摘要:

A hydrogen atomic beam generated by microwave discharge was successfully used for dissociating trimethylaluminum (TMA) at the low substrate temperature of 300 °C. The results indicate that ‘‘H’’ atomic beam can stimulate the clean and complete dissociation of organometallics at substrate temperatures only slightly above the onset of thermal pyrolysis of the organometallics. The clean dissociation of TMA is dependent on both thermal as well as plasma stimulation. Residual oxygen present in our system resulted in growth of high quality Al2O3films. In a clean metalorganic chemical vapor deposition system H atomic beam can be used to dissociate organometallics at significantly lower temperatures (300–400 °C) relative to the presently used growth temperature of 700 °C.

ISSN:0003-6951    

DOI:10.1063/1.103004

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Strained GaAs/InGaAs/AlGaAs quantum well structures have been grown by molecular beam epitaxy at substrate temperatures from 375 to 510 °C. The well layer thickness and In composition are analyzed by ion channeling and particle‐induced x‐ray emission as a function of growth temperature. Test structures for modulation‐doped field‐effect transistors grown at 375 and 510 °C under two different As4overpressures were also characterized by Van der Pauw measurements and low‐temperature photoluminescence. The observed differences in film quality can be explained by the influence of substrate temperature and As4flux on the cation surface mobility during growth of the InGaAs layer.

ISSN:0003-6951    

DOI:10.1063/1.103005

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this work, we demonstrate the firstinsitustudy of GaAs using grazing incidence x‐ray scattering during processing at 100 Torr prior to epitaxial film growth. Our results indicate that the native oxide is removed and a well‐ordered surface reconstruction is established at temperatures as low as 500 °C in hydrogen. In contrast, no surface reconstruction is observed when the substrate is heated in nitrogen at temperatures as high as 585 °C while comparable heating in hydrogen causes surface degradation in the form of liquid Ga droplets. These results suggest that a chemically induced surface transformation occurs rather than oxide evaporation during thermal treatment.

ISSN:0003-6951    

DOI:10.1063/1.103006

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report the effect of relatively weak (classical) magnetic fields on the gate current in heterostructure field‐effect transistors. With a three‐terminal device using the measured dependences, we deduce an average electron velocity and the average concentration of the two‐dimensional (2‐d) electron gas in the channel. In addition, our results clearly show the onset of velocity saturation in high electric field. The value of the effective saturation velocity in the channel is of the order of 105m/s, in agreement with the values deduced from device transconductance.

ISSN:0003-6951    

DOI:10.1063/1.103007

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We apply time‐resolved four‐wave mixing as a novel tool to study resonant tunneling of carriers in semiconductor heterostructures. The polarization decay of excitons in a quantum well is much faster when the alignment of the electron levels in adjacent wells leads to resonant tunneling and subsequent scattering of the carriers.

ISSN:0003-6951    

DOI:10.1063/1.103008

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have prepared highly oriented bismuth cuprate thin films (c‐axis textured) on yttria‐stabilized zirconia (100) YS‐ZrO2substrates at 650 °C by pulsed excimer laser ablation in a 200 mTorr oxygen ambient. The films were annealed in oxygen and nitrogen atmospheres to study the effect on zero‐resistance transition temperature (Tc0). Thin films deposited at 650 °C exhibited aTc0of 68 K with onset at 110 K. Post‐annealing for 1 h at 400 °C in oxygen improved theTc0to 82 K, while the onset remained the same. X‐ray diffraction, scanning electron microscopy, and Rutherford backscattering channeling studies were performed on these films for correlations between crystal structure, microstructure, and superconducting properties. X‐ray diffraction patterns indicated a 2212 type phase witha=5.39 A˚ andc=30.76 A˚; preferential orientation of thecaxis perpendicular to the substrate was observed. The lattice parameters and x‐ray diffraction patterns were found to be invariant with annealing treatments that improvedTc0from 68 to 82 K.

ISSN:0003-6951    

DOI:10.1063/1.103009

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A novel method for measurement of magnetic field, based upon magnetically modulated resistance in superconductors, is described and demonstrated with microwave absorption data in a Bi0.9Pb0.1SrCaCu2Oxsample. The method can be implemented as a vector magnetometer and, with suitable superconductors, could provide absolute magnetic field measurements.

ISSN:0003-6951    

DOI:10.1063/1.103010

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The irreversibility line in high‐temperature oxide superconductors is theoretically investigated from a viewpoint of dependence on the flux‐pinning strength and a general relation between the effective pinning potential and the critical current density is derived. It is shown that the irreversibility magnetic field at 77 K in strongly pinned oxide superconductors is sufficiently high for application.

ISSN:0003-6951    

DOI:10.1063/1.103011

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Magnetic flux relaxation data on a composite specimen of YBa2Cu3O7−&dgr;+Ag shock compacted at 167 kbar show that the flux‐pinning energy at 70 K and 10 kOe is enhanced by a factor of 2–3. This enhancement persists after annealing at 890 °C in oxygen for 24 h. The increased pinning energy is apparently caused by shock‐induced ⟨100⟩ and ⟨110⟩ line defects having a density up to 1012cm−2. Annealing replaces line defects with (001) extrinsic stacking faults having displacement vectors of 1/6[031]. The average separation of the stacking faults in the [001] direction is 0.04 &mgr;m, which is comparable to the separation between fluxoids at 10 kOe. These results indicate that intragranular critical current densities can be increased significantly in bulk materials by shock processing. High densities of flux‐pinning sites could be induced efficiently in industrial‐scale quantities of bulk highTcoxides by explosives.

ISSN:0003-6951    

DOI:10.1063/1.103231

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A simple technique for peparing magnetic tips used in a scanning force microscope is described. In this technique a thin layer of ferromagnetic material is deposited on an etched tungsten tip via sputtering or galvanic deposition. Images of magnetic domains obtained with these tips are presented, demonstrating a lateral resolution of the order of 50–100 nm.

ISSN:0003-6951    

DOI:10.1063/1.102991

出版商:AIP    

年代:1990

数据来源: AIP