Knowledge integration for problem solving in the development of complex aerospace systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Engineering Systems Division, 2008.Includes bibliographical references (p. 238-248).The development of complex products requires widespread knowledge interactions among a significant number of individuals and teams designing numerous interrelated components. Increasing product complexity typically leads to a corresponding increase in the types and sources of knowledge that need to be tapped during development, and a common strategy for managing product complexity is to outsource parts and components to external suppliers. As a result, the knowledge required for development is increasingly specialized and distributed across multiple boundaries spanning large-scale organizational networks, thus requiring the subsequent integration of this knowledge in order to accomplish the development task. A framework for knowledge integration in the development of complex systems in a large-scale organizational context is proposed in this thesis using an extensive review of the pertinent literature. The framework consists of the main channels, strategies, practices and mechanisms most commonly used to transfer, share and apply knowledge in the course of complex technical problem solving. The framework is progressively refined using empirical data collected through several rounds of interviews and a questionnaire instrument administered across three major aircraft programs in the defense aerospace industry. We find that knowledge integration in routine problem solving situations is most efficiently and effectively accomplished through extensive transfer and sharing of codified information using formal mechanisms such as information systems, while knowledge integration for major non-routine troubleshooting events requires extensive integration of individual expertise and know-how through both formal and informal advice sharing as well as direct assistance across internal and external organizational boundaries.(cont.) A principal contribution of this research is in demonstrating how different characteristics of the engineering artifact defined in terms of product complexity, architecture and technology newness, and different aspects of problem solving including problem type and novelty, drive the knowledge integration process and the organizational system. We conclude that permeability of cross-program boundaries, direct relationships with functional groups and rich tacit knowledge flow from suppliers are critical for countering rampant firefighting in complex product development.by Marc George Haddad.Ph.D

InGaAs/InP hot electron transistors grown by chemical beam epitaxy

In this letter, we report on the dc performance of chemical beam epitaxy grown InGaAs/InP hot electron transistors (HETs). The highest observed differential β (dIC/dIB) is over 100. The HETs have Pd/Ge/Ti/Al shallow ohmic base contacts with diffusion lengths less than 300 Å. Furthermore, we also demonstrated ballistic transport of electrons in an InGaAs/InP HET by obtaining an energy distribution of electrons with ∼60 meV full width at half maximum. The measured conduction band discontinuity of InGaAs/InP is 250.3 meV, which is 39.8% of the band gap difference.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70748/2/APPLAB-61-2-189-1.pd

Technical aspects of InGaAs MOMBE -- shutter action, system drift, and material quality

Lattice matched InxGa1 - x As films were deposited on InP substrates using metalorganic molecular beam epitaxy (MOMBE) with trimethylindium (TMIn), triethylgallium (TEGa) and a solid arsenic source. The effect of growth temperature and molecular beam composition on growth rate and crystal composition was investigated. A long term drift of the molecular beam composition and an increasing difference between temperature readings of the thermocouple and the pyrometer were observed. The corrected data show a linear dependence of crystal composition on molecular beam composition. Shutter action on TMIn and TEGa was investigated. The results show the adverse effect of solely using the shutters to control the metalorganic molecular beam, leading to inferior material quality and rough surface morphology. Material grown in the optimized process consistently showed electron mobilities of [mu]300 [approximate] 9000 cm2/V[middle dot]s and [mu]77 [approximate] 35,000 cm2/[middle dot]s at 1.2 x 1015 cm-3 n-type background concentration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29421/1/0000497.pd

