HgCdTe on Si: Present status and novel buffer layer concepts

We discuss buffer-layer concepts for the synthesis of low defect-density HgCdTe epilayers on Si for both hybrid and monolithically integrated, infrared focal-plane arrays (IRFPAa). The primary technical problems to overcome include the 19% lattice-parameter mismatch between HgCdTe and Si, and the (211)B surface orientation required for molecular-beam epitaxy (MBE), the growth technique of choice for HgCdTe. We provide a general overview of IRFPAs, motivations for realizing HgCdTe on Si, the current state-of-the-art parameters as a baseline, and three novel buffer-layer concepts and technologies based on (1) obedient GeSi films on SiO₂, (2) wafer bonding, and (3) chalcogenides.     

Convex hulls of objects bounded by algebraic curves

We present an0(n ·d ^o(1)) algorithm to compute the convex hull of a curved object bounded by0(n) algebraic curve segments of maximum degreed.     

Photoluminescence, Raman, and infrared diagnosis of GaAs-AlGaAssuperlattices for intersubband infrared detection

We present optical diagnosis of GaAs-AlGaAs superlattices grown by molecular beam epitaxy for use as 8-20 μm infrared detectors, that combines photoluminescence, Raman, and Fourier transform infrared spectroscopies. Various structural and physical parameters were obtained by theoretical analysis of the optical results. The use of multiple optical techniques offers comprehensive characterization and further understanding of the physics of long wavelength infrared detectors     

Photo-induced excess low frequency noise in HgCdTe photodiodes

We have investigated the properties of excess low frequency noise in illuminated mid wavelength infrared and long wavelength infrared HgCdTe photodiodes at zero bias. The current power spectrum (S_i) dependence is usually close to inverse frequency (f), but substantial variations have been observed. The magnitude of l/f spectra is voltage independent for small bias voltages, but is proportional to the square of the photocurrent (I). Consequently, the l/f knee increases, with photocurrent. Variable area device studies indicate that the noise sources are more closely associated with the device area (A_j) than perimeter, indicating bulk limitations. The power spectrum can be represented by an empirical relationship of the form S_i=α_phI ²/fA_j. This defines a figure of merit, α_ph which takes into the account the relationship between current dependence and device geometry. α_ph is device dependent, suggesting that randomly distributed defects play a role in the difference. This is also supported by noting that devices fabricated in material grown on lattice matched substrates have lower α_ph (10⁻¹⁶ cm²) than those fabricated in material grown on nonlattice matched substrates (10⁻¹⁴ cm²), which usually have two orders of magnitude larger dislocation density. We conclude that photo-induced l/f noise can be reduced via defect reduction and is not fundamental. Data on our best devices indicates that α_ph is somewhat lower for smaller band gap material. The temperature dependence of photo-induced excess low frequency noise is much weaker than that of bias induced excess low frequency noise, indicating unrelated generation mechanisms. In addition, photo-induced l/f adds in quadrature with bias induced l/f noise and is not well correlated in magnitude with either bias induced l/f noise or detector dark currents.     

Time-varying contour topology

The contour tree has been used to compute the topology of isosurfaces, generate a minimal seed set for accelerated isosurface extraction, and provide a user interface to segment individual contour components in a scalar field. In this paper, we extend the benefits of the contour tree to time-varying data visualization. We define temporal correspondence of contour components and describe an algorithm to compute the correspondence information in time-dependent contour trees. A graph representing the topology changes of time-varying isosurfaces is constructed in real-time for any selected isovalue using the precomputed correspondence information. Quantitative properties, such as surface area and volume of contour components, are computed and labeled on the graph. This topology change graph helps users to detect significant topological and geometric changes in time-varying isosurfaces. The graph is also used as an interactive user interface to segment, track, and visualize the evolution of any selected contour components over time.     

Parameterization in Finite Precision

Certain classes of algebraic curves and surfaces admit both parametric and implicit representations. Such dual forms are highly useful in geometric modeling since they combine the strengths of the two representations. We consider the problem of computing the rational parameterization of an implicit curve or surface in a finite precision domain. Known algorithms for this problem are based on classical algebraic geometry, and assume exact arithmetic involving algebraic numbers. In this work we investigate the behavior of published parameterization algorithms in a finite precision domain and derive succinct algebraic and geometric error characterizations. We then indicate numerically robust methods for parameterizing curves and surfaces which yield no error in extended finite precision arithmetic and, alternatively, minimize the output error under fixed finite precision calculations. Received January 8, 1997; revised August 27, 1998.     

Topologically correct reconstruction of tortuous contour forests

Motivated by the need for correct and robust 3D models of neuronal processes, we present a method for reconstruction of spatially realistic and topologically correct models from planar cross sections of multiple objects. Previous work in 3D reconstruction from serial contours has focused on reconstructing one object at a time, potentially producing inter-object intersections between slices. We have developed a robust algorithm that removes these intersections using a geometric approach. Our method not only removes intersections but can guarantee a given minimum separation distance between objects. This paper describes the algorithm for geometric adjustment, proves correctness, and presents several results of our high-fidelity modeling.