Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y11: Superlattices and Nanostructures - Electronic Properties |
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Sponsoring Units: DCMP Chair: Stefan Badescu, Naval Research Laboratory Room: 305 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y11.00001: Finding structures with specific properties in complex configurational spaces using multi-target inverse band structure approach Paulo Piquini, Alex Zunger The conventional strategy to look for materials with desired properties is to use physical intuition to select some candidates among an enormous number of possibilities.Apart the very special cases, the solutions to these search problems are far from obvious. The inverse band structure (IBS) approach, on the other hand, search for the desired electronic structures (instead of atomic configurations) from the beginning. Here we illustrate the power of this inverse approach by applying it to the simultaneous engineering of multi-target problems, which encompass huge configurational spaces: (i) the search of a specific band gap in the quaternary (In,Ga)(As,Sb) semiconductors(a) lattice-matched to InP and, (ii) the stacking sequence of (In,Ga)As/InP superlattices leading to band gaps and strains within the range suitable for thermophotovoltaic applications(b). \\[3pt] (a) P. Piquini, P.A. Graf, and A. Zunger, Phys. Rev. Lett. {\bf 100}, 186403 (2008); \\[0pt] (b) P. Piquini and A. Zunger, Phys. Rev. B {\bf 78}, 161302 (2008) [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y11.00002: First-principles studies of structure and electronic level alignment at nanoscale CdSe/CdTe heterojunctions Shenyuan Yang, David Prendergast, Jeffrey Neaton Group II-VI semiconductor nanostructured heterojunctions with type-II interfacial band offsets have many potential applications in nanoscale optoelectronics and photovoltaics. A key open question is how the electronic level alignment at nanoscale heterojunctions differs from that at their bulk counterparts, and whether bulk intuition can be used to understand their electronic behavior. We use density functional theory and beyond to study the structure and electronic properties of CdSe/CdTe interfaces in bulk-planar and nanowire form. Both periodic ``superlattice'' geometries and slabs finite along the surface normal are compared. We compute interface atomic and electronic structure, and for small-diameter nanowires, we discuss the impact of quantum confinement, intrinsic strain, and organic ligand passivation on the electronic properties of the interface. Our results are discussed in the context of recent experiments. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y11.00003: \textit{P}-type InSb and In$_{x}$Ga$_{1-x}$As quantum wells remotely doped with Be Chomani Gaspe, Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos CMOS circuits require $p-$type transistors with high hole mobility, in addition to \textit{n{\-}}type transistors with high electron mobility. We have observed room-temperature hole mobilities of 100 and 600 cm$^{2}$/Vs in In$_{x}$Ga$_{1-x}$As and InSb quantum wells, respectively. The In$_{x}$Ga$_{1-x}$As wells are remotely doped with Be in In$_{x}$Al$_{1-x}$As barrier layers, and grown on InP substrates. The InSb wells are remotely doped with Be in Al$_{x}$In$_{1-x}$Sb barrier layers, and grown on GaAs substrates. We will discuss the effects of strain, structural parameters, and defect density on hole mobility in InSb and In$_{x}$Ga$_{1-x}$As quantum wells. In zinc-blende semiconductors, a narrower band gap leads to smaller effective masses for electrons and holes. Strain and confinement increase the energy splitting between holes with light in-plane mass and those with heavy in-plane mass. This work was supported by the NSF Grants DMR-0808086 and DMR-0520550. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y11.00004: Predicted Band Structures of Wurtzite III-V Semiconductors Based on Empirical Pseudopotentials Amrit De, Craig Pryor The electronic properties of III-V semiconductor nanowhiskers present a problem since the nanowhiskers often crystallize in wurtzite (WZ) form while the corresponding bulk materials are zincblende. Using empirical pseudopotentials, including spin-orbit coupling, we have computed the bulk band structures for non-nitride III-V semiconductors in WZ form, which may be used for electronic structure calculations of nanowhiskers. The calculations make use of transferable model pseudopotentials fit to the zincblende form. We find that due to the stronger breaking of inversion symmetry in the WZ form, there are larger zero field spin splittings than in the corresponding zincblende materials, making WZ nanowhiskers good candidates for novel spin based devices. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y11.00005: Fundamental properties of TiO$_{2}$ nanostructures: the effects of the size confinement and the surface coverage Giovanni Cantele, Amilcare Iacomino, Fabio Trani, Domenico Ninno, Ivan Marri, Stefano Ossicini The titanium dioxide (TiO$_{2}$) complexes are widely investigated for their multipurpose capabilities. We discuss here a characterization of anatase TiO$% _{2}$ 0D clusters (NCs) and 1D nanowires (NWs) in the framework of ab initio DFT calculations. Based on both theoretical and experimental evidences, we defined an anatase TiO$_{2}$ NC by modifying a perfect bipyramidal morphology and then used this NC as a chain repetition unit in the anatase NW. We studied the size confinement and analyzed the effect of surface coverage by functionalization with simple water-derived adsorbates. We found that the structural reconstruction fit the available experimental data, that the band gap shift depends on the crystallinity and that the hydration is important in stabilizing the nanostructures. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y11.00006: Ion scattering from Au nanoclusters formed by buffer layer assisted growth Snjezana Balaz, Jory Yarmoff Ion scattering is used to probe the atomic and electronic structure of Au nanocrystals grown by Buffer Layer Assister Growth (BLAG). Amorphous solid water (ASW) was adsorbed as a buffer layer onto SiO$_2$/Si(111) at liquid nitrogen temperature. Au was then evaporated onto the buffer layer to form nanoclusters. The samples were subsequently annealed to room temperature, causing the water to desorb and the clusters to deposit directly onto the substrate. Time-of-flight (TOF) spectroscopy was used to measure 2 keV $^7$Li$^+$ and $^{39}$K$^+$ ions scattered from Au atoms both at low temperature when the clusters reside atop the buffer layer, and after desorbing the water. Small Au depositions yielded a sharp single scattering peak that indicates single layer structures. Following larger depositions, multiple scattering features were present indicating the formation of multilayer nanoclusters. The neutral fraction of scattered K$^{+}$, which provides an indication of the filled quantum states, starts at $\sim $50{\%} for small Au coverages and decreases with further deposition, indicating changes in the quantum state occupancy with cluster size. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y11.00007: Cyclotron Resonance of Two-Dimensional Hole Systems in InSb Quantum Wells James Coker, R. Doezema, M. Edirisooriya, T. Mishima, Mike Santos, X. Pan, G. Sanders, C. Stanton, L. Tung, Y.-J. Wang In order to realize high-performance InSb CMOS circuits, p-type InSb QW transistors with a high room-temperature mobility are necessary. We report on an experimental study of cyclotron resonance in InSb QWs with Al$_{0.20}$In$_{0.80}$Sb barriers doped with Be. Magnetic fields up to 17.5T were applied perpendicular to the QWs, at a temperature of 4.2K. At fields less than 4T, we deduce a hole effective mass of 0.05$\sim $0.1m$_{o}$ for densities of 2$\sim $5$\times $10$^{11}$cm$^{-2}$, which suggests that high hole mobilities are possible. At higher fields, we observe separate features for different spin-conserving transitions between neighboring Landau levels. The energies of the features depend on the levels' spin index and Landau level indices. The energies and intensities are explained by a modified Pidgeon-Brown model that explicitly incorporates pseudomorphic mechanical strain. This work was supported by the NSF under Grants DMR-0808086 and DMR-0520550. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y11.00008: Structural and electronic properties of crystals of thiolate-capped Au nanoparticles doped with donor and acceptor molecules: first-principles calculations Ronaldo Batista, Jonathan Martins, H\'elio Chacham Structural and electronic properties of crystals of etiolate- capped Au nanoparticles doped with donor and acceptor molecules: first-principles calculations We perform first-principles calculations for crystals composed of periodic assemblies of Au$_{38}$ nanoparticles that are capped with methylthiol molecules. We consider fcc structures, consistent with recent experimental results [1]. We also consider that the nanoparticle crystals can be doped with either donor (tetrabutylammonium) or acceptor (hexafluorophosphate) molecules, also consistent with recent experiments [2]. We find that the most stable positions of the dopant molecules are located near the nanoparticle surfaces, and not near interstitial positions. We also find that the electron chemical potential changes linearly with the impurity concentration, up to a critical concentration, consistent with recent experiments [2]. Above that critical concentration, a new regime is observed due to a partial charge transfer between dopants and nanoparticles. These features are reproduced by a simple capacitive model for the crystal. [1] Ab\'ecassis et al. Phys. Rev. Lett. 100, 115504 (2008) [2] Boettcher S. W. et al. Nature Mater. 