摘要:

The electromechanical response of a polyurethane elastomer was investigated at room temperature and in the temperature range near its glass transition. It was found that the Maxwell stress contribution to the strain response can be significant at temperatures higher than the glass transition temperature. In addition, the material exhibits a very high electrostrictive coefficientQ,about two orders of magnitude higher than that of polyvinylidene fluoride. It was also found that in a polymeric material, the chain segment motions can be divided into those related to the polarization response and those related to the mechanical response and the overlap between the two yields the electromechanical response of the material. In general, the activation energies for different types of motion can be different, resulting in different relaxation times in the dielectric, the elastic compliance, and the electrostrictive data, as observed in the polyurethane elastomer investigated. The experimental results indicate that at the temperatures investigated, the activation energy for the mechanical related segment motions is higher than that of nonmechanical related segment motions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363981

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Thin-film ferroelectric capacitors consisting of PbZr0.53Ti0.47O3sandwiched between La0.5Sr0.5CoO3electrodes have been deposited using pulsed laser deposition. The combination of oxidic perovskite-type materials results in capacitors with a coercive field(Ec)which is comparable with values for bulk ceramics. Textured thin-film capacitors with a columnar microstructure show lower switching voltages than epitaxial films. No thickness dependence ofEcand a good endurance up to 1011cycles have been observed for epitaxial as well as textured capacitors with oxidic electrodes. In contrast, capacitors with a metallic top electrode show an increase ofEcwith decreasing thickness of the ferroelectric layer. We show that charge injection can explain the experimentally observed increase ofEcwith decreasing ferroelectric layer thickness. An overview is given of the growth conditions needed for PbZr0.53Ti0.47O3films, because the precise stoichiometry is of the utmost importance for the capacitor quality. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363961

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have analyzed the mechanisms and the efficiency of the 1.54 &mgr;m electroluminescence from Er-doped crystalline Si. Optical doping of a 0.25 &mgr;m deepp+−n+junction was achieved by multiple Er and O implants which realize a uniform concentration of 1019Er/cm3and 1020O/cm3from 0.2 to 0.9 &mgr;m from the surface. It has been found that, for the same current density passing through the device, the room temperature electroluminescence signal is 2–10 times higher under reverse bias at the diode breakdown than under forward bias. Detailed analyses of the spectrum line shape, temperature, and current density dependencies and modulation performances under both forward and reverse bias allowed us to elucidate the reasons for this difference. In forward bias, in spite of the large effective excitation cross section (>6×10−17cm2at 300 K), the efficiency of room temperature electroluminescence is limited by the small number of excitable sites (∼1&percent; of the total Er concentration) and by the efficiency of nonradiative de-excitation processes of the Er ions. Furthermore, since in forward bias Er ions are excited by electron–hole recombination at an Er related level in the Si band gap, the electroluminescence yield is also reduced by competitive carrier recombinations at the residual defects left over after diode processing. On the other hand, under reverse bias, Er ions are excited by hot carrier impact excitation in the thin (∼70 nm) depletion layer. In this case all of the Er atoms in the depletion region are excitable and nonradiative de-excitation processes, such as Auger de-excitation to free electrons, are inhibited. This allows one to achieve an internal quantum efficiency of 1.5×10−4at 300 K. Moreover, fast modulation of the diode can be achieved. At the diode turn-off, the excited Er ions are embedded in the heavily doped (∼1019/cm3) neutral regions of the diode where Auger-type de-excitation processes produce fast decay of the Er ions thus allowing to achieve modulation frequencies higher than 80 kHz. The major limitations to the achievement of a higher efficiency under reverse bias are the thin excitable region and the limited fraction of hot carriers having enough energy to impact excite the Er ions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363935

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Cu-rich CuGaSe2films grown on [001] oriented GaAs substrates by molecular beam epitaxy show a remarkable low temperature photoluminescence emission peaked at 1.620 eV. This emission can be attributed to a free-to-bound recombination accompanying a donor-to-acceptor pair recombination. The ionization energies of these states are found to be 3.4 and 108 meV, respectively, via temperature dependent photoluminescence. The 108 meV state is attributed to a donor and the 3.4 meV state to an acceptor state observed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363936

