Epitaxial Gold Films Research Articles

Normal incidence X-ray standing wave analysis of thin gold films

Normal incidence X-ray standing wave (NIXSW) analysis has been successfully performed on epitaxial gold films on mica substrates using reflection from the (1 1 1) planes parallel to the surface. We show that NIXSW can be used to monitor the decrease in order within the gold film caused by annealing, and the position of sulfur within a monolayer of methyl thiolate (CH 3S–) on the surface. The Au–S layer spacing was found to be 2.54 ± 0.05 Å, in close agreement with previous work on a single crystal system.

Effective Use of Focused Ion Beam (FIB) in Investigating Fundamental Mechanical Properties of Metals at the Sub-Micron Scale

AbstractRecent advances in the 2-beam focused ion beams technology has enabled researchers to not only perform high-precision nanolithography and micro-machining, but also to apply these novel fabrication techniques to investigating a broad range of materials' properties at the sub-micron and nano-scales. In our work, the FIB is utilized in manufacturing of sub-micron cylinders, or nano-pillars, as well as of TEM cross-sections to directly investigate plasticity of metals at these small length scales. Gold nano-pillars, ranging in diameter between 200 nm and several micrometers were fabricated from bulk gold and epitaxial gold films on MgO substrates and subsequently compressed using a Nanoindenter fitted with a custom-fabricated diamond flat punch. We show convincingly that fundamental mechanical properties like flow stress, yield strength, and stiffness strongly depend on the sample size, as some of our smaller specimens were found to plastically deform in uniaxial compression at stresses as high as 800 MPa, a value ~50 times higher than for bulk gold.We believe that these high strengths are hardened by dislocation starvation. In this mechanism, once the sample is small enough, the mobile dislocations have a higher probability of annihilating at a nearby free surface than of multiplying and being pinned by other dislocations. Therefore, plasticity is accommodated by the nucleation and motion of new dislocations rather than by interactions of existing dislocations, as is the case for bulk crystals. To validate this mechanism, direct observation of dislocations was accomplished by utilizing the Omniprobe micromanipulator, coupled with FIB-milling and Pt deposition, for fabrication of site-specific TEM specimens. Preliminary TEM images show the lack of mobile dislocations in deformed pillars, which agrees with the proposed dislocation starvation mechanism, as discussed.

Resistivity and phonon softening in ion-irradiated epitaxial gold films

The influence of ion irradiation-induced defects on the temperature dependence of the resistivity of epitaxial, thin (25nm), and ultrathin (7nm) gold films was investigated. To include surface scattering properly, the analysis was performed with the classical size-effect model of Fuchs–Sondheimer. Values for the residual resistivity, the specularity parameter p, and the Debye temperature were obtained. It turned out that ion irradiation not only leads to an expected increase of the resistivity but also to a modification of electron-phonon scattering. With increasing defect density, the effective Debye temperature was significantly reduced. This reduction was less pronounced for He+ as compared to Ar+ irradiation pointing towards vacancy clustering in the latter case. In ultrathin films (7nm), the Debye temperature is reduced already in the as-prepared state due to an increased surface-to-volume ratio, and ion irradiation-induced defects do not lead to a further reduction.

Size effect of the resistivity of thin epitaxial gold films

The temperature dependence of the electrical resistivity of thin, epitaxial, and flat (111)-oriented gold films with thickness between 2 and $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ is investigated. The quality of these Au films is superior to epitaxial Au films grown on mica, therefore the investigation of well-defined thin films was possible. The experiments are analyzed within the frame of classical size-effect theories of the resistivity. It turns out that for thin films, the characteristic Debye temperature is strongly decreased as compared to the bulk value. In contrast to the more physically motivated approach of Soffer, the original model of Fuchs-Sondheimer describes the experimental data in a physically consistent way.

Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study.

