Increase In Buffer Layer Thickness Research Articles

High-performance numerical modeling of toxic-free CZTS solar cell structure

A systematic numerical study of a toxic-free CZTS solar cell model based on the ZnO buffer layer is numerically investigated and analysed using the SCAPS-1D simulation platform. The proposed toxic-free CZTS model exhibits an improvement in the current density (Jsc) of about 1.41 mA/cm2 and 6.33% in quantum efficiency (QE) compared to the conventional toxic structure. The simulation results of the eco-friendly model reveal an optimum efficiency of about 19.96% at a bandgap energy of 1.42 eV and with absorber thickness as low as 1600 nm. It has been observed that increasing the doping level from 1015 cm−3 to 1018 cm−3 results in an efficiency enhancement of about 2.5%. To maintain the high-performance of the device, the ZnO buffer layer must be as thin as 100 nm with a bandgap energy of 3.3 eV. The increase in buffer layer thickness above 100 nm leads to deterioration in the device performance. This might be ascribed to the increase of the recombination rate due to the increase of carrier diffusion length away from the junction at thicker ZnO thicknesses.

Influence of the thickness of ZrO2 buffer layer on the electrical and optical properties of VO2 films

In this study, high-quality thermochromic vanadium dioxide (VO2) thin films were grown on ZrO2/glass by pulsed laser deposition and were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), double beam spectrophotometry and Hall Effect measurement system. Based on XRD results, it can be inferred that the crystal structure of ZrO2 buffer layer can be transformed between monoclinic and tetragonal structures with the increase of buffer layer thickness. Photoelectric performance tests showed that the introduction of ZrO2 buffer layer considerably reduced the phase transition temperature (Tc) and hysteresis loop width (ΔH) of the VO2 films and allowed the visible light transmittance (Tlum) and solar modulation ability (ΔTsol) of the films to be tuned effectively. Especially, when the thickness of the ZrO2 buffer layer was 50 nm, the Tc of the VO2 films reached the minimum value of 48 °C. When the ZrO2 buffer layer thickness increased to 125 nm, the sheet resistance change of the VO2 film attained two orders of magnitude, and their Tlum-90°C and ΔTsol were as high as 50.01% and 7.88%, respectively. These results showed that the ZrO2 buffer layer considerably affected the growth of VO2 films on amorphous glass substrates and was beneficial for the application of VO2 in energy-saving smart windows.

Morphological and electrical properties of RF sputtered calcium copper titanate thin films with the incorporation of intermediate layer

In this research work, an attempt has been undertaken to improve the structural, morphological and electrical properties of RF sputtered CCTO thin films with the incorporation of HfO2 as a buffer layer and Zn as an intermediate layer. RF sputtering technique was used to deposit all the films with the optimized sputtering parameters. XRD pattern of HfO2 buffered CCTO thin films have shown an increase in (220) peak intensity with the enhancement in buffer layer thickness from 2.5 nm to 20 nm. Field emission scanning electron microscope images depicted the formation of smooth surface morphology. In addition, improved electrical and interface properties are observed with the increase in buffer layer thickness. CCTO films, deposited on 20 nm of HfO2 buffer layer, have shown lower leakage current of 3.15 × 10−7 Amp. In addition, reduced Dit value of 5.3 × 1010 eV−1cm−2 has been observed for film with 20 nm buffer layers. Improved morphological and electrical properties in CCTO films are attributed to suppression of defect states due to the presence of HfO2 buffer layer. Furthermore, the effect of different position of Zn intermediate layer, sandwiched between two CCTO layers, was also investigated. The appearance of splitted (220) peak has confirmed the incorporation of Zn in CCTO lattice. The surface morphology was found to be strongly dependent upon the multilayer geometry. Improved dielectric constant of 1078 at 100 kHz frequency is observed for the film configuration of 40-10-60, in which the Zn layer is situated towards the middle position.

