bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of Stoichiometry on Defect Distribution in Cubic GaN Grown on GaAs by Plasma-Assisted MBE
Authors: Ruvimov S. , Liliental-Weber Z. , Washburn J. , Drummond T. J. , Hafish M. , Lee S. R. 
Published: April 2011

ABSTRACTHigh resolution electron microscopy has been applied to characterize the structure of β-GaN epilayers grown on (001) GaAs substrates by plasma-assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N2 flow for fixed rf power). The best quality of GaN layers was achieved by “stoichiometric” growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the “major flat” of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN-GaAs interfaces.
GaN Crystals Grown in the Increased Volume High-Pressure Reactors
Authors: Porowski S. , Bockowski M. , Lucznik B. , Wroblewski M. , Krukowski S. , Grzegory I. , Leszczynski M. , Nowak G. , Pakula K. , Baranowski J. 
Published: April 2011

ABSTRACTGaN single crystals are grown from the solution of atomic nitrogen in liquid Ga at N2 pressure up to 20 kbar. The crystals reaching dimensions of 1cm, with dislocation density of 103 - 105 cm-2 are currently obtained and successfully used for homoepitaxy by MOCVD and MBE.The increase of the maximum size of GaN crystals with stable morphology requires the increase of the volume of the crucible. Beside the obvious geometric factors, the increase of the volume of the furnace and crucible allows to achieve much better control of supersaturation profiles in the solution and therefore better control of growth process. It helps to avoid morphological instabilities and leads to the growth of transparent, inclusion free crystals.In the paper, the results of crystallization of substrate quality GaN crystals obtained with the use of large volume high pressure reactor will be presented. The crystals were characterized by High Resolution X-ray Diffraction and Atomic Force Microscopy. It will be shown that the quality of GaN crystals does not deteriorate with the increasing size and that epi-ready surfaces of GaN substrates can be obtained. Two-dimensional growth by propagation of monoatomic steps of GaN homoepitaxial layers will be presented as a verification of the quality of GaN substrates.
Material and Device Characteristics of MBE-Grown GaN Using a New rf Plasma Source
Authors: Beresford R. , Stevens K. S. , Cui Q. , Schwartzman A. , Cheng H. 
Published: April 2011

ABSTRACTA new rf plasma nitrogen source has been characterized for growth of GaN on basal-plane sapphire by molecular beam epitaxy. For rf power of 500 W and N2 flow rate of 2 sccm, a maximum GaN growth rate of 0.80 μm/hr is obtained, implying a source efficiency greater than 5%. It is found that the GaN surface roughness is extremely sensitive to V:Il ratio near unity and independent of growth rate in the range 0.3-0.8 μm/hr. Roughness as small as 1.0 nm (rms) is measured by atomic-force microscopy. Microstructure of the high-growth-rate films is similar to other GaN films, as observed in cross-section transmission electron microscope images. The electroluminescence spectra from homojunction light-emitting diodes exhibit a band of nearultraviolet emissions corresponding to the energy separation of the intentional donor and acceptor levels on the two sides of the junction. The intensity of these emissions relative to the visible spectrum increases with drive current density, implying saturation of deep trap levels responsible for the visible light output.
Nitrogen Plasma Pretreatment of Sapphire Substrates for the GaN Buffer Growth by Remote Plasma Enhanced MOCVD
Authors: Hong Kim Min , Sone Cheolsoo , Hyung Yi Jae , Ok Heur Soun , Yoon Euijoon 
Published: April 2011

ABSTRACTLow-temperature GaN buffer layers with smooth surfaces and high crystallinity could be prepared by a remote plasma enhanced metalorganic vapor deposition after the pretreatment of substrates with rf nitrogen plasma. Smooth AIN thin layer was formed on the (0001) sapphire substrate by the nitrogen plasma pretreatment for an hour. The AIN layer provided the nucleation sites for the subsequent buffer layer growth, thus highly preferred (0001) GaN buffer layers could be grown on the pretreated substrate. Formation of the AIN layer on sapphire and the surface smoothness were affected by pretreatment parameters such as exposure time, temperature, and rf power.
Persistent Photoconductivity in n-Type GaN
Authors: Wickenden A. E. , Beadie G. , Koleske D. D. , Rabinovich W. S. , Freitas J. A. 
Published: April 2011

