Threshold energies in the light emission characteristics of silicon MOS tunnel diodes

Hot electron luminescence of MOS tunnel structures with sub-3 nm oxide layer on p-Si is experimentally studied. Radiation spectra are shown to exhibit thresholds whose positions depend on the initial energy E of injected electrons. Simultaneously, a threshold-like increase of the light intensity at a selected wavelength as a function of E is revealed, and attributed to the onset of different luminescence mechanisms.     

Electrical properties of MOS diodes In/TiO2/p-CdTe

In/TiO₂/p-CdTe MOS diodes, which have a rectification coefficient of K = 6 × 10³ at an external bias of 2 V, are fabricated for the first time by means of the inexpensive spray-pyrolysis method. It is established that tunnel-recombination processes in the MOS structures under investigation for forward and reverse voltages with the participation of levels at an energy depth of 0.25 eV are the dominant current-flow mechanism. The features of the voltage-capacitance characteristics of In/TiO₂/p-CdTe MOS diodes testify to a sharp decrease in the resistance of the TiO₂ high-resistance layer at forward bias, which is caused by the relation between the energy parameters of components of the MOS structure under investigation.     

Enhanced cooperative near-infrared quantum cutting in Pr3+-Yb3+ co-doped phosphate glass

Pr3+and Yb3+co-doped phosphate glasses are prepared to study their optical properties.Excitation and emission spectra and decay curves are used to characterize their luminescence.We demonstrate that upon excitation of Pr3+ion with one high energy photon at 470 nm,two near-infrared(NIR)photons are emitted at 950-1100 nm(Yb3+:2F 5/2 →2F 7/2)through an efficient cooperative energy transfer(CET)from Pr3+to Yb3+.The maximum energy transfer efficiency(ETE)and the corresponding quantum efficiency approach up to 90.17%and 190.17%,respectively.The glass materials might find potential application for improving the efficiency of silicon-based solar cells.     

Photoluminescence in CuGaSe2

The photoluminescence spectra of CuGaSe₂ have been investigated at 76 K in the range 0.65 < λ < 1.1μ. The crystals were grown by iodine transport, and had p-type conductivity. The photoexcitation was provided by the single-line outputs of a CW Ar⁺ laser, and the luminescence spectra were analyzed using a m grating monochromator.The luminescence observed consisted of a rather sharp structure near the energy gap, plus two broad peaks at considerably smaller energies. The broad peaks, centered around 1.52 and 1.30 eV respectively, are attributed to transitions involving deep localized states. The sharper structure near the energy gap is resolved into two peaks, one at 1.72 eV and another at 1.68 eV. This structure is explained by radiative recombination of free excitons (peak at 1.72 eV), and by recombination of free electrons with bound holes (peak at 1.68 eV).A thermal treatment done on the CuGaSe₂ crystals changed both their electrical properties and their luminescence spectra. These changes produced by sample treatment, plus measurements of the excitation power dependence and the polarization of the luminescent radiation, are in agreement with the interpretation presented.     

Optical and thermal cross-section of interface states from photocapacitance measurements

The photodepopulation transient of the interface states has been analyzed in order to obtain the capacitance variation under illumination in terms of the interface state distribution throughout the band gap, the temperature and the photon energy. From these results, two methods are reported to measure directly the majority carrier photoionization cross-section and the interface state density in MOS capacitors. Both methods are based on the analysis of the initial slope of the capacitance variation. The first method likens values obtained at different illumination delay times after bringing the MOS into depletion. The second uses the values measured at two near temperatures. In both cases, excellent agreement is obtained between optical and non-optical measurements of the interface state density. However, the optical cross-section of the interface states can be deduced without it being necessary to know their density previously. Likewise, from analysis of the dark capacitance transient during the delay time, the thermal cross section, σ_p^t, can also be deduced. Experimental values have been measured using AlSiO₂Si(p) structures. In agreement with other works, we observe that the profile of interface states does not depend on the temperature, and σ_p^t increases exponentially as the energy value approaches that of the midgap and approaches a constant value of 1.6 × 10⁻¹² cm² for energies greater than about 0.45 eV. The optical cross-section shows behaviour similar to those reported for n-type Si, GaAs and InP. For energy values smaller than about 1.0 eV, σ_p⁰ can be fitted according to σ_p⁰ ∝ (hv - hv₀)ⁿ with n about 3. For photon energy greater than 1.0 eV, the σ_p⁰ values deduced show a strong rise.     

