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A growth experiment aimed at growing two selenium-doped gallium arsenide crystals, each of which are one inch in diameter and 3.45 inches in length, is described. Emphasis is placed on the effect of microgravity on the segregation behavior of electronic materials. The lessons learned from the 1975 ASTP mission have been incorporated in this experiment.
A two-dimensional 128 x 128 detector array for the 1.0 - 1.7 micron spectral region has been demonstrated with indium gallium arsenide. The 30 micron square pixels had 60 micron spacing in both directions and were designed to be compatible with a 2D Reticon multiplexer. Dark currents below 100 pA, capacitance near 0.1 pF, and quantum efficiencies above 80 percent were measured. Probe maps of dark current and quantum efficiency are presented along with pixel dropout data and wafer yield which was as high as 99.89 percent (7 dropouts) in an area of 6528 pixels and 99.37 percent (103 dropouts) over an entire 128 x 128 pixel region.
The Crystal Growth Furnace (CGF) system configuration for the First United States Microgravity Laboratory (USML-1) mission is reviewed, and the planned on-orbit experiments are briefly described. The CGF is configured to accommodate four scientific experiments involving crystal growth which are based on the classical Bridgman method and CVT method, including vapor transport crystal growth of mercury cadmium telluride; crystal growth of mercury zinc telluride by directional solidification; seeded Bridgman growth of zinc-doped cadmium telluride; and Bridgman growth of selenium-doped gallium arsenide.
This experimental study evaluates the potential of a cesium plasma as an emitter for a thermophotovoltaic (TPV) energy conversion system. A cesium plasma, as a result of the ground-state transitions of its single outer-shell electron, produces large amounts of radiation in the 850-890-nm wavelength region. This would provide excellent coupling to silicon, gallium arsenide, and indium phosphide photovoltaic cells. Measurements of the radiative efficiency, the sum of the power at the 852 and 894 nm wavelengths relative to the total emitted power, were made and correlated to the plasma operating variables. It was determined that, for atomic density in the range (3-6) x 10 exp 21/cu cm and electron temperature in the range 2000-3000 K, radiative efficiencies in excess of 70 percent are attainable. This would indicate that a cesium plasma with its selective emission characteristics and low electron operating temperatures of 2000-3000 K would be an excellent candidate as an emitter in a TPV system.
Gallium arsenide concentrator cells have been irradiated with both electrons and protons with a wide variety of energies. The cells are made using OM-VPE growth process with a junction depth of a half micron. All data are taken with bare cells without coverglasses or shielding. Performance data are given at the designed concentration level of 100X AMO. Results are presented in a number of ways, including performance of electrical parameters (Pmax, Isc, and Voc) as a function of fluence for different electron and proton energies. Critical fluences (defined at a degradation of 25 percent in Pmax) are calculated for each energy level and presented for both electron and proton irradiations.
Apparatus performs correlation between input image and reference image in real time by means of degenerate four-wave mixing in photorefractive crystal, which serves as real-time holographic medium. Gallium arsenide chosen to be photorefractive material in this application because at frame rate and level of illumination used in experiments, offers adequate diffraction efficiency. Frame rates as high as 1,000 s to negative 1st power achievable.
To provide electrical power during an exploration mission to Mars, a deployable tent-shaped structure with a flexible photovoltaic (PV) blanket is proposed. The array is designed with a self-deploying mechanism utilizing pressurized gas expansion. The structural design for the array uses a combination of cables, beams, and columns to support and deploy the PV blanket. Under the force of gravity, a cable carrying a uniform load will take the shape of a catenary curve. A catenary-tent collector is self shading which must be taken into account in the solar radiation calculation. The shape and the area of the shadow on the array was calculated and used in the determination of the global irradiance on the array. The PV blanket shape and structure dimensions were optimized to achieve a configuration which maximizes the specific power (W/kg). The optimization was performed for three types of PV blankets (silicon, gallium arsenide over germanium, and amorphous silicon) and two types of structural materials (carbon composite and arimid fiber composite). The results show that the catenary shape of the PV blanket corresponding to zero end angle at the base with respect to the horizontal results in the highest specific power. The tent angle is determined by optimizing the specific mass and the output power for maximum specific power. The combination of carbon fiber structural material and amorphous silicon blanket produces the highest specific power.
Indium phosphide (InP) solar cells are more radiation resistant than gallium arsenide (GaAs) and silicon (Si) solar cells, and their growth by heteroepitaxy offers additional advantages leading to the development of light weight, mechanically strong, and cost-effective cells. Changes in heteroepitaxial InP cell efficiency under 0.5- and 3-MeV proton irradiations have been explained by the variation in the minority-carrier diffusion length. The base diffusion length versus proton fluence was calculated by simulating the cell performance. The diffusion length damage coefficient, K(sub L), was also plotted as a function of proton fluence.
