bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion Beam Deposition
Authors: Appleton B.R. , Zuhr R.A. , Noggle T.S. , Herbots N. , Pennycook S. J. , Alton G.D.
Published: August 2016

Ion beam processing of materials has a tradition at Oak Ridge National Laboratory that is as old as the laboratory itself. Consequently, when we began looking for a competitive way to participate in the excitement and new physics beginning to emerge from the fabrication and study of artificially structured materials, it was natural to look for a growth technique that incorporated ion beam processing. Our division, the Solid State Division, has a variety of ion implantation and ion beam analysis accelerators which are integrated with pulsed-laser sources into ultrahigh vacuum (UHV) surface analysis and processing chambers. These facilities allow us to do ion beam and laser processing of materials in UHV at temperatures from liquid helium to several hundred degrees centigrade and to study these alterations in situ by a variety of ion beam (ion scattering, ion channeling, nuclear reactions, etc.) and surface analysis (low energy electron diffraction, Auger, etc.) techniques. Since isotope separation has been done continually at ORNL for almost 45 years, the idea and advantages for altering this technique to do materials fabrication in UHV were immediately obvious. In the following article we will briefly review the history of the ion beam deposition (IBD) concept, describe our preliminary apparatus, and point out the inherent advantages of IBD for fabricating and studying artificially structured materials. Recent results obtained by IBD will be presented.
Up Close: Materials Division of NASA-Lewis Research Center
Authors: Abel Phillip
Published: August 2016

The Materials Division in the Aerospace Technology Directorate of the National Aeronautics and Space Administration (NASA)-Lewis Research Center has a distinguished history of contributions to advanced materials research and development. Located in Cleveland, Ohio, the Lewis Research Center was originally built for aircraft piston engine research during World War II as a part of NASA's predecessor, the National Advisory Committee on Aeronautics. After the war, with its need for immediate engineering solutions ended, attention at Lewis turned to a variety of more fundamental research problems. As early as the 1950s, a portion of the experimental effort at Lewis sought new materials to withstand the extremely high temperatures within turbojet engines. The needs for materials to withstand ever more severe temperature/environment extremes continue, and recognition of these needs, in part, motivates the current materials research effort at NASA-Lewis. The Materials Division structure reflects the strengths as well as the diversity of research areas being investigated. Each of the eight branches making up the Materials Division is briefly sketched below.

The Microgravity Materials Science Laboratory is a unique facility with the purpose of allowing industry, university, and government researchers to prepare for materials processing experiments to be done in the “microgravity” environment aboard the Space Shuttle.
Nondestructive Depth-Profiling of Multilayer Structures by Spectroscopic Ellipsometry
Authors: Vedam K.
Published: August 2016

Spectroscopic ellipsometry (SE) is the newest nondestructive and nonperturbing technique for characterizing surfaces, interfaces, and multilayer structures. The technique was originally developed and perfected by Aspnes of Bell Laboratories and a commercial instrument is currently available. After a brief description of the basic principles involved in this technique, one of its many applications — the depth profiling of multilayer structures — is described. Further details about SE and its other applications can be found elsewhere.

The automated spectroscopic ellipsometer that has been built in our laboratory is based on the design of Aspnes and Studna. A schematic diagram of the instrument is shown in Figure 1. It is basically a rotating analyzer ellipsometer operated by an on-line Apple computer, and it has spectroscopic scanning capability. Plane polarized monochromatic light is allowed to be incident on the sample at a chosen angle of incidence. The characteristics of the elliptically polarized reflected light is analyzed by a rotating analyzer. As Budde has shown, Fourier analysis of the detector output in a rotating analyzer ellipsometer yields the desired ellipsometric parameters Δ and ψ characterizing the material under study. Aspnes has pointed out that with proper design, alignment and operation, the precision attained is very high, and accuracy of the data is also as high as that attainable with null ellipsometers. The results of our own studies confirm these conclusions. Such measurements are carried out at a number of discrete wavelengths (˜100) distributed uniformly in the UV-visible-near IR spectral range.
1986 Von Hippel Lecture: The Many-Body Approach to Materials Science
Authors: Balkanski Minko
Published: August 2016

I would like to express my gratitude to the Awards Committee and its chairman for bringing me here today. One of the effects, and not the least, of this award is to call on my earliest recollections from the United States. In the Fall of 1956, I arrived in Boston at the Massachusetts Institute of Technology (MIT) as a postdoctoral fellow in the laboratory of Prof. von Hippel. My stay in the Materials Research Laboratory of MIT was a determining influence in my future work.

The many-body problem is the study of the effects of interaction between bodies on the behavior of a many-body system. The importance of the many-body problem derives from the fact that almost any real physical system is composed of a set of interacting particles. Another essential aspect is that the many-body problem is not a branch of solid-state or atomic or nuclear physics but deals with general methods applicable to all many-body systems.

