Analysis of precursors for crystal growth of YBaCuO thin films in magnetron sputtering deposition

We correlated the crystallinity of YBaCuO films prepared by magnetron sputtering deposition using Ar/O₂ mixture gas with the atomic and molecular composition in the gas phase. YBaCuO films were deposited on MgO substrates at 670 °C. Two-dimensional distributions of Y, Ba, Cu, YO, BaO, and CuO densities and one-dimensional distribution of O density were measured by laser-induced fluorescence spectroscopy. The Y and Ba densities decreased significantly with the increase of the O₂ partial pressure, and they were below the detection limit at an O₂ flow ratio of 10% and a total gas pressure of 53 Pa. The decrease in the Y and Ba densities was compensated by an increase in the YO and BaO densities. The decrease in the Cu density with the increase of the O₂ partial pressure was less significant, while the CuO density was below the detection limit at all the discharge conditions. The O density was evaluated to be 10¹²-10¹³ cm^− 3, which was much higher than the Cu density. On the other hand, YBaCuO films with high crystallinity were obtained at total gas pressures of 53-80 Pa and O₂ flow ratios of 50-70%. Therefore, it is concluded that the precursors for the deposition of YBaCuO films with high crystallinity are Cu, YO, BaO, and O.     

Nonequilibrium and bolometric responses of YBaCuO thin films to high-frequency modulated laser radiation

Picosecond nonequilibrium and slow bolometric responses to infrared radiation from a patterned high-T _c superconducting (HTS) film in resistive and normal states deposited onto LaAlO₃, NdGaO₃, and MgO substrates were investigated using both pulse and modulation techniques. The response time of 35 ps to a laser pulse of 17 ps FWHM has been observed. The intrinsic response time of the fast process is expected to be about a few picoseconds. The modulation technique, being free from the disadvantages of pulse methods (poor sensitivity, limited dynamic range), makes the detailed study of a number of relaxation processes possible. Besides the nonequilibrium response, two kinds of bolometric processes, namely phonon transport through the film-substrate interface and phonon thermal diffusion in a substrate, manifest themselves in certain frequency dependences.     

Photoinduced changes of the electrical conductivity in modified Y-Ba-Cu-O thin films

The conductivity changes of Y₁Ba₂Cu₃O_7– thin films under r.f. field and white light treatment at 77 K and under laser irradiation at 300 K were studied. The modified films were found to recover partially after prolonged laser illumination. The kinetics of photoactivation was investigated. The observed nonthermally induced changes were associated with free carrier trapping and local disordering of the lattice.     

Some spectral response characteristics of ZnTe thin films

Zinc telluride thin films have been grown at room temperature and higher temperature substrates by thermal evaporation technique in a vacuum of 10^-6 torr. A main peak in the photocurrent is observed at 781 nm (1.58 eV) with two lower amplitude peaks on the lower wavelength side and one on higher wavelength side. The evaluated thermal activation energy is found to correspond well with the main spectral peak. From these studies it can be inferred that temperatures up to 453 K is still in the extrinsic conductivity region of the studied ZnTe thin films. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Encryption of nonlinear optical signals in ZnO:Al thin films by ultrashort laser pulses

Described herein is the effect of optical annealing on the third-order non-linear optical properties exhibited by nanostructured Al-doped ZnO thin films. The samples were synthetized by an ultrasonic spray pyrolysis method. The optical annealing process was carried out by laser pulses at 532, 835 and 1064 nm wavelengths with, ps, fs and ps pulse duration, respectively. The optical non-linearity of the films was measured by the z-scan method with three different irradiations of excitation: 100 fs at 835 nm, 120 ps at 532 nm, and 150 ps at 1064 nm. The as-grown samples showed a saturable optical absorption that evolves into two-photon absorption transitions by a picosecond optical annealing phenomenon induced at 532 nm wavelength. Potential applications for developing optical encryption functions were considered.     

