A reactive molecular dynamics study of the hydrogenation of diamond surfaces

At a Glance

Metadata	Details
Publication Date	2021-09-14
Journal	Computational Materials Science
Authors	Eliezer Fernando Oliveira, Mahesh R. Neupane, Chenxi Li, Harikishan Kannan, Xiang Zhang
Institutions	DEVCOM Army Research Laboratory, Universidade Estadual de Campinas (UNICAMP)
Citations	9
Analysis	Full AI Review Included

Core Value Proposition: This study uses fully atomistic reactive molecular dynamics (FARMD) to identify optimal diamond surface orientations for hydrogenation, a critical step in achieving robust p-type surface conductivity required for high-frequency electronic devices (FETs).
Optimal Surfaces Identified: The (001), (110), and (113) surfaces are the most efficient for hydrogen passivation, providing homogeneous p-type surface conductivity robust under extreme conditions.
Hydrogen Coverage Threshold: The (013) and (113) surfaces incorporate the highest number of hydrogen atoms (up to 17 H/nm² at 1200°C) due to their high initial dangling bond densities.
Passivation Efficiency: Maximum hydrogenation efficiency reaches 67% at 1200°C; 100% coverage is physically impossible across all facets due to steric repulsion between terminated hydrogen atoms.
Promising Candidate: The (113) surface is highlighted as the most attractive candidate, combining high hydrogen coverage with previously reported low electrical resistance in boron-doped films.
Functional Group Control: The presence of CH₂ groups, particularly high on the (013) surface (up to 37% at 900°C), provides a mechanism for tuning the Work Function (WF) and achieving more negative Electron Affinity (EA).

Parameter	Value	Unit	Context
Simulation Method	FARMD (ReaxFF)	N/A	Fully atomistic reactive molecular dynamics
Temperature Range (Hydrogenation)	500 to 1200	°C	Range used for hydrogenation simulations
H Saturation Time Threshold	~50	ps	Minimum exposure time required to reach saturation
Max H Coverage (113)	17	H/nm²	Highest observed coverage at 1200°C
Max H Coverage (013)	16	H/nm²	Coverage at 1200°C
Max Passivation Efficiency	67	%	Achieved by (001), (110), and (113) surfaces at 1200°C
Dangling Bond Density (113)	30	per nm²	Highest initial density (up to 900°C)
Bulk C-C Bond Length (RT)	1.55 ± 0.03	Angstrom	Characteristic of sp³-sp³ bonds
Reconstructed C-C Bond Length (001)	1.42 ± 0.02	Angstrom	sp²-sp² bond (dimers) at Room Temperature (RT)
CH₂ Group Formation (013)	37	%	Percentage of incorporated H forming CH₂ groups at 900°C
Negative Electron Affinity (NEA)	Down to -1.2	eV	Expected for fully hydrogenated diamond surface (literature value)

Surface Selection and Modeling: Five pristine diamond surfaces—(001), (013), (110), (113), and (111)—were modeled as square slabs (~5.4 x 5.4 nm², eight layers thick).
Simulation Environment: All simulations used the ReaxFF force field implemented in the LAMMPS computational code.
Structural Constraints: The carbon atoms in the bottom two layers were constrained to their bulk positions to mimic a bulk-like slab structure. The remaining six layers were allowed to move freely.
Thermal Equilibration: Bare surfaces underwent energy minimization followed by thermal equilibration (NVT ensemble) for 400 ps across temperatures ranging from RT up to 1200°C to analyze structural stability and reconstruction.
Hydrogenation Setup: An atmosphere of randomly distributed atomic hydrogen was created above the thermalized diamond surfaces. The hydrogen concentration was set to 120% of the available dangling bonds.
Hydrogenation Dynamics: FARMD runs were executed for 2.0 ns in the NVT ensemble at 500°C, 700°C, 900°C, and 1200°C. Periodic boundary conditions were applied in the x and y directions, with hard Lennard-Jones walls confining the z-direction.
Saturation Analysis: The number of incorporated hydrogen atoms over time was fitted using an exponential function to determine the saturated hydrogen limit (#H_o) and the percentage of initial dangling bonds that were successfully passivated.

High-Frequency Field-Effect Transistors (FETs): Hydrogenated diamond is essential for creating the p-type surface conductivity necessary for high-speed, high-frequency FETs and MOSFETs.
Robust Power Electronics: Diamond’s high carrier mobility and thermal conductivity make it ideal for next-generation power electronics and devices operating in extreme environmental conditions (e.g., high-temperature or high-radiation environments).
Surface Engineering for Negative Electron Affinity (NEA): Controlling surface orientation and hydrogenation temperature allows for tuning the NEA and Work Function (WF), which is critical for optimizing device performance and creating efficient hole channels.
Advanced Diamond Film Synthesis: The data supports prioritizing the growth of diamond films with high content of exposed (001), (110), and especially (113) facets to maximize hydrogen coverage and achieve superior p-channel transistor performance.
Tuning Electronic Properties via Functional Groups: The ability to control the formation of CH and CH₂ groups (e.g., by selecting the (013) surface) provides a pathway for fine-tuning the electronic properties of the surface layer.

2019 - Boron-oxygen complex yields n-type surface layer in semiconducting diamond [Crossref]
2017 - Deep depletion concept for diamond MOSFET [Crossref]
2018 - High-mobility diamond field effect transistor with a monocrystalline h-BN gate dielectric [Crossref]
2020 - Oxidized Si terminated diamond and its MOSFET operation with SiO 2 gate insulator [Crossref]
2019 - Radiofrequency performance of hydrogenated diamond MOSFETs with alumina [Crossref]
2021 - Study of the structural phase transition in diamond (100) & (111) surfaces [Crossref]
1996 - Hydrogen-terminated diamond surfaces and interfaces [Crossref]