Showing papers in "Physica Status Solidi (a) in 2001"
TL;DR: In this paper, the mechanisms of information storage and subsequent read-out are discussed, however, the present understanding of the structures and reactions of the radiation-induced point defects involved in these processes are critically discussed.
Abstract: BaFBr doped with Eu 2+ is the most successful X-ray storage phosphor used for digital radiography so far. The mechanisms of information storage and subsequent read-out are, however, not yet fully understood. The present understanding of the structures and reactions of the radiation-induced point defects involved in these processes are critically discussed. The shortcomings of X-ray storage phosphor screens based on BaFBr:Eu 2+ are the inferior spatial resolution compared to conventional X-ray films. Also reviewed are therefore alternative storage phosphors such as alkali halides doped with Eu 2+ . Ga + , In + , or Tl + or elpasolites doped with Ce 3+ or Eu + which are also promising with high figures of merit and potentially higher spatial resolution. A completely new approach is the use of suitably doped glasses and glass ceramics. First results on fluorozirconate glass ceramics with good storage and read-out properties are reviewed.
158 citations
TL;DR: The first layer SERS effect is usually attributed to dynamical charge transfer (DCT) between metal and absorbate, following a model by Persson as discussed by the authors, but experiments show that there is no first-layer SERS at smooth noble metal surfaces, in spite of the surface resistance caused by adsorbates, which is also caused by DCT according to Persson.
Abstract: The first layer SERS effect is usually ascribed to dynamical charge transfer (DCT) between metal and absorbate, following a model by Persson. Experiments show that there is no first layer SERS effect at smooth noble metal surfaces, in spite of the surface resistance caused by adsorbates, which is also caused by DCT according to Persson. On the other hand, both SERS and SEIRA (surface enhanced infrared absorption) of ethylene on cold-deposited Cu display at low coverages exactly the same Raman active modes, which can be only understood by DCT. The apparent difference is attributed to special events of double scattering by DCT and atomic scale surface roughness, which prevent dephasing of the optical currents in the electromagnetic resonances. The double scattering model explains the observed change of the SERS background and SERS phonon spectra during annealing of the atomic scale roughness. Single molecule SERS is assigned to molecules within narrow and small contact zones of metal particle. The vibrations are effectively excited by the optical currents flowing through the molecule, without becoming dephased. “Blinking” (sudden changes in intensity and band positions) is assigned to thermally activated atomic scale jumps of the molecule, frozen out at low temperatures in agreement with experimental observations.
149 citations
TL;DR: In this paper, the growth and properties of GaN films grown on Si(111) substrates by molecular beam epitaxy using ammonia were reported. But the growth procedure is very efficient in order to overcome the difficulties encountered during the growth of nitrides on silicon substrates: first, no nitridation of the silicon substrate is observed at the interface between the AIN buffer laver and the silicon surface.
Abstract: We report on the growth and properties of GaN films grown on Si(111) substrates by molecular beam epitaxy using ammonia. The properties of the layers show that our growth procedure is very efficient in order to overcome the difficulties encountered during the growth of nitrides on silicon substrates: first, no nitridation of the silicon substrate is observed at the interface between the AIN buffer laver and the silicon surface: second. there is no Si autodoping coming from the substrate and resistive undoped GaN layers are obtained; and, also, strain balance engineering allows one to grow thick GaN epilayers (up to 3 mum) without formation of cracks. The optical, structural and electrical properties of these films are studied. In order to evaluate the potentialities of III-V nitrides grown on silicon substrates, we have grown heterostructures to realize light emitting diodes (LEDs), photodetectors and high electron mobility transistors (HEMTs).
148 citations
TL;DR: In this article, the performance of a number of relevant exhaust gas components (H 2, HC, CO, NO) was tested with the test gas concentrations and the composition of background gases were chosen to simulate exhaust gas emissions from lean-burn engines.
