Symposium Organizers
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 Abstract9:45 AM - Q1.1
Raman Spectroscopy of Single Crystal ZnGeN2.
Timothy Peshek 1 , Kathleen Kash 1 , John Angus 2 , Tula Paudel 1 , Walter Lambrecht 1
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States, 2 Chemical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractA 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
1 , Soft-Impact, Ltd, St.Petersburg Russian Federation, 2 , STR, Inc, Richmond, Virginia, United States
Show AbstractBulk 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
1 Institut fuer Chemie, TU Berlin, Berlin Germany
Show AbstractSolid 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
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States
Show AbstractRecently 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 [1010] is parallel to Si[110], 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
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
Show AbstractFor 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 [1].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.[1]M. P. Attfield and A. W. Sleight, Chem. Comm. 1998, 601.[2]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
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
Show AbstractFor 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
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom
Show AbstractWe 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
1 , CNSE, Albany, New York, United States, 2 Global Research Centre, General Electric, Niskayuna, New York, United States
Show AbstractA 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
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractA 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
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
Show AbstractMonday 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
1 Institute of Physical Chemistry, Justus-Liebig-University Giessen, Giessen Germany
Show Abstract3:00 PM - Q2.2
Seeded Growth of AlN on m-plane Seed.
Peng Lu 1 , Rafael Dalmau 1 , Zlatko Sitar 1
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractSeeded 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
1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand
Show AbstractThe 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 [111] 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
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
Show Abstract (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 [1]. 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 [3], and that the VN is responsible for the improvement of the electronic contact properties [2]. 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 [3]. 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
1 Department of Physics and Electrotechniques, University of Bremen, Bremen Germany, 2 Department of Materials Science, University of Erlangen, Erlangen Germany
Show Abstract4:45 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
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
Show AbstractThe 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 demonstrated the enhancement of light extraction efficiency in GaInN blue LEDs by using a graded-refractive-index AR coating, which is deposited by reactive RF magnetron co-sputtering of TiO2 and SiO2 targets. The refractive index of the coating varies linearly from the refractive index of TiO2, index matched to GaN, to that of SiO2, index-matched to an epoxy encapsulant. The linear index grading is achieved by adjusting the electrical power to the respective targets to their appropriate values in order to control the relative composition, and hence the refractive index of the mixture of TiO2 and SiO2. Normal-incidence and angle-dependent transmittance measurements are taken using a white light source and a He-Ne laser, respectively. A transmittance enhancement larger than 10% is shown for graded-refractive-index AR coating on GaN compared to conventional single-layer AR coating over a broad-band range of wavelengths. The graded-refractive-index AR coatings are incorporated into GaInN blue LEDs. The enhancement of light extraction efficiency and patterning of the graded-refractive-index AR coatings for further enhancement will be discussed in terms of theoretical and analytical considerations.
5:00 PM - Q2.8
HRTEM Observation of Dislocations in AlN Films.
Yuki Tokumoto 1 , Naoya Shibata 2 , Teruyasu Mizoguchi 2 , Masakazu Sugiyama 3 , Yukihiro Shimogaki 3 , Takahisa Yamamoto 1 , Yuichi Ikuhara 2
1 Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba, Japan, 2 Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo, Japan, 3 Department of Materials Science, The University of Tokyo, Bunkyo, Tokyo, Japan
Show AbstractGroup III-nitride wide-gap semiconductors, including AlN, GaN, InN and their alloys, have been widely studied because of their potential applications for new, high performance optical and electronic devices. However, it has been known that the group III-nitride films epitaxially grown on the substrate normally contain a high density of threading dislocations (TDs), and these TDs have deleterious effects on the optical and electrical properties of the films. Therefore, reduction of the TDs is a key issue for the group III-nitride thin film growth. The generation mechanism of the TDs, especially edge dislocations, is expected to have relation with the mosaic growth mode of the film. In the mosaic growth mode, the nuclei formed on the substrate are slightly misoriented, and the films eventually consist of sub-grains slightly misoriented with respect to each other. Consequently, at the sub-grain boundaries, dislocations are introduced to compensate small angle deviations. Although the mosaic growth model has been supported by many studies, the atomic structures of threading dislocations have not well been characterized in connection with the mosaic growth model. In the present study, we characterize the structures of threading dislocations in AlN films mainly using high-resolution transmission electron microscopy (HRTEM), in order to understand the generation mechanism of the dislocations.AlN films were grown on (0001) sapphire by metal-organic chemical vapor deposition (MOCVD) using trimethylaluminum (TMAl) and ammonia (NH3) as precursors for Al and N, respectively. Plan-view specimens for TEM observations were prepared by conventional methods including mechanical polishing and argon-ion-beam thinning. Conventional and high-resolution TEM observations were performed using JEOL JEM-2010HC (200kV) and JEOL JEM-4010 (400kV), respectively. As a result of conventional TEM observations, the density of TDs in the AlN films fabricated in this study was estimated to be about 5×1010cm-2. HRTEM observations confirmed that the threading dislocations are mostly edge-type dislocations, which are perfect dislocations with the Burgers vector of b=⅓<11-20>. The edge dislocations were periodically introduced and formed sub-grain boundaries, indicating that these dislocations are introduced to compensate the small angle mismatch between the sub-grains. Thus, the arrays and directions of the threading dislocations strongly suggest the validity of the mosaic growth model. The detailed generation mechanism and atomic core structures of the threading edge dislocations will be discussed in the presentation.
5:15 PM - Q2.9
Epitaxial Lateral Overgrowth of Thick AlN Layers by Migration Enhanced Metalorganic Chemical Vapor Deposition.
R. Jain 1 , J. Zhang 1 , W. Sun 1 , X. Hu 1 , M. Shatalov 1 , J. Deng 1 , I. Shtrum 1 , A. Lunev 1 , Y. Bilenko 1 , J. Yang 1 , R. Gaska 1
1 , Sensor Electronic Technology, Inc., Columbia, South Carolina, United States
Show AbstractIt has been shown that internal quantum efficiency and reliability of III-nitride visible and ultraviolet (UV) light emitting diodes (LEDs) is strongly affected by the material quality of buffers and subsequent layers of device structure. Therefore, for deep UV LED we developed epitaxial lateral overgrowth (ELOG) of thick AlN layers over sapphire substrates by using a combination of conventional metal-organic chemical vapor deposition (MOCVD) and our proprietary migration-enhanced MOCVD (MEMOCVD) processes. In this paper we will present our recent results on growth of high quality thick AlN layers over sapphire substrates that were used as templates for deep UV LED structure growth. We also will report on fabrication and characterization of deep UV LED structures grown over these high quality thick AlN layers.For this study, high-quality thick c-plane AlN layers were grown over sapphire substrates by a combination of conventional MOCVD and proprietary MEMOCVD processes. Initially, 2 μm to 5 μm thick AlxGa1-xN (x=0.9-1) films were heteroepitaxially deposited on sapphire substrates by MOCVD. Deep grooves for lateral overgrowth were formed in the AlGaN layer along <1-100> direction by using standard photolithography and reactive ion etching in BCl3/Cl2/Ar plasma. Fully coalesced 15 - 20 μm thick AlN films were laterally overgrown on the grooved templates at growth rates ranging from 1 to 5 μm/hr. Cracking, typical for thick AlN films due to the differences in lattice constants and thermal expansion coefficients of epilayer and of substrate, was eliminated by accommodating the strain in grooved AlGaN templates resulting in completely crack-free layers. V/III ratio was found to be the key growth parameter in improving layer coalescence and surface morphology. Preliminary structural and optical characterization was carried out confirming high quality of AlN layer. Thick low-defect density AlN layers were used as templates for subsequent growth of AlInGN based deep UV LED device structures. Standard UV LEDs were fabricated using reactive ion etching to access n-AlGaN layers combined with electron beam deposition for n- and p-contact metallization. Packaged devices with peak emission at 310 nm exhibited CW output power of 0.6 mW at 20 mA. Reliability tests of packaged devices show variation of the output power of about 10 % of initial value after 400 hours of operation at 20 mA CW. Based on the results of ongoing reliability measurements, the device operation lifetime of the order of 10,000 h at 20 mA CW can be estimated.
5:30 PM - Q2.10
Large Area Aluminum Nitride Substrates for UV Optoelectronics.
Robert Bondokov 1 , Kenneth Morgan 1 , Stephan Mueller 1 , Sandra Schujman 1 , Glen Slack 1 , Leo Schowalter 1
1 , Crystal IS, Inc. , Green Island, New York, United States
Show AbstractFree-standing, nitride-based substrates with dislocation densities of less than 1000 per sq. cm, high crystallinity, and uniformity are a highly desirable platform for the epitaxial growth of the next generation of III-nitride devices. These devices meet the increasing market demand for deep ultra-violet (UV) optoelectronics as well as high frequency applications. The choice of Aluminum Nitride (AlN) as substrate to fill this need offers the advantage of lower lattice and thermal expansion mismatch to Gallium Nitride (GaN) and AlGaN alloys than e.g. sapphire or silicon carbide. Additionally, AlN substrates exhibit high radiation hardness and a thermal conductivity superior to GaN. In this study we discuss the development of large area AlN substrates cut from bulk crystals, grown from the vapor phase by the sublimation-recondensation technique. Parameters influencing the growth rate and crystal quality include the thermal gradient across the growth cell, the nucleation conditions and the seed quality. By optimizing these parameters we have been able to grow large AlN boules for commercial production of AlN wafers up to 2” diameter. A clear advantage of cutting AlN substrates from a bulk crystal is the possibility to prepare wafers with different orientations, i.e. non-polar or quasi-polar orientations, which are not readily obtainable by epitaxial growth on foreign substrates. The lack of both piezoelectric and spontaneous polarization fields, for nitride heterostructures grown in non-polar directions, has recently created significant interest in high quality, non-polar nitride substrates to boost the performance of light emitting diodes (LEDs) and laser diodes.AlN boules and wafers have been characterized by X-ray Laue backscattering, diffraction and rocking curves measured in double axis configuration. We demonstrate AlN substrates with a high crystallinity, showing a full width at half maximum (FWHM) of 28 arcsec and 32 arcsec for the symmetric and asymmetric rocking curves, respectively. The high material quality was also confirmed by measurements of the etch pit densities (EPD) for AlN substrates oriented parallel to the c-, m-, and a-planes. The etch pits were revealed by a preferential chemical etching technique using molten Potassium and Sodium hydroxides in a previously reported setup. The EPD for all AlN substrate orientations varied in the range 100 - 10E4 per sq. cm. The impurity levels were measured by a glow discharge mass-spectrometry (GDMS) and secondary ion mass spectroscopy (SIMS). The oxygen content as measured by SIMS showed values of < 10E18 per cc. The room temperature UV transparency indicates absorption coefficients ≤ 50 cm-1 @ 280 nm and the high thermal conductivity measured by the laser flash method of ~ 270 W/mK correlates well with the low oxygen content and high crystallinity of the AlN substrates.