Features of the mechanoluminescence of thin metal films excited by long and short laser pulses

We have studied the strain-induced luminescence from thin fine-grained films of various metals under the action of short (subnanosecond) and long (millisecond) laser pulses. The possible mechanism of luminescence excitation is considered, according to which the emission is due to the interaction between grain boundary dislocations and impurities occurring in the intergranular region.     

Thickness dependence of microcrack formation in YBa2Cu3O6 + x thin films on NdGaO3 (001) substrates

Thickness dependence of parallel microcrack formation in YBa₂Cu₃O_6 + x thin films prepared by pulsed laser deposition from nano- (n) and microcrystalline (μ) targets on NdGaO₃ (001) is systematically investigated. Atomic force microscope and x-ray diffraction measurements show parallel microcracks normal to uniaxial twin boundaries. The amount of microcracks increases with film thickness. Superconducting properties of the films decrease very strongly with film thickness as a result of microcrack formation. The n-films have more rigid lattice and thus show more extensive cracking than μ-films. It is found that the μ-films have a thickness threshold (∼ 70 nm) where the first signs of cracking appear.     

Transition from dislocation controlled plasticity to grain boundary mediated shear in nanolayered aluminum/palladium thin films

Nanolayered materials consisting of alternate layers of two different metals offer enhanced mechanical properties such as hardness but the strengthening mechanism is not well understood when the bilayer thickness approaches a few nanometers. Here, we report on the uniaxial compression of aluminum/palladium pillars (900 nm diameter) with bilayer thickness = 2, 20 and 80 nm. We observe that the deformation behavior of these pillars depends on the value of bilayer thickness, changing from dislocation driven plasticity at large bilayer thickness to shear due to grain rotation via grain boundary sliding at small bilayer thickness. The transition occurs at about a bilayer thickness of 20 nm where a mixture of the two mechanisms is apparent.     

Growth of diborides thin films on different substrates by pulsed laser ablation

The growth of scandium, titanium and zirconium diborides thin films by pulsed laser ablation technique on different substrates has been studied. In situ reflection high energy electron diffraction and ex situ X-ray diffraction analyses indicate that the films are strongly c-axis oriented on all the substrates and also epitaxial, apart from Si(111), where the in plane orientation is poor. Atomic force microscopy imaging reveals a flat surface in all the epitaxial samples, with roughness lower than 1 nm. The results on silicon carbide and sapphire are very promising for using these materials as buffer layers in magnesium diboride thin films growth, especially to improve epitaxy and to prevent oxygen diffusion from the substrate, and also to study the influence of lattice strain on MgB₂ critical temperature.     

Modification of AlN thin films morphology and structure by temporally shaping of fs laser pulses used for deposition

We studied the effect of temporally pulse shaping upon the properties of thin layers synthesized by pulsed laser deposition with fs laser pulses generated by a Ti-sapphire laser source. We showed that the film morphology and structure can be gradually modified when applying mono-pulses of different duration or passing to a sequence of two pulses of different intensities.     

Influence of the substrate nature on the properties of ZnO thin films

Zinc metallic films, deposited onto different substrates, were submitted to a thermal oxidation process, in air, in order to obtain ZnO thin films. X-ray diffraction patterns revealed that as-obtained ZnO thin films were polycrystalline and have a wurtzite (hexagonal) structure. The temperature dependences of the electrical conductivity during some heating/cooling cycles were studied and the conduction mechanism was interpreted in terms of Seto model. The sensitivity of ZnO thin films, at five gases, was investigated and it was established that ethanol is the test gas that produces the most significant changes in the electrical resistance of all the studied films. Some correlations between the oxidation temperature and the substrate nature and the parameters which characterize the structural and electrical properties of ZnO thin films have been established.     

