Microstructure and Anisotropic Order Parameter of Boron-Doped Nanocrystalline Diamond Films

At a Glance

Metadata	Details
Publication Date	2022-07-25
Journal	Crystals
Authors	Somnath Bhattacharyya
Institutions	University of the Witwatersrand
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research investigates the unconventional superconductivity and topological features in Heavily Boron-Doped Nanocrystalline Diamond Films (HBDDF), focusing on the critical role of grain boundary microstructure.

Unconventional Superconductivity: HBDDF exhibits non-s-wave superconductivity (p-wave/d-wave components) driven by the unique grain boundary (GB) structure, which breaks translational symmetry.
Rashba Spin-Orbit Coupling (RSOC): Ultra-High Resolution TEM (UHRTEM) confirms a layered, superlattice-like GB structure. This broken symmetry intrinsically generates RSOC, eliminating the need for heavy magnetic elements.
Topological Phase Evidence: The microstructure, characterized by distorted hexagonal rings and triangular shapes (similar to a Kagome lattice), suggests the formation of a topologically protected system, potentially realizing a Zak phase.
Transport Signatures: Electrical measurements show a transition from a Bosonic Insulator (BI) phase to superconductivity. Magnetoresistance (MR) exhibits a crossover from Weak Localization (WL, triplet state) to Weak Anti-Localization (WAL, singlet state) around 500 mK, confirming spin-orbit interaction effects.
Anisotropic Order Parameter: Angle-dependent MR is highly anisotropic (90° periodicity), with resistance peaks corresponding to GB intersection angles (45° and 72°), providing strong evidence for an anisotropic superconducting order parameter.
Quantum Device Potential: The findings support the realization of a diamond-based topological qubit, leveraging the controllable junction properties and long-range triplet Josephson current induced by the GBs.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Method	MPCVD	N/A	Microwave Plasma-Enhanced Chemical Vapor Deposition
Substrate Temperature	850	°C	Film growth temperature
Chamber Pressure	~80	Torr	Film growth pressure
Microwave Power	1.4	kW	Plasma generation power
Carbon Source Ratio	1% CH₄ in H₂	N/A	Gas mixture for deposition
Boron Doping Source	4000 ppm TMB to CH₄	N/A	Trimethylborane (TMB) concentration
Boron Concentration (Achieved)	2.8 x 10²¹	cm^-3	Heavily doped, above Mott transition
Mott Metallic Transition Threshold	~3 x 10²⁰	cm^-3	Concentration required for metallicity
Grain Size (Nanocrystals)	50-70	nm	Size of individual diamond grains
Film Thickness	~100	nm	Columnar growth thickness
Electrical Measurement Range (T)	0.3 to 5	K	Cryogenic transport measurements
Magnetic Field Range (B)	0 to 5	Tesla	Magnetoresistance measurements
WL/WAL Crossover Temperature	~600	mK	Transition between triplet (WL) and singlet (WAL) states
MR Anisotropy Peak Angles	45 and 72	°	Corresponds to grain boundary intersection angles
Boron Acceptor Level (Proposed)	~37	meV	Associated with B-C sheets mixing with valence band
B-C Bond Length	1.6	Angstrom	Longer than C-C bonds (1.54 Angstrom)

Key Methodologies

The HBDDF samples were synthesized using Microwave Plasma-Enhanced Chemical Vapor Deposition (MPCVD) and characterized using advanced microscopy and low-temperature electrical transport techniques.

Film Synthesis (MPCVD)

Substrate Preparation: Fused quartz substrates were pre-cleaned and seeded with diamond nanoparticles.
Gas Mixture: A gas phase mixture of 95% CH₄ (Methane) in H₂ (Hydrogen) was used, with 4000 ppm Trimethylborane (TMB) added to the CH₄ flow for heavy boron doping.
Growth Parameters: The substrate temperature was maintained at 850 °C, pressure at ~80 Torr, and microwave power at 1.4 kW.
Doping Level: The process achieved a boron concentration of 2.8 x 10²¹ cm^-3, ensuring the film was well into the metallic regime.

Microstructure Characterization

SEM: A JEOL JEBL-7001FLV field emission SEM was used to analyze the grain distribution and the columnar growth morphology of the ~100 nm thick films.
UHRTEM/STEM: Ultra-High Resolution TEM and High-Angle Annular Dark-Field (HAADF) imaging were performed at the Diamond Light Source (UK) to analyze the atomic structure of the grain boundaries (GBs).
- Revealed sharp crystal twinning (Sigma=3, Sigma=9) and layered stacking faults.
- Identified distorted five- and six-fold rings and triangular microstructures, suggesting a Kagome lattice structure.
Sample Preparation: TEM lamella samples were prepared using ion beam milling at Carl Zeiss Microscopy.

