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	University of Tsukuba, National Institute for Materials Science
Citations	51
Analysis	Full AI Review Included

Technical Analysis & Documentation: High-Mobility Diamond FETs

Executive Summary

This research paper analyzes the factors limiting charge-carrier mobility in high-performance hydrogen-terminated (H-terminated) diamond Field-Effect Transistors (FETs) utilizing hexagonal boron nitride (h-BN) gate dielectrics.

High Performance Baseline: Diamond FETs achieved high hole mobility exceeding 300 cm²V^-1s^-1 at room temperature, significantly higher than typical H-terminated diamond surface conductivity (< 100 cm²V^-1s^-1).
Dominant Limiting Factor: Theoretical modeling confirmed that surface charged impurity scattering is the primary mechanism limiting mobility, not acoustic phonons or surface roughness.
Impurity Density Quantification: The density of surface charged impurities (n_imp) responsible for the current mobility limit was calculated to be approximately 1.0-1.5 × 10¹² cm^-2.
Path to Ultra-High Mobility: The study provides a clear pathway to achieving mobility exceeding 1000 cm²V^-1s^-1 by reducing n_imp below ≈1 × 10¹¹ cm^-2.
Material Requirements: Achieving this ultra-high mobility requires using atomically flat CVD diamond surfaces (average roughness Δ ≈ 0.3 nm) and implementing post-growth treatments like vacuum annealing to minimize surface adsorbates.
Device Relevance: The results confirm diamond FETs are superior to p-type Si MOSFETs for developing next-generation electronic devices requiring low loss and high-speed operation.

Technical Specifications

Extracted and calculated parameters critical to diamond FET performance and material quality.

Parameter	Value	Unit	Context
Achieved Hole Mobility	> 300	cm²V^-1s^-1	H-terminated diamond FET with h-BN gate dielectric
Target Hole Mobility	> 1000	cm²V^-1s^-1	Achievable by reducing surface impurities
Intrinsic Bulk Mobility	≈ 4000	cm²V^-1s^-1	Theoretical intrinsic mobility of bulk diamond
Dominant Impurity Density (n_imp)	1.0-1.5 × 10¹²	cm^-2	Limiting current device performance
Target Impurity Density (n_imp)	≈ 1 × 10¹¹	cm^-2	Required for > 1000 cm²V^-1s^-1 mobility
Operating Temperature (T)	300	K	Room temperature calculation
Required Surface Roughness (Δ)	≈ 0.3	nm	Average roughness for consistent modeling
Correlation Length (Λ)	2	nm	Used in surface roughness scattering model
Nitrogen Donor Concentration (N_D)	0.5	ppm	Measured in HPHT IIa substrate
Boron Acceptor Concentration (N_A)	5	ppb	Measured in HPHT IIa substrate
Spin-Orbit Gap Energy (Δ^SO)	6	meV	Used in split-off hole calculations
Gate Dielectric Thickness (t_hBN)	7	nm	Used in V_GS calculation

Key Methodologies

The study relied on advanced material fabrication combined with rigorous quantum mechanical modeling to isolate scattering mechanisms.

Substrate Selection: High-purity HPHT IIa Single Crystal Diamond (SCD) substrates were used, characterized by low background doping (N_D = 0.5 ppm, N_A = 5 ppb).
Surface Termination: The diamond surface was H-terminated to induce a p-type two-dimensional hole gas (2DHG) via the transfer doping model.
Gate Dielectric: Monocrystalline hexagonal boron nitride (h-BN) was used as the gate dielectric, chosen for its low intrinsic charged impurity density compared to amorphous oxides (e.g., Al₂O₃).
Theoretical Modeling: Mobility was calculated by solving the Schrödinger and Poisson equations self-consistently to determine the distribution of carriers across the Heavy Hole (HH), Light Hole (LH), and Split-Off (SO) valence bands.
Scattering Analysis: The total scattering rate was calculated using the Mathiessen rule, accounting for four mechanisms:
- Surface charged impurity scattering (dominant).
- Background ionized impurity scattering.
- Acoustic phonon scattering.
- Surface roughness scattering.
Surface Optimization Recommendation: To achieve mobility > 1000 cm²V^-1s^-1, the paper recommends using atomically flat diamond surfaces prepared by Chemical Vapor Deposition (CVD) with low methane concentration and subsequent vacuum annealing to reduce surface adsorbates.

6CCVD Solutions & Capabilities

6CCVD specializes in providing the high-quality MPCVD diamond materials and precision engineering services necessary to replicate and exceed the performance benchmarks established in this research. Our capabilities directly address the material limitations identified (surface purity and roughness).

Research Requirement	6CCVD Solution & Capability	Technical Advantage
High-Purity Substrates (Low N_D/N_A)	Optical Grade Single Crystal Diamond (SCD)	Our MPCVD process yields SCD with ultra-low nitrogen and boron incorporation, ensuring minimal background ionized impurity scattering (impbulk) and maximizing the intrinsic mobility potential (≈4000 cm²V^-1s^-1).
Atomically Flat Surfaces (Δ ≈ 0.3 nm)	Precision Polishing Service (Ra < 1 nm)	We guarantee SCD surfaces polished to an average roughness (Ra) significantly below the 0.3 nm requirement. This minimizes surface roughness scattering (sr), critical for high carrier densities (> 4 × 10¹² cm^-2).
Custom CVD Surface Preparation	Engineering Support for H-Termination Precursors	6CCVD’s PhD team can assist researchers in developing custom CVD recipes, including low methane concentration growth, specifically designed to produce the atomically flat, high-quality surfaces required for optimal H-termination and reduced surface charged impurities (n_imp).
Custom Metal Contacts (Au/Ti)	In-House Custom Metalization (Ti, Au, Pt, Pd, W, Cu)	We offer internal deposition of multi-layer metal stacks (e.g., Ti/Au) essential for ohmic contacts and gate structures in diamond FETs, ensuring precise thickness control and high adhesion for reliable device fabrication.
Large-Area Device Scaling	Custom Dimensions up to 125mm	We provide SCD and PCD plates/wafers in custom dimensions up to 125mm, supporting the scaling and mass production feasibility studies of high-mobility diamond power electronics.

Engineering Support: 6CCVD’s in-house material scientists and engineers are experts in controlling the MPCVD growth environment and post-processing steps (including surface termination and annealing protocols) necessary to achieve the ultra-low impurity densities (n_imp < 1 × 10¹¹ cm^-2) required for > 1000 cm²V^-1s^-1 mobility in Diamond Field-Effect Transistor (FET) applications.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

References

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2018 - High-mobility diamond field effect transistor with a monocrystalline h-BN gate dielectric [Crossref]