Trimethylboron Research Articles

Chemical vapor deposition of amorphous boron carbide coatings from mixtures of trimethylboron and triethylboron

Amorphous BxC films were deposited from the coreaction of triethylboron (TEB) and trimethylboron (TMB) at 700 °C in H2. We observed that combining both precursors allows us to balance their deposition kinetics and yields higher growth rates. Quantitative analysis by x-ray photoelectron spectroscopy shows that a wide range of B/C ratios between 0.7 and 4.1 could be obtained by varying the TEB:TMB ratio. Raman spectroscopy was used to assess the bonding in the films that gradually evolved from a structure similar to that of a-B, to a mixture of half-icosahedra embedded in a carbon matrix to a graphitic structure, as the carbon content increased. The addition of TMB in the gas phase was found to result in a decrease in elasticity and hardness but an improved adhesion, resulting in complex crack patterns upon cleaving, such as sinusoidal cracks and loops. On the one hand, the incorporation of carbon from TMB leads to an increasing contribution of the softer carbon matrix, to the detriment of polyhedral B–C structures, which in turn decreases Young’s modulus and hardness. On the other hand, it suggests that near the film-substrate interface, the presence of the carbon matrix affords a high density of strong carbon-based bonds, resulting in improved adhesion and preventing delamination of the coatings.

Nanocrystalline silicon-oxygen based tunneling recombination junctions in perovskite/silicon heterojunction tandem solar cells

The tunneling recombination junction (TRJ) of efficient perovskite/silicon heterojunction tandem cells should possess excellent optoelectronic characteristics to minimize light parasitic absorption and carrier transport/recombination losses. One effective approach is the utilization of hydrogenated nanocrystalline silicon-oxygen (nc-SiOx: H) materials, which characteristics can be controlled by adjusting the flow rate of CO2 and doped gas during the fabrication process. By carefully screening the material properties, the suitable n/p-doped nc-SiOx: H materials can be identified. For n-nc-SiOx: H, it is necessary to use a lower CO2 flux and perform moderate doping, while for p-nc-SiOx: H, strict control of CO2 and Trimethylboron (TMB) fluxes at lower levels is essential to preserve the electrical properties. Compared to the TRJ based on nc-Si: H, the TRJ based on nc-SiOx: H exhibits notable improvements in the open-circuit voltage (VOC) and short-circuit current density (JSC) of the tandem cell. The VOC increases by 38 mV (from 1.822 V to 1.864 V), and the JSC gains 0.82 mA/cm2 (from 19.00 mA/cm2 to 19.82 mA/cm2). Consequently, the conversion efficiency of the tandem cell, with an aperture area of 0.5091 cm2, is enhanced from 25.70% to 27.16%. Additionally, by considering the distinct lateral and vertical growth characteristics of nc-SiOx: H, the fill factor (FF) experiences a significant enhancement when applied to the TRJ of large-area tandem cells. Specifically, the FF increases from 55.21% to 69.68%. As a result, for a tandem cell with an aperture area of 12.5 cm2, the conversion efficiency improves from 17.87% to 23.39%.

Reduction in Residual Impurities in Chemical Vapor Deposition–Grown Hexagonal Boron Nitride Thin Films

The residual impurities in chemical vapor deposition (CVD)‐grown hexagonal boron nitride (h‐BN) on Al2O3 substrate grown by CVD using BN‐molded susceptor with B2H6 as a boron precursor are investigated. The Si, C, and O residual impurities of 2.6 × 1016, 3.4 × 1018, and 2.0 × 1017 cm−3 are achieved. These concentrations are two orders of magnitude lower than those when grown on the SiC‐coated graphite susceptor using trimethylboron (TMB) and the lowest values in the CVD‐grown h‐BN thin films. The cathodoluminescence spectrum has 5.5 eV as a broad peak and 4 eV as several peaks. The free excitonic recombination at the higher energy emission (5.79 eV) is owing to the reduction in residual impurities. The absorption coefficient of 1 × 104 cm−1 is observed in the energy range below 5 eV only when using the SiC‐coated graphite susceptor, which is most probably associated with Si contamination in the BN layer. The negligible absorption coefficient when using the BN‐molded susceptor presents high‐purity h‐BN signature.