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Optical properties of Cu-richCuInSe2thin films prepared by the selenization of Cu/In/Cu alloys in aH2Seatmosphere have been studied by photoluminescence (PL) spectroscopy. PL spectra of as-grown samples were dominated by transitions due to intrinsic defect levels, which are ascribed toVIn(24 meV),CuIn(75 meV), andCui(53 meV). After chemical etching (10&percent; KCN), emission lines attributed to free and bound excitonic emissions and their LO phonon replica became visible. Accurate analysis of the peak positions revealed values of 4.3 meV, 1.0441 eV, and 28.7 meV for the binding energy of the free exciton, the band gap, and the value for the LO phonon, respectively. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364306

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A comprehensive study is reported of the photoluminescence properties of ZnS thin films between 1.6 and 320 K grown by metalorganic molecular beam epitaxy and chemical beam epitaxy on GaAs substrates. Both heavy- and light-hole free excitons were observed at low temperatures with linewidths of 7.0 and 5.3 meV, respectively, as well as donor- and acceptor-bound excitons and free-to-bound recombination along with their longitudinal optical (LO) phonon replicas. The free exciton emission was observed up to 320 K, and enabled the room temperature band gap of ZnS to be unambiguously determined as 3.723 eV. The temperature dependence of the peak position, intensity, and linewidth was well described by the conventional empirical relations and by Toyozawa’s exciton line shape theory. The bound exciton peak positions were found to follow the temperature dependence of the band gap whereas the free-to-bound recombination feature was displaced by (1/2)kTabove the band gap energy. Thermal quenching of the donor-bound exciton was described by a one-step quenching process with an activation energy of 14.4 meV. The self-activation (SA) center was also observed at 2.846 eV with a linewidth of 410 meV. The temperature dependence of the SA emission was well described by the configuration coordinate model. From the thermal broadening of the SA emission, an average phonon energy of 47.5 meV was determined, in good agreement with the LO phonon energy. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363937

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A theoretical expression for the brightness of powder phosphor is derived in order to investigate the effects of phosphor’s properties on low-energy cathodoluminescence. It contains the particle size, surface recombination rate, lifetime of holes, carrier diffusion coefficients, and anode voltage. As such, it can be applied to systematically optimize the design parameters of phosphors for maximum efficiency at low-voltage excitation. Use of the given expression is illustrated and a typical case is analyzed in this study. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364307

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Photoluminescence measurements in a magnetic field between 0 and 13.6 T were carried out onCuPtB-ordered GaInP at liquid-helium temperature. Four samples of different degrees of ordering (&eegr;, varying from 0 to 0.54) were studied. Experimental results show that the ordering not only induces a band-gap reduction and valence-band splitting, it also causes changes in effective masses and an effective mass anisotropy. By measuring and analyzing the magnetoluminescence with the magnetic field aligned along both ordering (the [1¯11]) and growth (the [001]) directions, we demonstrate that, for the band-edge excitonic state, the reduced mass in the plane perpendicular to the ordering direction is smaller than that in the ordering direction and also smaller than that of a disordered alloy. The exciton binding energy is found nearly independent of the degree of ordering, in agreement with theoretical predictions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363938

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report on the observation of two-photon absorption for the direct gap in intrinsic germanium at room temperature using two independently tunable picosecond, mid-infrared light pulses. The two-photon absorption coefficient is determined for different wavelengths, and carrier densities down to about1013 cm−3could be detected at a sample length of 700 &mgr;m. Two-photon absorption in germanium is demonstrated to be a versatile tool for zero-point determination in time resolved pump-probe investigations and permits a precise measure of the time resolution of the laser system. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364308

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Structure of GaAs-SiO2composite thin films prepared by magnetron rf co-sputtering technique were investigated by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron microscopy. The results show that GaAs microcrystallites with an average size of 10 nm were uniformly dispersed in SiO2thin film. Quantum confinement effect was observed in the film by the measurement of absorption spectrum, and the blueshift of absorption edge was measured to be 0.5 eV. Nonlinear optical absorption and nonlinear optical refraction were studied byZ-scan technique using single Gaussian beam of He-Ne laser (632 nm). An enhanced third-order nonlinear optical absorption coefficient and nonlinear optical refractive index were achieved in the thin film, and measured to be 2×10−1and 4×10−6cm2/W, respectively. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363939

出版商:AIP    

年代:1997

数据来源: AIP