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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 42 MULTILAYER STRUCTURES Synthesis of multilayer structures with nanometer-scale thicknesses has been achieved by various atomic and molecular deposition processes, such as evaporation, sputtering, molecular-beam epitaxy, electrodeposition, chemical vapor deposition, cluster beam deposition, and gas-condensation methods. Molecular-Beam Epitaxy Molecular-beam epitaxy (MBE) is an extremely well-controlled deposition technique to grow single-crystal films on selected substrates at relatively low temperatures in ultrahigh vacuum. Undesirable impurities can be reduced to below 1014 atoms cm-3, and desirable dopants can be tailored to achieve atomically sharp junctions. Both line and plane defects can be eliminated, provided that the lattice mismatch and the epilayer thickness are not too large. Because of the control that can be achieved in both lattice structure and composition at the atomic level, many artificially structured materials with submicron dimension along the growth direction have been produced, as well as ordered compounds with atomic periodicities. MBE has been most successful in its application to III-V compounds because of the great flexibility it offers in control of alloy composition and lattice match. More recently, first attempts have been made to fabricate metal-ceramic and metal-metal multilayers. One example is the synthesis of a Co-WC carbide multilayer with periodicity in the 2 to 10 nm range. Such a layered structure exhibits a supermodulus effect, with a peak in the modulus occurring at a composition-modulation wavelength of about 2 nm. Use of reactive gas phase etching in conjunction with microlithography can create extremely high- surface-area materials for catalytic applications. Electrodeposition Artificially layered materials can also be made in a single electroplating bath by modulating either the cathodic current or potential. Ag and Pd multilayers have been produced by this technique. The layer thicknesses, however, were rather large--only composition modulations with a repeat length greater than 100 nm could be made because of electrokinetic limitations. Recently, multilayers with modulation as small as 5 nm have been obtained. A two-bath technique has been adapted successfully to the production of metallic multilayers (Goldman et al., 1986). By rotating a substrate either in front of two windows connected to baths of different compositions or above two jets of solutions with different composition, multilayers with composition-modulation wavelengths as small as 2 nm have been produced.
