Showing papers by "Solid State Physics Laboratory published in 1990"

The energy of an array of dislocations: Implications for strain relaxation in semiconductor heterostructures

Abstract: An exact closed-form formula is given for the energy of an array of dislocations, arranged periodically on the interface between an epitaxial layer and its substrate, when these are modelled as elastically isotropic with the same elastic constants. In the course of the formulation, the exact equivalence of equilibrium theories originating in the work of Frank and van der Merwe and of Matthews and Blakeslee is established. The new formula allows the exact assessment of arrays of dislocations in equilibrium, for layers thicker than critical. This demonstrates that previous approximations may be seriously in error.

Structure of liquid equiatomic KSn and CsSn.

Abstract: The structure factors of liquid equiatomic KSn and CsSn alloys have been determined by neutron diffraction. A distinct peak at about 1 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, the first sharp diffraction peak (FSDP), occurs in all measured structure factors. The temperature dependence of the structure has been studied for KSn: apart from the FSDP, whose position shifts toward smaller wave vector and magnitude decreases with increasing temperature, the structure factor remains essentially unaltered. The radial distribution function clearly shows three maxima in the first coordination shell. The simultaneous presence of the FSDP and several maxima in the first coordination shell are indications of formation of polyatomic species in the liquid. Application of two simple structural models, taking into account the presence of structural units, yields reasonable agreement with the measured structure factors.

Optical absorption of ZnGa 2 S 4 and ZnGa 2 S 4 : Co 2 + crystals

Abstract: Polycrystals of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ and ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ were prepared from high-purity elements at 1270 \ifmmode^\circ\else\textdegree\fi{}C. The optical-absorption spectra of these polycrystalline powders were measured in the wavelength region 300--3000 nm with use of an integrating sphere and photoacoustic spectrophotometer at 298 K. The optical energy gap is found to be 3.18 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ crystal and 2.60 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal. The optical-absorption peaks in the measurement of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal absorption spectrum at 3790, 6049, 13 725, 20 704, and 22 779 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are found to be the electron transition of the ${\mathrm{Co}}^{2+}$ ion with ${\mathit{T}}_{\mathit{d}}$ symmetry from the ground state $^{4}$${\mathit{A}}_{2}$${(}^{4}$F) to the excited states $^{4}$${\mathit{T}}_{2}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$P), $^{2}$${\mathit{T}}_{2}$${(}^{2}$G), and $^{2}$E${(}^{2}$G). The crystal-field parameter and the Racah parameter, which were obtained from the optical-absorption peaks, are found to be Dq=379 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and B=560 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The $^{4}$${\mathit{T}}_{1}$${(}^{4}$P) state of the ${\mathrm{Co}}^{2+}$ ion splits into three state of ${\mathrm{\ensuremath{\Gamma}}}_{6}$, ${\mathrm{\ensuremath{\Gamma}}}_{8}$, and ${\mathrm{\ensuremath{\Gamma}}}_{8}$+${\mathrm{\ensuremath{\Gamma}}}_{7}$ by first-order spin-orbit coupling effects at 298 K. The value of the spin-orbit coupling parameter was \ensuremath{\lambda}=-436 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

Exchange interactions in insulators and semiconductors: I. The cation-anion-cation three-center model

Abstract: In this paper we discuss the theory of superexchange. We show that a large number of (super)exchange mechanisms, which are used to explain the basic magnetic interactions in insulating and semiconducting transition metal and rare-earth compounds, can be derived from the same equation by using a single principle: in transition metal compounds the exchange takes place predominantly on the anion while in rare-earth compounds exchange takes place almost predominantly on the cation. In this way we get a unified theory for the exchange interaction between magnetic ions in these systems.

Experimental investigation of angular dependence of photon induced L-shell X-ray emission intensity

Abstract: The angular dependence of emission intensity of L shell X-rays induced by 59.57 keV photons in Pb and U is investigated by measuring the normalized intensities of the resolved L X-ray peaks at different angles varying from 40° to 120°. It is observed that while L l and Lα X-ray peaks (originating fromJ = 3/2 state) show some anisotropic angular distribution, the emission of Lβ and Lγ X-ray peaks is isotropic. The present results contradict the calculations of Co-oper and Zare (1969) that after photoionization of inner shell, the vacancy state has equal population of magnetic substates and the subsequent X-ray emission is isotropic but confirm the predictions of Fluggeet al (1972) that the atomic inner shell vacancies produced after photoionization are aligned and the x-ray emission from the filling of vacancies in state withJ ⩾ 3/2 is anisotropic.