The growth of InAlP using trimethyl amine alane by chemical beam epitaxy

The growth of InAlP and related compounds such as InGaP lattice matched to GaAs has attracted a great deal of interest for optoelectronic devices emitting in the range from 638 to 700 nm and for electronic devices such as the heterojunction bipolar transistor. Although some gas source MBE work has been performed in this material system, very little CBE work has been done, largely attributable to the lack of a suitable aluminum source. This is the first report of trimethyl amine alane (TMAA) being used to grow InAlP. TMAA offers advantages of less carbon incorporation and less oxygen sensitivity compared to triethyl aluminum, tri-isobutyl aluminum, or trimethyl aluminum. Trimethyl amine alane has been used to grow AlGaAs HBTs and more recently to grow InAlAs/InGaAs HEMTs by CBE. One of the principal strengths of CBE is its ability to handle phosphorus based compounds efficiently, offering excellent interface control. InAlP films with carbon concentrations below 7 x 1017 cm-3 lattice matched with good surface morphology have been grown. Double crystal X-ray diffraction exhibits a single epi-peak with a full width at half max of 46 arc sec with multiple Pendellosung fringes. The epitaxial films are semi-insulating, completely depleted for thicknesses up to 1.6 [mu]m. Oxygen levels measured by secondary ion mass spectroscopy are comparable to levels measured in InAlAs films (~2.5 x 1018 cm-3) lattice matched to InP. The likely source of this oxygen is the hydride precursor as has been shown for the growth of InAlAs. As the substrate temperature is raised, the films become increasingly indium-rich. Breaking the growth rate down into its constituent binaries indicates an enhanced TMI incorporation rate. The quality of the films as measured by X-ray full width at half max and the surface morphology is extremely sensitive to substrate temperature. A very narrow window exists for the growth of good quality material in the range from 535 to 545[deg]C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30971/1/0000644.pd

Parametric investigation of InGaAs/InAlAs HEMTs grown by CBE

The InAlAs/InGaAs high electron mobility transistor offers excellent high frequency, low noise operation for amplifiers. While this material system has been grown primarily by conventional MBE, other growth techniques have been examined for improved throughput. The flexibility of chemical beam epitaxy offers semi-infinite sources, good source stability, efficient phosphorus utilization, and extended uptime (more than 560 growth runs over 1.5 years). However, CBE has only recently been shown to produce excellent quality InAlAs suitable for the growth of InAlAs/InGaAs HEMTs [1]. This is the first parametric investigation of the properties of InAlAs/InGaAs HEMTs grown by CBE. A series of lattice matched, pulse doped HEMTs have been grown in which the dopant dose, spacer layer, and channel thickness were systematically varied. Low field 300 K Hall mobilities as high as 8700 cm2/V[middle dot]s for a sheet carrier concentration of 3x1012 cm-2have been measured. This mobility is somewhat lower than uniformly doped HEMTs, which have shown mobilities over 10,000 cm 2/V[middle dot]s at room temperature. A figure of merit, the low field conductivity, has been correlated among the device structure, gateless saturation currents, and DC and microwave device performance. Its applicability as a rough predictor of device performance will be discussed. For a given spacer thickness, the mobility improves as the pulse dose is decreased up to a mobility somewhat below that for uniformly doped structures. As the dopant to channel thickness is increased, this saturated mobility also increases. Secondary ion mass spectroscopy has shown no increase in carbon or oxygen levels at the dopant pulse. This has led to speculation that interface scattering at the top InAlAs/InGaAs interface may be important; however, initial SIMS results do not conclusively show intermixing of the Group III elements at this interface. It is possible that a reduction in the substrate temperature during growth may improve any interface roughness. Results of this modification in growth conditions shall be reported. Self-aligned 0.15 [mu]m HEMTs fabricated from these layers have shown external DC transconductances over 1000mS/mm, unity current gain cutoff frequencies as high as 190 GHz and unity power gain frequencies above 300 GHz. These results and those of more conventional 0.1 [mu]m gate length HEMTs demonstrate the potential of InAlAs/InGaAs HEMTs grown by CBE.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30970/1/0000643.pd

InAlAs/InGaAs/InP sub-micron HEMTs grown by CBE

The InAlAs/InGaAs/InP high electron mobility transistor (HEMT) lattice matched to InP offers excellent high frequency, low noise operation for MMICs and low-noise amplifiers. The InP channel in the InP/InAlAs HEMT offers the advantages of improved high field velocity and higher breakdown voltages (the potential for higher power applications) over InGaAs channel HEMTs. InAlAs has been grown for the first time by CBE using TMAA producing InGaAs/InAlAs and InP/InAlAs HEMTs. Sub-micron InGaAs/InAlAs HEMTs with planar Si doping have been fabricated with ft values of 150 GHz and fmax values of 160 GHz. This device showed excellent pinch-off charateristics, with a maximum transconductance of 890 mS/mm. The planar doped InGaAs channel HEMT had a higher ft than a similar uniformly doped device. However, the non-optimized structure of the planar doped device resulted in a large output conductance of 120 mS/mm, limiting fmax for that device. A sub-micron InP channel device was grown with a quantum well channel and double-sided planar Si doping. A sheet charge density of 4.4 x 1012 cm-2 and associated room temperature mobility of 2800 cm2/V[middle dot]s were achieved; however, the saturation current was low. The most likely causes for this are diffusion of the planar doping beneath the channel and the poor quality of the InP on InAlAs interface at the bottom of the quantum well channel.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30057/1/0000425.pd

