Walter R. L. Lambrecht Case Western Reserve University
Kevin Smith Boston University
H. Joseph Trodahl Victoria University of Wellington
Andreas Ney Universitaet Duisburg-Essen
I1: Rare-earth Nitride Growth
Monday PM, November 29, 2010
Room 301 (Hynes)
9:30 AM - **I1.1
Physical Properties of MBE-Grown GdN Thin Films and of ErAs and GdN Nanoparticles Embedded in III-V Semiconductors.
Michael Scarpulla 1 2 3 , Roberto Myers 4 , Jing Yang 4 , Chad Gallinat 3 , Hong Lu 2 , Shawn Mack 3 , James Speck 3 , Trevor Buehl 3 , Brent Melot 3 , Rajesh Chopdekar 5 6 , Kin Yu 6 , Arthur Gossard 3 Show Abstract
1 Materials Science & Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 3 Materials Department, UC Santa Barbara, Santa Barbara, California, United States, 4 Materials Science and Engineering, Ohio State University, Columbus, Ohio, United States, 5 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 6 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Similarities between the rocksalt crystal structure of RE-V (RE = Sc, Y, and La-Lu; V = N, P, As, Sb) and the zincblende and wurtzite III-V (III = Al, Ga, In) semiconductors allow RE-V compounds to be epitaxially grown on or embedded in III-V structures. Epitaxy of RE-Vs on III-V semiconductors was pursued in the early 1990s in order to achieve low-defect density epitaxial metal-semiconductor interfaces. Since then, unique epitaxial growth modes enabling the epitaxial embedding of RE-V nanoparticles in III-V matrices have been identified and exploited leading to a number of unique device demonstrations. Here we report on experiments aimed at elucidating the intrinsic physical properties of GdN layers and of embedded GdN and ErAs nanoparticles in III-Vs using molecular beam epitaxy (MBE). Although the bandstructure of GdN is difficult to determine unambiguously, it displays pure ferromagnetism making it attractive for nitride-based spintronics. High-quality samples are important for determining its intrinsic properties and usefulness for spintronic applications. The properties of epitaxial GdN layers grown on GaN will be discussed and constraints placed on possible bandstructures for GdN. GdN is demonstrated to grow readily on (0 0 0 1) GaN by MBE using both plasma and ammonia N sources, however the crystal quality is poor which is caused in large part by the presence of two rotational variants on the hexagonal GaN surface. For applications involving spin polarized transport, defects are expected to decohere spins thus more structurally perfect structures are desirable. RE-V nanoparticles embedded in III-V hosts allows some properties of these materials to be exploited without oxidation and with smaller volumes over which structural perfection must be maintained. We describe the intrinsic electrical, magnetic, and optical properties of 2 nm ErAs nanoparticles embedded in (Al, Ga, In)As and discuss the crystalline and electronic structure at the interface with the matrix. Studies of the strong near-infrared absorption in these nanocomposites have proven especially important in understanding their electronic structure. Although their small size suggests the possibility of a confinement-induced semimetal-semicondcutor transition, an explanation in terms of surface plasmon resonances on nanoparticles with semi- or full-metallic carrier densities is more consistent with all of the available data. This explanation does suggest that extreme defect densities exist at the ErAs / III-V interface on these nanoparticles which is interesting in light of their observed structural perfection and the original intentions of forming defect-free epitaxial interfaces. Lastly we will discuss the implications of these studies on arsenide RE-V/III-V nanocomposites on analogous nitride systems as well as preliminary results from GdN nanoparticles embedded in GaN.
10:00 AM - I1.2
Plasma Enhanced Atomic Layer Deposition (PE-ALD) of Gadolinium Nitride.
Richard Potter 1 , Ziwen Fang 1 , Paul Williams 2 1 , Peter Heys 2 Show Abstract
1 Engineering, The University of Liverpool, Liverpool, Merseyside, United Kingdom, 2 , SAFC Hitech, Wirral, Merseyside, United Kingdom
Rare-earth metal nitrides have been predicted and shown to possess a range of interesting electronic and magnetic properties. Many of these nitrides are ferromagnetic semiconductors, although over the years there has been conflicting reports about their electrical properties, which, for any one material may range from half-metallic to semiconducting to insulating. Of the rare-earth nitrides, gadolinium nitride has attracted significant attention due to the large spin moment of this metal and the resulting magnetic properties. A significant challenge for the future exploitation of rare-earth nitrides within nano-scale spintronic devices is the deposition technique used to form the high-quality thin film layers that are likely to be required for very large scale integration (VLSI) based fabrication process. From recent developments in CMOS integrated circuits, we can see moves towards high-aspect ratio structures to increase device packing densities and it is reasonable to assume that spintronic based devices will need to follow a similar strategy in the future. While physical based deposition techniques such as PVD and MBE are ideal for producing high quality nitride films, they are severely limited for conformal coatings on the high-aspect ratio structures. Atomic layer deposition (ALD) in contrast, is able to produce highly conformal coatings on very high aspect ratio structures with angstrom-scale control of thickness, it is therefore an ideal candidate for thin film coatings on nano-scale devices. In this paper, we report on the first ALD based deposition processes for successful deposition of gadolinium nitride at low temperatures. Films are deposited on Si(100) substrates using a cyclic PE-ALD sequence involving alternating doses of a cyclopentadienyl based gadolinium source and remote nitrogen plasma exposures separated by gas purges. The layers are capped with TaN to prevent post-deposition oxidation of the layers upon exposure to air. Films were deposited at temperatures between 150 and 300°C. Films grown at 200°C have good nitrogen incorporation, with relatively low oxygen and carbon incorporation. This work has been supported by the Engineering and Physical Sciences Research Council (Grant number: EP/E048560/1). The authors would like to acknowledge the help and support of staff at the Daresbury Laboratory MEIS facility (EPSRC: EP/E003370/1), Oxford Instruments Plasma Technology and Prof. Hyeongtag Jeon from Hanyang University, Korea for AES measurements.
10:15 AM - I1.3
Recent Development in the Epitaxial Growth of Selected Rare-earth Nitrides for Spintronics Applications.
F. Natali 1 , N. Plank 1 , B. Ruck 1 , J. Trodahl 1 , M. Azem 1 , F. Semond 2 , S. Sorieul 3 , L. Hirsch 4 Show Abstract
1 School of Chemical and Physical Sciences, Victoria University of Wellington,, MacDiarmid Institute of Advanced Materials and Nanotechnology,, Wellington New Zealand, 2 , CRHEA-CNRS, Valbonne France, 3 , Centre d'Etudes Nucléaires de Bordeaux-Gradignan IN2P3, Gradignan France, 4 , Laboratory of Integration from Materials to Systems , Pessac France
Up to now, most of the studies of the electrical and magnetic properties of the rare-earth nitrides (REN) have been devoted to polycrystalline bulk and thin films. As a matter of fact, there are to date only scarce reports on the epitaxial growth of REN compounds. The advantages of epitaxial over polycrystalline REN could be numerous at the nanometre scale: a better control and improvement of the interfaces, diminution of grains boundaries…. The few attempts at growing GdN, the prototypical REN compounds, have been achieved on YSZ(100)  and MgO(100)  due to the NaCl structure of GdN. But, due the propensity of GdN to be strongly textured in the (111) direction, growth efforts have been recently performed onto wurtzite GaN(0001) surfaces . Silicon, on the other hand, can be well-considered as an alternative to these substrates because of its properties as low-cost, large scale availability and high thermal and electrical conducting properties. Nevertheless, the main problem related to the growth of RE-based compounds on Si substrates is the strong reaction of the RE atoms with silicon surface. This leads to the formation of a GdSix layer with a surface of poor quality preventing the growth of high quality GdN layers. We show that this hindrance can be overcome by using a wurtzite (0001) AlN buffer layer on top of the silicon (111) substrate . This AlN layer can also act as template for the integration of GdN not into the silicon technology but also in the group III-nitrides, a technologically important family for the fabrication of optoelectronic devices and high power transistors. We will present the structural, electrical and magnetic properties of such thin films. In our recent work, we have extended this study to EuN and SmN thin layers and the first results will be presented. References: 1. B. M. Ludbrook et al., J. Appl. Phys. 106, 063910 (2009).2. J. W. Gerlach et al., Appl. Phys. Lett. 90, 061919 (2007).3. M. A. Scarpulla et al., J. Cryst. Growth 311, 1239 (2009).4. F. Natali et al., arXiv:1004.5428v1
10:30 AM - I1.4
Ferromagnetic State of GdN Thin Films with Variable Carrier Densities.
Natalie Plank 1 , Franck Natali 1 , Jan Richter 1 , Mark Simpson 1 , Ben Ruck 1 , H. Joe Trodahl 1 Show Abstract
1 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, Wellington Region, New Zealand
We have investigated GdN thin films grown at room temperature with a thin insulating layer of AlN to prevent decomposition in atmosphere. Ex-situ we have studied the electrical behaviour of the films and shown that the conductivity of the samples can be altered by varying the nitrogen pressure during growth, thereby changing the number of vacancies in the films. These polycrystalline films are semiconducting with carrier concentrations lower than epitaxial GdN films [1,2,3]. The resistivity anomaly at the Curie temperature scales by as much as 10 times as the nitrogen pressure is varied, however the Curie temperature (Tc) is always between 60-70K. It is striking that the low carrier concentration GdN films show structure in the magnetisation below the Tc, showing the onset of a stronger magnetic moment at around 40K (an effect not present for the higher carrier concentration epitaxial films). This is extremely interesting in view of the current debates surrounding the Tc of GdN [4, 5]. S. Granville et al., Phys. Rev. B., 73, 235335 (2006) B.M. Ludbrook et al., J. Appl. Phys., 106, 063910 (2009) F. Natali et al., arXiv:1004.5428v1 A. Sharma and W. Nolting, Phys. Rev. B., 81, 125303 (2010) C. Mitra and W. R. L. Lambrecht, Phys. Rev. B., 78, 134421 (2008)
10:45 AM - I1.5
Growth of Rare Earth Doped Gallium Nitride by Modified Metal Migration Epitaxy.
Mingyu Zhong 1 , Andrew Steckl 1 Show Abstract
1 Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, Ohio, United States
Metal migration epitaxy is a technique for the MBE growth of in-situ doped GaN (and III-N alloys) films wherein the metal fluxes (Ga, Al, Mg etc.) are modulated in a short periodic fashion, while maintaining a continuous nitrogen plasma flux. It was previously reported to provide high p-type GaN:Mg conductivity by reducing the compensation effect by nitrogen vacancies. We have modified this technique to a more general case based on understanding the doping process and the relationship between doping concentration and doping efficiency. This has proven to be a useful and robust method for growing rare earth (RE)doped GaN films. GaN:RE films were grown on AlGaN-coated p-Si (111) substrates with this modified metal modulated epitaxy (M3E) technique in a Riber 32 MBE system. SIMS depth analysis was used to determine the doping level and a He-Cd laser was used for above bandgap activation in PL measurement. During GaN:Eu growth, high Ga flux is maintained and Ga and Eu shutters are opened and closed periodically (not necessarily synchronous) while keeping the N flux constant, such that the Ga and Eu coverage on the surface during each cycle varies in a controlled way. The high Ga flux is used to smooth the surface during the metal shutter ON period. After the metal shutter closes, the N plasma fully saturates the film to prevent droplet buildup and reduce N vacancies in the film.We have found that Eu3+ ions doped during the period of low Ga coverage exhibit poor PL efficiency, probably due to surface segregation and accumulation of Eu atoms. Optimization of M3E is obtained by opening the Eu shutter only when the Ga coverage is sufficiently high. With the optimized M3E growth scheme, the PL efficiency of Eu3+ ion shows 50% enhancement compared to conventional (but optimized) MBE grown samples. The M3E technique will be useful in growing GaN films doped with other REs and with transition metals and will ultimately be very important for the fabrication of GaN:RE photonic and spintronic devices.
