摘要:

57Fe Mo¨ssbauer spectra were taken at 25 and 293 K on four representative specimens of compounds of the series R2Fe14B (R=Ce, Nd, Gd, and Y). In all these cases it proved possible to analyze the spectra in terms of six subspectra associated with the six crystallographically nonequivalent Fe sites in the tetragonal R2Fe14B unit cell. Assuming a linear relation between the observed hyperfine fields and magnetic moments it follows from our analysis that, although the average Fe moment in R2Fe14B is equal to that of &agr;‐Fe, there are Fe sites that give rise to moments much higher and much lower than the average value of 2.2 &mgr;B/Fe.

ISSN:0021-8979    

DOI:10.1063/1.334865

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Metal‐carbon‐metal structures were fabricated using either gold or aluminum evaporated electrodes and rf plasma (methane) deposited ‘‘diamondlike’’ carbon films. ac‐conductance and capacitance versus voltage and frequency (10 Hz to 13 MHz) data were taken to determine the dielectric properties of these films. Conductance versus frequency data fit a generalized power law, consistent with both dc and hopping conduction components. The capacitance versus frequency data are well matched to the conductance versus frequency data, as predicted by a Kramers‐Kronig analysis. The dielectric loss tangent is nearly constant at 0.5 to 1.0% over the frequency range from 1 to 100 kHz. The dc resistivity is above 1013&OHgr; cm, and the dc breakdown strength is above 8×106V/cm in properly prepared samples.

ISSN:0021-8979    

DOI:10.1063/1.334866

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The optical properties of undoped &agr;‐Si:H films prepared using magnetron sputtering at different deposition conditions were studied by measuring their transmittance and reflectance between &lgr;=0.25 and &lgr;=1.5 &mgr;m and their thickness. The extracted optical constants are interpreted to give values of the band gap. Values of dark conductivity and activation energy are also obtained. The study has been extended to structures SnO2/&agr;‐Si:H/substrate. From measurements of transmittance and reflectance of the system optical constants of the components can be extracted.

ISSN:0021-8979    

DOI:10.1063/1.334867

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Interfacial elastic strain induced by the lattice parameter mismatch between epilayer and substrate results in significant energy–band‐gap shifts for III‐V alloys. The epilayers used in this study are GaxIn1−xAs on (100) InP and GaxIn1−xP on (100) GaAs prepared by organometallic vapor phase epitaxy. For layer thicknesses between 1 and 1.5 &mgr;m, and &Dgr;as.f./a0≤3.5×10−3the misfit strain is assumed to be accommodated elastically. The energy–band‐gap shifts are determined by comparing the photoluminescence peak energies of the epilayers with the best experimental relation of band gap versus composition for unstrained layers. A calculation of the energy–band‐gap shift due to biaxial stress made for GaxIn1−xAs is found to agree with the photoluminescence measurements. In addition, a comparison of the energy–band‐gap shift for GaxIn1−xP shows a clearly different dependency for tensile and compressive strain, in good agreement with calculated results.

ISSN:0021-8979    

DOI:10.1063/1.334817

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A formalism to study the effect of alloy disorder and interface roughness on the linewidths of excitonic emission spectra in quantum‐well structures is developed. The study includes the cases where the alloy forms (a) the barrier region, (b) the well region, and (c) both the barrier and well regions of the quantum‐well structures, and demonstrates the importance of alloy quality in all three cases. The relative importance of the effects of alloy disorder and interface roughness on the excitonic linewidths is discussed. As an illustration, the formalism is applied to AlGaAs/GaAs, InP/InGaAs, and InAlAs/InGaAs quantum‐well structures and the results compared with the available experimental data.

ISSN:0021-8979    

DOI:10.1063/1.334818

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The spectral distribution of photoconductivity and infrared excited electroluminescence has been determined in GaP light emitting diodes at low temperatures by conventional infrared spectroscopic techniques. The observed photoresponse was found to be caused by ionization of shallow donors only, showing a peak sensitivity of 20 mA/W at 13‐&mgr;m wavelength. Spectral structures for photon energies lower than the binding energies of shallow donors are attributed to electric field assisted photoionization. An external quantum efficiency of 3×10−3was obtained being largely independent on the bias voltage and the wavelength of infrared stimulation.

