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NASA NTRS1999

Proton Effects and Test Issues for Satellite Designers: Displacement Effects - Section 4

Marshall, Cheryl J.↗

NASA NTRS2000

Magnetic Field Suppression of Flow in Semiconductor Melt

One of the most promising approaches for the reduction of convection during the crystal growth of conductive melts (semiconductor crystals) is the application of magnetic fields. Current technology allows the experimentation with very intense static fields (up to 80 KGauss) for which nearly convection free results are expected from simple scaling analysis in stabilized systems (vertical Bridgman method with axial magnetic field). However, controversial experimental results were obtained. The computational methods are, therefore, a fundamental tool in the understanding of the phenomena accounting during the solidification of semiconductor materials. Moreover, effects like the bending of the isomagnetic lines, different aspect ratios and misalignments between the direction of the gravity and magnetic field vectors can not be analyzed with analytical methods. The earliest numerical results showed controversial conclusions and are not able to explain the experimental results. Although the generated flows are extremely low, the computational task is a complicated because of the thin boundary layers. That is one of the reasons for the discrepancy in the results that numerical studies reported. Modeling of these magnetically damped crystal growth experiments requires advanced numerical methods. We used, for comparison, three different approaches to obtain the solution of the problem of thermal convection flows: (1) Spectral method in spectral superelement implementation, (2) Finite element method with regularization for boundary layers, (3) Multiquadric method, a novel method with global radial basis functions, that is proven to have exponential convergence. The results obtained by these three methods are presented for a wide region of Rayleigh and Hartman numbers. Comparison and discussion of accuracy, efficiency, reliability and agreement with experimental results will be presented as well.

Fedoseyev, A. I.↗

NASA NTRS1999

Proton Effects and Test Issues for Satellite Designers

Microelectronic and photonic systems in the natural space environment are bombarded by a variety of charged particles including electrons, trapped protons, cosmic rays, and solar particles (protons and other heavy ions). These incident particles cause both ionizing and non-ionizing effects when traversing a device, and the effects can be either transient or permanent. The vast majority of the kinetic energy of an incident proton is lost to ionization, creating the single event effects (SEES) and total ionizing dose (TID) effects. However, the small portion of energy lost in non-ionizing processes causes atoms to be removed from their lattice sites and form permanent electrically active defects in semiconductor materials. These defects, i.e., "displacement damage," can significantly degrade device performance. In general, most of the displacement damage effects in the natural space environment can be attributed to protons since they are plentiful and extremely energetic (and therefore not readily shielded against). For this reason, we consider only proton induced displacement damage in this course. (Nevertheless, we identify solar cells as an important example of a case where both electron and proton damage can be important since only very light shielding is feasible.) The interested reader is encouraged to explore the three previous NSREC and RADECS short courses which also treat displacement damage issues for satellite applications. Part A of this segment of the short course introduces the space environment, proton shielding issues, and requirements specifications for proton-rich environments. In order to exercise the displacement damage analysis tools for on-orbit performance predictions, the requirements document must provide the relevant proton spectra in addition to the usual total ionizing dose-depth curves. Ion-solid interactions and the nature of the displacement damage they generate have been studied extensively for over half a century, yet they still remain a subject of investigation. In this section, a description of the mechanisms by which displacement damage is produced will be followed by a summary of the major consequences for device performance in a space environment. Often the degradation of a device parameter can be characterized by a damage factor (measured in a laboratory using monoenergetic protons) that is simply the change in a particular electrical or optical parameter per unit proton fluence. In addition, we will describe the concept of a non-ionizing energy loss rate (NIEL) which quantifies that portion of the energy lost by an incident ion that goes into displacements. It has been calculated as a function of proton energy, and is analogous to (and has the same units as) the linear energy transfer (LET) for ionizing energy. We will discover that, to first order, the calculated NIEL describes the energy dependence of the measured device damage factors. This observation provides the basis for predicting proton induced device degradation in a space environment based on both the calculated NIEL and relatively few laboratory test measurements. The methodology of such on-orbit device performance predictions will be described, as well as the limitations. Several classes of devices for which displacement damage is a significant (if not the dominant) mode of radiation induced degradation will be presented.