On the Electronic Transport Properties of Boron Carbide

Abstract: The problem of the electronic transport properties of boron carbide is generalized to the icosahedral boron-rich solids. It is shown that their semiconductor property, which is unique because of the odd electron number of the boron atoms, as well as other common structural and electronic properties can be attributed to the Jahn-Teller effect distorting the icosahedron, reducing its symmetry from Ih to D3d thus explaining the preferred crystal symmetry R3m and leading to a separation of occupied and unoccupied electronic states. The electronic transport properties of boron carbide including several hitherto surprising results are qualitatively interpreted within this band structure model.

Glassy dynamics of pinned charge-density waves.

Abstract: Stretched-exponential relaxation behavior observed in a one-dimensional model of pinned charge-density waves is shown to arise from anomalous deterministic diffusion confined to a closed portion of phase space Simulation results and scaling arguments are used to relate the index of the stretched exponential to the mean-field value of the anomalous-random-walk exponent on directed percolation clusters

Analytic approach to the a.c. conductance method for rapid characterization of interface states in MOS structures

Abstract: Nicollian and Goetzberger's well known integral expression for the equivalent parallel conductance due to interface states G p / ω has been given a simple analytical representation which is valid under the condition that interface potential fluctuation parameter σ > 1.5. Previous methods of conductance analysis based upon numerical and/or graphical construction are provided with alternative analytic relations. The earlier parametric numerical relations like ξ p , f w , f D and ln ( ξ + / ξ − ) vs σ are described very well by analytic expressions in this paper. A new relation is presented to determine interface state time constant from the width of conductance peak. A new method is presented to determine dependence of σ on interface band bending from G p / ω vs bias measurements at just two frequencies. The expedient method of Nicollian, Goetzberger and Lopez utilises the same two curves to determine distribution of interface state density N ss and capture cross section σ p across the band gap. Therefore using the present new method for σ determination and other analytical relations for N ss and σ p determination, the expediency of their method is greatly enhanced. Complete conductance analysis for rapid characterization of the interface is shown to become now a simple straight forward affair, and the amount of data required is also substantially reduced.

Laser-melted amorphous and crystalline Fe-B alloys.

Abstract: The structure of nanosecond-pulse laser-melted surfaces of different evaporated layers and bulk ingots has been studied using room-temperature conversion-electron M\"ossbauer spectroscopy in the iron-boron system between 17 and 56 at. % B content. Considerable decrease (\ensuremath{\sim}10%) of the width of the iron hyperfine field distribution was observed for the laser-melted surfaces compared with the evaporated layers, indicating an enhanced disorder in the latter. Amorphous alloys were obtained in the 17--33 at. % B composition range by laser melting. No amorphous alloy was obtained by the laser melting of the equiatomic FeB ingot or the evaporated amorphous ${\mathrm{Fe}}_{44}$${\mathrm{B}}_{56}$ film. In the latter case the laser melting has resulted in the low-temperature crystalline \ensuremath{\alpha}-FeB modification, indicating the important role of surface nucleation.

Quasicrystals

Abstract: After some introductory remarks about quasicrystals, ME spectra of icosahedral AlMnFe-, AlCuFe- and TiNiFeSi-alloys are summarised. The results are interpreted in terms of the current understanding of the structure of quasicrystals. Further a ME-application to the stability criteria of quasicrystals is discussed. A dominant factor is the band splitting factor or the Hume-Rothery rule.

Cathodoluminescence determination of the temperature rise in CdS under an electron beam spot

Abstract: Different models for calculation of the temperature rise under an electron beam spot are reviewed. From a study of the cathodoluminescence (CL) shift as a function of temperature in CdS, a model for determining the temperature rise is proposed.

