bluuebunny/crossref_metadata_2025_split huggingface dataset:
Retardation in the Oxidation Rate of Nanocrystalline Silicon Quantum Dots
Authors: Omachi J. , Nakamura R. , Nishiguchi K. , Oda S. 
Published: April 2011

AbstractUsing very-high-frequency (VHF) plasma decomposition of SiH4 and pulsed gas technique, we have successfully prepared nanocrystalline silicon (nc-Si) quantum dots having average diameter of 8 nm and dispersion of 1 nm. The role of natural oxide is very important. It controls the size of nc-Si dots. Of particular interest is that the oxidation of these dots can be self limited, due to the stress induced near Si/oxide interface, which would allow further reduction of size and improvement in dispersion. This paper deals with the systematic study of oxidation process of nc-Si dots. Nc-Si dots formed in an Ar plasma with SiH4 gas pulses are deposited onto a Pt mesh The dots are then oxidized at 750, 800 and 850°C from 20 minutes to 15 hours. The dimensions of the residual nc-Si and the grown oxide are measured directly from the TEM micrographs and analyzed. For comparison, field oxide is investigated using ellipsometry. Retardation in the oxidation rate of nc-Si is observed. The mechanism of the reduction of oxidation rate in nc-Si is discussed taking into account the effect of stress.
Identification of the luminescence center of porous silicon under low temperature thermal oxidation
Authors: Goda Kazuo , Tanaka Ryuji , Honda Yukako , Inoue Kazuhisa , Ohno Hideki 
Published: April 2011

AbstractThe relationship between chemical states and optical properties for porous silicon (PS) under initial oxidation by heating at low temperatures (150, 200, 250 °C) in air was investigated using an infrared (IR) spectroscopy and photoluminescence (PL) measurements. The measurements in the IR region show that the IR absorption peaks for the Si-H stretching band (2050-2150 cm−1) change with appearance of the Si-O-Si-H stretching band (2100-2300 cm−1) when the thermal oxidation time is increased. On the other hand, the peak in the PL spectrum shows a blue shift from 820 nm to 710 nm with the oxidation time. The observed blue shift of the PL spectrum is due to the decrease in the initial PL peak intensity at 820 nm and the increase at 710 nm. Moreover, the peak intensities in the PL spectra at 820 nm and 710 nm have clear relationship to the amounts of the Si-H bonds and Si-O-Si-H bonds, respectively, as a function of the oxidation time.These results indicate that luminescence center (LC) for as-prepared PS is ascribed to complexes including Si-H bonds. Also the LC for oxidized PS under the oxidation process at low temperatures is ascribed to complexes including Si-O-Si-H bonds covering the inner surface of PS.
Fabrication of Silicon Nano-Crystal Dots on SiO2 by Ultrahigh-Vacuum Chemical Vapor Deposition
Authors: Oishi Masato , Kawashima Takayuki , Mashiro Supika , Sakai Junro , Madhukar Sucharita , Nguyen Bich-Yen , White Bruce E. 
Published: April 2011

AbstractWe report a process for Silicon (Si) nano-crystal dots fabrication using a cold-wall Ultrahigh-Vacuum Chemical Vapor Deposition (UHV-CVD) system. Si2H6 gas was used as the pre-curser and irradiated upon SiO2 film on Si wafer to form Si nano-crystal dots.In our system, nucleation, growth, and coalescence phases of nano-crystal dots on SiO2 were found to be related with the optical pyro-meter's read-out curve.At first, the optimum gas irradiation time which gives the highest dot density without coalescence was decided for every gas irradiation condition by using an optical pyro-meter. Then, the dependence of optimum gas irradiation time, dot diameter and dot density on gas flow rate and wafer temperature were investigated. A decrease of wafer temperature or an increase of gas flow rate during the nucleation and growth phase results in a decrease of dot diameter and an increase of dot density. The optimum gas irradiation time was prolonged by decreasing wafer temperature or gas flow rate.Finally, a reproducible process of forming non-coalesced, small size, and high-density Si nano-crystal dots of about 5.5nm in diameter with density of 1x1012 dots/cm2 were obtained. Typical process time to get such dot formation characteristics was about 10 minutes, which were long enough for ensuring reproducibility.
Stimulated Blue Emission and Second Harmonic Generation From Films of Ultrasmall Si Nanoparticles
Authors: NAYFEH Munir H. , Therrien Joel , Belomoin Gennadiy , Akcakir Osman , Barry Nicolas , Gratton Enrico 
Published: April 2011

