Boron-doped Layers Research Articles

Impact of firing temperature on fire-through metal contacts to P-doped (n) and B-doped (p) poly-Si

Screen-printed contacts to doped polysilicon (poly-Si) layers with high-temperature fire-through pastes is one of the key enablers for widespread industrial acceptance of passivating contact solar cells. In recent years, rapid progress has been made in the development of pastes for contacting doped poly-Si layers. However, there are still gaps in understanding the behavior of fire-through contacts, especially regarding damage to poly-Si layers. In this work we present a comprehensive study of metal contacts formed with fire-through pastes to both phosphorus-doped ( n + ) poly-Si and boron-doped ( p + ) poly-Si layers. The contacts are fired at different temperatures and characterized by evaluating specific contact resistivity and recombination current density under the metal contacts. The metal crystallites formed at the interface between metal fingers and poly-Si is quantitatively analyzed to determine their area fraction. It is observed that the area fraction of the crystallites tends to saturate at higher temperatures for n + poly-Si, however such behavior is not observed for p + poly-Si within the range of firing temperatures relevant for industrial solar cells. Metal crystallites appear to agglomerate at higher temperatures for contacts to both n + and p + poly-Si layers which explains increased recombination due to aggressive etching of poly-Si. For Ag contacts to n + poly-Si best results are achieved when the metal crystallites covered 50% of the surface area at the optimum firing temperature. For Ag–Al contacts to p + poly-Si the crystallites covered 30% of the area, but resulted in higher recombination under the contacts. n-type bifacial solar cells are fabricated with n + poly-Si passivating contacts on the rear side and fired at different temperatures. The solar cell results followed the trends shown by the contacts and efficiency reached maximum for an optimized firing temperature. However, the change in efficiency was dominated by the front side contact characteristics. The results obtained on solar cells validate the observations about contact formation and metal recombination. Etch pits on p + poly-Si observed under SEM at different peak firing temperatures. Different splits represent the peak firing temperatures. A) 710 B) 730 C) 750 D) 770 E) 790. • In depth analysis of fired-through contacts formed to 150 nm doped poly-Si layers at different temperatures. • Contact resistivity improved with increasing temperature, attributed to increased number of metal crystallites. • Excellent contact resistivity ≈0.5 mΩ/cm 2 and ≈4 mΩ/cm 2 achieved for Ag and Ag–Al fired through contacts respectively. • The J 0,metal increased with increasing temperature due to increasingly aggressive etching of the poly-Si layer. • An ideal metal paste should result in optimum number of crystallites that do not extensively etch the poly-Si layer.

Sol-gel Synthesis of Boron Doped TiO2/hollow Glass Bubbles Composite Powders for Photocatalytic Degradation of Azophloxine

Background: B-TiO2 was supported on the surface of iM16K glass bubbles to achieve a suitable density for the B-TiO2/iM16K composite hollow spheres. Aeration or stirring in the wastewater can lead to thorough mixing of photocatalyst and wastewater. Solid-water separation is quite easy because the materials can float on the water surface while stopping aeration or stirring. Methods: The iM16K glass bubbles were used to prepare boron-doped B-TiO2/iM16K composite hollow spheres through a sol-gel route. The materials were characterized by X-ray diffraction, scanning electron microscope, Fourier transform infrared spectroscopy, UV-Visible diffuse reflectance spectrometry, and N2 adsorption-desorption techniques. Photocatalytic degradation of azophloxine on the composites was determined. Results: The bandgap energy of the B-TiO2/iM16K composite was slightly less than 3.0 eV when the calcination temperature was below 500oC. The sample calcined at 350oC had a BET surface area of 88.6 m2 /g, while the value of the sample calcined at 800oC was 1.2 m2 /g. The maximum photocatalytic degradation efficiency was obtained for the sample calcined at 450oC, and nearly all of the original azophloxine molecules were decomposed after 120 min of irradiation. Photocatalytic degradation efficiency after 30 min of irradiation was enhanced from 18.8% to 47.9% when B-TiO2 dosage increased from 100 to 800 mg/L. Conclusion: Crystallization of anatase TiO2 was temperature dependent, and the properties of B-TiO2/iM16K composite hollow spheres were affected by phase composition of the boron-doped TiO2 layer. The change in calcination temperature can have significant effect on photocatalytic degradation of azophloxine. The production of hydroxyl radical depended on photocatalytic activity of the B-TiO2/iM16K composite hollow spheres.

