Chemical effects on the L-shell X-ray fluorescence parameters of Ta and W compounds

The L shell X-ray production cross-sections, the intensity ratios_, and the average fluorescence yields of pure Ta, W and their compounds have been analysed. The targets were irradiated with γ-photons at 59.5 keV from a ²⁴¹Am annular source and detected using an Ultra-LEGe detector with resolution of 150 eV at 5.9 keV. The experimental results were compared with the other theoretical and experimental results. The obtained results were interpreted according to the chemical effect and influence of removing electrons from the shells above the L shell.     

Schottky-type polycrystalline CdZnTe X-ray detectors

The polycrystalline CdZnTe:Cl thick films which have high resistivity about 5 × 10⁹ Ω cm are grown by thermal evaporation method. The leakage currents of as-deposited CdZnTe layers are still too high to operate as medical applications. The blocking layer of Schottky type was formed on the stoichiometric surface of polycrystalline CdZnTe layers to suppress the leakage current of polycrystalline CdZnTe X-ray detectors. The polycrystalline CdZnTe Schottky barrier diodes with indium contact exhibit the low leakage current (14 nA/cm²) at 40 V due to its high barrier height (ϕ_b = 0.80 eV). In X-ray image acquisition with Schottky-type linear array polycrystalline CdZnTe X-ray detector, we have obtained the promising results and proved the possibility of polycrystalline CdZnTe for applications as a flat panel X-ray detector.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Effect of temperature on the current–voltage characteristics of GaAs/AlGaAs quantum cascade photodetectors

Transport of electrons within a quantum cascade photodetector structure takes place with the help of the scattering of electrons by phonons. By calculating scattering rates of the electrons mediated by longitudinal optical phonons (the dominant scattering mechanism), current–voltage characteristic of a quantum cascade photodetector is calculated. The results indicate that with the increase of bias voltage dark current increases rapidly, then the increase becomes slow at higher voltages, whilst photocurrent remains approximately constant with only slight variations in its magnitude. With the increase of temperature from 80 K to 160 K dark current increases by about two orders of magnitude while photocurrent varies slightly, so that at the illuminating power of 1 mW/m² photocurrent density increases in mean from 1.10×10⁻⁹ A/cm² at 80 K to 1.14×10⁻⁹ A/cm² at 160 K and then decreases to 1.03×10⁻⁹ A/cm² at 240 K. Thus the responsivity of the detector varies only slightly with temperature. However owing to the decrease in the resistivity of the photodetector with the increase of temperature, Johnson noise limited detectivity decreases considerably.     

A low noise 2 × 16 Ge:Sb focal-plane array: Paving the way for large format FPAs for far IR astronomy

Future astronomical instruments call for large format and high sensitivity far infrared focal-plane arrays to meet their science objectives. Arrays as large as 128 × 128 with sensitivities equal to or better than 10⁻¹⁸ W/√Hz are set as targets for the far IR instruments to be developed within the next 10 years. These seemingly modest goals present a not-so-modest quantum leap for far IR detector technology whose progress is hampered by a number of complexities; chief among them the development of low noise readouts operating at deep cryogenic temperatures and a viable hybridization scheme suitable for far IR detectors. In an effort to incrementally develop large-format photoconductor arrays, we have fabricated a 2 × 16 Ge:Sb array using the SBRC190 readout – a cryogenic 1 × 32 CTIA readout multiplexer initially developed for SOFIA’s AIRES instrument. In this paper we report the results of the extensive parametric tests performed on this array showing an impressive noise performance of 2.2 × 10⁻¹⁸ W/√Hz and a DQE of 0.41 despite some design limitations. With such an encouraging performance, this prototype array will serve as a platform for our future developmental effort.     

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast ³He and ⁴He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.     

Band engineered HOT midwave infrared detectors based on type-II InAs/GaSb strained layer superlattices

We report on heterostructure bandgap engineered midwave infrared photodetectors based on type-II InAs/GaSb strained layer superlattices with high operating temperatures. Bandgap and bandoffset tunability of antimonide based systems have been used to realize photodiodes and photoconductors. A unipolar barrier photodiode, pBiBn, and an interband cascade photovoltaic detector have been demonstrated with a 100% cutoff wavelength of 5 μm at 77 K. The pBiBn detector demonstrated operation up to room temperature and the cascade detector up to 420 K. A dark current density of 1.6 × 10⁻⁷ A/cm² and 3.6 × 10⁻⁷ A/cm⁻² was measured for the pBiBn and interband cascade detector, respectively, at 80 K. A responsivity of 1.3 A/W and 0.17 A/W was observed at −30 mV and −5 mV of applied bias for pBiBn and cascade detector, respectively, at 77 K. The experimental results have been explained by correlating them with the operation of the devices.     

