Library

Notes: The crystal structure and microstructures of a high Tc BiSrCaCuO (BSCCO) compound were characterized by transmission electron microscopy, x-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Convergent beam electron diffraction (CBED) analysis established that the crystal is of mm2 symmetry. Combined with electron diffraction, XRD, and EDS data, the high Tc superconducting BSCCO compound was found to be of the Pnn2 space group, orthorhombic in structure with a=0.540 nm, b=2.689 nm, and c=3.040 nm and with an approximate composition of Bi2Sr2CaCu2Oy. Diffraction contrast analysis indicated that the dislocations are predominantly of screw character with [010] or [001] Burgers vectors. However, edge type dislocations and dislocations with [100] Burgers vector were also observed. The magnitude of the Burgers vector along the [100] direction was determined to be 1/2 [100] by high-resolution lattice imaging. Stacking faults and small-angle grain boundaries were frequently observed. The presence of a high density of defects in the superconducting oxide may facilitate the processing of the material and serve as effective flux line barriers.

Notes: Reflection high-energy electron diffraction oscillations have been studied during the growth of strained InxGa1−xAs on GaAs by molecular beam epitaxy and migration-enhanced epitaxy. The oscillations decay rapidly for x(approximately-greater-than)0.2 during molecular beam epitaxy, while they persist for a long while during migration-enhanced epitaxy. We believe that the altered surface reconstruction pattern in the latter case changes the growth mode from three-dimensional to a near perfect two-dimensional mode for high strain values. Using migration-enhanced epitaxy, we demonstrate improved channel mobility and performance of GaAs-based modulation-doped field-effect transistors and narrower linewidths in the low-temperature excitonic photoluminescence of In0.1Ga0.9As/Al0.3Ga0.7As quantum wells.

Notes: The carrier recombination rate in GaAs-AlGaAs single quantum well layers is investigated using a small-signal technique for carrier densities from 1017 to 1019/cm3. For carrier densities up to mid 1018/cm3, the inverse of the differential carrier lifetime, 1/τd, increases linearly with the carrier density. The differential rate, however, saturates at higher carrier densities and remains nearly constant for carrier densities higher than 1019/cm3. The deviation from the bulk recombination behavior is due to a portion of the injected carriers populating the semicontinuum states where the rate for the radiative transition is much smaller. The experimental data indicate that the runaway increase of threshold current with decreasing cavity length commonly observed in the short-cavity lasers is mainly due to the loss of carrier confinement at high carrier densities rather than due to fast carrier-depleting processes, such as Auger recombination.

Notes: We study the behavior of a hysteretic current-biased Josephson junction in the vicinity of its return to the zero-voltage states, with primary though not exclusive emphasis on the limit of weak damping (βJ(very-much-less-than)1), and under the assumption that the zero-point and thermal energies are both small compared to Icφ0 so that fluctuation effects are important only very close to the return point. We consider in detail the resistively shunted junction (RSJ) and quasiparticle-tunneling models, and also make predictions for more general models. Denoting the value of imposed current I at which return to the zero-voltage state would take place in the absence of fluctuations by Ir, we study in particular (a) the dc current-voltage characteristic in the running state for I−Ir(very-much-less-than)Ir, and (b) the first-passage-time statistics of the return to the zero-voltage state induced by both classical and quantum fluctuations. With regard to (b), we express our results in the form of a prediction of the width σ of the distribution of retrapping events as a function of imposed current; this prediction extends down to zero temperature and can be compared directly with the experimentally measured widths.Our two principal results are as follows: (a) In the running state, for I(very-much-less-than)Ir, the current-voltage characteristic should be given quite generally by the formula (I−Ir)/Ir =[(AV0/V)+B]exp−V0/V, where A and B are constants specific to the model, and V0 is a characteristic voltage which for the simplest models is given in the weak-damping limit by V0=ωJφ0+0(β2J) , with ωJ the junction plasma resonance frequency at zero current bias. (b) The square σ2 of the width of the retrapping distribution plotted as a function of I/Ir is given to within logarithmic factors by σ2(T)=const μf(T), where μ≡(h-dash-bar)ωJ/Icφ0, the constant is of order 1, and f(T) is a function which tends to 1 as T→0 and is proportional to T in the limit of high T; it is computed explicitly for the RSJ model. We also suggest an explanation (other than lead effects) of the "forbidden voltage regions'' which appear to be a characteristic of many high-quality junctions. We discuss the application of our results to the determination of the parameters of Josephson junctions necessary for the investigation of quantum effects on the macroscopic level.

Notes: Effects of an Ag2O addition in Y0.9Ca0.1Ba2Cu4O8 on the contact characteristics of Y0.9Ca0.1Ba2Cu4O8/Ag interfaces have been investigated by the measurement of contact resistivity. The lowest ohmic-contact resistivity about 4.55×10−5 Ω cm2 at 300 K and less than 10−8 Ω cm2 below Tc for Ag2O-free system are observed by annealing at 600 °C for 1 h under one oxygen atmosphere. With addition of 25 wt % Ag2O in Y0.9Ca0.1Ba2Cu4O8, the optimal annealing condition for the lowest ohmic-contact for Y0.9Ca0.1Ba2Cu4O8/Ag interface can be achieved without any thermal annealing just by the addition of 25 wt % Ag2O in Y0.9Ca0.1Ba2Cu4O8. These results indicate that the addition of Ag2O can upgrade effectively the characteristics of Y0.9Ca0.1Ba2Cu4O8/Ag interfaces to get the better ohmic contact even without annealing, resulting in a lower annealing temperature and contact resistivity. © 1994 American Institute of Physics.

Notes: The effectiveness of suppressing Be out-diffusion from a Be-doped GaAs layer by strained InGaAs layers using secondary ion mass spectroscopy has been evaluated. The experimental structures consist of an 800 A(ring) Be-doped (∼1×1019 cm−3) GaAs layer sandwiched between 80 A(ring) InxGa1−xAs (x=0,0.1,0.25) layers. The samples were subjected to rapid thermal annealing (RTA) at 750 °C for 6 min. It is clearly observed that Be diffusion beyond the InGaAs layers is the fastest for the structure with x=0 and the slowest for the structure with x=0.25.

Notes: The Hall factor for holes in relaxed p-type Si1−xGex alloys has been determined from mobility measurements at magnetic fields up to 7 T at 290 K. Our data together with previously published values for Si and Ge suggest that r for holes in SiGe varies between 0.73 and 1.7 with a possible strong bowing.

Notes: 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. © 1996 American Institute of Physics.

Notes: We report the operation of new photo-pumped blue-green ZnCdSe/ZnCdMgSe graded-index separate confinement heterostructure single quantum well lasers grown lattice matched on InP substrates. Laser emission at 512 nm was observed. The T0 value is 150 K at room temperature. These materials are proposed as alternative materials for the fabrication of visible semiconductor lasers. © 1997 American Institute of Physics.

Notes: 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 Hz−0.5 and 3×109 cm Hz0.5 W−1, respectively. © 1996 American Institute of Physics.