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We demonstrate the first long-wavelength (=20) quantum well infrared photodetector using non-lattice matched InGaAs/GaAs materials system. High optical gains (low capture probabilities) were achieved by using GaAs as a barrier material in this system.
The far-infrared and submillimeter absorption properties of random aggregates of conducting spheres have been studied as a possible source for the long-wavelength absorption of interstellar dust grains. The complex shapes of random aggregates have various fractal dimensions, depending on the process that forms them. The long wavelength absorption versus frequency is affected more by the shape of a conducting grain than by its composition.
The longest-wavelength quantum-well IR photodetector ever achieved, with a cutoff wavelength of 19 microns, is demonstrated. Detailed measurements and an in-depth analysis of the noise, optical gain, detectivity, quantum efficiency, and quantum-well escape probability are also discussed.
Utilizing the low temperature silicon molecular beam epitaxy (MBE) growth of degenerately doped SiGe layers on Si, long wavelength stacked SiGe/Si heterojunction internal photoemission (HIP) infrared detectors with multiple SiGe/Se layers have been fabricated and demonstrated.
The performance of integrating detectors with either destructive read-out or nondestructive direct read-out in a space environment subject to charge-particle and radiation hits was investigated by determining the optimal integration time for maximizing the performance in the presence of a given hit rate. The performance relative to that expected for background-limited performance was evaluated using a combination of analytic techniques and Monte Carlo simulations. Results are used to estimate the performance degradation, due to cosmic rays, of the long-wavelength spectrometer of the Space Infrared Telescope Facility. It was found that its performance may be a factor of 1-5 worse than the background limited performance.
Success has been achieved in extending the spectral wavelength of GaAs/Al(x)Ga(1-x)As quantum-well infrared photodetectors to significantly longer wavelengths of 11-15 micron. High responsivity of 0.5 A/W, high quantum efficiency of 12 percent, and high detectivity as well as an excellent noise equivalent temperature difference of 4 mK have been achieved at T = 50 K. High performance of 19 mK has also been achieved at an even higher temperature of 60 K.
Spectral types and photometry ranging from 0.4 to 18 microns are given for 54 late-type stars, including 21 supergiants. The infrared observations of the supergiants confirm the dependence of the strength of the 10-micron silicate emission feature on the spectral type and luminosity class of the star. The long-wavelength data are also used to determine the interstellar extinction by fitting an appropriate blackbody to the infrared observations. The deviation from the blackbody at the shorter wavelengths is then attributed to interstellar absorption and absorption by TiO. This method is particularly useful when the short-wavelength photometry is contaminated by a close companion. The M supergiant AH Sco has excess radiation between 1.5 and 8 microns similar to the NML Tauri-type infrared stars. This excess energy resembles free-free emission.
For potential applications in space infrared astronomy missions such as the Space Infrared Telescope Facility and the Large Deployable Reflector, integrated arrays of long-wavelength detectors are desired. The results of a feasibility study which developed a design for applying integrated array techniques to a long-wavelength (gallium-doped germanium) material to achieve spectral coverage between 30 and 200 microns are presented. An approach which builds up a two-dimensional array by stacking linear detector modules is presented. The spectral response of the Ge:Ga detectors is extended to 200 microns by application of uniaxial stress to the stack of modules. The detectors are assembled with 1 mm spacing between the elements. Multiplexed readout of each module is accomplished with integration sampling of a metal-oxide-semiconductor (MOS) switch chip. Aspects of the overall design, including the anticipated level of particle effects on the array in the space environment, a transparent electrode design for 200 microns response, estimates of optical crosstalk, and mechanical stress design calculations are included.
An analytical investigation is performed to assess the feasibility of long-wavelength power converters for the direct conversion of IR radiation onto electrical power. Because theses devices need to operate between 5 and 30 um the only material system possible for this application is the HgCdTe system which is currently being developed for IR detectors. Thus solar cell and IR detector theories and technologies are combined. The following subject areas are covered: electronic and optical properties of HgCdTe alloys; optimum device geometry; junction theory; model calculation for homojunction power cell efficiency; and calculation for HgCdTe power cell and power beaming.
A sample of 47 previously uncatalogued objects located above a Galactic latitude of 50 deg, and detected at 12 microns by the IRAS, is studied using near-infrared photometry. Ground-based observations show that the objects consist primarily of late-type M giant stars with long-wavelength excesses probably due to emission from dust associated with mass loss. The sample contains one oxygen-rich giant star undergoing rapid mass loss; an extremely cool (1230 K) carbon star 12560-1656 that may be as far as 10 kpc away; and a luminious quasar 13349-2438. The absence of nearby, low-luminosity infrared sources in this sample limits the space density of field brown dwarf stars. The fact that almost all the IRAS 12 micron sources have stellar counterparts visible on both the red and blue Palomar Observatory Sky Survey prints provides a tool for discriminating ordinary red stars.
