摘要:

As a liquid wire migrates through a solid in a thermal gradient, the cross‐sectional shape of the liquid is deformed from the symmetric equilibrium shape. A force model of the thermomigration process is derived to calculate both the nonequilibrium shape and the effect of wire size on the migration velocity for thermal gradients of different magnitude and orientation. The forces are related to the thermal gradient, diffusion in the liquid, and interface kinetics. At small wire sizes, the effects of interface kinetics become relatively more important; consequently, it is more difficult for liquid wires to migrate in the direction of the thermal gradient. In the case of a thermal gradient applied to a silicon crystal in the ⟨100⟩ direction, the shape becomes triangular, and the migration direction is sensitive at small wire sizes to fluctuations in the rate of reaction at the interface, while for a thermal gradient in the ⟨111⟩ direction, the shape becomes an unsymmetric flattened hexagon in which the migration direction at small wire sizes deviates from the direction of the thermal gradient. At sufficiently large wire sizes, nonuniform thermal gradient forces result in an unstable concave shape. The shape of a migrating wire depends only on the distribution of interface forces over the various facets of the solid‐liquid interface and on the cross‐sectional area of the liquid wire.

ISSN:0021-8979    

DOI:10.1063/1.325157

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Detailed analysis by reflection electron diffraction (RED), x‐ray diffraction (XRD), and scanning electron microscopy (SEM) was performed on thin ZnO layers which were formed under reactive and nonreactive rf‐sputtering conditions. A variety of textures and morphologies were observed. 100&percent; reproducible piezoelectric layers, preferred oriented with [002] perpendicular to the layer within 7°, could be obtained by reactive sputtering from a zinc target at rf power of 150 W, oxygen‐argon atmosphere of 8×10−3Torr with 35&percent; O2, and with the glass substrate being kept at room temperature by a cooling device. No differences in the surface‐acoustic‐wave properties were found between reactively and nonreactively sputtered ZnO layers which had similar texture and morphology. SEM techniques proved to be extremely misleading in the study of this type of layer; there is no relationship between an observed columnar structure and the texture of the layer which is determined by RED, and also between the column thickness (∼1 &mgr;m) and the actual crystallite size (∼200 A˚) which is determined by XRD. It is shown that the observed columns must be polycrystalline.

ISSN:0021-8979    

DOI:10.1063/1.325158

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The prediction of the effective electrical conductivity of a one‐dimensional polycrystal constructed from an anisotropic single crystal is described. The least upper and greatest lower bounds on the effective conductivity are derived for the isotropic polycrystal. The upper bound is equal to or less than the arithmetic mean of the principal conductivities, and the lower bound is equal to or greater than the geometric mean of the principal conductivities.

ISSN:0021-8979    

DOI:10.1063/1.325159

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Closed‐form solutions (integral representations) are obtained for the charge density and current density in the vicinity of a dislocation or a surface which pierces a junction at right angles. The total reduction in luminescence and increase in junction current are obtained as functions of two dimensionless ratios, &egr;, the recombination‐length/diffusion‐length ratio, and &eegr;, the radius‐of‐dislocation/diffusion‐length ratio, where recombination lengtha=D/sis the diffusion constant over surface‐recombination velocity.

ISSN:0021-8979    

DOI:10.1063/1.325160

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Infrared‐sensitive focal plane arrays based on extrinsic silicon, which integrate the detection and signal‐processing functions onto a single chip, are currently being developed at several laboratories. For imaging in the 3–5‐&mgr;m atmospheric window, highly doped Si : In is a leading candidate due to its spectral range, quantum efficiency, and moderate cooling requirements (50–60 K). The effects of residual boron impurities in the Si : In detector must, however, be compensated by donor concentrations to achieve these operational temperatures, so that precision compensation is a key factor for the production of uniform high‐responsivity detector material. We report here the successful use of thermal‐neutron irradiation for transmuting a small fraction of the silicon atoms into a known concentration of phosphorus donors in order to compensate Si : In detector material. Czochralski‐grown Si : In starting material of ⟨100⟩ orientation was evaluated by variable‐temperature Hall effect studies to containNIn=2.5×1017cm−3,NB=1.6×1014cm−3, andNP=0.6×1014cm−3. Samples were irradiated in a light‐water reactor and then thermally annealed for lattice damage recovery. Detector measurements indicate detectivitiesD* which approach background‐limited performance at temperatures of 55 K at a photon flux of 1015cm−2 sec−1. Maximum photoconductive lifetimes of 3 nsec were determined from responsivity measurements and are consistent with the known phosphorus concentration (2.2×1014cm−3) introduced by the transmutation of silicon and the measured impurity concentrations in Czochralski starting material. No attempts were made to optimize the compensation density to produce improved responsivity material, but the absence of any anomalous behavior is encouraging for future efforts at more precise compensation. Measurements of thermal‐carrier concentrations in both conventionally compensated and neutron‐compensated material also reveal an additional deep‐lying acceptor level at 0.11 eV above the valence band edge in Si : In. Concentrations of (3–6) ×1014cm−3were observed. A spectral dependence beyond the indium cutoff wavelength which can be attributed to a 0.11‐eV level in the Si : In material is observed.

