Thermally Stable, High Performance Transfer Doping of Diamond using Transition Metal Oxides

At a Glance

Metadata	Details
Publication Date	2018-02-14
Journal	Scientific Reports
Authors	Kevin G. Crawford, Dongchen Qi, Jessica C. McGlynn, Tony Ivanov, Pankaj B. Shah
Institutions	DEVCOM Army Research Laboratory, Centre National de la Recherche Scientifique
Citations	63
Analysis	Full AI Review Included

Thermally Stable Diamond Transfer Doping: 6CCVD Technical Analysis

This document analyzes the research paper, “Thermally Stable, High Performance Transfer Doping of Diamond using Transition Metal Oxides,” focusing on the material requirements and process parameters relevant to 6CCVD’s advanced MPCVD diamond capabilities.

Executive Summary

This research validates a robust methodology for achieving thermally stable, high-performance surface transfer doping in hydrogen-terminated diamond (H-diamond) using Transition Metal Oxides (TMOs).

Core Achievement: Demonstrated significantly enhanced thermal stability of H-diamond surface conductivity up to 300 °C using MoO₃ and V₂O₅ TMO acceptor layers.
Critical Process Step: Performing a 400 °C in-situ anneal prior to TMO deposition was verified as essential for maximizing long-term doping stability over 17 days.
Stability Mechanism: Encapsulation of the TMO layer with Hydrogen Silsesquioxane (HSQ, ~600 nm) dramatically improved stability in ambient air, isolating the diamond:metal-oxide interface from environmental degradation.
Material Foundation: The study relied exclusively on high-quality, optical grade Single Crystal CVD (SCD) diamond substrates, a core product line of 6CCVD.
Device Relevance: These findings confirm the viability of TMO-doped H-diamond for robust, high-power electronic devices, addressing historical limitations related to environmental sensitivity.
Performance Metrics: Achieved stable sheet resistance in the 5000-6000 Ω/square range and carrier concentrations exceeding 4 x 10¹³ cm^-2 when properly annealed and encapsulated.

Technical Specifications

The following hard data points were extracted from the experimental methodology and results:

Parameter	Value	Unit	Context
Substrate Material	Type IIa [001] SCD CVD Diamond	N/A	Optical grade, high purity
Substrate Dimensions	4.5 x 4.5	mm	Used for Van der Pauw (VDP) testing
H-Termination Temperature	600	°C	Microwave hydrogen plasma treatment
Critical In-Situ Annealing Temp.	400	°C	Performed for 1 hour prior to oxide deposition
TMO Film Thickness	100	nm	MoO₃ and V₂O₅ films
TMO Deposition Vacuum	2 x 10^-6	mbar	Thermal evaporation environment
Encapsulation Material	Hydrogen Silsesquioxane (HSQ)	N/A	Protective dielectric layer
Encapsulation Thickness	~600	nm	Spun layer thickness
Maximum Test Temperature	300	°C	High temperature sheet resistance measurements
Stable Sheet Resistance (Annealed/Encapsulated)	5000 - 6000	Ω/square	Measured after 17 days (Fig. 4c)
Stable Carrier Concentration (Annealed/Encapsulated)	4.0 - 4.5 x 10¹³	cm^-2	Measured after 17 days (Fig. 4a)
SCD Surface Roughness (Required)	Ra < 1	nm	Implied requirement for stable H-termination

Key Methodologies

The experiment focused on optimizing the surface preparation and encapsulation of the TMO acceptor layers on H-diamond.

Substrate Cleaning: Optical grade Type IIa [001] CVD diamond substrates were cleaned using sequential acid treatments (HNO₃:HCl followed by H₂SO₄:HNO₃) to remove metallic and organic adsorbents.
Hydrogen Termination: Surfaces were terminated using a microwave hydrogen plasma for 30 minutes at 600 °C.
VDP Contact Fabrication: Ohmic contacts were formed using silver paste applied to the corners of the samples to minimize processing contamination.
Crucial In-Situ Annealing: Select substrates received a 1-hour in-situ anneal at 400 °C in vacuum immediately prior to TMO deposition to ensure desorption of residual surface adsorbates.
TMO Deposition: 100 nm thick films of MoO₃ or V₂O₅ were deposited via thermal evaporation in a high vacuum (2 x 10^-6 mbar).
Dielectric Encapsulation: Select samples were encapsulated with a ~600 nm layer of Hydrogen Silsesquioxane (HSQ) applied via spin coating (2000 rpm) and baked at 80 °C.
Characterization: Samples were analyzed using Hall measurements for carrier concentration and mobility, and four-probe sheet resistance measurements were performed up to 300 °C in ambient air or low vacuum (60 mTorr).

