The role of SiO2 buffer layer in the growth of highly textured LiNbO3 thin film upon SiO2/Si by pulsed laser deposition

Highly c-axis-oriented lithium niobate (LiNbO₃) thin films have been grown on Si with optimum thickness of the SiO₂ buffer layer by pulsed laser deposition technique. The amorphous SiO₂ buffer layer was formed on Si (100) wafer by thermal oxidation method. The crystallinity and c-axis orientation of LiNbO₃ films were strongly influenced by the thickness of amorphous SiO₂ buffer layers. The optimum thickness of the amorphous SiO₂ buffer layer was found to be about 230 nm for the growth of highly c-axis-oriented LiNbO₃ films. The achieved films have smooth surface and sharp interface. The prism coupler method indicates that the prepared LiNbO₃ films have great potential for optical waveguide device.     

Gasochromic switching of reactively sputtered molybdenumoxide films: A correlation between film properties and deposition pressure

Molybdenumoxide (MoO_x) thin films can change their optical properties upon exposure to hydrogen. Since the film properties strongly depend on process parameters we have studied how the films are affected by the total pressure during deposition. Stoichiometric and sub-stoichiometric MoO_x films were prepared by reactive direct current magnetron sputtering in an atmosphere of argon and oxygen. Substoichiometric films were coated with platinum as a catalyst and were colored in diluted hydrogen atmosphere and bleached in air. Optical spectroscopy, X-ray reectometry, spectroscopic ellipsometry and simulations of the measured spectra were used to characterize the films ex situ. In situ switching characteristics as revealed by optical spectroscopy and changes in stress were measured as well. We find that the total pressure during sputter deposition has a strong influence on the optical constants, the film density, and the sputter rate. The mechanical stresses and switching Preprint submitted to Elsevier Science 10 March 2006 cycles during the film coloration and bleaching also strongly depend on the total pressure. The influence of the sputter pressure on film properties is explained by the kinetics during the sputter process.     

Optical properties of Cd1−xZnxS thin films for CuInGaSe2 solar cell application

The photovoltaic Cd_1−xZn_xS thin films, fabricated by chemical bath deposition, were successfully used as n-type buffer layer in CuInGaSe₂ (CIGS) solar cells. Comprehensive optical properties of the Cd_1−xZn_xS thin films were measured and modeled by spectroscopic ellipsometry (SE), which is proven to be an excellent and non-destructive technique to determine optical properties of thin films. The optical band gap of Cd_1−xZn_xS thin films can be tuned from 2.43 eV to 3.25 eV by controlling the Zn content (x) and deposition conditions. The wider-band-gap Cd_1−xZn_xS film was found to be favorable to improve the quantum efficiency in the wavelength range of 450-550 nm, resulting in an increase of short-circuits current for solar cells. From the characterization of quantum efficiency (QE) and current-voltage curve (J-V) of CIGS cells, the Cd_1−xZn_xS films (x = 0.32, 0.45) were demonstrated to significantly enhance the photovoltaic performance of CIGS solar cell. The highest efficiency (10.5%) of CIGS solar cell was obtained using a dense and homogenous Cd_0.68Zn_0.32S thin film as the buffer layer.     

The effects of buffer growth parameters on heteroepitaxial ZnO films grown by pulsed laser deposition

The effects of buffer layer deposition conditions on subsequent ZnO epitaxy on sapphire (0001) were examined. An initial ZnO buffer layer improves surface roughness for a wide range of buffer layer growth temperatures and pressures. Changes in buffer layer growth pressure and temperature have a moderate effect on the roughness of subsequent film growth. However, the conditions for buffer layer deposition have a large impact on crystallinity of subsequent films. In particular, the out-of-plane X-ray diffraction rocking curve full-width half-maximum decreased as buffer deposition temperature or O₂/O₃ pressure increases. Carrier mobility in the subsequent thick ZnO film was enhanced with increase in buffer layer deposition temperature. Carrier concentration decreased with increasing buffer layer deposition pressure.     

Control of ZnO thin film surface by ZnS passivation to enhance photoluminescence

To enhance the optical property of zinc oxide (ZnO) thin film, zinc sulfide (ZnS) thin films were formed on the interfaces of ZnO thin film as a passivation and a substrate layer. ZnO and ZnS thin films were deposited by atomic layer deposition (ALD) using diethyl zinc, H₂O, and H₂S as precursors. Investigations by X-ray diffraction and transmission electron microscopy showed that ZnS/ZnO/ZnS multi-layer thin films with clear boundaries were achieved by ALD and that each film layer had its own polycrystalline phase. The intensity of the photoluminescence of the ZnO thin film was enhanced as the thickness of the ZnO thin film increased and as ZnS passivation was applied onto the ZnO thin film interfaces.     

