Basic ammonothermal growth of Gallium Nitride – State of the art,challenges, perspectives

Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5?×?10⁴ cm^?2. Crystals with different types of conductivity, n-type with free electron concentration up to 10¹⁹ cm^?3, p-type with free hole concentration of 10¹⁶ cm^?3, and semi-insulating with resistivity exceeding 10¹¹ Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown.     

Electrochemical characterizations and the effect of glycerol in biopolymer electrolytes based on methylcellulose-potato starch blend

ABSTRACT

Polymer electrolytes have been prepared by blending methylcellulose (MC)-potato starch, doped with lithium perchlorate (LiClO₄) and plasticized with glycerol. The blend of 60 wt% MC-40 wt% starch was found to be the most suitable ratio to serve as polymer host. Fourier transform infrared (FTIR) spectroscopy analysis proved the interaction among the components. X-ray diffraction (XRD) analysis indicated that the conductivity enhancement is due to the increase in amorphous content. The highest ionic conductivity obtained at room temperature was (4.25 ± 0.82) × 10^?4 S cm^?1 for MC-starch-LiClO₄-20 wt% glycerol. The highest conducting samples in both systems were found to obey Arrhenius rule. Dielectric study further strengthens the conductivity result.     

The Effect of Plasticization on Conductivity and Other Properties of Starch/Chitosan Blend Biopolymer Electrolyte Incorporated with Ammonium Iodide

This work focuses on polymer electrolytes composed of a starch-chitosan blend host, ammonium iodide (NH₄I) and glycerol. Fourier transform infrared (FTIR) analysis confirms the interaction of starch-chitosan-NH₄I-glycerol. The highest room temperature conductivity is (1.28 ± 0.07) × 10^?3 S cm^?1, obtained by a sample containing 30 wt% glycerol. Dielectric studies showed that the electrolytes obeyed non-Debye behavior. The total ionic transference number for the 30 wt% glycerol sample was 0.991, and the conduction mechanism for this sample followed the quantum mechanical tunneling (QMT) model. Linear sweep voltammetry (LSV) showed that this sample was electrochemically stable up to 1.90 V. The highest conducting sample was used in the fabrication of an electrical double layer capacitor (EDLC) cell.     

Comparison of the laser Raman and far-ir spectra of dibenzyl and poly-p-xylylene

The infrared and laser Raman spectra of dibenzyl and poly-p-xylylene in the 20–650 cm^?1 region are compared with each other. New laser Raman data for poly-p-xylylene below 200 cm^?1 have been obtained using an argon-ion laser (4880 Å) and photon counting detection techniques.     

The optical constants of quartz,vitreous silica and neutron-irradiated vitreous silica (I)

The infrared reflectivity of three forms of silica, α-quartz, vitreous silica and neutron-irradiated vitreous silica (≈ 2.7 × 10²⁰ neutrons cm^?2) has beenmeasured from 400–2000 cm^?1. These data have been analysed by a Kramers-Kronig transform to give the real and imaginary parts of the complex dielectric constant. Considerable care has been taken to identify and minimize errors arising in the measurements of the reflectivity spectra and in the subsequent analysis. Data are presented for the optical constants, oscillator frequencies, band strengths and halfwidths of each band. The spectra for vitreous silica and neutron-irradiated silica show two regions of absorption which are not present in the crystalline form — a strong band is observed near 950 cm^?1 and a broad band from 600–800 cm^?1. A difference spectrum obtained by subtracting the spectrum of the imaginary part of the dielectric contrast for vitreous silica from the corresponding data for neutron-irradiated silica reveals more detailed structure in the form of a weak but sharp band at 620 cm^?1. Interpretation of these results is contained in a companion paper.     

Properties of AgGa1‐xInxSe2 single crystals grown by Bridgman method

High‐pure and single‐phase AgGa_1‐xIn_xSe₂ (x=0.2) polycrystalline was synthesized by the mechanical and temperature oscillation method. Adopting the modified Bridgman method an integral AgGa_1‐xIn_xSe₂ single crystal with diameter of 14 mm and length of 35 mm has been obtained at the rate of 6 mm/day. It was found that there is a new cleavage face which was (101), and observed the four order X‐ray spectrum of the {101} faces. By the method of DSC analysis the melting and freezing points of the AgGa_1‐xIn_xSe₂ (x=0.2) single crystal were about 828°C and 790°C. The transmission spectra of the AgGa_1‐xIn_xSe₂ (x=0.2) sample of 5×6×2 mm³ were obtained by means of UV and IR spectrophotometer. The limiting frequency was 774.316nm and the band gap was 1.6eV. It can be found in the infrared spectrum that the infrared transmission was above 60% from 4000cm^‐1 to 600cm^‐1. The value of α in 5.3µm and 10.6µm were 0.022cm^‐1 and 0.1cm^‐1 respectively. All results showed that the crystal was of good quality. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solution-Processable Low Voltage Organic Transistors of Thieno[3,2-b]thiophene-Based Conducting Polymer

