Narrow Results

Density functional theory method is used to explore the mechanism of dissociative adsorption of methane (CH₄) on S_A type stepped Si(100) surface. Two reaction paths are described that produce CH₃ and hydrogen atom fragments adsorbed on the dimer bonds present on each terraces. It has been found that, in the initial stage of the carbonization of stepped Si(100) surface, the CH₃ and H fragments bound to the Si dimer atoms by following the first reaction path.

IZO films were deposited onto polyethylene terephthalate (PET) substrate at room temperature by the inclination opposite target type DC magnetron sputtering equipment. In this paper, SiO or SiON thin films of about 20nm thickness were introduced as buffer layers between the IZO thin films and the PET substrate. Electrical resistivity, transmittance and surface uniformity properties were investigated. It is clear that the film surface roughness of multilayer thin films was less than that of IZO monolayer thin films. The surface average roughness (Ra) of the multilayer film with 230nm IZO layer thickness was about 1.6nm, and that of IZO monolayer film was 2.4nm. All of the multilayer samples had high optical transmittance (about 90%) in the visible region. Of particular note, the transmittance of multilayer films was slightly higher than that of monolayer films when the IZO layer was thicker than 200nm. High electrical conductivity was also achieved with thick films.

An Y₂O₃ film, as seed layer, was directly deposited on the meter long length rolling assisted biaxially textured substrate (RABiTS) NiW tapes by the reactive magnetron sputtering process. Yttria stabilized zirconia (YSZ) and CeO₂ films were further carried out as the barrier and cap layers, and YBCO layer was prepared by magnetron sputtering technique as well. X-ray diffraction was employed to characterize the texture of film. The results showed that on NiW tapes highly textured films were obtained.

A double buffer of EU₂CuO₃/YSZ has been used to grow highly epitaxial thin films of YBa₂Cu₃O_y (YBCO) on Si. These films showed enhanced superconductivity and improved crystallinity in comparing with that of films grown on Si directly. Well defined interfaces with no immediate layers were found. The YBCO film surface was more smooth and stable. The results obtained indicate that highly epitaxial YBCO thin films can be successfully grown on Si wafers, demonstrating advantages of such a double buffer structure.

A composite buffer of CeO₂/YSZ/ Y₂O₃ was investigated on the biaxially textured NiW long tape for YBCO coated conductor with magnetron sputtering technique. Every layer's surface morphology was observed by scanning electron microscopy. The seed layer Y₂O₃ film was full coverage of the NiW substrate. The cap layer CeO₂ showed a smooth and crack-free surface and good crystallinity. The roughness of CeO₂ surface was measured by atom force microscopy. The transmission electron microscopy was used to analyze the cross-section of buffer layers and YBCO layer.

ZnO thin films were prepared by an unbalanced magnetron sputter on silicon substrates and glass slides. An ITO layer was applied on Si substrate as a buffer layer. Microstructure and mechanical deformation behaviors of the ZnO films were investigated by XRD, SEM and nanoindentation methods. Results showed that ITO buffer layer plays an important role for ZnO heteroepitaxy growth on Si substrates. The strains at the interface induced by the lattice mismatch of Si and ZnO are repressed. As a result, ZnO films with the buffer layers showed larger grain size and better crystallinity. The hardness and modulus of ZnO films with buffer layer decreased. Continuous stiffness measurement (CSM) technique was also used to investigate the effects of buffer layer and substrate materials on the mechanical performance of the prepared ZnO films. The relationship between microstructure and mechanical properties of ZnO films are discussed based on the experimental results.

In order to improve the photoelectric transformation efficiency of thin-film solar cells, one plausible method was to improve the transparent conductive oxides (TCO) material property. In-doped tin oxide (ITO) was an important TCO material which was used as a front contact layer in thin-film solar cell. Using magnetron sputtering deposition technique, we prepared preferential orientation ITO thin films on quartz substrate. XRD and SEM measurements were used to characterize the crystalline structure and morphology of ITO thin films. The key step was adding a ZnO thin film buffer layer before ITO deposition. ZnO thin film buffer layer increases the nucleation center numbers and results in the (222) preferential orientation growth of ITO thin films.