Dependence of optimum V/III ratio on substrate orientation, and influence of buffer layer on MOVPE grown InSb/GaSb quantum dots

The dependence of optimum V/III ratio on substrate orientation and the influence of buffer layer on the growth of InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this study. The misorientation on the (1 0 0) substrate is suggested to enhance the formation of surface steps, which provides low energy growth sites and hence promotes island nucleation. An increase in V/III ratio from 1.0 to 3.0 resulted in a significant reduction in dot density on both (1 0 0) and (1 1 1) substrates, a phenomenon attributed to the reduced number of indium adatoms available on the growth surface, which simultaneously stimulates a decrease in the migration length of indium species. The shape and size of the dots were observed to be sensitive to the thickness of the buffer layer. Likewise, an increase in buffer layer thickness from 50 nm to ∼300 nm shows the onset of surface corrugation for buffer layer thickness of ∼150 nm, which persisted up to a buffer thickness of ∼250 nm. The corrugated buffer surfaces were observed to introduce order in the arrangement of the dots, which formed preferentially inside the troughs. A reduction in growth rate (from 0.21 to 0.17 nm/s) is presumed to induce sufficient adatom migration and strong bonding between the adsorbate atoms which results in an even buffer layer surface due to an enhancement in surface coverage of adatoms.

High electron mobility in epitaxial SnO2−x in semiconducting regime

We investigated the electronic transport properties of epitaxial SnO2−x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2−x [010]//Al2O3[12̄10]. When the SnO2−x films were grown in the oxygen pressure of 30 mTorr, we have found the electron mobility of the 30 nm thick SnO2−x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the electron mobility of values increased from 23 cm2 V−1 s−1 to 106 cm2 V−1 s−1 and the carrier density increased from 9 × 1017 cm−3 to 3 × 1018 cm−3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the electron mobility of SnO2−x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 47.5 mTorr, the carrier density was found to decrease to 9.1 × 1016 cm−3 and the electron mobility values to 13 cm2 V−1 s−1, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2−x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy was drawn. The excellent oxygen stability and high electron mobility of low carrier density SnO2−x films demonstrate its potential as a transparent oxide semiconductor.

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30nm)/Al2O3 and AlN(150nm)/Al2O3, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5nm to 2.3nm, while the FWHM value of GaN film rises up from 20.28arcmin to 84.6arcmin and then drops to 31.8arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.

X-ray analysis of strain distribution in two-step grown epitaxial SrTiO3 thin films

Epitaxial SrTiO3 (STO) thin films were grown on (001)-oriented LaAlO3 (LAO) substrates using a two-step growth method by ion beam sputter deposition. An STO buffer layer was initially grown on the LAO substrate at a low temperature of 150 °C prior to growing the STO main layer at 750 °C. The thickness of the STO buffer layer was varied at 3, 6, and 10 nm, while the total film thickness was kept constant at approximately 110 nm. According to x-ray structural analysis, we show that the STO buffer layer plays an essential role in controlling the strain in the STO layer grown subsequently. It is found that the strains in the STO films are more relaxed with an increase in buffer layer thickness. Moreover, the strain distribution in two-step grown STO films becomes more homogeneous across the film thickness when compared to that in directly grown STO film.

Effects of the ZnO buffer layer and Al proportion on AZO film properties

To evaluate the influence of the ZnO buffer layer and Al proportion on the properties of ZnO: Al (AZO)/ZnO bi-layer films, a series of AZO/ZnO films are deposited on the quartz substrates by electron beam evaporation. The X-ray diffraction measurement shows that the crystal quality of the films is improved with the increase of the film thickness. The electrical properties of the films are investigated. The carrier concentration and Hall mobility both increase with the increase of buffer layer thickness. However, the resistivity reaches the lowest at about 50 nm-thick buffer layer. The lowest resistivity and the maximum Hall mobility are both obtained at 1 wt% Al concentration. But the optical transmittance of all the films is greater than 80% regardless of the buffer layer thickness with Al concentration lower than 5 wt% in the visible region.

