Zn incorporation and band gap shrinkage in p-type GaAs

Dimethylzine (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium and arsine (AsH3) as source materials, The hole carrier concentrations and zinc (Zn) incorporation efficiency are studied by using the Hall effect, electrochemical capacitance voltage profiler and photoluminescence (PL) spectroscopy, The influence of growth parameters such as DMZn mole fraction, growth temperature, and AsH, mole fraction on the Zn incorporation have been studied. The hole concentration increases with increasing DMZn and AsH3 mole fraction and decreases with increasing growth temperature. This can be explained by vacancy control model. The PL experiments were carried out as a function of hole concentration (10(17)-1.5 x 10(20) cm(-3)). The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(p)(eV) = 1.15 x 10(-8)p(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Zn doped GaAs as a function of hole concentration. The value of Delta E-g(eV) = -2.75 x 10(-8)p(1/3), indicates a significant band gap shrinkage at high doping levels, These relations are considered to provide a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing AsH3 mole fraction and the main peak is shifted to a lower energy side. This can be explained also by the vacancy control model. As the hole concentration is increased above 3.8 x 10(18) cm(-3), a shoulder peak separated from the main peak was observed in the PL spectra and disappears at higher concentrations. (C) 1997 American Institute of Physics.

Porous wide band gap BiNbO4 ceramic nanopowder synthesised by low temperature solution-based method for gas sensing applications

In this study, we report the gas sensing behavior of BiNbO4 nanopowder prepared by a low temperature simple solution-based method. Before the sensing behaviour study, the as-synthesized nanopowder was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-diffuse reflectance spectroscopy, impedance analysis, and surface area measurement. The NH3 sensing behavior of BiNbO4 was then studied by temperature modulation (50-350 degrees C) as well as concentration modulation (20-140 ppm). At the optimum operating temperature of 325 degrees C, the sensitivity was measured to be 90%. The cross-sensitivity of as-synthesized BiNbO4 sensor was also investigated by assessing the sensing behavior toward other gases such as hydrogen sulphide (H2S), ethanol (C2H5OH), and liquid petroleum gas (LPG). Finally, selectivity of the sensing material toward NH3 was characterized by observing the sensor response with gas concentrations in the range 20-140 ppm. The response and recovery time for NH3 sensing at 120 ppm were about 16 s and about 17 s, respectively.

Measurement of the band bending and surface dipole at chemically functionalized Si(111)/vacuum interfaces

The core-level energy shifts observed using X-ray photoelectron spectroscopy (XPS) have been used to determine the band bending at Si(111) surfaces terminated with Si-Br, Si-H, and Si-CH₃ groups, respectively. The surface termination influenced the band bending, with the Si 2p_3/2 binding energy affected more by the surface chemistry than by the dopant type. The highest binding energies were measured on Si(111)-Br (whose Fermi level was positioned near the conduction band at the surface), followed by Si(111)-H, followed by Si(111)-CH₃ (whose Fermi level was positioned near mid-gap at the surface). Si(111)-CH₃ surfaces exposed to Br₂(g) yielded the lowest binding energies, with the Fermi level positioned between mid-gap and the valence band. The Fermi level position of Br₂(g)-exposed Si(111)-CH₃ was consistent with the presence of negatively charged bromine-containing ions on such surfaces. The binding energies of all of the species detected on the surface (C, O, Br) shifted with the band bending, illustrating the importance of isolating the effects of band bending when measuring chemical shifts on semiconductor surfaces. The influence of band bending was confirmed by surface photovoltage (SPV) measurements, which showed that the core levels shifted toward their flat-band values upon illumination. Where applicable, the contribution from the X-ray source to the SPV was isolated and quantified. Work functions were measured by ultraviolet photoelectron spectroscopy (UPS), allowing for calculation of the sign and magnitude of the surface dipole in such systems. The values of the surface dipoles were in good agreement with previous measurements as well as with electronegativity considerations. The binding energies of the adventitious carbon signals were affected by band bending as well as by the surface dipole. A model of band bending in which charged surface states are located exterior to the surface dipole is consistent with the XPS and UPS behavior of the chemically functionalized Si(111) surfaces investigated herein.

Measurement of filling-factor-dependent magnetophonon resonances in graphene using Raman spectroscopy

We perform polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T. This reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E2g phonons. This is explained with a model of Raman scattering taking into account the effects of spatially inhomogeneous carrier densities and strain. Random fluctuations of strain-induced pseudomagnetic fields lead to increased scattering intensity inside the anticrossing gap, consistent with the experiments. © 2013 American Physical Society.

Gap states and microstructure of microcrystalline silicon thin films

The density of states (DOS) above Fermi level of hydrogenated microcrystalline silicon (mu c-Si H) films is correlated to the material microstructure. We use Raman scattering and infrared absorption spectra to characterize the structure of the films made with different hydrogen dilution ratios. The DOS of the films is examined by modulated photocurrent measurement. The results have been accounted for in the framework of a three-phase model comprised of amorphous and crystalline components, with the grain boundary as the third phase. We observed that the DOS increases monotonically as the grain boundary volume fractions f(gb) is increased, which indicates a positive correlation between the DOS and the grain boundary volume fraction.

