摘要:

The ferromagnetic resonance (FMR) line shapes of Gd-substituted calcium vanadium garnets Y1.6−zCa1.4GdzFe4V0.4Zr0.6O12(0.8⩽z⩽1.4) have been studied as the function of the Gd content. The samples had been synthesized by two different routes, with porosities varying from 0.23&percent; to 8.9&percent;. The FMR linewidth,&Dgr;H, in low porosity samples is small (15–20 Oe) and becomes seven to eight times higher in high porosity samples. The linewidth can be explained on the basis of Schloemann’s theory of anisotropy and porosity broadening in polycrystalline materials. The line shape is Lorentzian for low porosity and Gaussian for high porosity samples. This variation of the FMR line shape with porosity is explained on the basis of the stochastic theory. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364015

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The one-dimensional vibration equations of crystal strips of AT-cut quartz with narrow width and finite length are given by Lee and Wang [J. Appl. Phys. 75, 7681 (1994)]. The mechanical vibrations of such strips have been studied by neglecting the coupling due to piezoelectric effect. In the present study, the one-dimensional equations are further expanded to include the piezoelectric effect. New sets of coupled equations which are much larger than the ones for the mechanical vibrations are obtained. By neglecting the components of high-order displacements and electric potentials, particular attention is given to the thickness-shear vibrations of the strips. Analytical solutions of the free vibrations, including components of thickness-shear, flexural, width-shear, width-stretch, width-flexural, and the zeroth- and first-order electric potentials, are obtained. The dispersion relations, frequency spectra, and mode shapes of the free vibrations are given as functions of the ratios of length to thickness and width to thickness. The forced vibrations are studied with these solutions, and forced response and capacitance ratios are obtained. Comparisons are made with known experimental results. A quartz strip resonator is studied with this seven modes model. The effect of the length of the electrodes, the width of the crystal, and the forcing frequency on the resonance frequency, vibration mode shapes, and capacitance ratios are obtained. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364042

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Two photoluminescent defects associated with Er+3-doped Si are (i) a high-temperature defect (which appears after annealing at ∼900 °C and produces five photoluminescence lines), and (ii) a low-temperature defect (which is created at lower annealing temperatures in addition to the high-temperature defect and yields four photoluminescence lines). We conclude that both defects are at Er sites of tetrahedral symmetry, one substitutional and the other interstitial, and that the crystal-field split ground-state levels of Er+3at either site are of the symmetries (in order of increasing energy) &Ggr;6, &Ggr;8, &Ggr;8, &Ggr;7, and &Ggr;8. We predict that the highest &Ggr;8level of the low-temperature defect has not yet been resolved, which is why that defect exhibits only four levels. The symmetries of the excited (initial) state levels, in order of increasing energy, are predicted to be &Ggr;8, &Ggr;6, &Ggr;6, &Ggr;8, and &Ggr;7. We speculate that the high-temperature defect’s Er is at an interstitial site, while the low-temperature defect’s Er is substitutional. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364043

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A calculation on intersubband optical transitions including heavy and light hole bands to spin-split-off band in heavily dopedp-type GaAs/AlGaAs multi-quantum well (MQW) structures is reported. The analysis is focused on elucidating the recent experimental observation of normal incidence absorption and photocurrent at photon wavelengths of 2–3 &mgr;m, in addition to the already-observed absorption at around 8 &mgr;m in heavily dopedp-type GaAs/AlGaAs MQW structures. The calculation is an extension of our previous study which only includes transitions between heavy and light bands. The analysis, in which Hartree and exchange-correlation many-body interactions are taken into account within one-particle local density approximation, shows that normal incidence absorption occurs in two wavelength regions over the transition energy range higher than the barrier height forp-type GaAs/AlGaAs superlattices with well doping of 2×1019cm−3. One region has broad absorption peaks with coefficients of about 5000 cm−1at around 8 &mgr;m, and the other has two rather sharp peaks at 2.7 and 3.4 &mgr;m with 1800 and 1300 cm−1, respectively. The result agrees reasonable well with the experimental observation in general absorption features. The existence of multiabsorption wavelengths in heavily doped GaAs/AlGaAs MQWs may allow the design of multicolor normal incidence photodetectors. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364044

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A detailed study of the refractive indices of the narrow-band-gap III–V semiconductors is presented. The calculation is based on a numerical Kramers–Kronig analysis of the carrier-related imaginary part of the dielectric function. The near-band-gap refractive index spectra of InSb, InAs, and GaSb for different free-carrier densities are obtained. The density dependence of the refractive index is analyzed and a comparison with the results from the Drude theory is performed. The refractive indices of In1−xGaxSb and InAsxSb1−xternary alloys are also investigated. The obtained results are expected to be useful in the design optimization of the phase modulators and the laser diodes for midinfrared applications. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365360

