Hierarchically Structured Materials

W/C multilayers were prepared on unheated Si substrates by RF sputtering method. Raman scattering, x-ray diffraction and Auger depth profile were used to characterize the structure of the as-prepared and annealed multilayers in the range of 300 to 800 C. The results were compared in samples subjected to three different annealing conditions: 1) in air, 2) in high purity Ar atmosphere and 3) in evacuated and sealed (approximately 10(-4) torr) ampules. The result of Auger profiles indicates that the penetration depth of oxygen in the films depends on the annealing conditions, annealing temperature and layering structure. It also shows the loss of compositional modulation and the oxidation of W in the region reached by the oxygen. Raman scattering from the oxidized top layer(s) displays a spectrum with lines associated with crystalline tungsten oxide and segregated microcrystalline graphite particles

Zinccadmium telluride (Zn x Cd1−x Te ) solid solutionfilms with 0≤x≤0.12 were deposited by the close spaced vapor transport method and characterized using photoluminescence, x‐ray diffraction, and scanning electron microscopy. The two former techniques indicate that films with high crystalline quality can be prepared with moderate substrate temperatures and low argon pressures. Under these conditions deposition rates of up to 1000 Å/s are achieved and Zn concentration in the film is the same as that of the source. The electron micrographs show grain sizes comparable to the film thickness.

Beryllium coatings with varying thicknesses and columnar grain sizes were deposited by low temperature magnetron sputtering and wet chemically etched to enhance diffuse absorption of light. After etching these coatings exhibited a matte black surface finish and low specular reflectance (below 2%) in the IR up to a critical wavelength dependent upon the original grain size of the coating. Extremely thick coatings (350 j.tm) with original grain sizes of 10 to 12 j.m were produced which exhibited specular reflectances below 0.5% up to 50 p.m wavelength and a Lambertian BRDF at 10.6 p.m averaging 4.3x103 ster1. Scanning electron micrographs are presented for etched and unetched beryllium coatings which showed the etching process produces roughness and porosity over several size scales simultaneously with the maximum size scale limited by the initial coating grain size and thickness. This technique for producing diffuse absorbing baffle materials has great versatility in choice of coating material and substrate and can be expected to provide optical system designers with a variety of material options for stray light management.

The crystalline quality of Zn x Cd1−x Te single crystals prepared by a modified Bridgman method with 0≤x≤0.05 has been analyzed using photoluminescence. The spectrum of a typical sample is dominated by lines originating from the recombination of free and bound excitons. Lines due to free excitons in their ground and first excited states are observed in both the pure CdTe and the mixed crystals. Excitons bound to Cd vacancies are observed in the pure CdTe crystal but not in the mixed crystal. Weaker and broader features appearing at energies below the exciton emission range were associated with transitions involving free‐to‐bound and bound‐to‐bound levels. The origin of the various lines in the spectra was deduced from the detailed measurements of the dependence of the spectrum on temperature and excitation intensity.

Large grain polycrystalline and single crystals of Zn x Cd1−x Te grown by a modified Bridgman method were studied using the photoluminescence and photoconductivity techniques. The temperature dependence of the band gap, as determined by photoluminescence, follows the Varshni equation for measuring temperature in the range of 15–290 K. One of the fitted parameters, the Debye temperature, monotonically decreases with the increase of the atomic zinc concentrations. A a close correlation between the photoluminescence and photoconductivitymeasurements is also found. Samples in which the photoluminescence spectra exhibit emission bands associated to cadmium vacancies and other structural defects, show a photoresponse curve which includes, in addition to the intrinsic band, another broad band at lower energies. Using the ionization energies of the defect related bands in the photoluminescence spectra we have identified the second band in the photoresponse curve due to the photoexcitation of trapped carriers at levels related with the structural defects.

Thin film deposition techniques currently being used to produce multilayer x-ray optics (MXOs) have difficulty producing smooth, uniform multilayers with d-spacings less than about twelve angstroms. We are investigating atomic layer epitaxy (ALE) as an alternative to these techniques. ALE is a chemical vapor deposition technique which deposits an atomic layer of material during each cycle of the deposition process. The thickness of a film deposited by ALE depends only on the number of cycles. Multilayers deposited by ALE should be smooth and uniform with precise d-spacings which makes ALE an excellent technique for producing multilayer x-ray optics. We have designed and built an ALE system and we have used this system to deposit ZnSe using diethyl zinc and hydrogen selenide.