Marshall, Cheryl J.↗

NASA NTRS1995

Design of a Three-Layer Antireflection Coating for High Efficiency Indium Phosphide Solar Cells Using a Chemical Oxide as First Layer

It is well known that the behavior of III-V compound based solar cells is largely controlled by their surface, since the majority of light generated carriers (63% for GaAs and 79% for InP) are created within 0.2 microns of the illuminated surface of the cell. Consequently, the always observed high surface recombination velocity (SRV) on these cells is a serious limiting factor for their high efficiency performance, especially for those with the p-n junction made by either thermal diffusion or ion implantation. A good surface passivation layer, ideally, a grown oxide as opposed to a deposited one, will cause a significant reduction in the SRV without adding interface problems, thus improving the performance of III-V compound based solar cells. Another significant benefit to the overall performance of the solar cells can be achieved by a substantial reduction of their large surface optical reflection by the use of a well designed antireflection (AR) coating. In this paper, we demonstrate the effectiveness of using a chemically grown, thermally and chemically stable oxide, not only for surface passivation but also as an integral part of a 3- layer AR coating for thermally diffused p(+)n InP solar cells. A phosphorus-rich interfacial oxide, In(PO3)3, is grown at the surface of the p(+) emitter using an etchant based on HNO3, o-H3PO4 and H2O2. This oxide has the unique properties of passivating the surface as well as serving as a fairly efficient antireflective layer yielding a measured record high AM0, 25 C, open-circuit voltage of 890.3 mV on a thermally diffused InP(Cd,S) solar cell. Unlike conventional single layer AR coatings such as ZnS, Sb2O3, SiO or double layer AR coatings such as ZnS/MgF2 deposited by e-beam or resistive evaporation, this oxide preserves the stoichiometry of the InP surface. We show that it is possible to design a three-layer AR coating for a thermally diffused InP solar cell using the In(PO3)3 grown oxide as the first layer and Al2O3, MgF2 or ZnS, MgF2 as the second and third layers respectively, so as to yield an overall theoretical reflectance of less than 2%. Since chemical oxides are readily grown on III-V semiconductor materials, the technique of using the grown oxide layer to both passivate the surface as well as serve as the first of a multilayer AR coating, should work well for essentially all III-V compound-based solar cells.

Moulot, Jacques↗

NASA NTRS1999

Design of Ceramic Springs for Use in Semiconductor Crystal Growth in Microgravity

Segregation studies can be done in microgravity to reduce buoyancy driven convection and investigate diffusion-controlled growth during the growth of semiconductor crystals. During these experiments, it is necessary to prevent free surface formation in order to avoid surface tension driven convection (Marangoni convection). Semiconductor materials such as gallium arsenide and germanium shrink upon melting, so a spring is necessary to reduce the volume of the growth chamber and prevent the formation of a free surface when the sample melts. A spring used in this application must be able to withstand both the high temperature and the processing atmosphere. During the growth of gallium arsenide crystals during the GTE Labs/USAF/NASA GaAs GAS Program and during the CWRU GaAs programs aboard the First and Second United States microgravity Laboratories, springs made of pyrolytic boron nitride (PBN) leaves were used. The mechanical properties of these PBN springs have been investigated and springs having spring constants ranging from 0.25 N/mm to 25 N/mm were measured. With this improved understanding comes the ability to design springs for more general applications, and guidelines are given for optimizing the design of PBN springs for crystal growth applications.

