Transport Properties of the Two-Dimensional Hole Gas for H-Terminated Diamond with an Al2O3 Passivation Layer

At a Glance

Metadata	Details
Publication Date	2022-03-14
Journal	Crystals
Authors	Cui Yu, Chuangjie Zhou, Jianchao Guo, Zezhao He, Mengyu Ma
Institutions	Hebei Semiconductor Research Institute, Xi’an Jiaotong University
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: H-Terminated Diamond 2DHG Transport

Executive Summary

This analysis focuses on the transport properties of the Two-Dimensional Hole Gas (2DHG) in H-terminated diamond, a critical structure for next-generation high-power and high-frequency Field Effect Transistors (FETs).

Core Finding: Carrier mobility in H-terminated diamond 2DHG is primarily limited by two mechanisms: Ionic Impurity (IM) scattering and Surface Roughness (SR) scattering across the 90 K to 300 K operating range.
Material Performance: Room temperature carrier mobility reached up to 152 cm²/V·s before passivation, with sheet densities (N_s) ranging from 4.7 x 10¹² to 14.3 x 10¹² cm^-2.
Passivation Impact: High-temperature Atomic Layer Deposition (ALD) of Al₂O₃ (300 °C to 450 °C) was found to enhance IM scattering, resulting in a significant decrease in carrier mobility (down to 77.9 cm²/V·s).
Material Requirement: Achieving high-performance H-diamond FETs necessitates ultra-high purity Single Crystal Diamond (SCD) substrates with exceptional surface smoothness to mitigate both IM and SR scattering.
6CCVD Value Proposition: 6CCVD provides the necessary high-purity, precision-polished MPCVD SCD and PCD substrates, optimized to minimize intrinsic defects and surface scattering, thereby maximizing 2DHG mobility.

Technical Specifications

The following hard data points were extracted from the analysis of H-terminated diamond samples (SCD and PCD) with and without Al₂O₃ passivation.

Parameter	Value	Unit	Context
Maximum Carrier Mobility (μ)	152	cm²/V·s	Room Temp, Before Al₂O₃ Deposition (Sample A)
Minimum Carrier Mobility (μ)	77.9	cm²/V·s	Room Temp, After Al₂O₃ Deposition (Sample B, 400 °C ALD)
Sheet Density (N_s) Range	4.7 to 14.3	10¹² /cm²	Room Temp, Before Al₂O₃ Deposition
Sheet Resistance (R_s) Range	5380 to 9271	Ω/sq	Room Temp, Before Al₂O₃ Deposition
ALD Deposition Temperature Range	300, 400, 450	°C	High-temperature Al₂O₃ passivation
MPCVD Growth Temperature	900	°C	Homoepitaxial SCD growth
H₂ Plasma Treatment Temperature	800	°C	H-termination process
Dominant Scattering Mechanisms	Ionic Impurity (IM), Surface Roughness (SR)	N/A	Limiting 2DHG mobility (90 K to 300 K)
SCD Growth Pressure	100	Torr	MPCVD process
H₂ Plasma Treatment Pressure	5	kPa	MPCVD process

Key Methodologies

The experimental procedure relied heavily on high-quality MPCVD diamond growth and precise surface treatments.

Diamond Growth: Samples were prepared using Microwave Plasma CVD (MPCVD) technique.
- SC-Epitaxial Samples: Homoepitaxial growth at 900 °C, 100 Torr chamber pressure, 1 kW microwave power, and a CH₄/H₂ ratio of 1% (500 sccm total flow).
- PC-H Samples: Polycrystalline diamond with a grain size of approximately 100 µm.
H-Termination: All samples underwent H₂ plasma treatment in the MPCVD system.
- Treatment parameters: 800 °C, 5 kPa chamber pressure, 40 minutes duration.
Passivation Layer Deposition: Al₂O₃ films were deposited via Atomic Layer Deposition (ALD).
- Deposition temperatures varied: 300 °C, 400 °C, and 450 °C.
Electrical Characterization: Electrical properties (sheet resistance R_s, sheet density N_s, and carrier mobility μ) were measured using the Van der Pauw-Hall method.
- Measurement range: Low temperature (90 K) to room temperature (300 K).
Scattering Analysis: Mobility temperature dependence was fitted considering four mechanisms: Ionic Impurity (IM), Acoustic Phonon (AC), Optical Phonon (OP), and Surface Roughness (SR) scattering.

