摘要:

In this review article a comprehensive analysis of the developments in ultraviolet (UV) detector technology is described. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further considerations are restricted to modern semiconductor UV detectors, so the basic theory of photoconductive and photovoltaic detectors is presented in a uniform way convenient for various detector materials. Next, the current state of the art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main efforts are currently directed to a new generation of UV detectors fabricated from wide band‐gap semiconductors the most promising of which are diamond and AlGaN. The latest progress in development of AlGaN UV detectors is finally described in detail. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362677

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

From a knowledge of the characteristic constant of an antenna or scatterer, simple expressions are found for the modal bandlimits and near‐field sample spacing in planar, cylindrical and spherical coordinates required to reconstruct the fields everywhere outside the antenna or scatterer (except possibly within a small fraction of a wavelength from discontinuities in the surface of the antenna or scatterer). For resonant antennas and scatterers the modal bandwidths can be much larger and the required sample spacings much smaller than for nonresonant antennas and scatterers. The modal bandwidths also determine upper bounds on the gain and effective area of antennas, and on the total and bistatic scattering cross sections of scatterers. The maximum possible radius of the significant reactive fields of an antenna or scatterer is shown to equal the radius of the maximum possible effective area of an antenna, or the radius of the maximum possible total scattering cross section of a scatterer. For resonant antennas and scatterers, this radius can be much larger than the radius of the sphere circumscribing the physical antenna or scatterer. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362683

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Guided modes propagating in dielectric material surrounded by periodic two‐dimensional photonic band‐gap material are theoretically studied using the plane‐wave expansion and a supercell. Two opposite boundary designs heavily affecting optical feedback at zone edge (Bragg reflection) are investigated. Limits to the validity range of simple bi‐mode coupling schemes in these periodic guides are emphasized and coupling coefficients presented in adequate cases. Mode symmetries are studied and related to dispersion and frequency‐width relationship. Finally, the smallest guide thickness allowing for a midgap mode is found to be much smaller than the analogous one‐dimensional case, raising prospects for enhanced light‐matter interaction. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362419

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present a simple experimental procedure that uses a slowly rotating e´talon to measure simultaneously the electro‐refraction and electro‐absorption in a poled polymer. Both effects generally contribute to the measured signal from such material systems and can be distinguished by rotating the sample and observing asymmetric peaks in the signal. The experimental results show the expected increase in both electro‐refraction and electro‐absorption as the probe wavelength approaches the absorption band of the chromophore. Furthermore, the dispersion of the complex electro‐optic coefficient displays a periodic variation that we attribute to multiple‐e´talon interference. The stratified nature of the thin‐film structure causes the multiple‐reflection interference. This artifact will pollute most of the standard electro‐optic characterization techniques for poled‐polymer films. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362420

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Light‐emitting diodes have been fabricated from self‐assembled multilayers of poly(p‐phenylene vinylene) (PPV) and two different polyanions; polystyrene sulfonic acid (SPS) and polymethacrylic acid (PMA). The type of polyanion used to assemble the multilayer thin films was found to dramatically influence the behavior and performance of devices fabricated with indium tin oxide and aluminum electrodes. Light‐emitting devices fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m2, a thickness dependent turn‐on voltage and classical rectifying behavior with rectification ratios greater than 105. In sharp contrast, the devices based on SPS/PPV exhibited near symmetric current–voltage curves, thickness independent turn‐on voltages and much lower luminance levels. The significant difference in device behavior observed between these two systems is primarily due to a doping effect induced either chemically or electrochemically by the sulfonic acid groups of SPS. It was also found that the performance of these devices depends on the type of layer that is in contact with the Al top electrode thereby making it possible to manipulate device efficiency at the molecular level. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362421

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present a rigorous and convenient approach for estimating the resonant states from the pole of the scattering matrix (reflection coefficient) in the complex energy plane and for determining the resonant tunneling time from the spacing of the energy doublets arising at resonance. The numerical calculation is much faster than numerical integration of the Schro¨dinger equation. When we apply this method to the coupling of two identical quantum wells, it is shown that the usual and naive estimation of resonant tunneling time through the Wentzel‐Kramers‐Brillouin method is invalid. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362422

