Publications

2013
Altfeder, I., Yi, W. & Narayanamurti, V., 2013. Spin-polarized scanning tunneling microscopy of the room-temperature antiferromagnet c-FeSi. PHYSICAL REVIEW B , 87.Abstract
Antiferromagnetic spin ordering has been revealed by room-temperature spin-polarized scanning tunneling microscopy (SP-STM) in thin epitaxial films of c-FeSi on Si(111). Spin polarization of tunneling current for unoccupied states is found to be unusually large I-up arrow up arrow/I-down arrow up arrow = 3.8. Atomically sharp spin-frustration domain walls, developing on the surfaces of nanoscale islands, have been observed on SP-STM images. Our results suggest that antiferromagnetism in c-FeSi is driven by Mott-Hubbard transition, and the atomically narrow domain walls are caused by local insulator-to-metal breakdown.
Narayanamurti, V. "V.", Odumosu, T. & Vinsel, L., 2013. RIP: The Basic/Applied Research Dichotomy. Issues in Science and Technology , XXIX (2).
2012
Gurwitz, R., et al., 2012. Bandgap and band discontinuity in wurtzite/zincblende GaAs homomaterial heterostructure. APPLIED PHYSICS LETTERS , 100.Abstract
A wurtzite GaAs epilayer grown on a zincblende GaAs substrate by metalorganic chemical vapor deposition is studied by surface photovoltage spectroscopy. The wurtzite structure of the epilayer is disclosed by scanning electron microscope images of surface pits, where the pits are seen to change their structure from a rectangular into a hexagonal shape. The wurtzite phase is also revealed in x-ray diffraction showing a < 0002 > diffraction alongside the main (200) diffraction, suggesting a ``c{''} lattice constant of 0.668 nm. A comparison of room temperature surface photovoltage spectra taken from the epilayer sample and from an epilayer-etched substrate suggests a type II heterostructure with valence band difference of about 15 meV and bandgap difference of about 70 meV between the zincblende and the wurtzite GaAs polytypes. (C) 2012 American Institute of Physics. {[}http://dx.doi.org/10.1063/1.4712562]
Narayanamurti, V. & Odumosu, T., 2012. Toward a Common Wireless Market. ISSUES IN SCIENCE AND TECHNOLOGY , Winter 2012. Full_Text
Narayanamurti, V., 2012. Review of The Idea Factory: Bell Labs and the Great Age of American Innovation by Jon Gertner. physicstoday , 65 (9) , pp. 51-52. Full_Text
2011
Anadon, D.L., et al., 2011. Transforming U.S. Energy Innovation, Belfer Center for Science and International Affairs.
Likovich, E.M., et al., 2011. High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes. ADVANCED MATERIALS , 23 , pp. 4521+.Abstract
Films of semiconductor quantum dots (QDs) are promising for lighting technologies, but controlling how current flows through QD films remains a challenge. A new design for a QD light-emitting device that uses atomic layer deposition to fill the interstices between QDs with insulating oxide is introduced. It funnels current through the QDs themselves, thus increasing the light emission yield.
Likovich, E.M., et al., 2011. Narrow band defect luminescence from Al-doped ZnO probed by scanning tunneling cathodoluminescence. APPLIED PHYSICS LETTERS , 99.Abstract
We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor. (C) 2011 American Institute of Physics. {[}doi:10.1063/1.3647622]
Likovich, E.M., et al., 2011. Scanning tunneling microscope investigation of local density of states in Al-doped ZnO thin films. PHYSICAL REVIEW B , 83.Abstract
The electrical properties of grain boundaries in technologically relevant oxide thin films are the subject of both applied and fundamental research. Here we present an investigation of the local density of states (LDOS) in sputtered Al-doped ZnO using a scanning tunneling microscope. We observe a pronounced difference in the tunneling conductivity recorded on- and off-grain, with the grain boundary LDOS peaked similar to 600 meV below the Fermi level. This provides a direct measurement of the distribution of charge traps that is of relevance in advancing understanding of carrier conduction in this transparent conducting oxide.