Kaforey, M. F.↗

NASA NTRS2000

Development of Electronics for Low Temperature Space Missions

The operation of electronic systems at cryogenic temperatures is anticipated for many future NASA space missions such as deep space probes and planetary surface exploration. For example, an unheated interplanetary probe launched to explore the rings of Saturn would reach an average temperature near Saturn of about -183 C. In addition to surviving the deep space harsh environment, electronics capable of low temperature operation would contribute to improving circuit performance, increasing system efficiency, and reducing payload development and launch costs. Terrestrial applications where components and systems must operate in low temperature environments include cryogenic instrumentation, superconducting magnetic energy storage, magnetic levitation transportation system, and arctic exploration. An on-going research and development program on low temperature electronics at the NASA Glenn Research Center focuses on the development of efficient power systems capable of surviving and exploiting the advantages of low temperature environments. Inhouse efforts include the design, fabrication, and characterization of low temperature power systems and the development of supporting technologies for low temperature operations, such as dielectric and insulating materials, semiconductor devices, passive power components, opto-electronic devices, as well as packaging and integration of the developed components into prototype flight hardware.

Patterson, Richard L.↗

NASA NTRS2001

Development of Electronics for Low-Temperature Space Missions

Electronic systems that are capable of operating at cryogenic temperatures will be needed for many future NASA space missions, including deep space probes and spacecraft for planetary surface exploration. In addition to being able to survive the harsh deep space environment, low-temperature electronics would help improve circuit performance, increase system efficiency, and reduce payload development and launch costs. Terrestrial applications where components and systems must operate in low-temperature environments include cryogenic instrumentation, superconducting magnetic energy storage, magnetic levitation transportation systems, and arctic exploration. An ongoing research and development project for the design, fabrication, and characterization of low-temperature electronics and supporting technologies at NASA Glenn Research Center focuses on efficient power systems capable of surviving in and exploiting the advantages of low-temperature environments. Supporting technologies include dielectric and insulating materials, semiconductor devices, passive power components, optoelectronic devices, and packaging and integration of the developed components into prototype flight hardware. An overview of the project is presented, including a description of the test facilities, a discussion of selected data from component testing, and a presentation of ongoing research activities being performed in collaboration with various organizations.

Patterson, Richard L.↗

NASA NTRS2001

Synthesis of ZnO:As Films Using Off-Axis Sputtering Deposition

As a novel oxide semiconductor material, ZnO is interesting for use in many applications. For fabricating electronic devices, it is important to have n- and p- type ZnO materials. Arsenic has been proven to be one of the p-type dopants for ZnO materials. However, information in studying the ZnAsO ternary compound films has been scarce. In order to investigate the morphology, structure and electrical properties of ZnAsO ternary compounds, ZnO:As films have been synthesized using off-axis sputtering deposition on various substrates including (100) Si and (0001) sapphire crystals. Films are grown under various growth conditions. ZnO:As targets with the atomic weight ratios of arsenic to zinc from 0.01 to 0.10 are used for film synthesis. The growth temperatures and pressures range from 350 to 550C and 5 to 150 mTorr, respectively. Argon to oxygen gas ratio for film growth is varied to examine the film quality as well. Film surface morphology, crystal structure, and compositions, are characterized using atomic force microscopy, x-ray diffraction, and energy dispersive spectroscopy, respectively. The compositions of target material and ZnO:As films grown under various conditions are then assessed. The electrical properties were also measured. The detail of these measurements will be discussed in the presentation.

Zhu, Shen↗

NASA NTRS2001

Fabrication of Very High Efficiency 5.8 GHz Power Amplifiers using AlGaN HFETs on SiC Substrates for Wireless Power Transmission

For wireless power transmission using microwave energy, very efficient conversion of the DC power into microwave power is extremely important. Class E amplifiers have the attractive feature that they can, in theory, be 100% efficient at converting, DC power to RF power. Aluminum gallium nitride (AlGaN) semiconductor material has many advantageous properties, relative to silicon (Si), gallium arsenide (GaAs), and silicon carbide (SiC), such as a much larger bandgap, and the ability to form AlGaN/GaN heterojunctions. The large bandgap of AlGaN also allows for device operation at higher temperatures than could be tolerated by a smaller bandgap transistor. This could reduce the cooling requirements. While it is unlikely that the AlGaN transistors in a 5.8 GHz class E amplifier can operate efficiently at temperatures in excess of 300 or 400 C, AlGaN based amplifiers could operate at temperatures that are higher than a GaAs or Si based amplifier could tolerate. Under this program, AlGaN microwave power HFETs have been fabricated and characterized. Hybrid class E amplifiers were designed and modeled. Unfortunately, within the time frame of this program, good quality HFETs were not available from either the RSC laboratories or commercially, and so the class E amplifiers were not constructed.

