Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G44: Focus Session: Defects in Semiconductors: Characterization |
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Sponsoring Units: DMP FIAP Chair: Yong Zhang, University of North Carolina at Charlotte Room: Mile High Ballroom 4C |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G44.00001: Angstrom resolved imaging of charge percolation through the interface between phosphorous doped crystalline silicon and silicon dioxide Kapildep Ambal, Philipp Rahe, Clayton C. Williams, Christoph Boehme Using a high resolution ($\approx$100fm/$\sqrt{\mathrm{Hz}}$ spectral noise density) scanning probe at T$\approx$4K, we measure currents through the interface between phosphorus doped ([P] $\approx$ 10$^{17}$-10$^{18}$ cm$^{-3}$) crystalline silicon and a native silicondioxide layer as a function of either the lateral cantilever position or the applied cantilever bias voltage (c-AFM imaging). These measurements visualize the percolation of charge through the interface and they show that local current maxima exist in patch-like structures of $\approx$30nm diameter, randomly distributed with an average distance between the centers of 30-40 nm. We associate these with P donor electron states. Within the patch-like structures, we observe additional, extremely localized ($\approx$5{\AA}), current maxima. We associate those to silicon dangling bonds at the interface or within the silicondioxide. The hypothesized association of these very reproducible features is tested by current-voltage (I-V) measurements. For any randomly chosen surface position, these measurements reveal one of only four qualitatively distinct I-V responses, each of which is identified with charge percolation from P donors to the cantilever either with or without different kinds of silicon dangling bond involvement. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G44.00002: Imaging of the native inversion layer in Silicon-On-Insulator wafers via Scanning Surface Photovoltage: Implications for RF device performance Daminda Dahanayaka, Andrew Wong, Philip Kaszuba, Leon moszkowicz, James Slinkman Silicon-On-Insulator (SOI) technology has proved beneficial for RF cell phone technologies, which have equivalent performance to GaAs technologies. However, there is evident parasitic inversion layer under the Buried Oxide (BOX) at the interface with the high resistivity Si substrate. The latter is inferred from capacitance-voltage measurements on MOSCAPs. The inversion layer has adverse effects on RF device performance. We present data which, for the first time, show the extent of the inversion layer in the underlying substrate. This knowledge has driven processing techniques to suppress the inversion. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G44.00003: Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy Neil Curson, Kitiphat Sinthiptharakoon, Steven Schofield, Philipp Studer, Veronika Brazdova, Cyrus Hirjibehedin, David Bowler We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sample at 77 K, using STM and STS. We observe a number of different dopant related features that fall into two classes, which we call As1 and As2. The As1 features are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D$+)$ through electrostatic ionisation when imaged under empty state conditions. DFT calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The appearance of the As2 features surprisingly suggests they are negatively charged at all biases. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G44.00004: Optical and electrical manipulation of a single bi-stable Si-atom in GaAs Paul Koenraad, Erwin Smakman We will show that a Si atom in the outermost layer of GaAs has a bi-stable character much alike the well-known DX-center in Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As. In the ground state the Si atom is negatively charged and in the excited metastable state it is positively charged. These two charge states are related to a modification of the bond configuration of the Si atom in the GaAs surface layer. The voltage dependence of this bi-stable character can be used to bring the Si atom in either of the two states while probing it with an STM tip. The electrical excitation and relaxation processes were studied by analyzing the current and voltage dependence of the observed Random Telegraph Noise. We have successfully used this to create a memory element based on a single impurity atom. Our low T STM setup allows to illuminate the tunneling area and/or to collect tunneling induced photons from the area below the STM tip. We will show our recent results with the optical manipulation of the bond configuration and corresponding charge state of a single bi-stable Si atom as a function of the excitation wavelength (E.P. Smakman et al. PRB \textbf{87} 085414 (2013)). This allowed us to unravel different pathways for the excitation and relaxation processes that are involved in this optical manipulation. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G44.00005: Observation of the exciton in low-temperature-grown GaAs using four-wave mixing Daniel Webber, Luke Hacquebard, Murat Yildirim, Sam March, Reuble Mathew, Angela Gamouras, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna, Kimberley Hall Low-temperature-grown (LT) semiconductors are the materials of choice in optoelectronic devices such as fast photodetectors and THz sources and detectors owing to their unique photoconductive properties tied to the presence of antisite defects . Recent experiments have provided insight into the carrier trapping processes responsible for the subpicosecond recovery times in these systems, as well as the relevance of band-tail transitions in the vicinity of the band gap; However, little is known about the coherent interband response in low-temperature-grown systems. Here we report the application of femtosecond four-wave mixing techniques to LT-GaAs. Our experiments reveal a clear response associated with bound excitons despite the absence of any such feature in linear spectroscopy studies on LT-GaAs. Experiments performed over a wide range of conditions indicate that carrier-carrier scattering dominates dephasing for carriers above the band gap, and that the exciton response tied to excitation-induced dephasing may be quenched in the presence of a prepulse with a sufficiently high fluence. Our findings provide new insight into the optical response of LT-GaAs in the ultrafast nonlinear regime applicable to the operating conditions of optoelectronic devices. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G44.00006: Evolution of transition metal dopant properties near the GaAs surface Anne Benjamin, Jay Gupta As electronic devices get smaller, individual defects become more important, and surface layers and interfacial regions constitute a greater proportion of the material in the device. We use scanning tunneling microscopy (STM) to examine the layer dependence of electronic properties of near-surface level 3d transition metals in gallium arsenide. Some transition metal dopants have been shown to have layer-dependent properties; manganese atoms are deep defects in surface layers and shallow defects in the bulk, while surface-layer zinc atoms can be shallow or deep defects depending on the proximity of other, subsurface zinc defects. By depositing a selected metal on our GaAs sample, and annealing to diffuse defects into the first few layers, we can study the layer dependence of a wide range of defects. At present, the different surface and subsurface properties of defects in semiconductors can only be studied after doping during growth or through individual placement of atoms. A diffusion method will allow for greater flexibility in the type and density of defects able to be studied with STM. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G44.00007: Sub-surface minority carrier lifetime mapping in photovoltaic materials Invited Speaker: P. James Schuck The minority carrier lifetime is considered one of the most critical and variable parameters in photovoltaic materials. However, accurately measuring its value is one of the great challenges in evaluating unconventional semiconductor materials for PV applications. I will describe our two-photon time-resolved photoluminescence decay measurements, which allow us to decouple surface and bulk recombination processes even in unpassivated samples. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10X and show that two-photon excitation more-accurately measures the bulk lifetime. I will finish by discussing how this technique enables the generation of three-dimensional minority carrier lifetime and charge collection efficiency maps that will be useful in identifying efficiency bottlenecks for new and conventional (e.g. CdTe {\&}~CIGS) thin film PV materials. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G44.00008: Imaging Long-Range Carrier Diffusion Across Grains in Polycrystalline CdTe Kirstin Alberi, Brian Fluegel, Helio Moutinho, Ramesh Dhere, Jian Li, Angelo Mascarenhas The use of polycrystalline semiconductors in electronic devices enables low cost fabrication on large area substrates. Understanding the extent to which structural defects and impurities influence carrier transport in these materials is increasingly important as device performance is maximized, but most conventional characterization techniques often cannot directly probe their effects. We have applied a novel photoluminescence imaging technique to directly observe carrier diffusion in the presence of grain boundaries and impurities in poly-CdTe films. Our results show that the grain boundaries in this material are relatively transparent to free carrier and exciton diffusion as compared to poly-GaAs. Furthermore, a network of inhomogeneously distributed impurity states is found to mediate hole transport across multiple grains to distances greater than 10 microns from the point of photogeneration. These results underscore the importance of controlling the concentration and distribution of impurity states in poly-CdTe thin film solar cells. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G44.00009: Atomic-resolution study of dislocation structures and interfaces in poly-crystalline thin film CdTe using aberration-corrected STEM Tadas Paulauskas, Eric Colegrove, Chris Buurma, Moon Kim, Robert Klie Commercial success of CdTe-based thin film photovoltaic devices stems from its nearly ideal direct band gap which very effectively couples to Sun's light spectrum as well as ease of manufacturing and low cost of these modules. However, to further improve the conversion efficiency beyond 20 percent, it is important to minimize the harmful effects of grain boundaries and lattice defects in CdTe. Direct atomic-scale characterization is needed in order identify the carrier recombination centers. Likewise, it is necessary to confirm that passivants in CdTe, such as Cl, are able to diffuse and bind to the target defects. In this study, we characterize dislocation structures and grain boundaries in poly-crystalline CdTe using aberration-corrected cold-field emission scanning transmission electron microscopy (STEM). The chemical composition of Shockley partial, Frank and Lomer-Cottrell dislocations is examined via atomic column-resolved X-ray energy dispersive (XEDS) and electron energy-loss spectroscopies (EELS). Segregation of Cl towards dislocation cores and grain boundaries is shown in CdCl2 treated samples. We also investigate interfaces in ultra-high-vacuum bonded CdTe bi-crystals with pre-defined misorientation angles which are intended to mimic grain boundaries. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G44.00010: Defect Energy Levels in GaAsBi and GaAs Grown at Low Temperatures Patricia Mooney, Keelan Watkins, Zenan Jiang, Alberto Basile, Ryan Lewis, Vahid Bahrami-Yekta, Mostafa Masnadi-Shirazi, Daniel Beaton, Thomas Tiedje GaAs$_{1-x}$Bi$_x$ alloys have the potential to extend conventional III-V semiconductor devices to longer infrared wavelengths. The bandgap energy decreases as the Bi fraction is increased, but with a small increase in lattice constant, thus reducing lattice mismatch constraints for GaAsBi/GaAs heterostructures. However, Bi is incorporated into GaAs films grown by molecular beam expitaxy (MBE) only at T$_G$ $<$400 $^{\circ}$C, making defects a concern. DLTS measurements show that trap concentrations in Si-doped (n-type) GaAs layers grown at standard temperatures are $<$4x10$^{13}$ cm$^{-3}$. They increase to 2x10$^{16}$ cm$^{-3}$ when T$_G$ is 390 $^{\circ}$C and to $\sim$10$^{18}$ cm$^{-3}$ when T$_G$ is 330 $^{\circ}$C, where the energy level of the dominant defect is E$_C$-0.40 eV. When only 0.3\% Bi is incorporated into n-type GaAs at 330 $^{\circ}$C, formation of the E$_C$-0.40 eV trap is suppressed. Other electron traps, including the dominant traps having energy levels at E$_C$-0.66 eV and E$_C$-0.80 eV, are present in similar concentrations in both GaAs and GaAsBi layers grown at 330 $^{\circ}$C and, therefore, result from the low growth temperature. The dominant traps are both point defect complexes involving an arsenic atom on a gallium lattice site (AsGa). [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G44.00011: A Single Molecule Approach to Defect Studies in ZnO N.R. Jungwirth, Y.Y. Pai, H.S. Chang, E.R. MacQuarrie, G.D. Fuchs Single molecule investigations are a powerful tool for understanding molecular systems with inhomogeneous behavior that is either broadened or completely washed out of ensemble measurements. Here we apply single molecule microscopy methods to defects in ZnO. In addition to its status as an emerging optoelectronic material, ZnO hosts point defects which may have useful quantum properties akin to those of nitrogen-vacancy centers in diamond, which are promising as single photon sources and solid-state qubits. We present confocal fluorescence measurements of single defects in ZnO nanoparticles and sputtered films that are selectively excited by sub-bandgap light. The resulting 560-720 nm emission often exhibits two broad spectral peaks separated by approximately 100 meV. Photon correlation measurements yield both antibunching and bunching, indicative of single photon emission from isolated defects with a metastable shelving state. Excited state lifetimes span 1-13 ns and are uncorrelated with doping concentration. We report discrete jumps in the fluorescence intensity between a bright and dark state. The dwell times are exponentially distributed in each state and the average dwell time in the bright (dark) state does (may) depend on the power of the excitation laser.~ [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G44.00012: Room-Temperature Plasticity in ZrC: Role of Crystal Anisotropy Christian Ratsch, S. Kiani, A.M. Minor, S. Kodambaka, J.M. Yang NaCl structure Group IV and V transition-metal carbides are hard, stiff, and high-melting solids with excellent wear, abrasion, and corrosion resistances, and are commonly used in advanced high-temperature structural applications. In this talk, we report results obtained from in situ transmission electron microscopy (TEM) studies and density functional theory calculations of uniaxial compression of ZrC(100) and ZrC(111) single crystals. In situ TEM observations show that dislocation motion and tangling lead to plastic deformation in ZrC(111), while slip along \textbraceleft 110\textbraceright \textless 1-10\textgreater is dominant in ZrC(100). We find that the yield strengths of ZrC crystals increase with decreasing size. Interestingly, yield strengths of uniaxially compressed ZrC(111) crystals are lower than those of ZrC(100), unexpected for NaCl-structured compounds. Based upon density-functional theory calculations, we attribute the orientation-dependent yield strengths to relatively lower energy barrier for shear along \textbraceleft 001\textbraceright \textless 1-10\textgreater compared to \textbraceleft 110\textbraceright \textless 1-10\textgreater . Our results provide important insights into the effects of crystal size and orientation on room-temperature plasticity. We expect that similar phenomena are likely to exist in other cubic-structured transition-metal carbides and nitrides. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G44.00013: Carrier dynamics in sulfur-hyperdoped silicon studied by time-resolved terahertz spectroscopy Meng-Ju Sher, Christie Simmons, Austin Akey, Mark Winkler, Daniel Recht, Tonio Buonassisi, Michael Aziz, Aaron Lindenberg Intermediate-band photovoltaics have been proposed to enhance efficiencies of solar cells by harvesting additional energy from sub-bandgap photons. One proposed method for fabricating an IB material is by introducing deep-level dopants at concentrations above the insulator-to-metal transition (IMT). Theory suggests that as the dopant states become delocalized, the non-radiative recombination is suppressed and the lifetime recovery enables photo-generated carriers to be harvested. We use optical-pump/terahertz-probe spectroscopy to study carrier dynamics of sulfur-hyperdoped silicon and test whether lifetime recovery is possible in this material system. S-hyperdoped silicon exhibits strong sub-bandgap light absorption and IMT at S concentration above $2 \times 10^{20}$ cm$^{-3}$. Previous photoconductivity study suggests the lifetime is less than 130 ps for samples at concentrations below IMT. Time-resolved THz spectroscopy is suitable for studying carrier dynamics on short time scales. We use a 400-nm fs-laser pulse to generate carriers and by monitoring the transmission of the THz probe as a function of time, we extract the carrier dynamics and mobility all-optically. [Preview Abstract] |
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