ABSTRACTPersistent photoconductivity has been observed in n-type Si-doped GaN grown on sapphire substrates by metalorganic vapor phase epitaxy. The effect has been seen both in films which are of electrically high quality, based on low temperature photoluminescence (PL) and Hall analysis, and in films which either appear to be compensated or which exhibit strong donor-acceptor pair recombination. The photoconductivity has been observed to persist for several days at room temperature (300K). Modeling of the resistance recovery by a stretched exponential treatment may suggest a distribution of deep level defect centers in the Si-doped GaN films.
Studies of Electrically and Recombination Active Centers in Undoped GaN Grown by OMVPE
Authors: Polyakov A. Y. , Govorkov A. V. , Smirnov N. B. , Shin M. , Skowronski M. , Greve D. W. 
Published: April 2011

ABSTRACTDeep centers were studied in GaN samples grown by organometallic vapor phase epitaxy (OMVPE). Electron traps 0.2 eV and 0.5 eV below conduction band edge and 0.25 eV and 0.50.85 eV above the valence band edge were detected by means of deep levels transient spectroscopy (DLTS), photoelectron relaxation spectroscopy (PERS) and thermally stimulated current spectroscopy (TSC). The photoconductivity at low temperature is shown to be persistent and the magnitude of photosensitivity is dependent on the way the samples are grown. Microcathodoluminescence (MCL) and electron beam induced current (EBIC) measurements indicate that the density of deep recombination centers near the dislocation walls between the misoriented GaN domains is lower than inside the domains. Spatially resolved PERS measurements show that the concentration of the 0.85 eV level is higher in the low angle grain boundary regions that produce bright contrast in EBIC and MCL.
Photoluminescence of Fe-Complexes in GaN
Authors: Thurian P. , Hoffmann A. , Eckey L. , Maxim P. , Heitz R. , Broser I. , Pressel K. , Meyer B-K. , Schneider J. , Baur J. , Kunzer M. 
Published: April 2011

ABSTRACTWe report a photoluminescence (PL) and photoluminescence excitation (PLE) investigation of the deep iron acceptor in hexagonal GaN. Codoped samples give rise to a new zero-phonon line (ZPL) at 1.268 eV. The spectral shape of its phonon side band is very similar to that of the Fe3+ -spectrum. A lifetime of 2.6 ms was measured for this ZPL which indicates a spinforbidden transition. In contrast to isolated Fe3+, no Zeeman-splitting is observed. PLE spectra of the Fe3+ (4T1-6A1) zero-phonon line at 1.299 eV in semi-insulating GaN samples reveal intracenter excitation processes via excited states of the Fe3+ center. Three excited Fe3+ crystal field states at 1.299 eV (4T1), 2.01 eV (4T2), and 2.731 eV (4E) above the 6A1, ground state were identified. PLE spectroscopy for the 1.268 eV zero-phonon line reveals resonances at 2.3 eV and 2.65 eV. The 1.268 eV ZPL is tentatively attributed to a Fe-complex with a nearby donor.
Raman Analysis of Electron-Phonon Interactions in GaN Films
Authors: Bergman L. , Bremser M. D. , Christman J. A. , King S. W. , Davis R. F. , Nemanich R. J. 
Published: April 2011