ANALYTICAL MODELING AND SIMULATION OF THE CHEMICAL ETCHING PROCEss OF THE BORON-DOPED SILICON LAYERS FROM BBr3 SOURCE in the micromachining technology

The boron-doped silicon layers commonly used to fabricate various micro-mechanical elements, particularly silicon membranes, are efficient stop-etching barriers, so their control is crucial in the bulk micromachining technology. As the properties of the boron doped layers depends on the type of the doping source and on the diffusion depth in the silicon bulk, a particular analysis should be applied in each specific case. In this paper there are reported the results of an analytical modeling of the boron diffusion profile in silicon, which are applied to simulate the boron diffusion profile at high diffusion temperatures (1050°C, 1100°C, 1150°C and 1200°C), emphasizing a dependence of the diffusion coefficient as a square root of the boron diffusion concentration. It is shown that the comparison of the theoretical results and some experimental diffusion data after diffusion at 1050°C shows a very good agreement, well supporting the analytical modeling. On this basis, the chemical etching rate and the etching time are simulated as a function of the boron diffusion depth in silicon for various etching solutions and etching conditions, providing suitable guiding curves for practical applications in the bulk micromachining technology

Properties of boron-doped (113) oriented homoepitaxial diamond layers

Although significant efforts have been dedicated to optimize the quality of epitaxial diamond layers, reported properties of diamond based electronic devices do not yet approach expected theoretical values. Recent works indicate that boron-doped and phosphorous-doped diamond can be grown on atomically stepped (113) surfaces (A. Tallaire et al., 2016; M.-A. Pinault-Thaury et al., 2019), however the electrical properties of these layers have not been studied in detail. In this work, we report on structural and electrical properties of boron-doped epitaxial diamond layers grown on (113) substrates. Properties of the diamond layers have been investigated by means of scanning electron microscopy, atomic force microscopy, Hall effect, secondary ion mass spectrometry and Raman spectroscopy. Our results show that boron-doped diamond layers can be grown on (113) substrates at high deposition rates with atomically flat surfaces, excellent electrical properties and high boron incorporation efficiency.

Impact of methane concentration on surface morphology and boron incorporation of heavily boron-doped single crystal diamond layers

The methane concentration dependence of the plasma gas phase on surface morphology and boron incorporation in single crystal, boron-doped diamond deposition is experimentally and computationally investigated. Starting at 1%, an increase of the methane concentration results in an observable increase of the B-doping level up to 1.7×1021 cm−3, while the hole Hall carrier mobility decreases to 0.7±0.2 cm2 V−1 s−1. For B-doped SCD films grown at 1%, 2%, and 3% [CH4]/[H2], the electrical conductivity and mobility show no temperature-dependent behavior due to the metallic-like conduction mechanism occurring beyond the Mott transition. First principles calculations are used to investigate the origin of the increased boron incorporation. While the increased formation of growth centers directly related to the methane concentration does not significantly change the adsorption energy of boron at nearby sites, they dramatically increase the formation of missing H defects acting as preferential boron incorporation sites, indirectly increasing the boron incorporation. This not only indicates that the optimized methane concentration possesses a large potential for controlling the boron concentration levels in the diamond, but also enables optimization of the growth morphology. The calculations provide a route to understand impurity incorporation in diamond on a general level, of great importance for color center formation.