AbstractWe describe a procedure for dispersion bulk Si into ultrasmall nanoparticles, much smaller than what is available today, with high throughput and excellent size definition and control. The quantum dots are ∼ 1 nm in diameter and contain 29 Si atoms[1-2]. We show most unexpected and totally surprising phenomenon. Unlike bulk Si, an optically inert indirect gap material, the particles are extremely active optically in the blue, exceeding the activity of fluorescein, such that single particles are readily detected and imaged, using two-photon near- infrared femto second excitation. This comes nearly a decade after Canham first surprised the scientific community by reporting red luminescence from anodized Si[3]. In addition to being ultrabright, the particles have other useful properties that are unlike those of bulk including being a source of stimulated emission and collimated beam emission[4-5], and harmonic generation[6]. Single electron charging and the confinment energy spacing, both are much larger than the thermal energy[7]. They can be formed into colloids, crystals, films, and collimated beams for applications in the electronics, optoelectronics and biomedical industries. The synthesis utilizes straightforward, low-cost, environmentally benign present-day commercial technologies and material: abundant Si, common chemicals and electricity. The procedure is readily amenable to large-volume production, easy recovery, and controlled delivery.Using density functional with exchange correlation, configuration interaction and Monte Carlo theory, we constructed a structural propotype for 29 atoms (magic number for the Td symmetry and spherical shape), giving a size, an energy gap and an absorption spectrum that agree with experiment[8]. In the prototype, twenty four atoms form a network of reconstructed Si-Si species on the surface, with five atoms constituting a tetrahedral core.The results suggest that the surface reconstructed network is the source of the new properties. In these particles, the elasticity drops, atoms become amenable to large displacement, and are subjected to unballanced atomic forces, conditions under which novel molecular structure or configuration, distinct from bulk, and which otherwise does not exist may form. This structure may constitute a transition between the bulk and atomistic phases.
Structural Evolution of Ni/Au Contact on GaN(0001)
Authors: Kim Chong Cook , Kim Jong Kyu , Lee Jong-Lam , Yi Min-Su , Kim Jin-Woo , Noh Do Young , Hwu Yeukuang , Ruterana Pierre , Je Jung Ho 
Published: April 2011

ABSTRACTWe investigated the structural behavior of the Ni/Au contact on GaN(000l) during annealing in N2, using in-situ x-ray diffraction, anomalous x-ray scattering, and high resolution electron microscopy. Thermally activated atomic mobility caused the two metal atoms, Au and Ni, to interdiffuse during annealing and form solid solutions. At temperature higher than 500°C, GaN decomposition and reactions occurred mostly along GaN dislocations. By decomposed nitrogen reacted with Ni, interestingly, epitaxial Ni4N phase was formed. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[0 1 1]//GaN[0211] (M= Ni4N, Au, and Ni).
Chemical ordering in AlGaN layers grown by MOCVD
Authors: Ruterana P. , Omnes F. 
Published: April 2011

ABSTRACTOne of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller (∼ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time.In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 - 15%, we have found that three processes can operate:1. Ordering into AlN/GaN as two simple hexagonal sublattices.2. The 3:1 ordering which has been recently reported to occur in InGaN.3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5:1 ordering.
Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication
Authors: Visconti P. , Reshchikov M. A. , Jones K. M. , Yun F. , Wang D. F. , Cingolani R. , Morkoç H. , Litton C. W. , Molnar R. J. 
Published: April 2011

ABSTRACTPhotoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (∼1×10 9cm−2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ∼1[.proportional]m high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smooth free-feature etched surface. Some highly resistive samples that cannot be etched under normal conditions because the band bending is too small to confine the holes to the surface for them to participate in the PEC process, can be etched with the application of a voltage to the sample. In this case, the etch rate depends on both the polarity and the magnitude of the voltage applied. In an MBE-grown sample with an AlN/GaN superstructure inside, we report on high selectivity between AlN and GaN (AlN is an etch stop); the selectivity is due to the etching mechanism of the PEC process.
Atomic Force Microscopy Study of GaN Grown on Al2O3(0001) by LP-MOVPE
Authors: Xu K. , Lim D. H. , Liu B. L. , Du X. L. , Yu G. H. , Jia A. W. , Yoshikawa A. , Takahashi K. 
Published: April 2011

ABSTRACTThere is currently a high degree of interest in understanding the diverse mechanisms that determine the growth morphology of epitaxial films. To understand and control over these mechanisms, it is essential to know how the growth mechanisms are correlated with morphologies. In GaN MOVPE processes, growth temperature has a remarkable effect on the film morphologies and properties. In present work, the temperature dependency of surface morphology of GaN epilayers grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) was studied using atomic force microscopy. It was demonstrated that dislocations strongly influence the growth mechanisms and the morphology of the films. Three growth modes were identified at different growth temperature ranged from 1030°C to 1100°C, which are, (1) spiral growth dominated by dislocation with screw type, (2) monolayer step flow, and (3) nucleation-assisted step flow growth. The significant effect of dislocation on the surface morphologies and growth mechanisms may be attributed to the high dislocation density and higher activation energy for a step overcoming the dislocation pinning in GaN. Because few dislocations will be introduced under monolayer step flow growth mode, we can infer the useful information about GaN initial growth.
A Comparison of Magnesium and Beryllium Acceptors in GaN Grown by rf-Plasma Assisted Molecular Beam Epitaxy
Authors: Ptak A.J. , Myers T.H. , Wang Lijun , Giles N.C. , Moldovan M. , Da Cunha C.R. , Hornak L.A. , Tian C. , Hockett R. A. , Mitha S. , Van Lierde P. 
Published: April 2011

ABSTRACTStep-doped structures of both magnesium and beryllium were grown in GaN and analyzed using secondary ion mass spectrometry. Dopant incorporation was studied as a function of substrate temperature and dopant flux for Ga-polarity and N-polarity GaN. Incorporation is different for each polarity, with Mg incorporating by up to a factor of 20 times more (30 times more with atomic hydrogen) on the Ga-face, while Be incorporates more readily on the N-face. The effect of atomic hydrogen on the incorporation kinetics of both Mg and Be is also discussed. Mg and Be both undergo surface segregation during growth. Photoluminescence measurements suggest that Be is a p-type dopant with an optical activation energy of approximately 100 meV.
Growth of quaternary AlInGaN/GaN Heterostructures by Plasma Induced Molecular Beam Epitaxy with high In Concentration
Authors: Lima A. P. , Miskys C. R. , Görgens L. , Ambacher O. , Wenzel A. , Rauschenbach B. , Stutzmann M. 
Published: April 2011

ABSTRACTGrowth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford Backscattering Spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775 to 665°C. X-Ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers.
Phase Separation in Multiple ZnO /Cubic- MgxZn1−xO Superlattice Heterostructures Observed Via High Resolution Transmission Electron Microscopy
Authors: Kvit A. , Dusher G. , Sharma A. K. , Jin C. , Narayan J. , Muth J. , Teng C.W 
Published: April 2011

