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Epitaxial liftoff is an alternative to lattice-mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin-film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self-bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin-film atomic lattices can be simultaneously imaged, showing only a thin (20–100 Å) amorphous layer in between.

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Crystals of the three Bi-based cuprates of general formula Bi2Sr2Ca, -&Cu"O~with n 1, 2, and 3 have been isolated and their structural and physical properties investigated. The structures are similar, diR'ering only in the number of Cu02-Ca-Cu02 slabs packed along the c axis. The insertion of one and two slabs increases c from 24.6 to 30.6 and 37.1 A. Transmission electron microscopy shows there are stacking faults within the crystals in agreement with our x-ray data and its analysis. Resistivity, ac susceptibility, and dc magnetization measurements demonstrate superconductivity in the n 1, 2, and 3 phases at 10, 85, and 110 K, respectively. The observed transition temperatures and the stacking fault densities are dependent upon sample processing, in particular, the annealing temperatures and cooling rates. The transition temperature is, within the accuracy of our chemical titration, independent of the average copper valency that was determined to be 2.15+'0.03 for each of the three compounds.

Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed-packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 Å thick, whose lateral dimensions vary from 12 to 90 Å depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six-membered rings. The optical absorption spectra of PbI2 and BiI3 can be quantitatively understood in terms of the measured cluster size distributions.