Deposition of Boron-Doped Thin CVD Diamond Films from Methane-Triethyl Borate-Hydrogen Gas Mixture

Boron-doped diamond is a promising semiconductor material that can be used as a sensor and in power electronics. Currently, researchers have obtained thin boron-doped diamond layers due to low film growth rates (2–10 μm/h), with polycrystalline diamond growth on the front and edge planes of thicker crystals, inhomogeneous properties in the growing crystal’s volume, and the presence of different structural defects. One way to reduce structural imperfection is the specification of optimal synthesis conditions, as well as surface etching, to remove diamond polycrystals. Etching can be carried out using various gas compositions, but this operation is conducted with the interruption of the diamond deposition process; therefore, inhomogeneity in the diamond structure appears. The solution to this problem is etching in the process of diamond deposition. To realize this in the present work, we used triethyl borate as a boron-containing substance in the process of boron-doped diamond chemical vapor deposition. Due to the oxygen atoms in the triethyl borate molecule, it became possible to carry out an experiment on simultaneous boron-doped diamond deposition and growing surface etching without the requirement of process interruption for other operations. As a result of the experiments, we obtain highly boron-doped monocrystalline diamond layers with a thickness of about 8 μm and a boron content of 2.9%. Defects in the form of diamond polycrystals were not detected on the surface and around the periphery of the plate.

Switching Transient Analysis and Characterization of an E-Mode B-Doped GaN-Capped AlGaN DH-HEMT with a Freewheeling Schottky Barrier Diode (SBD)

This paper presents a systematic study of Al0.23Ga0.77N/GaN/AlxGa1−xN double-heterojunction high-electron-mobility transistors (DH-HEMTs) with a boron-doped P+ GaN cap layer under the gate. The boron-doped GaN cap layer shows great potential to form a high-bandgap Schottky gate in DH-HEMT devices to increase the resistivity of the GaN cap with excellent structural characteristics. Thus, the polarization-induced field in the GaN cap layer can be used to raise the conductive band of the device in the normally OFF operation. In this paper, these AlGaN/GaN power-switching devices with freewheeling Schottky barrier diodes are examined in their working states. In comparison with conventional HEMT power devices, the HEMT with a B-doped GaN cap offers the lowest switching charges, area-specific ON-state resistance, and energy losses. Therefore, this study clearly shows the advantage of GaN transistors for power electronics applications.

Boron-doped graphene synthesis by pulsed laser co-deposition of carbon and boron

Incorporating dopants, such as boron, in graphene, is crucial for many applications in electrochemistry, sensors, photovoltaics, and catalysis. Many routes have been investigated for the preparation of B-doped graphene (BG) films, including chemical processes. A different way to obtain boron-doped layers to better control the concentration of boron in the doped graphene film, is pulsed laser co-ablation of C and B solid sources followed by rapid thermal heating of the B-doped carbon film deposited on a metal catalyst. Amorphous a-C:B films, containing 2 at. % boron, are synthetized by pulse laser deposition onto a nickel film catalyst. Rapid thermal annealing at 1100 °C leads to the formation of boron-doped graphene films, characterized by Raman, XPS, FEG-SEM, HRTEM and AFM. The results confirm the production of 1–4 layer boron doped graphene films, with a similar 2 at. % boron concentration to that of the a-C:B used as the graphene solid precursor. Boron doping does not modify the nano-architecture of graphene, but increases the concentration of defects in the films. Our results pave a new way for boron doped graphene synthesis using laser processing in a controlled and reproducible way, in particular to achieve designed electrical and chemical properties in various electronic and electrochemical applications.

Unique Layer-Doping-Induced Regulation of Charge Behavior in Metal-Free Carbon Nitride Photoanodes for Enhanced Performance.

Photoinduced charge carrier behavior is critical in determining photoelectrocatalytic activity. In this study, a unique layer-doped metal-free polymeric carbon nitride (C3 N4 ) photoanode is fabricated by using one-pot thermal vapor deposition. With this method, a photoanode consisting of a phosphorus-doped top layer, boron-doped middle layer, and pristine C3 N4 bottom layer, was formed as a result of the difference in thermal polymerization kinetics associated with the boron-containing H3 BO3 -melamine complex and the phosphorus-containing H3 PO4 -dicyandiamide complex. This layer-doping fabrication strategy effectively contributes to the formation of dual junctions that optimizing charge carrier behavior. The ternary-layer C3 N4 photoanode exhibits significantly enhanced photoelectrochemical water oxidation activity compared to pristine C3 N4 , with a record photocurrent density of 150±10 μA cm-2 at 1.23 V vs. RHE. This layer-doping strategy provides an effective means for design and fabrication of photoelectrodes for solar water oxidation.

