Ruediger Kniep Max-Planck-Institute for Chemical Physics of Solids
Francis J. DiSalvo Cornell University
Ralf Riedel Technische Universitaet Darmstadt
Zachary Fisk University of California
Yoshiyuki Sugahara Waseda University
Monday PM, November 26, 2007
Back Bay B (Sheraton)
9:30 AM - Q1
Opening Remarks by Symposium Organizers Show Abstract
9:45 AM - Q1.1
Raman Spectroscopy of Single Crystal ZnGeN2.
Timothy Peshek 1 , Kathleen Kash 1 , John Angus 2 , Tula Paudel 1 , Walter Lambrecht 1 Show Abstract
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States, 2 Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
A convenient picture of the ZnGeN2 lattice can be realized by replacement of the Ga atoms in the GaN lattice by alternating Zn and Ge atoms. This replacement results in a 2x2x1 orthorhombic lattice accompanied by a slight distortion of cell shape, and bond angles, and a bimodal distribution of bond lengths.1 GaN and ZnGeN2 have nearly identical lattice constants2 of the underlying wurtzite lattice, and nearly identical band gaps.3 However, we find that the Raman spectra for the two materials are substantially different. The period doubling in two directions in the c-plane compared to the wurtzite lattice results in 78 phonon modes for ZnGeN2, all of them Raman active,4 versus 6 Raman-active modes for GaN. Here, we present polarized Raman spectra on individual, oriented single crystal ZnGeN2 for several scattering geometries. The crystallites, grown by a vapor-liquid-solid method using NH3 and elemental Zn and Ge, are single crystal rods, as determined by electron diffraction, of lengths of the order of 100 μm and approximately hexagonal cross sections of up to 6 μm in width. A comparison of our results with a previously published unpolarized micro-Raman spectrum for polycrystalline material reveals major differences2. These include our observation of many individually resolved Raman peaks, and the absence of spectral weight for frequencies above 850 cm-1. The previously published spectrum shows a strong Raman signal in the entire region from 850-1300 cm-1. This portion of the spectrum, which, again, is absent in our spectrum, has been attributed tentatively to second harmonic overtones of the single phonon spectrum.4 A comparison with theory, including a discussion of the relevant selection rules for different scattering geometries, will be presented.This work was supported partially by grants from the Department of Education ( APR P200A030186), the National Science Foundation (DMR-0420765) and the Air Force Office of Scientific Research (F49620-03-1-0010).1 S. Limpijumnong, S.N. Rashkeev and W.R.L. Lambrecht, MRS Internet J. Nitride Semicond. Res. 4S1, G6.11 (1999).2 R. Viennois, T, Taliercio, V. Potin, A. Errebbahi, B. Gil, S. Charar, A. Haidoux, and J.-C. Tédenac, Mat. Sci. Eng. B82, 45 (2001).3 K. Du, C. Bekele, C.C. Hayman, J.C. Angus, P. Pirouz, and K. Kash, “Synthesis and Characterization of ZnGeN2 grown from Elemental Zn and Ge Sources”, submitted to J. Crystal Growth.4 Walter R.L. Lambrecht, Erik Alldredge and Kwiseon Kim, Phys. Rev. B72, 155202 (2005).
10:00 AM - Q1.2
Characteristic Features of PVT Growth of Bulk AlN and SiC Crystals: Modeling Analysis and Optimization.
