Judith A. Ruffner

Effects of low substrate temperature and ion assisted deposition on composition, optical properties, and stress of ZnS thin films.

ZnS thin films were deposited with and without ion assisted deposition onto substrates held at temperatures ranging from -120 to 50 degrees C. Effects on chemical and crystalline composition, film microstructure, refractive index, stress, and waveguide losses were investigated. We observed that minimum stress and minimum losses occurred at the same deposition temperature (-50 degrees C) in ZnS films, which also corresponded to a quenching of crystallinity.

Epitaxial growth and surface structure of (0001) Be on (111) Si

We report the growth of epitaxial single‐crystal (0001) hcp‐Be on (111) Si substrates using molecular beam epitaxy. The Be is oriented with Be[1010]∥Si[110] and Be[1120]∥Si[211]. Crystalline quality improves with increasing deposition temperature T, where T=100, 200, 300, 400, and 500 °C for the results reported here. The films deposited at T≤300 °C are smooth while those deposited at T≥400 °C are rough. A superstructure was observed on the surface, probably √3×√3, R30°, for films grown at T=300 °C. These epitaxial beryllium films are of much better quality than those we previously reported on α‐Al2O3.

Epitaxial growth of Be on α‐Al2O3

We report the growth of epitaxial single‐crystal (0001) hcp‐Be on (0001) α‐Al2O3 substrates using molecular beam epitaxy. Thin films were characterized in situ with reflection high energy electron diffraction, and ex situ with ion beam analysis, electron microscopy, atomic force microscopy, and a variety of x‐ray diffraction techniques. The in‐plane orientation of films grown at substrate temperatures T in the range 10 °C<T<270 °C is Be[1100] ∥α‐Al2O3[1210] and Be[1210] ∥α‐Al2O3[1100], while at T=500 °C the Be is aligned with the substrate. At T=10 °C the films are smooth, but the roughness increases with increasing T. At T=500 °C the crystal perfection improves dramatically but the Be forms large islands.

Propagation Losses Of Thin Film Waveguides

Propagation losses of ZnS thin film waveguides have been investigated. The effects of oxygen and water backpressures during the deposition of ZnS thin films on waveguide performance and microstructure have been observed. These effects dominated losses caused by surface finish. A decrease in losses, and a change in preferential crystal orientation from (111) faces parallel to the substrate to (220) was observed for increased H2O partial pressures. Increasing 02 partial pressure resulted in lower waveguide losses possibly from attachment to unbonded Zn and void filling. Because waveguide scattering losses are Rayleigh-like, a quadratic dependence on crystallite grain size is expected. (220) oriented crystallites had smaller grain sizes than the (111) and overall lower losses.

Status of focal plane arrays (FPAs) for space-based applications

The past decade has seen extensive development of strategic IR focal plane arrays, with the result that surveillance, tracking, and interceptor sensors are a much more credible force for national and theater defense. Investment in IR detector materials, especially HgCdTe, has resulted in breakthrough improvements in array sensitivity, uniformity, and size, making these materials viable for the vital strategic defense systems currently envisioned for deployment. Development of silicon impurity band conductor detector arrays has resulted in arrays for the very long-wave IR that approach theoretical limits for performance in the surveillance applications of tomorrow. Programs for the development of readout circuitry have allowed array sizes to increase dramatically, while permitting longer operational lifetimes in space radiation environments with reduced electronics noise. Efforts under these development programs to cut array costs, while improving yield and performance, are preparing us for programs to manufacture the number required at a cost that will allow the surveillance system to be affordable. This paper presents an overview of space sensor missions, technical progress from recently completed programs, status of ongoing efforts, and speculation about development needs and directions for the future.