ABSTRACTRaman analysis of film quality, phonon-plasmon coupling, and phonon-exciton interaction in GaN films with varying Si doping levels is presented. The films exhibit a small stress component ∽0.1 GPa, calculated from the frequency shift of the E2 mode. No correlation between the stress and the doping concentration was found. No forbidden Raman lines were detected in the spectra, implying high quality oriented films. The Raman lineshape of the E2 mode is a Lorentzian with similar linewidths at room temperature and at 10K indicating a homogeneous lifetime broadening mechanism which is not significantly affected by the change in temperature. The linewidth is also independent of Si concentration. The phonon-plasmon mixed frequency modes were calculated to be at ω−.=86 cm−1 and at ω+=741 cm−1 The modes are not present in the spectra and the only effect of the plasmons is a change of the Al(LO) lineshape. Analysis of the A1(LO) line indicated a uniform spatial doping distribution. A resonance effect was observed for the symmetry-allowed A1(LO) mode at T=10K with sub bandgap excitation light. The resonance interaction is consistent with the free exciton model.
Electronic Structure of Biaxially-Strained Wurtzite Crystals GaN and AlN
Authors: Majewski J. A. , Städele M. , Vogl P. 
Published: April 2011

ABSTRACTWe present first-principles studies of the effect of biaxial (OOOl)-strain on the electronic structure of wurtzite GaN, and A1N. We provide accurate predictions of the valence band splittings as a function of strain, which may facilitate the interpretation of data from strained samples. The conduction and valence band effective mass tensors for A1N and GaN are also presented. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials are in accord with available experimental data. We show that the numerically computed band energies can be excellently represented in terms of a 6-band k · p model. The present calculations are based on the first-principles pseudopotential method within the local-density formalism and include the spin-orbit interactions non-perturbatively.
Theory of Interfaces in Wide-Gap Nitrides
Authors: Nardelli M. Buongiorno , Rapcewicz K. , Briggs E. L. , Bungaro C. , Bernholc J. 
Published: April 2011

ABSTRACTThe results of theoretical studies of the bulk and interface properties of nitrides are presented. As a test the bulk properties, including phonons of GaN at the Γ-point, are calculated and found to be in excellent agreement with the experimental data. At interfaces, the strain effects on the band offsets range from 20% to 40%, depending on the substrate. The AlN/GaN/InN interfaces are all of type I, while the Al0.5Ga0.5N on A1N zinc-blende (001) interface is of type II. Further, an interface similar to those used in the recent blue laser diodes is of type I and does not have any electronically active interface states. The valence band-offset in the (0001) GaN on A1N interface is -0.57 eV and the conduction band-offset is 1.87 eV.
Reactivation of Acceptors and Trapping of Hydrogen in GaN/InGaN Double Heterostructures
Authors: Pearton S. J. , Bendi S. , Jones K. S. , Krishnamoorthy V. , Wilson R. G. , Ren F. , Karlicek R. F. , Stall R. A. 
Published: April 2011

ABSTRACTThe apparent thermal stability of hydrogen passivated Mg acceptors in GaN is a function of the annealing ambient employed, with H2 leading to a reactivation temperature approximately 150°C higher than N2. The dissociation of Mg-H complexes and the loss of hydrogen from GaN are sequential processes, with reactivation occurring at ≤700°C for annealing under N2, while significant concentrations of hydrogen remain in the crystal even at 900°C in implanted samples. The hydrogen is gettered to regions of highest defect density such as the InGaN layer in a GaN/InGaN double heterostructure. The addition of an accelerating potential for 2H+ ions in the plasma did not greatly affect the deuterium profiles.
Nano-Structured Silicon-Based Films with Visible Light Emission Synthesized by Laser Ablation
Authors: Makimura T. , Kunu Y. , Ono N. , Murakami K. 
Published: April 2011

AbstractApplying laser ablation technique, we have synthesized two types of SiO2 films that include nanometer-sized Si particles. One is synthesized by alternative deposition of Si nanoparticles layers and SiO2 layers. The synthesized film exhibits red photoluminescence (PL) with a peak energy below 1.5 eV. The other is synthesized by annealing at 1000°C of SiOx films, which are formed by laser ablation in diluted O2 gas. We find that there is a narrow range of composition for efficient red PL. Based on the experimental results, we tentatively discuss a possible model for the origin of the red PL.
Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Phases
Authors: Nirmal M. , Brus L. E. 
Published: April 2011

AbstractWe review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 Å in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ∼0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.
Photoluminescence Mechanism of Silicon Quantum Dots and Wells
Authors: Kanemitsu Y. , Okamoto S. 
Published: April 2011

