Boron-doped Zinc Oxide Thin Films Research Articles

Boron-Doped Thin Films Fabricated by the Spin Coating Method: The Effect of Doping Concentrations

This work examined the impact of different levels of B-doping on the structural, morphological, optical, and electrical characteristics of ZnO thin films. Boron-doped zinc oxide thin films were deposited on glass substrates using the spin-coating technique. The B concentrations employed were 1, 2, 3, 4, and 5 at. %. The systematic characterizations manifest that the properties of the deposited films were heavily influenced by changing concentrations of B doping. It was found that as the concentration of B-doping increases, the values of grain size decrease. In addition, it was observed that ZnO thin films containing a lower concentration of B dopant exhibited higher transparency. Finally, it was figured out that the resistivity of the films declines dramatically with a higher content of B-doping. The results of our research may initiate further inquiries into the creation of superior thin films.

Boron induced c-axis growth and ammonia sensing signatures of spray pyrolysis deposited ZnO thin films – Relation between crystallinity and sensing

Boron (B) doping in zinc oxide (ZnO) using spray pyrolysis has always led to deterioration of host crystal structure. Unlikely, here, we report the optimized spray pyrolysis deposition parameters for the development of c-axis oriented boron-doped ZnO thin films. As the B concentration increases from 2 to 10 mol%, the hexagonal crystal structure exhibited a highly oriented (002) plane with an increased texture co-efficient. In particular, 2 mol% B dopants promoted the growth of the (101) and (100) planes and increased the crystallite size. Tensile stress in the c-axis plane of 10 mol% B-doped ZnO films leads to the red shift in the band gap (from wide to narrow). Furthermore, 10 mol% B dopants resulted in the enhanced ammonia sensing performance at room temperature. The sensing performance of all the deposited B-doped ZnO films revealed that influence of enhanced planes of host structure energetically favoured the ammonia adsorption.

Growth of low resistivity and high transparency boron-doped zinc oxide film by pulse laser deposition

0.5 wt% boron-doped zinc oxide (BZO) films were fabricated by utilizing pulse laser deposition under different growth temperature ranged from 250 to 450 °C. The effect of the growth temperature on the structural, optical, and electrical properties was investigated and discussed. The crystal structure and orientation of BZO thin films were examined by X-ray diffraction. All of the BZO films under various growth temperatures had strong c-axis (002)-preferred orientation. Optical transparency was high (>80%) over a wide spectral range from 400 nm to 700 nm, and the optical bandgap value of BZO are found to be in the range from 3.18 to 3.47eV. According to the experimental data, the resistivity of the BZO film was optimized at ~1.13 × 10−3 Ω-cm and significantly influenced by the growth temperature.

A contrast in the electronic structures of B ion implanted ZnO thin films grown on glass and silicon substrates by using x-ray absorption spectroscopy

Fabrication of suitable c-axis oriented boron doped zinc oxide (BZO) thin films is vital for many applications. The properties of films critically depend on film preparation methods and the type of substrate. Furthermore, ion implantation at a specific position or depth of a sample substantially affects the properties of films. In this study, a contrast between the electronic structure of ZnO thin films grown on glass and silicon (Si) substrates and then implanted with B ions at 40 keV with a concentration of 5 × 1013 ions/cm2 was investigated. The B ions were implanted using a 3 MV tandem accelerator. Secondary ion mass spectrometry, atomic force microscopy, and x-ray diffraction were used to study the distribution and depth of B ions, surface morphology, and the crystallinity of ZnO and BZO thin films. The preferential orientation, texture, surface roughness, and grain size of the ZnO thin films were affected by B ion implantation. O K-, Zn L3-, and K-edge x-ray absorption near-edge structure (XANES), and Zn K-edge extended x-ray absorption fine structure techniques were used to investigate the electronic structure of the BZO thin films and to determine the substrate influence. O K- and Zn L3-edge XANES spectra of the BZO/glass thin films revealed an enhanced electron transfer from Zn to O as compared to the BZO/Si thin film, implying that B implantation increased the negative effective charge on the O ions. Interestingly, in contrast to the definition of electronegativity, an increase (decrease) in the number of O 2p (Zn 3d/4sp) unoccupied states due to an enhanced O 2p–Zn 3d/4sp hybridization is observed in the BZO/Si thin film as compared to the BZO/glass thin film, suggesting an increase in the number of O 2p-dangling bonds. This would affect the conductivity and luminescent behavior of the ZnO/Si thin films after B doping, which will be useful in optoelectronic applications.

