Effect of annealing on structure and optical properties of Ga5Se95 films

To investigate the effect of annealing on the structural and optical properties of a binary compound Ga₅Se₉₅, thin films of Ga₅Se₉₅ have been deposited on quartz substrates at room temperature by the thermal evaporation technique. X-ray diffraction patterns showed that the films before and after annealing at 573 K have polycrystalline texture and exhibit tetragonal structure. The dependences of the optical constants, the refractive index n and extinction coefficient k were studied in the spectral range of 200 nm to 2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple–DiDomenco single-oscillator model. Analysis of absorption index data reveals that as-deposited Ga₅Se₉₅ films has indirect transitions with optical energy gap of 1.685 eV.     

Photoluminescence properties of a near-stoichiometric CuGaSe2 film

Epitaxial films of CuGaSe₂ were grown on (001)GaAs substrates by an MBE technique. A near-stoichiometric film with chemical compositions consistently varied from Cu- to Ga-rich was prepared by growing the film without the substrate rotation. A series of PL spectra was obtained by directing a focused laser beam point-by-point across the boundary separating the Cu- and Ga-rich regions. Distinct features of these spectra were noted. On the Cu-rich side, optical emissions peaked at 1.71, 1.67, 1.63, and 1.59 eV were observed in a PL spectrum. The peak at 1.71 eV was due to the emission of bound exciton, while the peak at 1.67 eV was caused by the free-to-bound transition. The other two peaks were identified to be the donor-to-acceptor emissions. Further annealing experiments performed in different environments suggested that the peaks at 1.67, 1.63, and 1.59 eV were associated with the optical transitions of CB→Cu_Ga, Cu_i→Cu_Ga, and Cu_i→V_Ga, respectively. On the Ga-rich side, a dominant donor-to-acceptor emission peaked at 1.62 eV was observed, which was determined to be the Ga_Cu→V_Cu transition. The two defects with opposite charge states resulted in a highly compensated material with high resistivity.     

Determination of trapping parameters of thermoluminescent glow peaks of semiconducting Tl2Ga2S3Se crystals

Thermoluminescence (TL) properties of Tl₂Ga₂S₃Se layered single crystals were researched in the temperature range of 290–770 K. TL glow curve exhibited two peaks with maximum temperatures of ~373 and 478 K. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of the trapping centers associated with these peaks. Applied methods were in good agreement with the energies of 780 and 950 meV. Capture cross sections and attempt-to-escape frequencies of the trapping centers were reported. An energy level diagram showing transitions in the band gap of the crystal was plotted under the light of the results of the present work and previously reported papers on photoluminescence, thermoluminescence and thermally stimulated current measurements carried out below room temperature.     

Optical energy gaps of undoped and Co-doped ZnIn2Se4 single crystals

Single crystals of undoped and Co-doped ZnIn₂Se₄ were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10–300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn₂Se₄ and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy gaps were given by 1.774 and 1.413 eV for undoped ZnIn₂Se₄ and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation.     

Accumulation layer and interface effects in doped nonabrupt GaAs/AlxGa1 − xAs single quantum wells

The electron energy levels in doped nonabrupt GaAs/Al_xGa_1 − xAs single quantum wells 100 Å wide are calculated. Interface widths varying from zero to four GaAs unit cells are taken into account, as well as band bendings of 0–90 meV. It is shown that interface effects on the energy levels are important and sensitive to the level of doping. When interfaces of only two GaAs unit cells and a band bending of 40 meV are considered, the ground-state (first excited state) energy level shifts toward energies as high as 4 meV (20 meV).     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

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.     

Photoluminescence study of polycrystalline CsSnI3 thin films: Determination of exciton binding energy

We report on the determination of exciton binding energy in perovskite semiconductor CsSnI₃ through a series of steady state and time-resolved photoluminescence measurements in a temperature range of 10–300 K. A large binding energy of 18 meV was deduced for this compound having a direct band gap of 1.32 eV at room temperature. We argue that the observed large binding energy is attributable to the exciton motion in the natural two-dimensional layers of SnI₄ tetragons in this material.     

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.     

Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities

Photoluminescence (PL) related to rare-earth (RE) impurities (Ho, Er and Eu) in AgGaS₂ and CuGaS₂ crystals has been studied. In Ho-doped AgGaS₂ and CuGaS₂, two series of PL lines are observed in 1.86–1.92 eV region and 2.24 eV region, and they are assigned to ⁵F₃–⁵I₇ and ⁵S₂–⁵I₈ transitions of the Ho³⁺ ion, respectively. Similarly, in Er-doped AgGaS₂ and CuGaS₂, Er³⁺-related two PL series are observed: 1.83–1.88 eV region (⁴F_9/2–⁴I_15/2) and 2.22–2.26 eV region (⁴S_3/2–⁴I_15/2). For both Ho and Er impurities, the profile of the PL spectrum in AgGaS₂ is complex, and PL exhibited large number of lines compared with that in CuGaS₂. The differences in PL spectra between this two compounds are related to the crystal field at the cation site and the local atomic arrangement of the RE impurities. This work also refers to the PL band at 2.28 eV observed for the Eu-doped AgGaS₂ crystal.     

