James R. Waterman

Control of interface stoichiometry in InAs/GaSb superlattices grown by molecular beam epitaxy

The InAs/GaSb materials system, with different species for both cations and anions, allows one to envision the construction of heterojunctions with either InSb‐ or GaAs‐like interfaces. As a result, this system provides a unique opportunity to explore the limits of interfacial control that can be achieved at the monolayer level by vapor phase growth techniques. Using migration‐enhanced epitaxial techniques, we have prepared a series of InAs/GaSb superlattices with both types of interfaces. The large differences in bond lengths and vibrational properties of InSb and GaAs interfaces allow x‐ray diffraction and Raman spectroscopy to be sensitive probes of interfacial structure. The x‐ray and Raman measurements reveal that it is possible to grow superlattices with almost pure InSb‐like or GaAs‐like interfaces.

Large temperature changes induced by molecular beam epitaxial growth on radiatively heated substrates

We have performed optical transmission measurements on radiatively heated GaAs substrates as a function of molecular beam epitaxial growth of InAs, GaSb, AlSb, and GaAs films. The energy gap of the GaAs substrate is observed to decrease strongly in energy when materials with band gaps smaller than GaAs are deposited. This decrease in energy gap is a consequence of a substantial increase in growth temperature induced by the deposition of the film. We have observed increases in temperature of over 150 °C from the temperature measured before film growth. Because the thermocouple is weakly coupled to the radiatively heated substrate, conventional temperature controllers are ineffective at measuring or accounting for this change in temperature.

Variations in substrate temperature induced by molecular-beam epitaxial growth on radiatively heated substrates

Large changes in substrate temperature that occur during molecular‐beam epitaxial growth of materials with energy gaps smaller than that of the substrate have been observed. These changes in temperature, which are not detected by the conventional thermocouple, have been measured by observing the changes in the infrared transmission spectrum of the radiatively heated substrate. In addition to describing the experimental arrangement used for these measurements, the advantages and disadvantages of using this technique for substrate temperature determination are discussed.

Reflection high‐energy electron diffraction study of Sb incorporation during molecular‐beam epitaxy growth of GaSb and AlSb

The incorporation of antimony into GaSb and AlSb during molecular‐beam epitaxy growth has been investigated using reflection high‐energy electron diffraction (RHEED). RHEED intensity oscillations limited by the incorporation rate of Sb were observed. The RHEED oscillation measurements together with beam equivalent pressure measurements demonstrate that four Sb atoms are incorporated for each Sb4 molecule incident on the surfaces of GaSb and AlSb for substrate temperatures ranging from 350 to 530 °C and 420 to 610 °C, respectively. A new Ga stabilized (4×3) surface reconstruction was observed for (100) GaSb.

Interface composition control in InAs/GaSb superlattices

Abstract The GaSb/InAs materials system, with two species of both cations and anions, permits the construction of heterostructures with either InSb- or GaAs-like interfaces. As a result, this system provides an opportunity to explore the limits of interfacial control that can be achieved by vapor phase growth techniques. We have characterized a series of superlattices (SLs) prepared with both types of interfaces by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and photoconductivity. The large differences in bond lengths, electronic properties and vibrational properties of InSb and GaAs interfaces allow these techniques to be sensitive probes of interfacial structure. By carefully measuring and minimizing the group V cross-contamination in the SL layers, we are able to unambiguously demonstrate the growth of SLs with almost pure InSb-like or GaAs-like interfaces.

INTERFACIAL ROUGHNESS IN INAS/GASB SUPERLATTICES

Using high‐resolution transmission electron microscopy (HRTEM), we have studied InAs/GaSb superlattices grown by molecular beam epitaxy. Our HRTEM observations indicate that the apparent interface width is on the order of 1 monolayer for InSb‐like interfaces, and on the order of 2 monolayers for GaAs‐like interfaces. The combination of these results with x‐ray diffraction and Raman scattering measurements leads us to conclude that these interface widths are principally due to roughness rather than to interfacial diffusion.

