Microscopic Evaluation of Al<sub>2</sub>O<sub>3</sub>/p-Type Diamond (111) Interfaces Using Scanning Nonlinear Dielectric Microscopy

At a Glance

Metadata	Details
Publication Date	2022-05-31
Journal	Materials science forum
Authors	Yu Ogata, Kohei Yamasue, Xufang Zhang, Tsubasa Matsumoto, Norio Tokuda
Institutions	Tohoku University, Kanazawa University
Citations	3
Analysis	Full AI Review Included

Microscopic Evaluation of Al₂O₃/p-Type Diamond (111) Interfaces Using Scanning Nonlinear Dielectric Microscopy

Executive Summary

Core Problem Addressed: Low channel mobility in diamond inversion channel MOSFETs, primarily attributed to high interface defect density (D_it) at the Al₂O₃/diamond (111) interface.
Methodology: Scanning Nonlinear Dielectric Microscopy (SNDM) was employed, utilizing conventional dC/dV imaging, local Capacitance-Voltage (CV) profiling, and time-resolved local Deep Level Transient Spectroscopy (DLTS) for nanoscale analysis.
D_it Ranking by Termination: Local CV profiling established a clear ranking of interface quality: D_it was lowest for Al₂O₃/H-diamond (111), followed by Al₂O₃/OH-diamond, and highest for Al₂O₃/O-diamond.
Spatial Non-Uniformity: Significant non-uniform spatial fluctuations in interface properties (dC/dV maps) were observed across all samples, particularly the Al₂O₃/OH-diamond interface.
Impact of Surface Flatness: The flatness of the diamond surface strongly affects D_it distribution: atomically flat regions showed line-shaped D_it features (potentially related to atomic steps), while rough areas exhibited clustered high D_it regions.
Conclusion: The flattening process of the diamond surface is crucial for controlling the distribution and size of interface defects, which is necessary for improving MOSFET performance.

Technical Specifications

Parameter	Value	Unit	Context
Insulating Layer Material	Al₂O₃	N/A	Dielectric formed by ALD
Insulating Layer Thickness	50	nm	ALD Al₂O₃ thickness
Substrate Material/Orientation	p-type diamond	(111)	HPHT synthesized
Substrate Boron Concentration	10¹⁷	cm^-3	p-type doping level
Ohmic Contact Layer Thickness	200	nm	High B concentration p⁺ CVD diamond
Gold Electrode Thickness	200	nm	Vacuum deposited layer
SNDM Tip Radius	2	µm	Pt-Ir coated conductive cantilever
H-diamond Max CV Slope	68	aF/V	Measured at -24 V (indicates lowest D_it)
OH-diamond Max CV Slope	51	aF/V	Measured at 21 V (intermediate D_it)
O-diamond Max CV Slope	10	aF/V	Measured at 11 V (indicates highest D_it)
Local DLTS D_it Average	10¹³	cm^-2eV^-1	Order of magnitude for Al₂O₃/OH-diamond
Local CV Pulse Amplitude (H-diamond)	55	V_pp	Triangular voltage pulse amplitude
Local CV Pulse Amplitude (O/OH-diamond)	70	V_pp	Triangular voltage pulse amplitude
Local CV Sweep Rate	Tens of	kHz	High-frequency measurement regime
Local DLTS Accumulation Pulse	5	µs	Rectangular voltage pulse length

Key Methodologies

Substrate Synthesis and Doping: High-pressure high-temperature (HTHP) synthesized p-type diamond (111) substrates were used, featuring a boron concentration of 10¹⁷ cm^-3. A highly doped p⁺ CVD diamond layer (200 nm) was included for ohmic contact formation.
Surface Flattening: Anisotropic diamond etching was performed based on a carbon solid solution reaction into Ni. This process was chosen over conventional plasma etching to achieve atomically flat regions and minimize plasma-induced damage.
Surface Termination: Three distinct interface termination processes were applied to the diamond surface prior to dielectric deposition: Hydrogen (H), Oxygen (O), and Hydroxyl (OH) termination.
Dielectric Deposition: A 50 nm Al₂O₃ insulating layer was deposited using Atomic Layer Deposition (ALD).
Conventional SNDM (dC/dV Imaging): A small sinusoidal voltage was applied to the sample, and a lock-in amplifier was used to obtain the voltage derivative of capacitance (dC/dV), mapping spatial fluctuations of interface properties.
Time-Resolved SNDM (Local CV Profiling): Large amplitude triangular voltage pulses (up to 70 V_pp) were applied at high sweep rates (tens of kHz). The resulting capacitance responses were averaged (1000 times) across a 5 µm x 5 µm area to generate local CV profiles.
Time-Resolved SNDM (Local DLTS): A 5 µs rectangular accumulation voltage pulse was applied, stepping down from a reference level (-5 V) to a target level (+5 V). The resulting transient capacitance responses were averaged (1000 times) to map the microscopic distribution of D_it in 1 µm x 1 µm areas.

Commercial Applications

High-Performance Power MOSFETs: The primary application is the realization of inversion channel p-type diamond MOSFETs with improved channel mobility, crucial for achieving high power, high breakdown voltage, and normally-off operation.
High-Frequency Devices: Diamond’s inherent high carrier mobility and high thermal conductivity make these devices suitable for high-speed and high-frequency power switching applications.
Gate Stack Optimization: The findings provide actionable data for optimizing the gate dielectric/semiconductor interface (Al₂O₃/Diamond). Controlling the surface termination (H vs. OH vs. O) and ensuring surface flatness are key engineering steps to reduce D_it and enhance device efficiency.
Thermal Management Solutions: Diamond’s superior thermal properties are leveraged in power devices where heat dissipation is a major constraint, enabling operation at higher current densities and temperatures.
6ccvd.com Relevance: This research focuses on overcoming fundamental material limitations (interface defects) in diamond semiconductors, directly supporting the development and commercialization of high-quality CVD diamond substrates and advanced electronic devices.

View Original Abstract

Improvement of channel mobility is required to improve the performance of the inversion channel MOSFETs using diamond. The previous studies have suggested that high interface defect density ( D it ) at the Al 2 O 3 /diamond (111) interface has a significant impact on the carrier transport property on a channel region. To investigate the physical origins of the high D it , especially from microscopic point of view, here we investigate Al 2 O 3 /p-type diamond (111) interfaces using scanning nonlinear dielectric microscopy (SNDM). We find the high spatial fluctuations of Al 2 O 3 /hydroxyl (OH)-terminated diamond (111) interface properties and their difference by the flatness of the diamond surface.

Tech Support

Original Source

DOI: https://doi.org/10.4028/p-n0z51t

References

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