Low-temperature time-resolved vacuum ultraviolet spectroscopy of self-trapped excitons in KH<Subscript>2</Subscript> PO<Subscript>4</Subscript> crystals

A complex investigation of the dynamics of electronic excitations in potassium dihydrophosphate (KDP) crystals is performed by low-temperature time-resolved vacuum ultraviolet optical luminescence spectroscopy with subnanosecond time resolution and with selective photoexcitation by synchrotron radiation. For KDP crystals, data on the kinetics of the photoluminescence (PL) decay, time-resolved PL spectra (2–6.2 eV), and time-resolved excitation PL spectra (4–24 eV) at 10 K were obtained for the first time. The intrinsic character of the PL of KDP in the vicinity of 5.2 eV, which is caused by the radiative annihilation of self-trapped excitons (STEs), is ascertained; σ and π bands in the luminescence spectra of the STEs, which are due to singlet and triplet radiative transitions, are resolved; and the shift of the σ band with respect to the π band in the spectra of the STEs is explained.     

Electronic excitations and luminescence in CsLiB6O10 crystals

The paper presents the results of a complex investigation into the dynamics of electronic excitations in the CsLiB₆O₁₀ crystal (CLBO) by low-temperature luminescence VUV spectroscopy with subnanosecond time resolution under photoexcitation by synchrotron radiation. Strong broad-band low-temperature photoluminescence (PL) of the CLBO crystal has been revealed. Data on the PL decay kinetics, time-resolved PL and PL excitation spectra, and reflectance spectra at 9.3 and 295 K are obtained. It is shown that the intrinsic PL of CsLiB₆O₁₀ in the 3.5-eV range is caused by radiative annihilation of self-trapped excitons. The channels of creation and decay of relaxed and unrelaxed excitons in cesium lithium borate are discussed. The band gap of CLBO is estimated as E _g≈8.5 eV. A monotonic increase in the excitation efficiency of intrinsic CLBO luminescence at exciting photon energies above 19 eV is identified as the photon multiplication process.     

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.     

Luminescence of Thin Bi2W2O9 Films

The luminescence spectra of thin Bi₂W₂O₉ films have been investigated. The spectra were separated into elementary components by the Alentsev–Fock method. The radiation band with a maximum at 2.43 eV in the luminescence spectrum of Bi₂W₂O₉ has been assigned to the Frenkel autolocalized excitons. The luminescence bands with maxima at 2.10 and 1.90 eV have been assigned to the emission of the centers whose energy levels are located in the forbidden band. The luminescence of the Bi₂W₂O₉ films is due to the emission of the WO₆ complex.     

Luminescence and electronic excitations in crystals K<Subscript>2</Subscript>Al<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K₂Al₂B₂O₇ crystals have been discussed.     

Electron excitations in LiB3O5 crystals with defects: Low-temperature time-resolved luminescence VUV spectroscopy

The dynamics of electron excitations and luminescence of LiB₃O₅ (LBO) single crystals was studied using low-temperature luminescence vacuum ultraviolet spectroscopy with a subnanosecond time resolution under photoexcitation with synchrotron radiation. The kinetics of the photoluminescence (PL) decay, the time-resolved PL emission spectra, and the time-resolved PL excitation spectra of LBO were measured at 7 and 290 K, respectively. The PL emission bands peaking at 2.7 eV and 3.3 eV were attributed to the radiative transitions of electronic excitations connected with lattice defects of LBO. The intrinsic PL emission bands at 3.6 and 4.2 eV were associated with the radiative annihilation of two kinds of self-trapped electron excitations in LBO. The processes responsible for the formation of localized electron excitations in LBO were discussed and compared with those taking place in wide-gap oxides.     

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Luminescence of excitons and antisite defects in Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce³⁺ ion) in phosphors based on Lu₃Al₅O₁₂:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu _Al ³⁺ type and vacancy-type defects. The luminescence band with λ_max = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λ_max = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λ_max = 253.5 nm occurs predominantly near Lu _Al ³⁺ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λ_max = 325 nm (τ = 540 ns), which is excited in the band with λ_max = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu _Al ³⁺ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce³⁺ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce³⁺ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce³⁺ ions with λ_max = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce³⁺ ions is significantly smaller due to a low concentration of these types of defects.     

Emission characteristics of Dy3+ ions in lead antimony borate glasses

Glasses with the composition 30PbO–25Sb₂O₃–(45?x)B₂O₃–xDy₂O₃ for x=0 to 1 were prepared in steps of 0.2 by the melt-quenching method. Various physical parameters, viz., density, molar volume, and oxygen packing density, were evaluated. Optical absorption and luminescence spectra of all the glasses were recorded at room temperature. From the observed absorption edges optical band gap, the Urbach energies are calculated; the optical band gap is found to decrease with the concentration of Dy₂O₃. The Judd–Ofelt theory was applied to characterize the absorption and luminescence spectra of Dy³⁺ ions in these glasses. Following the luminescence spectra, various radiative properties, like transition probability A, branching ratio β and the radiative life time τ for different emission levels of Dy³⁺ ions, have been evaluated. The radiative lifetime for the ⁴F_9/2 multiplet has also been evaluated from the recorded life time decay curves, and the quantum efficiencies were estimated for all the glasses. The quantum efficiency is found to increase with the concentration of Dy₂O₃.     

