Performance improvement in tensile-strained In/sub 0.5/Al/sub 0.5/As/In/sub x/Ga/sub 1-x/As/In/sub 0.5/Al/sub 0.5/As metamorphic HEMT

This paper proposes a In/sub 0.5/Al/sub 0.5/As/In/sub x/Ga/sub 1-x/As/In/sub 0.5/Al/sub 0.5/As (x=0.3-0.5-0.3) metamorphic high-electron mobility transistor with tensile-strained channel. The tensile-strained channel structure exhibits significant improvements in dc and RF characteristics, including extrinsic transconductance, current driving capability, thermal stability, unity-gain cutoff frequency, maximum oscillation frequency, output power, power gain, and power added efficiency.     

BCB-bridged distributed wideband SPST switch using 0.25-/spl mu/m In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As metamorphic HEMTs

In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As metamorphic high-electron mobility transistor (mHEMT) dc-30 GHz distributed single-pole-single through (SPST) switches were designed and fabricated using the low-/spl kappa/ benzocyclobutene (BCB) bridged technology. The current gain cutoff frequency, and the electron transit time of In/sub 0.5/Al/sub 0.5/As--In/sub 0.5/Ga/sub 0.5/As mHEMTs have been investigated. By analyzing the extrinsic total delay time, the effective velocity of electrons can be estimated, and the average velocity is 2.3/spl times/10/sup 7/cm/s. The dc-30 GHz distributed wideband SPST switch exhibits an insertion loss of less than 5.5 dB, and an isolation larger than 30 dB, which is the first demonstration of the high-isolation of InAlAs-InGaAs mHEMTs monolithic switch. As to the power performance, this switch can handle the power up to 12 dBm at 2.4 GHz. After over 250 h of 85-85 (temperature =85/spl deg/C, humidity =85%) environmental evaluation, this BCB passivated and bridged microwave and monolithic integrated circuit switch demonstrates reliable RF characteristics without any significant performance change, which proves that this process using the low-/spl kappa/ BCB layer is attractive for millimeter-wave circuit applications.     

High breakdown voltage (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P/InGaAs quasi-enhancement-mode pHEMT with field-plate technology

A high breakdown voltage and a high turn-on voltage (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P/InGaAs quasi-enhancement-mode (E-mode) pseudomorphic HEMT (pHEMTs) with field-plate (FP) process is reported for the first time. Between gate and drain terminal, the transistor has a FP metal of 1 /spl mu/m, which is connected to a source terminal. The fabricated 0.5/spl times/150 /spl mu/m/sup 2/ device can be operated with gate voltage up to 1.6 V owing to its high Schottky turn-on voltage (V/sub ON/=0.85 V), which corresponds to a high drain-to-source current (I/sub ds/) of 420 mA/mm when drain-to-source voltage (V/sub ds/) is 3.5 V. By adopting the FP technology and large barrier height (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P layer design, the device achieved a high breakdown voltage of -47 V. The measured maximum transconductance, current gain cutoff frequency and maximum oscillation frequency are 370 mS/mm, 22 GHz , and 85 GHz, respectively. Under 5.2-GHz operation, a 15.2 dBm (220 mW/mm) and a 17.8 dBm (405 mW/mm) saturated output power can be achieved when drain voltage are 3.5 and 20 V. These characteristics demonstrate that the field-plated (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P E-mode pHEMTs have great potential for microwave power device applications.     

Preparation and growth of thin film Pb/sub 2/Sr/sub 2/Ca/sub 0.5/Y/sub 0.5/Cu/sub 3/O/sub 8+ delta / superconductors by laser ablation

For the first time, the preparation of thin films of superconducting Pb/sub 2/Sr/sub 2/Y/sub 0.5/Ca/sub 0.5/Cu/sub 3/O/sub 8+ delta / material using standard bulk target preparation conditions and the laser ablation technique is reported. In common with much of the bulk characteristics, it is found that the width of the superconducting transition is large, extending from T/sub c,onset/ between 70-83 K and T/sub c,zero/ between 16-20 K.<>     

Room-temperature self-organised In/sub 0.5/Ga/sub 0.5/As quantum dot laser on silicon

The first room-temperature operation of In/sub 0.5/Ga/sub 0.5/As quantum dot lasers grown directly on Si substrates with a thin (/spl les/2 /spl mu/m) GaAs buffer layer is reported. The devices are characterised by J/sub th//spl sim/1500 A/cm/sup 2/, output power >50 mW, and large T/sub 0/ (244 K) and constant output slope efficiency (/spl ges/0.3 W/A) in the temperature range 5-95/spl deg/C.     

