摘要:

An analysis has been carried out of optical heterodyne conversion with an interdigitated‐electrode photomixer made from low‐temperature‐grown (LTG) GaAs and pumped by two continuous‐wave, frequency‐offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0‐&mgr;m‐wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2‐&mgr;m‐wide electrodes and gaps would have a temperature‐limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 &OHgr; load resistance and pumped ath&ngr;=2.0 eV by a total optical power of 50 mW. The predicted 3‐dB bandwidth (193 GHz) of this photomixer is limited by both the electron‐hole recombination time (0.6 ps) of the LTG‐GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.

ISSN:0021-8979    

DOI:10.1063/1.353222

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An analysis of the microwave ( f≳1 GHz) properties of field‐emitter arrays (FEAs) and several representative medium power (10–100 W) microwave amplifiers employing FEAs is presented. The FEA analysis is limited to parallel‐plate structures having discrete pointlike vertical emitter tips and gate apertures aligned to each tip. A transmission line analysis of wave propagation in this structure is presented and used to evaluate the geometries and materials needed for microwave operation. This analysis is used to investigate the performance capabilities and emitter requirements of both modulated‐emission linear beam tubes and microdevices based on FEAs. Specific microtriode designs are used to investigate practical problems such as space charge and thermal effects. Competitive performance should be achievable in gated‐emission linear beam tubes by using FEAs that perform at levels previously reported by several laboratories. Existing FEA technology (currents of 10 &mgr;A per emitter, transconductances of 1 &mgr;S per emitter, 1 &mgr;m oxide thickness, and 3 &mgr;m emitter spacing) is suitable for use in cavity klystrodes(r)at frequencies through 10 GHz, and in moderately bunched beam (bunch width of 180°), octave‐bandwidth traveling‐wave‐tube applications through 3 GHz. Extending the operating frequency and/or reducing the bunch width will require a larger ratio of transconductance to current. Microtriodes operating at 10 GHz will benefit from a modified FEA structure and improved emitter performance.An extra acceleration electrode must be added above the gate aperture to alleviate problems due to space charge between the gate and collector, and the gate oxide thickness must be increased to at least 2 &mgr;m. A FEA incorporating these features and capable of producing 5 &mgr;S and 100 &mgr;A per emitter could generate 130 W from a 5‐mm‐wide device with 8.6 dB gain, 7% bandwidth, and 36% power added efficiency. To allow higher gain and wideband operation, the transconductance at a given current must be increased. A FEA capable of producing 5 &mgr;S at only 10 &mgr;A per emitter would result in a microtriode with more than 1 octave bandwidth, 45 W output power, 10 dB gain, and 34% power added efficiency. Anode‐to‐case temperature differences of less than 100 °C appear possible in this device if BeO is used as the dielectric.

ISSN:0021-8979    

DOI:10.1063/1.353223

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A mechanism for highly selective etching of phosphorus‐doped polycrystalline silicon (n+poly‐Si) on SiO2by employing a Cl2magnetron plasma reactor at low wafer temperatures was investigated. Only the SiO2etch rate drops rapidly in the magnetron plasma at low wafer temperatures below 0 °C. X‐ray photoelectron spectroscopy analysis revealed that only the SiO2surface etched at lower temperatures was covered with silicon chloride or/and oxychloride compounds. The highly dense magnetron plasma decomposes the etched products into unsaturated molecules such as SiCl, SiCl2, and their oxides. These species have a large dipole moment and a higher sticking probability on SiO2than on Si because the SiO2bond is also ionic, and thus attracts a dipole molecule by Coulomb force. Thus, only the SiO2surface was protected by a thin film from chlorine ion bombardment at a certain temperature range in the magnetron plasma. This protection film suppressed SiO2etching, and a high selectivity ofn+poly‐Si/SiO2has been achieved.

ISSN:0021-8979    

DOI:10.1063/1.353224

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Numerical modeling of a novel quantum wire structure formed by the confinement of electrons between lateral quasi‐two‐dimensional (Q2‐D)p–njunctions in a corrugated GaAs/AlGaAs heterostructure is reported on. Such a quantum wire may be realized at the tip of a Si‐doped AlGaAs overgrownVgroove in a SI–GaAs substrate due to the surface orientation dependence of Si doping. The two‐dimensional conduction and valence band potential profiles for the electron and hole charge densities are solved within a semiclassical Thomas–Fermi screening model. The quantized electronic wire states at the heterointerface are then obtained by solving the two‐dimensional effective mass Schro¨dinger equation using the calculated potential profile. The parameter space of the one‐dimensional electron system is explored to establish which features of the structure are dominant factors in controlling the electronic states. It is demonstrated that the energy level spacing of the quantum wire depends primarily on the lateral confinement width in then‐type region at the tip of theVgroove. The ground state energy of the wire is shown to depend on both the lateral confinement width and the vertical heterointerface confinement width. The results of our initial calculations are also reported on to incorporate lateral gates on the surface to obtain direct control of the quantum wire transport properties. The advantages of fabricating quantum wires with this structure compared to conventional methods of electrostatic confinement are discussed.