Sullivan, Gerry↗

NASA NTRS2001

Etching in Chlorine Discharges Using an Integrated Feature Evolution-Plasma Model

Etching of semiconductor materials is reliant on plasma properties. Quantities such as ion and neutral fluxes, both in magnitude and in direction, are often determined by reactor geometry (height, radius, position of the coils, etc.) In order to obtain accurate etching profiles, one must also model the plasma as a whole to obtain local fluxes and distributions. We have developed a set of three models that simulates C12 plasmas for etching of silicon, ion and neutral trajectories in the plasma, and feature profile evolution. We have found that the location of the peak in the ion densities in the reactor plays a major role in determining etching uniformity across the wafer. For a stove top coil inductively coupled plasma (ICP), the ion density is peaked at the top of the reactor. This leads to nearly uniform neutral and ion fluxes across the wafer. A side coil configuration causes the ion density to peak near the sidewalls. Ion fluxes are thus greater toward the wall's and decrease toward the center. In addition, the ions bombard the wafer at a slight angle. This angle is sufficient to cause slanted profiles, which is highly undesirable.

Hwang, Helen H.↗

NASA NTRS1995

Advances in Detector Technology for Infrared Astronomy

Progress in semiconductor materials and processing technology has allowed the development of infrared detector arrays with unprecedented sensitivity, for imaging and spectroscopic applications in astronomy. The earlier discrete-detector approach has been replaced by large-element (up to 1024 x 1024 pixel), multiplexed devices. Progress has been made against a number of key limiting factors, such as quantum efficiency, noise, spectral response, linearity, and dark current. Future developments will focus on the need for even larger arrays, which operate at higher temperatures.

McCreight, Craig↗

NASA NTRS1994

Cell Libraries

A NASA contract led to the development of faster and more energy efficient semiconductor materials for digital integrated circuits. Gallium arsenide (GaAs) conducts electrons 4-6 times faster than silicon and uses less power at frequencies above 100-150 megahertz. However, the material is expensive, brittle, fragile and has lacked computer automated engineering tools to solve this problem. Systems & Processes Engineering Corporation (SPEC) developed a series of GaAs cell libraries for cell layout, design rule checking, logic synthesis, placement and routing, simulation and chip assembly. The system is marketed by Compare Design Automation.

Source record↗

NASA NTRS2002

Development of Sic Gas Sensor Systems

Silicon carbide (SiC) based gas sensors have significant potential to address the gas sensing needs of aerospace applications such as emission monitoring, fuel leak detection, and fire detection. However, in order to reach that potential, a range of technical challenges must be overcome. These challenges go beyond the development of the basic sensor itself and include the need for viable enabling technologies to make a complete gas sensor system: electrical contacts, packaging, and transfer of information from the sensor to the outside world. This paper reviews the status at NASA Glenn Research Center of SiC Schottky diode gas sensor development as well as that of enabling technologies supporting SiC gas sensor system implementation. A vision of a complete high temperature microfabricated SiC gas sensor system is proposed. In the long-term, it is believed that improvements in the SiC semiconductor material itself could have a dramatic effect on the performance of SiC gas sensor systems.

Hunter, G. W.↗

NASA NTRS2004

Quantification Of 4H- To 3C-Polymorphism In Silicon Carbide (SiC) Epilayers And An Investigation Of Recombination-Enhanced Dislocation Motion In SiC By Optical Emission Microscopy (Oem) Techniques

