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LaBella Group
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Nanoscale Schottky Barrier Visualization Utilizing Computational Modeling and Ballistic Electron Emission Microscopy,
Westly Nolting, Chris Durcan, Steven Gassner, Joshua Goldberg, Robert Balsano, Vincent P. LaBella,
Journal of Applied Physics, 123 245302 (2018).
ABSTRACT: The electrostatic barrier at a metal semiconductor interface is visualized using nanoscale spatial and meV energetic resolution. A combination of Schottky barrier mapping with ballistic electron emission microscopy and computational modeling enables extraction of the barrier heights, the hot electron scattering, and the presence of localized charges at the interface from the histograms of the spectra thresholds. Several metal semiconductor interfaces are investigated including W/Si(001) using two different deposition techniques, Cr/Si(001), and mixed Au-Ag/Si(001). The findings demonstrate the ability to detect the effects of partial silicide formation in the W and Cr samples and the presence of two barrier heights in intermixed Au/Ag films upon the electrostatic barrier of a buried interface with nanoscale resolution. This has potential to transform the fundamental understanding of the relationship between electrostatic uniformity and interface structure for technologically important metal semiconductor interfaces.
(full text and abstract) DOI: 10.1063/1.5029913

Fermi Level Manipulation through Native Doping in the Topological Insulator Bi2Se3,
Lee A. Walsh, Avery J. Green, Rafik Addou, Westly Nolting, Christopher R. Cormier, Adam T. Barton, Tyler R. Mowll, Ruoyu Yue, Ning Lu, Jiyoung Kim, Moon J. Kim, Vincent P. LaBella, Carl A. Ventrice, Stephen McDonnell, William G. Vandenberghe, Robert M. Wallace, Alain Diebold, Christopher L. Hinkle,
ACS Nano, 12 6310 (2018).
ABSTRACT: The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in Bi2Se3 and achieving Fermi levels (EF) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the Bi2Se3 studies in the literature report strongly n-type materials with EF in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic Bi2Se3 with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune EF from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for EF control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states Bi2Se3.
(full text and abstract) DOI: 10.1021/acsnano.8b03414

Detection of Silicide Formation in Nanoscale Visualization of Interface Electrostatics,
Westly Nolting, Chris Durcan, Vincent P. LaBella,
Applied Physics Letters, 110 141606 (2017).
ABSTRACT: The ability to detect localized silicide formation at a buried metal semiconductor Schottky interface is demonstrated via nanoscale measurements of the electrostatic barrier. This is accomplished by mapping the Schottky barrier height of the Cr/Si(001) interface utilizing ballistic electron emission microscopy (BEEM). Monte-Carlo modeling is employed to simulate the distributions of barrier heights that includes scattering of the electrons that traverse the metal layer and a distribution of electrostatic barriers at the interface. The best agreement between the model and the data is achieved when specifying two barrier heights less than 60 meV from one other instead of a singular barrier. This provides strong evidence that localized silicide formation is occurring that would be difficult to observe in an averaged BEEM spectra or conventional current voltage measurements.%In this work, we perform BEEM on Cr/Si Schottky contacts to visualize the interface electrostatics to nanoscale dimensions. This is accomplished by acquiring tens of thousands of spectra on a regularly spaced grid and fitting the results to determine the local Schottky barrier height. Barrier heights of \phib = 0.6 eV for n-Si(001) and \phib = -0.59 eV for p-Si(001) were measured from fitting to the averaged spectra. A Monte-Carlo model is then utilized to simulate the distributions of barrier heights that includes effects from the interface and transport of the hot electrons. The agreement between the measurements and modeling provides strong evidence that localized silicide formation is occurring that is difficult to observe in the average spectra or conventional current voltage measurements.
(full text and abstract) DOI: 10.1063/1.4979874

Economical rotatable holder for magneto-transport measurements,
Avyaya J. Narasimham, Daniel Pennock, Graham J. Potter, Brian Taylor, Vincent P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 35 023201 (2017).
ABSTRACT: A low cost rotatable holder is designed and fabricated for the Quantum Design VersaLab that enables rotation of the device over 90° without the need for rewiring. This allows the magnetic field to be oriented from in plane to perpendicular to plane for the devices, enabling angle dependent measurements. Several measurements are performed to test performance against the standard sample mounting puck that comes with the system. In addition, Anomalous Hall measurements are performed on Si|SiO2|Ta (5 nm)|CoFeB (1 nm)|MgO (1.6 nm)|Ta (Cap) devices with the field perpendicular and parallel to the plane of the sample to demonstrate rotation.
(full text and abstract) DOI: 10.1116/1.4974488

Nanoscale Schottky barrier mapping of thermally evaporated and sputter deposited W/Si(001) diodes using ballistic electron emission microscopy,
Westly Nolting, Chris Durcan, Avyaya J. Narasimham, Vincent P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 34 04J110 (2016).
ABSTRACT: Ballistic electron emission microscopy (BEEM) has been utilized to demonstrate differences in the interface electrostatics of tungsten-Si(001) Schottky diodes fabricated using two different deposition techniques; thermal evaporation using electron-beam heating and magnetron sputtering. A difference of 70 meV in the Schottky barrier heights is measured between the two techniques for both p-type and n-type silicon even though the sum of n-type and p-type Schottky barrier heights agrees with the band gap of silicon. Spatially resolved nanoscale maps of the Schottky barrier heights are uniform for the sputter film and are highly disordered for the e-beam film. Histograms of the barrier heights show a symmetric Gaussian like profile for the sputter film and a skewed lognormal distribution for e-beam film. A Monte-Carlo model is developed to simulate these histograms which gives strong indication that localized elastic scattering is causing this skewing as forces the hot electrons to need a greater total energy to surmount the barrier. These differences are attributed to silicide formation from the unintentional substrate heating during the e-beam deposition, which is confirmed with transmission electron microscopy.
(full text and abstract) DOI: 10.1116/1.4958721

Relating Spatially Resolved Maps of the Schottky Barrier Height to Metal/Semiconductor Interface Composition,
Robert Balsano, Chris Durcan, Akitomo Matsubayashi, Avyaya J. Narasimham, Vincent P. LaBella,
Journal of Applied Physics, 119 095302 (2016).
ABSTRACT: The SBH is mapped with nanoscale resolution at pure Au/Si(001) and mixed Au/Ag/Si(001) interfaces utilizing beem by acquiring and fitting spectra every 11.7 nm × 11.7 nm over a 1 μm × 1 μm area. The SBH distributions for the mixed interfaces are about four times broader than the pure gold interface and have a complex structure arising from the incomplete intermixing and its effects on the scattering of hot electrons. The maps of the barrier heights and amplitudes for the mixed samples are inhomogeneous with localization attributed to the disordered mixture of the Au and Ag. A method to calculate ratio of the Au to Ag Schottky barrier heights is presented and discussed in relationship to the interface composition. These results demonstrate how the Schottky barrier height is a function of position at the interface and how mapping can capture the electrostatic nature of these and possibly other mixed metal-semiconductor interfaces.
(full text and abstract) DOI: 10.1063/1.4942659

Time dependent changes in Schottky barrier mapping of the W/Si(001) interface utilizing Ballistic Electron Emission Microscopy,
Chris A. Durcan, Robert Balsano, Vincent P. LaBella,
Journal of Applied Physics, 117 245306 (2015).
ABSTRACT: The W/Si(001) Schottky barrier height is mapped to nanoscale dimensions using BEEM over a period of 21 days to observe changes in the interface electrostatics. Initially the average spectrum is fit to a Schottky barrier height of 0.71 eV and the map is uniform with 98% of the spectra able to be fit. After 21 days, the average spectrum is fit to a Schottky barrier height of 0.62 eV and the spatial map changes dramatically with only 27% of the spectra able to be fit. Transmission electron microscopy shows the formation of an ultra-thin tungsten silicide at the interface which increases in thickness over the 21 days. This increase is attributed to an increase in electron scattering and the changes are observed in the BEEM measurements. Interestingly, little to no change is observed in the I-V measurements throughout the 21 day period.
(full text and abstract) DOI: 10.1063/1.4922972

