Equilibrium helium films under the influence of surface roughness

No Heading Equilibrium helium films adsorbed on solid substrates are investigated. Due to their thickness these films are mainly in the retardation regime where the influence of the roughness of the substrates, (x), can be strong enough to be observed. For the definition of (x) we use a simple corrugation model. This model is supported by experimental results using the surface plasmon resonance technique to determine the thickness of helium films grown on different Ag surfaces.PACS numbers: 67.70.+n, 68.15.+e, 68.43.–h, 68.55.–a     

Whole-field determination of surface roughness by speckle correlation

A whole-field method of double-exposure speckle photography is employed to determine metal surface roughness by correlation between two speckle patterns. A movable rectangular aperture that is mounted before an image lens is shifted between the exposures, which results in a decrease in the contrast of the reconstructed Young's fringes with increasing roughness. The technique permits evaluation of the roughness of particular points on a surface as well as the average roughness of an entire surface. Four sets of random surfaces that were prepared by different machine-finishing processes and with roughnesses ranging from 0.6 to 13 μm have been tested. Different methods have been carried out to process the test data, and a practical method for the evaluation of surface roughness is proposed.     

Back-scattering energy loss parameter measurements in thin metal films

The back-scattering energy loss parameter [S] has been determined directly from evaporated metal layers of Ti, V, Ni, Mo and Ta for 2.0 MeV He⁺ ions and a back-scattering angle θ = 168°. Reasonable agreement has been found with [S] values calculated from experimental dE/dx results tabulated by Ziegler and Chu. Discrepancies in [S] values occurred for thin Mo and Ta layers (< 500 Å) and for Ti and V layers evaporated in a poor vacuum. Variations are thought to be due to film growth effects and impurity incorporation during deposition. A correlation of [S] values with layer superconducting properties has been observed.     

Ellipsometric studies of microscopic surface roughness of CdS thin films

Analysis of changes in surface roughness of CdS thin films with preparation temperature was carried out using variable angle spectroscopic ellipsometry (VASE). The films studied were prepared by spray pyrolysis technique, in the substrate temperature range 200–360°C. The VASE measurements were carried out in the visible region below the band gap (E _g=2·4eV) of CdS so as to reduce absorption by the film. The thickness of the films was in the range 500–600 nm. Bruggeman’s effective medium theory was used for analysis of the surface roughness of the film. The roughness of the film had a high value (∼ 65 nm) for films prepared at low temperature (200°C) and decreased with increase in substrate temperature. This reached minimum value (∼ 27 nm) in the temperature range 280–300°C. Thereafter roughness increased slowly with temperature. The growth rate of the films was calculated for different temperature ranges. It was found that the deposition rate decreases with the increase in substrate temperature and have an optimum value at 300°C. Above this temperature deposition rate decreased sharply. The scanning electron micrograph (SEM) of the film also showed that the film prepared at 280–300°C had very smooth surface texture.     

Effect of surface roughness on nanoindentation test of thin films

By using the two-dimensional quasicontinuum method, the nanoindentation process on a single crystal copper thin film with surface roughness is simulated to study the effect of surface morphology on the measurements of mechanical parameters. The nanohardness and elastic modulus are calculated according to Oliver-Pharr’s method. The obtained results show a good agreement with relevant theoretical and experimental results. It is found that surface roughness has a significant influence on both the nanohardness and elastic modulus of thin films determined from nanoindentation tests. The effect of such factors as the indenter size, indentation depth and surface morphology are also examined. To rule out the influence of surface morphology, the indentation depth should be much greater than the characteristic size of surface roughness and a reasonable indenter size should be chosen. This study is helpful for identifying the mechanical parameters of rough thin films by nanoindentation test and designing nanoindentation experiments.     

Experimental determination of the symmetry of the order parameter in YBCO

At present, the symmetry of the order parameter in the high temperature superconductor YBCO is quite controversial. Recent experiments using SQUIDs and Josephson junctions appear to support competing theories, with some experiments supporting a d_x2_–y2 pairing symmetry for the order parameter and others a s-wave pairing symmetry. We note that a number of factors such as trapped flux, magnetic field gradients and SQUID asymmetries could lead such measurements astray. We use a Scanning SQUID Microscope and a time-reversal invariance test to resolve these experimental problems. We find the order parameter in YBCO has a time-reversal invariant d_x2–_y2 symmetric component. We estimate the amplitude of anyimaginary s-wave symmetric component to be less than 4% and anyreal s-wave component to be less than 82%.     

Nanometer surface roughness effect on the magnetic properties of CoFeHfO thin films

This article reports the effects of nanometer surface roughness on the magnetic properties of CoFeHfO thin films, as deposited on Si (100) substrates. The surface roughness was controlled via the working pressure during the sputtering time. When the working pressure increases from 0.5 to 3 mT, the surface roughness (R) of CoFeHfO thin films, formed by islands with the average high R, increases from 0.25 nm to 4.66 nm, respectively. At surface roughness (R) = 4.66 nm, coercivity (H(c)) reaches the highest value of 0.42 Oe and magnetic anisotropy (H(k)) drops to the lowest value of 33 Oe. This suggests that the quality of the soft magnetic properties of thin film decrease due to the increase in surface roughness. However, at very low working pressure, thin films become a homogeneous structure which also exhibits poor soft magnetic properties. The optimum value, with H(c) of 0.10 Oe and H(k) of 50 Oe, were obtained at 1.5 mT of working pressure. The model of the roughness effect on the magnetic properties is introduced and discussed.     

