Volkmar Dierolf Lehigh University
Yasufumi Fujiwara Osaka University
Uwe Hommerich Hampton University
Pierre Ruterana CIMAP
John Zavada North Carolina State University
D2: Nitrides I
Tuesday AM, December 02, 2008
Room 201 (Hynes)
9:30 AM - **D2.1
Development of Rare-earth Doped III-Nitride and its Application for Optoelectronic Devices.
Akihiro Wakahara 1 , Hiroshi Okada 1 Show Abstract
1 , Toyohashi University of Technology, Toyohashi Japan
10:00 AM - D2.2
The Dependence of the Luminescence Efficiency of Eu-Implanted GaN on the Implantation Fluence and Post-Annealing Temperature.
Iman Roqan 1 , Kevin O'Donnell 1 , Carol Trager-Cowan 1 , Katharina Lorenz 2 , Eduardo Alves 2 , Michal Bockowski 3 Show Abstract
1 Physics Department, SUPA, University of Strathclyde, Glasgow United Kingdom, 2 Department of Physics, Instituto Tecnologico e Nuclear, Sacavem Portugal, 3 , Institute of High Pressure Physics Polish Academy of Sciences, Warsaw Poland
Studies of implantation damage and its effects are a pre-requisite for successful implementation of ion implantation in the production of electronic and optoelectronic devices. This implantation damage affects the crystal structure and therefore the optical properties of the materials; bombardment of heavy ions (such as RE) into III-nitrides creates collision cascades and host atom displacements. This can create point defects, such as Ga and N vacancies, Ga and N interstitials, and extended defects (such as stacking faults and point defect clusters). High temperature annealing is usually necessary to remove the implantation damage and thereby activate rare earth ions optically. Previously it was shown that a thin AlN layer grown on top of GaN successfully protects the GaN surface from dissociation during high temperature annealing up to 1200 oC and a strong increase of Eu-related luminescence with annealing temperature was observed for samples implanted with a fluence of 1×1015 cm-2. We will further show in this paper that the optimum annealing temperature for optical activation increases as the implantation fluence increases, being 1000 oC and 1100 oC for 1×1013 and 1×1014 cm-2, respectively. Above the optimum temperatures, a substantial decrease in Eu3+ luminescence is observed together with the appearance of a broad yellow defect band; this band is not observed for samples implanted with higher implantation fluence of 1×1015 cm-2. Surprisingly, this broad band is not seen for GaN samples without AlN caps. In further investigations, unimplanted AlN capped and uncapped GaN samples annealed at high temperature are studied to investigate the annealing effect. The yellow band increases for AlN capped samples which indicates that new defects related to the AlN cap can suppress RE3+ luminescence in such materials. Results are compared to GaN samples annealed at 1 GPa nitrogen overpressure at temperatures up to 1400 oC without AlN cap. Optical measurements will be accompanied with Rutherford Backscattering Spectrometry (RBS) to investigate the structural properties of these films.
10:15 AM - D2.3
Optical Characterization of Nanocrystallized AlN and AlN: Er Films Prepared by Magnetron Sputtering.
Valerie Brien 1 , Syed Sajjad Hussain 1 , Hervé Rinnert 2 , Nolwenn Tranvouez 1 , Manuel Dossot 3 , Bernard Humbert 3 , Philippe Pigeat 1 Show Abstract
1 LPMIA UMR CNRS 7040, CNRS/Nancy-University, Vandœuvre-les-Nancy France, 2 LPM UMR CNRS 7556, Nancy-University/CNRS , Vandœuvre-les-Nancy France, 3 LCPME UMR 7564 CNRS, CNRS/Nancy-University, Villers-les-Nancy France
10:30 AM - **D2.4
GaN Doped with Neodymium by Plasma-Assisted Molecular Beam Epitaxy for Potential Lasing Applications.
Eric Readinger 1 , Grace Metcalfe 1 , Hongen Shen 1 , Michael Wraback 1 , Naveen Jha 2 , Nathaniel Woodward 2 , Pavel Capek 2 , Volkmar Dierolf 2 Show Abstract
1 , US Army Research Laboratory, Adelphi, Maryland, United States, 2 Physics Department, Lehigh University, Bethlehem, Pennsylvania, United States
Solid state laser applications using crystals doped with Nd are quite successful (e.g., Nd:YAG). What these crystals lack is the ability to dissipate thermal energy, with thermal conductivities on the order of 5-15 W/m-K, which limits their high power operation. By changing the matrix to a wide bandgap semiconductor such as GaN (or AlGaN), with a thermal conductivity of 100-300 W/m-K, the improvement in heat extraction could allow for major gains in solid state laser technology. It has been shown previously that rare-earth (RE) dopants such as Er, Pr, Tm, and Eu are well-suited to III-nitride semiconductors. Although there have been difficulties in preserving optical quality while achieving adequate concentrations, light emission from RE-doped GaN has been demonstrated by photoluminescence, electroluminescence, and cathodoluminescence. We provide an investigation of in situ doping of GaN with the RE element Nd by plasma assisted-molecular beam epitaxy (PA-MBE). GaN epilayers are grown on c-plane sapphire and GaN substrates and the Nd doping is controlled by an effusion cell. The ideal growth conditions for Nd incorporation in GaN are investigated and the crystal quality is verified with x-ray diffraction studies. The optical absorption characteristics are evaluated to ensure that the GaN:Nd epilayer remains transparent at the Nd emission wavelength of interest. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectroscopy data indicate ~5 atomic percent in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation with up to ~1 atomic percent Nd. The highest luminescence intensities correspond to doping of ~0.5 atomic percent with the strongest emission occurring at 1.12 eV (1106 nm). We also present the Stark energy sublevels of Nd3+ ions in GaN as determined by luminescence spectra. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). We correlate the photoluminescence spectra with transitions from the 4F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above (325 nm) and below (836 nm) bandgap excitation. Spectral correlation of the Nd emission multiplets in addition to site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site. Laser structures utilizing GaN:Nd epilayers will be grown and evaluated.