A low-operating voltage and high performance polymeric field effect transistors using octadecylphosphonic acid-treated high-k AlOx and HfO₂ hybrid dielectrics were demonstrated. High-k metal oxide hybrid dielectrics were prepared by oxygen plasma treatment of deposited Al film for AlOx and by spin coating of solution-processable HfO₂ sol-gel solution for HfO₂ in combination with phosphoric acid-based self-assembled monolayer (SAM), resulting in high capacitance (10 nF/cm² for SiO₂, 600 nF/cm² for AlOx and 580 nF/cm² for HfO₂). With phosphoric acid-based SAM on high-k metal oxide and thermal annealing of thieno[3,2-b]thiophene-based conducting polymer, the device performance was significantly enhanced. The highest mobility of the transistors using ODPA-treated AlOx as a gate dielectric is 2.3 × 10^?2 cm² V^?1 s^?1 in the saturation region with the source-drain of ?2 V. In ODPA-treated HfO₂ hybrid dielectric, the saturated mobility is 1.1 × 10^?2 cm² V^?1 s^?1 and the threshold voltage was measured to be ?0.31 V, which is at least one order lower than SiO₂ hybrid dielectric (?3 V).     

Study of the surface state density and potential in MIS diode Schottky using the surface photovoltage method

ABSTRACT

The effects of surface preparation and illumination on electric parameters of Au/GaN/GaAs Schottky diode were investigated. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses of GaN layers (0.7 – 2.2 nm). In order to study the electrical characteristics under illumination, we use an He-Ne laser of 632 nm wavelength. The I(V) current- voltage, the surface photovltage SPV measurement were plotted and analysed taking into consideration the influence of charge exchange between a continuum of the surface states and the semiconductor. The barrier height Ф_Bn, the serial resistance R_s and the ideality factor n are respectively equal to 0.66 eV, 1980 Ω, 2.75 under dark and to 0.65 eV, 1160 Ω, 2.74 under illumination for simple 1 (GaN theckness of 0.7 nm). The interface states density N_ss in the gap and the excess of concentration δn are determined by fitting the experimental curves of the surface photovltage SPV with the theoretical ones and are equal to 4.5×10¹² eV^?1 cm^?2, 5×10⁷ cm^?3, respectively, for sample 1 and 3.5×10¹² eV^?1 cm^?2, 7×10⁸ cm^?3 for sample 2 (GaN theckness of 2 nm). The results confirm that the surface photovoltage is an efficient method for optical and electrical characterizations.     

Polycrystalline Synthesis and Single Crystal Growth of AgGaS2

A modified two temperature vapor transport procedure has been used to synthesize AgGaS₂ polycrystalline materials at 1060 °C from high-purity Ag, Ga, S elements. The results showed that the polycrystalline materials are pure single phase AgGaS₂ by X-ray powder diffraction analysis. The polycrystalline ingot includes a few interior voids and has a high mass density. The AgGaS₂ ingot with 12 mm in diameter and 20 mm in length has been grown by the modified Bridgman technique in two-zone vertical furnace. As-grown AgGaS₂ ingot was characterized by the X-ray diffraction technique, obtained the rocking curve of the crystal (011) face, it is shown that the ingot is AgGaS₂ single crystal. The infrared transparent of the crystal at 2 ∼ 10μm is 49% (and absorption coefficient is 0.74 cm^—1).     

Infrared spectra of glassy Se containing small amounts of S,Te, As,or Ge

The infrared transmission spectra of glassy Se containing 2.5 and 5.0 at % S, Te, As or Ge as well as pure Se were measured in the wavenumber region 400-60 cm^?1 at room temperature. Besides the absorption bands reported already for pure Se, well-defined bands were founds at 355 cm^?1 and 168 cm^?1 for the addition of S, and at 205 cm^?1 for Te. These new absorption bands attributed to Se₃S₅ and Se₅Te₃ mixed rings, respectively. For As, a strong absorption band appeared at 240 cm^?1. The band near 135 cm^?1 began to broaden and shift to lower frequency with As content. Two shoulder bands near 307 cm^?1 and 278 cm^?1 and a separated band at 195 cm^?1 were found for Ge. With increasing Ge concentration the band at 135 cm^?1 broadened and shifted to lower frequency. An interpretation for the new bands is presented on the basis of a molecular model.     

