Recovery of low temperature electron irradiation-induced damage in n-type gaas

Undoped n-type GaAa was irradiated near 5 and 77 °K with electrons having incident energies between 0.46 and 1.30 MeV. The recovery of the electrical resistivity and the Hall coefficient upon annealing from 4 to 520 °K was monitored. Changes which occurred upon annealing below 200 °K could be reversed by ionizing radiation. A small amount of irreversible ionization-induced recovery was observed after irradiation near 5 °K. Major irreversible recovery stages were centered near 235 (stage I), 280 (stage II) and 520 °K (stage III). Recovery in stage I and II obeyed first order kinetics. The activation energies of stages I and II were determined as 0.72 and 0.83 eV, respectively. The carrier concentration changes per unit irradiation dose corresponding to the three recovery stages differed in their energy dependence indicating that the defects which are removed in stage III have the lowest threshold energy. The carrier concentration changes per unit irradiation dose corresponding to stages I and III were higher for irradiation near 5 °K than for irradiation near 77 °K.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

Application of BaF2 scintillator to off-line gamma-ray spectroscopy

Total cross sections of the monitoring reaction²⁷Al(d, 3p 2n)²⁴Na at 3·0 AGeV and 3·65 AGeV were determined from direct gamma-ray counting of irradiated targets with a spectrometer using a large volume BaF₂ scintillation crystal. Cross sections of 14·1 ± 1·3 mb and 14·7 ±± 1·2 mb are compared with previous data at other energies.     

Optical absorption and electrical conductivity of flash evaporated CuGaTe2 thin films

The optical absorption edge of CuGaTe₂ thin films in the energy range 1 to 2·3 eV was studied. The characteristic band gaps were found to be 1·23 eV and 1·28 eV whereas the acceptor ionization energy was 170 meV. Electrical conductivity measurements were carried out in the temperature range 300–550 K and two acceptor states with ionization energies 400 meV and 140 meV were found. The origin of acceptor states is explained based on covalent model.     

Pinning of size-selected Co clusters on highly ordered pyrolytic graphite

Deposition and implantation of size-selected Co⁺ _50±5 cluster ions on/in highly ordered pyrolytic graphite (HOPG) have been performed. Cobalt clusters were produced by laser ablation using the second harmonic (532 nm) of a Nd:YAG laser. They were deposited/implanted with energies of 250–4850 eV/cluster and examined using scanning tunneling microscopy (STM). For the highest energies the clusters created craters and wells with residual clusters at the bottom of the wells. Decrease of the impact energy led to formation of bumps whichconsist of damaged graphite areas mixed with fragmented cobalt clusters. Further decrease of the impact energy to 250–450 eV/cluster probably corresponds to the so-called pinning regime, when the impacting cluster creates defects in the surface layer and becomes bound to them. The transition from implantation to pinning with a decrease of impact energy was confirmed by etching experiments showing the depth of the damage introduced by the cluster collisions with HOPG.     

Nuclear level widths in 113Sb from particle-X-ray coincidences

The particle-X-ray coincidence method is applied in (p, p₀) and (p, p′) reactions on ¹¹² Sn at 7.138, 10 and 12 MeV proton energy to measure the width of compound nuclear states in ¹¹³Sb. The united-atom K X-ray intensities observed in coincidence with inelastic proton scattering (35⁺⁷⁸_?35, 80 ± 50 and 250 ± 80 counts at the respective energies) yield mean level widths for ¹¹³Sb of ?3.9 eV, 16.6 ± 15.1 eV, and 19.1 ± 11.1 eV at the excitation energies 10.1, 13 and 15 MeV, respectively. From X-ray coincidences with elastic proton scattering, upper limits for the compound elastic component are derived. No united-atom X-rays were detected in short runs at 7 MeV on ⁹²Mo and ¹⁰⁶Cd. A comparison of nuclear state widths from X-ray method, crystal blocking technique and fluctuation averaging is given.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Electron stimulated desorption of carbon monoxide from a (111) niobium surface

Electron stimulated desorption of CO from the (111) face of a Nb single crystal produced both CO⁺ and O⁺ ions after adsorption at 150°K on a clean surface. When the surface was heated to above 250 °K only O⁺ ions were observed, and this current disappeared as the temperature was increased to 700 °K. Readsorption (at 150 °K) was inhibited following the 700 °K heating. These data indicate the formation on heating of a tightly bound surface phase with very low ionic desorption cross section. Threshold energies for CO⁺ and O⁺ ion production were 10.0 ± 0.5 eV and 19.0 ± 0.5 eV, respectively. The cross section for electron stimulated depopulation of the O⁺ producing phase was (4 ± 1) × 10^?18 cm² for 100 eV electrons.     

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.     

Influence of the surface on the quantum P.E.M. effect in InP

The photoelectromagnetic effect of InP is studied in quantizing magnetic fields at 4·2 K in an energy range 1·4–1·5 eV for linearly polarized light. Depending on the sample surface condition two types of spectral oscillations may appear, those associated with interband transitions between Landau levels or the LO phonon type usually seen in photoconductivity. An analysis of the spectral oscillations gives: E₀ = 1·423±0·001 eV; Δ₀ = 0·102±0·006 eV; hω_L = 0·036 eV.     