Pulsed-N2 assisted growth of 5-20 nm thick β-W films,
Avyaya J. Narasimham, Richard J. Matyi, Avery Grene, Prasanna Khare, Tuan Vo, Alain Diebold, Vincent P. LaBella,
AIP Advances, 5 117107 (2015).
ABSTRACT: A technique to deposit 5-20 nm thick β-phase W using a 2-second periodic pulse of 1 sccm-N2 gas on Si(001) and SiN(5 nm)/Si(001) substrates is reported. Resistivity, X-ray photoelectron spectroscopy and X-ray reflectivity were utilized to determine phase, bonding and thickness, respectively. X-ray diffraction patterns were utilized to determine the crystal structure, lattice constant and crystal size using the LeBail method. The flow rate of Nitrogen gas (continuous vs. pulsing) had significant impact upon the crystallinity and formation of β-phase W.
(full text and abstract) DOI: 10.1063/1.4935372

Synthesis and properties of ferromagnetic nanostructures embedded within a high-quality crystalline silicon matrix via ion implantation and nanocavity assisted gettering processes,
Girish Malladi, Mengbing Huang, Thomas Murray, Steven Novak, Akitomo Matsubayashi, Vincent LaBella, Hassaram Bakhru,
Journal of Applied Physics, 116 054306 (2014).
ABSTRACT: Integrating magnetic functionalities with silicon holds the promise of developing, in the most dominant semiconductor, a paradigm-shift information technology based on the manipulation and control of electron spin and charge. Here, we demonstrate an ion implantation approach enabling the synthesis of a ferromagnetic layer within a defect free Si environment by exploiting an additional implant of hydrogen in a region deep below the metal implanted layer. Upon post-implantation annealing, nanocavities created within the H-implanted region act as trapping sites for gettering the implanted metal species, resulting in the formation of metal nanoparticles in a Si region of excellent crystal quality. This is exemplified by the synthesis of magnetic nickel nanoparticles in Si implanted with H+ (range: 850nm; dose: 1.51016cm2) and Ni + (range: 60nm; dose: 21015cm2). Following annealing, the H implanted regions populated with Ni nanoparticles of size (1025nm) and density (1011/cm2) typical of those achievable via conventional thin film deposition and growth techniques. In particular, a maximum amount of gettered Ni atoms occurs after annealing at 900C, yielding strong ferromagnetism persisting even at room temperature, as well as fully recovered crystalline Si environments adjacent to these Ni nanoparticles. Furthermore, Ni nanoparticles capsulated within a high-quality crystalline Si layer exhibit a very high magnetic switching energy barrier of 0.86eV, an increase by about one order of magnitude as compared to their counterparts on a Si surface or in a highly defective Si environment.
(full text and abstract) DOI: 10.1063/1.4892096

Microstructure fabrication process induced modulations in CVD graphene,
Akitomo Matsubayashi, Zhenjun Zhang, Ji Ung Lee, Vincent P. LaBella,
AIP Advances, 4 127143 (2014).
ABSTRACT: The systematic Raman spectroscopic study of a ``mimicked'' graphene device fabrication is presented. Upon photoresist baking, compressive stress is induced in the graphene which disappears after it is removed. The indirect irradiation from the electron beam (through the photoresist) does not significantly alter graphene characteristic Raman peaks indicating that graphene quality is preserved upon the exposure. The 2D peak shifts and the intensity ratio of 2D and G band, I(2D)/I(G), decreases upon direct metal deposition (Co and Py) suggesting that the electronic modulation occurs due to sp2 C-C bond weakening. In contrast, a thin metal oxide film deposited graphene does not show either the significant 2D and G peaks shift or I(2D)/I(G) decrease upon the metal deposition suggesting the oxide protect the graphene quality in the fabrication process.
(full text and abstract) DOI: 10.1063/1.4905068

Fabrication of 5-20 nm thick b-W films,
Avyaya J. Narasimham, Manasa Medikonda, Akitomo Matsubayashi, Prasanna Khare, Hyuncher Chong, Richard J. Matyi, Alain Diebold, Vincent P. LaBella,
AIP Advances, 4 117139 (2014).
ABSTRACT: A technique to fabricate 5 to 20 nm thick sputter deposited β W films on SiO2 and Si substrates is presented. This is achieved by growing tungsten on a 5 nm SiO2 layer or in an oxygen controlled environment by flowing 2 sccm of O2 during deposition. Resistivity, X-ray photoelectron spectroscopy, X-ray diffraction and reflectivity studies were performed to determine the phase and thickness of tungsten films. These results demonstrate a technique to grow this film on bare Si or a SiO2 substrate, which can enable growth on the bottom of a write unit in a non-volatile spin logic device.
(full text and abstract) DOI: 10.1063/1.4903165

Nanoscale Mapping of the W/Si(001) Schottky Barrier,
Chris A. Durcan, Robert Balsano, Vincent P. LaBella,
Journal of Applied Physics, 116 023705 (2014).
ABSTRACT: The W/Si(001) Schottky barrier was spatially mapped with nanoscale resolution using BEEM and BHEM using n-type and p-type silicon substrates. The formation of an interfacial tungsten silicide is observed utilizing TEM and RBS. The BEEM and BHEM spectra are fit utilizing a linearization method based on the power law BEEM model using the PL fitting exponent. The aggregate of the Schottky barrier heights from n-type (0.71 eV) and p-type (0.47 eV) silicon agree with the silicon band gap at 80 K. Spatially resolved maps of the Schottky barrier are generated from grids of 7225 spectra taken over a 1 μm × 1 μm area and provide insight into its homogeneity. Histograms of the barrier heights have a Gaussian component consistent with an interface dipole model and show deviations that are localized in the spatial maps and are attributed to compositional fluctuations, nanoscale defects, and foreign materials.
(full text and abstract) DOI: 10.1063/1.4889851

Schottky barrier height measurements of Cu/Si(001), Ag/Si(001), and Au/Si(001) interfaces utilizing ballistic electron emission microscopy and ballistic hole emission microscopy,
Robert Balsano, Akitomo Matsubayashi, Vincent P. LaBella,
AIP Advances, 3 112110 (2013).
ABSTRACT: The Schottky barrier heights of both n and p doped Cu/Si(001), Ag/Si(001), and Au/Si(001) diodes were measured using BEEM and BHEM, respectively. Measurements using both forward and reverse BEEM and BHEM injection conditions were performed. The SBHs were found by fitting to a linearization of the power law form of the Bell-Kaiser BEEM model. The sum of the n-type and p-type barrier heights are in good agreement with the band gap of silicon and independent of the metal utilized. The Schottky barrier heights are found to be below the region of best fit for the power law form of the BK model, demonstrating its region of validity.
(full text and abstract) DOI: 10.1063/1.4831756

Characterization of Metal Oxide Layers Grown on CVD Graphene,
Akitomo Matsubayashi, Joseph Abel, Dhiraj Prasad Sinha, Ji Ung Lee, Vincent P. LaBella,
Journal of Vacuum Science and Technology A, 31 021506 (2013).
ABSTRACT: Growth of a fully oxidized aluminum oxide layer with low surface roughness on graphene grown by chemical vapor deposition is demonstrated. This is accomplished by the deposition of a 0.2 nm thick titanium seed layer on the graphene prior to the deposition of the aluminum under ultra highvacuum conditions, which was subsequently oxidized. The stoichiometry and surface roughness of the oxide layers were measured for a range of titanium and aluminum depositions utilizing ex situ x-ray photoelectron spectrometry and atomic force microscopy. These fully oxidized films are expected to produce good dielectric layers for use in graphene based electronic devices
(full text and abstract) DOI: 10.1116/1.4792068

Signatures of the semiconductor crystallographic orientation on the charge transport across non-epitaxial diodes,
John J. Garramone, Joseph R. Abel, Salvador Barraza-Lopez, Vincent P. LaBella,
Applied Physics Letters, 100 252102 (2012).
ABSTRACT: The hot electron attenuation length of Ag is measured utilizing ballistic electron emission microscopy on nanoscale Schottky diodes for Si(001) and Si(111) substrates. Marked differences in the attenuation length are observed at biases near the Schottky barrier depending upon the substrate orientation, increasing by an order of magnitude only for Si(001). These results provide clear evidence that the crystallographic orientation of the semiconductor substrate and parallel momentum conservation affect the charge transport across these interfaces. A theoretical model reproduces the effect that combines a free-electron description within the metal with an ab-initio description of the electronic structure of the semiconductor.%The attenuation length for the (001) semiconductor orientation can be ascribed to the presence of a focused and dispersion-less ballistic channel at the bottom of the Si(001) conduction band edge that is not present on Si(111).
(full text and abstract) DOI: 10.1063/1.4729622

