Diamond and III-nitride wide-bandgap semiconductors - a research journey

At a Glance

Metadata	Details
Publication Date	2025-09-05
Journal	Functional Diamond
Authors	Yasuo Koide
Institutions	Meijo University
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Wide-Bandgap Semiconductors

Reference: Koide, Y. (2025). Diamond and III-nitride wide-bandgap semiconductors: a research journey. Functional Diamond, 5:1, 2554126.

Executive Summary

This review confirms the critical role of MPCVD diamond in developing next-generation extreme electronics, focusing on high-power, high-frequency (6G), and deep-ultraviolet (DUV) applications.

Ohmic Contact Breakthroughs: Established fundamental guidelines for achieving low specific contact resistance (ρ_c) on p-type Boron-Doped Diamond (BDD) by utilizing carbide-forming metals (Ti, Mo, Cr) or solid-solution forming metals (Pd, Co).
Thermally Stable Contacts: Demonstrated that Ohmic contacts require metal reaction with diamond (forming carbides like TiC) at elevated temperatures, while Schottky contacts require non-reactive metals, providing a crucial guideline for device stability.
High-Performance MOSFETs: Successfully fabricated H-terminated diamond Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) using extreme-k dielectrics (e.g., ZrO₂/Al₂O₃), achieving high maximum drain current (I_DSmax up to -224.1 mA/mm).
Extreme-k Dielectrics: Confirmed that high-k and extreme-k materials (k up to 306 for AlO_x/TiOy nanolaminates) are effective for controlling high-density hole carriers in the H-terminated diamond channel.
Solar-Blind DUV Detection: Developed thermally stable deep-ultraviolet (DUV) photodetectors (190-260 nm) using WC Schottky contacts on homoepitaxial diamond, achieving a visible-blind ratio of nearly 10⁸.
Future Applications: The research directly supports the hybridization of diamond and III-nitrides for 5G/6G high-frequency (100-330 GHz) and high-power integrated circuits, leveraging diamond’s superior thermal conductivity and dielectric breakdown field.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Growth Method	MPCVD	N/A	Polycrystalline B-doped diamond synthesis
B-doped Diamond Thickness	1	µm	Layer used for contact resistance study
Undoped Diamond Thickness	4 - 8	µm	Layer used for contact resistance study
Lowest Specific Contact Resistance (ρ_c)	2 - 3 x 10^-3	Ω-cm²	Ni/Au contact on p-GaN (annealed in N₂/O₂)
Schottky Barrier Height (SBH) for p-Diamond	0.48 ± 0.05	eV	Achieved using carbide-forming metals (Ti, Mo, Cr)
DUV Photodetector Wavelength Range	190 to 260	nm	Solar-blind detection range
DUV Photoconductor Visible-Blind Ratio	Nearly 10⁸	N/A	Highest reported value for diamond PDs at the time
MOSFET Maximum Drain Current (I_DSmax)	-224.1	mA/mm	SD-ZrO₂/ALD-Al₂O₃/H-diamond MOSFET
MOSFET Effective Mobility (µ_eff)	217.5 ± 0.5	cm²-V^-1-s^-1	SD-ZrO₂/ALD-Al₂O₃/H-diamond MOSFET
Highest Dielectric Constant (k)	306	N/A	ALD-AlO_x/TiOy nanolaminate layer
Contact Annealing Temperature (Ti, Mo, Cr)	400 - 600	°C	Optimization for lowest ρ_c on p-diamond

Key Methodologies

The research relied heavily on advanced material synthesis and interface engineering techniques, particularly for diamond and electrode formation:

MPCVD Diamond Growth: Polycrystalline Boron-Doped Diamond (BDD) layers were grown on n-type Si (100) substrates using Microwave Plasma Chemical Vapor Deposition (MPCVD).
Controlled Doping: Boron doping was achieved by dissolving H₃BO₃ in a mixed solution (CH₃COCH₃ and CH₃OH), allowing precise control of the B/C atomic ratio via the mass fraction of H₃BO₃.
Surface Treatment: Post-growth processing included heating at 600 °C in air, followed by a 40-minute boiling acid etch (1:1:1 solution of HNO₃:H₂SO₄:HClO₄) to remove amorphous carbon and the surface conductive layer (H-termination).
Metal Contact Deposition: Metal films (Ti, Mo, Cr, Pd, Co) were deposited via electron beam evaporation under high vacuum (base pressure below 4x10^-5 Pa). Au films were subsequently evaporated via resistance heating to reduce sheet resistance.
Thermal Annealing for Ohmic Contacts: Contacts were annealed in evacuated quartz tubes (base pressure below 1x10^-4 Pa) at temperatures up to 600 °C for specific durations (e.g., 5 to 60 minutes) to induce metallurgical reactions (carbide formation) necessary for low contact resistance.
High-k Dielectric Integration: Metal-Insulator-Semiconductor (MIS) structures utilized high-k dielectrics (e.g., ZrO₂, Al₂O₃, HfO₂, Ta₂O₅) deposited using Sputter Deposition (SD) and Atomic Layer Deposition (ALD) techniques on H-terminated diamond surfaces.

