摘要:

Undoped GaAs epilayers grown by metalorganic chemical vapor deposition on Zn‐ and Si‐doped GaAs substrates were studied using the technique of photoreflectance (PR) spectroscopy. Oscillations were observed in the PR spectra of the samples grown onn‐type substrates below the fundamental band gap of GaAs. The origin of these oscillations was investigated in terms of interference between the light beam reflected from the air/film interface and the modulated beam reflected from the film/substrate interface. The latter is due to the modulation of the index of refraction of the substrate. The thickness of each film was calculated by a model which was used to fit the experimental spectra of the differential change in reflectance (&Dgr;R/R) versus energy. The observed changes in the period of these oscillations as a function of film thickness were in good agreement with the calculated results. Scanning electron microscope measurements of the thicknesses of the films supported these calculations.

ISSN:0021-8979    

DOI:10.1063/1.346176

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Silicon dioxide layers of 250 A˚ thick were grown on Si at 1000 °C in a dry O2/TCA ambient. Thermal nitridation of the samples was performed in a pure ammonia ambient at temperatures from 900 to 1100 °C with one hour time intervals up to a maximum of 4 h. The fixed charge state densities at the interface of the samples were determined from high frequencyC‐Vmeasurements, and the breakdown fields fromI‐Vcurves. Secondary ion mass spectroscopy depth profiles show low levels of contaminants, and high levels of nitrogen at the interface for samples annealed at temperature greater than or equal to 1000 °C and for periods longer than 2 h. Post metal annealing of the nitrided samples appears to help in reducing the trapped charges. Better quality films with lowerQfandVFBshifts, and higher breakdown fields were achieved for samples annealed at 1100 °C. Metal‐oxide‐semiconductor device quality nitrided films with aQfof 1010/cm2were achieved by optimizing the process conditions at 1100 °C. The fixed charge build up for lower nitridation temperatures (<1000 °C) and times (<2 h)is due to the dissociation of Si—O bonds in the presence of hydrogen, and is in accordance with the earlier results in the literature. However, the reduction in the fixed charge buildup at 1100 °C, we believe, is due to the increased levels of nitrogen.

ISSN:0021-8979    

DOI:10.1063/1.346177

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The problem of calculating the valence‐band structure of strained‐layer quantum wells in the effective‐mass approximation is reviewed. Using the spherical approximation and exploiting the simplicity of the infinitely deep well model we show that the in‐plane effective mass is determined by two factors—a splitting contribution which is dominant at large strains, and a quantum confinement contribution. A model for finite‐depth wells is presented which gives analytic expressions for the zone‐center in‐plane mass and associated nonparabolicity factor, and it is applied to the system InxGa1−xAs/GaAs. The model allows the computation of valence‐band structure using no more than a pocket calculator. It is shown to give results in reasonable agreement with experiment.

ISSN:0021-8979    

DOI:10.1063/1.346178

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A simple linear electrical circuit model is developed to study the ac characteristics of a doping superlattice with selective contacts. Small signal ac measurements at different frequencies were carried out on silicon doping superlattices under reverse bias. Parameters such as capacitance, doping profile, ac conductance, and leakage resistance of a doping superlattice calculated from the model are shown to agree well with the experiment.

ISSN:0021-8979    

DOI:10.1063/1.346179

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This paper presents the investigation of low‐temperatureinsitusurface cleaning of oxide‐patterned wafers by an Ar/H2plasma prior to the epitaxial growth on exposed silicon windows. Ar/H2plasma sputter cleaning was carried out at 2.5 or 20‐W rf power and a susceptor dc bias from 100 down to 0 V. Epitaxial layers were grown immediately after theinsitusurface cleaning by ultralow‐pressure chemical vapor deposition from SiH4/H2in a high‐vacuum system (base pressure, 10−7Torr). Process temperatures were varied from 800 down to 500 °C. The epitaxial films were characterized by cross‐sectional transmission electron microscopy, secondary‐ion‐mass spectroscopy, Nomarski optical microscopy, and ion channeling Rutherford backscattering spectroscopy. It was found that a highly structural epitaxial layer can be grown down to 600 °C and an epitaxial layer can be grown at 500 °C by utilizing the preepitaxialinsituAr/H2plasma sputter cleaning.

ISSN:0021-8979    

DOI:10.1063/1.346180

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The transport mechanisms in ZnO/CdS/CuInSe2solar cells prepared by ARCO (now Siemens) Solar Inc. have been analyzed by measurements of current versus voltage at different temperatures in the dark, short‐circuit current versus open‐circuit voltage at different temperatures in the light, spectral response of quantum efficiency, and junction capacitance. In the dark, recombination in the depletion region and/or thermally assisted tunneling are the dominant transport mechanisms. The observation of a smaller open‐circuit voltage than would be predicted from the dark transport parameters is the result of a small change in the transport parameters under illumination, probably without a change in transport mechanism.

