Michael Lamvik

MOCVD of Bi2Te3, Sb2Te3 and their superlattice structures for thin-film thermoelectric applications

The characteristics of metalorganic chemical vapor deposition (MOCVD) of Bi2Te3, Sb2Te3 and their superlattice structures are discussed in this paper. We have grown c-oriented films on both hexagonal sapphire and fcc GaAs substrates, with specular morphology and occasional stacking faults. Single crystallinity was confirmed by X-ray diffraction and low-energy electron diffraction (LEED). The stoichiometry (Bi:Te = 2:3, Sb:Te = 2:3) of the films were confirmed by X-ray photo-emission spectroscopy (XPS) and Rutherford back-scattering. We have also attempted to grow short-period (∼ 10 to 80 A) superlattice structures in the Bi2Te3Sb2Te3 materials system. X-ray diffraction data indicating the quality of these layered structures is presented. The advantages offered by the nature of chemical bonding in these materials, along the growth direction, for obtaining abrupt interfaces is discussed. The electrical transport properties of the MOCVD-grown p-type Bi2Te3Sb2Te3 structures and other thermoelectric properties including thermal conductivity and Seebeck coefficient are discussed. The initial results on the performance parameter known as figure-of-merit of the superlattice structures, measured parallel to the plane of the superlattice interfaces, are significantly higher than in conventional bulk materials. These initial results suggest a significant potential for MOCVD-based materials technology for high-performance, thin-film, thermoelectric refrigeration.

Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics

We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 A. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random inte...

Image Analysis Method (IAM) for Measurement of Particle Size Distribution and Mass Availability on Carpet Fibers

Exposure to particles that have deposited on surfaces is common in occupational and residential environments. Lack of an accurate tool for assessing particle size distribution and loading (mass per unit area) on carpet fibers available for exposure contributes to the uncertainty associated with current risk assessment models. This research presents a new, direct image analysis method (IAM) for measuring particle size distribution and loading on carpet fibers. New and old carpet fibers loaded with Arizona Test Dust were used to test the method. Carpet fibers were removed from the bulk carpet, mounted on substrates, and scanning electron microscopy (SEM) images were collected. Particle size distribution and total mass were calculated from the processed images. The Arizona Test Dust (ATD) size distribution on fibers from two different carpets had mass median diameters of 3.6 ( ± 1.2) and 4.1 (±0.7) μm, similar to that for bulk ATD, 4.0 (± 0.5) μm. Total ATD mass available on new carpet fibers calculated by IAM were statistically correlated with the mass collected on micro-vacuum samples (R 2 = 0.95). Direct comparison of the aerodynamic diameters measured by IAM with those measured automatically by the SEM showed a slight negative bias due to image resolution problems for the smallest particles.

Polymer light guide arrays for tiling large-scale displays

We demonstrate arrays of light guides that are fabricated in sheets and laminated together to form tapered plastic fiber optic bundles that can be used to produce large seamlessly tiled displays.

Methods of automatic seamless tiling for rapid deployment of projection displays

A large seamless display that could be set up rapidly, operated effectively, and moved quickly would be a great value to the military. Such a display system would be even more beneficial if it consisted entirely of easily available, commercial, off-the-shelf components, not dependent on a single supplier. We have designed a seamlessly tiled projection system to meet these needs. The system consists entirely of easily available, commercial, off-the-shelf equipment. We created software that records the location of projected images as detected by a digital camera, computes the movements necessary to align the images, and drives the projection lenses to align the seamlessly tiled display. We developed a method for rapid mechanical mounting of the projectors and camera. We chose commercial folding stands and a mirror-based alignment method that accelerated the mechanical setup. In field trials, our prototype was operational within one-half hour of opening the shipping cases.

