摘要:

Measurements of the hole density in carbon‐doped GaAs and AlxGa1−xAs as a function of the annealing temperature are presented. It is shown that after sample annealing at low temperatures (T<550 °C), the hole concentration increases in all samples doped ≥1×1019cm−3with a simultaneous decrease in the hole mobility. However, sample annealing at higher temperatures (T≳600 °C) results in a reduction of the hole concentration in all samples doped with carbon at concentrations higher than ≊5×1019cm−3. The reduction in hole concentration is also accompanied by an increase in lattice parameter of the carbon‐doped epilayer. The observed changes in the electrical and microstructural properties are explained in terms of two different mechanisms: (1) the passivation of carbon acceptors by the incorporation of hydrogen during growth, and (2) the change in the lattice site location of carbon atoms upon annealing. Direct determination of the lattice site location of carbon in samples doped ≥5×1019cm−3showed that the fraction of interstitial carbon after annealing atT≳600 °C is at least 70% higher than in the as‐grown samples.  

ISSN:0021-8979    

DOI:10.1063/1.352017

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A method of evaluating interface states for coupled superlattices has been developed. Explicit solutions for these states have been obtained within the framework of the Kronig–Penney model for the case of two semiconductor superlattices coupled by a tunnel barrier. The interface states are shown to be largely dependent on the coupling strength if the constituent superlattices have identical structure parameters, while insensitive to the tunnel barrier thickness if their structure parameters differ. In the latter case, electrons behave as if the system were composed of two decoupled semi‐infinite superlattices.

ISSN:0021-8979    

DOI:10.1063/1.352356

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A new quantum‐confined Stark shift structure, the step‐coupled well, is introduced. The feature of the new structure is using a square well of only one quantum state to enhance the Stark shift of the step well. The design rule of 10‐&mgr;m modulator is suggested. The transfer matrix technique is used for numerical calculation. Numerical results show that the new structure produces a larger Stark shift (13 meV) than the step well (7 meV) under the same bias (20 kV/cm). A lower electric field is thus needed for the step‐coupled well to produce the same Stark shift, so that the leakage problem can be minimized and a high switching speed can be achieved.  

ISSN:0021-8979    

DOI:10.1063/1.351967

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Electron tunneling spectroscopy has been used to study the phonon modes of the GaAs electrode and the AlGaAs barrier of single barrier GaAs/AlGaAs/GaAs heterostructures. The barriers were spiked doped with Si or Be to determine whether defects or impurities in the barrier have an effect on the measured line shapes. The phonon line shapes and intensities have been observed to change after shining light on the devices to photoionize defects in the barrier. The results demonstrate that the charge state of defects in a heterostructure barrier can affect the interaction between a tunneling electron and the phonon modes of a tunnel barrier.

ISSN:0021-8979    

DOI:10.1063/1.351968

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

YBa2Cu3O7films can be grown on different substrates by the off‐axis ac magnetron sputtering method. Films in theaborientation tend to grow at a lower substrate temperature thanc‐axis films. However, as the temperature is lowered, all the substrates we have tested, except for SrTiO3, form amorphous phases that degrade the quality of the films being made. In this manuscript it is shown that if a thin layer ofc‐axis film is deposited on the substrate first, then it effectively reduces the formation of the amorphous phases and improves the film quality significantly.  

ISSN:0021-8979    

DOI:10.1063/1.351969

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Thin films of Y‐Ba‐Cu‐O wereinsituprepared with the use of a simple conventional inexpensive vacuum system. No thickness monitor or control system is required. A pulverized mixture of Y, BaF2, and Cu constituents weighed in the atomic proportion was evaporated from resistively heated source onto a MgO substrate. The substrates temperature was then raised to 700 °C after evaporation. Oxygen was injected through a nozzle placed near the substrate surface, thus raising the pressure to about 8 Pa (60 mTorr). The film was kept at this stage for about 15 min, after which it was gradually cooled to room temperature and the pressure raised to atmospheric pressure. The obtained films with no further heat treatment were found to be superconductors with zero resistance at 85 K detected by four‐probe dc measurements.

ISSN:0021-8979    

DOI:10.1063/1.351970

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Current‐voltage characteristics were investigated for YBa2Cu3Oy/La1−xCaxMnOzor La1−xSrxMnOz(200 nm)/YBa2Cu3Oyjunctions. The results are discussed together with electrical and magnetic properties of the barrier materials. Supercurrents passed through a 200‐nm‐thick magnetic barrier for certain values ofx. The barrier materials did not necessarily have high electric conductivity in this region, but the ferromagnetism nearly disappeared. The results suggested the possible mechanism of a novel kind of proximity effect concerned with magnetism of the barrier.

ISSN:0021-8979    

DOI:10.1063/1.351971

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Low‐temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of highTcthin films. In the presented article we have given some experimental observations on the spatial variation of the critical current density of two kinds of highTcthin films using a LTSEM. The results show that in the polycrystalline TlBaCaCuO film prepared by dc magnetron sputtering, the maximum local critical current density is about 3–4 times greater than the minimum local critical current density. The microstructure analysis implies that the strong inhomogeneous critical current density distribution in the TlBaCaCuO film is due to its granular structures and the existence of several structural phases. Forc‐axis oriented epitaxial YBaCuO thin film depositedinsituby laser ablation, the results indicate a less inhomogeneous critical current density distribution. The maximum local critical current density is less than 1.5 times the minimum local critical current density in the epitaxial thin film.

ISSN:0021-8979    

DOI:10.1063/1.351972

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The resistivity data of ferromagnetic alloys has been re‐evaluated taking into account that the conduction electrons in 3dferromagnetics are predominantly polarizedd‐like electrons with high net polarization. This drastically changes the previously derived values of solute cross sections and scattering rates ratios of the majority and minority electrons. In particular, it removes the perplexing result of previous analyses that the spin‐up and spin‐down cross sections of sp solute atoms were not equal. Moreover, since past analyses of scattering, transport effects, etc., in 3dferromagnetics have assumed that the conduction electrons aresp‐like, all these results should also be critically re‐examined to determine their validity.

ISSN:0021-8979    

DOI:10.1063/1.351973

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A measuring procedure for the determination of the switching behavior of hard magnetic particles is presented. For this end, the remanence vector is determined for increasing fields. In the first part of the presented analysis, the angular dependence of the switching field is calculated, neglecting the switching field distribution. In the second part, the portion of switching particles depending on the strength of the applied field and its orientation with respect to the particle easy axes is determined. This switching portion includes information on the angular dependence of the switching field and the switching field distribution for each particle orientation. The method is demonstrated on a magnetically diluted barium ferrite sample with noninteracting single‐domain particles.

ISSN:0021-8979    

DOI:10.1063/1.351974

出版商:AIP    

年代:1992

数据来源: AIP