11:00 AM - I1:RE growth
I2: Rare Earth Nitrides: Band Structure Spectroscopy and Devices.
Monday PM, November 29, 2010
Room 301 (Hynes)
11:30 AM - **I2.1
Electronic Structure of the Rare-earth Nitrides.
Ben Ruck 1 , Andrew Preston 2 1 , Joe Trodahl 1 , Bart Ludbrook 1 , Louis Piper 2 , Jan Richter 1 , James Downes 3 , Steve Durbin 4 , Roger Reeves 4 , Claire Meyer 5 , Kevin Smith 2 , Walter Lambrecht 6 Show Abstract
1 The MacDiarmid Institute, Victoria University of Wellington, Wellington New Zealand, 2 Department of Physics, Boston University, Boston, Massachusetts, United States, 3 Department of Physics, Macquarie University, Sydney, New South Wales, Australia, 4 The MacDiarmid Institute, Canterbury University, Christchurch New Zealand, 5 , Institut Neel (CNRS and UJF), Grenoble France, 6 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
The rare-earth nitrides (RE-N) represent a class of materials displaying especially strong coupling between their electronic and magnetic properties. They lie on the boundary between metals and insulators, and treating the strongly correlated 4f electrons within band theory is a challenge for the developing theoretical methods [1,2]. Various treatments disagree about such fundamental issues as whether the members of the RE-N series are metals or insulators, and of equal interest to the possibility of half-metals are predictions that some are intrinsic ferromagnetic semiconductors, with the same spin polarisation in both the valence and conduction bands.Experimental studies addressing the theoretical predictions have been limited, due largely to the lack of quality samples, and the propensity of the RE-Ns to react with atmosphere. We have recently made advances in the growth and passivation of thin RE-N films, and using a combination of optical and synchrotron-based x-ray spectroscopy, along with magnetic and transport measurements, we have begun to elucidate their electronic structure [3-5]. We have found that GdN, SmN, and DyN are semiconducting, with GdN in particular showing very strong coupling between magnetic and transport properties. By contrast, EuN is likely metallic, and possibly half-metallic. The results suggest that interesting spintronics devices could be made based on these materials. M. Aerts, P. Strange, M. Horne, W.M. Temmerman, Z. Szotek, and A. Svane, Phys. Rev. B 69, 045115 (2004). P. Larson, W.R.L. Lambrecht, A. Chantis, and M. van Schilfgaarde, Phys. Rev. B 75, 045114 (2007). S. Granville, B.J. Ruck, F. Budde, A. Koo, D. Pringle, F. Kuchler, A.R.H. Preston, D.H. Housden, N. Lund, A. Bittar, G.V.M. Williams, and H.J. Trodahl, Phys. Rev. B 73, 235335 (2006). A.R.H. Preston, B.J. Ruck, W.R.L. Lambrecht, L.F.J. Piper, J.E. Downes, K.E. Smith, and H.J. Trodahl, Appl. Phys. Lett. 96, 032101 (2010).  H.J. Trodahl, A.R.H. Preston, J. Zhong, B.J. Ruck, N.M. Strickland, C. Mitra, and W.R.L. Lambrecht, Phys. Rev. B 76, 085211 (2007).
12:00 PM - I2.2
Physical and Electronic Properties of Rare Earth Nitride Thin Films.
Josh Brown 1 , James Downes 1 , Chris McMahon 1 , Kevin Smith 2 , Andrew Preston 2 , Louis Piper 2 , Alex DeMasi 2 Show Abstract
1 Physics, Macquarie University, Sydney, New South Wales, Australia, 2 Physics, Boston University, Boston, New York, United States
The rare earth nitride (REN) series form a fascinating class of materials due to their unique electronic and magnetic properties. Theoretical predictions yield a variety of conductive states from metallic to insulating amidst a variety of magnetic states including both ferro- and antiferromagnetism, depending on the rare earth element involved. Compared with DMS materials, the intrinsic ferromagnetism present in RENs represents a system in which a high degree of spin-polarisation is achievable, without the loss of control over the free carrier concentration. As such, these materials have emerged as strong candidates for use in spintronics applications. Unfortunately, experimental support for these predictions is scarce due to the materials’ strong tendency to oxidise when exposed to ambient atmosphere, and the difficulty in producing high-quality, stoichiometric samples. We have overcome these difficulties and have successfully deposited high-purity, polycrystalline thin films of NdN, SmN, EuN, DyN, HoN, ErN, TmN and YbN using low-energy IAD, and passivated the samples with YSZ capping layers. We present a systematic study of these samples, identifying trends in the physical and electronic properties across the series. Bandstructure information is extracted from x-ray emission and absorption spectroscopy which probe the occupied and unoccupied states at the N k-edge. Crystal structure, growth orientation and lattice constants from XRD are compared with theoretical predictions. The optical bandgaps are identified using UV-visible absorption spectroscopy. By studying the evolution of these properties across the series we gain a valuable understanding of the driving mechanisms involved, enabling validation of the current theoretical approach to modelling these strongly correlated systems and providing vital experimental data used to refine the correction parameters in the LSD+U model. We acknowledge funding provided by the ISAP managed by the Australian Synchrotron. 1.Larson et al., Phys. Rev. B 75, 045114 (2007). 2.Trodahl et al., Phys. Rev. B 76, 085211 (2007).
12:15 PM - I2.3
Electronic Band Structure of CrN and GdN from Soft X-ray Spectroscopy.
Andrew Preston 1 , Louis Piper 1 , Jim Partridge 3 , Bo Chen 1 , James McNulty 1 , Alexander DeMasi 1 , Steve Durbin 3 , Ben Ruck 4 , Joe Trodahl 4 , Walter Lambrecht 2 , Kevin Smith 1 Show Abstract
1 Department of Physics, Boston University, Boston, Massachusetts, United States, 3 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand, 4 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand, 2 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
In this talk we present the results of synchrotron based soft X-ray absorption (XAS), emission (XES), and photoemission measurements of thin films of chromium nitride (CrN) and gadolinium nitride (GdN). These nitrides are to a certain extent correlated. For example, recent theoretical results suggest CrN is either a Mott insulator, or very close to being one, and the band gap of GdN is not well understood either. We compare our measurements with electronic structures calculated under the LSDA+U approximation. Additionally we detail the results of resonant XES (RXES) measurements, and compare them to spectra calculated under the Kramers-Heisenberg formalism.
12:30 PM - I2.4
Spin Dependent P-N Diode Based on GdN.
Thomas Minnee 1 , Ben Ruck 1 , H. Joseph Trodahl 1 , Ulrich Zuelicke 2 Show Abstract
1 The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington New Zealand, 2 The MacDiarmid Institute for Advanced Materials and Nanotechnology, Massey University, Palmerston North New Zealand
Rare-earth nitrides (RE-N’s) are a class of materials that have been largely ignored in spintronic device proposals , despite the consensus the series contains intrinsically ferromagnetic semiconductors [2,3]- materials the field is calling for. The ability to dope such semiconductors either N- or P-type independent of the magnetism presents a unique opportunity to create homogenous spin-dependent semiconductor devices that could potentially circumvent some of the controversy regarding spin transport across junctions. This talk will describe a theoretical investigation of a P-N diode based on GdN, a material for which theory and experiment indicate an approximately 0.5 eV band gap with around 0.5 eV spin splitting of both the valence and conduction bands [3,4]. Also significant is that the spin splitting in the conduction and valence bands is of opposite sign, meaning that the band edges which brace the band gap are of the same spin polarisation. The primary model for analysis is based on the classical models of drift-diffusion and thermionic emission used in conventional electronics, but implemented on four possible carrier bands (i.e. valence and conduction bands for each spin polarisation) as opposed to the normal two. Interestingly, when this simple model is applied to a P-N junction of anti-aligned ferromagnetic polarisation, the ratio of spin currents through the diode is a function of applied voltage, thus potentially providing a strikingly simple way to control spin dynamics.  I. Zutic, J. Fabian, and S. C. Erwin, Bipolar spintronics: from spin injection to spin-controlled logic, J. Phys.: Condens. Matter 19, 165219 (2007).  M. Aerts, P. Strange, M. Horne, W.M. Temmerman, Z. Szotek, and A. Svane, Half-metallic to insulating behavior of rare-earth nitrides, Phys. Rev. B 69, 045115 (2004). P. Larson, W.R.L. Lambrecht, A. Chantis, and M. van Schilfgaarde, Electronic structure of rare-earth nitrides using the LSDA+U approach: Importance of allowing 4f orbitals to break the cubic crystal symmetry, Phys. Rev. B 75, 045114 (2007). H.J. Trodahl, A.R.H. Preston, J. Zhong, B.J. Ruck, N.M. Strickland, C. Mitra, and W.R.L. Lambrecht, Ferromagnetic red shift of the optical gap in GdN, Phys. Rev. B 76, 085211 (2007).
12:45 PM - I2.5
Ga1-xGdxN-Based Spin Polarized Light Emitting Diode.
Tahir Zaidi 1 3 , Andrew Melton 1 , Muhammad Jamil 1 , Tianming Xu 1 , Ian Ferguson 1 2 Show Abstract
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 , Center for Advanced Studies in Engineering (CASE), Islamabad Pakistan, 2 Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina, United States
To date, the ability to successfully produce ferromagnetic semiconductors based optoelectronic devices that experience a change in polarization/spin upon exposure to a magnetic field with Curie temperatures above room temperature remains a challenge. In this work ferromagnetic Ga1-xGdxN based LED device structures were grown by metalorganic chemical vapor deposition (MOCVD) and studied for their feasibility in spin polarized optoelectronic applications. Gadolinium doped GaN thin films have been extensively grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) and room temperature ferromagnetism of varying magnitude in these films has been reported by a number of research groups. However, no published report exists of use of these films in a practical device structure because of their high resistivity and the inability to form p-type material. Gadolinium doped GaN thin films were grown on 2µm GaN templates using Trimethylgallium (TMGa), ammonia (NH3) and Gd(thd)3 as group-III, group–V and Gd dopant precursors respectively. These thin films were also co-doped with Si and Mg to achieve n-type and p-type materials. Thin Ga1-xGdxN films as well as device structures were characterized for their structural, optical, electrical and magnetic properties. Co-doping of the Ga1-xGdxN films with Si gave an n-type material improving further the resistivity of the films. Doping with Mg gave a p-type material with an increase in the resistivity of the material. In order to confirm the p-type behavior, p-n diodes were fabricated followed by fabrication of LED using the Ga1-xGdxN films. The I-V curves of theses structures confirms the p-type behavior of the films. An increase in series resistance was observed in these devices due to increased compensation in the Mg doped Ga1-xGdxN layers. For comparison, a baseline standard GaN LED was also fabricated having the same QW structure in the active region. The spin LED was shown not only to emit spin-polarized light but also change the polarization ratio with the application of an external magnetic field. The spin-polarized LED was subject to a number of functional characterizations to establish the manipulation of its spin-polarized emission by an external magnetic field.
I3: Rare Earth Doping of GaN
Monday PM, November 29, 2010
Room 301 (Hynes)
2:45 PM - **I3.1
Ferromagnetism in Lightly Gd Doped GaN: The Role of Defects.
