摘要:

Periodic submicrometer dot structures were fabricated by laser‐induced surface electromagnetic wave (SEW) etching ofn‐GaAs substrates using the holographic exposure system of the frequency‐tripled Nd:YAG laser. The shorter and longer diameters were about 310 and 540 nm, respectively. Spatial periods along two perpendicular axes were 370 and 545 nm, respectively. These periods were nearly in agreement with the calculation. The period of SEW grating depended on not only the laser wavelength, but also the number density of the laser‐induced quasifree carriers.

ISSN:0021-8979    

DOI:10.1063/1.353161

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Fe—Ni alloys were prepared by mechanical attrition of elemental powders in a planetary ball mill in argon atmosphere. A nonequilibrium phase diagram of Fe100−xNix, mechanically alloyed at various milling intensities, is presented. Disordered body‐centered‐cubic and/or face‐centered‐cubic supersaturated solid solutions with a two‐phase region are found. The maximum solubilities are 30 at. % Ni in Fe and 40 at. % Fe in Ni and decrease slightly with increasing milling intensity, i.e., processing temperature. Mechanically alloyed Fe—Ni is in a nanocrystalline state with an average grain size of 25–35 nm. The grain refinement is accompanied by an increase in atomic‐level strain up to about 1% root‐mean‐square. This high internal strain affects predominantly the coercivity which is at least 4 A/cm even for single‐phase alloys. The kinetics of the martensitic &ggr;→&agr;’transformation in mechanically alloyed nanocrystalline Fe—Ni are significantly modified in comparison to coarse‐grained as‐cast alloys. The nonequilibrium microstructure is assumed to hinder the growth of martensite which is confirmed by the investigation of the magnetic field‐induced martensitic transformation.

ISSN:0021-8979    

DOI:10.1063/1.353162

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We have investigated laser‐assisted chemical vapor deposition of W on GaAs to get insight into the deposition mechanism involved. Depositions were performed with an ArF excimer laser aligned parallel to the substrate and with WF6and H2as precursor and reducing agents, respectively. Samples were analyzed by Rutherford backscattering spectrometry and the sheet resistance was measured with a four‐point probe. We investigated the temperature dependence of the deposition rate up to 400 °C for both laser‐assisted and thermal deposition. At 350 °C and a laser repetition frequency of 20 Hz, we observed a deposition rate for laser‐assisted deposition of 5.3 A˚/s, whereas without laser irradiation no deposition occurred. At this temperature, the film resistivity was 11 &mgr;&OHgr; cm. For deposition temperatures above 350 °C, anomalous behavior was observed in both deposition rate and film resistivity. Experiments in which W coverage was measured as a function of laser pulse repetition frequency and as a function of time revealed that laser‐assisted deposition at 300 °C consists of a two‐step process. The first phase consists of exclusively laser‐induced deposition at a rate of 3.3×1015atoms/cm2/min/Hz, until a W coverage of approximately 15×1015atoms/cm2is reached. Laser‐independent growth was not observed probably through insufficient nucleation. The second phase consists of combined laser‐induced and laser‐independent deposition on the initial W layer. The laser‐induced growth rate was 3.2×1015atoms/cm2/min/Hz for repetition frequencies in excess of 5 Hz. At 20 pulses per second, laser‐induced deposition formed 47% of the total deposition rate. Below 5 Hz, the laser‐induced rate was higher than above 5 Hz but nonlinear with pulse frequency. The growth rate by laser‐independent deposition was 72×1015atoms/cm2/min at 300 °C, which corresponds well with theory for thermally activated deposition of W on W.

ISSN:0021-8979    

DOI:10.1063/1.353163

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Tungsten was deposited on GaAs employing WF6as precursor gas. A comparison is made between molecular and atomic hydrogen as reducing agent. Atomic hydrogen is producedinsituby dissociating molecular hydrogen on a hot (2100 °C) filament. Tungsten deposition was not observed only when molecular hydrogen and the substrate were available as reducing agents. Introduction of atomic hydrogen did lead to W deposition on GaAs.

ISSN:0021-8979    

DOI:10.1063/1.353164

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A laser method for studying the ignition processes of a pyrotechnic mixture is presented. The experimental equipment consists mainly of a 250 W CO2laser, an explosive chamber, a power meter, and a mechanical shutter. Using the laser ignition apparatus a parametrical study of the ignitability of a pyrotechnic mixture (Mg/NaNO3) was conducted. The influence of the following parameters on the ignition energy was studied: pulse width, beam radius, particle size, different sorts of Mg, and gas pressure. These measurements are complemented by some ignition measurements on a Mg/BaO2mixture where the influence of catalysis is studied. Furthermore some high‐speed photography of the ignition process is presented. The thermal explosion theory is then applied to these measurements for a calculation of some of the important parameters followed by a discussion of the ignition mechanisms. The activation energy (Ea=148 kJ/mol), frequency factor (K0=7.1×1017s−1), and absorption coefficient (&agr;=3.1104m−1) were calculated from the measurements. Using the experimentally determined parameters a 2D finite‐element computer modeling of the ignition was carried out, showing that the ignition energy can be calculated with a fair degree of accuracy but that the time to ignition cannot be modeled with a pure heat transfer model. The study is concluded with a discussion of ignition mechanisms for pyrotechnic mixtures, which especially points out that the ignition mechanism for Mg/NaNO3is a melting of the oxidizer followed by a decomposition of the oxidizer, diffusion of the produced oxygen through the oxide layer on the magnesium particles with a consecutive reaction of oxygen and magnesium.

