摘要:

As an example of a novel class of glass‐forming liquid crystals, compound (I) was synthesized and characterized to possess a nematic mesophase betweenTgandTcas the pristine crystal was heated beyond itsTmfollowed by quenching to below room temperature. Differential scanning calorimetry (DSC) and x‐ray diffraction techniques were employed to investigate its morphological stability. It was found that the nematic mesophase persists upon annealing for a period of up to 22 h without the appearance of new phases. However, after annealing in the nematic regime over a longer period of time, thermally activated phase transformations were observed, resulting in a new crystalline phase plus the pristine crystalline phase based on DSC thermal transition data and x‐ray diffraction patterns. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113170

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Shear piezoelectric properties of a vinylidene fluoride trifluoroethylene copolymer [P(VDF/TrFE)] have been studied in a temperature range from 10 to 350 K. The shear electromechanical coupling factorsk15andk24at 290 K are found to be 0.19 and 0.20, respectively, the values being largest among the piezoelectric polymers known at present. The factorsk15andk24remain constant below 200 K, whilek24increases andk15decreases above 200 K. The effectiveness of P(VDF/TrFE) as a transverse ultrasonic transducer material was demonstrated by measurements of transverse sound velocities and absorptions in aluminum and poly(methyl methacrylate) with P(VDF/TrFE) shear transducers. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113171

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The surfaces of rubbed polyimide films for aligning liquid crystal have been studied by atomic force microscopy. The unrubbed films consisted of randomly distributed polyimide clusters of different sizes. On the rubbed surface, however, the clusters are aligned in long chains along the rubbing direction. The cluster chains were separated by about 100 nm for small rubbing strength. For higher strength the cluster chains coalesced into wider ones. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113172

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Long wavelength (∼1.55 &mgr;m) GaxIn1−xAs multiple‐quantum‐wire (MQWR) lasers have been grown by a single‐step molecular beam epitaxy technique. The MQWR heterostructure was fabricatedinsituusing the strain‐induced lateral‐layer ordering process. The wire formation was confirmed by cross‐sectional transmission electron microscopy and polarized photoluminescence spectroscopy. The 77 K threshold current densities for the MQWR laser diodes with laser cavities along [110] and [1¯10] directions show an anisotropy ratio of ∼10. Lasers with contact stripes aligned perpendicular to the quantum wire direction consistently show a lower threshold current density than those with stripes aligned parallel. The typical threshold current density for the MQWR laser with a stripe perpendicular to the quantum wires is ∼1 kA/cm2at 300 K. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.114148

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The features present in the capacitance– and conductance–voltage characteristics of [100]‐ and [111]‐oriented InGaAs/GaAs multi‐quantum well (MQW)p‐i‐nstructures under illumination are described. Sequential tunneling of electrons and holes is proposed to explain the different peaks observed, as they occur at voltages corresponding to resonant alignment of energy levels of adjacent wells. Two kinds of piezoelectric [111] devices are analyzed, with positive or negative average electric fields in the MQW region. The voltage corresponding to zero average electric field in the MQW is detected by a feature in the dark device capacitance. For the sample with negative average electric field, the capacitance characteristics clearly reflect the presence and evolution with voltage of a long range screening of the piezoelectric fields by the photogenerated carriers. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113173

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Pseudomorphic Si1−xGex/Si multi‐quantum well structures for waveguide applications have been grown on (110) Si by molecular beam epitaxy. In a series of samples the Ge fractionxhas been varied fromx=0.25 tox=0.37 andx=0.50, respectively. We have used photocurrent spectroscopy on mesa diodes to demonstrate that the absorption edge of the strained Si1−xGexquantum wells can be tuned from 1.3 to 1.55 &mgr;m by the Ge fractionx. Realization and characterization of single‐mode (110) Si1−xGexrib waveguides with end facets of high optical quality, prepared by cleaving, is reported. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113174

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this letter we report on measurements of the diffusion coefficient for Ni in unrelaxed amorphous (&agr;) Si at temperatures between 270 and 435 °C using secondary ion mass spectrometry. A diffusion coefficient of ∼3×10−3 exp[−1.30(eV)/kT] cm2/s is obtained, which is approximately six to eight orders of magnitude lower than that for interstitial diffusion of Ni in crystalline (c) Si. The diffusion process in &agr;‐Si is described by a model invoking trap‐retarded interstitial migration; the main difference between the diffusion coefficients in &agr;‐Si andc‐Si is attributed to the presence of intrinsic traps in the amorphous phase with a binding enthalpy of ∼0.83 eV. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113175

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Optical second harmonic generation has been used to study oxide‐nitride‐oxide/Si(111) interfaces. The second harmonic (SH) intensity is measured as a function of an azimuthal rotation angle at a 45° incidence angle. The SH intensities change for wet oxidation, nitridation and reoxidation. The SH intensity from wet oxide/Si(111) interface decreases as the temperature during wet oxidation increases. With subsequent nitridation, this SH intensity of the oxidized interface then decreases. However, the SH intensity increases again with reoxidation. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113176

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Small angle reflectance difference spectroscopy (RDS) has been employed to study the interface states of ZnSe/GaAs heterojunctions. A sharp resonance of &Dgr;R/Rnear 2.70 eV is observed. The resonance is due to the crossover electronic transition from the valence band of ZnSe to the quantum well states of the heterojunction, as is confirmed by two‐photon second harmonic generation spectra. The resonance persists even when the thickness of the ZnSe layer is beyond the critical thickness, suggesting that the interface remains largely intact after strain relaxation. The advantages of small angle RDS in studying buried heterojunctions over other techniques are discussed. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113177

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Results are reported of measurements of depth profiles and stability against redistribution with annealing up to 800 or 900 °C, for implanted Be, C, Mg, Si, S, Zn, Ge, and Se as dopants in GaN. The results confirm the high‐temperature stability of dopants in this material up to temperatures that vary from 600 to 900 °C. S redistributes for temperatures above 600 °C, and Zn and Se, for temperatures above 800 °C. All of the other elements are stable to 900 °C. These results indicate that direct implantation of dopants rather than masked diffusion will probably be necessary to define selective area doping of III–V nitride device structures based on these results for GaN. ©1995 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.113178

出版商:AIP    

年代:1995

数据来源: AIP