摘要:

The strength of the surface field on GaAs epitaxial layers of various thicknesses and doping concentrations grown by metalorganic chemical vapor deposition on a semi‐insulating GaAs substrate was studied as a function of applied bias using modulation spectroscopy. While for small applied potentials the square of the field strength increases linearly with respect to reverse bias, for larger values of bias it saturates. It is shown that this saturation appears when the width of the depletion layer in the epilayer approaches the width of the epilayer itself.

ISSN:0021-8979    

DOI:10.1063/1.350521

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Solutions to the main problems in operating a spin‐polarized scanning tunneling microscope are discussed. Preliminary experimental results obtained in the course of implementing these solutions are reported. Atomic resolution on Si(111) and Si(100) is achieved with a scanning tunneling microscope (STM) using chromium and iron tips. Fabrication of antiferromagnetic tips of Cr, MnNi, and MnPt is described. A technique of preparation of clean (100) surfaces of Fe3O4(magnetite) is given. Low‐energy electron diffraction patterns were obtained on Fe3O4for the first time. The first STM experimental results obtained on magnetite in air and in ultrahigh vacuum are reported. Atomic resolution is obtained on Fe3O4(100) with an ultrahigh vacuum scanning tunneling microscope using iron and tungsten tips. This is the first successful observation of atomic resolution on a ferromagnetic sample using a ferromagnetic tip.

ISSN:0021-8979    

DOI:10.1063/1.350522

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The influence of the crystallographic phase transformation on the work function was studied on a Co(0001) surface by a precise measurement of the contact potential difference. The characteristic hysteresis of the work function due to the hcp&rlarr2;fcc allotropic transformation was found. The work function changes abruptly by about 2 meV with the phase transformation while decreasing monotonically with temperature except in the transformation range. The contributions of the surface dipole barrier and the chemical potential of electrons to the work function are discussed from the point of view of cellular approximation.

ISSN:0021-8979    

DOI:10.1063/1.350523

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Properly gettered and annealed siliconp‐njunctions are either well described by Shockley’s diffusion theory (the ideal case) or they have (the almost ideal case) a reverse voltage–current characteristic constituted by three parts: a voltage‐independent contributionIr, smaller than the saturation diffusion current, and by two contributions, one,igr, of generation–recombination and the other,ioh, ohmic, which are both due to the same defect centers characterized by equal activation energy and by a null charge. In a recent work we have shown that such experimental findings may be ascribed to four‐state traps (FST) which contain a ion dipole and which may be empty of carriers (the ground state), or filled by an electron or a hole, or both (the stimulated state), and to different Frenkel–Poole effects for electron and hole in their field‐assisted thermal emission from trap states. Here, firstly, the model is simplified and improved (i) by considering the most likely emission (capture) of a carrier from (into) the only states containing (empty of) it, (ii) by taking into account that the transition between the ground and the stimulated states is thermally activated and assisted by both the electric field and the tunnel effect, and (iii) by considering the effects on the current of the dispersionsD&lgr;andD&thgr;of the length and direction, respectively, of the ion dipole and of thatDVmof the energy barrier height crossed by the electron in the above mentioned tunnel transition.In this way, the FST kinetic equations are greatly simplified, the whole model is improved and generalized and, at the same time, the physical meaning of the analytical results becomes more simple and straightforward. As a matter of fact we find that (a)Iris due to the field‐assisted tunnel effect in the transition between the ground and the stimulated states and it depends onD&lgr;,D&thgr;, andDVm, (b)igrandiohderive from the Frenkel–Poole effect on the carrier emission from the stimulated state and the (ioh, practically) are independent ofD&lgr;,D&thgr;, andDVmand (c) the activation energies ofigrandiohare equal and the value is given by the smallest of the energies required to bring each of the carriers from the ground state, through the stimulated state, into the respective band. Finally, the physical origin of the FSTs is found in groups of localized states with different positions and energy levels, due to the oxygen, whose donor levels, related to SiyOxclusters of a few hundred atoms of oxygen, are destroyed and generated at the annealing temperature of 650 and 800 °C, respectively, at which the ideal and almost idealp‐njunctions are obtained, respectively.

ISSN:0021-8979    

DOI:10.1063/1.350524

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Linear and nonlinear characteristics of superlattice perpendicular conduction are examined from a balance‐equation theory recently developed for miniband transport [Lei, Horing, and Cui, Phys. Rev. Lett.66, 3277 (1991)]. Analytical expressions are given for linear mobilities due to impurity and due to optic‐phonon scatterings. Attention is focused on the temperature dependence of low‐field miniband mobility and the behavior of drift velocity versus electric field, derived from the balance‐equation theory, as well as from Esaki–Tsu and Boltzmann theories. Good qualitative agreements are obtained in comparison with experiments, but more accurate measurements are needed to appreciate the differences between these two types of theories.

