摘要:

Stoichiometry issues in lithium tantalate were studied through Czochralski growth of single crystals and vapor transport equilibration (VTE) of crystalline samples to specified Li/Ta compositions. Crystals were characterized through measurement of the ferroelectric Curie temperatureTc. The VTE experiments indicated a linear variation ofTcwith Li/Ta ratio across the investigated range of 47.5–49.5 mol % Li2O. By control of initial melt stoichiometry, crystals representing melt fractions ≳90% were grown having compositional uniformity within ±0.015 mol % Li2O. Growth of the compositionally uniform crystals was determined to take place through incongruent solidification, with lithium rejection from the growth interface compensating for lithium volatilization from the melt surface. With the growth process applied, a melt composition of 48.39 mol % Li2O yielded grown crystals of a uniform 47.85 mol % Li2O solid composition. The lithium tantalate congruent composition was determined to lie at a value less than or equal to 47.70 mol % Li2O. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359811

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Praseodymium (Pr) emissions were investigated for Pr‐implanted GaAs and AlxGa1−xAs as a function of Al mole fraction, sample temperature, and anneal temperature using photoluminescence (PL) spectroscopy. Two groups of major PL peaks have been observed near 1.6 and 1.3 &mgr;m, which can be attributed to the crystal‐field‐split spin‐orbit level transitions of3F3→3H4and1G4→3H5of Pr3+(4f2), respectively. The PL intensity of Pr3+varies dramatically with the Al mole fraction in Pr‐implanted AlxGa1−xAs. For GaAs, the PL peak intensity near 1.3 &mgr;m is strong and the peak intensity near 1.6 &mgr;m is weak, whereas the PL peak intensity near 1.6 &mgr;m is much stronger than that of 1.3 &mgr;m for AlxGa1−xAs. Furthermore, Al0.15Ga0.85As shows particularly strong luminescence peaks near 1.6 &mgr;m. PL emissions of Pr were detected for sample temperatures as high as 100 K, and several hot lines were identified. The optimal anneal temperatures for the Pr‐implanted GaAs and AlxGa1−xAs are between 725 and 775 °C. Dual implantations of Pr and Er into Al0.15Ga0.85As have also been made, but the PL results showed that the emission intensities of both ions did not increase compared to those of identically prepared singly implanted samples. Possible excitation processes are explored in light of these results.

ISSN:0021-8979    

DOI:10.1063/1.359812

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The luminescence properties of Cr3+ions and the energy transfer between Cr3+and Nd3+ions have been studied quantitatively in yttrium aluminum garnet (YAG). The energy level diagram and the emission spectrum of4T2→4A2transition of Cr3+ions were calculated in this crystal using the single‐configurational‐coordination model and the absorption data. Our calculated &Dgr; value (the energy difference between the2Ezero‐vibrational level and the crossover point of2Eand4T2states), ∼730 cm−1at room temperature, in contrast to the previously estimated value of ∼1000 cm−1, accounts for a variety of experimental results more satisfactorily. Both the4T2and2Estates of Cr3+ions were found to contribute to the energy transfer to Nd3+ions. The contribution of the former dominates at high temperatures and the latter at low temperatures. The thermal population distribution between the4T2and2Estates of Cr3+ions was shown to be responsible for the temperature dependences of the Cr3+luminescence properties and the Cr3+to Nd3+energy transfer processes. An electric dipole–dipole interaction mechanism was found to be adequate to describe the energy transfer process for temperatures up to about 400 K. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359813

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A simulation of alternating‐current thin‐film electroluminescent device operation with positive space charge present in the phosphor layer of the device is presented. The simulation is based on modeling the space‐charge distribution using a single‐sheet charge model. The simulation is performed for two cases of space‐charge creation: by impact ionization of deep levels in the phosphor or by field emission from traps in the phosphor. Results of the simulation show that space‐charge creation by either mechanism is capable of causing overshoot in both capacitance‐voltage and internal charge‐phosphor field (Q‐Fp) plots. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359814

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The complex dielectric function, &egr;(E)=&egr;1(E)+i&egr;2(E), of hexagonal CdSe has been measured by spectroscopic ellipsometry in the photon‐energy range between 1.2 and 5.3 eV at room temperature. The measured spectroscopic‐ellipsometry data are analyzed on the basis of a simplified model of the interband transitions. The model is based on the Kramers–Kronig transformation and includes theE0(E0&agr; ; &agr;=A,B,C),E1(E1&agr; ; &agr;=A,B,C), andE0′gaps as the main dispersion mechanisms. The recent SE data of cubic, zinc‐blende‐type CdSe have also been analyzed with the same model by considering the critical points for the cubic phase (i.e.,E0,E0+&Dgr;0,E1,E1+&Dgr;1, andE2). Results are in satisfactory agreement with the experimental data over the entire range of photon energies. To facilitate design of various optoelectronic devices, dielectric‐function‐related optical constants, such as the complex refractive index, absorption coefficient, and normal‐incidence reflectivity, of these crystals are also presented. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359815