Abstract: The paper reports on novel gas sensing devices based on III-nitride materials. Both platinum GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AIGaN heterostructures with catalytically active platinum gates were investigated. The performance of these devices towards a number of relevant exhaust gas components (H 2 , HC, CO, NO) was tested. The test gas concentrations as well as the composition of background gases were chosen to simulate exhaust gas emissions from lean-burn engines. We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO. Furthermore a strikingly dissimilar gas sensing behavior has been observed with respect to saturated hydrocarbons on the one hand and to hydrogen and unsaturated hydrocarbons on the other hand.
141 citations
TL;DR: In this article, high-electron-mobility transistors (HEMTs) based on AlGaN/GaN heterostructures were successfully tested as chemically sensing devices.
Abstract: High-electron-mobility transistors (HEMTs) based on AlGaN/GaN heterostructures were successfully tested as chemically sensing devices. Exposing the unprotected polar GaN surface in the gate area of a HEMT to liquids of different polarity, milliampere changes in the source-drain current could be detected. These sensing effects are likely to arise from chemical interactions with a sheet of ionic charge on the free GaN surface which compensates the electronic charge of a two-dimensional electron gas at a subsurface AlGaN/GaN interface.
136 citations
TL;DR: In this paper, the diamond field effect transistors have operated in electrolyte solution for the first time since the hydrogen-terminated diamond surfaces are stable enough for the use as an electrochemical electrode, the diamond surface channels are exposed to the electrolyte in the transistor structure.
Abstract: Diamond field effect transistors have operated in electrolyte solution for the first time Since the hydrogen-terminated diamond surfaces are stable enough for the use as an electrochemical electrode, the diamond surface channels are exposed to the electrolyte in the transistor structure A perfect pinch-off and saturated current-voltage characteristics have been obtained for bias voltages within the potential window The threshold voltages are almost constant in electrolytes with different pH values of 7-13, indicating pH insensitiveness of the hydrogen-terminated diamond surface Based on this pH insensitive surface, ion selective regions can be fabricated to form transistor-based biosensors
125 citations
TL;DR: In this paper, the performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output-current-voltage (L-I-V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm.
Abstract: The performance of high-power AlInGaN light emitting diodes (LEDs) is characterized by light output–current–voltage (L–I–V) measurements for devices with peak emission wavelengths ranging from 428 to 545 nm. The highest external quantum efficiency (EQE) is measured for short wavelength LEDs (428 nm) at ≈29%. EQE decreases with increasing wavelength, reaching ≈13% at 527 nm. With low forward voltages ranging from ≈3.3 to ≈2.9 V at a drive current density of 50 A/cm2, these LEDs exhibit power conversion efficiencies ranging from ≈26% (428 nm) to ≈10% (527 nm).
115 citations
TL;DR: In this paper, an InGaN multi-quantum-well (MQW) structure was fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy.
Abstract: Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5 x 10 8 cm -2 . The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the UV-LED was operated at a forward-biased current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-biased current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.
103 citations
TL;DR: In this paper, the authors consider the case of ultrathin CdSe insertions in wide gap ZnMgSSe matrices and show that the nominal thickness is chosen below the critical thickness for three-dimensional (3D) island formation.
Abstract: Ultrathin insertions of a narrow band-gap material in wide band-gap matrices represent a challenging medium in view of aspects of growth phenomena, unique optical properties, and non-trivial approaches for structural characterization. In a very general case ultrathin submonolayer insertions may form arrays of islands due to the principally discrete nature of the growth front. If the islands are large enough, these islands may act as locally formed quantum well (QW) insertions. If, however, the islands' size is comparable to the Bohr radius and the band-gap difference between the insert and the matrix material is large enough, quantum dots (QD) are formed. Realization of the first or the second regime depends on the surface properties of the substrate and the deposit, particularly, on the tensors of the intrinsic surface stress of both materials and on the lattice mismatch. In this work we consider in detail the case of ultrathin CdSe insertions in wide gap ZnMgSSe matrices: that the nominal thickness is chosen below the critical thickness for three-dimensional (3D) island formation. We give an overview of the experimental results available for these structures obtained by submonolayer or about-one monolayer CdSe depositions. A comparison with similar phenomena observed in conventional III-V and III-N systems is given and possible growth scenarios are discussed. We also discuss practical device applications of the structures based on ultrathin insertions for non-traditional devices. Examples of resonant waveguiding and lasing in edge geometry, of surface emitting lasers with low finesse cavities, and of broad-miniband high-frequency Esaki-Tsu anti-dot superlattices are given.
