摘要:

The encapsulation of liquid crystals in a polymer matrix produces a material with interesting and potentially useful optical properties. The earliest efforts generated a dispersed phase whose dimensions were comparable to optical wavelengths and therefore afforded materials which were highly scattering. These materials have already been found useful for display applications. We have found that when these materials are fabricated so that the dimensions of the dispersed phase are substantially smaller than the incident radiation wavelengths, the material is far less scattering. dc Kerr measurements on these liquid‐crystal composites yield quite large values, ranging as high as 30 000 times CS2This activity is shown to be a strong function of the liquid‐crystal loading and the temperature.

ISSN:0021-8979    

DOI:10.1063/1.344939

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Photoluminescence spectra, carrier lifetime, and laser action of Hg1−xCdxTe (x=0.5) layers grown by liquid‐phase epitaxy on a CdZnTe substrate were studied as a function of temperature. The minority‐carrier lifetime was measured from the recovery of the luminescence signal following pulsed photoexcitation. Photoluminescence spectra of the annealed sample exhibited two lines, 15 meV apart, whereas in an as‐grown film one broad line was observed. The transition mechanisms determining the temperature variation of the peak energy and the linewidth are discussed. The luminescence intensity increased rapidly from 300 to 50 K, but leveled off at low temperatures suggesting carrier freezeout in accord with resistivity measurements. Lasing was observed from a cleaved stripe of the wafer using photopumping with a pulsed GaAs laser. The nominal threshold power density increased from 37 W/cm2 &mgr;m at 12 K to 380 W/cm2 &mgr;m at 130 K, in agreement with threshold calculations. Far‐field pattern widths perpendicular and parallel to the film plane of 3.2° and 2.8°, respectively, were measured. Lateral guiding in this device is discussed. The laser wavelength decreased slowly with the photopumping power, suggesting gain pinning.

ISSN:0021-8979    

DOI:10.1063/1.344940

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Resonant second‐order Raman scattering of zone‐center longitudinal‐optical phonons [2LO(&Ggr;)] in Hg0.8Cd0.2Te and Hg0.7Cd0.3Te is reported. Three 2LO(&Ggr;) components are seen, which are assigned as LO1+LO1,LO1+LO2and LO2+LO2combinations (where 1 denotes CdTe‐like and 2 denotes HgTe‐like). The intensity of these bands is very sensitive to crystal damage, making this a very useful probe of near‐surface crystal order. The detection of Te inclusions by Raman microscopy is reported and a variation in peak position is attributed to different inclusions being under varying tensile strain. Bromine‐methanol etching of Hg1−xCdxTe is shown to lead to a buildup of Te on the surface, while an increase in intensity of the 2LO(&Ggr;) modes of the Hg1−xCdxTe reflects removal of surface damage.

ISSN:0021-8979    

DOI:10.1063/1.344941

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper time‐resolved measurements of Mn2+luminescence in ZnS electroluminescent layers grown by atomic layer epitaxy are reported. The Mn concentration ranges from 0.03 to 1.4 mol %. The intensity of the luminescence versus time is cast in the form of a kinetic equation instead of an algebraic formula. This allows one to determine on a purely phenomenological basis the kinetic rates of mono‐ and bimolecular processes responsible for the relaxation. We show by a Laplace transform analysis technique that the long‐time behavior is not dominated by the lifetime of the single Mn2+ion in the host matrix. The influence of the electrical properties is evidenced on the second‐order kinetic rate. The methods used provide a general framework for investigating complex decay processes.

ISSN:0021-8979    

DOI:10.1063/1.344942

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Synchrotron radiation photoemission studies show that thin Yb overlayers (0.3–4.1) monolayers) enhance the oxidation of GaAs(110) surfaces. The magnitude of the promotion effect varies as a function of Yb coverage. The oxidation reaction products involve several nonequivalent oxidation states of As and Ga. The specific catalytic activity of the pure divalent Yb overlayers in promoting GaAs oxidation appears lower than that of Sm overlayers containing both Sm2+and Sm3+species. The spectroscopic signature of the oxidation reaction products, instead, is compellingly similar for the two rare‐earth promoters. We propose that the oxidation promotion mechanism is related, in both cases, to the decomposition of metal/semiconductor interface reaction products upon exposure to oxygen, and that the rare‐earth atomic valence has only a limited influence on the promotion mechanism.

ISSN:0021-8979    

DOI:10.1063/1.344943

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Photoresist etching mechanisms in O2abd SF6microwave plasmas are investigated using x‐ray photoelectron spectroscopy (XPS) and etch rate measurements. Experiments are performed in a microwave multipolar plasma using an electron cyclotron resonance at 2.45 GHz and independent rf biasing at 13.56 MHz. The photoresist etch rates are studied as a function of the parameters of the plasma polymer interaction. As in an O2plasma, the etch rate in SF6exhibits a two‐step evolution with ion energy as well as a monolayerlike adsorption of atomic fluorine on photoresist. The relationship between the surface mechanisms deduced from the etch kinetics and the surface compositions analyzed by XPS is explored. The effect of reactive species concentration, intensity of ion bombardment, and surface temperature on etching and/or degradation of the photoresist is investigated. In particular, the phenomenon of resist damage, described as a graphitization of the polymer layer, is shown to appear when the mechanical effects of ion bombardment become significant with respect to the chemical effects.

