Hydrogen-terminated Surface Research Articles

Surface Treatments Effects for NO2 Gas Responsivity of p-Type Surface Conductive Layer on Diamond Films

ABSTRACTThe low resistance layer called p-type Surface Conductive Layer (PSCL) is formed when the nitrogen dioxide (NO2) was absorbed onto hydrogen-terminated surface, although the diamond is generally isolator. The PSCL conductance is dependent on NO2 concentration in the atmosphere, and this reaction has reversibility. The diamond having these characteristics can apply to gas sensor. In this study, the gas responsivity of PSCL was improved by surface treatment. The changes in the responsivity were evaluated from serial measurements of the conductance in the gas atmosphere. From this evaluation, it was observed that the adsorption and desorption of NO2 were faster via the surface conditions variation by treatment.

<i>In Situ</i> ATR-FTIR Observation about Surfactant/Hydrogen-TerminatedSi(111) Interface in Solution

Development of novel functional devices has been expected by modification for Si surface. This study investigated immobilization and roles of the Si surface with flowing surfactant by in situ ATR-FTIR method. This result suggested that the surfactant prevented oxidation of the hydrogen-terminated Si surface from the higher concentration in aqueous solution. These would guard the Si surface against H2O molecules.

Improvement of Minority Carrier Lifetime and Si Solar Cell Characteristics by Nitric Acid Oxidation Method

The minority carrier lifetime of single crystalline Si increases by the formation of an ultrathin silicon dioxide (SiO2) layer by use of the nitric acid oxidation of Si (NAOS) method. With the NAOS method using 68 or 98 wt% HNO3 solutions, large increases in the lifetime are observed while the enhancement of the lifetime is low for the 40 wt% HNO3 NAOS method. In the case of the hydrogen-terminated Si surfaces formed by hydrofluoric acid (HF) treatment, the initial minority carrier lifetime is high, but it returns to the value before the HF treatment by keeping in air for 5 days. In the case of the NAOS method, on the other hand, the enhanced lifetime remains unchanged with time kept in air. This stability is attributed to high atomic density of the NAOS SiO2 layer which prevents diffusion of oxidizing species. The atomic density of the NAOS SiO2 layer increases with the HNO3 concentration, which leads to an increase in the valence band discontinuity energy at the SiO2/Si interface. When the NAOS SiO2 layer is inserted between the Si substrate and a silicon nitride anti-reflection layer for n-Si-based pn-junction solar cells, the conversion efficiency is increased from 17.2 to 18.9%. The internal quantum efficiency in the short wavelength region (300∼600 nm) is improved by the NAOS SiO2 layer, indicating the front surface is effectively passivated by the NAOS SiO2 layer.

Electrochemical oxidation and electroanalytical determination of xylitol at a boron-doped diamond electrode

Xylitol is a reduced sugar with anticariogenic properties used by insulin-dependent diabetics, and which has attracted great attention of the pharmaceutical, cosmetics, food and dental industries. The detection of xylitol in different matrices is generally based on separation techniques. Alternatively, in this paper, the application of a boron-doped diamond (BDD) electrode allied to differing voltammetric techniques is presented to study the electrochemical behavior of xylitol, and to develop an analytical methodology for its determination in mouthwash. Xylitol undergoes two oxidation steps in an irreversible diffusion-controlled process (D=5.05 × 10(-5)cm(2)s(-1)). Differential pulse voltammetry studies revealed that the oxidation mechanism for peaks P1 (3.4 ≤ pH ≤ 8.0), and P2 (6.0 ≤ pH ≤ 9.0) involves transfer of 1H(+)/1e(-), and 1e(-) alone, respectively. The oxidation process P1 is mediated by the (•)OH generated at the BDD hydrogen-terminated surface. The maximum peak current was obtained at a pH of 7.0, and the electroanalytical method developed, (employing square wave voltammetry) yielded low detection (1.3 × 10(-6) mol L(-1)), and quantification (4.5 × 10(-6) mol L(-1)) limits, associated with good levels of repeatability (4.7%), and reproducibility (5.3%); thus demonstrating the viability of the methodology for detection of xylitol in biological samples containing low concentrations.