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The incorporation of high concentrations (≳1019cm−3) of Si, Be, and C in InxGa1−xAs relaxed layers has been studied as a function of In content (x≤0.16) by Raman spectroscopy of local vibrational modes (LVM). The frequencies of the Raman peaks resulting from the convolution of several split modes shift to lower values as the In content is increased, the carbon acceptor (CAs) local mode frequency showing the strongest dependency onx. Features attributed exclusively to the influence of In on second‐neighbor sites are identified only in the Si‐doped samples. The transverse and longitudinal modes expected from the splitting of a LVM of CAswith one In first neighbor are not observed in layers withxup to 0.085. A new calibration for the BeGa, CAs, and Si‐related LVM leads to the conclusion that In increases the total electrical activation of Si and favors its incorporation on group‐III sublattice sites. In contrast, no influence of In on the Be or C doping activation has been detected. The analysis of the CAsLVM spectra supports the view that in InxGa1−xAs CAsis preferably surrounded by Ga instead of In atoms forx≤0.085. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359816

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Pulsed laser irradiation of nanosecond duration is used in a variety of applications, including laser deposition of thin films and micromachining. Of fundamental interest is the prediction of the evaporative material removal rates, as well as the velocity, density, and temperature distributions of the ejected particles as functions of the laser‐beam pulse energy, temporal distribution, and irradiance density on the target material surface. In order to address these issues, the present work establishes a new computational approach for the thorough treatment of the heat transfer and fluid flow phenomena in pulsed laser processing of metals. The heat conduction in the solid substrate and the liquid melt is solved by a one‐dimensional transient heat transfer model. The ejected high‐pressure vapor generates shock waves against the ambient background pressure. The compressible gas dynamics is computed numerically by solving the system of Euler equations for mass, momentum, and energy, supplemented by an isentropic gas equation of state. The aluminum, copper, and gold targets considered were subjected to pulsed ultraviolet excimer laser irradiation of nanosecond duration. Results are given for the temperature distribution, evaporation rate, and melting depth in the target, as well as the pressure, velocity, and temperature distributions in the vapor phase. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359817

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The epitaxial silicon layer selectively grown on the reactive ion etched (RIE) silicon substrate using CF4, CHF3and Ar etching gases has been studied. Defects and contaminants induced by the RIE process result in a rough epilayer, and degrade the current–voltage (I–V) characteristics. An interfacial carbide layer is present between the epilayer and the RIE treated substrate. Using an efficient and convenient after‐etching treatment with a CF4/O2low‐energy plasma, we obtain a clean Si surface in the patterned oxide windows for selective epitaxial growth, and the electrical characteristics are significantly improved. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359818

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We calculate the anisotropic elastic coefficient of composites of oriented ellipsoids in the effective medium approximation. The theoretical prediction is compared with experimental results for the longitudinal and transverse Young’s modulus of injection molded polymer blends of an amorphous thermoplastic polyester and an ethylene‐propylene‐diene rubber at different compositions. The different ‘‘percolation limits’’ experimentally observed are reproduced. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359819

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Insituwafer curvature measurements were performed to study mechanical stress in amorphous SiO2during Xe, Ne, and Er ion irradiation at energies in the 0.27–4.0 MeV range. Three phenomena are observed: network compaction, radiation‐induced viscous flow, and a nonsaturating anisotropic deformation phenomenon. The radiation‐inducedviscosityis shown to be inversely proportional to the energy density deposited into atomic displacements. The relation between radiation‐induced flow and diffusion is discussed in the context of the Stokes–Einstein relation. Viscous flow serves to relax stress, yet a continuous nonsaturatinganisotropicdeformationeffect causes the stress in the irradiated layer to saturate at nonzero values: Xe irradiation at an energy below 3.6 MeV results in a tensile saturation stress; for higher energies a compressive stress builds up. These effects are explained in terms of competing bulk and surface deformation processes resulting from local heating of the SiO2around the ion tracks. The macroscopic effect of deformation phenomena is illustrated by showing the surface morphology after 4.9 MeV Er irradiation of silica through a contact implantation mask. Finally, aninsitustress study of an alkali borosilicate glass is presented. In this case a fourth radiation induced effect is observed, namely, the generation and annihilation of volume occupying point defects. These defects are shown to anneal out at room temperature, following a broad spectrum of activation energies. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359820

出版商:AIP    

年代:1995

数据来源: AIP