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Spatially nonuniform optical illumination of semiconductors containing metastable defects, such as GaAs:EL2 and DX‐related defects in III–V and II–VI semiconductors, induces metastable photorefractive and free‐carrier gratings. Detailed calculations of the space‐charge gratings in these materials are made as functions of fringe spacing and optical exposure. In both defect cases a uniquenonequilibriumFermi level, obtained from a charge balance approximation, can be defined that describes the spatial charge transfer and the resulting modulation of the optical properties of the semiconductor, without resorting to multiple quasi‐Fermi levels. The process of writing holographic gratings with low intensities is analogous to adiabatic modulation doping. The transfer of charge from photoquenched regions to nonphotoquenched regions determines the highest spatial resolution that can be supported in the materials. In the case of EL2, the modulation doping produces ap‐i‐p‐idoped superlattice. The DX defects support smaller Fermi level modulation, but have larger optical effects based on persistent photoconductivity and free‐carrier gratings. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362423

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This investigation focuses on low‐pressure S2molecular rf discharges for use as diffuse radiators. Cylindrical fused silica S2–Ar and S2–Ne lamps have been tested at 13.56 MHz and the absolute radiative efficiencies have been measured. The radiation is emitted primarily between 2800 and 3500 A˚. The UV radiative efficiencies were studied as a function of input power, buffer gas pressure, discharge radius, and applied external cooling. The maximum observed efficiency for a low power density, diffuse rf discharge is 7%. The measured UV efficiencies of the 13.56 MHz discharges are somewhat lower than those of low‐pressure S2–Ar dc discharges. A reentrant lamp design has been tested as well and no improvements in efficiency were observed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362424

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Aluminum metal‐induced crystallization and doping of hydrogenated amorphous silicon (a‐Si:H) have been investigated. Aluminum was evaporated onto device qualitya‐Si:H films deposited in an ultrahigh vacuum plasma‐enhanced chemical vapor deposition system. These Al/a‐Si:H structures were annealed in the 100–300 °C range. Electrical, surface morphological, and chemical characterizations of the material were performed. The transmission line model technique was used for electrical characterization. Raman spectroscopy showed that crystallization of the interacteda‐Si:H film underneath Al pads initiates at temperatures as low as 180 °C. X‐ray diffraction analysis showed very good polycrystallinity of the interacted film. Electrical measurement, Hall measurement and x‐ray photoelectron spectroscopy analysis results revealed thata‐Si:H film in contact with Al becomes heavily doped by Al during crystallization as a result of annealing at relatively low temperatures. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362425

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in a high‐energy ball mill for different times. Structure refinement of x‐ray powder diffraction data was performed to study the structural transformations induced by mechanical and subsequent thermal annealing treatments. The mechanical alloying (MA) process induces a progressive dissolution of aluminum phase into the bcc iron phase. After 32 h of MA a single‐phase Fe(Al) bcc extended solid solution, with lattice parametera0=2.891 A˚, average coherent domain size ⟨D⟩≊50 A˚, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored the aluminum dissolution in &agr;‐iron and the precipitation of the Al5Fe2phase, whereas a nanostructured B2 FeAl intermetallic compound was observed in the annealed samples which were previously milled for 8, 16, and 32 h. In the same specimens a minority cubic phase Fe3AlCX, anti‐isomorphous with perovskite, derived from contamination of ethanol and introduced in the steel vial as a lubricant agent, was also observed. Anelasticity measurements have shown the occurrence of two main transient effects during the first thermal run. The first one occurring at 500 K in all mechanically alloyed specimens was attributed to thermally activated structural transformations, whereas the second at about 700 K was attributed to a magnetic order–disorder transition. During the second run of anelasticity measurements a relaxation peakP1in the nanostructured B2 FeAl intermetallic compound, attributed to grain‐boundary sliding mechanisms and with an activation energy of 1.8±0.2 eV was observed. In specimens milled for 8–32 h a second small peakP2at the low‐temperature tail of theP1peak was observed and tentatively attributed to a Zener‐type relaxation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362408

出版商:AIP    

年代:1996

数据来源: AIP