An investigation of radiation shielding properties of boron doped ZnO thin films

In recent years, the production of alternative materials instead of lead in the environmentally sensitive and non-toxic material research in radiation shielding has been gained importance. In this study, transparent pristine and boron (B) doped zinc oxide (ZnO) thin films were synthesized by spray pyrolysis method. The crystalline structure, morphology and optical properties of the produced films were investigated with X-ray diffractometer (XRD), scanning electron microcopy (SEM) and Uv–vis spectrometer. The wurtzite structure of pure ZnO was deformed by boron doping found from XRD results. The hexagonal rods have been worsened with boron doping observed from SEM analysis. The factors related to radiation shielding; mass and linear attenuation coefficients, half value layer, mean free path and tenth value layer of different percentage (5, 10 and 20%) B doped ZnO thin films were examined. To perform these measurements, PTW® electron detector and irradiation have been utilized with 6 MeV energized electrons by using VARIAN®. The obtained results from these measurements can be useful to understanding of radiation shielding performance of ZnO:B thin films.

A study of superstrate amorphous silicon thin film solar cells and modules on flexible BZO glass

Flexible thin film silicon solar modules on heat-resistant transparent flexible substrates are promising to achieve high efficiency by a combination of high-quality silicon thin films and fully monolithic series integration. In this work, performance of superstrate hydrogenated amorphous silicon (a-Si:H) thin film solar cells and modules on flexible glass using boron-doped zinc oxide (BZO) front electrode have been investigated. Compared with conventional glass, BZO thin films on flexible glass exhibited mixed structure of large-sized pyramid and small-sized grain, preferential crystalline orientations of (100) and (110), relatively lower surface roughness and scattering ability. Accordingly, compared with conventional BZO glass, a-Si:H thin film solar cells on flexible BZO glass exhibited a relative increase in open-circuit voltage, fill factor, and efficiency of 2.0, 5.5, and 3.4%, respectively. Finally, ∼50 cm2 flexible a-Si:H thin film solar modules were prepared by fully monolithic series integration using laser scribing, and relatively higher efficiency was achieved by improving thin films uniformity.

Enhanced optical and electrical properties of boron-doped zinc-oxide thin films prepared by using the sol-gel dip-coating method

Undoped ZnO and B-dopoed ZnO (BZO) thin films with 0 to 2.5 at.% B were prepared by using sol-gel synthesis. Their optical and electrical properties and surface morphology were investigated using scanning electron microscopy, photoluminescence (PL), and van der Pauw Hall-effect measurements. All of the thin films were deposited appropriately onto quartz substrates; they exhibited a fibrous root morphology, with structures that changed in size with increasing B concentration. The PL spectra showed near-band-edge (NBE) emissions and deep-level emissions (DLE). The NBE emission and the DLE for the BZO thin films were more blue-shifted than those for the undoped ZnO thin film, and the blue shift increased the efficiency of the NBE emission of the BZO thin films. The Hall-effect data suggested that B doping also improved the electrical properties, such as the carrier concentration, Hall mobility, and resistivity, of the thin films. The resistivity and the Hall mobility decreased with increasing B concentration and were inversely proportional to the carrier concentration.

Improved optical and electrical properties of sol–gel-derived boron-doped zinc oxide thin films

Sol–gel spin-coating was used to grow zinc oxide (ZnO) thin films doped with 0–2.5 at.% B on quartz substrates. The structural, optical, and electrical properties of the thin films were investigated using field-emission scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), ultraviolet–visible spectroscopy, and van der Pauw Hall-effect measurements. All the thin films had deposited well onto the quartz substrates and exhibited granular morphology. The average crystallite size, lattice constants, residual stress, and lengths of the bonds in the crystal lattice of the thin films were calculated from the XRD data. The PL spectra showed near-band-edge (NBE) and deep-level emissions, and B doping varied the PL properties and increased the efficiency of the NBE emission. The optical transmittance spectra for the undoped ZnO and boron-doped zinc oxide (BZO) thin films show that the optical transmittance of the BZO thin films was significantly higher than that of the undoped ZnO thin films in the visible region of the spectra and that the absorption edge of the BZO thin films was blue-shifted. In addition, doping the ZnO thin films with B significantly varied the absorption coefficient, optical band gap, Urbach energy, refractive index, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator strength, average oscillator wavelength, dielectric constant, and optical conductivity of the BZO thin films. The Hall-effect data suggested that B doping also improved the electrical properties such as the carrier concentration, mobility, and resistivity of the thin films.

Effects of Doping Ratio and Thermal Annealing on Structural and Electrical Properties of Boron-Doped ZnO Thin Films by Spray Pyrolysis

Boron-doped zinc oxide (BZO) thin films have been fabricated by spray pyrolysis on a glass substrate. The morphology and electrical properties of the thin films were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed. It was found that [B]/[Zn] ratio altered both the microstructure and concentration of the BZO thin films. The film grain size was reduced by increasing the [B]/[Zn] ratio. The highest Hall mobility was 3.65 cm2 V-1 s-1 for the undoped ZnO thin film, and the highest carrier concentration of 1.0×1019 cm-3 was achieved for the as-deposited BZO thin film with [B]/[Zn] = 1.5 at. %. Conductivity was determined at different measurement temperatures and shallow donors provided the dominate conduction mechanism for the as-deposited BZO thin films. After 600 °C annealing, shallow level reduction and donors with a high activation energy of 129±6 meV in the BZO thin films were characterized, and the shallow donors that dominate the carrier concentration for the as-deposited spray-pyrolized BZO thin film were eliminated.

