Thermal Activation Measurements Research Articles

Thermal activation, cathodo- and photoluminescence measurements of rare earth doped (Tm,Tb,Dy,Eu,Sm,Yb) amorphous/nanocrystalline AlN thin films prepared by reactive rf-sputtering

Abstract We present comprehensive cathodo- and photoluminescence measurements from thin amorphous/nanocrystalline (a/nc-) AlN films doped with rare earths. The (a/nc-) AlN films were prepared by reactive rf-sputtering using a high purity (5 N) aluminium disk in high purity nitrogen atmosphere (5 N, pressure ca. 1.3 mbar). The rare earth doping (Tm, Tb, Dy, Eu, Sm, Yb, concentrations were below 1%) was performed by placing respective rare earth metal pieces of appropriate size onto the aluminium disk. The rare earth ion emissions cover the blue (Tm), green (Tb), yellow (Dy), red (Sm, Eu) and infrared (Yb). The optical and related structural properties of the films are correlated by means of high resolution transmission electron microscopy in combination with cathodoluminescence measurements in a scanning electron microscope. In addition, the corresponding compositions are determined by energy-dispersive X-ray analysis. The cathodo- and photoluminescence spectra of the rare earth 3+ ions are recorded in the visible at 300 K in the as-grown condition (Tm, Tb, Dy, Sm, Eu, Yb) and after annealing treatments in the temperature range from 300 to 1100 °C by steps of 150 °C (Tb, Sm, Eu, Yb). The anneal-related changes in the cathodo- and photoluminescence emission spectra and in the microstructure of the films are addressed. The AlN films show nanocrystalline structure almost independent of the annealing treatment. Optimal annealing temperature for emissions of Sm and Eu doped (a/nc-) AlN were derived to be 900 °C whereas Tb 3+ and Yb 3+ emissions increase at least up to 1100 °C.

The impact of possible modifications to the DS86 dosimetry on neutron risk andrelative biological effectiveness

The current DS86 dosimetry system for the Japanese bomb survivors indicatesthat neutron doses were so low that they prevent the direct derivation of anyuseful estimates of neutron risk. However, the large body of thermal neutronactivation measurements carried out over many years in Hiroshima and Nagasakiappear to indicate that current DS86 neutron doses may have been significantlyunderestimated in Hiroshima. An earlier companion paper has provided an updateof neutron activation measurements. While a large body of data appears tosupport a significant increase, there is ongoing debate and review regarding itsvalidity. However, as yet, there are no detailed, peer-reviewed, publishedrefutations of the neutron activation data which appear to support an increase inneutron doses. In this paper, we consider the impact of possible futurerevisions in the DS86 dosimetry on radiation risk estimates. We considerthe extreme range of possibilities from maintaining the existing DS86values, to changes in neutron doses in accord with the majority of existingneutron activation data. We have used the latest cancer incidence dataand cancer mortality data for the A-bomb survivors, and neutron doseshave been modified using a neutron revision factor (NRF) in line withthe latest thermal neutron activation measurements in Hiroshima. Incontrast to previous analyses, a nonlinear relationship between log(NRF)and slant range has been used which better represents the data beyondslant ranges of ∼1 km. The impact on the evaluation of neutron relativebiological effectiveness (RBE) and gamma radiation risk estimates has beenassessed. While DS86 neutron doses are too low to allow any useful directevaluation of neutron risk or neutron RBE, it becomes possible to derive moremeaningful values if neutron doses are increased in Hiroshima in linewith the broad range of thermal neutron activation measurements. Theuncertainties are smallest for the cancer incidence data. The best estimates ofneutron RBE give upper 95% confidence limits of about 6 for all solidtumours for the incidence data and about 28 for the mortality data. Theuncertainties in neutron RBE for leukaemia incidence are larger, andestimation at doses below about 0.1 Gy is not possible. There is no significantchange in the excess relative risk for gamma radiation for all solid tumourstaken together, compared with the current DS86 dosimetry. The resultspreclude neutron RBE values significantly greater than the current ICRPradiation weighting factors, which range between 5 and 20, depending onenergy. Whether or not the Japanese bomb survivors can indeed form thebasis for useful, directly determined neutron risks clearly depends on theveracity of existing neutron activation data. This is currently the subjectof careful international scrutiny and the outcome is eagerly awaited.