Subhabrata Dhar 1 , Jitendra Mishra 1 Show Abstract
1 Physics, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India
Recently, an interesting magnetic behavior has been observed in lightly Gd-doped GaN layers. The material is found to exhibit ferromagnetism above room temperature even with a Gd concentration as low as 7x10^15 cm^-3. Moreover, the effective magnetic moment per Gd ion is found to be as high as 4000 μ_B. This colossal moment has also been reported for Gd-implanted GaN layers where It is found to be higher as compared to that of a GaN:Gd layer doped in situ and found to decrease upon annealing. These results strongly suggest that native point defects play an important role for the ferromagnetism in this material. So far, however, there has been no direct evidence for the presence of these defects in GaN:Gd layers doped in situ. Here, we show by a variety of experimental techniques that a large density of defects is formed in GaN during Gd incorporation in NH_3 molecular beam epitaxy. These defects are also shown to be directly involved in shaping the magnetic properties of this material. GaN layers with a Gd concentration ranging from 7x10^15 to 2x10^17 cm^-3 were grown directly on 6H-SiC(0001) substrates. Wavelength dependent photoconductivity measurement conducted on these samples shows the existence of a broad defect feature peaking at approximately 450 mev below the band edge. The integrated area of the defect band, which represents the concentration of defects, is found to increase with the Gd concentration. Thermally stimulated current (TSC) spectroscopy shows the existence of a broad defect band appearing approximately 200 mev below the band edge. The band is found to be shifted to higher energies as the concentration of Gd increases. The density of defects is estimated to be much higher than that of Gd in these samples. It is also found to increase with the Gd concentration. It has to be noted that no such defect feature could be found in undoped reference sample by either photoconductivity or TSC spectroscopy. Moreover, x-ray diffraction study shows a systematic increase of the rocking curve band width as the concentration of Gd increases in these samples. These findings clearly indicate that a large density of defects is generated in GaN during Gd incorporation. Magneto-resistance (MR) measurement is carried out on these samples either in dark or illuminated (with a blue LED) conditions. A negative MR indicating a ferromagnetic behavior is found in these samples even at room temperature. MR is found to be more negative in illuminated condition than that is found in dark which indicates a change in the ferromagnetic property of these samples as the charge state of these defects is changed due to the illumination. There was no sign of negative MR or any change in MR due to illumination in case of undoped reference sample even when it is illuminated by an above band-gap light. These finding clearly suggests that the magnetic property of this materials system must be governed by the defects which are generated during Gd incorporation.
3:15 PM - I3.2
Identifying Magnetic Species in Gd-doped GaN via Element Specific Magnetometry.
T. Kammermeier 1 , V. Ney 1 , F. Wilhelm 2 , A. Rogalev 2 , M. Roever 3 , J. Malindretos 3 , A. Rizzi 3 , Andreas Ney 1 Show Abstract
1 Experimentalphysik, Universitaet Duisburg-Essen, Duisburg Germany, 2 , European Synchrotron Radiation Facility , Grenoble France, 3 IV. Physikalisches Institut, Georg August Universität, Göttingen Germany
Gd-doped GaN was reported to exhibit colossal effective magnetic moments per Gd atom in the ultradilute limit and magnetic order far above room temperature . We have demonstrated by means of x-ray linear dichroism (XLD) that for moderate Gd doping levels the Gd atoms are located predominantly on Ga substitutional sites . Further we could show that the element specific magnetic properties as measured with x-ray magnetic circular dichroism (XMCD) significantly deviate from the integral ones as measured by conventional SQUID magnetometry . Finally, at higer Gd concentrations either paramagnetism or superparamagnetism stemming from GdN inclusions were found . Here we will demonstrate how the various magnetic constituents in these types of Gd-doped GaN epitaxial films can be identified by quantitatively analyzing element specific magnetization measurements using XMCD. This analysis demonstrates the onset of phase separation as already indicated by magnetic resonance measurements . However, it leaves the question regarding the origin of room temperature ferromagnetism unanswered.  S. Dhar et al., Phys. Rev. Lett. 94, 037205 (2005);  A. Ney et al., Appl. Phys. Lett. 90, 252515 (2007);  A. Ney et al., J. Magn. Magn. Mater. 322, 1162 (2010);  T. Kammermeier et al., phys. stat. solidi (a) 205, 1872 (2008).
3:30 PM - I3.3
Ab-initio Calculations on Gd and Nd Doped Bulk GaN.
Vijay Kumar 1 Show Abstract
1 , Dr. Vijay Kumar Foundation, Gurgaon, Haryana, India
Doping of rare earths in bulk GaN is important for optical applications as well as from the point of view of dilute magnetic semiconductors. We have performed ab initio calculations on Gd and Nd doped bulk GaN using a 3x3x3 supercell within projector augmented wave pseudopotential formalism and spin-polarized generalized gradient approximation for the exchange-correlation energy. We find doping of Gd or Nd to be energetically favorable on Ga sites in GaN and there is a large magnetic moment on the dopant. The rare earth dopant creates significant strain in the GaN bond lengths. In the case of doping of two rare earth atoms in the unit cell we studied substitution on different Ga sites and obtained the optimal configuration. In general the magnetic coupling between the rare earths at the most favorable sites is found to be ferromagnetic.Acknowledgements: I gratefully acknowledge the support provided by US Army International Technology Center-Pacific (ITC-PAC) and discussions with Dr. J.P. Singh and John M Zavada.
3:45 PM - I3.4
Site Dependence of Electronic Structure of Gd Impurities in GaN.
Tawinan Cheiwchanchamnangij 1 , Atchara Punya 1 , Walter Lambrecht 1 Show Abstract
1 Physics, Case Western Reserve University, Cleveland, Ohio, United States
The origins of magnetism in Gd doped GaN are still unclear. While recent interest has focused on intrinsic defects, such as Ga vacancies and N or O interstitials, the potential role of Gd clustering is unclear. Here we study Gd doped on the N site and the nearest neighbor pair of Gd doped on Ga and N sites. Such defects were recently proposed to be required to explain the X-ray linear dichroism signals in Gd L-edge spectra by Ney et al. JMMM 322, 1162 (2010). We use the full-potential linearized muffin-tin orbital method in the LSDA+U (local spin density approximation with Hubbard-U corrections). We find strong outward relaxation near Gd on N site. Unlike Gd on the Ga site, Gd on a N site produces a spin-polarized band in the gap and a partial filling of the conduction band. Compared to the 7 Bohr magneton expected from the 4f half-filled shell of the Gd^3+ ion, we find an additional 2-3 Bohr mangeton, which are localized partially not only in the Gd d-like defect level but also spread over the nearest Ga and second nearest neighbor N shells. These delocalized spin-polarized electrons would be expected to play an important role in contributing to the magnetism. However, we find the energy of formation of Gd on N site to be higher than Gd on Ga site by several eV, making it a very unlikely site, even in N-poor, Ga-rich conditions. The preference for parallel or antiparallel orientation of spins in various types of Gd-pairs will be discussed.
4:00 PM - I3:RE doping
4:30 PM - **I3.5
Ferromagnetic Properties of Rare-earth Doped III-nitrides.
Neeraj Nepal 1 , John Zavada 2 Show Abstract
1 Power Electronic Materials Section, U.S. Naval Research Laboratory, Washington, District of Columbia, United States, 2 Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Transition metal and rare-earth (RE) doping of III-nitride semiconductor materials is an attractive approach to form a dilute magnetic semiconductor (DMS) material for room temperature (RT) ferromagnetic (FM) devices. Recently, manipulation at RT FM on Mn-doped GaN based devices has been demonstrated. The FM was controlled by depletion of the holes in the GaMnN/p-GaN/n-GaN multilayer structures. Alternatively, isovalent RE dopants can be incorporated in III-nitride semiconductors such that the unpaired 4f electrons can align along the magnetic field to induce FM ordering. Fairly low concentrations of RE elements in GaN can induce ferromagnetism with a Curie temperature above 360 K.We will present recent developments on the FM properties of III-nitride semiconductors (GaN, AlN, and InN) doped with RE elements, including Nd, Eu, Gd, Er, and Tm. The RE-doped III-nitride films were prepared by different synthesis techniques, including ion implantation and in situ doping during epitaxial growth. Magnetic, optical, and structural properties of the films were examined and all films exhibited FM behavior at RT as determined by hysteresis and magneto-resistance measurements. A photo-enhanced FM as well as an anomalous Hall Effect was observed at RT for Nd, Eu, and Er-doped films. Prospects of multifunctional devices based on RE-doped III-nitride will be also presented.
5:00 PM - I3.6
Zeeman Splittings of the 5D0–7F2 Transition Lines of Eu3+ Ions Implanted into GaN.
Kevin O'Donnell 1 , Vyacheslav Kachkanov 4 2 , C. Rice 2 , Daniel Wolverson 2 , Robert Martin 1 , Katharina Lorenz 3 , Eduardo Alves 3 , Michal Bockowski 5 Show Abstract
1 Physics, Strathclyde University, Glasgow United Kingdom, 4 , DIAMOND, Didcot United Kingdom, 2 Physics, University of Bath, Bath United Kingdom, 3 , ITN, Sacavem Portugal, 5 , Unipress, Warsaw Poland
Luminescence produced by rare earth ions in III-nitride semiconductors has attracted considerable interest due to the possibility of the electrical excitation of sharp emission lines, which are relatively independent of the operating temperature . Recently, a GaN:Eu-based light emitting diode was realised by Nishikawa and co-workers . We report here the splitting in an external magnetic field of three photoluminescence emission lines assigned to the 5D0–7F2 transition of GaN:Eu3+. The samples were prepared by ion implantation followed by high pressure (1 GPa), high temperature (1450 °C) annealing. The application of a magnetic field in the Faraday geometry (B||c) leads to a splitting of each line into two components. The Zeeman splittings increase linearly with magnetic field up to 5 Tesla. In contrast, a magnetic field applied in the Voigt geometry (B ⊥ c) does not influence the PL spectra. The estimated g-factor in the c-axis direction varies slightly from sample to sample with mean values of g|| ~2.8, ~1.6 and ~0.9 for emission lines at 621.00 nm, 621.90 nm and 622.75 nm respectively. The general tendency is the larger energy of emission line the greater its g-factor. The implications of these observations will be discussed.  K. P. O’Donnell and B. Hourahine, Eur. Phys. J.: Appl. Phys. 36, p. 91 (2006). A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, and Y. Fujiwara, Appl. Phys. Express 2, 071004 (2009) V. Katchkanov, K. P. O’Donnell, S. Dalmasso, R. W. Martin, A. Braud, Y. Nakanishi, A. Wakahara, and A. Yoshida, Phys. Status Solidi B 242, p. 1491 (2005).
5:15 PM - I3.7
Optical and Magneto-optical Properties of Rare Earth Doped Gallium Nitride Doped Epilayers.
Nathaniel Woodward 1 , Volkmar Dierolf 1 , H. Jiang 2 , J. Lin 2 , John Zavada 3 , A. Nishikawa 4 , Y. Fujiwara 4 , Eric Readinger 5 Show Abstract
1 Physics Department, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, United States, 3 Department of Electrical and Computer Engineering,, North Carolina State University, Raleigh, Texas, United States, 4 Graduate School of Engineering, Osaka University, Osaka Japan, 5 Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland, United States
Following the commercial availability of high power III-nitride light emitting diodes in the 370-540 nm range, a strong desire for efficient rare earth-doped (In)GaN epilayers has developed to produce light source and electrically pumped optical amplifiers in the visible and IR spectral region. To this end, Er and Eu-doped GaN epilayers have been grown by OVMPE with excellent crystal quality [1, 2]. Similar success has been achieved with in-situ doped Nd:GaN samples grown by plasma-assisted MBE . For all three dopants, we have studied incorporation site-specific excitation pathways using combined excitation-emission spectroscopy (CEES) along with emission spectroscopy under excitation with above bandgap laser light or an electron beam. We find that defect-trap related rare earth centers exhibit strongly enhanced energy transfer efficiency from electron-hole pairs to the rare earth ion. Aside from application in novel light sources, there has been an increasing interest in the magnetic properties of the dilute RE-dopants which introduce ferromagnetism to the GaN host . To contribute to the clarification of the underlying coupling mechanism, we have investigated the effects of applied magnetic fields on rare earth spectra and were able (using CEES) to determine Zeeman splittings and the associate effective g-values of the ions’ excited and ground states. We find linear Zeeman splitting behavior for Nd ions while for Er ions a strong mixing of states takes place due to the rather small crystal field splitting of the ground state. Utilizing the observed Zeeman splitting as a measure for the applied and remnant magnetic fields we studied the ferromagnetic hysteresis behavior for all dopants under variation of dopant concentration, initial conditions, and temperature. We further investigated the optical excitation pathway under the influence of the magnetic field. Supported by NSF-grant DMR-705217 and ECCS-0854619.A. Sedhain et al. Appl. Phys. Letters 95 (2009) 041113.A. Nishikawa, et al. Appl. Phys. Express 2 (2009) 071004G. D. Metcalfe, J. Appl.Phys. 105, 05310, 2009.e.g.: N. Nepal et al. Appl. Phys. Letters 95 (2009) 022510.
