Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C23: Focus Session: Dopants and Defects in Semiconductors III |
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Sponsoring Units: DMP Chair: Matt McCluskey, Washington State University Room: 325 |
Monday, March 18, 2013 2:30PM - 3:06PM |
C23.00001: Highly Efficient Defect Emission from ZnO:Zn and ZnO:S Powders Invited Speaker: Henry Everitt Bulk Zinc Oxide (ZnO) is a wide band gap semiconductor with an ultraviolet direct band gap energy of 3.4 eV and a broad, defect-related visible wavelength emission band centered near 2 eV. We have shown that the external quantum efficiency can exceed 50{\%} for this nearly white emission band that closely matches the human dark-adapted visual response. To explore the potential of ZnO as a rare earth-free white light phosphor, we investigated the mechanism of efficient defect emission in three types of ZnO powders: unannealed, annealed, and sulfur-doped. Annealing and sulfur-doping of ZnO greatly increase the strength of defect emission while suppressing the UV band edge emission. Continuous wave and ultrafast one- and two-photon excitation spectroscopy are used to examine the defect emission mechanism. Low temperature photoluminescence (PL) and PL excitation (PLE) spectra were measured for all three compounds, and it was found that bound excitons mediate the defect emission. Temperature-dependent PLE spectra for the defect and band edge emission were measured to estimate trapping and activation energies of the bound excitons and clarify the role they play in the defect emission. Time-resolved techniques were used to ascertain the role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and non-radiative traps. In unannealed ZnO we find that defect emission is suppressed and UV band edge emission is inefficient (\textless\ 2{\%}) because of reabsorption and non-radiative recombination due to a high density of non-radiative bulk traps. By annealing ZnO, bulk trap densities are reduced, and a high density of defects responsible for the broad visible emission are created near the surface. Interestingly, nearly identical PLE spectra are found for both the band edge and the defect emission, one of many indications that the defect emission is deeply connected to bound excitons. Quantum efficiency, also measured as a function of excitation wavelength, closely mirrors the PLE spectra for both emission bands. Sulfur-doped ZnO exhibits additional PLE and X-ray features indicative of a ZnS-rich surface shell that correlates with even more efficient defect emission. The results presented here offer hope that engineering defects in ZnO materials may significantly improve the quantum efficiency for white light phosphor applications. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C23.00002: Optical and Magnetic Resonance Studies of Na-Diffused ZnO Bulk Single Crystals E.R. Glaser, N.Y. Garces, N.S. Parmar, K.G. Lynn Photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24 GHz were performed on bulk ZnO crystals after diffusion of Na impurities that were explored as an alternate doping source for p-type conductivity. PL at 2K revealed strong bandedge excitonic recombination at 3.361 eV and a broad ``orange'' PL band at 2.17 eV with FWHM of $\sim$0.5 eV. This ``orange'' emission is very similar to that reported previously\footnote{D. Zwingel and F. Gartner, Solid State Commun. 14, 45 (1974).} from thermoluminescence measurements of intentionally Na-doped bulk ZnO and, thus, strongly suggests the incorporation and activation of the Na-diffused impurities. ODMR performed on this ``orange'' PL revealed two signals. The first was a sharp feature with g-value of $\sim$1.96 and is a well-known ``fingerprint'' of shallow donors in ZnO. The second signal consisted of a pair of lines with an intensity ratio of $\sim$3:1 and with g-tensors (g$_{\parallel}$,g$_{\perp}$$\sim$2.008-2.029) very similar to ESR signals attributed previously\footnote{Ibid.} to holes bound to Na impurities located at the axial and non-axial Zn host lattice sites in Na-doped ZnO. Thus, the ``orange'' PL can be tentatively assigned to radiative recombination between residual shallow donors and deep Na-related hole traps. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C23.00003: Quantum Emission from Defects in ZnO N.R. Jungwirth, E.R. MacQuarrie, H.S. Chang, G.D. Fuchs Single defects in wide bandgap semiconductors, such as nitrogen-vacancy centers in diamond, are promising candidates for solid state qubits and single photon sources.~ Additionally, single defect studies provide an opportunity to probe properties and dynamics that are washed out of ensemble measurements. Despite the wealth of available semiconducting hosts, investigations of isolated defects in semiconductors other than diamond are limited. Here we present confocal photoluminescence measurements of ZnO nanocrystals that are excited with below bandgap light to selectively address individual deep levels. In addition to wavelength-resolved and time-resolved photoluminescence measurements, we report photon anti-bunching that is consistent with quantum emission from isolated defects. These measurements, made at the single and few defect level, enable insight into long-standing questions that surround defect emission in ZnO. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C23.00004: Rare-earth doped Si-rich ZnO for multiband near-infrared light emitting devices Emanuele Francesco Pecora, Thomas Ian Murphy, Luca Dal Negro Transparent Conductive Oxides (TCOs) are a broad class of organic and inorganic materials exhibiting both optical transparency and electrical conductivity simultaneously. TCOs are utilized as top-con tact passive layers in a number of optoelectronic devices, including flat panel displays and solar cells. Recently, they are also attracting considerable attention as an active platform for a wide range of novel device applications. Zinc oxide (ZnO) is the most promising candidate for optoelectronic integration due to its low cost and Si compatibility. Moreover, it is a biocompatible material and possibly biodegradable. We fabricated rare earth-doped Si-rich ZnO thin films through magnetron sputtering and we investigate their near-infrared emission properties under both optical and electrical injection. Er and Nd efficient (3ms RT lifetime) radiative transitions were simultaneously activated due to energy transfer via the ZnO direct bandgap and its luminescent defect centers. Moreover, by incorporating Si atoms, we demonstrate Si-mediated enhancement of light emission in Er-doped ZnO, and electroluminescence. We fabricated a proof-of-concept 1.55$\mu $m-electroluminescent device with record low turn-on voltage (\textless 1.5V) in Er-doped Si-rich ZnO at room temperature. These results pave the way to novel Si-compatible light emitters that leverage the optically transparent and electrically conductive ZnO matrix for multiband near-IR telecom and bio-compatible applications. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C23.00005: Synthesis of ZnO:Ge Thin Films via Plasma Gas Condensation Abdullah Ceylan, Janan Ali, Sadan Ozcan we introduce a new method for the synthesis of Ge nanoparticle embedded ZnO thin films that are considered to be a potential candidate for photovoltaic applications. As opposed to current techniques, for the independent preparation of Ge nanoparticles, Cluster Deposition Source (CDS) utilising gas condensation of sputtered Ge atoms is used. For the synthesis of ZnO thin film host material conventional sputtering is employed. In the proposed technique independently synthesized Ge nanoparticles and ZnO thin films are combined into a composite structure on Si. XRD patterns of the samples have revealed that Ge nanoparticles preferentially settle on (113) planes on top of the (002) oriented ZnO layer. It is realized that Ge nanoparticles with sizes ranging from 16 nm to 20 nm could be embedded into a well-defined ZnO matrix. In fact, TEM studies performed on Ge nanoparticles captured on a Cu grids have manifested that Ge reach to ZnO matrix as clusters composed of particles with sizes of about 7-8 nm and then eventually grow larger due to substrate heating implemented during capping layer deposition. Optical absorption measurements have revealed that Ge nanoparticle inclusion lead to an additional absorption edge at about 2.75 eV along with 3.17 eV edge resulting from ZnO host. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C23.00006: Identification of a N-related shallow acceptor and related EPR center in ZnO: N2 on Zn site Walter R.L. Lambrecht, Adisak Boonchun While the deep level of N$_{\rm O}$ makes it unsuitable for p-type doping of ZnO, a shallow level at about 165$\pm40$ meV above the VBM related to N is known to exist in ZnO (Zeuner et al. 2002). Here we show that a N$_2$ molecule on the Zn site behaves as a shallow acceptor. First-principles calculations show that when N$_2$ is placed on a Zn site, two electrons are removed from the $\sigma_g^+$ HOMO. The molecular levels line up with the ZnO band strucure in such a way that the $\sigma_g^+$ level forms a resonance near the VBM. In contrast, for N$_2$ on the O-site, two extra electrons occupy the $\pi_g$ LUMO of the N$_2$ molecule and form a donor level. The 0/- transition level of the acceptor is found at $\sim$0.2 eV above the VBM. When singly occupied the defect corresponds to a N$_2^+$ radical. We show that the $g$-factor, calculated within a simple tight-binding model, of this radical agrees better with an observed EPR center by Garces et al. (2003) than the N$_2^-$ radical. The $\sigma_g$ nature of the defect wavefunction for N$_2$ on Zn is consistent with a significant isotropic hyperfine interaction, while the $\pi_g$ character of N$_2$ on O is not. The lower value of $A_{iso}$ compared to the isolated molecule is consistent with the shallow nature of the defect. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C23.00007: Quantum Monte Carlo calculation of point defect thermal and optical ionization levels: application to magnesium oxide and zinc oxide Elif Ertekin, Lucas Wagner, Jeffrey Grossman From electronics to optoelectronics to photovoltaics, point defects influence and dominate the properties of semiconducting materials. Quantitative descriptions of the effect of point defects on electronic, optical, and transport properties are critical to enabling point-defect engineering for materials design. However, accurate prediction of point-defect energetics, thermal ionization energies, and optical transition energies from first principles remains a challenge. We present an approach to calculation of point defect optical and thermal ionization energies based on the highly accurate quantum Monte Carlo methods, and demonstrate it for the oxygen vacancy in the binary ionic compound magnesium oxide and the substitutional nitrogen impurity in zinc oxide. The use of quantum Monte Carlo, an inherently many--body theory that directly treats electron correlation, offers many improvements: it can help overcome the band gap problem in density functional theory and obviate the need for ad-hoc corrections. Our computed optical and thermal ionization energies are in excellent agreement with experimental and/or other high-accuracy results. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C23.00008: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 4:30PM - 4:42PM |