Environments that impose operational constraints on conventional silicon-(Si) based semiconductor devices frequently appear in military- and space-grade applications. These constraints include high temperature, high power, and high radiation environments. Silicon carbide (SiC), an alternative type of semiconductor material, has received abundant research attention in the past few years, owing to its radiation-hardened properties as well as its capability to withstand high temperatures and power levels. However, the growth and manufacture of SiC devices is still comparatively immature, and there are severe limitations in present crystal growth and device fabrication processes. Among these limitations is a variety of crystal imperfections known as defects. These imperfections can be point defects (e.g., vacancies and interstitials), line defects (e.g., edge and screw dislocations), or planar defects (e.g., stacking faults and double-positioning boundaries). All of these defects have been experimentally shown to be detrimental to the performance of electron devices made from SiC. As such, it is imperative that these defects are significantly reduced in order for SiC devices to become a viable entity in the electronics world. The NASA Glenn High Temperature Integrated Electronics & Sensors Team (HTIES) is working to identify and eliminate these defects in SiC by implementing improved epitaxial crystal growth procedures. HTIES takes two-inch SiC wafers and etches patterns, producing thousands of mesas into each wafer. Crystal growth is then carried out on top of these mesas in an effort to produce films of improved quality-resulting in electron devices that demonstrate superior performance-as well as fabrication processes that are cost-effective, reliable, and reproducible. In this work, further steps are taken to automate HTIES' SiC wafer inspection system. National Instruments LabVIEW image processing and pattern recognition routines are developed that are capable of quantifying and mapping defects on both the substrate and mesa surfaces, and of quantifying polymorphic changes in the grown materials. In addition, an optical emission microscopy (OEM) system is developed that will facilitate comprehensive study of recombination-enhanced dislocation motion (REDM).

Speer, Kevin M.↗

NASA NTRS2001

Atomically Flat Surfaces Developed for Improved Semiconductor Devices

New wide bandgap semiconductor materials are being developed to meet the diverse high temperature, -power, and -frequency demands of the aerospace industry. Two of the most promising emerging materials are silicon carbide (SiC) for high-temperature and high power applications and gallium nitride (GaN) for high-frequency and optical (blue-light-emitting diodes and lasers) applications. This past year Glenn scientists implemented a NASA-patented crystal growth process for producing arrays of device-size mesas whose tops are atomically flat (i.e., step-free). It is expected that these mesas can be used for fabricating SiC and GaN devices with major improvements in performance and lifetime. The promising new SiC and GaN devices are fabricated in thin-crystal films (known as epi films) that are grown on commercial single-crystal SiC wafers. At this time, no commercial GaN wafers exist. Crystal defects, known as screw defects and micropipes, that are present in the commercial SiC wafers propagate into the epi films and degrade the performance and lifetime of subsequently fabricated devices. The new technology isolates the screw defects in a small percentage of small device-size mesas on the surface of commercial SiC wafers. This enables atomically flat surfaces to be grown on the remaining defect-free mesas. We believe that the atomically flat mesas can also be used to grow GaN epi films with a much lower defect density than in the GaN epi films currently being grown. Much improved devices are expected from these improved low-defect epi films. Surface-sensitive SiC devices such as Schottky diodes and field effect transistors should benefit from atomically flat substrates. Also, we believe that the atomically flat SiC surface will be an ideal surface on which to fabricate nanoscale sensors and devices. The process for achieving atomically flat surfaces is illustrated. The surface steps present on the "as-received" commercial SiC wafer is also illustrated. because of the small tilt angle between the crystal "basal" plane and the polished wafer surface. These steps are used in normal SiC epi film growth in a process known as stepflow growth to produce material for device fabrication. In the new process, the first step is to etch an array of mesas on the SiC wafer top surface. Then, epi film growth is carried out in the step flow fashion until all steps have grown themselves out of existence on each defect-free mesa. If the size of the mesas is sufficiently small (about 0.1 by 0.1 mm), then only a small percentage of the mesas will contain an undesired screw defect. Mesas with screw defects supply steps during the growth process, allowing a rough surface with unwanted hillocks to form on the mesa. The improvement in SiC epi surface morphology achievable with the new technology is shown. An atomic force microscope image of a typical SiC commercial epilayer surface is also shown. A similar image of an SiC atomically flat epi surface grown in a Glenn laboratory is given. With the current screw defect density of commercial wafers (about 5000 defects/cm2), the yield of atomically free 0.1 by 0.l mm mesas is expected to be about 90 percent. This is large enough for many types of electronic and optical devices. The implementation of this new technology was recently published in Applied Physics Letters. This work was initially carried out in-house under a Director's Discretionary Fund project and is currently being further developed under the Information Technology Base Program.

