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This paper presents a theoretical study of an adaptive microwave phase shifter based on a nanomechanical resonator. The phase shifter can be tuned to provide an arbitrary phase rotation in the range between –90° and 90° by adjusting the bias voltage applied to the resonator. An analytical model is developed to reveal the underlying mechanism of the phase shifter. It is also analytically demonstrated that the operating frequency can be selected by adjusting the bias voltage. In addition, it is found that the bandwidth is tunable via the quality factor for the cantilever. [ABSTRACT FROM AUTHOR]

It has been possible to use low temperature molecular beam epitaxy to grow (Ga,Mn)As films with high nominal Mn concentration. It is concluded that the growth of high Mn concentration epitaxial (Ga,Mn)As could be possible by pushing forward the material science of this material.

We investigate the influence of naturally grown 2-5 nm thin surface layer of antiferromagnetic (AFM) Cr2O3 on the half metallic ferromagnet CrO2 by measuring the magnetic behavior of a single micro-crystal. The temperature variation of the magnetic stray fields of the micro-crystal measured by micro-Hall magnetometry shows an anomalous increase below ∼60 K. We find clear evidence that this behavior is due to the influence of the AFM surface layer. The average amplitude of the Barkahausen jumps exhibits a similar temperature dependence indicating that the AFM surface layer plays a role in defining the potential landscape seen by the domain configuration in the ferromagnetic grain. [ABSTRACT FROM AUTHOR]

Variation in stacking fault energy with annealing at 1173 K were identified in Czochralski-grown silicon crystals heavily doped with n- or p-type dopant atoms. In n-type crystals, the energy decreased with increasing annealing time. The higher the concentration of dopant atoms, the larger the degree of the decrease. On the other hand, the energy was unchanged during annealing in p-type and nondoped crystals. These results imply that n-type dopant atoms segregate nearby a stacking fault, via their thermal migration, under an electronic interaction leading to a reduction in the stacking fault energy. [ABSTRACT FROM AUTHOR]

Optical properties of wurtzite ZnO bulk single crystals in which an arbitrary number (typically 109–1010 cm-2) of fresh dislocations were introduced intentionally by the plastic deformation at elevated temperatures (923–1073 K) were examined. Deformed specimens showed excitonic light emission with photon energies of 3.100 and 3.345 eV, as well as their LO phonon replicas at 11 K. The light intensities increased with increasing dislocation density. The activation energy for a thermal quenching of the 3.100 or 3.345 eV emission band, which corresponds to the depth of the localized energy level associated with the emission band, was estimated to be 0.3±0.1 or 0.05±0.01 eV, respectively. The origin of the energy levels was proposed as point defect complexes involving dislocations. The introduction of the dislocations at the elevated temperatures above 923 K did not influence the intensities of the emission bands except the dislocation-related emission bands. [ABSTRACT FROM AUTHOR]

The mechanical strength of bulk single crystal wurtzite ZnO was investigated at elevated temperatures by means of compressive deformation. The yield stress of ZnO in the temperature range of 650–850 °C was found to be around 10–20 MPa, i.e., extremely lower than that of GaN, a typical wide band-gap semiconductor. On the basis of the observed temperature dependence of yield stress, the activation energy for dislocation motion at elevated temperatures in ZnO is deduced to be 0.7–1.2 eV, which follows the relation of activation energy for dislocation motion versus band-gap energy known in a variety of semiconductors. [ABSTRACT FROM AUTHOR]

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350 nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs. [ABSTRACT FROM AUTHOR]

The resonant tunneling through single InAs quantum dots embedded in an n-GaAs/i-Al[sub 0.38]Ga[sub 0.62]As/n-GaAs diode has been studied by using microscopic electrophotoluminescence spectroscopy. Many sharp luminescence lines which originated from single quantum dots were observed by injecting resonant electrons from the emitter to the dots. Bias dependence of a single luminescence line was investigated. The peak intensity showed triangular dependence which was similar to the current–voltage characteristics of electron resonant tunneling from three dimension to zero dimension. When the bias voltage was increased, the peak energy slightly shifted to a lower energy indicating the existence of Stark effect, and the linewidth slightly increased. The higher the luminescence energy was, the broader the linewidth was. This result agrees with the calculated resonant level width. The lifetime of resonant states was estimated to be 2.4–27 ps for luminescence linewidth of 250–22 μeV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The spin-dependent scattering in ferromagnet/nonmagnet/ferromagnet (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As trilayer structures was studied. An increase of sheet resistance was observed when the magnetizations of the two ferromagnetic (Ga,Mn)As layers were aligned anti-parallel, which was realized by the different coercivity of the two (Ga,Mn)As layers with different compositions. This is the first demonstration of spin-dependent scattering in magnetic multilayer structures made of semiconductor-materials alone. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An arbitrary number (3×108-1×1010 cm-2) of edge dislocations on (1100) prismatic planes, with the Burgers vector of (a/3)[1120] and with the dislocation lines nearly parallel to [0001], were introduced intentionally in wurtzite ZnO bulk single crystals at elevated temperatures of 923-1073 K, and the optical properties were examined. After the introduction of the dislocations, the intensity of the intrinsic emissions existing in pre-dislocated crystals, i.e., near-band edge emissions and deep level emissions, was almost unchanged, and donor-acceptor pair (DAP) emissions with photon energies of 2.20 and 2.50 eV at temperature of 12 K appeared. The intensity of the DAP emissions increased with increasing the dislocation density. The origin of the DAP emissions was determined as acceptor levels of 0.9 and 1.2 eV depth introduced with the dislocations. [ABSTRACT FROM AUTHOR]