Alexander Segal 1 , Denis Bazarevskiy 1 , Mark Ramm 1 , Yuri Makarov 2 Show Abstract
1 , Soft-Impact, Ltd, St.Petersburg Russian Federation, 2 , STR, Inc, Richmond, Virginia, United States
Bulk AlN and SiC crystals are currently grown using Physical Vapor Deposition (PVT) technique. This technique has some characteristic features as compared to various Chemical Vapor Deposition (CVD) methods, related to growth of the crystals in more or less tightly closed crucibles. In this case, total pressure and vapor composition in the crucible cannot be directly controlled but are spontaneously determined by the conditions of mass and species exchange between the crucible and the ambient. This effect essentially worsens controllability of the PVT technique and hampers its modeling analysis and optimization.We have developed original models of the AlN and SiC PVT growth that describe this effect and provide the correct prediction of the total pressure and vapor composition in both the hermetically closed and somewhat open crucibles. The models give also detailed description of the interplaying processes of heat exchange (conductive, convective, and radiant), gas flow dynamics, multi-component species diffusion, and surface chemical kinetics. It is essential that they allow modeling the process evolution during long growth of large crystals, including gradual movement of the evaporation/crystallization fronts and shift of the initial growth conditions. The models are validated using the available experimental data and embodied as the commercial computational codes Virtual Reactor AlN TM and Virtual Reactor SiC TM.The developed codes are applied to find the optimal technological conditions for growth of bulk AlN and SiC crystals. Computations with the codes revealed some unexpected effects, including possible high jumps of the species partial pressures at the Knudsen layers on the reactive surfaces, considerable increase of the total pressure in the crucible due to the diffusion of inert carrier gas from the ambient, complicated movement of the evaporation/crystallization fronts relative to the isotherms, disappearance of the species diffusion in the strictly stoichiometric vapor, and others. The results of computations were applied to study of particular machines for sublimation growth of bulk AlN and SiC crystals. Optimization of the processes allowed growing the crystals of the desired slightly convex shape that favors high quality of the crystals.
10:15 AM - **Q1.3
Solids with Mobile Nitrogen Ions.
Martin Lerch 1 Show Abstract
1 Institut fuer Chemie, TU Berlin, Berlin Germany
Solid electrolytes are an important class of materials used in fuel cells, sensors, or batteries. In the case of anion conductors solids with high oxygen mobility are most prominent. Zirconia doped with aliovalent oxides such as yttria or scandia is commonly used in Solid Oxide Fuel Cells or automotive oxygen sensors. These zirconia-based materials can be described as anion-deficient fluorite-type structures. This structure type tolerates a large amount of oxygen vacancies and an activation energy of around 1 eV was found for the jump process of the oxygen ions. Thinking about solids with mobile nitrogen ions, fluorite-type based nitride oxides are promising candidates from a structural point of view. Nitriding zirconium dioxide doped with small amounts of yttria leads to nitride oxides with randomly distributed anion vacancies. For detailed investigations of these potential nitrogen conductors large single crystals are necessary. A special method for the growth of nitride oxide crystals was developed, the ‘reactive skull melting’. Impedance spectroscopy studies resulted in ionic conductivities of the Y-Zr-O-N samples comparable to commonly known YSZ ceramics (Y-Zr-O) but with increased activation energy for the conduction process. This was confirmed by tracer diffusion and neutron diffraction experiments. Whereas the activation energy for oxygen ions is nearly the same for nitride oxide as for oxide (YSZ) materials (1 eV), the barrier for nitrogen ions is 2 eV. The described nitride oxides are mixed oxygen/nitrogen ion conductors but limited to a nitrogen concentration of 15 anion-%. Consequently, another class of nitride oxides with a higher nitrogen amount was investigated. Beta tantalum nitride oxide shows the monoclinic baddeleyite-type structure at ambient temperature and can be considered as nitrogen-rich analogue of zirconium dioxide. Partial substitution of tantalum by yttrium ions leads to cubic anion-deficient fluorite-type phases with randomly distributed vacancies. The vacancy concentration can be optimized by varying the amount of yttrium. Nevertheless, compared to the above described zirconia-based materials, which are stable up to more than 2000 K in nitrogen, the thermal stability of the cubic tantalum nitride oxides is poor (decomposition at 1200 K). Consequently, no dense ceramics or single crystals can be prepared. In general, the thermal stability of the fluorite-type nitride oxides decreases with increasing nitrogen content.A nitrogen ion conductor without additional oxygen conductivity must base on a totally different concept. Mayenite (Ca12Al14O33), a good oxygen ion conductor, can be described as a framework structure in which 32 of the 33 oxygen anions are tightly bound, forming large cages, 1/6 of them being filled randomly by the remaining “free” and mobile oxygen. First results on the preparation and characterization of N-mayenite, where the “free” oxygen is substituted by nitrogen, are presented and discussed.
10:45 AM - Q1.4
MOCVD Growth of Hexagonal Nitride on Si(100).