11:30 AM - D2.5
Different Behavior for AlN and GaN during Medium Energy Range Rare Earth Ion Implantation and Annealing.
Pierre Ruterana 1 , Marie Pierre Chauvat 1 , Florence Gloux 1 , Katharina Lorenz 2 , Eduardo Alves 2 Show Abstract
1 CIMAP, CNRS, Caen France, 2 , Instituto Tecnológico e Nuclear, Sacavem Portugal
The damage induced in either bare GaN thin films by 300 keV rare earth ions implantation at room temperature, or through AlN epilayer capped GaN has been investigated by transmission electron microscopy (TEM) and compared with AlN thin films implanted in similar conditions. The AlN capping layer was shown to protect the GaN surface during implantation and annealing for low fluence implantation ~2x1015 at/cm2 but failed for higher fluences and temperature exceeding ~1200C. Through a 25 nm thick AlN layer, at the lowest fluence 6x1015at/cm2, the damage build-up consists in the formation of nanocrystalline pockets in GaN just below the AlN cap. AFM and TEM analysis suggest that the origin of these pockets are cracks within the AlN-capping layer. When the fluence increases to 1.2x1016at/cm2, an entirely buried nanocrystalline layer forms below the AlN cap. The AlN cap itself stays crystalline up to the highest investigated fluence of 3x1016at/cm2. When the implantation is carried out in AlN thick films, it is necessary to increase the ion fluence to more than 1017 at/cm2 in order to attain the formation of an amorphous layer. This layer forms at the ion projected range and grows by extending at the same time towards the bulk and the surface, in agreement with many materials such as silicon, but in contrast to GaN in which the highly damaged layer forms from the surface and is not amorphous, but nanocrystalline. Damage evolution during high temperature annealing will also be discussed.
11:45 AM - D2.6
Spatially Resolved Site Selective Optical Spectroscopy on Nd Doped GaN Epitaxial Layers.
Nate Woodward 1 , Naveen Jha 1 , Volkmar Dierolf 1 , Eric Readinger 2 , Grace Metcalfe 2 , Michael Wraback 2 Show Abstract
1 Physics, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Sensors and Electronic Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland, United States
Due to its favorable electronic and thermal properties GaN has been considered as a rare-earth host material for solid state laser applications. To this end, we performed spatially resolved combined excitation emission spectroscopy (CEES) on Nd ions which were in-situ-doped into GaN epitaxial films grown by plasma assisted molecular beam epitaxy (PA-MBE) on c-plane sapphire substrate. For a wide range of concentration (up to 8at%) we find in the emission a dominant incorporation site, which can be identified with good certainty as a substitutional 'Ga' site. Energy levels and electron-phonon coupling to a localized mode can be identified. The frequency of the observed mode is similar but not identical to that observed for Eu-ions in GaN. While resonant excitation yields strong emission signals even at high temperatures indicating good intrinsic quantum efficiency, above bandgap excitation of the dominant incorporation site is rather inefficient. This conclusion is further supported by the observation that in the spectra under above band-gap excitation the minority sites (with better excitation efficiencies) become more pronounced. For the majority site, confocal and NSOM imaging under selective excitation show changes in emission intensity, excitation and emission wavelength on a submicron length scale. These observations are consistent with an interpretation that the changes are due to fluctuations in Nd-concentration that will create fluctuation in the local strain fields that are caused by the substitution of the small Ga ion by a larger Nd ion (0.62Å vs 0.99Å). These results are a nice demonstration of the use of rare earth ions as probes for local perturbation. Supported by Lehigh-ARL collaborative agreement W911NF-07-2-0064 and by NSF-DMR-0705217
12:00 PM - D2.7
Luminescence and Excitation Mechanisms of Eu-doped GaN Phosphor.
Wojciech Jadwisienczak 1 , Tiju Thomas 2 , Michael Spencer 2 , Nelson Garces 3 , Evan Glaser 3 , Krzysztof Wisniewski 4 Show Abstract
1 School of EECS, Ohio University, Athens, Ohio, United States, 2 School of ECE, Cornell University, Ithaca, New York, United States, 3 Code 6877, Naval Research Laboratory, Washington, District of Columbia, United States, 4 Institute of Experimental Physics, University of Gdansk, Gdansk Poland
Applications of rare earth doped III-Nitrides are of considerable interest in different optoelectronic applications including phosphors for inorganic electroluminescent devices. In this presentation we have investigated Eu-doped GaN powder synthesized through the reaction between metals gallium, bismuth and europium in ammonia ambient . The resulting single phase powder is luminescing efficiently at 621 nm due to emission from the Eu3+ ion 5D0-7F2 transition. Temperature dependent spectroscopic studies including photo- and cathodoluminescence revealed that the emission at 621 nm is thermally quenched by one order of magnitude when ambient temperature changed from 10 K to 400 K. The excitation and quenching mechanisms responsible for the observed reduction of radiative recombination were investigated by means of photoexcitation spectroscopy (PLE), electron paramagnetic resonance (EPR) and high pressure photoluminescence (PL). It was found that Eu3+ ions are very effectively excited through excitonic recombination when excited above bandgap. Furthermore, PLE shows a possible excitation channel involving shallow donors when excited below bandgap. The presence of shallow donors was confirmed by the EPR study and their possible involvement in the energy transfer between the GaN host and Eu ion centers was studied by the high pressure PL.  C. B. Poitras, H. Wu, A. C. Turner, M. G. Spencer and M. Lipson, Appl. Phys. Lett. 89 111912 (2006).
12:15 PM - D2.8
Development of RE-Doped III-Nitride Nanomaterials for Laser Applications.