Electrical properties and electronic structure of Cu1−xZnxInSe2 and Cu1−xZnxInS2 single crystals

Temperature dependence of the electrical conductivity of CuInS₂–ZnIn₂S₄ and CuInSe₂–ZnIn₂Se₄ solid solutions possessing n-type conductivity has been studied. It has been established that when the temperature decreases down to ~100 to 27 K, the hopping mechanism of electrical conductivity with a variable jumping length between localized states positioned in a narrow energy band near the Fermi level becomes dominant. The main parameters of the hopping conductivity have been determined. At higher temperatures (150–300 K), in the CuInSe₂–ZnIn₂Se₄ single crystals containing 15 and 20 mol% ZnIn₂Se₄ the thermally activated conductivity with activation energy of 0.018 and 0.04 eV, respectively, is detected. Among the CuInSe₂–ZnIn₂Se₄ single crystals, samples with 5 and 10 mol% ZnIn₂Se₄ were found to be close to degenerate semiconductors. Temperature dependences of the electrical conductivity of CuInS₂–ZnIn₂S₄ single crystals are described by a more complicated function that may indicate a competition of several conduction mechanisms in these compounds. For the CuInS₂–ZnIn₂S₄ solid solutions, X-ray photoelectron core-level and valence-band spectra have been measured for both pristine and Ar⁺ ion-bombarded surfaces. Our results indicate that the Cu_1−xZn_xInS₂ single-crystal surfaces are sensitive to Ar⁺ ion-bombardment. Additionally, for the Cu_1−xZn_xInS₂ crystal with the highest ZnIn₂S₄ content, namely 12 mol% ZnIn₂S₄, the X-ray emission bands representing the energy distribution of the Cu 3d, Zn 3d and S 3p states have been measured and compared on a common energy scale with the X-ray photoelectron valence-band spectrum.     

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo,W) heterostructures

The electronic structure of the strained g-C₂N/XSe₂ (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C₂N/MoSe₂ heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C₂N/WSe₂ heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C₂N, but it comes from XSe₂ (X=Mo, W) above 6%. Our results show that the g-C₂N/XSe₂ heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.     

Role of heat treatment on structural and optical properties of thermally evaporated Ga10Se81Pb9 chalcogenide thin films

Amorphous chalcogenides, based on Se, have become materials of commercial importance and were widely used for optical storage media. The present work deals with the structural and optical properties of Ga₁₀Se₈₁Pb₉ ternary chalcogenide glass prepared by melt quenching technique. The glass transition, crystallization and melting temperatures of the synthesized glass were measured by non-isothermal DSC measurements at a constant heating rate of 30 K/min. Thin films of thickness 4000 Å were prepared by thermal evaporation techniques on glass/Si (1 0 0) wafer substrate. These thin films were thermally annealed for two hours at three different annealing temperatures of 345, 360 and 375 K, which were in between the glass transition and crystallization temperatures of the Ga₁₀Se₈₁Pb₉ glass. The structural, morphological and optical properties of as-prepared and annealed thin films were studied. Analysis of the optical absorption data showed that the rules of the non-direct transitions predominate. It was also found that the optical band gap decreases while the absorption coefficient, refractive index and extinction coefficient increase with increasing the annealing temperature. Due to the higher values of absorption coefficient and annealing dependence of the optical band gap and optical constants, the investigated material could be used for optical storage.     

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.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

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.     

Temperature-dependent photoluminescence of CuInS2 quantum dots

Temperature-dependent photoluminescence (PL) spectroscopy of CuInS₂ core and CuInS₂/ZnS core–shell quantum dots (QDs) was studied for understanding the influence of a ZnS shell on the PL mechanism. The PL quantum yield and lifetime of CuInS₂ core QDs were significantly enhanced after the QD surface was coated with the ZnS shell. The temperature dependences of the PL energy, linewidth, and intensity for the core and core–shell QDs were studied in the temperature range from 92 to 287 K. The temperature-dependent shifts of 98 meV and 35 meV for the PL energies of the QDs were much larger than those of the excitons in their bulk semiconductors. It was surprisingly found that the core and core–shell QDs exhibited a similar temperature dependence of the PL intensity. The PL in the CuInS₂/ZnS core–shell QDs was suggested to originate from recombination of many kinds of defect-related emission centers in the interior of the cores.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).