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A small (1-V) modulation of the spacecraft potential is observed on the SCATHA satellite through its effects on the data from four instruments: two particle detectors and two field detectors. It is shown that there is a strong causal link between the modulation of the potential at this 1-V level and a nonuniform distribution of the photoemissive properties of the conducting material on the surface of the satellite.
The exponential charge decay time for particulates constituting the spokes in Saturn's B ring is presently proposed as a dynamical time scale for their evolution. Mean particulate radius is determined to be 0.01 + or - 0.03 microns, on the basis of a consideration of solar UV photoemission as the charge decay mechanism which leads to the determination of a dynamical constraint that can be placed on the particulate sizes.
Results are presented from a Spherical Retarding Potential Analyzer and a spherical Langmuir Probe which were flown on the Spacelab-2 mission of the Space Shuttle Orbiter in August 1985. The probes were mounted in such a position that for certain Orbiter orientations, the wake edge formed by the starboard payload bay sill swept across the probes at a roll rate of once per orbit about an inertially stabilized pointing direction. Profiles of O(+) number density and electron number density across the wake edge showed that the charge particle number densities declined into the wake at a faster rate than that predicted by theoretical models. At angles of more than 10 deg into the wake from its geometric edge, two electron populations are found, with different mean random energies and electrical potentials. One of these populations is suggested to be the result of selective filtering of the high energy tails of the ambient ionospheric population, the other, a result of photoemission from the payload bay surfaces.
A method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structures of semiconductors is proposed. The HgTe-CdTe system is used as an example in which it is found that the valence-band maximum of HgTe lies 0.35 + or - 0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the X-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication on the heterojunction band lineup and the electronic structures of semiconductor alloys, are discussed.
The results of the photoreflectance, photoconductivity, and photoluminescence studies on Mn(x)Cd(1-x)Te single crystals are discussed from the viewpoint of the utility of these compounds as gap components in multijunction solar cells. Attention is also given to recent results on CuInSe2 and on quaternary I-III-VI2 alloys based on CuInSe2. Recent Kelvin probe and X-ray photoemission spectroscopy studies of the surface properties of CuInSe2 and Mn(x)Cd(1-x)Te, respectively, are described. In addition, recent results on the radiation damage and annealing kinetics of Mn(x)Cd(1-x)Te are considered in the context of future work on the sputtering and ion beam deposition of thin films of this material.
Data from the Pioneer Venus Langmuir probe, collected since 1979 (and thus, including the period between solar maximum in 1979-1980 and solar minimum in 1986-1987) are examined. Calculations show that about 51 percent of the solar emission at Venus is due to Lyman alpha (1216 A), 46 percent is produced by wavelengths between 550 and 1100 A, and less than 3 percent is due to wavelengths longer than Lyman alpha. The photocurrents were found to exhibit variations related to the solar cycle and solar rotation, as well as a major 7.2-month periodicity. Three different indices of solar EUV behavior at Venus were derived, which include the photoemission current itself, the total EUV flux, and an F(10.7)-like solar index, and are compared with related measurements made simultaneously at earth.
Interfacial morphology and Fermi level pinning behavior at the interfaces of Al, Ag, and Pt with UHV-cleaved CdTe and ZnTe have been studied using X-ray and ultraviolet photoemission spectroscopies. Results are compared to metal/HgCdTe interface formation, where the weak HgTe bond and consequent ease of Hg loss strongly influence semiconductor disruption and metal-semiconductor intermising. For Al/CdTe, the strong Al-Te reaction yields a significantly more extensive Al-Te reacted region than has been observed for HgCdTe. The Al/ZnTe interface is observed to be more abrupt than Al/CdTe. The final Fermi level pinning positions, Ef-Evbm for Al, Ag, and Pt on p-type CdTe and p-ZnTe have been determined. Efi is found to be roughly the same for both CdTe and ZnTe, with the value for ZnTe lying approximately 0.2 eV closer to the VBM for all three metals. From these results, one would expect Schottky barriers of about the same height for these metals on p-CdTe and p-ZnTe; and also that, in principle, metal interfaces with the two alloys HgCdTe and HgZnTe would have the same properties. Comparisons and implications for electrical behavior of metal contacts to the alloys are discussed.
