Void defects were demonstrated to form away from the substrate-epifilm interface during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride substrates. These were smaller in size compared to voids which nucleated at the substrate-epifilm interface, which were also observed. Once nucleated, voids usually replicated all the way to the surface even if the flux ratios were modified to prevent additional nucleation of voids. Occasionally, void defects which close before reaching the top surface without leaving any perturbations on the surface, have also been observed. The voids which form away from the substrate-epi interface, nucleate on defects, frequently hillocks, introduced into the film already grown, leading to formation of defect complexes. These voids can be smaller than 1 µm and appear almost indistinguishable from unaccompanied simple voids. However, these void-hillock complexes displayed a nest of dislocations decorating these defects, which become apparent upon dislocation etching, whereas unaccompanied simple voids did not. The nests could extend as much as 25 µm from the individual void-hillock complex. The density of dislocations within the nest exceeded 5 x 10 6 cm -2 , whereas the dislocation density outside of the nest could decrease to < 2 x 10 5 cm -2 .
The initial interaction between mercury vapor and gold film surfaces was determined from the response of gold-coated piezoresistive silicon cantilevers maintained at a constant temperature. It was found that for low mercury vapor concentration the cantilever response, indicative of interaction strength, exhibits weak dependence on mercury concentration and that the interaction remains confined to the gold film surface, with no detectable alteration of either its solid surface or bulk properties. A Freundlich isotherm [H. Freundlich, Colloid and Capillary Chemistry (E.P. Dutton, New York, 1922)] can approximate several characteristics of this interaction. For high mercury vapor concentration, the interaction extends to the entire volume of the gold film and deviates strongly from a Freundlich isotherm dependence. The initial interaction rate decreases slowly with decreasing mercury vapor concentration as determined from the response rate of change of a piezoresistive cantilever at equilibrium with condensed mercury. The mercury concentration could be confirmed independently of this process down to 0.09 part per billion. With continued cooling of the condensed mercury source, the initial interaction rate continues to decrease slowly, indicating probable detection at concentrations ranging to less than 1 part per quadrillion, though no independent confirmation of the mercury partial pressure is possible in this range. The cantilever response also increases strongly with decreasing platform surface temperature. Similar results were obtained for the interaction of ethanol vapor and the surface of a positive photoresist layer coating on piezoresistive silicon cantilevers. These characteristics will permit the development of new sensors in detecting vapors at ultra low concentrations.
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