Diamond Metal-Oxide-Semiconductor Field-Effect Transistors on a Large-Area Wafer

At a Glance

Metadata	Details
Publication Date	2023-07-21
Authors	Jiangwei Liu, Hirotaka Ohsato, Bo Da, Yasuo Koide
Institutions	National Institute for Materials Science
Analysis	Full AI Review Included

Diamond Metal-Oxide-Semiconductor Field-effect Transistors on a Large-area Wafer: Engineering Analysis

Executive Summary

This study successfully demonstrates the fabrication and electrical characterization of hydrogen-terminated (H-diamond) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) on large-area diamond wafers (8 x 8 mm²), a critical step toward commercial viability.

Large-Area Fabrication: MOSFETs were fabricated on 8 x 8 mm² Ib-type diamond substrates, significantly larger than the typical 3 x 3 mm² research size, enabling the integration of 720 devices per wafer.
T-type Performance Superiority: The T-type MOSFET geometry demonstrated superior performance compared to the planar-type, achieving a maximum drain current (I_D,max) of -3.0 mA/mm and an on-resistance (R_ON) of 3.0 x 10³ Ω mm (at L_G = 3.0 µm).
R_ON Reduction: The T-type structure achieved an R_ON approximately four times lower than the planar-type (1.2 x 10⁴ Ω mm) by eliminating the highly resistive H-diamond surface interspaces between the source/drain and gate electrodes.
Gate Stack Engineering: A high-k Al₂O₃/HfSiO₂ bilayer was employed as the gate insulator. Atomic Layer Deposition (ALD) Al₂O₃ (4.0 nm) served as a buffer layer to protect the sensitive H-diamond surface during subsequent Sputtering Deposition (SD) of HfSiO₂.
P-type Characteristics: Both device types exhibited clear p-type characteristics and distinct pinch-off behavior, confirming effective modulation of the two-dimensional hole gas (2DHG) channel.

Technical Specifications

Parameter	Value	Unit	Context
Wafer Size	8 x 8	mm²	Ib-type diamond (100) substrate
Substrate Thickness	0.3	mm	Ib-type diamond (100) substrate
Epitaxial Layer Thickness	~150	nm	H-diamond homoepitaxial layer
H₂ Flow Rate (CVD)	500	sccm	Microwave plasma-enhanced CVD
CH₄ Flow Rate (CVD)	0.5	sccm	Microwave plasma-enhanced CVD
Deposition Temperature	900-940	°C	H-diamond epitaxial growth
Deposition Pressure	80	Torr	H-diamond epitaxial growth
Gate Width (W_G)	100	µm	Standardized width for all 720 devices
Gate Length (L_G) Range	3.0 to 31.0	µm	Tested range for planar MOSFETs
Planar S/D-Gate Interspace	3.5	µm	Distance between contacts (Planar type)
T-type S/D-Gate Interspace	0	µm	Distance between contacts (T-type)
ALD-Al₂O₃ Thickness	4.0	nm	Buffer layer contacting H-diamond
T-type I_D,max (L_G=3.0 µm)	-3.0	mA/mm	Measured at V_GS = -20.0 V
Planar I_D,max (L_G=3.0 µm)	-0.8	mA/mm	Measured at V_GS = -20.0 V
T-type R_ON (L_G=3.0 µm)	3.0 x 10³	Ω mm	On-resistance at V_GS = -20.0 V
Planar R_ON (L_G=3.0 µm)	1.2 x 10⁴	Ω mm	On-resistance at V_GS = -20.0 V

Key Methodologies

The H-diamond MOSFETs were fabricated using a multi-step process involving CVD growth, plasma etching, and electron-gun evaporation for contacts.

Substrate Cleaning: Ib-type diamond (100) substrate (8 x 8 mm²) was cleaned in H₂SO₄+HNO₃ acid solution at 300 °C for 3 hours.
Epitaxial Growth: H-diamond homoepitaxial layer (~150 nm thick) was grown using microwave plasma-enhanced CVD at 900-940 °C and 80 Torr, utilizing H₂ (500 sccm) and CH₄ (0.5 sccm) for 1.5 hours.
Mesa Structure Formation: The H-diamond was etched in an O₂ ambient using a capacitively-coupled plasma Reactive Ion Etching (RIE) system (100 sccm O₂, 10 Pa, 50 W) for 90 seconds.
Ohmic Contact Formation: Source/drain electrodes (Pd/Ti/Au, 10/20/200 nm) were formed via electron-gun evaporation.
Gate Oxide Deposition:
- A 4.0 nm Al₂O₃ buffer layer was deposited directly onto the H-diamond surface using Atomic Layer Deposition (ALD).
- A high dielectric constant HfSiO₂ layer was subsequently deposited via Sputtering Deposition (SD).
Gate Contact Formation: Gate electrodes (Ti/Au, 10/200 nm) were formed using electron-gun evaporation.
Characterization: Electrical properties were measured using a four-probe system at room temperature.

Commercial Applications

The development of large-area diamond MOSFETs leverages diamond’s intrinsic properties (wide band gap, high breakdown field, high carrier mobility, and large thermal conductivity) for extreme environment and high-efficiency electronics.

High-Power Electronics:
- Power switching devices (MOSFETs, IGBTs) for electric vehicles (EVs) and hybrid electric vehicles (HEVs).
- High-voltage converters and inverters for smart grids and renewable energy systems (solar/wind power).
High-Frequency/RF Applications:
- High-frequency amplifiers and switches for 5G/6G base stations, radar systems, and satellite communications, benefiting from diamond’s high carrier mobility.
Extreme Environment Operation:
- Electronics designed for high-temperature environments (e.g., aerospace, geothermal drilling) due to diamond’s wide band gap and thermal stability.
Thermal Management:
- Integration into high-density electronic modules where diamond’s superior thermal conductivity (highest known material) is essential for heat dissipation.

View Original Abstract

Diamond is promising for high-power, highfrequency, and high-temperature applications.By now, most of diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) are fabricated on small-area diamond wafers (3 × 3 mm 2 ).In order to push forward the diamond electronic devices for future practical application, it is necessary to investigate the electrical properties of them on the large-area wafers.In this study, we fabricate planar-type and T-type hydrogen-terminated diamond MOSFETs on a large-area wafer (8 × 8 mm 2 ).Electrical properties of them are investigated and discussed.

Diamond Metal-Oxide-Semiconductor Field-Effect Transistors on a Large-Area Wafer

At a Glance