Investigations on the damage caused by ion etching of SiO2 layers at low energy and high dose

The radiation damage caused by argon ion bombardment during ion etching of thermally grown SiO₂ films at an energy below 1 keV and a dose of about 10¹⁸ cm^?2 has been studied by evaluating MOS C-V curves, FET characteristics, as well as Rutherford ion backscattering spectra. The bombarded samples revealed that ion beam etching in this energy range causes a damaged layer of 5–10 nm thickness at the single crystal silicon surface. Moreover, traces of metal atoms are found in the damaged layer together with argon atoms (≈ 10²¹cm^?3).     

The role of impact ionization in the formation of reverse current-voltage characteristics of Al-SiO2-n-Si tunnel structures

Physical processes responsible for the reverse current-voltage (I-V) characteristics of Al-SiO₂-n-Si structures with 1.2–3.2-nm-thick SiO₂ and a silicon doping level of 10¹⁴-10¹⁸ cm^?3 were analyzed. A new model describing the evolution of the hot-electron energy in structures of this kind is suggested. The roles played by Auger ionization and impact ionization are differentiated. The turn-on voltages of a tunnel MOS structure are studied both theoretically and experimentally. The turn-on voltage is shown to decrease with increasing oxide layer thickness.     

The effects of gamma irradiation on low-pressure chemical-vapor-deposited silicon dioxide metal-oxide-silicon structures

The effects of gamma irradiation on as-deposited, oxygen-annealed, and dual-dielectric gate (undoped polysilicon/oxide) low-pressure chemical-vapor-deposited (LPCVD) silicon dioxide (SiO₂) metal-oxide-silicon (MOS) structures were investigated. As-deposited LPCVD SiO₂ MOS structures exhibit the largest shift in flatband voltage with gamma irradiation. This is most likely due to the large number of bulk oxide traps resulting from the nonstochiometric nature of as-deposited LPCVD SiO₂. Dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures exhibit the smallest shift in flatband voltage and increase in interface state density compared to as-deposited and oxygen-annealed LPCVD SiO₂ MOS structures. The interface state density of dual-dielectric MOS structures increases from 5 × 10¹⁰ eV cm⁻² to 2–3 × 10¹¹ eV cm⁻² after irradiation to a gamma total dose level of 1 Mrads(Si). This result suggests that the recombination of atomic hydrogen atoms with silicon dangling bonds, either along grain boundaries or in crystallites of the undoped polysilicon layer in dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures, probably reduces the number of atomic hydrogen atoms reaching the Si/SiO₂ interface to generate interface states.     

Luminescence Properties of Cobalt-Doped ZnO Films Prepared by Sol–Gel Method

Pure ZnO and Co-doped ZnO films have been deposited on coverslip substrates by sol–gel spin coating. The morphological, structural, and optical properties of the films were investigated. The microstructure of the ZnO films became increasingly fine and the crystalline size decreased with Co doping. Analysis of x-ray diffraction (XRD) and Raman spectra reveals that Co²⁺ ions are substituted for Zn²⁺ ions in the ZnO lattice without changing its wurtzite structure. Co doping induces a decrease of the band-gap energy and fluorescence quenching of the emission bands. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra. Photoluminescence (PL) spectra under different excitation energies and PL excitation spectra for the visible emissions suggest that the orange–red emission and green emission could be related to interstitial zinc (Zn_i) shallow donors and oxygen vacancy (V _O) deep donors, respectively. The red emission of Co-doped ZnO film could be assigned to the radiative transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal after band-to-band excitation. A consistent explanation for the pure and Co-doped ZnO films is that the red emission under the excitation energy below the band gap is probably associated with extended Zn_i states.     