NASA plans for space-based studies of the portion of the electromagnetic spectrum spanning about 30 through 1000 microns are reviewed. The scientific goals of such investigations are examined, and the relevant missions are discussed, including the Space Infrared Telescope Facility (SIRTF), the Submillimeter Wave Astronomy Satellite (SWAS), the Submillimeter Imager and Line Survey (SMILS), the Large Deployable Reflector (LDR), and the lunar submillimeter interferometer. Performance comparisons for these long wavelength missions are presented.
Two modules of gallium-doped germanium (Ge:Ga) infrared detectors with integrated multiplexing readouts and supporting drive electronics were designed and tested. This development investigated the feasibility of producing two-dimensional Ge:Ga arrays by stacking linear modules in a housing capable of providing uniaxial stress for enhanced long-wavelength response. Each module includes 8 detectors (1x1x2 mm) mounted to a sapphire board. The element spacing is 12 microns. The back faces of the detector elements are beveled with an 18 deg angle, which was proved to significantly enhance optical absorption. Each module includes a different silicon metal-oxide semiconductor field effect transistor (MOSFET) readout. The first circuit was built from discrete MOSFET components; the second incorporated devices taken from low-temperature integrated circuit multiplexers. The latter circuit exhibited much lower stray capacitance and improved stability. Using these switched-FET circuits, it was demonstrated that burst readout, with multiplexer active only during the readout period, could successfully be implemented at approximately 3.5 K.
IRAS flux densities are presented for all galaxies in the Center for Astrophysics magnitude-limited sample (mB not greater than 14.5) detected in the IRAS Faint Source Survey (FSS), a total of 1544 galaxies. The detection rate in the FSS is slightly larger than in the PSC for the long-wavelength 60- and 100-micron bands, but improves by a factor of about 3 or more for the short wavelength 12- and 25-micron bands. This optically selected sample consists of galaxies which are, on average, much less IR-active than galaxies in IR-selected samples. It possesses accurate and complete redshift, morphological, and magnitude information, along with observations at other wavelengths.
Measurements at a wavelength of 1.25 mm have been obtained for 17 IRAS galaxies selected on the basis of high far-infrared luminosity. These measurements are used to estimate the lower and upper limits to the mass of cold dust in infrared galaxies. As a lower limit on dust mass, all of the galaxies can be successfully modeled without invoking any dust colder than the dust responsible for the 60 and 100 micron emission that was detected by IRAS. As an upper limit, it is possible that the dust mass in a number of the galaxies may actually be dominated by cold dust. This large difference between the lower and upper limits is due primarily to uncertainty in the long-wavelength absorption efficiency of the astrophysical dust grains.
While experimentally there is great regularity below 1 deg K in the behavior of a particular thermal property for all amorphous dielectrics it is not understood why these properties should differ from those of crystalline dielectrics, since it would seem that at low temperatures long-wavelength elastic waves, similar in both cases, would determine the thermal properties. A model involving systems having very few levels is used in the present study, although the relation between the model's systems and the nature of the glassy state is not known. It is shown, among other effects, that: specific heat measurements above 0.1 K indicate a distribution of local modes independent of energy; ultrasonic velocity measurements give information about phonon-local mode coupling parameters; and thermal expansion and far infrared experiments indicate a phonon-assisted tunneling model.
A general process for creating complete, continuous, and absolutely calibrated 1-35 micron stellar spectra is described. The method is demonstrated by constructing an absolutely calibrated 1-35 micron spectrum of Alpha Tau and the method is independently validated using new observations. The absolute calibration of the IRAS low resolution spectrometer (LRS) data base is investigated by comparing the observed spectrum of Alpha Tau with that assumed in the original LRS calibration scheme. An analysis of asteroidal LRS spectra results in an independent assessment of the calibration problems with the LRS and provides a natural complement to the effort based on Alpha Tau. A direct comparison of LRS stellar spectra with independently calibrated long-wavelength airborne and ground-based spectra is made.
Infrared photometry from 1 to 20 microns and spectroscopy at about 2 microns are presented for Nova Cygni 1975 for the period from 2 days before to 1 yr after maximum light. The data can be explained by a simple model in which the expanding gas expelled during the explosion is always a plasma at approximately 10,000 K. Initially the gas is optically thick; this phase clearly defines the time of onset of the nova. Later, as the gas continues to expand, it becomes optically thin. The temporal dependence of the observed flux suggests that in this phase the expanding cloud is in the form of a shell. After about 300 days, long-wavelength emission which may be attributable to thermal reradiation from dust is observed.
The morphology of Titan's infrared spectral features reveals a pattern of differences with respect to Saturn's bands. Titan's bands exhibit a progressive washing out with increasing wavelength relative to the corresponding bands in Saturn's spectrum. Unidentified absorption features of apparently gaseous origin appear in the long-wavelength wing of Titan's 1-micron CH4 band complex with considerably greater strength. Greater absorption is also observed in the 1.1 micron region. Not all of the phenomena can be explained by enhanced CH4 absorption, so Titan's spectrum implies the existence of another spectroscopically active gas. Several candidates for this gas are briefly considered.