Epitaxial growth of the Au/C/Ag system in ultrahigh vacuum

Carbon layers were evaporated onto silver single crystals (that had been previously exposed to air) at room temperature in an ultrahigh vacuum (UHV), and a gold film was subsequently evaporated onto the specimens without change of temperature. The gold films were examined by transmission electron microscopy to determine whether or not epitaxy had occurred. Three cases were observed: complete epitaxy when the average carbon film thickness was 1 Å or less; completely disordered structures for amorphous films 3 Å thick or more; and a partially oriented structure in the intermediate range. The fact that epitaxy is destroyed before the coalescence stage of the carbon film is reached suggests that the carbon islands block the mobility of the nuclei that are growing epitaxially; there is no need for carbon to cover the silver crystal completely for epitaxy to be destroyed. The experimental results obtained also indicate that not only is direct contact between substrate and evaporated film necessary for epitaxy but that the substrate surface must be “suitably” contaminated.     

Influence of substrate temperature and film thickness on the structure of reactively evaporated In2O3 films

Indium oxide films 25–550 Å thick were reactively evaporated at an oxygen pressure of about 0.27 Pa and at a substrate temperature between room temperature and 400°C. The dependence of the structure of the films on the substrate temperature and on the film thickness was studied using transmission electron microscopy and electron diffraction. It was found that thick films (about 550 Å) were amorphous at room temperature, partially crystallized at 50–125°C and crystalline at 150–400°C. The crystallinity of the films deposited at 150–250°C also depended markedly on the film thickness. Very thin films about 25 Å thick were quasi-amorphous, but with increasing film thickness the amorphous phase transformed into a crystalline phase.The thermal transformation of the amorphous films after deposition was also studied. Amorphous films about 550 Å thick deposited at room temperature and 100°C crystallized at 230°C and 210°C respectively.     

High temperature metal-induced crystallization of r.f. sputtered amorphous Si thin films

R.f. sputter-deposited amorphous Si (a-Si) films have been found to exhibit an interconnected fine structure of low and high density regions. The porosity decreases as the film thickness increases in agreement with results for evaporated a-Ge previously published by Donovan. At an annealing temperature of 800°C, a-Si films crystallize by the relatively slow diffusionless growth of crystallites nucleated within the a-Si matrix. Similar a-Si films containing a buried layer 30 Å thick of Al in an a-Si/Al/a-Si structure exhibited greatly enhanced nucleation and crystallization rates. Crystallite growth in these films occurred by two separate mechanisms both involving the precipitation of crystalline Si from an Al + Si liquid region. The first growth mechanism was observed during the early stages of crystalline formation in which crystalline islands grew around isolated liquid regions. Later, Al + Si liquid boundary layers were established between crystalline Si and a-Si regions, and further crystallization occurred by the motion of this boundary layer.     

Crystallization of amorphous antimony films deposited onto glass substrates in ultrahigh vacuum

The crystallization of amorphous antimony (a-Sb) films deposited onto glass substrates in an ultrahigh vacuum of 10^?6?10^?7 Pa is investigated through in situ observation with an optical microscope camera. In comparison with the results for deposition in a conventional vacuum of 10^?4?10^?5 Pa, a marked reduction is observed in the critical thickness d_c for crystallization. For thicknesses less than the previous d_c value of about 250 Å, the dependence of the crystallization rate on the thickness is found to weaken drastically and the activation energy of the atoms for crystallization to increase. The antimony crystallites which nucleate in such thin a-Sb films do not take a simple spherical form.     

Lattice distortion energy spectra of thin evaporated silver films

Thin silver films (thickness, 12.5–160 nm) were evaporated under ultrahigh vacuum conditions onto clean glass substrates (substrate temperature, 225 K). During the growth process a large number of lattice defects were incorporated (condensation rate, 0.2 and 0.01 nm s^-1). The films were subjected to heat treatment (constant heating rate, 0.1 K s^-1) and the variation in the electrical resistance was measured as a function of temperature. Using Vand's theory the initial lattice distortion energy spectra of the films were determined from the resistance-temperature data. The lattice distortion energy function has maximum values. While the number of distortions with a decay energy of about 0.8 eV increases rapidly with decreasing film thickness, the number of distortions with a decay energy of about 0.93 eV varies only slightly.     