Influence of the Dopant Gas Precursor in P-Type Nanocrystalline Silicon Layers on the Performance of Front Junction Heterojunction Solar Cells

Silicon heterojunction solar cells can employ p-type hydrogenated nanocrystalline silicon nc-Si:H(p) on their front side, since these can provide better transparency and contact resistance compared to hydrogenated p-type amorphous silicon layers. We investigate here the influence of trimethyl boron (TMB) and BF3 as dopant source on the layer properties and its performance in solar cells. Both gases enable high efficiencies but yield a different crystallinity and effective doping. A high BF3 flow lowers the series resistance through a low activation energy of dark lateral conductivity and maintains a high crystallinity. This allows fill factors up to 83%, however with the apparition of a parasitic absorption in the UV. A low TMB flow enables simultaneously a high crystallinity and a low activation energy. As an illustration of this layer potential, a 23.9%-certified efficiency is achieved with a 2 × 2 cm2 screen-printed device. We finally suggest that similar transport versus transparency trade-offs can be reached for both dopant types for front junction application, while high BF3 flow allowing lower series resistance might be of interest when placed on the rear side.

Low-resistivity p-type a-Si:H/AZO hole contact in high-efficiency silicon heterojunction solar cells

Decreasing the contact resistance between hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxide film (TCO) is beneficial for achieving high efficiency silicon heterojunction (SHJ) solar cells. This study reports the implementation of trimethyl boron B(CH3)3 (TMB) doped p-type a-Si:H (a-Si:H(p)) film as hole transport layer contacting with indium-free aluminum doped zinc oxide (AZO) in SHJ solar cells. The influence of doping concentration on the nanostructure of a-Si:H(p), TCO/a-Si:H(p) contact resistivity as well as the resultant cell performance was systematically investigated. It was found that excessive TMB doping results in more carbon and voids inside the films and reduces the doping efficiency, lowering the conductivity and increasing the contact resistivity. a-Si:H(p) film with low defect density and high doping level was obtained at a moderate doping concentration, which facilitates tunneling transport for holes to overcome the high energy barrier at the a-Si:H(p)/AZO interface and results in a low contact resistivity down to 0.14 Ωcm2. The optimized low-resistivity a-Si:H(p)/AZO contact enables a fill factor above 81% and efficiency of 23.6% for M2 SHJ solar cells, which is comparable with 23.7%-efficient cells using traditional tin doped indium oxide (ITO). To our knowledge, this is the highest efficiency for AZO-implemented SHJ cells without double anti-reflection layer and silver back reflector. This work provides design principles on how to achieve high-efficiency SHJ cells with low resistive loss at the hole contact side via doping engineering.

Comparative Study of Boron Precursors for Chemical Vapor‐Phase Deposition‐Grown Hexagonal Boron Nitride Thin Films

Two different boron precursors, diborane (B2H6) and trimethyl boron ((CH3)3B, TMB), are investigated for chemical vapor‐phase deposition (CVD)‐grown hexagonal boron nitride (h‐BN) on α‐Al2O3 (0001) substrates. The BN layer grown using TMB includes a large amount (2 × 1020 cm−3) of carbon atoms, which is 60 times higher than that in the BN layer grown using B2H6. The X‐ray diffraction 2θ/ω scans for BN film grown using B2H6 exhibit the h‐BN (002) peak. The BN film obtained using TMB includes turbostratic BN (t‐BN). The E2g Raman peak frequencies in B2H6 and TMB h‐BN are observed at 1368.8 and 1369.7 cm−1, respectively. The Raman peak shift to a higher frequency indicates that a larger compressive strain is induced using TMB than using B2H6. The full width at half maximum of the B2H6 and TMB Raman peak frequencies is 21.8 and 42.7 cm−1, respectively. The cathodoluminescence spectra of B2H6 h‐BN show the band‐edge emissions at 225 and 232 nm, whereas only a 300 nm broadband is obtained in TMB h‐BN. It is suggested that the carbon atoms in TMB prevent the formation of highly crystalline h‐BN thin films.

Impact of growth temperature on the structural properties of BGaN films grown by metal-organic vapor phase epitaxy using trimethylboron

BGaN containing boron (B) atoms with a large neutron capture cross section is expected as a novel neutron detecting semiconductor material. Many studies of the BGaN growth mechanism have been conducted in order to fabricate high crystallinity BGaN films. In this study, BGaN films were prepared using trimethylboron (TMB) as a metal-organic B source to investigate the impact of growth temperature. When the growth temperature was increased, the formation and incorporation of cubic phase was observed, voids were formed in the BGaN film, and the growth rate decreased. In order to clarify these phenomena, the decomposition energy and the decomposition temperature in the desorption region of BGaN growth were examined from the evaluation of the temperature dependency of BGaN growth rate. Experimental results showed that the desorption rate of adatoms in BGaN growth is different from that of GaN growth. This could originate from a change in the desorption process at the surface due to different bonding states caused by the adsorption of B atoms.

