Thermal stress induced band gap variation of ZnO thin films

The growth temperature and post annealing-dependent optical and structural effect of RF magnetron sputtered ZnO thin films were examined. As the growth temperature increased, the lattice constant increased and approached the bulk value, suggesting a decrease in interfacial strain between the substrate and thin film. For the post annealed samples, the interfacial strain decreased further and was close to the bulk value regardless of the post annealing environments (in air and O₂). The optical properties of all ZnO thin films examined and revealed higher transparency (>90%). Furthermore, the optical band gap varied according to the growth temperature and post annealing environments due to a decrease in the interfacial strain effect.     

Thermal behavior of thin metal films irradiated by shaped femtosecond pulse sequences laser

A numerical solution of the two-temperature model has been performed up to the shaped femtosecond pulse sequences heated metal target. The two-temperature model is used to analyze the shaped femtosecond pulse sequences with the following major conclusions. We confirm the distinctly different results on the different shaped femtosecond pulse sequences. As the number of shaped femtosecond pulses increases, the nonequilibrium state between electrons and phonons gradually disappears, the highest transient electron temperature is lowered and the thermolization time is prolonged, the electron heat conductivity remains higher because of the effect of incubation on the electron temperature, which preserves the advantages of ultrashort lasers. The shaped femtosecond pulse sequences can increase the efficiency in ablation and micromachining.     

Doppler-shifted cyclotron resonance in thin metal films

The velocities of electrons contained in a thin slab are quantized because the component of momentum transverse to the slab faces is quantized. For a free electron gas the transverse velocity is given by |v_H| = l(/m) (π/d) where l = 1, 2, 3, …. If a magnetic field is applied normal to the slab, the wave number and frequency dependent conductivity consists of a series of resonant terms. The resonances occur at the Doppler-shifted cyclotron resonance frequencies |ω_c| = ω ± p(/m) (π/d)² where l = 1, 2, 3, …. It is shown that these resonances in the conductivity result in an absorption in pure thin films at low temperatures which is periodic in magnetic field. The semi-classical expression for the absorption is in substantial agreement with the corresponding quantum calculation, and has the virtue that it may be readily extended to non-spherical Fermi surfaces.     

Elasto-plastic behaviour of thin metal films

By extracting the variation of the plastic strain rate from measurements of the stress–strain curves of thin films of varying thickness, the large extent of the microdeformation stage was determined for tensile deformation of free-standing thin films, as well as for films on substrates deformed by cyclic heating. The stress varies dramatically with strain during this stage. It is demonstrated that this behaviour is common to most fine-grained polycrystals, and that the extent of the microdeformation stage is much larger than the conventional 0.2% proof strain, and depends both on the material as well as on the measurement technique. Therefore a careful analysis of this stage is essential in measuring the mechanical behaviour of these materials.     

Optically induced transverse voltages in thin metal films

Results of experiments on laser induced voltages in metal films are summarized. The voltages are found to occur along certain directions in the plane of slant-angle vapour deposited films. A thermo-electric model based on periodic variations in the microscopic film structure, is described. The magnitude of the effect is proportional to the absorbed power/length between the contact points. Application of the films in the wavelength range 0.33–10.6μm is described and compared to more conventional detectors. A recent application, using the effect as a tool to profile non-uniformities in thin films, is also discussed.     

Nanostructure and conductivity of thin metal films

Amorphous silver, copper, gold, and iron films of a thickness between 6 and 350 nm are grown on polymeric substrates by vacuum evaporation. The nanostructure of the films is investigated. The dependence of the conductivity on the film thickness is obtained, and a correlation between the surface morphology and the conductivity is established.     

Inelastic electron scattering from thin metal films

I discuss the role of electron-hole pair excitation in inelastic scattering of slow electrons from thin metals films.     

Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

Thermal decomposition of nickel carbide thin films

Thin films of nickel carbide are produced by evaporating fourteen atomic layers (1.8 × 10¹⁶ atoms cm^?2) of nickel onto the (0001) surface of graphite at room temperature. The presence of nickel carbide is indicated by the characteristic carbide Auger electron signal. LEED shows that no ordered structural change takes place on the graphite (0001) surface when nickel carbide is produced in this manner. Isothermal heating of the sample leads to an irreversible change of the carbon Auger signal. The times required for this change range from 150 min at 150°C to 45 min at 185°C. The times required for decomposition yield an activation energy for decomposition equal to 50 kJ mole^?1. The decomposition of nickel carbide thin films obeys zero order kinetics. Depth profiling of the film after decomposition reveals that the observed decomposition is limited only to the top two or three atomic layers.     