5:45 PM - Q2.11
Sapphire Nano-Patterning and GaN Nano-heteroepitaxy.
Hongwei Li 1 , Jason Perkins 1 , Sreya Dutta 1 , Yik Khoon Ee 2 , Ronald Arif 2 , Nelson Tansu 2 , Richard Vinci 1 , Helen Chan 1 , Pavel Capek 3 , Naveen Jha 3 , Volkmar Dierolf 3
1 Center for Advanced Materials and Nanotechnology, Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Center for Optical Technologies, Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 3 Center for Optical Technologies, Department of Physics, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractGaN grown epitaxially on single crystal sapphire suffers both high density of misfit dislocations and internal stress due to lattice and thermal mismatch. Nano-heteroepitaxy has been shown as a promising means to reduce dislocations as well as stress in large lattice-mismatched epitaxial systems, but it is difficult to apply to the GaN/sapphire system due to challenges associated with sapphire patterning. Here a novel process of sapphire nano-patterning by Aluminum deposition, Growth of Oxide and Grain growth (AGOG) is demonstrated. Nanoscale aluminum patterns were first fabricated on sapphire substrates through E-beam lithography and lift-off processes, then the aluminum was oxidized in air and fully converted to single crystal sapphire through grain growth under high temperature. Successful conversion is evident in cross section TEM images and by SEM-based electron backscattered diffraction (EBSD). Heteroepitaxy of GaN on the conventional and patterned sapphire substrates was performed using metalorganic chemical vapor deposition (MOCVD) with a low-pressure vertical-type MOCVD system. The growth of the GaN template consists of a ~30 nm nucleation layer of low-temperature GaN layer at 535oC and a 2.8 μm GaN at 1080oC under H2 ambient gas. During the growth of the high-temperature GaN template layer, the flow rate of trimethylgallium (TMGa) and NH3 were 10 μmol/min and 2500 sccm, respectively, corresponding to a V/III ratio of about 1800. Photoluminescence (PL) structure comprising of 4 period In0.18Ga0.82N / GaN quantum wells were subsequently grown on the GaN virtual template. The peak and integrated room temperature (RT) photoluminescence of the quantum wells grown on nano-patterned sapphire shows ~2 times intensity improvement compared to the quantum wells grown on plain sapphire. The dislocation density in the nano-heteroepitaxially-grown GaN is on the order of 108/cm2, as determined from cross sectional TEM characterization. This is on the low end of the typical range for planar substrates, demonstrating that the patterned substrate may be positively influencing the GaN growth even though the processing has not yet been fully optimized. Further optimization on the growth and reducing the AGOG pattern dimensions will also be conducted.
Symposium Organizers
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
Tuesday AM, November 27, 2007
Back Bay B (Sheraton)
9:30 AM - Q3.1
Sublimation Growth and Defect Characterization of AlN Single Crystals.
Shaoping Wang 1 , Balaji Raghothamachar 2 , Michael Dudley 2 , Zaiyuan Ren 3 , Jung Han 3 , Andrew Timmerman 1
1 , Fairfield Crystal Technology, LLC, New Milford, Connecticut, United States, 2 Dept. of Materials Science & Engineering, State University of New York at Stony Brook, Stony Brook, New York, United States, 3 Dept. of Electrical Engineering, Yale University , New Haven, Connecticut, United States
Show AbstractAlN single crystal is a promising substrate material for high quality III-V nitride epitaxy, especially for fabrication of short wavelength (UV and blue) III-nitride-based devices employing high Al concentrations (e.g. 30% Al). This is because AlN has the same crystal structure as GaN, AlxGa1-xN epitaxial layers lattice match all crystal planes of AlN; that cannot be easily done with either sapphire or SiC. AlN also has a high thermal conductivity, III-nitride-based devices fabricated on AlN substrates will be better able to dissipate heat, which will contribute significantly to better efficiency, longer lifetime and lower degradation. In this paper, we report results from AlN single crystal growth experiments carried out using a sublimation physical vapor transport (PVT) technique at high temperatures. Stand-alone AlN single crystal boules up to 7mm in diameter were produced. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy and an etching technique. Selected AlN single crystals were studied using a high-resolution X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Polished surfaces of AlN single crystal wafers were studied using an AFM and a hydrogen etching technique. Major structural defects identified in AlN single crystals include dislocations, grain boundaries and cracks.
9:45 AM - Q3.2
Low-V-defect Blue and Green GaInN/GaN Light Emitting Diodes.
Mingwei Zhu 1 2 , Yong Xia 1 2 , Wei Zhao 1 2 , Yufeng Li 1 2 , Jayantha Senawiratne 1 2 , Shi You 1 2 , Theeradetch Detchprohm 1 2 , Christian Wetzel 1 2
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractGaInN/GaN material system has made rapid progress in recent years for their realization of blue and green light emitting diodes (LEDs) and laser diodes. In order to achieve longer emission wavelength in green and deep green range, higher indium composition is essential in the active region. However, as more indium is incorporated, more defects, including V-defects may be generated. It has been reported that V-defects interrupt the homogeneity of the QWs and act as the non-radiative recombination centers. Therefore, it is necessary to understand and suppress the generation of V-defects in LEDs.In this study, we characterized the structural defects in blue and green GaInN/GaN LEDs by cross-sectional and plan-view transmission electron microscopy (TEM). The samples were grown on bulk GaN or sapphire substrates by metal organic vapor phase epitaxy. LEDs grown on bulk GaN show low defect density in the active region. High resolution TEM measurements further suggest highly homogenous quantum wells and barriers with sharp edges.We observe two types of V-defects in blue and green LEDs. Both types were initiated by edge-type threading dislocations (TDs). Large V-defects with diameters around 500 nm were found in the blue LEDs on bulk GaN. They are initiated around the epitaxial growth boundary. The V-defects density is as low as 2E5cm-2. The optical output power of blue LEDs on bulk GaN is five times better than those on sapphire, which has one order of magnitude higher V-defect density. On the other hand, high density (2E9cm-2) of smaller V-defects with {1-101} facets was observed in the active region of green LEDs. The diameters of these smaller V-defects are approximately 150 to 200 nm. A second set of narrower QWs and barriers were grown on the sidewalls of these V-defects. The TDs that lead to the generation of such V-defects were mostly initiated during the growth of active region. After successfully suppressing the generation of these TDs and smaller V-defects, the optical output power is improved by one order of magnitude.This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860.
10:00 AM - Q3.3
Growth and Characterization of High-Performance GaN and AlxGa1-xN Ultraviolet Avalanche Photodiodes Grown on GaN Substrates.
Russell Dupuis 1 , Dongwon Yoo 1 , Jae Hyun Ryou 1 , Yun Zhang 1 , Shyh-Chiang Shen 1 , Jae Boum Limb 1 , Drew Hanser 2 , Edward Preble 2 , Keith Evans 2
1 School of ECE, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Kyma Technologies, Raliegh, North Carolina, United States
Show AbstractWide-bandgap GaN-based avalanche photodetectors (APDs), such as AlGaN p-i-n diodes, are excellent candidates for short-wavelength photodetection due to the capability of operating in the solar-blind UV spectral region, λ < 290 nm. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of Group III-nitride based APDs. We have demonstrated GaN p-i-n APDs with record-high gains grown on free-standing GaN substrates. In order to achieve intrinsically solar-blind APDs, the use of wider-bandgap material than GaN is required; but the growth of AlGaN APD structures on GaN substrates introduces technological challenges such as less perfect materials quality due to dislocations and strain and even strain-induced cracking, as well as a limitation in doping, etc.. In this study, we investigate on the growth and characterization of the AlGaN-based APDs on GaN substrates.Epitaxial layers of GaN and AlGaN p-i-n ultraviolet avalanche photodiodes on GaN substrates were grown using a Thomas Swan CCS 7x2 close-coupled showerhead MOCVD reactor. Improved crystalline and structural quality for GaN and AlxGa1-xN epitaxial layers were achieved by employing optimum growth parameters on low-dislocation-density bulk GaN substrates in order to minimize the defect density in epitaxially grown materials. In this work, GaN APDs with avalanche gains >10^5 have been obtained. For Al0.05Ga0.95N APDs, a compositionally graded layer from unintentionally doped GaN to Al0.05Ga0.95N was inserted as a strain-management layer for the crack-free growth. The epitaxial layer structure consists of an n-type Al0.05Ga0.95N:Si layer, followed by an unintentionally doped Al0.05Ga0.95N drift region (0.25 μm, n < 5×1016 cm-3), a p-type AlGaN:Mg+ layer, and topped with p-type GaN:Mg++ (heavily doped) contact layer. The devices were fabricated into 30μm- and 50μm-diameter circular mesas. The forward I-V characteristics and low reverse-bias voltage (up to -100V) I-V characteristics were measured. No microplasmas or side-wall breakdown luminescence was visually observed. The photocurrent was obtained using a UV lamp-monochromator operating at a peak emission wavelength of ~250 nm. The avalanche gain reaches a maximum value of ~50 at a voltage of 86.75V. For AlxGa1-xN APDs with higher Al-content, the crack-free growth of thick AlxGa1-xN on a bulk GaN substrate has been investigated by employing various strain management layers such as AlxGa1-xN (x > 0.5) interlayers and/or AlN/GaN multiple short-period superlattice structures. Growth of AlxGa1-xN (x>0.1) PIN structures with higher Al-content and APD device performance of will be reported.