Qian Sun 1 , Soon–Yong Kwon 1 , Jung Han 1 Show Abstract
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States
Recently two research groups have demonstrated hexagonal GaN-based LED and HEMT on offcut Si(100). The GaN material quality on Si(100) is at present much worse than that of GaN on sapphire or Si(111). In this paper we investigated the growth of AlN and Al0.13Ga0.87N on 4-deg offcut Si(100) to achieve single crystalline hexagonal phase. It is found that an optimum Al-pre-deposition and a high growth temperature play significant roles in the morphological and structural quality. V/III ratio during subsequent AlGaN growth is crucial in determining in-plane alignment. Al0.13Ga0.87N grown under a low V/III ratio shows a very rough surface with many misaligned grain boundaries prohibiting coalescence and x-ray phi-scan of the AlGaN (1011) shows two sets of 6-fold diffraction peaks with nearly equivalent intensity. As for the smooth AlGaN grown under high V/III, however, the (1011) x-ray diffraction intensity of the major type of AlGaN domains, whose  is parallel to Si, is much stronger than that of the minor type of domains. Moreover, room temperature photoluminescence of smooth Al0.13Ga0.87N epilayer obtained under high V/III condition presents a strong near band-edge emission around 334 nm, while the rough AlGaN gives only a broad deep level emission. Evolution of hetero-nucleation and AlGaN heterostructures will be reported.
11:30 AM - Q1.5
Negative or Zero Thermal Expansion in Silicon Dicarbodiimide, Si(NCN)2.
Peter Kroll 1 , Emanuel Ionescu 2 , Ralf Riedel 2 Show Abstract
1 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas, United States, 2 Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Darmstadt Germany
For many compounds the linear thermal expansion coefficient is not a constant of temperature, but becomes negative at some teperature. Thus, the crystal lattice parameter contracts as the temperature increases. Even diamond and other zinc-blende structures behave in such a way at low temperatures. While the technical importance is evident, usage of β-eucryptite in CERAN cooking tops is a major application, there are just a few compounds that exhibit an isotropic negative thermal expansion at ambient and elevated temperatures, resulting in a volume contraction upon heating. All of them are oxides of tungten, vanadium, and molybdynum; the standard reference being ZrW2O4 .Here we show computational results that indicate that silicon dicarbodiimide, Si(NCN)2, is the first non-oxide material that exhibits strong isotropic negative thermal expansion in a temperature range from 300-700 K. Our results are based on extensive ab initio molecular dynamics simulations. We investigated a wide field of parameters for temperature and volume to locate the points of lowest free energy. We find a negative thermal expansion coefficient of -1*10-5 K-1, comparable to that of ZrW2O4.Our subsequent experimental study using synchroton radiation shows that the thermal expansion of Si(NCN)2 is at least zero in a temperature range from 500-800 K. Further experimental studies with better quality crystals are currently under way. The property is mainly related to the combination of tetrahedral environment and the flexible carbodiimide, -NCN-, functional group present in Si(NCN)2. The discovered property, thus, is likely be a general phenomenon of all such carbodimide compounds.M. P. Attfield and A. W. Sleight, Chem. Comm. 1998, 601.R. Riedel, A. Greiner, G. Miehe, W. Dressler, H. Fuess, J. Bill, and F. Aldinger, Angew. Chem. Int. Ed. Engl. 1997, 36, 603.
11:45 AM - Q1.6
Analysis of Structural Defect Distributions in Aluminum Nitride (AlN) Bulk Crystals Grown by the Seeded Physical Vapor Transport (PVT) Technique.
Balaji Raghothamachar 1 , Michael Dudley 1 , Rafael Dalmau 2 , Ziad Herro 2 , Zlatko Sitar 2 , Raoul Schlesser 2 Show Abstract
1 Materials Science & Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
For III-nitride device technology, aluminum nitride (AlN) substrates are a more attractive choice compared to sapphire substrates that are currently used. This is due to several favorable properties such as crystal structure and chemical compatibility of AlN with device epilayers of gallium nitride (GaN) and AlGaN alloys, close lattice match, negligible thermal expansion difference and high thermal conductivity. Several efforts are ongoing to grown bulk AlN crystals from which substrate wafers can be obtained. Using the seeded physical vapor transport (PVT) method, we have grown AlN single crystal boules in a RF reactor. Wafers sliced and polished from these boules have been systematically imaged by synchrotron white beam x-ray topography (SWBXT) to map the defect distribution and trace the defect evolution down the length of the boule. High resolution x-ray diffraction (HRXRD) measurements have also been carried out to quantify the variation in structural defect distribution. Results show a marked change in defect density across the length of the boule as well as considerable variation within each wafer. The effect of polarity as well as other growth conditions on defect generation and distribution is discussed.
