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Layers of porous Si(x)Ge(1-x) buried within single crystals of Si formed by epitaxial growth of Si/Si(x)Ge(1-x)/Si structures followed by etching in solutions of HF:HNO3:H2O. Electroluminescence from these layers utilized in novel optoelectronic devices.
This presentation summarizes progress to date at GE Aircraft Engines in demonstration of a lean combustion system for the High Speed Civil Transport (HSCT). These efforts were supported primarily by NASA contracts, with the exception of initial size and weight estimates and development of advanced diagnostics which were conducted under GE Independent Research and Development projects. Key accomplishments to date are summarized below.
Particles from a J85-GE-5L turbojet engine were measured over a range of engine speeds at simulated altitude conditions ranging from near sea level to 45,000 ft and at flight Mach numbers of 0.5 and 0.8. Samples were collected from the engine by using a specially designed probe positioned several inches behind the exhaust nozzle. A differential mobility particle sizing system was used to determine particle size. Particle data measured at near sea-level conditions were compared with Navy Aircraft Environmental Support Office (AESO) particle data taken from a GE-J85-4A engine at a sea-level static condition. Particle data from the J85 engine were also compared with particle data from a J85 combustor at three different simulated altitudes.
We have measured the electrical conductivity of molten germanium-silicon Ge(0.95)Si(0.05) from the liquidus temperature (1050 C) up to 1220 C. The data were acquired with a unique apparatus which utilizes the standard four-probe technique. The basic unit consists of a fused silica enclosure that contains hermetic glass-to-tungsten seals that can support vacuum pressures down to 10(exp -7) Torr. With calibration, the measurement error for the low vapor pressure materials of this study was typically less than 7%. The temperature dependence of the electrical conductivity of a Ge(0.95)Si(0.05) melt was found to vary from approximately 22,300/ohm/cm at the liquidus temperature down to approximately 16,000/ohm/cm at 1220 C. The negative temperature coefficient as well as the magnitude of these data clearly indicate that the molten material is metallic in nature despite the semiconducting properties of the solid.
Results of a study of the effectiveness of using static magnetic fields to reduce convection in Ge-Si melts will be presented. Lenz's law causes a retardation of convection when a static magnetic field is applied to an electrically conducting liquid. However, during the solidification of a solid-solution system such as Ge-Si, the interface is neither isothermal nor isoconcentrational. The variation of temperature and chemical composition along the interface causes thermoelectric currents to be generated within the solidifying material (and the container if it is electrically conductive). These currents, in the presence of a magnetic field, can cause movement (stirring, convection) in the melt which can exceed convection induced by normal thermosolutal mechanisms. Crystals have been grown by both the Bridgman and floating-zone methods. Clear evidence for the existence of this thermoelectromagnetic convection, especially in the case of Si floating-zone growth, will be presented.
The wetting angles and the surface tension of Ge(1-x)Si(x) melts (0.02 less than x less than 0.13) have been measured on various substrate materials using the sessile drop technique. Fused quartz, sapphire, SiC, glassy carbon, pBN, AIN, and Si3N4 have been used as substrates. The highest and most stable wetting angles were found for pBN substrates with 164 +/- 8 deg., either under forming gas with an additional carbon getter in the system or under active vacuum. The surface tension measurements resulted in a value of +2.2 x 10(exp -3) N/m.at%Si for the concentration dependence delta(gamma)/(delta)C. For the composition range measured, the temperature dependence (delt)gamma/(delta)T showed values similar to those of pure Ge, on average -0.07 x 10(exp -3) N/mK.
Electrochemical studies were performed on LaNi (sub 5-x) Ge (sub x) metal hydride alloys with 0 <= x <= 0.5. We carried out single-electrode studies to understand the effects of the Ge substituent on the hydrogen absorption characteristics, the electrochemical capacity, and the electrochemical kinetics of hydrogen absorption and desorption.
