摘要:

Cathodoluminescence scanning electron microscopy is used to study AlxGa1−xAs epilayers grown on a nonplanar substrate by molecular beam epitaxy. Grooves parallel to the [011¯] direction were etched in an undoped GaAs substrate. Growth on such grooves proceeds on particular facet planes. We find that the aluminum concentration in the epilayers is dependent on the facet orientation, changing by as much as 35% from the value in the unpatterned areas. The transition in the aluminum concentration at a boundary between two facets is observed to be very abrupt.

ISSN:0003-6951    

DOI:10.1063/1.100711

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The magnitude of potential fluctuations due to remote ionized dopants is calculated for selectively doped heterostructures using unscreened and screened Coulomb potentials. Potential fluctuations are found to be minimized (corresponding to maximum carrier mobility) if the dopant distribution is &dgr;‐function‐like. Our experimental study of electron mobilities in selectively doped heterostructures grown by molecular beam epitaxy reveals that carrier mobility indeed increases as the thickness of the doped layer is reduced, in agreement with the calculation. A peak electron mobility of 5.5×106cm2/V s is obtained at low temperatures in a selectively &dgr;‐doped heterostructure.

ISSN:0003-6951    

DOI:10.1063/1.100712

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We grew In1−xGaxAsyP1−y/InP quantum wells (QWs) by low‐pressure metalorganic vapor phase epitaxy. The In1−xGaxAsyP1−ylayer was closely lattice matched to InP with a composition ofy=0.9 (x=0.47y). We investigated structural imperfections such as composition fluctuations, interface roughness, and nonperiodicity analyzing the low‐temperature photoluminescence linewidth. We found that the InGaAsP layer composition fluctuated, causing about 5 meV inhomogeneity in the exciton energy level in QWs wider than about 3 nm. Since we obtained very smooth interfaces with less than one monolayer of fluctuation and excellent periodicity by lowering growth temperature to 570 °C, the inhomogeneity of the exciton energy level could be held at 6 meV for 20‐period 10‐nm multiple QWs. As a result, despite composition fluctuations, a clear room‐temperature exciton optical absorption peak was observed at 1.5 &mgr;m for the first time to our knowledge.

ISSN:0003-6951    

DOI:10.1063/1.100713

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We describe a novel approach to determining absorption coefficients in thin films using luminescence. The technique avoids many of the difficulties typically encountered in measurements of thin samples, Fabry–Perot effects, for example, and can be applied to a variety of materials. We examine the absorption edge for GaAs/AlGaAs multiple quantum well structures with quantum well widths ranging from 54 to 193 A˚. Urbach parameters and excitonic linewidths are tabulated.

ISSN:0003-6951    

DOI:10.1063/1.100714

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The capacitance‐voltage (C‐V) and the current‐voltage characteristics of metal/ZnSe/p‐GaAs capacitors have been investigated; both epitaxial layers were grown by molecular beam epitaxy. In the capacitor structures highly resistive, stoichiometric ZnSe functioned as the pseudoinsulator on the doped GaAs layers. The capacitance‐voltage measurements demonstrated that the capacitors could be biased from accumulation through depletion, and into deep depletion, with current in the range of 10−8A cm−2. Very little frequency dispersion was observed in theC‐Vdata when measured from 1 kHz to 1 MHz. From the high‐frequencyC‐Vcurve, the surface state density as a function of position in the GaAs band gap was determined. Surface state densities were comparable to densities reported for (Al,Ga)As/GaAs heterojunctions.

ISSN:0003-6951    

DOI:10.1063/1.100715

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We prepared Pb‐doped Bi‐Sr‐Ca‐Cu‐O thin films by rf‐magnetron sputtering with multiple targets. A Bi2O3target compensated for lack of Bi and a PbO target was used to dope Pb into films in sufficient quantity. In thin films, Pb evaporates easily during sintering. We prepared heavily Pb‐doped films and they exhibited zero resistance at 94.5 K with sintering as short as 10 min. After 1 h of sintering, more than half of the Pb evaporated and the film showed a sharp transition to its superconducting state.Tcewas 106.5 K. Pb doping promotes highTcphase formation; however, the highTcphase does not always have to contain Pb.

ISSN:0003-6951    

DOI:10.1063/1.101401

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We have studied the effect of varying the postdeposition anneal conditions on the properties of thin films of YBa2Cu3O7prepared by coevaporation from Y, Cu, and BaF2. The temperature of this processing step determines both the microstructure of the films and the constituent phases. At low temperatures, BaF2is incompletely reacted and the predominant superconducting phase is Y2Ba4Cu8O20−x. At higher temperatures, the equilibrium distribution of phases, which includes YBa2Cu3O7, is formed. As the temperature is raised, the texture of the films also changes. The thermodynamics of the formation reaction and of the reaction between YBa2Cu3O7and various substrate materials is discussed.

ISSN:0003-6951    

DOI:10.1063/1.101402

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Microbridges with widths of about 10 &mgr;m were lift‐off structured from rf‐sputtered YBaCuO thin films and irradiated with microwaves at different temperatures. The bridges contain only a few grains with a typical size of 4 &mgr;m and are fullyc‐axis oriented. The observed current‐voltage characteristics exhibit sharp constant voltage steps up to 71 K. An oscillatory dependence of the critical currentI0and the step heights 2Inis observed, clearly revealing Josephson‐like behavior. From the grain structure, the critical current, and the microwave response, it is very likely that grain boundaries as superconductor‐normal conductor‐superconductor (SNS) contacts dominate the behavior of the bridges.

ISSN:0003-6951    

DOI:10.1063/1.101403

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The erase process in a directly overwritable magneto‐optical material is shown to be a domain collapse dominated process, not a reversed domain growth process as previously suggested. The results of two experiments which support this conclusion are described. The first is a static evaluation of the residual domains of unsuccessful erase attempts. The second is actual high‐speed observation of the dynamic process itself. Both experiments show that erasure in this material results from collapse of the previously written domain.

ISSN:0003-6951    

DOI:10.1063/1.100716

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Sulfur dimers were detected in sulfur‐hexafluoride plasma‐etching discharges using optical emission spectroscopy and laser‐induced fluorescence spectroscopy. Dimer densities were estimated to be on the order of 1013/cm3and appear to decrease rapidly with increasing oxygen content in the discharge.

ISSN:0003-6951    

DOI:10.1063/1.101404

出版商:AIP    

年代:1989

数据来源: AIP