Influence of p- and n-type doping gases on nanoparticle formation in SiH4/H2 radiofrequency plasma discharges used for polymorphous silicon thin film deposition

We study the effect of the addition of p- or n-type doping gases, trimethylboron (TMB) or phosphine, respectively, on cluster formation in the SiH4/H2 plasma used for the deposition of hydrogenated polymorphous silicon thin films. The formation of clusters is monitored using time-resolved measurements of the second harmonic of the radio frequency current J2. We show that the addition of PH3 does not change the cluster formation, while the addition of a small amount of trimethylboron strongly affects its charging behavior, preventing to some extent its agglomeration. The most ordered pm-Si:H thin films are obtained under conditions from which not only clusters but also larger silicon agglomerates are formed in the plasma and contribute to the deposition. The inhibiting role of TMB on the agglomeration and powder formation is evidenced by the smoothness of p-type films at high rates, as deduced from the sample surface topography obtained by atomic force microscopy measurements.

Rational design of nucleophilic amine sites via computational probing of steric and electronic effects

Accessibility of the nucleophilic site in organocatalysts is essential to ensure adequate catalytic activity. Gas-phase trimethylborane (TMB) Lewis basicity and Brønsted proton basicity of several amine based organocatalysts have been calculated using the CBS-QB3 model chemistry. This TMB basicity scale can, as opposed to the proton basicity scale, account for steric effects encountered in the initial nucleophilic attack of the nitrogen free electron pair on a substrate. Since such a step is the first one in several amine catalyzed reactions, severe steric hindrance of the nucleophilic center would render the catalyst ineffective. Comparing the TMB basicity and proton basicity with the experimentally observed catalytic activity of both homogeneous and heterogeneously supported amine sites found in literature for the aldol reaction of acetone with 4-nitrobenzaldehyde showed that, due to the inclusion of these steric effects, the TMB basicity scale is a much better predictor of catalytic activity than the proton basicity. According to this computational Lewis basicity scale, potential steric hindrance in alternative nitrogen containing active sites was probed. This resulted in 3-propylpyrrolidine being proposed among the most promising monofunctional amine groups and 1-(methylamino)propan-2-ol among the most promising bifunctional amine-hydroxyl groups for heterogeneous aldol reaction catalysts.

Uniform p-type doping of silicon nanowires synthesized via vapor-liquid-solid growth with silicon tetrachloride

Boron-doped silicon nanowires (SiNWs) grown by the vapor-liquid-solid growth mechanism using silicon tetrachloride (SiCl4) as the silicon precursor and trimethylboron (TMB) as the boron source were studied to understand the axial and radial doping uniformity. TMB-doped SiNWs with diameters up to 400 nm and lengths > 7.5 μm were integrated into a global back-gated test structure with multiple electrodes for electrical characterization. From gate modulated measurements, the SiNWs were confirmed to be heavily doped p-type. Multiple four point resistivity measurements across a total length of 7.5 μm were taken on as-grown SiNWs. Resistivity, corrected for surface charge, was determined to be 0.01 +/− 0.002 Ω cm along the entire length of the as-grown boron doped SiNWs. This was also observed in the axial direction for etched SiNWs, with corrected resistivity of 0.01 +/− 0.003 Ω cm, therefore confirming the uniform p-type doping of SiNWs using TMB and SiCl4 as precursors.

Gas Phase Chemistry of Trimethylboron in Thermal Chemical Vapor Deposition

Alkylboranes, such as trimethylboron (TMB) and triethylboron (TEB), are promising alternative precursors in low-temperature chemical vapor deposition (CVD) of boron-containing thin films. In this study, CVD growth of B–C films using TMB and quantum-chemical calculations to elucidate a gas phase chemical mechanism were undertaken. Dense, amorphous, boron-rich (B/C = 1.5–3) films were deposited at 1000 °C in both dihydrogen and argon ambients, while films with crystalline B4C and B25C inclusions were deposited at 1100 °C in dihydrogen. A script-based automatization scheme was implemented for the quantum-chemical computations to enable time efficient screening of thousands of possible gas phase CVD reactions. The quantum-chemical calculations suggest TMB is mainly decomposed by an unimolecular α-H elimination of methane, which is complemented by dihydrogen-assisted elimination of methane in dihydrogen.

