摘要:

The electric field induced phase transition behavior of lead lanthanum zirconate titanate stannate (PLZTS) ceramics was investigated. PLZTS undergoes a tetragonal antiferroelectric(AFETet)to rhombohedral ferroelectric(FERh)phase transition with the application of an electric field. The volume increase associated with this antiferroelectric (AFE)–ferroelectric (FE) phase transition plays an important role with respect to actuator applications. This volume increase involves an increase in both transverse and longitudinal strains. TheEfield at which the transverse strain increases is accompanied by an abrupt jump in polarization. The longitudinal strain, however, lags behind this polarization jump exhibiting a slight decrease at the onset of phase switching. This decoupling was related to the preferentially oriented AFE domain configuration, with its tetragonalc-axis perpendicular to the applied electric field. It is suggested that phase switching involves multiple steps involving both structural transformation and domain reorientation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365982

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The piezoelectric properties of relaxor based ferroelectric single crystals, such asPb(Zn1/3Nb2/3)O3–PbTiO3andPb(Mg1/3Nb2/3)O3–PbTiO3were investigated for electromechanical actuators. In contrast to polycrystalline materials such asPb(Zr,Ti)O3,morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients(d33’s)>2500 pC/N and subsequent strain levels up to>0.6&percent;with minimal hysteresis were observed. Crystallographically, high strains are achieved for ⟨001⟩ oriented rhombohedral crystals, although ⟨111⟩ is the polar direction. Ultrahigh strain levels up to 1.7&percent;, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to anE-field induced phase transformation. High electromechanical coupling(k33)>90&percent;and low dielectric loss<1&percent;,along with large strain make these crystals promising candidates for high performance solid state actuators. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365983

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We find that theCe3+ion in polycrystalline sputtered SrS:Ce thin films resides in a distorted octahedral environment, as opposed to the cubic host environment. Using electron paramagnetic resonance and x-ray diffraction analysis, we show that the degree of axial distortion is related to the preferential growth direction of the SrS films. To first order, the blue-emission properties (emission wavelength and decay times) of the SrS:Ce films do not appear to be affected by the amount of distortion in the localCe3+environment. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366284

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Er-doped GaAs/AlGaAs structures were grown by the molecular beam epitaxy technique with concentrations of Er in the range1017–2×1019 cm−3.Photoluminescence (PL) ofEr3+ions and Er-induced defects was studied at liquid helium and higher temperatures. A strong diffusion of erbium and interdiffusion of gallium and aluminum ions are observed [at the boundary of GaAs/AlGaAs quantum wells (QWs)] which leads at high erbium concentrations to degradation of the QW’s and macroscopic (average) leveling of erbium and aluminum concentrations over the whole semiconductor structure. From high-resolution PL spectra the existence of three types of Er centers is deduced, which differ by positions of fine structure lines, PL lifetimes, and temperature dependence. The results indicate that these centers are accompanied by the appearance of three types of carrier traps with binding energies of 20, 50, and about 400 meV, respectively. The experiments show evidence that carriers captured into these traps control the Auger excitation of Er ions assisted by multiphonon emission of local phonons. Temperature quenching of erbium PL is controlled by depopulation of defect states in the case of Auger excitation via the most shallow hole trap (20 meV) and by competition of multiphonon nonradiative capture with the Auger transitions in the case of the deepest defect (400 meV). De-excitation processes of excitedfelectrons accompanied by generation of electron-hole pairs were also considered. Such processes are especially important in the case of resonance excitation. Besides erbium PL at 1.54 &mgr;m, PL of erbium ions was observed from upper excited states at 0.82 and 0.98 &mgr;m, which demonstrates the possibility of realizing a three-level scheme of light emission. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365984