To clarify the mechanism of mercury adsorption on gold surfaces thin epitaxial gold films have been exposed to trace amounts of gaseous mercury under laboratory conditions for different periods of time. The changes in the surface morphology of the thin films caused by the exposure have been studied by scanning tunneling microscopy (STM). The evolution of the surface structures with time has been also investigated, in the course of a few days after the exposure. The adsorption of mercury on the gold surfaces has caused drastic changes in the morphology of the surfaces. Pits and islands of 2 to 30 nm in diameter have appeared on the surface, their size and density per unit area depending on the amount of exposure to mercury. The formation of pits and islands followed a certain path of events.

Preparation and characterization of epitaxial gold films deposited on mica by direct current magnetron sputtering

The preparation of epitaxial Au(111) films on mica by means of direct current magnetron sputtering is reported. We employed low-energy electron diffraction, scanning electron microscopy, and scanning tunneling microscopy to study the surface structure of sputtered gold films at various deposition temperatures and substrate cleaning procedures. It will be shown that by sputter deposition at 400 °C onto oxygen annealed mica substrates uniformly textured gold films can be obtained which typically exhibit step-free, atomically flat areas of about 150×150 nm2.

Influence of the substrate crystallographic orientation on the Auger signal intensity

In this paper, we show that for a single crystal sample, the Auger signal intensity depends on the atomic density of the crystalline plane exposed to the analysis. The theoretical approach, based on the Gallon and on the Seah models, is well confirmed by experimental study carried out during the growth of epitaxial gold films on palladium substrates.

The electrical and structural properties of gold films and mercury-covered gold films

The electrical properties and morphology of polycrystalline gold films prepared on alumina and silica substrates and epitaxial thin gold films deposited on mica are examined as a function of film thickness, substrate, structure and mercury adsorption. The resistivities were examined for these films and compared with an ideal, specular gold film in accordance with the Fuchs-Sondheimer (FS) model. According to the FS model, specular films should demonstrate optimum response to adsorbates which produce diffuse rather than specular conduction electron scatter. When the as-prepared polycrystalline films were exposed to mercury, the resistance increased for both the gold-alumina and gold-silica films, and it was found that the gold-alumina films had the greatest response and showed a pronounced thickness dependence. This is due to a rougher, more porous film produced on the polycrystalline alumina substrate which promotes diffusion of the mercury into the films' grain boundaries and the gold bulk. When the epitaxial gold films were exposed to mercury, the resistivity oscillated with increasing mercury coverage. This may be attributed to the formation of successive monolayer coverage, and a resulting oscillatory change in specularity.

High resolution images of single C 60 molecules on gold (111) using scanning tunneling microscopy

The electronic interactions of fullerene molecules with metals, with other molecules, and with themselves are important to the chemical and conductive properties of these materials. We demonstrate high resolution scanning tunneling microscopy images of C 60 molecules condensed on epitaxial gold (111) films on mica, in which the C 60 molecules are isolated from each other. The C 60 molecules were locked in position to the gold substrate by an ordered layer of methyl isobutyl ketone. The images of the C 60 molecules exhibit intramolecular contrast indicating a significant electronic interaction with the gold substrate. Current versus voltage measurements show that both the C 60 and the thin film of methyl isobutyl ketone have conductances comparable to that of the gold substrate.

In situ deposition of Au on plasma-prepared GaAs substrates

In situ deposition of single crystal epitaxial and textured polycrystalline gold films on plasma-cleaned or plasma-etched GaAs substrates is accomplished in an ultrahigh vacuum integrated processing facility. Au/GaAs samples are characterized using reflection high energy electron diffraction, Auger electron spectroscopy, and ion channeling. Au crystallinity in films deposited at 100° C is shown to strongly depend on the GaAs surface cleanliness after plasma processing. Heating the substrate to 250° C after plasma processing subsequently yields epitaxial Au films; omitting the heating procedure results in polycrystalline Au films. The substrate thermal treatment removes residual physisorbed gas molecules and reaction products from the GaAs surface. Epitaxial Au films contain significantly less Ga and As on the free surface of Au than polycrystalline films, and no interaction between epitaxial Au and GaAs is observed.

