Normally-Off Hydrogen-Terminated Diamond Field Effect Transistor With Ferroelectric HfZrOx/Al2O3Gate Dielectrics

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	IEEE Access
Authors	Kai Su, Zeyang Ren, Yue Peng, Jinfeng Zhang, Jincheng Zhang
Institutions	Xidian University
Citations	13
Analysis	Full AI Review Included

Executive Summary

This research demonstrates the first hydrogen-terminated diamond (H-diamond) Field Effect Transistor (FET) utilizing a ferroelectric HfZrO_x/Al₂O₃ stacked gate dielectric, achieving critical normally-off operation.

Normally-Off Operation: Achieved completely normally-off behavior in the saturation region (V_DS = -15 V), a crucial requirement for fail-safe systems and simplified logic circuits, overcoming the inherent normally-on nature of 2DHG H-diamond FETs.
Negative Capacitance (NC) Characteristics: The device exhibits a minimum subthreshold slope (SS) of 58 mV/decade (reverse sweep), which is less than the theoretical limit of 60 mV/decade for MISFETs, confirming NC behavior.
Non-Volatile Memory Potential: Robust ferroelectric properties are confirmed by a wide memory window (MW) of 7.3 V to 9.2 V, suggesting suitability for high-density non-volatile memory integration.
High Performance: Demonstrated a maximum on/off current ratio of 10⁹ in the linear region and a high saturation drain current of approximately -51 mA/mm.
Low-Temperature Fabrication: The ferroelectric HfZrO_x/Al₂O₃ gate stack was grown using Atomic Layer Deposition (ALD) at a low temperature of 300 °C, eliminating the need for high-temperature post-annealing.
Polarization Control: The shift to normally-off behavior in saturation is attributed to the polarization state of the HfZrO_x changing from uniform (linear region) to strongly non-uniform along the channel due to the large V_GS and V_GD difference.

Technical Specifications

Parameter	Value	Unit	Context
Gate Dielectric Stack	HfZrO_x (16 nm) / Al₂O₃ (4 nm)	nm	Total thickness 20 nm
Gate Length (L_G)	4	µm	Fabricated MFISFET
Maximum On/Off Ratio	10⁹	Ratio	Linear region (V_DS = -0.1 V)
On/Off Ratio (Saturation)	10⁸	Ratio	Saturation region (V_DS = -15 V)
Subthreshold Slope (SS)	58	mV/decade	Minimum value (reverse sweep)
Memory Window (MW)	7.3 - 9.2	V	Linear region (V_DS = -0.1 V)
Linear Mobility (µ)	18.7	cm²V^-1s^-1	Calculated at V_DS = -0.1 V
Max Saturation Drain Current (-I_D,sat)	~51	mA/mm	At V_GS = -7.0 V, V_DS = -15 V
On-Resistance (R_on)	175.1	Ω.mm	At V_GS = -7.0 V
Remnant Polarization (2P_r)	6.62	µC/cm²	MFM capacitor, 8 V sweep
Leakage Current Density (J_GS)	< 7.1 x 10^-5	A/cm²	At V_GS = -10 V
HfZrO_x Relative Permittivity (ε_r)	> 25.6	Unitless	Calculated from C_i (0.777 µF/cm²)
Diamond Band Gap	5.5	eV	Material property
Thermal Conductivity	22	W.cm^-1.K^-1	Material property (Highest in natural materials)

Key Methodologies

The device fabrication followed a standard Metal-Ferroelectric-Insulator-Semiconductor (MFIS) FET process flow, emphasizing low-temperature Atomic Layer Deposition (ALD) for the gate stack.