5:30 PM - I3.8
Excited Multiplets of Eu in GaN.
Ben Hourahine 1 Show Abstract
1 Department of Physics, SUPA, The University of Strathclyde, Glasgow United Kingdom
Europium centers in GaN are very bright lumophores in the red region of the spectrum, due to the 5Dx→7Fy family of intra-f optical transitions of the trivalent ion. The 7F0 ground state of these ions is not magnetic, however Eu-III doped material has been suggested as a dilute magnetic semiconductor.While modelling excited states of solids are usually out of reach of the usual choice of density functional theory (DFT), rare earth dopants in solids present a special opportunity. It is possible to find the lowest energy states of systems, as specified by symmetry or quantum numbers, which are distinct from the ground state. In thiscase the j, l and s quantum numbers of the 4f shell, where the problem of DFT's inability to produce eigenstates of spin operators is alleviated by the presence of spin-orbit coupling.By applying non-collinear spin polarized density functional based tight binding including both LDA+U corrections for electronic correlation and spin orbit coupling, the properties of both isolated Eu atoms at Ga sites and complexes with native vacancy defects are considered.Extending beyond the ground state, constrained calculations for the 5D0 and 7F2 multiplets of ions in both types of environment are presented for the first time.
5:45 PM - I3.9
Room Temperature Ferromagnetic Behavior in Rare-earth Doped GaN and AlN Semiconductor Thin Films.
Ratnakar Palai 1 , J. Wu 1 , A. Rivera 1 , K. Liu 2 , M. Shur 2 , H. Huhtinen 4 , C. Binek 3 , W. Jadwisienczak 5 Show Abstract
1 Dept. of Physics, University of Puerto Rico, San Juan United States, 2 Department of Physics, Rensselar Polytechnic Institute, Troy, New York, United States, 4 Department of Physics, University of Turku, Turku, Turku, Finland, 3 Department of Physics, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 5 School of Electrical Engineering & Computer Science, University of Ohio, Athens, Ohio, United States
Since last four decades the information and communication technologies are relying on the semiconductor materials. Currently a great deal of attention is being focused on adding spin degree-of-freedom into semiconductor to create a new area of solid-state electronics, called spintronics. In spintronics not only the current but also its spin state is controlled. Such materials need to be good semiconductors for easy integration in typical integrated circuits with high sensitivity to the spin orientation, especially room temperature ferromagnetism being an important desirable property. GaN-based materials have unique properties and are extremely promising for high power and high temperature sensor applications. Development of GaN-based RT ferromagnetic semiconductors is one of the major worldwide research and development programs. In the present work, thin films of Er and Yb-doped GaN and AlN were grown on Si (111) and sapphire (0001) substrates using plasma assisted molecular beam epitaxy (MBE) at different growth conditions. X-ray diffraction patterns of the films show c-axis orientation with high degree of crystallinity. The magnetic properties of GaYbN thin films measured using a superconducting quantum interference device (SQUID) shows room temperature ferromagnetism. The magnetization vs. temperature shows no paramagnetic contribution at very low temperature unlike it observed in Mn and Eu-doped GaN thin films. Magnetic force microscopy shows uniform distribution of magnetic domains through the surface of samples and rules out the presence of cluster or precipitation of magnetic ions. X-ray photoelectron spectroscopy shows 3-5% rare-earth doping in thin films depending on growth temperature. Photoluminescence spectra of Ga(1-x)YbxN thin films show the dominance of donor bound excitons and the reduction of transition line width with increasing Yb concentration. The average carrier concentration and mobility of Ga(1-x)YbxN thin filmds were calculated from the hall effect measurement and was found to be 8.29 x 1016 cm-3 and 760 cm2/V.s, respectively . On the other hand, YbAlN thin films show higher carrier concentration and lower mobility compared to the GaYbN thin films. All the films show room temperature ferromagnetism with excellent semiconducting properties, which will facilitate the realization of spintronic devices.  J. Wu, A. Rivera, A. Martinez, R. Palai, K. Liu, M.S. Shur, H. Huhtinen, W. M. Jadwisienczak, “Room temperature ferromagnetic behavior in MBE grown Yb-doped GaN semiconductor thin films”, Int. J. High Speed Electron Systems (in press)
Walter R. L. Lambrecht Case Western Reserve University
Kevin Smith Boston University
H. Joseph Trodahl Victoria University of Wellington
Andreas Ney Universitaet Duisburg-Essen
I7: Poster Session: Nitrides
Tuesday PM, November 30, 2010
Exhibition Hall D (Hynes)
I4: Rare Earth Nitride Magnetism
Tuesday PM, November 30, 2010
Room 301 (Hynes)
9:30 AM - **I4.1
Magnetic Properties of Rare Earth Nitrides.
Claire Meyer 1 2 , Ben Ruck 2 , Joe Trodahl 2 , Andrew Preston 2 , Simon Granville 2 , Bart Ludbrook 2 , Jan Richter 2 , Grant Williams 3 , Laurent Ranno 1 Show Abstract
1 Institut Néel, CNRS - Université Joseph Fourier, Grenoble France, 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington New Zealand, 3 MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research Ltd., Lower Hutt New Zealand
The magnetic properties of rare earth mononitrides (REN) have been extensively measured in the sixties, as potential magnetic semiconductors. Magnetic ordering at low temperature was established for most of the members of the RE series, in reasonable agreement with the cubic crystal field ground state of the trivalent atom. Expected exceptions were Ce, which is tetravalent thus non magnetic, Pr and Tm of singlet ground state, and Eu for which J=0. Either collinear or non-collinear ferromagnetic order was found for RE=Nd, Gd, Tb, Dy, Ho, Er. An antiferromagnetic order was suggested in SmN and YbN. Since then some controversy appeared about the nature of the magnetic order, even in the archetypal GdN. The main problem came from the lack of reliable reproducible preparation as for the Nitrogen stoichiometry, which is extremely difficult to achieve. Taking account of the carrier doping introduced by N vacancies, the exchange mechanism may be dependent on the material. For strictly stoichiometric REN, no conduction electron is expected to notably enhance the ferromagnetic order. We will present an overview of these magnetic properties, recalling some of these previous data acquired on bulk materials, and focusing on more recent results, especially obtained at Victoria University of Wellington, on highly stoichiometric thin films, polycrystalline or epitaxial. The GdN, DyN, ErN films are found to be ferromagnetic in agreement with the literature. But SmN is demonstrated to be ferromagnetic, contrary to the previous data. The nearly zero spontaneous magnetic moment is due to the near cancellation of its spin and orbital contributions, in the crystal field ground state. The experimental results obtained on ErN fit with the bulk data, and the (111) texturation of the films provides complementary information about the magnetic structure. X-Ray magnetic circular dichroism at the M4,5 and L2,3 edge of the rare earth is currently used on the films to probe the 4f and 5d shells polarization respectively. A magnetic transition is observed for EuN, involving a divalent contribution, while the trivalent part only shows Van Vleck paramagnetism as expected. The occurrence of mixed valence will be discussed.The magnetic ordering temperatures of the REN are systematically smaller than the Curie point of the metal, ruling out an exchange mechanism through conduction electrons only, in agreement with the calculated band structure predicting no rare earth 5d states at the Fermi level. However the Curie temperatures are by far too large to be accounted for superexchange only. Therefore theoretical calculation is of great help trying to elucidate the exchange mechanism in the rare earth nitrides. Various proposed models will be presented and trends will be discussed.
10:00 AM - **I4.2
Theories of the Electronic Structure of Rare Earth Nitrides.
Axel Svane 1 Show Abstract
1 Department of Physics and Astronomy, Aarhus University, Aarhus C Denmark
Two complementary electronic structure methods are applied to the rare earth mononitrides. On the one hand the self-interaction corrected local spin density (SIC-LSD) method is used to compute total energy aspects of the electronic structure. On the other hand electron excitation energies are calculated with the quasiparticle self-consistent GW (QSGW) approximation. Both approaches lead to a separate description of occupied and unoccupied f-states, hence gaps in the f manifold from the formation of upper and lower Hubbard bands. Trends in magnetism and gaps are discussed. The SIC-LSD approach is furthermore applied to dilute rare earth dopants in a GaN host.
10:30 AM - I4.3
Mechanism of Ferromagnetic Exchange Interaction in GdN.
Anand Sharma 1 2 , Wolfgang Nolting 2 Show Abstract
1 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 2 Institut fuer Physik, Humboldt-Universitaet zu Berlin, Berlin Germany
There are several proposed mechanisms behind ferromagnetism in GdN. It has been argued that it can be due to fourth-order cross process of d-f mixing and d-f exchange. Another explanation suggests an anti ferromagnetic interaction between Gd 'd' and N 'p' induced moments on the rock salt structure which aligns the nearest neighbor Gd 'f' moments ferromagnetically through the d-f exchange. Here we present results of Curie temperature in GdN as a function of carrier density calculated within our multiband modified RKKY-like exchange interaction. It includes realistic bandstructure of the 5d conduction band as an input for single particle energies. We study the possibility of carrier-mediated ferromagnetism in GdN and demonstrate a simple phenomenological model justifying the role of charge carriers.
10:45 AM - I4.4
First-principles Study of Nitrogen Vacancies in GdN.
Atchara Punya 1 , Tawinan Cheiwchanchamnangij 1 , Walter Lambrecht 1 Show Abstract
1 Physics, Case Western Reserve University, Cleaveland, Ohio, United States
Nitrogen vacancies are often assumed to be the main source of unintentional n-type doping of GdN. Here we calculate the electronic structure and energy of formation of nitrogen vacancies in GdN. The full-potential linearized muffin-tin orbital method is used in the local spin density approximation with Hubbard-U corrections (LDA+U) for 64 atom supercells. We find that the nitrogen vacancy produces two deep impurity levels in the gap, one of each spin. In the neutral charge state, the first two additional electrons fill these impurity levels and the third partially occupies the mostly majority spin lower conduction band, leading to a delocalized net magnetic moment of about 1 Bohr magneton per vacancy. In the 1+ charge state, there is no magnetic moment and in the 2+ charge state, the magnetic moment returns to 1 Bohr magneton but is more localized. The spin-polarized d-electrons are expected to be important for the magnetic properties. Previous work of our group predicted that pure semiconducting GdN would have a Curie temperature of only about 10 K while experimental values range from 58 to 70 K. The average first and second nearest neighbor Gd-Gd exchange interactions in GdN with N-vacancies are extracted from fitting the energies of different magnetic configurations (AFM-I, AFM-II, i.e. antiferromagnetic ordering along  and  and ferromagnetic FM) in the Heisenberg model. We find an increase mainly of the nearest neighbor interaction, sufficient to more than double the Curie temperature. This model however does not yet take into account the possibility of longer-range exchange interactions induced by the delocalized d-electrons, which could further enhance the Curie temperature.
11:00 AM - I4:RE mag
I5: TM Nitrides: Bulk
Tuesday PM, November 30, 2010
Room 301 (Hynes)
11:30 AM - **I5.1
ScN, CrN, TiN: Epitaxial Layer Growth and Optical and Electronic Transport Properties.