Vibrational spectra of vapor-deposited binary phosphosilicate glasses

Room temperature infrared transmission spectra in the range 4000-250 cm^?1 of binary phosphosilicate glass (PSG) films deposited by reacting argon- or nitrogen-diluted PH₃SiH₄O₂ mixtures on heated silicon substrates at 300–400° C have been obtained across the whole composition range. In all the as-deposited binary films, an absorption at ≈1300 cm^?1 characteristics of the P=O vibration was found to persist, together with a couple of broad absorptions in the regions 1200-900 cm^?1 and 500 cm^?1. Using a differential infrared technique the broad feature in the higher frequency region has been resolved into two well-defined bands at ≈1100 and 970 cm^?1. A detailed analysis shows that the intensity variation of the differential band at ≈1100 cm^?1 conforms well, at least to 50 mol% P₂O₅, to a simple structural model that yields an analytic distribution of POSi linkages as a function of composition by assuming chemical mixing in the vapor-deposited P₂O₅SiO₂ system. Furthermore, the system may be written as (P=O)₂ O₃SiO₂ in order to emphasize the similarity of its coordination scheme with that of the B₂O₃SiO₂ system studied earlier. The nature of these CVD films has also been elucidated by thermal and water treatments.     

Influence of microscopic defects on optical damage resistance of Zn:Fe:LiNbO3 crystals

Influence of defect structure on the infrared transmission spectra of OH^‐ in Zn:Fe:LiNbO₃ crystals with various ZnO concentration and different Li/Nb ratios was investigated. It indicates that above the Zn concentration threshold the OH^‐ absorptions bands successively shift from 3482cm^‐1 to 3504cm^‐1 and 3529cm^‐1. The intensity of the 3504cm^‐1 band increases with ZnO concentration increasing. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases rapidly when the ZnO concentration exceeds a threshold value. This result contributed to the site alteration from the Li sites to Nb sites due to Zn‐doping in crystal. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

The Absorption,Fluorescence and Phosphorescence and Phosphorescence of Single Crystals of 1,2,4,5-Tetrachlorobenzene and 1,4-Dichlorobenzene at Low Temperatures

Abstract

The emission spectra of ultra pure single crystals of 1, 2, 4, 5. tetrachlorobenzene (TCB) and 1, 4-dichlorobenzene (DCB) at temperatures from 4.2°K upwards are reported. In addition, by use of the phosphorescence excitation technique the singlet-triplet absorption spectrum at 4.2°K has been obtained.

The phosphorescence emission of TCB at 4.2°K occurs predominantly from a defect origin “X” situated 48 cm^?1 below the triplet exciton band. The triplet exciton energy level is at 26676 cm^?1 from both emission and absorption studies. This is the triplet emission origin when the crystal is above 12°K. The temperature dependence of the emission intensity from the defect has an activation energy of 40±8 cm^?1. This value is consistent with our suggestion of thermal depopulation of the traps.

Single crystals of DCB show strong excimer emission and weak triplet exciton emission. There is no evidence for trapping levels. The triplet exciton emission. There is no evidence for trapping levels. The triplet exciton origin is 27890 cm^?1 from both emission and absorption studies.

Weak fluorescence (φ < 10⁰²) is detected from both TCB and DCB.

Vibrational analyses are reported for the absorption, fluorescence and phosphorescence spectra.     

Observation of individual dislocations in 6H and 4H SiC by means of back‐reflection methods of X‐ray diffraction topography

SiC crystals of high structural perfection were investigated with several methods of X‐ray diffraction topography in Bragg‐case geometry. The methods included section and projection synchrotron white beam topography and monochromatic beam topography. The investigated 6H and 4H samples contained in large regions dislocations of density not exceeding 10³ cm^‐2. Most of them cannot be interpreted as hollow core dislocations (micro‐ or nano‐pipes). The concentration of the latter was lower than 10² cm^‐2. The present investigation confirmed the possibility of revealing dislocations with all used methods. The quality of presently obtained Bragg‐case multi‐crystal and section images of dislocation enabled analysis based on comparison with numerically simulated images. The analysis confirmed the domination of screw‐type dislocations in the investigated crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A mössbauer study of hematite dissolution and devitrification in float glass