6, 592–596 (2007). [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y11.00009: Interaction effects in conductance of quasi-1D channels formed from AlGaAs/GaAs 2DEG: Crossover from weakly-disordered Fermi liquid to Luttinger liquid Matthew Bell, Andrei Sergeev, Jonathan Bird, Vladimir Mitin, Aleksandr Verevkin We investigated the conductance of a long and narrow high-mobility channel and observed crossover from weakly-disordered multi-channel Fermi liquid [1] to Luttinger liquid with decreasing channel width to $\sim$100nm. Quasi-one-dimensional channels were formed from an AlGaAs/GaAs heterostructure using the split-gate technique. The lengths of the channels were 100 $\mu$m. The width of the channels were varied from a lithographic width of 500 nm to $\sim$50 nm by applying negative bias to the split-gate. The effective electron concentration and the channel widths were evaluated from magnetoresistance measurements. In the range of channel widths 500 - 100 nm, at temperatures 1-10 K we clearly observe the logarithmic temperature dependences of the conductance. These dependences are adequately explained by effects of electron-electron interaction in weakly-disordered quasi-one dimensional (with respect to the interaction) Fermi liquid [1]. When the width further decreases, the logarithmic dependences change to power-law dependences, which are typical for Luttinger liquid. This crossover takes place when the channel width corresponds to 2-3 one-dimensional subbands. [1] Sergeev et.al., Phys. Rev. B. 69, 075310 (2004). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y11.00010: Giant Orbital Paramagnetism in Nanometer Scale 2DEG Strips Michael Harrison An elementary calculation shows that Landau diamagnetism becomes significantly altered and very large paramagnetic effects emerge at low tempersatures in nanoscale 2DEG strips penetrated by a perpendicular applied magnetic field and bounded by a parabolic potential, such as may arise from negative voltage applied to a split gate. These novel results are described by an expression which manifests the total system magnetization as a difference between evolved orbital paramagnetism and altered diamagnetism. These predicted effects correspond to drift motion of electrons parallel to the strip length arising from Landau eigenstates that are non-degenerate in the combined presence of a perpendicular applied magnetic field and electric fields associated with a confining parabolic potential. A new heterostructured magnetic material based on orbital electronic motion in 2DEG strips is proposed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y11.00011: Excitonic condensation with different pairing symmetries in double quantum wells Christopher Jamell Double quantum wells with one containing electrons and the other containing holes as carriers are a promising candidate for condensation of dipolar excitons with lifetime much larger than lifetime of excitons in bulk semiconductors. When the inter-well distance is comparable to the interparticle distance within a single well, $d \leq r_s a_B$, inter-well coherence is expected to lead to an excitonic condensation. We explore the ground state of a balanced system as a function of inter-well distance $d$ and the carrier density $n_{2D}$. We present Hartree-Fock mean-field results for the quasiparticle and order parameter dispersion with different pairing symmetries. We obtain the quasiparticle density of states in each case. These results lay the ground work for mean-field study of excitonic condensate states with spontaneously broken translational symmetry. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y11.00012: Coulomb Blockade in a Field Emitting Freely Suspended Island Chulki Kim, Hyun S. Kim, Hua Qin, Robert H. Blick We observe staircase current-voltage characteristics from an isolated nanomechanical island. The island is fixed by CF$_{2}$ connections, which makes the structure suspended 1µm above SiO$_{2}$. The noteworthy difference to the ``orthodox'' single electron transistors is the fact that we observe Coulomb blockade in conjunction with field emission. We can trace and reproduce the transition from staircase in the high bias regime. A theoretical model based on field emission current reproduces the experimental data. The full profile of the current-voltage measurement shows the transition from Coulomb staircase current to island field emission current. [Preview Abstract] |
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