Because of the complexity of the many-body problem, one of the preferred solutions is simply to ignore it. One can always say, “Let us admit that the particles forming the system do not interact or that their interaction is so weak that the effect can be considered negligible.” In many cases, this method produced good results, and one of the great mysteries is why.
Growth Of High Quality GaN Thin Films By MBE On Intermediate-temperature Buffer Layers
Authors: Fong W.K. , Zhu C. F. , Leung B. H. , Surya Charles
Published: August 2016

We report the growth of high quality GaN epitaxial layers by rf-plasma MBE. The unique feature of our growth process is that the GaN epitaxial layers are grown on top of a double layer that consists of an intermediate-temperature buffer layer (ITBL), which is grown at 690°C and a conventional low-temperature buffer layer deposited at 500°C. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm2V−1s−1 for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. Our analyses of the mobility and the photoluminescence characteristics demonstrate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material resulting in improvement in the optoelectronic properties of the films. A maximum electron mobility of 430 cm2V−1s−1 can be obtained using this technique and further optimizing the growth conditions for the low-temperature buffer layer.
The Durability of Various Crucible Materials for Aluminum Nitride Crystal Growth by Sublimation
Authors: Liu B. , Edgar J.H. , Gu Z. , Zhuang D. , Raghothamachar B. , Dudley M. , Sarua A. , Kuball Martin , Meyer III H. M.
Published: August 2016

Producing high purity aluminum nitride crystals by the sublimation-recondensation technique is difficult due to the inherently reactive crystal growth environment, normally at temperature in excess of 2100 °C. The durability of the furnace fixture materials (crucibles, retorts, etc.) at such a high temperature remains a critical problem. In the present study, the suitability of several refractory materials for AlN crystal growth is investigated, including tantalum carbide, niobium carbide, tungsten, graphite, and hot-pressed boron nitride. The thermal and chemical properties and performance of these materials in inert gas, as well as under AlN crystal growth conditions are discussed. TaC and NbC are the most stable crucible materials with very low elemental vapor pressures in the crystal growth system. Compared with refractory material coated graphite crucibles, HPBN crucible is better for AlN self-seeded growth, as crystals tend to nucleate in thin colorless platelets with low dislocation density.
Time-resolved Spectroscopy of the violet luminescence of undoped AlN
Authors: Freitag R. , Thonke K. , Sauer R. , Ebling D. G. , Steinke L.
Published: August 2016

We report on the time-resolved luminescence of the defect-related violet band from undoped AlN epitaxial layers grown on sapphire and SiC. For both measurements in photoluminescence and in cathodoluminescence a decay of algebraic nature at long times is observed. This is typical for donor-acceptor pair transitions. We compare the behavior of this band to that of the generically yellow luminescence of GaN.
Modeling of InGaN MOVPE in AIX 200 Reactor and AIX 2000 HT Planetary Reactor
Authors: Talalaev R. A. , Yakovlev E. V. , Karpov S. Yu. , Makarov Yu. N. , Schoen O. , Heuken M. , Strauch G. , Juergensen Holger
Published: August 2016

Multiwafer Planetary Reactor is a promising system for large-scale production of heterostructures for LED's based on III-group nitrides. Analysis of chemical processes occurring in the reactor allows one to get insight into specific mechanisms governing growth of nitride based heterostructures. In the present paper results of modeling analysis of MOVPE of InxGa1−xN layers in AIX-200 Reactor and AIX 2000 HT Planetary Reactor are reported. The model used for MOVPE process analysis accounts for gas flow, heat transfer, and multicomponent mass transport along with gas phase and surface chemical reactions. Results of the modeling analysis of In transport and incorporation into the solid phase are compared with experimental data. It is shown that the model predicts reasonably well the In incorporation during MOVPE of InGaN under In/(In+Ga) ratio in the gas phase less than 20%.
Visible-Blind UV Digital Camera Based On a 32 × 32 Array of GaN/AlGaN p-i-n Photodiodes
Authors: Brown J.D. , Yu Zhonghai , Matthews J. , Harney S. , Boney J. , Schetzina J.F. , Benson J.D. , Dang K.W. , Terrill C. , Nohava Thomas , Yang Wei , Krishnankutty Subash
Published: August 2016

A visible-blind UV camera based on a 32 × 32 array of backside-illuminated GaN/AlGaN p-i-n photodiodes has been successfully demonstrated. Each of the 1024 photodiodes in the array consists of a base n-type layer of AlGaN (~20%) onto which an undoped GaN layer followed by a p-type GaN layer is deposited by metallorganic vapor phase epitaxy. Double-side polished sapphire wafers are used as transparent substrates. Standard photolithographic, etching, and metallization procedures were employed to obtain fully-processed devices. The photodiode array was hybridized to a silicon readout integrated circuit using In bump bonds. Output from the UV camera was recorded at room temperature at a frame rate of 30 Hz. This new type of visible-blind digital camera is sensitive to radiation from 320 nm to 365 nm in the UV spectral region.
Synthesis and Growth of Gallium Nitride by the Chemical Vapor Reaction Process (CVRP)
Authors: Callahan M. , Harris M. , Suscavage M. , Bliss D. , Bailey J.
Published: August 2016

A new process for synthesis and bulk crystal growth of GaN is described. GaN single crystal c-plane platelets up to 9mm by 2mm by 100μm thick have been grown by the Chemical Vapor Reaction Process (CVRP). The reaction between gallium and a nitrogen precursor is produced by sublimation of solid ammonium chloride in a carrier gas, which passes over gallium at a temperature of approximately 900°C at near atmospheric pressures. Growth rates for the platelets were 25-100 μm/hr in the hexagonal plane. Seeded growth in the c-direction was also accomplished by re-growth on previously grown c-plane platelets. The crystals were characterized by X-ray diffractometry, atomic force microscopy, secondary ion mass spectrometry, inert gas fusion, and room temperature Hall effect and resistivity measurements.
Electric Field Distribution in strained p-i-n GaN/InGaN multiple quantum well structures.
Authors: Cartwright A.N. , Sweeney Paul M. , Prunty Thomas , Bour David P. , Kneissl Michael
Published: August 2016