Influence Of ZnO Buffer Layer On Growth Of Sb Doped ZnO Nano Wires Using Nano Particle Assisted Pulsed Laser Deposition (NAPLD) Using Sb As Catalyst

Influence of ZnO buffer layer thickness on the structural and optical properties of the Sb catalyzed/doped ZnO nanowires synthesized using NAPLD has been investigated. Buffer layer with a thickness of 100 nm, 800 nm and 1600 nm coated with Sb are used as a substrate and pure ZnO was used as a target to synthesize Sb doped ZnO nanowires. Introduction of the buffer layer lead to the growth of vertically aligned along with horizontally grown ZnO nano wires. With the increase in buffer layer thickness, the core diameter of the vertically grown ZnO nano wires was subsequently increased. Nano wires synthesized with a buffer layer thickness of 1600 nm showed a significant change in the lattice constants, resulting in measurable lower angle of about 0.06o from XRD, widening of lattice fringe spacing of 0.54 nm from TEM and Suppression of A1T and E1(L0) modes in Raman Spectroscopic. In addition a strong UV emission with absence of visible emission was observed from the room temperature PL. This confirms the generation of Sb doped ZnO nano wire with high crystal quality. Copyright © 2012 VBRI Press.

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO thin films with different buffer layer thicknesses were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The effects of PS and buffer layer thickness on the structural and optical properties of ZnO thin films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The ZnO buffer layers, the intensity of the (002) diffraction peak for the ZnO thin films and its full width at half maximum (FWHM) decreased with an increase in the thickness of the ZnO buffer layers, indicating an improvement in the crystal quality of the films. On introducing PS as a substrate, the grain sizes of the ZnO thin films became larger and their residual stress could be relaxed compared with the ZnO thin films grown on Si. The intensity ratio of the ultraviolet (UV) to visible emission peak in the PL spectra of the ZnO thin films increased with an increase in buffer layer thickness. Stronger and narrower UV emission peaks were observed for ZnO thin films grown on PS. Their structural and optical properties were enhanced by increasing the buffer layer thickness. In addition, introduction of PS as a substrate enhanced the structural and optical properties of the ZnO thin films and also suppressed Fabry–Perot interference.

Effects of Buffer Layer on Structure and Ferromagnetism of Co-Doped SnO$_{2}$ Films

The thickness of SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layers has been found to considerably affect the structure and magnetic properties of Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films. As the buffer-layer thickness increases, the lattice constant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> and Co-O bond length of the Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films contract, which suggest the gradual relaxation of the accumulated tensile strain. Simultaneously, the amount of structural defects in Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films decreases significantly and the grain size increases with increasing buffer-layer thickness. All the Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films exhibit intrinsic ferromagnetism at room temperature and the saturated magnetic moment varies with the buffer-layer thickness. The dependence of ferromagnetism on the buffer-layer thickness can be explained on the basis of the variations in the density of structural defects and Co-O bond length in Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films.

The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.

Crystallinity and Photoluminescence Properties of ZnO Films on Zn Buffer Layers Deposited by rf Magnetron Sputtering

It is very desirable to grow ZnO epitaxial films on Si substrates since Si wafers with a high quality is available and their prices are quite low. Nevertheless, it is not easy to grow ZnO films epitaxially on Si substrates directly because of formation of an amorphous SiO2 layer at the interface of ZnO and Si. A Zn film and an undoped ZnO film were deposited sequentially on an (100) Si substrate by rf magnetron sputtering. The sample was annealed at 700°C in a nitrogen atmosphere. X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analyses were performed to investigate the cristallinity and surface morphology of the ZnO film. According to the analysis results the crystallinity of a ZnO thin film deposited by rf magnetron sputtering is substantially improved by using a Zn buffer layer. The highest ZnO film quality is obtained with a 110nm thick Zn buffer layer. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.

Effects of ZnO buffer layer thickness on the crystallinity and photoluminescence properties of rf magnetron sputter-deposited ZnO thin films

Highly c-axis oriented ZnO thin films were grown on Si(100)substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated using X-ray diffraction (XRD), photoluminescence (PL) and Atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by rf magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.