Invasiveness of LiTaO/sub 3/ and GaAs probes in external E-O sampling

Disturbances of signals on a coplanar waveguide (CPW) induced by the presence of LiTaO/sub 3/ and GaAs electrooptic probes in external electrooptic (EO) sampling have been simulated and compared quantitatively. The finite-difference-time-domain method is used to simulate the full wave field around a coplanar waveguide on a GaAs substrate in an external EO sampling configuration. The results indicate that the induced signal disturbance, or invasiveness, of a LiTaO/sub 3/ probe is almost ten times that of a GaAs probe in terms of the magnitude of S/sub 11/, but that LiTaO/sub 3/ yields about two times the EO response for a given S/sub 11/ and optical probing wavelength. The transparency of LiTaO/sub 3/ to shorter wavelengths, however, allows an even higher sensitivity for this material relative to GaAs. The results suggest that these probes do not exhibit significant invasiveness (magnitude of S/sub 11/ smaller than -40 dB), if they are removed from contact by the distance of CPWs center conductor width.<>     

Low-loss electrooptic BaTiO/sub 3/ thin film waveguide modulator

A strip-loaded electrooptic waveguide modulator based on an epitaxial BaTiO/sub 3/ thin film was fabricated and characterized for the first time. The strip-loaded waveguide structure greatly improves waveguide propagation and polarization-dependent loss performance. A propagation loss of 1.1 dB/cm and polarization dependent loss of 0.1 dB/cm were measured. The electrooptic waveguide modulator exhibited a half-wave voltage-interaction length product of 4.5 V /spl middot/ cm at a wavelength of 1542 nm. The measured effective electrooptic coefficient of the as-grown BaTiO/sub 3/ waveguide modulator was 38 pm/V. The experimental results indicate that a strip-loaded thin film waveguide modulator is suitable for photonic applications.     

Lasing at three-dimensionally quantum-confined sublevel of self-organized In/sub 0.5/Ga/sub 0.5/As quantum dots by current injection

A laser oscillation from self-organized In/sub 0.5/Ga/sub 0.5/As quantum dots is achieved at 80 K by current injection. Lasing at a three-dimensionally quantum confined sublevel of the In/sub 0.5/Ga/sub 0.5/As quantum dots is clearly demonstrated for the first time by electroluminescence and diamagnetic energy shift measurement. The results predict the possibility of ultra-low threshold current operation of quantum dot lasers.     

The microwave power performance comparisons of Al/sub x/Ga/sub 1-x/As/In/sub 0.15/Ga/sub 0.85/As (x=0.3, 0.5, 0.7, 1.0) doped-channel HFETs

The properties of doped-channel field-effect transistors (DCFET) have been thoroughly investigated on Al/sub x/Ga/sub 1-x/As/InGaAs (x= 0.3, 0.5, 0.7, 1) heterostructures with various Al mole fractions. In this study, we observed that by introducing a 200-/spl Aring/-thick Al/sub 0.5/Ga/sub 0.5/As (x=0.5) Schottky layer can enhance the device power performance, as compared with the conventional x=0.3 AlGaAs composition system. However, a degradation of the device power performance was observed for further increasing the Al mole fractions owing to their high sheet resistance and surface states. Therefore, Al/sub 0.5/Ga/sub 0.5/As Schottky layer design provides a good opportunity to develop a high power device for power amplifier applications.     

Effect of BaCeO/sub 3/ and BaSnO/sub 3/ additives on microstructural development and critical current density of melt textured YBa/sub 2/Cu/sub 3/O/sub 7-x/

Improvement of superconducting properties, such as critical current density J/sub c/ and trapped magnetic field, of melt textured YBa/sub 2/Cu/sub 3/O/sub 7-x/ (Y123) require introducing of effective pinning sites, e.g., nonsuperconducting inclusions, twin boundaries and other defects. It has been shown that addition of small quantities of BaCeO/sub 3/ (<0.5 wt%) into Y123 results in an increase of the J/sub c/. However, higher cerium concentrations affect the solidification process and inhibit the growth of melt textured Y123 single crystals. In this study, the effect of BaCeO/sub 3/ additions on the growth and microstructural development of melt textured Y123 single crystals were investigated. The relationship between the solidification kinetics, microstructural development and superconducting properties of Y123 melt textured single crystals with cerium additives is discussed.     

