/Pt/Au metalization to reduce contact resistance, confirming suitability for integrated quantum diamond devices.

Technical Specifications

The following hard data points were extracted from the research, highlighting the material quality achieved:

Parameter	Value	Unit	Context
Longest Spin Coherence Time ($T_2$)	1.62 $\pm$ 0.10	ms	Sample C1, Room Temperature
Highest Hall Mobility ($\mu$)	422	cm²/(Vs)	Sample B1, Room Temperature
Optimum Phosphorus Concentration ($C_P$)	~6 x 10¹⁶	cm^-3	Reported optimum for long $T_2$
Experimental $C_P$ Range	10¹⁶ - 10¹⁷	cm^-3	Measured by SIMS
NV Center Alignment	Perfectly aligned	N/A	Along the [111] direction
Substrate Orientation	(111)	N/A	Required for aligned NV centers
Substrate Dimensions Used	2 x 2 x 0.5 or 2 x 2 x 0.3	mm³	Ib-type HPHT substrates
Metalization Stack	Ti (30 nm)/Pt (30 nm)/Au (100 nm)	nm	Used for Hall electrodes
CVD Gas Pressure	25	kPa	Common growth pressure
Estimated Growth Rate	1 to 6	µm/h	Based on SIMS results

Key Methodologies

The n-type diamond films were synthesized using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD) with specific recipe controls:

Substrate Preparation: Ib-type (111) diamond substrates (HPHT grown) were used, with off-angles corresponding to 1°-3° along the [112] direction.
Source Gases:
- Main Carbon Source: ¹²C-enriched CH₄ (purity > 8N, ¹³C < 0.005 at%) or natural abundant CH₄ (purity: 6N).
- Dopant Source: tert-butylphosphine (TBP, purity: 3N3) diluted with H₂ (purity > 8N).
Impurity Control: Unintentional nitrogen incorporation was suppressed by incrementally increasing the total gas flow rates of H₂ and CH₄ (up to 400 sccm total flow).
Doping Control: TBP flow rate was adjusted to achieve phosphorus concentrations in the 10¹⁶-10¹⁷ cm^-3 range.
Electrode Fabrication: A selective growth method was used to deposit heavily phosphorus-doped diamond (concentration ~10²⁰ cm^-3) only in the electrode area to reduce contact resistance.
Metalization and Annealing: Ti/Pt/Au films were deposited via electron beam evaporation, followed by annealing at 420°C for 30 min in air to form TiC at the interface, further suppressing contact resistance.
Characterization: Samples were analyzed using Hall measurements (300-900 K, 0.5 T magnetic field) and Optically Detected Magnetic Resonance (ODMR) using a 532 nm laser to confirm $T_2$ and NV alignment.

6CCVD Solutions & Capabilities

This research validates the critical role of high-quality, doped, and oriented diamond for next-generation quantum devices. 6CCVD is uniquely positioned to supply the necessary materials and customization required to scale this research into commercial products.

Applicable Materials

To replicate and extend this research, 6CCVD recommends the following specialized materials:

6CCVD Material	Specification	Relevance to Research
Quantum Grade SCD (111)	Single Crystal Diamond, high purity, low nitrogen, (111) orientation.	Essential for achieving the perfect NV center alignment demonstrated (1.62 ms $T_2$).
Custom P-Doped SCD	Single Crystal Diamond, tailored phosphorus concentration (e.g., 10¹⁶ to 10¹⁷ cm^-3).	Matches the n-type doping required for stabilizing the NV charge state and achieving long coherence times.
Isotopically Purified SCD	SCD with ultra-low ¹³C concentration (< 0.005 at%).	Necessary to minimize nuclear spin noise, which is critical for achieving $T_2$ times exceeding 1 ms.

Customization Potential

The experimental requirements align perfectly with 6CCVD’s advanced fabrication and customization services, enabling researchers to move beyond small lab samples:

Large Area Substrates: While the researchers used small 2x2 mm³ chips, 6CCVD can supply SCD plates up to 10x10 mm and PCD wafers up to 125 mm in diameter, facilitating the scaling of quantum device manufacturing.
Precision Polishing: Achieving high-fidelity optical readout and low surface noise requires exceptional surface quality. 6CCVD guarantees Ra < 1 nm polishing for SCD, ensuring optimal performance for confocal microscopy and ODMR.
Advanced Metalization Services: The paper required a specific Ti/Pt/Au stack for low-resistance ohmic contacts. 6CCVD offers in-house custom metalization using Ti, Pt, Au, Pd, W, and Cu, allowing for precise replication or optimization of electrode designs.
Custom Dimensions and Thickness: 6CCVD provides SCD layers from 0.1 µm up to 500 µm thick, and substrates up to 10 mm, allowing precise control over the active layer depth and substrate handling.

Engineering Support

The successful synthesis of P-doped diamond requires precise control over gas flow ratios (CH₄/H₂) and dopant concentration (TBP/CH₄) to manage the carrier compensation ratio ($\eta = N_A/N_D$).

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth recipes for complex doping profiles and orientations. We provide expert consultation on:

Material Selection: Guiding the choice between SCD (111) and high-quality PCD for specific quantum diamond device applications.
Doping Optimization: Assisting in tuning precursor flow rates to achieve the ideal $C_P$ concentration (e.g., 6 x 10¹⁶ cm^-3) while minimizing unintentional nitrogen incorporation.
Interface Engineering: Designing custom metal stacks and annealing protocols to ensure robust, low-resistance ohmic contacts for Hall effect and device integration.

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

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

Original Source

DOI: https://doi.org/10.1063/5.0101215

References

2010 - Quantum Computation and Quantum Information
2013 - Solid-state electronic spin coherence time approaching one second [Crossref]