Fabrication of an Electrical Spin Transport Device utilizing a Diazonium Salt/Hafnium Oxide Interface Layer on Epitaxial Graphene grown on 6H-SiC(0001),
J. Abel, A. Matsubayashi, T. Murray, C. Dimitrakopoulos, D. B. Farmer, Ali Afzali, A. Grill, C. Y Sung, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 30 04E109 (2012).
ABSTRACT: Non-local Hanle spin precession devices are fabricated on wafer scale epitaxial graphene utilizing conventional and scalable processing methods. To improve spin injection and reduce contact related spin relaxation, hafnium oxide is utilized as an interface barrier between the graphene on SiC(0001) and ferromagnetic metal contacts. The hafnium oxide layer is deposited by ald utilizing an organic seed layer. Spin precession is observed in the epitaxial graphene.
(full text and abstract) DOI: 10.1116/1.4732460

Low Cost Cyrogenic High Vacuum Sample Holder for In-Plane Magneto-transport Studies,
Joseph Abel, Akitomo Matsubayashi, Vincent P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 30 043201 (2012).
ABSTRACT: Non-local Hanle spin precession devices are fabricated on wafer scale epitaxial graphene utilizing conventional and scalable processing methods. To improve spin injection and reduce contact related spin relaxation, hafnium oxide is utilized as an interface barrier between the graphene on SiC(0001) and ferromagnetic metal contacts. The hafnium oxide layer is deposited by ald utilizing an organic seed layer. Spin precession is observed in the epitaxial graphene.
(full text and abstract) DOI: 10.1116/1.4732506

Schottky barrier and attenuation length for hot hole injection in non-epitaxial Au on p type GaAs,
Ilona Sitnitsky, John J. Garramone, Joseph Abel, Peng Xu, Steven D. Barber, Matt L. Ackerman, J. Kevin Schoelz, Paul M. Thibado, Vincent P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 30 04E110 (2012).
ABSTRACT: Ballistic electron emission microscopy (BEEM) was performed to obtain current versus bias characteristics of non-epitaxial nanometer-thick Au on p-type GaAs in order to accurately measure the local Schottky barrier height. Hole injection BEEM data was averaged from thousands of spectra for various Au film thicknesses and then used to determine the attenuation length of the energetic charge carriers as a function of tip bias. We report an increase in attenuation length at biases near the Schottky barrier, providing evidence for the existence of coherent BEEM currents in Schottky diodes. These results provide additional evidence for the conservation of charge carriers's parallel momentum at the metal-semiconductor interface.
(full text and abstract) DOI: 10.1116/1.4734307

Temperature Dependent Spin Precession Measurements in Tri-Layer Graphene Utilizing Co/Graphene Contacts,
J. Abel, A. Matsubayashi, J. J. Garramone, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 30 03D115 (2012).
ABSTRACT: The temperature dependence of the spin lifetime and spin diffusion coefficient of exfoliated multi-layer graphene is measured using non-local spin detection and spin precession measurements. Low impedance cobalt contacts are utilized for spin injection and readout. A decrease in spin lifetime with increasing temperature is observed as well as an increase in the spin diffusion coefficient with increasing temperature. This observation provides some insight into the relevant spin relaxation mechanisms that are occurring in this tri-layer graphene sample.%and is attributed to an increase in the conductance of the metal/graphene interface, which enhances the contact induced relaxation. is attributed to an increase in the mean free path and carrier concentration in the multi-layer graphene channel.%The magnitude of the spin lifetimes combined with the low mobility of the graphene suggests that contact induced spin relaxation is the dominant mechanism.
(full text and abstract) DOI: 10.1116/1.4709768

Hot-Electron Transport Studies of the Ag/Si(001) Interface Using Ballistic Electron Emission Microscopy,
J. J. Garramone, J. R. Abel, I. L. Sitnitsky, V. P. LaBella,
Journal of Vacuum Science and Technology A, 28 643 (2010).
ABSTRACT: Ballistic electron emission microscopy has been utilized toinvestigate the hot-electron transport properties of theAg/Si(001) Schottky diode utilizing metal films deposited bothin situ and ex situ. The sbh is measured tobe \ESphiB\ and \ISphiB\ for the ex situ and insitu depositions, respectively. The metal overlayersdemonstrates typical Volmer-Weber growth when deposited on theSi(001) semiconducting substrate as seen in the stmimages. An enhancement in hot-electron transmission is measured for thein situ deposited metal films when compared to the ex situ films.Material Names: Gold, Silver, Silicon
(full text and abstract) DOI: 10.1116/1.3397795

Studies of Al2O3 Barriers for Use in Tunnel Junctions For NonLocal Spin Detection Experiments,
J. Abel, J. J. Garramone, I. L. Sitnitsky, V. P. LaBella,
Journal of Vacuum Science and Technology A, 28 702 (2010).
ABSTRACT: Aluminum oxide films were grown on Si under uhv conditionsfor use as tunnel barriers in spin injection studies. X-rayphotoelectron spectroscopy was performed to characterize thefilm stoichiometry. It was observed that all the aluminum wasbonded to the oxygen for the films grown in 1 nm steps. Whereasthe 2 nm sample grown in one 2 nm step left a partiallyun-oxidized aluminum film. Current - Voltage measurements wereperformed and fit to a tunnel model. The resistance areaproducts fall within the range needed for spin-injection andnon-local readout.Material Names: Aluminum Oxide, Silicon, Iron
(full text and abstract) DOI: 10.1116/1.3386589

Measurement of the Hot Electron Attenuation Length of Copper,
J. J. Garramone, J. R. Abel, I. L. Sitnitsky, L. Zhao, I. Appelbaum, V. P. LaBella,
Applied Physics Letters, 96 062105 (2010).
ABSTRACT: Ballistic electron emission microscopy isutilized to investigate the hot-electron scattering propertiesof Cu through Cu/Si(001) Schottky diodes. A SBH of \phiB\and a hot-electron attenuation length of \atten\ are measuredat a tip bias of \Vtip\ and a temperature of 80 K. Thedependence of the attenuation length with tip bias is fit to aFermi liquid model that allows extraction of the inelastic andelastic scattering components. This modeling indicates thatelastic scattering due to defects, grain boundaries, andinterfaces is the dominant scattering mechanism in this energyrange.
(full text and abstract) DOI: doi:10.1063/1.3299712

Hot Electron Transport Studies of the Cu/Si(001) Interface Using Ballistic Electron Emission Microscopy,
J. J. Garramone, J. R. Abel, I. L. Sitnitsky, R. L. Moore, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 27 2044 (2009).
ABSTRACT: The hot electron transport properties of the Cu/Si(001)interface have been studied using ballistic electron emissionmicroscopy (BEEM). The Schottky barrier height was measured tobe \phiB. The STM images provide evidence of Volmer-Webergrowth of the metal, while Rutherford backscatteringspectrometry (RBS) data corroborated with Auger depth profilingindicate distinct Cu and Si regions with little intermixing.Comparison with Au/Si(001) BEEM data provides some insight intothe hot electron transport and scattering properties of theCu/Si(001) interface.Keywords: copper, silicon, gold
(full text and abstract) DOI: 10.1116/1.3136761

Effect of interface band structure on hot-electron attenuation lengths in Au thin films,
A. J. Stollenwerk, E. J. Spadafora, J. J. Garramone, R. J. Matyi, R. L. Moore, V. P. LaBella,
Physical Review B, 77 033416 (2008).
ABSTRACT: The hot electron attenuation length in thin Au films has beendetermined by means of ballistic electron emission microscopy onAu/Si(111), Au/Si(001) and Au/GaAs(001) Schottky diodes. Theattenuation length measured for Au/GaAs(001) was ~ 10 ×shorter than attenuation lengths measured on the Si substrates. Inaddition, the slope of the attenuation length vs. tip bias decreasesabove the X conduction band minimum in GaAs. Theseobservations are attributed to differences in the amount of allowedparallel momentum at the interfaces of both semiconductors. Theseresults suggest that these apparent attenuation lengths are not anintrinsic property of the metal and can be utilized as a powerfulmethod to probe the band structure at metal-semiconductorinterfaces.
(full text and abstract) DOI: 10.1103/PhysRevB.77.033416