Electrical conductivity,thermoelectric power and optical absorption studies of amorphous and crystalline gallium antimonide thin films

Amorphous films (240–1740 Å thick) of gallium antimonide were formed by vacuum evaporation onto goldseal glass slides and onto sodium chloride crystals. They were annealed in successive cycles to obtain the ideal amorphous state. The heat treatment was continued until the crystalline phase was obtained. The electrical conductivity, the thermoelectric power with respect to bulk silver and the optical absorption were measured in all these films before and after crystallization. The electron diffraction patterns of the as-deposited, the annealed and the crystalline films were obtained using a transmission electron microscope. The activation energy for conduction ΔE, the amorphous-to-crystalline transition temperature, the density of states at the Fermi level N(_EF) and the optical energy gap E_o were evaluated.     

Surface structure of silver thin films on In2O3:Sn and Al2O 3 †

Surface structure of thin silver films (200 Å) on two technologically important films, indium tin oxide (ITO) and aluminium oxide, has been studied using scanning tunneling microscope. ITO films were prepared by reactive electron beam evaporation. Aluminium oxide films were prepared by oxidizing 2000 Å thick aluminium films evaporated on to H₂ terminated single crystal silicon substrates. The surface structure of silver on ITO and aluminium oxide appeared to be same and was characteristic of Stranski-Krastanov type. The observed asymmetry in the island shape was attributed to the anisotropic nature of the strain fields surrounding the nucleation centres.     

Diffusion in Au-Ni thin films

The objective of this work was to determine experimentally the values of the surface diffusion parameters of Au-Ni thin films obtained by vacuum evaporation and by sputter deposition. Thin film diffusion couples with an edge-to-edge interface arrangement were employed in order to define the surface diffusion mechanisms better. Experimental results show that the frequency factor (diffusion constant) for evaporated films (1.5×10^-8 cm² sec^-1) is higher than that for sputtered films (1.1×10^-8 cm² sec^-1) and bulk material (bulk diffusion data). The values obtained for the thermal activation energy in evaporated films were one order of magnitude less than those obtained for bulk material. Sputtered thin films were found to have an activation energy over 20% higher than that for evaporated films. This discrepancy apparently occurs because of partial incorporation of sputtered atoms into the glass substrate. Measurements of thin film adhesion showed the same effect.Examination of the structural characteristics of the specimens showed that both sputtered and evaporated films 300 Å and more in thickness become completely microscopically continuous. Some variations in grain size were also observed. Sputtered films were found to have crystallite grains twice as large as those in films prepared by evaporation. Microphotographs showed that for films 300 Å thick the “evaporation-condensation” effect occurs in the overlapping zone.     

Effect of a thin silver sublayer on the growth of antimony films deposited in vacuum of 10-5 Pa

The growth of antimony films on collodion covered with a predeposited thin silver sublayer was studied by electron microscopy. The mean thickness of the sublayer, which was composed of silver islands, ranged from 0.03 to 0.30 nm. The oblique angle φ of the antimony vapour beam with respect to the substrate surface was varied from 0° to 80°. The surface coverage of the substrate by the antimony film is markedly increased, independently of φ, by the predeposition of silver because the silver islands act as excess nucleation centres to capture antimony molecules, absorbing the momenta of the molecules along the substrate surface. The crystallization thickness d_c for Sb/Ag/collodion is much smaller than that for Sb/collodion and no sensitive dependence of d_c on φ is observed for the former system.     

Unusual thickness dependence of the dielectric constant of erbium oxide films

The thickness dependence of the dielectric constant of Er₂O₃ films was studied. It was found that for films less than 700 Å thick and greater than 1300 Å the dielectric constant ε shows the usual behaviour of increasing with increasing thickness and then assuming the limiting bulk value. However, for films of intermediate thicknesses (700–1300 Å) the dielectric constant first decreases, attains a minimum value and then increases to obtain the bulk value. It is suggested, based on electron microscope observations, that this curious thickness dependence of ε arises because of the transformations from (i) amorphous to crystalline and (ii) f.c.c. type to b.c.c. type crystalline phases in the films.     