Experimental determination of the thermal conductivity of thin films

Two techniques have been developed to determine experimentally the thermal conductivity of thin solid films of thickness 500 Å or more at low and high temperatures. The first technique is a steady state and is suitable for measurements above room temperature. The method enables the thermal conductivity of eight film specimens to be measured simultaneously. The second technique is a transient one (an adaptation of Ioffe's method for bulk materials) and is suitable for measurements in the temperature range 100–260 K. The two techniques have been used to make measurements of thin films of copper and various crystalline and amorphous semiconductors. The values of the thermal conductivity for thick copper films by both techniques agree quite well with the bulk values.     

In-situ quantification of the surface roughness for facile fabrications of atomically smooth thin films

This work presents an in-situ technique to quantify the layer-by-layer roughness of thin films and heterostructures by measuring the spectral profile of the reflection high-energy electron diffraction(RHEED).The characteristic features of the diffraction spot,including the vertical to lateral size ratio c/b and the asymmetrical ratio c₁/c₂ along the vertical direction,are found to be quantitatively dependent on the surface roughness.The quantitative relationships between them are established and discussed for different incident angles of high-energy electrons.As an example,the surface roughnesses of LaCoO₃ films grown at different temperatures are obtained using such an in-situ technique,which are confirmed by the ex-situ atomic force microscopy.Moreover,the in-situ measured layer-by-layer roughness oscillations of two LaCoO₃ films are demonstrated,revealing drastically different information from the intensity oscillations.The experiments assisted with the in-situ technique demonstrate an outstanding high resolution down to-0.1 A.Therefore,the new quantitative RHEED technique with real-time feedbacks significantly escalates the thin film synthesis efficiency,especially for achieving atomically smooth surfaces and interfaces.It opens up new prospects for future generations of thin film growth,such as the artificial intelligence-assisted thin film growth.     

Thickness determination of ultrathin metal films using the X-ray fluorescence technique

The X-ray fluorescence technique was used to determine the thickness of single-layered, double-layered and triple-layered films of copper, bismuth and gold on mylar substrates. An annular ¹⁰⁹Cd X-ray source of 5 mCi was used to excite hte characteristic X-rays. The background was much lower as well as flat in the present study in comparison with our earlier results an ²⁴¹Am exciter source. This resulted in a downward extension of the lower limit of thickness measurement of thin films coupled with an improved accuracy.     

Influence of the surface roughness on the properties of Au films measured by surface plasmon resonance and X-ray reflectometry

Thickness and refractive index of Au films thermally evaporated onto glass substrates and with an underlayer of Cr are determined from surface plasmon resonance. The results for the thickness are found to agree very well with those from X-ray reflectivity when a simple model of layers with flat interfaces is used. Plasmon propagation along thin films is influenced by radiative damping due to scattering from surface roughness. To study this influence the surface roughness of the glass substrate, Cr an Au layers are measured by X-ray reflectometry and the results used to introduce three intermediate layers with effective refractive indices and thicknesses corresponding to the roughness. Then Fresnel's equations are used to fit the reflectivity and to deduce the layer properties. It is found that the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm. In particular, the absolute value of real part of the dielectric constant decreases while its imaginary part increases when those effects are not taken into account.     

Surface roughness and transverse magnetic field dependence of the Hall coefficient and the magnetoresistance in thin metal films

Defining an effective relaxation time which depends on the root mean square (rms) surface roughness and on the angle of incidence of electrons and then using the Boltzmann transport equation general expressions have been derived for the Hall coefficient and conductivity in thin metal films subjected to à transverse magnetic field. In the weak- and strong-field limits simple analytical equations have been proposed which reveal slight size effects in the Hall coefficient and in the magnetoresistance as well as à weak field dependence of these transport parameters in agreement with previous experiments. The theoretical predictions of the present model have been compared with those of the mean free path (mfp) method which constituted an extension of the Coney model. 1n conclusion a correlation between the respective size parameters,A, in the present model and, µ, in the mfp method is proposed.[/p]     

Reduction of lossy surface waves in a double metal films structure

It has been demonstrated that the spontaneous emission rate can be enhanced dramatically by Surface Plasmon Polaritons (SPP) due to large density of states for photon near the SPP resonance frequency and very small mode volume. Some valuable works have been reported to verify such effect with a single metal layer above the semiconductor active layer. However, huge power dissipating to "lossy surface wave (LSW)" mode occurs in such monolayer structure because the enhancement decays with the distance away from the interface exponentially and therefore the active layer should be close enough to the metal film. LSW will convert the energy into heat and make the monolayer structure hardly be applied to obtain large enhancement of spontaneous emission. In this paper, we propose a double-layer structure with two metal films at the top and bottom of semiconductor layer. By assuming the dipole (classical model for the source) at the center of the active layer, which means the distance between the dipole and the interfaces is half of the thickness of active layer, the enhancements of spontaneous emission were calculated and analyzed for both monolayer and double-layer structures with different thickness of the metal film and active layer. The calculation results show that, for the double-layer structure, the critical thickness of the active layer, where the power to SPP exceeds the power to LSW, is much thinner than that in the monolayer structure. This means that the active layer can be set closer to the metal film in the double-layer structure without worrying about the LSW. Therefore, larger enhancement of spontaneous emission rate and higher available energy can be achieved simultaneously.