H-Terminated Diamond MOSFETs on High-Quality Diamond Film Grown by MPCVD

At a Glance

Metadata	Details
Publication Date	2023-08-08
Journal	Crystals
Authors	Wenxiao Hu, Xinxin Yu, Tao Tao, Kai Chen, Yucong Ye
Institutions	Nanjing University, Institute of Electronics
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research details the successful fabrication of high-performance H-terminated diamond MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) utilizing a high-quality, atomically flat epitaxial diamond layer grown by Microwave Plasma Chemical Vapor Deposition (MPCVD).

Enhanced Performance: The MOSFET fabricated on the thick epitaxial layer achieved a saturation current density of ~200 mA/mm, representing a 54% improvement over the reference device built on a polished substrate (~130 mA/mm).
Reduced Defects: The pre-etching and high-rate epitaxy process effectively recovered polishing damage and etching pits, leading to a significant reduction in surface defects and device traps.
Atomic Flatness: The root mean square (RMS) surface roughness was drastically reduced from 0.92 nm (polished substrate) to 0.18 nm (thick epitaxial layer, 7 µm).
High-Rate, High-Purity Growth: A high growth rate of ~7 µm/h was achieved without intentional nitrogen doping, confirmed by the absence of N-V related peaks in Photoluminescence (PL) spectra, ensuring high crystal purity.
Improved Reliability: Devices on the epitaxial layer showed significantly reduced hysteresis in the transfer curve and minimized threshold voltage shift, indicating fewer carrier traps associated with substrate defects.
Crystal Quality Improvement: The epitaxial process improved the crystal quality, evidenced by a reduction in the Raman FWHM from 3.11 cm^-1 (substrate) to 2.58 cm^-1 and the XRD FWHM from 0.017° to 0.015°.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Orientation	(100)	-	CVD Diamond
Substrate Disorientation Angle	3	°	-
Epitaxial Layer Thickness (Sample 3)	~7	µm	Thick layer, 1 h growth
Growth Rate	~7	µm/h	High-purity growth, no N-doping
RMS Roughness (Polished Substrate)	0.92	nm	5 µm x 5 µm area (Reference)
RMS Roughness (Thick Epitaxy)	0.18	nm	5 µm x 5 µm area (Best result)
Saturation Current Density (Reference)	~130	mA/mm	Polished substrate (Sample 1)
Saturation Current Density (Best Device)	~200	mA/mm	Thick epitaxial layer (Sample 3)
Performance Improvement	54	%	Sample 3 vs. Sample 1
On-Resistance (R_on) (Reference)	161	Ω·mm	Polished substrate (Sample 1)
On-Resistance (R_on) (Best Device)	95	Ω·mm	Thick epitaxial layer (Sample 3)
Raman FWHM (Substrate)	3.11	cm^-1	Crystal quality metric
Raman FWHM (Epitaxy)	2.58	cm^-1	Improved crystal quality
XRD FWHM (Epitaxy)	0.015	°	Improved crystal quality
Gate Dielectric Thickness	50	nm	Al₂O₃ deposited by ALD
Ohmic Contact Metal	50	nm	Au (EB evaporation)

Key Methodologies

The fabrication process focused on optimizing the diamond surface quality through a precise pre-etching and high-rate MPCVD epitaxy sequence before standard MOSFET processing.

