Study of band offsets in InN/Ge heterojunctions

InN layers were directly grown on Ge substrate by plasma-assisted molecular beam epitaxy (PAMBE). The valence band offset (VBO) of wurtzite InN/Ge heterojunction is determined by X-ray photoemission spectroscopy (XPS). The valence band of Ge is found to be 0.18 ± 0.04 eV above that of InN and a type-II heterojunction with a conduction band offset (CBO) of ~ 0.16 eV is found. The accurate determination of the VBO and CBO is important for the design of InN/Ge based electronic devices.     

Ultraviolet spectroscopy of Pr+3 and its use in making ultraviolet filters

Sputtered deposited thin films of AlN:Pr and GaN:Pr emit in ultraviolet–visible and visible regions of the spectrum, respectively, under electron excitation in cathodoluminescence apparatus. The goal is to study the ultraviolet emission from Pr⁺³ when doped in nitride semiconductor hosts. Luminescence peaks at a wavelength of 295 nm (4.2 eV), 335 nm (3.7 eV) and 385 nm (3.24 eV) are observed as a result from ¹S₀ → ¹G₄, ¹S₀ → ¹D₂ and ¹S₀ → ¹I₆ transitions, respectively. However the ¹S₀ → ¹G₄ and ¹S₀ → ¹D₂ transitions are not observed when Pr⁺³ is doped in GaN host. The bandgap of GaN absorbs the ultraviolet radiation emitted from Pr⁺³ and hence GaN can be used as ultraviolet filter for radiation shielding and protection purposes. AlN is transparent to ultraviolet due to its wide bandgap of 6.2 eV.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

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.     

Influence of growth temperature and deposition duration on the structure,surface morphology and optical properties of InN/YSZ (1 0 0)

InN films with the wurtzite structure have been grown directly on YSZ (1 0 0) substrate by the RF-magnetron sputtering technique. Strongly (0 0 2) oriented films with smooth surfaces (0.7–2.9 nm surface roughness depending on substrate temperature), were grown within 30 min. Films deposited for 60 min developed three-dimensional (3D) pyramidal islands on top of their surfaces, which diminished the residual elastic strain. The optical absorption edge and PL peak energy around 1.7 eV were found to redshift with increasing film thickness and substrate temperature.     

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.     

Optical and photoelectric properties of TlInS2 layered single crystals

Single crystals of the layered compound TlInS₂ were grown by direct synthesis of their constituents. The spectral and optical parameters have been determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. Absorption spectra of thin layers of TlInS₂ crystals are used to study the energy gap and the interband transitions of the compound in the energy region 2–2.4 eV. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transmitted region. The direct and indirect band gaps were determined to be 2.34 and 2.258 eV, respectively. Photoconductivity measurements at room temperature resolve the structure that can be identified with the optical transition.     

The role of torsional modes in the electronic absorption spectrum of acetone

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8 eV at a resolution of ～4 meV at 4 eV and ～10 meV at 10 eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708 eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper.     

Physical reasons of emission transformation in infrared CdSeTe/ZnS quantum dots at bioconjugation

The core/shell CdSeTe/ZnS quantum dots (QDs) with emission at 780–800 nm (1.55–1.60 eV) have been studied by means of photoluminescence (PL) and Raman scattering methods in the nonconjugated state and after conjugation to different antibodies (Ab): (i) mouse monoclonal [8C9] human papilloma virus Ab, anti-HPV 16-E7 Ab, (ii) mouse monoclonal [C1P5] human papilloma virus HPV16 E6+HPV18 E6 Ab, and (iii) pseudo rabies virus (PRV) Ab. The transformations of PL and Raman scattering spectra of QDs, stimulated by conjugated antibodies, have been revealed and discussed.The energy band diagram of core/shell CdSeTe/ZnS QDs has been designed that helps to analyze the PL spectra and their transformations at the bioconjugation. It is shown that the core in CdSeTe/ZnS QDs is complex and including the type II quantum well. The last fact permits to explain the nature of infrared (IR) optical transitions (1.55–1.60 eV) and the high energy PL band (1.88–1.94 eV) in the nonconjugated and bioconjugated QDs. A set of physical reasons has been analyzed with the aim to explain the transformation of PL spectra in bioconjugated QDs. Finally it is shown that two factors are responsible for the PL spectrum transformation at bioconjugation to charged antibodies: (i) the change of energy band profile in QDs and (ii) the shift of QD energy levels in the strong quantum confinement case. The effect of PL spectrum transformation is useful for the study of QD bioconjugation to specific antibodies and can be a powerful technique for early medical diagnostics.     