Geliang Sun 1 , Xiaofei Liu 1 , Stephen Tse 1 , Sudhir Trivedi 2 , Uwe Hommerich 3 , John Zavada 4 Show Abstract
1 Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey, United States, 2 , Brimrose Corporation, Baltimore, Maryland, United States, 3 Physics, Hampton University, Hampton, Virginia, United States, 4 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Conventional solid-state lasers utilize single crystal hosts, doped with rare-earth (RE) or transition metal active ion. However, key challenges in using single crystals include limited solubility of the dopant during growth, difficulties in growing large crystals, and mismatch in thermal properties. In the past decade, polycrystalline ceramic lasers, obtained by processing ceramic micropowders into bulk, have demonstrated lasing efficiencies equal to that of single crystal, e.g. Nd:YAG, while affording engineering of size and shape, as well as improving mechanical properties. By further employing nanopowders, potential exists for minimizing residual pores, sintering to near full density, enhancing thermal/mechanical properties, and increasing dopant concentration through metastable processing, thus ameliorating laser performance. Recently, III-Nitride compound semiconductors have been investigated for the development of high-power laser gain media, due to their higher thermal conductivities, hardnesses, and radiative and chemical stabilities compared to those of typical YAG.In this work, we investigate the synthesis and consolidation of GaN, AlN, and c-BN nanopowders, along with their doping using Nd, Er, and Yb. The powders are synthesized using a novel, aerodynamically-enhanced plasma process, with in-situ laser-based diagnosis of the gas-phase flow field and as-synthesized nanoparticles. Properties of the nanopowders, i.e. phase and crystallinity, morphologies and primary particle size, aggregate particle size, powder surface area, particle size distribution, and precursor conversion, are performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), Brunauer-Emmet-Teller (BET), nano-scanning mobility particle sizer (nano-SMPS), and thermogravimetric analysis (TGA). Cases where RE is doped directly during the plasma synthesis process and where undoped powders are post-treated with dopant exposure and nitridization are investigated and compared. Nanocomposite pellets are fabricated from the RE-doped III-Nitride nanopowders, and the technical pathway for producing such bulk ceramics is examined. Finally, both powders and ceramic pellets are characterized spectroscopically. Photoluminescence of the powders and absorption/emission characteristics and emission life times of the pellets are measured.
12:30 PM - **D2.9
Rare Earth Activated GaN Luminescent Powders by Combustion Synthesis and GaN:RE Thin Films Deposited by a Novel Hybrid PLD/MOCVD Technique.
Gustavo Hirata 1 Show Abstract
1 Center of Nanoscience and Nanotechnology, National University of Mexico, San Ysidro, California, United States
In this work we report on the fabrication and optical properties of rare earth-activated gallium nitride (GaN) luminescent powders and thin films. GaN and GaN:RE (RE=Eu,Tb) powders with excellent photoluminescent and cathodoluminescent properties were prepared via a low-temperature combustion synthesis method. These powders were pressed into 2 cm diameter targets for laser ablation experiments.On the other hand, a novel low-temperature method was developed by combining pulsed laser ablation and metal-organic chemical vapor deposition (MOCVD) in order to fabricate GaN thin films activated with rare earth (RE) ions. Laser ablation is used to remove material from the GaN target and, simultaneously europium atoms are injected from a MOCVD bubbler into the laser ablation plume and sprayed onto the film growth surface to form GaN:RE (RE=Eu,Tb) luminescent thin films. The films were deposited on GaN/Al2O3 substrates. The X-ray diffraction analysis of these GaN:RE samples showed that the films have the hexagonal phase of GaN and are polycrystalline with strong texture along the  direction. Cathodoluminescent measurements acquired at room temperature showed the band-edge emission of GaN and the corresponding red or green emission originated within the intra-shell transitions of either Eu3+ or Tb3+. The europium-doped GaN:Eu3+ thin films showed strong luminescence due to f-f transition lines within the Eu3+ (4f6) electron emission configuration. The largest peak emission line at around λ= 621 nm, is assigned to the hypersensitive 5D0→7F2 transition using a forced electric dipole transition mechanism. Furthermore, from the analysis of the luminescence spectra which are revealed by the local environment of the Eu3+, it is established that europium ions occupy low-symmetry sites. Lack of inversion symmetry at the cationic sites is favorable for observing the electric dipole transition as a forced transition due to the admixture of the odd parity states. For GaN:Tb3+ thin films various transitions originated from the 5D4 level of Tb3+ are observed including the classical green transition to the 7F5 level. These transitions were observed in GaN:Tb3+ powders, with the exception of the near-band-edge (NBE) at λ= 370 nm. This indicates that the lower defect level in the films improves the quality of the material.We have demonstrated that combining two techniques: pulsed laser deposition and MOCVD represent a novel approach for the synthesis of RE-activated GaN luminescent thin films.
D3: Nitrides II
Tuesday PM, December 02, 2008
Room 201 (Hynes)
2:30 PM - **D3.1
Ferromagnetism and Luminescence of Diluted Magnetic Semiconductors GaGdN and AlGdN.
Shuichi Emura 1 , Masahiro Takahashi 1 , Hiroyuki Tambo 1 , Tetsuya Nakamura 2 , Y. Zhou 1 , Shigehiko Hasegawa 1 , Hajime Asahi 1 Show Abstract
1 The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan, 2 , JASRI/SPring-8, Sayocho, Hyogo, Japan
Rare earths are well known as luminescent centers and as principal component of ferromagnetic materials. Research on the luminescent nature of these elements and their compounds is of a long history of over half century in modern science, and also development of new ferromagnetic materials containing rare earth elements as main carriers has old history over one century. As our society gets more and more complicated, multi-functional materials are expected to be invented to meet the needs of the infrastructure in the near future. Ga(Al)N based diluted magnetic semiconductors, for example GaGdN, show luminescence and room-temperature ferromagnetism . Therefore, these are one of the high potential materials for near future devices. GaGdN has unusual ferromagnetic behaviors. Even in very dilute Gd concentration (7x1015/cm3), it shows the ferromagnetic behavior at room temperature , and takes the second magnetic phase at 10 - 20K. Moreover, curious step-like temperature dependence in soft X--ray magnetic circular dichroism (SX-MCD) spectra is observed around 30 – 70K. GaGdN seems to show three different magnetic phases. With the aid of SX-MCD spectra along with X-ray absorption fine structure analysis, the unique ferromagnetism in GaGdN will be discussed. On the other hand, we found a luminescence at 652 nm in GaGdN. Gd center in the trivalent state usually presents a sharp luminescence around 318 nm (f – f transition), which corresponds to the transition to the lowest excited energy level in multiplet of f7 electron configuration, and is observable in AlGdN because the band gap of AlN (about 6eV) is beyond the luminescent energy. We present a model on the origin of the luminescent center. Application to devices combining the magnetic and luminescence properties of GaGdN will be proposed. Teraguchi et al., Solid state comm. 122 (2002) 65.  S.Dhar et al., Phys. Rev. Letters, 94 (2005) 037205-1,
3:00 PM - D3.2
First Principles Calculations for Gd doped GaN.