Influence of buffer layer thickness on the structure and optical properties of ZnO thin films

A series of ZnO thin films were deposited on ZnO buffer layers by DC reactive magnetron sputtering. The buffer layer thickness determination of microstructure and optical properties of ZnO films was investigated by X-ray diffraction (XRD), photoluminescence (PL), optical transmittance and absorption measurements. XRD results revealed that the stress of ZnO thin films varied with the buffer layer thickness. With the increase of buffer layer thickness, the band gap edge shifted toward longer wavelength. The near-band-edge (NBE) emission intensity of ZnO films deposited on ZnO buffer layer also varied with the increase of thickness due to the spatial confinement increasing the Coulomb interaction between electrons and holes. The PL measurement showed that the optimum thickness of the ZnO buffer layer was around 12 nm.

Residual stress in GaN films grown by metalorganic chemical vapor deposition

We have studied the growth of gallium nitride (GaN) on (0001) sapphire substrates in a commercial (CVD Equipment Corp.) metalorganic chemical vapor deposition reactor. High quality epitaxial GaN films were obtained by deposition at 1000 °C from trimethylgallium and ammonia precursors (1.5 slpm total flow rate with V/III molar flow ratio of 5800) on a 24-nm-thick GaN buffer layer grown at 500 °C. X-ray rocking-curve measurements of films grown under these conditions showed a full width at half maximum of 0.28°. The x-ray lattice parameter method was used to determine the residual stress in GaN films by measuring the c-axis and a-axis strains separately. The results indicate that residual stress in GaN film grown on sapphire is compressive and is reduced as the buffer layer thickness increases.

Microcrystalline n- and p-layers at the tunnel junction of a-Si:H/a-Si:H tandem cells

Completely microcrystalline layers have been incorporated at the tunnel junction of a tandem cell in the configuration SnO 2:F/p-a-SiC:H/i-a-Si:H/n- μc-Si:H/p- μc-Si:H/i-a-Si:H/n-a-Si:H/Ag and an efficiency of 9.89% has been achieved. We present here our experimental findings and our modelling predictions using the computer programme AMPS. A wide band gap a-Si:H buffer layer between p- μc-Si:H and i-layer a-Si:H was necessary for obtaining good performance of the bottom cell. The spectral response in the blue region is reduced with the increase of buffer layer thickness and thickness as small as 1.0 nm was necessary to achieve the best efficiency. Computer prediction shows that recombination in the p- μc-Si:H could be very high when no buffer is used because of the combination of low band gap and high (>10 17 cm −3) defect density. Buffer layer prevents electron back diffusion into the μc-Si:H p-layer. Computed light current voltage characteristics did not change significantly when tunnelling across the thin buffer layer was incorporated in the model. To achieve larger efficiencies in the tandem configuration, insertion of oxide layers between a-Si:H layer and n- μc-Si:H layer of the top cell and between n- μc-Si:H layer and p- μc-Si:H layer was necessary. Our simulations show that, (a) the recombination rate is increased at the junction due to lower mobility gap of the microcrystalline layers, (b) the density of trapped carriers is reduced with microcrystalline junction, not only in the p- and n-layers but also in the i-layers, (c) the recombination rate at the microcrystalline junction is increased with an oxide layer at the p- μc-Si:H and n- μc-Si:H interface showing an improvement of both FF as well as V oc.

Two‐dimensional numerical analysis of gaas MESFET with a p‐buffer layer

AbstractTo suppress the threshold voltage shift of GaAs MESFET's in a shorter channel region, a two‐dimensional numerical analysis of the FET structure with a p‐buffer layer is carried out. As a result of the analysis, it is clarified that the insertion of the p‐buffer layer is effective in preventing the carrier penetration in the substrate. In analyzing the electrical properties as a function of the buffer layer thickness, it is found that the substrate current which flows beneath the p‐channel layer, decreases with an increase in the buffer layer thickness and that the threshold voltage shift is suppressed with the increase. As the buffer layer thickness increases, the transconductance increases. Moreover, as the buffer layer thickness increases, the gate‐substrate capacitance increases. Therefore, to obtain a high cutoff frequency, it is necessary to optimize the buffer layer thickness.