100 citations
TL;DR: In this paper, the effect of boundary scattering on the thermal conductivity of a thermoelectric material was investigated and the relative magnitudes of the effects on the phonons and charge carriers were estimated for SiGe alloys, Bi-Sb alloys and Bi 2 Te 3 alloys.
Abstract: The lattice thermal conductivity of a thermoelectric material can be significantly reduced at ordinary temperatures by boundary scattering but the beneficial effect on the figure of merit can sometimes be more than offset by boundary scattering of the charge carriers. Here we extend the previous theory to include both weak and strong boundary scattering. We show that the relative magnitudes of the effects on the phonons and charge carriers can be estimated and illustrate our predictions by applying them to Si-Ge alloys, Bi-Sb alloys, and Bi 2 Te 3 alloys. Our theory does not distinguish between the effects of acoustic and optical phonons and may, therefore, be criticized when applied to materials with large unit cells. We present an alternative treatment for this case using a classical method for assessing the thermal conductivity due to the acoustic phonons. The new technique is applied to one of the half-Heusler alloys, Zr 0.5 Hf 0.5 NiSn.
99 citations
TL;DR: Scandium oxide was deposited epitaxially on (0001) GaN via gas-source molecular beam epitaxy (GSMBE) using elemental Sc and an electron cyclotron resonance (ECR) oxygen plasma.
Abstract: Scandium oxide was deposited epitaxially on (0001) GaN via gas-source molecular beam epitaxy (GSMBE) using elemental Sc and an electron cyclotron resonance (ECR) oxygen plasma. HXRD indicated that the Sc 2 O 3 has a lower defect density than similarly prepared films of Gd 2 O 3 . The scandium oxide was atomically smooth, with a rms roughness of 0.5-0.8 nm, and was uniform in stoichiometry as measured by Auger electron spectroscopy (AES) depth profiling. An interface state density of mid 10 11 eV -1 cm -2 was determined from capacitance-voltage profiling using the Terman method. This interface state density was roughly a factor of five lower than that obtained from similar diodes made from SiO 2 on GaN.
TL;DR: In this paper, the authors investigated the importance of localized QW excitons in their spontaneous emission mechanisms and verified the real-space variation of the luminescence peak energy by the spatially resolved monochromatic cathodoluminescence mapping method.
Abstract: Strained In x Ga 1-x N quantum wells (QWs) on thick GaN base layers were investigated to verify the importance of localized QW excitons in their spontaneous emission mechanisms. A strength of the internal piezoelectric field (F PZ ) across the QW increases with increasing x up to 1.4 MV/cm for x = 0.25, since the in-plain strain increases. For the QWs with the well thickness L greater than 3 nm, F PZ dominates the emission peak energy due to the quantum-confined Stark effect. Absorption spectra of both hexagonal and cubic InGaN QWs exhibited a broad band-tail regardless of the presence of F PZ normal to the QW plane. The luminescence peak energy of the 3 nm thick QWs was higher than the bandgap energy of the unstrained bulk crystal for x < 0.15, showing that doping of Si in barriers or injection of carriers effectively screens the field. The emission lifetime increased with increasing monitoring wavelength, Also, a temperature-induced change in the luminescence peak energy decreased with increasing x. The real-space variation of the luminescence peak energy was confirmed by the spatially resolved monochromatic cathodoluminescence mapping method. The localization depth increases with increasing x, The carrier localization is confirmed to originate from the effective bandgap inhomogeneity due to a fluctuation of the local InN mole fraction, which is enhanced by the large and composition-dependent bowing parameter of InGaN material.
TL;DR: In this paper, the authors describe silicon carbide MOSFET sensors and their performance and give examples of industrial applications such as monitoring of car exhausts and flue gases.
