HfAlOx/Al2O3 Bilayer Dielectrics for a Field Effect Transistor on a Hydrogen-Terminated Diamond

At a Glance

Metadata	Details
Publication Date	2022-01-07
Journal	Materials
Authors	Minghui Zhang, Fang Lin, Wei Wang, Feng Wen, Genqiang Chen
Institutions	Xi’an Jiaotong University
Analysis	Full AI Review Included

Executive Summary

This research details the fabrication and characterization of a hydrogen-terminated (H-terminated) diamond Field Effect Transistor (FET) utilizing a high-k HfAlO_x/Al₂O₃ bilayer dielectric stack.

Dielectric Innovation: The device employs a bilayer dielectric (4 nm Al₂O₃ protective layer + 30 nm HfAlO_x high-k layer) deposited via Atomic Layer Deposition (ALD) to stabilize the 2D Hole Gas (2DHG) channel.
High Current Density: The FET achieved a maximum drain source current density (I_DSmax) of -6.3 mA/mm, which is relatively high compared to previous diamond FET reports, attributed to the protective Al₂O₃ layer maintaining the 2DHG quality.
Normally-On Operation: The device exhibits normally-on (depletion mode) characteristics with a threshold voltage (V_TH) extrapolated at 8.3 V.
Low Leakage: The gate leakage current density (|I_GS|) was measured at a low value of 7.95 x 10^-7 A/cm² at V_GS = -6 V, suggesting good dielectric integrity.
High Carrier Concentration: A large carrier density (p) of 1.50 x 10¹³ cm^-2 was achieved, confirming the high quality of the H-terminated diamond channel.
Performance Metrics: The maximum transconductance (G_m) reached 0.73 mS/mm, and the maximum capacitance (C_ox) was 0.22 µF/cm².

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Single Crystal Diamond	N/A	High Temperature High Pressure (HPHT)
Homoepitaxy Layer Thickness	200	nm	Grown by MPCVD
Gate Dielectric Stack	HfAlO_x/Al₂O₃	N/A	Bilayer structure
HfAlO_x Thickness	30	nm	Deposited by ALD
Al₂O₃ Thickness	4	nm	Deposited by ALD (Channel protection layer)
HfAlO_x Composition (Hf:Al:O)	2:23:75	Atomic %	Evaluated by EDS
Effective Dielectric Constant (k)	8.45	N/A	Calculated value for the bilayer stack
Maximum Drain Source Current Density (I_DSmax)	-6.3	mA/mm	V_GS = -6 V, V_DS = -20 V
Threshold Voltage (V_TH)	8.3	V	Normally-on operation
Maximum Transconductance (G_m)	0.73	mS/mm	Transfer characteristic
Maximum Capacitance (C_ox)	0.22	µF/cm²	Measured at 1 MHz, V_GS = -2 V
Carrier Density (p)	1.50 x 10¹³	cm^-2	Evaluated at V_GS = -2 V
Gate Leakage Current Density (	I_GS	)	7.95 x 10^-7
Hysteresis Voltage	0.9	V	C-V curve shift
Fixed Negative Charge Density	1.25 x 10¹³	cm^-2	Deduced from C_ox and V_FB
Gate Length (L_G)	4	µm	Device dimension
Gate Width (W_G)	100	µm	Device dimension
Source-Drain Gap (L_SD)	20	µm	Device dimension

Key Methodologies

The H-terminated diamond FET fabrication utilized standard semiconductor processing techniques combined with specialized diamond growth and surface treatments:

Substrate Preparation: A High Temperature High Pressure (HPHT) single crystal diamond substrate was chemically cleaned using various solutions.
Homoepitaxy Growth: A 200 nm homoepitaxy layer was grown on the substrate using Microwave Plasma Chemical Vapor Deposition (MPCVD).
Ohmic Contact Formation: 150 nm Au source and drain electrodes (L_SD = 20 µm) were defined using photolithography, electron beam (EB) evaporation, and the lift-off technique.
Isolation: The H-terminated surface outside the active channel area was isolated using a 20 min UV/ozone treatment, which removes the 2DHG.
Bilayer Dielectric Deposition (ALD):
- A 4 nm Al₂O₃ film was deposited first to protect the H-terminated channel surface.
- A 30 nm HfAlO_x film was subsequently deposited, both using Atomic Layer Deposition (ALD).
Gate Electrode Deposition: A 150 nm Al gate electrode was deposited onto the dielectric stack (L_G = 4 µm, W_G = 100 µm).
Characterization: Electrical properties (I-V, C-V) were measured using an Agilent B1505A semiconductor analyzer.

Commercial Applications

Diamond FETs leveraging high-k dielectrics like HfAlO_x/Al₂O₃ are critical components for next-generation electronics requiring operation under extreme conditions, capitalizing on diamond’s superior material properties (5.47 eV bandgap, 10 MV/cm breakdown field, 22 W/cm·K thermal conductivity).

High Power RF Electronics: Diamond FETs are ideal for high-frequency, high-power amplifiers and switches due to high carrier mobility and large breakdown voltage, enabling compact and efficient radar and communication systems.
Extreme Environment Electronics: The high thermal conductivity and wide bandgap allow these devices to operate reliably in high-temperature environments (e.g., automotive, aerospace, geothermal drilling) where silicon or GaAs devices fail.
Power Switching Devices: Used in high-voltage power converters and inverters, where the high breakdown field of diamond minimizes switching losses and improves system efficiency.
Wide Bandgap Semiconductor Technology: This research contributes directly to advancing the performance and stability of diamond-based Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) for commercialization.
Radiation Hardened Systems: Diamond’s inherent stability makes these FETs suitable for use in nuclear, space, and military applications requiring resistance to ionizing radiation.

View Original Abstract

In this work, a hydrogen-terminated (H-terminated) diamond field effect transistor (FET) with HfAlOx/Al2O3 bilayer dielectrics is fabricated and characterized. The HfAlOx/Al2O3 bilayer dielectrics are deposited by the atomic layer deposition (ALD) technique, which can protect the H-terminated diamond two-dimensional hole gas (2DHG) channel. The device demonstrates normally-on characteristics, whose threshold voltage (VTH) is 8.3 V. The maximum drain source current density (IDSmax), transconductance (Gm), capacitance (COX) and carrier density (ρ) are −6.3 mA/mm, 0.73 mS/mm, 0.22 μF/cm2 and 1.53 × 1013 cm−2, respectively.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15020446

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