Sensitized anti-stokes luminescence centers in AgCl crystals

In the AgCl microcrystals with adsorbed methylene blue dye molecules, sensitized anti-Stokes luminescence centers formed as a result of a photostimulated low-temperature (77 K) process are observed. The emission is recorded on excitation of the samples by radiation with the flux density 10¹⁴?10¹⁵ photon cm^?2 s^?1 in the spectral range 620–750 nm corresponding to optical absorption in the adsorbed dye molecules and adsorbed Ag₁, Ag₂, and Ag₃ clusters. It is shown that the anti-Stokes luminescence centers formed by photo-stimulation present (adsorbed dye molecule)—(silver subnanocluster) hybrid nanostructures, in which the bonding between the components is weak but sufficient to provide the possibility for two-photon interband transitions that occur due to transfer of the electron’s excitation energy from the dye molecule to the silver cluster with subsequent photoionization of the cluster.     

Photoluminescence in semiconductor structures based on butyl-substituted erbium phthalocyanine complexes

The study is concerned with the luminescence properties of ensembles of semiconductor structures containing organic phthalocyanine molecules with erbium ions as complexing agents. The photoluminescence spectra of the structures of the type of erbium monophthalocyanine, bisphthalocyanine, and triphthalocyanine are recorded. The photoluminescence peaks are detected at the wavelengths 888, 760, and 708 nm (and photon energies 1.4, 1.6, and 1.75 eV) corresponding to electronic transitions within the organic complexes. It is found that, when a metal complexing agent is introduced into the molecular structure of the ligand, the 708 nm luminescence peak becomes unobservable. It is shown that, in the bisphthalocyanine samples, the photoluminescence signal corresponding to transitions from the 4F _9/2 level of erbium ions is enhanced.     

Dielectric properties in Au/SnO2/n-Si (MOS) structures irradiated under Co-γ rays

The metal-oxide-semiconductor (MOS) structures with insulator layer thickness range of 55-430 Å were stressed with a bias of 0 V during ⁶⁰Co-γ ray source irradiation with the dose rate of 2.12 kGy/h and the total dose range was 0-5×10⁵ Gy. The real part of dielectric constant ε′, dielectric loss ε″, dielectric loss tangent tanδ and the dc conductivity σ_dc were determined from against frequency, applied voltage, dose rate and thickness of insulator layer at room temperature for Au/SnO₂/n-Si (MOS) structures from C-V capacitance and G-V conductance measurements in depletion and weak inversion before and after irradiation. The dielectric properties of MOS structures have been found to be strongly influenced by the presence of dominant radiation-induced defects. The frequency, applied voltage, dose rate and thickness dependence of ε′, ε″, tanδ and σ_dc are studied in the frequency (500 Hz-10 MHz), applied voltage (−10 to 10 V), dose rate (0-500 kGy) and thickness of insulator layer (55-430 Å) range, respectively. In general, dielectric constant ε′, dielectric loss ε″ and dielectric loss tangent are found to decrease with increasing the frequency while σ_dc is increased. Experimental results shows that the interfacial polarization can be more easily occurred at the lower frequency and/or with the number of density of interface states between Si/SnO₂ interfaces, consequently, contribute to the improvement of dielectric properties of Au/SnO₂/n-Si (MOS) structures.     

Novel Sol–Gel Nano‐Glass–Ceramics Comprising Ln3+‐Doped YF3 Nanocrystals: Structure and High Efficient UV Up‐Conversion

Transparent glass‐ceramics containing Ln³⁺‐doped YF₃ nanocrystals are successfully obtained under adequate thermal treatment of precursor sol–gel glasses for the first time, to the best of our knowledge. Precipitation of YF₃ nanocrystals is confirmed by X‐ray diffraction and high‐resolution transmission electron microscopy images. An exhaustive structural analysis is carried out using Eu³⁺ and Sm³⁺ as probe ions of the final local environment in the nano‐structured glass–ceramic. Noticeable changes in luminescence spectra, related to relative intensity and Stark structure of band components, along with remarkably different lifetime values, allow us to discern between ions residing in precipitated YF₃ nanocrystals and those remaining in a glassy environment. A large fraction of optically active ions is efficiently partitioned into nanocrystals of small size, around 11 nm. Moreover, bright and efficient up‐conversion, including very intense high‐energy emissions in the UV range, due to 4‐ and 5‐infrared photon processes, are achieved in Yb³⁺–Tm³⁺ co‐doped samples. Up‐conversion mechanisms are analysed in depth by means of intensity dependence on sensitiser Yb³⁺ concentration and pump power.     