Chandrima Mitra 1 , Walter R.L Lambrecht 1 Show Abstract
1 Physics , Case Western Reserve University, Cleveland, Ohio, United States
3:15 PM - D3.3
Magnetotransport in Gd-implanted Wurtzite GaN/AlxGa1-xN High Electron Mobility Transistor Structures.
Fang-Yuh Lo 1 2 , Alexander Melnikov 2 , Dirk Reuter 2 , Yvon Cordier 3 , Andreas Wieck 2 3 Show Abstract
1 Department of Physics, National Dong-Hwa University, Hualien Taiwan, 2 Lehrstuhl für Angewandte Festkörperphysik, Ruhr-Universität Bochum, Bochum Germany, 3 Centre de Recherche sur l'Hétéro-Epitaxie et ses Applications, Centre National de la Recherche Scientifique, Valbonne France
GaN/AlxGa1-xN high electron mobility transistor (HEMT) structures grown by ammonia-source molecular beam epitaxy (MBE) are focused-ion-beam implanted with 300 keV Gd-ions at room temperature. The two-dimensional electron gas (2DEG) of these HEMT structures is located 27 nm underneath the sample surface. At 4.2 K, current-voltage characteristics across implanted rectangles show that the structures remain conducting up to a Gd-dose of 1×1012 cm-2. Anomalous Hall effect (AHE) and anisotropic magnetoresistance (AMR) are observed at T = 4.2 K for structures implanted with different doses of Gd. Measurements of AHE in the wide temperature range from 2.4 K to 300 K show that the magnetic ordering temperature of these structures is around 150 K. Therefore, these Gd-implanted HEMT structures containing the still conducting 2DEG, which is now embedded in a ferromagnetic semiconductor, open the possibility to polarize the electron spins. The support by the Deutsche Forschungsgemeinschaft in the framework of the Sonderforschungsbereich SFB 491 and the Schwerpunktprogramm Halbleiterspintronik SPP 1285, and by the Université Franco-Allemande (DFH/UFA) within the CDFA-05-06 are gratefully acknowledged.
3:30 PM - D3.4
Rare Earth Doping of GaN with Gadolinium by MOCVD.
Shalini Gupta 1 , Andrew Melton 1 , William Fenwick 1 , Hongbo Yu 1 , Ian Ferguson 1 2 Show Abstract
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Rare earth (RE) elements can be used as dopants for spintronics applications. Instead of relying on the d-shells of the transition metal (TM) as the magnetic element, the f-electrons from the RE elements are used. Despite the promising results obtained by molecular beam epitaxy for Ga1-xGdxN, till date there are no metal organic chemical vapor deposition (MOCVD) reports on Ga1-xGdxN. This paper presents the first report on MOCVD doping of GaN with gadolinium (Gd). Ga1-xGdxN thin films (x = 0-24%, calculated per molar flow) were grown using the precursor Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) gadolinium (III); all growths were carried out on 2” sapphire substrates under typical GaN temperatures and pressures. The growth rate was maintained at 0.2 µm/hr to allow enough time for adatom diffusion and to prevent clustering. Although Gd is a large atom, X-ray diffraction (XRD) measurements did not show any shift in the GaN peak or the presence of any gross secondary phase. Co-doping with silane and magnesium did not cause any change in the XRD peaks either. Furthermore, atomic force microscopy (AFM) measurements revealed smooth Ga1-xGdxN films with topography similar to that of GaN. Hall measurements showed that the Ga1-xGdxN has typical n-GaN background carrier concentration and that this carrier concentration can be increased by silane co-doping (1016 cm-3 to 1018 cm-3). Magnesium co-doping of 1019 cm-3 results in a decrease in carrier concentration (measured: 1015 cm-3) and thereby increases the resistivity, making it hard to get reliable hall measurements. These films are optically active and photoluminescence measurements on Ga1-xGdxN showed, in addition to the GaN peak, the presence of peaks in the range of 3.1 eV-3.3 eV which can be attributed to the internal transition associated with Gd3+. Moreover, a peak is seen at 1.74 eV which could possibly be attributed to Gd3+. Room temperature magnetization data for Ga1-xGdxN showed that as the Gd concentration is increased, a transition from diamagnetism to ferromagnetism occurs, and a magnetization strength of 20 emu/cm3 is obtained for ~12% Gd. Moreover, co-doping with silane and magnesium results in an increase in magnetization strength, with the maximum magnetization obtained for the p-type Ga1-xGdxN (~500 emu/cm3 for x = 12 %, and p-doping of 1019 cm-3). It is interesting to note that the magnetization strength can be enhanced by co-doping and provides the possibility that the mechanism associated with the observed ferromagnetism could be due to carrier mediated ferromagnetism. This is the first MOCVD report on Ga1-xGdxN, and the results obtained are promising, as a large magnetic signal is obtained for conducting Ga1-xGdxN films which could prove to be a beneficial dilute magnetic semiconductor for spintronic applications.
4:00 PM - **D3.5
Gd-doped III-nitride Dilute Magnetic Semiconductor Materials.