Pressure dependent structural and optical properties of silicon carbide thin films deposited by hot wire chemical vapor deposition from pure silane and methane gases

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition technique from silane (SiH₄) and methane (CH₄) gas precursors. The effect of deposition pressure on structural and optical properties of SiC films was investigated. Various spectroscopic methods including Fourier transform infrared spectroscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and UV–Vis–NIR spectroscopy were used to study these properties. Films deposited at low deposition pressure were Si-rich, and were embedded with nano-crystals of silicon. These films showed strong absorption in the visible region and had low energy band gaps. Near stoichiometric SiC film, were formed at intermediate deposition pressure and these films were transparent in the visible region and exhibited a wide optical band gap. High deposition pressures caused inhomogeneity in the film as reflected by the increase in disorder parameter and low refractive index of the films. This was shown to be due to formation of sp ² carbon clusters in the film structure.     

Enhancement of photosensitivity by annealing in Bi2S3 thin films grown using SILAR method

Bi₂S₃ thin films were grown by successive ionic layer adsorption and reaction method (SILAR) onto the glass substrates at room temperature. The as prepared thin film were annealed at 250 °C in air for 30 min. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurement systems. The X-ray diffraction patterns reveal that Bi₂S₃ thin film have orthorhombic crystal structure. SEM images showed uniform deposition of the material over the entire glass substrate. The optical energy band gap observed to be decreased from 1.69 to 1.62 eV for as deposited and annealed films respectively. The I–V measurement under dark and illumination condition (100 W) show annealed Bi₂S₃ thin film gives good photoresponse as compared to as deposited thin film and Bi₂S₃ thin film exhibits photoconductivity phenomena suggesting its useful in sensors device. The thermo-emf measurements of Bi₂S₃ thin films revealed n-type electrical conductivity.     

High quality ZnO films deposited by radio-frequency magnetron sputtering using layer by layer growth method

Three-layered ZnO films were deposited on Si substrates by radio-frequency magnetron sputtering using layer by layer growth method. The Raman scattering confocal analysis confirms that ZnO film quality is improving at increasing the number of ZnO layers at film deposition.Applied method of deposition was used to realize homoepitaxial growth of ZnO films on c-Al₂O₃, Si, SiN_x/Si, glass and ITO/glass substrates. In order to improve the film quality we increased the number of deposition stages up to 5. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmittance measurements were used to testify the quality of grown five-layered ZnO films. XRD results showed that all five-layered ZnO films have (002) texture. The second order diffraction peak (004) on XRD spectra additionally testifies to the high quality of all five-layered ZnO films. SEM results demonstrated that no defects such as cracks and dislocations caused by interruption of deposition ZnO films were observed. Transmittance measurement results showed that ZnO films deposited on transparent substrates have abrupt absorption edge and high optical transmission in the visible region of the spectrum.     

YBa2Cu3O7 − δ thin films with enhanced film properties grown on sapphire using Y2O3/CeO2 bi-layer buffer

Highly epitaxial, microcrack-free thin films of YBa₂Cu₃O_7 − δ (YBCO) have been grown by pulsed laser deposition on sapphire substrates with a double buffer of Y₂O₃/CeO₂. The Y₂O₃ layer, which has excellent compatibility with CeO₂ and YBCO, has been found to be beneficial in reducing the surface porosity of YBCO films as well as inhibiting a-axis-oriented epitaxial growth. The reduction in porosity is attributed to the presence of the Y₂O₃ layer which acts as a suitable barrier for the chemical reaction occurring at high deposition temperatures between YBCO and CeO₂. Due to the improvement in film quality and surface morphology, enhancement of the self-field critical current density (J_c, 77.3 K) by as high as 30% was obtained for a 650-nm thick YBCO film deposited on Y₂O₃/CeO₂ bi-layer buffer compared to simultaneously-deposited YBCO film on CeO₂ single-layer buffer only.     

Multilayer antireflective and protective coatings comprising amorphous diamond and amorphous hydrogenated germanium carbide for ZnS optical elements

To effectively protect and improve the transmittance of ZnS optical elements in the far infrared band, combined amorphous diamond (a-D) and amorphous hydrogenated germanium carbide (a-Ge_1−xC_x:H) films have been developed. The optical interference coatings were designed according to the layer optics theory. The a-D films, of which refractive index and film thickness were controlled by changing substrate bias and deposition time respectively, were deposited by filtered cathodic vacuum arc technology. The a-Ge_1−xC_x:H films were prepared by radio frequency sputtering technology. During this process their refractive index was modulated by changing the gas flow rate ratio and their film thickness was controlled by the flow rate ratio and deposition time. It has been shown that the combined films are superexcellent antireflective and protective coatings for ZnS optical elements.     