ISSN:0021-8979    

DOI:10.1063/1.353165

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Measurements confirm the ability of a numerical model to simulate basic features of electron flow and subsequent radiation output from EPA (extended planar‐anode) diodes on HERMES III. HERMES III is a 19 MV, 700 kA, 25 ns electron accelerator. These diodes simultaneously terminate a coaxial MITL (magnetically insulated transmission line), and produce intense bursts of bremsstrahlung. The measurements and modeling show that (a) the EPA diode provides an impedance match to the MITL, (b) parapotential flow theory and the range of H−ions generated at the cathode can be used to measure the time‐dependent voltage pulse and peak voltageVP, respectively, and (c) that ∼95% of the electrical energy transmitted to the diode can be converted to kinetic energy of electrons incident on bremsstrahlung targets. Further, it is demonstrated that the forward radiation fluence can be described byDA/Q=(76±11)V˜1.61over theVPrange 13–19 MV.DA,Q, andV˜ refer to the forward radiation fluence measured in Mrad(CaF2) cm2, the incident charge measured in C, and the median current‐weighted accelerating voltage measured in MV, respectively. The dependence onVPof the beam characteristics at the targets is shown and the detailed kinetic‐energy distribution there is discussed.

ISSN:0021-8979    

DOI:10.1063/1.353166

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Process conditions for fabricating Si‐O impurity‐induced layer disorder defined AlxGa1−xAs‐GaAs buried heterostructure quantum well lasers utilizing a fully self‐aligned planar process and conventional As free open‐tube‐furnace annealing are presented. An SiO2layer, deposited by sputtering, was used as a diffusion source for Si and O impurities as well as a source for Ga vacancies that enhance impurity diffusion and allow for a reduction in the required annealing temperature and time. A self‐aligned native oxide of the AlxGa1−xAs cladding layer was used to form a Zn diffusion mask and dielectric layer. Lasers fabricated using this process exhibited threshold currents as low as 2.72 mA and external differential quantum efficiencies of 79% at room temperature in continuous operation.

ISSN:0021-8979    

DOI:10.1063/1.353142

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We report on synchronous operation of series arrays of inductively coupled superconducting quantum interference devices (SQUIDs). Each array consisted ofN=3 or 11 dc SQUIDs with common inductances providing a strong interaction between neighboring cells. Externally shunted (&bgr;c≊1) trilayer Nb—AlOx—Nb Josephson junctions were used. Coherent radiation was detected in the frequency rangef=67–73 GHz while the array voltage wasVN=Nf&Fgr;0(&Fgr;0=2.07×10−15V s). The stability of the coherent state was influenced by the external dc magnetic field. The linewidth depended on the external loading. For small loading it wasNtimes smaller than expected for a single cell. The influence of the inductive coupling mechanism on the operation of discrete Josephson junction circuits and the similarity to the coupling in layered structures of long Josephson junctions is discussed.

ISSN:0021-8979    

DOI:10.1063/1.353143

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A mixture of Si2H6and C2H2was used for deposition of SiC by ArF‐excimer‐laser chemical vapor deposition. Optical emission spectroscopy was employed to study the radicals CH, C2, SiH, and Si atoms in the photolyzed gas. The line intensities were measured changing the disilane/acetylene flow ratio. The concentrations of excited CH and C2were very low without disilane, however they rapidly increased with the addition of disilane. This is attributed to the reactions of silicon‐based radicals with C2H2or C2H to form CH and C2. A measurement on the deposition rates of the films agreed well with the result.

ISSN:0021-8979    

DOI:10.1063/1.353144

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We describe aninsitunondestructive method to measure the intergranular coupling in magnetic thin film recording media based on Stoner–Wohlfarth switching theory [Philos. Trans. Roy. Soc. London A240, 599 (1948)]. The method uses recording write and read heads and a disk test stand to measure a medium’s remanence curves from saturation and demagnetization. Using this technique, we have measured Henkel plots [Phys. Status. Solidi7, 919 (1964)] for single layer and multiple layer longitudinal media, as well as a Henkel plot for a perpendicular thin film with a highly permeable underlayer, which cannot be obtained by the conventional vibrating sample magnetometer method.

ISSN:0021-8979    

DOI:10.1063/1.353145

出版商:AIP    

年代:1993

数据来源: AIP