Degradation of three types of commercially available perfluoroalkyl polyethers (PFPE)-Demnum S200, Fomblin Z25, and Krytox 16256-by X-ray irradiation was studied by using X-ray photoemission spectroscopy (XPS) and a mass spectrometer under ultra-high-vacuum conditions. The carbons in the polymers were characterized by chemical shifts of Cls binding energies. Gaseous products containing COF2 and low-molecular-weight fluorocarbons were formed. From Fomblin Z25, which has acetal linkages (-OCF2O-), a large quantity of COF2 gas was evolved. Liquid products became tacky after a long irradiation time, and some did not dissolve in Freon. High-pressure liquid chromatography (HPLC) showed that molecular weight distribution became broader and that higher molecular weight polymers were formed from Demnum and Krytox. We concluded from these results that degradation and cross-linking took place simultaneously. Demnum crosslinked more easily than the other fluids. The time dependence of both XPS spectra of Cls and mass spectra showed that C-O-bonded carbons in PFPE'S were removed faster than other carbons. There was no substrate effect on the degradation reaction because the first-order rate constants calculated from the change of gaseous products were similar when stainless steel (440C) and gold-coated surfaces were used. Metal fluorides were formed on stainless steel during the reaction. A mechanism for the degradation of PFPE'S is discussed on the basis of their molecular structures.
A nonaqueous chemical etch, with Br as the active ingredient, is described which removes the insulating hydroxides and carbonates that form on high-temperature superconductor surfaces as a result of atmospheric exposure. X-ray photoemission spectra have been recorded before and after etching YBa2Cu3O(7-x) films. It is found that, after the etch, the high binding energy O 1s and Ba 3d peaks associated with surface contaminants are greatly reduced, the Y:Ba:Cu ratio is close to the expected 1:2:3, and the oxidation state of the Cu(2+) is not affected. The resistance of an etched film reaches zero at 78 K, compared to 81 K for a similar unetched film. The suitability of other nonaqueous halogen-based etches is discussed, as is the applicability of this etch to other high-Tc superconductors.
Interfacial morphology and Fermi-level pinning behavior at the interfaces of Al, Ag, and Pt with UHV-cleaved CdTe and ZnTe are studied using X-ray photoelectron and ultraviolet photoemission spectroscopies. Results are compared to metal/HgCdTe interface formation. For Al/CdTe, a case is found where significantly greater intermixing occurs in CdTe than seen on HgCdTe. The Al/ZnTe interface is also more abrupt than Al/CdTe. Band bending results for interfaces of all three metals with p-CdTe and p-ZnTe are presented and implications for metal/HgZnTe interface formation are considered.
Spacecraft charging phenomena in the cometary environment of Giacobini-Zinner are less dramatic than expected. The potential of the ICE-probe is less than +1V in the vicinity of Giacobini-Zinner, while the potential may rise up to +6V in the solar wind environment. The paper presents results of PIC simulations that show the dominant influence of photoemission (photoelectrons or impact-induced electrons) in the presence of the solar wind core and halo electrons. Secondary electrons are also important in the cometary environment to explain positive potentials.
The optical constants of epitaxial films of CoSi2 and NiSi2, grown by molecular beam epitaxy on Si(111), in the energy range of 0.9-4.0 eV have been measured. The behavior of the optical constants is characteristic of metals. It is Drude-like in low energy region and deviates from Drude behavior as interband transitions set in. Interband transitions are found to have already set in at 1 eV. The absorption varies significantly with energy, which has implications for photoresponse studies of internal photoemission in these material systems.
This paper describes an X-ray polarimeter that will be flown on the Spectrum X-Gamma mission. The instrument exploits three distinct physical processes to measure polarization: Bragg reflection from a graphite crystal, Thomson scattering from a metallic lithium target, and photoemission from a cesium iodide photocathode. These three methods allow polarization measurements over an energy band from 0.3 to 12 keV. The polarimeter will make possible sensitive measurements of several hundred known X-ray sources. X-ray polarization measurements will make it possible to constrain the geometry of gas flow in X-ray binaries, identify nonthermal emission in supernova remnants, test current models for X-ray emission in radio pulsars, determine the radiation mechanisms in active galactic nuclei, and search for inertial frame dragging (Lense-Thirring effect) around the putative black hole in Cygnus X-1.