GeMoW refractory ohmic contacts to n-type GaAs with In/sub 0.5/Ga/sub 0.5/As cap layer

GeMoW is presented as a refractory ohmic contact to n-type GaAs with an In/sub 0./5Ga/sub 0.5/As cap layer. The contact exhibits ohmic behaviour over a wide annealing temperature range from 300 to 700 degrees C. A minimum contact resistance of 0.176 Omega mm was obtained following furnace annealing at 500 degrees C.<>     

Free-space electrooptic sampling of terahertz radiation from optically excited superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films

The characteristics of photogenerated terahertz radiation from a current-biased superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) bow-tie antenna were investigated using a free-space electrooptic sampling technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. The dynamics of the quasi-particles optically induced by the ultrafast laser pulse primarily determines the performances of the transient terahertz radiation generated under different operating parameters. The results indicate a characteristic quasi-particle relaxation time of about 2.5 ps close to the critical temperature T/sub c/, and a faster time at lower temperatures.     

In/sub 0.5/Ga/sub 0.5/As/GaAs quantum dot infrared photodetectors grown by metal-organic chemical vapor deposition

We report the growth by low-pressure metal-organic chemical vapor deposition, fabrication, and characterization of ten-layer In/sub 0.5/Ga/sub 0.5/As/GaAs quantum dot infrared photodetectors. Normal incidence photoresponse of the detector was obtained at 5.9 /spl mu/m. The 77-K peak responsivity was 5.6 mA/W with the detectivity D/sup */ of 1.2/spl times/10/sup 9/ cm/spl middot/Hz/sup 1/2//W at the bias of 0.4 V.     

(Al/sub 0.5/Ga/sub 0.5/)/sub 0.65/In/sub 0.35/P/Ga/sub 0.65/In/sub 0.35/P double-heterostructure orange light-emitting diodes

Visible light-emitting diodes (LEDs) emitting at 615 nm and employing the AlGaInP/GaInP double heterostructure (DH) grown on a lattice-matched GaAs/sub 0.7/P/sub 0.3/ substrate have been fabricated for the first. The external quantum efficiency of 0.156% for the orange LEDs can be achieved by introducing the GaP material as the current spreader and window layer for the DH LEDs.<>     

Effect of Mg doping on the superconducting properties of Cu/sub 1-x/Tl/sub x/Ba/sub 2/Ca/sub 3-y/Mg/sub y/Cu/sub 4/O/sub 12-/spl delta//

Superconducting properties of Cu/sub 1-x/Tl/sub x/Ba/sub 2/Ca/sub 3-y/Mg/sub y/Cu/sub 4/O/sub 12-/spl delta// (Cu/sub 1-x/Tl/sub x/Mg/sub y/-1234) material have been studied in the composition range y=0,1.5,2.25. The zero resistivity critical temperature [T/sub c/(R=0)] was found to increase with the increased concentration of Mg in the unit cell; for y=1.5 [T/sub c/(R=0)]=131 K was achieved which is hitherto highest in Cu/sub 1-x/Tl/sub x/-based superconductors. The X-ray diffraction analyses have shown the formation of a predominant single phase of Cu/sub 0.5/Tl/sub 0.5/Ba/sub 2/Ca/sub 3-y/Mg/sub y/Cu/sub 4/O/sub 12-/spl delta// superconductor with an inclusion of impurity phase. It is observed from the convex shape of the resistivity versus temperature measurements that our as-prepared material was in the region of carrier over-doping, and the number of carriers was optimized by postannealing experiments in air at 400/spl deg/C, 500/spl deg/C, and 600/spl deg/C. The T/sub c/(R=0) was found to increase with postannealing and the best postannealing temperature was found to be 600/spl deg/C. The mechanism of increased T/sub c/(R=0) is understood by carrying out infrared absorption measurements. It was observed through softening of Cu(2)-O/sub A/-Tl apical oxygen mode that improved interplane coupling was a possible source of enhancement of T/sub c/(R=0) to 131 K.     

