Chen-Chia Wang

Antimonides with the half-Heusler structure: New thermoelectric materials

The thermoelectric properties near ambient temperature of half-Heusler alloys based on LnPdSb, where Ln=Ho, Er, and Dy are reported. The Seebeck coefficients are large, between 60 and 250 μV/K, and the materials are p type. The resistivities are between 0.6 and 20 mΩ cm. Thermal conductivities are between approximately 5.0 and 3.5 W/mK at 300 K, and are smallest in intentionally disordered materials. The highest ambient temperature ZT obtained is 0.06. Band-structure calculations are presented for LuPdSb. It is suggested that half-Heusler alloys with 18 electrons per formula unit may represent a large class of thermoelectric materials.

Cost-effective low timing jitter passively Q-switched diode-pumped solid-state laser with composite pumping pulses

A novel scheme that combines gain switching with passive Q switching of a miniature diode-pumped solid-state laser is proposed and implemented. A composite pumping pulse, consisting of a long, low-intensity pulse and a following short, high-intensity pulse, is used to reduce the timing jitter. A greater-than-tenfold reduction in timing jitter is demonstrated.

Simple laser velocimeter that uses photoconductive semiconductors to measure optical frequency differences.

The dc photocurrents generated by steady-state moving space-charge fields inside photoconductive semiconductors containing deep level donors and traps can be used to determine the relative frequency differences between the two interfering optical fields that establish the space-charge fields. A simple laser velocimeter that uses a semi-insulating GaAs:Cr sample to detect the Doppler frequency shift between two laser beams is demonstrated.

Standoff photoacoustic detection of explosives using quantum cascade laser and an ultrasensitive microphone

Standoff detections of explosives using quantum cascade lasers (QCLs) and the photoacoustic (PA) technique were studied. In our experiment, a mid-infrared QCL with emission wavelength near 7.35 μm was used as a laser source. Direct standoff PA detection of trinitrotoluene (TNT) was achieved using an ultrasensitive microphone. The QCL output light was focused on explosive samples in powder form. PA signals were generated and detected directly by an ultrasensitive low-noise microphone with 1 in. diameter. A detection distance up to 8 in. was obtained using the microphone alone. With increasing detection distance, the measured PA signal not only decayed in amplitude but also presented phase delays, which clearly verified the source location. To further increase the detection distance, a parabolic sound reflector was used for effective sound collection. With the help of the sound reflector, standoff PA detection of TNT with distance of 8 ft was demonstrated.

Recent Progress in the Development of Neodymium-Doped Ceramic Yttria

Solid-state lasers play a significant role in providing the technology necessary for active remote sensing of the atmosphere. Neodymium-doped yttria (Nd:Y2O3) is considered to be an attractive material due to its possible lasing wavelengths of ~914 and ~946 nm for ozone profiling. These wavelengths, when frequency tripled, can generate ultraviolet (UV) light at ~305 and ~315 nm, which is particularly useful for ozone sensing using differential absorption light detection and ranging (LIDAR) technique. For practical realization of space-based UV transmitter technology, ceramic Nd:Y2O3 material is considered to possess a great potential. A plasma melting and quenching method has been developed to produce Nd3+-doped powders for consolidation into Nd: Y2O3 ceramic laser materials. This far-from-equilibrium processing methodology allows higher levels of rare earth doping than can be achieved by equilibrium methods. The method comprises two main steps: 1) plasma melting and quenching to generate dense, and homogeneous doped metastable powders and 2) pressure-assisted consolidation of these powders by hot isostatic pressing to make dense nanocomposite ceramics. Using this process, several in 1times1 ceramic cylinders have been produced. The infrared transmission of a 2-mm-thick undoped Y2O3 ceramic was as high as ~75% without antireflection coating. In the case of Nd:Y2O3, ceramics infrared transmission values of ~50% were achieved for a similar sample thickness. Furthermore, Nd:Y2O3 samples with dopant concentrations of up to ~2 at.% were prepared without significant emission quenching.

