Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V7: Dopants and Defects in Semiconductors: Silicon and GermaniumFocus Session
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Sponsoring Units: DMP FIAP Chair: Leonard Feldman, Rutgers University Room: 303 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V7.00001: Scanning Tunneling Spectroscopy Study of Single Layer Step Edges on Si (100) Surfaces. Xiqiao Wang, Pradeep Namboodiri, Kai Li, Xiao Deng, Richard Silver Advanced Hydrogen lithography enables the fabrication of atomically precise donor-based quantum devices on Si(100) surfaces. Understanding the defect and step edge interaction with local electronic and geometric structures is needed to properly interpret device measurement results. Low temperature Si epitaxy, used to encapsulate devices, introduces island growth and step edges near/above buried donor nanostructures, presenting a real challenge in relocating and characterizing buried donor devices using Scanning Tunneling Microscopy/Spectroscopy (STM/STS). We present spatially resolved STS results across single layer steps on Si(100) surfaces. While the electronic properties across SA steps were found to be very similar to that on flat terraces, we observed an edge induced gap state on rebonded SB step edges, which was assigned to the unpaired dangling bond state at the lower edge atom of the rebonded SB steps. In addition, we used computational simulation within Bardeen's formalism to probe the influence of subsurface doping density profiles on the observed STS features over step edges and other defects. This study will help to elucidate the role played by surface step edges and subsurface doping densities in characterizing surface and subsurface nanostructures using STS/STM. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V7.00002: Structural and electrical properties of silicon hyperdoped with gold Jay Mathews, Yining Liu, Girish Malladi, Harry Efstathiadis, Jeffrey Warrender Recent advances in the field of laser hyperdoping have produced a new class of materials that could lead the way to silicon-based, CMOS-compatible infrared detectors. Using the method of ion implantation followed by pulsed laser melting (II-PLM), silicon films with impurities at concentrations well above the solid solubility limit can be fabricated. Recent work has centered around Si:Au, from which prototype IR detectors have been successfully fabricated, but there are still many questions about the structural, electrical, and optical properties of this material. In order to enhance the infrared absorption and achieve high-efficiency devices, a thorough understanding of these properties is necessary, and the processes for device fabrication must be optimized. In this work, we explore the structural and electrical properties of Si:Au hyperdoped films. Si:Au films were annealed at various temperatures, and RBS channeling was used to measure the fraction of Au atoms sitting at substitutional sites. Additionally, transmission line method (TLM) and van der Pauw (VDP) test structures were fabricated in order to investigate formation of Ohmic contacts on the hyperdoped films and to study the electrical properties of Si:Au. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V7.00003: Intersubband scattering in modulation-doped Si two-dimensional electron gases Yi-Hsin Su, Jiun-Yun Li, Leonid Rokhinson, James Sturm A bilayer of modulation doped two-dimensional electron gas (2DEG) is of great interest to probe Coulomb drag. For bottom-doped Si 2DEGs, impurity scattering due to poor phosphorus (P) turn-off results in low carrier mobility. Here we demonstrate a record-high electron mobility of 470,000 cm$^{\mathrm{2}}$/V-s at 0.3 K in a bottom-doped 2DEG, comparable to that in top-doped structures. The power-law exponent of mobility vs. density was also evaluated for different P turn-off slopes. With fast turn-off, the power is 1.5, indicative of dominant remote doping scattering. The power decreases with slower P turn-off due to the enhanced scattering from the segregated P atoms. Further, for the first time, we report the second subband occupancy and intersubband scattering in a single Si quantum well, supported by the Shubnikov-de Haas oscillation data. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V7.00004: Extending silicon's infrared response through laser hyperdoping with gold Jeffrey Warrender, Quentin Hudspeth, Harry Efstathiadis, Elif Ertekin, Jay Mathews Pulsed laser melting of silicon ion-implanted with gold has recently been shown to form a highly crystalline layer with a significantly greater-than-equilibrium gold concentration.[1] Rudimentary devices made with such a laser-doped layer exhibit device response at room temperature under illumination by infrared photons with wavelengths out to 2200 nm.[2] The external quantum efficiency in the infrared is approximately 10$^{\mathrm{-4}}$. In this presentation, we will describe efforts to increase the quantum efficiency and avert the high cost and time of ion implantation. We study the effect of varying the gold implantation dose on the resultant gold concentration in the layer and the optoelectronic properties of the layer. Additionally, we show that an alternative approach to incorporating the gold, through deposition of a thin gold layer onto the silicon surface prior to laser melting, achieves gold concentrations comparable to those achievable by ion implantation, approximately 2 x10$^{\mathrm{19}}$ atoms per cubic centimeter. We perform optoelectronic measurements on layers fabricated in this way and compare to the results obtained when using the preparation method detailed in [1] and [2]. [1] Recht \textit{et al.}, \textit{J. Appl. Phys. }\textbf{114}, 124903 (2013) [2]Mailoa \textit{et al.