10:15 AM - **Q3.4
Synthesis and Physical Properties of LixZrNCl Superconductors.
Yasujiro Taguchi 1
1 , Institute for Materials Research, Tohoku Universiy, Sendai Japan
Show Abstractβ-ZrNCl and β-HfNCl have been known to become superconductors when doped with electrons by means of alkali-metal intercalation. These superconductors can be thought of as two-dimensional analogue of well-known superconductors, ZrN and HfN with NaCl-type structure. Doped electrons are accommodated into ZrN or HfN double-honeycomb layers, and form a rather simple, two-dimensional electronic state according to recent band calculations. Upon the change in crystal structure and electronic state from three-dimensional one to two-dimensional one, the Tc increases from 10.7 K to 15.2 K, and from 8.8 K to 25.5 K in the case of Zr- and Hf-based materials, respectively. It should be noted that the Tc value of 25.5 K is even higher than those of the materials that are currently used for practical applications, such as NbTi and Nb3Sn. Despite the high Tc value, the difficulties in synthesizing single phase materials and in treating the samples with extreme sensitivity to the air have thus far prevented the systematic investigations on the fundamental properties of these materials. Recently, we have successfully developed a method to obtain single phase samples of LixZrNCl with controlled doping levels, and clarified the electronic phase diagram as well as doping-evolution of physical properties of the materials[1].The as-intercalated samples of LixZrNCl in the lightly doped region of x<0.1 tend to be easily phase-separated into Li-doped and pristine phases, but we found that single phase samples can be obtained by high temperature annealing at 873 K. For thus obtained samples, we carefully confirmed the formation of solid solution, which is a very rare case from a view point of materials chemistry of intercalation compounds. As the doping concentration is reduced from x=0.4, the Tc value remains almost constant, but rapidly increases below x=0.12, taking the maximum value of 15.2 K at x=0.06. At x=0.05, the Tc suddenly disappears and the material turns to an Anderson insulator. Another remarkable feature of the present material is that the electronic specific heat coefficient is much smaller than other superconductors having the similar value of Tc[2]. In the presentation, I would like to discuss about the doping-evolution of the fundamental properties of the present superconductors.This work was done in collaboration with T. Takano, T. Kishiume, A. Kitora, T. Kawabata, M. Hisakabe and Y. Iwasa. [1] Y. Taguchi, A. Kitora, and Y. Iwasa, Phys. Rev. Lett. 97, 107001(2006).[2] Y. Taguchi, M. Hisakabe, and Y. Iwasa, Phys. Rev. Lett. 94, 217002 (2005).
10:45 AM - Q3.5
Investigation on Origin of Efficiency Droops in InGaN-based High-Power Blue Light Emitting Diodes.
Min-Ho Kim 1 2 3 , Martin Schubert 1 2 , Jong-Kyu Kim 1 2 , E. Schubert 1 2 , Hee Seok Park 3 , Yong Jo Park 3 , Joachim Piprek 4
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Electrical, Computer, & Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Central R&D Institute , Samsung Electro-Mechanics, Su-Won, Gyunggi-Do, Korea (the Republic of), 4 , NUSOD Institute LLC, Newark, Delaware, United States
Show AbstractRecently, excellent progress in the III-nitride semiconductor light-emitting devices (LEDs) has paved the way for the use of III-nitride LEDs as a light source suitable for headlamps in automobiles and lighting systems. Such applications generally demand a high optical flux as well as a high luminous efficiency, which mandates a great effort to realize LEDs operating at higher forward currents. It should be noted, however, that most high-power III-nitride LEDs show a severe external quantum efficiency (EQE) droop of about 30~40%, as the forward current increases to 350 mA. The physical origin of the efficiency droop is not yet clearly understood. In this paper, we present simulations on band diagrams, carrier distribution, and light output-current-voltage (L-I-V) characteristics of InGaN/GaN multiple quantum well (MQW) blue LEDs with various parameters including hole mobility, band offset, and polarization effect in order to identify the origin of EQE droop. The LED structure employed in the simulation consists of 5 period MQWs of 3 nm-thick In0.2Ga0.8N wells and 18 nm-thick GaN:Si barriers and a p-type Al0.13Ga0.87N electron blocking layer, followed by a p-type GaN layer. These reference high-power blue LED chips show an optical output power of about 250 mW at a forward current of 350 mA (J = 35 A/cm2). The simulated L-I-V characteristics of the reference LED structure are in excellent agreement with experimental results.Our simulations have shown that the electron overflow over the EBL indeed occurs even at a low forward current, and increases linearly with increasing forward current, resulting in the external quantum efficiency (EQE) droop of ~ 25%. It is also shown that when the hole mobility decreases from 10 to 1 cm2/Vs, the efficiency droop further increases from 25% to 37% due to a significant increase in electron overflow. Furthermore, our simulations reveal that piezoelectric polarization fields introduced in the EBL and the MQW region have significant role in the EQE droop. Based on our findings, a promising LED structure with minimized efficiency droop will be presented and discussed in detail.
11:45 AM - Q3.7
Growth and Characterization of Non-polar GaN Multi-Quantum-Well Structures on LiAlO2.
H. Behmenburg 1 , A. Alam 1 , Y. Dikme 1 , B. Dlugosch 2 , C. Sommerhalter 2 , N. Rzheutski 3 , R. Schreiner 1 , E. Lutsenko 3 , A. Gurskii 3 , G. Yablonskii 3 , M. Heuken 1
1 , AIXTRON AG, Aachen Germany, 2 , AIXTRON Inc., Sunnyvale, California, United States, 3 , National Academy of Sciences of Belarus, Minsk Belarus
Show Abstract12:00 PM - Q3.8
Green Light Emitting Diodes under Photon and Electron Beam Modulation.
Yufeng Li 1 2 , Jayantha Senawiratne 1 2 , Yong Xia 1 2 , Mingwei Zhu 1 2 , Wei Zhao 1 2 , Theeradetch Detchprohm 1 2 , Christian Wetzel 1 2
1 Future Chips Constellation, Rensselaer Polytechinic Institute, Troy, New York, United States, 2 Physics, Applied Physics and Astronomy, Rensselaer Polytechinic Institute, Troy, New York, United States
Show AbstractEfficiency is a subject of continued debate in green and deep light emitting diodes (LEDs) that employs active regions of GaInN/GaN multiple quantum wells (MQWs). It is now widely recognized that piezoelectric (PZ) fields play a dominant role in the spectral emission properties of such strained MWQs structures. It therefore must be expected, that piezoelectric properties should also control their power performance and efficiency. Here we therefore present a study of the efficiency of green (520-545 nm) GaInN/GaN MQWs LEDs under external modulation of bias illumination and electron beam excitation. In this way we explore the role of the electrical field inside the quantum well and its impact on the optical power of the device. We modulate the electroluminescence performance in operational devices under a bias of laser and electron beam excitation and use lock-in technique for sensitive detection. As a function of drive current, we find a modulation of the electroluminescence (EL) power that well supersedes photoluminescence (PL) and cathodoluminescence (CL) by itself. At low diode current (1 mA), resonant laser excitation (488 nm) is found to increase LEDs efficiency by 19% in higher light-output dies (A) and 7% in lower light-output dies (B). The enhancement increases with the driving current up to 4-5 mA. The enhanced intensity at 5 mA is 4 times larger than that of PL. We attribute the extra luminescence to an improved efficiency of the device. The same behavior is observed in a commercial green LED. Under electron beam modulation, the same phenomenon occurs but with a different magnitude. The resulting effect is not a simple modulation of the electrical transport properties as concluded from the current-voltage characteristics. Upon further investigation we expect clues as to the origin and mechanisms of green LED performance sloop. This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860.
12:15 PM - Q3.9
Nanoporous GaN p-n Junctions Fabricated by a Simple Chemical Vapor Deposition Approach.