Optical properties of photochromatic sulfur-doped chlorosodalite

Synthetic.photochromic sulfo-chlorosodalite, 6(NaAlSiO₄) ·2 NaCl(S), has been thoroughly investigated by measurements of optical absorption, photo-luminescence and cathodoluminescence. Depending on the sulfur ion form and concentration, the doped sodalite exhibits either sensitive tene-brescence or photoluminescence with long wavelength UV excitation. The photo-induced color absorption peaks at 5260A at 300°K with absorption coefficient, Δα_max >200 cm⁻¹ . This is by far the highest photo-induced absorption observed for synthetic chlorosodalite. At 80°K, the peak position of the absorption does not show significant shift within instrumental accuracy. In photoluminescence, the emission spectra as well as the excitation spectra are studied at both 300 and 78°K. Four characteristic spectral bands (IR, blue, red, and a band with oscillation in wavelength) are observed. The oscillatory S₂ ^- ion emission band starting about 2.35 eV and extending to lower energy and the IR band peaked at 1.4 eV are most efficiently excited by 3660A (3.4 eV), whereas the blue luminescence peaked at 2.7 eV has an excitation threshold of 3.9 eV. The red band is often masked by the oscillatory band and can be observed by higher energy excitation. The red and blue bands are also observable in the cathodoluminescence measurements of the sulfur-doped samples but not the undoped samples. Correlating the absorption, luminescence, and excitation spectral results, a quantitative model is derived to interpret the nature and the role of sulfur ions in the photochromic chlorosodalite material.     

Luminescence properties of light-emitting diodes based on GaAs with the up-conversion Y<Subscript>2</Subscript>O<Subscript>2</Subscript>S:Er,Yb luminophor

Y₂O₂S luminophors doped with Er³⁺ and Yb³⁺ ions are produced by means of solid-phase synthesis and deposited onto standard AL123A infrared light-emitting diodes. When excited with 940 nm radiation from a light-emitting diode, the structures exhibit intense visible up-conversion luminescence. A maximal brightness of 2340 cd/m² of green and red up-conversion luminescence at corresponding wavelengths around 550 and 600 nm is observed for the Y₂O₂S compound doped with 2 at % Er³⁺ ions and 6 at % Yb³⁺ ions. The ratio of the intensity of green (or red) up-conversion luminescence to the intensity of infrared Stokes luminescence increases with increasing applied voltage. The efficiency of visible emission of the light-emitting diode structures is η = 1.2 lm/W at an applied voltage of 1.5 V.     

Radiative recombination in Zn1-x MnxTe/Zn0.59Mg0.41Te quantum well structures: Exciton emission and intracenter luminescence

The luminescence spectra controlled by excitons and intracenter 3d emission of Mn²⁺ ions are studied for a series of Zn_1-x Mn_xTe/Zn_0.59Mg_0.41 Te quantum well (QW) structures that differ in manganese content and QW width. It is shown that the relative intensities of exciton emission of the QWs and barriers and the dependences of the intensities on the optical excitation level are controlled mainly by the manganese content in the QWs that affect the efficiency of excitons transfer of to the 3d shell of the Mn²⁺ ions. The effects of QW width and manganese content on the decay kinetics of the intracenter luminescence of the Mn²⁺ ions are studied.     