Measurement of polar C-plane and nonpolar A-plane InN/ZnO heterojunctions band offsets by x-ray photoelectron spectroscopy

The valence band offsets of the wurtzite polar C-plane and nonpolar A-plane InN/ZnO heterojunctions are directly determined by x-ray photoelectron spectroscopy to be 1.76 +/- 0.2 eV and 2.20 +/- 0.2 eV. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 0.84 +/- 0.2 eV and 0.40 +/- 0.2 eV. The difference of valence band offsets of them mainly attributes to the spontaneous polarization effect. Our results show important face dependence for InN/ZnO heterojunctions, and the valence band offset of A-plane heterojunction is more close to the "intrinsic" valence band offset.

Comparison between double crystals X-ray diffraction micro-Raman measurement on composition determination of high Ge content Si1_xGex layer epitaxied on Si substrate

It is important to acquire the composition of Si1-xGex layer, especially that with high Ge content, epitaxied on Si substrate. Two nondestructive examination methods, double crystals X-ray diffraction (DCXRD) and micro-Raman measurement, were introduced comparatively to determine x value in Si1-xGex: layer, which show that while the two methods are consistent with each other when x is low, the results obtained from double crystals X-ray diffraction are not credible due to the large strain relaxation occurring in Si1-xGex layers when Ge content is higher than about 20%. Micro-Raman measurement is more appropriate for determining high Ge content than DCXRD.

Competition between band gap and yellow luminescence in undoped GaN grown by MOVPE on sapphire substrate

Undoped GaN epilayer on c-face (0 0 0 1) sapphire substrate has been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal-type low-pressure two-channel reactor. Photoluminescence (PL) as a function of temperature and excitation intensity have been systematically studied, and the competition between near band gap ultraviolet (UV) and defect-related yellow luminescence (YL) has been extensively investigated, It is revealed that the ratio of the UV-to-YL peak intensities depends strongly on the excitation intensity and the measurement temperature. The obtained results have been analyzed in comparison with the theoretical predications based on a bimolecular model. (C) 2001 Elsevier Science B.V. All rights reserved.

RF system design and measurement of HIRFL-CSRe

An RF system for the CSRe (cooling storage experimental ring) is designed and manufactured domestically. The present paper mainly describes the RF system design in five main sections: ferrite ring, RF cavity, RF generator, low level system and cavity cooling. The cavity is based on a type of coaxial resonator which is shorted at the end with one gap and loaded with domestic ferrite rings. The RF generator is designed in the push-pull mode and the low level control system is based on a DSP+FGPA+DDS+USB inter...

Empirical measurement of illicit tobacco trade in the Philippines.

Cigarette smuggling reduces the price of cigarettes, thwarts youth access restrictions, reduces government revenue, and undercuts the ability of taxes to reduce consumption. The tobacco industry often opposes increases to tobacco taxes on the claim that greater taxes induce more smuggling. To date, little is known about the magnitude of smuggling in the Philippines. his information is necessary to effectively address illicit trade and to measure the impacts of tax changes and the introduction of secure tax markings on illicit trade. This study employs two gap discrepancy methods to estimate the magnitude of illicit trade in cigarettes for the Philippines between 1994 and 2009. First, domestic consumption is compared with tax-paid sales to measure the consumption of illicit cigarettes. Second, imports recorded by the Philippines are compared with exports to the Philippines by trade partners to measure smuggling. Domestic consumption fell short of tax-paid sales for all survey years. The magnitude of these differences and a comparison with a prevalence survey for 2009 suggest a high level of survey under-reporting of smoking. In the late 1990s and the mid 2000s, the Philippines experienced two sharp declines in trade discrepancies, from a high of $750 million in 1995 to a low of $133.7 million in 2008. Discrepancies composed more than one-third of the domestic market in 1995, but only 10 percent in 2009. Hong Kong, Singapore, and China together account for more than 80 percent of the cumulative discrepancies over the period and 74 percent of the discrepancy in 2009. The presence of large discrepancies supports the need to implement an effective tax marking and tobacco track and trace system to reduce illicit trade and support tax collection. The absence of a relation between tax changes and smuggling suggests that potential increases in the excise tax should not be discouraged by illicit trade. Finally, the identification of specific trade partners as primary sources for illicit trade may facilitate targeted efforts in cooperation with these governments to reduce illicit trade.

Three-dimensional macroporous silicon photonic crystal with large photonic band gap

Three-dimensional photonic crystals based on macroporous silicon are fabricated by photoelectrochemical etching and subsequent focused-ion-beam drilling. Reflection measurements show a high reflection in the range of the stopgap and indicate the spectral position of the complete photonic band gap. The onset of diffraction which might influence the measurement is discussed.

