Collision damage processes due to energetic heavy ions in silver chloride

The optical absorption band induced by energetic heavy ions implanted into AgCl single crystals at 77°K must be connected with the formation of silver colloids. The experimental (primitization and E.P.R. spectra) and theoretical study, makes it possible to obtain an approximate value of cluster radius. The colloid concentration is correlated with nuclear stopping power. It is shown that the collision efficiency is modified by a chemical effect. Finally the conditions of cluster formation are analysed.     

Spectral photoconductivity of tellurium in the CO2 laser wavelength range

Infrared photoconductivity measurements on tellurium single crystals and evaporated films at 90 K yielded a maximum response at 0.126 ± 0.002 eV using a low power CO₂ laser. The frequency dependence of the response was slow and could not be interpreted as vast intervalence band transitions.     

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.     

Photoelectronic and electrical properties of Tl2InGaS4 layered crystals

Tl₂InGaS₄ layered crystals are studied through the dark electrical conductivity, space charge limited current and illumination- and temperature-dependent photoconductivity measurements in the temperature regions of 220-350 K, 300-400 K and 200-350 K, respectively. The space charge limited current measurements revealed the existence of a single discrete trapping level located at 0.44 eV. The dark electrical conductivity showed the existence of two energy levels of 0.32 eV and 0.60 eV being dominant above and below 300 K, respectively. The photoconductivity measurements reflected the existence of two other energy levels located at 0.28 eV and 0.19 eV at high and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 330 K. The illumination dependence of photoconductivity is found to exhibit supralinear recombination in all the studied temperature ranges. The change in recombination mechanism is attributed to exchange in the behavior of sensitizing and recombination centers.     

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor-acceptor pairs (DAP) with a maximum at about 4200 Å has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to E_D = = 0.045?0.065 eV and E_A = 0.155?0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 Å) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton-phonon interaction (LO ? 18.7 meV) has been determined.     

Pressure dependence of orthopositronium lifetime and bubble parameters in liquid helium

Orthopositronium lifetime in liquid He-4 at 4.2 K is found to decrease from 100 nsec to 50 nsec under pressures 1–60 atm. The radius and potential well-depth of the positronium bubble are estimated to vary from 14 Å to 7 Å and from 0.25 eV to 0.55 eV.     

Study of the 1.65 eV pair recombination in cinnabar powders: Thermal quenching and total light decay

We have studied the luminescence of cinnabar powders excited by an electron beam. The shape of the emission spectrum is a broad band, whose bandwidth is about 320 meV and which is peaked at 1.65 eV at liquid helium temperature. The study of thermal quenching leads to a donor depth E_D ～ 170 meV; the Bohr radius of this level is approximately 4 Å.Time resolved spectroscopy curves, which have been published in a previous paper, are theoretically analyzed and we derive the maximum recombination probability W_max ～ 10⁷ s^-1, i.e. the theoretical recombination probability extrapolated to the limit of very close pairs.From the above results we compute the theoretical decay curves of the total light emission and we compare them to the experimental curves. A reasonable agreement is observed between theory and experiments.     

Space-charge-limited currents and photoconductive properties of Tl2InGaSe4 layered crystals

The extrinsic electronic parameters of Tl₂InGaSe₄ layered crystals were investigated through measurement of the temperature-dependent dark conductivity, space-charge-limited currents and photoconductivity. Analysis of the dark conductivity reveals the existence of two extrinsic energy levels at 0.40 and 0.51 eV below the conduction band edge, which are dominant above and below 260 K, respectively. Current–voltage characteristics show that the one at 0.51 eV is a trapping energy level with a concentration of (4.8–7.7) × 10¹⁰ cm^?3. Photoconductivity measurements reveal the existence of another energy level located at 0.16 eV. In the studied temperature range, the photocurrent increases with increasing temperature. The dependence of the photoconductivity on the incident light intensity exhibits a linear recombination character near room temperature and a supralinear character as the temperature decreases. The change in recombination mechanism is attributed to an exchange in the behavior of sensitizing and recombination centres.     

Laser characteristics of KCL:O-2

Amplified spontaneous emission and laser action have been observed at 77 K in KCL crystals containing the superoxide ion (O^-₂). The laser operates in two bands of width ～50 Å centered at 6350 Å and 5984 Å. These bands are the (lattice) phonon sidebands of vibronic transitions of the superoxide ion. At lower temperatures (～6 K) amplified spontaneous emission is observed in the zero phonon line at 6294 Å.     

Thermally depolarized current and photoconductivity in GaS: Ho single crystals

Research results of thermally depolarized current and photoconductivity in GaS: Ho single crystals are given in the report. TSD is shown to be of inverse character. Impurity photoconductivity is found in the crystals under study. Such levels as E_ti = 0.70, 0.80, 0.91 eV are shown to be responsible for the thermally stimulated depolarization current and for photoconductivity.     

Optical properties of GaSe: Mn single crystals

To identify the manganese related defect levels in GaSe, GaSe:Mn single crystals were grown with various Mn dopant levels using the Bridgman technique and the photoconductivity and photoluminescence properties were investigated. Peaks introduced by the manganese related defects were observed at 1.916 and 1.724 eV in the photoconductivity spectra and at 1.804 eV in the photoluminescence spectra at 80 K. These results allow the calculation of the energies of the A₁ and A₂ centers at 0.348 and 0.156 eV, respectively, above the valence band and a donor level at 0.112 eV below the conduction band. Also, we find that the A₁ and A₂ centers are pinned within the conduction band from measurements of the temperature dependence of the photoconductivity spectra.     

