The comparison between CdS thin films grown on Si(111) substrate and quartz substrate by femtosecond pulsed laser deposition

Two kinds of cadmium sulfate (CdS) thin films have been grown at 600 °C onto Si(111) and quartz substrates using femtosecond pulsed laser deposition (PLD). The influence of substrates on the structural and optical properties of the CdS thin films grown by femtosecond pulsed laser deposition have been studied. The CdS thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman spectroscopy. Although CdS thin films deposited both on Si(111) and quartz substrates were polycrystalline and hexagonal as shown by the XRD , SEM and AFM results, the crystalline quality and optical properties were found to be different. The size of the grains for the CdS thin film grown on Si(111) substrate were observed to be larger than that of the CdS thin film grown on quartz substrate, and there is more microcrystalline perpendicularity of c-axis for the film deposited on the quartz substrate than that for the films deposited on the Si substrate. In addition, in the PL spectra, the excitonic peak is more intense and resolved for CdS film deposited on quartz than that for the CdS film deposited on Si(111) substrate. The LO and TO Raman peaks in the CdS films grown on Si(111) substrate and quartz substrate are different, which is due to higher stress and bigger grain size in the CdS film grown on Si(111) substrate, than that of the CdS film grown on the amorphous quartz substrate. All this suggests that the substrates have a significant effect on the structural and optical properties of thin CdS films. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 78.67.-n; 42.62.-b     

Structure and Surface Morphology of Cd<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Epitaxial Films

Cd_1–xMn _x Se (х = 0.03) epitaxial films are produced by the molecular beam condensation method in a vacuum chamber with a residual pressure of 10^–4 Pa on mica and glass substrates. It is established that at room temperature and at a substrate temperature of T = 573 K films of polycrystalline structure grow on the mica substrates, but films of both polycrystalline and amorphous structure grow on the glass substrates. It is shown that the polycrystalline Cd_1–xMn _x Se (х = 0.03) films, unlike the bulk crystals, have a sphalerite-type structure with a lattice parameter of a = 6.05 Å. Increasing the substrate temperature to 673 K leads to epitaxial growth with the direction [111]. Dark aggregates, observed on the film surface, are removed using a source of compensating Se vapors during the growth process. The optimal conditions for the production of structurally perfect epitaxial films are defined.     

不同衬底和CdCl2退火对磁控溅射CdS薄膜性能的影响

在科宁7059玻璃, FTO, ITO, AZO四种衬底上磁控溅射CdS薄膜, 并在CdCl₂+干燥空气380 ℃退火, 分别研究了不同衬底和退火工艺对CdS薄膜形貌、结构和光学性能的影响. 扫描电子显微镜形貌表明: 不同衬底原位溅射CdS薄膜的形貌不同, 退火后相应CdS薄膜的晶粒度和表面粗糙度明显增大. XRD衍射图谱表明: 不同衬底原位溅射和退火CdS薄膜均为六角相和立方相的混相结构, 退火前后科宁7059玻璃, FTO, AZO衬底上CdS薄膜有 H(002)/C(111) 最强衍射峰, ITO衬底原位溅射CdS薄膜没有明显的最强衍射峰, 退火后出现 H(002)/(111) 最强衍射峰. 紫外-可见分光光度计分析表明: AZO, FTO, ITO, 科宁7059玻璃衬底CdS薄膜的可见光平均透过率依次减小, 退火后相应衬底CdS薄膜的可见光平均透过率增大, 光学吸收系数降低; 退火显著增大了不同衬底CdS薄膜的光学带隙. 分析得出: 上述结果是由于不同衬底类型和退火工艺对CdS多晶薄膜的形貌、结构和带尾态掺杂浓度改变的结果. 关键词： CdS薄膜 磁控溅射 退火再结晶 带尾态     

Optoelectronics and formaldehyde sensing properties of tin-doped ZnO thin films

Sn-doped ZnO thin films were deposited on clean glass substrates using the chemical spray pyrolysis technique. XRD analyses confirm stable ZnO hexagonal wurtzite structure of the films with crystallite size in the range of 20–28 nm. The surface roughness of the films increases on Sn doping, which favors to higher adsorption of oxygen species on the film surface, resulting in higher gas response. Optical studies reveal that the band gap decreases on Sn doping. All the films show near band edge emission, and on Sn doping the luminescence peak intensity has been found to increase. Photocurrent in the 1.5 at.% doped film enhances about three times to that observed in the undoped ZnO film. Among all the films examined, the 1.5 at.% Sn-doped film exhibits the maximum response (～94.5 %) at the operating temperature of 275?°C for 100 ppm concentration of formaldehyde, which is much higher than the response (～35 %) in the undoped film. The gas response of the film is attributed to the chemisorption of oxygen on the film surface and the subsequent reaction between the adsorbed oxygen species and the formaldehyde molecules.     