The effect of 5% hematite on the devitrification behaviour of float glass has been measured as functions of time and temperature by Mössbauer spectroscopy and X-ray diffraction (XRD). The devitrification products are devitrite (Na₂O·3CaO·6SiO₂), cristobalite, quartz and an iron-rich pyroxene, aegirine ((Na Ca)(Fe, Mg) Si₂O₆) for which XRD data are tabulated. The Mössbauer parameters for glassy and crystalline materials are interpreted in terms ofthe aegirine structure, whose site assignment is discussed in terms of next-nearest neighbour effecs. The bulk dissolution data obtained by Mössbauer and XRD are used to generate diffusion coefficients for Fe³⁺ in the glass melt, using a moving boundary model for diffusion controlled dissolution. The diffusion coefficients at 800°C are D_{Fe³⁺ (XRD)} = 3.52 × 10^?15 cm²/s and D_{Fe³⁺ (MS)} = 1.27 × 10^?15 cm²/s.     

Influence of doping on OH absorption in LiNbO3 crystals

Undoped, Cr doped and Mg, Cr codoped LiNbO₃ crystals were grown by conventional Czochralski technique. Comparative study was carried out using Fourier transform infrared (FTIR) and UV‐Visible spectroscopy. Infrared optical absorption for OH^– ion has been used to study the effect of dopants on the crystals. The peak position of OH^– shift to 3535 cm^‐1 for Mg, Cr codoped crystals compared to 3484 cm^‐1 for undoped and Cr doped crystals. Prominent absorption bands are found in the visible region centered at 480 nm (20833 cm^‐1) and 653 nm (15313 cm^‐1) in Cr doped crystals. Whereas in Mg, Cr codoped crystals these broad absorption bands are red shifted to 517 nm (19342 cm^‐1) and 678 nm (14749 cm^‐1). UV cutoff in Cr doped crystals shift towards higher wavelength compared to undoped LiNbO₃ crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Semiconducting Properties of TI2Te3 Single Crystals

Electrical conductivity and Hall effect measurements were performed on single crystals of TI₂Te₃ to have the general semiconducting behaviour of this compound. The measurements were done at the temperature range 160–350 K. All crystals were found to be of p-type conductivity. The values of the Hall coefficient and the electrical conductivity at room temperature were 1.59 × 10³ cm³/coul and 3.2 × 10⁻² ω⁻¹ cm⁻¹, respectively. The hole concentration at the same temperature was driven as 39.31 × 10¹¹ cm⁻³. The energy gap was found to be 0.7 eV where the depth of impurity centers was 0.45 eV. The temperature dependence of the mobility is discussed.     

FTIR,powder XRD,TEM and dielectric studies of pure and zinc doped nano‐hydroxyapatite

Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ or HAP, is an important bio‐material, which is having application in bone implants and dentistry. In the present study, zinc doped nano‐hydroxyapatite (Zn‐HAP) was synthesized via chemical precipitation route using surfactant mediated approach. The doping of zinc was confirmed by EDAX. The powder X‐ray diffraction (XRD) pattern revealed the typical hydroxyapatite pattern with broadening and extra peaks were observed for higher concentration. The average crystallite size was calculated by applying the Scherrer's formula to powder XRD pattern and was found in the range of 16 to 33 nm. The morphology of synthesized nano‐particles was also confirmed using TEM. FTIR spectroscopy was used to confirm the presence of various bonds. The dielectric study was carried out at room temperature within the frequency range from 10² Hz to 10⁷ Hz and the variations of dielectric constant with frequency of applied field as well as with the concentration of zinc were studied. It was found that as the concentration of zinc increased the dielectric constant increased. The variations of dielectric loss and a.c. conductivity with frequency of applied field were studied. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and Crystal Structure of Dimethyl-7-oxabicyclo[2.2.1]hept-5-ene exo,exo-2,3-dicarboxylate

Dimethyl-7-oxabicyclo[2.2.1]hept-5-ene exo,exo-2,3-dicarboxylate (1) was prepared by Fisher esterification of 7-oxabicyclo[2.2.1]-hept-5-ene exo-2,3-dicarboxylic anhydride (2) in methanol. The colorless plates obtained were characterized by FT-IR, ¹H- and ¹³C-NMR, TGA-DSC, and single crystal X-ray diffraction. The material crystallizes in space group P2₁/c, with a = 9.2375(14) Å, b = 12.8757(18) Å, c = 9.4608(15) Å, β = 115.327(3)°, V = 1017.1(3) Å³, and Z = 4. Chains of hydrogen-bonded molecules along the c-axis interact in the b direction to form layers parallel to the bc plane.