Cr(sub 11)Ge(sub 19) belongs to a large family of materials known as the Nowtny chimney-ladder compounds, some of which were recently proposed as advanced thermoelectric materials because of their low thermal conductivity related to their relatively complex crystal structure. In order to assess the potential of the compound Cr(sub 11)Ge(sub 19) for thermoelectric applications, we grew large single crystals of this peritectic compound by a vertical gradient freeze technique and measured the thermoelectric properties of the crystals.
We report a detailed evaluation of Ge-substituted LaNi(sub 5) for electrochemical application as a negative electrode inalkaline rechargeable cells. Alloys with small substitutions of Ge for Ni show operating pressures, chargeability, cyclic lifetime, and kinetics for hydrogen absorption and desorption all superior to those found in many substituted LaNi(sub 5) alloys.
Large samples of Ru(sub 2)Ge(sub 3) were grown from Ge-rich off-stoichiometric melts at a temperature close to 1460 C by a vertical gradient freeze method in graphite and glassy carbon crucibles.
A photoresponse model has been developed for the Si_(1-x)Ge_x/Si heterojunction internalphotoemission (HIP) infrared detector at wavelengths corresponding to photon energies less than theFermi energy. A Si_(0.7)Ge_(0.3)/Si HIP detector with a cutoff wavelength of 23 micrometers andan emission coefficient of 0.4 eV^(-1) has been demonstrated. The model agrees with the measureddetector response at lambda greater than 8 micrometers. The potential barrier determined by themodel is in close agreement (difference similar to 4 meV) with the potential barrier determined by theRichardson plot, compared to the discrepancies of 20-50 meV usually observed for PtSi Schottkydetectors.
Current collaborative research with General Electric Aviation on Open Rotor propulsion as part of the Subsonic Fixed Wing Project Ultra High Bypass Engine Partnership Element is discussed. The Subsonic Fixed Wing Project goals are reviewed, as well as their relative technology level compared to previous NASA noise program goals. The current Open Rotor propulsion research activity at NASA and GE are discussed including the contributions each entity bring toward the research project, and technical plans and objectives. GE Open Rotor propulsion technology and business plans currently and toward the future are also discussed, including the role the NASA SFW UHB partnership plays toward achieving those goals.
High efficiency III-V multijunction solar cells deposited on metal foil or even polymer substrates can provide tremendous advantages in mass and stowage, particularly for planetary missions. As a first step towards that goal, poly-crystalline p/i/n GaAs solar cells are under development on polycrystalline Ge substrates. Organo Metallic Vapor Phase Epitaxy (OMVPE) parameters for pre-growth bake, nucleation and deposition have been examined. Single junction p/i/n GaAs photovoltaic devices, incorporating InGaP front and back window layers, have been grown and processed. Device performance has shown a dependence upon the thickness of a GaAs buffer layer deposited between the Ge substrate and the active device structure. A thick (2 m) GaAs buffer provides for both increased average device performance as well as reduced sensitivity to variations in grain size and orientation. Illumination under IR light (lambda > 1 micron), the cells showed a Voc, demonstrating the presence of an unintended photoactive junction at the GaAs/Ge interface. The presence of this junction limited the efficiency to approx.13% (estimated with an anti-refection coating) due to the current mismatch and lack of tunnel junction interconnect.
Skutterudites have proven to be a useful thermoelectric system as a result of their enhanced figure of merit (ZT1), cheap material cost, favorable mechanical properties, and good thermal stability. The majority of skutterudite interest in recent years has been focused on binary skutterudites like CoSb3. Binary skutterudites are often double and triple filled, with a range of elements from the lanthanide series, in order to reduce the lattice component of thermal conductivity. Ternary and quaternary skutterudites, such as Co4Ge6Se6 or Ni4Sb8Sn4, provide additional paths to tune the electronic structure. The thermal conductivity can further be improved in these complex skutterudites by the introduction of fillers. The Nd (sub z) Fe (sub x) Co (sub 4-x) Sb (sub 12-y)Ge (sub y) system has been investigated as a p-type thermoelectric material, and is stable up to 600 degrees Centigrade. The influence of Fe and Ge content, along with filler Nd, was investigated on thermoelectric transport properties. In addition to the chemical influence on properties, some processing details of the system will also be addressed.