Dominique Drouin 1 , Juan Carvajal 2 , M. Aguilo 2 , Arnaud Beaumont 1 , F. Diaz 2 , J. Rojo 3
1 Nanofabrication and nanocharacterization research center, Electrical and computer engineering, Universite de Sherbrooke, Sherbrooke, Quebec, Canada, 2 Física i Cristalolografia de Materials, Universitat Rovira i Virgili, Tarragona Spain, 3 , GE Global Research, Niscayuna, New York, United States
Show AbstractThe unique properties that porous semiconductor materials exhibit compared to their bulk counterparts have propelled the utilization of these materials in the fabrication of devices with enhanced functionality for advanced microelectronics, sensors, interfacial structures and catalysis. Among these materials, wide bandgap semiconductors, and specially GaN are expected to potentially contribute to the advancement of novel technologies in magnetism, catalysis, and biotechnology. The actual application of these materials does, however, critically hinge on the development of processing methods able to precisely control the optical and electrical properties of the resulting porous materials. A major difficulty for the application of these materials also appears with the lack of simple routes for their integration on the most common semiconductor technologies, based on silicon.Using a simple chemical vapor deposition approach based on the direct reaction of gallium and ammonia, we have been able to grow n-type nanoporous GaN particles with pore sizes below 100 nm on Si (111) and Si (100) substrates. SEM analysis of the nanoporous GaN crystalline micron-size particles reveals an almost regular array of nanopores closely aligned along the [0001] crystallographic direction. By introducing Mg3N2 during the growth process we were able to dope GaN particles with Mg while still maintaining the nanoporosity, which allowed producing p-type GaN in a simple and costless synthetic route. Furthermore, in a two step growth process we were able to produce nanoporous GaN p-n junctions, by producing first n-type GaN nanoporous particles, and then, in a second step, growing Mg-doped GaN on these nanoporous particles. The resulting particles with a p-n junction in them and with diameters around 1.5 um still exhibited porosity in one of their surfaces.Cathodoluminescence studies of these structures revealed a sharp and well defined band-gap emission in pure GaN porous particles that shifted to longer wavelengths and broadened, while the intensity of the peak decreased. The p-n junctions fabricated following this simple approach showed emission at around 395 nm. In all cases, the porous faces of the samples showed stronger emission that the rest of their flat faces, allowing a better efficiency of light extraction when compared with conventional flat GaN p-n junctions, with benefits for applications in high bright white LEDs that require increased external quantum efficiency.Compared to other reported approaches, this process is unique in that it results in the formation of nanoporous p-n junctions already integrated on Si substrates during the growth process without requiring any post-growth treatment.
12:30 PM - Q3.10
Photoluminescence of Gallium Nitride in Air with Acidic and Basic Vapors.
Vidhya Chakrapani 1 , John Angus 1 , Kathleen Kash 1 , Chandrashekar Pendyala 2 , Mahendra Sunkara 2
1 , Case Western Reserve University, Cleveland, Ohio, United States, 2 , University of Louisville, Louisville, Kentucky, United States
Show AbstractIn humid air the yellow band luminescence increases in intensity and is blue-shifted in the presence of HCl vapors; in the presence of NH3 the intensity decreases and is red-shifted. The intensity of the near-band-edge luminescence decreases in HCl vapors and increases in NH3 vapors. These observations are interpreted as arising from electron transfer between the GaN and an electrochemical redox couple in an adsorbed water film on the GaN surface. In equilibrium the surface Fermi level of the GaN is equal to the electron chemical potential in the water film, which in turn is fixed by the oxygen redox couple,O2 + 4H+ + 4e− = 2H2O. The electron chemical potential of the redox couple is a function of pH and spans the energy range of the midgap states. At low pH the chemical potential is low in the gap and midgap states are emptied; at high pH the electron chemical potential is higher in the gap, filling midgap states. This process directly mediates the intensity and average photon energy of the yellow band emission. This interpretation of the effect of ambient on the luminescence from GaN is consistent with prior observations [1-4] and may explain disparate results obtained from experiments done in air. Electrochemically mediated charge transfer is not unique to GaN and is believed to be responsible for the unusual p-type surface conductivity observed in hydrogen-terminated diamond [5-7]. In humid air, adsorbed water films are ubiquitous [8] and related effects may occur in other semiconductors as well.[1] M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005).[2] M. A. Reshchikov, M. Zafar Iqbal, D. Huang, L. He, and H. Morkoc, Mat. Res. Soc. Symp. Proc. 743, L11.2.1 (2003).[3] M. Z. Iqbal, M. A. Reshchikov, L. He, and H. Morkoc, J. Electron. Mater. 32, 346 (2003).[4] U. Behn, A. Thamm, O. Brandt, and H. T. Grahn, J. Appl. Phys. 87, 4315 (2000). [5] M. I. Landstrass, and K. V. Ravi, Appl. Phys. Lett., 55, 975 (1989).[6] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472-3475.[7] V. Chakrapani, S. C. Eaton, A. B. Anderson, M. Tabib-Azar, and J. C. Angus, Electrochem. and Solid State Lett. 8 (2005) E4-E8[8] A.W. Adamson, "Physical Chemistry of Surfaces," John Wiley, NY, 4th edition, 1982.
Tuesday PM, November 27, 2007
Back Bay B (Sheraton)
3:00 PM - Q4.2
Molecular Beam Epitaxy of Nonpolar Cubic AlxGa1-xN/GaN Epilayers.
Donat As 1 , Stefan Potthast 1 , Joerg Schoermann 1 , Elena Tschumak 1 , Marcio de Godoy 1 , Klaus Lischka 1
1 Department of Physics, University of Paderborn, Paderborn Germany
Show AbstractGroup III-nitrides crystallize in the stable wurtzite structure or in the metastable zincblende structure. An important difference between these material modifications is the presence of strong internal electric fields in hexagonal (wurtzite) III-nitrides grown along the polar c-axis, while these “build-in” fields are absent in cubic (zincblende) III-nitrides. Since polarization fields can limit the performance of devices some attention has been focused recently on the growth of wurtzite structures with nonpolar orientations e.g., growth along a, m or R directions and also on cubic nitrides. The cubic III-nitride polytype is metastable and can only be grown successfully in a narrow window of process conditions. For the fabrication of electronic devices it is essential to realize AlxGa1-xN epilayers with a well defined Al mole fraction x which determines e.g. the barrier height of heterojunctions.Nonpolar cubic AlxGa1-xN films were grown by molecular beam epitaxy on freestanding 3C-SiC (001) substrates with an Al mole fraction of x = 0 to 1. Using the intensity of a reflected high energy electron beam as a probe we find optimum growth conditions of c-AlGaN when a one-monolayer gallium coverage is formed at the growing surface. Clear reflection high energy electron diffraction oscillations during the initial growth of AlxGa1-xN/GaN layers were observed. The growth rate was about 177 nm/h. We find that the aluminium mole fraction is only determined by the aluminium flux, and that the AlxGa1-xN growth rate is independent on the aluminium content. Atomic force microscopy exhibits smooth surfaces with a RMS roughness of about 5 nm on 5x5 µm2 areas. Cathodoluminescence spectroscopy revealed clear band edge emission up to an aluminium mole fraction of x = 0.52, showing a linear relation between the band gap energy and the Al composition.
3:15 PM - Q4.3
Two- and Three-Dimensional Design of InGaN White Light Emitting Diodes Nanostructures.
Zhiwen Liang 1 , Edwin Garcia 1
1 Materials Engineering, Purdue University, West Lafayette, Indiana, United States
Show Abstract3:45 PM - Q4.5
Multifunctional Ultracomposites: Piezoelectric Materials Grown on Binary Metallic Glasses.
Michael Brougham 1 2 , Colin Ophus 1 2 , Steven Melenchuk 1 2 , Jia Luo 1 2 , Erik Luber 1 2 , Mohsen Danaie 1 2 , Fraser Forbes 1 , Velimir Radmilovic 3 , Zonghoon Lee 3 , David Mitlin 1 2
1 Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, 2 , National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 NCEM, Lawrence Berkeley National Laboratory, University of California, Berkeley, California, United States
Show AbstractAs MEMS devices enter the regime of the nanoscale, it becomes increasingly important to effectively scale each component. The present study explores the feasibility and advantages of a new class of "multifunctional ultracomposites" by examining the characteristics of a key piezoelectric, AlN, grown on a novel metastable alloy, Al-32 at.% Mo. Using a parameter optimization procedure and advanced tools such as XRD and TEM, we have demonstrated the following: the ultrasmooth surface of AlMo combined with its incredibly high nucleation site density together enable a competing growth pattern whereby the desired piezoelectric (0002) orientation dominates from the very outset of growth. Our initial results also suggest a third contributing factor and new physical phenomenon namely local epitaxial growth on nanocrystallites through the enhanced mechanical compliancy of the amorphous matrix. Taken together, this novel approach opens new possibilities in the NEMS-scale miniaturization of both actuation and detection capabilities with applications including self-sensing nanorobotics and scanning probe microscopy.
4:30 PM - Q4.6
GaN Nanowalls Grown by RF-plasma Assisted Molecular Beam Epitaxy.
Akihiko Kikuchi 1 2 , Takayuki Hoshino 1 , Shunsuke Ishizawa 1 2 , Hiroto Sekiguchi 1 2 , Katsumi Kishino 1 2
1 Engineering of Electrical and Electronics, Sophia University, Tokyo Japan, 2 , CREST, JST Japan
Show AbstractSemiconductor nanocrystals are promising candidates for future nano-photonics and nano-electronics applications. Recently, there are many repots on growth and device application of semiconductor nanocrystals. We also have been reported on GaN nanocolumns, that is self-assembled one-dimensional columnar nanocrystals [1] with superior optical characteristics [2] and their LED applications [3-4].
In this study, we demonstrated growth of two-dimensional GaN nanocrystals that is nanowalls. GaN nanowalls with the width of 300 nm, height of 1000 nm and length of over 100 um were successfully grown by rf-plasma assisted molecular beam epitaxy (RF-MBE) on GaN template substrate. The GaN nanowalls were grown c-axis (0001) perpendicular to the substrate surface and having very smooth side planes. The width of the nanowalls was same from the bottom to the top. It is considered that for the GaN nanowalls along the [1_100] direction have a-plane (11_20) as side-planes and for the nanowalls along [11_20] direction have m-plane (1_100) as side-planes.
The GaN nanowalls were grown on GaN template. The GaN template was grown by a metal-organic vapor deposition (MOCVD) on a (0001) sapphire substrate. Prior to the growth, titanium metal was deposited on the MOCVD-GaN template with electron beam deposition and stripe windows with ~300 nm width was open by electron beam lithography and dry-etching technique to appear the GaN surface.
