Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D12: Focus Session: Dopants and Defects in Semiconductors: Compound Semiconductors II |
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Sponsoring Units: DMP Chair: Mao-Hua Du, Oak Ridge National Laboratory Room: D223/224 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D12.00001: Defect energy distribution in GaN/AlGaN heterostructures grown in Ga-rich and ammonia-rich conditions Tania Roy, Yevgeniy Puzyrev, Enxia Zhang, Daniel Fleetwood, Ronald Schrimpf, Sokrates Pantelides We use low-frequency noise measurements to estimate energy distributions of electrical-stress-induced defects in AlGaN/GaN high electron mobility transistors from 85 K to 450 K. The devices were grown under Ga-rich and ammonia-rich conditions using molecular beam epitaxy. The Ga-rich devices show a positive shift in pinch-off voltage and a decrease in gate leakage current with stress under a gate voltage of -3.6 V and a drain voltage of 20 V. These changes in response are associated with hydrogenated Ga vacancies in AlGaN. The ammonia-rich devices show a negative shift in pinch-off voltage and an increase in gate leakage current under the same stress conditions; these changes in device response are caused by N-antisite defects. The excess drain voltage power spectral density of the low frequency 1/$f$ noise peaks at $\sim $ 100 K in both device types, which corresponds to a trap energy of 0.2 eV. We attribute this to N-vacancy-related defects, which are also observed in proton damage experiments. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D12.00002: Thermal electron capture rate by Fe acceptor in GaN J. Dashdorj, M.E. Zvanut, T. Paskova, K. Udwary Doping GaN with Fe compensates the main residual impurities such as O and Si to produce semi-insulating substrates. Electron paramagnetic resonance measurements were made on GaN grown by hydride vapor phase epitaxy and doped with 1.5x10$^{17}$ to 1.6x10$^{18}$ cm$^{-3}$ Fe. The Fe$^{3+}$ spectra, angular dependence, and concentrations are consistent with literature and secondary ion mass spectroscopy data. During illumination with photon energies greater than 1.2 eV, the Fe$^{3+}$ signal increased in the lowest doped sample, but decreased in the more highly doped samples. One possible interpretation of the results is that the Fe$^{2+/3+}$ and Fe$^{3+/4+}$ levels are about 1.2 eV below the conduction band. Due to our measurement resolution, the spectral separation between the levels cannot be determined. The time-dependence of the Fe$^{3+}$ signal recovery after removal of 2.64 eV was recorded at temperatures between 3.5 and 297 K. Analysis show that capture rate of electrons by Fe$^{3+}$ decreases from 6x10$^{-16}$ to 5x10$^{-17}$ cm$^{3}$/s with an inverse-square-root temperature dependence. The work is supported by the NSF. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D12.00003: Direct atomic imaging of Mn in the GaN growth surface: High-density, Two-dimensional, Striped Superstructures Kangkang Wang, Noboru Takeuchi, Abhijit Chinchore, Wenzhi Lin, Arthur Smith A class of novel well-ordered striped superstructures have been observed by depositing submonolayer Mn onto GaN(0001)-``1$\times$1'' surface. These superstructures consist of stripe domains along [1$\bar{1}$00]$_{GaN}$ with various widths, while scanning tunneling microcopy images resolved a common local $\sqrt{3}$$\times$$\sqrt{3}$-R30$^{\circ}$ structure for the stripes. Combined with first-principles calculations, a new two-dimensional structural model is proposed having a dense Mn$_{x}$Ga$_{1-x}$ surface layer. Mn atomic sites within the GaN surface are directly identified. A spin-induced asymmetry in the Mn electronic structure is revealed in real-space for the narrow stripes. These findings explain the behavior of Mn atoms in the GaN growth surface and herald the development of magnetic nanostructures on GaN surfaces. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D12.00004: Initial Phase of Sub-monolayer Iron Growth on GaN(0001) pseudo-1$\times $1-1+1/12 Surface Studied Using Scanning Tunneling Microscopy and First Principles Theoretical Calculations Wenzhi Lin, Noboru Takeuchi, Kangkang Wang, Abhijit Chinchore, Meng Shi, Arthur Smith, Hamad Albrithen Iron/gallium nitride bi-layer structures have potential use for spintronic applications. Therefore, we have carried out an investigation of the initial phase of sub-monolayer iron growth on GaN(0001) pseudo-1$\times $1-1+1/12 surface. To begin with, we verified an atomically smooth GaN growth surface with the assistance of \textit{in situ} reflection high energy electron diffraction. STM shows smooth terraces separated by single and double height bilayer atomic steps. About 0.4 ML iron was deposited on the smooth GaN, and the subsequent STM images reveal Fe islands with a height of $\sim $ 2 {\AA} growing in a two-dimensional step-flow mode outward from the GaN step edges of the pseudo-1x1-1+1/12 surface. A clear 6 $\times $ 6 structure is observed for the islands. First principles theoretical calculations are being carried out in order to interpret the experimental results. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D12.00005: Atomic displacements in proton-irradiated AlGaN/GaN heterostructures Yevgeniy Puzyrev, Tania Roy, Enxia Zhang, Ronald Schrimpf, Daniel Fleetwood, Sokrates Pantelides We report results of quantum molecular dynamics calculations of atomic recoils in AlGaN and GaN. The recoil energy required to create defects in a perfect AlGaN/GaN lattice is known to be over 40eV. However, drastic changes in atomic configuration occur when defect atom itself recoils with than 10eV. We show that both N antisite defects and N atoms near Ga vacancy require less than 10 eV to introduce N vacancies, divacancies and N interstitials. This phenomenon leads to additional donors that can account for a positive shift in threshold voltage, observed in our electrical measurements in AlGaN/GaN devices irradiated by 1.8 MeV protons.\footnote{T. Roy, et. al.,~\textit{IEEE Trans. Nucl, Sci.}, 2010. accepted} In addition, divacancies and N vacancies have an electron transition level near the Fermi level in AlGaN which also provides explanation for the experimentally observed increase in 1/f noise after proton irradiation.\footnote{T. Roy, et al,~\textit{Microelectron. Reliab.}, 2010, accepted.} [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D12.00006: Nanoscale Potential Fluctuations in (GaMn)AsGaAs Heterostructures: From Individual Ions to Charge Clusters and Electrostatic Quantum Dots Paul Koenraad, Ineke Wijnheijmer, Jens Garleff, Oleg Makarovsky, Laurence Eaves, Richard Campion, Bryan Gallagher During growth of the dilute p-type ferromagnetic semiconductor GaMnAs, interstitial manganese is formed when the Mn concentration exceeds 2{\%}. This interstitial Mn acts as a double donor which compensates the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of interstitial Mn in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual interstitial Mn ions, and show that the ions are doubly charged. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D12.00007: Controlled layer-by-layer depth-profiling of GaAs(110) using scanning tunneling microscopy David Gohlke, Donghun Lee, Jay Gupta The electronic properties of dopants in semiconductors such as GaAs vary depending on proximity to interfaces. We utilize a low temperature (5K) scanning tunneling microscope to realize a layer-by-layer peeling technique on p-GaAs(110). We apply positive voltage pulses near As vacancies to desorb surface-layer Ga and As atoms. Subsequent motion of the STM tip peels away the first layer from this starting point, fully exposing sections of the subsurface layer. The second and further layers can be readily peeled away by the same technique. This newly created pit allows depth-profiling of subsurface defects with STM. Funded by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (DMR-0820414). http://www.physics.ohio-state.edu/$\sim$jgupta [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D12.00008: Time-Resolved Far-Infrared Magnetospectroscopy of Electron Relaxation in GaAs S.N. Gilbert, G.L. Carr We report time-resolved magnetospectroscopy results for (S-I) GaAs at T$\sim$10K and fields up to 10T. A pulsed Ti:sapphire laser produces photoelectrons with energy $\sim$10 meV above the conduction band minimum that are subsequently probed by far-infrared transmission spectroscopy. Both free electrons and exciton transitions are observed, including transitions involving Landau levels when the magnetic field is applied. We also observe a time-dependent change in these transitions and discuss a model for the relaxation of a warm (non-equilibrium) distribution of electrons on a $\sim$500 ps time scale. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D12.00009: Tunable control over the ionization state of single Mn acceptors in GaAs with defect-induced band bending Donghun Lee, Jay Gupta The continuous miniaturization of semiconductor devices will ultimately reach a point where control over the properties of single dopants is necessary. Recent STM studies have demonstrated the ability to control the ionization state of single dopants through tip-induced band bending. This change in ionization state appears in STM images as a ring-like feature centered on the dopant, whose diameter depends on voltage, tip-sample distance, and tip termination. Here we demonstrate an additional degree of freedom for controlling the charge state of single Mn acceptors in GaAs by utilizing nearby charged defects which can be positioned with atomic precision. Systematic changes in the ring diameter with the separation between Mn and defect allows us to separately extract contributions from ~defect-induced and tip-induced band bending. These methods provide non-volatile control over the ionization state of single dopants, even in the absence of probe electrodes or STM tip. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D12.00010: Identification of the major cause of endemically poor mobilities in SiC/SiO$_{2}$ structures Xiao Shen, Sokrates T. Pantelides Mobility degradation at semiconductor-dielectric interfaces is generally attributed to defects at the interface or inside the dielectric, as is the case in Si/SiO$_{2}$ structures. In the case of SiC/SiO$_{2}$ structures, a decade of research focused on reducing or passivating interface and oxide defects, but low mobilities have persisted. It is known that during oxidation of Si, Si atoms are emitted into the substrate, but they do not form strongly-bonded complexes and their effects are usually benign. In contrast, during oxidation of SiC, C atoms are emitted into the substrate and they can form strongly-bonded carbon complexes. Here we identify one particular complex that explains a range of experimental defect signatures and electrical measurements. We propose that this complex is a major cause of the poor mobility in SiC/SiO$_{2}$ structures. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D12.00011: Electronic properties of Si-C interfaces Xiang-Guo Li, Hai-Ping Cheng In this work, we report our investigations of interfacial properties of Si-C systems. Electronic properties of Fe-doped carbon on silicon surfaces, Si-Fe-C layered structures and Si-graphene-Si junctions have been studied using first-principles calculations. Charge transfer at the interfaces, densities of states, and magnetization are fully analyzed. These problems are important because recent experiments show that Fe@C-Si materials have giant electro-resistance and magneto-resistance highly sensitive to the external magnetic field. The non-magnetic feature leads to very small magnetic noise. In addition, photovoltaic effects were also observed in some of these systems. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D12.00012: Polariton Formation Enhances Lifetimes of Dense Exciton Gasses in Cuprous Oxide by Suppressing Two-Exciton Decay N. Laszlo Frazer, R.D. Schaller, J.I. Jang, S.E. Mani, J.B. Ketterson Collective excitonic states form at high densities, but in dense gases the long lifetime of excitons in cuprous oxide (Cu$_{2}$O) is compromised by two-exciton annihilation processes. Using the picosecond streak camera spectroscopy facility at the Center for Nanoscale Materials at Argonne National Laboratory we directly measured the decay of orthoexciton-polaritons generated by two photon absorption. The two-body decay lifetime is an order of magnitude longer than for excitons uncoupled to photons. The extended lifetime opens opportunities for experiments that manipulate polariton collective states. Unlike time-averaged measurements of two-body processes, streak camera detection shows decay suppression without effects from production efficiency or additional density dependent processes. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D12.00013: Ultrafast carrier dynamics in Bi2Se3 thin films Keliang He, Liguo Zhu, Chen Xia, Brian Kubera, Jie Shan Bismuth Selenide (Bi2Se3), a group V-VI narrow gap layered semiconductor, is a well-known efficient solid thermoelectric material at room temperature. It has recently also attracted much research attention due to its interesting topological properties. The carrier dynamics and charge transport, electron-phonon coupling, and its role in the transport properties in Bi2Se3 are fundamental issues in understanding its thermoelectric and topological properties. In this work, we employ the optical-pump terahertz-probe technique to study the transient photoconductivity in Bi2Se3 thin crystalline films as a function of the pump-probe delay time and the excitation fluence. The photoconductivity spectrum ranging from 0.3 to 1.9 THz reveals both a Drude and a Lorentz contribution. The former is attributed to a free electron response with a scattering time of 0.7 ps; and the latter, with both its amplitude and peak frequency dependent on the carrier density, arises from a coupled LO phonon-plasmon mode. The nature of this mode and its role in transport will be discussed. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D12.00014: Oxygen in B$_{2}$O$_{3}$ covered Czochralski-grown Ge Toshinori Taishi, Hideaki Ise, Yu Murao, Takayuki Ohsawa, Yuki Tokumoto, Yutaka Ohno, Ichiro Yonenaga Ge has been regained keen interest in applications of Ge for ultra-fast CMOS and PV devices. High quality Ge crystals should be demanded for realization of such devices with higher performances in these circumstances. Oxygen impurity can be expected to enhance thermo-mechanical stability of Ge crystals due to dislocation locking similar to oxygen in Si. For the purpose, we grew oxygen-enriched Ge crystals by the Czochralski method from B$_{2}$O$_{3}$ covered melt added with GeO$_{2}$ powder in a silica crucible. To evaluate precious knowledge oxygen behavior in Ge, local vibrations of oxygen were evaluated by FT-IR spectroscopy. Concentrations of interstitially dissolved oxygen impurity in the crystals were in the range between 8.5 $\times $ 10$^{15}$ and 5.5 $\times $ 10$^{17}$ cm$^{-3}$ determined from the FT-IR absorption at 855 cm$^{-1}$ originating in local vibration of Ge-O$_{i}$-Ge quasi-molecules. Absorption peaks relating to GeO$_{x}$, SiO$_{x}$ and Si-Oi-Si were not detected in the as-grown crystals. By prolonged annealing at 350{\_}C, an absorption peak developed at 780 cm$^{-1}$, indicating formation of oxygen related thermal donors. Such donors disappeared by annealing at 550{\_}C. [Preview Abstract] |
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