Das Emissionsspektrum gebundener Excitonen in Zinkoxydkristallen in Abhängigkeit von Temperatur und uniaxialer Verspannung

The recombination radiation of bound excitons in ZnO-crystals (spectral range 3,38–3,35 eV at 4.2 °K) and the adjacent phonon-assisted spectrum have been studied at temperatures between 4.2 °K and 90 °K. The phonon-assisted spectrum changes its structure at about 30 °K. With increasing temperature the exciton lines shift toward smaller quantum energies (ΔE～T ²) and their half widthH increases (H～T ²). The line shift is explained by a band gap variation due to deformation potential coupling of the holes toTA phonons. The line width is explained by broadening of an infinitely sharp line under the influence of the mean square fluctuation of thermally generated crystal deformations. The influence of uniaxial stress on the spectral positions of the lines has been studied.     

Annealing of γ-ray irradiated lithium compensated p-type silicon

Abstract

Annealing behavior of electrical properties and photoluminescence spectra both at 77 °K in electron-irradiated melt-grown n-GaAs were investigated. Defects electrically active in the Hall mobility and carrier removal anneal through two stages centered at 250° and 460 °K. From the temperature dependence of carrier concentration the existence of a defect level located near 0.15 eV below the conduction band is supposed. Several emission bands are resolved at 1.51, 1.47, 1.415, 1.305 and ～1.2 eV in photoluminescence experiments. Electron irradiation (1.5–2.0 MeV) causes a remarkable decrease in emission intensity of 1.51 and ～1.2 eV bands. Recovery of emission intensity occurs remarkably when samples are annealed to 520 °K which would correspond to the 460 °K annealing stage for carrier concentration and Hall mobility. The 250 °K annealing stage is not observed in photoluminescence experiments. The 1.415 eV peak appears clearly after irradiation and grows remarkably with the 520 °K annealing, especially in Si-doped samples, resulting in large reverse annealing. This band is tentatively speculated to be a complex of Si on As site with As vacancy. Moreover, in samples doped with Te a new emission band at 1.305 eV (9500 Å) is observed after 470°–620 °K annealing.     

Growth-morphology-luminescence correlation in ZnO-containing nanostructures synthesized in different media

Zinc hydroxide particles were prepared by a two-step process employing zinc nitrate hexahydrate, urea, ethylene glycol, water and p-toluene-sulfonic acid monohydrate (p-TSA). We used different concentrations of the reactants as well as different volume ratios of the solvents. ZnO particles were obtained by thermal treatment of the reaction products at two different temperatures: 350 °C and 500 °C. The samples were characterized by scanning field emission electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, BET analysis, thermogravimetry (TG) analysis and photoluminescence (PL) spectroscopy. It was found that after the thermal treatment particles become smaller, with the p-TSA concentration strongly affecting the morphology of the particles. Luminescence properties of the samples probed by PL at 8 K and room temperature exhibited a remarkable correlation with specimens′ nanomorphology. Luminescent features at ～2.0–2.2 eV, ～2.4–2.5 eV, ～2.65 eV, ～2.9 eV, ～3.0–3.1 eV and ～3.3 eV were observed in most specimens, although their relative intensity and temperature dependence were specific to an individual group of samples vis-à-vis their growth history and morphology.     

Determination of the indirect band edge of GaAs by quantum-well bandfilling (Lz∼100Å)

Photoexcitation-bandfilling of a quantum-well Al_xGa_1?xAs-GaAs-Al_xGa_1?xAs (x～0.6, L_z～100Å) heterostructure is used to generate visible-red recombination radiation that cuts-off at high energy (1.864 eV, 4.2°K) due to electron transfer to indirect minima. This leads to an estimate of ΔE = E_L?E_Γ～310 meV.     

Polycrystalline GaSb films prepared by the coevaporation technique

Gallium antimonide (GaSb) films were deposited onto fused silica and n-Si (100) substrates by coevaporating Ga and Sb from appropriate evaporation sources. The films were polycrystalline in nature. The size and the shape of the grains varied with the change in the substrate temperature during deposition. The average surface roughness of the films was estimated to be 10 nm. Grain boundary trap states varied between 2×10¹² and 2.2×10¹² cm^?2 while barrier height at the grain boundaries varied between 0.09 eV and 0.10 eV for films deposited at higher temperatures. Stress in the films decreased for films deposited at higher temperatures. XPS studies indicated two strong peaks located at ～543 eV and ～1121 eV for Sb 3d_3/2 and Ga 2p_3/2 core-level spectra, respectively. The PL spectra measured at 300 K was dominated by a strong peak located ～0.55 eV followed by two low intensity peaks ～0.63 eV and 0.67 eV. A typical n-Si/GaSb photovoltaic cell fabricated here indicated V _oc～311 mV and J～29.45 mA/cm², the density of donors (N _d)～3.87×10¹⁵ cm^?3, built in potential (V _bi)～0.48 V and carrier life time (τ)～28.5 ms. Impedance spectroscopy measurements indicated a dielectric relaxation time ～100 μs.     