Ryan Davies 1 , Jennifer Hite 1 , Rachel Frazier 1 , Brent Gila 1 , Gerald Thaler 1 , Cammy Abernathy 1 , Stephen Pearton 1 , Christopher Stanton 2 , John Zavada 3 Show Abstract
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 2 Physics, University of Florida, Gainesville, Florida, United States, 3 Electronics Division, U.S. Army Research Office, Durham, North Carolina, United States
Wide band gap dilute magnetic semiconductor (DMS) materials are being investigated as potential materials for spintronic devices. Since these materials demonstrate magnetic behavior at room temperature in addition to their semiconducting characteristics, they possess the potential for larger integration density, less power consumption than their electronic counterparts, and room temperature generation of polarized optical signals. Gd-doped III-nitrides are particularly interesting due to the low Gd concentrations needed to achieve ferromagnetic ordering at room temperature and to the large magnetic moments obtained in this system. However, one area of concern with these materials is the apparent absence of magneto-optical effects at room temperature. Although the Kerr effect has been demonstrated for magnetic dopants in the smaller band gap arsenides, this has not been the case for the larger band gap nitrides. This along with the absence of an anomalous Hall effect at room temperature raises questions regarding the source of the observed magnetic ordering. Various models have been proposed suggesting defect involvement and experimental evidence is beginning to point in this direction as well. This talk will review the current understanding of the role of co-dopants, growth conditions, processing and post-growth irradiation in the context of elucidating the ordering mechanism. Comparison with similar effects in GaMnN will also be presented. Finally, implications of the ordering mechanism on future device development will be discussed.
4:30 PM - D3.6
Ultraviolet Luminescence from Thullium-doped Aluminum Gallium Nitride Epilayers.
Neeraj Nepal 1 , John Zavada 1 , Ei Ei Brown 2 , Uwe Hommerich 2 , Ashok Sedhain 3 , Jingyu Lin 3 , Hongxing Jiang 3 , Dong Lee 4 , Andrew Steckl 4 Show Abstract
1 ECE, North Carolina State Univ, Raleigh, North Carolina, United States, 2 Physics, Hampton University, Hampton, Virginia, United States, 3 Physics, Kansas State University, Manhattan, Kansas, United States, 4 ECE, University of Cincinnati, Cincinnati, Ohio, United States
Incorporation of rare-earth (RE) atoms into a semiconductor host has received wide spread attention due to potential optoelectronic applications including displays, optical amplifiers, and light-emitting devices. Thulium is an important RE element for such applications since in its trivalent charge state (Tm3+) prominent luminescence in the visible and infrared regions has been observed in glass fibers. In this paper we report on ultraviolet (UV) luminescence found in AlGaN epilayers grown by solid-source molecular beam epitaxy. The UV luminescence appears only if the bandgap of the AlGaN layer exceeds the photon energy of the corresponding 4f shell transition. Using above band gap optical excitation prominent, narrow emission lines, at 298 and 358 nm, were observed in epilayers with high Al content. With below band gap excitation broad emission bands were found at these wavelengths. Temperature dependence and lifetimes of the UV emissions have been measured. The data indicate the presence a defect level related to the Tm incorporation and a model is presented to explain the UV properties.
4:45 PM - D3.7
Europium Doping of Cubic (Zincblende) GaN by Ion Implantation.
Katharina Lorenz 1 2 , N. Franco 1 2 , E. Alves 1 2 , I. Roqan 3 , K. O'Donnell 3 , C. Trager-Cowan 3 , R. Martin 3 , D. As 4 , M. Panfilova 4 Show Abstract
1 UFA, Instituto Tecnologico e Nuclear, Sacavem Portugal, 2 CFNUL, University of Lisbon, Lisbon Portugal, 3 Department of Physics, SUPA, University of Strathclyde, Glasgow United Kingdom, 4 Department of Physics, University of Paderborn, Paderborn Germany
Eu-doped wurtzite GaN (W-GaN) has been widely studied due to its intense red light emission at ~ 621 nm in epitaxial films or powders. Ion implantation is a useful technique to achieve the incorporation of optically active rare earth dopants into pre-grown W-GaN templates. On the other hand very few reports exist on rare earth doping of cubic (zincblende) GaN (ZB-GaN) and on ion implantation in this material. In this study Eu was implanted at different fluences between 1013 and 1015 at/cm2 into ZB-GaN layers grown by Molecular Beam Epitaxy (MBE). Detailed structural characterization before and after implantation was performed by X-ray diffraction (XRD) and Rutherford Backscattering/Channeling (RBS/C) spectrometry. RBS/C spectra of the as grown templates reveal a relatively strong dechannelling with depth probably caused by wurtzite phase inclusions. XRD pole figures confirm that the as-grown samples contain wurtzite phase inclusions whose <001> plane is aligned with the <111> plane of the cubic lattice. Implantation causes an expansion of the lattice parameter in the implanted region similar to that observed for W-GaN. While implantation in W-GaN causes the formation of a highly damaged nano-crystalline surface layer, this surface damage is absent in implanted ZB-GaN. By thermal annealing the bulk implantation damage in ZB-GaN could partly be removed, however, an increase of the wurtzite phase fraction was observed at the same time. Although the photoluminescence (PL) emission is dominated by luminescence from Eu ions incorporated into the W-GaN fraction, selective PL and PL excitation spectroscopy revealed several additional emission lines, which may be attributed to Eu-related optical centers in ZB-GaN.
5:00 PM - D3.8
Current-injected 1.54 μm Emitters based on Er Doped GaN.
Rajendra Dahal 1 , Cris Ugolini 1 , Ashok Sedhain 1 , John Zavada 2 , Jingyu Lin 3 , Hongxing Jiang 3 Show Abstract
1 Physics, Kansas State University, Manhattan, Kansas, United States, 2 Electrical & Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Nano Tech Center and Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, United States
Rare earth elements, in particular erbium (Er) doped III-nitride semiconductors has been widely explored aiming to achieve photonic devices with multiple functionalities in photonic integrated circuits which are not possible with either Er doped silica glasses or narrow band gap semiconductors like InGaAsP. Emitters and optical amplifiers operating at 1.54 μm based on Er doped semiconductors are expected to be electrically pumped, integratable, temperature insensitive and high signal gain with low noise. These properties are very attractive for next generation optical network system where multiple amplification steps are required. We will report here on the fabrication of a chip size current injected 1.54 μm emitters by heterogeneously integrating metal organic chemical vapor deposition (MOCVD) grown Er doped GaN with 365 nm nitride light-emitting diodes. The emitted intensity at 1.54 μm varied almost linearly with input forward current. Further, the propagation loss at 1.54 μm of Er doped GaN waveguide amplifier will also be discussed. The feasibility of electrically pumped optical amplifiers for photonic integrated circuit with advantages of both semiconductor optical amplifiers and Er-doped fiber amplifiers will also be discussed.