The design of an ECR plasma system and its application to InP grown by CBE

An electron cyclotron resonance (ECR) plasma system has been designed for the purpose of using an excited beam of gases during CBE growth. The system was designed to use hydrogen, nitrogen and argon. An ECR plasma system has the ability to ignite a low pressure and low temperature plasma with very low ion energies, which should minimize any damage to the growing layer. The motivation behind using a plasma during growth is the ability of atomic hydrogen to remove contaminants from the growing layer and to enhance the decomposition of organometallic precursors at low substrate temperatures. InP grown with a hydrogen plasma showed an n-type background carrier concentration of 6.0X1016 cm-3, with a rough surface and a strong photoluminescence peak at 1.378 eV. A control sample grown with excess hydrogen but without the plasma had a background carrier concentration of 1.0X1015 cm-3, a 77 K mobility of 65,000 cm2/V[middle dot]s and a very weak photoluminescence peak at 1.378 eV. The most likely cause for the layer degradation during plasma growth is an intrinsic defect such as an antisite defect or a vacancy. The n-type nature of the layer and the relatively high carrier concentration would seem to exclude the possibility of carbon or any other unintentional impurities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29336/1/0000403.pd

The optimization of InxGa1-xAs and InP growth conditions by CBE

Minimization of the number of experiments needed to fully characterize and optimize the growth of epitaxial material is the first important step in realizing state of the art device structures. While widely used in some fields such as chemical engineering, response surface modeling (RSM) has been little used in crystal growth applications. Using RSM, input parameters such as substrate temperature hydride injector temperature and V/III ratio, were simultaneously adjusted to characterize the crystal growth process. This technique identified interactions among parameters, minimized the number of experiments necessary to understand and optimize the process, and minimized the variability of the growth process. RSM has been applied to the CBE growth of InGaAs and InP with the purpose of generating an operating point at which both good surface morphology and high mobility material can be produced. Although the best 77 K InP mobility was 70,000 cm2/V...s, in order to improve the surface quality the input parameters were changed so that the final mobility was 37,000 cm2/V...s. Although the quality of the InGaAs layers showed a dependence on the reactor history, there did not appear to be any sensitivity to variations made in the operating conditions. The best 77 K InGaAs mobility was 62,500 cm2/V...s.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29335/1/0000402.pd

Pan-Cancer Analysis of lncRNA Regulation Supports Their Targeting of Cancer Genes in Each Tumor Context

Long noncoding RNAs (lncRNAs) are commonly dys-regulated in tumors, but only a handful are known toplay pathophysiological roles in cancer. We inferredlncRNAs that dysregulate cancer pathways, onco-genes, and tumor suppressors (cancer genes) bymodeling their effects on the activity of transcriptionfactors, RNA-binding proteins, and microRNAs in5,185 TCGA tumors and 1,019 ENCODE assays.Our predictions included hundreds of candidateonco- and tumor-suppressor lncRNAs (cancerlncRNAs) whose somatic alterations account for thedysregulation of dozens of cancer genes and path-ways in each of 14 tumor contexts. To demonstrateproof of concept, we showed that perturbations tar-geting OIP5-AS1 (an inferred tumor suppressor) andTUG1 and WT1-AS (inferred onco-lncRNAs) dysre-gulated cancer genes and altered proliferation ofbreast and gynecologic cancer cells. Our analysis in-dicates that, although most lncRNAs are dysregu-lated in a tumor-specific manner, some, includingOIP5-AS1, TUG1, NEAT1, MEG3, and TSIX, synergis-tically dysregulate cancer pathways in multiple tumorcontexts