摘要:

The Stransky–Krastanow metalorganic vapor phase epitaxy growth of self-organized ZnTe islands on homoepitaxial (001)GaAs is demonstrated. The−7.4&percent;lattice mismatch of the ZnTe/GaAs heterostructure leads to a strain-driven distribution of nanoscale ZnTe islands on top of a two-dimensionally (2D) grown wetting layer. Atomic force microscopy and Rutherford backscattering spectrometry are used to determine the island dimensions and the thickness of the wetting layer. The density of the islands, their average diameter, and aspect ratio turn out to be about520 &mgr;m−2,13.6 nm, and 0.20, respectively, for a 1.2 ML thick 2D layer. Furthermore, the average aspect ratio of the islands decreases by increasing the thickness of the wetting layer, as expected by the progressive extinction of the strain-driven island nucleation. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120736

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

InAs self-organized quantum dots inIn0.53Ga0.47AsandIn0.52Al0.48Asmatrices have been grown on InP substrates by molecular beam epitaxy. The dot size in InGaAs has been found to be 3–4 times larger, but the areal density about an order of magnitude smaller than that in InAlAs. Low-temperature photoluminescence (PL) of the InAs/InGaAs quantum dots is characterized by a narrow (35 meV) PL line as compared to that of InAs/InAlAs quantum dots (170 meV). Quantum dot formation increases the carrier localization energy as compared to quantum well structures with the same InAs thickness in a similar manner for both InAs/InGaAs and InAs/InAlAs structures. The effect of the barrier band gap on the optical transition energy is qualitatively the same for quantum well and quantum dot structures. The results demonstrate a possibility of controlling the quantum dot emission wavelength by varying the matrix composition. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120737

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Strong defect-specific low-frequency peaks are detected in low-temperature Raman spectra of hexagonal GaN grown by molecular beam epitaxy on sapphire substrate. The intensity of these peaks is found to be enhanced by excitation in resonance with yellow luminescence transitions. The validity of the assignment to electronic Raman scattering (ERS), as proposed before for their counterparts in cubic GaN on GaAs [M. Ramsteiner, J. Menniger, O. Brandt, H. Yang, and K. H. Ploog, Appl. Phys. Lett.69, 1276 (1996)], is confirmed. Our results imply that the observed ERS peaks are related to shallow donors which are not necessarily hydrogenic. One Raman peak at very low frequency (11.7 meV) is alternatively explained by a pseudo-localized vibrational mode. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120738

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Temperature dependent measurements of the noise parameters of a pseudomorphic high electron mobility transistor show the minimum noise figure vanishes at low temperatures for frequencies from 2 GHz up to 18 GHz and for most currents. We propose that the exponential decrease seen in the noise figure with temperature is due to a reduction of thermally activated noise sources with an activation energy of ∼95 meV. Other noise-parameter data suggest a flattening of the noise figure paraboloid as temperature is decreased. These results demonstrate the feasibility of designing a near 0.1 dB noise figure amplifier for high-frequency operation over a wide range of currents. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120739

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Light-induced metastable defects in a-Si:H are proposed to be silicon dangling bonds accompanied by pairs of hydrogen atoms breaking a silicon bond, forming a complex with two Si-H bonds. This supports the model of Branz. These defects are the analog of theH2*defect in c-Si and their energy correlates with the bond-angle strain. Several features of the annealing are well described by this defect complex. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120740

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Ballistic electron emission spectroscopy has been used to measure the electron transmittance function of both half- and quarter-electron-wavelength (using optics terminology) quantum-interference filters under zero applied voltage bias. At the design energy, these devices exhibit constructive and destructive interference, respectively. Second-derivative spectra from current versus voltage measurements clearly show both tunneling and above-barrier quasibound energy states. The spectra accurately reproduce the transmittance functions of the designed structures, attaining nearly the temperature-limited resolution at 77 K and 300 K. The presence of the above-barrier resonances has been confirmed conclusively by measurements on these complementary half- and quarter-wavelength device structures. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120741

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We have fabricated and tested two stacks of strongly coupled long Josephson junctions with access to the intermediate electrode so that they can be independently biased. In these samples, we find four families of voltage-locked states in zero external magnetic field. One of these families is identified with the zero field steps already observed and interpreted in the case of weaker coupling; the others appear to be new phenomena. The voltage spacing of one of them suggests the necessity of a refinement of the current model to obtain a full description of the dynamical states in stacked junctions. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120742

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present structural and magnetic properties of epitaxial Sm–Co/Co superlattice films prepared via magnetron sputtering. X-ray diffraction and cross-sectional transmission electron microscopy show that the films are structurally coherent. The oriented nature of the interleaved ferromagnetically “hard” and “soft” layers comprising the superlattice provides a realization of the ideal nanostructure of exchange-spring magnets as well as a model system to study layer thickness dependences of the magnetic properties. The superlattice films have an effective fourfold, in-plane magnetic anisotropy. Room-temperature hysteresis loops are relatively square and the demagnetization of the Co is reversible, as expected of exchange-spring magnets with aligned hard magnet layers.

ISSN:0003-6951    

DOI:10.1063/1.120743

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Sequential position readout from a microfabricated array of eight cantilever-type sensors (silicon technology) is demonstrated. In comparison with single sensors we find that mechanical disturbances from noise, such as from vibrations, turbulent gas flow, or abrupt pressure changes, can be effectively removed in array sensors by recording difference signals with respect to reference cantilevers. We demonstrate that chemically specific responses can be extracted in a noisy environment using a sensor to detect specific chemical interactions and an uncoated cantilever as reference. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120749

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A new technique to probe a single nanometer-scale magnetic particle by measuring the tunneling current through point-contact electrodes is described. The tunnel junctions are formed by a bulk cobalt bottom electrode, a monolayer of ultrasmall Co clusters sandwiched between twoAl2O3barriers and a point-contact top electrode, defined by e-beam lithography and permitting to study the quantum transport through an isolated cluster. We report on single electron charging effects such as Coulomb blockade and Coulomb staircase which are undoubtedly consistent with the tunneling through a single Co cluster. One of our devices exhibits a Coulomb gap of 380 mV which is, to our knowledge, one of the largest values ever reported in metallic granular systems. ©1998 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120744

出版商:AIP    

年代:1998

数据来源: AIP