Spectroscopic characterizations of individual single-crystalline GaN nanowires in visible/ultra-violet regime

Spectroscopic investigations of individual single-crystalline GaN nanowires with a lateral dimensions of ～30–90 nm were performed using the spatially resolved technique of electron energy-loss spectroscopy in conjunction with scanning transmission electron microscope showing a 2-Å electron probe. Positioning the electron probe upon transmission impact and at aloof setup with respect to the nanomaterials, we explored two types of surface modes intrinsic to GaN, surface exciton polaritons at ～8.3 eV (～150 nm) and surface guided modes at 3.88 eV (～320 nm), which are in visible/ultra-violet spectral regime above GaN bandgap of ～3.3 eV (～375 nm) and difficult to access by conventional optical spectroscopies. The explorations of these electromagnetic resonances might expand the current technical interests in GaN nanomaterials from the visible/UV range below ～3.5 eV to the spectral regime further beyond.     

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 °C for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V), respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V), 0.72 eV (C–V) for the samples annealed at 200 °C, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 °C. Further increase in annealing temperature to 500 °C the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 °C and 400 °C. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 °C.     

Optical properties of InN—the bandgap question

The recent controversy on the bandgap of InN is addressed, with reference to optical data on single crystalline thin film samples grown on sapphire. The optical absorption spectra deduced from transmission data or spectroscopic ellipsometry are consistent with a lowest bandgap around 0.7 eV in the low doping limit. Further, these data from a number of different independent authors and samples give values for the absorption coefficient within a factor 2 well above the absorption edge, supporting an intrinsic direct bandgap process. The presence of Mie resonances due to In inclusions in the InN matrix affects the shape of the absorption above the edge, but is less relevant for the discussion of the bandgap for pure InN. The alternative model of a deep level to conduction band transition requires the presence of a deep donor at a concentration close to 10²⁰ cm⁻³; in addition this concentration has to be the same within a factor 2 in all samples studied so far. This appears implausible, and no such deep donor could so far be identified from SIMS data in the highest quality samples studied. The line shape of the photoluminescence spectra can be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band, including the Coulomb effects of the impurity potentials. A value of 0.69 eV for the bandgap of pure InN is deduced at 2 K. For samples that appear to be only weakly degenerate n-type two narrow peaks are observed in the photoluminescence at low temperature, assigned to conduction band—acceptor transitions. These peaks can hardly be explained in the deep level model. Recent cathodoluminescence data on highly n-doped InN films showing that the emission appears to be concentrated around In inclusions can also be explained as near bandgap recombination, considering the plausible enhancement due to interface plasmons. Finally, recent photoluminescence data on quantum structures based on InN and InGaN with a high In content appear to be consistent with moderate upshifts of the emission from a 0.7 eV value due to electron confinement.     

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 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.     

Oxygen species on the silver surface oxidized by MW-discharge: Study by photoelectron spectroscopy and DFT model calculations

A polycrystalline silver surface has been studied by synchrotron radiation photoelectron spectroscopy after deep oxidation by microwave discharge in an O₂ atmosphere. Oxidized structures with high oxygen content, AgO_x with x > 1, have been found on the silver surface after oxidation at 300–400 K. The line shapes observed in the O1s spectra were decomposed into five components and indicated that complex oxidized species were formed. An analysis of the oxidized structures with binding energies, Е_b(O1s), greater than 530 eV pointed to the presence of both Ag–O and O–O bonds. We have carried out a detailed experimental study of the valence band spectra in a wide spectral range (up to 35 eV), which has allowed us to register the multicomponent structure of spectra below Ag4d band. These features were assigned to the formation of Ag–O and O–O bonds composed of molecular (associative) oxygen species. DFT model calculations showed that saturation of the defect oxidized silver surface with oxygen leads to the formation of associative oxygen species, such as superoxides, with electrophilic properties and covalent bonding. The high stability of oxygen-rich silver structures, AgO_x, can be explained by the formation of small silver particles during the intensive MW oxidation, which can stabilize such oxygen species.     