Abstract: The development and field-testing of high-temperature sensors based on silicon carbide devices have shown promising results in several application areas. Silicon carbide based field-effect sensors can be operated over a large temperature range, 100-600 °C, and since silicon carbide is a chemically very inert material these sensors can be used in environments like exhaust gases and flue gases from boilers. The sensors respond to reducing gases like hydrogen, hydrocarbons and carbon monoxide. The use of different temperatures, different catalytic metals and different structures of the gate metal gives selectivity to different gases and arrays of sensors can be used to identify and monitor several components in gas mixtures. MOSFET sensors based on SiC combine the advantage of simple circuitry with a thicker insulator, which increases the long term stability of the devices. In this paper we describe silicon carbide MOSFET sensors and their performance and give examples of industrial applications such as monitoring of car exhausts and flue gases. Chemometric methods have been used for the evaluation of the data.
TL;DR: In this article, a GaN HEMT with a breakdown voltage of 1050 V was fabricated with a specific on-resistance of 3.4 mΩ cm 2 and current density of 4 kA cm -2.
Abstract: SiC devices have been the focus for several years as potential high voltage switches working at high temperature with very low on-resistance. Another competitive candidate for this application is the GaN HEMT (high electron mobility transistor). GaN has slightly wider bandgap, higher electric strength, and higher saturated velocity than SiC. The most prominent is that by utilizing the AlGaN/GaN heterojunction, the GaN HEMT has much higher charge density (up to 2 × 10 13 cm -2 ) and mobility (up to 2200 cm 2 /Vs) in the channel compared to SiC MOSFET or JFET, yielding much lower on-resistance than SiC devices. GaN grown on SiC substrate also takes the advantage of high thermal conductivity of SiC. A GaN HEMT with a breakdown voltage of 1050 V was fabricated with a specific on-resistance of 3.4 mΩ cm 2 and current density of 4 kA cm -2 . State-of-art power device figure of merit of V 2 BR /R on = 3.24 × 10 8 [V 2 Ω -1 cm -2 ] was achieved on this device. Projected performance of GaN HEMTs is also discussed and compared with SiC devices in this paper.
TL;DR: In this article, the complex refractive indices of thick plasma-enhanced chemical vapour deposited silicon nitride and oxynitride films were determined within the infrared spectral region (4000-400 cm -1, i.e. 2.5-25 μm) and used further to obtain their complex dielectric response functions.
Abstract: The complex refractive indices of thick plasma-enhanced chemical vapour deposited silicon nitride and oxynitride films were determined within the infrared spectral region (4000-400 cm -1 , i.e. 2.5-25 μm) and used further to obtain their complex dielectric response functions. The imaginary part, i.e. the so-called energy-loss-function was analysed to get accurate phonon data of the amorphous layer. This way, TO-phonon frequencies, half-widths, and intensities of characteristic infrared absorptions were determined for each film. The dependence of the obtained data upon the variation of chemical/physical structure of the amorphous lattice was discussed.
TL;DR: In this article, the results of reliability tests performed on undoped AlGaN/GaN HEMTs on SiC under dc and rf stress conditions were presented, and the degradation in device performance induced by hot electron and thermal effects have been observed.
Abstract: Undoped AlGaN/GaN HEMTs grown on SiC substrates have recently demonstrated record output power density of 10.7 W/mm (cw) and total output power of 10 W at 10 GHz (L. F. Eastman, Joint ONR/MURI Review (5/15–16, 2001), CA (USA) [1]). In this paper, we present the results of reliability tests performed on undoped AlGaN/GaN HEMTs on SiC under dc and rf stress conditions. Undoped AlGaN/GaN HEMTs on SiC substrates have been submitted to on-wafer dc and rf stress conditions at a room temperature and the degradation in device performance induced by hot electron and thermal effects have been observed.
TL;DR: In this paper, a study on the material properties of GaN films grown on (111) silicon substrates by low-pressure metalorganic chemical vapour deposition using AlN buffer layers is presented.