The presence of piezoelectric fields within p-i-n GaN/InGaN multiple quantum well structures is discussed. Time integrated and time-resolved photoluminescence measurements and theoretical calculations of the effect of these fields is presented. Furthermore, a description of how these fields influence the carrier dynamics and a discussion of how the piezoelectric field effects the design of GaN/InGaN devices is presented.
New buffer sublayers with crystal structure of cubic syngony for growing the heteroepitaxial films of nitride compounds of type AIIIBV on sapphire substrates
Authors: Kotelyanskii M.J. , Kotelyanskii I.M. , Kravchenko V.B.
Published: August 2016

We propose the use of a buffer sublayer made of materials with crystal structure of cubic syngony to eliminate 30° in-plane rotation of (0001) heteroepitaxial wurtzite type AIIIBV nitride films with respect to the (0001) or (110) working surface of the sapphire substrate. In these cases, the lattice parameter mismatch between the sapphire substrate surface and the semiconductor film is much smaller, and the cleavage planes of the sapphire and the semiconductor films with wurtzite structure forming the active region of a heterolaser are parallel. It is shown experimentally that using, for instance, Nb on (0001) Al2O3 or NbN on (110) Al2O3, allows the elimination of the 30° in-plane rotation of the (0001)AlN film with respect to the (0001) or (110) working surface of the sapphire substrate.
Effect of internal absorption on cathodoluminescence from GaN
Authors: Knobloch Klaus , Perlin Piotr , Krueger Joachim , Weber Eicke R. , Kisielowski Christian
Published: August 2016

We have studied optical properties of GaN grown on sapphire by metalorganic chemical vapor deposition in the near band-edge energy range by cathodoluminescence. A large shift of the band-edge luminescence to lower energies is induced by increasing the beam energy. The free exciton position shifts about 20 meV when the beam energy is increased from 5 keV to 25 keV at room-temperature. The effect is explained by internal absorption caused by an exponential absorption tail at the band-edge. An Urbach parameter of about 30 to 40 meV for the exponential band-tail in our samples is estimated by comparing experimental with simulated spectra.
Effect of Magnesium and Silicon on the lateral overgrowth of GaN patterned substrates by Metal Organic Vapor Phase Epitaxy
Authors: Haffouz S. , Beaumont B. , Gibart Pierre
Published: August 2016

Metalorganic vapor phase epitaxy was used to achieve selective regrowth of undoped, Mg- and Si-doped GaN on a silicon nitride patterned mask, capping a GaN epitaxial layer deposited on (0001) sapphire substrate. Hexagonal openings in the mask defined into 10 µm diameter circles separated by 5µm were used as a pattern for the present study. Uniform undoped and Mg-doped GaN hexagonal pyramids, delimited by C (0001) and R {101} facets, were achieved with a good selectivity. Si-doped GaN hexagonal pyramids delimited by vertical {100} facets and (0001) top facet were obtained for a high SiH4 flow rate in the vapor phase. We found that the GaN growth rates VR and VC, measured in the R <101> and C <0001> directions respectively, were drastically affected by the Mg and Si incorporation. By adjusting the Mg partial pressure in the growth chamber, the VR/VC ratio can be increased. Hence, the delimiting top C facet do not vanish as usually observed in undoped GaN selective regrowth but conversely expands. On the other hand, under proper growth conditions, 20µm-high Si-doped GaN columns were obtained.
Spectroscopic Ellipsometry on GaN: Comparison Between Hetero-epitaxial Layers and Bulk Crystals
Authors: Zauner A. R. A. , Devillers M. A. C. , Hageman P. R. , Larsen P. K. , Porowski S.
Published: August 2016

In the present study spectroscopic ellipsometry was used for characterising GaN bulk crystals obtained at high pressure and thin films grown on sapphire. The undoped GaN films grown by MOCVD show interference fringes below the fundamental gap. The ellipsometric data (Δ and Ψ), measured in the wavelength range between 500 nm and 680 nm, were analysed using a multilayer description in the transfer matrix formalism. For the thin films grown on sapphire the model includes a buffer layer, a GaN epilayer, and a hypothetical overlayer. Furthermore, both, the real and imaginary part of the complex refractive index were taken into account. The real part of the refractive index n1 was found to follow a Cauchy-type of dispersion . For bulk crystals n1 was found to be 2.337 ± 0.010 at the wavelength of 632.8 nm. This value compares well with MOCVD grown GaN where n1(λ = 632.8 nm) = 2.328 ± 0.003.
Optical properties of electron-irradiated GaN
Authors: Buyanova I. A. , Wagner Mt. , Chen W. M. , Lindström L. , Monemar B. , Amano H. , Akasaki I.
Published: August 2016

The electronic structure of defects produced by 2.5-MeV electron irradiation and their effect on optical properties of GaN are investigated using photoluminescence (PL) and optically detected magnetic resonance (ODMR) techniques. The electron irradiation is shown to produce, in particular, a deep PL band with a no-phonon line at around 0.88 eV followed by a phonon-assisted sideband. We suggest that this emission is caused by an internal transition between excited and ground state of a deep defect. The excited state is a multiple-level state, as revealed from temperature dependent PL and level anti-crossing experiments. The electronic structure of the 0.88 eV defect is shown to be sensitive to the internal strain in the GaN epilayers. The ODMR studies reveal that the principal axis of the defect coincides with the c-axis of the host lattice and should therefore be either an on-site point defect or an axial complex defect along the c-axis.
Polarization and band offsets of stacking faults in AlN and GaN
Authors: Majewski J. A. , Vogl P.
Published: August 2016

