Ultraheavy boron doping of diamond by ion implantation for low-resistance layer formation - Effect of hot implantation

At a Glance

Metadata	Details
Publication Date	2025-10-08
Journal	Journal of Applied Physics
Authors	K. Imamura, Yuhei Seki, Yasushi Hoshino
Institutions	Kanagawa University, Hokkaido University
Analysis	Full AI Review Included

Technical Documentation & Analysis: Ultraheavy Boron Doping in Diamond

This document analyzes the research paper “Ultraheavy boron doping of diamond by ion implantation for low-resistance layer formation: Effect of hot implantation” and outlines how 6CCVD’s advanced MPCVD diamond materials and customization services can support and extend this critical research in high-performance diamond electronics.

Executive Summary

This research successfully demonstrates a method for creating ultra-low-resistance, p-type diamond layers while maintaining high crystal quality, a crucial step for next-generation power electronics.

Core Achievement: Formation of a low-resistance (metallic-like) p-type layer in diamond via Boron (B) ion implantation at high substrate temperatures (hot implantation).
Low Resistivity: Specific resistance as low as $\approx 10^{-3}$ $\Omega$cm was achieved, comparable to that of graphite, but within a crystalline diamond structure.
Crystallinity Preservation: Hot implantation (400 °C and 800 °C) successfully suppressed the formation of graphitized layers, which typically occurs during Room Temperature (RT) implantation at high doses.
Doping Limit Extension: The critical B doping concentration limit for preserving diamond crystallinity was increased by more than an order of magnitude (from $1.0 \times 10^{20}$ cm^-3 at RT to $3.5 \times 10^{21}$ cm^-3 at 400 °C/800 °C).
Mechanism: Elevated substrate temperature during implantation is effective in suppressing vacancy accumulation, thereby preventing the structural transition from diamond (sp³) to graphite (sp²).
Application: These findings are vital for developing localized, low-resistance contact layers necessary for high-performance diamond-based electronic devices and power semiconductors.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Type IIa CVD Diamond	Optical Grade	3 x 3 mm² size, Element Six source
Implantation Temperatures	RT, 400, 800	°C	Investigated effect of hot implantation
Target B Concentration (Max)	$3.5 \times 10^{21}$	cm^-3	Approximately 2 at.%
Minimum Specific Resistance	$\approx 10^{-3}$	$\Omega$cm	Achieved at $3.5 \times 10^{20}$ cm^-3 (RT, graphitized) and $3.5 \times 10^{21}$ cm^-3 (Hot, crystalline)
Activation Energy (Low Doping)	0.37	eV	Corresponds to B acceptor ionization energy
Activation Energy (Heavy Hot Doping)	< 0.02	eV	Suggests metallic-like conduction
Activation Annealing Temperature	1300	°C	Performed in Ar atmosphere for 2h
Ohmic Contact Annealing	600	°C	Performed in vacuum
Ohmic Contact Metal Stack	Ti/Pt/Au	N/A	500 µm diameter electrodes
Critical B Doping (RT, Crystalline)	$1.0 \times 10^{20}$	cm^-3	Maximum concentration before graphitization at RT
Critical Vacancy Concentration	$2.5 \times 10^{22}$ to $7 \times 10^{22}$	cm^-3	Threshold for irreversible graphitization

Key Methodologies

The experimental process focused on precise control of implantation temperature and subsequent annealing to manage crystal damage and dopant activation.

Substrate Preparation:
- CVD-grown Type IIa diamond substrates (3 x 3 mm²) were polished to an atomic layer level.
- Chemical Wet Cleaning (CWC) performed using H${2}$SO${4}$/HNO${3}$ (300 °C) and NH${3}$/H${2}$O${2}$ (80 °C).
Boron Ion Implantation:
- B ions implanted from the normal direction using eight different energies (5 to 200 keV) to achieve a uniform B concentration profile up to hundreds of nm in depth.
- Implantation performed at Room Temperature (RT), 400 °C, and 800 °C.
- TRIM simulation (SRIM2013) used to adjust ion dose for uniform concentration.
Protective Layer Deposition:
- Thin SiO$_{2}$ film ($\approx 100$ nm) deposited by RF sputtering to protect the surface from graphitization during high-temperature annealing.
Activation Annealing:
- Samples annealed at 1300 °C in an Ar atmosphere for 2 hours.
Surface Treatment and Metalization:
- SiO$_{2}$ layer removed using diluted Hydrofluoric (HF) acid solution, followed by CWC.
- Four Ohmic electrodes (500 µm diameter) composed of Ti, Pt, and Au layers deposited for electrical measurements.
Final Annealing and Characterization:
- Samples annealed at 600 °C in vacuum to optimize Ohmic contacts.
- Electrical properties (resistivity, Hall effect) measured using the van der Pauw (vdP) geometry.
- Crystallinity characterized using Raman scattering spectroscopy (532 nm green laser) and Rutherford Backscattering Spectrometry (RBS-channeling, 1 MeV proton beam).