Structural,electrical and optical properties of Cdx Zn1−xSe thin films

X-ray diffraction and electron diffraction techniques indicate that Cd_x Zn_1–xSe thin films on glass substrates have a polycrystalline nature, with sphalerite structure for x0.5 and wurtzite structure for x0.6. The crystalline size in each composition increases with increasing the film thickness. The room temperature dark resistivity varies from one composition to another showing a transition at x=0.55The temperature dependence of of the deposited films revealed two conduction mechanisms, one below 352 K due to shallow levels, surface states, and defects introduced during the film growth, and over 352 K due to deep-level ionization following the ordinary semiconducting behaviour.The thermal activation energy of the free charge carriers decreases linearly with increasing the molar fraction x of the CdSe content up to x=0.55, above which it increases with increasing x.The optical constants of Cd_x Zn_1–xSe thin films of different compositions were determined in the spectral range 400–2000 nm. The analysis of the absorption coefticient at and near the absorption edge indicates the existence of allowed direct transition energy gaps decreasing with increasing x.     

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)(110)+(111) random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant. PACS 77.55.+f; 77.22.Ch; 81.15.Fg     

Pulsed laser deposition of Pb(Zr0.52Ti0.48)O3 thin film on cobalt ferrite nano-seed layered Pt(111)/Si substrate: effect of oxygen pressure

The effect of oxygen pressure during pulsed laser deposition of Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films on CoFe₂O₄ nano-seed layered Pt(111)/Si substrate was investigated. The PZT film deposited at oxygen pressure lower than 25 mTorr is identified as both perovskite and pyrochlore phases and the films deposited at high oxygen pressure (50-100 mTorr) show the single-phase perovskite PZT that has a perfect (111)-orientation. In addition, the film deposited at P_O2 of 50 mTorr has a uniform surface morphology, whereas the film deposited at P_O2 of 100 mTorr has a non-uniform surface morphology and more incompacted columnar cross-section microstructure. The polarization of film deposited at 100 mTorr is higher than that deposited at 50 mTorr, but shift of the hysteresis loop along the electrical field axis in the film deposited at P_O2 of 100 mTorr is larger than that of the film deposited at P_O2 of 50 mTorr.     

Desorption and crystallization kinetics in nanoscale thin films of amorphous water ice

The amorphous to crystalline ice phase transition is studied by measuring the water desorption rate from nanoscale thin films of water vapor deposited on Au(111) and Ru(001) single crystal metallic substrates. The desorption kinetics are substrate dependent and suggest strongly that the film morphology is governed by the hydrophilicity of the substrate. The crystallization kinetics are independent of substrate but depend strongly on both temperature and film thickness and are consistent with a spatially random nucleation and isotropic growth model.     

Selective deposition of Au films on GaAs by projection-patterned excimer laser doping combined with electroless plating

Resistless microfabrication of Au thin films on n-type GaAs by projection-patterned laser doping using a KrF excimer laser and a 10% SiH₄-He gas is described. Gold thin films with a linewidth as narrow as 1.57 m are deposited selectively on the doped regions by electroless plating in a commercial Au-24s aqueous solution. The isotropic growth of the deposited films is discussed by comparing the linewidth of the deposited films with that of the doped regions. Furthermore, the dependence of the deposition characteristics of Au thin films on the laser irradiation conditions is investigated.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Effect of substrate temperature and oxygen partial pressure on microstructure and optical properties of pulsed laser deposited yttrium oxide thin films

Yttrium oxide thin films were deposited on Si (1 1 1) and quartz substrates by pulsed laser deposition technique at different substrate temperature and oxygen partial pressure. XRD analysis shows that crystallite size of the yttrium oxide thin films increases as the substrate temperature increases from 300 to 873 K. However the films deposited at constant substrate temperature with variable oxygen partial pressure show opposite effect on the crystallite size. Band gap energies determined from UV-visible spectroscopy indicated higher values than that of the reported bulk value.     

In situ STM of pulsed laser nanostructured deposits: First stages of film formation

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance.We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units.     