IR absorption spectra of SiO2 films grown by photo-CVD

Abstract: The infrared absorption spectra of photo-CVD grown SiO2 are investigated. By virtue of the low temperature growth, the stress at the SiSiO2 interface is absent in these films as shown by the absorption spectra. An attempt is made to infer the structure of these films on the basis of results of IR spectra, etching rate, and refractive index measurements. Es werden Infrarotabsorptionsspektren von mittels Photo-CVD aufgebrachtem SiO2 untersucht. Durch den Niedertemperaturwachstumsprozes ist keine Spannung an der SiSiO2-Grenzflache in diesen Schichten vorhanden, wie es durch die Absorptionsspektren gezeigt wird. Es wird versucht, die Struktur dieser Schichten, auf der Grundlage der Ergebnisse der IR-Spektren, der atzrate und der Messungen des Brechungsindex zu verbessern.

The structure of li-ga

Abstract: Des mesures de diffraction de neutrons pour deux echantillons du systeme Li-Ga (Li 0,5 Ga 0,5 et Li 0,77 Ga 0,23 ) indiquent une forte mise en ordre dans le liquide induite par transfert electronique des atomes de Li aux atomes de Ga

Local iron symmetry in quasicrystals

Abstract: Iron sites in quasicrystalline Al(TFe)-alloys have been investigated with Mossbauer spectroscopy. The quadrupole splitting of these materials is about 0.4 mm/s, showing that the iron atom is not in an icosahedral\((m\bar 3\bar 5)\) site. A measurement in an external magnetic field ofHext≈5 T yields the asymmetry parameter η=(Vxx−Vyy)/Vzz. The value of η enables one to decide between the applicability of some mathematical structure models of quasicrystals.

Superexchange in insulators and semiconductors .2. the range of the exchange interaction

Abstract: In this paper we discuss the range of exchange interactions in magnetic semiconductors It is shown that the range depends strongly on the energy required to excite an electron from the fully occupied valence band to the empty magnetic d orbitals The range of the exchange interaction and of the spin-density distribution is the same when this excitation energy is large It is found that kinetic exchange and correlation exchange are the dominant contributions The usual perturbation theory breaks down when the dp excitation energy becomes of the order of the band width In this case one has to calculate the exchange interaction in a more rigorous way This limit is discussed using two-particle Green functions It is found that these results deviate strongly from the ones obtained using perturbation theory We derive a path model from an analysis of the effective transfer between two magnetic orbitals Such a model can explain the long-range exchange interactions reported in the last decades in a large number of magnetic semiconductors and semimagnetic semiconductors This path model can explain the anisotropy of long-range spin-density distributions found in doped semiconductors

Growth of large diameter lithium niobate single crystals by czochralski method

Abstract: An attractive combination of piezoelectric and optical properties has made lithium niobate one of the most extensively studied materials. Nowadays, application of these crystals in various fields such as acoustic wave propagation, electro-optic light modulation, optical second harmonic generation and parametric oscillation demands the bulk single crystal production. Solid State Physics Laboratory has grown large diameter (2 inch) lithium niobate (LiNbO3) single crystals successfully. Colourless, transparent, good optical quality crystals weighing up to 1.3 kg have been grown by Czochralski method. A computer controlled crystal puller equipped with R.F. heating has been used to pull these crystals from melt. An after heater wound with Kanthal A1 wire has been used. The growth parameter to develop crack free, good optical quality crystals have been optimized and will be discussed in detail. Grown crystals were sliced into 1 mm thick wafer and polished. Grown crystal surfaces were observed through o...

Some aspects of bias current tuning of W-band IMPATT diodes mounted in a reduced-height waveguide circuit

Abstract: The bias current tuning of W-band IMPATTs in reduced-height waveguide circuits have been discussed in details. The measured tuning response can be increasing, decreasing or practically insensitive with bias current depending on the device and circuit parameters. The observed results are explaine with a simple device-circuit interaction model.

Cathodoluminescence and X-ray analysis of defect reaction in Ag—CdS polycrystalline layers

Abstract: The diffusion of silver in the surface region of evaporated CdS polycrystalline films and single crystal has been carried out at 300°C for diffusion times between 5 min and 60 h. The diffusion profiles were studied by cathodoluminescence (CL) and X-ray analysis. The very sensitive CL modulation across the diffusion front correlates with EPMA; both techniques show anomalous oscillating profiles. Based on self-compensation theory and self-induced field effects, as well as on complex formation between impurities, a phenomenological model is proposed.

