Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A23: Focus Session: Dopants and Defects in Semiconductors I |
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Sponsoring Units: DMP Chair: Marek Skowronski, Carnagie Melon University Room: 325 |
Monday, March 18, 2013 8:00AM - 8:36AM |
A23.00001: Defects in Carbon-Based Materials Invited Speaker: Gerd Duscher Two distinctly different carbon based semiconducting materials were investigated as to how point defects can influence the electric properties. SiC is a high power electronic material with high bulk mobility. The interface between SiC and SiO$_2$ is generally considered to be the cause for the reduced mobility of SiC devices compared to bulk SiC. We investigated this interface with atomic resolution Z-contrast and electron energy-loss spectroscopy. We come to the conclusion that the previously observed interface layer is due to the miscut and does not exhibit any stoichiometric change. The structure of the interface which is limiting the device performance is caused by the steps and facets at the interface introduced by the miscut. We observed a high number of carbon in the oxide right next to the interface. Aberration corrected transmission electron microscopy enabled the investigation of the atomic structure of this highly stepped interface and the impact of geometry and chemistry on the electronic properties of this material. Graphene is an emerging electronic material also with high mobility. We investigated the defects and dopants in graphene were investigated. We observed point and extended defects in this 2D material. Due to the clear observation of all atoms involved, this material can serve as a model material to study point defects directly. We observe a electronegativity doping of substitutional Si. We observed a remarkable resistance to oxidation of a variety of point defects of elements that readily oxidize in normal circumstances. Boron and nitrogen doped graphene was investigated and the exact nature of the dopant sites and interactions will be shown. Generally speaking modern electron microscopy can directly visualize the full atomic structures in geometrically simple materials like graphene. The knowledge of point defects can be the basis to understand the electronic property structure relationship of structurally complex materials like SiC. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A23.00002: Characterization of the oxide-semiconductor transition layer in NO, P, and N-plasma passivated 4H-SiC/SiO$_2$ structures using transmission electron microscopy Joshua Taillon, Joonhyuk Yang, Claude Ahyi, John Williams, John Rozen, Leonard Feldman, Tsvetanka Zheleva, Aivars Lelis, Lourdes Salamanca-Riba The 4H-SiC/SiO$_2$ interface in MOSFET devices contains a high density of electrically active traps. Recent work has revealed an inverse relationship between the SiC-SiO$_2$ transition layer width and FET channel mobility. Interfacial N and P, introduced by nitric oxide (NO) anneals, nitrogen plasma (N2P), or phosphosilicate glass (PSG) passivations improve carrier mobility, but a relationship to transition layer width is lacking. We present a characterization of the SiC/SiO$_2$ transition layer as a function of NO anneal time using high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and electron energy-loss spectroscopy (EELS). The transition layer was measured with HRTEM and HAADF-STEM and characterized by the evolution of the C/Si and O/Si composition ratios and the Si-\textit{L}$_{2,3}$ edge in the EEL spectra across the interface. We show an inverse relationship of NO anneal time and transition layer width, which correlates with improved channel mobility, increased N interfacial density, and reduced interface trap density. No excess C was noted at the interface. NO annealed samples are compared to N2P and PSG passivations. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A23.00003: Study of surface potential variation in p-/n-type 4H-SiC using scanning kelvin probe microscopy Jung-Joon Ahn, Lin You, Liangchun Yu, Sang-Mo Koo, Joseph Kopanski We report surface potential images of p-n junctions in 4H-SiC measured using scanning kelvin probe microscopy (SKPM) and relate them to the local dopant concentration. SKPM has been demonstrated on various semiconductor materials to examine crystalline defects and doping profiles. SKPM measured surface potential depends on the local dopant concentration and clearly differentiates between n-type and p-type materials. As opposed to scanning capacitance microscopy, which requires a good quality surface insulating layer, SKPM requires a clean surface and the lack of a screening oxide might result in higher spatial resolution. For the measurement, partially de-processed SiC high power LMOSFETS were used. The p-n junctions were formed from 4H-SiC wafers having a p-epilayer on p-substrate that was ion-implanted with nitrogen and annealed to build a shallow n-type region. The samples were observed in plan-view and in cross-section. Amplitude modulated, double pass SKPM was implemented with a commercial AFM. We conducted a detailed study of various data acquisition parameters and it seems that the lateral resolution of the potential difference can be enhanced by applying higher ac modulation amplitude and small tip-sample scanning height. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A23.00004: Near-infrared luminescent cubic silicon carbide nanocrystals for in vivo biomarker applications: an ab initio Study Adam Gali, Viktor Z\'olyomi, B\'alint Somogyi Small molecule-sized fluorescent emitters are needed as probes to image and track the locations of targeted nano-sized objects with minimal perturbation, and are much sought-after to probe biomolecules in living cells. For in vivo biological imaging, fluorescent biomarkers have to meet the following stringent requirements: (i) they should be non-toxic and bioinert, (ii) their hydrodynamical size should be sufficiently small for clearance, (iii) they should be photo-stable. Furthermore, it is highly desirable that (iv) they have intense, stable emission in the near-infrared range, and (v) they can be produced in relatively large amount for biological studies. Here we report time-density functional calculations on SiC-based QDs in the aspect of in vivo biological imaging applications. We find that Si-vacancy, divacancy, as well as single metal dopants such as Vanadium (V), Molybdenum (Mo) and Tungsten (W) in molecule-sized (1-2~nm) SiC QDs emit light efficiently in the near-infrared range. Furthermore, their emission wavelength varies on the size of host SiC QDs at less extent than that of pristine SiC QDs, thus sharper emission spectrum is expected even in a disperse size distribution of these QDs. These fluorescent SiC QDs are paramagnetic in the ground state. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A23.00005: The Search for Sub-Bandgap Optoelectronic Response in Silicon Hyperdoped with Gold Jonathan Mailoa, Austin Akey, Jay Mathews, David Hutchinson, Christie Simmons, Joseph Sullivan, Mark Winkler, Dan Recht, Peter Persans, Jeffrey Warrender, Michael Aziz, Tonio Buonassisi Deep-level dopants have been long known as the lifetime-killer in microelectronic devices. Nevertheless, it has been shown that deep-level donor can facilitate strong absorption of light with energy below the semiconductor bandgap. Due to this strong sub-bandgap absorption, it is possible to engineer silicon devices exhibiting sub-bandgap optoelectronic response, such as silicon-based infrared photodetectors and intermediate-band solar cells. In this work, we show the optoelectronic response of silicon doped with a gold concentration surpassing the equilibrium solubility limit (gold-hyperdoped silicon, Au:Si). We fabricated Au:Si by ion implantation followed by nanosecond pulse laser melting, achieving a gold dopant concentration of over 10$^{19}$ cm$^{-3}$. UV-VIS spectrophotometry was performed to measure sub-bandgap light absorption in the Au:Si layer. Our samples with the highest gold concentration have 10-15{\%} absorption of sub-bandgap light. We will present and discuss the sub-bandgap optoelectronic response of this gold-doped silicon. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A23.00006: Recombination lifetimes in laser hyperdoped Si layers measured via microwave photoconductive decay Jay Mathews, David Hutchinson, Ryan McAvoy, Mark Winkler, Daniel Recht, Austin Akey, Jonathan Mailoa, Michael Aziz, Tonio Buonassisi, Peter Persans, Jeffrey Warrender Silicon hyperdoped with impurities via ion implantation followed by pulsed laser melting has attracted much attention lately due to potential for forming an intermediate band. Such materials have shown significant optical absorption well below the band gap of Si and are being explored for applications in photovoltaics and infrared detection. However, while optical absorption can be increased, high dopant concentration generally leads to a substantial decrease in recombination lifetime, which can detrimentally affect the performance of detectors and solar cells. In this work, we use microwave photoconductive decay ($\mu $-PCD) to explore the transient behavior of Si hyperdoped with S at various levels. Excitation is achieved via a pulsed Nd:YAG laser at 355 nm (FWHM $\sim$ 5 ns), ensuring that carriers are generated only in the hyperdoped region. Decay times were found to decrease monotonically with increasing S concentration, and the highest concentrations do not show measureable photoconductivity, which could indicate unacceptably low lifetimes. Additional $\mu $-PCD measurements are presented on Si hyperdoped with Au, which are promising despite the fact that Au is typically a ``lifetime killer,'' as well as Si hyperdoped with Ti, which has been previously shown to exhibit lifetime recovery. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A23.00007: Insulator-to-metal transition with deep-level impurities in silicon achieved by compensated hyperdoping Christie Simmons, Austin Akey, Mark Winkler, Jacob Krich, Joseph Sullivan, Daniel Recht, Michael Aziz, Tonio Buonassisi Hyperdoping (achieved via nanosecond pulsed laser melting and rapid resolidification) allows the substitutional incorporation of impurities at concentrations orders of magnitude beyond the equilibrium solubility limit. This technique opens the door for studying the insulator-to-metal transition (IMT) in silicon doped with impurities for which the critical concentration necessary to drive the transition is inaccessible by conventional doping techniques; specifically, impurities that introduce deep, highly localized states. IMTs have already been observed for silicon hyperdoped with sulfur and with selenium. It may be possible to use these deep impurities to create an intermediate band semiconductor in which there is a delocalized band of impurity states isolated within the conventional band gap. We will discuss the possible nature of these IMTs (impurity band merging with the conduction band vs. closing of the Hubbard gap), and we will present further observations of a metal-to-insulator transition in highly compensated sulfur-doped samples. Sulfur is a double donor in silicon, and by adding varying concentrations of boron, a shallow acceptor, we demonstrate a tunable depletion of the impurity band as evidenced by the materials' optoelectronic properties. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A23.00008: Optoelectronic Characterization of Impurity Supersaturated Silicon Junctions David Hutchinson, Joseph Sullivan, Jay Mathews, Daniel Recht, Aurore J. Said, David J. Lombardo, Christie Simmons, Tonio Buonassisi, Jeffrey M. Warrender, Michael J. Aziz, Peter D. Persans Intermediate band semiconductors have been proposed as a path to high efficiency photovoltaics. Silicon doped to high levels with impurities such as S, Se, Au, and Ti which can produce deep levels, may fulfill this promise. We report here on the optoelectronic properties of diode structures prepared by implantation of 10$^{15}$ to 10$^{16}$ impurity atoms/cm2 into a p-type or n-type wafers, followed by nanosecond pulsed laser melting and resolidification. Experimental results from wavelength and temperature dependent diode response, spatial quantum efficiency mapping, intensity dependent efficiency, and current-voltage characterization will be reported. Current-voltage measurements under photoexcitation yield information on the built in voltage and absorption mechanisms. Most devices show maximum quantum efficiency for excitation wavelengths between 900 and 1000 nm. The drop in quantum efficiency for short wavelengths can yield the minority carrier diffusion length in the hyperdoped material. Long wavelength response elucidates photocarrier excitation mechanisms. The fundamental properties of the junction and the supersaturated material will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A23.00009: Cross-spectrum noise spectroscopy for characterization of deep-levels in nanoscale devices Deepak Sharma, Sergiy Krylyuk, Abhishek Motayed, Qiliang Li, Albert Davydov Applications of traditional methods to study deep-levels, such as deep-level transient spectroscopy, or photo-induced current transient spectroscopy, often become impractical for nanoscale devices. In low frequency noise spectroscopy, the accurate measurements of the noise signal in low-current nanowire devices are extremely challenging because the device noise, which is proportional to the dc current, becomes comparable with the measurement setup noise. To overcome these issues, we have implemented a LFN measurement method based on dual-channel cross-spectrum analysis technique, which reduced the power spectral density (PSD) by three orders of magnitude by reducing the parasitic background 1/f noise, enabling high sensitivity measurements. The method was applied to probe deep-levels in n- and p-type Si nanowires grown by Ni and Au catalysts. Temperature-dependent noise measurement clearly showed Lorentzian peaks due to the generation-recombination (G-R) process via the deep levels introduced by Ni and Au atoms diffused into the Si nanowires during the growth. Important parameters such as trap energies and concentrations of the deep levels, minority carrier life times, hole and electron capture cross sections were calculated for both Ni and Au deep-levels. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A23.00010: Defect engineering of complex semiconductor alloys: Cu$_{\mathrm{2-2x}}$M$_{\mathrm{x}}$O$_{\mathrm{1-y}}$X$_{\mathrm{y}}$ Stephan Lany, Vladan Stevanovic The electrical properties of semiconductors are generally controlled via doping, i.e., the incorporation of dilute concentrations of aliovalent impurity atoms, whereas the band structure properties (gap, effective masses, optical properties) are manipulated by alloying, i.e., the incorporation of much larger amounts of isovalent elements. Theoretical approaches usually address either doping or alloying, but rarely both problems at the same time. By combining defect supercell calculations, GW quasi-particle energy calculation, and thermodynamic modeling, we study the range of electrical and band structure properties accessible by alloying aliovalent cations (M $=$ Mg, Zn, Cd) and isovalent anions (X $=$ S, Se) in Cu$_{\mathrm{2}}$O. In order to extend dilute defect models to higher concentrations, we take into account the association/dissociation of defect pairs and complexes, as well as the composition dependence of the band gap and the band edge energies. Considering a composition window for the Cu$_{\mathrm{2-2x}}$M$_{\mathrm{x}}$O$_{\mathrm{1-y}}$X$_{\mathrm{y}}$ alloys of 0 $\le $ (x,y) $\le $ 0.2, we predict a wide range of possible band gaps from 1.7 to 2.6 eV, and net doping concentrations between p $=$ 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ and n $=$ 10$^{\mathrm{17}}$cm$^{\mathrm{-3}}$, notably achieving type conversion from p- to n-type at Zn or Cd compositions around x $=$ 0.1. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A23.00011: Ab-Initio Study of Defect Physics for Layered LaCuChO and BaCuChF (Ch=\{S,Se,Te\}) Structures Jason Vielma, David H. Foster, Guenter Schneider Layered oxychalcogenides LnCuChO (Ln = \{La,Pr,Nd\}, Ch = \{S,Se,Te\}) and isostructural layered fluorochalcogenides BaCuChF have drawn much interest in recent years as p-type wide bandgap semiconductors with applications in transparent electronics and photovoltaics. Previous experimental and computational studies concluded for both LaCuChO, with a bandgaps between 2.4-3.1 eV, and BaCuChF, with optical bandgaps between 2.8-3.5 eV, that p-type conductivity is primarily due to copper vacancies. We report a comparative {\it{ab-initio}} computational study of the defect physics for both families of materials. Point defects and defect complexes are taken into account and previously omitted corrections have been included.\footnote{A. Zakutayev, J. Tate, G. Schneider. \emph{Phys. Rev. B.} \textbf{82}, 195204, (2010)}$^,$\footnote{H. Hiramatsu, T. Kamiya, T. Tohei, E. Ikenaga, T. Mizoguchi, Y. Ikuhara, K. Kobayashi, H. Hosono. \emph{J. Am. Chem. Soc.} \textbf{132}, 15060, (2010)} Accurate chemical potential stability diagrams and formation energies are calculated using the GGA+U method and fitted elemental-phase reference energies.\footnote{V. Stevanovic, S. Lany, X. Zhang, A. Zunger. \emph{Phys. Rev. B.} \textbf{85}, 115104, (2012)} [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A23.00012: X-ray absorption spectroscopy to investigate the doping mechanism in amorphous Cu$_2$ZnSnS$_4$ thin films Sin Cheng Siah, Rupak Chakraborty, Peter Erslev, Glenn Teeter, Chenjun Sun, Tonio Buonassisi Recently, Teeter \textit{et al.} at NREL have discovered that Cu$_{2}$ZnSnS$_{4}$ thin films, of interest for photovoltaics, are amorphous (a-CZTS) when grown at room temperature and the film resistivity can be tuned over a wide range by controlling the Cu:Sn ratio. Tetrahedrally-coordinated amorphous semiconductors belong to an interesting class of compounds that are predicted to have the ability of being doped both $p$- and $n$-type. The four-fold coordination plays a critical role in unpinning the Fermi level to allow effective control over doping levels in a disordered structure. We performed extended X-ray absorption fine structure spectroscopy at the $K$-edges of Cu, Zn and Sn to determine the extent of structural disorder and tetrahedral coordination in a-CZTS films grown with varying Cu:Sn content. All films exhibit a high degree of structural disorder beyond the cations' first coordination shell. Both Cu and Zn atoms have high degree of tetrahedral coordination with respect to S atoms while the average coordination number of Sn decreases with increasing Sn content, indicative of either the favorable formation of sulfur vacancies around Sn atoms or the presence of Sn-related secondary phase. We combine these results with conductivity measurements to understand the relationship between the structural and electrical properties of this new material. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A23.00013: Phonon-induced spin-spin interactions in diamond nanostructures: application to spin squeezing Steven Bennett, Norman Yao, Johannes Otterbach, Peter Zoller, Peter Rabl, Mikhail Lukin We propose a novel mechanism for long-range spin-spin interactions in diamond nanostructures. The interactions are mediated by the coupling of electronic spins, associated with nitrogen vacancy centers, to the vibrational mode of a diamond mechanical nanoresonator. This results in phonon-mediated effective spin-spin interactions that can be used to generate squeezed states of a spin ensemble. We develop an approach combining spin echo techniques and coherent mechanical driving to suppress spin dephasing and relaxation, and find that substantial squeezing is possible under realistic experimental conditions. Our results have implications for spin-ensemble magnetometry, as well as phonon-mediated quantum information processing with spin qubits. [Preview Abstract] |
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