Fluorine-Terminated Polycrystalline Diamond Solution-Gate Field-Effect Transistor Sensor with Smaller Amount of Unexpectedly Generated Fluorocarbon Film Fabricated by Fluorine Gas Treatment

At a Glance

Metadata	Details
Publication Date	2022-04-19
Journal	Materials
Authors	Yukihiro Shintani, Hiroshi Kawarada
Institutions	Waseda University, Chiba Institute of Technology
Citations	3
Analysis	Full AI Review Included

Fluorine-Terminated Polycrystalline Diamond SGFET Analysis

Executive Summary

This research introduces a superior method for fabricating fluorine-terminated (C-F) polycrystalline diamond solution-gate field-effect transistor (SGFET) sensors by utilizing direct fluorine gas (F₂) treatment, addressing critical issues associated with conventional plasma methods.

Core Innovation: Direct F₂ gas treatment replaces conventional Inductively Coupled Plasma (ICP) fluorination using fluorocarbon gases (e.g., C₃F₈).
Contaminant Reduction: Analytical results (XPS and TOF-SIMS) confirm that the F₂ gas method eliminates or significantly reduces the formation of unwanted fluorohydrocarbon (C_xF_y) film on the diamond surface.
Improved Device Quality: The C-F BDD SGFET fabricated via F₂ treatment exhibits nearly ideal drain-source current (I_ds) vs. drain-source voltage (V_ds) characteristics, closely matching metal-oxide-silicon semiconductor FET (MOSFET) theory.
Surface State Confirmation: TOF-SIMS depth profiling showed that C_xF_y fragments (like CF₃⁺) were undetectable or constant away from the surface in F₂-treated samples, confirming a clean termination layer.
Electrical Performance: Partial fluorine termination resulted in a negative threshold voltage (V_th) shift of 0.8 V compared to C-H diamond, while maintaining ideal SGFET operation.
Process Advantage: The F₂ gas treatment effectively fabricates C-F diamond sensors without the unexpected semiconductor damage often caused by ICP processes.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Substrate	Highly (110)-oriented	Polycrystalline	Boron-Doped Diamond (BDD)
F₂ Treatment Pressure	ca. 101	kPa	Direct F₂ gas reaction
F₂ Treatment Time	30 min to 24	h	Direct F₂ gas reaction
ICP Treatment Pressure	3	Pa	Conventional C₃F₈-ICP method
ICP Gas Flow (C₃F₈)	20	sccm	Conventional C₃F₈-ICP method
ICP Power	100-500	W	Conventional C₃F₈-ICP method
I_ds (C-H, V_gs=0V)	-22	µA/mm	V_ds = -1.0 V
I_ds (C-F, V_gs=0V)	-11	µA/mm	F₂-treated C-F diamond; 50% reduction vs C-H
I_ds (C-H, V_gs=-1.0V)	-78	µA/mm	V_ds = -1.0 V
I_ds (C-F, V_gs=-1.0V)	-24	µA/mm	F₂-treated C-F diamond; 30% reduction vs C-H
Threshold Voltage (V_th) Shift	0.8	V	Negative shift due to partial F-termination
XPS C-F₃ Binding Energy	292.8	eV	F₂-treated surface coverage: 2.2%
XPS C-C(sp³) Coverage	62.4	%	F₂-treated surface
Electrolyte Solution	1 mM PBS	pH 7.4	Phosphate-Buffered Saline buffer
Measurement Temperature	ca. 20	°C	Room temperature

Key Methodologies

The fabrication process focuses on achieving a clean, fluorine-terminated diamond surface using a novel direct gas reaction method, followed by SGFET characterization.

Substrate Preparation: Highly (110)-oriented polycrystalline BDD substrates were cleaned using ultrapure water, ethanol, acetone, and isopropyl alcohol.
Hydrogen Termination (C-H): A nearly full C-H surface was achieved using an “ASTeX-type” microwave plasma chemical vapor deposition system.
Direct Fluorine Gas (F₂) Treatment (Novel Method):
- C-H diamond substrates were placed in a lab-made Nickel (Ni) reactor.
- The reactor was purged with Argon (Ar) gas, followed by decompression.
- F₂ gas, generated by the electrolysis of potassium diacid fluoride (KF₂HF) melted at approximately 100 °C, was introduced.
- Reaction conditions were maintained at ca. 101 kPa pressure for 30 minutes to 24 hours.
- Post-treatment involved cooling the reactor and purging the F₂ gas with Ar.
Conventional Comparison (C₃F₈-ICP Treatment): For comparison, some samples were treated using ICP with perfluoropropane (C₃F₈) gas at 3 Pa pressure, 20 sccm flow, and 100-500 W power for up to 30 seconds.
Surface Analysis: XPS was used to quantify chemical bonds (C-F, C-F₂, C-F₃, C-CF) and coverage. TOF-SIMS was used for depth profiling to confirm the absence of C_xF_y fluorohydrocarbon films.
SGFET Characterization: BDD SGFETs were immersed in 1 mM PBS buffer (pH 7.4). Electrical characteristics (I_ds-V_ds) were measured in common-source mode using an Ag/AgCl reference gate electrode, varying V_gs from 0 V to -1.0 V.

Commercial Applications

The clean, damage-free fluorination technique and the resulting high-performance C-F diamond SGFETs are highly relevant for several advanced sensing and material science applications.

Chemical and Biosensing:
- Development of highly stable, low-drift solution-gate FET sensors for monitoring ions, chemicals, and biological molecules in aqueous environments.
- Sensors requiring high chemical inertness and hydrophobicity at the sensing interface.
Electrochemical Devices:
- Fabrication of diamond electrodes with increased overpotential for hydrogen evolution reactions, improving sensor selectivity and operational window in electrolyte solutions.
pH-Insensitive Reference Electrodes:
- C-F terminated diamond surfaces exhibit reduced pH sensitivity, making them ideal for use as stable reference electrodes in complex chemical sensing systems.
Biocompatible Materials:
- C-F diamond’s chemical and biochemical inertness makes it suitable for implantable medical sensors and bio-interfaces where surface stability is critical.
Advanced Surface Engineering:
- Providing a clean, non-damaging method for diamond surface functionalization for applications requiring low coefficient of friction or protective, hydrophobic coatings.

View Original Abstract

In this study, a partially fluorine-terminated solution-gate field-effect transistor sensor with a smaller amount of unexpectedly generated fluorohydrocarbon film on a polycrystalline diamond channel is described. A conventional method utilizing inductively coupled plasma with fluorocarbon gas leads the hydrogen-terminated diamond to transfer to a partially fluorine-terminated diamond (C-F diamond); an unexpected fluorohydrocarbon film is formed on the surface of the diamond. To overcome this issue, we newly applied fluorine gas for the fluoridation of the diamond. Analytical results of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry suggest that the fluorocarbon film does not exist or only a smaller amount of fluorocarbon film exists on the diamond surface. Conversely, the C-F diamond fabricated by the conventional method of inductively coupled plasma with a perfluoropropane gas (C3F8 gas) source possesses a certain amount of fluorocarbon film on its surface. The C-F diamond with a smaller amount of unexpectedly generated fluorohydrocarbon film possesses nearly ideal drain-source-voltage vs. gate-source-current characteristics, corresponding to metal-oxide-silicon semiconductor field-effect transistor theory. The results indicate that the fluorine gas (F2 gas) treatment proposed in this study effectively fabricates a C-F diamond sensor without unexpected semiconductor damage.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15092966

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