Abstract: We present a study on the material properties of GaN films grown on (111) silicon substrates by low-pressure metalorganic chemical vapour deposition using AlN buffer layers. This buffer layer is optimised with respect to growth temperature and time for the optical and structural properties of the GaN epilayers. The insertion of a Si x N y intermediate layer significantly increases the optical and structural properties. It results in a reduction of the D 0 X FWHM to 10 meV and in a 2.5-fold increase of its luminescence intensity. The FWHM of symmetric and asymmetric ω-scans are reduced from 832 to 669 arcsec and from 702 to 547 arcsec, respectively.
TL;DR: In this article, the extinction coefficient k and the refractive index n of polycrystalline β-In2S3 thin films have been studied and the optical band gap is shown to be 2.8 eV.
Abstract: Thin films of indium sulphide containing oxygen have been synthesized following a dry physical process. The constituents are deposited by thermal evaporation on glass substrates and then annealed under argon flow. Polycrystalline β-In2S3 containing oxygen thin films are obtained as soon as the temperature of annealing is between 623 and 723 K. In this paper, these β-In2S3 thin films have optically been studied. The optical band gap is direct. Its value is not dependent on the temperature of annealing. It is about 2.8 eV, which is higher than that of β-In2S3 single crystal. This high value is related to the presence of oxygen in the films. The extinction coefficient k and the refractive index n of the films have also been found independent of the annealing temperature. These optical properties make the films studied good candidates to be substituted to CdS in Cu(In,Ga)Se2 based solar cells.
Cet article est consacre a l'etude des proprietes optiques de couches minces de sulfure d'indium (In2S3) realisees par evaporation thermique sous vide suivie d'un recuit sous flux d'argon. L'influence de la temperature de traitement thermique sur le coefficient d'extinction et sur l'indice de refraction des couches a ete etudiee. La largeur de la bande interdite des couches traitees a 623, 673 ou 723 K est de 2,8 eV, mettant en jeu une transition de nature directe. Une bande interdite aussi large, celle du monocristal etant de 2,0 eV, a deja ete trouvee par des auteurs a partir de couches obtenues par des processus chimiques et est liee a la presence d'oxygene. Ces proprietes font de ces films d'excellents candidats a la substitution du CdS comme couche tampon dans les cellules solaires a base de Cu(In,Ga)Se2.
TL;DR: In this article, the authors review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), TREL, transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, sub-microscopic TRpl using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra.
Abstract: Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.
TL;DR: In this paper, a low frequency dielectric study was performed on lead-tree non-stoichiometric perovskite ceramics of Ba 1-x Bi 2x/3 □ x/3 TiO 3 (0 ≤ x ≤ ≤ 0.09 ≤ x < 0.15).
Abstract: Crystallographic and low frequency dielectric studies were performed on lead-tree non-stoichiometric perovskite ceramics of Ba 1-x Bi 2x/3 □ x/3 TiO 3 (0 ≤ x ≤ 0.15) composition. A classical ferroelectric behaviour was shown for x lower than 0.09. A relaxor behaviour appears for higher values of x (0.09 ≤ x < 0.15). The corresponding relaxor characteristics increased as the composition deviated from BaTiO 3 . Such kind of materials are of interest for environmental-friendly applications, in particular the ceramic with composition corresponding to x = 0.15, whose maximum of permittivity occurs at 280 K. i.e. close to room temperature.
TL;DR: In this article, B-doped homoepitaxial diamond thin films were grown by chemical vapor deposition (CVD) by using trimethylboron (TMB) as a B source gas, showing a sharp Raman shift peak at 1332 cm -1, a strong free-excitonic emission at room temperature, and high Hall mobility as high as 1000 cm 2 /Vs or more.
