摘要:

The temperature dependence of the terahertz (THz) output power and spectra from biased photoconductive switches was measured for several antenna gap widths and applied biases. The spectrally integrated THz output had a nonmonotonic temperature dependence in all cases with the value increasing by a factor of 3 from room temperature to 150 K for low biases and 100 K at high biases. An abrupt decrease in output power occurs below 90 K, and the spectrum shifts to lower frequencies as the temperature is lowered. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121329

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Optical and electron spin resonance spectroscopies have been used to investigate the thermal stability of lasers based onCr3+:LiSr0.8Ca0.2AlF6,a material in which the onset of critically inhibiting nonradiative decay at temperatures below 300 K might be expected based on results from otherCr3+-activated disordered gain media. The influence of disorder on theCr3+environment is shown to be very small inLiSr0.8Ca0.2AlF6so that the4T2→4A2fluorescence transition is not broadened relative to that inLiSrAlF6and the onset of significant nonradiative decay is also held above room temperature. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121330

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We report on experiments in which picosecond ultrasonic techniques are used to investigate the modification of interfacial bonding that results from ion implantation. The bonding is studied through measurements of the acoustic reflection coefficient at the interface. This method is nondestructive and can be used to create a map of the variation of the bonding over the area of the interface. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121276

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A type of high strain piezoelectric ceramic actuator, namely, pseudoshear multilayer actuator, is described. In this structure, a stack of prepoled rectangular piezoelectric transducer ceramic sheets are conductively bonded at alternate ends, while the bottom layer bonded on a fixed base. When driven, alternate layers elongate or shrink in the same direction through converse piezoelectric effect, which results in the actuator structure developing a strong shear motion about the face perpendicular to the bonding direction. Experimental results indicate that more than 50 &mgr;m displacement can be achieved from the top layer for an actuator consisting of 18 layers with dimensions of25.57 mm×4.02 mm×0.51 mm(1×w×t).By reducing ceramic sheet thickness and using more layers, even large displacement can be obtained, and driving voltage can also be reduced while keeping the same field level. Nonlinear piezoelectric response under high driving field further enhances the displacement level. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121331

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This letter presents experimental evidence of an anisotropic compositional modulation upon annealing in(Ni/Au)nmultilayers andAu–Nialloy ultrathin films embedded in Au layers. These materials were grown by molecular beam epitaxy, on a (001)Au buffer layer. The influence of temperature, local composition, and strain has been investigated by high-resolution electron microscopy and x-ray diffraction. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121267

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The cold-cathode electron emission properties of amorphous tetrahedral diamondlike carbon are promising for flat-panel display and vacuum microelectronics technologies. The onset of electron emission is, typically, preceded by “conditioning” where the material is stressed by an applied electric field. To simulate conditioning and assess its effect, we combined the spatially localized field and current of a scanning tunneling microscope tip with high-spatial-resolution characterization. Scanning force microscopy shows that conditioning alters surface morphology and electronic structure. Spatially resolved electron-energy-loss spectroscopy indicates that the predominant bonding configuration changes from predominantly fourfold to threefold coordination. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121332

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A combination of atomic force microscopy and scanning capacitance microscopy was used to investigate the relationship between the surface morphology and the near-surface electrical properties of GaN films grown onc-axis sapphire substrates by metalorganic chemical vapor deposition. Local regions surrounding the surface termination of threading dislocations displayed a reduced change in capacitance with applied voltage relative to regions that contained no dislocations. Capacitance–voltage characteristics obtained from these regions indicated the presence of negative charge in the vicinity of dislocations. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121268

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We have measured the thermal conductance,G,of≈1&mgr;m thick low stress silicon nitride membranes over the temperature range,0.06<T<6K, as a function of surface condition. ForT>4K,Gis independent of surface condition indicating that the thermal transport is determined by bulk scattering. ForT<4K, scattering from membrane surfaces becomes significant. Membranes which have submicron sized Ag particles glued to the surface or are micromachined into narrow strips have aGthat is reduced by a factor as large as 5 compared with that of clean, solid membranes with the same ratio of cross section to length. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121269

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We have investigated the formation of polymer walls for high polymer content liquid crystal (LC) formulations, using a patterned electric field to induce phase separation. The effect of this field on the phase separation temperature of a LC/monomer mixture is studied as a function of the photopolymerizable monomer concentration. The phase separation temperature increases with the patterned field strength. The application of a patterned field results in segregation of the LC molecules in the high electric field regions, i.e., pixels, whereas the monomers segregate in the low-field regions, i.e., interpixels. Subsequent photopolymerization results in the formation of polymer walls around the pixels. The structure of the polymer walls was investigated using scanning electron microscopy. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121333

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The current–voltage (I-V) characteristics for a metal-insulator-heavily-doped semiconductor structure are computed numerically by solving the time-dependent Schro¨dinger equation. The Fowler–Nordheim tunneling expression was found to be inappropriate to estimate the barrier potential nor found to fit the experimental results at both high and low applied fields. It is shown also that the computedI–Vcharacteristic curves agree well with the recently published experimental data forTa-Sn-OandTa2O5films at the high as well as low fields.©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.121270

出版商:AIP    

年代:1998

数据来源: AIP