Shallow energy levels induced by γ rays in standard and oxygenated floating zone silicon

Shallow defect levels in floating zone (FZ) and diffusion oxygenated FZ (DOFZ) silicon, before and after irradiation with a ⁶⁰Co γ-source up to 300 Mrad, have been studied by thermally stimulated currents (TSC) and deep level transient spectroscopy (DLTS) in the temperature range 4.2–110 K. Besides vacancy oxygen (VO) and interstitial-substitutional carbon (C_iC_s) emissions, several TSC peaks have been observed. A trap with an activation energy of 11 meV has been observed at 6 K only in irradiated DOFZ. Two hole traps at 80 meV and 95 meV have been observed both in irradiated FZ and DOFZ, while a trap at 100 meV, related to an interstitial-oxygen (IO₂) complex, has been revealed only in irradiated DOFZ. A TSC peak close to 24 K has been resolved into two components, whose concentrations are independent of irradiation fluence: a trap at 55 meV and a level which remains charged after emission at 80 meV. Our measurements confirm the formation, only in DOFZ, of a radiation induced donor at 230 meV. It appears to be responsible for the improved radiation hardness of oxygenated Si together with the suppression of deep acceptors, since no shallower radiation-induced donors have been detected in DOFZ samples. PACS 71.55.Cn; 29.40.Wk; 61.80.Hg; 61.82.Fk     

Trapping centers and their distribution in Tl2Ga2Se3S layered single crystals

Thermally stimulated current (TSC) measurements with current flowing perpendicular to the layers were carried out on Tl₂Ga₂Se₃S layered single crystals in the temperature range of 10-260 K. The experimental data were analyzed by using different methods, such as curve fitting, initial rise and isothermal decay methods. The analysis revealed that there were three trapping centers with activation energies of 12, 76 and 177 meV. It was concluded that retrapping in these centers was negligible, which was confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. The capture cross section and the concentration of the traps have been also determined. An exponential distribution of electron traps was revealed from the analysis of the TSC data obtained at different light illumination temperatures. This experimental technique provided values of 10 and 88 meV/decade for the traps distribution related to two different trapping centers.     

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu₃Ga₅Se₉ layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46 eV and the temperature range of 15–115 K (PL) and 10–170 K (TSC). An infrared PL band centered at 1.42 eV was revealed at T = 15 K. Radiative transitions from shallow donor level placed at 20 meV to moderately deep acceptor level at 310 meV were suggested to be the reason of the observed PL band. TSC curve of Cu₃Ga₅Se₉ crystal exhibited one broad peak at nearly 88 K. The thermal activation energy of traps was found to be 22 meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.     

Determination of trapping parameters in n-Ga4Se3S by thermally stimulated current measurements

Thermally stimulated current (TSC) measurements were carried out on as-grown n-Ga₄Se₃S layered crystals in the temperature range 10–300 K. The experimental data were analysed by using different heating rates, initial rise, curve fitting and isothermal decay methods. The results of the analysis showed good agreement with each other. The measurements revealed the presence of one trapping level with activation energy of 23 meV. The corresponding capture cross-section and concentration of traps were found to be 2.2 × 10^?23 cm² and 2.9 × 10¹¹ cm^?3, respectively. TSC experiments showed the presence of an exponential distribution of electron-trapping states in Ga₄Se₃S crystal. The variation of one order of magnitude in the trap density for every 34 meV was obtained.     

Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part II: Application to CdGa2S4

Results of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments on CdGa₂S₄ are reported. In the lower temperature region of the TSl curve a decay of luminescence is observed that is probably due to donor-acceptor pair recombination since no accompanying conduction is measured. The TSL and TSC curves of the undoped material further consist essentially of two peaks. If trap depths are calculated with various methods from the literature it turns out that for one peak different values are obtained. These differences cannot be explained with the conventional model consisting of one trapping level and one recombination level. If the results are compared with more complex models it follows that thermal quenching, the presence of a trap distribution, thermally disconnected traps and recombination via excited states, are important. This is most clearly demonstrated in the cases with excess Ga and with Ag or In as dopant. Although this approach probably yields valuable information about CdGa₂S₄, no definite conclusions may be drawn since some essential questions remain unsolved.     