12:00 PM - Q1.7
Free-Standing Zinc-Blende (Cubic) GaN Substrates Grown by a Modified Molecular Beam Epitaxy Process.
Anthony Kent 1 , Sergei Novikov 1 , Nicola Stanton 1 , Richard Campion 1 , Charles Foxon 1 Show Abstract
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom
We demonstrate bulk, free-standing, zinc-blende (cubic) GaN substrates grown by a modified molecular beam epitaxy process. We have grown free-standing cubic GaN layers up to 60 microns in thickness. Even though our growth rate is currently not particularly fast, it is already comparable with the growth rate for bulk wurtzite GaN crystals from the liquid Ga at high pressure. We present measurements, which confirm the cubic nature of the GaN wafers and show that the hexagonal content of the material is less than about 10%. Cubic (001) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c-axis wurtzite GaN, therefore, our free standing GaN wafers make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high power and high frequency electronic applications.
12:15 PM - Q1.8
Development of Homoepitaxially Grown GaN Thin Film Layers on Freestanding Bulk m-plane Substrates by Metalorganic Chemical Vapor Deposition (MOCVD).
Vibhu Jindal 1 , James Grandusky 1 , Mihir Tungare 1 , Neeraj Tripathi 1 , Fatemeh Shahedipour-Sandvik 1 , Peter Sandvik 2 Show Abstract
1 , CNSE, Albany, New York, United States, 2 Global Research Centre, General Electric, Niskayuna, New York, United States
A design of experiment approach is used to investigate the growth space for optimization of homoepitaxial m-plane GaN films on freestanding HVPE m-plane substrates by metalorganic chemical vapor deposition. Under optimized c-plane GaN growth conditions, the homoepitaxy resulted in large areas without nucleation along with a high density of defects. These structural defects were mainly of arrow head shape caused by the difference in growth rates in a- and c- crystallographic directions. The growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure to obtain smooth and coalesced epitaxial layers on bulk substrates. For example, growth at lower temperature resulted in increased nucleation, with a rough surface morphology. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature, lower V/III ratio and lower pressure decreased the surface roughness of GaN thin films with better optical properties, as measured by photoluminescence, on m-plane substrates as compared to standard c-plane growth conditions.
12:30 PM - Q1.9
Phonons in Zn-IV-N2 Semiconductors.
Tula Paudel 1 , Walter Lambrecht 1 Show Abstract
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
A family of materials closely related to wurtzite GaN van be formed by substituting Ga by Zn and a group IV element: Si, Ge or Sn. Some of these materials, which all share the same ordering pattern of the cations, have recently been grown in thin film form and their properties start to be explored. Here we present a study of the lattice vibrations and structural properties using the linear response pseudopotential plane wave approach implemented in the ABINIT code using the local density approximation as well as the generalized gradient approxiation. Using the calculated Born effective charges and phonon eigenvectors, the oscillator strengths for infrared absorption have been calculated for all optically active modes for all three materials. The LO-TO splittings were also obtained. A detailed comparison with experiment is made for ZnSiN2 for the b1 modes, the only case for which experimental data are currently available. Good agreement (peak positions to within 5-8 %) is obtained although the interpretation is somewhat different from the experiment in the sense that some observed rather broad peaks are apparently superpositions of two modes. Interestingly, ZnSiN2 does not show a clear separation in frequency range of optic and acoustic type modes. A high oscillator strength mode occurs in the region where only low optical activity folded acoustic modes are expected. This mode, however, has not yet been observed, possibly because of a short lifetime due to mode coupling. High frequency and static dielectric constant tensors are also obtained and found to be in good agreement with experimental data for ZnSiN2. For ZnGeN2 and ZnSnN2 this low frequency mode is weaker and the spectrum splits into a region of 6 strongly active higher frequency modes and 5 weaker modes in the low frequency region.