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The temperature dependence of the optical excitation cross section of Er implantedn-type GaN was studied using photoluminescence excitation spectroscopy. Due to the large 3.4 eV band gap of GaN, it was possible to probe two Er absorption lines using a tunable Ti:sapphire laser in the 770–1010 nm range. Photoluminescence excitation spectra exhibiting several Stark splittings revealed a complex dependence upon temperature. The largest excitation cross section in the third excited state was1.65×10−20 cm2at an excitation wavelength of 809.4 nm when measured at 77 K. This value is roughly three times larger than the cross section in the second excited state at4.8×10−21 cm2when pumping at 983.0 nm. The Er-related photoluminescence was reduced between 1.5 and 4.8 times when going from 77 K to room temperature, except when pumping around 998 nm. At this excitation wavelength the room temperature photoluminescence was stronger by a factor of 1.26 compared to that at 77 K. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365985

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A study of the time-integrated reflectivity of Al targets during laser-induced back ablation by 1064 nm, 10 ns laser pulses is reported. The Al target reflectance data is analyzed using a model which includes not only Al surface removal but also the possible production of Al plasma. Calculations based on a sharp Al film ablation fluence(Jth≈710 mJ/cm2)threshold agree with the reflectance data. Plasma reflectance contribution is found to be negligible for plasma electron densities of less than 0.3 critical density. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366289

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report on the influence of doping, temperature, porosity, band gap, and oxidation on the photoluminescence (PL) properties of anodically etched porousa-Si:H anda-Si:C:H thin films. Only boron-doped,p-typea-Si:H samples exhibited visible photoluminescence. Two broad PL peaks at∼1.6and∼2.2 eVare apparent in room temperature PL spectra. The intensity of the 2.2 eV peak as well as the nanovoid density in the unetcheda-Si:H layers both correlate well with boron concentration. We see evidence of discrete defect or impurity levels in temperature-dependent luminescence measurements, where we observe multiple luminescence peaks. Unlike in porous crystalline silicon, the luminescence energy in porous amorphous silicon does not change with porosity. We do, though, observe a correlation of luminescence energy with band gap of the startinga-Si:C:H films. Oxidation, either native or anodic, reduces photoluminescence intensity. We discuss the implications of these observations on the nature of the luminescence mechanism. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365986

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

New luminescent thermally stable lanthanide-organic complexes, namely europium tris(4-methoxy benzoylbenzoate),Eu(MeOBB)3, and terbium tris(4-methoxy benzoylbenzoate),Tb(MeOBB)3, were synthesized and characterized. The UV absorption bands in these compounds are attributed to the ligand while the emission originates from the lanthanide ions’ electronic transitions. The quantum efficiency of photoluminescence, &Fgr;, measured in5×10−4M methylene chloride solutions is relatively large in both compounds: &Fgr;=0.27 forTb(MeOBB)3and &Fgr;=0.16 forEu(MeOBB)3. The narrow-band electroluminescence from these complexes is studied by fabricating and characterizing single- and multi-layer organic light emitting diodes. Energy transfer between the organic components of these devices is also discussed. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.365987

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electrogenerated chemiluminescence (ECL) from films of the title compounds in an electrolytic solution were studied. Poly(3-hexylthiophene) and poly(2-methoxy-5-dodecyloxy-p-phenylenevinylene) possess ECL predominantly during the anodic and cathodic scan, respectively, in the double potential step electrolysis which alternately produces negatively and positively charged excited species of the polymers. The chronoabsorptometric measurements indicated that the polarity dependence of the ECL is originated from the difference in the kinetics of the electrochemicalp- andn-type doping/undoping reactions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365988

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A quantitative theoretical comparison between two photothermal techniques—the photomodulated reflectance (PMR) and the photothermal infrared radiometry (PTR)—from the standpoint of their relative sensitivity to the thermal and carrier plasma waves in semiconductors is presented. The coefficients representing the relative contributions from the thermal and plasma waves to the total PMR and PTR signals arising as a result of the same temperature increase and photoinjected excess carrier concentration are calculated for three crystalline semiconductors: Si, Ge, and GaAs. The PTR signal is found to be extremely sensitive to the plasma-wave effects exhibiting up to five orders of magnitude higher carrier plasma-to-thermal contrast than that of the PMR method. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365989

出版商:AIP    

年代:1997

数据来源: AIP