ISSN:0021-8979    

DOI:10.1063/1.353225

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A nonequilibrium phase transformation in stoichiometric Ni3Sn2during mechanical impact in a high‐energy ball mill was monitored by the measurements of high‐field magnetization, ac susceptibility, and x‐ray diffraction. X‐ray diffraction showed that the material transformed from the low‐temperature Ni3Sn2phase with Ni3Sn2‐type orthorhombic structure to the high‐temperature Ni3Sn2phase with partially filled‐up B8‐type structure after 40 h of milling. Quenching was used to confirm the phase transformation during ball milling. The magnetization of the ball‐milled high‐temperature phase (metastable) is much higher than that of the quenched high‐temperature phase, where both ball‐milled and quenched high‐temperature phases have higher magnetization than the low‐temperature phase. The Curie temperature of our metastable high‐temperature phase is quite close to that of the equilibrium phases (high‐ and low‐temperature phases). The result is discussed in terms of atomic disordering induced by mechanical milling.

ISSN:0021-8979    

DOI:10.1063/1.353226

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Excited state absorption‐induced up conversion at 452 and 480 nm in Tm:Eu:ZBLAN fiber was observed for a single pump laser source at 650 nm. Compared to Tm:ZBLAN fiber, codoping with Eu3+increases the 452 nm emission while the 480 nm emission is reduced. The effective lifetime of the terminal3H4level of the 452 nm transition in Tm3+is shortened by energy transfer to the7F6level in Eu3+. This energy transfer scheme is useful in discriminating radiative transitions.

ISSN:0021-8979    

DOI:10.1063/1.353227

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

To make high‐responsivity YBa2Cu3O7−xmicrobolometers with response times in the tens of microseconds, heat capacity and hence physical size must be small. We measured the responsivities and response times of 2.5×2.5 &mgr;m and 10×10 &mgr;m YBa2Cu3O7−xmicrobolometers fabricated on ZrO2substrates and illuminated by an argon‐ion laser focused to a 2 &mgr;m spot. Responsivities were found to be 975 and 700 V/W, respectively, with response times of 10 and 40 &mgr;s.

ISSN:0021-8979    

DOI:10.1063/1.353228

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Photoemission oscillation from GaAs(001) surfaces illuminated by a deuterium lamp is studied for a highly As‐rich 2×4‐&ggr; phase during molecular beam epitaxy. The 2×4‐&ggr; phase shows higher oscillation amplitude than the As‐poorer 2×4‐&agr; phase. Oscillation of 2×4‐&ggr; occurs as a result of two‐dimensional nucleation, which is confirmed by the suppression of oscillation on highly misoriented GaAs(001) surfaces. Furthermore, the oscillation has its maxima at nearly half‐monolayer growth, which correspond to maximum step‐edge density. For this particular 2×4‐&ggr; phase, this implies that the surface with a higher step‐edge density emits more photoelectron and has a lower ionization energy threshold.

ISSN:0021-8979    

DOI:10.1063/1.353229

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An improved ‘‘tight‐binding’’ Hamiltonian matrix which accounts for the effects of a space‐dependent effective‐mass is formulated. The matrix elements are derived for a single‐particle effective‐mass Hamiltonian in the nearest‐neighbor approximation. In contrast to previous work, the derivation given here strictly preserves the Hermicity of the original continuous‐space Hamiltonian from which the tight‐binding Hamiltonian is derived.

ISSN:0021-8979    

DOI:10.1063/1.353230

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A simple and interesting technique for measuring theHc1of the superconducting grains in bulk highTcsuperconductor (HTSC) samples is presented herein. This technique takes advantage of interesting history effects observed in the nonlinear magnetic response of HTSC samples. The hysteretic second harmonic magnetization shows sharp structure at low applied fields due to the magnetic flux trapped inside the grains. This feature is used to track the penetration of magnetic field inside the superconducting granular regions.

ISSN:0021-8979    

DOI:10.1063/1.354049

出版商:AIP    

年代:1993

数据来源: AIP