Photoluminescence of the ZnSe single crystals doped by thermal diffusion of nitrogen

Photoluminescence (PL) spectra of ZnSe single crystals annealed in different ambients containing molecular nitrogen are investigated. The compensating activity of N impurity in n-ZnSe crystals is shown. It is caused by the formation of N_Se acceptor centers, having 101-108 meV activation energy. The intensity of amplification of both long-wave luminescence spectra bands and the edge luminescence spectra bands caused by the presence of nitrogen in annealing medium is investigated. The presented results allow one to assign the long-wave luminescence to deep acceptors caused by uncontrollable impurities, and the relevant bands of the edge luminescence spectra to the excitons bound with the same deep acceptors. The model explaining the transformations of the luminescent properties of ZnSe crystals by means of nitrogen impurity doping is proposed. The model considers the presence of donors having 75 meV activation energy, acceptors having 220-720 meV activation energy and centers having levels localized near the middle of the band gap.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

Ionoluminescence and photoluminescence studies of Ag ion irradiated kyanite

Ionoluminescence (IL) of kyanite single crystals bombarded with 100 MeV swift Ag⁸⁺ ions with fluences in the range 1.87-7.5×10¹¹ ions/cm² has been studied. A pair of sharp IL peaks at ∼689 and 706 nm along with broad emission in the region 710-800 nm are recorded in both crystalline and pelletized samples. Similar results are recorded in Photoluminescence (PL) of pelletized kyanite bombarded with same ions and energy with fluences in the range 1×10¹¹-5×10¹³ ions/cm² with an excitation of 442 nm laser beam. The characteristic pair of sharp emission peaks at 689 and 706 nm in both IL and PL is attributed to luminescence centers activated by Fe²⁺ and Fe³⁺ ions. The reduction in IL and PL bands intensity with increase of ion fluence might be attributed to degradation of Si-O (2ν₃) bonds, present on the surface of the sample.     

Optical characterization of gallium antimonide highly doped with manganese

Gallium antimonide crystals highly doped with Mn were prepared by a liquid-phase-electroepitaxy growth method. The crystals exhibited high hole concentrations up to 6×10¹⁸ cm⁻³. Photoluminescence (PL) and transmission techniques were used for their investigation. Spectral line-shapes typical for highly doped semiconductors were observed. The lines revealed the features corresponding to band gap narrowing and valence-band filling phenomena. Values of the band-gap narrowing ΔE_g and the degree of the valence-band filling ΔE_F were estimated from the PL spectra. The ionization energy of the Mn acceptor E_i was estimated to be approximately 15.1-15.6 meV. At low temperatures, the PL maxima shifted relatively strongly towards higher energy with temperature. The shifts most probably resulted from a dramatic change in the electron density of states near the bottom of the conduction band. The extent of low-energy tails of the PL bands correlates with the doping levels. The transmission spectra exhibited an absorption band centred at around 774-780 meV. The band most probably originated in electron transitions from the level of spin-orbit splitting to the top of the valence band.     

A comparative analysis of infra-red luminescence spectra of ZnSe crystals doped with Yb,Gd or Cr impurities

Infra-red (IR) photoluminescence (PL) spectra of ZnSe crystals doped with Yb, Gd rare-earth impurities and Cr impurity are investigated. The influence of stoichiometric deviation on the spectra is studied and the structure of complex IR PL bands is analysed. The good coincidence between the structures of IR PL spectra of the samples doped with Yb, Gd, and Cr is shown. Correlation between the component parts of the bands at 1 and 2 μm is found and possibility to control the composition of IR PL spectra by enrichment of the samples with Zn or Se is discussed. The models that explain the formation of complexes based on rare-earth and background Cr and Cu impurities, responsible for IR PL bands, are proposed. Keywords: IR luminescence, ZnSe, Rare-earth impurities, Cr impurity.     

Radiative recombination of indirect exciton in type-II ZnSeTe/ZnSe multiple quantum wells

The tunability of the emission energy, oscillator strength and photoluminescence (PL) efficiency by varying the well thickness and excitation density was demonstrated in the ZnSe_0.8Te_0.2/ZnSe multiple quantum wells. A significant blueshift about 260 meV of the PL peak energy was observed as the well width decreased from 5 to 1 nm. An extraordinary long lifetime (300 ns) of the recombination for the widest sample was detected. The binding energy of the indirect excitons is determined as 12 meV for the thinnest sample. The reduction of PL efficiency by thermal energy is greatly suppressed by employing a high excitation power.     