2010
Hummon, M.R., et al., 2010. Measuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope. PHYSICAL REVIEW B , 81.Abstract
We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage (IV) tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, (61.4 +/- 2.4) meV, and analysis of the DBTJ, show trap states similar to 1.09 eV below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.
Yi, W., et al., 2010. Probing semiconductor band structures and heterojunction interface properties with ballistic carrier emission: GaAs/AlxGa1-xAs as a model system. PHYSICAL REVIEW B , 81.Abstract
Utilizing three-terminal tunnel emission of ballistic electrons and holes in a planar tunnel transistor with a Mott-barrier collector, we have developed a method to self-consistently determine the energy gap of a semiconductor and band discontinuities at a semiconductor heterojunction without using a priori material parameters. Measurements are performed on lattice-matched GaAs/AlxGa1-xAs (100) single-barrier double heterostructures with AlxGa1-xAs as the model ternary III-V compounds. Electronic band gaps of the AlGaAs alloys and band offsets at the GaAs/AlGaAs (100) interfaces are measured with a resolution of several meV at 4.2 K. The direct-gap Gamma band offset ratio for the GaAs/AlGaAs (100) interface is found to be 59:41 (+/-3%). Reexamination of our previous experiment {[}W. Yi et al., Appl. Phys. Lett. 95, 112102 (2009)] revealed that, in the indirect-gap regime, ballistic electrons from direct tunnel emissions probe the X valley in the conduction band, while those from Auger-like scattering processes in the metal base film probe the higher-lying L valley. Such selective electron collection may be explained by their different momentum distributions and parallel momentum conservation at the quasiepitaxial Al/GaAs (100) interface. We argue that the present method is in principle applicable to arbitrary type-I semiconductor heterostructures.
Yi, W., et al., 2010. Room-temperature photoresponse of Schottky photodiodes based on GaNxAs1-x synthesized by ion implantation and pulsed-laser melting. APPLIED PHYSICS LETTERS , 97.Abstract
The spectral responsivity for Schottky photodiodes based on the GaNxAs1-x alloys synthesized using nitrogen (N) ion implantation followed by pulsed-laser melting and rapid thermal annealing is presented. An N-induced redshift up to 250 meV (180 nm) in the photocurrent onset energy (wavelength) is observed. The N concentration dependence agrees with the values measured by photomodulated reflectance and ballistic electron emission microscopy, and with the calculation by the band anticrossing model for the splitting of the conduction band in GaNxAs1-x. (C) 2010 American Institute of Physics. {[}doi:10.1063/1.3500981]
Russell, K.J., et al., 2010. Scattering-assisted tunneling: Energy dependence, magnetic field dependence, and use as an external probe of two-dimensional transport. PHYSICAL REVIEW B , 82.Abstract
For more than three decades, research on tunneling through planar barriers has focused principally on processes that conserve momentum parallel to the barrier. Here we investigate transport in which scattering destroys lateral momentum conservation and greatly enhances the tunneling probability. We have measured its energy dependence using capacitance spectroscopy, and we show that for electrons confined in a quantum well, the scattering enhancement can be quenched in an applied magnetic field, enabling this mechanism to function as an external probe of the origin of the quantum Hall effect.