Initial studies of plasma facing component surface conditioning in the national spherical tokamak experiment upgrade with the materials analysis particle probe

An innovative PFC diagnostic, the Materials Analysis Particle Probe (MAPP) was used to study the chemistry of Plasma Facing Components (PFCs) in the National Spherical Tokamak Experiment Upgrade (NSTX-U). NSTX-U used boronization as conditioning strategy during the 2015–2016 experimental campaign. Deposition with ∼9.1g of deuterated tri-methyl boron (d-TMB) in a helium glow discharge resulted in coatings with an equivalent thickness of 7.0nm at the lower outer divertor region. MAPP was used to capture for the first time the in-vacuo evolution of the chemical state of ATJ graphite PFCs after boronization and plasma exposure via X-ray Photoelectron Spectroscopy (XPS) on a day-to-day basis. The XPS data shows the formation of B4C on the PFCs surface during boronization. We observed the gradual increase of the B2O3 fraction in the coatings with plasma exposures. In contrast, we measured the formation of non-stoichiometric oxides when the samples were only exposed to residual gases and an argon vent. MAPP data revealed erosion and oxidation of deposited boron coatings over the course of tens of shots (a time resolution improved several orders-of-magnitude compared to post-campaign PFC characterization) that is correlated with the transitory nature of plasma performance enhancement with boron conditioning.

Advances in boronization on NSTX-Upgrade

Boronization has been effective in reducing plasma impurities and enabling access to higher density, higher confinement plasmas in many magnetic fusion devices. The National Spherical Torus eXperiment, NSTX, has recently undergone a major upgrade to NSTX-U in order to develop the physics basis for a ST-based Fusion Nuclear Science Facility (FNSF) with capability for double the toroidal field, plasma current, and NBI heating power and increased pulse duration from 1–1.5s to 5–8s. A new deuterated tri-methyl boron conditioning system was implemented together with a novel surface analysis diagnostic. We report on the spatial distribution of the boron deposition versus discharge pressure, gas injection and electrode location. The oxygen concentration of the plasma facing surface was measured by in-vacuo XPS and increased both with plasma exposure and with exposure to trace residual gases. This increase correlated with the rise of oxygen emission from the plasma.

Towards the controlled CVD growth of graphitic B–C–N atomic layer films: The key role of B–C delivery molecular precursor

Graphene-like, ternary system B–C–N atomic layer materials promise highly tunable electronic properties and a plethora of potential applications. However, thus far, experimental synthesis of the B–C–N atomic layers normally yields a microscopic phase-segregated structure consisting of pure C and BN domains. Further, growing the truly ternary B–C–N phase layers with homogenous atomic arrangements has proven to be very challenging. Here, in designing a bettercontrolled process for the chemical vapor deposition (CVD) growth of B–C–N atomic layer films with the minimized C and BN phase segregation, we selected trimethyl borane (TMB), a gaseous organoboron compound with pre-existing B–C bonds, as the molecular precursor to react with ammonia (NH3) gas that serves as the nitrification agent. The use of this unique B–C delivery precursor allows for the successful synthesis of high-quality and large-area B–C–N atomic layer films. Moreover, the TMB/NH3 reactant combination can offer a high level of tunability and control of the overall chemical composition of B–C–N atomic layers by regulating the relative partial pressure of two gaseous reactants. Electrical transport measurements show that a finite energy gap can be opened in the as-grown B–C–N atomic layers and its tunability is essentially dependent on the relative C to BN atomic compositions. On the basis of carefully controlled experiments, we show that the pre-existing B–C bonds in the TMB molecular precursor have played a crucial role in effectively reducing the C and BN phase segregation problem, thereby facilitating the formation of truly ternary B–C–N phase atomic layers.

Selective secondary nucleation controlled (001)-texture in boron doped diamond films by increasing the concentrations of tetramethylsilane and trimethylborane

Diamond shows anisotropic physical and chemical properties, depending on its crystallographic orientations. Owing to the lack of large-scale single crystalline diamond substrates, however, the growth of textured diamond film becomes a popular way in tailoring its optical, electrical, mechanical, and thermal properties for specific applications. In the present study, we show that the (001)-texture of boron doped diamond (BDD) thin film can be controlled by a selective secondary nucleation process. To achieve this, a low concentration of tetramethylsilane (TMS) was introduced to induce a high secondary nucleation rate on the non-{001} diamond facets, hindering their lateral growth. In contrast, the continuous growth and expansion of the {001} facets are kept, which leads to the formation of (001)-textured diamond film. In addition, the orientation of the film can be continuously improved by increasing the concentration of either TMS or trimethylborane (TMB) in the gas phase within a relatively large parameter window. This study not only presents a new mechanism in controlling the textured of diamond, but also sheds some light on the fundamental research regarding the diamond growth.