Untersuchung von Halbleiter-Sperrschichtzählern bis 4,2 °K

The behaviour of Semiconductor Surface Barrier Detectors has been studied in the temperature range from 300 °K to 4.2 °K. Measurements of the resolution forα- andβ-particles are given. The mean energy? required for producing an electron-hole-pair in Si is found to be 3.54±0.10 eV at 279 °K, and 3.72±0,11 eV at 78 °K and 4.2 °K respectively. Measured rise time of detector impulses is compared with theoretical values.     

Surface photoemission in the 4d band from polycrystalline silver surfaces

Surface induced local d-band states in the upper 4d band between ～ 4 and ～ 5.2 eV below E_Fermi have been identified for polycrystalline silver films in photoemission experiments using synchroton radiation. A thin over-coat (10 å) by an Al film leads to a depression of these surface induced local states whereas a change from s- to p-polarized excitation leads to an enhancement. Deposition of additional silver (～ 3 Å) at 120 K induces additional emission 4.2 eV below E_F with a FWHM of only ～ 0.4 eV.     

Ionization equilibrium and radiative energy loss rates for C,N, and O ions in low-density plasmas

The results of calculations of the ionization equilibrium and radiative energy loss rates for C, N and O ions in low-density plasmas are presented for electron temperatures in the range 10⁴–10⁷ °K (～1–10³ eV). The ionization structure is determined using the steady-state corona model, in which electron impact ionization from the ground states is balanced by direct radiative and dielectronic recombination. Using an improved theory, detailed calculations are carried out for the dielectronic recombination rates in which account is taken of all radiative and autoionization processes involving a single-electron electricdipole transition of the recombining ion. The radiative energy loss processes considered are electron-impact excitation of resonance line emission, direct radiative recombination, dielectronic recombination, and electron-ion bremsstrahlung. For all three elements, resonance line emission resulting from 2s?2p transitions produces a broad maximum in the energy loss rate near 10⁵°K(～ 10 eV).     

Anomalous temperature dependence of the energy gap of AgGaS2

The lowest energy gap Eg of AgGaS₂ in the temperature range from 4.2 to 300° K was determined from the reflectivity, photoluminescence and absorption measurements. Below ～ 80° K the temperature coefficient of the energy gap is +6 × 10^-5eV/°K. Above ～80° K the sign of the coefficient reverses and the value is -1.8 × 10^-4eV/°K. The positive value is explained with the lattice dilation effect being the dominant mechanism for the energy gap variation at lower temperatures than ～80°K.     

Hyperfine fields in erbium metal

The hyperfine Splitting of the 80.6-keVγ transition in Er¹⁶⁶ has been measured in erbium metal between 4.2 °K and 40 °K using the Mössbauer effect. There is evidence for a unique magnetic field and electric fieldgradient at all nuclei in erbium metal. The magnetic field decreases from (7.55±0.20)·10⁶ Oe at 4.2 °K to (6.10±0.40)·10⁶ Oe at 40 °K. Extrapolation to 0 °K yieldsH (0 °K)=(7.60±0.20)·10⁶ Oe. The quadrupole interaction energy for the 80.6-keV state iseQ V _Z′Z′/4=(0.95±0.20)·10^?6 eV at 4.2 °K. These results are discussed and compared with other measurements.     

UV absorption of RbAg4I5-RE (Sm, Yb) thin-film systems

UV spectra of samples prepared by vacuum deposition of Sm and Yb thin films on 100–200-nm thick films of the RbAg₄I₅ solid electrolyte (SE) at 300–350 K contain strong absorption bands peaking at about 4.3 and 5.0 eV. After deposition of ～5 nm of Sm, the ionic conductivity σ of the samples decreases from σ ₀ to ≈0.9 σ ₀, and the SE lattice parameter, from 11.24 to ≈11.15 Å, with the x-ray reflection halfwidth increasing from 0.5 to 0.8°. Further growth of Sm concentration in the samples changes the x-ray diffraction pattern, the absorption at 4.3 and 5.0 eV increases, a new absorption edge forms at 3.8 eV, and σ decreases down to ～10^?2 σ ₀. It is conjectured that the strong UV absorption bands in heavily defected silver halides of the RbAg₄I₅-Sm(Yb) system is genetically related to the 4d ¹⁰→4d ⁹5s electronic transitions in free Ag⁺ ions.     