ISSN:0021-8979    

DOI:10.1063/1.325161

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The charge transfer associated with the presence of photosensitizing dyes (fluorescein, erythrosin B, and rose bengal) was investigated on single‐crystal ZnO surfaces employing surface photovoltage spectroscopy. It was found that the charge transfer takes place via electron transitions from the surfaces to the conduction band. The energy necessary for these transitions is transferred to the surface states from excited states of the photosensitizing dyes. The surface states were found to be related to chemisorbed oxygen and their density is (2–4) ×1010/cm2under atmospheric pressure at room temperature.

ISSN:0021-8979    

DOI:10.1063/1.325162

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Using a modified Gaussian approximation to the depth distribution of the energy dissipation function for electron bombardment, an analytical expression was derived for electron‐beam‐induced current (EBIC) at a Schottky barrier parallel to the bombarded surface. Comparison of theory and experiment for the voltage dependence of EBIC for 14 specimens (includingp‐ andn‐type GaAs and Si) provided values for the diffusion length of excess carriers, the thickness of the metal layer of the Schottky barrier diode, and the average energyErequired to generate a hole‐electron pair. Diffusion lengths ranging from 0.41 to 55 &mgr; were determined with an estimated accuracy of ±20&percent;. The values ofEwere found to be 3.75±0.11 eV for Si and 4.68±0.14 eV for GaAs. The unique advantages of this method for measuring small diffusion lengths are emphasized.

ISSN:0021-8979    

DOI:10.1063/1.325163

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The electric‐field‐dependent longitudinal diffusion coefficient for charge carriers in a two‐valley semiconductor is calculated by modeling the carrier movement as two simultaneous coupled random walks. The first random walk represents carrier scattering from one valley to another, and the second describes the motion of a carrier within a single valley, with the carrier position in the first walk determining the parameters of the second walk. The calculated intervalley transfer diffusion coefficientDtrcontains earlier calculations ofDtras special cases. An agreement between calculated and measured low‐field diffusion coefficients for electrons inn‐GaAs at room temperature is obtained when the random time interval that a carrier spends in a valley is assumed to have a gamma distribution.

ISSN:0021-8979    

DOI:10.1063/1.325164

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

It has been the usual practice to solve the continuity equation for the case of steady excitation and presume that the same results are valid for pulse excitation of the photoexcited carriers. However, it has been shown in the present analysis that this assumption is valid for the exciting pulse which is of longer duration than the mean lifetime &tgr; of the photoexcited carriers. In the case of excitation of photoconductivity by picosecond pulses, which are now available from the mode‐locked laser system, the pulse duration is much smaller than &tgr; and the results of steady‐state excitation are no longer applicable. Moreover, the magnitude of the photoconductivity depends on the pulse durationTand is independent of &tgr;. In fact, the photoconductivity is found to exhibit a transient behavior.

ISSN:0021-8979    

DOI:10.1063/1.325165

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A theoretical investigation of the photovoltage produced in insulated CdS‐type crystals excited by strongly absorbed radiation has been carried out, taking into account trapped electrons and trapped holes in addition to free electrons. Expressions for the photovoltage dependence on intensity for isolated crystals have been obtained over a wide range of excitation levels. A plot of typical density profiles for the free electrons, trapped electrons, and trapped holes as functions of position is given. It is shown that the results can readily be extended to nonisolated crystals.

ISSN:0021-8979    

DOI:10.1063/1.325166

出版商:AIP    

年代:1978

数据来源: AIP