Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A12: Focus Session: Electricity-to-Light Conversion: Solid State Lighting I |
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Sponsoring Units: GERA DMP Chair: Daniel Koleske, Sandia National Laboratory Room: D223/224 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A12.00001: Surface phase matched templates for GaN hetroepitaxial growth Praveen Kumar, Jithesh Kuyyalil, Shivaprasad SM Surface structural modifications are performed on Si(111)-7x7 surface, to find the appropriate template for high quality GaN growth. Adsorption of Ga forms stable superstructural phases of (1x1), (6.3x6.3) and (rt3xrt3) at 1.5ML, 0.8ML and 0.33ML respectively on a (7x7) reconstructed Si(111) surface. Using PA-MBE system, GaN of 0.75microns is grown at a relatively low temperature of 450oC on each of these phases. The films formed grow in the wurtzite phase with c-axis perpendicular to the Si(111) substrate surface. Now XRD, PL, XPS, AFM, FESEM and RHEED are employed to evaluate the structural, optical, compositional and morphological aspects of the GaN films. It is clearly observed that the 0.33ML (rt3xrt3) Ga phase results in the best quality GaN films, followed by the (6.3x6.3) phase and then the (1x1) phase. The (rt3xrt3) unit cell dimension matches with 2xa of GaN unit cell size, and thus GaN grows epitaxially on this surface with oriented single crystal grains. Thus, the results clearly demonstrate the possibility of employing low coverage metal induced surface phases as templates to form matched GaN films of high structural {\&} optical quality. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A12.00002: Quantum Photovoltaics via Coherent Drive Konstantin Dorfman, Anatoly Svidzinsky, Marlan Scully We study the fundamental limit to photovoltaic efficiency that is widely thought to be due to detailed balance between radiative recombination and radiative absorption. Quantum coherence in fact can break the detailed balance yielding vanishing emission of incident resonant radiation with nonzero absorption. This results in the enhancement of the quantum efficiency of the photovoltaic (PV) cell as compared to the ``two-level'' system. Similar to lasing without inversion and photo-Carnot quantum heat engine, in a quantum dot PV cell with coherently driven doublet in the excited state it is possible to suppress the radiative recombination and increase the quantum limit of photovoltaic operation compare to classical one. Our approach is consistent and does not violate the laws of thermodynamics. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A12.00003: New Type of Core-Shell Nanocrystal Quantum Dots for Applications in Light Emitting Diodes (LEDs) B.N. Pal, S. Brovelli, Y. Gosh, V.I. Klimov, J.A. Hollingsworth, H. Htoon We demonstrate a proof of principle for LEDs based on giant nanocrystal quantum dots (g-NQDs). These dots consist of a CdSe core overcoated with a thick CdS shell built one monolayer at a time. Our device structure is composed only of a PEDOT:PSS coated indium-tin oxide (ITO) anode and a LiF-Al cathode. These simple devices exhibit a maximum external quantum efficiency (EQE) and luminance of 0.12{\%} and 1000 Cd/m2 respectively when 16 shell g-NQDs are used for the active layer. This performance is already comparable to that of more sophisticated all-inorganic NQD LEDs. Thick shell ($>$13 monolayer) g-NQD devices show EQEs about one order of magnitude higher than those of thin-shell (4 monolayer) NQD devices, as well as much greater stability for operation under ambient conditions. Although current g-NQD devices do not set any new performance records, this work demonstrates a significant potential of g-NQDs for LED applications. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 9:12AM |
A12.00004: Lattice-mismatched phosphide-based LEDs for color mixing white light applications Invited Speaker: The most promising means of achieving high efficiency white light emitting diodes (LEDs) with high color rendering indices (CRI) is to combine individual red (615 nm), yellow (573 nm), green (535 nm) and blue (459 nm) solid-state LEDs in a four color RYGB architecture. Due to their high bandgaps and the availability of bulk substrates, phosphide-based alloys are currently leading candidates for achieving the longer wavelengths, of which AlGaInP lattice-matched to GaAs has been extensively explored. In a departure from this approach, we investigate phosphide alloys at compositions that are lattice-mismatched with respect to GaAs for color mixing white light applications. Lifting the lattice-matching requirement extends the options for active and cladding layer design and optimization, thereby providing additional avenues for reducing carrier loss pathways and improving device efficiency. This talk covers our work on issues central to the success of this technology: metamorphic growth of high quality epilayers, the competing trade-off between operating wavelength and intervalley carrier transfer loss, and the availability of optimal cladding layers for high power operation. Support from the DOE EERE-SSL and BES-DMS programs and the ~LDRD program at NREL is gratefully acknowledged. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A12.00005: Ab initio study of MOCVD synthesis of InN and GaN Weronika Walkosz, Peter Zapol, Matthew J. Highland, Paul H. Fuoss, Gregory B. Stephenson A detailed understanding of MOCVD growth of group III nitrides is important for improved control over their properties and performance in a wide range of applications. Because of the relative instability of InN, chemically active precursors such as NH$_{3}$ are typically used to provide the high nitrogen activity needed for growth. Our goal is to understand the mechanism and species involved in active nitrogen formation on the growth surface. Here we present results of density functional theory calculations for the decomposition of NH$_{3}$ on InN and GaN (0001) surfaces through reaction intermediates such as adsorbed NH$_{2}$ and NH. The calculated equilibrium surface structures along with the reaction barriers for the dissociation pathways of NH$_{3}$ on these surfaces are described. Kinetic modeling based on the calculated barriers to determine reaction mechanisms and effective nitrogen activities is discussed. The results will be used to elucidate chemical kinetics on GaN and InN (0001) surfaces under MOCVD growth conditions with the aim to optimize synthesis conditions and precursors for effective growth of metastable nitrides. Work supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A12.00006: Band Gap Tuning and Structural Transformation in GaN through Equi-biaxial In-plane Strains and Alloying with InN Liang Dong, S. Pamir Alpay Gallium nitride (GaN)-based semiconductor devices play a key role in modern optoelectronics and photovoltaics. Structural and electronic properties of the GaN can be tuned through external/internal stresses or by alloying it with InN. In this study, we present an \textit{ab initio} analysis using density functional theory to understand the effects of equi-biaxial strains and indium additions to the crystallographic structure, electronic properties, and polarization of GaN and band bending in GaN-InN heterostructures. It is shown that internal strains in GaN may result in significant changes in the band gap and may even give rise to structural transformations from wurtzite to a graphite-like semi-metallic phase. For the InGaN alloys, possible stable crystal structures (besides the prototypical wurtzite structure), lattice parameters, the band gap, and the spontaneous polarization are calculated as function of indium composition. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A12.00007: Structural and Optical properties of Si-doped AlN Sashikanth Majety, Bed Pantha, Ashok Sedhain, Jing Li, Hongxing Jiang, Jingyu Lin A lot of research has focused on controlling the conductivity in AlN by Silicon doping. AlN has the widest bandgap ($\sim $6.1 eV) among III-Nitride semiconductors and exhibits excellent properties such as high temperature stability, high thermal conductivity, and deep ultraviolet transparency. In the AlN material system, doping causes crystal imperfections which can affect the structural and optical properties of the AlN epilayers. In this work, we investigated the impact of Si incorporation on the structural and optical properties of AlN epilayers. The formation of edge dislocations in Si-doped AlN is explained by the built-up tensile stress in the epilayers as revealed by X-ray diffraction measurement. Photoluminescence (PL) studies revealed that the full width at half maximum of both band-edge emission and impurity related transitions are correlated with the density of screw dislocations, $N_{screw}$, which is found to increase with increasing doping concentration of Si ($N_{Si})$. In addition, it was formulated that the band-edge (impurity) PL emission linewidth increases linearly with increasing $N_{screw}$ at a rate of $\sim $3.3$\pm $0.7 meV/10$^{8}$ cm$^{-2}$ (26.5$\pm $4 meV/10$^{8}$ cm$^{-2})$, thereby establishing PL measurement as a simple and effective method to estimate screw dislocation density in AlN epilayers. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:24AM |
A12.00008: Causes of yellow luminescence in GaN Invited Speaker: Although GaN is already used in light-emitting diodes and laser diodes, the origins of a number of frequently observed sub-band-gap luminescence bands are still under debate. For instance, the broad yellow luminescence that is invariably seen in n-type GaN has been long attributed to Ga vacancies. However, its presence in semi-insulating or p-type material, in which the Ga-vacancy concentration is low, has remained unexplained. The yellow luminescence has also been associated with the presence of carbon impurities, yet no credible, C-related configuration has been suggested. Using first-principles calculations we investigate the electronic and structural properties associated with defects and impurities in GaN. We employ a hybrid functional method to overcome the well-known band-gap problem of density functional calculations, and