Characterization of luminescent chromophore obtained from silica spheres

Blue light emitting chromophores have been separated from silica spheres by soaking them into acetone for 120 days. The luminescent chromophores were not obtained from other solvents, including ether, methanol, ethanol, 2-propanol, chloroform and tetrahydrofuran. According to the Fourier transform infrared spectrum, the luminescent material is composed of C–OH, –CH₂, –CH₃, C=O, and Si–O–Si. UV–visible absorption peak of the chromophore is at 5.17 eV (240 nm). Field emission scanning electron microscope images show small cracks on the surface of aged spheres. The luminescence peak was at 2.81 eV (441 nm) for excitation energy between 3.88 and 3.35 eV and slightly shifted toward lower energy for excitation energy lower than 3.35 eV. The deconvoluted luminescent spectrum shows two emission bands at 3.08 and 2.74 eV, which are well-matched the oxygen deficient center model. Compared to the absorption peak (5.17 eV) and the emission peak (2.81 eV), large Stokes shift (2.36 eV) is observed.     

The comparison of Ag–As33S67 films prepared by thermal evaporation (TE), spin-coating (SC) and a pulsed laser deposition (PLD)

Chalcogenide thin films could be prepared by many experimental methods resulting in some differences in structure and physicochemical properties of prepared films. In this work, the As₃₃S₆₇ amorphous films were prepared by three different preparation techniques: vacuum thermal evaporation (TE), pulsed laser deposition (PLD) and spin-coating (SC). A silver film was deposited on the top of the As₃₃S₆₇ films and photodoped.The X-ray diffraction analysis showed significant differences in arrangement between bulk glass and thin films and also among films themselves. The Raman spectroscopy showed that the Raman spectra of PLD film and bulk glass are almost similar. On the other hand, TE films contain higher amount of homopolar bonds As–As and S–S. The value of refractive index of As₃₃S₆₇ bulk glass was 2.31. All prepared films have lower index of refraction contrary to bulk glass, i.e. TE∼2.27, PLD∼2.20 and SC∼1.90. The increase of refractive index with silver concentration is shown either. The optical bandgap of undoped As–S prepared films was different: TE∼2.42 eV, PLD∼2.45 eV and SC∼2.54 eV.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Improved electrical parameters of vacuum annealed Ni/4H-SiC (0 0 0 1) Schottky barrier diode

The reported work has been focused on the improvement of electrical parameters of Schottky diode using vacuum annealing at mild temperature in Ar gas ambient. Nickel Schottky barrier diodes were fabricated on 50 μm epitaxial layer of n-type 4H-SiC (0 0 0 1) substrate. The values of leakage current, Schottky barrier height (?_B), ideality factor (η) and density of interface states (N_SS) were obtained from experimentally measured current–voltage (I–V) and capacitance–voltage (C–V) characteristics before and after vacuum annealing treatment. The data revealed that ?_B, η and reverse leakage current for the as-processed diodes are 1.25 eV, 1.6 and 1.2 nA (at ?100 V), respectively, while for vacuum annealed diodes these parameters are 1.36 eV, 1.3 and 900 pA (at same reverse voltage). Improved characteristics have been resulted under the influence of vacuum annealing because of lesser number of minority carrier generation due to incessant reduction of number of available discrete energy levels in the bandgap of 4H-SiC substrate and lesser number of interface states density at Ni/4H-SiC (0 0 0 1) interface.     

Temperature dependence of the PbWO4:F,Eu luminescence

Luminescence of the PbWO₄:F,Eu single crystal was investigated in the temperature region of 10–300 K. Besides two well known “blue” (2.80 eV) and “green” (2.45 eV) luminescence bands an additional band at 2.25 eV was observed in the whole temperature region and was assigned to the WO₃F defect centres. Europium dopant evinced as a narrow weak luminescence band at 2.02 eV only at 300 K. Temperature dependence of the excitation spectra was simulated assuming existence of the two defect absorption bands located near the fundamental absorption edge.