Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D19: Focus Session: Dopants and Defects in Semiconductors I |
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Sponsoring Units: DMP Chair: Anderson Janotti, University of California, Santa Barbara Room: Morial Convention Center 211 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D19.00001: Stability and Dynamics of Frenkel Pairs in Silicon Invited Speaker: Extensive EPR experiments on electron irradiated \emph{p}-Si observe only signals identified as arising from isolated vacancies and interstitial impurities. Subsequent experiments and calculations demonstrated that isolated interstitials in \emph{p}-Si diffuse athermally according to a charge-carrier- mediated mechanism. The overall conclusion has been that Si FPs either rapidly recombine or dissociate, even at cryogenic temperatures. More recent X-ray scattering experiments, however, suggest that Si FPs persist at temperatures up to 150 K. We report first-principles calculations of Si FP properties and resolve the apparent conflict between experiments. We find that the vacancy and interstitial components of a proximal FP interact electronically, suppressing the previously identified athermal interstitial diffusion. Such proximal FPs are bound only by the presence of barriers to either recombination or dissociation. Further, metastable FPs may lower their energy by transferring electrons from the interstitial to the vacancy component. We show that EPR studies of FPs are likely unable to distinguish between FPs and isolated vacancies. In addition, calculated diffusion barriers for FP components indicate that FPs should anneal at temperatures similar to those for isolated vacancies: $\sim$150 K. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D19.00002: Defect Formation Energies without the Band-Gap Problem: Combining DFT and \textit{GW} for the Silicon Self-Interstitial P. Rinke$^1$, A. Janotti, C.G. Van de Walle, M. Scheffler For the self-interstitial in silicon, a defect of high technological relevance, density functional theory (DFT) in the widely applied local-density approximation (LDA) underestimates the formation energies of different configurations in the neutral charge state by $\sim$1.5 eV compared to diffusion Monte Carlo calculations [1,2]. We attriubte this to artificial self-interaction and the absence of the derivative discontinuity in the LDA exchange-correlation potential that give rise to the band-gap problem. We present a new formalism that combines LDA with quasiparticle energy calculations in the $G_0W_0$ approximation to overcome these deficiencies. The formation of the neutral defect is expressed as successive charging of its 2+ charge state, for which the defect level is unoccupied, permitting a decomposition into a lattice (LDA) and an electron addition part ($G_0W_0$) [3]. The $G_0W_0$ corrections increase the LDA formation energy by $\sim$1.1~eV. Moreover, the $G_0W_0$-corrected charge transition levels agree well with recent measurements [4]. [1] Batista {\it et al.} PRB {\bf 74}, 121102(R) (2006), [2] Leung {\it et al.} PRL {\bf 83}, 2351 (1999), [3] Hedstr\"om {\it et al.} PRL {\bf 97}, 226401 (2006), [4] Bracht {\it et al.} PRB {\bf 75}, 035211 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D19.00003: Density functional study of charged self-interstitials in silicon Amita Wadehra, John W. Wilkins, Richard G. Hennig The self-interstitials in silicon created by ion-beam processing determine migration rate of impurities, time evolution of dislocations and dopant-enhanced diffusion. The large mobility of interstitials makes their experimental observation difficult. Electron-assisted transport mechanism suggests that charged states with different migration barriers and minimum energy positions are responsible for this rapid migration. Therefore, it becomes necessary to identify the stable ground state charges for each geometry and electron chemical potential. Recent studies of energetics and migration of these defects have largely concentrated on neutral defects and a few on charged single interstitials. We present a density functional study of electronic structure and energetics of charged single-, di- and tri-interstitials in silicon. An analysis of stability of five different charged states in various geometries is provided through formation energies. The diffusion pathways of these charged interstitials are also discussed. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D19.00004: Large scale ab initio calculations for shallow acceptor levels in bulk Si Lin-Wang Wang Accurate calculation of shallow donor levels in conventional semiconductors is a long standing challenge due to the large supercell needed for such calculation. We have used the charge patching method and local density approximation (LDA) to study the acceptor levels in bulk Si, including B, Al, Ga, In and Tl acceptors. The atomic positions are relaxed under LDA using 512 atom cells, and 64,000 atom supercells are used to calculate the acceptor energy levels to achieve the converged results. The calculated impurity binding energies reproduce the experimental trend from B to Tl. However, there is still a significant difference between the calculated binding energies and the experimental results, especially for Tl. This raises the question of whether the LDA can be used to calculate the shallow impurity level accurately. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D19.00005: Relative stability of extended interstitial defects in silicon: large-scale classical MD and first-principles DFT. Hyoungki Park, John Wilkins Extensive simulations for defected Si reveal the relative thermal and temporal stability of extended interstitial defects: {311} and {111} rod-like defects, and Frank dislocation loops. Classical molecular dynamics simulations provide the atomic configurations of those defects, and show that the energetically favored structures change from {311} rod-like defects to Frank loops as the number of interstitals increases, which is consistents with the experimentally-observed transition from rod-like defects to Frank loops after long, high-temperature annealing processes [1,2]. This relative stability is validated with massively parallelized density-functional calculations of ~1500-atoms 2D supercells. Relaxation of experimentally-observed-size defect cluster demonstrates the energetic hierarchy is dependent on the number of interstitials in the defect clusters. [1] L. S. Robertson et al., J. Appl. Phys. 87, 2910 (2000). [2] G. Z. Pan et al., J. Non-Crystalline Solids 352, 2506 (2006). [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D19.00006: Ortho-para transition of interstitial H$_{2}$ in Si Michael Stavola, Chao Peng, Megan Lockwood Interstitial H$_{2}$ in Si is a nearly-free rotator and has ortho and para species with the nuclear spins of the two protons aligned either parallel or antiparallel [1]. If one waits a sufficiently long time at low temperature, H$_{2}$ will relax to its lower energy para state. The ortho-para (o-p) transition for H$_{2}$ in Si has been observed in recent Raman studies [2]. We have performed IR absorption experiments to investigate issues that have proved difficult to study by Raman. When a Si sample containing H$_{2}$ is stored for a month or more at 77K, the 3618.4 cm$^{-1}$ IR line assigned to o-H$_{2}$ [1] is reduced in intensity because, when the o-p transition occurs, p-H$_{2}$ is not seen by IR. When this sample was annealed at room temperature, the ortho population characteristic of room temperature was recovered with a time constant of $\approx $6 hrs. Our IR studies of the kinetics of the o-p transition complement recent Raman results and suggest that the cause of the o-p transition is not yet understood. \newline [1] M. Stavola \textit{et al.}, Physica B \textbf{340-342}, 58 (2003). \newline [2] M. Hiller \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 055504 (2007); \textbf{99}, 209901 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D19.00007: Carbon Impurity Effects on Structural and Magnetic Properties of Manganese Doped Silicon Joshua LaRose, Roger Pink, Tara P. Das, Mengbing Huang, Jian-Qing Wang The recent finding of room temperature ferromagnetism in Mn-doped Si may open up a promising route toward Si-based spintronics. In this work, we investigate effects of co-doped carbon on the structural and magnetic properties of Si:Mn, in a hope to identify possible microstructures responsible for ferromagnetism. Carbon atoms of 0.25 at. {\%} are uniformly doped within the 250-nm surface layer of Si(100) using ion implantation. The C-rich Si and the Si control are subsequently implanted at 300 \r{ }C with Mn ions, yielding a concentration profile of Mn ([Mn] $\sim $ 0.25 at. {\%}) within the depth of 160 nm. Post-implantation annealing is conducted in the range of 800-1000 \r{ }C. Ion channeling measurements suggest that Mn could occupy several lattice sites in Si including the tetrahedral interstitial and substitional site, with respective occupancy affected by carbon impurity and thermal annealing. The Hartree-Fock Cluster Method is used to calculate the binding energies of Mn for different lattice sites in Si. These structural information are compared with the results of superconducting quantum interference device (SQUID) experiments. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D19.00008: Thermal Stability and Laser Annealing of Si$_{1-y}$C$_{y}$ alloys Stefan Zollner, P. Grudowski, V. Dhandapani, G. Spencer, A. Thean Dilute alloys of silicon and carbon are metastable, but can be produced (up to 3{\%} C) using nonequilibrium growth techniques, such as chemical vapor deposition. In such alloys, carbon atoms are located at lattice sites (preferred for device applications) or at interstitial sites. Other impurities (such as H) can be introduced during the growth process. Since Si:C alloys are metastable, they usually do not survive typical source-drain dopant activation anneals (on the order of 1000C for 5s). Also, Si:C alloys implanted with NMOS dopants do not recrystallize during conventional source-drain dopant activation anneals. We report here that the vertical lattice constant of as-grown Si:C alloys (measured by XRD) decreases after laser annealing ($\sim $1 ms near the melting point), leading to an increase in the measured substitutional carbon content. This indicates a conversion of interstitial carbon defects into substitutional carbon or an evolution of hydrogen. We describe our results using a multiscale model applicable to thermal processing over a broad range of temperature and anneal times. Our model describes both solid-state regrowth and the loss or gain of substitutional carbon after annealing. We also present NMOS transistor results, where embedded Si:C alloy source-drain stressors lead to a reduction in the channel resistance. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D19.00009: Chalcogen dopants for infrared optoelectronic Si Jeffrey Warrender, Brion Bob, Michael Aziz, Supakit Charnvanichborikarn, James Williams, Malek Tabbal, Atsushi Kohno Doping Si with a chalcogen in excess of the solubility limit has been shown to result in subbandgap optical absorption and sensitive photodetection, suggesting potential for chalcogen-doped Si as an infrared optoelectronic material. We investigated optical absorption and photovoltaic energy conversion using S, Se, and Te as dopants. We achieved supersaturation of the chalcogen dopant by ion implantation followed by pulsed laser melting and rapid solidification. We observed broadband subbandgap absorption for all dopants over a wavelength range from 1 to 2.3 microns. The subbandgap absorption and photovoltaic response depended sensitively on the chalcogen dose, laser processing, and thermal annealing conditions. We correlate these observations with the corresponding influence of the processing conditions on the material's crystalline quality, chalcogen dopant depth profile, carrier concentration profile, and dopant activation. We found good agreement between the chalcogen depth profiles obtained from experiments and a 1-dimensional model for plane-front melting, solidification, liquid-phase diffusion, and kinetic solute trapping. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D19.00010: H-shuttling within a Hf-defect complex in Si/SiO2/HfO2 structures A.G. Marinopoulos , I. Batyrev, X. Zhou, R. Schrimpf, D. Fleetwood, S.T. Pantelides It was recently shown that, following irradiation of Si-SiO2/HfO2 structures by X-rays or constant-voltage stress, both oxide- and interface-trap densities exhibit oscillations with switch-bias annealing that are much larger than those previously observed in Si/SiO$_{2}$ devices. Here we describe a particular defect complex that can account for the observations. The complex comprises a suboxide Hf-Si bond and an interfacial dangling bond (P$_{b}$ center). With the aid of first-principles calculations we show that this defect possesses a symmetric double-well minimum and can provide trapping sites for H atoms near the interface. In the first site, the H atom passivates the dangling bond; in the second site the H atom resides near the center of the Hf-Si bond. A moderate intervening barrier (1.2 eV) suggests a relatively easy hopping of H atoms between these two energy minima, aided by the applied field and temperature. This shuttling mechanism can explain the observed oscillations in the interface trap densities during switched-bias conditions. This work was supported in part by the AFOSR and the DOE. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D19.00011: Transition from high to low $1/f$ noise regimes in Field Oxide Field Effect Transistors (FOXFETs) Xing Zhou, Daniel Fleetwood, Ronald Schrimpf, Laura Gonella, Federico Faccio The excess low frequency ($1/f)$ noise of parasitic field oxide FETs from a 130 nm technology has been found to vary by more than $\sim $6 orders of magnitude with gate voltage, above the nominally measured device threshold. We find that this variation is due to a transition from noisy subthreshold conduction to full conduction in strong inversion at a point that is more than 5 V above the standard extrapolated threshold voltage. This field oxide structure has a length of $\sim $ 1 micron and a width of $\sim $ 200 micron. We attribute the conduction below to a noisy, subthreshold (perhaps even percolative) path at lower voltages, with a significant contribution from the high density of defects at the Si/SiO$_{2}$ interface in this parasitic FOXFET structure. The noise above the ?true? threshold (as determined with assistance from the noise measurements) follows a standard number fluctuation model, when the subthreshold conduction regime is separated out in the analysis. This work was supported in part by the US Navy. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D19.00012: Effects of Aging and Humidity on Low-Frequency Noise of Metal-Oxide-Semiconductor (MOS) Transistors Aritra Dasgupta, S.A. Francis, D.M. Fleetwood Low frequency noise measurements can provide a non-destructive method of measuring radiation hardness and/or reliability of MOS transistors. We have been studying the effects of moisture exposure at elevated temperatures on MOS low frequency noise. The devices under test were manufactured in the 1980s and came from two different process lots. The results show that the normalized 1/$f$ noise magnitudes $K$ of the pMOS transistors increased significantly with exposure to humidity at elevated temperatures, while the changes in the 1/$f$ noise magnitudes of the nMOS transistors were mostly much less. To estimate the energy dependence of the defects responsible for the noise, we evaluated the gate voltage dependence (Vg-Vt) of the noise, where Vt is the threshold voltage. We find, for the pMOS transistors, Svd$\sim $(Vg-Vt)$^{-1}$, whereas, for the humidity exposed nMOS transistors, Svd$\sim $(Vg-Vt)$^{ -1.25}$ These deviations from an inverse square gate voltage dependence indicate a strong energy dependence of the defect distribution due to humidity exposure. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D19.00013: Disorder--recrystallization effects following low-energy beam--solid interactions Matthew J. Beck, D. M. Fleetwood, R. D. Schrimpf, S. T. Pantelides Classical MD simulations have shown that thermal-spike-related disorder, including local melting, should be widely expected following high energy ($>$1 keV) recoils resulting from beam-- solid interactions during ion-beam processing. In contrast, the formation of isolated point defects by direct atomic displacement is expected for low energy ($<$1 keV) recoils. Using state-of-the-art dynamical DFT calculations of \emph{c}- Si systems we show that recoils of much less than 1 keV result in highly disordered regions which persist for 100s of fs. Therefore, the production of beam-induced defects, as well as the post-implant yield of active dopants, following low-energy beam--solid interactions is controlled by dynamic recrystallization processes. This work was supported in part by the AFOSR through a MURI grant. [Preview Abstract] |
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