Measuring Spin Dependent Hot Electron Transport Through a Metal-Semiconductor Interface Using Spin-Polarized Ballistic Electron Emission Microscopy,
A. J. Stollenwerk, M. R. Krause, J. J. Garramone, E. J. Spadafora, V. P. LaBella,
Physical Review B, 76 195311 (2007).
ABSTRACT: Spin polarized ballistic electron transport has been studied inFe/Si(001) Schottky diodes using ballistic electron emissionmicroscopy. Spin dependent scattering of polarized ballisticelectrons injected from an Fe coated Au tip into the Fe films hasbeen shown to affect the BEEM current. The spin dependentattenuation lengths were determined by measuring this effect with Fethickness and found to be 1.8 ± 0.2 nm for the minority spinelectrons and 2.5 ± 0.3 nm for the majority spin electrons at atip bias of 1.5 eV. In addition, the attenuation lengths weremeasured as a function of tip bias, which indicated that theFe/Si(001) interface band structure has an effect on the hotelectron transport through the diode.
(full text and abstract) DOI: 10.1103/PhysRevB.76.195311

Measurement of the clustering energy for manganese silicide islands on Si(001) by Ostwald Ripening,
M. R. Krause, A. J. Stollenwerk, M. Licurse, V. P. LaBella,
Applied Physics Letters, 91 041903 (2007).
ABSTRACT: The rate of growth during annealing of manganese silicide islands inthe submonolayer coverage regime on the Si(001) surface has beenmeasured by scanning tunneling microscopy. The fourth power of thegrowth rate is linearly dependent upon the annealing time,consistent with a diffusion limited Ostwald ripening mechanism forisland growth. The growth rate has been determined for fourdifferent annealing temperatures to extract the activation energyfor clustering, which has been found to be 2.6 ± 0.2 eV.
(full text and abstract) DOI: 10.1063/1.2766681

Implantation Damage Study in Ferromagnetic Mn-implanted Si,
C. Awo-Affouda, M. Bolduc, V. P. LaBella,
Journal of Vacuum Science and Technology A, 25 976 (2007).
ABSTRACT: To investigate the influence of the residual implant damage and postimplant annealing upon the structure and magnetic properties ofMn-implanted Si, lattice disorder depth profiles were obtained fromRBS-channeling experiments on Mn implanted <100> oriented p-typeSi wafers. The defect concentrations profiles were extracted fromthe RBS spectra using the two beams model. These profiles reveal astrong influence of the post implant annealing temperatures upon thedefects generated from implantation. Specifically, above800 °C, the back-scattering yield from Si lattice defectsdecreases, which is coincident with a decrease in the magnetization.The evolution of the Mn concentration profiles and the magnetizationsuggest that the magnetization originates from Mn atoms located inthe least damaged region.
(full text and abstract) DOI: 10.1116/1.2713117

Electronic structure changes of Si(001) (2×1) from subsurface Mn observed by STM,
M. R. Krause, A. J. Stollenwerk, J. Reed, V. P. LaBella, M. Hortamani, P. Kratzer, M. Scheffler,
Physical Review B, 75 205326 (2007).
ABSTRACT: The deposition of Mn atoms onto the Si(001)-(2×1)reconstructed surface has been studied using scanning tunnelingmicroscopy (STM) and first-principles electronic structurecalculations. Room temperature deposition of 0.1 monolayers of Mngives rise to a disordered surface structure. After insitu annealing between 300-700 °C, most of the Mn isincorporated into three-dimensional manganese silicide islands, andSi dimer rows re-appear in the STM images on most of the substratesurface. At the same time, row-like structures are visible in theatomic-scale STM images. A comparison with calculated STM imagesprovides evidence that Mn atoms are incorporated into the rowstructures in subsurface interstitial sites, which is the lowestenergy position for Mn on Si(001). The subsurface Mn alters theheight and local density of states of the Si dimer atoms causingthem to appear 0.6 Å higher than a neighboring Si dimer with noMn below. This height difference that allows the detection thesubsurface Mn results from a subtle interplay of geometrical andelectronic effects.
(full text and abstract) DOI: 10.1103/PhysRevB.75.205326

Atmospheric oxygen in Mn doped GaAs/GaAs(0 0 1) thin films grown by molecular beam epitaxy,
J.F. Xu, P.M. Thibado, C. Awo-Affouda, R. Moore, V.P. LaBella,
Journal of Crystal Growth, 301-302 54 (2007).
ABSTRACT: Mn doped GaAs thin films were grown using molecular beam epitaxy at high and low substrate temperatures. The elemental concentration depth profiles in the thin films were determined by using Auger electron spectroscopy combined with ion etching. The Mn concentration is higher near the surface and then decreases with depth for films grown at high substrate temperatures. The Mn concentration profile is much more uniform when films are grown using a low substrate temperature. What was unexpectedly found are high levels of oxygen in the low substrate temperature grown thin films.
(full text and abstract) DOI: 10.1016/j.jcrysgro.2006.11.234

Probing the hot electron transport properties and interface band structure of Fe/Si(001) and Fe81C19/Si(001) Schottky diodes,
A. J. Stollenwerk, M. R. Krause, D. H. Idell, R. Moore, V. P. LaBella,
Physical Review B, 74 155328 (2006).
ABSTRACT: Ballistic electron emission microscopy (BEEM) has been performed onboth Au/Fe81C19/Si(001) and Au/Fe/Si(001) Schottky diodes at 80 K. The Schottky heights were measured to be 0.68 ± 0.02 eV and0.70 ± 0.02 eV for the Fe81C19/Si(001) and Fe/Si(001) interfaces, respectively. In addition, a second threshold voltage was observedfor the Fe/Si(001) interface at 1.29 ± 0.04 eV and attributed to anadditional conduction band minimum at the interface that arises fromthe bonding of the Fe to the Si. The hot electron attenuationlengths at 1.25 eV were measured to be 3.5 ± 1.0 nm and 3.0 ± 0.9 nm for Fe81C19 and Fe, respectively. The attenuation length of the Fe81C19 showeda decrease with increasing energy consistent with the universalcurve for electron-electron scattering. However, the attenuationlength for the Fe showed this decrease only until the onset of thesecond threshold after which it increased. It is proposed that thisincrease is attributed to the parallel momentum distribution of theadditional conduction band minimum at the Fe/Si(001) interface.
(full text and abstract) DOI: 10.1103/PhysRevB.74.155328

Ballistic electron transport properties of Fe-based films on Si(001),
A. J. Stollenwerk, M. R. Krause, D. H. Idell, R. Moore, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 24 2009 (2006).
ABSTRACT: Thickness dependent ballistic electron emission microscopy(BEEM)studies have been performed on Au/Fe81C19/Si(001) and Au/Si(001)Schottky diodes at 80 K. The Schottky height was measured to be0.70 ± 0.02 eV for the Fe81C19/Si(001) interface. Electron attenuationlengths were extracted from the slope of the semi-log BEEM currentversus the thickness of the Fe81C19 layers for electron energiesranging from 1.0 to 1.5 eV. In this range theattenuation length was found to decrease with increasing energy from4.1 ± 0.9 nm to 4.1 ± 0.9 nm, which indicates that someelectron-electron scattering is occurring in the metal overlayer.This decrease is slightly greater than predicted for a free electrongas system, resulting from the complex structure of the Fe81C19
(full text and abstract) DOI: 10.1116/1.2213264

Annealing Temperature Effects on the Structure of Ferromagnetic Mn-implanted Si,
M. Bolduc, C. Awo-Affouda, M. B. Huang, F. Ramos, V. P. LaBella,
Journal of Vacuum Science and Technology A, 24 1648 (2006).
ABSTRACT: The dependence of the magnetization of Mn-implanted Si on thepost-implant annealing temperature is studied. P-type Si wafers wereimplanted with 300-keV Mn+ ions at 350 °C to a fluenceof 1×1016 cm-2, and then annealed at500-900 °C for 5 min. Ferromagnetic hysteresis loops wereobtained at 10 K using a SQUID magnetometer. The saturationmagnetization increases with the post-implant annealing temperature,reaching an optimum field strength of 0.2 emu/g at 800 °C.An out-diffusion of Mn is observed at higher temperatures thatcoincides with a decrease in the saturation magnetization. Thecalculated point-defect profile that was generated by theimplantation process peaks around the Mn-depleted region, suggestingthat the residual implant damage may play a role in theferromagnetic behavior of Mn-implanted Si.
(full text and abstract) DOI: 10.1116/1.2194921