Formation of zintl-ions in alkali-lead alloys

Abstract: In the last years the liquid alkali-Pb alloys have been studied extensively. The measurements made include resistivity, structure factor, and the specific heat capacity. These alloys show anomalous behavior at either the octet composition with 80 at. % alkalimetal or at the equiatomic composition. From the resistivity measurements follows that a shift in stoichiometry occurs between Na-Pb and K-Pb. Whereas liquid Na-Ph, just as Li-Ph forms an octet compound, the K-Pb, Rb-Pb and Cs-Pb systems form only an equiatomic compound. In this paper we present the results of neutron diffraction and specific heat measurements. Together they constitute evidence for the formation of (Pb{sub 4}){sup 4-}-ions in liquid K-Pb, Rb-Pb and Cs-Pb.

Thermal-stability and microstructure of the electrodeposited amorphous fe80p18yb2 alloy

Abstract: The thermal stability and microstructure of the electrodeposited amorphous Fe80P18Yb2 alloy have been investigated by means of DSC and TEM. The microstructure observation reveals that during the crystallization process crystal grains grow at the cost of clusters. The crystallization kinetics parameters are obtained by J-M-A analysis based on DSC curves with different heating rates. In order to study the effect of adding the rare earth element Yb to amorphous FeP binary alloys, the relative fracture strain-brittleness parameter ϵf and Hruby's Kgl factor of our samples are also calculated in this paper. Results show that both thermal stability and ductility of the amorphous Fe80P18Yb2 alloy are improved considerably compared with that of the amorphous Fe81.5P18.5 alloy. This offers favorable perspectives for the application of amorphous alloys in the future.

Isomer-shift in binary iron-alloys

Abstract: The isomer shift of binary iron alloys is related to the itineracy of the valence electrons. The problem of a volume correction in disordered alloys is discussed and further avoided by using data of intermetallic compounds. From a large selection of possible parameters a linear combination of the electronegativityφ* and the electron densitynws in the Wigner-Seitz cell seems to be a good choice for fitting the data. The problem that these parameters are of minor importance for the cohesion in transition metal alloys is discussed.

Determination and study of field dependent defect density in thermal SiO2 using 1,1,1 trichloroethane (TCA)

Abstract: Based on the extreme value statistics and alternate and simpler method to determine the field dependent defect density from the mean breakdown voltage of MOS capacitors of two electrode areas is proposed. Using this approach the total defect density (field dependent) of trichloroethane (TCA) oxides is determined in the accumulation and inversion region for both, p- and n-type of silicon with varying molar concentrations of TCA. The results are compared with trichloroethylene (TCE) oxides grown under identical conditions to assess the feasibility of substituting a TCA source for the TCE. It is found that the defect density Dtot for TCA oxides when plotted against the TCA concentration has the following three regions: “passivation of surface states”, “corrosion”, and “alkali ion neutralization” region whereas TCE oxides have the following three regions: “corrosion”, “alkali-ion neutralization”, and “trapping” region. Auf der Grundlage der Extremwertstatistik wird eine alternative und einfachere Methode zur Bestimmung der feldabhangigen Defektdichte aus der mittleren Durchbruchspannung von MOS-Kondensatoren von zwei Elektrodenflachen vorgeschlagen. Mit diesem Verfahren wird die Gesamtdefektdichte (feldabhangig) von Trichlorathan (TCA) im Anreicherungs- und Inversionsbereich sowohl fur p- als n-leitendes Silizium bei variierender Molarkonzentration von TCA bestimmt. Die Ergebnisse werden mit Trichlorathylen (TCE)-Oxiden verglichen, die unter identischen Bedingungen hergestellt werden, um die Moglichkeit der Substitution der TCA-Quelle fur TCE einzuschatzen Es wird gefunden, das die Defektdichte Dtot fur TCA-Oxide uber die TCA-Konzentration aufgetragen drei Bereiche besitzt: “Passivierung der Oberflachenzustande”, “Korrosion” und “Alkaliionenneutralisierung”, wahrend TCE-Oxide folgende drei Bereiche zeigen: “Korrosion,” “Alkaliionenneutralisierung” und “Trapping”.

Influence of Cu+ doping on the thermoluminescence of KCl

Abstract: Etude pour des echantillons irradies par rayonnement γ. Observation de pics dus a la recombinaison des electrons mobiles ou des trous relâches de leurs pieges respectifs pour des temperatures ou ces pieges deviennent thermiquement instables