Annealing of Boron-Doped Hydrogenated Crystalline Silicon Grown at Low Temperature by PECVD.

We investigate low-temperature (<200 °C) plasma-enhanced chemical vapor deposition (PECVD) for the formation of p–n junctions. Compared to the standard diffusion or implantation processes, silicon growth at low temperature by PECVD ensures a lower thermal budget and a better control of the doping profile. We previously demonstrated the successful growth of boron-doped epitaxial silicon layers (p+ epi-Si) at 180 °C. In this paper, we study the activation of boron during annealing via dark conductivity measurements of p+ epi-Si layers grown on silicon-on-insulator (SOI) substrates. Secondary Ion Mass Spectroscopy (SIMS) profiles of the samples, carried out to analyze the elemental composition of the p+ epi-Si layers, showed a high concentration of impurities. Finally, we have characterized the p+ epi-Si layers by low-temperature photoluminescence (PL). Results revealed the presence of a broad defect band around 0.9 eV. In addition, we observed an evolution of the PL spectrum of the sample annealed at 200 °C, suggesting that additional defects might appear upon annealing.

Ohmic Contacts to CVD Diamond with Boron-Doped Delta Layers

Various methods for the formation of ohmic contacts to boron-doped δ layers in CVD-diamond epitaxial structures are investigated. In the first variant, an additional thin heavily doped layer was formed on the diamond surface to which an ohmic contact is formed. Then, the surface p+ layer between the contact pads is etched out; therefore, the current in the structure flows only through a buried δ layer. In the second approach, the doped diamond selectively grows in the contact windows under a metallic mask after preliminary etching of the undoped diamond layer (cap) up to the δ layer. In this case, the heavily doped p+ layer forms an end contact to the δ layer. These two variants differ in terms of the conditions of applicability, the complexity of the manufacturing technology, the value of the contact resistance, and can be used to solve problems in which it is necessary to have a different quality of the contacts, such as the formation of transistor structures or test cells for measuring physical characteristics.

Conductivity of boron-doped diamond at high electrical field

Electrical properties of epitaxial boron-doped diamond layers are studied at high electric fields in a wide range of boron concentration. Current-voltage characteristics co-planar concentric disk ring electrodes devices were measured using transmission line pulsed (TLP) characterization technique. Finite element method shows the electric field between test structures with co-planar plane parallel electrodes, used in previous studies, and concentric disk ring electrodes are comparable for inner ring electrode with a large diameter. The threshold electric field for the observed sudden increase of current, attributed to impurity impact ionization and/or self-heating, decreases from 130 kV·cm−1 to 70 kV·cm−1 with the increasing boron concentration in the epitaxial diamond layer. The measured quasi-static current grows exponentially with the electric field for energy dissipated in the devices limited below 2 μJ.

Highly passivating and blister-free hole selective poly-silicon based contact for large area crystalline silicon solar cells

Passivating the contacts of crystalline silicon (c-Si) solar cells with a poly-crystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) is currently sparking interest for reducing recombination at the interface between the metal electrode and the c-Si substrate. Hole-selective poly-Si/SiOx structures could be particularly relevant to passivate the rear side of mass-produced p-type c-Si solar cells (i.e. PERC solar cells). In this study, we elaborate on the optimization of a hole-selective passivating structure consisting of a boron-doped poly-Si layer on top of a chemically grown thin SiOx. The poly-Si layer is prepared by Plasma Enhanced Chemical Vapor Deposition, which enables single-side deposition. However, if not optimized, this deposition technique leads to degradation of the poly-Si layer through a “blistering” phenomenon due to high hydrogen incorporation in the layer. To tackle this, a study of the interplay between process parameters and blistering is undertaken in order to obtain highly passivating and blister-free poly-Si/SiOx structures. By addition of a hydrogenation step, the implied open circuit voltage (iVoc) provided by the structure is further improved, leading to a maximum value of 734 mV demonstrated on symmetrical samples made from large area wafers. We also conduct a Conductive-Atomic Force Microscopy (C-AFM) study with the aim of investigating the pinholes formation in the SiOx interfacial layer that could explain the transport of free charge carriers within the poly-Si/SiOx structure. We show that the current levels detected by C-AFM are affected by an oxide layer that grows at the poly-Si top surface. We also demonstrate that conductive spots detected by C-AFM are not likely to mirror conductive pinholes within the SiOx layer but are rather linked to the poly-Si layer.