Surface transfer doping of diamond by MoO3: A combined spectroscopic and Hall measurement study

Surface transfer doping of diamond has been demonstrated using MoO3 as a surface electron acceptor material. Synchrotron-based high resolution photoemission spectroscopy reveals that electrons are transferred from the diamond surface to MoO3, leading to the formation of a sub-surface quasi 2-dimensional hole gas within the diamond. Ex-situ electrical characterization demonstrated an increase in hole carrier concentration from 1.00 × 1013/cm2 for the air-exposed hydrogen-terminated diamond surface to 2.16 × 1013/cm2 following MoO3 deposition. This demonstrates the potential to improve the stability and performance of hydrogen-terminated diamond electronic devices through the incorporation of high electron affinity transition metal oxides.

Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations

The effect chlorine addition to the gas mixture has on the surface chemistry in the chemical vapour deposition (CVD) process for silicon carbide (SiC) epitaxial layers is studied by quantum-chemical calculations of the adsorption and diffusion of SiH2 and SiCl2 on the (000-1) 4H–SiC surface. SiH2 was found to bind more strongly to the surface than SiCl2 by approximately 100 kJ mol−1 and to have a 50 kJ mol−1 lower energy barrier for diffusion on the fully hydrogen-terminated surface. On a bare SiC surface, without hydrogen termination, the SiCl2 molecule has a somewhat lower energy barrier for diffusion. SiCl2 is found to require a higher activation energy for desorption once chemisorbed, compared to the SiH2 molecule. Gibbs free energy calculations also indicate that the SiC surface may not be fully hydrogen terminated at CVD conditions since missing neighbouring pair of surface hydrogens is found to be a likely type of defect on a hydrogen-terminated SiC surface.

Analysis of diamond surface channel field-effect transistors with AlN passivation layers

Diamond surface channel field effect transistors were passivated with thin AlN layers grown by metal-organic chemical vapor deposition in order to improve the chemical stability of the surface-near p-type channel. Electrical characterization showed that the surface-near conductivity in the diamond is preserved during AlN overgrowth if the process temperature does not exceed 800 °C. However, the sheet carrier density is decreased by a factor of about 5 compared to the unpassivated hydrogen-terminated surface. A combination of TEM and XPS analysis showed that this effect is not induced by a partial modification of the surface termination or by a polarization of the AlN passivation. The preserved, but reduced surface-near conductivity in the diamond can however be explained by a hydrogen double bond between the diamond and the AlN film. Field-effect transistor structures fabricated on the passivated diamond substrates showed stable operation up drain-source voltages to −70 V and might therefore be promising candidates for future high-voltage applications.

Structure, morphology, and photoluminescence of porous Si nanowires: effect of different chemical treatments

The structure and light-emitting properties of Si nanowires (SiNWs) fabricated by a single-step metal-assisted chemical etching (MACE) process on highly boron-doped Si were investigated after different chemical treatments. The Si nanowires that result from the etching of a highly doped p-type Si wafer by MACE are fully porous, and as a result, they show intense photoluminescence (PL) at room temperature, the characteristics of which depend on the surface passivation of the Si nanocrystals composing the nanowires. SiNWs with a hydrogen-terminated nanostructured surface resulting from a chemical treatment with a hydrofluoric acid (HF) solution show red PL, the maximum of which is blueshifted when the samples are further chemically oxidized in a piranha solution. This blueshift of PL is attributed to localized states at the Si/SiO2 interface at the shell of Si nanocrystals composing the porous SiNWs, which induce an important pinning of the electronic bandgap of the Si material and are involved in the recombination mechanism. After a sequence of HF/piranha/HF treatment, the SiNWs are almost fully dissolved in the chemical solution, which is indicative of their fully porous structure, verified also by transmission electron microscopy investigations. It was also found that a continuous porous Si layer is formed underneath the SiNWs during the MACE process, the thickness of which increases with the increase of etching time. This supports the idea that porous Si formation precedes nanowire formation. The origin of this effect is the increased etching rate at sites with high dopant concentration in the highly doped Si material.