obtain accurate, quantitative results for defect transition levels. We find that C substituting for N (C$_{N})$ is a deep acceptor in GaN, with an ionization energy of 0.90 eV, in contrast to the commonly accepted view that C$_{N}$ acts as a shallow acceptor. Incorporating C$_{N}$ will therefore not result in $p$-type conductivity [1]. By inspecting the calculated configuration coordinate diagrams, we find that the absorption and emission lines of C$_{N}$ are in remarkable agreement with the experimental results for yellow luminescence. This solves the longstanding puzzle regarding the nature of the defect responsible for yellow emission in C-containing GaN, and suggests that previous experimental data, analyzed under the assumption that C$_{N}$ acts as a shallow acceptor, should be revisited. Work performed in collaboration with J. L. Lyons and C. G. Van de Walle, and supported by the NSF and by the UCSB Solid State Lighting and Energy Center. \\[4pt] [1] J. L. Lyons, A. Janotti, and C. G. Van de Walle, Appl. Phys. Lett. 97, 152108 (2010). [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A12.00009: Hybrid functional calculations of DX centers in AlN, GaN and AlGaN Luke Gordon, John L. Lyons, Anderson Janotti, Chris G. Van de Walle The group-III nitrides have important commercial applications in optoelectronic devices. To achieve high-efficiency UV lasers and LEDs, AlN substrates and high Al-content AlGaN alloys will likely be required. A better understanding of the role of defects and impurities in AlN is crucial. One of the outstanding problems in the study of AlN and high-Al-content AlGaN is the formation of the so-called DX centers, which consist of donor impurities that self-compensate by turning to acceptors as the Fermi level approaches the conduction band. In this work, we employ density functional calculations using a hybrid functional to investigate the possibility of DX-center formation for Si and O donors in AlN and GaN. The functional includes a portion of Fock exchange and gives band gaps and lattice parameters very close to the experimental values, allowing for quantitative predictions of defect levels. Based on the analysis of the stability of DX centers in AlN and GaN, we discuss the onset of DX behavior in AlGaN alloys. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A12.00010: Role of nitrogen vacancies and related complexes in compensation and luminescence of Mg-doped GaN Qimin Yan, Anderson Janotti, Matthias Scheffler, Chris G. Van de Walle Using first-principles calculations with the hybrid functional method (HSE), we investigate the effects of nitrogen vacancies and related complexes on the electrical and optical properties of Mg-doped GaN. We obtain information about the expected defect concentration, stable charge states, and defect levels by calculating the formation energies of vacancies and Mg$-$vacancy complexes. The $3+$ state of the nitrogen vacancy and the $2+$ state of the complex are found to be most stable when the Fermi level is near the valence-band maximum (VBM). Our calculations also enable us to study the role of these defects in luminescence. Vacancy-dopant complexes (including Mg$_{\rm Ga}$$-$$V_{\rm N}$) have been proposed as the origin of a deep level involved in the red (1.8 eV) photoluminescence (PL) band often observed in Mg$-$doped GaN. We investigate the optical absorption and emission energies by calculating the configuration coordinate diagram for the vacancy and for the Mg$_{\rm Ga}$$-$$V_{\rm N}$ complex. The emission, in which an electron in the conduction band is transferred to (Mg$_{\rm Ga}$$-$$V_{\rm N})^{2+}$, resulting in (Mg$_{\rm Ga}$$-$$V_{\rm N})^+$, peaks at 1.81 eV. Our calculated emission lines thus indicate that Mg$_{\rm Ga}$$-$$V_{\rm N}$ is a likely source for the red luminescence observed in Mg-doped GaN. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A12.00011: Effects of strain on effective masses in GaN and AlN Cyrus E. Dreyer, Anderson Janotti, Chris G. Van de Walle Strain caused by lattice mismatch or alloying is present in almost all heterostructure-based semiconductor devices. One of the fundamental effects of strain on semiconducting materials is to alter their band gap and thus the effective mass of their carriers. Because of the lack of native substrates for GaN and the mismatch between different layers, these effects are particularly important in GaN/AlGaN based devices. Using first-principles calculations, we have investigated the effects of hydrostatic and $c$-plane biaxial strain on the band structure of GaN and AlN, specifically on the band gap and effective mass in the direction parallel and perpendicular to the $c$ direction. In general, the effective mass decreases with increased hydrostatic or biaxial tensile strain, as expected from k.p theory. However, the opposite trend is observed for the effective mass of AlN in the $c$ direction under biaxial strain. This is explained by analyzing the strained band structure of AlN using a two-band Kane model. [Preview Abstract] |
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