Powell, J. Anthony↗

NASA NTRS2005

Silicon Carbide Radioisotope Batteries

The substantial radiation resistance and large bandgap of SiC semiconductor materials makes them an attractive candidate for application in a high efficiency, long life radioisotope battery. To evaluate their potential in this application, simulated batteries were constructed using SiC diodes and the alpha particle emitter Americium Am-241 or the beta particle emitter Promethium Pm-147. The Am-241 based battery showed high initial power output and an initial conversion efficiency of approximately 16%, but the power output decayed 52% in 500 hours due to radiation damage. In contrast the Pm-147 based battery showed a similar power output level and an initial conversion efficiency of approximately 0.6%, but no degradation was observed in 500 hours. However, the Pm-147 battery required approximately 1000 times the particle fluence as the Am-242 battery to achieve a similar power output. The advantages and disadvantages of each type of battery and suggestions for future improvements will be discussed.

Rybicki, George C.↗

NASA NTRS2005

Review of high-throughput techniques for detecting solid phase Transformation from material libraries produced by combinatorial methods

High-throughput measurement techniques are reviewed for solid phase transformation from materials produced by combinatorial methods, which are highly efficient concepts to fabricate large variety of material libraries with different compositional gradients on a single wafer. Combinatorial methods hold high potential for reducing the time and costs associated with the development of new materials, as compared to time-consuming and labor-intensive conventional methods that test large batches of material, one- composition at a time. These high-throughput techniques can be automated to rapidly capture and analyze data, using the entire material library on a single wafer, thereby accelerating the pace of materials discovery and knowledge generation for solid phase transformations. The review covers experimental techniques that are applicable to inorganic materials such as shape memory alloys, graded materials, metal hydrides, ferric materials, semiconductors and industrial alloys.

Lee, Jonathan A.↗

NASA NTRS2003

Robotic Technology for Exploration of Venus

Venus, the "greenhouse planet", is a scientifically fascinating place. A huge number of important scientific questions remain to be answered. Venus is sometimes called Earth's "sister planet" due to the fact that it is closest to the Earth in distance and similar to Earth in size. Despite its similarity to Earth, however, the climate of Venus is vastly different from Earth's. Understanding the atmosphere, climate, geology, and history of Venus could shed considerable light on our understanding of our own home planet. The surface of Venus is a hostile environment, with an atmosperic pressure of over 90 bar of carbon dioxide, temperature of 450 C, and shrouded in sulphuric-acid clouds. Venus has been explored by a number of missions from Earth, including the Russian Venera missions which landed probes on the surface, the American Pioneer missions which flew both orbiters and atmospheric probes to Venus, the Russian "Vega" mission, which floated balloons in the atmosphere of Venus, and most recently the American Magellan mission which mapped the surface by radar imaging. While these missions have answered basic questions about Venus, telling us the surface temperature and pressure, the elevations and topography of the continents, and the composition of the atmosphere and clouds, scientific mysteries still abound. Venus is of considerable interest to terrestrial atmospheric science, since of all the planets in the solar system, it is the closest analogue to the Earth in terms of atmosphere. Yet Venus' atmosphere is an example of "runaway greenhouse effect." Understanding the history and the dynamics of Venus' atmosphere could tell us considerable insight about the workings of the atmosphere of the Earth. It also has some interest to astrobiology-- could life have existed on Venus in an earlier, pre-greenhouse-effect phase? Could life still be possible in the temperate middle-atmosphere of Venus? The geology of Venus also has interest in the study of Earth. surface robot will require new technologies; specifically, it will require electronics, scientific instruments, power supplies, and mechanical linkages designed to operate at a temperature above 450 C-hot enough to melt the solder on a standard electronic circuit board. This will require devices made from advanced semiconductor materials, such as silicon carbide, or even new approaches, such as micro-vacuum tube electronics. Such materials are now being developed in the laboratory.

Landis, Geoffrey A.↗