Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J12: Focus Session: Electricity-to-Light Conversion: Solid State Lighting II |
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Sponsoring Units: GERA DMP Chair: Fred Schubert, Rensselaer Polytechnic Institute Room: D223/224 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J12.00001: Surface origin of the LO phonon feature in the Raman spectrum of InN Esther Alarcon-Llado, Nate Miller, Marie Mayer, Joel W. Ager Wurtzite InN presents an intrinsic Fermi level pinning above the conduction band edge at the surface. For this reason, a large surface electron accumulation (SEA) occurs in InN. The interaction between the free electrons at the surface and the longitudinal optical (LO) phonon has been addressed by previous studies, but questions still remain. Here, we use the insulating nature of the Helmholtz layer that forms on a surface of an object in contact with an an aqueous electrolyte to apply potentials to n-type (undoped) and p-type (Mg-doped) InN. Applying a potential to the electrolyte changes the electric field and distribution of free carriers near the surface. This enables us to perform electrolyte gated Raman spectroscopy on differently doped InN layers and while simultaneously modulating and probing the SEA region in InN. We find that the intensity of the forbidden LO Raman feature is changed by the external potential, showing that this feature is due, at least in part, to the SEA. This is the first experimental evidence of the relation between the LO feature in InN and free electron accumulation at its surface. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J12.00002: Hydrogen incorporation in high hole density GaN:Mg M.E. Zvanut, Y. Uprety, J. Dashdorj, M. Moseley, W. Alan Doolittle We investigate hydrogen passivation in heavily doped p-type GaN using electron paramagnetic resonance (EPR) spectroscopy. Samples include both conventionally grown GaN (10$^{19}$ cm$^{-3}$ Mg, 10$^{17}$ cm$^{-3}$ holes) and films grown by metal modulation epitaxy (MME), which yielded higher Mg (1-4x10$^{20}$ cm$^{-3})$ and hole (1-40x10$^{18}$ cm$^{-3})$ densities than found in conventionally grown GaN. The Mg acceptor signal is monitored throughout 30 minute annealing steps in N$_{2}$:H$_{2}$ (92{\%}:7{\%})) and subsequently pure N$_{2}$. N$_{2}$:H$_{2}$ heat treatments of the lower hole density films begin to reduce the Mg EPR intensity at 750 $^{o}$C, but quench the signal in high hole density films at 600 $^{o}$C. Revival of the signal by subsequent N$_{2}$ annealing occurs at 800 $^{o}$C for the low hole density material and 600 $^{o}$C in MME GaN. The present work highlights chemical differences between heavily Mg doped and lower doped films; however, it is unclear whether the difference is due to changes in hydrogen-Mg complex formation or hydrogen diffusion. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J12.00003: Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers I-Wen Feng, Jing Li, Ashok Sedhain, Jingyu Lin, Hongxing Jiang, John Zavada Rare earth doped III-nitrides have been intensively studied due to their intra-4$f$ transitions covering the window from visible emissions to infrared wavelengths. Trivalent Erbium (Er$^{3+})$ has driven particular interests since the intra-4$f$ transition from its first excited ($^{4}$I$_{13/2})$ to the ground state ($^{4}$I$_{15/2})$ gives 1.54 $\mu $m emission, which sits in the low optical loss band of silica fibers and potentially affords light emitters and optical amplifiers at optical communication wavelength. Due to the structural and thermal stability of GaN, GaN appears to be the promising candidate as the host semiconductors. We prepared Er doped GaN (GaN:Er) samples by metal organic chemical vapor deposition. GaN:Er epilayers were simultaneously grown on different templates, including GaN/Al$_{2}$O$_{3}$, AlN/Al$_{2}$O$_{3}$, GaN/Si(111), and c-GaN bulk. The effects of stress, caused by the lattice mismatch between GaN:Er epilayers and the substrates, on the intensity of 1.54 $\mu $m emission were probed. The emission intensity at 1.54 $\mu $m increased with greater tensile stress in the c-direction of GaN:Er epilayers. The correlation between stress and 1.54 $\mu $m emission will be presented. The results implied the potential to design efficient photonic devices based on GaN:Er semiconductors. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J12.00004: Effect of electrostatic image charge effect on the photoluminescence in Gallium droplet coated AlGaAs-GaAs Single Quantum Wells Karol Gryczynski, Jie Lin, Tony LLopis, Zhiming Wang, G. Salamo, Arkadii Krokhin, Arup Neogi Gallium (Ga) droplets deposited on the cap layers of AlGaAs-GaAs single quantum wells (QWs) lead to a large blue shift in the observed photoluminescence of quantum wells compared to identical single wells without Ga droplets. Furthermore the intensity of the blue shifted emission peaks is enhanced with respect to the reference QWs. As the emission energies for all depths of QWs used (1.5nm to 10nm) exceed the plasmon resonance energy of the deposited Ga droplets, surface Plasmon polariton interactions cannot account for an increase in exciton recombination energy of about 20meV it is concluded that the blue shift and enhancement from the Ga droplets is not plasmonic in nature. The observed phenomena are described and modeled by applying an additional electrostatic potential to the confined excitons within the QW. An electrostatic attraction between the confined exciton and inhomogeneous nanoscale metal surfaces exert forces on the carriers both parallel and perpendicular to the surface of the well. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J12.00005: Polarization induced doping in graded AlGaN films Morgan Ware, Shibin Li, Vasyl Kunets, Michael Hawkridge, Paul Minor, Jiang Wu, Gregory Salamo The fixed polarization field which is intrinsic to nitride based III-V semiconductors in the wurtzite crystal phase can be manipulated during growth by varying the alloy composition. We report on initial experiments to use the space charge field which results from changing the internal polarization field of graded AlGaN films in a simple p-n junction device. Our devices are fabricated from films which are graded from GaN to AlGaN then reverse graded back to GaN without the intentional addition of impurity dopants. Structural characterization of the films is reported through X-Ray diffraction rocking curves and reciprocal space maps, and the rectifying behavior of the device is demonstrated through temperature dependent I-V measurements. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J12.00006: Electroluminescence from $n-n$ isotype heterojunctions of graded-band-gap ZnMgO:Al and ZnO films Jong-Gul Yoon, Sung Woo Cho, Woo Seok Choi, Dae Yeol Kim, Joonhee Lee, Chang Oh Kim, Hojoon Chang, Heonsu Jeon, Suk-Ho Choi, Tae Won Noh We report room temperature electroluminescence (EL) from $n-n$ isotype heterojunction composed of Al-doped graded-band-gap Zn$_{1-x}$Mg$_{x}$O ($g$-ZnMgO:Al) and ZnO films fabricated on Pt/Ti/SiO$_{2}$/Si substrates. The graded-band-gap of $g$-ZnMgO:Al film was investigated by spectroscopic ellipsometry and found to change contiuously from 3.22 to 3.56 eV. The EL emission spectra covered visible and near infrared regions under unipolar operation condition, with $g$-ZnMgO:Al as positive, at the operation voltages as low as 3-5 V. Impact ionization/excitation process in a narrow region of the graded layer was suggested as a possible origin of the EL. We discussed multistep excitation process mediated by defect-related deep levels and the effect of quasi-electric field in the graded-band-gap layer in conjunction with the apparent upconversion EL in the heterojuncition device. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J12.00007: Efficiency Droop in III-nitride LEDs: a differential carrier lifetime analysis Invited Speaker: GaN-based LEDs suffer from a phenomenon known as efficiency droop, which causes a (non- thermal) roll-over of the IQE at high current density, and whose underlying physical origin is not well understood. Identifying the correct process is of importance, as it dictates which strategies can be employed to quench or mitigate droop. Among the most often cited hypotheses are: localization effects related to InGaN alloy fluctuations, leakage effects, and Auger scattering. In this contribution, we will present recent experimental results which aim at testing these scenarios. We will first show why droop appears to be a bulk-like phenomenon, rather than transport-related. We will present PL measurements to