摘要:

A zero‐field spontaneous relaxor‐ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3(PST). This behavior is different from that of other relaxors, where such transitions occur only under the field. A highly disordered PST that has the wide relaxation spectrum typical of relaxors is shown to transform spontaneously into a macroscopic ferroelectric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Dielectric properties of PST, with and without defects, are analyzed. For the interpretation of the observed properties, a model invoking an additional nonpolar phase is proposed. This model does not imply a freezing in the system. At the low‐frequency limit, it is possible to account for the Vo¨gel–Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption of an exponentially wide relaxation time spectrum that shrinks on heating. The presented approach interprets the observed proximity between the ferroelectric phase transition temperature and that of the freezing temperature obtained from the VF relation.

ISSN:0021-8979    

DOI:10.1063/1.354300

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We have measured the number of photons/cm2over the energy range 1.95 to 3.35 eV, required to form a negative‐relief image in 2000 A˚ films of As2S3using 100 A˚ films of evaporated Ag, Ag2Te, Ag2Se, and AgBr. We have observed sensitivity below the band gap of As2S3in all cases. We have observed an increase in sensitivity by several orders of magnitude with increase of photon energy between 1.95 and 3.35 eV. The sensitivity decreases from Ag to the semimetals Ag2Te and Ag2Se to the wide band gap AgBr by the ratio 1:50:2500 at 2.4 eV. Assuming an internal photoemission model as proposed by Goldschmidt and Rudman, we extract barrier heights of 1.71, 2.10, 2.22 and 2.42 eV for Ag, Ag2Te, Ag2Se, and AgBr.

ISSN:0021-8979    

DOI:10.1063/1.354301

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The phenomenon of superlinear photoconductivity in which the photocurrent varies as a power of the photoexcitation intensity greater than one, corresponding to an increase in carrier lifetime with increasing intensity, has been well known in a variety of crystalline semiconductors for many years. Description of the phenomenon in these materials requires two kinds of competing recombination centers: (1) one with a very small capture cross section for the majority carrier and a large cross‐section ratio for minority carrier (Coulomb attractive) to majority carrier (neutral or Coulomb repulsive) capture, and (2) another with a larger capture cross section for majority carriers. This article describes a new mechanism for superlinear photoconductivity that involves only a single multivalent defect, such as the metastable dangling bond defect in amorphous silicon, which can give rise to superlinear photoconductivity provided that the capture cross‐section ratio of Coulomb attractive to neutral capture is sufficiently small. Careful examination of the variation of photoconductivity with photoexcitation intensity in samples suitably selected for dark Fermi level position is capable of providing additional information about the capture cross‐section ratios for metastable defects in amorphous silicon.  

ISSN:0021-8979    

DOI:10.1063/1.355310

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of the proximity of a bare barrier surface on the quantum efficiency of underlying GaAs/Al0.3Ga0.7As and In0.13Ga0.87As/GaAs quantum wells (QWs) is studied by low‐temperature photoluminescence. The quantum efficiency of the resonantly excited QWs diminishes with decreasing surface barrier thickness; the onset of the reduction in quantum efficiency of the InGaAs QW occurs for a barrier that is 50 A˚ thicker than for the GaAs QW. A simple model of carrier tunneling to the surface is formulated to explain the dependence of the quantum efficiency on surface barrier thickness and well width and height. This model shows good agreement with both sets of experimental data.

ISSN:0021-8979    

DOI:10.1063/1.354276

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electrical treeing is often responsible for the breakdown of insulating materials used in power apparatus such as high‐voltage transformers, cables, and capacitors. Insulation, such as crosslinked polyethylene (XLPE), used in underground high‐voltage cables usually operates at temperatures above ambient. This paper describes the characteristics of luminescence, emitted prior to electrical tree inception, at a crosslinked polyethylene‐semiconducting material (XLPE‐semicon) interface held above room temperature. Use of a sensitive light detection system showed that XLPE subjected to elevated temperatures emits luminescence even without voltage application. This light was attributed to thermoluminescence which decreased with the decrease in the concentration of the crosslinking by‐products present in the polymer. The spectra of thermoluminescence were only in the visible range. On the other hand, electroluminescence occurred when the XLPE‐semicon interface was held above room temperature and subjected to high electric stress. This light did not depend on the concentration of the crosslinking by‐products and the spectra of electroluminescence were in the visible and the ultraviolet ranges. It is proposed that XLPE‐semicon interface held at elevated temperature and subjected to long‐term voltage application initially emits both thermoluminescence and electroluminescence. As the crosslinking by‐products exude out of the polymer, thermoluminescence decreases with time and ultimately ceases, but electroluminescence occurs as long as the voltage is applied to the polymer. Although the intensity of electroluminescence emitted at high temperature was lower than that emitted at ambient, sufficient ultraviolet radiation was emitted. The ultraviolet radiation could photodegrade the polymer and lead to electrical tree inception.