Observation of Crystallite Formation in Ferromagnetic Mn-implanted Si,
C. Awo-Affouda, M. Bolduc, M. B. Huang, F. Ramos, K. A. Dunn, B. Thiel, G. Agnello, V. P. LaBella,
Journal of Vacuum Science and Technology A, 24 1644 (2006).
ABSTRACT: Mn implanted Si was investigated using transmission electronmicroscopy to gain insight into the structure of the implantedregion. Diffraction contrast images, selected area diffractionpatterns, and high resolution images of the samples were acquiredbefore and after post-implant annealing at 800 °C. The imagesof the annealed samples revealed the formation of nanometer sizeprecipitates distributed throughout the implanted region. Analysisof the selected area diffraction pattern determined that the mostprominent lattice spacing of the crystallites is 2.15 Å. Thisspacing indicates that the most probable phase of the crystallitesis MnSi1.7 and this is consistent with the Mn:Si binary phasediagram. This phase is paramagnetic at room temperature with a Curietemperature of 47 K and cannot readily account for the high Curietemperature of the material.
(full text and abstract) DOI: 10.1116/1.2189265

Hot Electron Transport Across Manganese Silicide Layers on the Si(001) Surface,
A. Stollenwerk, M. Krause, R. Moore, V. P. LaBella,
Journal of Vacuum Science and Technology A, 24 1610 (2006).
ABSTRACT: Ballistic electron emission microscopy (BEEM) has been performed onMnSi/Si(001) Schottky diodes at 80 K to study the hot electrontransport properties. The BEEM spectra best fit the thermallybroadening 5/2 power law model with two threshold heights at 0.71 eVand 0.86 eV indicating a complex interface band structure. Inaddition, the normalized BEEM current in the MnSi overlayer wasfound to be approximately 7 × less than is observed inAu/Si(001) samples of similar thickness, indicating a larger amountof hot-electron scattering in the MnSi/Si(001) samples.
(full text and abstract) DOI: 10.1116/1.2206195

Ostwald ripening of manganese silicide islands on Si(001),
M. R. Krause, A. Stollenwerk, M. Licurse, V. P. LaBella,
Journal of Vacuum Science and Technology A, 24 1480 (2006).
ABSTRACT: The deposition of Mn onto Si(001) in the submonolayer regime has been studied with scanning tunneling microscopy (STM) to gain insight into the bonding and energetics of Mn with Si. The as deposited Mn films at room temperature are unstructured. Upon annealing to 300-500°C no smooth films but three dimensional islands of Mn or MnxSiy form while between the islands the Si(001)-(2×1) reconstruction becomes visible. With increasing annealing time the density of islands per surface area decreases while the average height of the remaining islands increases. The large islands grow by the condensation of material diffused across the surcace from small decomposing islands. The time evolution of the coarsening process can be explained in the context of the Ostwald Ripening theory.
(full text and abstract) DOI: 10.1116/1.2167070

Redistribution of Mn upon Annealing in Ferromagnetic Mn-implanted Si,
M. Bolduc, C. Awo-Affouda, M. B. Huang, F. Ramos, V. P. LaBella,
Material Research Society Symposium Proceedings, 908e 3 (2006).
ABSTRACT: The redistribution of implanted Mn ions in Si after thermal annealing is studied. P-type Si wafers were implanted with 300-keV Mn+ ions at 350 °C to a dose of 1×1015 cm-2, and then annealed at 800 °C for 5 min. Ferromagnetic hysteresis loops were obtained at 10 K using a SQUID magnetometer both before and after annealing. The saturation magnetization increases by ~ 2 × after the post-implant annealing, while the Mn redistributes with sharp peaks in concentration. The calculated point-defect profile created during the implantation process peaks around the Mn-depleted region, suggesting that the residual implant damage may play a role in the ferromagnetic behavior of Mn-implanted Si.

Arsenic-Rich GaAs(001) Surface Structure,
Vincent LaBella, Michael Krause, Zhao Ding, Paul M. Thibado,
Surface Science Reports, 60 1 (2005).
ABSTRACT: This article discusses the past 40 years of research covering the equilibrium thermodynamic properties of the arsenic-rich GaAs(0 0 1) surface, which is the starting surface for producing the majority of optoelectronic devices worldwide. A coherent picture of the observed surface structures, theoretical calculations, and experimental results will be presented. The interplay in surface-free-energy-reduction between reconstruction transformation and roughening is now well understood for the GaAs(0 0 1) surface and will be discussed. The recent confirmations of the structural models for the (2×4) and c(4×4) reconstructions as well as the discovery of preroughening aid in this understanding.
(full text and abstract) DOI: 10.1016/j.surfrep.2005.10.001

Investigation of the Structural Properties of Ferromagnetic Mn-implanted Si,
M. Bolduc, C. Awo-Affouda, A. Stollenwerk, M. B. Huang, F. Ramos, V. P. LaBella,
Nuclear Instruments {\&} Methods in Physics Research; Section B (Beam Interactions with Materials and Atoms), 242 367 (2005).
ABSTRACT: The structural properties of Si crystals that were made ferromagnetic through Mn-ion implantation are studied. 300-keV Mn+ ions were implanted at 350 °C at doses of 1-10×1015 cm-2 reaching peak concentrations of 0.1-0.8 at. %. Ferromagnetic hysteresis loops were obtained using a SQUID magnetometer at temperature of 300 K, yielding a saturation magnetization of 0.2 emu/g after annealing at 800 °C for 5 min. The Curie temperature for all samples was found to be greater than 400 K. After annealing, a pronounced redistribution of Mn is observed in the depth profiles as measured through SIMS profiling. The quality of the resulting crystal structure has been investigated by RBS in the channeling mode.
(full text and abstract) DOI: 10.1016/j.nimb.2005.08.132

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: enabling atomic scale characterization of semiconductor surfaces and interfaces,
M. Krause, A. Stollenwerk, C. Awo-Affouda, B. Maclean, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 23 1684 (2005).
ABSTRACT: A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5 °C. In addition, the microscope can operate at temperatures as low as 4 K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.
(full text and abstract) DOI: 10.1116/1.1941167

Above room temperature ferromagnetism in Mn-ion implanted Si,
M. Bolduc, C. Awo-Affouda, A. Stollenwerk, M. B. Huang, F. G. Ramos, G. Agnello, V. P. LaBella,
Physical Review B, 71 033302 (2005).
ABSTRACT: Above room temperature ferromagnetic behavior is achieved in Si through Mn ion implantation. Three-hundred-keV Mn+ ions were implanted to 0.1% and 0.8% peak atomic concentrations, yielding a saturation magnetization of 0.3 emu/g at 300 K for the highest concentration as measured using a SQUID magnetometer. The saturation magnetization increased by  2× after annealing at 800 °C for 5 min. The Curie temperature for all samples was found to be greater than 400 K. A significant difference in the temperature-dependent remnant magnetization between the implanted p-type and n-type Si is observed, giving strong evidence that a Si-based diluted magnetic semiconductor can be achieved.
(full text and abstract) DOI: 10.1103/PhysRevB.71.033302

Magnetic and Structural Properties of Mn-implanted Si,
M. Bolduc, C. Awo-Affouda, A. Stollenwerk, M. B. Huang, F. Ramos, G. Agnello, V. P. LaBella,
Material Research Society Symposium Proceedings, 853E 4 (2005).
ABSTRACT: P-type Si wafers (~1019 cm-3) were implanted with 300 keV Mn+ ions at 350 °C to a dose of 1×1016cm-2, and then annealed at 800 °C for 5 min. The magnetic properties with dependence upon temperature were measured by using a Superconducting Quantum Interference Device (SQUID) magnetometer. The Mn-implanted Si compound shows ferromagnetic ordering above room temperature. The saturation magnetization increases by ~ 2 × after annealing and the Curie temperature is Tc> 400 K. The structural properties have been investigated by means of Secondary Ion Mass Spectroscopy (SIMS) depth profiling and Transmission Electron Microscope (TEM) imaging. Measurements showed that the Mn atoms redistribute in the Si crystal due to the thermal annealing and form a band layer composed of nanoscale structures such as crystallites or defects.