Daniel Gall 1 Show Abstract
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
(I) An atomistic understanding of the epitaxial layer growth of ScN, TiN, and CrN on MgO(001) is developed by combining results from scanning tunneling and transmission electron microscopy with ab initio density functional calculations. Activation energies for surface diffusion processes are determined from two-dimensional surface island growth and decay kinetics and are compared with calculated activation energies. Particularly interesting is the high anisotropy with a 7-orders-of-magnitude difference in adatom mobility between 001 and 111-surfaces that leads, in combination with atomic shadowing processes, to a range of self-organized nanostructures including nanopipes and nanostaircases. (II) The electronic structure of these nitrides is investigated using a combination of optical analyses, in situ valence band photoelectron spectroscopy, electronic transport measurements, and density functional calculations. ScN is a semiconductor with an indirect Γ to X bandgap of 1.3 eV and a direct transition of 2.4 eV. Sc1-x TixN alloys exhibit colors which continuously vary with x from golden yellow to orange to burgundy to blue-green to transparent, corresponding to a shift in the plasma frequency due to a linear increase in the carrier density from 1.0x1021 for ScN to 4.6x1022 cm-3 for TiN. Temperature dependent resistivity and Hall measurements are in excellent agreement with simulated responses on electric and magnetic fields using ab initio calculated bandstructures, and Raman intensities are in agreement with calculated phonon dispersion curves which provide an explanation for the drop in the transition temperature for superconductivity.(III) CrN exhibits a depletion of the density of states at the Fermi level based on photoelectron spectroscopy and electronic transport properties which indicate Mott and Efros-Shklovskii hopping at high and low temperature, respectively. The optical properties measured over a wavelength range from 250 nm–30 μm indicate the onset for direct transitions at 0.64 eV, as well as high joint density of states at 1.5 and 2.9 eV, and an upper limit for free carriers of 1019 cm-3. The splitting of transverse and longitudinal optical phonon frequencies at the zone center provide a value for the Born effective charge of 4.4±0.9, and support the conclusion that CrN is likely a Mott-Hubbard type insulator with a small to negligible indirect bandgap of 0.19±0.46 eV.
12:00 PM - I5.2
ScN Photoemission: Band Structure, Band Alignment to GaN, and Electron Accumulation.
Tim Veal 1 , Louise Bailey 1 , Chris McConville 1 , Jac Hall 2 , Tom Foxon 2 , Richard Campion 2 , Walter Lambrecht 3 Show Abstract
1 Physics, University of Warwick, Coventry United Kingdom, 2 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 3 Physics, Case Western Reserve University, Cleveland, Ohio, United States
In contrast to group IIIA-N semiconductors, (In,Ga,Al)N, not much is known about the group IIIB-N compounds. However, ScN has promising physical and electronic properties, for example, when alloyed with GaN, it offers the potential for light emission across the visible spectrum including the currently problematic green region. It has already been shown that hexagonal ScGaN can be grown for Sc contents of less than 30 % . The stable polymorph of pure ScN is a rock salt structure and is almost lattice matched to zinc-blende GaN. Pure ScN is predicted, with some experimental confirmation, to be an indirect semiconductor with an indirect gap of 0.9 eV and a direct gap of approximately 2.1 eV . In undoped ScN, the free electron density is typically in the range 1017 to 1020 cm-3.
In this work, ScN(111) films have been grown by molecular beam epitaxy on zinc-blende GaN buffer layers on GaAs(001) substrates . X-ray photoemission spectroscopy has been used to study the valence band density of states of ScN, the valence band offset between ScN and zinc-blende GaN and to determine the surface electronic properties of the ScN films. The bulk electronic properties were determined from infra-red reflectivity and Hall effect measurements. The valence band photoemission spectra were compared with the valence band density of states from the ScN band structure calculated within the local density approximation. ScN is shown to exhibit a surface electron accumulation layer, in contrast to the electron depletion layer usually observed at the surface of n-type III-V semiconductors. The surface Fermi level was found to be at 1.8 eV above the Γ point valence band maximum with the bulk Fermi level at about 1.0 eV. Therefore, the ScN samples have a downward band bending of approximately 0.8 eV. The band bending and carrier concentration profiles as a function of depth from the surface were evaluated by solving Poisson’s equation and the surface sheet density was estimated to be 2−5×1013 cm-2, higher than for InN . The tendency for unintentional n-type conductivity and presence of surface electron accumulation can be understood from the bulk band structure of ScN and the particularly low Sc 3d-derived conduction band minimum at the X point.
 C. Constantin, H. Al-Brithen, M. B. Haider, D. Ingram, and A. R. Smith, Phys. Rev. B 70, 193309 (2004).
 W. R. L. Lambrecht, Phys. Rev. B 62, 13538 (2000).
 J. L. Hall, M. A. Moram, A. Sanchez, S. V. Novikov, A. J. Kent, C. T. Foxon, C. J. Humphreys, and R. P. Campion, J.Cryst. Growth 311, 2054 (2009).
 I. Mahboob, T. D. Veal, C. F. McConville, H. Lu, and W. J. Schaff, Phys. Rev. Lett. 92, 036804 (2004).
12:15 PM - I5.3
Effect of Magnetic Disorder and Strong Electron Correlations on the Thermodynamics of CrN.
Bjorn Alling 1 , Tobias Marten 1 , Igor Abrikosov 1 Show Abstract
1 Department of Physics, Chemistry, and Biology, Linköping University, Linköping Sweden
Unlike what is sometimes assumed, most magnetic systems retain magnetic moments also above their critical Curie or Néel temperature. Indeed local moments are typically present although long-range order between them is lost. CrN is such a system where the experimentally observed structural (lattice spacing) and electronic properties (semiconducting behavior) of the paramagnetic cubic phase can not be even qualitatively reproduced by non-magnetic calculations. At the same time, when performing first-principles calculations modelling such disordered cases, ordered magnetic structures should not be used because they might give rise to order-specific features, like the well known mixing anomaly in the Fe-Cr system. This means that a disordered magnetic state must be considered in order to fully understand the physics of paramagnetic CrN at room temperature.We use first-principles calculations to study the effect of magnetic disorder and electron correlations on the structural and thermodynamic properties of CrN. We illustrate the usability of a special quasirandom structure supercell treatment of the magnetic disorder by comparing with coherent potential approximation calculations and with a complementary magnetic sampling method. The need of a treatment of electron correlations effects beyond the local density approximation is proven by a comparison of LDA+U calculations of structural and electronic properties with experimental results. When magnetic disorder and strong electron correlations are taken into account simultaneously, pressure and temperature induced structural and magnetic transitions in CrN can be understood.
12:30 PM - I5.4
Structural and Electronic Properties of Iron Nitride on Cu(100) Surfaces: DFT and Ab-initio Molecular Dynamics Studies.
Dodi Heryadi 1 , Udo Schwingenschlogl 1 Show Abstract
1 , King Abdullah University of Science and Technology, Thuwal Saudi Arabia
Due to their potential applications for magnetic storage devices, numerous experimental and theoretical studies of iron nitrides have been performed. In recent efforts nanostructured layers of these materials have been grown on Cu(100) surfaces. To investigate structural and electronic properties of iron nitride on these surfaces, we perform density functional theory calculations and ab initio molecular-dynamics simulations at 720 K. Results of geometries and electronic density of states from our calculations will be presented and compared with experimental data.
I6: RE and TM Doping
Tuesday PM, November 30, 2010
Room 301 (Hynes)
2:30 PM - **I6.1
Growth and Characterization of Transition-metal and Rare-earth Doped III-nitride Semiconductors for Spintronics.
Hajime Asahi 1 , Shigehiko Hasegawa 1 , Yi-Kai Zhou 1 , Shuichi Emura 1 Show Abstract
1 ISIR, Osaka University, Ibaraki, Osaka Japan
Since we reported the first observation of high temperature (> 400K) ferromagnetic characteristics of Cr-doped  and Gd-doped  GaN in 2002, we have been reporting important results. In this talk, we will describe these results in detail. Transition-metal (TR) and rare-earth (RE) doped III-nitride semiconductors were grown on sapphire substrates and GaN templates by plasma-assisted molecular-beam epitaxy (MBE). The substitutional incorporation of Cr and Gd atoms into cation sites were confirmed by X-ray absorption fine structure (XAFS) measurements. The observation of PL emission at room temperature is important to fabricate practical spintronics devices that control charges, photons and spins; this is contrast to GaMnAs and InMnAs, where no PL emission was observed. We also observed the tunnel magneto-resistance (TMR) effect in the GaCrN/AlN/GaCrN trilayer structure diodes at 77 K, which is the highest temperature ever reported for the fully semiconductor-based TMR devices. Performance will be much improved by realizing more flat interfaces and improving crystalline qualities and magnetic properties. The enhancement of magnetization and magnetic moment for GaGdN/GaN superlattice structures and Si-doped GaGdN layers was observed compared with those for undoped GaGdN single layers. This can be understood by carrier-induced ferromagnetism. It is considered that the origin of observed high temperature ferromagnetism is carrier-induced ferromagnetism. We also succeeded in the growth of high Gd content GaGdN layers by low temperature (LT) MBE and observed the enhancement of magnetization and magnetic moment. These LT-grown layers exhibit PL emission. We also succeed in the growth of Dy-doped GaN, GaDyN, layers and observed room temperature ferromagnetic characteristics. For this GaDyN, we have observed the significant enhancement of magnetic circular dichroism (MCD) signals compared with GaN, which confirms that observed ferromagnetic properties are really originated from the interaction between host semiconductor and doped magnetic ions. In order to fabricate the long wavelength spin-controlled photonic devices, we have grown InGaGdN layers. In the InGaGdN/GaN SLs, the enhancement of magnetization and magnetic moment was observed. On the contrary, the reduction of magnetization and magnetic moment was observed for the InGaN/GaGdN SLs. These results also support the carrier-induced ferromagnetism. M. Hashimoto, Y.K. Zhou, M. Kanamura and H. Asahi, Solid State Commun. 122 (2002) 37. N. Teraguchi, A. Suzuki, Y. Nanishi, Y.K. Zhou, M. Hashimoto and H. Asahi, Solid State Commun. 122 (2002) 651.
3:00 PM - I6.2
Ferromagnetic Properties of Cr-doped GaN Thin Films.
Alex Smirnov 1 , Saritha Nellutla 1 , Neeraj Nepal 2 , Nadia El-Masry 3 , Yuriy Semenov 2 , John Zavada 2 , Nicoleta Kaluza 4 , Yong Suk Cho 4 , Hilde Hardtdegen 4 Show Abstract
1 Chemistry, North Carolina State University, Raleigh, North Carolina, United States, 2 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 materials Science Engineering, North Carolina State University, Raleigh, North Carolina, United States, 4 Institute of Bio- und Nanosystems (IBN-1), Jülich Aachen Research Alliance (JARA), Forschungszentrum Jülich, Jülich Germany
There is wide spread research activity in the III-N semiconductor material systems for optoelectronic device applications . Recent studies have also focused on these materials as hosts of magnetic ions for spin-based devices [1-4]. Here we report on the growth of Cr-doped GaN thin films by MOVCD and characterization of their magnetic and spin properties by alternating gradient magnetometer (AGM) and electron paramagnetic resonance (EPR) measurements. Hysteresis curves, taken at room temperature using AGM, showed evidence of both in-plane and out-of-plane alignment of the ferromagnetism in these films. However, the saturation magnetization taken from the hysteresis curves increased five-fold for multilayer structures in which a p-type GaN layer was grown adjacent to the Cr-doped region. X-band (~9.5 GHz) EPR measurements yielded a sharp signal from 52Cr (I = 0) as well as four hyperfine lines which arise from the interaction of unpaired electrons with 53Cr (I = 3/2). Also, the value of the 53Cr hyperfine coupling constant A (~ 45 G) was independent of the film orientation in the applied magnetic field. This suggests that the origin of A is mainly from Fermi contact interaction; the electronic unpaired spins are delocalized, thus, averaging out the dipolar interactions with the 53Cr nuclear spin. These results indicate the important role the carriers play in the ferromagnetic ordering and are in agreement with data reported on Mn-doped GaN thin films [3,4].  S. C. Jain, M.Willander, J. Narayan, and R. van Overstraeten, J. Appl. Phys. 87, 965 (2000). T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 (2000). M. L. Reed, N. A. El-Masry, H. H. Stadelmaier, M. K. Ritums, M. J. Reed, C. A. Parker, J. C. Roberts and S. M. Bedair, App. Phys. Lett. 79, 3473 (2001). N. Nepal, M. Oliver Luen, J. M. Zavada, S. M. Bedair, P. Frajtag, and N. A. El-Masry, Appl. Phys. Lett. 94, 132505 (2009).