}, \textit{Nature Communications} \textbf{5}, 3011 (2014) [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V7.00005: Theoretical interpretation of donor wavefunctions STM images in silicon Belita Koiller, A. L. Saraiva, Rodrigo B. Capaz, M.J, Calder\'on, J. Salfi, B. Voisin, J. Bocquel, S. Rogge Single dopant wavefunctions in Si have recently been probed by scanning tunneling spectroscopy, revealing localized patterns of resonantly enhanced tunneling currents. We show that the shapes of the conducting splotches resemble cuts through Kohn-Luttinger (KL) hydrogenic envelopes, which modulate the interfering Bloch states of conduction electrons. All the non-monotonic features of the current profile are consistent with the charge density fluctuations observed between successive $\{001\}$ atomic planes, including a counter-intuitive reduction of the symmetry -- a heritage of the lowered point group symmetry at these planes. A model-independent analysis of the diffraction figure constrains the value of the electron wavevector to $ k_0=(0.82\pm0.03)(2\pi/a_{\rm Si})$. Unlike prior measurements, averaged over a sizeable density of electrons, this estimate is obtained directly from isolated electrons. We further investigate the model-specific anisotropy of the wave function envelope, related to the effective mass anisotropy. This anisotropy appears in the KL variational wave function envelope as the ratio between Bohr radii $b/a$. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V7.00006: Interstitial Functionalization in elemental Si Boris Kiefer, Edwin Fohtung Societies in the 21$^{\mathrm{st}}$ century will face many challenges. Materials science and materials design will be essential to address and master some if not all of these challenges. Semiconductors are among the most important technological material classes. Properties such as electrical transport are strongly affected by defects and a central goal continues to be the reduction of defect densities as much as possible in these compounds. Here we present results of interstitial Fe doping in elemental Si using first-principles DFT calculations. The preliminary results show that Fe will only occupy octahedral interstitial sites. The analysis of the electronic structure shows that the compounds are ferromagnetic and that a bandgap opens as interstitial Fe concentrations decrease, with a possible intermittent semi-metallic phase. The formation energy for interstitial Fe is unfavorable, as expected, by \textasciitilde 1.5 eV but becomes favorable as the chemical potential of Fe increases. Therefore, we expect that biasing the system with an external electrical field will lead to the formation of these materials. Thus, our results show that interstitial defects can be beneficial for the design of functionalities that differ significantly from those of the host material. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V7.00007: Isolation of dangling bond states on Si(100) surfaces for quantum information applications Peter Scherpelz, Giulia Galli Hydrogen resist lithography allows dangling bonds to be created and manipulated on Si(100) surfaces, both for use as a controlled quantum system, and as a step in the deterministic placement of dopants at the single-atom level. However, previous experiments and computations [1] have shown conflicting results on the location of dangling bond energy levels, which can impact their utility as qubits. Here we use large-scale density functional theory and many-body perturbation theory (GW) calculations to show that in clean, H-passivated Si(100)-(2x1) surfaces a singly-occupied dangling bond does not give rise to an electronic state isolated from the valence bands. However, very thin (1-3 nm) samples terminated by a (100) surface should provide isolated singly-occupied and doubly-occupied dangling bond states. We also explore the effect of strain, and consider novel uses of boron dopants. [1] See e.g. Bellec et al. Phys. Rev. B (2013), Ye et al. Surf. Sci. (2013), Schofield et al. Nat. Commun. (2013), Wieferink et al. Phys. Rev. B (2010). [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V7.00008: Hybrid functional calculations of Copper impurities and related complexes in Silicon Abhishek Sharan, Zhigang Gui, Anderson Janotti Copper impurities affect electronic and optical properties of semiconductors. Cu is an ubiquitous impurity and can be introduced unintentionally during various processing step. In silicon, the fast-diffusing interstitial Cu donor often passivates shallow-acceptor dopants, affecting the electronic characteristics of devices, while deep levels associated with other forms of the Cu impurity degrade device performance. Here we revisit the problem of the Cu impurity in Si using first principles calculation based on a hybrid functional. We discuss the relative stability of the substitutional and interstitial forms, as well as the formation of complexes with hydrogen and oxygen impurities. The results of our calculations will be compared with recent experiments on the electrical activity of Cu impurities in Si. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V7.00009: Impurity distribution in high purity germanium crystal and its impact on the detector performance. Guojian Wang, Mark Amman, Hao Mei, Dongming Mei, Klaus Irmscher, Yutong Guan, Gang Yang High-purity germanium crystals were grown in a hydrogen atmosphere using the Czochralski method. The axial and radial distributions of impurities in the crystals were measured by Hall effect and Photo-thermal ionization spectroscopy (PTIS). Amorphous semiconductor contacts were deposited on the germanium crystals to make detectors. Three planar detectors were fabricated from three crystals with different net carrier concentrations (1.7, 7.9 and 10x1010 cm-3). We evaluated the electrical and spectral performance of three detectors. Measurements of gamma-ray spectra from 137Cs, 241Am and 60Co sources demonstrate that the detectors have excellent energy resolution. The relationship between the impurities and detector's energy resolution was analyzed. Keywords: High-purity germanium crystal, High-purity germanium detector This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V7.00010: Interface effects on acceptor silicon qubits Jose Carlos Abadillo-Uriel, Maria Jose Calderon Recently, proposals of acceptor-based qubits have drawn considerable attention due to the long range strong dipolar inter-qubit coupling and the possibility of exploiting the spin-orbit interaction to couple spins to phonons or oscillating electric fields. Dopant-based quantum computing implementations often require the dopants to be situated close to an interface to facilitate qubit manipulation with local gates. Interfaces not only modify the energies of the bound states but also affect their symmetry. Making use of the successful effective mass theory we study the energy spectra of acceptors in Si or Ge taking into account the quantum confinement, the dielectric mismatch and the central cell effects. The presence of an interface puts constraints to the allowed symmetries and leads to the splitting of the bulk four-fold degenerate ground state in two Kramers doublets. We show that, as the acceptor gets closer to the interface, the entire spectrum is compressed. Inversion symmetry breaking also implies parity mixing which affects the allowed optical transitions. Consequences for acceptor qubits are discussed. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V7.00011: Quantum states of interacting point defects in silicon Steven Schofield, Holly Hedgeland, Manuel Siegl, David Bowler We investigate point defect induced quantum states on silicon surfaces using low temperature scanning tunneling microscopy and spectroscopy (STM/STS). We compare defect states produced at missing H atom sites on the Si(001):H monohydride surface and those formed at boron deficient sites of the B-saturated Si(111):B-$\sqrt3\times\sqrt3 R30^\circ$ surface. We find good agreement between measured differential conductance and first principles calculations of the states. Furthermore we explore the interaction of pairs of defects in a range of varying close proximity arrangements and find non-linear interference between the laterally extended excited states of the point defects. The results support the interpretation of interacting excited states as we have presented previously [Nature Communications 4 (2013) 1649]. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V7.00012: Quantum Point Contacts and Valley Filters on a 6-fold Degenerate Hydrogen-terminated Si(111) Surface Luke D. Robertson, Binhui Hu, B. E. Kane Hydrogen-terminated Si(111) surfaces preserve the 6-fold valley degeneracy and anisotropic electron mass predicted in bulk Si, providing a unique environment for 2-D electron systems (2DESs). Our group has demonstrated high mobility as well as the fractional quantum Hall effect for electrons confined on the Si(111) surfaces, establishing evidence that they are ideal platforms for 2DESs and lower dimensional systems. Recently, we have succeeded in fabricating high mobility ambipolar devices and have found that heavily p-doped regions can be used as lateral depletion gates for confinement of 2DESs induced by a top gate [1]. Here, we describe our efforts to extend this technology to the nanoscale and in particular towards the fabrication of quantum point contacts (QPCs). QPCs realized in materials with anisotropic electron mass may exhibit valley filter phenomena [2] leading to extreme sensitivity to single donor occupancy, and thus are of interest to measurement schemes for donor-based quantum information processing. Preliminary measurements and fabrication techniques will be discussed. [1] B. Hu, et al, arXiv, 1509.03849 (2015) [2] Gunawan et al, Phys. Rev. B, 74, 155436 (2006) [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V7.00013: Electron spin resonance and relaxation of defects and donors in Silicon Nanowires Marco Fanciulli, Matteo Belli, Stefano Paleari, Antonio Pizio The current status of the investigation of defects in silicon nanowires and at the interface between the group IV semiconductor and its oxide in 1D nanostructures is reviewed and discussed. The paper concentrates on nanowires produced by metal assisted chemical etching. Donors (such as P and As) and defects at the interface between the semiconductor and its oxide (namely, the Pb centers) are investigated by continuous wave (CW) and pulsed Electron Paramagnetic Resonance (pEPR). The role in the de-activation mechanism of donors played by hydrogen and Pb centers is discussed. The characteristic times, the spin-lattice T1 and spin-spin T2, of the Pb centers are also reported in this study. Their behavior as a function of the temperature is addressed in the framework of Two-Level-Systems. TLS are usually invoked wherever there is a disordered system, which in the case of Pb centers is represented by the amorphous oxide side. The model includes a low- and a high-temperature regime. It is worth noticing that bulk techniques such as CW and pEPR are applied to surface defects thanks to the enhanced surface-to-volume ratio. The results of the present investigation highlight a possible issue for the exploitation of nanostructures in fields like spin-based quantum computing, i.e. the spin-lattice relaxation and the decoherence induced by the TLS in the Pb centers. [Preview Abstract] |
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