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The dopant effect of In0.5Ga0.5P/GaAs epitaxial layers has been studied using Raman spectroscopy. Our study of the LO phonon-plasmon interaction shows one mode behavior even though In1−xGaxP alloys are known to have partial two model behavior. We attribute this apparent one mode behavior to enhanced charge transfer from In–P bonds to Ga–P bonds which is induced by the local lattice distortion originating from bond length mismatch. Our work also shows a TOmmode between the InP-like LO mode and the GaP-like LO mode. In terms of this TOmmode, we discuss the valley depth ratio,b/a, which was formerly attributed in the literature to CuPt-type ordering without clear explanation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364045

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Strained-layer multiple quantum well (MQW) InAsP/InP optical modulators have been fabricated from layers grown by metal-organic vapor phase epitaxy. The devices are a series ofp-i(MQW)-nphotodiodes in which the active core regions consist nominally of 25 periods of 10 nm InAsP quantum wells of 4.4&percent;, 10.0&percent;, 15.6&percent;, and 26.4&percent; As composition separated by 10 nm InP barriers. Structural parameters for the samples were obtained using high-resolution x-ray diffraction rocking curves and transmission electron microscopy. The series contains samples with both coherently strained and partially relaxed multi-layers where the relaxation is characterized by misfit dislocations. The band offsets for the heterostructures were determined by fitting the energy positions of the optical absorption peaks with those computed using the Marzin–Bastard model for strained-layer superlattices [as in M. Beaudoin &etal;, Phys. Rev. B53, 1990 (1996)]. The conduction band discontinuities thus obtained are linear in the As composition(7.56±0.08 meV per As &percent; in the InAsP layer) at low and room temperature for As concentrations up to 39&percent;, and up to 17&percent; average relaxation. Comparisons between the coherently strained and partially relaxed samples demonstrated a broadening of optical transition linewidths due to relaxation which appears to be of minor consequence for optical modulator devices as the essential optical and electrical properties remain intact. The electric field-dependent red-shift of then=1 electron-heavy hole transition was measured by a photocurrent method and found to be enhanced in structures with lower barrier heights. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365549

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present an ensemble Monte Carlo study of the thermalization and relaxation of photoexcited heavy and light holes in gallium doped p-type germanium for doping densities of1×1016and7×1016 cm−3. We analyze recently published data obtained by picosecond excite-and-probe absorption spectroscopy in the mid infrared, i.e., with excitation energies of 124 and136meV, and at lattice temperatures of 30, 80, and300K. The Monte Carlo simulations contain all types of intraband and interband hole–hole and hole–optical–phonon scatterings, as well as free-carrier-induced ionizations of neutral acceptors and captures of free holes by ionized acceptors. Good agreement is found between the simulated and the measured absorption changes as functions of time. Quite generally it turns out that the initially created nonthermal features in the heavy- and light-hole distribution functions persist to times of typically twice the pump–pulse duration, thereby noticeably contributing to the initial nonlinear response. For longer times, the practically thermalized distributions give absorption changes which exclusively reflect the cooling dynamics of the heavy holes. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364046

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effective dielectric function, effective absorption coefficient and effective refractive index for a model of porous silicon (PS) are calculated using the volume and surface averaging method. The model consists of periodic Si wires with different surface coverages. This approach allows to obtain analytical results within certain approximations. The method uses experimental parameters to characterize the bulk and the surface. We choose the bulkc-Si, and cover it with three different possible surface skins: siloxanes,a-Si:H and SiO2. The results are compared with PS experimental data and other theoretical approaches for silicon wires. We obtain good agreement for certain coatings. Our results emphasize the important role of surface coatings in the effective response of porous silicon. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364047

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effects of microwave fields on recombination processes, which are responsible for the optical detection of cyclotron resonance (ODCR) in 4H and 6H SiC epitaxial layers, have been investigated. We present experimental evidence indicating that the dominant mechanism of ODCR in SiC, at low temperatures and in a common range of microwave power (<200 mW), is microwave-induced lattice heating under the cyclotron resonance conditions. The results also show that at low temperatures and low microwave power the dominant scattering mechanism is impurity scattering, while carrier scattering by lattice phonons dominates under high microwave power conditions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364048

出版商:AIP    

年代:1997

数据来源: AIP