Influence of Surface Orientation on Electrochemical Properties of Boron-Doped Diamond

To study the influence of crystal orientation on the electrochemical properties of boron-doped diamond (BDD), electrodes comprising (100) and (111) homoepitaxial single-crystal layers of BDD were i...

Ohmic graphite-metal contacts on oxygen-terminated lightly boron-doped CVD monocrystalline diamond

A process to obtain ohmic contacts on oxygen-terminated lightly boron-doped CVD monocrystalline diamond films was developed. Samples were contacted by Ti/Au metallic pads in the transmission line model (TLM) configuration. The electric contacts were placed onto a mesa structure produced on the CVD boron-doped layer. One of the samples was additionally implanted with helium ions at 10 keV in order to induce the formation of a graphitic layer underneath the diamond surface before contacting so as to improve electrical conduction. The electrical performance of both devices was characterized by the TLM method and compared. As a result, the sample with metallic electrodes exhibited a small and non-linear electrical conduction, while the graphitic/metallic contacts showed an ohmic behaviour with an estimated specific contact resistance as low as 3.3 × 10−4 Ω·cm2 for a doping level of a few 1017 cm−3. This approach opens the way to more efficient ohmic contacts on intrinsic or low-doped diamond that are crucial for the development of electronic devices and detectors.

Ultra-thin atomic layer deposited aluminium oxide tunnel layer passivated hole-selective contacts for silicon solar cells

In this work, we use ultra-thin thickness-controllable spatial atomic layer deposited (ALD) aluminium oxide (AlOx) tunnel layers, which contain high negative fixed charges (Qf), capped by highly boron-doped polysilicon layers to form tunnel layer passivated contacts. The high Qf of the tunnel layers is expected to enhance the carrier selectivity of these passivated hole-extracting contacts. The dependence of the ALD AlOx tunnel layer contact passivation performance on its thickness is investigated. Furthermore, two different thermal charge activation conditions, i.e., fast firing using a belt furnace at 700 and 800 °C are compared. The best measured recombination current density J0 and implied open-circuit voltage iVoc of the developed AlOx/polysilicon passivated contacts with a symmetrical AlOx/SiNx stack passivation are 6.6 fA/cm2 and 723 mV, respectively. Based on the measured J0 and on the measured total contact resistivity of the passivated contact, the practical efficiency limit of a rear-side full area passivated contact solar cell with a conventionally diffused front side is calculated to be as high as 23.2%. Additionally, three rear-side metallization schemes: (1) thermally evaporated full-area silver contacts; (2) screen-printed non-firing-through aluminium contacts and (3) screen-printed firing-through silver-aluminium contacts, are compared. Finally, rear-emitter solar cells, using a rear-side hole-selective AlOx tunnel layer passivated contact, are fabricated, which shows an efficiency of up to 20.5%. While the proposed hole-selective passivated contact scheme appears to be promising based on the simulation prediction, the efficiency of the fabricated cells is largely limited by the non-optimized front-side reflectance and recombination losses as well as the use of non-optimized rear-side metallization schemes.