AbstractWe discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are very similar to those of Si quantum wells sandwiched by SiO2 layers. The size-dependence of PL properties and resonantly excited PL spectra of SiCb-capped Si dots and wells indicate that excitons are localized near the interface between the crystalline Si core and surface oxide layer, and the strong coupling of electronic and vibrational excitations causes the broad PL spectrum. The exciton localization plays an essential role in the efficient luminescence process in nanoscale Si/SiO2 systems.
Electronic States of Nanocrystalline Carbon
Authors: Lopinski G. P. , Merkulov V. I. , Lannin J. S. 
Published: April 2011

AbstractElectron energy loss spectroscopy (EELS) has been used to investigate the electronic states of isolated, nanocrystalline carbon particles. Small carbon nanocrys-tals were prepared via sputter deposition onto SiO2 substrates, followed by annealing to 700C. The structure and size distribution of the particles have been characterized by Raman scattering, Auger electron spectroscopy and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than lnm. In addition, hydrogen adsorption is found to significantly affect the electronic states of these particles, indicating that both finite size and dangling bond effects modify the properties of small carbon nanocrystallites.
Structure and Vibrational Properties of Isolated Carbon Nanocrystallites
Authors: Merkulov V. I. , Lannin J. S. , Cowley J. M. 
Published: April 2011

AbstractSmall isolated nanocrystallites of carbon have been prepared on amorphous SiO2 and studied by interference enhanced Raman scattering (IERS) and scanning transmission electron microscopy (STEM). A new mode of dark field STEM, using a thin annular detector, has allowed imaging of 1–2 nm graphite-like particles using (002) diffraction. For such small particles, the Raman spectra provide the first evidence for changes in the phonon density of states of a nanocrystalline system.
Raman Spectroscopy of Ge Nanocrystals Grown by Self-Organization Processes
Authors: Stella A. , Bottani C. E. , Cheyssac P. , Kofman R. , Milani P. , Tognini P. 
Published: April 2011

AbstractWe report Raman spectroscopy measurements on Ge nanocrystals with average radii ranging from about 65 Å down to 10 Å (with a size dispersion lower than 20 %).Ge has been deposited by UHV evaporation on an amorphous substrate, kept at such a temperature as to produce the Ge nanodroplets nucleation in the liquid phase.A nanocrystalline size dependence of the Raman spectra has been observed and explained in the framework of a phonon confinement model. We have observed the softening of the TO Raman peak predicted by the theory when the dimensions of the particles are decreased. Moreover the observed inhomogeneous broadening of the Raman lines has been correlated with the size distribution of the particles in the samples.Our results provide a characterization of Ge nanoparticles exhibiting a good crystalline nature, down to about 10 Å, and in conditions of substantial absence of perturbations of the environment.
Determination of Optical Properties of the Micro-Facetted InGaAs Quantum Wells and Quantum Wires Using Magnetophotoluminescence
Authors: Kim Sung-Bock , Ro Jeong-Rae , Lee El-Hang 
Published: April 2011

AbstractWe report optical properties of the micro-facetted InGaAs quantum wells and quantum wires on non-planar substrates employing magnetophotoluminescence (MPL). The InGaAs/GaAs structures were grown by chemical beam epitaxy on V-groove patterned GaAs substrates. In the presence of a magnetic field of 18 T, the diamagnetic shifts of exciton ground states of the (001)-and side-QWLs are ΔE=15.6 and 10.3 meV, respectively. In MPL of the facetted microstructure, we found that the different diamagnetic shifts strongly depend on the magnitude of the effective magnetic field as well as the quantum confinement. From comparing the intensities and full widths at half maximum, we easily found that side-QWLs are of higher quality than (OOl)-QWLs. We also fabricated InGaAs/GaAs quantum wires with a size of about 200 Å × (500–600) Å. By fitting the diamagnetic shifts (ΔΕ = 9.5 meV) of the exciton ground state with the calculated results of a variational method, we estimated that the reduced mass of the exciton is approximately 0.052 me.
The Investigation of Donor and Acceptor States in the Nanoparticles of PbI2 Layered Semiconductor
Authors: Lifshitz E. 
Published: April 2011