Researchers investigate the heterojunction internal photoemission (HIP) approach that potentially offers long wavelength infrared (LWIR) photovoltaic detector performance (single pixel) that is competitive with the best of other approaches being considered. Most significantly, this approach offers a relatively simple device technology that promises producible and uniform FPA's. Researchers emphasize an exciting process based on intervalence band absorption. They investigate both III-V and Si-based heterojunctions grown by molecular beam epitaxy (MBE) in which the barrier can be tailored to the desired cutoff wavelength. In addition, MBE allows one to optimize the device structure with precise control of doping profiles and layer thicknesses, and perform band structure engineering by control of composition and heterojunction strain. Researchers also consider free carrier absorption in heterojunctions. Acceptable absorption coefficients can be achieved in very heavily n(exp +) doped semiconductor layers (approx. equals 10(exp 20)cm(exp -3). However, in this case the appreciable filling of conduction band states leads to a Schottky-like photoresponse with a gradual (quadratic) turn-on above threshold. A more satisfactory approach would be to use p(exp +) doping so that with the higher density of states in the heavy hole valence band there would be a narrow band of occupied states. This gives the desirable effect of a more rapid (linear) turn-on above threshold. Unfortunately, the higher hole effective mass also reduces (inversely) the free carrier absorption. For this and other reasons, the intervalence band absorption process looks much more promising.
Predictions of local density functional (LDF) calculations of the electronic structure and transport properties of high T(sub c) superconductors are presented. As evidenced by the excellent agreement with both photoemission and positron annihilation experiments, a Fermi liquid nature of the 'normal' state of the high T(sub c) superconductors become clear for the metallic phase of these oxides. In addition, LDF predictions on the normal state transport properties are qualitatively in agreement with experiments on single crystals. It is emphasized that the signs of the Hall coefficients for the high T(sub c) superconductors are not consistent with the types of dopants (e.g., electron-doped or hole-doped) but are determined by the topology of the Fermi surfaces obtained from the LDF calculations.
SiGe/Si heterojunction internal photoemission (HIP) detectors have been fabricated utilizing molecular beam epitaxy of p(+)-SiGe layers on p(-)-Si substrates. Elemental boron from a high-temperature effusion cell was used as the dopant source during MBE growth, and high doping concentrations have been achieved. Strong infrared absorption, mainly by free-carrier absorption, was observed for the degenerately doped SiGe layers. The use of elemental boron as the dopant source allows a low MBE growth temperature, resulting in improved crystalline quality and smooth surface morphology of the Si(0.7)Ge(0.3) layers. Nearly ideal thermionic emission dark current characteristics have been obtained. Photoresponse of the HIP detectors in the long-wavelength infrared regime has been demonstrated.
Studies of the polarization dependence of the photoelectric effect produced by soft X-rays from CsI indicate that the geometrical effects in these experiments can often mimic the polarization signature. This paper presents a detailed calculation of these geometrical effects that are produced when the X-ray beam is not precisely aligned on a rotatable plane photocathode. The experimentally observed geometrical effects were used to precisely determine the realignment of the incident beam of polarized X-rays on a rotatable photocathode. The results allow determinations of the true polarization dependence of the photoemission from CsI. It is shown that the photoelectric effect in CsI depends on the polarization state of the X-rays.
Dust grains immersed in plasma become charged. The charge is determined by the plasma characteristics, by secondary and photoemission from the grain, by grain velocity, and at any given instant by the past time history of the charging currents. This charge affects the Coulomb drag on a grain moving through the plasma. It affects the motion of the grain in an electromagnetic field of a planetary magnetosphere, and it is involved in the formation of the spokes in Saturn's rings and in the erosion of the rings by micrometeorites. And finally, it affects the coagulation rate of dust into larger bodies.