Hydrogen-terminated Diamond Films Research Articles

Growth and surface structrue of hydrogen terminal diamond thin films

The conductivity of hydrogen-terminated diamond is a limiting factor in its application in field-effect transistor devices. The traditional preparation process hinders the improvement of the electrical properties of hydrogen-terminated diamond due to impurity elements in the diamond bulk and surface damage caused by processing near the diamond surface. To overcome this, researchers have explored the epitaxial growth of a high-purity and flat-surfaced diamond thin film on a diamond substrate. However, this approach still faces challenges in film characterization and achieving high surface smoothness. In this study, microwave plasma chemical vapor deposition technology is used to epitaxially grow a sub-micron thick diamond film on a nitrogen-doping chemical vapor deposition diamond substrate of 10 mm × 10 mm × 0.5 mm in size. The influence of methane concentration on the growth and conductivity of diamond film is investigated. The test results reveal that the growth thickness of the diamond film ranges from 230 to 810 nm, and the nitrogen concentration in the epitaxial layer is lower than 1×10<sup>16</sup> atom/cm<sup>3</sup>. Three growth modes are observed for the homoepitaxial growth of the diamond thin film under different methane concentrations. A methane concentration of 4% enables two-dimensional planar growth of diamond, resulting in a smooth and flat surface with a roughness of 0.225 nm (10 μm×10 μm). The formation of different surface morphologies is attributed to the growing process and etching process of diamond. Surface low-energy electron diffraction testing indicates that the surface of the diamond film undergoes a structural transition from oxygen terminal (1×1: O) to hydrogen terminal (2×1: H) when grown for a short period of time. X-ray photoelectron spectroscopy analysis reveals an extremely low ratio of oxygen element to nitrogen element, giving the grown diamond film P-type conductivity characteristics. The Hall test results demonstrate that the hydrogen-terminated diamond film grown with a methane concentration of 4% exhibits the highest conductivity, with a square resistance of 4981 Ω/square and a hole mobility of 207 cm<sup>2</sup>/(V·s). This enhanced conductivity can be attributed to the lower defect density observed under these specific conditions. The findings of this study effectively improve the electrical properties of hydrogen-terminated diamond, and contribute to the development and practical application of high-power diamond devices.

Annealing Temperature on Contact Properties between Nickel Film and Hydrogen-Terminated Single Crystal Diamond

Ohmic contact of nickel on hydrogen-terminated single-crystal diamond film was investigated with an annealing temperature ranging from room temperature to 750 °C in hydrogen atmosphere. Nickel film was deposited on a hydrogen-terminated single-crystal diamond surface with gold film in order to protect it from oxidation. Contact properties between nickel and hydrogen-terminated single crystal diamond were measured by a circular transmission line model. The lowest specific contact resistivity was 7.82 × 10−5 Ω cm2 at annealing temperature of 750 °C, indicating good ohmic contact, which reveals improved thermal stability by increasing temperature.

Effects of hydrogen termination of CVD diamond layers

This paper is focused on studies of hydrogen terminated diamond films deposited from CH3OH and H2 gas mixture.The polycrystalline diamond layers of different ratios of sp2/sp3 carbon phases and levels of H-termination were synthesized by Hot Filament Chemical Vapour Deposition (HF CVD) method. As-grown CVD diamond layers were naturally hydrogen terminated which results from the nature of the CVD growth process.It is well known that hydrogen is responsible for the hydrophobic properties of the diamond surface and for its p-type conductivity, however, knowledge about the influence of hydrogen on optically active centers is very limited.The performed photoluminescence studies have shown that chemically bonded hydrogen can passivate non-radiative defects making the radiative transition more probable.In present work the properties of diamond surface has been studied using SEM (morphology), X-ray Powder Diffraction (XRD), Raman and photoluminescence spectroscopies (structural quality, carbon phases composition, photoluminescence properties), FTIR spectroscopy (confirmation of H-termination), PL (photoluminescence), X-ray photoelectron spectroscopy (XPS) and contact angle (wetting properties-hydrogen termination).

Electronic structures of two-dimensional hydrogenated bilayer diamond films with Si dopant and Si-V center

Abstract The structural, electronic and optical properties of two-dimensional hydrogen-terminated bilayer diamond films (2D-HBDF) with silicon (Si) dopant and silicon-vacancy (Si-V) center are investigated by first-principles calculations. The two positions of substitutional Si are energetically favorable with similar band structures compared to the pristine 2D-HBDF. For Si-V center, the flat intermediate bands originated from the complex impurities appear in band gap. Especially, the ground state of paramagnetic or nonmagnetic for the case of Si-V is dependent on the distributions of Si dopant and vacancy. The calculated optical absorptions with relating to Si-V center show a high intensity in the visible light region. It reveals that the introducing vacancy (substituted Si) and their positions play an important role in modulating the electronic and optical properties of 2D-HBDF, which are helpful to construct novel low-dimensional diamond-based optoelectronic devices applied in extreme environments.