ISSN:0021-8979    

DOI:10.1063/1.334819

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Carrier lifetime has been measured by the luminescence decay of a 1.3‐&mgr;m‐InGaAsP layer excited by a mode locked YAG laser at 1.06 &mgr;m. The measurements have been done as a function of excitation intensity for nearly three orders of magnitude of carrier concentration (4×1016–2×1019cm−3) and for different temperatures (between 32 and 346 K). At low and moderate carrier density, the lifetime &tgr; follows the variation with excitation of the theoretical radiative lifetime. At high carrier density (above 1018cm−3) the carrier decay rate increases more rapidly than the radiative one and around room temperature this can be accounted for by an additional recombination mechanism whose variation with excitation is typical of an Auger process. The Auger coefficient (Ca=2.6×10−29cm6 s−1) does not vary with temperature within experimental uncertainty. This suggests that though Auger recombination is for a large part responsible for the lowT0value of 1.3‐&mgr;m InGaAsP lasers, the temperature dependence of the Auger coefficient does not contribute to it.

ISSN:0021-8979    

DOI:10.1063/1.334820

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The emission spectrum of Tb3+substituted in YAG to various concentrations (xin Y3−xTbxAl5O12ranges from 3×10−5to 3×10−1) has been measured on samples prepared by two different methods. These methods are powder preparation by coprecipitation and growth of crystalline layers by liquid‐phase epitaxy. The spectrum consists of two groups of lines, one group around 550 nm originating from the5D4level and one group around 450 nm from the5D3level. The intensity of the two groups of lines depends on the method of excitation. In the case of UV excitation (&lgr;=254 nm) the emission intensity for low Tb concentrations is mainly governed by the absorption of the incident radiation. With electron excitation, the energy loss of incoming electrons to ‘‘killer sites’’ in the lattice is found to influence the emission. The ratio, however, of5D4to5D3intensity is independent of the excitation mode. This ratio tends to a constant value at low concentrations and increases strongly with increasing concentration. These effects are explained by feeding of the5D3and5D4states from 5dlevels, followed by cross relaxation, i.e.,5D3to5D4excitation transfer.

ISSN:0021-8979    

DOI:10.1063/1.334821

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Physical mechanisms responsible for nonlinear phenomena and anomalous transient response of cooled extrinsic far‐infrared photoconductors are discussed. A simple model describing carrier generation, trapping, and impact ionization is presented, which describes the transient response on fast time scales 10−3to 10−4sec, neglecting changes in space charge. Carrier heating by a dc electric field produces relatively fast, damped oscillatory response to external excitation. A small‐signal analysis of these equations is a test of stability. An analysis of the role of ideal electrical contacts and space charge is also presented. The very slow (∼1 sec) overshoot and transient response commonly observed in cooled extrinsic photoconductors is explained by the dynamics of trapped space charge near the injecting electrical contact. A small‐signal analysis determines the characteristic time constants for these processes, which are typically ∼1 sec. Calculated examples of the recombination and ionization coefficients, dcI‐Vcurves, differential equation flow diagrams, and transient response are presented for parameters typical of  p‐type Ge photoconductors doped with shallow acceptor levels, and suggestions for the design of more stable photoconductors are presented.

ISSN:0021-8979    

DOI:10.1063/1.334822

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A study of ion‐implanted zinc in GaAs has been made using three annealing techniques:e‐beam, graphite strip heating, and furnace annealing in an arsine ambient. The highest hole concentrations, 7–8×1019cm−3, were obtained using electron‐beam annealing. Graphite strip heating and electron‐beam annealing were able to electrically activate 100% of the implanted dose. The effect of strain on the activation of the zinc has been demonstrated by comparing chemical‐vapor‐deposited Si3N4with reactively evaporated AlN encapsulants.

ISSN:0021-8979    

DOI:10.1063/1.334823

出版商:AIP    

年代:1985

数据来源: AIP