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Three Y1Ba2Cu3O7−δ superconducting thin films with 20%, 60%, and 100% coverage of precipitates were fabricated into microbridges of 25×650 μm2 by a conventional photolithography method. These microbridges exhibit a Tco(R=0) from 83.6 to 87.2 K and a transition width of about 1.8 K. These also have (dR/dt)max values of about 71–78 Ω/K. We find that the fewer the precipitates on the film, the better the thermal conduction, and a larger responsivity (S) can be achieved. The best responsivity of 783 V/W measured at Ib=8 mA, Top=86.8 K, and f=2 Hz for various wavelengths of light sources is obtained from the film with 20% precipitation. The photoresponse signals decay exponentially with the precipitation coverages, which indicates that these precipitates behave as thermal resistance layers that smooth out the ac incident signal, suppress the rise in microbridge temperature, and result in a smaller photoresponse signal for the bolometer.

A photodetector with high responsivity has been fabricated with a high quality Y1Ba2Cu3O7−δ film on a regular SrTiO3 (100) substrate that has no thinning or etching. The film exhibits a precipitation‐free morphology and has an extremely smooth surface having a rms roughness smaller than 5 nm. The film was patterned into a 25×650 μm2 long microbridge by a conventional photolithography technique. The Tco of the microbridge can be tuned by applying different bias currents, which results in a tunable operation temperature of the photodetector at 77.35±1 K. This makes it operationally more practical and economical. The noise voltage at 10 mA bias current is less than the resolution of our setup, 15 nV Hz−0.5. A high responsivity of 2.3×103 V/W was obtained when a bias current of 8 mA and a low power density He–Ne laser of 0.08 J/cm2 s chopped at 2 Hz were applied on the microbridge at 77.35 K. Under the same condition, the noise equivalent power (NEP) and detectivity (D*) have been measured to be 4.34×10−12 W...

Tunable, single longitudinal mode spectrum of a novel monolithic, laterally coupled semiconductor stripe laser has been demonstrated. A tuning range of 14.2 nm, which is the widest observed so far in a monolithic device, has been achieved.

A model is developed to investigate the domain pattern and piezoelectric response across the polymorphic phase transition in strained epitaxial bismuth ferrite films. The orientations of stripelike pattern of the mixed rhombohedral and tetragonal phases are predicted as the consequence of competition between elastic and depolarization energies. The abnormally large piezoelectric response is attributed to the strain-driven softening in dielectric stiffness. The results are in good agreement with recent experimental observations and provide a guidance for developing similar strain-driven polymorphic phase transition in other related perovskite material systems.

Magnetoelastic domains in ferromagnetic shape memory alloys evolve through either variant rearrangement or magnetization rotation, resulting in a large or a small magnetic field-induced strain depending on the magnitude of applied compressive stress. These phenomena are simulated in this letter using an unconventional phase-field model motivated by energy-minimizing multirank laminated domain structures. The results agree well with experiments, and confirm the analysis of Ma and Li [Appl. Phys. Lett. 90, 172504 (2007)] based on an energy minimization theory.

A nonconventional phase-field model is developed to predict ferroelectric domain structures. It employs a set of field variables motivated by multirank laminates to represent energy-minimizing domain configurations, giving rise to an explicit expression of the energy-well structure. The framework is applied to domain simulation in the rhombohedral phase assuming that polarization is close to the ground states. An electromechanical self-accommodation pattern consisting of eight rhombohedral variants and an engineered domain configuration are predicted and found in good agreement with those observed in experiment.

We present results on high‐speed GaAs p‐i‐n photodiodes grown on Si substrates by molecular beam epitaxy. In addition to the dc operating characteristics, we also report the first impulse response and microwave frequency response measurements of GaAs‐on‐Si photodiodes. Results include an impulse response pulse width of 45 ps and a modulation corner frequency >4 GHz at a reverse bias of −3 V.

We report the first observation of a lateral junction formed in an alloy due to an abrupt transition from segregated to random AlGaAs alloy compositions. Al0.25Ga0.75As epilayers were grown by molecular beam epitaxy on [011-bar] oriented grooves in a nonplanar (100) GaAs substrate. A quasi-periodic modulation of the aluminum concentration occurs spontaneously in material grown on the (111) facets of the groove, with a period of 50–70 A along the [111] direction. The compositional modulation is associated with a reduction of the band gap by 130 meV, with respect to the random alloy. While segregation of the AlGaAs alloy has been seen previously, this is the first observation of segregation of AlGaAs grown on a (111) surface. The compositional modulation terminates abruptly at the boundaries of the (111) facet, forming abrupt lateral junctions in the AlGaAs layers grown on a groove.

Single quantum well, graded refractive index separate confinement heterostucture (SQW GRINSCH) lasers with well thicknesses in the range of 65–480 A have been grown by molecular beam epitaxy (MBE) on (100) and off of (100) by 4° toward (111) A substrates. The threshold current density appears to be independent of the well thickness in the range of 65–165 A due to the compensating effects of volume of inversion and optical confinement. Under optimum growth conditions, the tilted substrates led to lower threshold current densities, the lowest value being 93 A/cm2 for a 520‐μm‐long cavity laser with a 125‐A‐thick well. To our knowledge, this is by far the best ever reported threshold current density obtained in a semiconductor injection laser. Deviations from optimum growth conditions drastically increased the threshold current density on (100) substrates whereas the degradation for those on the tilted substrates was much less pronounced.

Single quantum well AlGaAs/GaAs graded-index separate-confinement heterostructure lasers have been fabricated using MeV oxygen ion implantation plus optimized subsequent thermal annealing. A high differential quantum efficiency of 85% has been obtained in a 360-µm-long and 10-µm-wide stripe geometry device. The results have also demonstrated that excellent electrical isolation (breakdown voltage of over 30 V) and low threshold currents (22 mA) can be obtained with MeV oxygen ion isolation. It is suggested that oxygen ion implantation induced selective carrier compensation and compositional disordering in the quantum well region as well as radiation-induced lattice disordering in AlxGa1–xAs/GaAs may be mostly responsible for the buried layer modification in this fabrication process.

We report on the frequency response of quantum well lasers on Si substrates grown by molecular beam epitaxy. Ridge waveguide lasers of 10 μm×380 μm having threshold currents as low as 40 mA were used in this study. Measurements were performed up to a frequency of 4.5 GHz with a resultant modulation corner frequency of 2.5 GHz when the laser was operated about 20% above the threshold.

A new hetero‐Schottky structure for the study of Schottky barriers and heterojunctions is discussed. Photoelectric measurements have revealed a correlation between Au/GaAs and Au/AlxGa1−xAs Schottky barrier heights and GaAs/AlxGa1−xAs heterojunction band offsets. The conduction‐band discontinuity ΔEc in the GaAs/AlxGa1−xAs system is determined, and the ratio ΔEc:ΔEg is found to vary between 69±5% and 58±5% for x=0.15–0.48.

Low‐threshold graded‐refractive‐index GaAs/AlGaAs laser structures were grown on Si (100) by molecular beam epitaxy and tested at room temperature under a probe station. Broad area devices having widths of 110–120 μm and cavity lengths of ∼500–1210 μm exhibited threshold current densities as low as 600 A/cm2 and total slope efficiencies of as high as 0.75 W/A. The thresholds fell in the range of 600–1000 A/cm2 in three different wafers, and it is assumed that the quality of the facets accounts for most of the spread in results.