Luminescence studies on swift heavy ion irradiated nanocrystalline aluminum oxide

Pellets of nanocrystalline aluminum oxide synthesized by a combustion technique are irradiated with 120 MeV Au⁹⁺ ions for fluence in the range 5×10¹¹-1×10¹³ ions cm⁻². Two photoluminescence (PL) emissions, a prominent one with peak at ∼525 nm and a shoulder at ∼465 nm are observed in heat treated and Au⁹⁺ ion irradiated aluminum oxide. The 525 nm emission is attributed to F₂²⁺-centers. The PL intensity at 525 nm is found to increase with increase in ion fluence up to 1×10¹² ions cm⁻² and decreases beyond this fluence. Thermoluminescence (TL) of heat-treated and swift heavy ion (SHI) irradiated aluminum oxide gives a strong and broad TL glow with peak at ∼610 K along with a weak shoulder at 500 K. The TL intensity is found to increase with Au⁹⁺ ion fluence up to 1×10¹³ ions cm⁻² and decreases beyond this fluence.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO_3（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3（STO）（110） substrates.Inclined ZnO and cobalt-doped ZnO（ZnCoO） thin films are grown on unetched STO（110） substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO（110） surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E（2G）→ 4A2（4F） transition of Co 2＋ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2＋ ions at the lattice positions of the Zn 2＋ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.     

Observation of room temperature photoluminescence in proton irradiated SrTiO3 single crystal

We have irradiated SrTiO₃ single crystal with 3 MeV-proton (H⁺) beam and found that blue -, green - and infrared - frequency photoluminescence (PL) are induced simultaneously at room temperature. TEM and EELS analyses show that an oxygen-deficient amorphous layer is formed at the crystal surface by the proton irradiation. Possible origin of the PL-effect is discussed.     

Morphology and growth modes of metal-oxides deposited on SrTiO3

Classical molecular dynamics simulations are used to examine the growth of SrO and TiO₂ thin films on SrTiO₃ (STO). In particular, the simulations consider the deposition of SrO and TiO₂ molecules at incident energies of 0.1, 0.5, and 1.0 eV/atom onto the (0 0 1) surface of STO. The role of surface termination layer (SrO vs. TiO₂) is analyzed. In the case of SrO deposition, smooth, ordered films are produced for all incident energies considered and for both surface terminations. By contrast, in the case of TiO₂ deposition, three-dimensional islands are formed under all conditions. These predictions are in good agreement with experimental data. Importantly, the simulations explain why these differing morphologies are produced for SrO and TiO₂ deposition.     

Unconventional photoluminescence effect of SrTiO3 thin film under UV-illumination

We find that PL intensity I(t) of SrTiO₃ thin film measured under UHV condition increases with UV-laser illumination over long time scale of ∼ 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O₂ and N₂ gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent I(t). We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.     

Effects of crystallographic orientation on the oxygen exchange rate of La0.7Sr0.3MnO3 thin films

The oxygen surface exchange of La_0.7Sr_0.3MnO₃ (LSM) thin films was investigated using the electrical conductivity relaxation (ECR) method. Epitaxial (100)-, (110)-, and (111)-oriented LSM films were fabricated on corresponding SrTiO₃ (STO) substrates using pulsed laser deposition. The LSM films had well-controlled surface qualities, exhibited bulk-like steady-state electrical properties, and exhibited surface dominated responses in ECR. The chemical surface exchange coefficients (k_chem) were determined and varied from ≈ 1 × 10^− 6 to 65 × 10^− 6 cm/s, depending on temperature and orientation, with activation energies of between 0.8 and 1.2 eV. At 800 °C, a four fold variation is observed in the k_chem values, with (110)/(100) being the highest/lowest, explained well by the high activation energy for (110), ≈ 1.16 eV, and the low energy for (111) and (100), ≈0.83 eV.     

Silicon MIS diodes with Cr2O3 nanofilm: Optical, morphological/structural and electronic transport properties

In this work we report the optical, morphological and structural characterization and diode application of Cr₂O₃ nanofilms grown on p-Si substrates by spin coating and annealing process. X-ray diffraction (XRD), non-contact mode atomic force microscopy (NC-AFM), ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) spectroscopy were used for characterization of nanofilms. For Cr₂O₃ nanofilms, the average particle size determined from XRD and NC-AFM measurements was approximately 70 nm. Structure analyses of nanofilms demonstrate that the single phase Cr₂O₃ on silicon substrate is of high a crystalline structure with a dominant in hexagonal (1 1 0) orientation. The morphologic analysis of the films indicates that the films formed from hexagonal nanoparticles are with low roughness and uniform. UV-vis absorption measurements indicate that the band gap of the Cr₂O₃ film is 3.08 eV. The PL measurement shows that the Cr₂O₃ nanofilm has a strong and narrow ultraviolet emission, which facilitates potential applications in future photoelectric nanodevices. Au/Cr₂O₃/p-Si metal/interlayer/semiconductor (MIS) diodes were fabricated for investigation of the electronic properties such as current-voltage and capacitance-voltage. Ideality factor and barrier height for Au//Cr₂O₃/p-Si diode were calculated as 2.15 eV and 0.74 eV, respectively. Also, interfacial state properties of the MIS diode were determined. The interface-state density of the MIS diode was found to vary from 2.90 × 10¹³ eV⁻¹ cm⁻² to 8.45 × 10¹² eV⁻¹ cm⁻².     

