Characterization of boron-doped single-crystal diamond by electrophysical methods (review)
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-01-01 |
| Journal | Журнал технической физики |
| Authors | Zubkov V.I., Solomnikova A.V., Solomonov A.V., Koliadin A.V., Butler J.E. |
| Institutions | Almaz-Antey (Russia), Saint Petersburg State Electrotechnical University |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This review critically analyzes the electrophysical characterization of boron-doped single-crystal diamond (BDD), focusing on methods necessary for electronic-grade material diagnostics.
- Core Challenge: BDD exhibits an extremely low degree of boron ionization (< 1% at room temperature) due to the high acceptor activation energy (EA = 370 meV), complicating standard concentration measurements.
- Key Diagnostic Method: Admittance Spectroscopy (AS) and Capacitance-Voltage (C-V) profiling were used to measure dynamic characteristics, providing quantitative data on free charge carriers (holes, p) and impurity concentration (NA).
- Measurement Requirement: Accurate C-V profiling requires non-quasi-static conditions, necessitating the use of low frequencies (20-50 kHz) and high temperatures (> 330 K) to achieve near-full ionization.
- Activation Energy Reduction: The study experimentally confirmed a substantial reduction in EA from 325 meV (weakly doped) down to 100 meV (heavily doped, NA up to 4·1019 cm-3), consistent with the Pearson-Bardeen model.
- Hopping Conduction: For heavily doped samples (NA >= 5·1018 cm-3), a transition to hopping conductivity was observed below 150 K, characterized by a very low activation energy of 10-20 meV.
- Method Comparison: C-V measurements provide spatial distribution and operating condition data, offering advantages over volume-averaged FTIR measurements, which require normalization via Hall measurements.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Boron Ionization Energy (EA, Reference) | 370 | meV | Weakly-doped diamond (Optical/Hall) |
| EA (Weakly Doped, AS) | 285 to 325 | meV | Valence Band Conductance (High T) |
| EA (Heavily Doped, AS) | 100 to 170 | meV | Valence Band Conductance (High T) |
| EA (Hopping Conduction) | 10 to 20 | meV | NA >= 5·1018 cm-3, T < 150 K |
| Boron Concentration Range (NA) | 2·1016 to 4·1019 | cm-3 | BD Single-Crystal Diamond |
| Intrinsic Carrier Concentration | 10-27 | cm-3 | Room Temperature |
| HPHT Growth Temperature | 1400 ± 50 | °C | Synthesis Conditions |
| HPHT Growth Pressure | 5 to 6 | GPa | Synthesis Conditions |
| CVD Growth Rate | > 1 | µm/h | Epitaxial Film Deposition |
| CVD Substrate Temperature | 700 to 1100 | °C | Plasma Activation |
| CVD Epitaxial Layer Thickness | 2 to 2.7 | µm | Studied samples |
| HPHT Surface Roughness (Ra) | 1 to 3 | nm | Best morphology samples |
| Admittance Spectroscopy Frequency Range | 100 Hz to 2 | MHz | Measurement range |
| Admittance Spectroscopy Temperature Range | 15 to 475 | K | Measurement range |
| Hole Capture Section (σp) | 0.0002·10-13 to 1.3·10-13 | cm2 | BD Diamond (Table 3) |
Key Methodologies
Section titled “Key Methodologies”The study utilized a combination of synthesis techniques, contact fabrication, and advanced electrophysical diagnostics to characterize BDD samples.
1. Single-Crystal Diamond Synthesis
Section titled “1. Single-Crystal Diamond Synthesis”- High-Pressure High-Temperature (HPHT):
- Conditions: 1400 ± 50 °C and 5-6 GPa.
- Doping: Boron added directly to the growth cell.
- Result: Produced bulk single- and multi-sector plates (8-15 carats, 0.3-0.5 µm thickness) with excellent morphology (Ra = 1-3 nm).
- Chemical Vapor Deposition (CVD):
- Conditions: Cylindrical MPACVD reactor (2.45 GHz UHF), 700-1100 °C substrate temperature, 100-200 Torr pressure.
- Doping Source: Trimethyl borate (B(OCH3)3) dissolved in ethanol.
