Alexana Roshko

Epitaxial nucleation and growth of chemically derived Ba2YCu3O7−x thin films on (001) SrTiO3

The nucleation and growth kinetics of epitaxial Ba2YCu3O7−x thin films prepared on (001) SrTiO3 by postdeposition annealing of a chemically derived intermediate layer were investigated in specimens quenched from the growth anneal. The films were produced by spin‐on deposition of a metal‐organic precursor solution at room temperature and ambient pressure, and subsequent postdeposition annealing to form Ba2YCu3O7−x (BYC). Integrated x‐ray intensities of reflections from the majority, c‐axis‐normal epitaxial BYC in the films were analyzed as a function of time at annealing temperature. Data collected from specimens annealed at 730 and 750 °C were consistent with upward growth of the BYC from nuclei at the substrate surface with a constant growth rate. The linear transformation kinetics suggest the growth process is controlled by molecular processes at the BYC/intermediate interface, and is not rate limited by diffusion. The activation energy for film growth along its [001] axis was estimated to be ∼5 eV. The...

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9×103 cm/s for our silicon-doped GaN nanowires.

Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151...

Scanning tunneling microscopy of the surface morphology of YBa2Cu3Ox thin films between 300 and 76 K

Scanning tunneling microscopy (STM) images of YBa2Cu3Ox (YBCO) thin films show different growth mechanisms depending on the deposition method and substrate material. We present images of YBCO films sputter deposited onto MgO and SrTiO3, and laser ablated onto LaAlO3 showing screw dislocation and ledge growth mechanisms. At room temperature we observed an anomalous tunneling conductance near the edge of growth steps which causes a large apparent step‐edge height in the STM image. This effect decreases with decreasing temperature, so that the step height approaches the expected value for one unit cell of 1.2 nm at 76 K. This phenomenon reflects changes in either the surface tunneling barrier or tunneling density of states upon cooling.

Polarization-resolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy

Polarization- and temperature-dependent photoluminescence (PL) measurements were performed on individual GaN nanowires. These were grown by catalyst-free molecular beam epitaxy on Si(111) substrates, ultrasonically removed, and subsequently dispersed on sapphire substrates. The wires were typically 5–10μm in length, c-axis oriented, and 30–100nm in diameter. Single wires produced sufficient emission intensity to enable high signal-to-noise PL data. Polarized PL spectra differed for the σ and π polarization cases, illustrating the polarization anisotropy of the exciton emission associated with high-quality wurtzite GaN. This anisotropy in PL emission persisted even up to room temperature (4–296K). Additionally, the nanowire PL varied with excitation intensity and with (325nm) pump exposure time.

On-chip optical interconnects made with gallium nitride nanowires.

In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems.

Heteroepitaxial growth of TiO2 films by ion-beam sputter deposition

Heteroepitaxial TiO 2 films of the rutile and anatase phases have been grown using the ion-beam sputter deposition technique. The orientations of the highest-quality rutile films grown and their corresponding substrates are (100)/(0001)Al 2 O 3 , (101)/(1120)Al 2 O 3 , (001)/(1010)Al 2 O 3 , and (110)/(110)MgO. This is the first report of the heteroepitaxial growth of (001)/(1010)Al 2 O 3 and (110)/(110)MgO rutile films. Results indicate that the films are aligned both perpendicular and parallel to the plane of the film. Distinct surface morphologies are observed for each orientation. The (100) and (101) rutile orientations were also grown on (111)MgO and (1102)Al 2 O 3 , respectively. The (100) anatase grew on both (100)MgO and MgAl 2 O 4 . The growth mechanisms of several rutile films on Al 2 O 3 substrates were investigated, and the data suggest island or Volmer-Weber type growth.

Hybrid glass substrates for waveguide device manufacture

Hybrid glass substrates were prepared by a novel, low-temperature process joining active (Er-Yb codoped) and passive phosphate glass. The resulting hybrid substrates are chemically and physically robust; they can be cut, ground, and polished by conventional, water-based techniques. The entire substrate can be immersed in a molten-salt bath to produce waveguides simultaneously in the active and passive regions. A low reflectance of -34+/-2 dB was measured at the joint interface with 1531.2-nm light by optical low-coherence reflectometry. Further, a hybrid laser waveguide device exhibited a slope efficiency of 33% at 1540 nm when pumped at 975 nm.

Imaging of domain-inverted gratings in LiNbO3 by electrostatic force microscopy

Ferroelectric domains in LiNbO3 have been investigated by means of electrostatic force microscopy. Polarization-inverted gratings with 4 μm periodicity were fabricated by titanium diffusion into both +c and −c faces of single-domain LiNbO3 crystals. The distribution of the electric field in the vicinity of the sample surface was measured using scanning probe microscopy. The electrostatic force image was found to correlate with the shape of the domain-inverted profile observed by scanning electron and optical microscopies.

Surface degradation of superconducting YBa2Cu3O7−δ thin films

The surface degradation of c‐axis oriented YBa2Cu3O7−δ thin films due to air, CO2, N2, O2, and vacuum exposure has been studied with reflection high‐energy electron diffraction (RHEED), scanning tunneling microscopy, and contact resistivity measurements. The formation of an amorphous surface reaction layer upon exposure to air and CO2 is monitored with RHEED and correlated with an increase in contact resistivity. The contact resistivity of samples exposed to air increases with time t as ρc = (1.0 × 10−7 Ω cm2)e√t/640 min. Surfaces exposed to CO2 show a similar degradation while surfaces exposed to N2 showed a slightly different degradation mechanism. Vacuum exposed surfaces how little increase in contact resistivity, indicating no long‐term surface oxygen loss.