An enhanced two-dimensional hole gas (2DHG) C–H diamond with positive surface charge model for advanced normally-off MOSFET devices
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-03-10 |
| Journal | Scientific Reports |
| Authors | Reem Alhasani, Taichi Yabe, Yutaro Iyama, Nobutaka Oi, Shoichiro Imanishi |
| Institutions | Waseda University |
| Citations | 19 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Enhanced 2DHG C-H Diamond MOSFETs
Section titled “Technical Documentation & Analysis: Enhanced 2DHG C-H Diamond MOSFETs”Executive Summary
Section titled “Executive Summary”This research successfully demonstrates a critical pathway for achieving normally-off (enhancement mode) operation in p-channel C-H diamond MOSFETs, a key requirement for safe, high-power electronic devices.
- Core Achievement: Normally-off operation ($V_{th} = -3.5$ V) achieved in a 2DHG Al2O3/SiO2/C-H diamond MOSFET by controlling the fixed positive surface charge density at the interface.
- High Performance: The device exhibited exceptional performance metrics, including a maximum drain current density ($I_{DS,MAX}$) of -305.0 mA/mm (experimental) and a high breakdown voltage ($V_{BD}$) of 1275 V.
- Material Strategy: The methodology relies on precise co-doping of the diamond substrate with low Boron (acceptor) and controlled Nitrogen (deep donor, $E_D = 1.7$ eV) concentrations to fix the Fermi level and enable inversion channel formation.
- Interface Control: The use of a thin SiO2 layer (2 nm) beneath the Al2$O$_{3}$ gate insulator effectively introduces a positive charge, shifting the threshold voltage to the required negative value for enhancement mode operation.
- Application Potential: These results confirm the viability of diamond for next-generation high-power electronics, specifically complementary power FETs, smart inverter systems, and devices requiring high thermal conductivity (22 W/cm·K).
- 6CCVD Value Proposition: 6CCVD offers the necessary highly controlled Single Crystal Diamond (SCD) substrates, precise doping, and custom metalization services required to replicate and advance this high-performance device architecture.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context | | :--- | :--- | :--- | :--- | | Maximum Drain Current Density ($I_{DS,MAX}$) | -305.0 | mA/mm | Experimental result (SiO2/diamond) | | Breakdown Voltage ($V_{BD}$) | 1275 | V | Measured at $L_{GD} = 20$ µm | | Threshold Voltage ($V_{th}$) | -3.5 | V | Normally-off operation (Enhancement Mode) | | Transconductance ($g_m$) | 0.4 | mS/mm | Simulated (Positive Charge Model) | | Diamond Bandgap ($E_g$) | 5.5 | eV | Material Property | | Nitrogen Donor Activation Energy ($E_D$) | 1.7 | eV | Fixed Fermi level position | | Hole Mobility (Surface, $\mu_p$) | 100 | cm2/V·s | Modeling Parameter | | Substrate Nitrogen Concentration ($N_D$) | $2 \times 10^{16}$ | cm-3 | Bulk Doping (Deep Donor) | | Substrate Boron Concentration ($N_A$) | $2 \times 10^{15}$ | cm-3 | Bulk Doping (Acceptor) | | Positive Interface Charge Density ($Q_f$) | $1 \times 10^{11}$ | cm-2 | Simulation for Normally-Off | | Electron Affinity ($E_A$) | -1.3 | eV | H-terminated diamond surface | | Gate Dielectric Thickness (Al2O3) | 200 | nm | Passivation Oxide | | Gate Dielectric Thickness (SiO2) | 2 | nm | Interface Charge Control Layer |
Key Methodologies
Section titled “Key Methodologies”The experimental and simulation work focused on controlling the diamond’s bulk properties and the interface charge to achieve the desired enhancement mode operation.
- Substrate Doping and Orientation:
- Used Single Crystal Diamond (001) substrate.
- Co-doped the bulk diamond with a low concentration of Boron ($2 \times 10^{15}$ cm-3) and a higher concentration of Nitrogen ($2 \times 10^{16}$ cm-3).
