Jonathan C. Denyszyn

Synthesis and Characterization of Sr2MgMoO6 − δ An Anode Material for the Solid Oxide Fuel Cell

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition

We report the growth, fabrication and characterization of high-quality AlGaN/GaN solar-blind p-i-n and MSM photodetectors by low-pressure metalorganic chemical vapor deposition (MOCVD). The epitaxial layers were grown on double-polished c-plane (0001) sapphire substrates to allow for back-side illumination. The p-i-n photodiode structures typically consist of a 0.7 μm thick Al 0.58 Ga 0.42 N window layer, graded to a 0.2 μm thick Al 0.47 Ga 0.53 N n layer, a 0.15 μm thick Al 0.39 Ga 0.61 N i layer, a 0.2 μm thick Al 0.47 Ga 0.53 N p layer, and capped with a 25 nm GaN:Mg contact layer. At a 0 V bias, the processed p-i-n devices exhibit a solar-blind photoresponse having a maximum responsivity of 0.058 A/W at 279 nm, corresponding to an external quantum efficiency of ∼26%, uncorrected for reflections, etc. The MSM devices typically consist of an AIGaN x ∼ 0.58 window layer, and an undoped AlGaN x ∼ 0.44 absorbing layer. The MSMs exhibit an external quantum efficiency as high as ∼47% at a bias of 15 V with a peak response at 262 nm.

Synthesis and characterization of Sr2MgMoO6-δ

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

AlGaN-GaN UV light-emitting diodes grown on SiC by metal-organic chemical vapor deposition

We report the study of the electrical and optical characteristics of AlGaN-GaN quantum-well (QW) ultraviolet light-emitting diodes grown on SiC by metal-organic chemical vapor deposition. These devices exhibit room-temperature electroluminescence emission peaked at /spl lambda/ = 363 nm with a narrow linewidth of /spl Delta//spl lambda/ = 9 nm under high-current-density dc injection. We have also applied a Mg-doped AlGaN-GaN superlattice structure as a p-cladding layer and vertical-geometry hole conduction improvement has been verified. A comparative study of the performance of light-emitting devices with single-QW and multiple-QW structures indicates that the single-QW structure is preferred.

Synthesis and Characterization of Sr[sub 2]MgMoO[sub 6−δ]

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

Improved Performance of AlGaN/GaN Heterojunction Field-Effect Transistors Using Delta Doping and a Binary Barrier

The improved performance of AlGaN/GaN heterojunction field-effect transistors using a delta-doping approach along with an AlN binary barrier is reported. Low-pressure metalorganic chemical vapor deposition was used to grow the epitaxial heterostructures on semi-insulating SiC substrates. The maximum carrier mobility of µ=1,066 cm2/V-s and sheet carrier density of ns~2.30×1013 cm-2 yields a large nsµ product of 2.45×1016 V-s. Devices with 0.15 µm gate lengths exhibited a maximum current density of IDSmax=1.82 A/mm (at VG=+1 V) and a peak transconductance of gm=331 mS/mm. Furthermore, fT~55 GHz and fmax~115 GHz were measured.

GaN and AlxGa1-xN p-i-n high-voltage rectifiers grown by metalorganic chemical vapor deposition

We report the study of the electrical characteristics of GaN and Al x Ga 1-x N vertical p-i-n rectifiers grown by metalorganic chemical vapor deposition. The forward and reverse I-V characteristics as a function of device structure, device size and process approach have been experimentally investigated. A vertical-geometry GaN p-i-n rectifier with a relatively thin i region of 2 μm thickness exhibits a breakdown voltage over 420 V and a forward voltage as low as 6 V at 100 A/ cm 2 on a 60 μm diameter device. An Al 0.4 Ga 0.6 N p-i-n with a thin i region of 0.15 μm shows a blocking voltage in excess of 460 V and a forward voltage drop of 14 V at 100 A/cm 2 .