In generally, it was known that the selective growth of III-V semiconductors by MBE is quite difficult compare to the MOCVD method. But as shown in the our previous study on GaN nanocolumn [1], RF-MBE can grow high aspect nanocrystals under a optimum growth conditions. By use of nanocolumn growth condition for the metal masked GaN template with stripe windows, we could achieve a complete selective growth of GaN nanowalls by RF-MBE.
From the SEM observation, it was confirmed that the GaN nanowalls were quite uniform and aligned very well. For example, a hundred nanowalls with a 300 nm width, 1000 nm height and 150um length were ordered with 100nm spacing. We also demonstrated some other structures with GaN nanowalls such as single stripe, hexagonal ring cavity, and mesh pattern.
The well controlled GaN nanowalls wil be used for many kinds of nano-electronic and -photonic devices such as laser diodes, LEDs, optical circuits, FETs, and so on.
Acknowledgements:
This study was supported by a Grant-in-Aid for Scientific Research #18069010 and #18310079 from the MEXT, and NEDO Industrial Technology Research Grant #02A23041d.
References:
[1] M. Yoshizawa, A.Kikuchi, M.Mori, N.Fujita and K.Kishino, Jpn. J. Appl. Phys 36, L459 (1997).
[2] A. Kikuchi, K. Yamano, M. Tada and K. Kishino, phys. stat. sol. (b), 241, 2754 (2004).
[3] A. Kikuchi, M. Kawai, M. Tada and K. Kishino, Jpn. J. Appl. Phys. 43, L1524 (2004).
[4] A. Kikuchi, M. Tada, K. Miwa and K. Kishino, Proc. SPIE 6129, 6129-05 (2006).
4:45 PM - **Q4.7
Semiconducting and Metallic Perovskite Nitrides: Structures and Properties.
Rainer Niewa 1
1 Chemistry, TU Munich, Garching Germany
Show AbstractInverse Perovskite chemistry emerges to display all basic crystallographic features known from normal Perovskites. However, chemistry and physical properties differ substantially. Perovskite nitrides of the general composition (Ca3N)E with E = P, As, Sb, Bi, Ge, Sn Pb, Tl are known for some years. All these compounds crystallize as cubic Perovskites or distortion variants thereof [1, 2]. The compounds of group 15 elements obey the (8 - N) rule and accordingly show properties compatible with semiconductors, while the compounds with E = Tl and E = Ge, Sn, Pb were described as electron-deficient metals. Compounds (A3N)E with A = Sr, Ba and E = Sb, Bi crystallize as cubic (Sr) and hexagonal 2H (Ba) Perovskites and represent diamagnetic semiconductors [3]. On gradual substitution of Sr by Ba (i. e. the quaternary system (Sr3-xBaxN)Bi) partial order Sr-Ba leads to phases with 4H and 9R Perovskite structures. The intriguing order scheme of the alkaline-earth metal ions presents the possibility of stacking engineering of such hexagonal Perovskites [4]. On insertion of oxygen recently even the n = 1, 3 members of a Ruddlesden-Popper series with the general composition ”n (A3Z)Bi ● ABi” (Z = N, O) and Sr–Ba site preference were obtained [5]. By exchange of E = group 15 elements by the E = group 14 elements Sn, Pb exclusively cubic Perovskites (A3Nx)E with x in the range of 0.60 < x < 0.85 and intrinsic defects in the nitride substructure are obtained [6]. Electrical resistivity and magnetic susceptibility studies indicate these phases to exhibit metallic properties. The composition with x = 2/3 would obey the (8 – N) rule, consequently one might expect semiconducting properties. However, electronic band structure calculations on ordered superstructure models reveal metallic behavior and indicate the tendency to higher nitrogen site occupancy as was observed in experiments. This chemistry is prolonged on turning to rare-earth metal systems of nitrogen, carbon and oxygen as compared to the alkaline-earth metal systems with nitrogen. Here, typically metallic properties of Perovskite compounds containing rare-earth metal species in the electronic R3+ state are obtained. Eu and Yb present exceptions. The introduction of rare-earth metals in such systems leads to extensive magnetic order phenomena [7]. [1] M. Y. Chern, D. A. Vennos, F. J. DiSalvo, J. Solid State Chem. 1992, 96, 415.[2] R. Niewa, W. Schnelle, F. R. Wagner, Z. Anorg. Allg. Chem. 2001, 627, 365.[3] F. Gäbler, M. Kirchner, W. Schnelle, U. Schwarz, M. Schmitt, H. Rosner, R. Niewa, Z. Anorg. Allg. Chem. 2004, 630, 2292.[4] F. Gäbler, R. Niewa, Inorg. Chem. 2007, 46, 859. [5] F. Gäbler, Yu. Prots, R. Niewa, Z. Anorg. Allg. Chem. 2007, 633, 93.[6] F. Gäbler, M. Kirchner, W. Schnelle, M. Schmitt, H. Rosner, R. Niewa, Z. Anorg. Allg. Chem. 2005, 631, 397.[7] M. Kirchner, W. Schnelle, R. Niewa, Z. Naturforsch. 2006, 61b, 813 and references therein.
5:15 PM - Q4.8
LED it be.
Andries Meijerink 1 , Volker Bachmann 1 2 , Cees Ronda 1 2
1 Chemistry, Debye Institute, Utrecht Netherlands, 2 , Philips Research Labortaories, Aachen Germany
Show AbstractThe discovery of blue emitting GaN LEDs had an enourmous impact on the lighting industry because it makes it possible to generate white light with LEDs. The market for white light emitting LEDs is now expanding rapidly. From niche applications, like flashlights and traffic lights, white light LEDs are finding their way into general lighting applications. This has an impact on phosphor research. New phosphors are needed efficiently absorb in the near UV to blue spectral range and emit in the visible. The energy difference between excitation and emission wavelength is small which is good for the energy efficiency, but it lowers the choice of activator ions that can be used. Research is mainly focused on Eu2+ and Ce3+. For an LED phosphor to be applied in commercial products several criteria have to be met such as: high quantum efficiency, high thermal quenching temperature, and the possibility to adjust the color. In this contribution the luminescence properties of various LED phosphors will be discussed to understand the mechanism for thermal quenching and energy transfer processes that determine the luminescence characteristics. First, the widely applied phosphor YAG:Ce will be revisited. It is shown that the intrinsic luminescence quenching temperature is very high (above 700 K). The variation in the (lower) quenching temperatures reported in the literature is explained on the basis of thermally activated concentration quenching and the temperature dependence of the oscillator strength of the 450 nm absorption band.Next, a new class of LED-phosphors will be discussed: oxynitrides doped with Eu2+. The luminescence and luminescence quenching mechanism for the Eu2+ and Yb2+ luminescence in SrSi2O2N2 will be presented. The Eu2+ doped compound is a very promising material for application in white light LEDs due to the luminescence quantum efficiency (>90%), the stability and the high luminescence quenching temperature. Only above 600 K the luminescence quenches. Comparison with the (anomalous) luminescence of Yb2+ in the same host show that the quenching mechanism is thermally activated photoionization from the f-d excited state. Finally, color tuning will be discussed. It is important to shift the emission to longer wavelengths (orange or red) to change the cool white light from LEDs with YAG:Ce to a warmer white for home lighting. For SrSi2O2N2:Eu and YAG:Ce it will be shown how the emission color can be tuned by changing the chemical composition, while retaining good thermal quenching behavior. Finally an outlook will be given on the future of solid state lighting based on GaN LEDs based on the rapid developments in this field.
5:30 PM - Q4.9
Polarization Anisotropy in the Light Emission of Blue GaInN/GaN Light-emitting Diodes Grown on (0001) Oriented Sapphire Substrates.
Martin Schubert 1 , Sameer Chhajed 1 , Jong Kim 1 , E. Fred Schubert 1 2 , Jaehee Cho 3
1 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, Applied Physics, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Opto System Laboratory, Corporate R&D Institute, Samsung Electro-Mechanics, Suwoon Korea (the Republic of)
Show AbstractPolarized light-emitting diodes (LEDs) would be highly advantageous for applications such as liquid crystal display backlighting, sensing, imaging, and free space optical communications. Previously, partially polarized light emission has been reported for GaInN/GaN LEDs grown on non-polar or semi-polar substrates. However, the polarization of light emitted by LEDs grown on conventional polar sapphire substrates has not been thoroughly examined. Here we report measurements of the polarization of light emission by blue 460 nm GaInN/GaN LEDs with multi-quantum-well (MQW) active regions grown on (0001) oriented sapphire substrates. Light emitted from conventionally packaged devices and bare unpackaged chips is measured over a wide range of emission angles from vertically above the LED surface (φ = 0°) to nearly directly below it (φ = 160°). For the unpackaged chips, ray-tracing simulations are used to calculate the angular distribution for top emitted light in order to distinguish top-emitted light from light emitted through the chip facets. It is found that unpackaged chips emit a majority of light from the facets, and that this side emission is highly polarized with the electric field in the plane of the MQW. The ratio of intensity of in-plane polarized light to normal-to-plane polarized light reaches values as high as 7:1. When integrated over all emission angles, the total power of in-plane polarized light is found to be more than double the power of normal-to-plane polarized light. The packaged devices do not show any polarization anisotropy, and emit each polarization with equal intensity due to the design of the LED chip packaging.
Symposium Organizers
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
Wednesday AM, November 28, 2007
Back Bay B (Sheraton)
9:30 AM - Q5.1
Host Dependence of Photoluminescence from Eu-implanted AlGaN Alloys.
