Ronald F. Broom

Screw dislocation mediated growth of sputtered and laser-ablated YBa2Cu3O7−δ films

By imaging the as-grown surfaces of sputtered and laserablated YBa2Cu3O7−δ films with scanning tunneling microscopy (STM), we have directly observed spiral-shaped growth terraces which emanate from screw dislocations. The density of screw dislocations was observed to decrease with increasing growth temperature and substrate misorientation. The surface structures observed by STM together with cross-sectional transmission electron microscope (TEM) images provide insights into the mechanisms of crystal growth operative during the formation of YBa2Cu3O7−δ films grown using these two widespread techniques.

Niobium oxide-barrier tunnel junction

We describe the fabrication and electrical characteristics of niobium oxide-barrier tunnel junctions having counterelectrodes of either Pb alloys or Nb. The conditions necessary to obtain good tunnel barriers are discussed in detail. These include RF plasma etching of the Nb film before oxidation, the oxidation step itself and, in the case of Nb counterelectrodes, exposure of the oxide to an RF plasma of N2containing traces of hydrocarbon. The single-particle current of Nb/Pb junctions at 4.2 K is close to the value calculated from BCS theory in the voltage range0 < V < 2mV. With Nb counterelectrodes the subgap current is higher, but in the best examples it is comparable to Nb/Pb at voltages below 1 mV. Finally, it is shown that the junctions exhibit outstanding stability during storage, annealing, and thermal cycling between room temperature and 4.2 K.

Identification of epitaxial Y2O3 inclusions in sputtered YBa2Cu3O7 films: Impact on film growth

Y2O3 inclusions with typical sizes between 100 and 300 nm3, densely distributed (1016 cm−3) in sputtered YBa2Cu3O7 (YBCO) films on SrTiO3 substrates, have been identified by high‐resolution electron microscopy. The precipitates exhibit either cuboid or needlelike shapes and grow epitaxially within and on top of YBCO. The dominant orientation relationship corresponds to a situation where the two‐dimensional lattices are nearly coincident in the interfacial (001) plane. These precipitates may contribute to the generation of screw and edge dislocations. In addition, they provide a large number of potential pinning sites for magnetic flux lines, which may contribute to the observed high critical current densities.

Josephson junctions of small area formed on the edges of niobium films

Niobium‐oxide‐lead Josephson tunnel junctions formed on the edge of the Nb base electrode are found to have excellent tunnel characteristics up to Josephson current densities of at least 20 kA/cm2. They are made by growing an anodic oxide several hundred angstroms in thickness on the Nb followed by plasma etching of the sandwich using CF4+O2. Only the exposed edges of the Nb films are subsequently oxidized to form the tunnel barrier. Preliminary results show uniform and reproducible Josephson currents, with no evidence of leakage through the surface oxide. The structures combine small capacitance, a well‐defined area, and the ability to pattern large arrays of junctions and interferometers, properties which make them attractive for cyrogenic computers and other high‐frequency applications.

Gallium desorption during growth of (Al,Ga)As by molecular beam epitaxy

A new two‐phase model for gallium desorption can quantitatively explain the change of the desorption energy as a function of the III/V ratio. In this model, the gallium loss rate is described as a phenomenon of simultaneous desorption from a Ga fluid state and a GaAs solid state. This behavior was experimentally verified by accurate thickness measurements of epitaxial AlxGa1−xAs layers by transmission electron microscopy. The results show the Ga loss rate is directly dependent only on the Ga coverage on the surface, while the desorption energy is independent of the aluminum concentration. By varying the III/V ratio, we found that the Ga desorption energy increases from 3.1 eV at low arsenic flux to 4.7 eV at high arsenic flux.

Growth investigations of 1.3 μm GaInAsP/InP MQW laser diodes grown by chemical beam epitaxy

We report on growth investigations and results of low threshold current density GaInAsP/InP multiple quantum well (MQW) lasers emitting at a wavelength of 1.3 μm. Bulk layers and separate confinement heterostructure MQW laser diodes were grown by chemical beam epitaxy. At optimized growth conditions, broad-area lasers with as low a threshold current density J th as 160 A/cm 2 and an internal loss of 5 cm -1 were obtained

Observation of superconductivity in an indium‐silicon bistable device

Superconducting behavior characteristic of a weak link is found to occur in the low‐resistance state of a bistable nonvolatile switching device, formed by two closely spaced In contacts on Si.

Observation of a novel form of ordering in Al0.5In0.5As and (Ga,Al)0.5In0.5As

We have found a novel form of ordering in Al0.5In0.5As and (Ga,Al)0.5In0.5As as grown on (001) substrates by molecular beam epitaxy (MBE). Electron diffraction patterns reveal a four-fold periodicity along the [110] direction as well as the more usual two-fold periodicity along 〈111〉 directions. The morphology of the latter type of ordering is also different from its normal form.

Regrowth of InP by MOVPE on dry-etched heterostructures of InP-GaInAsP

Abstract The MOVPE growth behavior of InP on masked and dry-etched ridges in InP/InGaAsP heterostructures grown on (001)-oriented InP substrates has been studied by scanning electron and transmission electron microscopy. It is found that the orientation of the ridges is critical for obtaining good planarization. For ridges oriented along the [110] direction, the growth is uniform and defect-free, leading to a plane surface. In the orthogonal [110] direction, 60° twins are nucleated adjacent to the walls of the ridge. The resultant high density of (111)/(001) facets enhances the growth rate in these regions, leading to projecting walls at the sides of the ridge.

Measurement Of The Recombination Lifetime In Semiconductor Lasers Using rf Techniques

Two techniques have been used for determining the recombination lifetime in semiconductor lasers: A capacitance bridge at 10 MHz, or a network analyzer in the range from 45 to 245 MHz. The latter, although more tedious than the former, has the advantage of allowing detection and inclusion of parasitic contributions. The AlGaAs ridge graded-index separate confinement heterostructure lasers studied here are well described by a simple equivalent circuit consisting of an ideal laser and a series resistance. Below threshold, the inverse lifetime squared exhibits a linear dependence on the forward current, as expected from simple theory. The measurements yield non-radiative lifetimes of 2.5 ... 5 ns. The bimolecular recombination constant B = 0.24 ... 0.56 x 10-10 cm3/s is smaller than the bulk values; possible reasons are discussed.