Efficient modelling technique for fractal electromagnetic band-gap arrays

An efficient modelling technique is proposed for the analysis of a fractal-element electromagnetic band-gap array. The modelling is based on a method of moments modal analysis in conjunction with an interpolation scheme, which significantly accelerates the computations. The plane-wave and the surface-wave responses of the structure have been studied by means of transmission coefficients and dispersion diagrams. The multiband properties and the compactness of the proposed structure are presented. The technique is general and can be applied to arbitrary-shaped element geometries.

Sonochemical synthesis and measurement of optical properties of zinc sulfide quantum dots

A facile sonochemical method has been developed to prepare very small zinc sulfide nanoparticles (ZnS NPs) of extremely small size about 1. nm in diameter using a set of ionic liquids based on the bis (trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The structural features and optical properties of the NPs were determined in depth with X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS) analysis, and UV-vis absorption spectroscopy. The energy band gap measurements of ZnS NPs were calculated by UV-vis absorption spectroscopy. One of the interesting features of the present work is that the wide band gap semiconductor ZnS nanocrystals were prepared which are used in the fabrication of photonic devices.

L’étude de l’InP et du GaP suite à l’implantation ionique de Mn et à un recuit thermique

Cette thèse est dédiée à l’étude des matériaux InMnP et GaMnP fabriqués par implantation ionique et recuit thermique. Plus précisément nous avons investigué la possibilité de former par implantation ionique des matériaux homogènes (alliages) de InMnP et GaMnP contenant de 1 à 5 % atomiques de Mn qui seraient en état ferromagnétique, pour des possibles applications dans la spintronique. Dans un premier chapitre introductif nous donnons les motivations de cette recherche et faisons une revue de la littérature sur ce sujet. Le deuxième chapitre décrit les principes de l’implantation ionique, qui est la technique utilisée pour la fabrication des échantillons. Les effets de l’énergie, fluence et direction du faisceau ionique sur le profil d’implantation et la formation des dommages seront mis en évidence. Aussi dans ce chapitre nous allons trouver des informations sur les substrats utilisés pour l’implantation. Les techniques expérimentales utilisées pour la caractérisation structurale, chimique et magnétique des échantillons, ainsi que leurs limitations sont présentées dans le troisième chapitre. Quelques principes théoriques du magnétisme nécessaires pour la compréhension des mesures magnétiques se retrouvent dans le chapitre 4. Le cinquième chapitre est dédié à l’étude de la morphologie et des propriétés magnétiques des substrats utilisés pour implantation et le sixième chapitre, à l’étude des échantillons implantés au Mn sans avoir subi un recuit thermique. Notamment nous allons voir dans ce chapitre que l’implantation de Mn à plus que 1016 ions/cm2 amorphise la partie implantée du matériau et le Mn implanté se dispose en profondeur sur un profil gaussien. De point de vue magnétique les atomes implantés se trouvent dans un état paramagnétique entre 5 et 300 K ayant le spin 5/2. Dans le chapitre 7 nous présentons les propriétés des échantillons recuits à basses températures. Nous allons voir que dans ces échantillons la couche implantée est polycristalline et les atomes de Mn sont toujours dans un état paramagnétique. Dans les chapitres 8 et 9, qui sont les plus volumineux, nous présentons les résultats des mesures sur les échantillons recuits à hautes températures : il s’agit d’InP et du GaP implantés au Mn, dans le chapitre 8 et d’InP co-implanté au Mn et au P, dans le chapitre 9. D’abord, dans le chapitre 8 nous allons voir que le recuit à hautes températures mène à une recristallisation épitaxiale du InMnP et du GaMnP; aussi la majorité des atomes de Mn se déplacent vers la surface à cause d’un effet de ségrégation. Dans les régions de la surface, concentrés en Mn, les mesures XRD et TEM identifient la formation de MnP et d’In cristallin. Les mesures magnétiques identifient aussi la présence de MnP ferromagnétique. De plus dans ces mesures on trouve qu’environ 60 % du Mn implanté est en état paramagnétique avec la valeur du spin réduite par rapport à celle trouvée dans les échantillons non-recuits. Dans les échantillons InP co-implantés au Mn et au P la recristallisation est seulement partielle mais l’effet de ségrégation du Mn à la surface est beaucoup réduit. Dans ce cas plus que 50 % du Mn forme des particules MnP et le restant est en état paramagnétique au spin 5/2, dilué dans la matrice de l’InP. Finalement dans le dernier chapitre, 10, nous présentons les conclusions principales auxquels nous sommes arrivés et discutons les résultats et leurs implications.

Consumer-based brand equity conceptualisation and measurement: a literature review

Although there is a large body of research on brand equity, little in terms of a literature review has been published on this since Feldwick's (1996) paper. To address this gap, this paper brings together the scattered literature on consumer-based brand equity's conceptualisation and measurement. Measures of consumer-based brand equity are classified as either direct or indirect. Indirect measures assess consumer-based brand equity through its demonstrable dimensions and are superior from a diagnostic level. The paper concludes with directions for future research and managerial pointers for setting up a brand equity measurement system.