Optical spectra of lithium niobate

The specific features revealed in the optical spectra of lithium niobate crystals at temperatures of 90 and 120–125°C can be attributed to the change in the electronic subsystem and the related isostructural transformation of the crystal lattice. In the near-IR range, the optical absorption spectra of lithium niobate crystals exhibit bands (1.43 eV) assigned to polarons of large radius with a binding energy of 0.48 eV. The decrease observed in the absorption coefficient at the maxima of these bands with an increase in the temperature to 160°C can be explained by the decay of polarons of large radius at these temperatures.     

Mechanism of necking as revealed by ultrasonic cavitation of polyethylene single crystals

Ultrasonic cavitation of polyethylene single crystals and single-crystal aggregates in the form of cakes results in lamella fragmentation and necking involving the transformation of lamellar crystals into fibrillar crystals between 20 to 400 Å in diameter. The smaller fibrils (～20–30 Å) have a very smooth appearance, whereas the larger ones (～100–400 Å) contain a beady structure about 100 Å periodically spaced along the fibrils. The smoother microfibrils are suggested to contain extended chains that are formed by unfolding of molecules directly from the chain-folded lamellae as well as from the folded-chain crystals contained within the beady fibrils. The presence of the chain-folded crystals within the larger beady fibrils is shown in numerous instances to be due to incorporation of mosaic crystalline blocks originally present, but weakly connected to one another, in the lamellar single crystals. The necking process is deduced by observation to involve primarily a mechanical shearing of mosaic crystalline blocks along the c-axis plus a rotation into the fibril direction. Observation of extreme resistance of lamellae in the overgrowth regions to cavitation damage suggests the presence of tie molecules and/or interpenetrating cilia between these lamellae. The suggestion finds strong support from additional studies carried out on lamellae that have been tied together at the folds by cross-linking with γ rays.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Attenuation lengths of low-energy electrons in free-standing carbon films

Attenuation lengths for electrons passing through a free-standing 40-Å carbon film are reported for electron energies between 6 and 1200 eV. Attenuation lengths for inelastic scattering are found to decrease from approximately 20 Å at 1200 eV to 6 Å in a broad minimum at 40 eV and then to rise to 9 Å at 6 eV. The results are in satisfactory agreement with theory between 200 and 1200 eV where appropriate calculations are available.     

Phonon scattering and interstitial clusters in γ-irradiated LiF

The thermal conductivity of pure and Mg-doped LiF single crystals irradiated at room temperature in a Co⁶⁰ source (3–780 Mrad) was measured between 60mK and 70 K. Several phonon scattering mechanisms for extended defects are reviewed and it is shown that experimental results can be understood in terms of two types of interstitial clusters: (a) small spherical clusters whose number density and size depend on the residual impurity content; and (b) large non-spherical clusters of toroïdal shape with a major radius of several hundred Å and minor radius of some tens of Å. For increasing dose, the major radius decreases, the minor radius increases and the rings become increasingly opaque possibly due to the associated strain field.     

ZnSe晶体中Cu杂质深能级的ODLTS谱

用控制Cu杂质在ZnSe品格中占据位置的方法,成功地得到了Cu-G和Cu-R中心分别占优势的ZnSe:Cu晶体.首次用ODLTS方法测得与Cu-R和Cu-G中心相应的受主能级分别位于价带顶上0.72eV和0.30eV.     

Photoconductivity on europium oxyde single crystals and thin films

In this paper, results of photoconductivity measurements on four EuO samples are given. Low frequency photoconductivity versus temperature (10°K < T < 300°K) and magnetic field H is investigated for two wavelengths: 6600 Å and 9000 Å. The photoconductivity kinetic is also described, and is characterized by a distribution in decay times. Temperature, magnetic field and carrier concentration have small effects on this kinetic. Quenching effect is obtained by adding a continuous illumination (λ₂) to the weak modulated light (λ₁). The kinetic is strongly affected by quenching and becomes more simple. Quenching effect is maximum for the wavelength associated to the 4?⁷–4?⁶ 5d, transition. In contrast to the Penney-Kasuya[1] model we propose another one in which the conduction of equilibrium carriers as photo-excited carriers takes place in a broad band. The variation of low frequency photoconductivity versus temperature is attributed to the mobility variation. This variation agrees well with the model of mobility controlled by spin-disorder. The photoconductivity kinetic is interpreted by a three levels recombination model: the conduction band, the 4f levels and a distribution of trap levels. The lack of variation of photoconductivity decay in the range of metal-semiconductor transition is discussed.     

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.     

Ranges and range theories

Irradiation of ionic crystals causes the displacement of lattice ions and the formation of primary defects in the form of vacancies and interstitials. At high temperatures when these defects are mobile secondary defect reactions will produce various types of defect clusters. In some compounds clustering can lead to the formation of small particles of the metal constituent, referred to as colloids. A well-known example of this effect occurs in the alkali halides, where the colloids form as the result of large-scale aggregation of the primary F centres, so that the metallic region in this case derives from primary defects on the anion sublattice. The latent image of the photographic process in silver halides is also an example of the formation of a small metal colloid, and other crystals such as hydrides and azides can also be partially decomposed into metallic particles by irradiation with ionizing radiation. Recently metal colloids have been found as a result of displacement damage in the oxides Li₂ and Al₂O₃. This article reviews some of the background properties of colloids in ionic crystals and describes some examples of colloid formation by irradiation. Colloid growth in NaCl is described in more detail, since recent experimental and theoretical work provides a more complete picture than in other compounds. The Jain-Lidiard theory explains many features of the behaviour observed during high dose irradiation at high temperatures, and some comments are made about ways in which the theory could be developed further.