Defect characterization of Ga<Subscript>4</Subscript>Se<Subscript>3</Subscript>S layered single crystals by thermoluminescence

Trapping centres in undoped Ga ₄Se ₃S single crystals grown by Bridgman method were characterized for the first time by thermoluminescence (TL) measurements carried out in the low-temperature range of 15 ?300 K. After illuminating the sample with blue light (～470 nm) at 15 K, TL glow curve exhibited one peak around 74 K when measured with a heating rate of 0.4 K /s. The results of the various analysis methods were in good agreement about the presence of one trapping centre with an activation energy of 27 meV. Analysis of curve fitting method indicated that mixed order of kinetics dominates the trapping process. Heating rate dependence and distribution of the traps associated with the observed TL peak were also studied. The shift of peak maximum temperature from 74 to 113 K with increasing rate from 0.4 to 1.2 K /s was revealed. Distribution of traps was investigated using an experimental technique based on cleaning the centres giving emission at lower temperatures. Activation energies of the levels were observed to be increasing from 27 to 40 meV by rising the stopping temperature from 15 to 36 K.     

On the binding energies and configurations of vacancy and copper–vacancy clusters in bcc Fe–Cu:a computational study

The properties of trapping centres in – as grown – Tl₄GaIn₃S₈ layered single crystals were investigated in the temperature range of 10–300 K using thermoluminescence (TL) measurements. TL curve was analysed to characterize the defects responsible for the observed peaks. Thermal activation energies of the trapping centres were determined using various methods: curve fitting, initial rise and peak shape methods. The results indicated that the peak observed in the low-temperature region composed of many overlapped peaks corresponding to distributed trapping centres in the crystal structure. The apparent thermal energies of the distributed traps were observed to be shifted from ~12 to ~125 meV by increasing the illumination temperature from 10 to 36 K. The analysis revealed that the first-order kinetics (slow retrapping) obeys for deeper level located at 292 meV.     

Determination of trap depth and trap density in Se70Te30−xZnx thin films using thermally stimulated current measurements

In the present paper concentration of traps (N_t) and trap depth (E_t) have been calculated by thermally stimulated current (TSC) measurements in amorphous Se₇₀Te_30−xZn_x (x=2, 4) thin films. These measurements are carried out at three different heating rates. It is observed that the amount of thermally stimulated current gradually increases and the temperature (T_m), at which maxima in TSC occurs, shifts to higher temperatures with increasing heating rates (β) as expected. The trap depth is found to be quite different for x=2 and x=4. The concentration of traps also increases slightly at higher concentration of Zn.     

EPR,thermoluminescence, and thermally stimulated conductivity in BaWO4 crystals

In BaWO₄ crystals electrons and holes trapped at WO₄ complexes are identified by electron paramagnetic resonance (EPR) after X-irradiation at T = 80 K. The thermal decay of the intrinsic hole centres at about 100 K is accompanied by a simultaneous decrease of electron traps (WO₄)^3- and glow maxima of thermoluminescence (TL) and of thermally stimulated conductivity (TSC). This connection is explained by a thermally activated hopping of the $\text{(WO}{}_4\text{)}\text{3-}\text{2}$ hole centres followed by radiative recombination with electron traps (WO₄)^3-. A qualitative kinetic calculation based upon EPR data and the shift between TL and TSC glow peak confirms this model.     

Selective trap filling induced by electron pulse excitation during TSC measurement in PbI2

Thermally stimulated current (TSC) measurements have been performed on samples cut from a PbI₂ single crystal. After light excitation, several peaks have been observed in the range 77-320 K. It is demonstrated that excitation by means of a pulsed electron beam can be used to selectively fill either electron or hole traps. The results on PbI₂ confirm that most of the observed traps can be identified as hole traps.     