We have performed systematic first-principles pseudopotential local density functional calculations of stacking faults in GaN and AlN. Their band offsets and the charge accumulation at stacking fault interfaces has been investigated, taking fully into account the effects of lattice relaxation and electric polarization. We find the stacking fault junctions to be of type I in both materials. However, the intrinsic valence band offsets are close to zero, so that the conduction band offsets result mostly from the differences in the energy gaps between the cubic and wurtzite phases. The charge accumulated at the interface between the cubic and wurtzite phase is found to be 0.009 and 0.003 C/m2 for the AlN and GaN stacking fault, respectively.
Magneto-optical studies of shallow donors in MOCVD grown GaN layers in FIR
Authors: Witowski A. M. , Sadowski M. L. , Paku ,a K. , Suchanek B. , Stepniewski R. , Baranowski Jacek M. , Potemski M. , Martinez G. , Wyder P.
Published: August 2016

Far infrared magnetooptical investigations of shallow donors in epitaxial MOCVD GaN layers show two types of shallow donors. In relaxed layers, a donor with an ionization energy of 35 meV was found. In strained, undoped and Si doped samples, a donor with ionization energy 32.5 meV was observed. From the p state splitting in magnetic field, the cyclotron effective mass for conduction electrons was found to be m*=0.222 m0.
Schottky Diodes on MOCVD Grown AlGaN Films.
Authors: Polyakov A.Y. , Smirnov N.B. , Govorkov A.V. , Greve D. W. , Skowronski M. , Shin M. , Redwing Joan M.
Published: August 2016

Au Schottky diodes were prepared by vacuum evaporation or by plasma sputtering on n-AlGaN(Si) films with Al mole fractions of 0, 0.11 or 0.23. The barrier heights were deduced from C-V and I-T measurements. The difference between the C-V and I-T results was less than 0.1 eV for the barriers deposited at 300 °C on HF etched samles with prior in situ heating at 450 °C. For low deposition temperatures (about 150°C) C-V and I-T methods give results differing by some tenths of an eV. For deposition temperatures exceeding 450°C the diodes were very leaky. The barrier heights were 0.8 eV, 0. 9 eV and 1.1 eV for AlGaN with compositions of 0, 0.11 and 0.23. For plasma sputtered diodes on GaN and AlGaN (x=0.11) samples, the difference in C-V and I-T results was quite considerable and admittance spectroscopy indicated the presence of deep electron traps at 0.12-0.14 eV that were absent in vacuum evaporated diodes. For similar diodes on AlGaN(x=0.23) samples the results of C-V and I-T measurements were very close and no traps at 0.12-0.14 eV could be detected. This difference is most likely due to damage caused by low energy ions. More Al-rich films are less susceptible to such damage. Persistent photocapacitance was observed in n-AlGaN Schottky diodes after illumination at 85K.
The Emission Properties of Light Emitting Diodes using InGaN/AlGaN/GaN Multiple Quantum Wells
Authors: Yunovich A. E. , Kudryashov V. E. , Turkin A. N. , Kovalev A. , Manyakhin F.
Published: August 2016

Luminescence spectra of Light Emitting Diodes (LEDs) with Multiple Quantum Wells (MQWs) were studied at currents J = 0.15 μA - 150 mA. A high quantum efficiency at low J is caused by a low probability of the tunnel current J (which is maximum at Jm ≈ 0.5-1.0 mA). J(V) curves were measured in the range J= 10−12-10−1 A; at J > 10−3A they may be approximated by a sum of four parts: V= φk+ mkT·[ln(J/J0)+(J/J1)0.5] + J·Rs. The part V ~ (J/J1)0.5is the evidence of a double-injection into i-layers near MQWs. Their presence is confirmed by capacitance measurements. An overflow of carriers through the MQW causes a lower quantum efficiency at high J. A model of a 2D-density of states with exponential tails fits the spectra. The value of T in the active layer was estimated. A new band was detected at high J; it can be caused by non-uniformity of In content in MQWs.
Growth on GaN and GaAs on fianite by MOCVD capillary epitaxy technique
Authors: Buzynin A. N. , Osiko V. V. , Buzynin Yu. N. , Pushnyi B.
Published: August 2016

Heteroepitaxial GaN and GaAs films were grown by both conventional two-step MOCVD and the new “capillary epitaxy” technique on (001) and (111) fianite (YSZ) substrates. The capillary epitaxy technique was investigated for the example of GaAs films growth on a YSZ substrate. This technique allows both the reduction of the minimum thickness and the improvement of the quality of III-V films. PL spectra of undoped GaN films on YSZ were studied.
Morphology and optical properties of cubic phase GaN epilayers grown on (001) Si
Authors: Godlewski M. , Goldys E. M. , Philips M. R. , Bergman J. P. , Monemar B. , Langer R. , Barski A.
Published: August 2016

Optical properties of GaN epilayers of a cubic phase are studied. We show a strong influence of the sample morphology on intensity of the edge emission. Whereas edge luminescence is reduced at the grain boundaries, red emission is spatially homogeneous.
Aging Mechanisms of InGaN/AlGaN/GaN Light-Emitting Diodes Operating at High Currents
Authors: Manyakhin F. , Kovalev A. , Yunovich A. E.
Published: August 2016