Direct recognition of non-radiative recombination centers in semi-insulating LEC InP:Fe using double excitation photoluminescence

In order to observe the effect of intra-band gap excitation on the photoluminescence (PL) properties of undoped InP and iron doped InP (InP:Fe), PL measurements were performed in InP crystals with thickness of 360 μm and area of about 4×3 mm², grown by the liquid encapsulated Czochralski (LEC) technique upon excitation with both Ar-ion laser and 980 nm light. The PL intensities for InP:Fe under 980 nm wavelength light illumination relative to no illumination increased by about 52%, 33%, and 12% for the 1.337, 1.380, and 1.416 eV peaks, respectively, at 10 K, whereas there was no illumination effect for undoped InP. This is a strong indication that Fe centers play a role as non-radiative recombination centers to decrease the PL intensity. PL experiments were performed in the spectral range of 1320-1440 meV for InP in the sample temperature range of 10-160 K. The electron and hole photoionization cross-sections at 980 nm wavelength light illumination were calculated as and , respectively.     

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F₂, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16-150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16-200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F₂ laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼t⁻¹.For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups.Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects.The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.     

Photoluminescence spectra of nitrogen implanted GaSe crystals

GaSe single crystals were N-implanted along c-axis with ion beams of 10¹⁴ and 10¹⁶ ions/cm² doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states.     

Dark electrical conductivity and photoconductivity of Ga4Se3S layered single crystals

Ga₄Se₃S layered crystals were studied through the dark electrical conductivity, and illumination- and temperature-dependent photoconductivity in the temperature region of 100-350 K. The dark electrical conductivity reflected the existence of two energy states located at 310 and 60 meV being dominant above and below 170 K, respectively. The photoconductivity measurements revealed the existence of another two energy levels located at 209 and 91 meV above and below 230 K. The photoconductivity was observed to increase with increasing temperature. The illumination dependence of photoconductivity was found to exhibit linear and supralinear recombination above and below 280 K, respectively. The change in recombination mechanism was attributed to the exchange in the behavior of sensitizing and recombination centers.     

Radiative-recombination transitions in sulphur-doped GaSb

Photoluminescence (PL) of sulphur-doped gallium antimonide prepared by a liquid-phase-electro-epitaxy growth method was investigated. Pumping-intensity-dependent and temperature-dependent PL measurements were carried out, properties of individual spectral bands were studied, and their physical origin was specified in detail. Sulphur caused compensation in GaSb, which is usually p-type if it is undoped due to the high concentration of its characteristic native acceptor (NA). As a result of compensation, recombination occurred under the condition of a fluctuating potential and spectral properties characteristic for such a material state were observed. Three bands formed the low-temperature PL spectra. Band A_U, connected with the NA, exhibited extremely low peak energy for some samples (down to 765 meV). Together with the presence of a “moving” PL, with a moving rate of approximately 10 meV per decade of the pumping intensity, it is a direct consequence of perturbed energy bands. Band S, peaking at about 732 meV, is a characteristic one for sulphur-doped GaSb and is most probably connected with a sulphur-donor-to-valence-band transition. The thermal decay of the band agrees with this supposition. Intensity-dependent and temperature-dependent PL of band A_I (maximum at 705-710 meV) both indicate that the band is connected with the ionised NA. PL intensity of the peak is relatively high, because compensation enhances the concentration of such centres.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

Synthesis and fluorescence properties of pure and metal-doped spherical ZnS particles from EDTA-metal complexes

Monodispersed spherical ZnS particles as well as doped with Cu, Mn ions were synthesized from metal-chelate solutions of ethylenediamine tetraacetate (EDTA) and thioacetamide (TAA). The characterizations of the ZnS-based particles were investigated via TEM, SEM, XRD, TG/DTA and PL measurements. The sphere size was controlled from 50 nm to 1 μm by adjusting the nucleation temperatures and molar ratio of Zn-EDTA to TAA. The emission intensity continuously increased with the increase of the particle size. When the ZnS microspheres were annealed at 550-800 °C, there were two specific emission bands with the centers at 454 nm and 510 nm, which were associated with the trapped luminescence arising from the surface states and the stoichiometric vacancies, respectively. When Cu²⁺ was introduced into ZnS microspheres, the dominant emission was red-shifted from 454 to 508 nm, fluorescence intensity also sharply increased. However, for the Mn²⁺-doped ZnS, the emission intensity was significantly enhanced without the shift of emission site.     

