Suppression of High Threshold Voltage for Boron-Doped Diamond MOSFETs
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-01-30 |
| Journal | IEEE Transactions on Electron Devices |
| Authors | Jiangwei Liu, Tokuyuki Teraji, Bo Da, Yasuo Koide |
| Institutions | National Institute for Materials Science |
| Citations | 6 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research focuses on suppressing the high threshold voltage (VTH) in Boron-doped Diamond (B-diamond) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), a critical step toward realizing diamond complementary MOS (CMOS) circuits.
- Core Achievement: Successfully suppressed VTH to a minimum of 0.8 V, significantly lower than previously reported values (up to 63.2 V).
- Optimization Strategy: VTH reduction was achieved by carefully adjusting three key parameters: B-diamond epitaxial layer thickness (800 nm), boron doping concentration (1.36 x 1016 cm-3), and Al2O3 gate oxide thickness (45 nm).
- Mechanism of Reduction: The combination of a thinner channel layer, lower dopant concentration, and a large absolute flat band voltage (|VFB| = 19.5 V) contributed to the dramatic decrease in VTH.
- Device Performance: Fabricated devices demonstrated excellent switching characteristics, achieving on/off ratios greater than 106.
- Output Current: Maximum drain currents (ID,max) ranged from -2.4 to -4.3 µA/mm, indicating a trade-off between low VTH and high output current density.
- Interface Quality: The Al2O3/B-diamond heterojunction exhibited a Type II staggered band configuration with robust band offsets (VBO: 2.9 eV, CBO: 1.2 eV), resulting in low leakage current density (J) of approximately 10-8 A/cm2.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Lowest Threshold Voltage (VTH) | 0.8 ± 0.1 | V | MOSFET-III |
| Maximum Drain Current (ID,max) | -4.3 | µA/mm | Highest measured value (MOSFET-II) |
| On/Off Ratio | > 106 | N/A | All MOSFET types |
| Subthreshold Swing (SS) | 260 | mV/dec | Lowest value (MOSFET-III) |
| B-Diamond Epitaxial Thickness | 800 | nm | Channel layer |
| Boron Acceptor Concentration (NA) | 1.36 x 1016 | cm-3 | Deduced from C-2-V measurement |
| Gate Oxide Thickness (tox) | 45 | nm | Al2O3 film (ALD) |
| Gate Oxide Dielectric Constant (k) | 8.0 | N/A | Used for theoretical Cox calculation |
| Surface Roughness (RMS) | < 0.2 | nm | B-diamond channel |
| Flat Band Voltage ( | VFB | ) | 19.5 |
| Leakage Current Density (J) | ~10-8 | A/cm2 | At -19.0 V to 5.0 V |
| Valence Band Offset (VBO) | 2.9 | eV | Al2O3/B-diamond heterojunction |
| Conduction Band Offset (CBO) | 1.2 | eV | Al2O3/B-diamond heterojunction |
| Interface Trapped Charge Density (Dit) | 3.33 x 1012 | eV-1 cm-2 | Lowest value (MOSFET-III) |
Key Methodologies
Section titled “Key Methodologies”- B-Diamond Epitaxial Growth: A B-diamond epitaxial layer (800 nm thick) was grown on a well-polished Ib-type (100) diamond substrate using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD).
- Substrate Cleaning and Preparation: The substrate was acid-cleaned using H2SO4 + HNO3 at 300 °C for 3 hours.
- Channel Surface Termination: The initial hydrogen-terminated surface was modified to an oxygen-terminated surface using a mixture acid solution (H2SO4 + HNO3) to form the stable B-diamond channel.
- Source/Drain Ohmic Contact Formation: Ti/Au (10/150 nm) electrodes were deposited via high-vacuum evaporation, followed by lift-off. Ohmic contact was achieved by Rapid Thermal Annealing (RTA) at 550 °C for 20 minutes.
- Gate Oxide Deposition: A 45 nm-thick Al2O3 film was deposited using Atomic Layer Deposition (ALD) at 200 °C, utilizing Al(CH3)3 and water vapor precursors.
- Gate Electrode Formation: Ti/Au (10/150 nm) gate electrodes were subsequently formed.
- Contact Window Etching: The Al2O3 layer covering the source/drain contacts was etched using Capacitively Coupled Plasma Reactive-Ion Etching (RIE). Process parameters included 100 W plasma power, 10 sccm CHF3 flow, and 40 sccm Ar flow.
Commercial Applications
Section titled “Commercial Applications”The successful suppression of VTH in B-diamond MOSFETs addresses a major hurdle in diamond electronics, enabling new applications in high-performance and harsh-environment systems.
- High-Temperature Electronics: The use of oxygen-terminated B-diamond channels provides superior thermal stability compared to traditional hydrogen-terminated diamond, making these devices ideal for high-temperature operation (e.g., downhole drilling, aerospace).
- Diamond CMOS Logic Circuits: The achieved low VTH (0.8 V) is essential for designing energy-efficient diamond complementary MOS (CMOS) circuits, reducing the complexity and voltage requirements of the gate drive source.
- High-Power Switching Devices: Diamond’s intrinsic properties (wide bandgap, high breakdown field) are leveraged for high-voltage power electronics and switching applications, although further efforts are needed to increase ID,max.
- Radiation-Hardened Systems: Diamond’s inherent radiation hardness makes these MOSFETs suitable for use in nuclear, space, and defense applications where reliability under extreme conditions is paramount.
- Integrated Circuits (ICs): The low VTH facilitates the integration of diamond transistors into complex digital and analog ICs, advancing the development of all-diamond integrated systems.
View Original Abstract
Suppression of high threshold voltage ( <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>${V} _{\text {TH}}$ </tex-math></inline-formula> ) for the boron-doped diamond (B-diamond) MOSFETs plays a key role to design the diamond complementary MOS circuits with low gate drive sources. The <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>${V} _{\text {TH}}$ </tex-math></inline-formula> can be further suppressed by adjusting B-diamond epitaxial layer thickness, boron doping concentration, and gate oxide thickness. Three MOSFETs with different device structures are fabricated on the same oxygen-terminated B-diamond channel. Thickness and acceptor concentration for the B-diamond epitaxial layer are approximately 800 nm and <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>$1.36\times 10^{{16}}$ </tex-math></inline-formula> cm−3, respectively. A 45 nm-thick Al2O3 is deposited as the gate oxide by an atomic layer deposition technique. Maximum drain currents and ON/OFF ratios for the B-diamond MOSFETs are in the range of <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>$-2.4\sim - 4.3,,\mu \text{A}$ </tex-math></inline-formula> /mm and greater than <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>$10^{{6}}$ </tex-math></inline-formula> , respectively. Their <inline-formula xmlns:mml=“http://www.w3.org/1998/Math/MathML” xmlns:xlink=“http://www.w3.org/1999/xlink”> <tex-math notation=“LaTeX”>${V} _{\text {TH}}$ </tex-math></inline-formula> values are lower than 3.4 V with the lowest one of 0.8 V.