Gas-sensing behaviour of ZnO/diamond nanostructures.

Microstructured single- and double-layered sensor devices based on p-type hydrogen-terminated nanocrystalline diamond (NCD) films and/or n-type ZnO nanorods (NRs) have been obtained via a facile microwave-plasma-enhanced chemical vapour deposition process or a hydrothermal growth procedure. The morphology and crystal structure of the synthesized materials was analysed with scanning electron microscopy, X-ray diffraction measurements and Raman spectroscopy. The gas sensing properties of the sensors based on i) NCD films, ii) ZnO nanorods, and iii) hybrid ZnO NRs/NCD structures were evaluated with respect to oxidizing (i.e., NO2, CO2) and reducing (i.e., NH3) gases at 150 °C. The hybrid ZnO NRs/NCD sensor showed a remarkably enhanced NO2 response compared to the ZnO NRs sensor. Further, inspired by this special hybrid structure, the simulation of interaction between the gas molecules (NO2 and CO2) and hybrid ZnO NRs/NCD sensor was studied using DFT calculations.

<italic>V</italic>2O5MISFETs on H-Terminated Diamond

We report for the first time on the dc and RF performance of novel MISFETs fabricated on hydrogen-terminated (H-terminated) single crystal diamond film using vanadium pentoxide (V2O5) as insulating material. The active devices were characterized in terms of static $I$ – $V$ characteristics and static transconductance as well as of S-parameters for the calculation of the maximum cutoff frequency and the maximum oscillation frequency. Time stability of the drain current was evaluated overnight observing a maximum fluctuation of 7%. Investigations on temperature dependence of diamond-based MISFET were also performed up to 130 °C. The experimental results were compared with the better established diamond MESFET technology. Finally, the surface transfer doping of H-terminated diamond by very thin V2O5 insulator was also investigated in terms of conductivity, stability in air, and resistance to high temperatures.

Diamond based field-effect transistors with SiNx and ZrO2 double dielectric layers

A diamond-based field-effect transistor (FET) with SiNx and ZrO2 double dielectric layer has been demonstrated. The SiNx and ZrO2 gate dielectric are deposited by plasma-enhanced chemical vapor deposition (PECVD) and radio frequency (RF) sputter methods, respectively. SiNx layer is found to have the ability to preserve the conduction channel at the surface of hydrogen-terminated diamond film. The leakage current density (J) of SiNx/ZrO2 diamond metal-insulator-semiconductor FET (MISFET) keeps lower than 3.88×10−5A·cm−2 when the gate bias was changed from 2V to −8V. The double dielectric layer FET operates in a p-type depletion mode, whose maximum drain-source current, threshold voltage, maximum transconductance, effective mobility and sheet hole density are determined to be −28.5mA·mm−1, 2.2V, 4.53mS·mm−1, 38.9cm2·V−1·s−1, and 2.14×1013cm−2, respectively.

Gate-Source Distance Scaling Effects in H-Terminated Diamond MESFETs

In this paper, an analysis of gate–source and gate–drain scaling effects in MESFETs fabricated on hydrogen-terminated single-crystal diamond films is reported. The experimental results show that a decrease in gate–source spacing can improve the device performance by increasing the device output current density and its transconductance. On the contrary, the gate–drain distance produces less pronounced effects on device performance. Breakdown voltage, knee voltage, and threshold voltage variations due to changes in gate–source and drain–source distances have also been investigated. The obtained results can be used as a design guideline for the layout optimization of H-terminated diamond-based MESFETs.

Voltammetric and impedance behaviours of surface-treated nano-crystalline diamond film electrodes

The electrochemical performances of hydrogen- and oxygen-terminated nano-crystalline diamond film electrodes were investigated by cyclic voltammetry and AC impedance spectroscopy. In addition, the surface morphologies, phase structures, and chemical states of the two diamond films were analysed by scanning probe microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. The results indicated that the potential window is narrower for the hydrogen-terminated nano-crystalline diamond film than for the oxygen-terminated one. The diamond film resistance and capacitance of oxygen-terminated diamond film are much larger than those of the hydrogen-terminated diamond film, and the polarization resistances and double-layer capacitance corresponding to oxygen-terminated diamond film are both one order of magnitude larger than those corresponding to the hydrogen-terminated diamond film. The electrochemical behaviours of the two diamond film electrodes are discussed.

