The initial discovery by Bednorz and Muller1 of 35-K superconductivity in the La-Ba-Cu-O system has stimulated worldwide activity in searching for higher-temperature superconductors. Elemental substitution has proved to be most effective in raising transition temperature. Substitution of Sr for Ba has produced 40-K superconductivity2–5and substitution of Y for La has produced a new high-temperature superconductor with transition temperature above liquid-nitrogen temperature6. A class of superconducting compounds of the form RBa2Cu307-x has been explored by further substitutions of other rare earths (Y is considered in the rare-earth [RI category here) for Y7-13. To date, a rare earth, an alkaline earth, copper and oxygen have been required for all high-temperature superconductors14,15. (Zhanget al 14reported 90-K superconductivity in the Th-Ba-Pb(Zr)-Cu-O system. Panetal15reported 50-K superconductivity in the Y-Ba-Ag-O system. As Th is a member of the actinide series which belongs to the same Group 3B in the periodic table as the lanthanide series and Ag belongs to the same Group 1B as Cu, high-temperature supercon-ductors are still thought to be closed in the Group 3B—Group 2A-Group 1B—oxygen system. ) Only partial substitutions ha. e led to superconductors, but with no significant rise of transition tem-perature (the only exception is 40-K superconductivity in La2CuO4-x , refs 16, 17). Here we report superconductivity in the rare-earth-free TI-Ba-Cu-O system. We have obsened sharp drops of resistance starting above 90 K with zero resistance at 81 K in this system. Magnetic measurements have confirmed that these sharp drops of resistance in the TI-Ba-Cu-O samples origi-nate from superconductivity. The samples are stable in air for at least two months, and their preparation is easily reproduced.

Superconductivity in the rare-earth-free Tl–Ba–Cu–O system above liquid-nitrogen temperature

Long‐range ordering in a (GaP)2/(InP)2 short‐period superlattice and a Ga0.525In0.475P buffer layer grown on a (001)GaAs substrate by gas source molecular beam epitaxy were studied. Transmission electron microscopy and low‐temperature cathodoluminesence techniques were used to examine the microstructure of the short‐period superlattice and to determine its band‐gap energy. The superlattice layer was found to have a [001] long‐range ordered structure with a band gap narrowing of about 130 meV, while the Ga0.525In0.475P layer had a 37 meV band‐gap narrowing induced by spontaneous long‐range ordering in the [111] direction. The ordered superlattice layer was found to have a growth‐induced lateral periodic modulation of the composition along the [110] direction. Within the modulating bands, which had a 200 A periodicity, the In composition was found to vary from 42 to 56% while the Ga correspondingly varied between 58 and 44%.

Compositional modulation and long‐range ordering in GaP/InP short‐period superlattices grown by gas source molecular beam epitaxy

Lp-movpe Growth and Optical Characterization of Galnp/gaas Heterostructures: Interfaces, Quantum Wells and Quantum Wires

This study explores the dislocation dynamics of strain relaxation in graded composition buffers of InxGa1−xP grown on GaP (InxGa1−xP/GaP) by metalorganic vapor phase epitaxy. Transmission electron microscopy, cathodoluminescence imaging, atomic force microscopy, and triple-axis x-ray diffraction are applied to the characterization of InxGa1−xP/GaP with final compositions ranging from x=0.09 to x=0.39 and growth temperatures ranging from 650 to 810 °C. The previously reported escalation of defect density with continued grading of InxGa1−xP/GaP beyond x∼0.3 is discovered to be due to the formation of dislocation pileups. A new defect microstructure with a branching morphology and featuring sharp local strain fields, hereafter referred to as branch defects, is observed to pin dislocations and cause the dislocation pileups. Branch defect morphology varies strongly with growth temperature, becoming significantly stronger with increasing growth temperature and causing severe material degradation above 700 °C. F...

Evolution of microstructure and dislocation dynamics in InxGa1−xP graded buffers grown on GaP by metalorganic vapor phase epitaxy: Engineering device-quality substrate materials

Growth- and processing-induced defects in semiconductors

Transmission electron microscopic observation of InGaP crystals grown on (001)GaAs substrates by metalorganic chemical vapor deposition

Pseudomorphic n-InGaP/InGaAs/GaAs grown by MOVPE for HEMT LSIs

Uniform and abrupt InGaP/GaAs selectively doped heterostructures grown by MOVPE for HEMT ICs

We report the first successful multi-wafer growth of HEMT LSI quality AlGaAs/GaAs selectively doped heterostructures by atmospheric pressure MOCVD. Highly uniform AlGaAs films with variations in thickness and doping characteristics of less than ± 2.0% and ± 1.5 %, respectively, have been grown on simultaneously revolving and rotating substrates. A trial fabrication of HEMTs resulted in threshold voltage standard deviations as small as 23 mV for E-HEMTs and 35 mV for D-HEMTs distributed over an entire two-inch wafer.

Multi-Wafer Growth of HEMT LSI Quality AlGaAs/GaAs Heterostructures by MOCVD

Postamplifier and p-i-n/preamplifier modules suitable for 40-Gb/s short-haul and long-haul optical communication systems are presented. The postamplifier module consists of two stages of pseudomorphic high-electron mobility transistor (PHEMT) GaAs amplifiers mounted in a housing. A test fixture is used to provide bias supplies that control the output signal. The postamplifier has a 20-dB gain and 4.5-dB noise figure. The input voltage swing of the postamplifier can be varied from 10 to 148 mV p-p while providing an open output eye-pattern response over the complete range. The p-i-n/preamplifier module consists of a waveguide p-i-n/photodetector, with a polarization insensitive spot-size converter and a PHEMT-based GaAs transimpedance amplifier. The components are integrated inside a hermetically sealed compact package using hybrid techniques. The p-i-n/preamplifier module having 150-/spl Omega/ transimpedance exhibits a linear output voltage swing over 800 mV p-p and a responsivity of 0.8 A/W. The receiver module has a 3-dB bandwidth as high as 45 GHz, and the input equivalent noise current density is less than 15 pA//spl radic/(Hz). The electrical sensitivity of the postamplifier is found to be 7.8 mV p-p for a 40 Gb/s, 2/sup 7/-1 pseudorandom binary sequence nonreturn-to-zero signal. When used in combination with a 150-/spl Omega/ 40-Gb/s p-i-n/preamplifier module, a sensitivity of less than -11.2 dBm is achieved. These results indicate that the postamplifier module, in combination with the p-i-n/preamplifier module, can be used in 40-Gb/s short-haul systems, while the p-i-n/preamplifier module itself can be used for long-haul front-end receiver systems.

Masaru Takechi

Papers

Transmission electron microscopic observation of InGaP crystals grown on (001)GaAs substrates by metalorganic chemical vapor deposition

Pseudomorphic n-InGaP/InGaAs/GaAs grown by MOVPE for HEMT LSIs

Uniform and abrupt InGaP/GaAs selectively doped heterostructures grown by MOVPE for HEMT ICs

Multi-Wafer Growth of HEMT LSI Quality AlGaAs/GaAs Heterostructures by MOCVD

40 Gb/s postamplifier and p-i-n/preamplifier modules for next generation optical front-end systems