Electrical Transport Measurements

Setup: Measurements were conducted using a cryogen-free system (Cryogenic Ltd.) capable of reaching temperatures from 0.3 K to 5 K and magnetic fields up to 5 Tesla.
Geometry: A four-probe van der Pauw geometry was used for measuring longitudinal resistance (R_xx) and transverse resistance (R_xy).
Bias Dependence: Resistance vs. Temperature (R-T) was measured under varying bias currents (1 µA down to 10 nA) to study the evolution of the Bosonic Insulator (BI) peak and the metal-insulator-superconductor transition.
Magnetoresistance (MR): MR was measured as a function of temperature and magnetic field (B), observing hysteresis and the transition from Weak Localization (WL) to Weak Anti-Localization (WAL) around 600 mK.
Anisotropy: Angle-dependent resistance was measured by rotating the sample in a 2 Tesla field to confirm the anisotropic nature of the superconducting order parameter.

Commercial Applications

The demonstration of intrinsic RSOC and topological phases in a simple carbon-based system like boron-doped diamond opens pathways for next-generation quantum and spintronic devices.

Application Area	Specific Product/Function	Technical Advantage
Quantum Computing	Topological Qubits	Utilizes the topologically protected nature of the GB interface (Zak phase) for robust quantum information storage and processing.
Spintronics	Spin-Orbit Coupled Devices	Leverages intrinsic Rashba-type Spin-Orbit Coupling (RSOC) generated by broken lattice symmetry, enabling spin manipulation without heavy magnetic materials.
Superconducting Electronics	Long-Range Triplet Josephson Junctions	The anisotropic order parameter and spin-triplet superconductivity allow for the creation of long-range Josephson currents, critical for advanced superconducting circuits.
High-Performance Sensors	Anisotropic Magnetic Sensors	The highly anisotropic magnetoresistance (peaking at 45°/72°) can be exploited for highly directional magnetic field sensing.
Extreme Environment Electronics	Superconducting Components	Boron-doped diamond maintains superior thermal conductivity and mechanical hardness, making these superconducting films ideal for robust devices operating in harsh conditions.
Fundamental Research	Quantum Simulators	The layered GB structure (Shockley model) and associated quantum phenomena (e.g., Aharonov-Bohm rings, vortex structures) can be used as solid-state platforms for simulating complex quantum physics.

View Original Abstract

Unconventional superconductivity in heavily boron-doped nanocrystalline diamond films (HBDDF) produced a significant amount of interest. However, the exact pairing mechanism has not been understood due to a lack of understanding of crystal symmetry, which is broken at the grain boundaries. The superconducting order parameter (Δ) of HBDDF is believed to be anisotropic since boron atoms form a complex structure with carbon and introduce spin-orbit coupling to the diamond system. From ultra-high resolution transmission electron microscopy, the internal symmetry of the grain boundary structure of HBDDF is revealed, which can explain these films’ unconventional superconducting transport features. Here, we show the signature of the anisotropic Δ in HBDDF by breaking the structural symmetry in a layered microstructure, enabling a Rashba-type spin-orbit coupling. The superlattice-like structure in diamond describes a modulation that explains strong insulator peak features observed in temperature-dependent resistance, a transition of the magnetic field-dependent resistance, and their oscillatory, as well as angle-dependent, features. Overall, the interface states of the diamond films can be explained by the well-known Shockley model describing the layers connected by vortex-like structures, hence forming a topologically protected system.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12081031

References

2016 - Bosonic Anomalies in Boron-Doped Polycrystalline Diamond [Crossref]
2015 - Superconductivity in doped semiconductors [Crossref]
2008 - An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure [Crossref]
2016 - Formation of boron-carbon nanosheets and bilayers in boron-doped diamond: Origin of metallicity and superconductivity [Crossref]
2020 - Complex nanostructures in diamond [Crossref]
2013 - Observation of higher stiffness in nanopolycrystal [Crossref]
2003 - Influence of an interface double electric layer on the superconducting proximity effect in ferromagnetic metals [Crossref]
2009 - Three-dimensional topological phase on the diamond lattice [Crossref]
2014 - Entanglement of a 3D generalization of the Kitaev model on the diamond lattice
2019 - Finite bias evolution of bosonic insulating phase and zero bias conductance in boron-doped diamond: A charge-Kondo effect [Crossref]