Abstract: We review our recent results on properties of high quality B-doped homoepitaxial diamond thin films grown by chemical vapor deposition (CVD) by using trimethylboron (TMB) as a B source gas. The conventional (one-step) and the two-step growth methods were used for film preparation. The latter realized smooth surface without any non-epitaxial crystallites (UCs). The films showed a sharp Raman shift peak at 1332 cm -1 , a strong free-excitonic emission at room temperature, and high Hall mobility as high as 1000 cm 2 /Vs or more, indicating high-quality diamond. We also successfully realized resistivity control of the films in a wide range from 10 0 to 10 5 Ω cm due to low compensation ratio. Using the two-step growth method, Schottky junctions with the ideality factor n of 1.1 or less and undetectable leakage current could be prepared between various metals such as Al, Zn, Cr, Ni, Au or Pt and oxidized B-doped CVD diamond films. In particular, we successfully made nearly ideal Schottky junctions using highly B-doped films in the order of 10 17 cm -3 , indicating that the quality of the present B-doped films is comparable with or higher than those of conventional semiconductors such as Si and GaAs.
TL;DR: In this paper, the authors have fabricated n-Al 0.44 Ga 0.56 N/p-GaN heterojunction photodiodes with a cut-off wavelength of 275 nm.
Abstract: We have fabricated n-Al 0.44 Ga 0.56 N/i-Al 0.44 Ga 0.56 N/p-GaN heterojunction photodiodes with a cut-off wavelength of 275 nm. The multilayer device structure was grown by metal-organic vapor phase epitaxy using a low-temperature interlayer technique. Thanks to the high quality AIGaN, the responsivity dropped steeply by three, five and six orders of magnitude at the AIGaN bandedge of 275 nm, 600 nm, and 1 μm, respectively. The steep bandedge enables a minimum leakage of effective weak-flame luminescence between 250 and 280 nm. For flame detection, a visible cut-off filter to assist the selectivity to the flame luminescence was attached. In addition, the photodiode was operated at zero-bias so that the darkcurrent does not exceed the weak photocurrent induced by flame luminescence. This solar-blind photosensor thus successfully responded selectively to the flame luminescence regardless of whether the room was lighted or not.
TL;DR: In this article, a crack-free and low-dislocation-density Al x Ga 1-x N was achieved by low-temperature-deposited interlayer technique in combination with a lateral seeding epitaxy.
Abstract: Crack-free and low-dislocation-density Al x Ga 1-x N was achieved by low-temperature-deposited interlayer technique in combination with a lateral seeding epitaxy. We found that there was a strong correlation between the threading dislocation density and the PL intensity in the GaN/AIGaN MQWs. This new UV light emitting diode exhibits strong UV light output, having peak wavelength of 352 nm, a full width at half maximum as narrow as 6 nm and output power of 0.6 mW at 50 mA dc current injection.
TL;DR: In this paper, a MOCVD-grown crack-free GaN-based light emitters on Si with a layer thickness of 3.6 μm is presented. But the impact of the facet type on the growth rate and impurity incorporation is observed by scanning electron microscopy and cathodoluminescence measurements.
Abstract: We present MOCVD-grown crack-free GaN based light emitters on Si with a layer thickness of 3.6 μm. The crack free layer is grown on a thin predeposited GaN layer with fields defined by a Si x N y mask. In X-ray diffraction measurements a reduction in stress is observed for the patterned sample as compared to a similar unstructured sample. Lateral growth occurs at the edges of the fields with strongly differing growth rates for perpendicular directions. The impact of the facet type on the growth rate and impurity incorporation is observed by scanning electron microscopy and cathodoluminescence measurements. In electroluminescence the diode shows a bright blue emission at 421 nm.
TL;DR: The tensile strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy was investigated in this article, where X-ray diffraction and photoluminescence measurements were used to determine the effect of these AlN/GaN superlattices on the strain in subsequent GaN layer.
Abstract: The tensile strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy was investigated. Thick (0.9-2.5 μm) GaN layers without any crack were deposited on Si(111) using AIN/GaN superlattices as templates. X-ray diffraction and photoluminescence measurements were used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layer. Evolution of strain as a function of the GaN layer thickness is also determined. Dislocation reduction (from 10 10 to 2.5 x 10 9 cm -2 ) is observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed, with full width at half maximum of the I 2 line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) X-ray rocking curve attests the high crystalline quality of GaN on Si(111 ), when AlN/GaN superlattices are used.
TL;DR: In this paper, the optical anisotropy of differently reconstructed GaAs(001) surfaces has been analyzed both theoretically and experimentally, and the atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4).