Correlation of dielectric properties,Raman spectra and calorimetric measurements of β-cyclodextrin-polyiodide complexes (β-cyclodextrin)2 ·BaI7 ·11H2O and (β-cyclodextrin)2 ·CdI7·15H2O

The frequency and temperature dependence of real and imaginary parts of the dielectric constant (ε′,?ε″), the phase shift (?) and the ac-conductivity (σ) of polycrystalline complexes (β-CD)₂·BaI₇·11H₂O and (β-CD)₂·CdI₇·15H₂O (β-CD?=?β-cyclodextrin) has been investigated over the frequency and temperature ranges 0–100?kHz and 140–420?K in combination with their Raman spectra, DSC traces and XRD patterns. The ε′(T), ε″(T) and ?(T) values at frequency 300?Hz in the range T<330?K show two sigmoids, two bell-shaped curves and two minima respectively revealing the existence of two kinds of water molecule, the tightly bound and the easily movable. Both complexes show the transition of normal hydrogen bonds to flip-flop type at 201?K. In the β-Ba complex most of the eleven water molecules remain tightly bound and only a small number of them are easily movable. On the contrary, in the β-Cd case the tightly bound water molecules are transformed gradually to easily movable. Their DSC traces show endothermic peaks with onset temperatures 118°C, 128°C for β-Ba and 106°C, 123°C, 131°C for β-Cd. The peaks 118°C, 106°C, 123°C are related to the easily movable and the tightly bound water molecules, while the peaks at 128°C, 131°C are caused by the sublimation of iodine. The activation energy of Ba²⁺ ions is 0.52?eV when all the water molecules exist in the sample and 0.99?eV when the easily movable water molecules have been removed. In the case of β-Cd the corresponding activation energies are 0.57?eV and 0.33?eV. The Raman peaks at 179?cm^?1, 170?cm^?1 and 165–166?cm^?1 are due to the charge transfer interactions in the polyiodide chains.     

Thermally-induced changes in barrier heights of aluminium contacts to p- and n-type silicon

The barrier height of aluminium contacts to n-type silicon, φ_bn, increases continuously from ～-0.45 eV to ～-0.80 eV as a result of post-evaporation heat treatments in the range 100–550°C. Aluminium-p-type silicon contacts show complementary behaviour, with φ_bp progressively decreasing from ～-0.75 eV to ?0.40 eV over this same temperature range. Three mechanisms are believed to contribute to this behaviour.     

Low temperature photoconductivity in electron - irradiated p-type Si

The photoconductivity spectra of p-type silicon irradiated at ～15 °K with 1.2 MeV electrons were studied in the wavelength range from 1.2 to 5.5 μ at temperatures from 23 to 80 °K. The 3.9 μ photoconductivity band appears immediately after irradiation in all crystals already at low temperatures, giving further evidence that it is due to the divacancy formed directly during irradiation by electrons. Three main annealing stages of the photoconductivity have been observed; (a) below 160 °K, (b) 160–250 °K, and (c) 280–360 °K. A radiation-induced deep level at E_v , +(0.12±0.02 eV disappears upon annealing at stage b. The annealing behavior of the spectra depends strongly on the measuring temperature. The dependence of the spectra on chopper speed was also investigated.     

Luminescence and decay times of CsI:In+

Luminescence spectra of single crystals of CsI:In⁺ excited in the A(304 nm), B(288 nm), C(268 nm) and D(257 nm) absorption bands have been studied in the temperature range 4.2–300 K. Excitation in the A band at 4.2 K gives rise to the principal emission at 2.22 eV accompanied by a partly-overlapping weak band at 2.49 eV. An additional emission band at about 2.96 eV is observed on excitation in the B, C or D bands. Yet another emission band located at 2.67 eV is excited only in the D band. The relative intensities of the bands are very sensitive to excitation wavelength as well as to temperature. The origin of all these bands is assigned in terms of a model for the relaxed excited states (RES). All the luminescence spectra were resolved into an appropriate number of skew-Gaussian components. Moments analysis leads to a value of (1.35 ± 0.02) × 10¹³ rad s^-1 for the effective frequency (ω_eff) of lattice vibrations coupled to the RES. At the lowest temperature, the radiative decay times of each of the intracenter emission bands (2.22, 2.49 and 2.96 eV) show a slow decay ( ～ 10–100 μs) and a fast decay ( ～ 10–100 ns). The 2.96 eV band, which is assigned to an emission process which is the inverse of the D-band absorption, exhibits a single decay mode ( ～ 10 μs). The intrinsic radiative decay rates (k₁, k₂), the one-phonon transition rate (K) and the second-order spin-orbit splitting (D) for the RES responsible for the principal emission are: k₁ = (6.0±-0.3)×10³ s^-1, k₂ = (1.33±-0.06)×10⁵ s^-1, K = (2.4±-0.4)×10⁷ s^-1 and D = (13.8±-0.5) cm^-1.     