The performance of pigtailed annealed proton exchange LiNbO/sub 3/ modulators at cryogenic temperatures

Integrated optic modulators have been proposed for transmitting signals from electronic and electrooptic devices which must operate at 77 K and colder. In this letter, the V/spl pi/ of LiNbO/sub 3/ modulators at temperatures as cold as 10 K are measured. A /spl sim/9.25% increase over room temperature values is seen. This increase is attributed primarily to a temperature dependence in r/sub 33/. In addition, temperature-induced insertion loss changes are evaluated for two different pigtailing techniques and one is shown to be suitable for cryogenic operation.     

Bandpass traveling-wave Mach-Zehnder modulator in LiNbO/sub 3/ with domain reversal

A bandpass traveling-wave Mach-Zehnder modulator is demonstrated in a Z-cut LiNbO/sub 3/ substrate where quasi-phase-matching is achieved by using a crystal domain reversal and a simple uniform coplanar waveguide electrode structure. The domain reversal was created, by using electric-field poling, to implement the three-section alternating electrooptic interaction. At the operating wavelength /spl lambda/=1.32 /spl mu/m, the modulator has a 15 GHz-broad bandpass response centered at 25-GHz 3.6-dB fiber-to-fiber insertion loss, 12 V on-off voltage at the center frequency, and -33-dB extinction ratio.     

ARROW's in KTiOPO/sub 4/

Antiresonant reflecting optical waveguides (ARROW's) have been designed and fabricated in z-cut KTiOPO/sub 4/ (KTP) by e-beam direct writing and double Rb/spl rlhar2/K ion exchange. Strong diffusion anisotropy allows the realization of ARROW's in this electrooptical crystal. It was found, that argon-ion dry etching of the crystal surface prevents the ion exchange process totally. Using this as a new masking technique, antiresonant reflectors with lateral tolerances of less than 0.1 /spl mu/m could be realized. Typical quasi-single-mode waveguiding and attenuation of about 1.0 dB/cm were observed at /spl lambda/=860 mm, which are in good agreement with our simulations made by the vectorial finite element method and effective index approach.     

Characteristics of MOSFET Prepared on Si/MgO /spl dot/ Al/sub 2/O/sub 3//SiO/sub 2//Si Structure

A new silicon on insulator (SOI) wafer with epitaxial-Si/ epitaxial-MgO/spl dot/Al/sub 2/O/sub 3/ (0.1 /spl mu/m)/SiO/sub 2/(0.5 /spl mu/m)/     

Analysis and simulation of a mid-infrared P/sup +/-InAs/sub 0.55/Sb/sub 0.15/P/sub 0.30//n/sup 0/-InAs/sub 0.89/Sb/sub 0.11//N/sup +/-InAs/sub 0.55/Sb/sub 0.15/P/sub 0.30/ double heterojunction photodetector grown by LPE

A photovoltaic detector based on an N/sup +/-InAs/sub 0.55/Sb/sub 0.15/P/sub 0.30//n/sup 0/-InAs/sub 0.89/Sb/sub 0./$ d1/sub 1//P/sup +/-InAs/sub 0.55/Sb/sub 0.15/P/sub 0.30/ double heterostructure (DH) suitable for operation in the mid-infrared (MIR) spectral region (2 to 5 /spl mu/m) at room temperature has been studied. A physics based closed form model of the device has been developed to investigate the relative importance of the different mechanisms which determine dark current and photoresponse. The results obtained on the basis of the model have been compared and contrasted with those obtained from experimental measurements on DH detectors fabricated previously in our laboratory using liquid phase epitaxy (LPE). The model helps to explain the various physical mechanisms that shape the characteristics of the device under room temperature operation. It can also be used to optimize the performance of the photodetector in respect of dark current, responsivity and detectivity. A comparison of theoretical predictions and experimental results revealed that Shockley-Read-Hall (SRH) recombination is more important than Auger recombination in determining the room temperature detector performance when the concentration of nonradiative recombination centers in our material exceeds 10/sup 17/ cm/sup -3/. Furthermore, compositional grading in the cladding regions of the double heterostructure has been found to be responsible for the reduction of the detectivity of the device in the shorter wavelength region.