ABSTRACTWe have synthesized ZnMgO alloy of wurtzite (Mg content equals to 0.0=0.34) and cubic (Zn content equals to 0.0- 0.18) phases using nonequilibrium pulsed laser deposition method. Epitaxial films of ZnMgO wurtzite structure have been grown on (0001) sapphire substrates. Using JEOL-2010 field-emission transmission electron microscope equipped with STEM and Gatan image filter, we can perform atomic structure, STEM-Z, electron energy loss spectroscopy and imaging simultaneously. Such studies on the ZnO/MgZnO superlattices provide first direct evidence of phase-separation in the range 3 nm.
Microwave AlxGa1−xN/GaN Power HEMT's
Authors: Eastman Lester F. 
Published: April 2011

ABSTRACTGaN in its hexagonal Wurtzite crystal has 3.4 eV bandgap and is capable of withstanding over 3 MV/cm electric field intensity. It is grown on either sapphire or SiC and the lattice mismatches cause threading dislocations. If it is grown along the c-axis on its Ga face, a thin pseudomorphic AlxGa1−xN barrier layer grown on top causes a substantial electrical polarization, which induces a substantial 2DEG in the GaN [1]. With x =.3 this 2 DEG is about 1 × 1013/cm2 without any doping impurities. The ∼ 109/cm2 threading dislocations accept electrons from the low density ambient donors, yielding very low net electron densities (< 1014/cm3) in the GaN buffer layer used as the HEMT channel [2]. The 2DEG electron mobility can be 1500-1700 cm2/V-s with ≥ 1 × 1013/cm2 electron sheet density. This 2DEG sheet density charge is less than the electrical polarization charge of 1.7 × 1013/cm2 for Al.3Ga.7N/GaN, due to some surface depletion and due to the fact that each dislocation traps ≥ 2 × 103 electrons. The design, processing, and measured performance of such undoped, polarization-induced HEMT's are presented below.
Surface Morphology of SiGe Epitaxial Layers Grown on Uniquely Oriented Si Substrates
Authors: Ware Morgan E. , Nemanich Robert J. 
Published: April 2011

AbstractThe 4% lattice mismatch between Si and Ge creates strain in epitaxial layers of SiGe alloys on Si, and this strain can manifest itself in the morphological structure of the surface of the epitaxial layer. This study explores the relationship of the evolution of the surface morphology of SiGe layers grown on a range of Si surface orientations. We have grown thin, strained and thick, relaxed layers of Si0.7Ge0.3 by solid source molecular beam epitaxy on substrates with surface normals rotated from [001] towards [111] by angles of θ = (0, 2, 4, 10, 22) degrees. The surface morphology was investigated by atomic force microscopy, which showed considerable ordering of surface features on relaxed samples. These features evolve from hut-like structures at 0 degrees to large mesa-like structures separated by pits and crevices at 22 degrees. The organization of these features is also shown to vary with the substrate orientation. Each surface has characteristic directions along which features are aligned, and these directions vary continuously with the angle of rotation of the substrate. Transmission electron microscopy confirmed that misfit dislocations had formed along those same directions. The state of relaxation of each layer is quantified by Raman spectroscopy in order to make a direct correlation between residual strain and surface morphology.
Optical properties of self-organised SSMBE and GSMBE Ge nanostructures grown on SiGe template layers on Si (118)
Authors: Brémond G. , Ferrandis P. , Souifi A. , Ronda A. , Berbezier I. 
Published: April 2011