The effect of annealing on the photoluminescent and optical properties of porous anodic alumina films formed in sulfamic acid

Photoluminescent and optical properties of porous oxide films formed by two-step aluminum anodization at a constant potential of 30 V in sulfamic acid have been investigated after their annealing, ranging from room temperature up to 600 °C. X-ray diffraction reveals the amorphous nature of porous oxide films. Infrared and energy dispersive spectroscopy indicates the presence of sulfuric species incorporated in oxide films during the anodization. Photoluminescence (PL) measurements show PL bands in the range from 320 to 600 nm. There are two peaks in emission and excitation spectra. One emission peak is at constant wavelength centered at 460 nm and the other shifts from 390 to 475 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is at constant wavelength at 270 nm and the other also shifts to longer wavelengths while increasing emission wavelength. Upon annealing of the as-prepared oxide films PL increases reaching maximum value at about 300 °C and then decreases. The results indicate the existence of two PL centers, one placed at surface of the pore wall, while the other positioned inside the oxide films.     

Passive Q-switching of short-length Tm-doped silica fiber lasers by polycrystalline Cr:ZnSe microchips

We demonstrate passive Q-switching of short-length double-clad Tm³⁺-doped silica fiber lasers near 2 μm pumped by a laser diode array (LDA) at 790 nm. Polycrystalline Cr²⁺:ZnSe microchips with thickness from 0.3 to 1 mm are adopted as the Q-switching elements. Pulse duration of 120 ns, pulse energy over 14 μJ and repetition rate of 53 kHz are obtained from a 5-cm long fiber laser. As high as 530 kHz repetition rate is achieved from a 50-cm long fiber laser at ∼10-W pump power. The performance of the Q-switched fiber lasers as a function of fiber length is also analyzed.     

Infrared radiation of zinc selenide single crystals

Long-wave photoluminescence (PL) spectra of both as-grown and Au-doped n-ZnSe single crystals are studied in the temperature range from 81 to 300 K. A narrow band of infrared (IR) radiation centered at 878 nm (1.411 eV) manifests itself in the low-temperature PL spectrum. It is established that this band intensity first increases and then decreases with increasing concentration of doping impurity. With increasing excitation radiation intensity, spectral position of the IR PL band is unchanged and its intensity increases under the linear law. With increasing excitation radiation wavelength, the IR PL band intensity increases, it becomes narrower and shifts towards long wavelengths. It is shown that the observed IR radiation is caused by recombination of free electrons with holes localized on associative acceptors in the ZnSe:Zn:Au crystals or in the undoped crystals.     

First-principles study on electronic structures of BaWO4 crystals containing F-type color centers

The electronic structures of BaWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The optical transition energies are 2.449 and 3.101 eV, which correspond to the 507 and 400 nm absorption bands, respectively. It is predicted that 400-550 nm absorption bands originate from the F and F⁺ color centers in BaWO₄ crystals.     

Effects of Fe doping on crystalline and optical properties of yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) samples with different Fe concentrations were prepared to study the effects of Fe doping on crystalline and optical properties of YSZ. The former properties were determined by X-ray diffraction, while the latter properties were determined by diffuse reflectance (DR) and photoluminescence (PL) spectroscopies. Lattice contraction of YSZ caused by the Fe doping was observed. We revealed that the DR spectra of the 3 and 6 mol% Fe-doped YSZ samples originate from the Fe ions dissolved and undissolved in the YSZ, respectively. Moreover, two PL bands centered around 440 and 530 nm were observed for the YSZ sample, whereas one PL band centered around 440 nm was observed for the Fe-doped YSZ samples. The Fe doping reduced the PL intensity of YSZ and quenched the PL band around 530 nm. This could be explained by considering that the concentration of Fe ions near the surface of YSZ is much larger than that in the bulk of YSZ or by considering that the Fe doping enhances surface band bending of YSZ.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Photoluminescence and field emission properties of Sn-doped ZnO microrods

Sn-doped ZnO (SZO) microrods have been fabricated by a thermal evaporation method. Effect of Sn dopant on the microstructure, morphological and composition of as-prepared SZO microrods have been investigated by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The influence of the doping concentration on the morphological of the microrods has been investigated. Photoluminescence (PL) of these SZO microrods exhibits a weak ultraviolet (UV) emission peak at around 382 nm and the strong green emission peak at around 525 nm at room temperature. Field emission measurements demonstrate that the SZO possess good performance with a turn-on field of ∼1.94 V/μm and a threshold field of ∼3.23 V/μm, which have promising application as a competitive cathode material in FE microelectronic devices.