Indium phosphide solar cells are more radiation resistant than gallium arsenide and silicon solar cells, and their growth by heteroepitaxy offers additional advantages leading to the development of lighter, mechanically strong and cost-effective cells. Changes in heteroepitaxial InP cell efficiency under 0.5 and 3 MeV proton irradiations are explained by the variation in the minority-carrier diffusion length. The base diffusion length versus proton fluence is calculated by simulating the cell performance. The diffusion length damage coefficient K(L) is plotted as a function of proton fluence.
Programmable waveform generator synthesizes two independent waveforms simultaneously at frequencies up to 250 MHz. Can be in phase or out of phase with each other. Use of commercial integrated circuits helps keep cost low. Operation governed by BASIC source code enabling any user equipped with suitable personal computer to specify waveforms. User can modify source code to satisfy special needs. Other applications include simulation of Doppler waveforms for radar, and of video signals for testing color displays and computer monitors. With eventual substitution of gallium arsenide integrated circuits for its present silicon integrated circuits, instrument able to generate waveforms with 14-bit precision and sample rates as high as 2 GHz.
Thin films of cubic form of gallium sulfide (GaS) formed on surfaces of gallium arsenide (GaAs) substrates via metal/organic chemical vapor deposition (MOCVD). Deposited cubic GaS, crystalline lattice matched to substrate GaAs, neutralizes electrically active defects on surfaces of both n-doped and p-doped GaAs. Enabling important GaAs-based semiconducting materials to serve as substrates for metal/insulator/semiconductor (MIS) capacitors. Cubic GaS enables fabrication of ZnSe-based blue lasers and light-emitting diodes. Because GaS is optically transparent, deposited to form window layers for such optoelectronic devices as light-emitting diodes, solar optical cells, and semiconductor lasers. Its transparency makes it useful as interconnection material in optoelectronic integrated circuits. Also useful in peeled-film technology because selectively etched from GaAs.
The spectral noise characteristics of Aerojet gallium arsenide (GaAs) junction field effect transistors (JFET's) have been investigated down to liquid-helium temperatures. Noise characterization was performed with the field effect transistor (FET) in the floating-gate mode, in the grounded-gate mode to determine the lowest noise readings possible, and with an extrinsic silicon photodetector at various detector bias voltages to determine optimum operating conditions. The measurements indicate that the Aerojet GaAs JFET is a quiet and stable device at liquid helium temperatures. Hence, it can be considered a readout line driver or infrared detector preamplifier as well as a host of other cryogenic applications. Its noise performance is superior to silicon (Si) metal oxide semiconductor field effect transistor (MOSFET's) operating at liquid helium temperatures, and is equal to the best Si n channel junction field effect transistor (n-JFET's) operating at 300 K.
The NASA Lewis Research Center in Cleveland, Ohio, has developed the electronics for a free-space, direct-detection laser communications system demonstration. Under the High-Speed Laser Integrated Terminal Electronics (Hi-LITE) Project, NASA Lewis has built a prototype full-duplex, dual-channel electronics transmitter and receiver operating at 325 megabit S per second (Mbps) per channel and using quaternary pulse-position modulation (QPPM). This paper describes the integration and testing of the transmitter portion for future application in free-space, direct-detection laser communications. A companion paper reviews the receiver portion of the prototype electronics. Minor modifications to the transmitter were made since the initial report on the entire system, and this paper addresses them. The digital electronics are implemented in gallium arsenide integrated circuits mounted on prototype boards. The fabrication and implementation issues related to these high-speed devices are discussed. The transmitter's test results are documented, and its functionality is verified by exercising all modes of operation. Various testing issues pertaining to high-speed circuits are addressed. A description of the transmitter electronics packaging concludes the paper.
This report contains 4271 annotated references to reports and journal articles of Japaness intellectual origin entered into the NASA scientific and technical information system during 1992. Representative subject areas of interest include: adaptive control, antireflection coatings, fiber reinforced composites, gallium arsenide lasers, laser interferometry, reduced gravity (microgravity), and VHSIC (circuits).