6CCVD Solutions & Capabilities

The research highlights that maximizing 2DHG mobility requires minimizing both intrinsic impurities (IM scattering) and surface imperfections (SR scattering). 6CCVD’s expertise in high-purity MPCVD diamond growth and precision polishing directly addresses these critical material challenges.

Applicable Materials

To replicate and extend this high-performance H-terminated diamond research, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for the highest mobility devices (like samples A, B, C, D, E). Our SCD is grown via MPCVD with ultra-low nitrogen incorporation, minimizing the ionized impurity concentration (N_I) that limits mobility.
High-Quality Polycrystalline Diamond (PCD): Suitable for cost-effective, large-area applications. The paper noted that PC-H mobility is comparable to SC-H mobility, validating the use of high-quality PCD for certain FET designs. 6CCVD offers PCD wafers up to 125 mm diameter.

Customization Potential

6CCVD provides comprehensive material engineering services essential for developing H-diamond FETs:

Research Requirement / Challenge	6CCVD Solution & Capability	Technical Advantage
Mitigating Surface Roughness (SR)	Precision Polishing: Guaranteed Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD wafers.	Provides the atomically smooth surface necessary to minimize SR scattering, a dominant mobility limiter identified in the study.
Custom Substrate Dimensions	Custom Dimensions & Thickness: SCD/PCD plates up to 125 mm diameter. SCD/PCD thickness from 0.1 µm to 500 µm. Substrate thickness up to 10 mm.	Supports scaling from small R&D samples to larger, high-throughput device fabrication platforms.
Ohmic Contact Integration	In-House Metalization: Custom deposition of standard FET contact stacks (e.g., Ti/Pt/Au, W, Cu, Pd).	Facilitates rapid prototyping and ensures high-quality, low-resistance ohmic contacts required for H-diamond FET source and drain terminals.
Optimized H-Termination Foundation	Tailored Substrate Preparation: Substrates are delivered with specific orientations and surface preparations optimized for subsequent high-temperature H-termination (800 °C) and ALD passivation processes.	Ensures consistent starting material quality, minimizing variability in 2DHG formation and electrical properties.

Engineering Support

The paper clearly demonstrates that the electrical properties of the 2DHG are highly sensitive to processing conditions, particularly the high-temperature ALD step which enhances impurity scattering.

Scattering Mitigation Expertise: 6CCVD’s in-house PhD team specializes in diamond material science and can assist engineers in selecting the optimal SCD grade and surface finish to minimize IM and SR scattering for high-frequency and high-power diamond FET projects.
Process Integration Consultation: We offer consultation on how material specifications (purity, surface termination, defect density) impact subsequent high-temperature processing steps, helping researchers maintain high carrier mobility post-passivation.

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

View Original Abstract

Diamonds are thought to be excellent candidates of next-generation semiconductor materials for high power and high frequency devices. A two-dimensional hole gas in a hydrogen-terminated diamond shows promising properties for microwave power devices. However, high sheet resistance and low carrier mobility are still limiting factors for the performance improvement of hydrogen-terminated diamond field effect transistors. In this work, the carrier scattering mechanisms of a two-dimensional hole gas in a hydrogen-terminated diamond are studied. Surface roughness scattering and ionic impurity scattering are found to be the dominant scattering sources. Impurity scattering enhancement was found for the samples after a high-temperature Al2O3 deposition process. This work gives some insight into the carrier transport of hydrogen-terminated diamonds and should be helpful for the development of diamond field effect transistors.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst12030390

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