Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W32: Structure and Morphology: Semiconductor Surfaces |
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Sponsoring Units: DMP DCMP Chair: Steve Erwin, Naval Research Laboratory Room: E142 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W32.00001: Step-mediated island growth: a new ripening mechanism A.L. Chin, H.C. Kan, C.R. Lee, F.K. Men Two types of islands have grown on a Co-deposited Si(111)-(5$\times $2)/Au vicinal surface: islands grow on terraces (terrace islands) and on top of a step (step islands). The terrace islands follow the classical Ostwald ripening process. For step islands, their density remains unchanged as the average size grows to a limit of $\sim $ 650 \textit{nm}$^{3}$. Furthermore, an ordering of step islands gives a ratio of $\sim $ 0.25 between the average island width and island-island separation. We attribute this unique growth to the vertical lattice mismatch created by the height difference between two halves of an island separated by a step. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W32.00002: Gold on silicon: Structure and energetics of Si(111)-(5x2)-Au Steven Erwin, Ingo Barke, F. J. Himpsel Gold forms linear chain-like reconstructions on several vicinal surfaces of silicon, as well as on Si(111). We propose a new structural model for Si(111)-(5x2)-Au that incorporates three Au chains per surface unit cell. Five main theoretical results, obtained from first-principles total-energy calculations, support the model. (1) In the presence of silicon adatoms the periodicity of the gold rows spontaneously doubles, in agreement with experiment. (2) The dependence of the surface energy on the adatom coverage indicates that a uniformly covered phase is unstable and will phase-separate into empty and covered regions, as observed experimentally. (3) Theoretical scanning tunneling microscopy images are in excellent agreement with experiment. (4) The calculated band structure is consistent with angle-resolved photoemission data. (5) The calculated activation barrier for diffusion of silicon adatoms along the row direction is in excellent agreement with the experimentally measured barrier. These results for the flat Si(111)-(5x2)-Au surface also shed light on the widely studied Au chain systems that form on the vicinal surfaces Si(557) and Si(553), as will be discussed. [1] S.C. Erwin, I. Barke, and F.J. Himpsel, Phys. Rev. B 80, 155409 (2009). [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W32.00003: Si adatoms as catalyst for the growth of monolayer Al film on Si(111) Jing Teng, Lixin Zhang, Kehui Wu, Ying Jiang, Jiandong Guo, Qinlin Guo, Philipp Ebert, Toshio Sakurai, Enge Wang Recently, we reported the growth of atomically smooth Al(111) films on Si(111) with continuously controllable thickness down to the extreme level of 1 ML. Here, we study the underlying unexpected Si adatom-mediated clustering-melting mechanism by scanning tunneling microscopy and by the first-principles calculations. The Si adatoms in the initial Si(111)$\surd $3$\times \surd $3-Al surface act as seeds to form $SiAl_2 $ clusters. The clusters are then transformed into Al(111)1$\times $1 by incorporating further incoming Al atoms and spontaneously releasing the Si atoms, which then participate in the next cycle of the process. As a result, a two-dimensional growth of monolayer Al(111) is achieved. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W32.00004: On the feasibility of magnetic doping with a surface driven route: Manganese on group IV semiconcuctor surfaces and quantum dots Petra Reinke, Christopher Nolph, Kiril Simov The magnetic doping of group IV semiconductors and quantum dots is a critical to combine charge and spin driven devices. The incorporation of the magnetic element Mn is hampered by low solubility and competition with compound formation. The primary step in our surface driven approach to Mn-incorporation in a semiconductor matrix is the adsorption of Mn on the Si(100), Ge(100) and Ge(105), the quantum dot facet. The presentation will discuss the formation and bonding of Mn-wires to the Si(100) surface, and the subsequent thermally driven conversion to sub-surface bonding with an n-type characteristic. The adsorption of Mn on Ge-quantum dots leads to a roughening of the wetting layer, and the formation of Mn-clusters which are defined by the surface reconstruction of the QD facet. The consequences of cluster agglomeration, ripening and dissolution in the QDs as a function of temperature for the feasibility of magnetic doping will be discussed. The surface structures, Mn-wires and clusters can be stabilized by the deposition of a Si or Ge capping layer which in turn modifies the magnetism in these nanostructures. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W32.00005: One dimensionally modulated Ag films grown on Si(111) decorated by atomic chains Aaron Gray, Tai Chiang, Hawoong Hong, Manami Ogawa, Iwao Matsuda Recent work on metal films grown on semiconductor substrates has demonstrated that interfacial engineering in these systems can have profound effects on their properties. The structure of Ag films, grown on an array of atomic In chains on Si, has been shown to be modulated by the underlying chains, in the direction perpendicular to them. These modulations are found to persist in films with a thickness as great as 30 monolayers. It was proposed that a periodic insertion of stacking faults in the Ag film lattice allows the periodicity of its lattice structure to match that of these modulations [1]. We have conducted surface x-ray diffraction experiments to further elucidate the structure of this system. We have taken reflectivity data to provide detailed information about the out of plane structure and reciprocal space maps to determine the structure of the unit cell of these films for a number of different coverages. The results of this study confirm the existence of stacking faults and reveal further changes in the structure as the film thickness is increased.