illustrate how droop scales with carrier density, and biased-PL measurements which quantify the magnitude of the leakage current. In a second part, we will present differential carrier lifetime measurements, which aim at characterizing the various recombination processes in InGaN heterostructures. We will review recently published results, which show that droop is caused by the onset of a high-order non-radiative process, and confirm that lifetimes are quantitatively compatible with the hypothesis of Auger scattering. Finally, we will present new lifetime measurements on QW samples with various In contents, and discuss how the variations in droop can be explained by the impact of piezoelectric fields on the carrier lifetime. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J12.00008: MOCVD Growths of Linearly-Shaped Staggered InGaN Quantum Wells Light-Emitting Diodes Hongping Zhao, Jing Zhang, Takahiro Toma, Guangyu Liu, Jonathan Poplawsky, Volkmar Dierolf, Nelson Tansu High-efficiency InGaN-based quantum wells (QWs) light-emitting diodes (LEDs) play an important role in solid state lighting. However, the existence of both spontaneous and piezoelectric polarization fields in III-Nitride semiconductors leads to severe charge separation in InGaN QWs, which significantly reduces the electron-hole wavefucntion overlap ($\Gamma _{e\_hh})$ in InGaN QWs. In this work, the growths of linearly-shaped (LS) staggered InGaN QWs LEDs are investigated. The InGaN QWs with LS staggered In-content profile were grown by metalorganic chemical vapor deposition (MOCVD). The use of LS staggered In-contents in InGaN QWs results in improved electron-hole wavefunction overlap ($\Gamma _{e\_hh})$, in comparison to that of conventional InGaN QW. The power dependent cathodoluminescence (CL) measurement shows 2.5-3.5 times enhancement of CL intensity for LS staggered InGaN QWs as compared to that of the conventional InGaN QWs. Theoretical calculations using self-consistent 6-band \textbf{\textit{k.p}} method were performed for both LS staggered InGaN QWs and conventional InGaN QWs. The experimental measurements show good agreement with the theoretical simulation. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J12.00009: Indirect Auger recombination in nitride light emitters Emmanouil Kioupakis, Patrick Rinke, Kris T. Delaney, Chris G. Van de Walle Nitride-based light emitters suffer from an efficiency loss at high drive currents (droop), which limits their high-power performance. The origin of this efficiency droop is not fully understood, and several loss mechanisms have been suggested as its cause. One such mechanism is Auger recombination, a three-carrier non-radiative recombination process that dominates over the radiative one at high carrier densities. We have employed first-principles computational techniques to show that Auger recombination is strong in nitride materials and therefore a likely cause of the droop in nitride LEDs. The underlying microscopic Auger recombination processes occur in an indirect way, mediated by electron-phonon and alloy scattering. Our work elucidates the origin of the droop and suggests ways to improve the high-power efficiency of nitride LEDs. This work was supported by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. DOE, by the UCSB Solid State Lighting and Energy Center, and by the NSF MRSEC Program. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J12.00010: Micro- and Nanostructural Properties of Wide Bandgap Semiconductors Nabil Dawahre, Gang Shen, Shawn David Wilbert, Nick Harris, William Baughman, Lee Butler, Joseph Brewer, Seongsin Margaret Kim, Patrick Kung Wide bandgap semiconductor materials based on ZnO and GaN have attracted considerable attention in recent years because of the practical applications such as green and blue light emitting and laser diodes, solid-state lighting, photovoltaics, RF and microwave electronics, and gas sensors. However, the resulting device performance strongly depends on the quality, both compositionally and structurally, of the constituent materials. Here we report the combined use of high resolution transmission electron microscopy imaging, micro-Raman and micro-photoluminescence spectroscopy, and local electrode atom probe tomography to understand the micro- and nanostructural properties of materials