Changes of luminescence spectra and electrical properties of light-emitting diodes (LED’s) based on InGaN/AlGaN/GaN heterostructures were investigated over a long period of operation. Blue and green LED’s with InGaN single quantum wells were studied at currents up to 80 mA for 102−2.103 hours. An increase of luminescence intensity at operating currents of 15 mA was detected at the 1st stage of aging (100−800 hours) and a slow fall was detected in the 2nd stage. Greater changes of spectra were observed at low currents (< 0.15 mA). A study of charged acceptor distribution in the space charge region has shown that at the 1st stage their concentration grows, and in the 2nd stage, it falls. The models for the two stages are proposed: 1) activation of Mg due to destruction of residual Mg-H complexes; 2) formation of donor vacancies N. A model of defect formation by hot electrons injected into the quantum well is discussed.
High Frequency AlGaN/GaN MODFET's
Authors: Eastman L. , Chu K. , Schaff W. , Murphy M. , Weimann N. G. , Eustis T.
Published: August 2016

Short-gate MODFET's of AlGaN/GaN on Sapphire have been fabricated and characterized with gate lengths in the .12 - .25 μm range. Values of ft = 50 GHz and fmax = 100 GHz have been obtained. Analyzing the performance, the average electron transit velocity is shown to be 1.25 × 107 cm/s and in some cases well under that value. This compares with theoretical predictions of ~ 2.0 × 107 cm/s. The electron scattering effects of dislocations, which are charged, are modeled to explain the lower mobility. Ion bombardment or dry etching is used for mesa isolation. Ti/Al/Ti/Au sintered for 100 seconds at 800 °C is used to yield ohmic contacts of .5 - 1.0 Ω-mm. Pt/Au Schottky gates are used. A high breakdown voltage, exceeding 100 V even for short gate MODFET's, shows that ten times higher load resistance values are possible, compared with GaAs MODFET's. Normalized output power levels well over 10 W/ mm are thus projected for GaN MODFET's on SiC substrates, where the thermal conductivity is about 5W/cm-°C. with future integrated traveling-wave, power-combining circuits, output power > 100 W at 10 GHz is predicted.
XPS study of Au/GaN and Pt/GaN contacts
Authors: Sporken R. , Silien C. , Malengreau F. , Grigorov K. , Caudano R. , Sánchez F.J. , Calleja E. , Muñoz E. , Beaumont B. , Gibart Pierre
Published: August 2016

Au/GaN and Pt/GaN contacts have been studied with XPS. According to XPS depth profiling, the N signal is weak in the region below the metal contact and the Pt or Au signal decreases much more slowly than expected for a sharp interface. Next, we have performed in situ studies of the formation of Au contacts on GaN. In contrast to the results from depth profiling, we observe 2D growth and little or no chemical interaction between Au and GaN. This suggests that conventional calculations of sputtering yields and ion-beam-induced mixing cannot be applied to the analysis of noble metal/GaN depth profiles. Heating during or after Au deposition results in strong clustering, observed by both XPS and AFM. The Schottky barrier height measured by XPS is 1.15 eV.
Electron Beam Pumped MQW InGaN/GaN Laser
Authors: Kozlovsky V.I. , Krysa A.B. , Skyasyrsky Y.K. , Popov Y.M. , Abare A. , Mack M.P. , Keller S. , Mishra U. K. , Coldren L. , DenBaars Steven , Tiberi Michael D. , George T.
Published: August 2016

E-beam pumped lasers are attractive for Laser Cathode Ray Tubes (LCRT) in projection displays and a variety of applications typically associated with optically pumped lasers. For the first time an InGaN/GaN multiple quantum well (MQW) in-plane laser pumped by surface normal pulse and scanning electron beams was demonstrated. Pumping at room temperature (RT) and 80 K showed peak stimulated emission wavelengths of 402 and 409 nm with a full width half maximum (FWHM) of 0.6 nm and 1.2 nm, respectively. The threshold electron beam current densities have been estimated as 60 A/cm2 for 35 keV electron energy at 80 K using scanning e-beam pumping and 200-300 A/cm2 at RT using pulsed e-beam pumping with a maximum electron energy of 150 keV. At 80 K, light output of 150 mW was measured out of one facet at an e-beam current of 1.7 mA.
Characteristics of Indium-Gallium-Nitride Multiple-Quantum-Well Blue Laser Diodes Grown by MOCVD
Authors: Mack M.P. , Abare A. , Aizcorbe M. , Kozodoy Peter , Keller S. , Mishra U. K. , Coldren L. , DenBaars Steven
Published: August 2016

Room temperature (RT) pulsed operation of blue (420 nm) nitride based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates with lifetimes exceeding 6 hours have been demonstrated. Threshold current densities as low as 12.7 kA/cm2 were observed for 10×1200 μm lasers with uncoated reactive ion etched (RIE) facets. The emission is strongly TE polarized and has a sharp transition in the far field pattern above threshold.
Surface Treatment and Layer Structure in 2H-GaN Grown on the (0001)Si surface of 6H-SiC by MBE
Authors: Ruterana P. , Vermaut Philippe , Nouet G. , Salvador A. , Morkoç H.
Published: August 2016