Highly boron-doped germanium layers on Si(001) grown by carbon-mediated epitaxy

Smooth and fully relaxed highly boron-doped germanium layers were grown directly on Si(001) substrates using carbon-mediated epitaxy. A doping level of was measured by several methods. Using high-resolution x-ray diffraction we observed different lattice parameters for intrinsic and highly boron-doped samples. A lattice parameter of aGe:B = 5.653 Å was calculated using the results obtained by reciprocal space mapping around the (113) reflection and the model of tetragonal distortion. The observed lattice contraction was adapted and brought in accordance with a theoretical model developed for ultra-highly boron-doped silicon. Raman spectroscopy was performed on the intrinsic and doped samples. A shift in the first order phonon scattering peak was observed and attributed to the high doping level. A doping level of was calculated by comparison with literature. We also observed a difference between the intrinsic and doped sample in the range of second order phonon scattering. Here, an intense peak is visible at for the doped samples. This peak was attributed to the bond between germanium and the boron isotope 11B.

Enhanced lithium-storage performance of Li4Ti5O12 coated with boron-doped carbon layer for rechargeable Li-ion batteries

In this study, the B-doped carbon coating has been applied to enhance the lithium-storage performance of Li4Ti5O12 anode material for Li-ion batteries. The designed B-doped carbon improved Li4Ti5O12 composite (abbreviated as Li4Ti5O12@C-B) is successfully synthesized using a sol-gel approach followed by a microwave heating route. The XRD and HRTEM results demonstrate that the well-crystallized Li4Ti5O12 particles are uniformly coated with the B-doped carbon layer with a thickness of about several nanometers. The existence of boron doping in the carbon layer is also confirmed by XPS and EDX mapping. Compared with the undoped Li4Ti5O12@C, the Li4Ti5O12@C-B anode shows superior electrochemical performances such as higher reversible capacity and better cycling stability. The enhanced lithium-storage property can be attributed to the doped boron element which could bring many amazing improvements on the carbon coating. Therefore, this novel strategy of B-doped carbon coating can be widely used to improve the electronic conductivity of other electrode materials for electrochemical energy storage.

Heavily Boron-Doped Silicon Layer for the Fabrication of Nanoscale Thermoelectric Devices.

Heavily boron-doped silicon layers and boron etch-stop techniques have been widely used in the fabrication of microelectromechanical systems (MEMS). This paper provides an introduction to the fabrication process of nanoscale silicon thermoelectric devices. Low-dimensional structures such as silicon nanowire (SiNW) have been considered as a promising alternative for thermoelectric applications in order to achieve a higher thermoelectric figure of merit (ZT) than bulk silicon. Here, heavily boron-doped silicon layers and boron etch-stop processes for the fabrication of suspended SiNWs will be discussed in detail, including boron diffusion, electron beam lithography, inductively coupled plasma (ICP) etching and tetramethylammonium hydroxide (TMAH) etch-stop processes. A 7 μm long nanowire structure with a height of 280 nm and a width of 55 nm was achieved, indicating that the proposed technique is useful for nanoscale fabrication. Furthermore, a SiNW thermoelectric device has also been demonstrated, and its performance shows an obvious reduction in thermal conductivity.

One-pot environment-friendly synthesis of boron doped graphene-SnO2 for anodic performance in Li ion battery

High specific capacity, availability of tin oxide (SnO2) and improved electrical conductivity and rate capability of boron-doped graphene sheets (BG) promises the composite of the two as a potential anode material for Li ion battery. We report for the first time, one-pot synthesis of SnO2 dispersed boron-doped few layer graphene (SnO2/BG-I) in an environment friendly, hazardous-harmful reducing agent free, hydrogen assisted reduction technique in comparison to complicated two-step conventional process. In addition, a two-step, wet chemical free, greener approach is also employed to achieve SnO2 nanoparticles over BG support (SnO2/BG-II). Both the materials as anode showed enhanced performance compared to un-doped graphene supported SnO2 nanoparticles (SnO2/G). One-pot synthesized SnO2/BG-I as anode in Li ion battery shows comparable performance with a reversible capacity of 348 mAh g−1 at a current density 200 mA g−1, while SnO2/BG-II with higher amount of boron doping, exhibits higher stability over cycles with an excellent reversible capacity of 558 mAh g−1 due to the uniform distribution of SnO2 over boron-doped graphene sheets.