Bulletin of the American Physical Society
11th Annual Meeting of the Northwest Section of APS
Volume 54, Number 6
Thursday–Saturday, May 14–16, 2009; Vancouver, BC, Canada
Session B2: Condensed Matter Physics I |
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Chair: Michael Crosser, Linfield College Room: Hennings 202 |
Friday, May 15, 2009 1:00PM - 1:36PM |
B2.00001: Control of the Photodissociation of Molecules Adsorbed at Metal Surfaces Invited Speaker: This talk will discuss aspects of molecular photo- and electron- dissociation physics applied at material interfaces. We have studied several molecular systems that have well-known properties in the gas-phase phase to understand how the molecular properties are modified and can be controlled at the solid--vacuum interface. In the first instance we show how molecular orientation in a 2-D surface phase can be exploited to help understand the photodissociation dynamics of CH$_3$I in the near-UV range, and how the dissociation processes are modified near a metal surface. In the second instance we describe recent experiments on low energy dissociative electron attachment (DEA) of several halogenated molecules, and show how particular surface electron states (image states) can be used as an effectively monochromatic electron source to initiate DEA of the target molecules, using either photoelectrons from the metal surface or low energy electrons from an external source. [Preview Abstract] |
Friday, May 15, 2009 1:36PM - 1:48PM |
B2.00002: Deep level characterization in sulfur-doped GaPN layers grown by OMVPE A.F. Basile, S. Hatakenaka, H. Okada, A. Wakahara Dilute Nitrides based on GaP and its alloys can be lattice matched to Silicon, thus opening a window for optoelectronic circuits on Si wafers. Some of the intrinsic defects in GaPN alloys may either be related to displaced N atoms, or instead be introduced because of the nonequilibrium growth conditions, specifically low-temperature organo-metallic vapor phase epitaxy. Sulfur-doped GaPN layers on GaP substrate, with N content either at a low doping level of about 0.3{\%} or in the dilute-alloy range of 1{\%} were characterized by four electrophysical techniques: C-V, Hall, deep level transient spectroscopy and thermal admittance spectroscopy measurements. Two deep-level defects were observed. A shallow trap at $\sim $0.4eV, which increases in concentration with N content and displays Poole-Frenkel effects, corresponds to the N interstitial. A mid-gap level with a tendency to form complexes and occurring in high concentration, independent of N content, is believed to be the Ga interstitial and is likely to strongly affect the optical and transport properties. [Preview Abstract] |
Friday, May 15, 2009 1:48PM - 2:00PM |
B2.00003: Pl and structural characterization of InAsSb/InAs MQWs grown on GaSb for infrared detector applications David Lackner, Oliver J. Pitts, Tom Cherng, Michael Steger, Albion Yang, Michael L.W. Thewalt, Simon P. Watkins InAs$_{0.91}$Sb$_{0.09}$, epitaxially grown on GaSb, has received steady attention in the past few years for optical detectors in the 3-5micron range. Attempts to increase the detection wavelength by increasing the Sb mole fraction have been hindered by the lack of lattice-matched substrates. In this work we report the growth of strain balanced InAs/InAsSb superlattice structures strain-balanced to GaSb for potential application in photodetectors beyond 5 microns. The strain balanced method permits the incorporation of larger Sb mole fractions in the Sb layers, considerably extending the absorption cutoff. We find the PL-energy of the InAsSb/InAs MQW stack to depend linearly on the Sb mole fraction for samples with Sb compositions ranging from 14{\%} to 21{\%}. At the latter composition a PL energy of 175 meV (7 $\mu $m) is measured which is more than 100 meV lower than the calculated strained bandgap. Most likely this can be explained by a type II band-alignment. Also first detector results for a pin InAsSb device, lattice matched to GaSb, will be presented. [Preview Abstract] |
Friday, May 15, 2009 2:00PM - 2:12PM |
B2.00004: Elastically Strain-Relaxed GaAs/InGaAs/GaAs Heterostructures on GaAs(001) D.L. Owen, D. Lackner, O.J. Pitts, S.P. Watkins, P.M. Mooney Engineered substrates with surface regions having an in-plane lattice parameter different from that of available semiconductor wafers are of interest to extend the properties of semiconductor devices. This was achieved by bonding strain-relaxed GaAs/In$_{0.08}$Ga$_{0.92}$As/GaAs structures on GaAs(001) substrates. Pseudomorphic heterostructures having a lattice mismatch of 0.56{\%} were grown by metal-organic chemical vapor deposition (MOCVD) and patterned using conventional photolithography to define arrays of 10 or 20 micron square structures. A sacrificial AlAs layer was then removed by selective etching with HF. During etching, the strained structures relax elastically, without introducing misfit dislocations, and bond weakly in-place to the substrate. The degree of strain relaxation of the InGaAs layer is determined by the relative thickness of the GaAs and InGaAs layers in agreement with a force balance model.\footnote{G.M. Cohen, et al., App. Phys. Lett. \textbf{86}, 251902 (2005).} The bond was then strengthened by annealing at 400$^{\circ}$C for 2hrs, while stabilizing the surface with TBAs in the MOCVD reactor. The increase in the in-plane lattice parameter of the bonded structures compared to the GaAs substrate was 0.25-0.44{\%}. [Preview Abstract] |