Heteroepitaxy of hexagonal symmetry materials is more complicated than in the more usual case of cubic systems. In the growth of layers on the (0001) surfaces, the misfit dislocations always exhibit a screw component that leads to rotation of the epilayer in a 3 dimensional growth mode. The size of the islands will depend on many factors among which the substrate surface treatment, prior to growth, may be a predominant one. In this work, a comparative study is carried out for samples grown on plasma treated samples, with and without additional substrate annealing prior to epitaxy. It is found that the defect density can be brought below 109 cm−2, which is better than one order of magnitude in comparison to the layers grown on sapphire substrates. On top of the annealed substrates, the island growth is not obvious. Whereas, misorientations as large as a few degrees can be measured inside the layers on top of non annealed substrates, justifying the occurrence of high densities of threading dislocations.
Characteristics of an Electron Cyclotron Resonance Plasma Source for the Production of Active Nitrogen Species in III-V Nitride Epitaxy
Authors: Meyyappan M.
Published: August 2016

A simple analysis is provided to determine the characteristics of an electron cyclotron resonance (ECR) plasma source for the generation of active nitrogen species in the molecular beam epitaxy of III-V nitrides. The effects of reactor geometry, pressure, power, and flow rate on the dissociation efficiency and ion flux are presented. Pulsing the input power is proposed to reduce the ion flux.
Free Excitons in GaN
Authors: Monemar B. , Bergman J.P. , Buyanova I.A. , Li W. , Amano H. , Akasaki I.
Published: August 2016

Optical spectra on free exciton properties for GaN are presented and discussed, in particular the influence of epitaxial strain and temperature. The exciton-phonon coupling is also manifested via the temperature dependence of the LO phonon replicas of the free exciton.
Study of GaN films grown by metalorganic chemical vapour deposition
Authors: Van der Stricht W. , Moerman I. , Demeester P. , Crawley J. A. , Thrush E. J.
Published: August 2016

In this paper GaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. The effects of several growth parameters on the film quality are discussed. The results on n-type doping of GaN with SiH4 are presented. The GaN layers were evaluated by surface morphology studies, DC X-ray diffraction, electrical and optical characterisation.
Research on GaN MODFET's
Authors: Eastman L. , Burm J. , Schaff W. , Murphy M. , Chu K. , Amano H. , Akasaki I.
Published: August 2016

Initial results on 0.25 μm gate MODFET's have yielded ft=21.4 GHz and fmax=77.5 GHz. These devices have characteristics that agree with the gradual channel model dominated by the electron mobility. The AlGaN/GaN structure, grown on sapphire substrates, are polycrystalline, and thus yield low mobility (<100cm2/Vs) at low electron sheet density. Using a simple model, design optimization predicts electron sheet density values of 7.3 × 1012 cm−2 in thin, 3 nm quantum wells for single-sided doping with 5 nm spacer for use in future high frequency Al0.4Ga0.6N/In0.25Ga0.75N/GaN MODFET's with gate lengths of 0.10 μm. Double sided doping with 5 nm spacers would yield a sheet density of 1.4 × 1013cm−2 in such 3 nm quantum wells.
Surface morphology of as grown and annealed bulk GaN crystals
Authors: Nowak G. , Krukowski S. , Grzegory I. , Porowski S. , Baranowski Jacek M. , Pakula K. , Zak J.
Published: August 2016

GaN single crystals have been grown from Ga solution. The crystals grow in the form of platelets with their basal plane perpendicular to the c-axis. The two opposite crystal surfaces are not equivalent since one is N- and the other Ga-terminated. Atomic force microscopy has been applied to study surface morphology on both surfaces. It was found that one side is atomically flat. The other side consists of pyramid-like structures about 25 nm in size.

The influence of annealing in an NH3+H2 atmosphere in the temperature range from 600°C to 900°C was investigated. Depending on crystal face the results were drastically different. It was found that on the rough side, annealing yields an atomically flat surface with terraces of monolayer height. The size of the terraces depends on the temperature of the annealing. On the originally flat side the surface becomes rougher after annealing. The transformation of surface morphology begins at temperatures below 700°C. Preliminary results of annealing in a hydrogen atmosphere are also reported. These findings are crucial for the understanding and development of GaN homoepitaxy.
Growth Rate Reduction of GaN Due to Ga Surface Accumulation
Authors: Crawford D.E. , Held R. , Johnston A. M. , Dabiran A. M. , Cohen Philip I.
Published: August 2016

GaN(0001) has been grown on Al2O3 (0001) by molecular beam epitaxy where NH3 was used as the nitrogen precursor. Desorption mass spectroscopy and reflection high energy electron diffraction (RHEED) were used to monitor the relationship between growth rate and the incident fluxes during growth. Excess surface Ga decreases the GaN formation rate when the substrate temperature is too low or the Ga flux is too high. A simple rate equation is used to describe the observed behavior.
Photoluminescence study on GaN homoepitaxial layers grown by molecular beam epitaxy
Authors: Teisseyre H. , Nowak G. , Leszczynski M. , Grzegory I. , Bockowski M. , Krukowski S. , Porowski S. , Mayer M. , Pelzmann A. , Kamp Markus , Ebeling K. J. , Karczewski G.
Published: August 2016

