Temperature-dependent photoluminescence of GaN grown on beta-Si3N4/Si (111) by plasma-assisted MBE

Photoluminescence (PL) of high quality GaN epitaxial layer grown on beta-Si3N4/Si (1 1 1) substrate using nitridation-annealing-nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5-300 K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an `S-like' shape of free exciton (FX) emission peaks. Distant shallow donor-acceptor pair (DAP) line peak at approximately 3.285 eV was also observed at 5 K, followed by LO replica sidebands separated by 91 meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be similar to 27.8 +/- 0.7 meV from the temperature-dependent PL studies. Low carrier concentrations were observed similar to 4.5 +/- 2 x 10(17) Cm-3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2 eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy. (C) 2010 Elsevier B.V. All rights reserved.

Electrical properties of ferroelectric YMnO3 films deposited on n-type Si(111) substrates

YMnO3 thin films were grown on an n-type Si substrate by nebulized spray pyrolysis in the metal-ferroelectric-semiconductor (MFS) configuration. The capacitance-voltage characteristics of the film in the MFS structure exhibit hysteretic behaviour consistent with the polarization charge switching direction, with the memory window decreasing with increase in temperature. The density of the interface states decreases with increasing annealing temperature. Mapping of the silicon energy band gap with the interface states has been carried out. The leakage current, measured in the accumulation region, is lower in well-crystallized thin films and obeys a space-charge limited conduction mechanism. The calculated activation energy from the dc leakage current characteristics of the Arrhenius plot reveals that the activation energy corresponds to oxygen vacancy motion.

Dielectric properties of (110) oriented PbZrO3 and La-modified PbZrO3 thin films grown by sol-gel process on Pt(111)/Ti/SiO2/Si substrate

Highly (110) preferred orientated antiferroelectric PbZrO3 (PZ) and La-modified PZ thin films have been fabricated on Pt/Ti/SiO2/Si substrates using sol-gel process. Dielectric properties, electric field induced ferroelectric polarization, and the temperature dependence of the dielectric response have been explored as a function of composition. The Tc has been observed to decrease by ∼ 17 °C per 1 mol % of La doping. Double hysteresis loops were seen with zero remnant polarization and with coercive fields in between 176 and 193 kV/cm at 80 °C for antiferroelectric to ferroelectric phase transformation. These slim loops have been explained by the high orientation of the films along the polar direction of the antiparallel dipoles of a tetragonal primitive cell and by the strong electrostatic interaction between La ions and oxygen ions in an ABO3 perovskite unit cell. High quality films exhibited very low loss factor less than 0.015 at room temperature and pure PZ; 1 and 2 mol % La doped PZs have shown the room temperature dielectric constant of 135, 219, and 142 at the frequency of 10 kHz. The passive layer effects in these films have been explained by Curie constants and Curie temperatures. The ac conductivity and the corresponding Arrhenius plots have been shown and explained in terms of doping effect and electrode resistance.

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (111) by plasma-assisted MBE

The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics of a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements. (C) 2012 Elsevier Ltd. All rights reserved.

Valence band offset at GaN/beta-Si3N4 and beta-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure beta-Si3N4 (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using beta-Si3N4 layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/beta-Si3N4/ Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the beta-Si3N4 /Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 +/- 0.05 eV below that of beta-Si3N4 and a type-II heterojunction. The conduction band offset was deduced to be similar to 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/ beta-Si3N4 interface formation. (c) 2011 Elsevier B.V. All rights reserved.

Selenium adsorption on Au(111) and Ag(111) surfaces: adsorbed selenium and selenide films

Results of a high resolution photoemission and electrochemistry study of Se adsorption Au(111) and Ag(111) surfaces performed by immersion of pristine samples into an aqeuous solution of Na2Se are presented. Cyclic voltammetry on Au shows formation of selenium adsorbed species and the structures observed in reductive desorption are to the atomic and polymeric species observed in XPS. In the case of Au(111) XPS spectra in the Se(3d) region indeed show two main features attributed to Se chemisorbed atomically and polymeric Se-8 features.' Smaller structures due to other types of Se conformations were also observed. The Au(4f) peak line, shape does not show core level, shifts: indicative of Au selenide formation the case of silver, XPS spectra for the Ag(3d) show a broadening of the peak and a deconvolution into Ag-B bulk like Ag-Se components shows that the Ag-Se is located at a lower binding energy, an effect similar to oxidation and sulfidation of Ag. The Se(3d) XPS spectrum is found to be substantially different from the Au case and dominated by atomic type Se due to the selenide, though a smaller intensity Se structure at an energy similar to the Se-8 structure for Au is also observed. Changes in the valence band region. related to Se adsorption are reported.