Structural evolution of magnetron sputtered shape memory alloy Ni-Ti films

Near equiatomic and Ti-rich Ni-Ti polycrystalline films have been deposited by magnetron co-sputtering using a chamber installed at a synchrotron radiation beamline. The in situ X-ray diffraction studies enabled the identification of different steps of the structural evolution during film processing.The depositions on a 140 nm amorphous SiO₂ buffer layer heated at 520 °C (without applying bias voltage, V_b, to the substrate) led to a preferential growth of <100> oriented grains of the Ni-Ti B2 phase from the beginning of film growth until the end of the deposition. Films exhibiting a preferential growth of <110> oriented grains of the Ni-Ti B2 phase from the beginning of the deposition were obtained (without and with a V_b of −45 V) by using a TiN coating with a topmost layer formed by <111> oriented grains. Those trends have been observed for the growth of near equiatomic (≈50.0 at.% Ti-Ni) and Ti-rich (≈50.8 at.% Ti-Ni) Ni-Ti films.Additionally, an ion gun had been commissioned, which allows ion bombardment during sputter deposition or post-deposition ion irradiation. In this first series of experiments, a Ni-Ti film was irradiated with He ions after deposition (without exposing the film to the atmosphere, i.e., avoiding surface oxide formation), thus modifying deliberately the microstructure of the film locally.     

Growth of GaN on SiC/Si substrates using AlN buffer layer by hot-mesh CVD

GaN films were grown on SiC/Si (111) substrates by hot-mesh chemical vapor deposition (CVD) using ammonia (NH₃) and trimetylgallium (TMG) under low V/III source gas ratio (NH₃/TMG = 80). The SiC layer was grown by a carbonization process on the Si substrates using propane (C₃H₈). The AlN layer was deposited as a buffer layer using NH₃ and trimetylaluminum (TMA). GaN films were formed and grown by the reaction between NH_x radicals, generated on a tungsten hot mesh, and the TMG molecules. The GaN films with the AlN buffer layer showed better crystallinity and stronger near-band-edge emission compared to those without the AlN layer.     

Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient

ZnO films with improved crystallinity were grown on a Si (111) substrate by a two-step growth process using low-temperature ZnO buffer layers. The effect of the ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural properties of the films was investigated by X-ray diffraction (XRD), photoluminescence, and transmission electron microscopy. The use of argon as the ambient gas during the thermal treatment of the buffer layer leads to the enhancement of the (0002) diffraction peak intensity at 2θ ∼ 34.4° and the reduction of the full width at half maximum value in the XRD rocking curve, which means that well-defined and c-axis oriented ZnO film was obtained. The relationship between the thickness of the SiO₂ layer between the ZnO buffer layers and Si substrates and the structural and optical properties of the ZnO films is discussed.     

Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films

The influence of film thickness, type of buffer underlayer, and deposition substrate temperature on the crystal structure, microstructure, and morphology of the films of dicyanovinyl-substituted sexithiophene with four butyl-chains (DCV6T-Bu₄) is investigated by means of X-ray diffraction (XRD) and X-ray reflectivity methods. A neat Si wafer or a Si wafer covered by a 15 nm buffer underlayer of fullerene C₆₀ or 9,9-Bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) is used as a substrate. The crystalline nature and ordered molecular arrangement of the films are recorded down to 6 nm film thickness. By using substrates heated up to 90 °C during the film deposition, the size of the DCV6T-Bu₄ crystallites in direction perpendicular to the film surface increases up to value of the film thickness. With increasing deposition substrate temperature or film thickness, the DCV6T-Bu₄ film relaxes, resulting in reducing the interplane distances closer to the bulk values. For the films of the same thickness deposited at the same substrate temperature, the DCV6T-Bu₄ film relaxes for growth on Si to BPAPF to C₆₀. Thicker films grown at heated substrates are characterized by smaller density, higher roughness and crystallinity and better molecular ordering. A thin (up to about 6 nm-thick) intermediate layer with linear density-gradient is formed at the C₆₀/DCV6T-Bu₄ interface for the films with buffer C₆₀ layer. The XRD pattern of the DCV6T-Bu₄ powder is indexed using triclinic unit cell parameters.     

