Large microwave inductance of granular boron-doped diamond superconducting films

At a Glance

Metadata	Details
Publication Date	2021-06-14
Journal	Applied Physics Letters
Authors	Bakhrom Oripov, Dinesh Kumar, Cougar Garcia, Patrick Hemmer, T. Venkatesan
Institutions	National University of Singapore, Neocera (United States)
Citations	5
Analysis	Full AI Review Included

Executive Summary

Core Achievement: Measured the in-plane complex surface impedance of granular Boron-Doped Diamond (BDD) superconducting thin films in the microwave frequency range using a Parallel Plate Resonator (PPR) technique.
High Kinetic Inductance: The BDD films exhibit an exceptionally large zero-temperature magnetic penetration depth (λ(0)), measured up to 4.02 µm. This value is significantly larger than conventional superconductors (50 nm - 500 nm), confirming high kinetic inductance.
Superconducting Behavior: The temperature dependence of the magnetic penetration depth is consistent with a fully-gapped s-wave BCS superconductor, despite the material’s strong granular nature.
Critical Temperature: Optimal DC critical temperature (T_c,DC) reached 7.2 K. The microwave T_c (T_c,RF) extracted from the frequency shift data ranged from 1.54 K to 6.72 K, depending on the film and measurement mode.
Granularity Confirmation: The large penetration depth and high normal state resistivity (up to 40 mΩ cm) confirm the material’s highly granular microstructure, leading to a large effective screening length combining Meissner and Josephson screening.
Application Potential: The material is highly promising for applications requiring large kinetic inductance, such as compact microwave resonators, Microwave Kinetic Inductance Detectors (MKIDs), and high-impedance quantum circuits.

Technical Specifications

Parameter	Value	Unit	Context
Optimal DC T_c (Onset)	7.2	K	Highest reported T_c for BDD films
Microwave T_c (T_c,RF)	6.72 ± 0.001	K	Film A (PPR measurement, 7.39 GHz mode)
Zero-T Penetration Depth (λ(0))	2.19 ± 0.01	µm	Film A (PPR measurement, 7.39 GHz mode)
Film Thickness (t)	1.5 ± 0.2	µm	Film A
Normal State Sheet Resistance (R_N)	696.77 ± 46.21	mΩ	Derived from R_eff fit (Film A)
Normal State Resistivity (ρ_N)	2.32 ± 0.31	mΩ cm	Derived from R_N (Close to measured 5.5 mΩ cm)
Hall Concentration (n_h)	3.0 x 10²¹	cm^-3	Film A (Doping density)
Superconducting Gap (Δ(0))	924.38 ± 76.60	µeV	Derived from low-T exponential fit (Film A)
Superconducting Gap Ratio (Δ(0)/k_BT_c)	1.597	-	Less than weak-coupling BCS limit (1.764)
Zero-T Surface Resistance (R_eff(0))	64.22 ± 0.39	mΩ	Film A (Residual loss at 7.39 GHz)
Self-Kerr Coefficient (K₁₁)	15.25	mHz/photon	Film B (Nonlinearity measure at 100 mK)
Deposition Temperature	850	°C	HFCVD process
Deposition Pressure	~7	Torr	HFCVD process
B/C Ratio (Doping)	~10,000	ppm	HFCVD process

Key Methodologies

The BDD films were fabricated using Hot Filament Chemical Vapor Deposition (HFCVD), and their microwave properties were characterized using the Parallel Plate Resonator (PPR) technique.

Film Deposition (HFCVD):
- BDD films were grown on silicon substrates.
- Process gases included CH₄ (80 sccm), H₂ (3000 sccm), and B(CH₃)₃ for Boron doping.
- The B/C ratio was maintained at approximately 10,000 ppm.
- Substrate temperature was fixed at 850 °C, and chamber pressure was maintained at approximately 7 Torr.
Microwave Measurement (PPR):
- Two nominally identical BDD films were diced and placed face-to-face, sandwiching a Sapphire dielectric spacer (75 µm or 430 µm thickness) to form the PPR.
- The PPR assembly was placed in a metallic enclosure and cooled in a BlueFors dilution refrigerator down to a base temperature of 25 mK.
- Microwave signals were coupled capacitively using coaxial cables, avoiding direct electrical contacts.
- The complex transmitted signal (S₂₁ vs. frequency) was measured to extract the resonant frequency (f₀) and the loaded quality factor (Q).
Data Analysis:
- The temperature dependence of f₀(T) was fit using the BCS s-wave model (Eq. 1) to extract the zero-temperature penetration depth (λ(0)) and the microwave critical temperature (T_c,RF).
- The low-temperature data (T < T_c/3) was fit to an exponential activation model (Eq. 2) to determine the zero-temperature superconducting gap (Δ(0)).
- The effective surface resistance R_eff(T) was calculated from the measured Q(T) and fit using a model incorporating intrinsic BCS resistance and extrinsic finite-thickness contributions to determine the normal state sheet resistance (R_N).

Commercial Applications

The high kinetic inductance and robust superconducting properties of granular BDD films make them highly valuable for next-generation quantum and microwave technologies.

Quantum Information Science (QIS):
- High-Impedance Quantum Circuits: Used to create high-impedance structures (R₀ = h/(2e)² ≈ 6.5 kΩ) necessary for certain quantum circuit designs.
- Hybrid Quantum Devices: BDD can serve as a versatile platform (insulator, semiconductor, or superconductor) for building complex hybrid quantum devices, potentially incorporating nitrogen vacancy centers in diamond.
Microwave and Terahertz Detection:
- Microwave Kinetic Inductance Detectors (MKIDs): The large kinetic inductance enables the creation of highly compact and sensitive photon detectors, crucial for astronomy and high-frequency sensing.
- Superconducting Microresonator Bolometers: Used in sensitive thermal detection applications.
RF and Signal Processing:
- Compact Microwave Resonators: High inductance allows for significant miniaturization of resonant structures.
- Current-Tunable Delay Lines and Phase Shifters: Utilizing the kinetic inductance non-linearity for active control of microwave signals.
Advanced Materials Technology:
- Josephson Junctions: Potential for fabricating highly uniform vertical SNS (Superconductor-Normal-Superconductor) weak-link junctions solely using BDD by controlling doping concentration.
- Electroanalytics and Sensing: Leveraging the chemical stability and electrical properties of BDD films for robust sensing platforms.

View Original Abstract

Boron-doped diamond granular thin films are known to exhibit superconductivity with an optimal critical temperature of Tc=7.2 K. Here, we report the measured in-plane complex surface impedance of boron-doped diamond films in the microwave frequency range using a resonant technique. Experimentally measured inductance values are in good agreement with estimates obtained from the normal state sheet resistance of the material. The magnetic penetration depth temperature dependence is consistent with that of a fully gapped s-wave superconductor. Boron-doped diamond films should find application where high kinetic inductance is needed, such as microwave kinetic inductance detectors and quantum impedance devices.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0051227

References

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