Correlation of in-situ reflectance spectra and resistivity of GaN/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interfacial layer in metalorganic chemical vapor deposition

The correlation between the resistivity of an undoped GaN/Al₂O₃ interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an ∼0.25-μm-thick layer near the GaN/Al₂O₃ interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the O and Al diffused from Al₂O₃, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., ∼2.5 μm).     

Fabrication of solid-state dye-sensitized TiO2 solar cells combined with polypyrrole

A solid-state solar cell was fabricated by photoelectrochemical polymerization of pyrrole on porous nanocrystalline TiO₂ electrode sensitized by the Grätzel dye, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium (II) dihydrate, [RuL₂(NCS)₂]), or a newly synthesized cis-Ru(dcb)₂(pmp)₂ (pmp=3-(pyrrole-1-ylmethyl)-pyridine). Polypyrrole successfully worked as a hole-transport layer with improvement of the cell characteristics when the TiO₂ cell with cis-Ru(dcb)₂(pmp)₂ was compared with the similarly fabricated cells using [RuL₂(NCS)₂]. The improvement by using Ru(dcb)₂(pmp)₂ can be explained as due to direct molecular wiring of the polymer-chain to the excited metal center of the complex.     

Single-Cycle Terahertz Pulse Generation from OH1 Crystal via Cherenkov Phase Matching

OH1 crystal is an organic nonlinear optical crystal with a large nonlinear optical constant. However, it has dispersion of refractive indices in the terahertz (THz) frequency. This limits the frequencies that satisfy the phase matching conditions for THz wave generation. In this study, we addressed the phase matching conditions for THz wave generation by combining an OH1 crystal with prism-coupled Cherenkov phase matching. We observed the generation of single-cycle THz pulses with a spectrum covering a frequency range of 3 THz. These results prove that combining prism-coupled Cherenkov phase matching with nonlinear optical crystals yields a THz wave generation method that is insusceptible to crystal dispersion.     

A case study in computer-aided co-design of embedded controllers

We present an application of the methodology and of the various software tools embedded in the POLIS co-design system. The application is in the realm of automotive electronics: a shock absorber controller, whose specification comes from an actual product. All aspects of the design process are closely examined, including high level language specification and automatic hardware and software synthesis. We analyze different software implementation styles, compare the results, and outline the future developments of our work.This work was partially supported by SRC Contract DC-324-028 and by MURST Research Project VLSI Architectures.     

High-performance 770-nm AlGaAs-GaAsP tensile-strained quantum-welllaser diodes

Experimental results on tensile-strained Al_0.3Ga_0.66As-GaAs_0.78P_0.22 separate-confinement-heterostructure single-quantum-well (SCH-SQW) laser diodes are reported. Threshold current density as low as 260 A/cm² for broad-stripe lasers with a cavity length of 1500 μm has been observed. Broad-stripe devices have operated cw at room temperature with output power as high as 620 mW/facet. Ridge-waveguide lasers have exhibited cw threshold currents as low as 13.5 mA and output power of 90 mW. The output of the tensile-strained lasers is TM polarized