Kevin O'Donnell 1 , Ke Wang 1 , Benjamin Hourahine 1 , Robert Martin 1 , Katharina Lorenz 2 , Eduardo Alves 2 , Ian Watson 3
1 Physics, University of Strathclyde, Glasgow United Kingdom, 2 , ITN, Sacavem Portugal, 3 Photonics, University of Strathclyde, Glasgow United Kingdom
Show AbstractThe photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted and annealed AlxGa1-xN alloy are obtained over the whole composition range for the first time. A single Eu centre, which can be photoexcited both above and below the bandgap, dominates the PL of the 5D0-> 7F2 band, which is found near 622 nm in pure GaN. The emission band broadens and then narrows, similar to an alloy exciton, but downshifts by only 7 meV (~2.2 nm) as x increases from 0 to 1. The trans-gap PLE spectrum of Eu emission measures the bandgap energy of AlGaN from 3.5 towards 6.2 eV. Below gap, we identify 2 related excitation bands: X1 has previously been observed in GaN at 380 nm (3.25 eV); it upshifts linearly by 0.23 eV as x increases to 1. A second band, X2, with a very similar energy shift with composition, emerges when x increases above 0.5. In AlN, the energy of X2 is 4.59 eV. We identify X1,2 as bound-exciton like complexes of Eu emitting centres, formally equivalent to charge transfer states. Hence the emission mechanism of Eu doped III-nitride luminescence is clarified and the need for “RE-related deep defects” to “sensitise” or “activate” that luminescence is eliminated by these studies.
9:45 AM - Q5.2
Electron Band Structure of MnGaN.
Dimiter Alexandrov 1 , Nikolaus Dietz 2 , Ian Ferguson 3 , Hang Yu 1
1 Electrical Engineering, Lakehead University, Thunder Bay, Ontario, Canada, 2 Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia, United States, 3 School of Elect. & Comp. Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractTheoretical determination of the electron band structure of MnGaN semiconductor compound alloy is performed in this paper. The authors model the MnGaN alloy on Mn content. Segregated Mn species in wurtzite GaN containing Mn are not considered, only alloyed species with Mn substituting on the Ga site. For this arrangement a new ternary semiconductor MnGaN is identified and it has two binary constituents – GaN (the existing binary crystal) and MnN that exists in the structure of MnGaN. It allows that MnN primitive cell satisfying the following conditions to be introduced: a) Mn atom substitutes for a Ga atom saving the tetrahedral shape of the crystal cell; b) Mn atom has valence equal to 2 and the N atom has valence equal to 5; c) Mn-N bond is ion-covalent. Using the conditions a), b) and c) the authors have found that the 2p orbital of N atom attracts an electron from the 3d orbital of Mn atom and this electron replaces on the 4p orbital of Mn atom. In this way Mn-N bond becomes of sp3 type. The following parameters of MnN binary constituent are found: 1) The energy band gap of tetrahedral MnN is 6.10 eV. 2) Charge transfer between 3d-orbital of Mn atom and 4p-orbital of the same atom is one electron and as result the valence electron cloud of Mn atom is replaced in direction to nitrogen atom, i.e. donor behavior can be expected from the alloyed Mn atoms. 3) Using the Hartree-Fock method it is found that the electron energy of the 4p orbital of Mn atom, which forms MnN tetrahedral cell, is -3.41 eV. 4) Using the valence orbital radii of both atoms Mn and N and considering the tetrahedral shape of MnN cell, the distance between Mn and N atoms is found to be 1.56 Angstrom. LCAO electron band structure of wurtzite MnGaN for points Γ is calculated in term of optical properties and in consideration of the following ratio Mn:Ga forming quasi-elementary cells: 1) GaN quasi-elementary cell. 2) mixed Mn-GaN quasi-elementary cell containing 1.5 atoms of Ga and 0.5 atoms of Mn; 3) mixed Mn-GaN quasi-elementary cell containing 1 atom of Ga and 1 atom of Mn; 4) mixed Mn-GaN quasi-elementary cell containing 0.5 atoms of Ga and 1.5 atoms of Mn; 5) pure MnN quasi-elementary cell. This electron band structure has the following features: a) Inter-band optical absorption can be observed for photon energy ~1.60 eV, which is close to the experimentally observed absorption band ~1.50 eV in the samples of MnGaN without silicon impurities; b) Tunnel optical absorption can be observed for photon energies ~1.59 eV and greater than 1.86 eV. c) Peaks of the PL spectra can be observed for energies 1.60 eV, 2.05 eV that is close to the experimental value of 2.20 eV, and ~2.67 eV, which is close to the experimental value of 2.70 eV. The exact value of the PL peak depends on the technological circumstances of the ratio Mn:Ga forming mixed quasi-elementary cells in the structure of MnGaN. The same is valid for domination of either inter-band or tunnel optical absorption.
10:00 AM - Q5.3
Deep Ultraviolet Photoluminescence Studies of AlN Epilayers Grown on Different Substrates.
Neeraj Nepal 2 , B. Pantha 2 , T. Tahtamouni 2 , J. Li 2 , M. Nakarmi 2 , J. Lin 2 , H. Jiang 2 , J. Zavada 3
2 Department of Physics, Kansas State University, Manhattan, Kansas, United States, 3 , U. S. Army Research Office, Durham, North Carolina, United States
Show AbstractAlN is an ideal material for applications in ultraviolet (UV) emitters and detectors active in the spectral wavelength region down to 200 nm. The increased importance of AlN for various applications and the diverse choices of substrates call for a systematic comparative study concerning the basic structural and optical properties of AlN epilayers grown on different substrates, which however has been highly challenging because the growth conditions of AlN epilayers on different substrates have to be independently optimized. We report on the epitaxial growth, and DUV photoluminescence (PL) and x-ray diffraction (XRD) studies of AlN epilayers grown on sapphire, SiC, Si, and AlN bulk single crystal substrates. The variations of the free exciton emission peak position and lattice constant with respect to the growth substrate have been systematically probed and correlated. It was confirmed by PL and XRD results that AlN epilayers grown on AlN bulk substrates (or homo-epilayers) have the same lattice parameters as AlN bulk crystals and strain-free, while AlN hetero-epilayers grown on sapphire substrates experience a compressive strain and those grown on SiC and Si substrates undergo a tensile strain. Based on the variations of the FX emission peak position and lattice constant with respect to the growth substrate, a linear relationship between the FX transition and in-plane stress was obtained and a value of 45 meV/GPa for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced, which is about 88% larger than that in GaN. The work here establishes PL as an alternative simple and effective method for monitoring the substrate induced biaxial stress in AlN epilayers.* Presently at Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC.
10:15 AM - **Q5.4
Shock Wave Synthesis and Exploration of High-pressure Nitrides and Related Materials.
Toshimori Sekine 1
1 , National Institute for Materials Science, Tsukuba Japan
Show AbstractShock wave achieves high pressure and high temperature states that give us unique conditions for material synthesis. We have carried out shock wave synthesis and exploration of high-pressure nitrides and related systems recently. We investigated the high-pressure phases by shock recovery experiments and in situ measurements of shocked materials. The samples obtained after shock-loadings were characterized by the techniques of XRD, TEM, NMR, DTA and the Hugoniot data were determined by the inclined mirror method to record wave velocities. These results will be reviewed. We have successfully developed a method to manufacture spinel-type Si3N4, which can be synthesized only above ~15 GPa, and also a chemical treatment method to separate the spinel-type phase from the low-pressure phase. Similar methods could be applied for the SiAlON systems. In order to explore high-pressure nitrides and oxynitrides, we extended toward the system Si3N4-AlN-Al2O3. According to the results of in situ measurements of the high-pressure behavior up to pressures of 200 GPa, there appears to be post-spinel phase in the system. This is consistent with the results from the first principles calculations. However we could not obtain the post-spinel phase by the shock recovery experiments at present. We also carried out shock recovery experiments on carbon nitrides. We employed several reactions including the followings: (1) X CBr4 + 4X NaN(CN)2 = [C3N4]3X + 4X NaBr , (2) 3 C2N4H4 = 2 C3N4 + 4 NH3 , (3) C3N6H6 = C3N4 + 2 NH3 .Nitrogen-rich starting materials, included a C-N-O amorphous precursor, dicyandiamide, melamine, and a mixture of carbon tetrahalide and sodium dicyanoamide, were used and the recovered samples were investigated by X-ray diffraction technique, elemental analysis, transmission electron microscopy and so on. Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock pressure of 37 GPa, and production of amorphous C-N materials with a highest N/C ration of 1.26 from the reaction between carbon tetrahalide and sodium dicyanoamide. We extended toward the system C3N4-Si3N4 after taking into account the results on shock synthesis of spinel-type nitrides.
10:45 AM - Q5.5
Superfluorescence in Green Emission GaInN/GaN Quantum Well Structures under Pulsed Laser Excitation.
