ALBERT

Notes: We report on the room temperature optical characterization of single crystal hexagonal GaN films on (0001) sapphire grown by metalorganic chemical vapor deposition. The energy gap of GaN was determined to be 3.400 eV by photoreflectance and the possible origin of the photoreflectance signal is discussed. Photocurrent measurement exhibited a peak at 3.351 eV and a continued photoresponse through the ultraviolet region. We found that the intensity of photocurrent was dependent upon the chopper frequency in the measurement. Absorption coefficient and film thickness were obtained from the optical transmission spectra. © 1995 American Institute of Physics.

Notes: We have studied the deep levels in the lattice-matched InP/(InAlGa)As heterojunction system. Five p-n junction samples were grown by metalorganic chemical vapor deposition with varied Al composition of 0%, 16%, 36%, 45%, and 55%. A vacancy-related deep electron trap was found at the InP/(InAlGa)As interface. This trap has less concentration in the sample with higher Al composition, and is not observed when Al composition is greater than 55%. Its activation energy within the band gap increases with the increasing Al composition. The trap can be eliminated by rapid thermal annealing at 700 °C. Current-voltage characteristics show that this trap acts as recombination centers under forward bias condition. © 1995 American Institute of Physics.

Notes: Deep levels have been measured in a molecular beam epitaxy grown Ga0.51In0.49P/GaAs heterostructure by double correlation deep level transient spectroscopy. Gold (Au) and aluminum (Al) metals were used for a Schottky contact. A contact-related hole trap with an activation energy of 0.50–0.75 eV was observed at the Al/GaInP interface, but not at the Au/GaInP interface. To our knowledge, this contact-related trap has not been reported before. We attribute this trap to oxygen contamination, or a vacancy-related defect, VIn or VGa. A new electron trap at 0.28 eV was also observed in both Au- and Al-Schottky diodes. It depth profile showed that it is a bulk trap in a GaInP epilayer. The temperature dependent current-voltage characteristics show a large interface recombination current at the GaInP surface due to the Al contact. The energy distribution of the interface state density showed a maximum at EV+0.85 eV within the band gap. Concentration of the interface trap and the magnitude of recombination current are both reduced by a rapid thermal annealing at/or above 450 °C after aluminum deposition. The Al Schottky barrier height also increased after a 450 °C annealing.

Notes: An aligned nematic droplet in the axial configuration is modeled as a hollow spheric scatterer for studying its small angle light scattering characters. When the thickness of the shell is reduced by an external field, calculations by anomalous diffraction approximation suggest that scattering intensity would decrease and scattering peak would drift to a smaller angle. Measured angular scattering intensities show good agreement with calculations. As the shell part originates from anchoring of the nematic molecules to the surrounding medium, angular scattering intensity can be associated with correlation length. © 1996 American Institute of Physics.

Notes: Deep levels in lattice-matched Ga0.51In0.49P/GaAs heterostructure have been investigated by thermal-electric effect spectroscopy (TEES) and temperature-dependent conductivity measurements. Four samples were grown by molecular-beam epitaxy with various phosphorus (P2) beam-equivalent pressures (BEP) of 0.125, 0.5, 2, and 4×10−4 Torr. A phosphorus vacancy (VP) -related deep level, an electron trap, was observed located at EC−0.28±0.02 eV. This trap dominated the conduction-band conduction at T(approximately-greater-than)220 K and was responsible for the variable-range hopping conduction when T〈220 K. Its concentration decreased with the increasing phosphorous BEP. Successive rapid thermal annealing showed that its concentration increased with the increasing annealing temperature. Another electron trap at EC−0.51 eV was also observed only in samples with P2 BEP less than 2×10−4 Torr. Its capture cross section was 4.5×10−15 cm2. This trap is attributed to VP-related complexes. © 1995 American Institute of Physics.

Notes: The 257 nm photodissociation dynamics of CH3I adsorbed on a MgO(001) surface is studied using classical molecular dynamics method. The substrate is modeled by a 6×6×3 slab of movable ions surrounded by a semi-infinite array of static ions. A single adsorbate molecule is aligned with the surface normal, the methyl end pointed either toward or away from the substrate. The system is equilibrated by using a Monte Carlo method to obtain the starting configuration. Fragment final state distributions are calculated for kinetic energy, angle of departure, and rovibrational states. Upon photodissociation of the adsorbate with the methyl end pointed toward the surface, the methyl fragments experienced vibrational cooling, in agreement with experimental results. Some rotational excitation is predicted for fragments produced from the methyl down orientation. The kinetic energy distributions of both the methyl and iodine fragments are qualitatively similar to those obtained by experiment. The results are compared with those obtained by the same model for CH3I adsorbed on LiF(001). Trapping of iodine atoms by the surface has also been investigated in this simulation. © 1995 American Institute of Physics.

Notes: Today, many bright photon beams in the ultraviolet and x-ray wavelength range are produced by insertion devices installed in specially designed third-generation storage rings. There is the possibility of producing photon beams that are orders of magnitude brighter than presently achieved at synchrotron sources, by using self-amplified spontaneous emission (SASE). At the Advanced Photon Source (APS), the low-energy undulator test line (LEUTL) free-electron laser (FEL) project was built to explore the SASE process in the visible through vacuum ultraviolet wavelength range. While the understanding gained in these experiments will guide future work to extend SASE FELs to shorter wavelengths, the APS FEL itself will become a continuously tunable, bright light source. Measurements of the SASE process to saturation have been made at 530 and 385 nm. A number of quantities were measured to confirm our understanding of the SASE process and to verify that saturation was reached. The intensity of the FEL light was measured versus distance along the FEL, and was found to flatten out at saturation. The statistical variation of the light intensity was found to be wide in the exponential gain region where the intensity is expected to be noisy, and narrower once saturation was reached. Absolute power measurements compare well with GINGER simulations. The FEL light spectrum at different distances along the undulator line was measured with a high-resolution spectrometer, and the many sharp spectral spikes at the beginning of the SASE process coalesce into a single peak at saturation. The energy spread in the electron beam widens markedly after saturation due to the number of electrons that transfer a significant amount of energy to the photon beam. Coherent transition radiation measurements of the electron beam as it strikes a foil provide additional confirmation of the microbunching of the electron beam. The quantities measured confirm that saturation was indeed reached. Details are given in Milton et al., Science 292, 2037 (2001) (also online at www.sciencexpress.org as 10.1126/science. 1059955, 17 May 2001), and Lewellen et al., "Present Status and Recent Results from the APS SASE FEL," to be published in the Proceedings of the 23rd International Free-Electron Laser Conference, Darmstadt, Germany, 20–24 August 2001. © 2002 American Institute of Physics.

Notes: Lead zirconate titanate (PZT) thin films on platinized silicon were fabricated and their structural development upon annealing was characterized by x-ray diffraction and transmission electron microscopy (TEM). The amount of a transient intermetallic phase Pt3Pb was found initially to increase with annealing time and to decay after reaching a maximum. The kinetic process of growth and decay was simulated by using the Avrami equation. The Avrami coefficient n and growth rate constant k were determined by comparing the experimental results and the simulated curves, from which activation energies of 40 and 145 kJ/mol were obtained for the growth and decay of the intermetallic Pt3Pb phase, respectively. The perovskite PZT was found by using TEM to nucleate epitaxially on top of the Pt3Pb phase. Evidence is presented that the Pt3Pb phase plays a major role in determining the crystallite's orientation at the nucleation stage of the perovskite PZT. This depends strongly on the annealing temperature and the orientation changes little during the following growth process. © 1999 American Institute of Physics.