Diamond Electronics and Related Wideband Gap Semiconductors for High Temperature Applications

At a Glance

Metadata	Details
Publication Date	2023-11-07
Journal	IMAPSource Proceedings
Authors	A. Christou
Institutions	University of Maryland, College Park
Analysis	Full AI Review Included

Diamond Electronics for Extreme Environments: Technical Analysis and 6CCVD Solutions

This document analyzes the research presented at the 2022 HiTEN Conference regarding diamond field effect transistors (FETs) designed for high-temperature and radiation-hard applications, and outlines how 6CCVD’s advanced MPCVD diamond materials and processing capabilities can support and extend this critical research.

Executive Summary

Application Focus: Development of robust diamond FETs leveraging two-dimensional (2D) carrier transport systems for use in high-temperature and radiation-intensive environments.
Architectures Demonstrated: Successful fabrication and testing of both hydrogen-terminated surface 2DHG (Two-Dimensional Hole Gas) FETs and advanced subsurface boron delta-doped channel FETs.
Radiation Hardness: Devices exhibited exceptional stability, maintaining key electrical parameters (drain current, threshold voltage) after exposure to gamma irradiation doses up to 100 kRad.
Thermal Stability: The double delta-doped FET structure demonstrated high thermal stability, maintaining performance up to 450 °C.
Material Requirement: Requires ultra-high quality, low-dislocation density (100) Single Crystal Diamond (SCD) substrates, polished to an RMS roughness of Ra < 3 Å (0.3 nm).
Processing Complexity: Successful integration relies on precise MPCVD epitaxy for ultra-thin (nm scale) boron delta-doped layers and complex multi-layer metalization (e.g., Ti/Pt/Au).

Technical Specifications

The following hard data points were extracted from the research detailing material properties and device performance metrics.

Parameter	Value	Unit	Context
Substrate Material	Undoped (100) Type IIa	N/A	HPHT SCD used for epitaxy/devices
Surface Roughness (RMS)	< 3	Å	Required post-polishing and plasma etch
H-Termination Temperature	> 700	°C	Required for Negative Electron Affinity (NEA)
Gamma Irradiation Dose (Stability)	Up to 100	kRad	Electrical parameters remained stable
Thermal Stability (Double Delta FET)	Up to 450	°C	High-temperature operating limit reported
$\delta_{2}$ Channel Depth	26	nm	Depth from top surface (FET Channel)
$\delta_{2}$ Channel Thickness	1.85	nm	Boron delta-doped layer thickness
$\delta_{2}$ Boron Concentration	1.20 x 10²¹	cm^-3	High concentration required for channel
$\delta_{1}$ Ohmic Aid Thickness	0.75	nm	Top surface layer for contact enhancement
$\delta_{1}$ Boron Concentration	4.96 x 10²⁰	cm^-3	Concentration for Ohmic aid layer
2DHG Density (Double Delta)	> 10¹²	cm^-2	Estimated carrier density in $\delta_{2}$ layer
Ti/Pt/Au Contact Thicknesses	50/50/150	nm	Deposited contact metals

Key Methodologies

The following steps outline the critical material preparation and fabrication processes used to achieve the high-performance diamond FETs:

Substrate Preparation: Commercially supplied (100) HPHT SCD substrates were selected for their very low dislocation density. Substrates were polished and etched using a low power plasma to achieve an RMS roughness Ra < 3 Å.
Hydrogen Termination (H-FET): The substrate surface was exposed to a hydrogen plasma at temperatures above 700 °C to achieve hydrogen termination, resulting in a Negative Electron Affinity (NEA) surface and the formation of a conductive 2DHG.
Epitaxial Growth (Delta-Doped FET): An intrinsic diamond epi layer was grown on the HPHT substrate via MPCVD, incorporating two distinct, ultra-thin boron delta-doped layers ($\delta_{1}$ and $\delta_{2}$) at precise depths and concentrations to form the FET channel and Ohmic contact aid, respectively.
Device Isolation: For H-terminated devices, O₂ plasma etch was employed to replace C-H bonds with C-O bonds between devices, effectively eliminating the 2DHG channel in isolation regions.
Ohmic Contact Formation:
- H-FET: KI/I₂ wet etchback was performed to form discrete Ohmic contacts from a blanket Au layer (100-150 nm).
- Delta-Doped FET: Boron ions were implanted directly underneath the intended contact pads to aid conduction to the $\delta_{2}$ channel.
Gate Stack Deposition:
- H-FET: 25 nm of Al₂O₃ was deposited by Atomic Layer Deposition (ALD), followed by 100 nm of Al gate metal using e-beam and liftoff processes.
- Delta-Doped FET: Ti/Pt/Au contact metals (50/50/150 nm) were deposited.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational materials and advanced processing required to replicate and advance this research into high-temperature and radiation-hard diamond electronics. Our MPCVD capabilities directly address the stringent requirements for ultra-smooth surfaces, precise doping, and custom metalization.

Applicable Materials

Research Requirement	6CCVD Material Recommendation	Rationale
Substrate: Undoped (100) Type IIa, ultra-low dislocation.	Optical Grade Single Crystal Diamond (SCD)	Provides the necessary high purity, low-strain, and low-dislocation density required to minimize carrier scattering and maximize mobility in 2D channels.
Epitaxy: Precise, high-concentration boron delta doping (10²⁰ - 10²¹ cm^-3).	Heavy Boron-Doped Diamond (BDD) Epitaxy	Our MPCVD systems offer exceptional control over gas flow and temperature, enabling the growth of ultra-thin (nm scale) BDD layers with concentrations up to 10²¹ cm^-3, essential for replicating the $\delta_{1}$ and $\delta_{2}$ structures.

Customization Potential

The success of these devices hinges on tight dimensional and surface control, areas where 6CCVD excels:

Ultra-Smooth Polishing: The research demands Ra < 3 Å (0.3 nm). 6CCVD’s standard SCD polishing achieves Ra < 1 nm. We offer custom ultra-smooth polishing services to meet or exceed the Ra < 0.3 nm requirement, crucial for minimizing surface scattering and ensuring high-quality ALD gate dielectric deposition.
Custom Metalization Stacks: The paper utilized complex Ti/Pt/Au (50/50/150 nm) and Au/Al stacks. 6CCVD offers in-house metalization services including Au, Pt, Pd, Ti, W, and Cu. We can deposit these multi-layer stacks to exact thickness specifications, streamlining the Ohmic contact formation process for researchers.
Dimensions and Thickness: While the paper used standard HPHT substrates, 6CCVD can provide SCD plates up to 500 µm thick and offers PCD wafers up to 125mm for future scaling efforts.

Engineering Support

6CCVD’s in-house PhD team specializes in wide bandgap semiconductor material science. We can assist researchers with material selection, doping profile design, and surface preparation optimization for similar High-Temperature and Radiation-Hard Diamond FET projects. Our expertise ensures that the starting material meets the rigorous specifications necessary for achieving high transconductance and long-term stability in extreme environments.

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

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.

Tech Support

Original Source

DOI: https://doi.org/10.4071/001c.89939