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality, precisely engineered diamond materials for advanced surface transfer doping applications. 6CCVD is uniquely positioned to supply the necessary substrates and customization services to replicate and extend this work into high-power device fabrication.

Applicable Materials

To replicate or extend this research into high-performance FETs and diodes, the following 6CCVD materials are required:

Optical Grade Single Crystal Diamond (SCD): The foundation of this study. 6CCVD supplies high-purity, low-defect Type IIa [001] SCD wafers, essential for achieving the high hole mobility and stable H-termination required for transfer doping.
Ultra-Polished SCD Substrates: Achieving stable surface doping requires an atomically smooth interface. 6CCVD guarantees SCD polishing to Ra < 1 nm, minimizing interface scattering and maximizing 2DHG performance.
Custom Thickness SCD: 6CCVD offers SCD thicknesses ranging from 0.1 µm (for thin film studies) up to 500 µm, and robust substrates up to 10 mm for high-power applications requiring superior thermal management (thermal conductivity >20 W/cm·K).

Customization Potential

The experimental work utilized small, custom-sized samples and temporary contacts. 6CCVD provides the necessary industrial-grade customization to transition this research to scalable device manufacturing:

Research Requirement	6CCVD Customization Service	Benefit to Researcher/Engineer
Custom Dimensions	Precision Laser Cutting & Wafer Sizing	6CCVD provides SCD plates and wafers in custom sizes, including large-area PCD up to 125 mm diameter, enabling scale-up for commercial device prototyping.
Robust Electrical Contacts	In-House Metalization Stacks	The paper used Ag paste. 6CCVD offers reliable, high-temperature metal stacks (e.g., Ti/Pt/Au, W/Au, Pd) deposited directly onto H-diamond surfaces, crucial for stable ohmic contacts in high-power FETs.
Alternative Doping	Boron-Doped Diamond (BDD)	For comparison or hybrid doping strategies (as mentioned in the paper), 6CCVD supplies both lightly and heavily Boron-Doped Polycrystalline (PCD) and Single Crystal (SCD) diamond films.
Global Logistics	DDU and DDP Shipping	Global shipping services ensure rapid, reliable delivery of custom materials to research facilities worldwide.

Engineering Support

The successful implementation of TMO transfer doping relies heavily on precise material specifications and surface preparation (e.g., the critical 400 °C in-situ anneal). 6CCVD’s in-house PhD engineering team specializes in diamond surface chemistry and material selection for high-power electronic devices, such as Field Effect Transistors (FETs) and high-frequency devices. We provide consultation on:

Optimizing SCD crystallographic orientation and surface termination for maximum transfer doping efficiency.
Selecting appropriate metalization schemes for high-temperature operation (up to 300 °C and beyond).
Material specifications for projects requiring enhanced thermal stability and environmental robustness.

Call to Action: For custom specifications or material consultation regarding transfer doping, high-power electronics, or advanced diamond substrates, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract We report on optimisation of the environmental stability and high temperature operation of surface transfer doping in hydrogen-terminated diamond using MoO 3 and V 2 O 5 surface acceptor layers . In-situ annealing of the hydrogenated diamond surface at 400 °C was found to be crucial to enhance long-term doping stability. High temperature sheet resistance measurements up to 300 °C were performed to examine doping thermal stability. Exposure of MoO 3 and V 2 O 5 transfer-doped hydrogen-terminated diamond samples up to a temperature of 300 °C in ambient air showed significant and irreversible loss in surface conductivity. Thermal stability was found to improve dramatically however when similar thermal treatment was performed in vacuum or in ambient air when the oxide layers were encapsulated with a protective layer of hydrogen silsesquioxane (HSQ). Inspection of the films by X-ray diffraction revealed greater crystallisation of the MoO 3 layers following thermal treatment in ambient air compared to the V 2 O 5 films which appeared to remain amorphous. These results suggest that proper encapsulation and passivation of these oxide materials as surface acceptor layers on hydrogen-terminated diamond is essential to maximise their environmental and thermal stability.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-018-21579-4