The Role of Weak Interactions in the Phase Transition and Distinct Mechanical Behavior of Two Structurally Similar Caffeine Co-crystal Polymorphs Studied by Nanoindentation

Although weak interactions, such as C-H center dot center dot center dot O and pi-stacking, are generally considered to be insignificant, it is their reorganization that holds the key for many a solid-state phenomenon, such as phase transitions, plastic deformation, elastic flexibility, and mechanochromic luminescence in solid-state fluorophores. Despite this, the role of weak interactions in these dynamic phenomena is poorly understood. In this study, we investigate two co-crystal polymorphs of caffeine:4-chloro-3-nitrobenzoic acid, which have close structural similarity (2D layered structures), but surprisingly show distinct mechanical behavior. Form I is brittle, but shows shear-induced phase instability and, upon grinding, converts to Form II, which is soft and plastically shearable. This observation is in contrast to those reported in earlier studies on aspirin, wherein the metastable drug forms are softer and convert to stable and harder forms upon stressing To establish a molecular level understanding, have investigated the two co-crystal polymorphs I and II by single crystal X-ray diffraction, nanoindentation to quantify mechanical properties, and theoretical calculations. The lower hardness (from nanoindentation) and smooth potential surfaces (from theoretical studies) for shearing of layers in Form II allowed us to rationalize the role of stronger intralayer (sp(2))C-H center dot center dot center dot O and nonspecific interlayer pi-stacking interactions in the structure of II. Although the Form I also possesses the same type of interactions, its strength is clearly opposite, that is, weaker intralayer (sp(3))C-H center dot center dot center dot O and specific interlayer pi-stacking interactions. Hence, Form I is harder than Form IL Theoretical calculations and indentation on (111) of Form I suggested the low resistance of this face to mechanical stress; thus, Form I converts to II upon mechanical action. Hence, our approach demonstrates the usefulness of multiple techniques for establishing the role of weak noncovalent interactions in solid-state dynamic phenomena, such as stress induced phase transformation, and hence is important in the context of solid-state pharmaceutical chemistry and crystal engineering.

An early in-situ stress signature of the AlN-Si pre-growth interface for successful integration of nitrides with (111) Si

The integration of Metal Organic Chemical Vapor Deposition (MOCVD) grown group III-A nitride device stacks on Si (111) substrates is critically dependent on the quality of the first AlN buffer layer grown. A Si surface that is both oxide-free and smooth is a primary requirement for nucleating such layers. A single parameter, the AlN layer growth stress, is shown to be an early (within 50 nm), clear (<0.5 GPa versus > 1GPa), and fail-safe indicator of the pre-growth surface, and the AlN quality required for successful epitaxy. Grain coalescence model for stress generation is used to correlate growth stress, the AlN-Si interface, and crystal quality. (C) 2013 AIP Publishing LLC.

Evidence of Low Field Magnetoimpedance Effect in (111) Oriented Fe3O4 Thin Films

Thin film of Fe3O4 is grown on (100) oriented GaAs substrate using pulsed laser deposition (PLD). The films were found to be (111) oriented. They were characterized morphologically as well as magnetically. They show Verwey transition at 120 K with the coercivity of 220 Oe at room temperature. These films were found to show magnetoimpedance effect with the ac magnetoresistance (MRac) of the order of similar to -6% at 10 MHz, at room temperature. The MRac was found to increase in low field-low frequency regime and decrease in low field high frequency regime.