ISSN:0021-8979    

DOI:10.1063/1.346148

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Josephson voltage standards utilize microwave‐induced constant voltage steps in the dc characteristic of Josephson tunnel junctions. This paper describes the design and operation of array circuits with 108 and 2000 junctions connected in series. In contrast with similar realizations, simpleQ‐band equipment is used for the microwave supply. The microwave attenuation of 1000 junctions was about 1 dB. The version with 2000 junctions generated Josephson voltages up to 1.2 V when operated at 35 GHz. The stability times of the quantized levels were, under normal laboratory conditions (unshielded room), better than 10 min.

ISSN:0021-8979    

DOI:10.1063/1.346149

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The height of the velocity‐matching step (VM step) of a flux‐flow type Josephson oscillator is studied systematically by numerical simulations. It is defined by the phase velocity of the propagating fluxons equaling that of the electromagnetic wave in the junction structure. The height of VM step (&ggr;m) is an important parameter for the practical application of such a device. The results presented on &ggr;mare important data in order to save computer time in the simulation of electromagnetic properties, such as the output voltage, at the velocity‐matching condition. The detailed simulations were performed using the perturbed sine‐Gordon equation (PSGE) with a uniformly distributed bias current &ggr;. The PSGE was solved numerically by using an implicit finite‐difference scheme. The distributions of electromagnetic fields in an overlap, long Josephson junction in the flux‐flow state are mapped. The parameters influencing &ggr;mare discussed. We show that &ggr;mincreases almost linearly with an external magnetic field &bgr;eor with the quasiparticle damping &agr;, while it is less dependent on surface loss &bgr;. &ggr;mincreases slowly with junction length above a minimum length, and it levels out at long lengths. We also present an approximate expression for &ggr;mwhich fits numerical results well.

ISSN:0021-8979    

DOI:10.1063/1.346150

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Domains written thermomagnetically in TbFeCo thin films are studied with Lorentz transmission electron microscopy (TEM) and scanning electron microscopy with polarization analysis (SEMPA). Four different rare‐earth/transition‐metal compositions TbxFeyCo1−x−yare examined. The domain structures observed with both techniques are similar even though TEM Lorentz only detects the transverse component of the net magnetic field along the electron’s trajectory through the sample, while SEMPA detects the surface electron‐spin polarization (magnetization). We find that the magnetic contrast in the SEMPA measurements is proportional to the magnetization of the transition‐metal (TM) subnetwork which is antiferromagnetically coupled to the rare‐earth (RE) subnetwork. This allows high‐contrast SEMPA images to be acquired even when the system is magnetically compensated (Ms=‖MRE−MTM‖=0). The surface magnetization can be explained by assuming that the surface of the TbFeCo alloy consists of an outermost thin oxide layer followed by an Fe‐rich subsurface layer. The importance of the demagnetizing field on the switching and domain nucleation process for thermomagnetically written bits is examined.

ISSN:0021-8979    

DOI:10.1063/1.346151

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Systematic room‐temperature measurements of the aftereffect of the ac magnetic permeability have been performed on a Fe81B13.5Si3.5C2amorphous ribbon in order to get detailed information about the nature of the atomic ordering processes responsible for the magnetic relaxation. The magnetic aftereffect related to 180° domain‐wall motion has been measured by means of a specific technique allowing periodic domain‐wall displacements to be induced between two fixed, neighboring equilibrium positions by applying a square‐wave field of proper amplitude and frequency. In this way, the average direction of the magnetization vector is cyclically modified in all points where the studied directional ordering processes may occur. As a consequence, the kinetics of ordering is modified in a characteristic way, giving rise to relevant variations in the intensity of the magnetic aftereffect measured between fixed times (t1=2×10−3s andt2=10−1s), and in the value of the magnetic induction at the timet2. All measurements have been performed at constant applied field. The degree of reliability of this experimental technique has been analyzed in detail. The magnetic aftereffect, &Dgr;B=B(t1)−B(t2), and the magnetic inductionB(t2) have been measured as functions of the number of domain‐wall cycles, and after removing the square‐wave field for a variable timet*. The results of many independent measurements are reported and discussed.

ISSN:0021-8979    

DOI:10.1063/1.346152

出版商:AIP    

年代:1990

数据来源: AIP