Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B25: Focus Session: Electric-to-Light Conversion and Optics in Semiconductors I |
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Sponsoring Units: DMP GERA Chair: Sean O'Malley, Rutgers University, Camden Room: D135 |
Monday, March 15, 2010 11:15AM - 11:51AM |
B25.00001: GaInN-based LED structures on selectively grown semi-polar crystal facets Invited Speaker: In order to overcome the problems of reduced recombination probability due to internal piezoelectric fields in strained GaInN quantum wells, we have developed a method to grow semipolar GaInN-based LED structures on $\{1\bar{1}01\}$ side facets of selectively grown GaN. This enables to grow semipolar device structures over the full area of a 2'' wafer while still making use of the well developed growth of optimized c-plane GaN buffer layers. This talk will describe our recent work concerning the growth by metalorganic vapor phase epitaxy along with some sophisticated characterization studies. For the selective growth, we have studied stripe and hexagonally shaped geometries. We found a strongly facet dependent growth mechanism leading to very flat surfaces on $\{1\bar{1}01\}$ facets as opposed to their $\{11\bar{2}1\}$ counterparts along with a different indium incorporation efficiency. An increased indium uptake on semi-polar $\{1\bar{1}01\}$ facets as compared to conventional c-plane layers helped to shift the LED emission to longer wavelengths beyond 500\,nm in the green spectral range despite the significantly reduced field-dependent Stark shift. The significant reduction of the internal electric field could be verified by several methods. Hexagonally shaped mask geometries are more favorable for large area device applications and for the reduction of dislocations. However, by locally resolved cathodoluminescence, we found a quite strong local variation of the emission wavelength over the formed inverted pyramid facets which is also visible in locally resolved measurements of the carrier recombination times. First results on stripe periods on a sub-micrometer scale may pave the way for the direct realization of DFB lasers on c-plane GaN. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B25.00002: X-ray excited photoluminescence in InGaN/GaN MQW structures S.M. O'Malley, A. Kazimirov, P. Revesz, A.A. Sirenko Synchrotron-based x-ray radiation at CHESS (A2 beamline) was used to excite photoluminescence (PL) spectra in InGaN/GaN MQW structures. Both, cw and time-resolved techniques have been performed in detecting the PL signal. The peak of x-ray PL (XPL) for GaN layers coincides with that for conventional laser excitation, while the XPL peak for the InGaN active region has a 50 nm shift compared to spectra measured with laser-based PL and cathodoluminescence. Time-resolved measurements were done on XPL spectra using a streak camera. The temporal structure of the x-ray synchrotron beam permits exciton life-time measurements in a broad range from 0.5 ns up to a few microseconds. We determined that GaN exciton lifetime varies between 1.3 and 4.5 ns in different structures, while the InGaN exciton lifetime is on the microsecond scale due to higher defect density. Our experimental approach has been extended to the use of x-ray micro-beams. Using this approach a micron-size spot can probe InGaN/GaN device structures simultaneously measuring: (i) x-ray diffraction, (ii) photoluminescence spectra, and (iii) exciton life-times. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B25.00003: Polarized luminescence from GaN/AlN quantum dots subject to variable stress and excitation conditions Ofer Moshe, Daniel Rich, Benjamin Damilano, Jean Massies Nitride-based heterostructures often exhibit important effects caused by the presence of large piezo- and pyro-electric polarizations. External stress can distort the unit cell and alter the polarization field, electronic states, and optical properties. We study the effects of carrier excitation on the screening of the polarization field in GaN/AlN self-assembled quantum dots (QDs) grown on Si substrates. We apply stress perturbations found near naturally occurring micro-cracks to examine ensembles of QDs subject to varying stress configurations. Using cathodoluminescence, we have probed the optical properties of ensembles of QDs in vertically stacked layers for different stress configurations and excitation conditions. We find a strong spatial variation of the linear polarization of light emitted from the QDs in a region of transition between in-plane biaxial stress and uniaxial stress. When the QDs experience uniaxial stress, the in-plane polarization anisotropy also strongly depends on e-beam excitation conditions at room temperature and exhibits a much weaker dependence at low temperatures. These effects will presented and explained using a 3D self-consistent \textbf{\textit{k.p }}model including Fermi statistics for the calculation of the QD electron/hole energies and wavefunctions. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B25.00004: Giant Excitonic Transport in GaN/AlGaN Surface Quantum Wells Y.D. Glinka, S.V. Goupalov, T.V. Shahbazyan, H.O. Everitt, J. Roberts, P. Rajagopal, J. Cook, E. Piner, K. Linthicum A huge progressive spatial expansion of photoluminescence (PL) area over a few 100 $\mu $m with decreasing the GaN/Al$_{0.2}$Ga$_{0.8}$N surface quantum well width in the range of 2.9 - 1.51 nm has been observed by focusing 4.66 eV laser light (170 fs pulses) into the spot on the sample surface. The out-of-spot PL spectra reveal higher energy bands which are separated from the free exciton peak by multiples of optical phonon energy. The effect is explained as a dipolar excitonic explosion followed by nonequilibrium-optical-phonon-mediated lateral transport. The PL is observable due to radiative recombination of high-energy excitons accompanied by inelastic scattering with surface ionized acceptors. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B25.00005: Quantum-confined Stark effect in GaN/AlN quantum dots in nanowires Julien Renard, R. Songmuang, G. Tourbot, C. Bougerol, B. Daudin, B. Gayral GaN/AlGaN heterostructures have attracted much interest in the past few years due to their potential for optoelectronic applications such as UV light emission or telecommunication wavelength intersubband devices. More recently, many studies focused on nanowire heterostructures, because such geometry is very likely to lead to a dramatic reduction of the defect density thanks to the free surface in comparison with two dimensional (2D) structures. This reduction is indeed very appealing for light emission application. It is well known that the quantum confined Stark effect strongly influences the optical properties of 2D heterostructures. In contrast, the strain reduction and the presence of the surface could potentially lead to a very different picture for nanowire heterostructures. In InGaN/GaN nanowire heterostructures, it has been shown that the QCSE is strongly reduced [1] as a consequence of the decrease of the piezoelectric polarization. We addressed this issue for GaN/AlN nanowire heterostructures and evidenced the presence of a strong QCSE [2]. This demonstrates the importance of the spontaneous polarization in nanowire heterostructures. [1] Y. Kawakami et al, Appl. Phys. Lett. 89, 163124 (2006) [2] J. Renard et al, Phys. Rev. B 80, 121305 (R) (2009) [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 1:15PM |
B25.00006: InGaN Growth Morphology and Its Relationship to Luminescence for Solid State Lighting Invited Speaker: It has been suggested that InGaN quantum well (QW) thickness fluctuations, acting in conjunction with piezoelectric fields, are sufficient to produce exciton localization and reduce non-radiative recombination at dislocations.\footnote{D. M. Graham, et al., J. Appl. Phys. 97 103508 (2005).} Consequently, the study of InGaN step morphology and the control of InGaN/GaN interfaces may be crucial for understanding and improving blue and green LED quantum efficiency. To this end, a variety QW and thin film structures have been grown to determine how the surface roughens when indium is alloyed with GaN. Statistical analysis of the step height distributions from AFM images shows that the nominally single-layer step heights transition to multiple-layer step heights as InGaN is grown on GaN. Further analysis of the surface-roughness power spectral density suggests that the main smoothing mechanism changes from evaporation/recondensation for GaN growth to surface diffusion for InGaN growth. Exploiting two smoothing mechanisms allows the construction of otherwise identical MQW structures with smoother or rougher InGaN QWs. Initial studies of the correlations between QW interface roughness and luminescence intensity suggest that some degree of InGaN QW roughness leads to higher luminescence efficiency, lending credence to theories that propose a structural-based enhancement of exciton localization.\footnote{This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.} [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B25.00007: Gas phase interactions with Au nanoparticles decorated GaN nanowires by X-ray photoelectron spectroscopy I.B. Niraula, B.A. Fouetio, M. DeHart, M. Martinez, D.N. McIlroy, M.G. Norton Au nanoparticle coated GaN (Au-GaN) nanowires have been shown to exhibit gas sensing capabilities, yet virtually nothing is known about the interactions of gases at the surface of Au-GaN nanowires. The electronic surface structure and the gas adsorption behavior of the nanoparticles have been examined by X-ray photoelectron spectroscopy (XPS). The interaction of gold nanoparticles as a function of CO, H$_{2}$O and O$_{2}$ exposures was investigated by examining binding energy shifts of the Au 4f, C1s and O1s electron core level states at 298 K, 77 K and 14 K. XPS analysis reveals that for T= 298 K and T=77 K for all gases the interaction with the surface of the Au-GaN nanowires is too weak to be observed on the timescale of XPS. However at T=14K binding energy shifts of the Au 4f, C1s and O1s electron core level states occur, indicative of adsorption onto the Au-GaN nanowire surface. The temperature dependence of the gas adsorption indicates that the interaction of all the gases investigated is physisorption. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B25.00008: How does atomic microstructure of alloys affect electronic structure: The case of InGaN J.A. Chan, J.Z. Liu, A. Zunger The atomic microstructure of alloys is rarely perfectly random, exhibiting instead differently shaped precipitates, clusters, zig-zag chains, etc. While its expected that such microstructural features will affect the electronic structure carrier localization, band gaps), theoretical studies have until now been restricted to investigate artificial ``guessed'' microstructural features. We have {\it derived} the equilibrium alloy microstructure of InGaN using (i) density functional total energies of $\sim$50 ordered structures to construct a (ii) multi-body cluster-expansion, including strain effects to which we have applied (iii) Monte Carlo simulations, giving the T-x phase diagram of bulk and epitaxial alloys as well as their atomic microstructure [1].The ensuing electronic structure associated with the microstructure was then determined by applying empirical pseudopotentials. We catalogue the manner in which microstructure affects electronic structure and investigate how the hole localization and band level energies correlate with In concentration, strain and particular structural motifs formed in equilibrium. [1] J.Z. Liu, and A. Zunger, Phys. Rev. B, {\bf 77}, 205201 (2008), J.Z. Liu {\it et al}, Phys. Rev. Lett. {\bf 99}, 145501 (2007). [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B25.00009: Fundamental studies of lattice mismatched, strain-free Ga$_{x}$In$_{1-x}$P alloys for x $>$ 0.51 L. Bhusal, M. Steiner, B. Fluegel, A. Mascarenhas Fundamental electronic and optical properties of the Ga$_{x}$In$_{1-x}$P alloy system are investigated for applications requiring high bandgaps in the range of $\sim $1.9--2.2 eV, corresponding to the composition range 0.51$<$x$<$0.8. Samples were grown by metalorganic vapor phase epitaxy (MOVPE) on GaAs substrates. To achieve the nearly-strain-free, lattice-mismatched compositions with higher bandgaps, the samples were prepared by first growing a thick step-graded layer of GaAs$_{1-y}$P$_{y}$ to bridge the lattice misfit between the Ga$_{x}$In$_{1-x}$P layers and the GaAs substrate. Phenomena such as spontaneous long-range ordering and intermixing of $\Gamma $-L-X bands near the direct-indirect crossover were studied using high resolution x-ray diffraction, modulated electro/photo-reflectance, and time-resolved photoluminescence techniques at various temperatures. The results will be discussed within the context of using these high bandgap alloys as light emitting diodes (LEDs) emitting within the green gap. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B25.00010: Degradation of ZnS:Cu,Cl on Micron Scale Using Time-Lapse Microscopy Scott Medling, Frank Bridges, Sue Carter CuS forms conducting needle-shaped nanoprecipitates in ZnS along the (111) planes which amplify the local electric fields and enable electroluminescence (EL) for $\sim$50$\mu$m thick devices at AC voltages of order 100V, a factor of 100 less than for DC EL in bulk ZnS. However, EL only occurs from tiny points (diameter $<$2$\mu$m) near the CuS nanoprecipitates. Unfortunately, devices degrade relatively quickly through a process that is not understood. We study how the light output varies spatially within 25$\mu$m particles during a long degradation process. From photodiode measurements we find that the total device light output can be modeled as a sum of two decaying exponentials. However, time-lapse microscopy indicates that the emission from many individual points within a particle often decreases in large, sudden, discrete drops. Furthermore, emission at some points increases, sometimes gradually and sometimes suddenly, by significant amounts during degradation. This suggests that the local electric fields may vary with time or that it is possible that Cu or Cl atoms could be diffusing out from and back into the active regions near the tips of the CuS on macroscopic time scales. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B25.00011: Band-gap bowing of InGaN alloys: A DFT study Poul Georg Moses, Qimin Yan, Chris G. Van de Walle InGaN alloys are successfully being used in optical, electronic, and photovoltaic devices; a novel application is for photochemical water splitting. In order to further improve InGaN-based devices a detailed understanding of the materials properties as a function of alloy composition is needed. To obtain such insight we have investigated the band bowing of InGaN alloys and the strain effects in GaN and InN in the wurtzite phase using density functional theory. The HSE exchange correlation functional has been used in order to accurately calculate the electronic band structure [1]. We will discuss bowing effects in InGaN alloys based on accurate calculation of band gaps of InGaN alloys and on a analysis of experimental results using our calculated deformation potentials to disentangle the effect of strain and alloying on the band gap. \\[4pt] [1] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003) [Preview Abstract] |
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