Electron-beam evaporated cobalt films on molecular beam epitaxy prepared GaAs(001),
Z. Ding, P. M. Thibado, C. Awo-Affouda, V. P. LaBella,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 22 2068 (2004).
ABSTRACT: We have deposited Co films on the GaAs(001) surface by using an e-beam evaporation method. The thicknesses of the Co films are measured by using x-ray reflectivity and Rutherford backscattering. The magnetic properties of the films have been measured using superconducting quantum interference device. The magnetization of the films was found to decrease with increasing film thickness. The slight degradation of magnetic properties is attributed to increasing roughness on the Co surface or the Co/GaAs interface during the Co deposition.
(full text and abstract) DOI: 10.1116/1.1771674

Time-evolution of the GaAs(001) pre-roughening process,
Z. Ding, D. W. Bullock, P. M. Thibado, V. P. LaBella, K. Mullen,
Surface Science, 540 491 (2003).
ABSTRACT: The GaAs(001) surface is observed to evolve from being perfectly flat to a surface half covered with one-monolayer high spontaneously formed GaAs islands. The dynamics of this process are monitored with atomic-scale resolution using scanning tunneling microscopy. Surprisingly, pit formation dominates the early stages of island formation. In-sight into the nucleation process is reported.
(full text and abstract) DOI: 10.1016/S0039-6028(03)00916-6

Atomic-Scale Observation of Temperature and Pressure Driven Preroughening and Roughening,
Z. Ding, D. W. Bullock, P. M. Thibado, V. P. LaBella, K. Mullen,
Physical Review Letters, 90 216109 (2003).
ABSTRACT: Preroughening and roughening transitions are observed on the GaAs(001) surface using scanning tunneling microscopy. By tuning the substrate temperature or As4 pressure the surface morphology can be made free of islands, covered with one monolayer high islands or covered with islands on top of islands forming a wedding-caketype structure. These three distinct surface morphologies are classified as ordered flat (OF), disordered flat (DOF), and rough within the restricted solid-on-solid model. Here, the DOF phase is macroscopically flat; however, an up-down-up-down step pattern persists across the entire surface. Using this model we have determined the next-nearest-neighbor interaction energy to be about 0.05 eV.
(full text and abstract) DOI: 10.1103/PhysRevLett.90.216109

Role of aperiodic surface defects on the intensity of electron diffraction spots,
D. W. Bullock, Z. Ding, P. M. Thibado, V. P. LaBella,
Applied Physics Letters, 82 2586 (2003).
ABSTRACT: A random distribution of two-dimensional gallium arsenide (GaAs) islands is found to effect the intensity of the electron diffraction pattern from the GaAs(001) surface. By utilizing the spontaneous island formation phenomenon as well as submonolayer deposition, the island coverage is systematically changed. It is found that the intensities of the one-, two-, and three-quarter-order diffraction spots of the [110] azimuth decrease as the concentration of islands increases. In addition, only in the presence of islands, does the intensity of the half-order spot decrease as the grazing angle of the electron beam is decreased. A simple quantitative model is developed that provides insight into how an aperiodic arrangement of islands effects the electron diffraction patterns.
(full text and abstract) DOI: 10.1063/1.1568161

Dynamics of spontaneous roughening on the GaAs(001)-(2×4) surface,
Z. Ding, D. W. Bullock, W. F. Oliver, P. M. Thibado, V. P. LaBella,
Journal of Crystal Growth, 251 35 (2003).
ABSTRACT: The dynamics of a random distribution of spontaneously formed 2D GaAs islands are studied using scanning tunneling microscopy. The equilibrium concentration of islands is easily tuned from 0% to 50% coverage by only changing the As4 overpressure. Images taken during the early stages of island formation reveal the roughening transition primarily occurs through an intermediate pit formation phase. Interestingly, pit formation in the middle of an otherwise pristine terrace is overwhelmingly preferred to atom detachment from the edges of the terraces.
(full text and abstract) DOI: 10.1016/S0022-0248(02)02272-8

Simultaneous surface topography and spin-injection probability,
D. W. Bullock, V. P. LaBella, Z. Ding, P. M. Thibado,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 21 67 (2003).
ABSTRACT: A spin-polarized electron current is injected into a p-type GaAs(110) surface at 100 K using a polycrystalline ferromagnetic Ni scanning tunneling microscope tip. The injected electrons recombine to the valence band and emit circularly polarized light, and the degree of the light polarization is related to the degree of the electron polarization at the instant of recombination. Details of how to simultaneously measure the surface topography and obtain a pixel-by-pixel map of the spin-injection probability are discussed. The degree of light polarization is found to change when the electrons are injected into a clean, flat terrace versus over a step. However, the terrace-to-step polarization differences are systematically reduced as the energy of the electron is reduced.
(full text and abstract) DOI: 10.1116/1.1532022

Mapping the spin-injection probability on the atomic scale,
D. W. Bullock, V. P. LaBella, Z. Ding, P. M. Thibado,
Journal of Superconductivity, 15 37 (2002).
ABSTRACT: A spin-polarized electron current is injected into the GaAs(110) surface at 100 K by using a polycrystalline ferromagnetic Ni scanning tunneling microscopy (STM) tip. The injected electrons recombine to the valence band and emit circularly polarized radiation whose degree of light polarization is related to the polarization of the conduction-band electrons at the instant of recombination. When the polarized electrons are injected into clean, flat terraces an average polarization for the emitted radiation is found to be 6.79%, while over the 10-nm step region the polarization is reduced to 0.46%. This step scattering effect is studied further by varying the tunneling gap through adjusting the tunneling current. As the distance between the STM tip and sample decrease the spin-scattering effect of the step edge is enhanced.
(full text and abstract) DOI: 10.1023/A:1014023126179

Enhancing the Student-Instructor Interaction Frequency,
D. W. Bullock, V. P. LaBella, T. Clingan, Z. Ding, G. Stewart, P.M. Thibado,
The Physics Teacher, 40 535 (2002).
ABSTRACT: A 100-fold increase in the frequency of studentteacher interaction has been achieved in a large-enrollment classroom. Students answer in-class questions using personalized hand-held transmitters. Outside the classroom, personalized homework sets are generated and collected via the Internet.
(full text and abstract) DOI: 10.1119/1.1534821

Microscopic structure of spontaneously formed islands on the GaAs(001)-(2×4) reconstructed surface,
V. P. LaBella, Z. Ding, D. W. Bullock, C. Emery, P. M. Thibado,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 19 1640 (2001).
ABSTRACT: Islands are found to spontaneously form on the GaAs(001)-(2×4) reconstructed surface. The geometry and size of these islands are examined as a function of substrate temperature and island coverage. Both the islands'''''''' coverage and size increase with increasing temperature. The islands are elongated rectangles and the aspect ratio is independent of temperature. A relationship between the islands'''''''' aspect ratio and the step formation energies is presented. These results are also related to recent theoretical work on equilibrium island geometry.
(full text and abstract) DOI: 10.1116/1.1386376

Spatially resolved spin-injection probability for gallium arsenide,
V. P. LaBella, D. W. Bullock, Z. Ding, C. Emery, A. Venkatesan, W. F. Oliver, G. J. Salamo, P. M. Thibado, M. Mortazavi,
Science, 292 1518 (2001).
ABSTRACT: We report a large spin-polarized current injection from a ferromagnetic metal into a nonferromagnetic semiconductor, at a temperature of 100 K. The modification of the spin-injection process by a nanoscale step edge was observed. On flat gallium arsenide [GaAs(110)] terraces, the injection efficiency was 92%, whereas in a 10-nanometer-wide region around a [111]-oriented step the injection efficiency is reduced by a factor of 6. Alternatively, the spin-relaxation lifetime was reduced by a factor of 12. This reduction is associated with the metallic nature of the step edge. This study advances the realization of using both the charge and spin of the electron in future semiconductor devices.
(full text and abstract) DOI: 10.1126/science.292.5521.1518

Enabling electron diffraction as a tool for determining substrate temperature and surface morphology,
V. P. LaBella, D. W. Bullock, C. Emery, Z. Ding, P. M. Thibado,
Applied Physics Letters, 79 3065 (2001).
ABSTRACT: The reconstruction transitions for the GaAs(001) surface have been identified as a function of the band gap-derived substrate temperature and As4 beam equivalent pressure. Surface morphology measurements using in situ scanning tunneling microscopy reveal that the surface spontaneously forms a random distribution of two-dimensional islands. The onset of island formation is coincident with the reflected high-energy electron diffraction pattern changing from the β to α subphase of the (2×4) reconstruction. An electron diffraction-based method for determining the substrate temperature and engineering the surface morphology with a desired amount of roughness is presented.
(full text and abstract) DOI: 10.1063/1.1416477