Low-Temperature Transport Properties of Holes Introduced by Ionic Liquid Gating in Hydrogen-Terminated Diamond Surfaces

The surface conductivity of (111)- and (100)-oriented hydrogen-terminated diamonds was investigated at low temperatures for different carrier densities. The carrier density was controlled in a wide range in an electric double-layer transistor configuration using ionic liquids. As the carrier density was increased, the temperature dependences of sheet resistance and mobility changed from semiconducting to metallic ones: the sheet resistance and mobility for the (111) surface were nearly independent of temperature for a sheet carrier density of \({\approx}4\times 10^{13}\) cm-2, indicating metallic carrier transport. It was also found that the interface capacitance, determined from the gate voltage dependence of the Hall carrier density, depended significantly on the crystal orientation.

Diamond Nanoelectrode Arrays for the Detection of Surface Sensitive Adsorption

Nanocrystalline diamond nanoelectrode arrays (NEAs) have been applied to investigate surface-sensitive adsorption phenomena at the diamond-liquid interface. The adsorption of neutral methyl viologen (MV(0) ) was used as a model system. The adsorption of MV(0) was examined on hydrogen- and oxygen-terminated surfaces. On the hydrogenated nanoelectrode surface, a sharp anodic stripping peak was observed upon oxidation of MV(0) , revealing strong adsorption of MV(0) . In contrast, a sigmoidal voltammogram was recorded with an oxygenated electrode surface, indicating there was no MV(0) adsorption. The changes in the shapes of these voltammograms are due to the drastic changes that occur in the diffusion profiles during the transition. The diffusion profile changes from hemispherical diffusion on oxygen-terminated surfaces to thin-layer electrochemistry upon adsorption on hydrogen-terminated surfaces. Different types and concentrations of buffer solutions were then used to vary the interaction of MV(0) with diamond NEAs. The results suggest that the adsorption of MV(0) on hydrogen-terminated diamond NEAs is controlled by hydrophobic interactions. Therefore, diamond NEAs are ideal for the study of adsorption phenomena at the liquid-solid interface with voltammetry.

Construction and surface enhanced Raman scattering activity of gold nanoparticles array on boron doped diamond film

Surface functionalization of diamond with amine groups and immobilization of gold nanoparticles (AuNPs) on boron doped nanocrystalline diamond (BDND) films deposited by microwave plasma chemical vapor deposition were investigated. Hydrogen-terminated BDND film surfaces were activated through bonding with allylamine molecules under UV light irradiation. The resulting diamond surfaces were characterized by using X-ray photoelectron spectroscopy and water contact angle measurement. The amine groups were successfully bonded covalently on the BDND diamond surface via a direct photochemical reaction with allylamine. Gold nanoparticles with the average size of 15 nm were then further self-assembled on the amine-terminated diamond surface by immersing the film surface into the gold colloidal solution, and a dense and well distributed AuNPs array in two dimensions with controlled density was obtained. Standard Rhodamine 6G probe molecules were used to access the surface enhanced Raman scattering (SERS) activity of the prepared new SERS substrate based on AuNPs modified BDND film. The results indicated that such AuNPs modified BDND film showed an excellent and stable SERS activity in the low concentration detection of R6G due to the electromagnetic enhancement mechanism.

Hydrogen termination of CVD diamond films by high-temperature annealing at atmospheric pressure

A high-temperature procedure to hydrogenate diamond films using molecular hydrogen at atmospheric pressure was explored. Undoped and doped chemical vapour deposited (CVD) polycrystalline diamond films were treated according to our annealing method using a H2 gas flow down to ~50 ml∕min (STP) at ~850 °C. The films were extensively evaluated by surface wettability, electron affinity, elemental composition, photoconductivity, and redox studies. In addition, electrografting experiments were performed. The surface characteristics as well as the optoelectronic and redox properties of the annealed films were found to be very similar to hydrogen plasma-treated films. Moreover, the presented method is compatible with atmospheric pressure and provides a low-cost solution to hydrogenate CVD diamond, which makes it interesting for industrial applications. The plausible mechanism for the hydrogen termination of CVD diamond films is based on the formation of surface carbon dangling bonds and carbon-carbon unsaturated bonds at the applied tempera-ture, which react with molecular hydrogen to produce a hydrogen-terminated surface.