Properties of carbon as an acceptor in cubic GaN

Cubic gallium nitride epilayers were successfully doped with carbon using an e-beam evaporation source. Room temperature Hall measurements revealed hole concentrations up to 6×10 17 cm −3 and hole mobilities of 200 cm 2/V s. Low temperature (2 K) photoluminescence showed a donor–acceptor transition at 3.08 eV, which could clearly be assigned to the incorporation of a carbon acceptor. Thermal activation measurements were performed. They exhibited an activation energy of E A=215 meV both in photoluminescence and electrical measurements, respectively.

Rectifying properties of solid C 60/n-GaN, C 70/n-GaN and C 70/p-GaN heterojunctions

Current-voltage measurements show that C 60/n-GaN, C 70/n-GaN and C 70/p-GaN contacts have very good rectification characteristics, however, the polarities of the forward biases for C 60/n-GaN (C 70/n-GaN) and C 70/p-GaN are opposite to each other. By fitting forward current-voltage data, the relations of both the series resistance and ideality factor vs forward bias for C 60/n-GaN and C 70/n-GaN have been obtained. Thermal activation measurements at a fixed forward bias reveal exponential relations of currents vs the reciprocal of temperature. The effective barrier heights for C 60/n-GaN and C 70/n-GaN are determined to be 0.535 and 0.431 eV, respectively.

Electronic transport properties of nitrogen doped amorphous carbon films deposited by the filtered cathodic vacuum arc technique

Highly tetrahedral amorphous carbon thin films were deposited by the filtered cathodic vacuum arc technique at room temperature. Nitrogen was found to be a good n-type dopant of the tetrahedral amorphous carbon thin films. The Fermi level shifts from 0.91 eV above the valence band to 0.65 eV below the conduction band with increasing nitrogen flow rate from null to 16 sccm (nitrogen partial pressure from 0 to ). At the same time, the optical band gap drops from 2.7 to 1.8 eV. Three electronic transport mechanisms, namely, transport in extended states, in band tails by hopping and variable range hopping (VRH) near the Fermi level, were observed from the thermal activation measurements in the temperature range from 100 to 450 K. The VRH transport parameters for ta-C films are studied, and the density of states near the Fermi level extracted from the hopping transport parameters was found in the range of . The dominant doping configuration is the substitution in the coordination at low N concentration and adoption of bonding at high N concentration.

Heterojunctions of solid C70 and crystalline silicon: Rectifying properties and barrier heights

Heterojunctions of solid C70 and n- or p-type crystalline Si have been made. Current–voltage measurements show that both C70/n-Si and C70/p-Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n-Si and 0.27 eV for C70/p-Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high-frequency capacitance–voltage characteristics for Ti/C70/p-Si structures.

The DS86 neutron dosimetry enigma: An analysis of some missing pieces to the puzzle

Atom bomb survivors from Hiroshima and Nagasaki form, in principle, the most comprehensive database for the effects of radiation on man. The basic parameters required to define this database are the gamma-ray and neutron exposure doses experienced by each survivor. The importance of quantifying this database has long been recognized throughout the world. International programs conducted over the last four decades attest to the significance of this goal. Unfortunately, the quest for accurate gamma-ray and neutron exposure doses at the Hiroshima and Nagasaki sites has proven illusive. Efforts in the most recent of these programs, designated as Dosimetry System 1986 (DS86), have revealed a serious discrepancy between neutron transport calculations and thermal neutron activation measurements at the Hiroshima site. The magnitude of this discrepancy is especially difficult to explain in view of the extensive experimental and calculational efforts that have been expended to quantify this database. This serious and persistent discrepancy will be called the DS86 neutron dosimetry enigma. It is established that the DS86 neutron dosimetry enigma is a complex puzzle that precludes simple solutions. This conclusion is deduced through the identification and detailed description of a number of missing pieces of the puzzle. Each missing puzzle piece introduces distinct effects that must be addressed properly before there is any chance of resolving this enigma. While these missing puzzle pieces cannot resolve the DS86 enigma, they can be combined in different ways to provide insight into different aspects of the puzzle. Implications and conclusions that can be inferred from these missing puzzle pieces are presented and analyzed.