Magnetic relaxation spectra of YbPd2Si2

We present the results of inelastic neutron scattering experiments on the intermediate-valent system YbPd₂Si₂ to investigate the magnetic relaxation behaviour. We have performed measurements on polycrystalline samples with neutrons of incident energyE ₀=3.1 meV at temperatures between 1.5 K and 250 K, and withE ₀=12.7 meV andE ₀=50.8 meV at temperatures between 5 K and 50 K using time-of-flight spectrometers. At temperaturesT>50 K we find a pure quasielastic magnetic response with a rather broad linewidth typical for intermediate-valent systems. AtT50 K an inelastic excitation line appears at about 21 meV; its intensity increases with decreasing temperature. In the same temperature range (T<50 K) the quasielastic linewidth decreases rapidly and atT=5 K the quasielastic response has been apparently transformed to a second inelastic feature at about 4.7 meV. The width of this low-energy excitation fits well to the temperature dependence of the quasielastic linewidth forT>5 K.     

TSC study of deuteration of polystyrene

A comparative study of natural and deuterated polysterene has been performed by thermally stimulated currents (TSC). Around the glass-rubber transition temperature (T_g a TSC peak is observed whose relaxation times obey an Arrhenius equation. At 50° above T_g another TSC peak is observed whose relaxation time obeys a Vogel equation. Deuteration is found to increase the values of T_g.     

Thermally stimulated current measurements on vapour-grown HgI2

Two ways of performing the Thermally Stimulated Current (TSC) experiment have been used to investigate vapour grown HgI₂:i) under light excitation, four peaks are observed at 104, 158, 210, 240 K; ii) under bias excitation, two types of spectra have been obtained with peaks around 160, 240K (type I) and 182, 280 K (type II), respectively. The activation energies have been estimated to be 0.38 eV for the peak showing up at 182 K, and 1.05 eV for the one at 280 K.     

Photoconductive properties of HgGa2S4

Mercury thiogallate, HgGa₂S₄ is a defect chalcopyrite semiconductor with the space group S₄² which offers a combination of attractive properties for applications.In order to obtain information about the electron states in the energy gap, photoconductivity measurements are performed in the 80–300 K range. Photoconductivity spectra show two peaks related to intrinsic and extrinsic excitation at about 410 and 500 nm, respectively; these maxima show a temperature dependence similar to the linear coefficient of the energy gap. Thermally stimulated currents have been studied by exciting the samples with intrinsic light at different temperatures. For all excitation temperatures a single TSC peaks were obtained. The analysis of TSC curves allowed one to estimate the kinetics of the trap emptying, trap energy distribution and thermal activation energy.A model for the level distribution in the semiconductor energy gap is suggested which in good agreement with the results of a previous photoluminescence study.     

Growth and absorption properties of Dy-doped and undoped p-type TlGaSe2

2 and Dy-doped p-TlGaSe₂ (p-TlGaSe₂:Dy)single crystals were grown by the Bridgman–Stockbarger method. Absorption spectra were measured on freshly cleaved (001) surfaces. The freshly cleaved crystals had a mirror-like surface and there was no need for mechanical treatment. The absorption measurements were carried out in p-TlGaSe₂ and p-TlGaSe₂:Dy samples in the temperature range 10–320 K with a step of 10 K. The phonon energies calculated in p-TlGaSe₂ and p-TlGaSe₂:Dy are 23.0 meV and 21.0 meV, respectively. The direct band gaps of p-TlGaSe₂ and p-TlGaSe₂:Dy are 2.279 eV and 2.294 eV at 10 K, respectively. There is an abrupt change for the energy peak for p-TlGaSe₂ in the temperature ranges 105–120 K, 240–250 K, and for p-TlGaSe₂:Dy in the temperature ranges 100–110 K, 240–260 K. Received: 3 December 1997 / Accepted: 5 October 1998     