GaN epitaxial layers on GaN single crystals were grown using molecular beam epitaxy with an NH3 source. The deposited layers were examined by high resolution x-ray diffraction and photoluminescence (PL) spectroscopy. We observed strong and extremely narrow (half-widths of 0.5 meV) lines related to the bound excitons. In the higher energy range we observed three strong lines. Two of them are commonly attributed to free exciton transitions A (3.4785 eV) and B (3.483 eV). Their energetic positions are characteristic of strain-free GaN material.
GaN m-i-n LED grown by MOVPE
Authors: Da-cheng Lu , Xianglin Liu , Du Wang , Xiaohui Wang , Lanying Lin
Published: August 2016

Undoped and Zinc-doped GaN films have been grown using TMGa, DEZn and Ammonia by MOVPE. The GaN blue-green LEDs of m-i-n structure have been fabricated. They can be operated at forward bias less than 5 volts. The EL peak wavelength was from 455 nm to 504 nm.
Microstructure, growth mechanisms and electro-optical properties of heteroepitaxial GaN layers on sapphire (0001) substrates
Authors: Christiansen S. , Albrecht M. , Dorsch W. , Strunk H. P. , Zanotti-Fregonara C. , Salviati G. , Pelzmann A. , Mayer M. , Kamp Markus , Ebeling KJ
Published: August 2016

We investigate the structure, growth morphology and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy (GSMBE) and use transmismission electron microscopy, atomic force microscopy and scanning tunneling microscopy, photoluminescence (PL) and cathodoluminescence (CL). We find two types of specimens: one type which shows a strong UV luminescence (band-to-band transition at 358nm/3.46eV) in CL and PL and only faint yellow luminescence (Gaussian shaped CL/PL peaks at around 528nm/2.35eV), specimen ‘B’, and another type, which shows a strong UV and a comparably strong yellow luminescence, specimen ‘Y’. These two types of specimens have a rough layer surface, specimen ‘Y’ even an islanded one with, facetted hexagonal islands with a width of 1-2μm at a height of 50nm. A correlation of spectrally resolved CL images to the observed defect structure shows: (i) the yellow luminescence is homogeneously distributed over the complete specimen for ‘B’ and ‘Y’ specimens. Our investigations strongly suggest the yellow luminescence to be related to screw dislocations with , which are found randomly distributed in ‘B’ and ‘Y’ specimens with a high density of 1.3·109cm−2; (ii) the strong UV luminescence in ‘Y’ specimens is located in the troughs between adjacent surface islands, where dislocations essentially in small angle grain boundaries of edge type, i.e. with or are located; (iii) in the case of the ‘B’ specimens these dislocations are randomly distributed and so is the luminescence.
Epitaxial growth of cubic GaN and AlN on Si(001)
Authors: Barski A. , Rössner U. , Rouviere J. L. , Arlery M.
Published: August 2016

Thermal treatment under propane at 1300-1400 °C has been used to prepare Silicon (001) wafers for subsequent growth of cubic GaN and AlN by Electron Cyclotron Resonance Plasma Assisted Molecular Beam Epitaxy (ECRMBE). Thermal treatment of Silicon wafers under propane, used in this experiment, produced a very thin (40 Å) layer of cubic SiC on the Silicon (001) surface. Despite an extremely low thickness of as-produced SiC layer, high quality cubic GaN has been successfully grown. The cubic form of AlN grown on the SiC(40Å)/Si(001) surface has also been observed despite a very high density of stacking faults.
Electronic structure of biaxially strained wurtzite crystals GaN, AlN, and InN
Authors: Majewski J. A. , Städele M. , Vogl P.
Published: August 2016

We present first-principles studies of the effect of biaxial (0001)-strain on the electronic structure of wurtzite GaN, AlN, and InN. We provide accurate predictions for the valence band splittings as a function of strain which greatly facilitates the interpretation of data from samples with unintentional growth-induced strain. The present calculations are based on the total-energy pseudopotential method within the local-density formalism and include the spin-orbit interaction nonperturbatively. For a given biaxial strain, all structural parameters are determined by minimization of the total energy with respect to the electronic and ionic degrees of freedom. Our calculations predict that the valence band state Γ9(Γ6) lies energetically above the Γ7(Γ1) states in GaN and InN, in contrast to the situation in AlN. In all three nitrides, we find that the ordering of these two levels becomes reversed for some value of biaxial strain. In GaN, this crossing takes place already at 0.32% tensile strain. For larger tensile strains, the top of the valence band becomes well separated from the lower states. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials agree well with the available experimental data.
In-depth Analysis of the Impurities in GaN
Authors: Kovarsky A. P. , Strykanov V. S.
Published: August 2016

GaN epitaxial films were analyzed by Secondary Ion Mass Spectrometry (SIMS). Standard implanted samples were used to determine the appropriate analytical conditions for analysis of impurities. The dose and energy of implantation for selected elements (Mg, Al, Si, Zn, Cd, H, C and O) were chosen so the maximum impurity concentration was not more than 1020 atoms/cm3. The optimum analysis conditions were ascertained from the standards for each element, and the detection limits were deduced from the background levels of the implantation profiles. We demonstate that lower detection limits of 1015 atoms/cm3 with a dynamic range 103 − 105 are possible. Zn and Cd have low ion yields, so the minimum detection level for these elements is the background level of the detector. The detection limits of the other elements are determined by the contamination of an initial GaN matrix.
Optical Properties of Nitride-based Structures Grown on 6H-SiC
Authors: Tsvetkov D.V. , Zubrilov A.S. , Nikolaev V. I. , Soloviev V.A. , Dmitriev V.A.
Published: August 2016