- Result: Epitaxial layers (2-2.7 µm thick) grown on (100) HPHT substrates, with B/C ratios varying from 600 to 12000 ppm.
2. Contact Fabrication
Section titled “2. Contact Fabrication”- Material: Platinum (Pt) contacts applied via magnetron spraying.
- Geometry: Vertical geometry (Schottky diodes) for bulk samples; planar arrangement for epitaxial layers (due to undoped substrate).
- Annealing: Ohmic contacts annealed at 300 °C; rectifying contacts deposited at 70 °C.
3. Electrophysical Diagnostics
Section titled “3. Electrophysical Diagnostics”-
Admittance Spectroscopy (AS) / C-V Profiling:
- Setup: Cryogenic probe station (Janis CCR-10) and precision LCR-meter (Agilent E4980A).
- Procedure: Measured capacitance (C) and conductance (G) as a function of temperature (T), voltage (V), and frequency (ω).
- C-V Analysis: Used to calculate the apparent hole concentration (pcv) profile along the depth. Low frequencies (20-50 kHz) and high temperatures (> 330 K) were required to achieve quasi-static conditions and minimize frequency dispersion.
- AS Analysis: Used temperature and frequency spectra to determine the activation energy (EA) and hole capture cross-section (σp) based on the resonance condition G(ω, T) maximum occurring when the emission speed (ep) equals the test frequency (ω).
-
FTIR Spectroscopy:
- Purpose: Measured the volume-averaged concentration of partially compensated boron (NA - ND).
- Calibration: Used absorption bands (2802, 2454, 1290 cm-1) calibrated against Hall measurements.
Commercial Applications
Section titled “Commercial Applications”The unique properties of BDD, particularly its wide bandgap and high thermal conductivity, make it suitable for next-generation devices operating under extreme conditions.
- High-Power Electronics: Diamond-based semiconductor devices can significantly increase limit breakdown voltages and operating frequencies, reducing energy consumption compared to silicon or SiC.
- Extreme Environment Electronics: Suitable for high-temperature and high-radiation environments where conventional semiconductors fail.
- High-Frequency Unipolar Transistors: The ability to achieve ultra-high doping levels in narrow layers (delta-doping) is critical for creating powerful unipolar transistors.
- Optical and UV Detection: Used in two-spectrum photodetectors (deep UV and near-IR) and sensors for charged particles and radiation.
- Electrochemistry and Biosensorics: BDD’s exceptional chemical resistance, biocompatibility, and wide electrochemical window make it ideal for electrode materials and advanced biosensors.
- Micro-electromechanical Systems (MEMS): Diamond films are advantageous due to their low coefficients of adhesion and friction.
View Original Abstract
A critical analysis of the existing methods of controlling the concentration of impurity and majority charge carriers in wide bandgap semiconductors and the issues of improvement of modern diagnostics of the main electrophysical properties of single-crystal diamond are considered based on the results of our studies and the works of other authors. It was found that independent assessment of impurity concentration and concentration of free charge carriers is of fundamental importance for semiconductor diamond due to very low (less than 1%) degree of ionization of the introduced impurity. The advantages and prospects of admittance spectroscopy as a diagnostic method for ultrawide bandgap semiconductors are shown and solutions aimed at the correct interpretation of the experimental data are proposed. The high ionization energy of boron impurity in diamond (370 meV) results in a strong frequency dispersion of the measured barrier capacitance. It is shown that under disturbance of quasi-static conditions in capacitance-voltage measurements, low frequencies and high temperatures should be used for correct assessment of the charge carrier concentration. The results of electrophysical studies are compared with traditional measurements of impurity concentration in diamond by optical methods. A decrease of hole activation energy from the boron impurity level from 325 to 100 meV was found upon increasing the boron concentration NA from 2·10 16 to 4·10 19 cm -3 . The transition to the hopping mechanism of conductivity within the impurity (acceptor) band with thermal activation energy of 10-20 meV was registered for N A ≥5·10 18 cm -3 at temperatures of 120-150 K. Keywords: single-crystal diamond, boron impurity, charge carrier concentration, activation energy, admittance spectroscopy, capacitance-voltage measurements.