铟锡氧化物薄膜表面等离子体损耗降低的研究

本文采用直流磁控溅射方法在普通浮法玻璃基底上制备了立方多晶铁锰矿结构的铟锡氧化物(indium tin oxide, ITO)薄膜,并对其进行了结晶性、表面粗糙度、紫外-可见吸收光谱、折射率、介电常数及霍尔效应的测试.研究了溅射时基底温度的改变对于ITO薄膜的光电、表面等离子体性质的影响.随着基底温度由100?C升高至500?C,其光学带隙(3.64—3.97eV)展宽,减少了电子带间跃迁的概率,有效降低了ITO薄膜的光学损耗.与此同时,对应ITO薄膜的载流子浓度(4.1×10~(20)-—2.48×10~(21)cm~(-3))与迁移率(24.6—32.2 cm~2·V~(-1)·s~(-1))得到提高,电学损耗明显降低.     

Magnetic anisotropy properties of CoZrNb thin films deposited on PET substrate by magnetron sputtering

Soft magnetism and magnetic anisotropy properties of CoZrNb thin films deposited on polyethylene terephthalate (PET) substrate by magnetron sputtering were investigated. As the film thickness increases, the coercivity of films decreases from 7 to 4 Oe. It exhibits an in-plane uniaxial magnetic anisotropy as the thickness of CoZrNb thin films increases. An easy axis is observed in CoZrNb films along the direction transverse to the rolling direction of the polymer web.     

Controlling the size and alignment of ZnO microrods using ZnO thin film templates deposited by pulsed laser ablation

The production of dense arrays of well aligned ZnO nano- and microrods with a controllable distribution of diameters is demonstrated. The rods were grown using a hydrothermal method, on pre-deposited ZnO thin films exhibiting a range of different grain sizes. These template ZnO thin films were deposited by pulsed laser ablation, at 193 nm, in a low background pressure of O₂; average grain sizes ranged from 10 nm (room temperature deposition) to 50 nm when deposited at a substrate temperature (T_sub) of 500 °C. The morphology of the ZnO microrod arrays grown onto these ZnO thin films was found to depend on the T_sub used during film deposition. Increasing T_sub resulted in a lower density of larger nanorods, with a more pronounced [0001] alignment. The diameters of the rods produced are typically an order of magnitude greater than the grain size of the template ZnO film. Thus the rods cannot be a direct continuation of the grain structure of the template films. Rather, we suggest that the rod diameter is controlled by the density of sites at which the initial nucleation occurs, which is expected to be higher on the smaller grain size films deposited at lower temperatures. The factor controlling the final size of the rods is thus the available free space into which they can grow, which is smaller at the higher nucleation densities. The increased extent of [0001] texturing of the thin film templates deposited at higher T_sub is proposed as the reason for the improved [0001] alignment of rods grown on these templates. PACS 81.07.De; 81.10.Dn; 52.38.Mf     

Optimization of magnetron deposition process for formation of high-quality oriented ZnO films

The thickness distribution and structure of ZnO films deposited by DC-magnetron sputtering of a zinc target in argon-oxygen gaseous medium at substrate temperature of 27°C and gas pressure in the chamber within 5×10^?3 ? 5×10^?2 mm Hg was investigated. It was revealed that the use of a target with a certain depression in the sputtering zone allows depositing high quality c-oriented films at lower gas pressure than with a flat target. The dependence of film quality on geometric factors is interpreted on the basis of theoretical computations with the assumption that the film structure is improved when the flux of deposited Zn particles decreases while their energy increases.     

Ion tracks in ultrathin polymer films: The role of the substrate

Surface damage produced by single MeV-GeV heavy ions impacting ultrathin polymer films has been shown to be weaker than those observed under bulk (thick film) conditions. The decrease in damage efficiency has been attributed to the suppression of long-range effects arising from excited atoms lying deeply in the solid. This raises the possibility that the substrate of the films itself is relevant to the radiation effects seen at the top surface. Here, the role of the substrate on cratering induced by individual 1.1 GeV Au ions in ultrathin poly(methyl methacrylate) (PMMA) layers is investigated. Materials of different thermal and electrical properties (Si, SiO₂, and Au) are used as substrates to deposit PMMA thin films of various thicknesses from ∼1 to ∼300 nm. We show that in films thinner than ∼40 nm craters are modulated by the underlying substrate to a degree that depends on the transport properties of the medium. Crater size in ultrathin films deposited on the insulating SiO₂ is larger than in similar films deposited on the conducting Au layer. This is consistent with an inefficient coupling of the electronic excitation energy to the atomic cores in metals. On the other hand, the damage on films deposited on SiO₂ is not very different from the Si substrate with a native oxide layer, suggesting, in addition, poor energy transmission across the film/substrate interface. The experimental observations are also compared to calculations from an analytical model based on energy addition and transport from the excited ion track, which describe only partially the results.     

