摘要:

A nonoptical phase-locked shear-force microscope utilizing a quartz crystal tuning fork acting as a voltage-controlled oscillator in a phase-locked loop has been implemented. A tapered optical fiber is rigidly mounted on one of the prongs of the fork to serve as both a shear-force pickup and a near-field optical probe. The crystal is driven at its resonance frequency through positive feedback of the monitored current through the crystal. This signal is used as the voltage-controlled oscillator in a phase-locked loop. The scheme allows for scan speeds far beyond the Q-limited amplitude sensor bandwidth and exhibits excellent sensitivity for a high-Q resonator. Furthermore, given the small vibration amplitude of the tip(<0.5 nm) and the distance over which it is reduced(>6 nm), it is unlikely that the tip is making direct contact with the sample surface as has been suggested for the optical shear-force detection scheme. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118318

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Cathodoluminescence has been employed to investigate the luminescence and lateral transport properties of excited carriers at 8 K in GaAs-AlGaAs quantum well material and in submicron features fabricated in this material by magnetron reactive ion etching. A carrier diffusion length of 0.85±0.04 &mgr;m in quantum wells and a surface recombination velocity (5.4±0.8)×103m/s at etched sidewalls were measured. Also, the effect of feature size on luminescence efficiency was examined and compared with model calculations using the measured values of diffusion length and surface recombination velocity. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119124

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The development of a completely new class of artificially engineered phosphors for electroluminescent displays is proposed in which deep quantum wells embedded in a high band gap material act as radiative centers. The proof of concept of such phosphor activation by quantum wells (QWs) has been demonstrated in the CdSe/SrS multi-quantum well system prepared by atomic layer epitaxy. Various QW widths (3–15 nm) were studied. The reduction in QW width shifts the emission towards shorter wavelength and the emission spectra exhibit multiple peaks, believed to be the result of transitions from several QW levels. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118319

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Nonlinear photoconductivity effects at high excitation power in quantum well infrared photodetectors (QWIPs) are studied both experimentally and theoretically. The photoconductivity nonlinearity is mainly caused by a redistribution of the electric potential at high power, which leads to a decrease of electric field in the bulk of the QWIP. As a result of the decreased field, the photoexcited electron escape probability and drift velocity decrease resulting in a decrease of responsivity. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118320

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have used an atomic force microscope to analyze the surface of C60thin films, which have been grown epitaxially on a mica surface and illuminated with a 514.5 nm laser light at different temperatures. Two regimes of light-induced polymerization are observed. If the illumination temperatureTpis lower than 320 K, the illuminated surface shows polymer chains typically six molecules long with an intermolecular distance of 9.2±0.1Å . Because of the strain associated with the shorter inter-C60distance in the polymer compared to the bulk, the surface is buckled with a period of∼100 Å and amplitude of ∼5 Å resulting in a characteristic herringbone three-dimensional structure. For films, illuminated above 350 K, the surface shows primarily dimers and trimers with an unusually short inter-C60distance of8.8±0.1Å, this being the shortest interbuckyball distance reported so far. We also find large changes in the adhesive forces on the polymerized films compared to unpolymerized ones, suggesting that the polymer is substantially more hydrophyllic than normal C60.©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118321

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We discuss the relationship between microstructure and luminescence efficiency for heteroepitaxial films of GaN grown onc-axis sapphire substrates by metalorganic chemical-vapor deposition. We directly characterize the correlation between threading dislocations as observed by transmission electron microscopy, surface morphology as observed by atomic force microscopy, and wavelength-resolved cathodoluminescence imaging. We show that the inhomogeneity in the luminescence intensity of these films near band edge can be accounted for by a simple model where nonradiative recombination at threading dislocations causes a deficiency of minority carriers and results in dark regions of the epilayer. An upper bound for average diffusion length is estimated to be 250 nm. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118322

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have determined the densities of Si in the liquid,&rgr;l(T), and solid,&rgr;s(T), states as a function of temperature,T, by employing an image digitizing technique and numerical calculation methods in combination with an electrostatic levitator. The obtained density data can be fitted with the following equations:&rgr;l(T)=&rgr;l(Tm)−1.71×10−4(T−Tm)−1.61×10−7(T−Tm)2(g/cm3);&rgr;s(T)=&rgr;s(Tm)−2.63×10−5(T−Tm)(g/cm3),whereTmis the melting point, 1687 K, and&rgr;l(Tm)and&rgr;s(Tm)are 2.580 and 2.311 (g/cm3), respectively. The error involved in the determination is estimated to be±0.006(g/cm3). The&rgr;l(T)value smoothly varies throughTmand does not indicate a reported anomalous density variation. The&rgr;l(Tm)value is 2&percent; larger than the literature value and the coefficient of the linear temperature dependence is approximately half of a reported value. The&rgr;s(Tm)value closely agrees with the literature value. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118317

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Rutherford backscattering and channeling techniques have been used to study temperature-dependent segregation of Cu, Au, and Pd at Si–SiO2interfaces during high dose 15 keV O+bombardment of Si. Results illustrate a strong tendency for metals to be retained in an amorphous Si layer adjacent to the continuous SiO2layer formed by O+bombardment of Si. The magnitude of segregation depends on the bombardment temperature. We explain our temperature-dependent data in terms of a thermodynamic model whereby the segregation is driven by a large solubility difference between metals in amorphous Si and SiO2. Segregation is enhanced when the metal diffusivity in amorphous Si is high. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118170

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Absorption band countered on wave numbers 3504 and 6843cm−1have been observed in potassium niobate crystals. These absorption bands correspond to the transition energies of O–H stretching vibration, from the ground state to the first excited state and to the second excited state, respectively. Absorption bands around 3992 and 4476cm−1are attributed to the combination of O–H vibration plus libration. The integrated absorption intensities around 3504, 3992, and 4476cm−1are three dimensional polarization dependent which indicates the motion properties of hydrogen ions in the lattice. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118171

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Molecular dynamics simulations as well asab initioand nearab initioHartree-Fock calculations in crystalline silicon predict that the configuration of the hexavacancy that has a hexagonal ring missing from the crystal is remarkably stable. The energetics imply that it does form and is more likely to grow than to dissociate during heat treatments. Further, the energy eigenvalues and the charge distribution imply that it has no electrical or optical activity. However, it is a large void in the crystal and could be an efficient gettering center and a precursor of extended defects. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.118172

出版商:AIP    

年代:1997

数据来源: AIP