Charge Transport Across High Surface Area Metal/Diamond Nanostructured Composites

High surface area composites featuring metal nanostructures and diamond particles have generated a lot of interest in the fields of heterogeneous catalysis, electrocatalysis, and sensors. Diamond surfaces provide a chemically robust framework for active nanostructures in comparison with sp(2) carbon supports. The present paper investigates the charge transport properties of high surface area films of high-pressure, high-temperature diamond particles in the presence and absence of metal nanostructures, employing electrochemical field-effect transistors. Oxygen- and hydrogen-terminated surfaces were generated on 500 nm diamond powders. Homogeneously distributed metal nanostructures, with metal volume fractions between ca. 5 and 20%, were either nucleated at the diamond particles by impregnation or incorporated from colloidal solution. Electrochemical field-effect transistor measurements, employing interdigitated electrodes, allowed the determination of composite conductivity as a function of electrode potential, as well as in air. In the absence of metal nanostructures, the lateral conductivity of the diamond assemblies in air is increased by over one order of magnitude upon hydrogenation of the particle surface. This observation is consistent with studies at diamond single crystals, although the somewhat modest change in conductivity suggests that charge transport is not only determined by the intrinsic surface conductivity of individual diamond particles but also by particle-to-particle charge transfer. Interestingly, the latter contribution effectively controls the assembly conductivity in the presence of an electrolyte solution as the difference between hydrogenated and oxygenated particles vanishes. The conductivity in the presence of metal nanoparticles is mainly determined by the metal volume fraction, while diamond surface termination and the presence of electrolyte solutions exert only minor effects. The experimental trends are discussed in terms of the electrochemical formation of charge carriers in the diamond particles, percolation theory, and charge screening at the double layer.

Phosphorus atomic layer doping in Ge using RPCVD

Phosphorus atomic layer doping (P-ALD) in Ge is investigated at temperatures between 100°C and 400°C using a single wafer reduced pressure chemical vapor deposition (RPCVD) system. Hydrogen-terminated and hydrogen-free Ge (100) surfaces are exposed to PH3 at different PH3 partial pressures after interrupting Ge growth. The adsorption and reaction of PH3 proceed on a hydrogen-free Ge surface. For all temperatures and PH3 partial pressures used for the P-ALD, the P dose increased with increasing PH3 exposure time and saturated. The saturation value of the incorporated P dose at 300°C is ∼1.5×1014cm−3, which is close to a quarter of a monolayer of the Ge (100) surface. The P dose could be simulated assuming a Langmuir-type kinetics model with a saturation value of Nt=1.55×1014cm−2 (a quarter of a monolayer), reaction rate constant kr=77s−1 and thermal equilibrium constant K=3.0×10−2Pa−1. An electrically active P concentration of 5–6×1019cm−3, which is a 5–6 times higher thermal solubility of P in Ge, is obtained by multiple P spike fabrication using the P-ALD process.

Research on the diamond MISFET

Based on the hydrogen-terminated surface channel diamond material, a 1 μm gate length diamond metal—insulator—semiconductor field-effect transistor (MISFET) was fabricated. The gate dielectric A12O3 was formed by naturally oxidated thin Al metal layer, and a less than 2 pA gate leakage current was obtained at gate bias between −4 V and 4 V. The DC characteristic of the diamond MISFET showed a drain-current density of 80 mA/mm at drain voltage of −5 V, and a maximum transconductance of 22 mS/mm at gate—source voltage of −3 V. With the small signal measurement, a current gain cutoff frequency of 2.1 GHz was also obtained.

Computational investigation of the mechanical and tribological responses of amorphous carbon nanoparticles

Nanoparticles are a class of materials that have seen increasing use as friction and wear reducers in tribological applications. Amorphous carbon (a-C) films have been the subject of significant scientific and industrial interest for use as solid-state lubricants. Here, we present classical molecular dynamics simulations to investigate the mechanical and tribological responses of a-C nanoparticles that are subjected to external forces between hydrogen-terminated diamond surfaces. Over the range of a-C nanoparticle diameters (2–5 nm) and hydrogenation (0%–50%) considered, the simulations predict a consistent mechanical response where each nanoparticle is highly elastic. The simulations predict that the transition from elastic to plastic response is directly related to an increase in the percentage of carbon-carbon crosslinking within the individual nanoparticles. Contrarily, the simulations also predict that the tribological response is noticeably impacted by changes in diameter and hydrogenation. This is because during friction, hydrogen passivates the unsaturated carbon atoms near the nanoparticle's surface, which prevents interfacial bond formation and allows the nanoparticle to roll within the interface. From these findings, it is demonstrated that a-C nanoparticles are able to provide good tribological performance only when sufficient chemical passivation of the nanoparticles is maintained.