AbstractThe nanoparticles of PbI2 layered semiconductor, embedded in Sio2 films, were prepared by the sol-gel method. The low-temperature luminescence of PbI2 nanoparticles consists of a series of exciton lines in the region of ∼2.5eV and additional donor-acceptor recombination luminescent bands at lower energies, centered at 2.44eV (Green) and 2.07 eV (red), respectively. This paper reports the investigation of the nonexcitonic transitions, associated with of the Green and Red bands. The correlation between donor-acceptor recombination emission processes and lattice imperfections was examine, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The results identified the following imperfection sites of both the Green and Red bands: an acceptor site associates with an isotropie Lead vacancy defect, [V]Pb2+ and a donor site, associate with an anisotropie Iodine vacancy, [V°]jodine. However, the results pointed on differences in relaxation processes of these bands. The relaxation processes correspond to the existence of competitive nonradiative recombination, spin-lattice relaxation and thermalization among the spin substrates. Thus, the results suggest that the Red band corresponds to stoichiometric defects at the surface, while the Green band corresponds to the same stoichiometric defects at the interior part of the nanoparticles.
Atomic Layer Etching of Porous Silicon
Authors: Libon I. H. , Voelkmann C. , Petrova-Koch V. , Koch F. 
Published: April 2011

AbstractIn this work we describe the controlled shifting of the PL peak of p+ (10 mΩcm) porous silicon (PoSi) by means of atomic layer etching (ALEP). We hereby study the cluster-size dependence of the PL of this material. By this technique of repeated oxidation by H2O2 and stripping of the oxidized surface layer, we reduced the size of the crystallites layer by layer. In all previous reports the PoSi PL appeared to have a natural lower energy limit of ≈ 1.4 eV. We report for the first time a continuous shift of the PoSi PL peak between 1.01 and 1.20 eV. This observation allows us to draw conclusions for the luminescence mechanism: it proves that geometrical quantum confinement in Si crystallites is responsible for the efficient room-temperature PL in PoSi near the indirect bandgap of c-Si. Together with observations of size-independent PL peaks around 1.6 eV in thermally oxidized samples this result indicates that the PoSi PL cannot be described by one origin alone. Both the existence of molecular centers and the geometrical quantum confinement are valid in their specific range of etching and post-anodic treatment parameters.
Electron Time-of-Flight Measurements in Porous Silicon
Authors: Rao Prasanna , Schiff E. A. , Tsybeskov L. , Fauchet P. M. 
Published: April 2011

AbstractTransient photocurrent measurements are reported in an electroluminescent porous silicon diode. Electron drift mobilities are obtained from the data as a function of temperature. Electron transport is dispersive, with a typical dispersion parameter α≈ 0.5. The range of mobilities is 10−5 − 10−4 cm2Vs between 225 K amd 400 K. This temperature-dependence is much less than expected for multiple-trapping models for dispersion, and suggests that a fractal structure causes the dispersion and the small mobilities.
Nonlinear Electrical Functions of Porous Silicon Light-Emitting Diodes
Authors: Ueno K. , Ozaki T. , Koyama H. , Koshida N. 
Published: April 2011

AbstractSome nonlinear electrical characteristics in electroluminescent porous silicon (PS) diodes with a relatively thin PS layer (0.5–5 μm thick) are described. The experimental PS diodes were composed of a semitransparent Au film, a PS layer, p- or n-type Si substrate, and an ohmic back contact. The PS layers were prepared by anodizing Si wafers in an ethanoic HF solution. In some cases, the PS layers were treated by rapid thermal oxidization (RTO) process. When the bias voltage is applied, the PS diodes show the electrical behavior like the metal-insulator-semiconductor (MIS) diodes. The negative-resistance characteristics and memory effect are also observed. These results indicate that the quantum-structured nature of the PS layer appears not only in the optical properties but also in the electrical properties.