Friday, May 15, 2009 2:12PM - 2:24PM |
B2.00005: Oxygen annealing effects on optical properties of ZnO and TiO nanocluster thin film Ryan Souza, Hui Che, Yufeng Tian, You Qiang, Rene Rodriguez, Lisa Lau, Paul Turner, Dmitri Tenne Thin films of zinc oxide (ZnO) and titanium oxides (TiO) were deposited on Si (100) substrate by third generation nanocluster source. Post deposition Oxygen Annealing (OA) effects were evaluated by X-Ray diffraction (XRD), Raman spectroscopy, and photoluminescence (PL). As deposited, diameter of ZnO nanoclusters $\sim $24 nm and ZnO OA nanoclusters $\sim $30 nm. As deposited TiO clusters $\sim $10 nm in diameter, and TiO OA cluster $\sim $20 nm. XRD shows increased crystal quality and improved ZnO c-axis crystal growth. XRD shows improved stoichiometric TiO$_{2}$ clusters and preferred anatase phase. Intensity of ZnO PL spectrum has temperature dependence from 10K to 300K. ZnO OA produces red-shifted PL peak, and there are no below-bandgap PL peaks. TiO has room temperature PL with UV peak $\sim $388 nm and visible peak $\sim $426 nm. ZnO OA nanoclusters show narrower, red shifted peak by $\sim $0.07eV compared to as deposited ZnO nanoclusters. This indicates increased average grain size and improved size homogeneity after annealing. This is confirmed by Raman spectra showing blue-shifted and narrower peak of the A1 (LO) phonon peak in ZnO OA sample. DOE-EPSCoR (DE-FG02-04ER46142) DOE-BES (DE-FG02-07ER46386). [Preview Abstract] |
Friday, May 15, 2009 2:24PM - 2:36PM |
B2.00006: Synthesis and Optical Properties of Zn/ZnO core-shell nanoparticles Hui Che, Yufeng Tian, Ryan Souza, You Qiang, Chongmin Wang, Zheming Wang, Weilin Jiang Wide band-gap semiconductor ZnO is an important functional material due to its large exciton binding energy and unique optical properties. The structure of ZnO affects the properties. Zn/ZnO core-shell structured nanoparticles are produced in our lab by a sputtering-gas aggregation cluster source and deposited onto a Si substrate. During the aggregation the outer sphere of the nanoparticles is oxidized, giving Zn particles in ZnO shells. Post annealing was performed to some samples in a tube furnace in air. TEM, XRD and photoluminescence (PL) were performed. TEM results show that the size of Zn/ZnO particles can be controlled by adjusting the aggregation length and the flow rate of Ar gas. During post annealing, the particles were totally oxidized into ZnO and become polycrystalline so that the Zn phase can no longer be detected in XRD pattern. PL spectra of as prepared samples show a strong UV emission peak centered at 355nm and two peaks in visible region centered at 445nm (blue) and 530nm (green). Post annealing decreases these emissions. This research is supported by DOE-EPSCoR (DE-FG02-04ER46142) and DOE-BES (DE-FG02-07ER46386). Part of the work was performed at EMSL, a DOE user facility. [Preview Abstract] |
Friday, May 15, 2009 2:36PM - 2:50PM |
B2.00007: BREAK
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Friday, May 15, 2009 2:50PM - 3:26PM |
B2.00008: Atom movement in solids studied using PAC spectroscopy Invited Speaker: Perturbed angular correlation of gamma rays (PAC) is a nuclear hyperfine interaction spectroscopy that has been used to study solids for 50 years. In particular, local environments of radioactive probe atoms can be flagged by interactions between the probe's nuclear quadrupole moment and electric field gradients (EFGs) arising from nearby charges. Recently, it was shown that one can use PAC to study diffusion of probes through stochastic fluctuations of the EFG that lead to nuclear relaxation [Phys. Rev. Lett. 92, 225901 (2004)]. Jump-frequencies can be measured with much less effort than tracer diffusivities and have comparable precision. In this talk I will outline features of the method and survey recent investigations of highly-ordered intermetallic compounds made in our laboratory using the $^{111}$In/Cd probe. These include cubic phases having the Cu$_{3}$Au structure, in which the EFG fluctuates in orientation but not in magnitude, and of tetragonal phases having the Al$_{4}$Ba structure, in which the EFG fluctuates in magnitude but not in orientation. Furthermore, it will be shown that PAC can be used to determine whether vacancy diffusion in a binary phase A$_{n}$B$_{m}$ predominantly involves A-vacancies or B-vacancies [Phys. Rev. Lett., in press]. In a recent study of a large series of In$_{3}$R phases (R= rare-earth), it was found that In-vacancies control diffusion in heavy lanthanide indides and R-vacancies in light lanthanide indides. The change in diffusion mechanism was found to be correlated with lattice parameter, which decreases monotonically through the series. However, the change is, in itself, a surprise given the very similar chemistry of rare-earth elements. \textit{This work was supported in part by the NSF under grant DMR 05-04843 (Metals Program). Group web site: http://defects.physics.wsu.edu/}. [Preview Abstract] |
