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Fractal-like structures have been observed in sputter-deposited thin films of NbGe2. These structures exhibit a striking resemblance to those produced by computer simulations of diffusionlimited aggregation using the Witten-Sander model. An effective fractal dimensionality of about 1.9 has been determined from digitized photomicrographs. Our results indicate a two-stage growth process in which an initial structure with a fractal dimensionality of about 1.7 is thickened by a subsequent growth process.PACS numbers: 06.30.Bp, 61.10.Fr, 81.15.Cd Sputter deposition has come into use over the past twenty years as a major technique for preparing thin films for a variety of applications. Many materials and metastable phases not otherwise obtainable as thin films have been produced by this process. One of the major concerns with all thin films is the morphology of the surface, since it often strongly influences the electrical characteristics of devices made with such films.Recently considerable interest has also developed in a wide variety of growth and aggregation models. This work was stimulated by the discovery by Witten and Sander 1 that a simple diffusion-limited aggregation model in which particles are added, one at a time, to a growing cluster of particles via random-walk trajectories leads to scale-invariant structures with a fractal dimensionality 2 (D) which is considerably smaller than the Euclidean dimensionality of the space in which the growth process is occurring. Two-dimensional simulations lead to structures with a fractal dimensionality of about 1.7. 2 ' 3 This paper reports the existence of fractal-like structures on the surface of sputterdeposited NbGe 2 films and correlates their fractal dimensionality with the results of simulations to provide hypotheses for the growth mechanisms.In 1978, a unique sputter-deposition process was developed at the Naval Research Laboratory to produce films of Nb 3 Ge. This process initially produced films of NbGe 2 with a previously unobserved surface structure. Complex clusters were visible on the surface under an optical microscope at magnifications of a few hundred. Figure 1 shows a digitally reproduced image of a photomicrograph from one of these films. Changing the sputtering conditions to produce the desired material eliminated the surface features, and study of this phenomenon languished. Our interest in these systems was recently revived by the striking visual similarity between the surface structure found in the Nb-Ge thin films and pictures of twodimensional simulations of diffusion-limited aggrega-tion. 1,3 ' 4 This led to the hope that information concerning the mechanisms which yielded the structures might be obtained from a more quantitative analysis.The films were prepared by rf reactive sputtering of a niobium target in a mixture of argon and germane gases at pressures less than 100 mT. The quartz substrates were heated to 840 °C during deposition by a carbon-fiber-heated Mo table. The temperature was measured with an infrared pyrometer focused on th...

In this paper we show that the large number of approaches using apparently unrelated strainless increments for unsubstituted alicyclic hydrocarbons in the literature are neither mathematically nor conceptually unique. We additionally demonstrate that if the strain energy assigned to a compound by any three sets of increments is known, the strain energy any other approach would assign can automatically be determined without considering any further details of the structure of the compound. Equivalently, there are but three mathematically distinct, i.e. linearly independent, strainless incremental approaches for these compounds. Thus the choice of which method to employ in one's own reasoning relative to a chemical problem must be based on personal, rather than strictly chemical or mathematical criteria. We proceed by presenting our criteria and their molecular realization, the concept of diagonal reference states. Diagonal reference states are defined from hydrocarbons composed solely of the group of interest. In particular, >C<, >CH–, –CH2– and –CH3 are derived from diamond, cubane, cyclohexane and ethane with the accompanying enthalpies of formation, +1.9, –9.7, –20.6. and –42.0kJ mol−1. The virtues and debits of this method are presented in support of our conclusion that this approach is preferable because it is diagonal.

The rugate filter employs a sinusoidal refractive index depth profile to produce high reflection in a narrow band of wavelengths. Fabrication relies on a continuously variable index of refraction in the wavelength regime of interest. The near IR refractive index of amorphous silicon-nitrogen films decreases continuously as the composition varies from pure silicon to stoichiometric silicon nitride (Si(3)N(4)). Ion implantation was found unsuitable as a fabrication method for rugate filters. Homogeneous and inhomogeneous films up to 5 microm in thickness have been produced by simultaneous deposition of electron beam evaporated silicon and of energetic nitrogen particles arising from an ion beam. The relative fluxes of beam and evaporant are found to determine the ratio of nitrogen to silicon in the films and therefore to determine the index. Single-band reflection filters of the rugate design of high peak optical density were fabricated under computer control using a quartz crystal oscillator shielded from the beam to monitor the silicon evaporation and three suppressed Faraday cups to monitor the ion beam current.