No abstract

We explore the effects of surfactant-mediated epitaxy on the structural, electrical, and optical properties of fast metal-semiconductor superlattice photoconductors. Specifically, application of a bismuth flux during growth was found to significantly improve the properties of superlattices of LuAs nanoparticles embedded in In 0.53 Ga 0.47 As. These improvements are attributed to the enhanced structural quality of the overgrown InGaAs over the LuAs nanoparticles. The use of bismuth enabled a 30% increase in the number of monolayers of LuAs that could be deposited before the InGaAs overgrowth degraded. Dark resistivity increased by up to $15Â while carrier mobility remained over 2300 cm 2 /V-s and carrier lifetimes were reduced by >2Â at comparable levels of LuAs deposition. These findings demonstrate that surfactant-mediated epitaxy is a promising approach to enhance the properties of ultrafast photoconductors for terahert generation.

A high-resolution liquid-nitrogen-free mid-infrared tunable diode laser absorption spectroscopy (TDLAS) system was used to perform real-time measurement of acetaldehyde concentrations in human exhaled breath following ingestion of an alcoholic beverage. Acetaldehyde absorption features were measured near 5.79 mum (1727 cm(-1)) using a IV-VI semiconductor laser, a 100 m long path optical gas cell, and second- harmonic detection coupled with wavelength modulation. Acetaldehyde levels were measured with a minimum detection limit of 80 ppb for 5 s integration time. The variations in exhaled acetaldehyde levels over time were analyzed prior to and following ingestion of two different amounts of white wine. A method to calibrate acetaldehyde measurements internally using water vapor absorption lines was investigated to eliminate the need for system calibration with gas standards. The potential of a TDLAS system to be used as a noninvasive clinical tool for measurements of large volatile compounds with possible applications in cancer detection is demonstrated.

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °°°°C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20 °C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.

Epitaxial growth of PbSe/BaF 2 /CaF 2 heterostructures was carried out by molecular beam epitaxy ͑MBE͒ on Si͑111͒ wafers. Successful transfer of 3-m-thick PbSe epilayers was accomplished by bonding the MBE-grown samples face down to polished copper plates followed by the removal of the silicon substrate by dissolving the BaF 2 buffer layer in water. High-resolution x-ray diffraction measurements demonstrated that the PbSe epilayer maintained high-crystalline quality after transfer. In addition, optical Nomarski characterization of the exposed growth interface showed sets of parallel straight step lines consistent with glide of dislocations in the primary ͕100͖͗110͘ glide system. Such features are evidence of the large thermal expansion mismatch strain that occurred in these layers.