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6CCVD Research

Normally-off Hydrogen-Terminated Diamond Field-Effect Transistor with SnOx Dielectric Layer Formed by Thermal Oxidation of Sn

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-07-21
JournalMaterials
AuthorsShi He, Yanfeng Wang, Genqiang Chen, Juan Wang, Qi Li
InstitutionsXi’an Jiaotong University
Citations3
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study successfully demonstrates a normally-off (enhancement-mode) hydrogen-terminated diamond Field-Effect Transistor (H-diamond FET) utilizing a tin oxide (SnOx) dielectric layer formed via a simple, low-temperature thermal oxidation process.

  • Core Achievement: Realization of a normally-off H-diamond FET with a threshold voltage (VTH) of -0.5 V, crucial for system safety and energy efficiency in power electronics.
  • Simplified Fabrication: The SnOx dielectric is formed by thermal oxidation of a 5 nm Sn film at a low temperature (100 °C) in air, avoiding high vacuum or high-temperature processes that typically degrade the 2DHG channel.
  • High Performance: The device achieved a high effective hole mobility (”eff) of 92.5 cm2V-1s-1, suggesting a good interface quality between the SnOx film and the H-diamond surface.
  • Maximum Current Density: A peak drain current density (IDMAX) of -21.9 mA/mm was measured at VGS = -5 V and VDS = -10 V.
  • Low Leakage: The gate leakage current density is exceptionally low (1.6 x 10-4 A/cm2 at -8.0 V), resulting in a high on/off ratio of approximately 108.
  • Mechanism Insight: XPS analysis confirmed partial oxidation of the Sn film. The normally-off characteristic is suspected to be caused by the unoxidized metallic Sn component, whose outermost electrons deplete the holes in the channel.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Substrate TypeIIb-type (100)N/AHigh-Pressure High-Temperature (HPHT) Diamond
Threshold Voltage (VTH)-0.50VNormally-off operation
Maximum Drain Current (IDMAX)-21.9mA/mmAt VGS = -5 V, VDS = -10 V
Effective Mobility (”eff)92.5cm2V-1s-1Calculated from RON fitting curve
Maximum Transconductance (gm)5.6mS/mmAt VGS = -4.2 V
Gate Leakage Current Density (JG)1.6 x 10-4A/cm2At VGS = -8.0 V
On/Off Ratio~108N/AMeasured at VDS = -10 V
Oxide Capacitance (Cox)0.207”F/cm2Measured at 5 MHz
Fixed Charge Density (Qf)4.5 x 1011cm-2Calculated from C-V shift
Trapped Charge Density (Qt)2.39 x 1012cm-2Calculated from C-V hysteresis (1.85 V shift)
Gate Length (LG)8”mDevice dimension
Gate Width (WG)100”mDevice dimension

Key Methodologies

Section titled “Key Methodologies”

The fabrication process involved standard semiconductor techniques combined with low-temperature thermal oxidation for the dielectric layer.

  1. Substrate Preparation: HPHT single crystal diamond (100) substrate was cleaned using mixed acid (H2SO4:HNO3:HClO4) at 250 °C for 1 h, followed by mixed alkali cleaning at 80 °C for 10 min.
  2. Epitaxial Growth and H-Termination:
    • Substrate was treated with H-plasma for 20 min (cleaning).
    • Epitaxial layer (300 nm thick) was grown via MPCVD at 850 °C and 70 Torr, using H2 (500 sccm) and CH4 (5 sccm).
    • Post-growth H-plasma treatment was performed for 10 min to ensure H-termination.
    • The sample was exposed to air for 5 h to establish the 2D hole gas (2DHG).
  3. Source/Drain (S/D) Metallization: Au (100 nm thick) was deposited via photolithography and electron beam (EB) evaporation. S/D spacing (LSD) was 20 ”m.
  4. Device Isolation: UV/ozone irradiation was used to isolate the devices, protecting the channel area with photoresist.
  5. Dielectric Formation (SnOx):
    • 5 nm of Sn was deposited using an EB evaporator.
    • The Sn film was thermally oxidized on a hot stage at 100 °C for 24 h in air, forming the SnOx dielectric layer.
  6. Gate Metallization: 120 nm Al gate electrode was deposited directly onto the SnOx layer using a self-aligned process. Gate length (LG) was 8 ”m.

Commercial Applications

Section titled “Commercial Applications”

This technology, leveraging the extreme properties of diamond and a simplified fabrication method for normally-off operation, is highly relevant for next-generation power and high-frequency electronics.

  • Radio Frequency (RF) Power Amplifiers: Diamond FETs are ideal for high-frequency applications due to diamond’s high carrier mobility and high breakdown field (>10 MV/cm).
  • High-Power Switching Devices: The normally-off characteristic (enhancement mode) is mandatory for safe operation in power electronics (e.g., inverters, converters, and smart grids).
  • Extreme Environment Electronics: Diamond’s superior thermal conductivity (22 W/Kcm) allows devices to operate reliably at high temperatures and high power densities where silicon or GaN may fail.
  • Wide Band Gap Semiconductors: This research contributes to the development of wide band gap materials for next-generation high-power electronic devices, offering performance advantages over SiC and GaN.
  • Energy Saving Systems: Enhancement-mode devices inherently reduce standby power consumption, aligning with modern energy efficiency requirements.
View Original Abstract

SnOx films were deposited on a hydrogen-terminated diamond by thermal oxidation of Sn. The X-ray photoelectron spectroscopy result implies partial oxidation of Sn film on the diamond surface. The leakage current and capacitance-voltage properties of Al/SnOx/H-diamond metal-oxide-semiconductor diodes were investigated. The maximum leakage current density value at −8.0 V is 1.6 × 10−4 A/cm2, and the maximum capacitance value is measured to be 0.207 ÎŒF/cm2. According to the C-V results, trapped charge density and fixed charge density are determined to be 2.39 × 1012 and 4.5 × 1011 cm−2, respectively. Finally, an enhancement-mode H-diamond field effect transistor was obtained with a VTH of −0.5 V. Its IDMAX is −21.9 mA/mm when VGS is −5, VDS is −10 V. The effective mobility and transconductance are 92.5 cm2V−1 s−1 and 5.6 mS/mm, respectively. We suspect that the normally-off characteristic is caused by unoxidized Sn, whose outermost electron could deplete the hole in the channel.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2022 - An enhanced two-dimensional hole gas (2DHG) C-H diamond with positive surface charge model for advanced normally-off MOSFET devices [Crossref]
  2. 2006 - Diamond FET using high-quality polycrystalline diamond with fT of 45 GHz and fmax of 120 GHz [Crossref]
  3. 2022 - Over 1 A/mm drain current density and 3.6 W/mm output power density in 2DHG diamond MOSFETs with highly doped regrown source/drain [Crossref]
  4. 2021 - 345-MW/cm2 2608-V NO₂ p-Type Doped Diamond MOSFETs with an Al₂O₃ Passivation Overlayer on Heteroepitaxial Diamond [Crossref]
  5. 2021 - Progress in semiconductor diamond photodetectors and MEMS sensors [Crossref]
  6. 2019 - Enhanced ultraviolet absorption in diamond surface via localized surface plasmon resonance in palladium nanoparticles [Crossref]
  7. 2019 - Diamond Schottky barrier diodes with floating metal rings for high breakdown voltage [Crossref]
  8. 2021 - Surface transfer doping of diamond: A review [Crossref]
  9. 1996 - Hydrogen-terminated diamond surfaces and interfaces [Crossref]

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