5:15 PM - D3.9
Excitation Pathways of Rare Earth Ions by Energetic Electrons.
Samson Tafon Penn 1 , Zackery Fleischman 1 , Leon Maurer 1 , Volkmar Dierolf 1 Show Abstract
1 Physics, Lehigh University, Bethlehem, Pennsylvania, United States
The announcement of efficient electrically pumped luminescence of rare earth ions that are placed in the insulating layer of a MOS-type Si/SiO2 structure renewed the quest for a silicon-based light source . Similarly, electrically pumped rare earth emission and optical amplification have been found in rare earth doped wide bandgap semiconductors such as GaN[ 2]. For both cases, the energy transfer from the energetic electrons determines the critical limit of the device performance. We report on our comprehensive study of the electronic excitation pathways of rare earth ions in GaN, AlGaN, AlN, and LiNbO3. In order to simulate the electron injection in real devices, we used the electron beam within an electron microscope and probed the resulting cathodoluminescence. Manipulating the electron beam in terms of acceleration voltage, beam current, position, and sweep frequency allows studying the saturation behavior of the emission. We find that in many cases the saturation occurs for smaller beam currents than predicted from our photoluminescence studies on the same samples. Moreover, the typical time constants do not always reflect the lifetime of the involved rare earth ion transition indicating the presence of an intermediate step that can act as a bottleneck depending on the lifetime of such a (defect) trap and the efficiency of energy transfer to the rare earth ion. We find in LiNbO3 doped with Er, Eu, and Pr an intermediate trap state that has an effective lifetime in the order of 200µs and a rather low energy transfer rate. resulting in a very weak cathodoluminescence signal. We speculate that after the electronic excitation of the host material, polarons are formed through self-trapping and will constitute the trap state. The excitation pathways are significantly more efficient in the wide bandgap semiconductor hosts but a clear dependence on the incorporation site is observed by site-selective studies. In these hosts, we speculate that N-defects may act as the trap. In both classes of material we come to the conclusion that a direct excitation of the RE earth through either the direct impact or through free excitons is very inefficient. The presence of defects is needed to allow efficient electrically pumped rare earth emission!!. This important conclusion of our work may explain the inverse relation of device efficiency to its lifetime reported by Coffa et al. in the Si/SiO2 device structures .  M.E. Catagna, S. Coffa, M. Monaco, A. Muscara, L. Caristia, S. LOrenzi, and A. Messina, Mat. Sci&Eng B 105, 83 (2003) A.J. Steckl, J. Heikenfeld, D. Lee, M. Carter, C. Baker, Y. Wang, and R. Jones, IEEE Journal on Selected Topics n Quantum Electronics 8, 749 (2002).Supported by NSF-grant DMR-0602986, NSF-DMR-0705217, and collaborative agreement W911NF-07-2-0064 between the Army Research Lab and Lehigh University
5:30 PM - **D3.10
Optical Activation of Rare Earths Implanted into III-Nitrides.
Eduardo Alves 1 , Katharina Lorenz 1 , F. Gloux 2 , P. Ruterana 2 , Alan Braud 2 , Kevin Donnell 3 , Robert Martin 3 Show Abstract
1 Physics and Accelerators, ITN, Sacavem Portugal, 2 , CIMAP, UMR 6252 ENSICAEN-CNRS-CEA-UCBN, Caen France, 3 Department of Physics, SUPA, University of Strathclyde, Glasgow United Kingdom
Wide band-gap semiconductors, particularly III-nitrides, have become one of the most studied materials during the last decades. These compounds are the base of a new generation of optoelectronic devices operating in the UV-Blue region of the electromagnetic spectrum. Incorporation of rare-earth (RE) into nitrides creates new routes to build novel all-nitride electroluminescent devices, using the sharp intra 4f transitions of these elements. The introduction of the RE ions in the nitride lattice during the growth or by ion implantation creates defects which influence the behaviour of the optical active region.In this work we present our recent achievements on rare earth (RE) doped III-nitrides by ion implantation. A combination of techniques (Rutherford backscattering/Channeling, High Resolution X-Ray Diffraction and Electron microscopy, EXAFS, Photoluminescence and Cathodoluminescence) were used to assess the mechanism responsible for the optical and structural behaviour of the doped materials. The data clearly demonstrate how the optical activity of the RE could be enhanced by orders of magnitude by reducing the number of non-radiative paths related with defects and by the incorporation of RE ions into substitutional Ga lattice sites. The results suggest a correlation between the increase of substitutional RE ions and the Cathodoluminescence intensity.
D4: Poster Session
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - D4.1
Luminescence Enhancement in Eu-doped GaN Powder by Oxidative Passivationof the Surface.