A union of the real-space and reciprocal-space view of the GaAs(001) surface,
V. P. LaBella, Z. Ding, D. W. Bullock, C. Emery, P. M. Thibado,
International Journal of Modern Physics B, 15 2301 (2001).
ABSTRACT: A union of the real-space and reciprocal space view of the GaAs(001) surface is presented. An optical transmission temperature measurement system allowed fast and accurate temperature determinations of the GaAs(001) substrate. The atomic features of the GaAs(001)-(2×4) reconstructed surface are resolved with scanning tunneling microscopy and first principles density functional theory. In addition, the 2D lattice-gas Ising model within the grand canonical ensemble can be applied to this surface to understand the thermodynamics. An algorithm for using electron diffraction on the GaAs(001) surface to determine the substrate temperature and tune the nanoscale surface roughness is presented.
(full text and abstract) DOI: 10.1142/S0217979201005647

Microscopic view of a two-dimensional lattice-gas Ising system within the grand canonical ensemble,
V. P. LaBella, D. W. Bullock, M. Anser, Z. Ding, C. Emery, L. Bellaiche, P. M. Thibado,
Physical Review Letters, 84 4152 (2000).
ABSTRACT: A reversible 2D critical transition is observed on the GaAs(001) surface and modeled as a lattice-gas Ising system. Without depositing any material, 2D GaAs islands spontaneously form. The order parameter, four critical exponents, and coupling energies are measured from scanning tunneling microscope images of the microscopic domain structure and correlation functions as a function of temperature and pressure. Unprecedented insight into the domain structure of a 2D Ising system through the critical point and a complete Hamiltonian for modeling the GaAs(001) surface are presented.
(full text and abstract) DOI: 10.1103/PhysRevLett.84.4152

A Novel STM Imaging Mechanism is Used to Determine the Atomic Structure of the GaAs(001)-(2×4) Surface,
V. P. LaBella, D. W. Bullock, P. M. Thibado, P. Kratzer, M. Scheffler,
Omicron Newsletter, 4 4 (2000).

Monte Carlo derived diffusion parameters for Ga on the GaAs(001)- (2×4) surface: A molecular beam epitaxy-scanning tunneling microscopy study,
V. P. LaBella, D. W. Bullock, Z. Ding, C. Emery, W. G. Harter, P. M. Thibado,
Journal of Vacuum Science and Technology A, 18 1526 (2000).
ABSTRACT: The migration of individual Ga atoms on the technologically important GaAs(001)-(2×4) reconstructed surface has been studied as a function of substrate temperature and As4 pressure using a combined molecular beam epitaxy and scanning tunneling microscope ultrahigh vacuum multichamber facility. We have deposited 10% of a plane of Ga onto a GaAs(001) surface with a low defect density (<1% ) and with large terraces (>0.5 μ m) to avoid the influence of surface defects like step edges and vacancies. Both the island number density and the geometry are measured and compared to Monte Carlo solid-on-solid simulations. Basic diffusion parameters, such as the activation energy, directional hopping-rate ratio, directional sticking-probability ratio, etc., are reported.
(full text and abstract) DOI: 10.1116/1.582379

Reflection high-energy electron diffraction and scanning tunneling microscopy study of InP(001) surface reconstructions,
V. P. LaBella, Z. Ding, D. W. Bullock, C. Emery, P. M. Thibado,
Journal of Vacuum Science and Technology A, 18 1492 (2000).
ABSTRACT: The reconstructions of the InP(001) surface prepared by molecular beam epitaxy have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The growth chamber contains a highly accurate temperature measurement system and uses a solid-source, cracked phosphorus, valved effusion cell. Five InP(001) reconstructions are observed with RHEED by analyzing patterns in three principal directions. Under a fixed P2 flux, decreasing the substrate temperature gives the following reconstructions: c(2×8), (2×4), (2×1), (2×2), and c(4×4). In situ STM images reveal that only two of these reconstructions yields long-range periodicity in real space. InP(001) does not form the metal rich (4×2) reconstruction, which is surprising because the (4×2) reconstruction has been coined the universal surface reconstruction since all III-V(001) surfaces were thought to favor its formation.
(full text and abstract) DOI: 10.1116/1.582373

Atomic structure of the GaAs(001)-(2×4) surface resolved using scanning tunneling microscopy and first-principles theory,
V. P. LaBella, H. Yang, D. W. Bullock, P. M. Thibado, P. Kratzer, M. Scheffler,
Physical Review Letters, 83 2989 (1999).
ABSTRACT: The atomic arrangement of the technologically important As-rich GaAs(001)-(2×4) reconstructed surface is determined using bias-dependent scanning tunneling microscopy (STM) and first-principles electronic structure calculations. The STM images reveal the relative position and depth of the atomic scale features within the trenches between the top-layer As dimers, which are in agreement with the β 2(2×4) structural model. The bias-dependent simulated STM images reveal that a retraction of the topmost dangling bond orbitals is the novel electronic mechanism that enables the STM tip to image the trench structure.
(full text and abstract) DOI: 10.1103/PhysRevLett.83.2989

Role of As4 in Ga diffusion on the GaAs(001)-(2×4) surface: A molecular beam epitaxy-scanning tunneling microscopy study,
H. Yang, V. P. LaBella, D. W. Bullock, P. M. Thibado,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 17 1778 (1999).
ABSTRACT: The role of As4 molecules in Ga diffusion on the GaAs(001)-(2×4) reconstructed surface has been studied using a combined molecular beam epitaxy and scanning tunneling microscopy multichamber facility. We deposited 10% of a plane of Ga atoms onto an otherwise pristine surface, while exposed to two separate As4 beam equivalent pressures of 10-5 and 10-6 Torr. The higher As4 flux resulted in the production of fewer and larger islands, indicating that increasing the As4 flux increases the total interrogation area available to the Ga atoms before forming islands.
(full text and abstract) DOI: 10.1116/1.590825

Activation energy for Ga diffusion on the GaAs(001)-(2×4) surface: an MBE-STM study,
H. Yang, V. P. LaBella, D. W. Bullock, Z. Ding, J. B. Smathers, P. M. Thibado,
Journal of Crystal Growth, 201-202 88 (1999).
ABSTRACT: The pure migration of individual Ga atoms on the technologically important GaAs(001)-(2×4) reconstructed surface has been studied as a function of substrate temperature using a combined molecular beam epitaxy and scanning tunneling microscopy (STM) ultra-high vacuum, multi-chamber facility. We have successfully deposited 1/10 of a plane of Ga atoms onto a pristine GaAs surface under a constant As4 beam equivalent pressure of 10-6 Torr, at various substrate temperatures. After deposition the substrate was quenched to room temperature and transferred to the surface analysis chamber for STM imaging. A plot of the number density of islands formed as a function of deposition temperature follows an Arrhenius relationship. Assuming either a pure one-dimensional diffusion model or a pure isotropic two-dimensional diffusion model, the activation energy for diffusion is 2.3 or 1.7 eV, respectively.
(full text and abstract) DOI: 10.1016/S0022-0248(98)01296-2

Measurements of epitaxially grown Pt/CaF2/Si(111) structures by ballistic electron emission microscopy and scanning tunneling microscopy,
V. P. LaBella, Y. Shusterman, L. J. Schowalter, Jr., C. A. Ventrice, Jr.,
Journal of Vacuum Science and Technology A, 16 1692 (1998).
ABSTRACT: The hot electron transport properties and growth morphology of ultrathin Pt/CaF2/Si(111) metal-insulator-semiconductor structures have been characterized in situ by ballistic electron emission microscopy (BEEM) and scanning tunneling microscopy (STM). Platinum thicknesses from submonolayer to 50 Å and CaF2 thicknesses from 2 to 10 ML have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules, which nucleate at step edges and defect sites. Some BEEM spectra show an anomalous peak near 2 eV, which has not been observed in previous studies of other metal/CaF2/Si(111) structures. The localized nature of this peak indicates that it results from an interaction between the ballistic electrons and fluorine vacancies at the Pt-CaF2 interface.
(full text and abstract) DOI: 10.1116/1.581286