Blue and infrared cathodoluminescence induced by carbon-irradiation in SrTiO3 single crystal

We show that blue-CL can be induced in SrTiO₃ single crystal at room temperature by irradiating it with 60 KeV carbon ion (C⁻) beam. An infrared CL-emission is induced simultaneously. Transmission electron microscopy (TEM) measurement exhibits an 200 nm thick surface layer formed by the C⁻ irradiation. We show that the emitting region can be patterned into any desired shape by fabricating the STO surface using Pt-based micro-lithography technique.     

Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films

A series of Pr_0.5Sr_0.5MnO₃ (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT), LaAlO₃ (LAO) and SrTiO₃ (STO), and (0 1 1)-oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM-AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)-oriented STO as well as LAO substrates show FMM-AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM-AFI transition.     

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 10^19-20 cm⁻³. Current-voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A⁰X) emission and a donor-acceptor pair (DAP) emission due to doped nitrogen, respectively.     

The preparation of c-axis epitaxial Bi-2212 films on STO(1 0 0) by sol-gel method

Bi₂Sr₂Ca₁Cu₂O_8+δ (Bi-2212) films were grown on (1 0 0) oriented SrTiO₃ (STO) substrate using sol-gel spin-coating method. The effects of heat treatment conditions and coating times on the phase formation and surface morphology were investigated using thermal analysis, optical microscope, X-ray diffraction, and scanning electronic microscopy. Mixed phases were formed from 820 to 840 °C, and Bi-2212 single phase was obtained at 830 °C for 3 h. c-axis epitaxial films with smooth surfaces were obtained by drying at 600 °C and coating for 5 times.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.     

Large enhancement of photoluminescence in SrTiO3:Pr powders by fluorhydric acid treatment

We have investigated the effect of fluorhydric acid (HF) treatment on the photoluminescence (PL) properties of SrTiO₃:Pr³⁺ powders prepared by sol-gel method. The red emission intensity increased significantly up to 18 times when the powders were subjected into a water-diluted 5% HF solution for 10 min. The origin of the PL enhancement was ascribed to the increase of oxygen vacancies in HF-treated SrTiO₃:Pr³⁺ powders. This study also manifested that HF etching is more effective to improve the red PL intensity than vacuum-annealing for the sol-gel made SrTiO₃:Pr³⁺ powders.     

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 studies of γ-irradiated CaF2:Dy:Pb:Na single crystals

The optical absorption (OA) and photoluminescence (hereafter referred to as luminescence) studies were made on CaF₂:Dy:Pb:Na single crystals (Dy—0.005 at%, Pb—0.188 at% and Na—0.007 at%) before and after γ-irradiation. The unirradiated crystal exhibited a strong OA band around 6.36 eV attributed to the ‘A’ band absorption of Pb²⁺ ions. The γ-irradiated crystal exhibited OA bands around 2.06, 3.28, 3.75 (broad shoulder) and 2.48 eV. The first three bands could be tentatively attributed to M_Na centre when compared with that of the coloured CaF₂:Na. The origin of 2.48 eV band was not explicitly known. Luminescence emission and excitation of Pb²⁺ and Dy³⁺ ions were negligible in the unirradiated crystal. Irradiated crystal exhibited a strong excitation spectrum with overlapping bands, due to different colour centres, in the UV-vis region for the 2.15 eV emission characteristic of Dy³⁺ ion. When excited, the absorbed energy (may be a part) was transferred from a colour centre to nearby Dy³⁺ ions and Dy³⁺ characteristic emission was observed. Exciting the irradiated crystal around 3.28 eV yielded emission at 2.56, 2.15 and 1.76 eV. The first two emission bands were due to Dy³⁺ ions. The excitation spectrum for the 1.76 eV emission showed two prominent bands around 2.02 and 3.08 eV and hence the emission was attributed to the M_Na centre. The luminescence mechanism was described.     

