Metal films, epitaxial

We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

Many of the various techniques associated with metal film preparation have recently been reviewed by Klemperer (76). Much of the catalytic work with thick continuous films has used a cylindrical reaction vessel (Fig. 7a). This cylindrical geometry permits a cylindrical sleeve of mica sheet to be inserved and used as the film substrate for epitaxial film growth... 

The problem of surface magnetism and of the magnetic properties of thin metal films supported, or epitaxially grown onto single crystal substrates is also developing with an open interest for the possible exploitation of magnetic supports for information... 

Thin films of carbides and nitrides of Group 6 metals were synthesized by reaction of a metal film with a reactive gas at high temperature and by reactive sputtering. The phases obtained depended on the experimental conditions. High temperatures metastable phases (/i-WC, v and 6-MoC]. ) were obtained by reactive sputter deposition of films. The carbon concentration in such films depended on the temperature of the substrate and on the pressure. In some cases ordered sublattices of carbon and nitrogen were observed and epitaxial relationships between the deposit and the substrate were studied. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

The etch rate of silicon relative to other materials is also important in device fabrications. The data on relative etch rate, which is specific to a specific set of conditions, are widely spread in the literature. For example, the relative etch rate of silicon has been reported for Sii. GC , in NH4OH, boron monophosphide epitaxial layer in HF-HNO3, SiOj in HF solutions, SiOj in KOH, SiOj and nitride in TMAH, ° ° Si02 in different oxides and metallic films, various glasses... 

The supported Pd4 cluster has Pd-Pd distances which are elongated compared with those of the compound in the gas-phase. The optimum geometry of Pd4 adsorbed on the oxide anions of MgO has metal-metal distances of 2.93 A, somewhat shorter than the MgO lattice parameter. " The potential energy for the Pd-Pd stretch is, however, very flat with very little energy it is possible to expand the cluster to a pseudomorphic structure on the MgO surface. Because the clusters experimentally deposited on MgO are usually much larger than the species considered here, it is conceivable that a thin Pd metal film will grow epitaxially on MgO, at variance with Ni and Cu. 

Fig. 2.12 Principle arrangement of the metal evaporator for the epitaxial growth of thin metallic films. The current on the collector, due to metal ions during evaporation, facilitates a constant and reproducible flux with a deviation below 5% (from [2], used with permission)...

Growth experiments were carried out in two ultra high vacuum (UHV) cambers with sublimation sources of Si, Fe and Cr and quartz sensors of film thickness. Optical properties of the samples were studied in UHV chamber VARIAN (210 10Torr) equipped with differential reflectance spectroscopy (DRS) facilities. The samples surface was studied in the second UHV chamber (1 -10 9 Torr) equipped with LEED optics. Si(100) and Si(l 11) wafers were used as substrates for different series of the growth experiments. For the growth of silicide islands, metal films of 0.01-1.0 nm were deposited onto silicon surface. Silicon overgrowth with the deposition rate of 3-4 nm/min was carried out by molecular beam epitaxy (MBE) at 600-800 °C for different substrates. The samples were then analyzed in situ by LEED and ex situ by HRTEM and by... 

A major advance in technique has permitted the calorimetric determination of heats of adsorption of hydrocarbons on thin single-crystal surfaces of metals. " Films of about 200 mn thickness are grown epitaxially by vapour deposition on the surface of a sodium chloride crystal cut to expose the desired face it is then dissolved in water, and the film, the surface of which mimics that of the crystal, is then rescued. Its small thermal mass means that the heat liberated is rapidly equilibrated throughout the metal, and the temperature rise can be measured by a remote infrared detector. By using a pulsed molecular beam, simultaneous estimates of sticking probability can be made. Heats of adsorption of hydrocarbons on supported metals at various temperatures have also been reported (Section 4.5). 

Introduction Evaporation of gold onto various supports preceded SAMs by many years. Originally, the interest focused on the epitaxial growth of thin metal films on mica and other flat surfaces. This introduction, therefore, deals with general aspects of evaporated gold films, while the proceeding subchapters refer specifically to studies of SAMs on the various gold surfaces. 

The influence of the support material and the deposition temperature on the crystalKnity and orientation of an evaporated metal film was already studied in 1936 by Brtick [2]. A dependence of the orientation on the deposition temperature was also found by Rudiger [3]. Pashley [4] reviewed a number of studies on the epitaxial growth of fee metals on mica. Investigations on the epitaxy of gold deposited at high temperatures were carried out by Poppa et al. [5]. 

Coming back to the detailed analysis of diffraction patterns, we note that such efforts can be in practice more complicated for real samples for different reasons. First of all, the crystallites (grains) inside a polycrystalline sample might have a preferred orientation (texture), and accordingly, the Bragg reflexes of all other orientations are extremely suppressed in their intensity compared to those expected from calculated structure factors. Such a behavior can be expected e.g. in the case of epitaxially grown thin films that adopt the structure or at least the orientation of the substrate. This is observed e.g. for the passive films on iron discussed above [4], and in part also for those on Ni [19] but also for electrodeposited metal films. 

---