3:15 PM - I6.3
Magneto-optical Spectroscopy of the Nickel Doped Amorphous AlN Films.
Wojciech Jadwisienczak 1 , Hiroki Tanaka 1 , Gang Chen 2 , Martin Kordesch 2 , Aurangzeb Khan 3 Show Abstract
1 School fo EECS, Ohio University, Athens, Ohio, United States, 2 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States, 3 Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States
In this paper we investigate magneto-optics of nickel doped amorphous AlN layers (a-AlN) deposited by rf sputtering on Silicon (0001) substrates. As-grown material exhibits weak ferromagnetic behavior as evidenced by the magneto-optic Kerr effect (MOKE) measurement with Kerr rotation less than 100 μrad at room temperature. Significant enhancement of the polar Kerr rotation signal was observed (3400 μrad) after post-growth thermal annealing at 450 C in nitrogen. The abrupt decrease in material magnetization to ~400 μrad was observed after annealing at higher ambient temperature. The morphology of as-grown and annealed a-AlN:Ni films were characterized by small angle x-ray scattering. It is found that the as-deposited film contains nano-particles of different sizes with average diameters between 10 and 30 nm. The size distribution of nano-particles in the thermally annealed a-AlN:Ni films was studied as a function of annealing time and temperature, and the results correlate well with those obtained from the magnetism measurements. Furthermore, the morphological changes of a-AlN:Ni due to annealing, possibility of Ni atoms clustering, hybridization of Ni atoms and/or Ni clusters with immediate amorphous surrounding are studied in the paper. Finally, the magnetic domains boundary formation and their role in observed ferromagnetism are assessed and discussed.
3:30 PM - **I6.4
Dilute vs. Condensed Magnetic Nitrides.
Alberta Bonanni 1 Show Abstract
1 , Johannes Kepler University, Linz Austria
We summarize our recent growth, characterization, and interpretation works on the exchange interaction and magnetism in GaN and related systems doped with either Fe [1-5] and Mn  and co-doped with Si and Mg [3,4,6]. In particular, we discuss how the strong coupling  between the open d-shells of the transition metal impurities and the p-orbitals of neighboring anions renormalizes the hole spectrum and we highlight the consequences for carrier-mediated ferromagnetism. Furthermore, we show that a significant contribution of d orbitals to the bonding leads to the self-organized aggregation of magnetic cations resulting in nanoscale chemical or crystallographic phase separations [7,8]. Finally, we present results aiming at the determination of the coupling strength between Mn ions in GaN. A. Bonanni, M. Kiecana, C. Simbrunner, Tian Li, M. Sawicki, M. Wegscheider, M. Quast, H. Przybylinska, A. Navarro-Quezada, R. Jakiela, A. Wolos, W. Jantsch, and T. Dietl, Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N: The relationship between structural, electronic, and magnetic properties, Phys. Rev. B 75, 125210 (2007). W. Pacuski, P. Kossacki, D. Ferrand, A. Golnik, J. Cibert, M. Wegscheider, A. Navarro-Quezada, A. Bonanni, M. Kiecana, M. Sawicki, T. Dietl, Observation of strong-coupling effects in a diluted magnetic semiconductor (Ga,Fe)N, Phys. Rev. Lett. 100, 037204 (2008). A. Bonanni, A. Navarro-Quezada, Tian Li, M. Wegscheider, R.T. Lechner, G. Bauer, Z. Matej, V. Holy, M. Rovezzi, D’Acapito, M. Kiecana, M. Sawicki, and T. Dietl, Controlled aggregation of magnetic ions in a semiconductor. Experimental demonstration, Phys. Rev. Lett. 101, 135502 (2008). M. Rovezzi, F, D’Acapito, A. Navarro-Quezada, B. Faina, T. Li, A. Bonanni, F. Filippone, A. Amore Bonapasta, T. Dietl, Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy, Phys. Rev. B 79, 195209 (2009). A. Navarro-Quezada, W. Stefanowicz, Tian Li, B. Faina, M. Rovezzi, R. T. Lechner, T. Devillers, F. d’Acapito, G. Bauer, M. Sawicki, T. Dietl, and A. Bonanni, Embedded magnetic phases in (Ga,Fe)N: the key role of growth temperature, Phys. Rev. B 81, 205206 (2010).  W. Stefanowicz, D. Sztenkiel, B. Faina, A. Grois, M. Rovezzi, T. Devillers, A. Navarro-Quezada, T. Li, R. Jakiela, M. Sawicki, T. Dietl, and A. Bonanni, Magnetism of dilute (Ga,Mn)N, Phys. Rev. B 81, 235210 (2010). A. Bonanni, Topical Review, Ferromagnetic nitride-based semiconductors doped with transition metals and rare earths, Semicond. Sci. and Technol. 22, 41 (2007). A. Bonanni and T. Dietl, A story of high-temperature ferromagnetism in semiconductors, Chem. Soc. Rev. 39, 528 (2010).
4:00 PM - I6:REamp;TM doping
4:30 PM - **I6.5
On the Origin of Room Temperature Ferromagnetism in GaN Doped with Magnetic Ions.
Joerg Malindretos 1 Show Abstract
1 IV. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen Germany
An experimental study of ferromagnetism in GaN layers and heterostructures doped with magnetic ions of manganese or gadolinium is presented. All samples were grown by plasma-assisted molecular beam epitaxy on varying substrates. Based on a comprehensive analysis of structural, electrical and magnetic properties, the origin of the room temperature ferromagnetism observed in part of the samples is discussed and the prospect of GaN based dilute magnetic semiconductors is considered.After the theoretical prediction of high Curie temperatures in nitride semiconductors, GaMnN became a prominent candidate for future spintronic applications. However, a complete understanding of the relevant coupling mechanisms in this material is still lacking due to the complex interplay between growth conditions, the resulting transition metal atom configuration and the consequential magnetic behavior. Under optimized growth parameters, single GaMnN layers with Mn concentrations up to a few percent could be realized without any apparent crystallographic phase separation. Most samples exhibit a predominantly paramagnetic behavior, but also cases with different magnetic signatures were found. In order to investigate Fermi level effects in this material, AlGaN/GaMnN heterostructures were fabricated. The pronounced band bending at the interface is expected to influence the effective magnetic coupling and might also be a way to control phase separation and the type of predominant defects in the layer. A detailed analysis reveals three distinct contributions in the magnetic signal: a strong room temperature ferromagnetic signal from MnGa precipitates forming at the GaMnN/AlGaN interface, a superparamagnetic contribution from nanoscale clusters and a paramagnetic contribution, which shows indications of a transition to a ferromagnetic dilute magnetic semiconductor phase at around 12 K.The report of room temperature ferromagnetism and the observation of giant magnetic moments in highly diluted GaGdN layers have triggered a strong interest in this material. While recent theoretical works point out the possible role of defects in the magnetic coupling, several fundamental questions concerning the authenticity and the underlying physics of the giant magnetic moments remain unanswered. Unwittingly, sets of magnetic and non-magnetic samples were obtained during the growth campaign with nominally identical layer composition. Even at moderate Gd concentrations below the residual doping level, a change from n-type band activated transport to variable-range hopping within an impurity band of localized states is observed. It is shown that other acceptor defects than Gd atoms are responsible for the compensation of the native donors and for the variable range hopping transport behavior. Furthermore, it can be ruled out by positron annihilation spectroscopy that gallium vacancies are responsible for the formation of a ferromagnetic order in these layers.
5:00 PM - I6.6
Multivalent Magnetic States of Fe in GaN.
Paola Alippi 1 , Aldo Amore Bonapasta 1 , Francesco Filippone 1 , Vincenzo Fiorentini 2 3 Show Abstract
1 Istituto di Struttura della Materia, CNR, Monterotondo Stazione (Rome) Italy, 2 Physics, Cagliari University, Monserrato Italy, 3 , CNR-IOM SLACS, Cagliari Italy
Early predictions of high Curie temperatures in diluted nitrides has triggered much work on transition-metal (TM) impurities in GaN. TM in nitrides are also under scrutiny because of suspected effects of electron correlation, naturally expected due to electronic localization of both the impurity d shell and the valence states of the highly ionic host. As the applicability of gradient-corrected or local-density functionals may be dubious, here we study neutral and charged Ga-substitutional Fe impurities in GaN via spin-polarized density-functional-theory electronic structure calculations using the hybrid GGA+Hartree-Fock HSE functional in the PAW VASP implementation. The neutral impurity induces states of three kinds: a) strongly Fe-localized majority states in the lower part of the valence band; b) dispersed Fe-N- hybrid majority states in a 3-4 eV range below the valence top Ev; c) empty minority states resonant with the Ga-like conduction band bottom. Negatively charged Fe occupies the latter minority level, which shifts down decidedly into the gap, producing a ε(0/–1) transition level near the observed value, at Ev+2.4 eV. In the singly-positive state, an empty level appears just above the valence band. It is rather strongly localized on Fe-first-neighbor N’s, and originates from valence band Fe-N hybrids of the neutral impurity. Correspondingly, we obtain a hyper-deep ε(+1/0) donor level at Ev+0.3 eV, with a bound hole producing a nominal 4+ ionization of Fe. This is the first theoretical evidence of such a state, foreshadowed in earlier models and supported by indirect experiments. In actual fact, the 4+ ionization state -just as all other ionization states- can be attributed only nominally to Fe in itself. Firstly, the hole of this state is mostly localized on Fe-neighboring N’s. More generally, the analysis of the orbital-projected density of states shows that Fe:GaN nicely follows the Raebiger-Lany-Zunger ”charge self-regulation” rule. In the –1 state, the increased occupation of the gap state is counterbalanced by a de-occupation of the valence band states, and conversely in the +1 state, the added hole is an empty gap state which only appears upon charging. The total population of band-resonant and gap states remains constant, and the actual electronic charge residing on Fe does not change upon charging the impurity+host system. This invalidates any attribution of a well defined ionization state to the Fe impurity.Finally, the Fe properties just discussed may influence the stability of magnetic nano-phases apparently producing ferromagnetism in Fe-doped GaN. In particular, the repulsion of positively (or negatively) charged Fe centers would tend to prevent Fe clustering: this agrees with the observed suppression of nano-phase formation in intentionally n- and p-doped GaN.
5:15 PM - **I6.7
Optical, Electrical and Magnetic Properties of Tranition Metal Doped Indium Nitride.
Steve Durbin 1 2 3 Show Abstract
1 Department of Electrical Engineering, State University of New York at Buffalo, Buffalo, New York, United States, 2 Department of Physics, State University of New York at Buffalo, Buffalo, New York, United States, 3 , The MacDiarmid Institute for Advanced Materials and Nanotechnology, Christchurch New Zealand
Interest in ferromagnetic semiconductors having a Curie temperature above 300 K has recently led many to consider transition metal doping of a variety of materials, in particular column III nitrides. In roughly the same period, indium nitride garnered considerable attention worldwide as it became clear that its bandgap energy was approximately one-third of the commonly accepted value of 1.9 eV. Subsequently this neglected semiconductor drew continued attention for extreme bandbending on exposed surfaces, and unexpected quenching of luminescence efficiency upon doping. In this talk, an overview of what is currently understood about doped indium nitride will be presented, with particular emphasis on results of Mn and Cr doped samples.
5:45 PM - I6.8
Nitrogen Charge Sinks in InAsN and InN Quantum Dots.