P- and n-type microcrystalline silicon oxide (μc-SiOx:H) for applications in thin film silicon tandem solar cells

We report on the development and application of p- and n-type hydrogenated microcrystalline silicon oxide (μc-SiOx:H) alloys in tandem thin film silicon solar cells. Our results show that the optical, electrical, and structural properties of μc-SiOx:H can be conveniently tuned over a wide range to fulfil the requirements for solar cell applications. We have shown that adding of PH3gas during deposition tends to increase crystallinity of μc-SiOx:H layers, while additional trimethylboron (TMB) tends to suppress crystalline growth. When applied in tandem solar cells, both p- and n-type μc-SiOx:H lead to a remarkable increase in the top cell current. Taking advantage of low refractive index and high optical band gap of μc-SiOx:H allows the achievement of high efficiencies of 13.1% (initial) and 11.8% (stabilized).

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H2/CH4/CO2 and H2/CH4 gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO2/CH4 ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp3/sp2 ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO2 concentration. Higher sp3/sp2 ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO2. Incorporation of high concentrations of B (up to 1.75 × 1021 cm3) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.

The microstructure and electrochemical properties of boron-doped nanocrystalline diamond film electrodes and their application in non-enzymatic glucose detection

Boron-doped nanocrystalline diamond (BDND) films were deposited on Si(100) by microwave plasma chemical vapor deposition using trimethyl boron as boron source. The surface morphology, microstructure, and electrochemical properties of the BDND films were investigated. Cyclic voltammograms indicated that the BDND film electrode exhibited good reversibility and repeatability of electrode reaction using [Fe(CN)6]3−/4− as redox couple. The non-enzymatic glucose sensor based on the as-prepared BDND film electrode without any modification was developed, and the selective detection of glucose in alkaline solution containing interference species of ascorbic acid and uric acid was demonstrated. The results showed that glucose can be directly oxidized with a wide linear range and high sensitivity, and selectively detected in the presence of uric acid and ascorbic acid in alkaline solution using the as-prepared BDND film electrode.

Wide bandgap p-type window layer prepared by trimethylboron doping at high temperature for a-Si:H superstrate solar cell

Wide bandgap p-type window layer is necessary for silicon thin film solar cell to obtain excellent performance, such as high open-circuit voltage (VOC), and large short-circuit current density (JSC). Instead of the usually used material, SiCx:H or SiOx:H fabricated by incorporating C or O into the Si matrix, and nc-Si:H deposited at a very low temperature, wide bandgap p-type window layer was realized here by doping with trimethylboron (TMB) at a relatively high temperature via plasma enhanced chemical vapor deposition (PECVD). Excellent performance with VOC larger than 900mV was achieved for p–i–n superstrate solar cell on SnO2:F coated glass while the p-type window layer was deposited at 200°C to 250°C. By investigating the influence of the deposition temperature on the p-layer bandgap and microstructure further, it was found that the compromise between wide bandgap and good quality of the p-layer determined the solar cell performance.

Influence of deposition pressure on p-type a-Si:H window layer doped by trimethylboron for a-Si:H superstrate solar cell in plasma enhanced chemical vapor deposition

Wide bandgap (Eg) p-type window layer is very important for silicon based thin film solar cell to obtain high performance, especially high open-circuit voltage (VOC). In this work, the influence of the deposition pressure on the properties of p-type a-Si:H window layer doped by trimethylboron (TMB) in plasma enhanced chemical vapor deposition (PECVD) was investigated systematically by transmission, Raman, and Fourier transform infrared (FTIR) spectroscopies. As a result, high performance hydrogenated amorphous silicon (a-Si:H) p–i–n superstrate solar cell with VOC up to 927mV was successfully achieved on Asahi Type-U SnO2:F coated glass. In this case, excellent wide bandgap p-type a-Si:H window layer was fabricated under a mild deposition condition, including a low hydrogen dilution ratio (H2/SiH4) of 20, a relatively high deposition temperature of 220°C, which was also adopted for the i-layer and n-layer deposition, and a moderate deposition pressure of about 160Pa. We think it is the compromise between wide Eg and good microstructure quality of the p-layer that brings about the good solar cell performance. Such p-type window layer will be very helpful for the fabrication of a-Si:H solar cell, especially of the cell finished in a single PECVD chamber, due to its mild deposition condition.