Tiju Thomas 1 , Mvs Chandrashekhar 1 , Carl Poitras 1 , Junxia Shi 1 , Jesse Reiherzer 2 , Francis DiSalvo 2 , Michal Lipson 1 , Michael Spencer 1 Show Abstract
1 School of Electrical and Computer Engineering, Cornell University, Ithaca, New York, United States, 2 Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States
We show a method to increase the red luminescence of Eu doped GaN(GaN:Eu) powder by a factor of >3 using a nitric acid rinse, over powderreported previously in literature. The mechanism of this enhancement wasstudied using XPS, XRD and PL. The powder was synthesized using a lowcost, high yield rapid-ammonothermal process reported in literature. Theresulting powder is known to have a strong emission in the red. Despiteits strong luminescence. the powder prepared using this method is alsofound to be dark on account of several impurities in it. The dark color ofthe powder can be attributed to Europium Nitride, elemental Bi and Ga, asdetermined by XRD analysis. Due to its dark color, these impurities arestrongly absorbing, decreasing the luminescence of the GaN:Eu.These impurities were removed using a simple chemical rinse in an acidsolution. While the chemical rinse clears the powder of all theimpurities, we also observed that it enhances the photoluminescence of thepowder. Nitric acid, hydrochloric acid rinses of varying times andtemperatures were investigated.The XPS peak correponding to Ga showed achemical shift, indicating a Ga-O bond. This peak shifted continously withrespect to the duration over which the chemical rinse was performed beforereverting back to its unshifted position. This observation suggests thatour process continuously oxidizes the GaN surface, forming a GaN-oxideinterface. This additional shift is attributed to electrostatic potentialsat the interface arising from the valence band discontinuity betweenGaN/Ga2O3, in analogy with GaN/AlN interfaces reported earlier by Markoc et al. A valenceband offset of ~1 eV was estimated, in good agreement with known band parameters.As the oxide layer gets formed uniformly around the GaN particle, greater interface potentials develop, owing to the fact that pure Ga2O3 has alarger bandgap than GaN. We have found that a well defined GaN-oxideinterface was formed by rinsing in nitric acid for 4 hours at roomtemperature. The valence band offset attains the ideal GaN/Ga2O3 valuewhen the PL intensity of the powder is highest. We suggest that the higherintensities of Ga2O3 passivated GaN:Eu is because the surface states thatmight be responsible for reducing the luminiscence of these GaN particlesare eliminated when a clean GaN-oxide junction is formed. We alsospeculate that the removal of absorbing impurities in the as preparedpowder contributes to this increase. If the rinse is too short, theluminescence is not maximized due to incomplete oxide formation andimpurity removal. However, if the chemical rinse is performed for toolong, all the GaN gets consumed, quenching the luminiscence. Thisquenching is accompanied by a corresponding loss of interface potential,as observed by XPS.
9:00 PM - D4.10
Luminescence Properties from Si- and Ge-doped AlN:Eu Thin Films Prepared by RF Magnetoron Sputtering.
Naoki Iwata 1 , Shin-ichiro Uekusa 1 Show Abstract
1 School of Science and Technology, Meiji University, Kawasaki-shi Japan
The inorganic EL display are attracting significant attention as one of the most promising flat panel displays. The displays are low power consumption. Moreover, there is a feature that it is a high contrast because it emits spontaneously light. Recently the main of the research and development of inorganic EL is a sulfureted system. However, the sulfide is chemically unstable in wet atmosphere. Moreover, it has a negative environmental impact given as a problem. Then, we paid attention to AlN in stead of sulfide, because AlN have been studied as promising materials for light emitting diodes and EL devices, it has a wide band gap (6.2 eV), high thermal conductivity, and chemical inertness. Inorganic EL of nitride fluorescent body is few report. In the case of AlN thin films doped with Eu, red line emission is usually seen as a result of electron transition of Eu3+. Recently, it is reported to show blue luminescence from AlN:Eu powder that adds Si powder. The aim of this study is to obtain the strong blue luminescence from the Si doped AlN:Eu thin films comparison with Ge doped AlN:Eu thin films. To remove the surface oxidation of the n-Si(111) it was etched 5 % by using HF, subsequently loaded in the growth chamber for sputtering. The Eu chips (99.9 %) on the Al target (99.999 %) and several Si chips (99.9 %) were used, and Si doped AlN:Eu thin film were deposited. Moreover the Eu chips (99.9 %) on the Al target (99.999 %) and several Ge chips (99.9 %) were used, and Ge doped AlN:Eu thin film were deposited. Nitrogen and Argon gas were used for the preparing the sample. The prepared samples were annealed in the temperature range from 400 to 900oC for 30 minutes and 60 minutes for in the nitrogen and oxygen atmosphere. The luminescence of the samples was measured by a photoluminescent (PL). The Eu addition density of the sample was measured by Energy dispersive X-ray profile (EDX). Crystallinity of the samples was measured by X-ray diffraction (XRD). The thin films were prepared in the flowing quantity of N2 and Ar was 5.6 sccm and 2.4 sccm, respectively. Two main peaks were observed in PL spectra features. One main peak was observed around 530 nm, and an increase in the luminescence intensity was seen by increasing the annealing temperature up to 900oC. It has shifted to the short wavelength side. The result luminescence from Eu2+ in AlN:Eu thin film. The other main peak was observed at about 620 nm. This means luminescence from Eu3+. We systematically discuss the results in detail.
9:00 PM - D4.11
Optical and Luminescent Properties of Highly Oriented Nanocrystalline Gd2-xEuxO3 Thin Films.
Segundo Jauregui-Rosas 1 2 , Oscar Perales-Perez 3 , Maharaj Tomar 2 , Omar Vasquez 2 Show Abstract
1 Fisica, Universidad Nacional de Trujillo, Trujillo Peru, 2 Physics, University of Puerto Rico at Mayagüez, Raleigh, North Carolina, United States, 3 Department of Engineering Science and Materials, University of Puerto Rico at Mayagüez, Mayagüez, Puerto Rico, United States
Highly crystalline and transparent Eu-doped Gd2O3 thin films were produced by sol-gel method and spin coating process from starting acetic acid solutions with no need for any chelating agent. The effect of the atomic fraction of Eu3+ ions (‘x’ =0.05-0.30) on the structural, optical and luminescent properties has been determined. It was found that the oxide films exhibited a preferential growth along the (400) plane. No other than cubic phase was developed at all levels of dopant. UV-vis measurements revealed the high transparency of the films in the visible region and evidenced a negligible effect of Eu contents on the corresponding band gap value (5.3eV). It was also found that the luminescence properties were strongly dependent on both the excitation wavelength and europium content; the most efficient excitation conducive to red luminescence was achieved at the absorption band of the Gd2O3 host (229nm). Under this condition, all films exhibited strong red emission characteristic of the Eu3+ ions hosted by crystalline frameworks. The emission intensity was strongly dependent on the Eu content; the most intense luminescence was obtained for x=0.15. The drop in the luminescence intensity for ‘x’ values higher than 0.15 was attributed to the quenching concentration effect. Similar behavior was observed using the charge transfer band as excitation wavelength. Morphological and film roughness analyses of films using Atomic Force Microscopy (AFM) will also be presented and discussed.