Liudmila Pozhar 1 Show Abstract
1 Physics, University of Idaho, Moscow, Idaho, United States
Indium nitrides are among the most intensively studied semiconductors primarily due to their band gap (tunable from ultraviolet to infrared) that allows wide spectrum of device applications, including field effect transistors, photoelectrodes, solar blind photodetectors, and light emitting and laser diodes. Large and intermediate size quantum dots (QDs) and wires (QWs) of indium nitrides have been grown and investigated in recent years, but experimental synthesis of smaller indium nitride QDs/QWs remains a challenging task, despite of the growing demand. Thus, accurate, first principle theoretical assessment of electronic and magnetic properties of such small QDs/QWs systems is necessary to help further progress in experimental synthesis of In – N –based small nanoscale systems. In the work presented here a virtual (i.e., fundamental theory-based, computational) synthesis method was applied to derive the equilibrium electronic and magnetic properties of small clusters (molecules) composed of In, As and N atoms from their structure, chemistry, composition, geometry and those of the confinement. This approach uses the first-principle, many-body quantum theoretical methods, including the Hartree-Fock, configuration interaction (CI), complete active space (CAS) self-consistent field (SCF), and multiconfiguration SCF (MCSCF) approximations. The studied clusters were split into two groups to model their nucleation in the presence and absence of excluded volume effects due to quantum confinement. Nucleation in quantum confinement and substitution of some cluster atoms (doping) were also used to break the tetrahedral or hexagonal symmetry of the clusters. Such symmetry breaking is necessary to realize the first singlet excitations in such clusters which are optically forbidden for tetrahedral and hexagonal symmetry systems, but are very important to tune the clusters’ electronic properties as required for applications. Electronic charge and spin distributions (CDD and SDD, respectively) of the virtually synthesized clusters were studied in detail. The obtained results prove that the electron spins are localized on In atoms, while electron charge is delocalized over the “surfaces” and in N-rich regions of the studied clusters. Such charge re-distribution allows a system with broken symmetry minimize its total energy. This mechanism is also “responsible” for breaking the octet rule of the valence theory. In correspondence to the existing experimental findings, the results of this work prove that nitrogen atoms in the studied In-As-N and In-N QDs act as charge sinks accumulating the electron charge in “belts” of collectivized charge (that “stretch” into the space outside that occupied by cluster atoms), and thus define sensitivity of the direct optical transition energy (OTE) of the clusters to their nitrogen content.
I7: Poster Session: Nitrides
Wednesday AM, December 01, 2010
Exhibition Hall D (Hynes)
9:00 PM - I7.10
Theoretical Study of the Expected Potential of Nitride-based Photovoltaics.
Mariko Medlock 1 , Tomas Palacios 1 Show Abstract
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Nitride-based semiconductors have recently been proposed as ideal material for future photovoltaic systems. The broad bandgap control possible in InAlGaN alloys (i.e. from 0.6 eV in InN to 6.2 eV in AlN) could potentially enable the fabrication of solar cells with an absorption spectrum closely matched to the solar spectrum. However, although significant experimental work is currently focused on the demonstration of InGaN-based solar cells, very limited work has been published on the requirements and expected performance of nitride-based photovoltaics. This paper aims to analyze the expected potential performance of nitride-based solar cells and their optimum configuration.Theoretical limits on the efficiency of solar cells under direct illumination have been calculated. These limits are based on the principle of detailed balance and two assumptions: only incident photons with energy higher than the bandgap energy will produce power, and any energy above the bandgap energy is lost to heat. Ideally, using a range of bandgaps instead of only one would increase the power generated by incident photons with energy lower or higher than the single bandgap energy. Thus the theoretical efficiency increases with the number of semiconductor layers. In this paper we have examined, for the first time, the theoretical optimum bandgap combinations for a multi-junction solar cell with up to four semiconductor layers. Practical considerations, such as lattice-matching the semiconductors of adjacent layers, choosing transparent materials for the upper layers, achieving a continuous current through adjacent layers, or if each layer is contacted separately, achieving electrical isolation between adjacent layers, might seem to favor the use of certain semiconductors in building multi-junction solar cells. However, these constraints are specific to the device design and should not be considered in calculating the optimum bandgap combinations. In this paper it is found that the bandgap for silicon is very close to the ideal bandgap for a single layer solar cell (1.1 eV). A two-junction solar cell has ideal bandgaps of 1.4eV and 0.7eV, while an ideal three-junction solar cell has optimum absorption bandgaps of 1.8eV, 1.2eV, and 0.7eV, which are well covered by InGaAs and AlGaAs technology. For a four-junction solar cell, nitride-based photovoltaics could offer an advantage in efficiency, but that would require growth on InGaN ~50%, and offer less than 2% higher efficiency over a four-junction cell with all bandgaps restricted to be less than 2.0eV, in the ideal case.The wide bandgap of nitride-based solar cells could however have a much more significant impact in multiband solar cells. In this paper, several device structures for these new cells will be proposed and theoretically analyzed.Acknowledgements.- This work has been partially funded by the CAREER Award of the National Science Foundation.
9:00 PM - I7.2
Trends on Electronic and Chemical Properties of Rare-earth Lanthanoids in Gallium Nitride.
Glaura Caroena 1 , Wanda VM Machado 1 , Joao F. Justo 2 , Lucy VC Assali 1 Show Abstract
1 DFMT, Universidade de São Paulo (USP), São Paulo, SP, Brazil, 2 LME, Universidade de São Paulo (USP), São Paulo, SP, Brazil
Considerable effort has been focused on rare-earth (RE) doped materials for photonic applications over the last decade. In such context, RE doped III-V semiconductors emerged as leading candidates for light emission in the visible region. Erbium doped gallium nitride (GaN) has long been used as light emitting devices . Recent advances have allowed to get laser activity in Eu doped GaN at room temperature , which suggests that other lanthanoids could provide equivalent properties, but in different frequency ranges. Such optical transitions are believed to be associated with intra-f spin transitions. Although other RE dopants, such as Pr, Tb, and Tm, have also been considered in GaN for photonic applications, no comparative study on the properties of those impurities in this material have been performed so far. Nowadays spintronics has recently attracted much attention because of its potential to provide new functionalities and enhanced performance in conventional electronic devices. Wide band gap semiconductors such as GaN have continued to be forefront of spintronics research due to the demonstration of room-temperature ferromagnetism in this materials. The progression of the ferromagnetism metals incorporated into GaN has moved from transition metals to rare-earth such as lanthanoids .Here, we carried a theoretical investigation on the physical properties of isolated 4f RE impurities (from Gd to Tm) in the cation site of GaN. The calculations were performed within the framework of the density functional theory and the generalized gradient approximation plus a U-Hubbard potential (DFT/GGA+U), using the all-electron spin-polarized full-potential linearized augmented plane wave (FP-LAPW), implemented in the WIEN2k package . Additionally, the 4f electrons were treated as valence electrons, and atomic positions were fully relaxed with no constraints. The results allowed to establish a clear trend on the electronic and chemical properties and Hyperfine fields of these impurities. These results were compared with those obtained in recent theoretical investigations [5,6]. A. J. Steckl and J. M. Zavada, Mater. Res. Soc. Bull. 24, 33 (1999).  J. H. Park and A. J. Steckl, Appl. Phys. Lett. 85, 4588 (2004).  R. P. Davies, C. R. Abernathy, S. J. Pearton, D. P. Norton, M. P. Ivill, and F. Ren, Chem. Eng. Comm. 196, 1030 (2009). P. Blaha, et al., WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties. J. S. Filhol, R. Jones, M. J. Shaw, and P. R. Briddon,Appl. Phys. Lett. 84, 2841 (2004). B. Hourahine, S. Sanna, B. Aradi, C. Kohler, and T. Frauenheim, Physica B 376-377, 512 (2006).
9:00 PM - I7.3
Strain Effects on the Equi-spin-splitting Distribution near the Minimum-spin-splitting Surface in Bulk Wurtzite Materials.
Meng-En Lee 1 , Chieh-Lung Wu 2 , Hsiu-Fen Kao 2 , Jih-Chen Chiang 2 , Wan-Tsang Wang 2 , Ming-Hong Gau 2 , Ikai Lo 2 , Yu-Chi Hsu 2 , Der-Jun Jang 2 , Chun-Nan Chen 3 Show Abstract
1 Department of Physics, National Kaohsiung Normal University, Kaohsiung County Taiwan, 2 Department of Physics, National Sun Yat-sen University, Kaohsiung Taiwan, 3 Department of Physics, Tamkang University, Taipei County Taiwan
The spin-splitting energies in biaxially strained bulk wurtzite materials are calculated using the linear combination of atomic orbital (LCAO) method. It is found only three types of shapes of the minimum-spin-splitting (MSS) surface exist in the wurtzite Brillouin zone; that is, a hyperboloid of two sheets, a hexagonal cone and a hyperboloid of one sheet. The ellipsoidal MSS surface under compressively biaxial strain predicted by the two-band k.p (2KP) model does not exist, due to the MSS data points in this case are far from the Γ point. In addition, the equi-spin-splitting distribution in k-space near the MSS surface is illustrated, and the data are analyzed using the 2KP model to identify the magnitude of spin splitting induced by Rashba and Dresselhaus effects.
9:00 PM - I7.4
Atomic Bond-orbital Model for Zinc-blende Semiconductors.
Meng-En Lee 1 , Hsiu-Fen Kao 2 , Jih-Chen Chiang 2 , Wan-Tsang Wang 2 , Ikai Lo 2 , Yu-Chi Hsu 2 , Chieh-Lung Wu 2 , Der-Jun Jang 2 , Chun-Nan Chen 3 Show Abstract
1 Department of Physics, National Kaohsiung Normal University, Kaohsiung County Taiwan, 2 Department of Physics, National Sun Yat-sen University, Kaohsiung Taiwan, 3 Department of Physics, Tamkang University, Taipei County Taiwan
We develop a 16-band atomic bond-orbital model (16ABOM) which is able to compute the spin splitting induced by bulk inversion asymmetry. This model is derived from the linear combination of atomic orbital (LCAO) scheme such that the characteristics of real atomic orbitals can be preserved for spin-splitting calculations. We derive the Hamiltonian of 16ABOM by performing a similarity transform on the nearest-neighbor LCAO Hamiltonian, followed by taking a second-order Taylor series expansion over k-vector on matrix elements at the Γ point. The spin-splitting energies in bulk zinc-blendes are calculated using this newly developed model, and the results are in good agreement with LCAO calculations. In addition, it is found the spin-orbit coupling between anti-bonding and bonding p-like states, which can be evaluated directly by this 16ABOM, dominates the magnitude of the spin splitting of the lowest conduction bands in middle-bandgap and wide-bandgap materials.
9:00 PM - I7.5
Magneto-optical Kerr Effect of Rare Earth Doped GaN.
Wojciech Jadwisienczak 1 , H. Tanaka 1 , R. Palai 2 , J. Wu 2 , A. Rivera 2 , J. Zavada 3 , A. Andars 4 Show Abstract
1 School fo EECS, Ohio University, Athens, Ohio, United States, 2 Department of Physics, University of Puerto Rico, San Juan United States, 3 Department of ECE, North Caroline State University, Raleigh, North Carolina, United States, 4 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Recently, a significant amount of effort has been dedicated toward understanding the origin of ferromagnetism in transition metal (TM) and rare earth (RE) doped III-nitrides with respect to different dopants and growth parameters. Despite efforts to explain magnetization observed in Gd-doped GaN and other RE-doped III-nitrides, it is not clear yet what is the origin of the reported magnetization. The magneto-optical Kerr effect (MOKE) is a non-destructive ex situ characterization technique useful in studying the magnetization of localized area of bulk, thin and ultrathin magnetic layers. However, MOKE has not been widely used to exploit the magneto-optical parameters of RE-doped III-nitride diluted magnetic semiconductors. In this paper, we report on the preliminary results of magneto-optical studies on Yb, Er and Nd- doped GaN samples. RE-doping was done in situ by molecular beam epitaxy and by ion implantation. Selected samples were subjected to thermal treatment to stimulate structural changes affecting observed material magnetization. The MOKE signal was measured for RE-doped GaN with a maximum magnetic field of 1.2 Tesla at room temperature. Each studied sample show characteristic MOKE hysteresis loop. The ferromagnetic behavior of investigated samples was confirmed by superconducting quantum interference device (SQUID) magnetometry. Furthermore, we will discuss an approach for enhancing the magneto-optic effect in RE-doped III-nitrides by the cavity MOKE technique.