Epitaxial growth and characterization of GaP on insulating substrates

Heteroepitaxy of gallium phosphide on insulating substrates has been studied. Undoped epitaxial films with thicknesses up to 25 μm have been successfully grown on (0001) and (1012) sapphire and on (111) spinel by the reaction between trimethyl gallium and phosphine using hydrogen as the carrier gas. Gallium phosphide has also been grown on thin (2000–3000 A) silicon grown on sapphire, forming a multilayer structure. The epitaxial film-substrate composites have been characterized by X-ray diffraction, electron microscopy, and optical methods. Information on the growth characteristics, epitaxial orientation relationships, and surface structures have been obtained. The crystalline perfection of the epitaxial gallium phosphide has been examined by transmission X-ray topography. It has been found that the epitaxial GaP deposits are composed of crystallites which are misoriented by ±0.1° from the nominal orientation of the layer. Electrical properties of selected films have been assessed by Hall measurements. Electron mobilities up to 70 cm2/V-sec have been measured in unintentionally doped films grown on (0001) sapphire.

Growth and characterization of large stoichiometric magnesium aluminate spinel single crystals

Abstract Large stoichiometric magnesium aluminate spinel single crystals with sizes up to one inch on an edge have been grown at 1250 °C by a flux method. Purification of the crystals from microscopic flux inclusions was obtained by a post-growth vacuum annealing process. The crystals were characterized by wet chemical analysis, X-ray and electron diffraction, optical microscopy, emission and mass spectroscopy. Information on composition, homogeneity, impurity content and lattice parameter was obtained. The crystals were examined by Lang X-ray topography. They exhibit dislocation densities as low as 50 lines/cm 2 and are of superior quality when compared to Verneuil and Czochralski melt-grown spinel crystals which were observed to have dislocation densities in excess of 10 5 and 10 3 lines/cm 2 respectively. Stacking faults with fault vectors R = 1/4 〈101〉 and randomly scattered small volumes of precipitates were observed in most of the crystals. Growth striations were found in some crystals. They are probably due to the segregation of impurities at the {111} solid-liquid interfaces during growth.

Photocurrents in photoconductive semiconductors generated by a moving space-charge field

The behavior of photocurrents generated in the semiconductors InP:Fe, GaAs:Cr, and undoped GaAs inoptical two-wave mixing experiments as a function of the frequency difference between the two waves is characterized and verified experimentally. The sign, the concentration, and the ratio gammaR/micro of the dominant charge carriers and the effective concentration of trapping centers can be found from this dependence.

Enhanced detection bandwidth for optical doppler frequency measurements using moving space charge field effects in GaAs multiple quantum wells

Moving space charge field (photo-emf) effects are observed in standard and low temperature grown GaAs multiple quantum well (MQW) samples in which dc photocurrents are generated by constant speed moving optical interference patterns. Experiments reveal an enhanced detection bandwidth for optical doppler frequency measurements compared to previously studied bulk II-VI and III-V materials due to the higher quantum efficiency and optical absorption of GaAs MQWs, in spite of their reduced carrier mobility-lifetime product values resulting mainly from the significantly shorter free carrier lifetimes. The high defect density of low temperature grown MQWs can also yield large field-of-view and, for optical ultrasound detections, high cutoff frequency.

Signal strength enhancement and bandwidth tuning in moving space charge field photodetectors using alternating bias field

Enhancement of the photocurrent density generated by internal moving space charge electric fields in semi-insulating GaAs multiple quantum wells is observed when the sample is biased with a high speed, zero-mean square wave electric field which increases the internal space charge electric field strength. Tunability of the space charge field formation time is also demonstrated by adjusting the amplitude of the bias signal. This technique can be used to enhance the sensitivity of optical doppler frequency measurements based on moving space charge field effects in photoconductive semiconductors. Electronic tunability of the space charge field formation time also adds versatility in optical power spectrum analysis applications.