\\[4pt] [1] T. Uchihashi et al., Phys. Rev. Lett. 96, 136104 (2006) [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W32.00006: Growth of Iridium and Silver on Ge(111) and Ge(110) Studied by STM and LEEM Cory Mullet, Shirley Chiang, James Morad, Alice Durand We have used both scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) to characterize the growth of iridium and silver onto Ge(111) and Ge(110) as a function of coverage, deposition temperature, and annealing temperature. Ir deposited onto the Ge(111) c(2x8) surface forms a ($\surd $3x$\surd $3)R30$^{o}$ phase, with island size dependent upon substrate temperature during deposition. Deposition at a sample temperature of 670 C yields $\mu $m-scale regions of continuous $\surd $3 coverage, as seen by LEEM. Deposition at 400 C produces Ir islands of three different sizes, all of which are too small to be resolved in LEEM but are easily observed in room temperature STM images. Heating the sample over 640 C yields larger islands observable with LEEM, with island size dependent upon annealing temperature. For Ag/Ge(111) below 1 ML, the 4x4 phase grows preferentially from step edges, while the $\surd $3 phase grows on terraces. Above 1 ML coverage, Ag forms multilayer islands that induce changes in the Ge(111) surface. Ag/Ge(110) forms 1D islands, $\sim $100nm wide, $\sim $10 $\mu $m long at 9 ML coverage. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W32.00007: Dispersive resonance bands within the space charge layer of metal- semiconductor junction Shu-Jung Tang, Tay- Rong Chang, Chien-Chung Huang, Chang-Yeh Lee, Cheng-Maw Cheng, Ku-Ding Tsuei, H. -T. Jeng, Chung-Yu Mou Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)-$\sqrt 3 \times \sqrt 3 $ R30\r{ } structure, in addition to three bands resembling Ge bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to strong coupling between Pb and Ge, the NSO-like band gets weaker and disappears for larger thickness of Pb, which, when combined with ab initio calculations, indicates its localized nature within space charge layer. Our results are clearly important for designing electronics involved with metal-semiconductor contacts [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W32.00008: Island Formation on Nano-membranes Ken Elder, Zhi-Feng Huang Recent experimental studies of strained island formation on Si nano-membranes have yielded interesting results [1]. As the islands grow strain in the islands can be partially relieved by deforming the nano-membranes. The amount of strain relieved increases with decreasing Si membrane thickness. This phenomena allows the islands to grow coherently to larger sizes for thinner membranes, before dislocations nucleate. Perhaps more interestingly the strain field produced by an island on one side of membrane creates preferential locations for the nucleation of islands on the opposite side. This leads to local periodic ordering of the islands, giving rise to novel electronic properties. In this talk a binary phase field crystal model will be used to examine this phenomena. Predictions for the relationship between maximum island size, flux rate and nano-membrane thickness as well as a study of island ordering will be presented. \\[4pt] [1] H.-J. Kim et al., Phys. Rev. Lett. 102, 226103 (2009); M.-H. Huang et al., ACS Nano 3, 721 (2009). [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W32.00009: Effect of Surface Bonding on Semiconductor Nanoribbon Wiggling Structure Yu Zhang, Minrui Yu, Donald Savage, Max Lagally, Robert Blick, Feng Liu SiGe nanomembranes and nanowires provide one important class of stretchable electronic materials. We have investigated a very interesting wiggling phenomenon of SiGe nanoribbons bonded to Si substrate as experimentally observed in a Hall-bar structure. Based on continuum linear stability analysis, we establish a scaling rule between the wiggling period and surface bonding area, in relation to the ratio of strain energy over the interfacial bonding energy. We show that surface bonding plays an important role in morphological deformation of the SiGe nanoribbon structure. Our studies have significant implications in fabricating self-assembled nanostructures on surfaces, and their potential application in stretchable electronics. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W32.00010: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W32.00011: Islands as nanometric probes of strain distribution in heterogeneous surfaces Eldad Peretz, Henry Realpe, Noah Shamir, Moshe H. Mintz, Roni Z. Shneck, Yishay Manassen We presents a novel approach to the mapping of strain on heterogeneous surfaces, at the nanometric scale, using the islands on the surface that are the outcome of the strain, as local strain probes. Utilizing a stress-relieve model we show that the island size and shape reflect the strain variations on the surface. Hence, an STM analysis of the size and shape distributions of the islands may provide a probe to the strain variations on undulated surfaces, having the resolution of the island sizes and inter-distances. The different island sizes and aspect ratios play a significant role in the determination of the strain. To the best of our knowledge, this is the first time that such a mapping is performed. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W32.00012: Continuum Elasticity Modeling and Nonlinear Effects during Epitaxially Strained Island Evolution Champika G. Gamage, Zhi-Feng Huang The formation of surface nanostructures such as islands or quantum dots during strained film epitaxy has attracted continuing great interest. One of the underlying mechanisms has been attributed to the occurrence of morphological instability of the strained film, for which the coupling between film/substrate misfit strains, film deposition rate and growth temperature plays a major role. In this research we focus on the nonlinear evolution of strained surface nanostructures during epitaxy, via constructing a continuum elasticity model based on the 2nd order perturbation theory. The resulting nonlinear evolution equation for film morphology, which also incorporates some realistic factors such as wetting effects, yields a long-wavelength, dynamic description of surface islands or quantum dots. The morphological properties of the growing film and the self-organization process of the coherent surface islands are examined, with the dependence on various material parameters and growth conditions identified. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W32.00013: Low temperature epitaxial growth of Ge on Si(100)-(2x1) with excitation laser by pulsed laser deposition Ali Er, Hani Elsayed-Ali Low temperature epitaxy is important for Ge-Si device fabrication because it can lead to suppressing the introduction of defects such as dislocations and staking faults\textbf{.} The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QDs) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted under a pressure $\sim $1x10$^{-10}$ Torr. A Q-switched Nd:YAG laser ( $\lambda $ = 1064 nm, 10 Hz repetition) was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. \textit{In-situ} reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and \textit{ex-situ} atomic force microscopy (AFM) were used to study the morphology of the grown QDs. It was observed that excitation laser reduces the epitaxial growth temperature to 250 \r{ }C. Applying excitation laser to the substrate during the growth changes the QD morphology and island density, also enhances epitaxy, and improves the size uniformity of QDs at 390 \r{ }C and decreases the surface roughness at room temperature. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms due to a phonon kick following two hole localization could explain the results. Ongoing experiments using a femtosecond laser for excitation and \textit{in-situ} STM for detection of the early stages of island nucleation will be presented. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W32.00014: Spontaneous Nanoscale Faceting of Si(100) Surfaces During Aqueous Etching Marc Faggin, Kent Hallman, Brandon Aldinger, Ankush Gupta, Melissa Hines Although pyramidal texturing of Si(100) surfaces by wet chemical etching has been used industrially to enhance the efficiency of solar cells, the pyramid formation mechanism has eluded understanding. A number of wet chemical etchants have been shown to produce a low density micron-scale hillocks under certain conditions, but atomic-scale probes of these surfaces have revealed considerable roughness rather than perfect faceting. We report the complete chemical transformation of Si(100) surfaces into Si{\{}111{\}}- and Si{\{}110{\}}-nanofaceted surfaces by aqueous chemical etching as confirmed by both morphological probes (STM and AFM) as well as chemical probes (infrared absorption spectroscopy). The high homogeneity of the etched surfaces is unequivocally demonstrated by the very narrow linewidth of the surface vibrational spectrum and the absence of (100) modes. The temporal evolution of the morphology yields mechanistic insights into the hillock formation process; these will be presented as time permits. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W32.00015: Deciphering the Structure of Etched Si(100) Surfaces Brandon S. Aldinger, Ankush Gupta, Ian T. Clark, Marc F. Faggin, Richard G. Hennig, Melissa A. Hines A simple aqueous etchant has recently been shown to create near-atomically flat Si(100) surfaces. We use a combination of STM, vibrational spectroscopy, and density functional theory to propose a new model for the etched silicon surface structure. This model contradicts long-standing interpretations of the spectrum of H-terminated Si(100). Broadness in the absorbance bands previously attributed to roughened surfaces is actually caused by variations in the interadsorbate stress. Also, etchant pH is shown to have a surprisingly large effect on morphology. The low pHs used in many industrial processes are shown to cause pronounced roughening and nanoscale hillock formation. The interpretation of these relatively simple H/Si(100) spectra sheds light on the chemical mechanisms that lead to much more complicated Si(100) morphologies. [Preview Abstract] |
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