9:00 PM - I7.6
Enhanced Light Emission from Europium Complex Owing to the Localized Surface Plasmon of Silver Islands.
Seong Min Lee 1 , Woo Hyun Kim 1 , Kwan Hyun Cho 1 , Kyung Cheol Choi 1 Show Abstract
1 , kaist, Daejeon Korea (the Republic of)
The europium ion has been of great interest as a potential luminescent center of photoluminescent (PL) phosphors for use with White LEDs (WLEDs) and ACPDPs. Several studies have attempted to increase the light emission of rare-earth ions using a localized surface plasmon [1-3]. Hayakawa, et al. and Malta both reported enhanced light emission from a trivalent europium complex using the localized surface plasmon resonance of silver nanoparticles [3,4]. However, there have been few studies of the divalent europium complex incorporating the localized surface plasmon. Therefore, the present work investigates surface-plasmon-mediated light emission from the Eu(II) ion. To prepare the samples, silver islands on a glass substrate were fabricated using thermal deposition. The 1mM EuCl2 complex was dissolved in a 1% w/w solution of PMMA in toluene. The PMMA/Toluene solution including the Eu(II) complex was spun-coated onto the silver islands. To cure the PMMA/Toluene thin film, the samples were dried for 5 min in convection oven at 150 celsius. The emission characteristics of the Eu(II) ions were analyzed through the confocal fluorescence microscopy and photoluminescence measurements. The plasmon resonance of the silver islands was obtained via uv-vis spectroscopy. From the results, the effect of the silver islands on the emission intensity of the Eu(II) complex was investigated under the condition of a deposition thickness of silver islands. Up to a five-fold enhancement factor in the form of integrated emission intensity was obtained according to deposition thickness of silver islands. This occurred due to the resonant coupling that resulted from the localized surface plasmon of the silver islands. The extinction peak was increased and red-shifted as the deposition thickness of the silver islands was increased. This spectral overlap between the emission spectra of the Eu(II) ion and the plasmon peak of the silver islands likely induced the coupling between the localized surface plasmon and the 4f electrons of the Eu(II) ion. Consequently, the plasmon-enhanced luminescence of this rare-earth ion can potentially be applied in display devices in the form of White LEDs and ACPDPs.REFERENCES1.T. Hayakawa, S. T. Selvan, and M. Nogami, Applied Physics Letters 74, 1513-1515 (1999). 2.Jian Zhu, Materials Letters 59, 1413–1416 (2005)3.T. Hayakawa, S. Tamil Selvan, and M. Nogami, Journal of non-crystalline solids 259, 16-22 (1999).4.O.L. Malta, Chemical Physics Letters 174, 13-18 (1990).
9:00 PM - I7.7
Bandgap Engineering in MBE Grown Al(1-x)GaxN Epitaxial Columnar Nanostructures.
Ratnakar Palai 1 , Javier Wu 1 , W. Jadwisienczak 2 Show Abstract
1 Dept. of Physics, University of Puerto Rico, San Juan United States, 2 2School of Electrical Engineering & Computer Science, Ohio University, Athens, Ohio, United States
Wide bandgap semiconductors (WBSs) are capable of electronic functionality at much higher temperature than silicon, which could benefit a variety of important applications, especially in the automotive, aerospace, power generation (turbine engine), and energy production industries. The success of WBSs in optoelectronics and integrated circuits is largely attributed to the capability of energy-band engineering through composition modulation. Heterostructures with a 2D-film configuration having multiple or tunable bandgaps hold promise for many solid state device applications including semiconductor lasers, solar cells, transistors and sensors. The most appealing current application is the fabrication of multijunction solar cells for enhancing efficiency by absorbing the whole solar spectrum. Heterostructures with a 3D-configuration (columnar films/nanostructures) are very useful for high-speed and flexible optoelectronic devices because it could minimize the interfacial strain due to the lattice mismatch between the film and substrate. 3D columnar films often have anisotropic and enhanced magnetic, electric, piezoelectric, ferroelectric, and optical properties. Fabrication of 3D-epitaxial columnar nanostructures with multiple bandgaps is critical and not yet well understood, which is a challenging problem and currently a field of intense research. In the present work, heterostructures of Al(1-x)GaxN (x = 0.6, 0.8, and 0.9) were fabricated on Si (111) and Al2O3 (0001) substrates with AlN buffer layer using a plasma assisted molecular beam epitaxy (MBE) at different growth conditions. Cross-sectional scanning electron microscopy of the nanostructures shows three well distinct layers showing 3D-like columnar structure. No structural defects and imperfections have been found at the interfaces. X-ray diffraction shows Al(1-x)GaxN nanostructures are highly epitaxial with c-axis orientation. X-ray photoelectron spectroscopy shows all the fundamental electronic states of Al, Ga, and N. Cathodeluminescence imaging of Al(1-x)GaxN nanostructures show whitish corona surrounded by blue color at room temperature indicating existence of different bandgap layers in the fabricated structures. UV cathodoluminescence spectra measured in a wide temperature range (10-300 K) show three bands corresponding to the band-to-band transitions of each subsequent Al(1-x)GaxN epilayers. Samples grown at low N flux shows brighter blue emission implying oxygen related defects and/or nitrogen vacancies. The carrier concentration and mobility calculated from Hall effect measurements agrees with reported results.
Walter R. L. Lambrecht Case Western Reserve University
Kevin Smith Boston University
H. Joseph Trodahl Victoria University of Wellington
Andreas Ney Universitaet Duisburg-Essen
I8: TM Nitride Surface
Wednesday AM, December 01, 2010
Room 301 (Hynes)
9:45 AM - **I8.1
Gallium Nitride Surfaces with Magnetic Atoms Investigated Using Scanning Tunneling Microscopy.
Arthur Smith 1 Show Abstract
1 Nanoscale & Quantum Phenomena Institute, Ohio University, Athens, Ohio, United States
The doping of semiconductor thin films with magnetic atoms during epitaxial growth is a topic of on-going interest and excitement for the purposes of spintronics. Yet a full understanding of this phenomenon is not possible without understanding the structures formed at semiconductor surfaces with addition of magnetic atoms. The case of gallium nitride is influenced strongly by the fact that atomically smooth surfaces are grown typically under gallium-rich conditions in molecular beam epitaxy, and this has a strong effect on the way magnetic atoms bond at the surface. In this talk, new results will be presented regarding the formation of ordered structures on GaN surfaces. Many new results have recently been discovered in the case of manganese, but other magnetic atoms are also under investigation. Samples in the work to be presented are prepared using molecular beam epitaxy in a growth system which includes metal effusion cells and an rf plasma nitrogen source. Magnetic species are deposited in quantities of a fraction of a monolayer up to several monolayers on top of atomically clean gallium nitride surfaces. Following preparation, samples are transferred directly to the adjoining surface analysis chamber containing a custom scanning tunneling microscope system. Various well-ordered structures are found which depend on the growth conditions as well as the surface polarity. Prospects for magnetic/spintronic properties of these structures are also currently under investigation.
10:15 AM - I8.2
Magnetic and Two-point Correlation Function Measurements of Dilute MnScN(001) Films Grown by Molecular Beam Epitaxy.
Costel Constantin 1 , Kangkang Wang 2 , Abhijit Chinchore 2 , Kai Sun 3 , Han-Jong Chia 4 , John Markert 4 Show Abstract
1 Physics and Astronomy, James Madison University, Harrisonburg, Virginia, United States, 2 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 3 Material Science and Engineering, University of Michigan, Ann Arbor, 103, Maldives, 4 Physics Department, University of Texas at Austin, Austin, Texas, United States
Dilute magnetic semiconductor which exhibit above room temperature ferromagnetism has been a major focus for scientists in the last twenty years. In this study, MnxSc(1-x)N films (with x = 3, and 5%) were grown on ScN(001)/MgO(001) substrates by radio frequency plasma assisted molecular beam epitaxy. The ScN(001) buffer layer was grown on top of MgO(001) at Ts ~ 800 oC and with a thickness of ~50 nm. The magnetic measurements show that both samples [i.e., Mn0.03Sc0.97N, and Mn0.05Sc0.95N] have a ferromagnetic to paramagnetic transition at ~ 365 K. The calculated values for the magnetic moments per Mn-atom were μ(S450) = 0.0612 μB/ Mn-atom for Mn0.03Sc0.97N film, and μ(S452) = 0.0151 μB/ Mn-atom for the Mn0.05Sc0.95N film. A combination of experimental scanning transmission electron microscopy together with a statistical two-point correlation function method has been developed to look at the clustering properties of manganese populated areas. It has been found that manganese is distributed uniformly within the ScN lattice. Work supported by NSF.
10:30 AM - I8.3
Growth of Thin Transition Metal-Based Superconducting Films by Atomic Layer Deposition.
Jeffrey Klug 1 , Thomas Proslier 1 , Nicholas Becker 1 2 , Jeffrey Elam 3 , James Norem 4 , John Zasadzinksi 2 , Michael Pellin 1 Show Abstract
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Physics, Illinois Institute of Technology, Chicago, Illinois, United States, 3 Energy Systems Division, Argonne National Laboratory, Argonne, Illinois, United States, 4 High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois, United States
We report the use of atomic layer deposition (ALD) to synthesize a variety of transition metal-based superconducting thin films. ALD is a technique which exploits sequential self-terminating surface chemical reactions in order to produce uniform coatings with atomic scale control on substrates with arbitrary shape. The ALD process therefore offers the possibility of conformally coating complex shapes with precise, layered structures with tightly constrained morphology and chemical properties. Among other applications, such coatings may enable the production of superconducting radio frequency (SRF) structures with significantly better performance and yield than those obtained from bulk niobium. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and correlated electron phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of preliminary results of ALD-grown films of nitrides, carbides, and carbonitrides of niobium, titanium, and molybdenum. Our program looks both at the metallurgy and superconducting properties of these coatings, and also their performance in working SRF structures.Research supported by the U.S. Department of Energy, Office of High Energy Physics, under Award DE-PS02-09ER09-05
10:45 AM - I8.4
Molecular Beam Epitaxy and Scanning Tunneling Microscopy Studies of Iron onto N-polar Wurtzite Gallium Nitride (000-1).
Wenzhi Lin 1 , Abhijit Chinchore 1 , Yinghao Liu 1 , Kangkang Wang 1 , Arthur Smith 1 Show Abstract
1 Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Athens, Ohio, United States
GaN attracts tremendous attention, for its excellent electronic and optical properties. Furthermore, it is of great interest to add magnetic functionality to nitride semiconductors. One way is to add magnetic layers onto the surface of GaN. Using scanning tunneling microscopy (STM), we investigate sub-monolayer (ML) iron deposition onto N-polar GaN(000-1). The smooth N-polar GaN(000-1) surface is grown by molecular beam epitaxy after nitriding a sapphire substrate surface. Annealing of the GaN surface is applied to reduce the amount of excess Ga metal adatoms. The surface is monitored using in-situ reflection high-energy electron diffraction (RHEED) to verify the structure and quality. After an atomically smooth GaN surface with minimal excess Ga is verified, a fraction of ML of Fe is deposited. STM reveals a surface which has reconstructions which are similar to those found on clean GaN(000-1) but in many places not as highly ordered as the clean GaN surface. Within the reconstructed regions, atomically flat islands having typical widths of up to 200 Å and heights in the range 5- 10 Å are observed. These islands show clear atomic ordering. Besides the regions with islands, smooth, featureless terraces not containing any islands are also observed in the same sample.(This work is supported by the U.S. Department of Energy.)