摘要:

Direct conversion of thermal energy into electrical energy using a photovoltaic cell is called thermophotovoltaic energy conversion. One way to make this an efficient process is to have the thermal energy source be an efficient selective emitter of radiation. The emission must be near the band‐gap energy of the photovoltaic cell. One possible method to achieve an efficient selective emitter is the use of a thin film of rare‐earth oxides. The determination of the efficiency of such an emitter requires analysis of the spectral emittance of the thin film including scattering and reflectance at the vacuum‐film and film‐substrate interfaces. Emitter efficiencies (power emitted in emission band/total emitted power) in the range 0.35–0.7 are predicted. There is an optimum optical depth to obtain maximum efficiency. High emitter efficiencies are attained only for low (≤0.05) substrate emittance values, both with and without scattering. The low substrate emittance required for high efficiency limits the choice of substrate materials to highly reflective metals or high‐transmission materials such as sapphire.

ISSN:0021-8979    

DOI:10.1063/1.354184

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We present a study of the optical properties of highly strained CdSe/ZnSe quantum well system. A variety of CdSe/ZnSe samples containing single quantum well or multiple quantum wells grown by molecular beam epitaxy has been studied by using low‐temperature photoluminescence (PL), photoluminescence excitation, and photoreflectance measurements. The strong PL signals associated with excitonic emissions from the samples show that the CdSe/ZnSe heterostructure system is promising in the development of laser diodes and light‐emitting diodes operating in the blue‐green range. Linewidth narrowing of PL spectra with decreasing well width is observed and attributed to alloy formation at the interface due to lateral interdiffusion. The PL signal intensities and the pressure coefficients of interband transitions are also found to depend on the well width, which can be explained in terms of strain relaxation induced misfit dislocations and the critical thickness in the heterostructure system. Our results suggest that the critical thickness for a CdSe layer coherently grown on ZnSe is less than four monolayers.

ISSN:0021-8979    

DOI:10.1063/1.354185

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Experimental data are presented on the illumination dependence of the dynamic resistance of long‐wave infrared heterojunction photodiodes and the results are interpreted using a numerical drift/diffusion device model. The diodes are double‐layer heterojunctions grown by liquid‐phase epitaxy; nominal cutoff wavelength, zero‐bias quantum efficiency, and resistance area product are 10.6 &mgr;m, 76%, and 210 &OHgr; cm2(77 K), respectively. It is shown that for a single‐sidedp‐on‐nheterojunction, the illumination dependence is due to an increase in hole current resulting from a voltage‐dependent increase in the base layer diffusion current. Calculated results are provided as a function of relative hetero‐ andp‐njunction location and applied reverse bias. The results are compared with experimental data obtained on back‐illuminated HgCdTe heterojunction diodes. An interpretation based upon the voltage dependence of the hole concentration at the depletion edge is presented and contrasted with recent models appearing in the literature.

ISSN:0021-8979    

DOI:10.1063/1.354186

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new technique called nonlinear photothermal radiometry that applies the second‐harmonic thermal waves to material inspection and nondestructive evaluation is described. The measurement results of the second‐harmonic signal show more sensitivity and higher resolution than those of fundamental signal. A theoretical model is developed and some characteristics are discussed to aid in understanding the experiment results.

ISSN:0021-8979    

DOI:10.1063/1.354187

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This paper presents a thorough theoretical study of pulsed photothermal phenomena in semiconductors. The explicit expressions of the temporal distributions of plasma wave, thermal wave, and displacement field within a finite semiconductor sample, excited by an incidence of the pulsed laser, are derived analytically. We simulate numerically the time evolutions of the pulsed photothermal optical reflectance, the pulsed photothermal optical beam deflection (i.e., Mirage effect), and the pulsed photothermal displacement signals for different characterized parameters of semiconductor samples. The theoretical works are significant to provide a quantitative time‐ and space‐resolved study of dynamic deexcitation processes and characterization of parameters of semiconductor samples under normal experimental conditions by the pulsed photothermal techniques.