Abstract: The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.
TL;DR: The thermoluminescent properties of polycrystalline Dy doped strontium tetraborate are reported in this article, showing two spectral bands at approximately 480 and 580 nm.
Abstract: The thermoluminescent properties of polycrystalline Dy doped strontium tetraborate are reported. Its efficiency is at least five times larger than that of TLD-700 and therefore comparable to that of Cu doped lithium tetraborate. The isometric plot shows two spectral bands at approximately 480 and 580 nm like the emission spectra of Dy doped phosphors reported to date.
TL;DR: In this article, the properties of Niobium oxide films as a function of oxygen partial pressure have been studied systematically by Rutherford backscattering spectroscopy, X-ray diffraction, Xray reflectometry and optical spectroscopic analysis.
Abstract: Niobium oxide films have been prepared on glass and Si(100) substrates by reactive dc magnetron sputtering of a metallic Nb target in an argon-oxygen atmosphere. The properties of the films as a function of oxygen partial pressure have been studied systematically by Rutherford backscattering spectroscopy, X-ray diffraction, X-ray reflectometry and optical spectroscopy. Rutherford backscattering studies show that the films are substoichiometric at lower oxygen flow while stoichiometric films can be formed above 7.5 sccm O 2 flow. Grazing incidence X-ray diffraction studies show that the films are amorphous. X-ray reflectometry was used to determine the sputter rate, density and surface roughness of the films. The films prepared at lower oxygen flow rates had a higher density than those prepared at higher oxygen flow rates. For appropriate oxygen flow rates fully transparent niobium oxide films can be grown with rates up to 0.5 nm/s for a constant cathode current of 900 mA. From optical spectroscopy measurements of reflectance and transmittance we have determined the optical constants such as the refractive index (n), extinction coefficient (k) and band gap (Eg) as well as the film thickness. For the fully transparent films prepared above 7.5 sccm O 2 flow rate a band gap of 3.5 to 3.6 eV is obtained. Optical spectroscopy measurements show that the refractive index of the films decreases upon increasing oxygen flow rate and this finding is related to a decrease of density upon increasing oxygen flow.
TL;DR: In this article, a near-UV and violet emitting AlGaInN single quantum well LED structures were grown by MOCVD on sapphire substrates, with an output power at 40 mA between 1.4 mW and 6.7 mW at 420 nm.
Abstract: Near-UV and violet emitting AlGaInN single quantum well LED structures were grown by MOCVD on sapphire substrates. On-wafer tests before processing gave an output power at 40 mA between 1.4 mW at 380 nm and 6.7 mW at 420 nm. LED chips with wavelengths between 380 and 404 nm were selected for manufacturing radial UV LEDs and white emitting LEDs. The UV to white converters were prepared from broad band red, green and blue emitting powder phosphors with individual luminescence peaks near 610, 550 and 460 nm, respectively. White luminescence conversion (LUCO) LEDs with a warm hue and color temperatures in the range 4000-4300 K are demonstrated.
TL;DR: In this article, the room temperature ozone sensing properties of polycrystalline indium oxide (InO x ) thin films have been investigated and the sensitivity was studied for different ozone concentrations and at different temperatures.
Abstract: The room temperature ozone sensing properties of polycrystalline indium oxide (InO x ) thin films have been investigated. Films with thicknesses of 10 to 1100 nm were sputtered in a dc-magnetron system onto Corning 7059 glass at various substrate temperatures and sputtering atmospheres. Initially, as-grown films were brought to a high conducting state through a photoreduction process by UV light exposure and subsequently they were exposed to a controlled ozone atmosphere. By this treatment the sensitivity of the films could be monitored. The films exhibit resistivity changes of more than five orders of magnitude. The sensitivity was studied for different ozone concentrations and at different temperatures. The response of the films increased linearly with the ozone concentration and the highest sensitivity was achieved when the measurements were carried out at room temperature. Best results were achieved with very thin InO x films (<100 nm) deposited at room temperature in a pure oxygen atmosphere.