Luminescent properties of Al2O3:Ce single crystalline films under synchrotron radiation excitation

The paper is dedicated to study the luminescent and scintillation properties of the Al₂O₃:Ce single crystalline films (SCF) grown by LPE method onto saphire substrates from PbO based flux. The structural quality of SCF samples was investigated by XRD method. For characterization of luminescent properties of Al₂O₃:Ce SCFs the cathodoluminescence spectra, scintillation light yield (LY) and decay kinetics under excitation by α-particles of Pu²³⁹ source were used. We have found that the scintillation LY of Al₂O₃:Ce SCF samples is relatively large and can reach up to 50% of the value realized in the reference YAG:Ce SCF. Using the synchrotron radiation excitation in the 3.7–25 eV range at 10 K we have also determined the basic parameters of the Ce³⁺ luminescence in Al₂O₃ host.     

Growth and luminescent properties of Lu2SiO5 and Lu2SiO5:Ce single crystalline films

Single crystalline films (SCF) of Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce (LSO:Ce) silicates with thickness of 2.5-15 μm were crystallized by liquid phase epitaxy method onto undoped LSO substrates from melt-solution based on PbO-B₂O₃ flux. The scintillation and luminescence properties of LSO:Ce SCF were compared with the properties of LSO:Ce single crystal. The peculiarities of luminescence properties of LSO:Ce SCF in comparison with crystal analog can be due to different distribution of Ce³⁺ over the Lu1 and Lu2 positions of LSO host and are further influenced by Pb²⁺ flux-originated contamination.     

Luminescence characteristics of LuAG:Pr and YAG:Pr single crystalline films

Emission and excitation spectra and luminescence decay kinetics were studied for Pr³⁺-doped Lu₃Al₅O₁₂ and Y₃Al₅O₁₂ single crystalline films (SCF) grown by the liquid phase epitaxy method from a PbO-based flux. The influence of lead-induced centers on their scintillation characteristics was clarified. It was found that the influence of single Pb²⁺-based centers on the characteristics of Pr³⁺ centers due to the Pb²⁺ → Pr³⁺ energy transfer was weak. However, an overlap of the emission spectra of single and dimer lead-induced centers with the emission spectrum of Pr³⁺ ions, and especially a strong overlap of the 4f–5d₁ absorption band of Pr³⁺ ions with the slow emission of localized excitons in the 290 nm band had a considerable influence on the scintillation characteristics of the Pr³⁺-doped SCF.     

Intrinsic and Ce-related luminescence of YAG and YAG:Ce single crystals, single crystalline films and nanopowders

A comparative analysis of the luminescent properties of YAG and YAG:Ce nanopowders (NP) in comparison with single crystalline film (SCF) and single crystal (SC) analogues was performed under excitation by a pulsed synchrotron and X-ray radiation. It was shown that the natural defects concentration in NP was between the SC with a large (0.18–0.19 at.%) concentration of Y_Al antisite defects (AD) and SCF of these garnets where Y_Al AD were completely absent. At the same time, Ce³⁺ doped YAG NP showed luminescent properties close to those of YAG:Ce SCF.     

Surface morphology and properties of GaAs epilayers controlled by temperature difference method of liquid phase epitaxy

Epilayers of gallium arsenide were grown by using the steady-state temperature difference method of liquid phase epitaxy. The surface of grown layers was smooth and shiny. Carrier concentrations of films varying from 10¹⁶ to 10¹⁷ cm⁻³ could be obtained with good reproducibility. The surface morphology growth rate, carrier concentration and Hall mobility of the epilayers were studied. Several distinct types of surface features were also investigated and explained. A segregation coefficient for the net carrier concentration versus tin concentration in the growth melt was calculated as 1·84 × 10⁻⁴ at 700°C for (100) GaAs substrate. Thickness control for epilayers down to submicron can be obtained reproducibly.     

Optical and scintillation properties of SrI2:Yb2+

The luminescence and scintillation properties of SrI₂:0.5%Yb²⁺ have been investigated. SrI₂:Yb single crystals were grown by the vertical Bridgeman method from the melt. They showed a light yield of 38,400 ph/MeV and energy resolution of 12.5% for the 662 keV full absorption peak. Yb²⁺ photoluminescence intensity and decay time were studied between 78 and 600 K. Two emission bands centered at 418 and 446 nm were observed and ascribed to spin-allowed and spin-forbidden Yb²⁺ 5d-4f transitions, respectively. Their corresponding room-temperature decay time constants are 710 ns and 77 μs. Both, the emission intensities and the decay time constants vary with temperature. The obtained results were interpreted using a model of self-absorption of Yb²⁺ emission and a model of non-radiative relaxation of the electron from the low spin to the high spin 4f¹³5d Yb²⁺ excited states. The radiative lifetime of the low spin Yb²⁺ excited state was determined as 400 ns.     