Kesterite Cu₂ZnSnS₄ (CZTS) has recently attracted the intensive attention of researchers as a significant photovoltaic material for the scalable production of thin film solar cells. We have particularly focused on replacing the conventional CdS buffer layer with non-toxic and earth-abundant materials of zinc stannate (Zn₂SnO${}_4)$ in environmentally friendly thin-film solar cells based on CZTS. Zinc stannate (ZTO) with a wide bandgap energy of about 3.33eV can be a promising material to reduce photon absorption loss and improve the photovoltaic performance of the device. Thus, the absorber and buffer layers were practically prepared to determine the bandgaps of layers for citation in SCAPS-1D simulation program. We employed chemical methods to deposit the CdS and CZTS layers and succeeded to obtain a high-quality kesterite phase of CZTS. The common configuration of FTO/CZTS/buffer/ZnO/AZO was the basis of the simulations in which, the thicknesses of the absorber and buffer layers were optimized by using the SCAPS-1D. According to the outcome of the simulations, the ZTO buffer layer has a better performance than CdS.

Trapping of hot electron behavior by trap centers located in the buffer layer of a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. The simulated results show that trap centers are responsible for current collapse in GaN MESFET at low temperatures. These electrical traps degrade the performance of the device at low temperatures. On the other hand, at high temperatures, the electrical performances are improved due to electron emission from the trap centers. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including the trapping center effects show much closer agreement with the available experimental data than without trap center effects.

Al₂O₃-doped zinc oxide (in AZO, the Al₂O₃ contents are approximately 2 wt.%) films have been grown by radio frequency (RF) magnetron sputtering at room temperature under varied sputtering pressures ranging from 3.5–15 mTorr. The electrical resistivity of AZO films is about 2.22×10^-3 Ωcm (sheet resistance ~ 89 Ω/square for a thickness ~ 250 nm), and the visible range transmittance is about 80% at the argon sputtering pressure of 15 mTorr and a RF power of 100 W. This study analyzes the structural, morphological, electrical and optical properties of AZO thin films grown on soda-lime glass substrate with 2, 5, and 10 nm thick Al buffer layers (and SiO₂ buffer). For the films deposited on the 2 nm thick Al buffer layer, we obtained a c-axis-oriented AZO/Al thin film on glass with the XRD full-width at half maximum (FWHM) of 0.31 and root mean square (RMS) surface roughness of about 3.22 nm. The lowest resistivity of 9.46×10^-4 Ωcm (sheet resistance ~ 37.87 Ω/square for a thickness ~ 250 nm) and a high transmittance (80%) were obtained by applying a 2 nm thick Al buffer layer. In contrast, the resistivity was slightly increased by applying the SiO₂ buffer layer.

Recent advances in silicon technology have pushed the silicon properties to its theoretical limits. Therefore, wide band gap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN) have been considered as a replacement for silicon. The discovery of these wide band gap semiconductors have given the new generation power devices a magnificent prospect of surviving under high temperature and hostile environments. The primary focuses of this review are the properties of GaN, the alternative substrates that can be used to deposit GaN and the substitution of SiO₂ gate dielectric with high dielectric constant (k) film. The future perspectives of AlGaN/GaN heterostructures are also discussed, providing that these structures are able to further enhance the performance of high power devices.

This work presents the results of synthesis and characterization of polycrystalline $n$-type Bi₂S₃ thin films. The films were grown through a chemical reaction from co-evaporation of their precursor elements in a soda-lime glass substrate. The effect of the experimental conditions on the optical, morphological structural properties, the growth rate, and the electrical conductivity $(\sigma )$ was studied through spectral transmittance, X-ray diffraction (XRD), atomic force microscopy (AFM) and $\sigma$ versus $T$ measurements, respectively. The results showed that the films grow only in the orthorhombic Bi₂S₃ bismuthinite phase. It was also found that the Bi₂S₃ films present an energy band gap $(E_g)$ of about 1.38 eV. In addition to these results, the electrical conductivity of the Bi₂S₃ films was affected by both the transport of free carriers in extended states of the conduction band and for variable range hopping transport mechanisms, each one predominating in a different temperature range.

Epitaxial CeO₂ seed layer and La₂Zr₂O₇ (LZO) buffer layers were deposited on biaxially-textured Ni–5 at.% W (NiW) tape substrate by liquid-phase polymer assisted nanoparticles deposition (PAND) method. LZO layers deposited by PAND have consistently shown tilting of the $c$-axis toward the direction of the sample’s surface normal. A new approach increasing the sharpening of the buffer texture by magnetic structural processing (MSP) of buffer layers was tested. The LZO layers, deposited on the seed and buffer layers after MSP, have dense and smooth surface structure, and more importantly, significantly improved out-of-plane texture, compared with the LZO layers that were deposited on a layer without MSP. Transmission electron microscopy study confirmed the $c$-axis tilting of CeO₂ and LZO layers and revealed the absence of interfaces between LZO layers which have been grown on the layers after MSP. There are very small (2–4 nm) gated pores in the single-crystal structure of LZO layers that are not typical for structure of LZO layers obtained by liquid-phase methods. Thus the LZO buffer layers can serve as an effective metal-ion diffusion barrier.

