Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A8: Semiconductor Surfaces and Interfaces |
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Sponsoring Units: DCMP Chair: Francesca Cavallo, University of New Mexico Room: 006C |
Monday, March 2, 2015 8:00AM - 8:12AM |
A8.00001: Unusual Dramatic Surface Restructuring of Silicon Substrate during Epitaxy Tanya Gupta, Daniel Steingart, James Hannon Interfacial strain is unavoidable in heteroepitaxial growth and can have a profound impact on the morphology and properties of thin films. In fact, ``engineering'' thin-film strain is a critical component in many advanced technologies. For example, straining the silicon in advanced CMOS devices can increase the device speed by as much as 90 percent.order to control interfacial strain, its effects on growth must be understood. The common picture is that the growth substrate is essentially passive: its role is to provide the lattice mismatch that the growing film must respond to. As the film grows thicker, the stress in the film evolves, which can lead to morphological changes in the film, e.g. dislocations, or a change in growth mode from 2D, planar growth to 3D, quantum dot growth., in both of these examples, the action is in the growing film. In this work we describe a growth system that behaves in a completely unexpected manner that does not fit into this conventional picture. Interfacial strain that accompanies the growth of SiC nanoparticles is relieved by a dramatic restructuring of the *substrate* rather than the nanoparticles. The growth of nanoparticles induces a massive change in the substrate. In situ measurements of the Si mound formation was done with the use of LEEM. Using a simple, illustrative model with parameters taken from the literature, we show that the shapes and heights of the mounds are consistent with a strain-driven formation mechanism. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A8.00002: A joint first principles and Kelvin probe force microscopy study of stepped silicon surfaces with unprecedented resolution Stefan Wippermann, Carmen P\'erez Le\'on, Holger Drees, Michael Marz, Regina Hoffmann-Vogel Stepped well-ordered surfaces are important nanotemplates for the fabrication of one-dimensional nanostructures with intriguing electronic properties. The vicinal Si(7710) surface is an important model system of this kind. It contains (7x7) reconstructed areas equivalent to the well characterized and understood Si(111)-(7x7) surface. Thereby this system essentially contains its own calibration, providing an ideal testbed for surface characterization techniques and understanding in depth the rich morphology of the structural features present in this system. Here we present a joint experimental and theoretical investigation of the structural properties of the vicinal Si(7710) surface. We carried out Kelvin probe force microscopy (KPFM) measurements with unprecedented atomic resolution, and \emph{first principles} calculations of the local work function as a function of the lateral position of the tip above the surface. These calculations allowed us to interpret the experimental KPFM data in terms of specific structural features and electronic properties of surface states, such as e. g. defects, dangling bond angles and occupations of dangling bonds. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A8.00003: A novel ground state of the (2$\surd $3x2$\surd $3)R30$^{\circ}$ Sn double layer on Si(111) induced by modulation hole-doping Fangfei Ming, Daniel Mulugeta, Paolo Vilmercati, Hanno H. Weitering, Paul C. Snijders Charge doping provides a tuning knob of the delicate interactions between spin, orbital, charge and lattice degrees of freedom in low-dimensional systems, which dictate many intriguing quantum phenomena. Using scanning tunneling microscopy/spectroscopy, we characterize the (2$\surd $3x2$\surd $3)R30$^{\circ}$ Sn double layer grown on a ($\surd $3x$\surd $3)-B reconstructed Si(111) surface, where 1/3 monolayer B dopants resides in the subsurface layer without forming direct chemical bonds with the Sn layer. The B atoms donate holes to the Sn double layer and shift the Fermi level toward the valence band edge. Surprisingly, it further induces a fraction of the 2$\surd $3x2$\surd $3 phase to gradually transform to a new 4$\surd $3x2$\surd $3 phase below 80 K. The two phases coexist down to 2.5 K, indicating a phase-separated ground state for the hole doped Sn double layer, in contrast to a homogenous 2$\surd $3x2$\surd $3 phase for the undoped one. The new 4$\surd $3x2$\surd $3 phase has a larger band gap than the 2$\surd $3x2$\surd $3 phase and the valence band edge shifts a few tens meV away from the Fermi level to higher binding energy, suggesting that the transition to the 4$\surd $3x2$\surd $3 structure is accompanied by an electronic structure rearrangement. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A8.00004: Structure of the modulation hole doped (2$\surd $3x2$\surd $3)R30$^{\circ}$ Sn double layer on Si(111) Paul C. Snijders, Ying-Tzu Huang, Fangfei Ming, Daniel Mulugeta, Weisong Tu, Paul R.C. Kent, Renee D. Diehl, Hanno H. Weitering Over the years, a large collection of temperature dependent phase transitions has been identified in atomically thin metal overlayers on semiconductor surfaces. However, studying their doping dependence remains difficult due to the symmetry breaking nature of dopant atoms located on top of these structures, and the associated local lattice deformations localizing the doped carriers. Here we have used a subsurface modulation doping approach using B on Si(111) to hole-dope an overlaying Sn double layer. STM, LEED-I(E), and XPS were used to characterize the structure of a (2$\surd $3x2$\surd $3)R30 Sn double layer on a Si(111) ($\surd $3x$\surd $3)-B surface. STM images 4 atoms in the top Sn layer. Using the tip to remove this top layer, a second layer consisting of 3 triangular structures becomes visible, resulting in a total of 13 Sn atoms per unit cell, consistent with an XPS core level intensity analysis. An extensive LEED-I(E) analysis using 20 independent beams with an energy range of 7126 eV points to a structural model consistent with the STM and XPS data and having a Pendry R-factor of 0.34. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A8.00005: Ba termination of Ge(001) studied with STM Neil Curson, Wojciech Koczorowski, Tomasz Grzela, Marian Radny, Steven Schofield, Giovanni Capellini, Ryszard Czajka, Thomas Schroeder We use controlled annealing to tune the interfacial properties of a sub-monolayer and monolayer coverages of Ba atoms deposited on Ge(001), enabling the generation of either of two fundamentally distinct interfacial phases, as revealed by scanning tunneling microscopy (STM). Firstly we identify the two key structural phases associated with this adsorption system, namely on-top adsorption and surface alloy formation, by performing a deposition and annealing experiment at a coverage low enough (0.15 ML) such that isolated Ba-related features can be individually resolved. Subsequently we investigate the monolayer coverage case, of interest for passivation schemes of future Ge based devices, for which we find that thermal evaporation of Ba onto a Ge(001) surface at room temperature results in on-top adsorption. This separation (lack of intermixing) between Ba and Ge layers is retained through successive annealing steps up to 770 K with a gradual ordering of the Ba layer at 570 K and above and a decrease in Ba layer density. Annealing above 770 K produces the 2-D surface alloy phase accompanied by strain relief through monolayer height trench formation. At 1070 K the surface morphology changes again but remains a 2-D surface alloy. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A8.00006: The effect of Al content on the work function engineering at TiAlN/HfO$_{2}$ interface Geun-Myeong Kim, Young Jun Oh, Kee Joo Chang In high-k/metal gate stacks of metal-oxide-semiconductor field-effect transistors, it is important to control the metal work function such that it should be close to the valence and conduction band edges of Si in p- and n-channel devices. It was reported that depositing TiAl on top of TiN/HfO$_{2}$ stack in gate-last process can effectively induce the n-type shift of work function, while the work function is of p-type at TiN/HfO$_{2}$ stack. In this work, we perform first-principles density functional calculations to investigate the Schottky barrier height at TiAlN/HfO$_{2}$ interface. In bulk TiN, it is found that a substitutional Al is the most stable form of Al impurity. When substitutional Al atoms are introduced at TiN/HfO$_{2}$ interface, the effective work function tends to decrease. At TiAlN/HfO$_{2}$ interface, the n-type shift of the work function increases almost linearly with the Al content. This is attributed to the change of interface bonds by Al incorporation and the dipole field induced at the interface. On the other hand, relative thicknesses of TiAl and TiN at abrupt TiAl/TiN/HfO$_{2}$ interface do not significantly affect the effective work function. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A8.00007: Gaussian distribution of inhomogeneous barrier height in Al/p-GaAs Schottky Barrier Diodes (SBDs) Sahar Alialy, Semsettin Altindal The forward bias current-voltage (I-V) characteristics of Al/p-GaAs SBDs have been investigated in the temperature range of 240-360 K. The main electrical parameters such as zero-bias barrier height ($\Phi_{Bo})$, ideality factor ($n)$ and series resistance ($R_{s})$ determined from the forward bias I-V data. These values are strong function of temperature and voltage. The analysis of I-V data based on the thermionic emission (TE) mechanism show that while the $n$ decreases, the $\Phi_{Bo}$ and $R_{s}$ increases with increasing temperature. $\Phi_{\mathrm{Bo}}$ and n versus q/2kT plots were drown to obtain an evidence of GD of BH. The mean value of BH and standard deviation ($\sigma_{\mathrm{o}})$ values were found from the intercept and slope of $\Phi_{\mathrm{Bo}}$ vs q/2kT plot, respectively. Furthermore, the mean value of BH and the effective Richardson constant A* were obtained from the intercept and slope of the modified ln(I$_{\mathrm{o}}$/T$^{2})$-q$^{2}\sigma _{\mathrm{0}}^{2}$/2(k${\rm T})^{2}$ versus q/kT plot. The obtained value of A* is closed to theoretical value of p-GaAs. As a result, the I-V characteristics in Al/p-GaAs successfully have been explained based on TE theory with GD of BHs. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A8.00008: Reversible hydrogenation of silicene Jinglan Qiu, Huixia Fu, Lan Chen, Kehui Wu The hydrogenation of monoatomic silicene sheet on Ag(111) was studied by scanning tunneling microscopy and density functional theory calculations. It was observed that hydrogenation of silicene-4x4 structure at room temperature results in a perfectly ordered $\gamma $-4x4 superstructure. A theoretical model, which involves 7 H atoms and rearranged buckling of Si atoms, was proposed and agrees with experiments very well. Unlike silicene-4x4, the hydrogenation of ($\surd $7x$\surd $7)silicene/(2$\surd $3x2$\surd $3)R30$^{\circ}$Ag(111) superstructure at room temperature shows silicene-1x1 structure with $\surd $7x$\surd $7 reconstruction Moreover, by annealing to a moderate temperature, about 450 K, de-hydrogenation process occurs and the clean silicene surface can be fully recovered. Such reversible hydrogenation suggests that silicene may be a potential candidate as hydrogen storage materials. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A8.00009: The STM investigations of hydrogenation on monolayer silicene with r7xr7 superstructure Lan Chen The early investigations revealed the monolayer silicene with r7xr7 superstructure on Ag(111) can extend over the whole substrate. But this phase was considered to be highly defective due to the strong strain, which limit the applications of this phase. In this presentation, I will report our works about the hydrogenations on the monolayer silicene with r7xr7 superstructure on Ag(111) by scanning tunneling microscopy (STM). The STM images show the ordered structures with lattice identical to silicene-1x1 unit cell after hydrogenation on silicene at room temperature, which reveals that the original silicene with r7xr7 superstructure is an ideal defectless single-crystal monolayer film. Combined with density functional theory calculations, the structures of hydrogenation can be explained by the ``sub-lattice adsorption-picture,'' in which H atoms only adsorb on Si atoms in one sub-lattice of silicone. Moreover, by annealing to a moderate temperature, about 450 K, de-hydrogenation process occurs and the clean silicene with r7xr7 superstructure can be fully recovered. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A8.00010: The Role of a Buried Interface in the Growth of Metallic Nanocrystals: Quantum Size Effects in Ag/Si(111)7x7 Yiyao Chen, Michael Gramlich, Shawn Hayden, Paul Miceli It is shown that the buried interface between a metallic nanocrystal and its supporting substrate plays an essential role in understanding the stability of nanomaterials that grow on a wetting layer in the Stranski-Krastanov growth mode. These in situ x-ray scattering studies reveal a minimum tri-layer height for stable incommensurate FCC Ag islands that are in coexistence with a commensurate Ag wetting layer. The minimum height without an oscillating height preference is explained by electron quantum confinement effects, which is manifested differently for Ag(111) than for other metals, such as Pb(111), that can exhibit oscillations of height preference with thickness. The results suggest that quantum size-effects are broadly important for the growth of metallic nanocrystals. Support is gratefully acknowledged from NSF DMR-0706278 and DGE-1069091. The Advanced Photon Source at Argonne National Laboratory is supported by the US-DOE W-31-109-Eng-38. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A8.00011: A multi-scale approach to the electronic structure of doped semiconductor surfaces Ofer Sinai, Oliver T. Hofmann, Patrick Rinke, Matthias Scheffler, Georg Heimel, Leeor Kronik The inclusion of the global effects of semiconductor doping poses a unique challenge for first-principles simulations, because the typically low concentration of dopants renders an explicit treatment intractable. Furthermore, the width of the space-charge region (SCR) at charged surfaces often exceeds realistic supercell dimensions. We present a multi-scale technique that addresses these difficulties. It is based on the introduction of excess charge, mimicking free charge carriers from the SCR, along with a fixed sheet of counter-charge mimicking the SCR-related field. Self-consistency is obtained by imposing charge conservation and Fermi level equilibration between the bulk, treated semi-classically, and the electronic states of the slab/surface, which are treated quantum-mechanically. The method, called CREST - the Charge-Reservoir Electrostatic Sheet Technique - can be used with standard electronic structure codes. We validate CREST using a simple tight-binding model, which allows for comparison of its results with calculations encompassing the full SCR explicitly. We then employ it with density functional theory, obtaining insight into the doping dependence of the electronic structures of the metallic clean-cleaved Si(111) surface and its semiconducting (2x1) reconstructions. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A8.00012: First principles studies of the stability and Shottky barriers of metal/CdTe(111) interfaces Odkhuu Dorj, M.S. Miao, N. Kioussis, S. Tari, F. Aqariden, Y. Chang, C. Grein CdZnTe and CdTe based semiconductor X-Ray and Gamma-Ray detectors have been intensively studied recently due to their promising potentials for achieving high-resolution, high signal-to-noise ratios and low leakage current, all are desirable features in applications ranging from medical diagnostics to homeland security. Understanding the atomic and electronic structures of the metal/semiconductor interfaces is essential for the further improvements of performance. Using density functional calculations, we systematically studied the stability, the atomic and electronic structures of the interfaces between Cd-terminated CdTe (111) surface and the selected metals. We also calculated the Schottky barrier height (SBH) by aligning the electrostatic potentials in semiconductor and metal regions. Our calculations revealed the importance of intermixing between semiconductor and metal layers and the formation of Te-metal alloys at the interface. The obtained SBH does not depend much on the choice of metals despite the large variation of the work functions. On the other hand, the interface structure is found to have large effect to the SBH, which is attributed to the metal induced states in the gap. The position of such states is insensitive to the metal work functions, as revealed by the analysis of the electronic structures. [Preview Abstract] |
(Author Not Attending)
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A8.00013: Band offsets at the crystalline / hydrogenated amorphous silicon interface from first-principles Ebrahim Hazrati, Karol Jarolimek, Gilles A. de Wijs The heterojunction formed between crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H) is a key component of a new type of high-efficiency silicon solar cell. Since a-Si:H has a larger band gap than c-Si, band offsets are formed at the interface. A band offset at the minority carrier band will mitigate recombination and lead to an increased efficiency. Experimental values of band offsets scatter in a broad range. However, a recent meta-analysis of the results (W. van Sark et al.pp. 405, Springer 2012) gives a larger valence offset (0.40 eV) than the conduction offset (0.15 eV). In light of the conflicting reports our goal is to calculate the band offsets at the c-Si/a-Si:H interface from first-principles. We have prepared several atomistic models of the interface. The crystalline part is terminated with (111) surfaces on both sides. The amorphous structure is generated by simulating an annealing process at 1100 K, with DFT molecular dynamics. Once the atomistic is ready it can be used to calculate the electronic structure of the interface. Our preliminary results show that the valence offset is larger than the conduction band offset. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A8.00014: Interface Formation for a Ferromagnetic/Antiferromagnetic Bi-layer System Studied by Scanning Tunneling Microscopy and First Principles Theory Andrada-Oana Mandru, Jonathan Guerrero-Sanchez, Jeongihm Pak, Noboru Takeuchi, Arthur Smith We investigate the initial stages of interface formation for Fe/Mn$_{3}$N$_{2}$(001) ferromagnet/antiferromagnet bi-layer system down to the atomic scale using a combination of molecular beam epitaxy, scanning tunneling microscopy and first principles theoretical calculations. Sub-monolayer iron depositions onto manganese nitride nanopyramid surfaces induce the formation of one monolayer high islands that are present on all terraces. Unexpectedly, the chemical composition of the observed islands does not consist of Fe, as determined using Auger electron spectroscopy, conductance map imaging and theoretical models. Further theoretical calculations reveal how Fe atoms incorporate into specific subsurface layers. In addition, theory finds the magnetic alignment of the Fe atoms within a particular layer and with adjacent Mn$_{3}$N$_{2}$(001) layers. This study suggest that these complex structural arrangements at the interfaces between such magnetic bi-layer systems are important to take into account when considering the exchange coupling between the layers. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A8.00015: ABSTRACT WITHDRAWN |
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