Textured surface ZnO:B/(hydrogenated gallium-doped ZnO) and (hydrogenated gallium-doped ZnO)/ZnO:B transparent conductive oxide layers for Si-based thin film solar cells

Textured surface un-doped zinc oxide (ZnO) and boron-doped ZnO (ZnO:B) thin films with pyramid-like grains were directly deposited by metal organic chemical vapor deposition (MOCVD) technique and hydrogenated gallium-doped ZnO (HGZO) thin films with good electrical properties were grown by pulsed direct-current magnetron sputtering. In order to obtain high quality transparent conductive oxides (TCOs) for Si-based thin film solar cells, through combining the advantages of MOCVD–ZnO and sputter-HGZO thin films, bi-layer textured surface glass/ZnO:B/HGZO and glass/HGZO/ZnO:B TCO thin films with milky surface were fabricated and investigated in detail. The main research purposes are to explore appropriate surface morphology and improve the electrical properties of TCO layers. Compared with textured surface un-doped ZnO and ZnO:B thin films, both the bi-layer ZnO/HGZO and ZnO:B/HGZO thin films exhibit high electron mobility. The ZnO:B/HGZO thin films present modified pyramid-like surface morphology with high transparencies (T>85%) and excellent electrical properties (resistivity of ~9.28×10−4Ωcm and mobility of ~28.8cm2/Vs). Meanwhile, the HGZO/ZnO thin films exhibit large grain size (~500nm–1000nm) with preferred growth orientations along (100) and (002) crystallographic directions. Bi-layer HGZO/ZnO and HGZO/ZnO:B thin films exhibit relatively higher diffuse transmittance than that of corresponding ZnO and ZnO:B thin films.

Low temperature electrical transport properties of B-doped ZnO films

Low temperature electrical transport properties of boron-doped zinc oxide thin films prepared by metal-organic chemical vapor deposition technique were investigated. The temperature coefficient of resistivities is negative at low temperatures and becomes positive at relatively high temperatures. The magnetic field dependence of resistivity was measured at different temperatures from 2to30K. The samples exhibit negative magnetoresistivity at all measuring temperatures and the absolute values of the negative magnetoresistivities systematically increase with decreasing temperature at a given magnetic field. Weak-localization effect and two-band model were used to analyze the experimental results. Least-squares fits showed that the data can be well described by the theory. The inelastic scattering time is proportional to T−3∕2, suggesting that the electron-electron scattering is the dominant coherent backscattering breaker.

Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3×1019to2×1020cm−3.

Boron-doped zinc oxide thin films for large-area solar cells grown by metal organic chemical vapor deposition

Boron-doped zinc oxide (ZnO:B) films were grown by metal organic chemical vapor deposition using diethylzinc (DEZn), and H 2O as reactant gases and diborane (B 2H 6) as an n-type dopant gas. The structural, electrical and optical properties of ZnO films doped at different B 2H 6 flow rates were investigated. X-ray diffraction spectra and scanning electron microscopy images indicate that boron-doping plays an important role on the microstructure of ZnO films, which induced textured morphology. With optimized conditions, low sheet resistance (∼ 30 Ω/□), high transparency (> 85% in the visible light and infrared range) and high mobility (17.8 cm 2 V − 1 s − 1 ) were obtained for 700-nm ZnO:B films deposited on 20 cm × 20 cm glass substrates at the temperature of 443 K. After long-term exposure in air, the ZnO:B films also showed a better electrical stability than the un-doped samples. With the application of ZnO:B/Al back contacts, the short circuit current density was effectively enhanced by about 3 mA/cm 2 for a small area a-Si:H cell and a high efficiency of 9.1% was obtained for a large-area (20 cm × 20 cm) a-Si solar module.

Boron-doped zinc oxide thin films prepared by sol-gel technique

Multilayer transparent conducting boron-doped zinc oxide films have been prepared on glass substrates by the sol gel dip coating process. Zinc acetate solutions of 0.4 M in isopropanol stabilized by diethanolamine and doped with boron tri-i-propoxide were used. Each layer was fired at 400–650∘C in a conventional furnace for 30 min. Selected samples were vacuum annealed at 400–450∘C for 1 h to improve their electrical properties. The electrical resistivity curve with doping shows a minimum around 0.8 at.%. Excess boron caused a drop of the carrier mobility without acting as donors. Post-deposition annealing sequence was crucial for dopant partial regeneration. Films with an average optical transmittance exceeding 90% can be achieved reproducibly.