Luminescence due to exciton-exciton collision in GaAs

Emission spectra of high-purity GaAs have been studied at 4.2 K under N₂ laser excitation. The slope of the low-energy tail of the main band has been found to fit well with the theoretical prediction for radiative Auger recombination of free excitons. Measurements under electric field support the dominant contribution of this process in the luminescence of highly excited GaAs.     

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.     

X-ray and thermoluminescence of LiKB4O7 single crystals

The thermoluminescent (TL) and X-ray luminescent (XL) spectra of undoped LiKB₄O₇ (LKBO) single crystals had been investigated in the temperature range 80-300 K. It was found that in LKBO crystals, there are two intensive TL peaks at 112 and 132 K. The only one band emission spectra of sharply defined Gaussian shape, confirming the same mechanism of XL and TL by the radiation annihilation of the strongly localized self-trapped excitons (STE), had been observed in the TL and XL spectra. The possible models of these localization centers STE have been discussed.     

Influence of Oxygen Vacancies on the Luminescence Spectra of Y2O3 Thin Films

Luminescence spectra of Y₂O₃ thin films annealed in air and in vacuum are investigated. It is established that the presence of oxygen vacancies leads to a decrease in the intensity of the luminescence band with a maximum at 3.4 eV (related to emission of selflocalized Frenkel excitons describing the excited state of a molecular ion (YO₆)^9–) and of the luminescence band with a maximum at 2.9 eV (related to the anion sublattice). It is revealed that the oxygen vacancies also lead to a decrease in the luminescence intensity in the 2.60, 2.35, 2.10. 1.90, and 1.70 eV bands that are related to radiative recombination in the donor–acceptor Y³⁺–O^2– pairs. The donor–acceptor distances are calculated.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

Luminescence and electronic excitations in KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystals

This paper reports on a study of the dynamics of electronic excitations in KBe₂BO₃F₂ (KBBF) crystals by low-temperature luminescent vacuum ultraviolet spectroscopy with nanosecond time resolution under photoexcitation by synchrotron radiation. The first data have been obtained on the kinetics of photoluminescence (PL) decay, time-resolved PL spectra, time-resolved PL excitation spectra, and reflection spectra at 7 K; the estimation has been performed for the band gap E _g = 10.6−11.0 eV; the predominantly excitonic mechanism for PL excitation at 3.88 eV has been identified; and defect luminescence bands at 3.03 and 4.30 eV have been revealed. The channels of generation and decay of electronic excitations in KBBF crystals have been discussed.     

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract

The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F⁺- centres in pure Al₂O₃ are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.     

Luminescence of Sc3+ and La3+ isoelectronic impurities in Lu3Al5O12 single-crystal films

The time-resolved luminescence and luminescence excitation spectra, and luminescence decay kinetics at 8 and 300 K of Lu₃A₁₅O₁₂ (LuAG) single-crystal films doped with Sc³⁺ and La³⁺ isoelectronic impurities and excited by synchrotron radiation are investigated. It is established that the La³⁺ isoelectronic impurity in the ?ub;c?ub; positions of the garnet lattice forms La _Lu ³⁺ luminescence centers emitting in the band with λ_max = 280 nm and the decay time of the main component τ = 300 ns at 300 K. The Sc³⁺ isoelectronic impurity located in the ?ub;c?ub; and (a) positions of the LuAG lattice forms two luminescence centers, Sc _Lu ³⁺ and Sc _Al ³⁺ , emitting in the bands with λ_max = 290 nm and τ = 240 ns and λ_max = 335 nm and τ = 375 ns, respectively, at 300 K. It is shown that the luminescence excitation of the La³⁺ and Sc³⁺ isoelectronic impurities in LuAG single-crystal films occurs through radiative decay of excitons localized near La _Lu ³⁺ , Sc _Lu ³⁺ , and Sc _Al ³⁺ centers. The energies of formation of these excitons are determined to be 6.8, 6.88, and 7.3 eV, respectively. It was found that the excited state of the excitons genetically related to the La _Lu ³⁺ , Sc _Lu ³⁺ , and Sc _Al ³⁺ enters has two radiative levels with different transition probabilities. This configuration leads to the presence of fast (2.3–8.4 ns) and slow (150–375 ns) main components in the luminescence of the centers formed by isoelectronic impurities in garnets.     

Room temperature visible light emission from Si/SiO2 multilayers: Roles of interface electronic states and silicon phase

We have studied luminescence properties and microstructure of 20 patterns Si/SiO₂ multilayers. The photoluminescence spectra consist of two gaussian bands in the visible-infrared spectral region. It has been demonstrated that the strong PL band is caused by the radiative recombination in the Si/SiO₂ interfaces states, whereas the weaker band originates from radiative recombination in the nanosized Si layers. The peak shift of this latter band shows a discontinuity that corresponds to a crystalline-to-amorphous phase change when the Si layers are thinner than 30 Å. The peak energy as a function of the layer thickness is interpreted using a quantum confinement model in the case of amorphous Si layers.     

Luminescence centers in Sc2O3

Sc₂O₃ luminescence spectra are studied. The spectra are separated into elementary bands by the Alentsev–Fock method. It is established that the luminescence spectra consist of a number of overlapping bands with maxima at 3.5; 3.05; 2.65; 2.35, and 2.05 eV. The band at 3.5 eV is interpreted as emission of self-localized excitons, and the other bands, as defect-center recombination. L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 776–778, November–December, 1997.