Skutterudites have proven to be a useful thermoelectric system as a result of their enhanced figure of merit (ZT1), cheap material cost, favorable mechanical properties, and good thermal stability. The majority of skutterudite interest in recent years has been focused on binary skutterudites like CoSb3. Binary skutterudites are often double and triple filled, with a range of elements from the lanthanide series, in order to reduce the lattice component of thermal conductivity. Ternary and quaternary skutterudites, such as Co4Ge6Se6 or Ni4Sb8Sn4, provide additional paths to tune the electronic structure. The thermal conductivity can further be improved in these complex skutterudites by the introduction of fillers. The Nd(z)Fe(x)Co(4-x)Sb(12-y)Ge(y) system has been investigated as a p-type thermoelectric material, and is stable up to 600 C. The influence of Fe and Ge content, along with filler Nd, was investigated on thermoelectric transport properties. In addition to the chemical influence on properties, some processing details of the system will also be addressed.
As2Te3(GeTe) is MAX Phase-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three As2Te3(GeTe) sheets oriented in the (0, 0, 1) direction. Ge4+ is bonded to six equivalent Te2- atoms to form GeTe6 octahedra that share corners with six equivalent AsTe6 octahedra, edges with six equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. All Ge–Te bond lengths are 2.97 Å. As2+ is bonded to six Te2- atoms to form AsTe6 octahedra that share corners with three equivalent GeTe6 octahedra, edges with three equivalent GeTe6 octahedra, and edges with six equivalent AsTe6 octahedra. The corner-sharing octahedral tilt angles are 1°. There are three shorter (2.85 Å) and three longer (3.03 Å) As–Te bond lengths. There are two inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a 3-coordinate geometry to three equivalent As2+ atoms. In the second Te2- site, Te2- is bonded to three equivalent Ge4+ and three equivalent As2+ atoms to form a mixture of corner and edge-sharing TeGe3As3 octahedra. The corner-sharing octahedral tilt angles are 0°.
Bi12GeO20 crystallizes in the cubic I23 space group. The structure is three-dimensional. Ge4+ is bonded in a tetrahedral geometry to four equivalent O2- atoms. All Ge–O bond lengths are 1.80 Å. Bi3+ is bonded in a 5-coordinate geometry to five O2- atoms. There are a spread of Bi–O bond distances ranging from 2.11–2.66 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to three equivalent Bi3+ atoms. In the second O2- site, O2- is bonded to one Ge4+ and three equivalent Bi3+ atoms to form distorted corner-sharing OGeBi3 tetrahedra. In the third O2- site, O2- is bonded in a trigonal planar geometry to three equivalent Bi3+ atoms.
GeBi2Te4 is Caswellsilverite-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three GeBi2Te4 sheets oriented in the (0, 0, 1) direction. Ge2+ is bonded to six equivalent Te2- atoms to form GeTe6 octahedra that share corners with six equivalent BiTe6 octahedra, edges with six equivalent GeTe6 octahedra, and edges with six equivalent BiTe6 octahedra. The corner-sharing octahedral tilt angles are 6°. All Ge–Te bond lengths are 3.04 Å. Bi3+ is bonded to six Te2- atoms to form BiTe6 octahedra that share corners with three equivalent GeTe6 octahedra, edges with three equivalent GeTe6 octahedra, and edges with six equivalent BiTe6 octahedra. The corner-sharing octahedral tilt angles are 6°. There are three shorter (3.08 Å) and three longer (3.29 Å) Bi–Te bond lengths. There are two inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a 6-coordinate geometry to three equivalent Bi3+ atoms. In the second Te2- site, Te2- is bonded to three equivalent Ge2+ and three equivalent Bi3+ atoms to form a mixture of edge and corner-sharing TeGe3Bi3 octahedra. The corner-sharing octahedral tilt angles are 0°.