ISSN:0021-8979    

DOI:10.1063/1.354188

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Cathodoluminescence (CL) in the scanning electron microscope has been used to study the upper surface and cross‐sectional samples of chemical vapor deposited diamond films. The CL emission is mainly localized at the grain boundaries of the columnar grains. The concentration of dislocation related radiative centers is higher in boundaries parallel to the growth axis than in boundaries parallel to the sample surface. The opposite occurs with the concentration of centers related to the presence of nitrogen.

ISSN:0021-8979    

DOI:10.1063/1.354189

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The spectral and temporal behavior of absorption bleaching and of self‐diffraction in ZnxCd1−xTe polycrystalline films with a nominal compositionx=0.87 have been investigated. It has been found that the optical transmission increases 15 times at a wavelength of 575 nm for a laser light intensity of 0.2 GW/cm2. In a pump and test experiment, the buildup and the relaxation timesT1of the bleaching were measured. Their values, between 10 and 40 and 20 and 45 ps, respectively, depend on the wavelengths of the pump and the test beams. The saturation intensityIswas also determined (between 1.5 and 6 GW/cm2). This absorption saturation effect can be related to the high density of crystallite grain surface states as well as to other imperfections in the polycrystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photogenerated carriers. Then, in self‐diffraction experiments, the phase relaxation timeT2=(10±5) ps of the localized surface states has been determined. The long duration ofT2and the short energy lifetimeT1indicate that elastic scattering processes are highly suppressed in polycrystalline material when compared to monocrystals, while their coherence time is increased.

ISSN:0021-8979    

DOI:10.1063/1.354190

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The dependence of the bulk (&bgr;b) and surface (&bgr;s) absorption coefficients of AgClxBr1−xcrystals and of polycrystalline fibers at 10.6 &mgr;m on the composition (x) of the material were investigated. The bulk coefficient &bgr;bfor AgBr was about 0.5×10−4cm−1and increased gradually with increasing molar concentrationxof the Cl−ions up to 2.5×10−4for pure AgCl crystals. Luminescence properties of these crystals as well as effects of thermal treatments on the absorption, luminescence, and conductivity were also investigated. A correlation was found between the effects of these treatments on the infrared (IR) absorption, the luminescence, and conductivity of the various samples. Experimental results indicate that the absorption of the silver halide crystals at 10.6 &mgr;m is mainly due to cation vacancies bound to dislocations. The absorption of the fibers was found to be greater than that of the crystals of the same composition. This is apparently due to defects induced by the heating under pressure, applied during the production of the fibers by extrusion, and the subsequent fast quenching to room temperature. A simple method for reduction of the IR absorption is proposed.

ISSN:0021-8979    

DOI:10.1063/1.354191

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Photoluminescence and absorption properties of photo‐induced chemical vapor deposition produced SiO2films have been studied to identify the defects and the other imperfections. Two kinds of distinct absorption peak have been found which lie at 6.3 eV for low temperature deposition (around 30 °C), and at 7.6 eV for high temperature deposition (around 300 °C). Photoluminescence peaks at 2.7, 3.6–3.8, and 4.4 eV are found in the film deposited at high temperature, but the 4.4 eV peak is not seen in film deposited at low temperature. Intensity changes of the photoluminescence and absorption peaks have been obtained in various films which were deposited at different gas‐flow‐rate‐ratios of Si2H6/O2and were annealed in O2and N2ambients. It is considered from the treatment dependence of these peaks that the absorption peak at 7.6 eV and photoluminescence peak at 2.7 eV are attributable to oxygen‐vacancies, but the absorption peak at 6.3 eV relates to oxygen‐excess defects.

ISSN:0021-8979    

DOI:10.1063/1.354192

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Recombination and generation at slightly mismatched heterojunctions under optical excitation conditions are investigated with a model based on the ambipolar diffusion equation. This model is used to fit experimental photoluminescence power dependencies in order to analyze interface‐related parameters as a function of lattice mismatch. Power dependencies are derived for photoluminescence of directly excited surface layers and photoluminescence generated indirectly by carrier diffusion or photon recycling through the interface region. Experimental results are presented for AlGaAs/GaAs and GaInP/GaAs heterostructures.

ISSN:0021-8979    

DOI:10.1063/1.354193

出版商:AIP    

年代:1993

数据来源: AIP