12:45 PM - Q1.10
Electronic Properties of Mixed Conducting Solid Oxides Containing Nitride.
Hans Wiemhoefer 1 , Mustafa Dogan 1 , Vera Ruehrup 1 , Ilia Valov 2 , Juergen Janek 2 , Martin Lerch 3 , Eberhard Schweda 4 Show Abstract
1 Institute of Inorganic & Analytical Chem., University of Muenster, Muenster Germany, 2 Institute of Physical Chemistry, University of Giessen, Giessen Germany, 3 Institute of Chemistry, Technical University of Berlin, Berlin Germany, 4 Institute of Inorganic Chemistry, University of Tuebingen, Tuebingen Germany
Monday PM, November 26, 2007
Back Bay B (Sheraton)
2:30 PM - **Q2.1
Ion-conducting Nitride Oxides: Transport, Reactions and Electrochemistry.
Juergen Janek 1 , Ilia Valov 1 Show Abstract
1 Institute of Physical Chemistry, Justus-Liebig-University Giessen, Giessen Germany
3:00 PM - Q2.2
Seeded Growth of AlN on m-plane Seed.
Peng Lu 1 , Rafael Dalmau 1 , Zlatko Sitar 1 Show Abstract
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Seeded growth of AlN was achieved on m-plane AlN seeds by physical vapor transport (PVT). The seeds with high crystalline perfection were cut from freestanding AlN single crystals obtained by self-seeded growth. The seeded growth was performed at temperatures above 2200°C in N2 atmosphere at 500 Torr total pressure. Crystals were grown at a growth rate of 200 μm/hr in the [10-10] direction while the in-plane growth rates were highly anisotropic. The growth surface showed macroscopic prismatic facets parallel to the c-axis and each of these facets consisted of micro-steps. X-ray diffraction analysis confirmed seeded growth and a high crystalline quality of grown boules. The defects formed in m-plane AlN growth were studied by aqueous solution and molten KOH etching.
3:15 PM - Q2.3
Growth and Texturing of Rare-earth Nitride Thin Films.
Jianping Zhong 1 , Andrew Preston 1 , B. Ruck 1 , H. Trodahl 1 Show Abstract
1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand
The rare-earth nitrides combine extended band electrons with highly localized 4f electrons that carry large magnetic moments. They lie on the boundary between metals and insulators, and advanced electronic structure calculations have predicted that their numbers include half metals with potential applications in the field of spintronics. Experimentally, tests of the theoretical predictions have been hampered by the absence of stoichiometric samples, and by the propensity of the rare-earth nitrides to react with atmosphere. Thus, there is presently an imperative to explore growth techniques that provide high quality samples for study. We have grown a range of rare-earth nitride thin films at room temperature on silicon and sapphire by evaporating the rare-earth element in the presence of a partial pressure of high purity nitrogen gas. Rutherford backscattering spectroscopy shows that the films are stoichiometric, and x-ray diffraction has shown that they all possess the rock-salt structure with the expected lattice constant. For SmN, DyN, ErN, and LuN the films consist of approximately 10 nm randomly oriented crystallites, while GdN shows a strong  orientation independent of substrate. We have probed the electronic states through the temperature dependent resistivity, and find semiconducting behaviour for all of the samples.
3:45 PM - Q2.5
Contact Formation on GaN Investigated with Electron and Soft X-ray Spectroscopies.
