FEATURES OF THE TECHNOLOGY OF HOMOEPITAXIAL CHEMICAL DEPOSITION OF THIN DIAMOND LAYERS ON A NITROGEN-DOPED SINGLE-CRYSTAL DIAMOND SUBSTRATE
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-06-20 |
| Journal | IZVESTIYA VYSSHIKH UCHEBNYKH ZAVEDENIY KHIMIYA KHIMICHESKAYA TEKHNOLOGIYA |
| Authors | С. А. Тарелкин, Н. В. Корнилов, М. С. Кузнецов, Nikolay V. Luparev, S. Yu. Martyushov |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research investigates the critical role of substrate quality in fabricating efficient diamond betavoltaic energy converters (nuclear batteries) using homoepitaxial Chemical Vapor Deposition (CVD).
- Core Objective: To optimize the synthesis of low-defect homoepitaxial diamond layers on nitrogen-doped (n+) substrates for n-type Schottky diodes designed to convert 63Ni beta decay energy.
- Substrate Comparison: Two types of HPHT substrates were tested: high nitrogen (300 ppm) and low nitrogen (60 ppm) concentrations.
- Defect Inheritance: Substrates with high N content (300 ppm) exhibited a high density of extended structural defects (dislocations 106-107 cm-2). These defects were inherited by the subsequent CVD layer, resulting in high leakage currents and rendering the diodes unsuitable for energy conversion.
- Optimized Performance: Diodes fabricated on low N content (60 ppm) substrates showed significantly lower defect density (103-105 cm-2) and achieved low leakage current and efficient energy conversion.
- Key Achievement: The optimized diodes (60 ppm substrate) delivered a maximum electrical power of 170 pW with an efficiency of approximately 1%, maintaining stability at temperatures up to 500 °C.
- Technological Implication: The study confirms that precise control over nitrogen doping and crystalline quality of the HPHT substrate is essential for the successful creation of multi-layered PiN structures required for high-efficiency, durable radioisotope power sources.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate N Concentration (High) | 300 | ppm | HPHT Substrate #1 (High Defects) |
| Substrate N Concentration (Low) | 60 | ppm | HPHT Substrate #2 (Low Defects) |
| Dislocation Density (High N Substrate) | 106-107 | cm-2 | Inherited by homoepitaxial layer |
| Dislocation Density (Low N Substrate) | 103-105 | cm-2 | Inherited by homoepitaxial layer |
| Homoepitaxial Layer Thickness (i-layer) | 30 | µm | Calculated thickness |
| CVD Growth Rate | ~1.3 | µm/h | i-layer synthesis rate |
| CVD Growth Temperature | 850 ± 15 | °C | i-layer synthesis |
| Gas Ratio (H2/CH4) | 24/1 | Ratio | CVD process parameters |
| CVD Pressure | 180 ± 5 | mbar | CVD process parameters |
| Maximum Operating Temperature | 500 | °C | Efficient energy conversion (Substrate #2) |
| Maximum Electrical Power | 170 | pW | Achieved by Substrate #2 diode |
| Conversion Efficiency (Max) | ~1 | % | Achieved by Substrate #2 diode |
| Short Circuit Current (Ixx) @ 500 °C | 370 | pA | Substrate #2 performance |
| Optimal Load Voltage | 0.7 | V | Substrate #2 performance |
| Schottky Contact Thickness (Pt) | < 20 | nm | Minimizing beta absorption |
Key Methodologies
Section titled “Key Methodologies”- Substrate Synthesis (n+): Nitrogen-doped single-crystal diamond substrates were grown using the Temperature Gradient High-Pressure High-Temperature (TG-HPHT) method in a Fe-Co-C-N system. Nitrogen concentration (300 ppm or 60 ppm) was controlled using an Aluminum (Al) getter.
- Substrate Preparation: Rectangular substrates (3.5x3.5 mm2, 250 µm thick) with (001) orientation were polished, cleaned via hot acid etching (HCl:HNO3 3:1), and annealed at 680 °C for 20 minutes.
- Crystalline Quality Analysis: Substrate quality was assessed using UV photoluminescence mapping and Lang X-ray diffraction topography (220 reflection, AgKα1 radiation).