Fabrication and characterization of beryllium-based multilayer mirrors for soft x-rays

Beryllium has extremely low absorption (4/cm) for wavelengths below the carbon K absorption edge at 44 angstrom. This property makes Be a potentially useful spacer material for x-ray multilayer mirrors in the water window region of the spectrum (24 - 44 angstrom). In addition, Be based mirrors would have high reflectivities for wavelengths above the Be K edge ((lambda) > 114 angstrom), making them useful for applications in the spectral region around the Si L edge (at 124 angstrom). However, because thin films of this material have not been previously studied in detail, relatively little is known about the optimum conditions for Be growth. We have adapted a Riber 1000 Molecular Beam Epitaxy system, formerly used for epitaxy of GaAs, for deposition of beryllium films. We have grown hcp beryllium epitaxially on (001) sapphire substrates and have fabricated several beryllium/germanium and beryllium/bismuth multilayers. Ge and Bi were chosen as candidates for the absorber layers in our initial growth studies because of their expected low reactivity with Be as well as high predicted reflectivities (up to 53%) in the wavelength regions of interest. Characterization of our beryllium films and multilayers includes studies with reflection high energy electron diffraction, low and high-angle 0 - 20 x-ray diffraction, scanning electron microscopy, scanning tunneling microscopy, Auger depth profile analysis, Rutherford backscattering spectrometry, and ion beam channeling.

Structure and x-ray optical properties of MBE-grown multilayers (Invited Paper)

The structure, and therefore the performance, of multilayer x-ray mirrors is directly related to the growth mode of the constituent materials. We review some of our recent results from using molecular beam epitaxy (MBE) for the growth and study of materials for x-ray optics. This work involves the study of surfaces and interfaces important to improving conventional multilayer mirrors and development of superlattice mirrors. Recently, we have successfully grown hcp (0001) Be epitaxially on (0001) (alpha) -Al2O3 substrates. We find the orientation and morphology are strongly influenced by the substrate temperature during deposition. The discovery of a technique to grow Be epitaxially, coupled with the favorable optical constants of this material, gives Be-based multilayers great potential for the development of superlattice x-ray optics. In our preliminary studies of Be/Ge multilayers we found them to have a layered structure, making Be/Ge a potentially useful combination for soft x-ray mirrors. In another series of studies, we performed investigations of film growth and interface formation using a variety of surface analysis techniques for Mo on Si and for X on B (where X equals Pd, Ag, Si). These studies were carried out by depositing onto clean surfaces in ultra high vacuum, followed by characterization with in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and ex situ by scanning tunneling microscopy (STM) and x-ray diffraction. Continuous growth of multiple coverages on a single substrate was accomplished with a technique involving a moveable sample shutter. For Mo-Si, our data are consistent with a composition profile that has an atomically abrupt transition between Si and amorphous MoSix, where x equals 2 for the first 4 angstroms. The fraction of Si then decreases, with the composition approaching pure Mo after 15 - 20 angstroms depending upon the growth temperature. This silicide interlayer causes only a small reduction in the reflectivity of Mo/Si soft x-ray mirrors with (Lambda) > 60 angstroms, but has severe effects for smaller period structures. For Ag-B, the XPS, AES, and STM data are all characteristic of an island (three-dimensional) growth mode. At an Ag coverage 15 angstroms, the islands range in height from 100 to 200 angstroms and are 2000 to 3000 angstroms across. Although the interface between Ag and B may be sharp, the island growth mode causes too much roughness for the multilayers to be useful for short-wavelength optics. For Pd-B, the Pd reacts with the B to form an amorphous, Pd-rich boride. Although the absorber layer is an alloy, our calculations indicate that the ideal normal-incidence reflectivity of the reacted multilayers could be as high as 51% at 80 angstroms. For Si on B, the interface is quite sharp and the amorphous Si forms a smooth continuous layer. Si/B multilayers are useful as narrow bandpass mirrors for (lambda) > 125 angstroms.

Growth and surface structure of epitaxial Be thin films

We have investigated the growth of beryllium thin films on (alpha) - Al2O3, Si (111), and Ge (111). In all cases, epitaxial Be films were obtained under the proper conditions. The effects of substrate temperature T on crystalline quality and surface structure were also studied. Samples were analyzed in situ using reflection high energy electron diffraction and ex situ with ion beam analysis, scanning electron microscopy, atomic force microscopy, and a variety of x-ray diffraction techniques. Studies showed an increase in crystalline quality with increased T, as well as the presence of a surface superstructure, probably (root)3 X (root)3, R30 degree(s), for films deposited on Si at T >= 300 degree(s)C and films on Ge at T >= 200 degree(s)C. To date, the highest quality Be films are those grown on Ge (111) at T equals 300 degree(s)C.