9:00 PM - D4.12
Studies of III-Nitride Superlattice Structures and its Deformation upon Implantation with Lanthanide Ions.
Mohammad Ebdah 1 , W. Jadwisienczak 2 , H. Morkoc 3 , A. Anders 4 Show Abstract
1 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 School of Electrical Engineering and Computer Science, Ohio University, Athens, Ohio, United States, 3 Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States, 4 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Rare earth (RE) ions doped low dimensional III-Nitride structures including quantum wells (QW), superlattices (SL) and quantum dots (QD) have received recently increasing interest due to their potential applications in optoelectronics. In this work, AlN/GaN SLs of 20 periods with a fixed well/barrier thickness ratio were grown by chemical vapor deposition (CVD) technique on GaN/(0001) sapphire substrate without a capping layer on top. Implantation of selected rare earth ions was done at 150 keV and a dose of up to 1x1015 cm-2 at room temperature. Samples were given post implant isochronal thermal annealing treatment in 800 – 1200°C temperature range in NH3 and N2 using a conventional resistive furnace and rapid thermal annealing technique. The interfacial deformation between the SL layers before and after implantiation, as well upon annealing, has been investigated by X-ray diffraction (XRD) and the characteristic satellite peaks of SLs were measured for the (0002) reflection up to the second order in the symmetric Bragg reflections. Furthermore, the optical properties of RE ions implanted AlN/GaN SLs have been studied by means of ellipsometry, photo- and cathodoluminescence and photoluminescence excitation spectroscopy. The luminescence results demonstrate an increase of the radiative emission efficiency from RE-doped AlN/GaN SL when compared with RE-doped III-Nitride thin films. Furthermore, the possibility of using RE ions confined in III-Nitride low-dimensional quantum structures for optoelectronics devices is considered.
9:00 PM - D4.13
Similarities and Differences of Sensitization Mechanism of Er3+ in Si-rich SiO2 with and without Silicon Nanocrystals.
Oleksandr Savchyn 1 , Pieter Kik 1 , Ravi Todi 2 , Kevin Coffey 2 Show Abstract
1 CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida, United States, 2 AMPAC, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, Florida, United States
The indirect excitation of erbium in silicon-rich SiO2 has long been associated with the presence of silicon nanocrystals formed inside the silica matrix. However our recent studies showed that excess-silicon related luminescence centers (LC) are in fact the dominant source of Er3+ excitation in this material. The current study demonstrates that a significant fraction of the LC-mediated erbium excitation occurs directly into the first excited state with a LC-Er transfer time < 40 ns. This fast excitation process could allow for luminescence-center-sensitized Er3+ doped gain media with high maximum output power. Erbium-doped (0.63 at.%) Si-rich SiO2 films (Si excess 12 at.%) were prepared using sputter deposition technique. The subsequent annealing at different temperatures resulted in films with significantly different microstructure, namely (a) samples that do not contain detectable Si nanocrystals (annealed at 600oC), and (b) samples containing silicon nanocrystals (annealed at 1100oC). Modulated continuous-wave excitation at a wavelength of 351 nm was used to determine the effective absorption cross-section of the first excited state of Er3+ at sample temperatures in the range 15 – 300K. The obtained Er3+ absorption cross section was found to be approximately ~(0.9 - 1.0) x 10-15 cm2 for samples with and without silicon nanocrystals, and remained relatively constant in the considered temperature range. In contrast, the absorption cross section of Si nanocrystals is found to increase by a factor of ~ 2 as the temperature is increased from 15K to 300K. These results support the conclusion that Si-nanocrystal-mediated sensitization is not the dominant Er3+ excitation mechanism. Temperature dependent pulsed excitation measurements were conducted at an excitation wavelength of 355 nm. Under these conditions, samples not containing Si nanocrystals clearly show the presence of two different processes leading to the excitation of Er3+ ions into the first excited state: a fast luminescence center mediated excitation directly into the first excited state (τtransfer, fast < 40 ns), and a relatively slow excitation with a transfer time approximately equal to the 4I11/2 lifetime (τtransfer, slow ≈ 3 μs). The clear correlation of the excitation time constant and the lifetime of the Er3+ second excited state suggests that the slow Er3+ excitation process in low-temperature processed samples is related to the spontaneous relaxation from the 4I11/2 level into the 4I13/2 level. Based on the temperature-dependent magnitude of these two contributions and the measured time constants, the temperature dependence of the branching ratio of the second excited state is derived. The excitation processes of Er3+ in the samples with and without silicon nanocrystals are compared and the importance of these findings for the development of silicon compatible Er-based light sources is discussed.
9:00 PM - D4.14
Enhancement of Erbium Incorporation with Implantation into Nanoporous GaN.
Chew Beng Soh 1 , Sihui Sim 2 , Sudhiranjan Tripathy 1 , Soo Jin Chua 1 2 , Eduardo Alves 3 Show Abstract
1 , Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore Singapore, 2 , Centre of Optoelectronics, National University of Singapore, Singapore Singapore, 3 , ITN, Sacavém Portugal
9:00 PM - D4.15
First Eu3+@Organo-Si(HIPE) Hybrid Macro-Mesocellular Foams Generation and Associated Photonic Properties.
Nicolas Brun 1 5 , Béatriz Julian-Lopez 2 , Peter Hesemann 3 , Guillaume Laurent 4 , Hervé Deleuze 5 , Clément Sanchez 4 , Marie-France Achard 1 , Annick Babeau 1 , Rénal Backov 1
1 , Centre de Recherche Paul Pascal UPR 8641 CNRS, PESSAC France, 5 , Institut des Sciences Moléculaires UMR 5255 CNRS Université Bordeaux 1, TALENCE France, 2 , Departamento de Química Inorgánica y Orgánica ESTCE Universitat Jaume I, CASTELLÓN Spain, 3 , Institut Charles Gerhardt UMR 5253 Ecole Nationale Supérieure de Chimie, MONTPELLIER France,