Pinning Action of a Thin Ga Interfacial Layer in an Sb/Ga/GaAs Schottky Barrier Structure Grown by MBE

AbstractThe pinning action on the barrier height of a Ga interfacial layer in an Sb/Ga/GaAs structure prepared by molecular beam epitaxy has been tracked as a function of Ga layer thicknesses for both n and p-GaAs (100). The barrier height ÆBn for n-type GaAs (ND=3×1015cm−3), determined by thermal activation measurements and capacitance measurements, changes from 0.8 to 1.0 eV as the Ga layer is increased from zero to three monolayers. The barrier height ÆBp for p-type GaAs decreases in a converse fashion so that the sum ÆBn + ÆBp is equal to the GaAs energy gap. Up to a Ga layer thickness of about one monolayer the barrier height changes are about linearly proportional to thickness.The action seen is compatible with barrier height changes that are seen when Sb/GaAs junctions are prepared with the original surface alternatively As or Ga rich and subjected to thermal annealing at temperatures up to 250°C.

Characterization of oxide-supported ligand-protected osmium cluster catalysts

Os 6(CO) 18 has been impregnated on to silica and on to alumina from non-aqueous solution; the freshly impregnated materials acquire catalytic activity after thermal activation. Ultraviolet/visible diffuse reflectance spectroscopy and extended X-ray absorption fine-structure spectroscopy (EXAFS) have confirmed the presence of metal clusters in both the freshly impregnated materials and in the thermally activated materials. Confirmation has been obtained by examination of infrared absorption spectra in the carbonyl stretching region. The number and nature of the protecting ligands and the concentration of adsorption sites possessed by the catalytically active clusters have been evaluated by (i) the analysis of gases evolved during thermal activation and (ii) conventional adsorption measurements. Structures for the catalytically active clusters await further interpretation of the EXAFS spectra; this work is in progress.

Linear Models and the Kinetics of Thermally Activated Currents and Voltages

Experimental results from thermally activated measurements can be analyzed by different methods. In view of the difficulty of obtaining precise solutions of the differential equations describing the transport and trapping of charge carriers in solid dielectrics, various linearized theories have been developed. In the present paper the isothermal theory of conventional models is generalized and a nonisothermal model theory is established. Current transients generated by heating in short‐circuit and voltage‐transients generated by heating in open‐circuit are discussed. It is found that under nonisothermal conditions the concept of equivalency of models must be severely restricted and that the assumptions of charge invariance and absence of current reversals, implicit in many existing theories, cannot be taken for granted. One concludes that the interpretation of thermal activation measurements without the knowledge of the structure of the system under consideration and the underlying physical mechanism leads to ambiguities.

Electroabsorption by Substitutional Copper Impurities in GaAs

Previously, Hall effect and other thermal activation measurements have suggested that a copper atom substituted on a gallium site in GaAs is a double acceptor with its first ionization level at 0.145 eV and its second between 0.44 and 0.49 eV above the valence band. Also, previous photoluminescence measurements have revealed broad lines attributable to copper at the Stokes-shifted positions 0.155 eV and between 0.49 and 0.53 eV. Using an electroabsorption technique, we have studied the spectrum of substitutional copper in GaAs. We find an absorption line associated with a level at 0.463 eV above the valence band for temperatures of 103° and 300°K. We conclude from the selection rules that the wavefunction associated with this level transforms as the Γ6 representation of the double group Td. Except for a weaker additional absorption line (which we do not yet understand) occurring at 0.454 eV in a single p-type sample, we were able to find no other lines. These results supplement earlier work on the 0.145 eV state. We propose a simple model which we feel describes the substitutional copper impurity and apply it to our measurements on the 0.463 eV state and our earlier work on the 0.145 eV state. While this model is not adequate for a first-principles calculation, it does provide a framework in which to understand and discuss our results.