Sujitra Pookpanratana 1 , Marcus Baer 1 , Lothar Weinhardt 1 , Clemens Heske 1 , Ryan France 2 , Tao Xu 2 , Theodore Moustakas 2 , Oliver Fuchs 3 , Monika Blum 3 , Jonathan Denlinger 4 Show Abstract
1 Dept. of Chemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 Dept. of Electrical and Computer Engineering, Boston University, Boston, Massachusetts, United States, 3 Experimentelle Physik II, Universität Würzburg, Würzburg Germany, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
(Al, Ga, In)N-based semiconductors are of great interest for their applications in light emitting diodes (LEDs) and laser diodes. Traditionally, Ti-based metal contacts have been used for these types of materials. However, this becomes less than ideal for wide-gap nitrides with increasing AlN content . In that case, it has been shown that there is a significant improvement in ohmic character in the contacts when V replaces Ti [1,2]. The metal contacts implemented on some n-type nitrides use a layered stack formation consisting of Au/V/Al/V and subsequent rapid thermal annealing (RTA) in N2 atmosphere. It is hypothesized that the RTA treatment forms VN at the nitride interface , and that the VN is responsible for the improvement of the electronic contact properties . In order to understand the processes during contact formation between wide band gap nitrides and a V-based metal contact, we have used photoelectron spectroscopy (PES) and synchrotron-based x-ray emission spectroscopy (XES). These techniques give information about the chemical and electronic properties at or near a surface. Samples were measured before and after subsequent heat treatments using RTA as well as a conventional furnace at different temperatures. N-type GaN samples were grown by molecular beam epitaxy onto sapphire substrates and V-based contact stacks were deposited by e-beam evaporation with varying thickness. Our experiments allow us to paint a detailed picture of the contact formation, which indicates a rather complex behavior. While the PES spectra of Au/V/Al/V/GaN samples before annealing are dominated by Au photoemission lines, the PES signals of Al, V, Ga, and N can also be detected upon annealing. This indicates either pronounced intermixing processes or island formation induced by the heat treatment. The study of the respective morphology by atomic force microscopy, which is currently ongoing, will clarify these effects. N K XES spectra of RTA-annealed Au/V/Al/V/GaN samples show the formation of N-V bonds (in contrast to the samples annealed in the conventional furnace). At the same time, the emission feature indicating N-Ga bonds (i.e., of the GaN layer) disappears. Both findings support the previous hypothesis of the formation of VN . The V L emission spectra, however, show that the situation is more complicated, because they consist of superimposed emission features of V, VN, and VxOy. In-situ PES experiments of V deposition and controlled VN formation on clean GaN, which will also allow us to measure the work functions and the electronic level alignment precisely, are currently being conducted to shed further light on the various observed V species.1.A. Sampath et al., Mater. Res. Soc. Symp. Proc. 482, 1095 (1998).2.R. France et al., Appl. Phys. Lett. 90, 062115 (2007).3.I. Galesic and B. O. Kolbesen, Thin Solid Films 349, 14 (1999).
4:30 PM - Q2.6
AlN Thermal Expansion Coefficients Determined from Bulk Crystals.
Stephan Figge 1 , Hanno Kroencke 1 , Boris Epelbaum 2 , Detlef Hommel 1 Show Abstract
1 Department of Physics and Electrotechniques, University of Bremen, Bremen Germany, 2 Department of Materials Science, University of Erlangen, Erlangen Germany
4:45 PM - Q2.7
Enhancement of Light Extraction Efficiency in GaInN Blue Light-emitting Diodes by Graded-refractive-index Antireflection Coating of Co-sputtered Titanium Dioxide and Silicon Dioxide.
Frank Mont 1 2 , David Poxson 1 3 , Jong Kim 1 2 , E. Fred Schubert 1 2 3 , Arthur Fischer 4 , Mary Crawford 4 Show Abstract
1 Future Chips Constellation, Rensselaer Polytechic Institute, Troy, New York, United States, 2 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechic Institute, Troy, New York, United States, 3 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechic Institute, Troy, New York, United States, 4 Semiconductor Materials and Device Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States
The large refractive index (n) contrast between semiconductors (n = 2.5 to 3.5) and air (n = 1.0) results in low light extraction efficiencies in light-emitting diodes (LEDs) due to the total internal reflection and high Fresnel reflection losses. Single antireflection (AR) coatings are widely used to reduce reflections and thus to maximize transmitted light from the LED chip into the ambient. However, conventional AR coatings only function at a single wavelength and at normal incidence. In contrast, if the refractive index of an AR coating continuously varies from the substrate’s index to the ambient’s index, such graded-index optical coatings yield broad-band omni-directional AR characteristics with transmittance near 100% by complete elimination of Fresnel reflection. In this work, we have