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5eV and thermionic electron emission (TE) has been detected at temperatures less than 500°C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5×10−5A/cm2 at 500°C. In contrast, the initial TE current density of a film aged for 18months was ~1.8×10−9A/cm2 at 500°C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3×10−4mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2h of exposure with the sample at 500°C, the surface exhibited a stable emission current density of ~2.3×10−6A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5×10−8mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400°C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200meV, and the TE intensity increased by a factor of ~100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.

The influence of surface functionalisation on the electrical properties and thermal stability of nanodiamonds

Detonation nanodiamond (ND) has recently emerged as a useful new class of diamond material. However, to date there has been little investigation of the electrical properties of this material. Due to the nanoscale dimensions, the surface functionalisation of the individual ND is of particular importance to the characteristics of ND films. Here, hydrogen and oxygen termination of ND, verified using Fourier transform infrared spectroscopy, are shown to strongly influence the electronic properties of NDs. Hydrogen terminated ND exhibiting a far greater resilience to thermal decomposition when compared to the oxygen terminated NDs. Moreover, H-NDs also displayed so-called “surface conductivity,” a property displayed by hydrogen-terminated bulk diamond films, whilst O-NDs display properties high resistivity. These results indicate that under the correct conditions ND layers can display similar electrical properties to “bulk” diamond thin films.

Interface and interlayer barrier effects on photo-induced electron emission from low work function diamond films

Nitrogen-doped diamond has been under investigation for its low effective work function, which is due to the negative electron affinity (NEA) produced after surface hydrogen termination. Diamond films grown by chemical vapor deposition (CVD) have been reported to exhibit visible light induced electron emission and low temperature thermionic emission. The physical mechanism and material-related properties that enable this combination of electron emission are the focus of this research. In this work the electron emission spectra of nitrogen-doped, hydrogen-terminated diamond films are measured, at elevated temperatures, with wavelength selected illumination from 340 nm to 450 nm. Through analysis of the spectroscopy results, we argue that for nitrogen-doped diamond films on metallic substrates, photo-induced electron generation at visible wavelengths involves both the ultra-nanocrystalline diamond and the interface between the diamond film and metal substrate. Moreover, the results suggest that the quality of the metal–diamond interface can substantially impact the threshold of the sub-bandgap photo-induced emission. • Spectra of photo-induced electron emission from N-doped diamond is reported. • Separate energy thresholds are observed for photo-induced and thermionic emission. • Thresholds are attributed to variance in the samples’ electronic structure. • Photoelectron generation involves both the UNCD and the metal substrate.

Comparative study of friction properties for hydrogen- and fluorine-modified diamond surfaces: A first-principles investigation

The van der Waals corrected first-principles approach within density functional theory has been performed to calculate the interaction energies and friction coefficients for hydrogen- and fluorine-terminated diamond film systems. Comparative studies show that the fluorine-terminated diamond film is more stable and chemically inert than that of hydrogen termination. The repulsive interactions between two fluorine-terminated diamond films are stronger than that of hydrogen termination system, and the former system has a larger equilibrium adsorption heights than that of the latter. Quantitative calculations show that the hydrogen-terminated diamond film system has larger potential barriers and hence larger friction coefficients of about 0.15 under the loads of 0.5–3nN, which are almost twice larger than that of fluorine-terminated system (about 0.08). The results are in good agreement with experiments and other theoretical predictions.

Optimizing hydrogen plasma etching process of preferred (110)-textured diamond film

(110)-oriented diamond films were grown by hot-filament chemical vapor deposition method. Microwave assisted hydrogen plasma etching process was used to obtain hydrogen-terminated diamond films. The influence of gas pressure and substrate temperature during hydrogen plasma etching on structural and electrical properties of diamond film was investigated. The ultra-violet Raman spectra showed CHX bond increased with the gas pressure and the substrate temperature. The Hall Effect measurement results showed that the higher CHX bond density led to a higher sheet carrier density and lower sheet resistance, which was helpful for the fabrication of high-frequency and high-power field-effect transistors.

XPS study of polycrystalline diamond surfaces after annealing treatment

In many electronic applications, the surface properties of hydrogen-terminated diamond films determine the eventual performance of the electronic device. In this work, diamond films were grown by hot-filament chemical vapor deposition method and hydrogenated films were obtained by hydrogen plasma treatment. X-ray photoelectron spectroscopy analysis was carried out to evaluate the film surface after annealing treatment. The C1s XPS spectrum showed the C1s spectrum changed after annealing treatment. In air atmosphere, hydrogen decreased and oxygen absorption increased. The interaction on the surface changed drastically after annealed in argon atmosphere and phase transition would happen when the annealing temperature increased to 600°C.