Jayantha Senawiratne 1 2 , Stephanie Tomasulo 2 , Theeradetch Detchprohm 1 2 , Mingwei Zhu 1 2 , Yufeng Li 1 2 , Wei Zhao 1 2 , Yong Xia 1 2 , Peter Persans 2 , Christian Wetzel 1 2
1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractGaInN/GaN quantum well (QW) structures are the system of choice for UV and blue laser diodes and also hold the biggest promise for green laser diodes. We report optical investigation of green emission GaInN/GaN multi-QW (MQW) structures near lasing conditions under pulsed and continuum (cw) laser excitation. We compare the optical properties of c-plane grown MQW with m-plane MQW structures under strong laser field.GaInN/GaN MQW structures have been grown by metal organic vapor phase epitaxy on c-plane sapphire and m-plane on GaN substrates. The MQW consists of five GaInN/GaN quantum wells of nominal well width of 3 nm separated by barriers of nominal width 11 nm. High pump intensity measurements were carried out with a nitrogen laser (λ = 337.1 nm) with 10 ns pulse width and maximum pulse energy of 0.1 mJ. Low pump intensity measurements were carried out with a HeCd cw laser of wavelength 325 nm. Photoluminescence (PL) was collected in surface and edge emission configurations under variable excitation power. Edge emission PL of a sample grown on the c-plane of sapphire substrates blue shifts by 200 meV as the pump intensity is increased. In contrast, the PL blue shift measured under the same conditions is less than 10 meV for a sample grown on the nonpolar m-plane of the bulk GaN substrate. A large PL blue shift is explained by considering the screening of large piezoelectric polarization in the c-plane grown sample. Under intense pulse excitation, the full width half maximum of PL narrows to ~4 nm and couples out of the sample in two distinct modes. Variation of the PL intensity with excitation stripe length is consistent with superfluorescence.This work was supported by a DOE/NETL Solid-State Lighting Contract ofDirected Research under DE-FC26-06NT42860
11:30 AM - Q5.6
Magnetic Exchange Interactions in Mn-doped ScN.
Aditi Herwadkar 1 , Walter Lambrecht 2 , Mark Schilfgaarde 3
1 Department of Physics and Astronomy, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Department of Physics, Case Western Reserve University , Cleveland , Ohio, United States, 3 Department of Chemical and Materials Engineering, Arizona State University , Tempe, Arizona, United States
Show AbstractScN is a relatively little studied semiconductor with rocksalt structure. Here we study the possibility of using ScN as a diluted ferromagnetic semiconductor host using Mn doping. We present a study of the exchange interactions in Mn-doped ScN using the linear response approach implemented in the linear muffin-tin orbital Green's function method. In this approach, exchange interactions are calculated directly from the Green's function and the change in scattering t-matrix due to a rotation of a spin within the rigid spin approximation (maintaining constant spin density direction within each atomic sphere). The validity of the approach is tested by comparison with direct total energy calculations in small cells of 64 atoms with different collinear and non-collinear magnetic configurations. Special quasirandom structure cells of 256 and 432 atoms are used with different concentrations of Mn. We find that the exchange interactions are long-range and strongly influenced by the disorder in the system. It is found crucial to include a gap correction beyond the local density approximation. Without it ScN is a zero gap semiconductor and the exchange interactions turn out to become dominated by long-range antiferromagnetic interactions leading to a spin-glass state rather than a ferromagnet. This is confirmed by the fact that the lowest spin-wave excitation has negative energy. Including a gap correction, however, we find ferromagnetic interactions to be dominating up to a distance of about 3 cubic lattice constants. The concentration dependence is studied and the critical temperatures are estimated using a recently introduced extension of the cluster variation method for the Heisenberg Hamiltonian. We also study the effect of adding nitrogen vacancies which appears to further increase the critical temperature. Above room temperature Tc's appear feasible with modest Mn concentrations.
11:45 AM - Q5.7
Blue Light Emitting Diodes Based on ZnO/GaN Wafer Bonding.
Akihiko Murai 1 , Daniel Thompson 1 , Natalie Fellows 1 , Hitoshi Sato 1 , Umesh Mishra 1 , Shuji Nakamura 1 , Steven DenBaars 1
1 , University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractWe have investigated blue light emitting diodes (LEDs) based on a wafer bonding of a ZnO wafer to a III-nitride LED wafer for the purpose of obtaining high light extraction efficiency from a LED chip. The light extraction efficiency is critically important for improving external quantum efficiency of LEDs. A single crystal n-type ZnO wafer has the characteristic advantages of high transparency, electrical conductivity, and shape processing for using a transparent electrode to p-type GaN. For this study, we used a III-nitride LED layer grown on a (0001) sapphire wafer by metal-organic chemical vapor deposition and a commercially available n-type ZnO wafer. After surface cleaning, the two wafers were joined together and uniaxial pressure of 2 MPa was applied and thermal process (800 °C for 1 h in nitrogen) was performed. After this process, the two wafers were successfully wafer bonded. Then, the sapphire substrate was detached from the III-nitride LED film by a method of laser lift-off. An n-contact was formed on the exposed N-face GaN. A p-contact was formed on a ZnO. An anisotropic chemical etching was done on the O-face ZnO wafer to generate a hexagonal ZnO pyramid shape. This pyramidal shape is considered to have high light extraction efficiency. Fabricated samples were packaged. In this presentation, we will discuss the optical and electrical characteristics of these types of LED structures in detail.
12:00 PM - Q5.8
Alkaline-earth Nitrodocobaltates(I) Containing [CoIN2]5- Complexes.
Joanna Bendyna 1 , Peter Hoehn 1 , Walter Schnelle 1 , Ruediger Kniep 1
1 chemistry, Max-Planck-InstituteCPFS, Dresden Germany
Show Abstract12:15 PM - Q5.9
Comparison of UV, Blue- and Yellow-Band Micro-Photoluminescence Maps Near Defects in Semi-Insulating GaN.
Bruce Claflin 1 2 , David Look 1 2
1 Materials and Manufacturing Directorate, AFRL/MLPS, WPAFB, Ohio, United States, 2 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States
Show AbstractGrowth of semi-insulating (SI) GaN, either by Fe- or C-doping, has received considerable attention over the past few years in an effort to develop heteroepitaxial substrates with improved isolation for GaN-based electronic device applications. However, these SI materials have not been extensively characterized. Photoluminescence (PL) has been widely used to characterize unintentionally doped and n-type GaN to study the behavior of free and bound excitons, as well as to investigate the properties of extended- and point-defects. In the present work, room temperature micro-PL mapping of the near-band-edge (UV), blue- and yellow-band spectral regions is used to characterize structural defects in SI GaN. Intensity variations observed in the PL maps are correlated with structural features observed using plan-view scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PL spectra from these SI GaN samples are dominated by near-band-edge emission throughout. The yellow-band luminescence is more than 20x lower than the UV emission. Two SI GaN:Fe samples, one bulk and one epitaxial layer, contain low densities (< 5 cm-2) of macro-defects, 100-250 μm in diameter. The cores of these macro defects exhibit intense UV-PL, as much as 30x higher than the surrounding bulk material, suggesting that these sites getter radiative recombination centers. In the bulk SI GaN:Fe sample, these macro-defects are also correlated with an increase in yellow-band luminescence, although still well below the level of the UV-PL. In addition, for the bulk SI GaN:Fe sample, six-pointed-star patterns about 100 μm in diameter are observed that exhibit increased UV-PL along the crystallographic axes. It is possible that these star-patterns emanate from a dislocation at their center, however these patterns do not correlate with any structural features observed with SEM or AFM.
12:30 PM - Q5.10
Electronic and Magnetic Properties of Mixed Valence Monoclinic SrN.
Piotr Boguslawski 1 , Oksana Volnianska 1
1 , Institute of Physics PAN, Warsaw Poland
Show AbstractNitrides span a wide variety of compounds, beginning with the well-established semiconducting GaN and its alloys that currently are of fundamental importance in the microelectronics and blue photonics, and include also the recently investigated transition metal nitrides such as IrN2 and PtN2 [1] with exceptional material properties. Finally, first principles calculations predict that II-N nitrides with the rock salt structure, such as CaN and SrN, are ferromagnetic half-metals [2] due to the non-vanishing spin polarization of the p-shell of N ions, given by the Hund's rule, which persists in solids after formation of bonds. Recent synthesis of Sr nitrides [3], namely SrN2, SrN, and Sr2N, opens a question about their properties. They all share the monoclinic (m) crystal structure, with nitrogen anions occupying octahedral sites formed by Sr cations. Of particular interest is m-SrN, in which there are two non-equivalent types of nitrogen ions that occupy two non-equivalent lattice sites. The first type consists in isolated N ions, while in the second case two neighboring N form a N2 quasi-molecule, characteristic also for the transition metal nitrides [1] and SrN2 [3]. Consequently, one may expect that charge states of the two kinds of N ions are different. This conjecture is supported by calculations of the electronic and magnetic structure based on the density functional theory. We find that SrN is indeed a mixed valence compound. A comprehensive analysis of the results shows that (i) m-SrN is a metal with a pronounced, almost half-metallic spin polarization at the Fermi level, (ii) isolated N ions are in the 3- charge state with the closed-shell configurations, and their magnetic moment vanishes, and (iii) the N2 molecules are in the 2- charge state, and have a magnetic moment close to 1 μB, because of the partial population of the N2 antibonding molecular orbitals. The obtained results point out to the existence of the antiferromagnetic ordering of m-SrN at low temperatures. Presence of stable magnetic moments and their ordering in m-SrN are due to its mixed valence character. Differences between the properties of the monoclinic and rock salt phases of SrN are discussed. [1] J. C. Crowhurst et al., Science 311, 1275 (2006); A. F. Young et al., Phys. Rev. Lett. 96, 155501 (2006). [2] O. Volnianska and P. Boguslawski, Phys. Rev. B 75, 224418 (2007). [3] G. Auffermann, Yu. Prots, and R. Kniep, Angew. Chem. Int. Ed. 40, 547 (2001); G. Auffermann et al., Anal. Bioanal. Chem. 373, 880 (2002).
Wednesday PM, November 28, 2007
Back Bay B (Sheraton)
2:30 PM - **Q6.1
Thermodynamic and Kinetic Investigations in the System Ga-O-N.