6CCVD Solutions & Capabilities

The successful replication and extension of this high-temperature ion implantation research require ultra-high-quality, low-defect diamond substrates and precise material engineering capabilities—core competencies of 6CCVD.

Applicable Materials

To achieve the low-resistance layers and maintain the high crystallinity demonstrated in this paper, researchers require the highest quality substrates.

6CCVD Material Recommendation	Specification & Relevance to Research
Optical Grade Single Crystal Diamond (SCD)	Provides the lowest defect density (Type IIa equivalent) necessary for minimizing implantation damage and maximizing dopant activation efficiency. Essential for high-power device fabrication.
High-Purity SCD Plates	Available in thicknesses from 0.1 µm up to 500 µm, allowing researchers to select the optimal thickness based on the required drift layer depth and implantation energy profile.
Boron-Doped Diamond (BDD) Substrates	While the paper uses implantation, 6CCVD offers epitaxially grown BDD layers (in situ doping) as a benchmark or alternative for comparison against the electrical properties of the implanted layers.

Customization Potential

The complexity of the fabrication process (implantation, annealing, metalization) necessitates highly customized material preparation, which 6CCVD provides in-house.

Custom Dimensions and Geometry: The paper utilized small 3 x 3 mm² samples. 6CCVD can supply SCD plates in larger formats and PCD wafers up to 125mm in diameter, enabling scale-up and multi-device fabrication for industrial applications.
Advanced Polishing Services: The research emphasized substrates polished to the “atomic layer level.” 6CCVD guarantees ultra-smooth SCD surfaces (Ra < 1 nm) and inch-size PCD surfaces (Ra < 5 nm), critical for subsequent thin-film deposition and high-temperature processing stability.
Integrated Metalization Services: The experiment required a Ti/Pt/Au Ohmic contact stack. 6CCVD offers in-house custom metalization (Au, Pt, Pd, Ti, W, Cu) tailored to specific device architectures, eliminating the need for external processing steps.
Custom Substrate Thicknesses: 6CCVD can provide substrates up to 10 mm thick, offering robust support for high-temperature implantation and annealing processes (up to 1300 °C) without risk of warping or structural failure.

Engineering Support

The successful transition from RT implantation (which causes graphitization) to hot implantation (which preserves crystallinity) highlights the need for precise process control and material understanding.

PhD-Level Consultation: 6CCVD’s in-house PhD team specializes in MPCVD growth and post-processing of diamond semiconductors. We offer expert consultation on optimizing substrate selection, managing defect accumulation, and achieving high activation ratios for low-resistance layer formation projects.
Process Optimization: We can assist researchers in correlating TRIM simulation results (vacancy concentration profiles) with actual material properties (Raman, RBS) to fine-tune implantation parameters for specific high-power electronic device designs.

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

View Original Abstract

We performed heavy boron doping of diamond at high concentrations from 1019 to 1021cm−3 using ion implantation at various substrate temperatures to form a low-resistance p-type layer. This study details the electrical properties and crystallinity as a function of implantation temperature and discusses the potential for extremely heavy B doping. At room temperature, heavy B doping with a concentration of 3.5×1020cm−3 drastically reduced resistivity to approximately 10−3Ωcm, nearly comparable to that of graphite. Raman and Rutherford backscattering spectra confirmed the formation of a graphitized layer in the B-implanted region. In contrast, the crystallinity was maintained in samples B-implanted at 400 and 800 °C with the same doping concentration. A Raman peak at 1332 cm−1, corresponding to the diamond structure, was still observed in samples implanted at 400 and 800 °C even at a concentration of 3.5×1021cm−3 (2 at. %). The specific resistance was low (approximately 10−3Ωcm), and p-type conductivity was observed across all measured temperatures. These results clearly indicate that hot implantation above 400 °C is effective for achieving heavy doping of a few percent, forming a low-resistance layer while preserving the crystallinity of diamond.