Dissociative electron attachment to CO2

The 4.4 eV dissociative electron attachment peak in CO₂ was reinvestigated paying particular attention to (i) its structure associated with vibrational excitation and to (ii) the temperature dependence of the onset. For this purpose we have used two specially designed crossed beams machines, one having a trochoidal electron analyzer (TEM) and one with a hemispherical electron analyzer (HEM) for the production of the highly monochromatized electron beams (with FWHM’s down to 5 and 50 meV, respectively). The present results confirm the earlier findings in (i) interpreting the dominant structures of the 4.4 eV peak as being due to vibrationally excited states of CO in the reaction CO₂+e → O⁻+CO and (ii) assigning the much weaker and narrower structures to the intermediate CO ₂ ⁻ . In the temperature range between 300 and 245 K almost no temperature dependence of the onset can be seen in the present study. In comparison to NO and CO where the onset is vertical the CO₂ threshold behavior is less steep indicating that in the CO₂ case a different type of transition must be responsible for the onset of the O⁻ production. Besides DA to CO₂ we have for comparison and calibration purposes investigated also DA to CO and NO. Dedicated to Prof. Jan Janča on the occasion of his 60th birthday. Work was carried out within the Association EURATOM-OEAW and partially supported by the FWF, OENB, and BMWV, Wien, Austria.     

Thermally stimulated current measurements on vapour-grown HgI2

Two ways of performing the Thermally Stimulated Current (TSC) experiment have been used to investigate vapour grown HgI₂:i) under light excitation, four peaks are observed at 104, 158, 210, 240 K; ii) under bias excitation, two types of spectra have been obtained with peaks around 160, 240K (type I) and 182, 280 K (type II), respectively. The activation energies have been estimated to be 0.38 eV for the peak showing up at 182 K, and 1.05 eV for the one at 280 K.     

Electret properties of ethylene–propylene random co-polymer

Thermally stimulated current (TSC) spectra were examined for ethylene–propylene (EP) random co-polymer at different charging voltages V_p with positive and negative polarities. Observed TSC spectra showed two well-separated TSC bands, B_L and B_H, which respectively appeared in the temperature regions below and above 100 °C. Observed V_p dependence of B_L was quite different from that of typical polypropylene homo-polymer: As V_p increased, B_L band grew keeping its peak position same at 65 °C, and the band shape unchanged, as if the traps responsible for the B_L band are a single set of traps with the same trap depth and capture cross section. The trap depth of B_L was about 1.9 eV and 1.7 eV for positively charged EP and talc-containing EP samples, respectively. EP samples also showed unique TSC bands above 100 °C: one is a narrow TSC band peaked at 120 °C and the other is an unusual TSC band which was non-vanishing even at 165 °C just before destruction of samples by their melting. Consequently, the utmost stable charge density in EP co-polymer above 100 °C was found to be 3.5 × 10^?4 C/m² and 6.0 × 10 ^?4 C/m² for positively and negatively charged samples, respectively. These equivalent surface charge densities are much larger than those of usual polypropylene homo-polymer.     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (I)

Lattice dynamical calculations of ice VIII have been carried out by using a slightly modified set of force constants obtained recently for ice Ih (Li J C and Ross D K 1993 Nature 365 327). A weak interaction was introduced between the two interpenetrated sublattices in the ice VIII structure. The calculated results for H₂O and D₂O ice VIII are in reasonable agreement with the measured inelastic neutron scattering spectra. The eigenvectors of phonon modes in the range of translational and librational bands have been studied in order to understand the properties of the vibrational modes. It is found that the third peak at 26.7meV in the translation results from weak hydrogen bond interactions, and the first peak (14.7meV) is much higher than it is in ice Ih (～7.1meV), which is partially due to the interactions between the two sublattices.