Cu₂ZnSnS₄ (CZTS) materials have been widely investigated due to their excellent properties in solar cell applications. The common reference structure for CZTS cells is Al:ZnO(AZO)/i-ZnO/CdS/CZTS, but it is critical to find a suitable buffer layer material to replace toxic cadmium (Cd). In addition, the efficiency of CZTS cells is improved by improving the doping type (n or p) and doping concentration of MoS₂ generated during the manufacturing process. wxAMPS was used to simulate the performance of a CZTS battery with an Al:ZnO/i-ZnO/Zn(O,S)/CZTS/(MoS₂) structure. The performance of batteries using Zn(O,S) and CdS as buffer layers was compared. The optimal thickness of CZTS layer and the doping concentration of CZTS layer were calculated, and the doping type and concentration of MoS₂ layer were analyzed and the performance of the battery was improved by optimizing the solar cell parameters. This work provides novel ideas for designing and manufacturing higher performance solar cells.

Cu seed layer was deposited by chemical vapor deposition using new Cu precursor, Cu(dmamb)₂. The Cu layers still need the barrier layer to prevent the diffusion, so Ta and Ti were used for the barrier layer on Si(100). Low temperature (LT) copper buffer layer was introduced and the effect of the buffer on the Cu films was investigated. The grown Cu layers were analyzed using FESEM, XRD, and four point probe measurement. The Cu seed layers were successfully deposited using Cu(dmamb)₂ precursor. Better thickness uniformity was obtained in the Cu films with the LT Cu buffer, which lowered the electrical resistivity.

Magnetoelectric (ME) Ni/Pb(Zr_0.52Ti_0.48)O₃ bilayers have been successfully prepared by hydrothermal method using Ti as buffer layer. The hydrothermal mechanism of PZT thin film deposited onto Ni layer has been discussed. The structure and ferroelectric properties of the deposited PZT thin films are characterized by X-ray diffraction and ferroelectric testing. The ME voltage coefficient of the Ni/PZT bilayers gradually decreases as the thickness of buffer layer increases because the interface coupling of the Ni/PZT layers gradually decreases. The large ME coefficient makes these Ni/PZT bilayers possible for applications in multifunctional devices such as electromagnetic sensor, transducers and microwave devices.

Aluminum oxide (Al₂O₃) thin films with Al₂O₃ buffer layer were deposited on Si (100) and Si (111) substrates using RF magnetron sputtering of Al₂O₃ target in Ar atmosphere. The synthesized films were then annealed at the temperature of 400${}^{\circ }$C, 600${}^{\circ }$C and 800${}^{\circ }$C in nitrogen (N₂) environment for 6h. Structural properties and surface morphology are examined by using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM). XRD analysis indicated that different orientation of Al₂O₃ were formed with different intensities due to increase in the annealing temperature. From FESEM cross-section analysis results, it is observed that the thickness of films were increased as the annealing temperature increased. EDX analysis shows that the concentration of aluminum and oxygen on both the Si substrates increased with the increase in annealing temperature. The surface roughness of the films were found to be decreased first when the annealing temperature is increased to 400${}^{\circ }$C, yet the roughness increased when the annealing temperature is further increased to 800${}^{\circ }$C.

We have investigated the dependence of device characteristics of bulk-heterojunction organic thin-film solar cells on the concentration of glycerol and sorbitol addition in poly(3,4-ethylenedioxy thiophene):poly(4-styrene sulfonate) (PEDOT:PSS) solutions for fabricating buffer layers. The device structure is ITO/buffer/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid methylester (PCBM)/Al. Glycerol addition is effective for increasing power conversion efficiency (PCE) from 1.25 to 1.41% because of the increase in short-circuit current density (J_sc) without decreasing open-circuit voltage (V_oc). On the other hand, sorbitol addition decreases PCE from 1.25 to 1.04%, owing to the decrease in V_oc. This difference in V_oc behavior is ascribed to different work function of PEDOT:PSS with glycerol and sorbitol treatment.