Understanding anodic wear at boron doped diamond film electrodes

This research investigated the mechanisms associated with anodic wear of boron-doped diamond (BDD) film electrodes. Cyclic voltammetry (CV), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) were used to measure changes in electrode response and surface chemistry as a function of the charge passed and applied current density. Density functional theory (DFT) modeling was used to evaluate possible reaction mechanisms. The initial hydrogen-terminated surface was electrochemically oxidized at lower potentials than water oxidation (≤ 1.83V/SHE), and was not catalyzed by the hydrogen-terminated surface. In the region where water oxidation produces hydroxyl radicals (OH), the hydrogen-terminated surface may also be oxidized by chemical reaction with OH. Oxygen atoms became incorporated into the surface via reaction of carbon atoms with OH, forming both CO and C-OH functional groups, that were also detected by XPS measurements. Experimental and DFT modeling results indicate that the oxygenated diamond surface lowers the potential for activationless water oxidation from 2.74V/SHE for the hydrogen terminated surface to 2.29V/SHE for the oxygenated surface. Electrode wear was accelerated at high current densities (i.e., 500mAcm−2), where SEM results indicated oxidation of the BDD film resulted in significant surface roughening. These results are supported by EIS measurements that document an increase in the double-layer capacitance as a function of the charge passed. DFT simulations provide a possible mechanism that explains the observed diamond oxidation. DFT simulation results indicate that BDD edge sites (=CH2) can be converted to COOH functional groups, which are further oxidized via reactions with OH to form H2CO3(aq.) with an activation energy of 58.9kJmol−1.

Maximum hole concentration for Hydrogen-terminated diamond surfaces with various surface orientations obtained by exposure to highly concentrated NO2

NO2 exposure drastically increases the hole concentration on the surface of hydrogen (H)-terminated diamond. When the NO2 gas concentration is higher than 300ppm, the saturated hole sheet concentration ps stays the same. Therefore, the ps value is regarded as the high limit of the concentration of holes on H-terminated diamond surface, ps,max. In this work, we compared ps,max, mobility μ, and sheet resistance Rs for (100), (110), and (111) H-terminated surfaces of chemical-vapor-deposited single-crystal diamond. On (110), (111), (100) surfaces, the ps,max values are 1.717×1014 and 1.512×1014cm−2, and 0.981×1014, respectively. This result supports the first-principle calculations: the hole concentration depends on the energy difference between the valence band maximum and the unoccupied orbitals of adsorbent NO2 molecules. We have achieved Rs of 719.3 Ω/sq (ps=1.456×1014cm−2 and μ=59.6 cm2 V−1s−1), the lowest reported so far, on (111) surfaces under 20,000-ppm NO2 atmosphere.

Changes in Minority Carrier Lifetime of Hydrogen-Terminated Si Surfaces in Dry- and Wet-Air

Minority carrier lifetime of HF-treated Si(100) surfaces decreases in wet-air more rapidly than in dry-air. XPS measurements show that OH and O are mainly included in Si by keeping in wet-air and dry-air, respectively. OH cannot form network, and thus backbonded OH generates Si dangling bonds. On the other hand, backbonded O can form Si-O-Si networks, leading to the higher lifetime due to lower defect density. When the atmosphere is switched from dry-air to wet-air, the lifetime greatly decreases, and the workfunction increases. These changes are attributed to dominant formation of backbonded OH.

Simple fabrication of nanostructured silicon and photoluminescence

Porous Si (PS) was fabricated simply by electrochemical anodic etching of a Si wafer. By a combination of SEM, EDX and infrared spectrum measurements hydrogen-terminated crystalline Si surfaces were identified. Transparent Si nanocolloids were obtained via thermally-initiated hydrosilation of the hydrogenated Si surfaces reaction with unsaturated 1-undecene. In contrast to weak luminescence in the hydrogenated PS dispersed in toluene, intense luminescence was observed in the Si nanocolloid. In the Si nanocolloid, both strong luminescence and long-term stability can be correlated with alkylated passivation, as formation of Si–C bonds identified in infrared spectrum. Additionally, the oxidation effect was noticeable in affecting the luminescence of nano-Si during the process.