Substrate Preparation:
- Cleaning: Substrates were cleaned in 60° aqua regia for 40 min, followed by ultrasonic cleaning in acetone, alcohol, and deionized water.
- Pre-etching (Defect Removal): The reactor chamber was pumped to 2 x 10^-5 torr to minimize residual nitrogen. Substrates underwent H-plasma pre-etching to remove CMP damage and impurities.
  - Parameters: Microwave Power: 3200 W; Pressure: 250 torr; Duration: 30 min.
  - Mechanism: Etching pits formed at defect sites were later refilled during high-speed epitaxy.
Epitaxial Growth (MPCVD):
- A high-quality epitaxial diamond layer (up to 7 µm thick) was grown.
- Parameters: Microwave Power: 3600 W; Pressure: 300 torr; Gas Ratio: CH₄/H₂ = 2%; Duration: 1 h.
- Result: High growth rate (~7 µm/h) achieved without nitrogen doping, resulting in atomically flat surface morphology (RMS 0.18 nm).
H-Termination and 2DHG Formation:
- The epitaxial layer underwent a fast H-plasma treatment to form the H-terminated surface necessary for 2DHG (Two-Dimensional Hole Gas) formation.
- Parameters: Microwave Power: 2600 W; Pressure: 150 torr.
- 2DHG: Formed by subsequent exposure to air atmosphere for one day.
Device Fabrication:
- Ohmic Contacts: 50 nm Au was deposited via EB evaporation, patterned using photoresist, and unmasked Au was removed using potassium iodide (KI) solution.
- Device Isolation: Exposed diamond areas were treated with oxygen plasma for 5 min to achieve electrical isolation.
- Gate Stack: A 50 nm Al₂O₃ gate dielectric was deposited using Atomic Layer Deposition (ALD).

Commercial Applications

The successful demonstration of high-performance, low-defect diamond MOSFETs grown at a high rate addresses key manufacturing challenges, making this technology suitable for demanding electronic applications.

High Power RF Electronics: Diamond’s superior thermal conductivity and high critical breakdown field make these MOSFETs ideal for high-frequency, high-power amplifiers (PAs) and switches used in 5G/6G infrastructure and radar systems.
Power Conversion Systems: Suitable for high-efficiency power electronics (e.g., inverters, converters) where low R_on and high operating temperature capability are critical, such as in electric vehicles and smart grids.
Extreme Environment Electronics: Diamond devices maintain performance at high temperatures and in high-radiation environments, enabling use in aerospace, deep-well drilling, and nuclear facilities.
High-Frequency Devices: The enhanced current density and reduced defects support the development of high-speed logic and microwave devices operating in the GHz range.
Advanced Substrate Manufacturing: The optimized MPCVD recipe provides a pathway for producing high-quality, large-area diamond substrates necessary for commercial semiconductor manufacturing.

View Original Abstract

Diamond-based transistors have been considered as one of the best choices due to the numerous advantages of diamond. However, difficulty in the growth and fabrication of diamond needs to be addressed. In this paper, high quality diamond film with an atomically flat surface was grown by microwave plasma chemical vapor deposition. High growth rate, as much as 7 μm/h, has been acquired without nitrogen doping, and the root mean square (RMS) of the surface roughness was reduced from 0.92 nm to 0.18 nm by using a pre-etched process. H-terminated diamond MOSFETs were fabricated on a high-quality epitaxial diamond layer, of which the saturated current density was enhanced. The hysteresis of the transfer curve and the shift of the threshold voltage were significantly reduced as well.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst13081221

References

2002 - High Carrier Mobility in Single-Crystal Plasma-Deposited Diamond [Crossref]
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2020 - 1 W/mm Output Power Density for H-Terminated Diamond MOSFETs With Al2O3/SiO2 Bi-Layer Passivation at 2 GHz [Crossref]
2021 - 1.26 W/mm Output Power Density at 10 GHz for Si3N4 Passivated H-Terminated Diamond MOSFETs [Crossref]
2002 - Some aspects of diamond crystal growth at high temperature and high pressure by TEM and SEM [Crossref]
2002 - Device-grade homoepitaxial diamond film growth [Crossref]
2008 - Bond-Counting Rule for Carbon and its Application to the Roughness of Diamond (001) [Crossref]
2020 - Inversion channel MOSFET on heteroepitaxially grown free-standing diamond [Crossref]
2006 - Diamond FET using high-quality polycrystalline diamond with f/sub T/of 45 GHz and f/sub max/of 120 GHz [Crossref]
2018 - A High Frequency Hydrogen-Terminated Diamond MISFET With fT/fmax of 70/80 GHz [Crossref]