Double Gaussian distribution of barrier height observed in densely packed GaN nanorods over Si (111) heterostructures

GaN nanorods were grown by plasma assisted molecular beam epitaxy on intrinsic Si (111) substrates which were characterized by powder X-ray diffraction, field emission scanning electron microscopy, and photoluminescence. The current-voltage characteristics of the GaN nanorods on Si (111) heterojunction were obtained from 138 to 493K which showed the inverted rectification behavior. The I-V characteristics were analyzed in terms of thermionic emission model. The temperature variation of the apparent barrier height and ideality factor along with the non-linearity of the activation energy plot indicated the presence of lateral inhomogeneities in the barrier height. The observed two temperature regimes in Richardson's plot could be well explained by assuming two separate Gaussian distribution of the barrier heights. (C) 2014 AIP Publishing LLC.

A modified approach to numerical solution of Fredholm integral equations of the second kind

A modified approach to obtain approximate numerical solutions of Fredholin integral equations of the second kind is presented. The error bound is explained by the aid of several illustrative examples. In each example, the approximate solution is compared with the exact solution, wherever possible, and an excellent agreement is observed. In addition, the error bound in each example is compared with the one obtained by the Nystrom method. It is found that the error bound of the present method is smaller than the ones obtained by the Nystrom method. Further, the present method is successfully applied to derive the solution of an integral equation arising in a special Dirichlet problem. (C) 2015 Elsevier Inc. All rights reserved.

Simple crystallizations of 4-manifolds

Minimal crystallizations of simply connected PL 4-manifolds are very natural objects. Many of their topological features are reflected in their combinatorial structure which, in addition, is preserved under the connected sum operation. We present a minimal crystallization of the standard PL K3 surface. In combination with known results this yields minimal crystallizations of all simply connected PL 4-manifolds of ``standard'' type, that is, all connected sums of CP2, S-2 x S-2, and the K3 surface. In particular, we obtain minimal crystallizations of a pair of homeomorphic but non-PL-homeomorphic 4-manifolds. In addition, we give an elementary proof that the minimal 8-vertex crystallization of CP2 is unique and its associated pseudotriangulation is related to the 9-vertex combinatorial triangulation of CP2 by the minimum of four edge contractions.

Curvature Management in Buffer Layer for Device Quality GaN Growth on Si (111)

An efficient buffer layer scheme has been designed to address the issue of curvature management during metalorganic chemical vapour deposition growth of GaN on Si (111) substrate. This is necessary to prevent cracking of the grown layer during post-growth cooling down from growth temperature to room temperature and to achieve an allowable bow (<40 m) in the wafer for carrying out lithographic processes. To meet both these ends simultaneously, the stress evolution in the buffer layers was observed carefully. The reduction in precursor flow during the buffer layer growth provided better control over curvature evolution in the growing buffer layers. This has enabled the growth of a suitable high electron mobility transistor (HEMT) stack on 2'' Si (111) substrate of 300 m thickness with a bow as low as 11.4 m, having a two-dimensional electron gas (2DEG) of mobility, carrier concentration, and sheet resistance values 1510 cm(2)/V-s, 0.96 x 10(13)/cm(2), and 444 /, respectively. Another variation of similar technique resulted in a bow of 23.4 m with 2DEG mobility, carrier concentration, and sheet resistance values 1960 cm(2)/V-s, 0.98 x 10(13)/cm(2), and 325 /, respectively.

微碳深冲钢板的非｛111｝织构特征及其对r,△r值的影响

From St15 micro-carbon deep drawing steel sheets, two sets of samples with (r) over bar variant and Deltar constant, and (r) over bar constant and Deltar variant, were selected to carry out texture measurement and ODF analysis. A description of the texture characteristics and an investigation on the effect of the main textures on the (r) over bar and Deltar values were given. The results show that in the tested steel sheets no desired gamma < 111 > parallel to ND orientation line appears but gamma' orientation line located at <()over bar>=0-90 degrees, theta =19 degrees and phi =45 degrees, and L orientation line located around gamma < 111 > parallel to ND orientation line which spirally rotates from Psi =0 degrees, theta =54.7 degrees and phi =62.7 degrees to Psi =90 degrees, theta =40 degrees and phi =45 degrees occur. Among them, the L orientation line has a main influence on the (r) over bar value and the stronger the texture density, the higher the (r) over bar value is, but is somewhat detrimental to the Deltar value; at the same time, the gamma' orientation line has a major effect on the Deltar value in an opposite way, but is somewhat deteriorative to the (r) over bar value. A strong L orientation line superposed by a relatively strong gamma' orientation line may produce fine (r) over bar and Deltar values.