The integration of high quality, single crystal thin film gallium arsenide (GaAs) and indium phosphide (InP) based photonic and electronic materials and devices with host microstructures fabricated from materials such as silicon (Si), glass, and polymers will enable the fabrication of the next generation of micro-opto-mechanical systems (MOMS) and optoelectronic integrated circuits. Thin film semiconductor devices deposited onto arbitrary host substrates and structures create hybrid (more than one material) near-monolithic integrated systems which can be interconnected electrically using standard inexpensive microfabrication techniques such as vacuum metallization and photolithography. These integrated systems take advantage of the optical and electronic properties of compound semiconductor devices while still using host substrate materials such as silicon, polysilicon, glass and polymers in the microstructures. This type of materials optimization for specific tasks creates higher performance systems than those systems which must use trade-offs in device performance to integrate all of the function in a single material system. The low weight of these thin film devices also makes them attractive for integration with micromechanical devices which may have difficulty supporting and translating the full weight of a standard device. These thin film devices and integrated systems will be attractive for applications, however, only when the development of low cost, high yield fabrication and integration techniques makes their use economically feasible. In this paper, we discuss methods for alignment, selective deposition, and interconnection of thin film epitaxial GaAs and InP based devices onto host substrates and host microstructures.
The response time of an ampoule failure sensor exposed to a liquid or vapor gallium-arsenide (GaAs) is investigated. The experimental configuration represents the sample/ampoule cartridge assembly used in NASA's Crystal Growth Furnace (CGF). The sensor is a chemical fuse made from a metal with which the semiconductor material reacts more rapidly than it does with the containing cartridge. For the III-IV compound of GaAs, a platinum metal was chosen based on the reaction of platinum and arsenic at elevated temperatures which forms a low melting eutectic. Ampoule failure is indicated by a step change in resistance of the failure sensor on the order of megohms. The sensors will increase the safety of crystal growth experiments by providing an indication that an ampoule has failed. Experimental results indicate that the response times (after a known ampoule failure) for the 0.003 and 0.010 inch ampoule failure sensors are 2.4 and 3.6 minutes, respectively. This ampoule failure sensor will be utilized in the CGF during the second United States Microgravity Laboratory Mission (USML-2) and is the subject of a NASA patent application.
Advanced radar and communication systems rely heavily on state-of-the-art microelectronics. Systems such as the phased-array radar require many transmit/receive (T/R) modules which are made up of many millimeter wave - microwave integrated circuits (MMIC's). The heart of a MMIC chip is the Gallium Arsenide (GaAs) field-effect transistor (FET). The transistor gate length is the critical feature that determines the operating frequency of the radar system. A smaller gate length will typically result in a higher frequency. In order to make a phased array radar system economically feasible, manufacturers must be capable of producing very large quantities of small-gate-length MMIC chips at a relatively low cost per chip. This requires the processing of a large number of wafers with a large number of chips per wafer, minimum processing time, and a very high chip yield. One of the bottlenecks in the fabrication of MIMIC chips is the transistor gate definition. The definition of sub-half-micron gates for GaAs-based field-effect transistors is generally performed by direct-write electron beam lithography (EBL). Because of the throughput limitations of EBL, the gate-layer fabrication is conventionally divided into two lithographic processes where EBL is used to generate the gate fingers and optical lithography is used to generate the large-area gate pads and interconnects. As a result, two complete sequences of resist application, exposure, development, metallization and lift-off are required for the entire gate structure. We have baselined a hybrid process, referred to as EBOL (electron beam/optical lithography), in which a single application of a multi-level resist is used for both exposures. The entire gate structure, (gate fingers, interconnects and pads), is then formed with a single metallization and lift-off process. The EBOL process thus retains the advantages of the high-resolution E-beam lithography and the high throughput of optical lithography while essentially eliminating an entire lithography/metallization/lift-off process sequence. This technique has been proven to be reliable for both trapezoidal and mushroom gates and has been successfully applied to metal-semiconductor and high-electron-mobility field-effect transistor (MESFET and HEMT) wafers containing devices with gate lengths down to 0.10 micron and 75 x 75 micron gate pads. The yields and throughput of these wafers have been very high with no loss in device performance. We will discuss the entire EBOL process technology including the multilayer resist structure, exposure conditions, process sensitivities, metal edge definition, device results, comparison to the standard gate-layer process, and its suitability for manufacturing.
The response time of an ampoule failure sensor exposed to a liquid or vapor gallium-arsenide (GaAs) and the corresponding breach time of the containing cartridge is investigated. The experiments were conducted in niobium-hafnium (WC-103) cartridges with an exterior silicide coating. These cartridges were built to flight specifications that were used in NASA's Crystal Growth Furnace during the first United States Microgravity Laboratory (USML-1) mission. The ampoule failure sensor is a chemical fuse made from a metal with which the semiconductor material reacts more rapidly than it does with the containing cartridge. In these experiments a platinum metal was used for the manufacture of the sensors. This technical report discusses the response time of two different sensor designs. The first design utilizes a helical wrapped wire and the second uses a single bare wire element. Experimental results indicate that both sensors are adequate in sensing the presence of molten or vapor GaAs with the latter having a 2-minute longer response time. In both experiments, the containing cartridge was breached within 185 minutes after ampoule rupture.