Transport properties of hydrogen-terminated nanocrystalline diamond films

Hydrogen-terminated nanocrystalline diamond films of different granularity are investigated by van der Pauw measurement of the resistivity and Hall effect depending on the grain size. In agreement with the observation on the hydrogenated diamond single crystals, the measured transport quantities are surface related. The room temperature resistivity strongly increases when the mean grain size at surface varies from about 400nm down to 40nm while the Hall concentration is only weakly dependent on the granularity and exhibits values comparable with those found in hydrogenated single crystals. Using a two-dimensional geometrical model, we show that effective resistivity is controlled by the transversal resistance of the grain boundaries. Based on the mobility–concentration relation obtained by the surface adsorbate desorption, microscopic origin of the boundary resistance is discussed.

Charge-transfer doped intrinsic diamond electrochemical electrodes

ABSTRACTWe report electrochemical characteristics of hydrogen terminated charge-transfer doped intrinsic microcrystalline diamond films. Microcrystalline diamond was synthesized by Direct- Current Plasma Enhanced Chemical Vapor Deposition (DC-PECVD) in methane diluted by hydrogen. The diamond films were subjected to further treatment by microwave plasma in pure hydrogen to increase the hydrogen termination of the diamond surfaces and their negative electron affinity. When the diamond is exposed to the ambient moisture, valance electrons tend to tunnel from the first few atomic layers of the diamond surface to the adsorbed water adlayer. This charge transfer process results in the surface of hydrogen-terminated diamond behaving like a p-type semiconductor.Electrochemical characteristics of hydrogen-terminated diamond films were exposed to an air plasma for depleting the surface hydrogen atoms and then re-hydrogenated the same diamond films with atomic hydrogen. Cyclic voltammetry in 0.1M H2S04 aqueous solution and 0.01M Fe(CN)6-4/-3+0.1M KCl aqueous solution was applied to reveal high current density and wide potential window for hydrogen-terminated diamond grown on silicon substrates. The faceted surface morphology has been observed by SEM. The crystalline characteristics and carbon phases in the diamond film were examined by Raman spectroscopy.

Electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air

The electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air are investigated by scanning probe microscopy (SPM). The results indicate that for the hydrogen-terminated diamond surface a shallow acceptor above the valence-band-maximum (VBM) appears in the band gap. However, the oxygen-terminated diamond film exhibits a high resistivity with a wide band gap. Based on the density-functional-theory, the densities of states, corresponding to molecular adsorbate in hydrogenated and oxygenated diamond (100) surfaces, are studied. The results show that the shallow acceptor in the band gap for the hydrogen-terminated diamond film can be attributed to the interaction between the surface C–H bonding orbitals and the adsorbate molecules, while for the oxygen-terminated diamond film, the interaction between the surface C–O bonding orbitals and the adsorbate molecules can induce occupied states in the valence-band.

The electron transfer behavior of the hydrogen-terminated boron-doped diamond film electrode

To clarify the electron transfer behavior of the hydrogen-terminated boron-doped diamond film electrode in an electrolytic solution, its detailed electronic structures were investigated by using scanning probe microscopy and ab initio methods. The interface structure of the hydrogen-terminated diamond electrode and the electrolyte was also studied by means of cyclic voltammetry and AC impedance spectroscopy. The results showed that there exist shallow acceptors in the band gap of the hydrogenated diamond films. Neither the surface hydrogen alone nor the subsurface hydrogen could induce the shallow acceptors, though they interact strongly with the surface carbon atoms or subsurface carbon and boron atoms in the diamond film. It is the interaction of the surface adsorbates and the surface C–H bonding that gives rise to the shallow acceptors in the band gap of the hydrogenated diamond film. The surface shallow acceptors in the band gap of the hydrogenated diamond film could lower the energy barrier of the electron transfer between the diamond electrode and the electrolytic solution. Thus, electrochemical window for the hydrogenated diamond film is narrower and, its film resistance and capacitance are also smaller than those of the oxygenated one. In our work, the experimental data obtained by using scanning probe microscopy are in good agreement with the calculation results.

Electron emission suppression from hydrogen-terminated n-type diamond

By total photoelectron yield spectroscopy (TPYS), we have found a thermal instability of a negative electron affinity in hydrogen-terminated n-type diamond films, which has never been observed for intrinsic and p-type diamond films. Experimentally, we have succeeded in detecting surface defect states and surface valence band extended states on the same n-type sample after soft annealing from 100 to 300 °C, which allowed us to evaluate the surface Fermi level and surface band bending using phosphorus doping parameters. Our results show that “quasi-positive” electron affinity prevents electron emission from an n-type diamond film even though its surface has a negative electron affinity.