退火温度对Ge-SiO2薄膜结构的影响

用射频磁控溅射制备了Ge-SiO2薄膜，在N2的保护下对薄膜进行了不同温度的退火处理。使用傅里叶变换红外光谱分析（FTIR）技术，X射线衍射谱（XRD），X射线光电子能谱（XPS）分析样品的微结构，研究样品在退火中发生的结构，组分的变化，结果表明退火温度对薄膜的结构，尤其是薄膜中的GeO含量和GeO晶粒的大小的影响是显著的，并对其做了详尽的分析讨论。     

The contribution of grain boundary scattering versus surface scattering to the resistivity of thin polycrystalline films

Materials with nanometric dimensions exhibit higher electrical resistivity due to additional scattering centers for the conduction electrons, mainly from surfaces and grain boundaries. In this study we focus on the effect of grain boundaries by modeling the expected resistivity due to the observed log-normal distribution of boundaries, unlike the widely used model of Mayadas and Shatzkes that assumes a Gaussian distribution. The results of the model are then experimentally explored by correlating the resistivity of thin copper films with their grain size distribution. Applying a newly suggested analysis method solves the ambiguity in distinction between surface scattering and grain boundaries scattering. It is found that for the explored layers the increase in resistivity is dominated by the effect of grain boundaries.     

Influence of interfacial delamination on channel cracking of elastic thin films

Channeling cracks in brittle thin films have been observed to be a key reliability issue for advanced interconnects and other integrated structures. Most theoretical studies to date have assumed no delamination at the interface, while experiments have observed channel cracks both with and without interfacial delamination. This paper analyzes the effect of interfacial delamination on the fracture condition of brittle thin films on elastic substrates. It is found that, depending on the elastic mismatch and interface toughness, a channel crack may grow with no delamination, with a stable delamination, or with unstable delamination. For a film on a relatively compliant substrate, a critical interface toughness is predicted, which separates stable and unstable delamination. For a film on a relatively stiff substrate, however, a channel crack grows with no delamination when the interface toughness is greater than a critical value, while stable delamination along with the channel crack is possible only in a small range of interface toughness for a specific elastic mismatch. An effective energy release rate for the steady-state growth of a channel crack is defined to account for the influence of interfacial delamination on both the fracture driving force and the resistance, which can be significantly higher than the energy release rate assuming no delamination.     

Influence of irradiation on the switching behavior in PZT thin films

Spatially nonuniform imprint behavior induced by X-ray synchrotron, electron and neutron irradiation has been investigated in sol–gel Pb(Zr,Ti)O₃ thin films. The analysis of the switching current data reveals the strong influence of irradiation on the switching current shape. The obtained effects have been explained as a result of acceleration of the bulk screening process induced by irradiation. It was shown that the spatial distribution of the internal bias field is determined by the domain structure existing during irradiation. The changes in the structural characteristics during fatigue cycling have been reveled by high resolution synchrotron X-ray diffraction experiments on (1 1 1)-oriented PZT-based capacitors with a composition in the morphotropic region. From both ex situ and in situ measurements, microstructural changes with cyclic switching during fatigue have been evidenced and correlated with the evolution of the switching characteristics.     

Influence of substrate compliance on buckling delamination of thin films

A thin film subject to in-plane compressive stress is susceptible to buckling-driven delamination. This paper analyzes a straight-sided delamination buckle with a focus on the effects of substrate compliance, following earlier work by B. Cotterell and Z. Chen. The critical buckling condition, the energy release rate and the mode mix of the interface delamination crack are calculated as a function of the elastic mismatch between the film and substrate. The average energy release rate at the curved end of a tunneling straight-sided blister is also determined. The more compliant the substrate, the easier for the film to buckle and the higher the energy release rates. The effect becomes significant when the modulus of the substrate is appreciably less than that of the film. When the substrate modulus is comparable to that of the film, or higher, the usual assumption is justified to the effect that the film is clamped along its edges. When the substrate is very compliant the energy release rate at the curved front exceeds that along the straight sides.     

Influence of sputtering parameters on crystalline structure of ZnO thin films

The relations between the sputtering parameters and the crystalline microstructure of ZnO thin films are presented. The energetic bombardment of substrate by neutral atoms, ions and electrons during sputtering is characterized by total energy flux density which affects the film. This parameter can be estimated by RF power, substrate bias voltage and concentration of reactive gases. Substrate temperature and total energy flux density are the major parameters which have a significant influence on ZnO thin film crystalline structure.