Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X21: Spectroscopic Studies of Semiconductor Structure and Their Growth |
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Sponsoring Units: FIAP DCMP Chair: Yia-Chung Chang, Research Center for Applied Sciences, Academia Sinica Room: 323 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X21.00001: Electronic structure of Na$_{x}$CoO$_{2}$ investigated by X-ray absorption spectroscopy with Ab initio calculation Pao-An Lin, Jiunn-Yuan Lin, Ben Hsu, Horng -Tay Jeng, Chen-Shiung Hsue, Yia-Chung Chang The soft X-ray absorption spectra (XAS) of Na$_{x}$CoO$_{2}$ revealed marked and puzzling polarization dependence. It can not be explained by the degeneracy of e$_{g}$ states generally believed in Na$_{x}$CoO$_{2}$. We fabricated the thin films of x = 0.68 and x = 0.75 to investigate the polarization dependence of XAS. Within the first principles DFT calculations, we have explanations for this phenomenon. After the analysis of the DOS of Na$_{x}$CoO$_{2}$, we presume that the pre-edge peaks at 529 eV and 530 eV of Na$_{x}$CoO$_{2}$ O-$K$ edge may be not solely due to the unoccupied states of Co3+ and Co4+ eg states, but also due to the spacial asymmetry in the occupied Co 3d orbitals. Due to the hybridization between Co 3d {\&} O 2p orbitals, the p$_{x,y}$ and p$_{z}$ states will be non-degenerate. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X21.00002: Investigations of individual quantum dots of InAsP in InP nano-wires Mats-Erik Pistol, Niklas Sk\"old, Kimberley Dick, Craig Pryor, Jacob Wagner, Lars Samuelson We have grown InP quantum wires containing InAsP quantum dots by metal-organic vapor phase epitaxy. These structures were investigated by transmission electron microscop and photoluminescence spectroscopy and were modelled by six-band k.p-theory. We observe sharp emission lines from excitons, bi-excitons and tri-excitons. When we have observe tri-exciton emission we observe lines originating from the s-shell as well as from the p-shell. By changing the size of the dots we observe clear confinement effects. The wires have a wurtzite structure but were modelled (by necessity) using zinc-blende parameters. From the deviaiton between the theory and the experiments we can deduce rough values of the band-gap of the wurtzite InAs as well as the electron effective mass of wurtzite InAs. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X21.00003: Size dependent exciton g-factor in self-assembled InAs/InP quantum dots. Paul Koenraad, Niek Kleemans, Joost van Bree, Murat Bozkurt, Andrei Silov, Richard Notzel, Craig Pryor, Michael Flatte We have studied the size dependence of the exciton g-factor in self-assembled InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of these dots indicate a multimodal height distribution. Cross-sectional Scanning Tunneling Microscopy measurements have been performed and support the interpretation of the macro photoluminescence spectra. More than 160 individual quantum dots have systematically been investigated by analyzing single dot magnetoluminescence between 1200nm and 1600 nm. We demonstrate a strong dependence of the exciton g-factor on the height and diameter of the quantum dots, which eventually gives rise to a sign change of the g-factor. The observed correlation between exciton g-factor and the size of the dots is in good agreement with calculations. The results demonstrate that quantum dots emitting at 1.55 micrometer and showing no Zeeman splitting (g-factor = 0) can be constructed. This makes these does interesting for quantum information processing at optical telecommunication wavelengths. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X21.00004: Optical and Electrical Characterization of Melt-Grown Bulk Ternary In$_{x}$Ga$_{1-x}$As J. Wei, S. Guha, L. Gonzalez, P. Dutta, G. Rajagopllan, Y. K. Yeo, R.L. Hengehold Recent crystal growth technology breakthroughs led to successful growth of good quality bulk melt-grown ternary In$_{x}$Ga$_{1-x}$As single crystals. However, these bulk materials have not been well investigated compared to the epitaxial layers grown on a binary compound semiconductor, GaAs. Therefore, the optical and electrical properties of the bulk grown In$_{x}$Ga$_{1-x}$As have been investigated systematically as a function of temperature and In mole fraction x. The results show that the refractive index increases linearly with temperature from 100 to 300 K and also with In composition x from 0.0 to 0.9 for several IR wavelengths. Typical refractive index values are 3.388 and 3.376 for 4.6 and 10.6 $\mu $m, respectively, at 300 K for x=0.5. The results of Hall-effect measurements show that the electron concentrations increase monotonically with x, while the mobilities decrease as x increases from 0.5 to 1.0. Typical electron concentration and mobility at 300 K are 1.3x10$^{16}$/cm$^{3}$ and 9.1x10$^{3}$cm$^{2}$/V S, respectively, at x=0.75. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X21.00005: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X21.00006: Structural, Optical and Electrical Properties of Sputtered InGaN Alloy Thin Films. Mohammad Ebdah, Daniel Hoy, Joel Vaughn, Martin Kordesch Amorphous and polycrystalline InGaN alloy thin films were successfully fabricated using rf sputtering technique with a sputtering targets of metal In and Ga in pure Nitrogen. Films were deposited on Si and quartz substrates, with the ratio of In to Ga being varied from 0 to 1 in the alloy. Growth under different sputtering conditions has been examined, such as different temperatures, pressures, and substrate-target distances. The corresponding obtained structures have been studied using the x-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The compositions have been verified by means of energy dispersive x-rays (EDX) spectroscopy and Rutherford back scattering (RBS). Multiple crystallographic phases have been investigated upon growth at different temperatures, and the existence of Gallium Nitride (GaN) and Indium Nitride (InN) phases were investigated. Hall effect measurements were made in 0.55 T magnetic field for characterizing the electrical resistivity at room temperature and 77 K, the free carrier concentration, and mobility. The optical bandgap and optical properties were studied by spectrophotometric and spectroscopic ellipsometric (SE) techniques. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X21.00007: Atomic and electronic structure of AlN polar surfaces Maosheng Miao, Anderson Janotti, Chris Van de Walle We studied the stability and electronic structure of AlN (0001) and (000-1) polar surfaces using first-principles DFT methods. A plane-wave basis set and PAW potentials are employed surface calculations. ~In order to correct the band gap of AlN, we applied the hybrid functional in the HSE [1] framework. ~Using this approach, we obtained a band gap of 6.1 eV, and lattice constants in excellent agreement with experimental values. Under Al-rich conditions, the Al adatom at T4 sites on the Al-terminated (0001) surface was found to be the most stable (2x2) reconstruction.~ This reconstruction is characterized by occupied surface states (Al-Al bonding) at 3.0 eV below the conduction-band minimum (CBM) and unoccupied surface states (Al dangling bonds) at 1.1 eV below the CBM. ~Under Al-poor conditions, the N adatom at the H3 site is the most stable reconstruction, with occupied N-Al bonding states at 4.2 eV and an uncoccupied Al dangling-bond state at 1.1 eV below the CBM.~ For the N-terminated (000-1) polar surface, the structure with an Al adlayer is the most stable under Al-rich conditions. ~The impact of the surface states on the properties of materials and devices will be discussed. [1] J. Heyd, G.E.Scuseria, and M.Ernzerhof, J. Chem. Phys. 118, 8207(2003). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X21.00008: Micro-RDS to explore the spatial strain distribution in epitaxial AlN layers Chunhua Wang, Zhiyu Yang, Zhiqiang Yao, Wenjun Zhang A non-destructive method to observe the spatially resolved strain distribution in the sub-micrometer scale has been developed. By using micro-RDS to obtain the RA distribution on AlN films grown on Si and sapphire substrates, combining with the relation of the strain and optical anisotropy, we reveal the local strain distribution in the sub-micrometer scale. Strain domains several micrometers in size have been observed in AlN films grown epitaxially on Si without the amorphous interface layer. Each domain consists of hundreds of AlN grains. In films with many defects or grown on sapphire substrate there are no domains of dominant sizes, and the average strain is about 6 times smaller than the ones without the interface layer. The magnitude of the strain agrees well with the experimental values from the established methods such as XRD, TEM and Raman scattering. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X21.00009: Theoretical Spectra and Optimization of Geometries of GaAs Clusters Ajit Hira, John Auxier, Matilda Fernandez The goal of the current research is to expand our previous work on Ga$_{n}$As$_{n}$ clusters (n = 1 thru 12) and on gallium arsenide nanostructures. Our research group appears to be on the verge of making an original discovery about these clusters, which we want to disseminate through publication in a professional journal. So far in our work on nanotechnology, we used the hybrid ab initio methods of quantum chemistry to derive the optimal geometries of the Ga$_{n}$As$_{n}$ clusters for n going from 1 to 12. We also calculated binding energies, bondlengths, ionization potentials, electron affinities and HOMO-LUMO gaps, and IR spectra for these geometries. Of particular significance is the magic numbers for GaAs cluster stability that we found at n =8, 10, 12 and 16. Also, we recovered 32.7 {\%} of the bulk cohesive energy of 6.67 eV at 2.18 eV for n =10. However, we need to validate the accuracy of our results through more sophisticated computation and through experimental work. This is important; because materials containing controlled GaAs nanostructures provide the capability of preparing new classes of materials with enhanced optical, magnetic, chemical sensor and photo-catalytic properties. The second phase of the investigation will examine the effects of confinement on the optical properties the Ga$_{n}$As$_{n}$ clusters. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X21.00010: Relationship between Strain and Band Structure in Strained-Si Nanomembranes Feng Chen, Chanan Euaruksakul, Ming-Huang Huang, Don Savage, Bing-Jun Ding, Franz Himpsel, Max Lagally The influence of uniaxial and in-plane biaxial strain on the conduction bands of Si is explored using elastically strained single-crystal Si (SiNMs) and high-resolution x-ray absorption measurements (XAS). Strain alters the band structure and hence the mobility of charge carriers, as well as band offsets in heterostructures. In addition to the biaxial lattice-induced tensile strain in SiNMs, their extreme thinness ($<$100 nm) makes them flexible, allowing us to strain the membranes by mechanically bending them (on the host onto which they are transferred). We use UV Raman to determine the amount of strain and XAS with the Si2p-to-conduction band (CB) transition to measure energy shifts and the degeneracy splitting of several CB valleys. The strain-induced splitting of the CB minimum and the energy shifts of two higher CBs near L1 and L3 are clearly resolved. [1] CB shifts and 2p core level shifts for uniaxial strain in different directions and biaxial strain in SiNMs are measured and contrasted, and compared to theory where it exists. Supported by DOE and NSF [1] Euaruksakul, C., Lagally M., et al, P.R.L. 101, 147403(2008) [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X21.00011: Electronic gap-driven amorphization mechanism: a new paradigm in phase-change materials R.A. Nistor, D. Shakhvorostov, L. Krusin-Elbaum, G.J. Martyna, C. Cabral, S. Raoux, D.B. Shrekenhamer, D.N. Basov, M.H. Muser, D.M. Newns Phase change materials are materials that can be thermally interconverted between metallic (crystalline) and semiconducting (amorphous) phases. The interconversion process involves a change in local coordination number in some of the atomic constituents in these typically muticomponent materials. The electronic basis for the interconversion is still controversial. Here we report, in contrast to previous views, that the amorphization process is driven by an electronic reorganization in which lowering of the total energy by opening a Peierls-like gap drives the structural reorganization into the amorphous state, thereby explaining both the formation and semiconducting character of the amorphous phases. Our understanding of the process is based on phase transformation driven both thermally and by pressure, and in particular by analysis of long time {\it ab-initio} simulations of the amorphization process. We demonstrate the equivalence of thermal and pressure-driven interconversions in a system where vacancies are either at very low levels or enitrely absent. These discoveries open a new pressure-driven phase interconversion pathway. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X21.00012: Structural, optical, and thermal stability properties of CdZnO thin films grown by molecular-beam epitaxy (MBE) Zheng Yang, Lin Li, Sheng Chu, Jieying Kong, Jianlin Liu CdZnO thin films with near-band-edge emission from violet (3.07 eV) to orange (2.04 eV) were grown using MBE. The CdZnO thin films evolve from pure wurtzite (wz) structure to mixture of wz and rocksalt (rs) structures, and finally to pure rs structure, with increasing Cd concentration. Wz CdZnO shows a robuster thermal stability than the rs CdZnO. The temperature dependence of the CdZnO bandgap shrinkage was investigated and analyzed based on the empirical Varshni and Bose-Einstein fitting of the variable-temperature photoluminescence (PL) peak positions. The fitting parameters are not only useful for materials study, but also important for future device applications. The temperature-dependence of the integral PL intensity was fitted with and without considering the hopping term. Fitting with hopping term shows significant improvements to the data in the CdZnO thin films, but no evident change in a ZnO reference thin film, indicating the hopping process in the CdZnO thin films. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X21.00013: Electronic properties and stabilities of bulk, nano-cluster, and low-index surfaces of SnO in comparison with SnO$_{2}$: application to high-temperature gas sensor Yuhua Duan High-temperature gas sensors to detect various components of the gas flow in gasification technologies are highly desired. As one kind of the wide band-gap oxide semiconductors, tin oxides (SnO$_{2}$, SnO) are widely used as solid state sensor material, oxidation catalyst and transparent conductor. Due to the electronic structure and possibility of two different oxidation states of Sn$^{4+}$ and Sn$^{2+}$ and high thermal stability, tin oxides are very sensitive to oxidizing and reducing many kinds of gases, and therefore can be used to detect these gases with good sensitivity at high-temperature. In this study, based on density functional theory approach with an empirical correction of van der Waals interactions, the structural and electronic properties of the bulk, nano-cluster, and low-index surfaces of SnO$_{2}$ and SnO are obtained. Our results indicate that the differences between SnO$_{2}$ and SnO are significant and the convertible transition of Sn$^{4+} \quad \leftrightarrow $ Sn$^{2+}$ may have a great application in high-temperature sensor technology. In SnO, the van der Waals interactions play an important rule and may lead to more active sites for interacting with other molecules. By investigating the interactions between gas molecules (such as CO$_{2}$, C$_{2}$H$_{6}$, C$_{2}$H$_{5}$, \textit{etc}.) with the surfaces of SnO$_{2}$ and SnO, the sensing mechanism of tin oxides will be explored. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X21.00014: UV Induced Room Temperature Persistent Photocurrent in In$_{2}$O$_{3}$ Films Raghava Panguluri, A. Dixit, C. Sudakar, P. Kharel, Pushkal Thapa, I.A. Avrutsky, Alexander Efros, R. Naik, G. Lawes, B. Nadgorny We have investigated the effects of UV irradiation on the electrical and optical properties of In$_{2}$O$_{3}$ thin films. These films were grown on sapphire substrate by RF sputtering. Hall effect measurements carried out to measure the carrier concentrations, n$_{c}$, show n-type conduction, with n$_{c} \quad \sim $ 2 $\times $ 10$^{18}$ cm$^{-3}$. We find that UV illumination on In$_{2}$O$_{3}$ films leads to a dramatic increase in the charge carrier concentration, approximately one order of magnitude, and that these elevated carrier densities persist at room temperature on a timescale of days. Consequently, we observe distinct changes in the optical absorption spectra. Also, we studied the decay of the charge carrier density to the pre-illuminated state as a function of temperature. We will discuss possible mechanisms for persistent photoconductivity in these In$_{2}$O$_{3}$ thin films. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X21.00015: Effect of Oxygen Pressure on the Structure and Luminescence of Europium Doped Gadolinium Oxide Thin Films Patrick Wellenius, John F. Muth, Pae C. Wu, Henry O. Everitt, Eric R. Smith Gadolinium oxide has found uses as a dielectric or passivation layer for novel III-V materials and devices, but more recently has been the subject of study as a rare-earth host. It is believed that Gd$_{2}$O$_{3}$ makes a good host for these dopants due to the similarity in ionic radii between the gadolinium ion and the rare-earth dopants. The reported long radiative lifetimes of rare earth dopants in this material make it interesting for optically pumped laser materials. In this study, europium-doped gadolinium oxide (Eu:Gd$_{2}$O$_{3})$ polycrystalline thin films were deposited on sapphire substrates by pulsed laser deposition at 5 and 50 mTorr oxygen pressure. Changes in the crystal structure were observed by x-ray diffraction and photoluminescence. Low-temperature photoluminescence spectra of the $^{5}$D$_{0}-^{7}$F$_{0}$ and $^{7}$F$_{2}$ transitions in the europium ion were recorded with high resolution. Because the $^{5}$D$_{0}-^{7}$F$_{0}$ transition in europium is not subject to fine structure splitting, it provides a useful mechanism for investigation of the local environment. The $^{5}$D$_{0}-^{7}$F$_{2}$ transition is of interest as it results in the most intense emission, making europium doped material useful for red light-emitting phosphors. Radiative lifetimes of the observed transitions are also reported. [Preview Abstract] |
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