The luminescent properties of AlGaN epitaxial layers with AlN mole fractions up to 30% and various types of AlGaN/GaN-based heterostructures have been studied. The structures were grown on 6H-SiC substrates by MOCVD. The structures' cathodoluminescence and electroluminescence were measured. A “blue” shift of the edge luminescent peak position for AlGaN alloys was measured to be a non-linear function on the AlN mole fraction. For p-AlGaN/n-GaN double heterostructures (DH), the edge peak position was detected at 365 nm (300K). For a p-Al0.05Ga0.95N/n-Al0.03Ga0.97N heterostructure, the electroluminescent edge peak was observed at 355 nm (300K). The effects of temperature and forward current on the edge electroluminescence of theAlGaN/GaN DH's were investigated.
Determination of the dislocation densities in GaN on c-oriented sapphire
Authors: Pelzmann A. , Mayer M. , Kirchner C. , Sowada D. , Rotter T. , Kamp Markus , Ebeling K. J. , Christiansen S. , Albrecht M. , Strunk H. P. , Holländer B. , Mantl S.
Published: August 2016

We report on a comprehensive study of the defect structure in GaN grown on c-oriented sapphire by gas source molecular beam epitaxy and metal organic vapour phase epitaxy. Transmission electron microscopy is used to investigate the defect structures which are dominated by threading dislocations perpendicular to the sapphire surface and stacking faults. Additionally, dislocation densities are determined. For determination of dislocation densities by x-ray diffraction we employ a model that uses the linewidth of x-ray rocking curves for this purpose. Finally, Rutherford backscattering spectrometry is performed to complement the structural investigation.
GaN Layers Grown by HVPE on P-type 6H-SiC Substrates
Authors: Nikolaev A.E. , Melnik Yu.V. , Blashenkov M.N. , Kuznetsov N.I. , Nikitina I.P. , Zubrilov A.S. , Tsvetkov D.V. , Nikolaev V. I. , Dmitriev V.A. , Soloviev V.A.
Published: August 2016

Gallium nitride films were successfully grown by HVPE technique on p-type 6H-SiC substrate. The layers exhibit high crystal quality as was determined by X-ray diffraction. Photoluminescence (PL) of these films was measured. The PL spectra were dominated by band edge emission. Concentration Nd-Na in undoped epitaxial layers ranged from 2×1017 to 1×1019cm−3. Mesa-structures formed by reactive ion etching showed good rectifying current-voltage characteristics for GaN/SiC pn heterojunctions.
The Rate of Radiative Recombination in the Nitride Semiconductors and Alloys
Authors: Dmitriev Alexey V. , Oruzheinikov Alexander L.
Published: August 2016

The radiative recombination rates have been calculated for the first time in the wide band gap wurtzite semiconductors GaN, InN and AlN and their solid solutions GaxAl1−xN and InxAl1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We calculated the interband matrix elements for the direct optical transitions between the conductivity band and the valence one using the experimental photon energy dependence of the absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni. The calculations of the radiative recombination rates were performed in the wide range of temperature and alloy compositions.
Introducing recent medical graduates as members of Script Concordance Test expert reference panels: what impact?
Authors: Duggan Paul , Charlin Bernard
Published: August 2016

This article was migrated. The article was marked as recommended. The Script Concordance Test (SCT) is being increasingly used in professional development in clinical reasoning, with linear progression in performance in SCT's observed with increasing clinical experience. One of the limiting factors for the SCT is potential burnout in expert reference panel (ERP) members, which we have attempted to address by the introduction of recent medical graduates as panel members. We sought to evaluate the effect of introducing recent medical graduates in to our ERP's on pass/fail decisions in the final clinical reasoning examination of the 6-year undergraduate program of the University of Adelaide, Australia. We engaged an ERP comprising 50 faculty members from three collaborating universities and 13 recent medical graduates to answer on line an identical 20 case scenario, 50 question multidisciplinary SCT twice 6 months apart. The questions were used in high stakes end of year assessment of 5th year medical students (n=132). The pass mark set by the experienced, specialist members of the panel was 49.6% and this increased to 50.4% by addition of recent medical graduates to the panel. This difference would have had no effect on fail rates estimated from the data from the cohort of 132 medical student candidates. In the context of assessment of clinical reasoning in medical programs, recent medical graduates are suitable members of SCT ERP's, and their contribution can enrich the panel and might help to minimise risk of burnout of more experienced faculty.
Working with actors and non-actors as simulated patients: An advantage?
Authors: Viret Francine , Vadot Sara , Morend Laura , Christen Ariane , Bonvin Raphael
Published: August 2016

This article was migrated. The article was marked as recommended. Background: Actors as well as non-actors are currently hired as simulated patients (SPs) in medical education, and it is not clear whether one group performs better than the other. Aim: This study explores two facets of working with actor and non-actor SPs: their performance as perceived by students and tutors and how the two groups feel about being trained together. Method: A convergent parallel mixed-method approach was used, with a survey and focus groups. The survey was administered to tutors and third-year medical students after an SP encounter. They were asked to rate the SPs' performance, without information about whether the SP was a professional actor or not. Two focus groups with SPs were held, one with actors and one with non-actors, to explore the disadvantages and benefits of mixed training sessions. Results: The survey showed that actors and non-actors perform equally well from the viewpoint of the students and tutors and that the SPs valued being trained in mixed groups. Conclusion: When trained together for the same role, actors and non-actors exhibit the same quality of performance. Moreover, common training helps them improve their performance and standardize the play, and also contributes to fostering team spirit.