Analysis of 1 MeV electron irradiation-induced performance degradation in the germanium bottom cell of triple-junction solar cells using temperature-dependent photoluminescence measurements

Abstract

Temperature-dependent photoluminescence spectra of the germanium bottom cell of triple-junction solar cells unirradiated and irradiated with 1?MeV electrons were measured in the 10–300?K temperature range. In unirradiated germanium bottom cell, the spectra show that the PL intensity increases with temperature but slightly decreases at around 250?K because of the intrinsic defect. However, in irradiated germanium bottom cell, the spectra show that there are two negative thermal quenching processes (10–90?K and 200–270?K) and two usual thermal quenching processes (90–200?K and 270–300?K) as a result of the radiation-induced defects E_c ? 0.37?eV and E_c ? 0.12?eV.     

Electrical conductivity,absorption and luminescence measurements in cubic boron nitride

Electrical and optical measurements were carried out on tiny crystals of cubic boron nitride. The dark current i_D was found to change exponentially with T, with activation energies in the range 0·2–0·4 eV. A red electroluminescence, of intensity i_EL. proportional to i_D was observed. Upon illumination at low temperatures a photocurrent i_p proportional to the square root of the excitation intensity appeared. It varied exponentially with T, with an activation energy of 0.05 ± 0.01 eV. The crystals exhibited a red thermoluminescence with several unresolved peaks covering the temperature range 100–400K, and having activation energies in the range 0·15–0·40 eV.     

Positron escape from annihilation centers in electron-irradiated Si crystals

The mean positron lifetime in irradiated Si diminishes with increasing temperature between 77°K and 300°K. Isochronal high-temperature crystal anneals reduce the effect. Analysis suggests that thermally activated positrons escape after trapping in radiation-induced defects.     

Neutron diffraction study of anisotropy of crystal lattice compression in irradiated of fast neutron high-T c superconductors YBa2Cu3O6·95 and La1·83Sr0·17CuO4 under pressure

Abstract

The structural changes in the high temperature superconductors (HTSC) samples of the compounds YBa₂Cu₃O_6·95 and La_1·83Sr_0·17CuO₄ under pressure being unirradiated and irradiated by fast neutrons have been studied by the neutron diffraction method. The different anisotropy of crystal lattice compression was found for these compounds in the initial and irradiated states.     

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.     

Measurement of stored energy of KCl after 4·6°K reactor irradiation

Five KCl single crystals were irradiated at 4·6°K in the core of the Munich Nuclear Reactor for periods of 100 sec, 10 min, 1 hr or 10 hr, respectively. After irradiation the stored energy of the samples was measured in a differential-heat-flow calorimeter at two different heating rates: 0·29 and 1·1°K/min. At 0·29°K/min peaks of stored energy release were resolved at 21°K, near 27·5°K, at 32·5°K, at 42·5°K and near 50°K. An attempt was made to evaluate the corresponding activation energies using two different methods (see Table 1 in the text). The annealing stages at 21 and 32·5°K correspond to first order kinetics. The annealing stages at 42·5 and near 50°K are not of first order. A similar experiment was performed on KBr single crystals and is reported in the preceding paper. The results are compared with annealing studies on low temperature X-irradiated KCl and KBr crystals. From both our experiments it follows that also during reactor irradiation at 4·6°K ionization mechanisms of defect production should be responsible for the four observed low temperature recovery peaks; only defect recovery at higher temperatures may be at least partially explained by recombination of collision produced defects, i.e. the typical neutron irradiation effect as it takes place in metals.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Measurement of stored energy of KBr after 4·6°K reactor irradiation

Four KBr single crystals were irradiated at 4·6°K in the core of the Munich Nuclear Reactor for periods of 100 sec, 10 min or 1 hr, respectively. After irradiation the stored energy of the samples was measured in a differential-heat-flow calorimeter at heating rates of 0·29°K/min and 1·1°K/min. At 0·29°K/min peaks of stored energy release were resolved near 11 and 20°K and at 25, 38 and 45°K. An attempt was made to evaluate the corresponding activation energies using two different methods (see Table 1 in the text). The 25°K annealing stage is due to a first order recombination reaction. The anneling states near 20, at 38 and at 45°K do not correspond to first order reactions. Our results are compared with annealing studies on low temperature X-irradiated KBr crystals. A similar experiment was performed on KCl single crystals and is reported, together with a more detailed comparison and discussion of both experiments, in a later paper[14].     

Mössbauer studies of europium and ytterbium hydrides

Mössbauer absorption spectra were obtained for the 21·6 ke V transition of ¹⁵¹Eu in EuH₂ at various temperatures and for the 84·3 keV transition of ¹⁷⁰Yb in YbH₂ at 4·1°K. The isomer shift of EuH₂ relative to Eu³⁺: Sm₂O₃ is ? 12·1 ± 0·3 mm. sec^?1, and the magnetic hyperfine field equals ? 305 ± 5 kOe at saturation. The Curie temperature is found to be 16·2 ± 0·05°K, and the critical parameters of the transition are D = 1·17 ± 0·02 and β = 0·35 ± 0·01. The magnetic field is perpendicular to the principal axis of the electric field gradient and the values of the quadrupole hyperfine interaction e²qQ₀(3 cos² θ ? 1)/8 is ? 28 ± 4 Mc . sec^?1. A large increase of the resonance area (21%) occurs at the transitio to the ferromagnetic state. The isomer shift of YbH₂ relative to Yb: TmAl₂ is ?0·11 ± 0·01 mm . sec^?1. The value of the quadrupole coupling constant e²qQ_c/4is ? 91·5 ± 2 Mc . sec^?1 and the asymmetry parameter of the electric field gradient equals 0·89 ± 0·05. The data for EuH₂ and YbH₂ is shown to be consistent with the hydridic model for the rare earth hydrides.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.