Some d.c. properties of evaporated thin aluminium fluoride films

The measurements of some d.c. properties of samples of evaporated aluminium fluoride thin films sandwiched between metal electrodes are reported. The technique of Rutherford scattering of megaelectronvolt alpha particles was used to study the stoichiometry and impurity content of the films. Measurements were made in the temperature range 77–393 K, and the thickness of the films ranged from 800 to 4000 Å. At high temperatures and high fields the conduction process can be explained on the basis of the Poole-Frenkel mechanism in amorphous materials, as described by Hill. At low temperatures the conductivity fits the Mott relation $\text{σ ∝ exp(-T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. The breakdown electric field, measured as a function of film thickness d in the range 700–7500 Å, obeys the Forlani-Minnaja relation.     

Fiber texture and surface composition of evaporated gold films on quartz

The fiber texture, surface composition and homogeneity of polycrystalline films of gold 300–2000 Å thick evaporated onto quartz in the pressure range from 1.5 × 10⁻² to 2 × 10⁻⁷ Pa were investigated by means of X-ray diffraction, electron microscopy, Auger electron spectroscopy and gas sorption experiments. No qualitative differences are observed between the films characterized here and previous results reported for glass substrates: a (111) fiber orientation is observed and is enhanced as the substrate temperature, during or after the deposition, is increased. The percentage of gold present at the surface and the surface homogeneity are more sensitive to the cleaning and outgassing procedure than to the actual pressure during deposition. The uniformity of the quartz substrate has little influence on the surface homogeneity of the gold film.     

Quantum size effect in thin Cd3As2 films

An investigation has been carried out of the resistivity ?, Hall coefficient R_H and Hall mobility μ_H as functions of film thickness for vacuum-evaporated thin Cd₃As₂ films. The films were evaporated in a vacuum of 10^?5 torr onto mica substrates heated to a temperature t_s of about 160 °C at a rate of deposition v of about 40 Å s^?1. The thicknesses of the films were in the range 0.02–0.2 μm. An oscillating character of the resistivity and the carrier mobility was observed for thicknesses of 0.03–0.1 μm. No distinct oscillations of the Hall coefficient were found. The analysis of the results obtained was performed on the basis of theories of the quantum size effect (QSE). The period of the oscillations measured from the experimental data was about 100 Å, which is in good agreement with the calculated value of about 80 Å.     

Oxygen incorporation in hexagonal and amorphous nickel films

The incorporation of oxygen in r.f. sputtered nickel films 200–400 Å thick was observed by means of intensity analysis of electron spectroscopy for chemical analysis spectra from the Ni 2p_{$\text{3}\text{2}$} and O 1s energy levels. The films were of f.c.c, h.c.p. or amorphous structure depending on the deposition rate and temperature and also the vacuum annealing conditions. The structures were determined by electron diffraction. The surface regions of the metastable films of h.c.p. and amorphous structures contain excess oxygen which is incorporated at positions other than the octahedral coordination sites.     

Growth of monocrystalline CdS films on mica surfaces through amorphous interfacial layers of silicon monoxide and carbon by chemical transport reactions

The oriented crystallization of CdS through amorphous interfacial layers of silicon monoxide and carbon by the method of chemical transport reactions has been studied. Cleaved mica (muscovite) single crystals were used as substrates. The interfacial layer thicknesses ranged from 70 to 150 Å. The informative ability of the interfacial layers was checked using the method of decoration with anthraquinone. Epitaxial CdS films 3–15 μm thick were obtained on top of the amorphous interfacial layers, with the (0001) CdS plane parallel to the (001) cleavage plane of mica and the direction [10$\text{1}$0]_CdS coinciding with [110]_mica, which corresponds to the orientation of CdS films prepared directly on the mica surface. The structural perfection of CdS films obtained through interfacial silicon monoxide and carbon layers was practically the same as that of films grown on the mica surface with no interfacial layers. For interfacial layer thicknesses exceeding 120 Å for SiO and 100 Å for carbon, the informative properties of the layers vanished and the CdS films were polycrystalline. The suggestion is made that chemical transport reactions leading to oriented crystallization can proceed without direct contact of the reacting components with the surface of the single-crystal substrates.     