Barrier height imaging of magnetic films: Use for studying the initial growth of Co films and the surface structure of FePt films

Barrier-height (BH) imaging using scanning tunneling microscopy (STM) was used to study the growth of Co films on Au(001) surfaces. We have observed BH of metastable bcc Co film (> 1 ML) for the first time, and that showed a large BH value (~ 6 eV), whereas the observed BH of the Au(001) surface (~ 3.5 eV) was consistent with the previous results. The origin of the large BH was qualitatively understood by considering that 3d electrons for tunneling are dominant for the Co(001) surface. We have observed numerous islands with different sizes and heights after 0.15 ML Co coverage and successfully obtained, from the BH imaging, an element-specific contrast, i.e. recognizing aggregated Au islands and Co islands, and information about inhomogenities of BH with proper consideration of the artifacts near the step edges. The height modification by the large BH difference is discussed.STM/BH studies of FePt films revealed two kinds of monolayer heights, the sum which was equal to the c axis lattice constant of L1₀ FePt. Two different dI/dz signal levels were observed on atomically flat terraces.     

InGaAsP/InP photodetectors targeting on 1.06 μm wavelength detection

InP-based InGaAsP photodetectors targeting on 1.06 μm wavelength detection have been grown by gas source molecular beam epitaxy and demonstrated. For the detector with 200 μm mesa diameter, the dark current at 10 mV reverse bias and R₀A are 8.89 pA (2.2 × 10⁻⁸ A/cm²) and 3.9 × 10⁵ Ω cm² at room temperature. The responsivity and detectivity of the InGaAsP detector are 0.30 A/W and 1.45 × 10¹² cm Hz^1/2 W⁻¹ at 1.06 μm wavelength. Comparing to the reference In_0.53Ga_0.47As detector, the dark current of this InGaAsP detector is about 570 times lower and the detectivity is more than ten times higher, which agrees well with the theoretical estimation.     

Franck–Hertz effect in cathodo- and photoluminescence of wide-gap materials

A brief overview of previously obtained and novel data on the manifestations of an analogue of Franck–Hertz effect in photo- and cathodoluminescence of wide-gap inorganic materials is presented. On the example of NaCl:Tl⁺ and MgO:Cr³⁺ single crystals, the excitation processes of the luminescence of 6s² Tl⁺ ions and 3d³ Cr³⁺ ions by 5–15 keV electrons or 5–20 eV photons at 6–420 K have been studied. The rapid processes of the direct energy transfer to Tl⁺ by hot conduction electrons or to Cr³⁺ centers by hot electrons and/or hot valence holes have been separated from rapid excitonic and more inertial electron–hole processes.     

Mid-wave QWIPs for the [3–4.2 μm] atmospheric window

We report two approaches using Quantum Well Infrared Photodetectors for detection in the [3–4.2 μm] atmospheric window. Taking advantage of the large band gap discontinuity we demonstrated a strained AlInAs/InGaAs heterostructure on InP. The optical coupling in this structure has been experimentally and numerically investigated. The results show that the coupling is mainly due to guided modes. The second approach is based on double barrier strained AlGaAs/AlAs/GaAs/InGaAs active layers on GaAs. The segregation of the elements III in these structures has been investigated using a transmission electron microscope. The results show a strong modification of the conduction band profile. We demonstrate peak wavelengths at 3.9 μm for the InP based detector and 4.0 μm for the GaAs based detector. We report a background limited peak detectivity (2π field of view, 300 K background) at 4.0 μm of about 2 × 10¹¹ cm Hz^1/2 W^?1 at 77 K, and 1.5 × 10¹¹ cm Hz^1/2 W^?1 at 100 K.     

The unusual variations of photoluminescence and afterglow properties in monoclinic ZrO2 by annealing

The raw ZrO₂ is annealed at 600–1550 °C for 6 h. It is found that the emission at 492 nm increases greatly when the annealing temperature is higher than 1200 °C and its afterglow shows a small improvement at 1200–1450 °C and a large enhancement after annealing at 1550 °C. The results that are obtained indicate that the impurity Ti⁴⁺ in ZrO₂ is efficiently reduced to Ti³⁺ when the temperature is higher than 1200 °C, and the increase of Ti³⁺ centers contributes to the large improvement of emission at 492 nm. The thermoluminescence shows that at least two types of traps with different depths (0.65 eV and 1.46 eV) corresponding to oxygen vacancies exist in monoclinic ZrO₂. After annealing at 1200–1450 °C, some new trap clusters related to oxygen vacancies and Ti³⁺ form and causes the small improvement of afterglow at 1200–1450 °C. The large improvement of afterglow after annealing at 1550 °C originates from the sharp increase of proper shallow traps (0.65 eV) in ZrO₂. Accordingly, we present the feasible interpretations and luminescence mechanisms of monoclinic ZrO₂ for our observations.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

Photocapacitance study of GaSb: In,As for defect analysis in InAs/GaSb type-II strained layer superlattices

Steady-state photocapacitance measurements were used to characterize GaSb incorporated with In, As, and a control sample. Evidence of a trap level at 0.55 eV was observed for all samples. The change in the capacitance for the sample with indium was about half the change for the other samples, indicating that the addition of indium modified the near-mid-gap trap levels. Another change in capacitance starting at 0.71 eV was attributed to electrons from the valence band filling levels close to the conduction band.     