6CCVD Solutions & Capabilities

The research detailed by Koide (2025) underscores the need for highly customized, high-quality MPCVD diamond materials and advanced interface engineering. 6CCVD is uniquely positioned to supply the materials and processing services required to replicate and advance this research into commercial applications, particularly in high-power and 6G electronics.

Applicable Materials

To replicate the high-performance devices described, 6CCVD recommends the following materials from our catalog:

Boron-Doped Diamond (BDD): Essential for the p-type Ohmic contact studies and MOSFET channels. 6CCVD offers BDD layers (PCD or SCD) with precise, controlled acceptor concentrations (N_A) up to 10²⁰ cm^-3, matching or exceeding the doping levels required for low ρ_c.
Electronic Grade SCD: Required for the homoepitaxial DUV photodetector studies, ensuring minimal nitrogen (N) and boron (B) impurities in the active layer to maximize solar-blind performance.
Polycrystalline Diamond (PCD) Wafers: Available up to 125mm in diameter, ideal for scaling up the fabrication of large-area sensors or power device arrays.

Customization Potential

The success of the devices in this paper hinges on precise dimensional control and complex metal stacks. 6CCVD provides comprehensive customization capabilities:

Research Requirement	6CCVD Capability	Value Proposition
Custom Metal Stacks	Internal metalization capability: Au, Pt, Pd, Ti, W, Cu, and multi-layer stacks.	We can directly replicate the carbide-forming (Ti/Mo/Au) and refractory metal (WC/Ti/WC) contacts, ensuring thermally stable interfaces critical for high-temperature operation.
Precise Layer Thickness	SCD and PCD films available from 0.1 µm to 500 µm.	Allows researchers to precisely control the active layer thickness (e.g., the 1 µm B-doped layer) and substrate thickness (up to 10mm) for thermal management.
High-Quality Surface Finish	SCD polishing to Ra < 1nm; Inch-size PCD polishing to Ra < 5nm.	Ultra-smooth surfaces are necessary for minimizing interface defects when integrating high-k dielectrics (ZrO₂, Al₂O₃) via ALD/SD for MOSFET fabrication.
Patterning & Device Geometry	Custom laser cutting and patterning services.	Enables the fabrication of specific device geometries, such as the circular Transmission Line Method (TLM) patterns or the 4-5 µm channel length MOSFETs, without external processing delays.

Engineering Support

6CCVD’s in-house PhD team offers expert consultation to accelerate your research:

Interface Optimization: We provide engineering support for optimizing the critical surface treatments (e.g., H-termination, acid cleaning) and thermal annealing recipes necessary to achieve the low ρ_c and high mobility reported in this paper.
Material Selection for Power Electronics: Our experts can assist in selecting the optimal diamond grade (SCD vs. PCD) and doping profile for similar high-power and high-frequency (6G) projects, ensuring the material meets the stringent thermal and electrical breakdown requirements.
High-k Integration Guidance: Consultation on material compatibility and deposition techniques for integrating high-k and extreme-k dielectrics onto diamond surfaces to maximize carrier density and device performance.

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

View Original Abstract

My research progress to date is reviewed by focusing on III-nitride and diamond semiconductors within the scope of my limited experience. I have developed high-quality AlxGa1-xN epitaxial layers, Ohmic contact materials for GaN and diamond, and diamond optical and electronic devices. While my research themes have changed at each university and national laboratory, I have been involved in semiconductor crystal growth, electrode formation, processing, and device development. I believe that my broad experience in materials research will lead to new discoveries in a variety of semiconductor fields.

Tech Support

Original Source

DOI: https://doi.org/10.1080/26941112.2025.2554126

References

2005 - Thermally-stable visible-blind diamond photodiode using WC schottky contact