Substrate Preparation: Polycrystalline diamond plates (10 mm x 10 mm, 250-µm-thick) were used.
Hydrogen Termination (H-Diamond): Samples were treated using Microwave Plasma Chemical Vapor Deposition (MPCVD) for 15 minutes to form the 2DHG channel.
- Temperature: 850 °C.
- Pressure: 100 mbar.
- Gas Flow: 700 sccm (H₂).
- Microwave Power: 2 kW.
Ohmic Contact & Protection: 100 nm Au film was deposited via electron beam evaporation to provide ohmic contact and surface protection.
Device Isolation: Exposed diamond surfaces (after Au etching) were treated with oxygen plasma to form highly resistive oxygen-terminated diamond regions.
Gate Dielectric Deposition (ALD): The stacked dielectric was sequentially deposited at 300 °C (annealing-free process).
- Al₂O₃ (4 nm): Precursor was Trimethylaluminum (TMA).
- HfZrO_x (16 nm): Precursors were Tetrakis-dimethylamido-hafnium (TDMAHf) and Tetrakis-dimethylamido-zirconium (TDMAZr).
Gate Metallization: A 100 nm Aluminum (Al) layer was deposited via electron beam evaporation and patterned using a lift-off process.
Characterization: Polarization-Voltage (P-V) and Capacitance-Voltage (C-V) tests were performed on Metal/Ferroelectric/Metal (MFM) capacitors and the MFISFET to confirm ferroelectricity and electrical performance.

Commercial Applications

The unique combination of diamond’s extreme material properties (wide bandgap, high thermal conductivity) and the ferroelectric gate stack’s functionality (normally-off, NC, non-volatile memory) targets high-performance and harsh environment electronics.

High Power Electronics: Diamond’s high breakdown field (10 MV·cm^-1) and superior thermal conductivity enable high-power switching devices (e.g., power converters, grid infrastructure).
Harsh Environment Electronics: The robustness of diamond makes these FETs suitable for applications requiring operation at high temperatures or in high-radiation environments (e.g., aerospace, downhole drilling).
Non-Volatile Memory (NVM): The wide memory window (7.3-9.2 V) and ferroelectric switching confirm potential for high-density, low-power FeFET-based NVM integrated directly into logic circuits (Memory-in-Logic).
Ultra-Low-Power Logic (NCFETs): The sub-60 mV/decade subthreshold slope (SS) achieved via Negative Capacitance (NC) operation is critical for developing next-generation, ultra-low-power circuits that minimize energy consumption per switch.
High Frequency and RF Applications: Diamond’s high carrier mobility (4500 cm²V^-1s^-1 for electrons) supports high-frequency operation, suitable for 5G/6G base stations and radar systems.

View Original Abstract

A hydrogen-terminated diamond (H-diamond) Field effect transistor (FET) with a ferroelectric HfZrOx/Al2O3 stacked gate dielectric was demonstrated for the first time. The HfZrOx(16 nm)/Al2O3(4 nm) gate dielectric was grown by atomic layer deposition (ALD) at 300 °C. The bowknot-like capacitance-voltage hysteresis and the transfer characteristic curves in clockwise hysteresis loop directly illustrated the ferroelectricity of the device. A memory window as wide as 7.3-9.2 V, the maximum on/off ratio of 109 and the subthreshold slope (SS) of about 58 mV/decade was measured for the gate voltage sweeping between 10.0 to -10.0 V in the linear region. A completely normally-off behavior was observed in the saturation region because both threshold voltages (Vth ‘s) for forward and reverse sweeping transfer characteristic curves are negative at a drain voltage of -15 V. It is ascribed to that the polarization state of the HfZrOx dielectric along the channel changes from uniform in the linear region to strongly nonuniform in the saturation region. These results hint that HfZrOx/Al2O3/H-diamond FETs provide new possibility of diamond normally-off FETs, negative capacitance FETs and non-volatile memory of high density integration.

Tech Support

Original Source

DOI: https://doi.org/10.1109/access.2020.2968742

References

2018 - Integration and electrical properties of ferroelectric Hf0.5Zr0.5O2 thin film on bulk beta-Ga2O3(-201) substrate for memory applications
2019 - Ferroelectric negative capacitance [Crossref]