Manfred Martin 1
1 Institute of Physical Chemistry, RWTH Aachen University, Aachen Germany
Show AbstractThe wide band gap semi-conductors Ga2O3 and GaN are of interest to industrial applications, ranging from opto-electronic components to sensor materials. While there is high solubility of oxygen in gallium nitride only little is known on the solubility of nitrogen in gallium oxide and on the existence of gallium oxynitrides. Here we report on our thermodynamic and kinetic investigations in the system Ga-O-N. Samples were prepared by classical solid state reactions, by ammonolyis of gallium oxide, and by pulsed laser deposition (PLD). Structural and analytical characterization was performed by X-ray and neutron diffraction, X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and secondary ion mass spectromery (SIMS). The kinetics of the ammonolyis of gallium oxide was followed by means of in situ XAS at elevated temperatures. The optical band gap of the materials was investigated by ex situ and in situ optical absorption spectroscopy, and the electrical conductivity was measured using the van der Pauw technique. Density functional theory (DFT) was used to calculate electronic structures. Using our experimental and theoretical results we will discuss implications for the stability, the defect structure and the transport properties of some materials in the system Ga-O-N.
3:00 PM - Q6.2
Density Functional Theory Calculations for the Binding Energies and Adatom Diffusion on Strained AlN (0001) and GaN (0001) Surfaces.
Vibhu Jindal 1 , James Grandusky 1 , Neeraj Tripathi 1 , Mihir Tungare 1 , James Raynolds 1 , Fatemeh Shahedipour Sandvik 1
1 , CNSE, Albany, New York, United States
Show AbstractDensity functional theory calculations were carried out to study the binding energies and diffusion barriers for adatoms on AlN and GaN surfaces. The binding energies and diffusion barriers were calculated for Al, Ga and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculation of the diffusion paths and diffusion energy barriers for Al, Ga and N adatoms on AlN and GaN was made. Further the surfaces were subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5% to investigate the effect of strain on diffusion barriers. Deeper understanding of the effect of strain on these properties may help explain differences observed in growth conditions for homoepitaxy and heteroepitaxy on bulk III-Nitride substrates as opposed to III-Nitride template layers on foreign substrates.
3:15 PM - Q6.3
Light Emitting Diode and Lasers Beyond 1.55 μm with GaAsSbN/GaAs Single Quantum Wells
Kalyan Nunna 1 , Shanthi Iyer 1 , Jia Li 1 , Ward Collis 1
1 Electrical and Computer Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States
Show AbstractIn this report, GaAsSbN/GaAs single quantum well (SQW), separate confinement heterostructure has been successfully used as the emission region for a room temperature (RT) light emitting diode and laser diode operation beyond 1.55 μm. These diodes were grown by solid source molecular beam epitaxial (MBE) technique with an RF assisted plasma source for N. The emission wavelength of 1.61 μm was achieved for ~1.4% N in the quaternary active layer. The light emitting diodes exhibited a RT electroluminescence (EL) peak at 1.61 μm for 30 mA forward current, with a low turn-on voltage of ~500 mV and an abrupt reverse break-down voltage of ~7.5 V. A 1.61 μm pulsed mode laser operation was also achieved at RT with a threshold current density of ~0.94 kA/cm2 for a diode area of 1250 μm x 100 μm. Variation of the laser output power with the drive current as a function of stripe width and temperature will be presented.This work was supported by Army Research Office under the Grant No. W911NF-04-1-0025 and Contract No. W911NF-06-1-0396(Dr.Michael Gerhold, Technical Monitor)
3:30 PM - Q6.4
GaN Nanowires for Label-free Bio-sensing Applications.
Li-Chyong Chen 1 , Chin-Pei Chen 1 , Abhijit Ganguly 1 , Chen-Hao Wang 1 , Chih-Wei Hsu 2 , Yu-Kuei Hsu 2 , Kuei-Hsien Chen 1 2
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractA novel bio-sensing system based on GaN nanowires (NWs) is presented coupled with their electrochemical impedance and photoluminescence measurements. GaN is well established now for a variety of optoelectronic applications. However, while its inherent bio-compatibility has also been recognized, its application as bio-sensors has been surprisingly lacking till date. Meanwhile, one-dimensional nanostructures have attracted huge interest as potential building blocks for the future nanoelectronic devices. In this report, GaN NWs are used as a transducer for DNA-sensors, by immobilizing single-strand DNA (ssDNA) molecules through covalent binding using organosilane linker (MPTS). The MPTS-modified GaN NWs exhibited an electrochemical window remarkably wider than those of boron-doped diamond or carbon nanotubes reported to date. The immobilization of ssDNA and the subsequent hybridization to double-strand DNA (dsDNA) were confirmed using confocal microscope. Electrochemical impedance measurement showed that interfacial electron-transfer resistance (Ret), from solution to transducer surface, increased significantly when pristine GaN NWs were immobilized with ssDNA, along with a formation of additional semicircle region at lower frequency in Nyquist plot. The unique appearance of double-semicircle region for ssDNA-immobilized NWs, compared to single-semicircle region for pristine GaN NWs, leads to the idea of formation of double-capacitance layer in series. The phenomenon is more prominent by the appearance of double peaks in the plot of phase angle vs. frequency (Bode plot), the second peak, formed after ssDNA-immobilization, showed further increase under the hybridization to dsDNA, and consequently reduces the overall impedance. In contrast, characteristic impedance spectra remain the same when complimentary target DNA is replaced by mismatched DNA. Moreover, quenching behavior in photoluminescence of the GaN NWs was distinguishable for the ones immobilized with ssDNA and the same hybridized to dsDNA. Both the technique implies the ability of oligonucleotides, immobilized on the NW-surface, to interact with other biomolecules. The dual and label-free sensing capability in impedance and photoluminescence of GaN NWs makes them effective DNA transducers.
3:45 PM - Q6.5
Spinel-type Structured Gallium Oxonitride with Composition Ga3O3N.
Isabel Kinski 1 , Stefanie Hering 2 , Carmen Zvoriste 1 , Ralf Riedel 1 , Hubert Huppertz 2
1 Institute of Materials Science, Darmstadt University of Technology, Darmstadt Germany, 2 , Ludwig-Maximilians-Univeristaet Munich, Munich Germany
Show Abstract4:30 PM - Q6.6
Growth and Characterization of Zinc-blende InN Thin Films on R-plane Sapphire Substrates by Molecular Beam Epitaxy.
Ching-Lien Hsiao 1 , Ting-Wei Liu 2 , Hsu-Cheng Hsu 1 , Wen-Yu Hsiao 3 , Chih-Chung Yang 3 , Chia-Chun Chen 2 , Li-Chyong Chen 1 , Kuei-Hsien Chen 4 1
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan, 2 Department of Chemistry, National Taiwan Normal University, Taipei Taiwan, 3 Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei Taiwan, 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show AbstractIn the past decade, nonpolar III-nitrides have attracted much attention due to the absence of the spontaneous and piezoelectric polarizations that can limit the device performance. Cubic system with Zinc-blend (ZB) InN is expected to have high electron transport properties and low bandgap energy about 0.58eV. However, the growth window for ZB-InN is narrower than that of wurtzite (WZ) type because of its metastable phase and the lack of suitable substrate. Hence, the investigation on the electric and optoelectronic properties of ZB-InN is important for future device application. Here, we demonstrate a simple method for growing ZB-InN in MBE system by substrate nitridation. The InN thin films were grown on r-plane sapphire(0112) substrate by plasma-assisted molecular beam epitaxy (PAMBE). Substrate nitridation was performed before InN growth at 200oC for 30 to 120 min. Following the nitridation, the growth of InN films was carried out at 320oC for 3 hours. The structural evloution of the InN films was analyzed by the θ/2θ-scan x-ray diffraction (XRD) patterns. By increasing the initial nitridation time, the phase of the InN films changes from pure WZ-InN(1011) to mixture of WZ-InN and Zb-InN, to pure ZB-InN(002). However, the reciprocal space maps of the XRD analysis shows that a certain content of WZ-InN is still embedded in the pure phase InN film. Nevertheless, this result indicates that the interlayer produced by substrate nitridation plays an important role in controlling the InN phase. Optical characterization of InN films was analyzed by temperature-dependent photoluminescence (PL) spectroscopy using Nd+ YAG laser (532nm) as the excitation source. At room temperature, the PL peak emission at 0.670eV with a full width at half maximum (FWHM) of 130 meV is obtained from the pure zb-InN. At 22k, PL consists of an emission band at about 0.680 eV with a FWHM of 93 meV. With increasing temperature, the emission peak slightly broadened and shifted to lower energy. These PL results indicate that the optical quality of the ZB InN films is comparable to that of the high quality ZB InN films from other literatures. Furthermore, a S-shape shift of peak energy from temperature-dependent PL measurement is believed to be related to a type-II semiconductor behavior which was ever observed in type-II semiconductor with mixture phases of WZ and ZB-GaN films.
4:45 PM - Q6.7
High Stress PECVD Silicon Nitride Films For 65nm SOI Technology and Beyond.
Hartmut Ruelke 1 , Volker Jaschke 1 , Kai Frohberg 1 , Mihaela Balseanu 2 , Tsutomu Kiyohara 2 , Li-Qun Xia 2 , Derek Witty 2 , Hichem M'Saad 2 , Olaf Hiller 3 , Wolfgang Senninger 3
1 , Advanced Micro Devices, AMD Fab36 LLC& Co. KG, , Dresden Germany, 2 , Applied Materials, Sunnyvale, California, United States, 3 , Applied Materials, Dresden Germany
Show AbstractHigh stress nitride films have been used at 90nm node and beyond to increase transistor drive current performance for both NMOS (tensile stress) and PMOS devices(compressive stress). In this paper we discuss the mechanisms controlling the stress of the PECVD silicon nitride (SiN) films in the GPa range and present the drive current improvement induced by those films. Silicon Nitride films with tensile stress up to 1.7GPa can be produced at 400°C using PECVD deposition followed by UV cure in a separate chamber. The removal of hydrogen from the film b