Photoluminescence characterization of polycrystalline n-type Cu2O films

Electrodeposition was used to deposit Cu₂O thin films on ITO substrates. Photoresponse of the film clearly indicated n-type behavior of Cu₂O in photoelectrochemical cells. The temperature dependence of photoluminescence (PL) revealed that the spectra consist of donor-acceptor pair emissions and the recombination between electrons bound to donors and free holes. We observed that the dominant intrinsic defect, oxygen vacancies, creates a donor energy level at 0.38 eV below the bottom of the conduction band. As a result, this donor level acts as a center for both PL emissions and to produce n-type conductivity in the electrodeposited Cu₂O films. In addition, an acceptor energy level at 0.16 eV from the top of the valence band was observed.     

Surface segregation in Nb-doped BaTiO3 films

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO₃. The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-doped films. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO₃ ceramics and Nb-doped SrTiO₃ single crystal show higher Fermi level position indicating that our doped films are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy under variable temperature. Large amount of niobium is clearly observable at surface but the amount of dopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, we provide evidence of surface segregation which would explain the contrasted resistivity values reported in literature for such donor-doped films.     

Preparation and luminescence properties of SrTiO3:Pr,Al phosphor from the glycolate method

Spherical SrTiO₃:Pr³⁺,Al³⁺ phosphor with high crystallinity and uniform particle size distribution was formed from the glycolate precursor. The glycolate precursor was obtained by heating the mixed solution of metallic nitrates and titanium oxychloride in ethylene glycol up to 200 °C. The thermal decomposition of the glycolate precursor proceeded through three major stages, i.e., (i) evolution of glycols (∼200 °C), (ii) decomposition of glycolate precursor, and (iii) decomposition of strontium carbonate and crystallization of SrTiO₃:Pr³⁺,Al³⁺ phosphor.SrTiO₃:Pr³⁺,Al³⁺ phosphor exhibited a strong red emission, peaking at about 617 nm. SrTiO₃:Pr³⁺,Al³⁺ phosphor obtained from the glycolate complex has higher luminescent properties than the conventional solid state reaction and the Pechini method in terms of photoluminescence (PL) and cathodoluminescence (CL). High crystallinity, low residual carbon content and small grain size with uniform shape would enhance the luminescence intensity of phosphor by the glycolate method due to high surface area per unit volume and low organic content compared with the Pechini method. Also, Al³⁺ ion is more effective than Ga³⁺ ion to enhance PL intensity of SrTiO₃:Pr³⁺,Al³⁺ phosphor because of smaller Al³⁺ ion radius. Therefore, the glycolate method has been demonstrated to be a convenient and unique process for the production of muticomponent oxide with smaller grain size and higher crystallinity compared with the conventional mixed oxide reaction and the polymer precursor method.     

X-ray diffraction and photoelectron spectroscopy study of swift heavy ion irradiated Mn/p-Si structure

The electronic structure and interfacial chemistry of thin manganese films on p-Si (1 0 0) have been studied by photoelectron spectroscopy measurements using synchrotron radiation of 134 eV and from X-ray diffraction data. The Mn/p-Si structures have been irradiated from swift heavy ions (∼100 MeV) of Fe⁷⁺ with a fluence of 1 × 10¹⁴ ions/cm². Evolution of valence band spectrum with a sharp Fermi edge has been obtained. The observed Mn 3d peak has been related to the bonding of Mn 3d-Si 3sp states. Mn 3p (46.4 eV), Mn 3s (81.4 eV) and Si 2p (99.5 eV) core levels have also been observed which show a binding energy shift towards lower side as compared to their corresponding elemental values. From the photoelectron spectroscopic and X-ray diffraction results, Mn₅Si₃ metallic phase of manganese silicide has been found. The silicide phase has been found to grow on the irradiation.     

PLD of X7R for thin film capacitors

Thin film capacitors with a thickness of 200 nm were prepared on SrTiO₃ (1 0 0), (1 1 0) and (1 1 1) single crystal substrates at a temperature of 973 K by pulsed laser deposition (PLD) using a KrF excimer laser in an O₂-O₃ atmosphere with a gas pressure of 1 Pa using an X7R sintered target. As a result, perovskite BaTiO₃ solid solution films were obtained. In the X7R thin films on (1 0 0) and (1 1 0) SrTiO₃, only diffraction peaks with strong intensities from BaTiO₃ (1 0 0) and (1 1 0), respectively, were observed. X7R films on SrTiO₃ (1 1 1) were grown epitaxially oriented to the crystal plane direction of the substrate by inserting an initial homoepitaxial SrTiO₃ layer with a thickness of 4 nm. The X7R/SrTiO₃ film capacitors yielded a large volumetric efficiency of 50 μF/mm³ and a temperature coefficient of capacitance (TCC) of −1.3% to 1.3% which satisfies the EIA standard specifications for X7R.