ISSN:0021-8979    

DOI:10.1063/1.344944

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The first demonstration ofinsituelectron‐beam (EB) lithography is reported, where a photo‐oxidized surface thin layer of GaAs is used for a resist. TheinsituEB lithography sequence consists of five processes, i.e., preparation of a clean GaAs surface, photo‐oxidation for a resist film formation, direct patterning of the oxide resist by EB‐induced Cl2etching, Cl2gas etching of GaAs surface for pattern transfer, and thermal treatment in an arsenic ambient for resist removal and surface cleaning. The GaAs wafer is never exposed to air throughout all of the above processes to avoid an unintentional surface contamination. The minimum electron dose required for patterning of the GaAs oxide resist is about 5×1016cm−2. An overgrown layer on the patterned GaAs surface shows a good surface morphology, which strongly indicates that this technology makes it possible to repeat crystal growth and surface patterning.

ISSN:0021-8979    

DOI:10.1063/1.344945

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The study presented in this paper relates mostly to impact testing of polymers and similar materials to measure their crack resistance and dynamic properties. For some impact testers, then, at low impact velocities the initial transient forces can be very small and short lived so as to be insignificant. This is so that the specimen mostly experiences a steadily increasing strain until it fails. At higher impact velocities both transient and generally increasing strain are both significant in taking the specimen to its failure point. With very high impact velocities, the specimen can fail mostly due to the initial impact forces. It is possible in some cases to cushion the specimen from the initial higher impact transients and related secondary effects such as bounce at the contact points. This is if a steadily increasing strain to fail the specimen is the main requirement. Of course, this is only feasible when the impact forces are not too dominant. However, as material technology has rapidly advanced so dynamic as well as toughness properties of materials have much improved. Also, the variety and mix of properties now available in different materials has greatly increased. With these trends, testing that can combine well impact, strong dynamic forces and steadily increasing strain loading of the specimen is of increasing interest. This can be so that the force‐time curve of such testing to failure of a specimen can also be regarded as its dynamic performance signature. A problem is in arranging and monitoring the dynamic forces of such tests so as to be able to analyze effectively test results. These are the factors examined in this paper.

ISSN:0021-8979    

DOI:10.1063/1.344946

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A systematic Fourier‐transformed infrared‐spectroscopy study of oxygen and carbon in isolated form and as complexes in the silicon lattice has revealed a direct correlation between the decrease of substitutional carbon concentration and the decrease of interstitial oxygen concentration during 750 °C annealing. At a concentration exceeding 2 ppma, carbon was also found to enhance oxide precipitate growth. After completing a three‐step annealing (1100 °C+750 °C+1000 °C), an oxide‐precipitate‐related IR‐absorption band was observed. The changes of the IR‐absorption band were correlated with annealing‐induced changes in the state of carbon. A direct incorporation of carbon into oxide precipitates, and/or carbon interaction with silicon self‐interstitials generated during oxygen precipitation, are suggested to have an effect on reducing lattice strain associated with the oxygen‐precipitation process.

ISSN:0021-8979    

DOI:10.1063/1.344947

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Monitoring the reflectivity of optically thin Au films on the ends of multimode optical fibers during exposure to Hg vapor provides a new means of determining the nature and extent of chemisorption and reaction of Hg with Au. Upon Hg vapor exposure, a freshly deposited Au film shows an initial increase in reflectivity, followed after some time by a substantial reflectivity decrease. The initial reflectivity increase is a result of the chemisorption of Hg on the Au, augmenting the film’s optical thickness. The subsequent reflectivity decrease, which is observed only for saturated or near‐saturated Hg vapor concentrations (p/psat>0.1), is a consequence of amalgamation of the Au by the Hg. The amalgamation process, which begins at grain boundaries when multilayers of Hg exist on the surface of the Au, results in the formation of large voids in the Au film. Energy‐dispersive x‐ray fluorescence spectroscopy shows the resulting amalgam to have the approximate composition Au2Hg3. Reflectivity measurements have been made as functions of Hg partial pressure and of Au film thickness; models are presented which describe both behaviors. Detection of Hg vapor in the parts per 109concentration range has been demonstrated, suggesting that this effect could be used to monitor Hg vapor concentrations at the Occupational Safety and Health Administration exposure limits.

ISSN:0021-8979    

DOI:10.1063/1.344948

出版商:AIP    

年代:1990

数据来源: AIP