Photoluminescence of Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te based heterostructures grown by molecular-beam epitaxy

Photoluminescence (PL) of Hg_{1 − x} Cd _x Te-based heterostructures grown by molecular-beam epitaxy (MBE) on GaAs and Si substrates has been studied. It is shown that a pronounced disruption of the long-range order in the crystal lattice is characteristic of structures of this kind. It is demonstrated that the observed disordering is mostly due to the nonequilibrium nature of MBE and can be partly eliminated by postgrowth thermal annealing. Low-temperature spectra of epitaxial layers and structures with wide potential wells are dominated by the recombination peak of an exciton localized in density-of-states tails; the energy of this peak is substantially lower than the energy gap. In quantum-well (QW) structures at low temperatures, the main PL peak is due to carrier recombination between QW levels and the energy of the emitted photon is strictly determined by the effective (with the QW levels taken into account) energy gap.     

Photoelectric and luminescent properties of dysprosium-doped silver chloride

The influence of dysprosium doping on the photoelectric and luminescent properties of AgCl crystals is studied by methods of microwave photoconductivity and photoluminescence. Doping affects both the loss kinetics of photogenerated electrons and luminescence spectra and parameters of photostimulated burst of luminescence. It is shown that the charged [Dy_Ag^·· · V′_Ag]^· or neutral [Dy_Ag^·· · 2V′_Ag] ^x complexes are responsible for a new luminescence band peaked at 470 nm, which manifests itself at weight concentrations of the doping additive >10⁻⁶%. The long-wavelength shoulder at 570 nm in the photoluminescence spectra is attributed to intracenter transitions in the Dy³⁺ ions. The rate constant of the reaction of electron capture into the traps forming upon introduction of the dopant, k _t = (3–5) × 10⁻⁸ cm³ s⁻¹, is evaluated. It is assumed that the traps are Dy³⁺ dysprosium ions.     

Structural and electric properties of AlN substrates used for LED Heterostructures’ growth

The structural characteristics and electrical properties of bulk aluminum nitride crystals grown by sublimation and used as substrates for light emitting diode (LED) structures and AlGaN/GaN field effect transistors were studied. The crystalline perfection was assessed by selective chemical etching and by X-ray diffraction techniques. Electrical and optical properties were investigated using the temperature dependence of conductivity, admittance spectroscopy, high-temperature/low-frequency capacitance voltage measurements and by photoinduced transient current spectroscopy (PICTS), microcathodoluminescence (MCL) spectra and MCL imaging techniques. It was established that the studied samples were single crystals with a large grain substructure, with characteristic grain size of several hundred microns and a dislocation density of 10²–10⁴ cm^?2 inside the grains. The electrical characteristics of the crystals were governed by the compensation of residual donors with a level near E _c—0.3 eV by deep centers with activation energy of 0.7 eV, both centers manifesting themselves in the temperature dependence of conductivity and in admittance spectra. In addition, deep centers responsible for the luminescence band with the peak energy of 3.3 eV and associated with low-angle grain boundaries were also observed.     

Au/SnO2/n-Si (MOS) structures response to radiation and frequency

Metal-insulator-semiconductor (MOS) structures with insulator layer thickness of 290 Å were irradiated using a ⁶⁰Co (γ-ray) source and relationships of electrical properties of irradiated MOS structures to process-induced surface defects have been investigated both before and after γ-irradiation. The density of surface state distribution profiles of the sample Au/SnO₂/n-Si (MOS) structures obtained from high-low frequency capacitance technique in depletion and weak inversion both before and after irradiation. The measurement capacitance and conductance are corrected for series resistance. Series resistance (Rs) of MOS structures were found both as function of voltage, frequency and radiation dose. The C(f)-V and G(f)-V curves have been found to be strongly influenced by the presence of a dominant radiation-induced defects. Results indicate interface-trap formation at high dose rates (irradiations) is reduced due to positive charge build-up in the semiconductor/insulator interfacial region (due to the trapping of holes) that reduces the flow rate of subsequent holes and protons from the bulk of the insulator to the Si/SnO₂ interface. The series resistance decreases with increasing dose rate and frequency the radiation-induced flat-band voltage shift in 1 V. Results indicate the radiation-induced threshold voltage shift (ΔV_T) strongly dependence on radiation dose rate and frequency.