Hole polaron of small radius in assemblies of hydrated mono-protonated meso-tetraphenylporphine dimers at 77 K

Polaron theory is often used for the study of electrons and holes mobility in semiconductors when longitudinal optical (LO) phonons are generated upon the charge carriers moving. The polaron theory was applied to explain long-wavelength absorptions observed nearby Soret band in the electronic spectra of assemblies of mono-protonated meso-tetraphenylporphine dimer (TPP₂H⁺) that are interpreted as LO-phonons originated due to proton movement. The energy of hole polaron is found to be 1.50 eV at 77 K. Energy of Franck–Condon transitions of LO-phonons generated by hole polaron moving through water confined in the assemblies with distortions of O–H bonds is 0.2653 eV (2138 cm⁻¹). A broad band around 2127 cm⁻¹ corresponding the same energy of O–H bonds vibrations is observed in IR spectra of the assemblies consisting of water and mainly of TPP₂H⁺ species in the solid state indicating the presence of similar distortions of the hydrogen bonds. The radius of protonic sphere of 0.202 Å, which was estimated as a polaron quasiparticle moving through the confined water at 77 K, is found in agreement with earlier evaluated one of 0.265 Å that was obtained for proton diffusion at 298 K in similar assemblies.     

Optical properties of layered III–VI semiconductor γ-InSe:M (M=Mn,Fe, Co,Ni)

Indium selenide belongs to layered III–VI semiconductors with highly anisotropic optical and electronic properties. Energy gap of 1.32 eV makes this material very attractive for solar energy conversion. We investigated the influence of 1% 3-d transition metals M=Mn, Fe, Co, Ni, used as dopants, on energy levels of InSe:M in the range 1.4–6.5 eV and especially in the range of energy gap <1.4 eV by means of ellipsometric measurements. It was concluded that at ambient temperature foregoing dopants, all divalent, with 4s² valent electrons, in the similar way influenced on blue-shift of energy levels in valent zone, but did not influence on the fundamental energy gap. Photoluminescence measurements confirmed blue-shift of the valent zone energy levels and an existence of deep impurity levels.     

Energy relaxation through the π*-state in C,F and O K-shell excited CF3COCH3

Total photoabsorption spectra of CF₃COCH₃ were measured in the C, F and O K-shell regions and the peak assignments were tentatively given. The K-shell electrons of C, F and O atoms were selectively excited into the π^* orbital. The kinetic energy (KE) distribution of CF₃⁺ formed through the π^* states gave the maxima at KE = 0 and 0.43 eV. The yield of CF₃⁺ with KE = 0 eV increased from 10 to 50% by changing the excitation sites from F 1s to O 1s. This finding was reasonably understood by considering that intramolecular energy flows from the initially excited K-shell electron to vibrational modes of CF₃ group. The KE distribution of CH₃⁺ showed a mirror image of that for CF₃⁺.     

Optical properties of Cr/Fe(t)/MgO (0 0 1) thin films

In this work we report on the optical properties of single-crystalline iron thin films. For this, Cr-capped Fe films with thickness, t, in the range 30–300 Å were prepared on MgO (0 0 1) by DC magnetron sputtering, and then studied by optical absorption technique within the range from 1.0 to 3.6 eV. All measurements were carried out at room temperature using a fiber optics spectrophotometer. The intensity of the transmitted light decreases with increasing film thickness. The optical constants of the films are deduced from a model that considers the transmission of light by two absorbing films on an absorbing substrate. The absorption coefficient of the Fe films is also calculated from the transmission data. The absorption spectra show the following characteristics: (i) two large absorption peaks centered at about 1.20 and 2.65 eV; and (ii) a sharp step near 1.40 eV. These structures are associated with conventional interband transitions of the iron film.     

A wide-range Parallel Radial Mirror Analyzer for scanning electron/ion microscopes

This paper presents the design of a wide-range Parallel Radial Mirror Analyzer (RMA) for use as an attachment inside the specimen chambers of scanning electron/ion microscopes. The range of energies for the PRMA typically varies by a factor of 50, and it is predicted to have second-order focusing properties for all electrons/ions that are detected. For a polar angular spread of ±3°, the simulated energy resolution at an energy of 100 eV is around 0.65%, and it drops to less than 0.2% for energies between 300 eV and 5000 eV. The PRMA is predicted to have a transmittance of over an order magnitude better than previous wide-range parallel energy analyzer designs.     

Measurement of the sputter yield after mild ion erosion of a pristine Cu(001) surface

Using the STM technique we have determined the sputter yield on a pristine Cu(001) surface after mild (fluence less than 0.044 ions per surface atom) bombardment of the pristine surface with 800 eV Ar⁺ions at normal incidence. The experiments have been performed at substrate temperatures ranging from 200 to 350 K. Making use of the positional correlation of adatoms and surface vacancies, at 200 K and 325 K, we concluded that about 1/3 of the surface adatoms originate from interstitials arriving at the surface and they give a direct indication of the buried bulk vacancies. A careful analysis of the different areas for surface vacancies and adatom then allowed a quantitative evaluation of the sputter yield at 1.2 Cu atoms per 800 eV Ar⁺ ion.