Enhanced magnetic properties of chemical solution deposited BiFeO3 thin film with ZnO buffer layer

Magnetic properties of BiFeO₃ films deposited on Si substrates with and without ZnO buffer layer have been studied in this work. We adopted the chemical solution deposition method for the deposition of BiFeO₃ as well as ZnO films. The x-ray diffraction measurements on the deposited films confirm the formation of crystalline phase of BiFeO₃ and ZnO films, while our electron microscopy measurements help to understand the morphology of few micrometers thick films. It is found that the deposited ZnO film exhibit a hexagonal particulate surface morphology, whereas BiFeO₃ film fully covers the ZnO surface. Our magnetic measurements reveal that the magnetization of BiFeO₃ has increased by more than ten times in BiFeO₃/ZnO/Si film compared to BiFeO₃/Si film, indicating the major role played by ZnO buffer layer in enhancing the magnetic properties of BiFeO₃, a technologically important multiferroic material.     

Optical study of zinc blend SnS and cubic In2S3:Al thin films prepared by chemical bath deposition

Multilayers of zinc blend SnS crystalline thin film have been deposited onto glass substrates by a chemical bath deposition (CBD) method. The envelope method, based on the optical transmission spectrum taken at normal incidence, has been successfully applied to determine the layer thickness and to characterize optical properties of thin films having low surface roughness. Optical constants such as refractive index n, extinction coefficient k, as well as the real (??_r) and imaginary (??_i) parts of the dielectric constant were determined from transmittance spectrum using this method. Obtained low value of the extinction coefficient in the transparency domain is a good indication of film surface smoothness and homogeneity. To perform the heterojunction structure based on SnS absorber material, cubic In₂S₃:Al was deposited on SnO₂:F/glass as window layer using CBD with different aluminum content. Optical properties of these films were evaluated.     

High aspect ratio contact hole etching using relatively transparent amorphous carbon hard mask deposited from propylene

In this study, a high aspect ratio contact pattern, beyond 70 nm technology, in a very-large-scale integrated circuit, was achieved using hydrogenated amorphous carbon (a-C:H) film as the dry etching hard mask. The effect of temperature on the a-C:H deposits prepared by plasma enhanced chemical vapor deposition was studied. The a-C:H films resulting from propylene (C₃H₆) decomposition exhibited high transparency incorporated rich hydrogen concentration with a decreasing deposition temperature. A matrix of dispersed cross-linked sp³ clusters in a-C:H films, which has an increasing optical band gap and higher hydrogen content, is attributed to reduce the defect density of status and obtain high transmittance rate. Moreover, the higher transparency of a-C:H films could afford lithographic aligned capability as well as compressive stress and dry etching resistance. These explorations provided insights into the role of hydrogen in a-C film and also into the practicality of its future nano-scale device applications.     

Preparation of Double Perovskite Ca2NiWO6 as a Buffer Layer for YBa2Cu3O7−δ Coated Conductors

Ca₂NiWO₆ films were deposited on the Y-stabilized ZrO₂ (100) (YSZ) single crystalline substrates by pulsed laser deposition process. X-ray diffraction reveals that the Ca₂NiWO₆ film is coherently strained due to the small lattice mismatch between Ca₂NiWO₆ and YSZ substrate. With this c-axis orientated Ca₂NiWO₆ film as buffer layer, YBa₂Cu₃O_7?δ film has been successfully prepared by pulsed laser deposition. The subsequently deposited YBa₂Cu₃O_7?δ film has a preferred c-axis orientation and a smooth surface. The temperature dependence of resistance indicates a superconducting critical temperature higher than 82 K. Our results demonstrate that the double perovskite Ca₂NiWO₆ is a promising buffer layer candidate for coated conductors.     

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E) = ε₁(E) + iε₂(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5-4.5 eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4 nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.     

Effect of a ZnO buffer layer on the characteristics of MgZnO thin films grown on Si (100) substrates by radio-frequency magnetron sputtering

This is a report on the effect of a ZnO buffer layer on the microstructures and optical properties of MgZnO thin films grown on Si (100) substrates by radio frequency magnetron sputtering. For the sample without the ZnO buffer layer, the microstructural analyses carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the formation of Mg₂Si in the interface between the Si substrate and the MgZnO thin film. Mg₂Si induced the random oriented polycrystalline MgZnO thin film. For the sample with the ZnO buffer layer, a few Mg₂Si were observed. An epitaxial relationship between the Si substrate and the MgZnO thin film was formed. In both samples, the photoluminescence (PL) investigation showed a small blue shift of the emission peak, which was owing to the incorporation of Mg atoms in ZnO by co-sputtering the MgO and ZnO targets. In addition, the sample with the ZnO buffer layer showed the enhanced PL intensity, when compared with the sample without the buffer layer.