- Nitrogen acts as a deep donor ($E_D = 1.7$ eV), pinning the Fermi level close to the conduction band minimum to facilitate inversion.
- Surface Termination:
- Applied Hydrogen termination (C-H) to the diamond surface to induce surface p-type conduction (2DHG) and negative electron affinity.
- Gate Stack Engineering:
- A thin SiO2 layer (2 nm) was placed between the C-H diamond surface and the ALD-Al2O3 (200 nm) gate insulator.
- The SiO2 layer acts as a source of fixed positive charge, which cancels the inherent negative charge of the C-H surface, shifting $V_{th}$ to a negative value (normally-off).
- Contact Formation:
- Source and Drain contacts utilized Ti/Au metalization, relying on TiC formation for Ohmic properties.
- Device Modeling:
- Simulations were performed using the Silvaco Atlas TCAD 2D drift-diffusion model, incorporating the incomplete ionization of impurities model for the freeze-out region and various fixed interface charge sheet models (positive, negative, neutral).
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and customization services required to replicate and scale this high-performance normally-off MOSFET technology.
Applicable Materials
Section titled “Applicable Materials”To achieve the precise electronic properties demonstrated in this research, high-quality, controlled doping is essential.
| Research Requirement | 6CCVD Solution | Technical Advantage |
|---|---|---|
| Substrate Material | High-Purity Single Crystal Diamond (SCD) | SCD (001) orientation available, ensuring high crystalline quality necessary for high carrier mobility (4500 cm2/V·s for electrons, 3800 cm2/V·s for holes). |
| Doping Profile | Custom N/B Co-Doped SCD | We offer precise, controlled MPCVD doping to achieve the required $N_D$ ($2 \times 10^{16}$ cm-3) and $N_A$ ($2 \times 10^{15}$ cm-3) concentrations, critical for Fermi level pinning at $E_D = 1.7$ eV and enabling the normally-off inversion channel. |
| Surface Quality | Optical Grade Polishing (Ra < 1 nm) | Essential for subsequent C-H termination and gate dielectric deposition (Al2O3/SiO2) to minimize interface defects and maximize 2DHG stability. |
| Thickness/Dimensions | Custom Substrates up to 10 mm | While the paper used a 4 µm layer, 6CCVD can supply robust SCD substrates up to 500 µm thick, or bulk substrates up to 10 mm, enabling higher mechanical stability and superior thermal management (22 W/cm·K). |
Customization Potential
Section titled “Customization Potential”The fabrication of these advanced FETs relies on specific dimensions and metal contacts, all of which 6CCVD can provide in-house.
- Custom Metalization: The device utilized Ti/Au contacts for the source/drain. 6CCVD offers internal metalization capabilities, including Ti, Au, Pt, Pd, W, and Cu, allowing researchers to optimize Ohmic contact formation and reduce source resistance, which the paper notes is key to improving current saturation behavior.
- Large Area Devices: 6CCVD specializes in large-area diamond growth. We can provide Polycrystalline Diamond (PCD) plates up to 125 mm in diameter, suitable for scaling up high-power devices or for applications where the high $V_{BD}$ (1275 V) is critical across a large area.
- Precision Processing: We offer laser cutting and shaping services to meet exact device geometry requirements, such as the specific gate length ($L_G = 4$ µm) and gate width ($W_G = 25$ µm) used in the modeling.
Engineering Support
Section titled “Engineering Support”The successful replication of this normally-off operation requires deep expertise in diamond surface physics and doping control.
- Application Focus: 6CCVD’s in-house PhD team can assist with material selection and specification for similar p-channel diamond MOSFET, complementary power FET, and smart inverter projects.
- Interface Optimization: We provide consultation on achieving optimal surface termination (C-H) and selecting appropriate gate dielectric stacks (e.g., Al2O3/SiO2) to control the fixed interface charge density ($Q_f$) and ensure reliable enhancement mode operation.
- Global Supply Chain: We offer global shipping (DDU default, DDP available) to ensure rapid delivery of custom diamond materials worldwide, supporting time-sensitive research and development cycles.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.