Effects of oxygen partial pressure on resistive switching characteristics of ZnO thin films by DC reactive magnetron sputtering

Cu/ZnO/n⁺-Si structures were prepared by magnetron sputtering of a layer of ZnO thin film onto heavily doped silicon substrate, followed by thermal evaporation of a thin layer of metallic Cu. The resistive switching characteristics of Cu/ZnO/n⁺-Si structures were investigated as a function of oxygen partial pressure during ZnO deposition. Reproducible resistive switching characteristics were observed in ZnO thin films deposited at 20%, 33% and 50% oxygen partial pressure ratios while ZnO thin film deposited at 10% oxygen partial pressure ratio did not show resistive switching behavior. The conduction mechanisms in high and low resistance states are dominated by space-charge-limited conduction and ohmic behavior respectively, which suggests that resistive switching behaviors in such structures are related to filament formation and rupture. It is also found that the reset current decreases as oxygen partial pressure increases, due to the variation of oxygen vacancy concentration in the ZnO thin films.     

Influence of target substrate distance on the properties of transparent conductive Si doped ZnO thin films

Si doped zinc oxide (SZO, Si3%) thin films are grown at room temperature on glass substrates under argon atmosphere, using direct current magnetron sputtering. The influence of the target substrate distances on structure, morphology, optical and electrical properties of SZO thin films is investigated. Experimental results show that the target substrate distances have a significance impact on the growth rate, crystal quality and electrical properties of the films, and have little impact on the optical properties of the films. SZO thin film samples grown on glasses are polycrystalline with a hexagonal wurtzite structure and have a preferred orientation along the c-axis perpendicular to the substrate. When the target substrate distance decreases from 76 to 60 mm, the degree of crystallization of the films increased, the grain size increases, and the resistivity of films decreases. However, when the distance continuously decreases from 60 to 44 mm, the degree of crystallization of the films decreased, the grain size decreases, and the resistivity of the films increases. SZO(3%) thin films deposited at a target substrate distance of 60 mm show the lowest resistivity of 5.53 × 10⁻⁴ Ω cm, a high average transmission of 94.47% in the visible range, and maximum band gap of 3.45 eV under 5 Pa of argon at sputtering power of 75 W for sputtering time of 20 min.     

Molecular dynamics and experimental studies on deposition mechanisms of ion beam sputtering

Molecular dynamics (MD) simulation and experimental methods are used to study the deposition mechanism of ionic beam sputtering (IBS), including the effects of incident energy, incident angle and deposition temperature on the growth process of nickel nanofilms. According to the simulation, the results showed that increasing the temperature of substrate decreases the surface roughness, average grain size and density. Increasing the incident angle increases the surface roughness and the average grain size of thin film, while decreasing its density. In addition, increasing the incident energy decreases the surface roughness and the average grain size of thin film, while increasing its density. For the cases of simulation, with the substrate temperature of 500 K, normal incident angle and 14.6 × 10⁻¹⁷ J are appropriate, in order to obtain a smoother surface, a small grain size and a higher density of thin film. From the experimental results, the surface roughness of thin film deposited on the substrates of Si(1 0 0) and indium tin oxide (ITO) decreases with the increasing sputtering power, while the thickness of thin film shows an approximately linear increase with the increase of sputtering power.     

The influence of dielectric films on the superconductivity of thin Sn and Tl films

The change in transition temperature for thin Sn and Tl films has been measured after deposition of dielectric substances. The change in transition temperature is inversely proportional to the thickness of the superconducting film and relatively independent of the thickness of the dielectric film. When SnS or Tl₂Se is deposited on Sn films, the transition temperature is depressed. When TICl or Tl₂Se is deposited on Tl films, the transition temperature is increased. When S is deposited on Tl films, there is no initial change inT _c ; however,T _c is appreciably increased after annealing. A possible explanation for this effect is the reduction of the free electron concentration in the metal through the formation of the contact potential between the dielectric and the metal.