synthesized by chemical vapor deposition, including defects and impurities. Growth and doping process, sample preparation, and analysis results will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J12.00011: Auger Recombination in Defect-Free III-Nitride Nanowires Meng Zhang, Wei Guo, Pallab Bhattacharya, Junseok Heo, Animesh Banerjee Defect free InGaN nanowires (NWs) and InGaN/GaN dot-in-nanowires (DNWs) were grown on (001) Si by plasma assisted molecular beam epitaxy. The nanowires have a density of $\sim $ 1x10$^{11 }$cm$^{-2}$ and exhibit photoluminescence emission peak at $\lambda \sim $500 nm. The Auger recombination coefficients of these nanowires are determined by excitation power dependent photoluminescence and time-resolved photoluminescence techniques. The measured Auger coefficients are 6.1x10$^{-32}$ cm$^{6}\cdot $s$^{-1}$ and 4.1x10$^{-33}$ cm$^{6}\cdot $s$^{-1}$, in the NW and DNW samples, respectively, which are nearly two orders of magnitude lower than those measured in InGaN/GaN quantum wells and agree very well with theoretical calculations. This suggests that the abnormally high Auger coefficients measured in traditional wide bandgap nitride materials is related to the high density of dislocations. InGaN NW and InGaN/GaN DNW light emitting diodes are demonstrated. The external quantum efficiency does not decrease up to an injection current density of 400A/cm$^{2}$. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J12.00012: Effects of oxygen annealing treatment on formation of ohmic contacts to n-GaN Wenting Hou, Theeradetch Detchprohm, Christian Wetzel Low-resistance ohmic contacts are essential for the fabrication of efficient light emitting diodes (LEDs). A commonly used ohmic contact to n-type GaN is a layer sequence of Ti/Al/Ti/Au, followed by rapid thermal annealing (RTA) in nitrogen ambient at a high temperature. We present an ohmic contact on n-GaN by a surface treatment process of rapid thermal annealing (RTA) in oxygen ambient before the n-metal deposition. As deposited n-contacts are not linear. After RTA in nitrogen ambient, ohmic contact are obtained. The annealed n-contact degrades during the p-metal anneal in oxygen ambient. However, if the sample is annealed in oxygen ambient prior to the metal deposition, the as-deposited n-contact on the treated surface is ohmic, similar to or even better than processes optimized for single-type contacts. This benefit was seen for both, mesa-etched and as-grown n-GaN epi layers. Contacts improvement is also found on unintentionally doped GaN (u-GaN). Oxygen ambient is crucial in the treatment and RTA in nitrogen ambient fail to give ohmic contacts. XPS analysis of the surface shall give us more information on the mechanism of the treatment. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J12.00013: Strong non-plasmonic mechanism of light emission from semiconductor quantum well Arkadii Krokhin, Antony Llopis, Arup Neogi, Sergio Pereira, Ian Watson We report a new mechanism of enhancement of light emission from InGaN/GaN quantum wells (QW). This mechanism is due to electrostatic attraction of the carriers to gold nanoparticles (NP) imbedded within a QW. Metal equally attracts electrons and holes, causing the carriers to concentrate near its surface. Since the probability of e-h recombination is proportional to carrier densities, the QW with NPs generates a stronger emission than the same QW without the NP. We observed roughly a $60\%$ ($80\%$) enhancement with NPs at room temperature (77K). In these nitride heterostructures, dislocations result in hexagonal pits at the surface. Gold NPs were incorporated inside the pits with no effect on the quality. The same nitride material system used to demonstrate {\it plasmonic} enhancement [1]. It has been shown that gold film {\it do not} enhance the emission because of mismatch of surface plasmon energy and the emission energy of the QW. Here we observe the enhancement caused by electrostatic mechanism that does not require energy matching. This mechanism provides another means for enhancing the efficiency of solid- state emitters. This work is supported by the DOE grant \# DE-FG02-06ER46312. \\[4pt] [1] K.Okamoto, \textit{et al.}, \textit{Nature Mat.} \textbf{3}, 601 (2004). [Preview Abstract] |
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