ISSN:0021-8979    

DOI:10.1063/1.354277

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Thermoreflectance spectroscopy (TR) has been used to analyze Cd1−xFexTe thin films grown by radio‐frequency sputtering on glass substrates. Films with different Fe concentrationxin the range 0.05≤x≤0.15 were grown under the same conditions of substrate temperature and Ar pressure. To follow the growth of the ternary CdFeTe compound, the evolution of theE0point in the spectrum was monitored for different films. Compared to a CdTe film grown under the same conditions, the CdFeTe films show a shift in the spectrum of theE0point to higher energies forx=0.05, 0.10, and 0.15. Besides, both CdTe and CdFeTe films show evidence of the presence of a band of localized states below the band gap, probably related to the growth mechanism. For anxvalue of 0.10, the TR spectrum shows a line shape related to the presence of an extra transition that we have interpreted as due to additional levels arising from the Fe 3dorbitals.

ISSN:0021-8979    

DOI:10.1063/1.354278

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Optical damage tests have been performed on a pair ofa‐Si film onc‐Si substrate samples to determine their respective damage‐threshold values. The thermal properties of the two samples have been determined previously in a noncontact, nondamage fashion [B. S. W. Kuo, J. C. M. Li, and A. W. Schmid, Appl. Phys. A55, 289 (1992)], thus providing opportunity to relate the damage threshold to the thermal properties while controlling other factors. The samples have similar film thermal conductivity but much different interface thermal resistance. The damage test results show that the one with higher interface resistance is more vulnerable to high laser‐pulse energy. A heat‐transfer model involving both film thermal conductivity and interface thermal resistance has been developed to predict the damage‐threshold dependence on film thickness. The result using predetermined thermal properties agrees with experimental data qualitatively. It also indicates that the interface property is the dominant factor here, while the impurity‐dominant model cannot be applied, since no difference would be predicted for the two samples.

ISSN:0021-8979    

DOI:10.1063/1.354279

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Copper grows as continuous films on various silicides and metals at substrate temperatures of 310–385 °C, total pressures of 2–10 Torr, and precursor vessel temperatures of 60–80 °C (precursor mole fractions of 0.004–0.35) with hydrogen using copper (II) hexafluoroacetylacetonate [Cu(HFA)2] as the precursor. However, on TiW, TiW:N, and TiN substrates, that were previously exposed to air, copper formed only small crystalline aggregates, not continuous films, perhaps because of a thin surface layer of oxide. This conclusion was supported by Auger electron spectroscopy and scanning electron microscopy analyses.

ISSN:0021-8979    

DOI:10.1063/1.354280

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The growth of diamond thin films on a scratched silicon crystal surface by a chemical‐vapor deposition technique is reported. The substrate was bombarded by sputtered carbon from a graphite target in a helium dc glow discharge, and subsequently exposed to atomic hydrogen generated by a hot tungsten filament. The resulting diamond films were characterized by Raman spectroscopy and scanning electron microscopy. Deposited film quality and growth rate were studied as functions of carbon and atomic hydrogen exposure. An increase in growth rate of diamond was observed with atomic hydrogen exposure. We also observe that only the first monolayer of carbon deposited with each exposure appears to be utilized. These observations suggest that the diamond growth is a surface reaction. Further, calculations based upon the carbon utilization in traditional hot filament reactors indicate that a gas‐phase reaction process can account for neither the growth rate nor the saturation behavior observed. Based on this work it is proposed that the growth of diamond films is governed by surface reactions, and that the necessity of gas‐phase precursors can be precluded.

ISSN:0021-8979    

DOI:10.1063/1.354281

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new algorithm that determines the evolution of a surface eroding under reactive‐ion etching is presented. The surface motion is governed by both the Hamilton–Jacobi equation and the entropy condition for a given etch rate. The trajectories of ‘‘shocks’’ and ‘‘rarefaction waves’’ are then directly tracked, and thus this method may be regarded as a generalization of the method of characteristics. This allows slope discontinuities to be accurately calculated without artificial diffusion. The algorithm is compared with ‘‘geometric’’ surface evolution methods, such as the line‐segment method.  

ISSN:0021-8979    

DOI:10.1063/1.354282

出版商:AIP    

年代:1993

数据来源: AIP