Epitaxial grown K1−xRbxTiOPO4 films with extremely flat surfaces for waveguiding

We report on epitaxial {1 0 0} K_1−xRb_xTiOPO₄ waveguide films for the visible spectral range grown on KTiOPO₄ substrates by liquid phase epitaxy. Using the m-line technique a refractive index increase of Δn_x≈0.007 and Δn_z≈0.004 for TM and TE polarisation has been determined for a K_0.78Rb_0.22TiOPO₄ film. Optical transmission and nearfield distribution are comparable to conventional ion-exchanged waveguides. Typical attenuation of about 1 dB/cm for both TM and TE polarisation was obtained at λ=532 and 1064 nm. Energy-dispersive X-ray spectrometry reveals solid-solution films with graded rubidium composition profiles. X-ray rocking curve analyses confirm the epitaxial growth process and indicate perfect and relaxed K_1−xRb_xTiOPO₄ films. Atomic force microscopy investigations reveal regular step structures with step heights Δh<1.3 nm resulting in rms-roughness values of ≈0.4 nm.     

掺铈钇铁石榴石(Ce:YIG)的合成

采用共沉法合成掺铈钇铁石榴石前驱体,并在900℃的温度下,煅烧合成钇铁石榴石相。利用XRD,SEM,XPS对材料的物相,形貌和Ce离子的价态进行了分析。XRD分析表明,Ce：YIG是纯的YIG相;SEM分析表明,试样颗粒呈球状,尺寸小于0.5μm,且有一定程度的团聚;XPS分析表明,Ce：YIG中Ce离子以正三价形式存在。     

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated. Paper presented at the poster session of MRSI AGM VI. Kharagpur. 1995     

Characterization of polysiloxane organic scintillators produced with different phenyl containing blends

Aiming at the fabrication of elastomeric organic scintillators for the detection of ionizing particles and neutrons with good light yield, mechanical robustness and radiation resistance, several samples of polysiloxane added with suitable amounts of fluorophores, such as 2,5-diphenyloxazole (PPO) and Lumogen Violet (LV), have been herein produced starting from either the copolymer polydiphenyldimethylsiloxane with 22 mol% of diphenyl groups or from blends of this precursor with different amounts of the homopolymer polymethylphenylsiloxane, thereby ultimately obtaining, after room temperature vulcanization (RTV), siloxane scintillators bearing different amounts of phenyl side groups. The scintillators have been characterized as for optical properties by excitation and fluorescence spectroscopy, while their performances as radiation detectors have been derived from light yield measurements upon irradiation with α particles. Ion beam-induced luminescence (IBIL) has been also applied using a proton beam of 2 MeV to compare the behavior of the different compositions by observing the in-situ degradation rate of the emitting species under ion irradiation. The samples and commercial scintillators (EJ-212 and EJ-200) used as a standard underwent heavy irradiation with γ-rays from a ⁶⁰Co source at different doses, up to 54 kGy. Then, the ex-situ light yield toward α particles for each scintillator was collected twice again: immediately after the irradiation stage and after one month, in order to characterize the stability and the radiation hardness of scintillators produced with the different blends.     

Effect of substrate conductivity on the thickness and composition of GaAlSb epitaxial layers grown by liquid phase epitaxy

It is shown experimentally that the thickness and composition of Ga_1 − xAl_xSb layers grown by liquid phase epitaxy on Gasb substrates depend on the conductivity type of the substrates. To avoid experimental uncertainties that naturally happen from an experiment to another one, such as changes in the exact liquid composition, growth temperature, temperature gradients, etc., the Ga_1 − xAl_xSb layers have been grown simultaneously from the same liquid solution on N and P type GaSb substrates. The X-ray rocking curves show that for every couple of layers, grown on N and P type substrates, there is a consistent difference between their thickness and composition. The thickness difference has also been verified by optical microscopy. A likely explanation of this effect is a change in surface energy of the substrate induced by the surface electric field normally present in semiconductors.     

Fast UV luminescence in Pr-doped eulytite double phosphates

The paper presents the synthetic procedure and the structural characterisation of Pr³⁺-doped eulytite double phosphates Sr₃La(PO₄)₃ and Ba₃Lu(PO₄)₃. The luminescence properties of these materials were studied employing time-resolved VUV spectroscopy upon excitation with synchrotron radiation. The 5d–4f emission of Pr³⁺ ions was detected and assigned. It was shown that energy transfer from host to Pr³⁺ 5d states is quite inefficient. At the same time the materials demonstrate unwanted defect-related emission that presents main path for relaxation of host relaxation excitations.     