Ballistic electron emission microscopy measurements of epitaxially grown Pt/CaF2/Si(111) structures,
V. P. LaBella, Jr., C. A. Ventrice, Jr., L. J. Schowalter,
Applied Surface Science, 123-124 213 (1998).
ABSTRACT: The electronic transport properties and surface morphology of ultrathin Pt/CaF2/Si(111) metal-insulator-semiconductor structures have been characterized in-situ by ballistic electron emission microscopy (BEEM) and scanning tunneling microscopy (STM). Platinum thicknesses from 2-53 Å and CaF2 thicknesses of 5 and 15 Å have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules. The BEEM spectra from the thicker Pt coverages show an onset near the conduction band minimum (CBM) of the CaF2. The BEEM spectra from the thinner Pt coverages show an additional peak near 2 eV, which has not been observed in previous studies of metal/CaF2/Si(111) structures. This extra peak has been attributed to quantum mechanical resonant tunneling through the ultrathin MIS structure.
(full text and abstract) DOI: 10.1016/S0169-4332(97)00543-6

Scanning tunneling microscope and ballistic electron emission spectroscopy studies of molecular beam epitaxially grown Pt/CaF2/Si(111) structures,
V. P. LaBella, L. J. Schowalter, Jr., C. A. Ventrice, Jr.,
Journal of Vacuum Science and Technology B (Microelectronics and Nanometer Structures), 15 1191 (1997).
ABSTRACT: The electronic and morphological properties of ultrathin Pt/CaF2/Si(111) metal insulator semiconductor structures have been characterized in situ by scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM). Platinum thickness from 2 Å to 20 Å grown on 5 Å epitaxial CaF2 have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules. These nodules have been observed to coalesce into bigger grains when left at room temperature under ultrahigh vacuum conditions for over 24 h. For ultrathin Pt coverages (2 AA) the STM images at different tip biases reveal different topography, depending on the value of the tip bias relative to the conduction band minimum (CBM) of the CaF2 intralayer (3.3 eV). The STM images at biases well above the CBM of the CaF2, show similar features to bare CaF2/Si(111), while images at the CBM show features of the deposited Pt. BEEM spectra of the 10 Å sample show a peak at  4.5 eV due to the density of states of the CaF2, intralayer and an additional peak at 2 eV, which has not been observed in previous studies of metal/CaF2/Si(111) structures.
(full text and abstract) DOI: 10.1116/1.589437

Design of a scanning tunneling microscope for in situ topographic and spectroscopic measurements within a commercial molecular beam epitaxy machine,
Jr., C. A. Ventrice, Jr., V. P. LaBella, L. J. Schowalter,
Journal of Vacuum Science and Technology A, 15 830 (1997).
ABSTRACT: A scanning tunneling microscope that performs scanning tunneling microscopy, scanning tunneling spectroscopy, and ballistic electron emission microscopy measurements on 2-in. wafers has been designed and constructed. The instrument is incorporated into the Si preparation chamber of a cryo-pumped Fisons V90H Si/III-V molecular beam epitaxy machine. Its design uses two commercial Burleigh inchworms: one for performing scanning tunneling microscopy measurements and a second for making a front contact which is necessary for the ballistic electron emission microscopy measurements. The substrate holder for the V90H system is designed to handle wafers up to 6 in. in diameter. Therefore, a custom 6-in.-diam. holder has been constructed which supports two 2-in. holders: one for performing reflection high energy electron diffraction measurements and a second which allows transfer of the wafer to the scanning tunneling microscope and also incorporates a removable shadow mask for growing metal/semiconductor Schottky diodes. Although the chamber vibrations generated by the cryopump and its compressor are quite severe, atomic-resolution images have been obtained with all of the system''''s pumps in operation. The enhanced stability of our design is attributed to our unique support system of the inchworm and wafer which are both manufactured from machinable ceramic, the use of Be-Cu springs with an extension length of 16 in. for vibration isolation, and the use of silicone O-rings for vibration damping.
(full text and abstract) DOI: 10.1116/1.580716

The Effect of Strain Relaxation Mechanisms on the Electrical Properties of Epitaxial CaF2/Si(111) Heterostructures,
L.J. Schowalter, B.M. Kim, T.G. Thundat, Jr., Carl A. Ventrice, Jr., V.P. LaBella,
Material Research Society Symposium Proceedings, 466 21 (1997).

Hot-electron scattering at Au/Si(100) Schottky interfaces measured by temperature dependent ballistic electron emission microscopy,
Jr., C. A. Ventrice, Jr., V. P. LaBella, G. Ramaswamy, H. -P. Yu, L. J. Schowalter,
Applied Surface Science, 104-105 274 (1996).
ABSTRACT: Ballistic electron emission microscopy (BEEM) measurements have been performed on n-type Au/Si(100) interfaces as a function of STM tip bias, Au film thickness, and temperature. From these measurements, the attenuation length, λ a, of the BEEM electrons in the metal overlayer has been determined to be 133±2 W at room temperature (RT) and 147±6 A at 77 K for tip biases from -1.20 V to -0.92 V. The ratio of the zero thickness BEEM transmittances at 77 K to that at RT, I0(77 K)/I0(RT), was determined to be 1.79±0.09. Within the experimental uncertainties of these measurements, no energy dependence of h, or I0(77 K)/I0(RT) was observed. The large increase in the BEEM transmittance and the relatively small increase in λ a, at 77 K indicate that the primary temperature dependent scattering mode affecting BEEM electron transport is phonon absorption in the Si substrate. Images with large reductions in the BEEM current at topographic locations which have a large surface gradient have been obtained at RT. Our calculations, which assume that the probability of transmission across the interface is independent of the electron''''s transverse momentum, correlate well with the experimentally observed reductions. This result indicates that the BEEM electrons remain forward focused with very little broadening as they pass through the Au overlayer, and also implies that strong scattering must occur at the Au/Si interface to explain the previously documented non-conservation of transverse momentum at Au/Si interfaces.
(full text and abstract) DOI: 10.1016/S0169-4332(96)00215-2

Measurement of hot-electron scattering processes at Au/Si(100) Schottky interfaces by temperature-dependent ballistic-electron-emission microscopy,
Jr., C. A. Ventrice, Jr., V. P. LaBella, G. Ramaswamy, H. -P. Yu, L. J. Schowalter,
Physical Review B, 53 3952 (1996).
ABSTRACT: Ballistic-electron-emission microscopy measurements have been performed on n-type Au/Si(100) interfaces for injection energies up to 1.2 eV over a range of Au overlayer thicknesses from  65 to  340 Å at both room temperature and 77 K. Hot-electron attenuation lengths in the Au overlayer have been determined to be 133±2 Å at room temperature and 147±6 Å at 77 K over the energy range of 0.92-1.20 eV above the Fermi level. The lack of energy dependence and the relatively small temperature-dependent change in the attenuation lengths that have been measured indicate that electron scattering with defects is the dominant mechanism affecting hot-electron transport in these Au overlayers. The ratio of the zero-thickness collection current at 77 K to that at room temperature has been measured to be 1.79±0.09. This large increase in the collection efficiency at 77 K is attributed primarily to the large temperature dependence: of the transverse acoustic-phonon population in Si. Images with significant reductions in the collection current at topographic locations that have a large surface gradient have been obtained at room temperature. Calculations, which assume that the probability of transmission across the interface is independent of the transverse momentum of the electron, correlate well with the experimentally observed reductions. This result indicates that the injected electrons remain forward focused with little broadening as they pass through the Au overlayer, which implies that elastic scattering at the Au/Si interface accounts for the observation from previous Au/Si ballistic-electron-emission microscopy studies that transverse momentum is not conserved.
(full text and abstract) DOI: 10.1103/PhysRevB.53.3952

Anomalous attenuation of spin-entropy waves in superfluid 3He-A1,
M. Bastea, Y. Okuda, V. LaBella, H. Kojima,
Physical Review Letters, 73 1126 (1994).
ABSTRACT: The propagation of spin-entropy waves was observed in superfluid 3He-A1 at a pressure of 22.7 bars and magnetic fields up to 5 T. A large increase in attenuation was observed within the A1 phase near its lower transition temperature to the A2 phase over an unexpectedly large temperature width. The observed effects are suggested as evidence for the presence of a minority (down-moment pair) condensate population predicted by Monien and Tewordt (1985)
(full text and abstract) DOI: 10.1103/PhysRevLett.73.1126