- Homoepitaxial CVD Growth (i-layer): The intrinsic (i) layer was synthesized using a Plassys BJS 150 CVD system under the following conditions:
- Temperature: 850 ± 15 °C.
- Microwave Power: 2.7 kW.
- Gas Mixture: H2/CH4 ratio of 24/1.
- Pressure: 180 ± 5 mbar.
- Gas Purity: High-purity H2 (< 1 ppb impurities) and CH4 (99.9999%) were used to ensure low impurity content in the i-layer.
- Ohmic Contact Fabrication: A Ti/Pt metal stack was deposited on the bottom (n+) side and annealed at 700 °C to form a TiC layer, ensuring a high-quality ohmic contact.
- Schottky Contact Preparation: The top surface was annealed at 650 °C, followed by 20 minutes of SF6 plasma treatment to achieve fluorine termination, which maximizes Schottky barrier height uniformity.
- Schottky Contact Deposition: A thin (< 20 nm) Platinum (Pt) contact (9 mm2 area) was deposited via magnetron sputtering.
- Betavoltaic Testing: Current-voltage (IV) characteristics were measured under irradiation from a closed 63Ni beta source (4x4x0.1 mm3, 20% enrichment) using a Keithley 4200A-CSC system, with heating up to 600 °C in an argon atmosphere.
Commercial Applications
Section titled “Commercial Applications”The successful development of high-quality, low-defect homoepitaxial diamond layers on nitrogen-doped substrates directly supports several high-value technological fields:
- Radioisotope Power Sources (Betavoltaics): Creating highly efficient, long-life nuclear batteries for applications requiring decades of power without maintenance, such as deep-space probes, remote monitoring stations, and critical medical implants (e.g., pacemakers).
- High-Temperature Electronics: Manufacturing robust semiconductor devices (diodes, transistors) capable of operating reliably at extreme temperatures (up to 500 °C and potentially higher), suitable for automotive, aerospace, and industrial process control.
- Radiation Detection and Sensing: Utilizing the wide bandgap and radiation hardness of diamond for highly stable radiation detectors and dosimeters in nuclear facilities, particle accelerators, and medical imaging.
- Advanced PiN Structures: The established growth recipe for the i-layer on n+ substrates is a foundational step toward realizing high-efficiency diamond PiN diodes, which theoretically offer conversion efficiencies up to 29% for betavoltaics.
- High-Power RF and Microwave Devices: The low defect density and high thermal conductivity inherent in the optimized CVD diamond layers are crucial for fabricating high-frequency, high-power electronic components.
View Original Abstract
In this paper, we study the formation and properties of diamond structures designed to convert the beta decay energy of the radioactive isotope 63Ni into electrical energy. Diamond substrates with different nitrogen concentrations (300 ppm and 60 ppm) were grown using the temperature gradient high-pressure thermonuclear technique (TG-HPHT). Analysis showed that a high nitrogen content leads to a significant increase in the number of structural defects in the substrates. Homoepitaxial layers with a low impurity content were synthesized using chemical vapor deposition on the diamond substrates. Then n-type Schottky diodes were manufactured by metal contacts deposition. The electrical properties of the diodes were studied when exposed to beta radiation. It was shown that substrates with a high nitrogen concentration inherit structural defects, which lead to high leakage currents and make such substrates unsuitable for creating efficient energy converters. At the same time, substrates with a nitrogen concentration of 60 ppm provide low leakage current and efficient energy conversion at temperatures up to 500 °C. The maximum electrical power was 170 pW with an efficiency of about 1%. The results demonstrate the importance of optimizing the nitrogen doping level and the crystalline quality of the substrates for creating efficient and durable radioisotope energy converters. For citation: Tarelkin S.A., Kornilov N.V., Kuznetsov M.S., Luparev N.V., Martyushov S.Yu., Timoshenko V.O. Features of the technology of homoepitaxial chemical deposition of thin diamond layers on a nitrogen-doped single-crystal diamond substrate. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2025. V. 68. N 9. P. 60-65. DOI: 10.6060/ivkkt.20256809.11y.