Diamond Electronics and Related Wideband Gap Semiconductors for High Temperature Applications
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-11-07 |
| Journal | IMAPSource Proceedings |
| Authors | A. Christou |
| Institutions | University of Maryland, College Park |
| Analysis | Full AI Review Included |
Technical Analysis of Diamond Electronics for High Temperature Applications
Section titled âTechnical Analysis of Diamond Electronics for High Temperature ApplicationsâExecutive Summary
Section titled âExecutive SummaryâThis research focuses on developing highly stable and radiation-hardened diamond Field Effect Transistors (FETs) utilizing two-dimensional (2D) carrier systems.
- Core Technology: Investigation of 2D carrier transport in diamond, specifically through surface transfer doping (2DHG) and subsurface boron delta-doped (δ-doped) structures.
- Radiation Hardness Demonstrated: Surface-transfer-doped MOSFETs showed excellent stability against gamma irradiation, maintaining electrical parameters (drain current, threshold voltage) up to 100 kRad after an initial drop at 1 kRad.
- Thermal Stability Achieved: Double delta-doped FET structures exhibited high transconductance and thermal stability up to 450 °C, suitable for extreme environment applications.
- Advanced Channel Design: The double delta-doped structure incorporates two boron layers: a deeper layer (δ2) forming the FET channel and a shallower layer (δ1) aiding in low-resistance Ohmic contact formation.
- Material Quality: High-quality, low-dislocation-density undoped (100) type IIa HPHT single crystal diamond substrates were used, polished to an RMS roughness of less than 3 A.
- Future Direction: The research aims to apply 2D conduction channels to other ultra-wide bandgap (UWBG) semiconductors and understand dominant failure mechanisms under ionizing and non-ionizing radiation.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | (100) Type IIa | N/A | HPHT single crystal diamond |
| Surface Roughness (RMS) | less than 3 | A | After polishing and plasma etching |
| H-Termination Temp | above 700 | °C | Required for Negative Electron Affinity (NEA) surface |
| Gate Dielectric | 25 | nm | Al2O3 deposited by ALD |
| Gate Metal Thickness | 100 | nm | Al (E-beam/liftoff) |
| Gamma Radiation Source | 60Co | N/A | Panoramic source (UMRF) |
| Maximum Gamma Dose Tested | 100 | kRad | Surface-transfer-doped FETs |
| δ2 Channel Depth | 26 | nm | From top surface of double delta-doped epi |
| δ2 Channel Thickness | 1.85 | nm | Boron doped layer (FET channel) |
| δ2 Boron Concentration | 1.20 x 1021 | cm-3 | FET Channel (SIMS verified) |
| δ1 Contact Thickness | 0.75 | nm | Boron doped layer (Ohmic contact aid) |
| δ1 Boron Concentration | 4.96 x 1020 | cm-3 | Ohmic Contact Aid |
| Expected 2DHG Density (P2DHG) | greater than 1012 | cm-2 | Expected for δ2 channel |
| Thermal Stability | up to 450 | °C | Double delta-doped devices |
| Contact Metal Stack (Ti/Pt/Au) | 50/50/150 | nm | Thicknesses for delta-doped devices |
Key Methodologies
Section titled âKey MethodologiesâThe research employed distinct fabrication and characterization methodologies for two types of diamond FETs: surface-transfer-doped and double delta-doped structures.
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Substrate Preparation:
- Commercially supplied undoped (100) type IIa HPHT diamond substrates were selected for low dislocation density.
- Substrates were polished and etched using a low-power plasma to minimize carrier scattering centers, achieving an RMS roughness of less than 3 A (verified by AFM).
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Hydrogen Termination and 2DHG Formation (Surface-Transfer-Doped Devices):
- The substrate surface was exposed to a hydrogen plasma at temperatures above 700 °C.
- This process resulted in hydrogen termination, creating a Negative Electron Affinity (NEA) surface.
- The NEA surface subsequently loses electrons to atmospheric moisture/contaminants (surface transfer dopants), forming a conductive two-dimensional hole gas (2DHG) near the surface.
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Surface-Transfer-Doped MOSFET Fabrication:
- A blanket layer of Au (100-150 nm) was evaporated.
- Device mesas were defined using KI/I2 wet etchback.
- Device isolation was achieved using O2 plasma etch, which replaced C-H bonds with C-O bonds, effectively eliminating the 2DHG channel between devices.
- The gate stack consisted of 25 nm of Al2O3 deposited by Atomic Layer Deposition (ALD), followed by a 100 nm Al gate deposited using e-beam evaporation and liftoff.
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Double Delta-Doped FET Fabrication:
- Diamond epitaxial layers were grown on HPHT substrates, incorporating two distinct boron delta-doped layers (δ1 and δ2).
- The δ2 layer (1.85 nm thick, 1.20 x 1021 cm-3 B concentration) was positioned 26 nm from the surface to form the FET channel.
- The δ1 layer (0.75 nm thick, 4.96 x 1020 cm-3 B concentration) was incorporated at the top surface to aid Ohmic contact formation.
- Boron ions were implanted directly underneath the contact pads to enhance conduction to the δ2 layer (implantation profile determined by TRIM simulations).
- Contact metals (Ti/Pt/Au, 50/50/150 nm) were deposited.
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Characterization and Testing:
- Materials and defect characterization techniques were used to locate and monitor radiation-induced defects over various timescales.
- Devices were subjected to low-dose gamma irradiation (up to 100 kRad) using a 60Co source.
Commercial Applications
Section titled âCommercial ApplicationsâThe development of radiation-hardened, high-temperature diamond FETs utilizing 2D carrier systems is critical for several demanding engineering sectors:
- Aerospace and Defense: Components requiring extreme reliability and stability in high-radiation environments (e.g., satellite electronics, deep space probes, military systems).
- Nuclear Power and Fusion: Monitoring and control electronics operating within reactor cores or near high-flux radiation sources, leveraging the inherent radiation hardness of diamond.
- High Power/High Frequency Electronics: Utilizing the wide bandgap properties of diamond for high-efficiency power switching devices and RF amplifiers that must operate at elevated temperatures (up to 450 °C).
- Geothermal and Oil/Gas Exploration: Downhole logging tools and sensors that require electronics capable of functioning reliably at extreme subterranean temperatures and pressures.
- Ultra-Wide Bandgap (UWBG) Semiconductor Development: The process technology developed for 2D conduction channels in diamond is directly applicable to advancing transistor designs in related UWBG materials.
View Original Abstract
This presentation is focused on understanding the basic science of two-dimensional carrier systems in diamond-based semiconductor electronics. Over the course of the presentation, we will present the following: ⢠Process technology for diamond power field effect transistors (FETs) based on two-dimensional (2D) carrier transport in subsurface boron delta-doped structures ⢠The theory of carrier transport in diamond FETs with 2D conducting channels ⢠The results from materials and defect characterization techniques to locate, characterize, and monitor radiation-induced defects in these structures over various timescales ⢠New research directions in applying 2D conduction channels to ultra-wide bandgap semiconductor transistors based on diamond materials as well as related wide band gap semiconductors, ⢠Understand the dominant failure mechanisms operative in diamond-based FETs exposed to ionizing and non-ionizing radiation.