210 research outputs found

    Electronic Materials with Wide Band Gap: Recent Developments

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    The development of semiconductor electronics is shortly reviewed, beginning with the development of germanium devices (band gap Eg=0.66E_g=0.66 eV) after world war II. Quickly a tendency to alternative materials with wider band gap became apparent, starting with silicon (Eg=1.12E_g=1.12 eV). This improved the signal/noise ratio for classical electronic applications. Both semiconductors have tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or several anions and cations other semiconductors with wider EgE_g are obtained, that are transparent for visible light and belong to the group of wide band gap semiconductors. Nowadays some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue spectral region. Oxide crystals, such as ZnO and β\beta-Ga2_2O3_3, offer similarly good electronic properties but suffer still from significant difficulties in obtaining stable and technically sufficient pp-type conductivity.Comment: 25 pages, 8 figures, 4 table

    Growth of Oxide Compounds under Dynamic Atmosphere Composition

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    Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10^{-6} bar, independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga2O3, or Fe{1-x}O can be grown from the melt.Comment: 4 pages, 3 figures, talk on CGCT-4 Sendai, May 21-24, 200

    Pressure dependence of phase transitions in the quasi one-dimensional metal-insulator transition system beta-Na1/3V2O5

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    The pressure dependence of phase transitions in the quasi one-dimensional vanadium oxide β\beta-Na1/3_{1/3}V2_2O5_5 has been studied by magnetic susceptibility and electrical resistivity measurements. The pressure dependence of the various transition temperatures is quite differently. The transition at T=240 K, previously reported and attributed to ordering on Na sites, and a second transition at T≈222T \approx 222 K, reported here for the first time and attributed to a further increase of order on Na sites, are almost independent of pressure. On the other hand, the metal-insulator (MI) transition at TMI=130T_{MI}=130 K shifts to lower temperatures, while the magnetic transition at TN=24T_N=24 K shifts to higher temperatures with increasing pressure. We discuss the different pressure dependencies of TMIT_{MI} and TNT_N in terms of increasing interchain coupling and the MI transition to be of Peierls type.Comment: 5 pages, 5 figure

    On the Crystallization of Terbium Aluminium Garnet

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    Attempts to grow terbium aluminium garnet (Tb3Al5O12, TAG) by the Czochralski method lead to crystals of millimeter scale. Larger crystals could not be obtained. DTA measurements within the binary system showed that TAG melts incongruently at 1840 deg. C. The perovskite (TbAlO3, TAP) with a congruent melting point of 1930 deg. C is the most stable phase in this system. The region for primary crystallization of TAP covers the chemical composition of TAG and suppresses the primary crystallization of the terbium aluminium garnet.Comment: 6 pages, 2 figure

    Thermodynamic modeling of the LiF-YF3 phase diagram

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    A thermodynamic optimization of the LiF-YF3 binary phase diagram was performed by fitting the Gibbs energy functions to experimental data that were taken from the literature, as well as from own thermoanalytic measurements (DTA and DSC) on HF-treated samples. The Gibbs energy functions for the end member compounds were taken from the literature. Excess energy terms, which describe the effect of interaction between the two fluoride compounds in the liquid phase, were expressed by the Redlich-Kister polynomial function. The calculated phase diagram and thermodynamic properties for the unique formed compound, LiYF4, are in reasonable agreement with the experimental data.Comment: 4 pages, 3 figure

    Phase diagram analysis and crystal growth of solid solutions Ca_{1-x}Sr_xF_2

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    The binary phase diagram CaF2_2--SrF2_2 was investigated by differential thermal analysis (DTA). Both substances exhibit unlimited mutual solubility with an azeotropic point showing a minimum melting temperature of T_\mathrm{min}=1373^{\circ}CforthecompositionCaC for the composition Ca_{0.582}SrSr_{0.418}FF_2$. Close to this composition, homogeneous single crystals up to 30 mm diameter without remarkable segregation could be grown by the Czochralski method.Comment: accepted for publication in J. Crystal Growt
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