C23.00009: Non-stoichiometric Mn doped ZnO clusters: First principles calculations Sachin P. Nanavati, Shailaja Mahamuni, S.V. Ghaisas, Vijay Kumar It has been reported that cage like hollow clusters of (ZnO)$_n$ with $n$ = 12 \& 34 are stable and hence {\it magic}. Doping Mn impurity in ZnO (ZnO:Mn) clusters is a well studied problem. In most of the studies, single Mn doping has been achieved by substituting it on a surface Zn site, leading to a stoichiometric configuration of Zn$_{n-1}$MnO$_n$ and a large magnetic moment of 5 $\mu_B$. However, we show that using first principles methods, Mn doping would lead to O rich, non-stoichiometric clusters with significantly reduced magnetic moment. Specifically, we show that clusters of configuration Zn$_{12}$MnO$_{15}$ and Zn$_{34}$MnO$_{37}$, obtained when Mn is substituted in (ZnO)$_n$ ($n$ = 13 \& 35) cages, become magic. The magnetic moments in these clusters is reduced to 1 $\mu_B$. These clusters can also be considered as a composite structure where a MnO$_x$ ($x$ = 1 to 4) molecule is attached to ZnO$_n$ ($n$ = 12 \& 34) cages from outside.\footnote{S. P. Nanavati et al. Phys. Rev. B (in press, 2012).} We believe that these results would have important implications for the understanding of magnetism in ZnO:Mn nanostructures as well as thin films, for which recent experiments suggest mixed and higher oxidation states of Mn, {\it viz.,} Mn$^{+3}$ and Mn$^{+4}$. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C23.00010: Theoretical study of preferred dopants for n-type transparent conducting oxides Su-Huai Wei, Chong Li, Jingbo Li, Wanjian Yin, Yanfa Yan Traditionally, it is believed that the conduction band edges of $d^{0}$ or $d^{10}$ oxides are derived mostly from cation states, thus substitutional doping on anion sites is expected to cause less perturbation and produce shallow donor levels in these materials. Using first-principles calculations, we show that although this paradigm is applicable for more covalent oxides such as SnO$_{\mathrm{2}}$ where F$_{\mathrm{O}}$ is a better n-type dopant than Sb$_{\mathrm{Sn}}$, for more ionic oxides such as ZnO, the conduction band edge actually contains a considerable amount of O $s$ orbitals, thus F$_{\mathrm{O}}$ in ZnO causes larger perturbation and consequently produces deeper donor levels than cation site doping such as Al$_{\mathrm{Zn}}$. This observation can be explained by coupling of cation state with high lying oxygen orbitals. The origin of the preferred n-type dopability of oxides, the potential of oxygen vacancy as n-type dopant, and the selection of chemical potential for n-type doping will also be discussed. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C23.00011: Strong enhancement of the luminescence decay time of isoelectronic centers in GaP:N at low temperatures Philippe St-Jean, Gabriel Ethier-Majcher, Alaric Bergeron, Sebastien Francoeur Using time-resolved photoluminescence, the recombination dynamics of excitonic states bound to isoelectronic centers formed by either one or a pair of nitrogen atoms in GaP is investigated as a function of internuclear distance and temperature. Depending on their symmetry, centers formed by a pair of atoms exhibit several optical transitions that are, according to the excitonic state involved, either linearly polarized or unpolarized. At 4 K, for all nitrogen pairs studied, relatively long lifetimes approaching 1 $\mu$s are observed. Interestingly, these lifetimes vary considerably between excitonic states and ranges from 500 to 800 ns. This strong variation decreases with temperature, leading to similar lifetimes. Furthermore, as the temperature is increased to 30 K, all lifetimes decrease by about an order of magnitude, down to 60-90 ns, as previously reported. A thermodynamic model of the evolution of excitonic populations shows that a thermally activated process of about 2.5 meV characterizes this temperature behavior. This activation energy corresponds to an inter-level transfer between excitonic states. These findings enhance our understanding of the dynamics of carriers bound to isoelectronic centers, which are promising candidates for atomic-sized charge storing device. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C23.00012: Mechanism for conduction in polycrystalline p-type indium oxide films Jolanta Stankiewicz, Francisco Villuendas We report (i)- results from {\it ac} impedance measurements obtained for intrinsic indium oxide films, grown under O$_2$-rich conditions, (ii)- current-voltage (I-V) curves for {\it p-n} homojunctions fabricated by sequential growth of a 200 nm thick {\it p}--type In$_2$O$_3$ layer on a 400 nm thick {\it n}--type In$_2$O$_3$, and (iii)- capacitance-voltage (C-V) curves for these junctions. Impedance as well as I-V and C-V measurements were performed under UV irradiation and in darkness. We find two distinct contributions to the {\it ac} conductivity. One of them is brought about by grain boundaries, and the other one by inversion layers, which are on grain surfaces. In addition, we have found that photocurrents relax extremely slowly in these films. All of this fits consistently within a model in which mobile holes in inversion layers are responsible for {\it p}-type {\it dc} conductivity in intrinsic indium oxide films grown under O$_2$-rich conditions. Such mechanism might be important in other polycrystalline thin films which have a large number of oxidizing defects at grain boundaries. [Preview Abstract] |
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