Dickey, M.D., et al., 2010. Transistors Formed from a Single Lithography Step Using Information Encoded in Topography. SMALL , 6 , pp. 2050-2057.Abstract
This paper describes a strategy for the fabrication of functional electronic components (transistors, capacitors, resistors, conductors, and logic gates but not, at present, inductors) that combines a single layer of lithography with angle-dependent physical vapor deposition; this approach is named topographically encoded microlithography (abbreviated as TEMIL). This strategy extends the simple concept of `shadow evaporation' to reduce the number and complexity of the steps required to produce isolated devices and arrays of devices, and eliminates the need for registration (the sequential stacking of patterns with correct alignment) entirely. The defining advantage of this strategy is that it extracts information from the 3D topography of features in photoresist, and combines this information with the 3D information from the angle-dependent deposition (the angle and orientation used for deposition from a collimated source of material), to create `shadowed' and `illuminated' regions on the underlying substrate. It also takes advantage of the ability of replica molding techniques to produce 3D topography in polymeric resists. A single layer of patterned resist can thus direct the fabrication of a nearly unlimited number of possible shapes, composed of layers of any materials that can be deposited by vapor deposition. The sequential deposition of various shapes (by changing orientation and material source) makes it possible to fabricate complex structures-including interconnected transistors-using a single layer of topography. The complexity of structures that can be fabricated using simple lithographic features distinguishes this procedure from other techniques based on shadow evaporation.
Ruzmetov, D., et al., 2010. Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer. JOURNAL OF APPLIED PHYSICS , 107 (11) , pp. 114516.
2009
Likovich, E.M., et al., 2009. Direct injection tunnel spectroscopy of a p-n junction. APPLIED PHYSICS LETTERS , 95.Abstract
We demonstrate spectroscopic measurements on an InGaAs p-n junction using direct tunnel injection of electrons. In contrast to the metal-base transistor design of conventional ballistic electron emission spectroscopy (BEES), the base layer of our device is comprised of a thin, heavily doped p-type region. By tunneling directly into the semiconductor, we observe a significant increase in collector current compared to conventional BEES measurements. This could enable the study of systems and processes that have thus far been difficult to probe with the low-electron collection efficiency of conventional BEES, such as luminescence from single-buried quantum dots.
Likovich, E., et al., 2009. Magnetoresistance in an asymmetric Ga1-xMnxAs resonant tunneling diode. PHYSICAL REVIEW B , 80.Abstract
In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shift of negative differential resistance features to higher bias as the relative alignment of the GaMnAs layer magnetizations is changed from parallel to antiparallel. Our observations agree with recent predictions from a theoretical analysis of a similar n-type structure by Ertler and Fabian, and our results suggest that further investigation into ferromagnetic RTD structures may result in significantly enhanced magnetoresistance.
Likovich, E.M., et al., 2009. Weak localization and mobility in ZnO nanostructures. PHYSICAL REVIEW B , 80.Abstract
We conduct a comprehensive investigation into the electronic and magnetotransport properties of ZnO nanoplates grown concurrently with ZnO nanowires by the vapor-liquid-solid method. We present magnetoresistance data showing weak localization in our nanoplates and probe its dependence on temperature and carrier concentration. We measure phase coherence lengths of 50-100 nm at 1.9 K and, because we do not observe spin-orbit scattering through antilocalization, suggest that ZnO nanostructures may be promising for further spintronic study. We then proceed to study the effect of weak localization on electron mobility using four-terminal van der Pauw resistivity and Hall measurements versus temperature and carrier concentration. We report an electron mobility of similar to 100 cm(2)/V s at 275 K, comparable to what is observed in ZnO thin films. We compare Hall mobility to field-effect mobility, which is more commonly reported in studies on ZnO nanowires and find that field-effect mobility tends to overestimate Hall mobility by a factor of 2 in our devices. Finally, we comment on temperature-dependent hysteresis observed during transconductance measurements and its relationship to mobile, positively charged Zn interstitial impurities.
Yi, W., Stollenwerk, A.J. & Narayanamurti, V., 2009. Ballistic electron microscopy and spectroscopy of metal and semiconductor nanostructures. SURFACE SCIENCE REPORTS , 64 (5) , pp. 169-190.
Ruzmetov, D., et al., 2009. Hall carrier density and magnetoresistance measurements in thin-film vanadium dioxide across the metal-insulator transition. PHYSICAL REVIEW B , 79 (15) , pp. 153107.

Pages