The electrical resistivity of gold films

We present measurements of the d.c. electrical resistance of three classes of pure epitaxial gold films in the thickness range 30 to 900 nm. The combination of diverse morphological techniques with temperature-dependent data from 2 to 300 K enables us to apply a new theory based on those of Mayadas and Shatzkes for grain-boundary scattering, and of Soffer for surface scattering. Gold, evaporated and annealed on mica substrates, produced (111) films which gave a mean microscopic surface roughness to Fermi wavelength ratio, r, of 0.05 and a grain-boundary reflexion coefficient, Rg, of 0.45. On KBr substrates, samples, prepared similarly, formed two distinct types of (100) film. Use of reflexion high energy electron diffraction and electron microscopy showed that the grain structure of these types of film differed; however, both gave an r of 0.1. An R g of 0.10 was determined for one type but remained unknown for the other. Our results show how previous workers, often relying on the validity of Fuchs’s theory and the misapplication thereof, have failed to present convincing evidence for specular surface scattering. We demonstrate the necessity for, experimentally, morphological observations and measurements over a wide temperature range, and, theoretically, the use of a method that combines the effects of both grain-boundary and angular-dependent surface scattering.

Elektrotransportversuche mit epitaktischen Goldschichten

Abstract Electrotransport Experiments with Epitaxial Gold Films Electrotransport in gold films of 60 nm thickness grown epitaxially onto hot (100) NaCl substrate is determined from the growth rate of voids forming in the temperature gradient in short specimens observed in the transmission electron microscope while loaded with direct current densities of several 106 A/cm2 . For the temperature range 631 -1214 K an activation energy (1,19 ± 0,05) eV is found.

The structural and electron transport properties of CdS grown by molecular beam epitaxy

Epitaxial films of CdS were grown by the molecular beam epitaxy method on single-crystal spinel substrates and on epitaxial films of gold and aluminium. Epitaxial films with a wurtzite structure were grown on the spinel substrates at a substrate deposition temperature of 420 °C and with film growth rates of 0.3–15 μm h -1. The resistivity of the undoped films was approximately 10 5 Ω cm and it exhibited an exponential decrease with temperature with an activation energy of 1.6 eV. Extrinsic n-type doping was carried out with indium, and films with donor concentrations of 10 18 cm -3 were produced with Hall mobilities of 65 cm 2 V -1 s -1. Epitaxial films of CdS were also grown on epitaxial gold and aluminium films (grown on mica) at a substrate deposition temperature of approximately 300 °C. The films grown on gold had a wurtzite structure and were found to be n type from measurements of the thermo-e.m.f. Films grown on aluminium were of a cubic or sphalerite structure with a (111) orientation and the measured thermo-e.m.f. indicated that they were p type. This result is thought to be due to an inversion layer within the CdS which arises from a thin layer of Al 2O 3 sandwiched between the epitaxial aluminium and the CdS. The junction characteristics of the Al/Al 2O 3/CdS structures were measured and the forward-bias current was found to be proportional to exp( qv/ nkT) where n = 2.2; the thickness of the depletion region was approximately 0.3 μm from capacitance-voltage measurements. The polarity of the bias voltage for forward conduction indicated that a p-n junction may exist within the CdS film.

