摘要:

High‐quality ultraviolet photoconductive detectors have been fabricated using GaN layers grown by low‐pressure metalorganic chemical vapor deposition on (11⋅0) sapphire substrates. The spectral responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped sharply by almost three orders of magnitude for wavelengths longer than 365 nm. The kinetics of the photoconductivity have been studied by the measurements of the frequency‐dependent photoresponse and photoconductivity decay. Strongly sublinear response and excitation‐dependent response time have been observed even at relatively low excitation levels. This can be attributed to redistribution of the charge carriers with increased excitation level. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115385

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We use molecular beam epitaxy to grown coherently strained InGaAs islands on (100) GaAs substrates. The islands show room‐temperature photoluminescence at 1.3 &mgr;m with a full width at half‐maximum of only 28 meV. The integrated photoluminescence intensity is comparable to that of a quantum well. The islands are formed by depositing 22 monolayers of In0.3Ga0.7As with alternating beams of In, Ga, and As2. Atomic force microscopy measurements show that the islands are ellipsoidal sections with an average peak height of 24 nm. The intersection of the islands with the (100) plane is an ellipse whose major axis is along [011¯] and has a mean length of 54 nm, and whose minor axis is along [011] and has a mean length of 36 nm. The islands form a dense array with an areal coverage of about 40%. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115386

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Our previous modeling and simulation demonstrated that a considerable increase in Si single‐crystal back surface field (BSF) solar cell performance is possible by modifications to give a so‐called delta‐BSF device. In our first measurements a widened absorption (up to &lgr;≤3200 nm) and infrared photoconductivity (where the upper wavelength limit &lgr;≤1900 nm was imposed by the emitting lamp) have been observed. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115387

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report on the laser‐induced permanent changes of the critical current (Ic) and normal resistance (Rn) of YBa2Cu3O7−x(YBCO) step‐edge Josephson junctions. The 2‐ to 20‐&mgr;m‐wide junctions were prepared from a 200‐nm‐thick YBCO film deposited by a pulsed KrF excimer laser onto 300‐nm‐high steps etched in the LaAlO3substrate. The laser modification experiments were performed by illuminating the junctions at 50 K with a focused Ar‐ion laser beam of various intensities. Depending on the illumination power density, either increase or decrease of the junctionIchas been observed. In particular, after illumination at the 0.6×105W/cm2power level, a 75% enhancement ofIcand increase of theIcRnproduct up to 25% were obtained without a measurable change in the junction critical temperature. The laser‐induced modifications were very reproducible and remained unchanged even after a subsequent room‐temperature/helium thermal cycling of the sample. Photoassisted, thermally activated oxygen redistribution in the YBCO grain boundary region is proposed to explain the observed behavior. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115388

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Ultrafine Fe powders were synthesized by mechanochemical solid‐state reduction of FeCl3by Na and subsequent removal of the reaction by‐products. X‐ray diffraction, Mo¨ssbauer spectroscopy, and transmission electron microscopy measurements showed that Fe particles with a relatively uniform particle size ∼10 nm were formed during the mechanical milling process. The powders exhibited coercivity values of 350–550 Oe, consistent with that expected for separated nanosized particles. This study demonstrates that mechanochemical processing has significant potential for the synthesis of ultrafine metal powders in an economic and efficient way. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115389

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Heating chemically prepared nanocrystals of Cu80Co20causes Co to precipitate preferentially on the surface of the Cu nanocrystals. X‐ray diffraction and x‐ray photoelectron spectroscopies show that the Co significantly retards the oxidation of the Cu nanocrystals. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115390

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new continuously tunable submillimeter source for spectroscopy and other high‐resolution applications has been developed. In this source the optical spectrum of a mode‐locked femtosecond laser is downconverted into the submillimeter region by the demodulation process of a photoconductive switch. The power generated is subsequently radiated into free space by an antenna which is integrated along with the switch on low‐temperature grown GaAs. The very high resolution is ultimately traceable to the cavity length of the laser and the stable mode‐lock frequency which results. Among the most important attributes of the sources are straightforward absolute frequency calibration, very high spectral purity, and the potential for spectral multiplexing. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115391

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report a significant decrease in the temperature coefficient of the dielectric constant (TCK) for polycrystalline ceramics of barium–strontium–titanate through admixture with tetragonal bronze‐type barium strontium niobate. For Ba0.5Sr0.5TiO3ceramics, a 10% admixture of the niobate decreases TCK by a factor of 2.5 at 10 MHz, with negligible degradation of the dielectric loss; a 37.5% admixture decreases TCK by a factor of more than 100. Dielectric constants for the mixtures are in the range of 200–500. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115392

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Atomic force microscopy (AFM) images using carbon‐coated probe tips are presented for hydrogen‐passivated Si and SiO2surfaces. Tapping mode measurements on these surfaces demonstrate that the image quality and probe tip wear characteristics are dramatically improved for scans performed with the carbon coated tips. The reasons for the improved quality of AFM imaging and reduced tip damage are discussed. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.115393

出版商:AIP    

年代:1995

数据来源: AIP