Charge-carrier mobility in hydrogen-terminated diamond field-effect transistors

At a Glance

Metadata	Details
Publication Date	2020-05-14
Journal	Journal of Applied Physics
Authors	Yosuke Sasama, Taisuke Kageura, Katsuyoshi Komatsu, Satoshi Moriyama, JUN-ICHI INOUE
Institutions	National Institute for Materials Science, University of Tsukuba
Citations	51
Analysis	Full AI Review Included

Executive Summary

This study investigates the factors limiting the charge-carrier mobility in high-performance hydrogen-terminated diamond Field-Effect Transistors (FETs) utilizing a monocrystalline hexagonal boron nitride (h-BN) gate dielectric.

High Experimental Mobility: The fabricated diamond FETs achieved a high mobility exceeding 300 cm²V^-1s^-1 at room temperature.
Dominant Limiting Factor: Theoretical calculations, based on self-consistent Schrödinger-Poisson and Mathiessen rule modeling, identified surface charged impurity (SCI) scattering as the dominant mechanism limiting mobility.
Impurity Density: The density of SCIs (n_imp) responsible for the current mobility is estimated to be approximately (1.0-1.5) x 10¹² cm^-2.
Acoustic Phonon Immunity: Acoustic phonon scattering is not dominant, leading to a high theoretical mobility limit due to diamond’s high phonon velocity and crystal mass density.
Mobility Projection: Reducing the SCI density to approximately 1 x 10¹¹ cm^-2 is projected to increase the device mobility beyond 1000 cm²V^-1s^-1.
Technological Advantage: The resulting high mobility, significantly greater than that of p-type Si MOSFETs, is crucial for developing low-loss, high-speed electronic devices.
Mitigation Strategy: Suggested methods for reducing SCIs include vacuum annealing and using atomically flat diamond surfaces prepared by low methane concentration CVD.

Technical Specifications

Parameter	Value	Unit	Context
Experimental Mobility (h-BN FET)	> 300	cm²V^-1s^-1	Measured at Room Temperature (RT)
Target Mobility (Projected)	> 1000	cm²V^-1s^-1	Achievable by reducing n_imp
Intrinsic Bulk Diamond Mobility	≈ 4000	cm²V^-1s^-1	Theoretical maximum
Limiting SCI Density (n_imp)	(1.0 - 1.5) x 10¹²	cm^-2	Dominant scattering source in current devices
Target SCI Density	≈ 1 x 10¹¹	cm^-2	Required for > 1000 cm²V^-1s^-1 mobility
Gate Dielectric Material	Monocrystalline	h-BN	Used to achieve low charged impurity density
Gate Dielectric Thickness (t_hBN)	7	nm	Assumed thickness for V_GS calculation
Acoustic Deformation Potential (D_ac)	8	eV	Used in acoustic phonon scattering model
Crystal Mass Density (ρ)	3515	kgm^-3	Diamond material parameter
Longitudinal Acoustic Phonon Velocity (u_l)	17536	ms^-1	Diamond material parameter
Spin-Orbit Gap (ΔSO)	6	meV	Used for split-off hole calculation
Assumed Surface Roughness (Δ)	≈ 0.3	nm	Used for surface roughness scattering model
Assumed Correlation Length (Λ)	2	nm	Used for surface roughness scattering model
Substrate Donor Concentration (N_D)	0.5	ppm	Nitrogen concentration (used in N_depl calculation)
Substrate Acceptor Concentration (N_A)	5	ppb	Boron concentration (used in N_depl calculation)

Key Methodologies

The study relied on theoretical calculations combined with experimental data fitting to identify the mobility-limiting mechanism.

Self-Consistent Modeling: The electronic structure of the two-dimensional hole gas (2DHG) was determined by solving the Schrödinger and Poisson equations self-consistently for the hydrogen-terminated diamond surface.
Multi-Band Mobility Calculation: Mobility calculations were performed separately for the three valence bands: heavy hole (HH), light hole (LH), and split-off hole (SO), using effective masses derived from Luttinger parameters for the (111) surface.
Scattering Rate Determination: The total scattering rate (τ^-1) was calculated using the Mathiessen rule, summing contributions from four mechanisms:
- Surface charged impurity scattering (τ_imp^-1).
- Background ionized impurity scattering (τ_impbulk^-1).
- Acoustic phonon scattering (τ_ac^-1).
- Surface roughness scattering (τ_sr^-1).
Impurity Density Assumption: Unlike models for surface conductivity where impurity density equals carrier density (p_2D), the FET model treated the surface charged impurity density (n_imp) as a constant, independent of the gate-controlled carrier density.
Experimental Fitting: The calculated total mobility was fitted to the measured Hall mobility of the h-BN/diamond FETs by adjusting the constant n_imp value, confirming that n_imp ≈ (1.0-1.5) x 10¹² cm^-2 provided the best agreement.
Surface Roughness Parameterization: Surface roughness parameters were assumed (Δ = 0.3 nm and Λ = 2 nm), values comparable to those used for Si MOSFETs and AlGaN/GaN heterostructures.

Commercial Applications

The demonstrated high mobility and robust material properties of hydrogen-terminated diamond FETs make them highly suitable for advanced electronic applications:

Power Electronics: Enabling reduced energy loss and higher efficiency in power switching devices due to high mobility and wide bandgap.
High-Frequency Amplification: Suitable for high-output radio frequency (RF) applications requiring high-speed operation.
Extreme Environment Electronics: Diamond’s inherent properties (high thermal conductivity, wide bandgap, high breakdown field) allow operation at high temperatures (e.g., 400 °C) and high voltages (e.g., > 2000 V).
Compact High-Voltage Devices: The ability to resist high voltage allows for significant reduction in device size compared to conventional semiconductors.
Next-Generation Transistors: Provides a high-performance alternative to traditional p-type Si MOSFETs, especially where low loss and high speed are critical design requirements.

View Original Abstract

Diamond field-effect transistors (FETs) have potential applications in power electronics and high-output high-frequency amplifications. In such applications, high charge-carrier mobility is desirable for a reduced loss and high-speed operation. We recently fabricated diamond FETs with a hexagonal-boron-nitride gate dielectric and observed a high mobility above 300cm2V−1s−1. In this study, we identify the scattering mechanism that limits the mobility of our FETs through theoretical calculations. Our calculations reveal that dominant carrier scattering is caused by surface charged impurities with a density of ≈1×1012cm−2 and suggest that an increase in mobility over 1000cm2V−1s−1 is possible by reducing these impurities.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0001868

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