A high temperature high purity source for metal beam epitaxy

We report the first use of modulated beam mass spectrometry to assess the purity of metal beams generated from two types of Knudsen effusion oven used in metal beam epitaxy. One of these types contained sapphire components in contact with tantalum heater windings. In this case considerable contamination of gold and silver beams by Al 2O and aluminum was observed over the temperature range 1000–1400°C. Thermodynamic considerations support the view that these contaminants originated from contact reduction of the sapphire by the tantalum metal. The condensation of these impurities on an (001) indium phosphide surface at not greater than 40°C was detected by Auger electron spectroscopy. In the second type of oven all oxide components were replaced by pyrolytic boron nitride and the tantalum windings by molybdenum windings. Gold and silver beams generated from this oven were of high purity with no detectable oxide species or other beam contaminants. Aluminium beams contained Al 2O as a transitory beam impurity due to atmospheric oxidation of the aluminium charge prior to loading. The implications of the beam impurities from the first type of source are discussed, and the performance of the second type of source in generating epitaxial silver, aluminium and gold films on indium phosphide at growth temperatures not greater than 40°C is reviewed.

The epitaxy of gold on sodium chloride with a continuous carbon intermediate film

There are conflicting reports in the literature as to whether epitaxial films can be grown over intermediate thin continuous carbon layers. A series of experiments was carried out on the NaClAu system in which the effect of intermediate carbon films on the epitaxy of the gold was studied. For a very limited thickness range of carbon intermediate layers, good epitaxial gold films were formed. The corresponding carbon films were stripped, and examination in the electron microscope showed that the films were continuous and free from holes greater than 10 Å in diameter. A model based on “pseudomorphic islands” in the carbon film is proposed to account for the observations.

On the dislocation structure of low angle (110) twist boundaries and its relationship to the O-lattice

Several (110) twist boundaries in gold were examined by transmission electron microscopy. Thin film bicrystal specimens were used which were prepared by welding together single crystal epitaxial (110) gold films. Twist angles, theta, of 1/sup 0/ to 5/sup 0/ were used. The dislocation arrays were examined at 125 kV and imaged using (anti 1 11), (1 anti 1 1), (002) and (2 anti 2 0) reflections. The results emphasize the fact that the O-Lattice approach to the structure of grain boundaries may not be straightforward. In general, both simple homogeneous transformations and inhomogeneous transformations must be considered and there is no simple way to predict which may actually apply. Such procedures are equivalent, of course, to searching for the dislocation network of minimum energy.

1172. Structural investigations of epitaxial gold films with the aid of ion-graphical method: G Gotz et al,wiss zeitsch F schiller Univ Jena math-naturwiss rer, 22 ( [formula omitted]), 1973, 87–97 ( in German)

1172. Structural investigations of epitaxial gold films with the aid of ion-graphical method: G Gotz et al,wiss zeitsch F schiller Univ Jena math-naturwiss rer, 22 ( [formula omitted]), 1973, 87–97 ( in German)

The epitaxy of gold evaporated onto rocksalt in an argon atmosphere

The presence of argon gas in the vacuum chamber during deposition was found to yield epitaxial gold films, of extremely low defect content, on both vacuum-and air-cleaved rocksalt surfaces. The influence of the argon involves the formation of remarkably well oriented and faceted gold nuclei in the earliest observable stages of growth. The results are thought to be of commercial importance.

Twin-Twin Double Diffraction in Epitaxially Deposited Single Crystal and Polycrystalline Thin Films

Dark field electron microscopy of single crystal epitaxially deposited gold films has shown that some of the anomalous spots in the diffraction pattern from such films arise through a double diffraction mechanism involving two different twin orientations. The same sort of mechanism is shown to be a possible explanation of the so-called "forbidden 110 ring" previously observed in ring patterns from polycrystalline epitaxial gold films.

Temperature Variation of the Resistivity of Epitaxial Gold Films

Measurements of the resistivity of epitaxial gold films vacuum deposited upon mica were made over the temperature range of 4.2–300 K. A computerized data analysis scheme, in which the operator played the part of a judgment element in the feedback loop, was used to fit the Fuchs-Sondheimer theory of size effects to the experimental data. The specularity parameter could be determined from the data for a single film. The results for films thinner than 4000 Å are that the specularity, p, varied between 0.5 and 0.95 whereas the static lattice imperfection resistance, ρi, varied between 5.2 and 9.0×10−8 Ω-cm.