Understanding chromaticity shifts in LED devices through analytical models

Abstract This paper demonstrates that chromaticity shifts in light-emitting diode (LED) devices arise from multiple mechanisms that produce chemical changes in the materials used to construct the LED devices. Each chromaticity change is shown to proceed over a finite period of time, and there is a limit on the impact of each shift. For example, chromaticity shifts in LED devices usually start with a fast-acting component that quickly reaches a maximum value, followed by one or more slower acting component(s). This behavior can be modeled analytically with a bounded exponential component to describe the fast-acting component, followed by one or more generalized logistic models. These analytical models contain several key parameters, including the limiting value of each chromaticity shift (A for the upper asymptote and L for the lower asymptote) and the rate of the change k. This approach to chromaticity modeling is demonstrated with analytical models of the chromaticity shifts caused by the irreversible degradation of phosphors. These analytical models provide insights into the kinetic processes responsible for green and red chromaticity shifts caused by phosphor degradation. A green shift is produced by the surface oxidation of the nitride phosphor that changes the emission profile to lower wavelengths. As the surface oxidation reaction proceeds, surface reactants are consumed thereby slowing the reaction rate, and the bulk oxidation processes become more prevalent. A red chromaticity shift can arise from quenching of the green phosphor which shift the emission in the red direction. This paper concludes by discussing the implications of these models for projecting chromaticity for different operational conditions.

Modeling the impact of thermal effects on luminous flux maintenance for SSL luminaires

Meeting the longevity requirements of solid-state lighting (SSL) devices places extreme demands on the materials and designs that are used in SSL luminaires. Therefore, understanding the aging characteristics of lens, reflectors, and other materials is essential to projecting the long-term performance of LED-based lighting systems. Overlooking these factors at either the design or product specification stage can result in premature failure of the device due to poor luminous flux maintenance and/or excessive chromaticity shifts. This paper describes a methodology for performing accelerated stress testing (AST) on materials intended for use in SSL luminaires. This test methodology, which consists of elevated temperature and humidity conditions, produces accelerated aging data that can be correlated to expected performance under normal luminaire operating conditions. The correlations can then be leveraged to produce models of the changes in the optical properties of key materials including transmittance versus wavelength of lenses and reflectance versus wavelength for housings and other reflectors. This information has been collected into a lumen maintenance decision support tool (LM-DST) and together with user supplied inputs (e.g., expected operation conditions) can provide guidance on lifetime expectations of SSL luminaires. This approach has been applied to a variety of materials commonly found in SSL luminaires including acrylics, polycarbonates, and silicones used for lenses and paints, coatings, films, and composites used for reflectors.

P-68: Plano-Eucentric Photometry System for Micro-Display Metrology

We describe a unique micro-display metrology system in which the stage can be rotated arbitrarily and the area of interest does not leave the field of view. Similarly, the sample can be tilted up to 60 degrees and the area of interest stays continuously in focus. The mechanism operates instantaneously and does not depend on computers or motors to achieve correct centering. This capability facilitates the rapid measurement of micro-display characteristics over a wide range of angles.

A multiaxis stage configured for rapid observations of plane samples at multiple angles

We describe a mechanical configuration that allows any point on a flat sample to be moved to any angle of tilt and rotation (within the limits of which the stage is capable) while maintaining the area of interest at a constant focus and within the field of view of a fixed optical system. We describe eight axes of movement required to obtain the desired stability of focus and view area. We demonstrate the proposed concept by assembling an instrument using mostly existing parts. This optical-microscope-based instrument allows sample tilts to 60° and unlimited rotation while maintaining the point of interest within better than 17μm in x, y, and z of the center of the field of view of a 100× optical system.

Rapid automatic alignment of a tiled digital projection system

We have developed a mechanical, optical and digital system based only on commercially available equipment to allow a seamlessly tiled projection display system to be set up and aligned very rapidly. Using a digital camera and motorized projectors, our prototype system can be unpacked, aligned, and put into use within a half-hour. To minimize the number of projectors for a given pixel count, we do not overlap the projected fields and we do not move the images by digital displacement within the projected field. We use motorized lens mounts to displace the images to proper positions for tiling, a method that does not induce keystone distortion. The movement is under control of a digital camera using an image-processing system, making it unnecessary for an operator to control the movement of the lenses.