Influence of hydrogen absorption on stress changes in thin catalytic metal films dedicated for sensors application

In the paper results of investigation of the influence of low concentration hydrogen on stress changes in thin catalytic metal films were discussed. The concentration of H₂ was altered from 6 ppm to 1% of hydrogen (6N) in nitrogen (6N). Silicon beams covered with palladium or platinum films of various thicknesses were anchored at one end and their deflection at the other end was measured by atomic force microscope. Stress changes were determined by application of modified Stoney formula and compared with results of computer modelling. The influence of stress caused by hydrogen absorption on the alteration of output characteristics of AIII-N based hydrogen sensors was excluded. The time dependence of stress in metallic films for various hydrogen concentrations indicated dissociation limited mechanism of hydrogen absorption.     

Temperature of phase transitions in thin two-layer metal films

It is experimentally established that the melting temperatures of Pb/Bi, Pb/Sn, Mg/Sn, Sn/Zn, and In/Zn two-layer films decrease with a decrease in their thickness.     

Effect of stress in copper ferrite thin films

Cubic copper ferrite thin films, obtained by rf sputtering on quartz and subsequent post-annealing and quenching, show a large coercivity of about 300–600 Oe. Stress measurements using X-ray diffraction show high value of stress of about 400–1000 MPa. Both the stress and coercivity are found to increase with the decrease of the thickness of the films. There appears to be a contribution of the stress to the coercivity of the films, in the in-plane M–H loops.     

Spintronics in metal–insulator nanogranular magnetic thin films

Magnetic and electrical properties of metal–insulator nanogranular thin films are overviewed, from the spintronics point of view, by presenting our recent results. (1) The metal-rich ferromagnetic ones possess ultrahigh-frequency (MHz–GHz) permeabilities μ′, μ″, mainly owing to large induced magnetic anisotropy field H_k. They are useful for various magnetic devices, such as thin-film inductors and noise suppressors. (2) The insulator-rich superpramagnetic ones exhibit tunnel-type magnetoresistance, being large in (Mg or Al)-fluoride base films. The Granular-In-Gap (GIG) devices with improved field sensitivity are useful for very low magnetic-field sensors.     

Conducting and reflecting properties of thin metal films

Amorphous silver, copper, gold, and iron films with a thickness of 6–60 nm have been grown on a polymer substrate by the method of vacuum deposition. The dependences of the specific conductivity and the microwave reflection coefficient on the film thickness are obtained and a relation between these values is established.     

Ordered ultra thin ZnO films on metal substrate

Ultra thin ZnO films were prepared on metal Mo(1 1 0) substrate under ultrahigh vacuum conditions either by depositing Zn in 10⁻⁵ Pa oxygen or by oxidizing pre-deposited Zn films. The films were characterized in situ by various surface analytical techniques, including Auger electron spectroscopy, X-ray and ultraviolet photoelectron spectroscopies, low energy electron diffraction and high resolution electron energy loss spectroscopy. The results indicate that a long-range ordered and stoichiometric ZnO films are formed along its [0 0 0 1] direction. The annealing experiments show that as-prepared ZnO films are thermal stable until 800 K. This study provides constructive information to further understand the growth mechanism of ZnO films on different substrates.     

Crystallization of sputter deposited amorphous metal thin films

We have studied the crystallization of DC sputter deposited amorphous W-Si alloys under three separate conditions: free standing films, amorphous films deposited on Si substrates, and films with one of four different overlayers. In the case of the free standing films the crystallization temperature versus composition was measured by differential thermal analysis (DTA) for films with Si contents from 5 at% to 38 at%. Films with Si concentrations of 38% to 22% were amorphous as deposited, while those with less Si were crystalline. The crystallization temperature was a strong function of composition with a maximum T_c of 915°C (at a heating rate of 20 K/min) in alloys with 28% Si. Overlayers of W, Cu, Au and Al were investigated. Both the W and Cu overlayers had little effect on the stability of the underlying W-Si, while both the Au and Al reduced the crystallization temperature by at least 100°C. The results reported here reinforce the observation that the choice of overlayer plays a critical role in determining the overall stability of metallization systems that include amorphous layers as diffusion barriers.     

Thermal conductivity of nanoscale polycrystalline ZnO thin films

Thermal conductivities (TCs) of ZnO thin films of thickness 80-276 nm prepared by sol-gel method are measured by the transient thermoreflectance (TTR) system. The obtained TCs ranging from 1.4 to 6.5 W/m K decrease while the thickness decrease. The measured TCs are much smaller than those of bulk ZnO, which is about 100 W/m K. The possible reasons for the decrease are the grain boundary and defects. The latter is the dominating one from the analysis.