Wide-FOV FPAs for a shipboard distributed aperture system

The Navy faces an ever evolving threat scenario, ranging from sub-sonic sea skimming cruise missiles to newer, unconventional threats such as that experienced by the USS Cole. Next generation naval technology development programs are developing “stealthy” ships by reducing a ships radar cross section and controlling electromagnetic emissions. To meet these threat challenges in an evolving platform environment, ONR has initiated the “Wide Aspect MWIR Array” program. In support of this program, Raytheon Vision Systems (RVS) is developing a 2560 X 512 element focal plane array, utilizing Molecular Beam Epitaxially grown HgCdTe on silicon detector technology. RVS will package this array in a sealed Dewar with a long-life cryogenic cooler, electronics, on-gimbal power conditioning and a thermal reference source. The resulting sub system will be a component in a multi camera distributed aperture situation awareness sensor, which will provide continuous surveillance of the horizon. We will report on the utilization of MWIR Molecular Beam Epitaxial HgCdTe on Silicon material for fabrication of the detector arrays. Detector arrays fabricated on HgCdTe/Si have no thermal expansion mismatch relative to the readout integrated circuits. Therefore large-area focal plane arrays (FPAs) can be developed without concern for thermal cycle reliability. In addition these devices do not require thinning or reticulation like InSb FPAs to yield the high levels of Modulation Transfer Function (MTF) required by a missile warning sensor. HgCdTe/Si wafers can be scaled up to much larger sizes than the HgCdTe/CdZnTe wafers. Four-inch-diameter HgCdTe/Si wafers are currently being produced and are significantly larger than the standard 1.7 inch x 2.6 inch HgCdTe/CdTe wafers. The use of Si substrates also enables the use of automated semiconductor fabrication equipment.

Passive shortwave infrared broadband and hyperspectral imaging in a maritime environment

This work offers a comparison of broadband shortwave infrared, defined as the spectral band from 0.9 to 1.7 μm, and hyperspectral shortwave infrared imagers in a marine environment under various daylight conditions. Both imagers are built around a Raytheon Vision Systems large format (1024×1280) indium-gallium-arsenide focal plane array with high dynamic range and low noise electronics. Sample imagery from a variety of objects and scenes indicates roughly the same visual performance between the two systems. However, we show that the more detailed spectral information provided by the hyperspectral system allows for object detection and discrimination. A vessel was equipped with panels coated with a variety of paints that possessed spectral differences in the 0.9 to 1.7 μm waveband. The vessel was imaged at various ranges, states of background clutter, and times of the day. Using a standard correlation receiver, it is demonstrated that image pixels containing the paint can be easily identified. During the exercise, it was also observed that both bow waves and near-field wakes from a wide variety of vessel traffic provide a spectral signature in the shortwave infrared waveband that could potentially be used for object tracking.

Dependence of InAs phonon energy on misfit-induced strain

The transverse‐optical (TO) phonon energy in strained InAs quantum wells has been investigated by using far‐infrared absorption. We observe that the TO phonon energy decreases when the misfit‐induced biaxial tension in the InAs single quantum well is increased. Our result shows a stronger phonon energy dependence on the strain than the one reported by Cerdeira et al. [Phys. Rev. B 5, 580 (1972)]. The discrepancy may be explained by stress relaxation near the surface in their experiment. The application of our result will be discussed.

Characterization of the AWARE 10 two-gigapixel wide-field-of-view visible imager

System requirements for many military electro-optic and IR camera systems reflect the need for both wide-field-of-view situational awareness as well as high-resolution imaging for target identification. In this work we present a new imaging system architecture designed to perform both functions simultaneously and the AWARE 10 camera as an example at visible wavelengths. We first describe the basic system architecture and user interface followed by a laboratory characterization of the system optical performance. We then describe a field experiment in which the camera was used to identify several maritime targets at varying range. The experimental results indicate that users of the system are able to correctly identify ~10 m targets at between 4 and 6 km with 70% accuracy.