ISSN:0021-8979    

DOI:10.1063/1.350525

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The analysis of dark current‐voltage (I‐V) characteristics of Al/chlorophylla/Ag cells at room temperature with respect to the elucidation of conduction mechanisms and evaluation of cell parameters is presented. It is seen that the presence of series and shunt resistances can considerably affect theI‐Vplots. It is therefore important to remove their effects for correct and meaningful analysis of theI‐Vcurves. The results suggest that for Al/Chla/Ag cells with microcrystalline Chla∼3000 A˚ thick, the conduction mechanism for voltages between 0.53 and 1 V can be described by a modified Shockley equation from which the values ofRs,Rsh,n, andI0obtained are 3.2×104&OHgr;, 1.7×109&OHgr;, 1.74, and 2.4×10−15A, respectively.Rsis most likely due to the combined effect of bulk Chlaand the electrodes, particularly the insulating layer of Al2O3that is formed as a result of oxidation of Al in air. For higher forward biases, i.e., between 1 and 2 V, the current transport is due to the space‐charge‐limited current in presence of exponentially distributed traps. The density of traps obtained is ∼1018cm−3. Because of the breakdown of Al/Chla/Ag cells at fields higher than 107V m−1, the transition voltage to observe the trap‐filled‐limit situation was not possible. For a low‐bias region, the conduction mechanisms seem to be dominated by Schottky emission over an Al/Al2O3barrier; however, the temperature dependence ofI‐Vcharacteristics is necessary to confirm this. The barrier Al/Al2O3calculated is ∼1.17 eV.

ISSN:0021-8979    

DOI:10.1063/1.350526

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

It has been found that AlAs, AlGaAs, and AlInAs layers grown on 2° misoriented (111)BGaAs substrates by molecular beam epitaxy are much smoother than corresponding (111)GaAs layers. The quality of Al containing (111)Blayers is further demonstrated by the narrow (65 A˚) photoluminescence peak from an AlAs/Al0.5In0.5As strained multiquantum well structure. The photoluminescence spectra of this structure is considered in detail, with particular attention paid to Stark effects associated with strain‐generated internal electric fields. Spectral movement of the photoluminescence peak as a function of excitation intensity suggests the attainment of self‐biased strained quantum wells.

ISSN:0021-8979    

DOI:10.1063/1.350527

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The structure and properties of NiFe‐N films, prepared by rf reactive sputtering process using nitrogen in the range from 0 to 40% in the nitrogen‐argon gas mixture during sputtering, have been studied. The concentration of nitrogen and the resistivity of the NiFe‐N films were determined as a function of nitrogen partial pressure. Films with good soft magnetic properties were obtained when the nitrogen in the nitrogen‐argon gas mixture during sputtering was in the range 0–10%. The resistivity of these films was low and x‐ray diffraction results indicated only an fcc structure of &ggr;‐NiFe alloy. For films deposited with more than 10% of nitrogen in the gas mixture there is a transition region where resistivity and coercivity started to increase. Films deposited with 20% nitrogen mixed with argon consist of a mixture of &ggr;‐NiFe alloy and (Ni,Fe)4N phases. With further increase of nitrogen above 30% during sputtering, a (Ni,Fe)3N phase was observed. The resistivity and coercivity of these films were high.

ISSN:0021-8979    

DOI:10.1063/1.350528

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

High‐quality gallium nitride (GaN) film was obtained using a GaN buffer layer on a sapphire substrate. Using low‐temperature Hall measurements, we obtained a maximum mobility about 3000 cm2/V s, at around 70 K. This mobility value is the highest reported, to our knowledge, for GaN films. The infrared radiation transmission intensity oscillations, which were caused by interference effects, were observed by means of an infrared radiation thermometer during GaN growth. The growth process of GaN film with GaN buffer layers was almost the same as that of GaN film with AlN buffer layers except when the thickness of GaN buffer layers was small. When the thickness of GaN buffer layers was small, an additional new growth process, in which the surface of GaN film became rough during the growth, was observed. The GaN growth with GaN buffer layers had a tendency to improve the surface morphology even if it became poor due to excess Si doping or low buffer layer thickness.

ISSN:0021-8979    

DOI:10.1063/1.350529

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

To calculate the complex conductivity &sgr;=&sgr;1−j&sgr;2, and thus the surface resistanceRsand penetration depth &lgr;L, of highTcsuperconducting thin films from microwave power transmission measurements, new expressions are derived taking into account the film thickness. Numerical examples are given to show that, compared with the calculations which neglect the film thickness, corrections in &sgr;1, &sgr;2,Rs, and &lgr;Lare a few percent for film thickness ranging between 0.65 and 0.46 &lgr;Land that corrections in &sgr;1and &sgr;2are greater than those inRsand &lgr;L. Using the newly derived expressions, maximum errors inRsand &lgr;Lare estimated to be about 30% and 3%, respectively, if errors between −10% and +10% are tolerated in the measurements of the magnitude and phase shift of microwave transmission.

ISSN:0021-8979    

DOI:10.1063/1.350530

出版商:AIP    

年代:1992

数据来源: AIP