Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H12: Focus Session: Dopants and Defects in Semiconductors: Silicon |
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Sponsoring Units: DMP Chair: Bonna Newman, Massachusetts Intitute of Technology Room: D223/224 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H12.00001: Isotopic Fingerprints in the Luminescence of Deep Defects in Silicon Invited Speaker: In a series of recent papers [1, 2] we have shown that the dramatic improvements in spectral resolution made possible in highly enriched $^{28}$Si can provide surprising new information on the detailed constituents of deep luminescence centers. The `isotopic fingerprints' reveal the presence, and number, of different chemical species involved in the deep centers. While many of these luminescence centers have been studied for decades, this new technique revealed that \textit{none} of these was what it was thought to be. Armed with this new information, many new centers have been discovered, containing either four or five atoms chosen from among: Cu, Ag, Au, Pt and Li. There is at present no theoretical explanation for the stability and ubiquity of these centers in rapidly thermally quenched silicon. \\[4pt] [1] M. Steger et al., Phys. Rev. B 81, 235217-1-6 (2010). \\[0pt] [2] M. Steger et al., Phys. Rev. Lett. 100, 177402-1-4 (2008). [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H12.00002: Theoretical study of the Cu$_{\rm PL}$ defect in Si Alexandra Carvalho, Stefan K. Estreicher Copper is a common contaminant in Si processing. When in supersaturation, a fraction of 1\%\ of the Cu in the sample forms an electrically-active defect easily seen by photoluminescence. This Cu$_{\rm PL}$ defect in Si has a no-phonon line at 1014 meV. It has long been believed to consist of an interstitial copper (Cu$_i$) weakly bound to a substitutional copper (Cu$_s$) : The \{Cu$_s$Cu$_i$\} pair. However, PL studies in isotopically pure $^{28}$Si crystals have shown that the defect contains not two but four copper atoms [1]. We examine the possibility that the core of the defect consists of not one but two adjacent substitutional Cu atoms. This core traps two Cu$_i$ atoms, resulting in defect with $D_{3d}$ symmetry. We will discuss its formation mechanism and stability, and show that they are consistent with the conditions at which Cu$_{\rm PL}$ is observed. If this model is correct, then then DLTS lines associated with Cu$_s$ should be re-assigned to \{Cu$_s$Cu$_s$\}. \\[4pt] [1] M. Steger {\it et al}., Phys. Rev. Lett. 100, 177402 (2008) [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H12.00003: EPR parameters of the dangling bond defect in crystalline and amorphous silicon: A DFT-study Gernot Pfanner, Christoph Freysoldt, J\"{o}rg Neugebauer Thin-film a-Si:H solar cells are considered as low-cost alternatives to bulk crystalline silicon (c-Si) solar cells. A disadvantage of these devices is that their efficiency is severely limited by light-induced defects (Staebler-Wronski effect). In this context, electron-paramagnetic resonance (EPR) is a key technique to probe for the local atomic structure of defects with unpaired spins such as the silicon dangling bond. However, the assignment of the EPR signal to a specific defect structure requires comparison to theoretical models. Using density-functional theory, we address structure-property relationships by combining systematic studies for idealized dangling-bond models in c-Si with a statistical analysis of a variety of dangling bonds in a-Si:H supercells. Our studies reveal the influence of the local geometry on sp-hybridization and delocalization. Yet, the structural variability of a-Si:H cannot be captured by these idealized defect models alone. Rather, our calculations indicate that a relatively broad distribution of dangling-bond like structures gives rise to the experimental signal supporting a recent re-evaluation of EPR parameters from multifrequency EPR. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H12.00004: Electrically Detected Pulsed ENDOR in Phosphorus-Doped Silicon Felix Hoehne, Lukas Dreher, Hans Huebl, Martin Stutzmann, Martin S. Brandt We demonstrate the electrical detection of X-band electron nuclear double resonance (ENDOR) in phosphorus-doped silicon at 4\,K. A pulse sequence analogous to Davies ENDOR in conventional electron spin resonance is used to measure the nuclear spin transition frequencies of the $^{31}$P nuclear spins, where the $^{31}$P electron spins are detected electrically via spin-dependent transitions through Si/SiO$_2$ interface states. In addition, electrical detection of coherent nuclear spin oscillations is shown, demonstrating the feasibility to electrically read out the spin states of possible nuclear spin qubits. Combining the enhanced sensitivity of electrically-detected magnetic resonance and the wide range of applications of pulsed ENDOR, this techniques could be a versatile tool to study paramagnetic defects in semiconductor nanostructures. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H12.00005: Electroelastic Hyperfine Tuning of Phosphorus Donors in Silicon Lukas Dreher, Timon A. Hilker, Andreas Brandlmaier, Sebastian T.B. Goennenwein, Hans Huebl, Martin Stutzmann, Martin S. Brandt We demonstrate an electroelastic control of the hyperfine interaction between nuclear and electronic spins opening an alternative way to address and couple spin-based qubits. The hyperfine interaction is measured by electrically detected magnetic resonance in phosphorus-doped silicon epitaxial layers employing a hybrid structure consisting of a silicon-germanium virtual substrate, a piezoelectric actuator, and a loop-terminated coplanar strip line for on-chip microwave magnetic-field generation. By applying a voltage to the actuator, the hyperfine interaction is changed by up to 0.9~MHz, which would be enough to address spin-qubits in isotopically purified $^{28}$Si with a sufficient fidelity under optimized conditions. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H12.00006: Characterizing Individual Group V donors in Silicon Cyrus F. Hirjibehedin, Philipp Studer, Steven R. Schofield, Veronika Brazdova, David R. Bowler, Neil J. Curson The study of dopants in silicon has been rapidly growing in importance because the dimensions of semiconductor devices have now decreased to the point where their functionality relies upon only a few atoms. Group V donors are especially interesting due to their potential application in spintronics and quantum computing. Whereas P dopants have been extensively studied, comparatively little is known about the characteristics of other group V donors. Using a combination of ion implantation and cross-sectional scanning tunneling microscopy (XSTM) and, we study individual Bi and Sb atoms in the cleaved Si(111)2x1 surface. High-resolution STM topography images and scanning tunneling spectroscopy (STS) data allow us to probe the structural and electronic properties of these individual dopants in silicon. Density functional theory (DFT) calculations further support our structural assignments. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H12.00007: Ab initio shallow impurity level calculations in semiconductors Gaigong Zhang, Andrew Canning, Niels Jensen, Stephen Derenzo, Lin-Wang Wang Binding energies of B, Al, Ga, In and Tl shallow acceptors in bulk Si were calculated using a GW + Semi-empirical procedure. Within the procedure, both density functional theory calculation within local density approximation (LDA) and GW calculation were performed. In the LDA calculation, a large supercell containing tens of thousands of Si atoms and the center impurity atom was constructed from a potential patching procedure. The central potential of this system was further corrected by 64 atom GW calculations. The folded spectrum method was used to calculate the eigen energies of this large supercell containing the center impurity. The calculated binding energies show good agreement with experimental impurity binding energies. This procedure represents an efficient approach to study shallow impurity levels which are important for semiconductor devices. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H12.00008: First principles study of phosphorus and boron defects in Si-XII Brad D. Malone, Marvin L. Cohen We present a first-principles study of phosphorus and boron substitutional defects in Si-XII, a polytype of silicon in the R8 structure. Recent results from nanoindentation experiments reveal that this phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. We examine the formation energies of the B and P defects at the two distinct atomic sites in the R8 structure. We also calculate the thermodynamic transition levels of each defect in its relevant charge states. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H12.00009: Stability of donor-pair defects in $Si_{1-x}Ge_x$ alloy nanowires Ji-Sang Park, Byungki Ryu, K.J. Chang Semiconductor nanowires (NWs) have attracted much attention because of the quantum confinement effect, large surface-to- volume ratio, and compatibility with the existing Si technology. Although impurity doping is important for applications to optoelectronic devices, it is generally difficult to dope nanostructures due to segregation of dopants to the surface, high activation energies induced by the surrounding low dielectric medium, and compensation by defects such as surface dangling bonds. Furthermore, compared with bulk Si, electrically deactivating donor-pair defects are energetically more favorable than isolated shallow donors in NWs. In this work, we perform first-principles density functional calculations to study the stability of donor-pair defects and the doping efficiency in $Si_{1-x}Ge_x$ alloy NWs doped with P impurities. The stability of donor-pair defects is enhanced as the Ge concentration increases. Consequently, the doping efficiency in $Si_{1-x}Ge_x$ alloy NWs is expected to be suppressed by the formation of donor-pair defects, similar to previous calculations for Si NWs with small diameters. The effects of reduced dimensionality, Ge chemical bonding, and strain on the stability of donor-pair defects in alloy NWs are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H12.00010: New electronic effects observed on n-type Si(111)2x1 using cross-sectional STM Neil J. Curson, Philipp Studer, Steven R. Schofield, Greg Lever, David R. Bowler, Cyrus Hirjibehedin Cross-sectional scanning tunneling microscopy (XSTM) of in-situ cleaved semiconductor surfaces has two distinct advantages over STM experiments where studies are performed on the surface of the annealed and/or sputtered semiconductor wafers. Firstly, the cleaving process exposes a clean surface without the usual need for high temperature annealing, thus revealing a surface that has not been driven to its thermodynamic minimum energy state. Secondly, the surface being imaged is perpendicular to the surface of the original wafer, which is of particular value for the study of implanted or epitaxially overgrown wafers. We use XSTM measurements, spatially resolved scanning tunneling spectroscopy (STS) and density functional theory (DFT) to study the electronic properties of the cleaved (111)2x1 surface of silicon. We examine bulk-doped, and ion-implanted samples. Our studies reveal new, long range, electronic effects that have implications for future nanoscale devices in silicon. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H12.00011: Charged Defects in the Si(001) Surface Steven Schofield, Philipp Studer, Cyrus Hirjibehedin, Neil Curson, Gabriel Aeppli, David Bowler The Si(001) surface has been the subject of intense research for decades due to its ubiquitous use in the semiconductor industry, its applicability as a model semiconductor surface, and proposals for its use in novel quantum devices. Surprisingly, atomic-scale investigations using scanning tunneling microscopy and spectroscopy (STM/STS) continue to produce new insights into the structural and electronic properties of this deceptively simple semiconductor surface. Tip- and charge-induced band bending are generally considered to play only minor roles in measurements of silicon surfaces due to Fermi level pinning by surface states and defects. However, such effects become important when investigating charged defects and/or surfaces that have had their surface states removed through chemical passivation. We present high resolution STM images and spectroscopy data of defects in the Si(001) surface. We include band bending and charge state in the discussion of the results. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H12.00012: Near surface dopant depletion in UHV prepared H-Si(100): spectroscopic and imaging effects Jason Pitters, Robert Wolkow Dangling bonds (DBs) have been shown to be useful in directing chemical reactions on silicon and for atom scale electronics such as quantum cellular automata. One enabling aspect of DBs is that they can assume various charge states depending on the type and level of crystal doping. We have found that for degenerately doped n-type silicon, the scanning tunneling spectroscopy (STS) and imaging characteristics H-Si(100) surfaces and DBs varies depending on the preparation method. Samples heated to 1050\r{ }C were found to have a consistent level of doping throughout the bulk and near surface regions. Samples heated to 1250\r{ }C showed a reduced dopant concentration in the near surface region. STS showed shifted I/V spectra. The loss of degeneracy was indicated by the loss of tunneling through dopant states in the band gap. These results show that UHV prepared silicon does not have a consistent dopant profile and that the bulk dopant density should not be assumed in the near surface region. This has important ramifications for DB imaging and modeling. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H12.00013: Ab initio study of water molecule dissociation on the hydrogenated Si(100) surface Marilia J. Caldas, Regina Lelis-Sousa The reaction mechanisms for the H$_2$O molecule dissociation at the Si surface, and the resulting oxidation sites, are still object of debate. Here, we present a detailed theoretical investigation of the reaction pathways for the dissociation of water on the Si(100)(2x1):H surface, starting from different initial ``attack'' sites and leading to different final, oxidized configurations. We use extended-surface (slab) models, working within DFT, with pseudo-potentials and plane wave basis set in the quantum-espresso code. The pathways were mapped using the CI-NEB method with both local and gradient corrected exchange-correlation functionals in order to obtain a fair estimate of energy barriers. Our results indicate that the oxidation routes suggested by earlier experimental works are not favored. We propose two new oxidation routes, with simultaneous release of one H2 molecule: one related to chemisorption of the oxygen atom on the Si-Si dimer bond, and another related to the absorption on the back-bond. Analysis of energy barriers showed that these two new possibilities are both kinetically and energetically viable. We also present analyses of the profiles obtainable through STM for the investigated structures, which should help experimental identification. [Preview Abstract] |
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