Über den Einfluß von Zwischengitterionen auf die Absorption von Gitterlücken in KBr-Kristallen

Detailed measurements have been made of the optical properties of α-centers (anion vacancies) in KBr: KH crystals irradiated monochromatically in the U band at 58°K. The absorption band α, normally at 6·14 eV is under these conditions shifted to shorter wavelengths. In terms ofFritz's model for the infrared absorption of U₁ centers, this shift can be interpreted as the influence of the nearby interstitial hydride ions on the a absorption. The nearest observed interstitial ion, stable to 103°K, shifts the α band to 6·31 eV; the next nearest one (stable to 150°K) to 6·19 eV; heating above 150°K leaves only the normal α absorption at 6·14 eV. Data are presented on the quantum efficiency for the recombination under uv irradiation and on the oscillator strengths for the different α centers.     

Direct and modulated reflectance spectra of MnO,CoO, and NiO

Direct reflectance, thermoreflectance, and electroreflectance have been measured for MnO, CoO, and NiO above the fundamental edge. Spectra of all three materials support a model containing both localized and one-electron band states. In MnO peaks with temperature coefficients of ～ 10^?3 eV/K were observed at 5.7 and 6.9 eV, temperature-independent structure occurred at 5.4, 6.3, and 7.2 eV, and spectral features with indeterminate temperature dependence were seen at 4.6 and 5.5 eV. The temperature-dependent structure was assigned to one-electron interband transitions associated with anion 2p and cation 4s states; the temperature independent structure was assigned to crystal field split localized interionic transitions between the 3d-states of neighboring Mn ions. Thermoreflectance spectra for CoO exhibited temperature dependent structure (9.5 × × 10^?4 eV/K at 5.0, 6.0, and 7.2 eV. A strong, temperature dependent electroreflectance oscillation was seen in NiO near 6.2 eV. On the basis of these data the interband gap between the anion 2p and cation 4s bands was determined to be 5.7 eV in MnO, 6.0 eV in CoO, and 6.2 eV in NiO.     

The effect of carbon content on the beginning of slip in single crystals of Fe-3·2 wt%Si alloy

Specimens with 18 ppm and 100 ppm carbon of different orientations were deformed by compression (?～1·1×10^?4 s^?1) at the temperatures 150 °K, 195 °K, 293 °K, 393 °K and 483 °K. The measuredΨ(χ) andτ(χ) curves are presented. The changes of the curves caused by increased carbon content are discussed from the point of view of the low temperature induced cross-slip. Deviations from the Schmid law of critical resolved shear stress are found for both carbon contents. The dependence of the CRSS on temperature for specimens of standard purity has a slightly different course for orientationχ=?30° than for orientationsχ=0° andχ=+30°. These deviations are discussed in terms of the influence of normal stress on the slip. The course ofΔτ (difference between the CRSS in the MRSS plane for specimens with 100 ppm and 18 ppm of carbon) onχ is discussed using different models of lattice hardening due to interstitial impurities. These models can also be applied to the explanation of deviations ofτ(χ) curves from the Schmid law.     

Die Verfestigungserholung von plastisch verformten Steinsalzeinkristallen

Cylindrical rock salt single crystals have been plastically deformed by compression in the [001]-direction at room temperature to shear stresser τ _E of 200 N/cm² and 350 N/cm², respectively. Isochronal annealing experiments reveal, that workhardening recovers at >300° C. The characteristic annealing temperature was found between 400° C and 450° C. At 600° C the residual workhardening still amounts to 15–20%. The isochronal reduction of screw dislocation density between 400 and 600° C shows qualitatively the same behaviour as recovery of workhardening. From the isothermal annealing curves of the samples deformed to 200 N/cm² the activation energy for recovery of workhardening was found to be about 1 eV. Assuming that the kinetics of recovery can be explained by processes distributed in activation energy, an approximate spectrum of activation energies (with a maximum arising at ～1 eV) has been evaluated. The results show that recovery of workhardening after low deformation (stage I of the stress strain curve) is mainly due to the dislocations.     

Temperature dependance of the fundamental absorption edge in CdTe

Optical measurements made on CdTe put in light an extrinsic transition previously used to determine the interband edge of this compound. The donor level involved has a depth of 30 meV inside the bandgap at 77°K and of 60meV at 300°K. The value obtained for its temperature coefficient, suggests an association of this level with the L minimum of the conduction band.Also, determined is the true optical bandgap of CdTe between many conflicting results. One obtains: E_g = 1.529 meV with a temperature coefficient of ?3.10^?4eV/°K.