YAG(Ce) crystal characterization with proton beams

A YAG(Ce) crystal has been characterized with a proton beam up to 100 MeV. Tests were performed to investigate the possibility of using this detector as a proton calorimeter. A crystal size has been chosen that is able to stop up to 200 MeV. Energy resolution and light response have been measured at Laboratori Nazionali del Sud with a proton beam up to 60 MeV and a spatial homogeneity study of the crystal has been performed at Loma Linda University Medical Center with a 100 MeV proton beam. The YAG(Ce) crystal showed a good energy resolution equal to 3.7% at 60 MeV and measurements, performed in the 30-60 MeV proton energy range, were fitted by Birks' equation. Using a silicon tracker to determine the particle entry point in the crystal, a spatial homogeneity value of 1.7% in the light response has been measured.     

Fabrication of ZnO nanowire arrays by cycle growth in surfactantless aqueous solution and their applications on dye-sensitized solar cells

Large-scale ZnO nanowire arrays vertically aligned on the substrates were achieved from cycle growth without surfactants. The 8 μm long ZnO nanowire arrays were prepared by 20 cycles. The aspect ratio of ZnO nanowire can be increased with increasing the growth cycle. As displayed by microstructures and photoluminescence (PL) analysis, the ZnO nanowire was good single crystal and the defects in the as-prepared ZnO nanowire arrays were controlled at a low concentration. By increasing the length and aspect ratio of ZnO nanowire, the performances of dye-sensitized solar cells based on the ZnO nanowire arrays were improved. As-prepared ZnO nanowire arrays have potential applications in fabricating next generation nanodevices.     

1.3μmInGaAsP／InP增益耦合型分布反馈激光器

采用两次液相外延技术制备了１．３μｍＩｎＧａＡｓＰ／ＩｎＰ内含吸收光栅的增益耦合型分布反馈激光器，成功地利用湿法腐蚀光栅技术，控制吸收光栅的形状和占空比，实现了增益耦合型ＤＦＢ激光器在室温下的脉冲激射，器件表现出了ＤＦＢ模式的单模工作特性。     

用于光通信的磁性石榴石薄膜特性

采用液相外延法制备了ＹＩＧ磁性石榴石薄膜，研究了材料配方及退火对薄膜的光吸收、法拉第旋转角及感生各异性的影响。利用双层膜结构波导可有效地减小薄膜的线性双折射，实现单模传输。     

Structural characterization of TiNxOy/TiO2 single crystalline and nanometric multilayers grown by LP-MOCVD on (110)TiO2

TiO₂/TiN_xO_y superlattices were grown by Low Pressure-Metal-Organic Vapor Phase Epitaxy (LP-MOVPE) technique at deposition temperatures ranking from 650 to 750°C. The growth was performed on top of TiO₂(110) rutile substrates. Intense peaks observed in the X-rays rocking curves and θ-2θ diffraction patterns show the presence of crystalline epilayers. The TiN_xO_y layers were grown in a (200) cubic structure on the (110) quadratic TiO₂ epilayer structure. Transmission electron microscopy confirmed the XRD results and showed the formation of periodic and well structured epilayers.     

Surface roughness studies on 4H-SiC layers grown by liquid phase epitaxy

The surface roughness on 4H-SiC layers grown by LPE was systematically studied. The surface morphology was investigated by scanning electron microscopy and atomic force microscopy. Depending on the substrate orientation two different types of growth morphology were found: terraced and step flow growth. The height of growth steps was found to depend on the angle of the substrate off-orientation. The higher the degree of substrate off-orientation the higher the step height. The terrace height depends on the impurity concentration in LPE layers. With the decrease of the impurity level, the terrace height has been significantly decreased.     

Zn diffusion of In0.5Ga0.5P investigated by photoluminescence measurements

Zn diffusion of an In_0.5Ga_0.5P layer grown on semi-insulating GaAs substrate by the liquid phase epitaxy technique has been investigated using photoluminescence (PL) measurements. The PL spectrum shows a characteristic emission peak of In_0.5Ga_0.5P at 1.934 eV after diffusion. From temperature-dependent studies of this peak and depth profiling of this luminescence, it was found that this peak behaves like D-A (donor-acceptor) pair recombination and is associated with the interstitial Zn donor to substitutional Zn acceptor band transition. Also it was found that recombination due to interstitial Zn donor is dominant near the surface and decreases with increasing depth. The calculated activation energy of substitutional acceptor was found to be 47 meV.     

InGaAsp-i-n photodiodes for fibre-optic communication

High purity layers of In_{1 −x} Ga _x As have been grown by liquid phase epitaxy using a novel impurity gettering technique with rare earth atoms. The electron concentration could thus be decreased from 3 × 10¹⁸ cm⁻³ to 2·4 × 10¹⁵ cm⁻³ and the mobility increased from 7 110 cm²/Vs to 18,981 cm²/Vs (100 K). The excellent quality of the layers has been evidenced by X-ray diffraction and photoluminescence measurements. The fabrication ofp-i-n photodiodes using this technique is described and reliability aspects addressed.