Surface Transfer Doping in MoO3–x/Hydrogenated Diamond Heterostructure
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-02-01 |
| Journal | The Journal of Physical Chemistry Letters |
| Authors | Liqiu Yang, Ken‐ichi Nomura, Aravind Krishnamoorthy, Thomas Linker, Rajiv K. Kalia |
| Institutions | University of Southern California, SLAC National Accelerator Laboratory |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research investigates Surface Transfer Doping (STD) of hydrogenated diamond (H-diamond) using molybdenum oxide (MoO3-x) to enable high-performance electronic devices.
- Core Value Proposition: MoO3-x is confirmed as an effective surface electron acceptor, providing a viable, non-substitutional method for p-type doping of diamond, which is notoriously difficult to dope traditionally.
- Mechanism Confirmation: The deposition of MoO3-x extracts electrons from the H-diamond valence band, creating a quasi-two-dimensional subsurface hole gas (2DHG) necessary for FET operation.
- Oxygen Vacancy Control: Charge transfer efficiency is found to be monotonically dependent on the oxygen vacancy level (x). Higher Mo oxidation states (lower x, e.g., MoO2.9) result in significantly higher charge transfer (up to 2.0 electrons net transfer).
- Transport Properties: The doped holes are primarily localized around the hydrogen atoms at the interface, and the hole density is spatially extended, which is consistent with the excellent transport properties reported experimentally for MoO3-x/H-diamond interfaces.
- Engineering Guidance: The monotonic relationship between vacancy level and charge transfer provides a direct pathway for engineering the MoO3-x material to maximize doping capability during the STD process.
- Methodology: The study utilized a robust, first-principles-informed approach combining Reactive Molecular Dynamics (RMD) simulations (using ReaxFF) to model deposition and interfacial structure, followed by Density Functional Theory (DFT) calculations for electronic structure and charge analysis.
Technical Specifications
Section titled “Technical Specifications”The following specifications relate to the intrinsic properties of diamond and the quantitative results derived from the simulation of the MoO3-x/H-diamond heterostructure.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Bandgap | 5.5 | eV | Wide bandgap material, crucial for high-power applications. |
| Electron Mobility (Diamond) | 4,500 | cm2/V s | High intrinsic carrier mobility. |
| Hole Mobility (Diamond) | 3,800 | cm2/V s | High intrinsic carrier mobility. |
| Breakdown Electric Field | >10 | MV/cm | Extremely high dielectric strength. |
| Thermal Conductivity | 22 | W cm-1 K-1 | Excellent heat dissipation capability. |
| Highest Net Charge Transfer (x=0.1) | 2.0 | electrons | Net electron transfer to MoO2.9 (lowest vacancy level). |
| Lowest Net Charge Transfer (x=0.7) | 1.6 | electrons | Net electron transfer to MoO2.3 (highest vacancy level). |
| Fermi Level (EF) Shift (x=0.1) | -0.19 | eV | EF relative to Valence Band Maximum (VBM) after MoO2.9 deposition. |
| Fermi Level (EF) Shift (Pristine) | -0.69 | eV | EF relative to VBM before deposition (H-diamond only). |
| RMD Melting Temperature | 3,300 | K | Temperature used in RMD to generate amorphous MoO3-x structure. |
| DFT Plane Wave Cutoff | 450 | eV | Energy cutoff used in VASP calculations. |
Key Methodologies
Section titled “Key Methodologies”The interfacial structure and electronic properties were determined using a three-step simulation workflow combining RMD, QMD, and DFT.
-
Reactive Molecular Dynamics (RMD) Simulation:
- Purpose: Generate fully thermalized, amorphous interfacial structures of MoO3-x on H-diamond (111).
- Potential: ReaxFF interatomic potential, designed to describe both material properties and chemical reactions.
- Procedure: MoO3-x (with O/Mo ratios 2.3, 2.6, 2.9) was heated to 3,300 K and melted. A “momentum mirror” was used to prevent premature chemical bonding with the H-diamond surface during melting. The resulting amorphous oxide was then deposited and relaxed at 10 K.
-
Quantum Molecular Dynamics (QMD) Optimization:
- Software: VASP (Vienna Ab initio Simulation Package).
- Purpose: Further optimize the RMD-generated interfacial structure using DFT-level accuracy.
- Constraints: The bottom three layers of carbon and all hydrogen atoms in the H-diamond were kept fixed during optimization.
- Functionals: Projector-Augmented Wave (PAW) method and PBE Generalized Gradient Approximation (GGA) functional.
-
Electronic Structure Calculation (DFT):
- Purpose: Investigate the electronic density-of-states (DOS) alignment and quantify charge transfer.
- Analysis Tools: Bader charge analysis was used to calculate the charge difference per atom before and after deposition. Charge density difference (Δρ) was computed to visualize electron accumulation (in oxide) and hole accumulation (in diamond).
- K-point Mesh: A fine 3 x 3 x 1 Monkhorst-Pack k-point mesh was used for electronic structure calculations.
Commercial Applications
Section titled “Commercial Applications”The successful and controllable p-type doping of diamond via Surface Transfer Doping (STD) using MoO3-x is critical for realizing high-performance diamond-based semiconductor devices.
- High-Power Field-Effect Transistors (FETs):
- Diamond’s exceptional breakdown field (>10 MV/cm) and thermal conductivity enable the creation of highly efficient power switching devices (e.g., MOSFETs, MESFETs) capable of handling extreme power densities far exceeding silicon or SiC.
- High-Frequency RF Electronics:
- The high carrier mobility and the formation of a stable 2DHG layer are essential for high-speed operation in RF power amplifiers and switches, critical for 5G/6G wireless communication infrastructure and radar systems.
- Extreme Environment Devices:
- Diamond’s inherent resistance to radiation and high temperatures allows these doped devices to be deployed in aerospace, nuclear, and deep-well drilling applications where traditional semiconductors fail.
- Integrated Thermal Management:
- The ability to build functional electronic layers directly on the highly conductive diamond substrate facilitates integrated heat dissipation, simplifying packaging and improving device reliability in dense microelectronic systems.
- Sensors and Detectors:
- The controlled surface conductivity achieved through STD can be leveraged for highly sensitive chemical or biological sensors operating under harsh conditions.
View Original Abstract
Surface transfer doping is proposed to be a potential solution for doping diamond, which is hard to dope for applications in high-power electronics. While MoO<sub>3</sub> is found to be an effective surface electron acceptor for hydrogen-terminated diamond with a negative electron affinity, the effects of commonly existing oxygen vacancies remain elusive. We have performed reactive molecular dynamics simulations to study the deposition of MoO<sub>3-<i>x</i></sub> on a hydrogenated diamond (111) surface and used first-principles calculations based on density functional theory to investigate the electronic structures and charge transfer mechanisms. We find that MoO<sub>3-<i>x</i></sub> is an effective surface electron acceptor and the spatial extent of doped holes in hydrogenated diamond is extended, promoting excellent transport properties. Charge transfer is found to monotonically decrease with the level of oxygen vacancy, providing guidance for engineering of the surface transfer doping process.