Friday, May 15, 2009 3:26PM - 3:38PM |
B2.00009: Jump frequencies of tracer atoms on Al-sites in Al$_{4}$Ba phases Randal Newhouse, Gary S. Collins The Al$_{4}$Ba structure has two inequivalent Al sites with collinear electric field gradients (EFGs) of unequal magnitude. Nuclear quadrupole interactions (NQIs) were measured at $^{111}$In/Cd probe atoms in Al$_{4}$Ba, In$_{4}$Ba and Al$_{4}$Eu phases using perturbed angular correlation spectroscopy (PAC). The probes were found to occupy both Al-type sites. At low temperature, two NQI frequencies were detected that, with increasing temperature, approached each other and merged at $\sim $400 \r{ }C, above which only a single NQI was observed. This is attributed to rapid jumping of probe atoms between the Al-sites, leading to motional averaging of the EFGs. Merging occurs at the temperature for which the jump frequency equals the difference between static NQI frequencies. Since differences in static frequencies were all about 20 Mrad/s, we conclude that the jump frequencies equaled about 3 MHz at $\sim $400\r{ }C in each phase. This type of motional averaging differs from motional averaging through reorientation of EFGs observed in previous work [Phys. Rev. Lett. 92, 225901 (2004)]. \textit{This work was supported in part by the NSF under grant DMR 05-04843 (Metals Program).} [Preview Abstract] |
Friday, May 15, 2009 3:38PM - 3:50PM |
B2.00010: Calculation of formation energies of intrinsic point defects in In$_{3}$La John P. Bevington, Gary S. Collins We use \textit{perturbed angular correlation of gamma rays}, or PAC, to measure jump frequencies of $^{111}$In/Cd probe atoms in In$_{3}$La and related phases (see Phys. Rev. Lett. 92, 225019 (2004)). To interpret the jump frequencies, first principles calculations were used to determine energies of intrinsic vacancy and antisite atom defects. In$_{3}$La has the L1$_{2}$ structure in which four possible combinations of vacancies and antisite atoms can be thermally activated that maintain the homogeneity of the phase. Formation energies of the defect combinations were calculated using the electronic structure code WIEN2k that is based on density functional theory. Partial formation energies were found to be least for La$_{In}$, at 0.71 eV, and V$_{In}$, at 1.47 eV, with much greater energies for In$_{La}$ and V$_{La}$ defects. Formation energies per defect of the four defect combinations all had similar values, but the antisite pair, La$_{In}$ + In$_{La}$, had the lowest value. \textit{This work was supported by the NSF under grant DMR 05-04843 (Metals Program).} [Preview Abstract] |
Friday, May 15, 2009 3:50PM - 4:02PM |
B2.00011: Soft Magnetic Films Made of Iron-Iron Oxide Core-Shell Nanocluster Impact for High Frequency Applications Qi Yao, M. Kaur T. Singh, Ryan Souza, You Qiang High-frequency soft magnetic thin films, are increasingly in demand with rapid improvement and miniaturization in electromagnetic devices. To achieve high permeability in high frequency up to GHz, high resistivity, saturation magnetization and uniaxial anisotropy field are required. Accordingly, Fe/Ferrite core-shell nanocluster thin films are prepared at room temperature directly on the top of CMOS by novel energetic cluster impact. To avoid high-temperature field deposition or post-annealing, bias voltage up to 20 kV is applied to the oblique substrate to induce in-plane shape anisotropy. The high magnetic moment of Fe cores and the insulated Fe oxide shells jointly cause the combination of high saturation magnetization and high resistivity of the nanocluster thin films. Consequently, the thin films exhibit high permeabilities in up to 1.5 GHz, which are measured by a shorted transmission-line perturbation method. Moreover, the core fraction and the surface density are adjusted, which shows remarkable effects on the thin films' high-frequency permeabilities. [Preview Abstract] |
Friday, May 15, 2009 4:02PM - 4:14PM |
B2.00012: Phase transitions on the surface of an individual carbon nanotube Zenghui Wang, Peter Morse, Jiang Wei, Oscar Vilches, David Cobden Noble gases adsorbed on the surface of graphite exhibit two-dimensional (2D) analogs of 3D gas, liquid and solid phases, as well as commensurability phenomena related to the periodic substrate potential. Here, by employing individual suspended single-walled carbon nanotubes as vibrating mass balances with a sensitivity of a few atoms, we demonstrate that analogous phases occur on the cylindrical nanotube surface. As theoretically predicted we find that the binding of both Ar and Kr is weaker on the surface of a nanotube than on graphite, and we see evidence for formation of 1D atomic chains in grooves between two bundled nanotubes. We observe a 2D melting transition in Ar, and a commensurate phase preempting a liquid-vapor transition in Kr. The transitions are very sharp, indicating that they act as very uniform substrates. The experiments open up new regimes of statistical mechanics to experiment and offer excellent prospects of methodically investigating the interplay between adsorbates and substrate electrons. This work was supported by NSF grant number 0606078. [Preview Abstract] |
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