AbstractThis work reports on the photoluminescence properties of self-organized fully strained Ge dots fabricated using two different growth techniques: gas source molecular beam epitaxy (GS-MBE) and solid source molecular beam epitaxy (SS-MBE). Variable temperature photoluminescence (PL) measurements were carried out on Si/(n)Ge/SiGe/Si structures (n varying from 1 to 7 monolayers) consisting in double layer structures (Ge(n)/Si1-xGex) deposited on (118) oriented Si substrates. The process used consists in realising in a first step a Si1-xGex template layer with a “self-patterned” morphology. Such patterning, based on periodic morphological modulation of the surface is used to confine and organise the Ge dots in a second deposition step. Similar series of experiments with various growth temperatures, Ge coverage levels and Si1-xGex concentrations (x) were done by both GS-MBE and SS- MBE. The PL from the 2D wetting layer in the case of n = 3 ML has been found to be more intense in GS-MBE thanks to the passivating role of the hydrogen atoms. The 2D to 3D growth transition is accompanied by the occurrence of a red-shifted broad PL band (L) attributed to the Ge dots. While broadened luminescence is obtained from dots directly deposited on the Si substrate, a narrower band is obtained from dots deposited on the template layer. Moreover the red-shift of the (L) band observed in the latter case is attributed to higher Ge concentration in the dots. On the other hand, there is no effect of the hydrogen on the formation of the islands which show similar optical and structural properties in both growth techniques. The main difference between GS-MBE and SS-MBE concerns a low energy shift in the PL of the SSMBE Ge dots we interpret as due to size, dispersion and Ge concentration or strain effects
Strain dependence and magnetic anisotropy in chromium dioxide thin films
Authors: Spinu L. , Srikanth H. , Li X. W. , Gupta A. , Xiao Gang 
Published: April 2011

AbstractField-dependent transverse susceptibility measurements based on a novel tunnel-diode oscillator (TDO) method were used to probe the magnetic anisotropy and switching in epitaxially grown CrO2 films on TiO2 substrates. The static field was applied in three different orientations (0°, 45° and 90° ) with respect to the magnetic easy axis. Singular peaks in the transverse susceptibility were observed that are associated with the anisotropy and switching fields in CrO2. Theoretical calculations based on a coherent rotation model display remarkable agreement with the experimental data. A new peak emerges at low temperatures and is identified to be a consequence of temperature-dependent magnetoelastic contributions resulting from the in-plane tensile strain in the films due to varying coefficients of thermal expansion in the film and the substrate.
Pulsed laser deposited nanocrystalline ZnO thin films
Authors: Thareja R. K. , Mitra A. , Ganesan V. , Gupta A. , Sahoo P. K. , Kulkarni V.N. 
Published: April 2011

AbstractWe report on random laser emission in pulsed laser deposited nanocrystalline ZnO thin films. The deposition was done on silicon and glass substrates in ambient oxygen pressure ranging from 10 mTorr to 1 Torr at room temperature. The deposited films were characterized using XRD, RBS, PL and AFM. The films grown at pressures less than 300 mTorr are found to be preferentially oriented along (002) plane. Morphology of the films deposited at different background pressures showed that the films deposited at lower pressures are significantly smoother than those at higher back ground pressure. PL intensity depends on the stoichiometry of the films and hence on oxygen pressure. Laser action was observed on optically pumping the films with 355 nm radiation. At excitation intensity above threshold, very narrow peaks are observed in the emission spectrum. The dependence of the laser emission on the size of nanocrystallites at different pressures of the ambient gas is presented.
Mesoscopic Patterning Induced by Co-deposition of C60 and Ni on the MgO(100) Single Crystal
Authors: Vacik Jiri , Naramoto Hiroshi , Narumi Kazumasa , Yamamoto Shunya , Myashita Kiyoshi 
Published: April 2011

AbstractWe report on an unusual phenomenon of pattern formation in the mixture of Ni and C60 codeposited on the single crystal of MgO(100). Under certain deposition kinetics an array of periodic stripes was formed. The stripes consist of sub-half-micrometer Ni droplets (embedded in the complex Ni-C base) and encompassed with a polymerized C60 rind. The stripes are bound to the thin double-layer platform consisting of the epitaxial Ni and amorphous C (a-C) layers. During co-deposition the C60 molecules partly disintegrate and transform towards the a-C structures. Accumulation of a-C (immiscible with Ni) may induce thermodynamic instability in the system and trigger the process of spontaneous partitioning. As a principal mechanism of the stripe formation a sequential drift and co-ordinate release of the incited instability was proposed.