2.25 Thickness dependent effects of thermal ageing in thin conductive films

Al and Zn films, 110–900 Å thick, are dc or rf sputtered; SnO₂ films in the 1500–8000 Å thickness-range are rf sputtered. Thickness dependence variations in electrical resistance are consistent with a two-layer model: the first layer, less than 150 Å thick, is the thinner continuous layer that may be obtained; the second layer has a surface smoothness increasing with thickness up to a value near 1300 Å, above this thickness the surface ordering progressively decreases. Mayadas-Shatzkes conduction (with a constant grain diameter) occurs within metal films; thus the electronic reflection factor on grain boundaries and the electronic specular reflection factor on interfaces are determined. Thermal ageing slightly modifies the grains but improves the interface specular factor and induces thickness dependent ageing effects. Variations in the tcr of metallic films are consistent with the two-layer model and the approximate M-S equations.     

Some optical properties of intrinsic and doped UO2 thin films

Thin films of depleted uranium dioxide were made using the sol–gel approach. Films were deposited onto sapphire and MgO substrates that were 0.1 mm thick. Film thickness ranged from 1540 Å to 2470 Å. Intrinsic films were made as were films doped with aluminum, fluorine, phosphorus, nitrogen, and boron. Dopant concentrations ranged from 10¹⁹ to 10²¹ atoms/cc. Optical transmission data obtained on these samples revealed a range in bandgap from 2.0 to 2.5 eV identifying this as a wide bandgap semiconductor.     

The effects of metallic underlayers on Properties of permalloy films

Thin Permalloy films of zero-magnetostrictive composition were evaporated on a variety of metal film underlayers of various thicknesses (which were deposited on glass substrates) and also on smooth metallic substrates. In contrast to Prosen et al., however, the observed uniaxial magnetic anisotropy is not zero. In thin Permalloy films (100-1000 Å) deposited on high melting-point metal films (Mo, Ti, Pd, and Cr), essentially the same anisotropy field is obtained as is normally observed on glass substrates. In these films the coercivity and the angular dispersion increase slightly as the underlayer thickness increases. Permalloy properties on low melting-point metals (Au, Ag, Cu, and Al) depend strongly upon the underlayer thickness. At a given substrate temperature, a maximum in coercivity and angular dispersion is found in ∼100 Å thick underlayers of Au, Ag, and Cu. In Al underlayers, the values of coercive force along the easy and hard axis increase rapidly as a function of thickness. Large values of the easy-axis skew are obtained in all metal underlayers where the direction of the skew depends upon the geometrical arrangement of the vapor source (Permalloy) and the substrate. These effects are attributed to the microstructure and morphology of the underlayers. Electron microscopy studies are presented in confirmation of these surface geometrical effects.     

The variation in electrical resistance with temperature for Bi/Ag bilayer films

The variation in the electrical resistance of vacuum-evaporated Bi/Ag bilayers with different layer thicknesses was studied as a function of temperature. A silver overlayer 50 Å thick increased the variation in R_T/R with temperature of the bismuth film (R_T is the resistance of the film at temperature T and R the resistance at room temperature). It increased the resistance of the film and made the temperature coefficient of resistance at room temperature more negative. In addition, the resistance minimum was shifted to higher temperatures. The variation in resistance with temperature is explained on the basis of the Kaidanov and Regal model. When the total thickness of the bilayer film was kept constant (approximately 1000 Å), the variation in resistance on annealing was dependent on the thickness of the silver layer. The rapid rise in resistance above 100 °C observed in films with silver layers between 100 and 600 Å thick is explained on the basis of diffusion at the interface and aggregation of the silver film on the surface. By controlling the thickness of the layers it is possible to keep the variation in resistance with temperature of the film to a minimum.     

Characteristic X-ray excitation in thin films by 4–50 keV electron bombardment

X-ray excitation by electron bombardment has been used to analyse thin films, the main objective being non-destructive thickness determination. In a specially designed, but otherwise conventional, electron probe the thin film targets were irradiated with 4–50 keV electrons. Al, Mg, Cu and Zn films evaporated and/or sputtered onto aluminium and copper substrates were measured and the influence of the excitation energy on the sensitivity was studied in the thickness range from 20 to 5000 Å. Typical sigmoid-shaped calibration curves are found, provided X-ray excitation by the primary electrons is the dominating process. Good agreement can then be expected with calculations based on the depth distribution of characteristic X-ray generation. This is not true, however, of film-substrate systems in which X-ray excitation by electrons backscattered from the substrate becomes comparable with direct excitation (e.g. Al on Cu). Artificial enhancement of the ionization density by thin surface layers is demonstrated and applications of this effect are discussed with particular reference to energy-dispersive measurements.