摘要:

Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undopedn‐AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen‐like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow‐deep transition occurs at aboutx=0.2 in the low conductivity AlxGa1−xN series and at aboutx=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors. For the yellow luminescence band at 2.25 eV it is demonstrated that this band consists of two overlapping bands and that the dominant band is due to a transition between the native donors and a carbon‐related deep center. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363653

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A band offset diagram for the heterojunction InyGa1−yPzAs1−z/AlxGa1−xAs based on the transitivity rule and our measured band offset for In0.5Ga0.5P/GaAs is given. A carrier leakage analysis is developed and explains the experimental observations in 670 nm visible InyGa1−yPzAs1−z/AlxGa1−xAs double heterostructure (DH) lasers. The analysis based on the performance of this laser verifies that our band offset is more accurate than previous values. In contrast to GaAs/AlxGa1−xAs, InGaPAs/InP and InGaP/AlGaInP DH lasers, we found that the leakage of holes, rather than of electrons, is responsible for the high threshold current density of this type of laser. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363654

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The conduction mechanism and origin of the electrical stress‐induced leakage current (SILC) in thin silicon dioxide (SiO2) films thermally grown on silicon substrate were clarified from various electrical properties. The properties examined consisted of the I‐V characteristics, the oxide trap charge buildup, the generation of the Si/SiO2interface states, and the generation of the neutral oxide traps. The electrical properties were obtained from films of different oxide thicknesses fabricated by different oxidation processes. The conduction mechanism of SILC was investigated from the viewpoint of oxide thickness dependence, using 92‐ and 56‐A˚‐thick oxide films. From the oxide‐thickness‐dependent studies it was found that the SILC phenomenon was not correlated with the oxide trap charge buildup and interface state generation, but rather closely correlated with neutral electron trap generation. The conduction mechanism for nonequilibrium SILC was theoretically deduced from one‐dimensional ballistic triangular barrier tunneling that occurred only during the filling process. The tunneling was directed from a leakage spot at the electron‐injecting cathode to neutral electron trap sites uniformly generated within the oxide at a trap level (≊1.17 eV from the cathode conduction band and ≊2.0 eV from the SiO2conduction band) lower than the SiO2barrier height during only the filling process. The origin of the SILC was also investigated from the viewpoint of oxidation process dependence, using both wet and dry oxides of 86 and 50 A˚ thicknesses. The oxidation‐process‐dependent studies revealed that the SILC associated with a wet oxide after the stress application was less than that of a stressed dry oxide. The oxide trap charge buildup and the interface state generation associated with a wet oxide after the stress application was, however, greater than that of a stressed dry oxide. This result suggested that the SILC originated not from water‐related chemical reactions, but from the distortion of the thermally grown SiO2bond structure during electrical stressing. The SILC of both wet and dry oxides after the application of stress were well fitted by Fowler‐Nordheim lines, confirming that the leakage conduction mechanism is independent of the oxidation process. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363655

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The influence of the electromagnetic properties of the substrate upon the size effect in hard superconductors is theoretically studied. Within the frame of the critical state model we calculate the distribution of the magnetic induction and the electric field inside a superconducting plate subject to unilateral electromagnetic excitation. This calculation allows to obtain the dependences of surface impedance and absorptivity on the amplitude of the external signal. We find that the manifestation of the size effect in the ac response of the superconducting plate depends strongly on the value of the dielectric constant of the substrate. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363656

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

YBa2Cu3O7−&dgr;superconductor–normal–superconductor junctions in step‐edge geometry are fabricated by aninsituprocess using off‐axis sputter deposition. Gold is used as the normal metal interlayer. A special inhibiting layer is introduced to ensure proper separation of the superconducting electrodes. The temperature dependence of the critical currentIcis nearly linear. The junctions show high normal resistances up toRN=10 &OHgr; and highIcRNproducts up to 8.8 mV at 4.2 K.I–Vcharacteristics exhibit a large excess currentIexover the whole temperature range, even for electrically narrow junctions. The power dependence of Shapiro step heights shows characteristic deviations from the resistively and capacitively shunted junction model. Calculations using the time‐dependent Ginzburg–Landau equations are carried out to model the measuredI–Vcharacteristics with and without microwave irradiation. A point‐contact‐like superconductor–constriction–superconductor model is suggested to explain the excess current and the measured temperature dependence ofIcandIex. Time‐dependent Ginzburg–Landau equations are used to account for the observed dynamic characteristics. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363715

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 &mgr;m. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (&khgr;⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility &khgr;∥acand the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which ⟨&khgr;⟩=1.35. In very low fields (<0.5 mT) &khgr;∥acis small (∼0.1), but it increases toward a constant value of order 1 when the field exceeds a value of order 1 mT which is inversely proportional to the particle size. There is a corresponding increase in loss angle which is attributed to weak pinning of domain walls at surface defects. The volume fraction of single‐domain and multidomain powder are deduced as a function of average particle size both in the unmagnetized and remanent states, from an analysis of high‐field magnetization curves on isotropic powder. For example, about 87% of the volume of powder with an average size of 6.7 &mgr;m is multidomain in the unmagnetized state, but only 43% of the volume is multidomain in the remanent state. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363638

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The stress distribution around a 50% blind‐hole pit in a steel pipe with a 9 mm wall has been studied using high‐resolution magnetic Barkhausen noise (MBN) measurements. A magnetic disk read‐head is used as the pick up coil in the MBN probe. The study shows a stress concentration factor of ∼2 at the defect edge perpendicular to the direction of applied stress and ∼−0.6 at the edge parallel to the same. The experimental results are consistent with the analytical solutions obtained by the Airy’s stress function approach. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363657

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The dielectric properties of nematic liquid crystal (4‐cyano‐4′‐n‐pentylbiphenyl: 5CB) cells in the ultralow frequency regime was investigated. A dielectric relaxation, whose relaxation time is 160 s, is observed at 303 K, and the dielectric relaxation is found to be independent of applied electric field. It is shown that the dielectric relaxation is caused by the Helmholtz double layer formed by the adsorption of impurity ions in 5CB onto the electrode surfaces of the cells, and hence the thickness of the double layer is comparable to the radius of impurity ions in 5CB. The dielectric relaxation obeys the empirical Cole‐Cole circular arc law, indicating that dielectric relaxation times are distributed. The distribution of dielectric relaxation times can be explained in terms of distributed thicknesses of the Helmholtz double layer. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363658

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An equilibrium domain theory is used to calculate the thickness and temperature dependencies of relative domain populations in tetragonal ferroelectric thin films epitaxially grown on cubic substrates. The relative coherency strain between the film and substrate that governs domain configurations is evaluated with the account of strain accomodation caused by the generation of misfit dislocations at the growth temperature. The threshold coherency strain, which provides the energetic equivalency of thecandasingle‐domain films and plays an important role in predicting multidomain patterns, is computed as a function of temperature with the aid of the Landau–Ginsburg–Devonshire‐type thermodynamic theory. Theoretical predictions are compared with the experimental measurements for PbTiO3films grown on (001)‐oriented MgO and KTaO3substrates and found to be in good agreement in both the thickness and temperature dependence of thec‐domain volume fraction in these epitaxial heterostructures. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363659

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The polarization induced by thermopoling the alternating vinylidene cyanide/vinyl acetate copolymer has been studied by the thermal pulse technique. The mean polarization over the thickness and its spatial profile were determined as functions of the poling variables: electric field, time, and temperature. Additionally, the thermal stability of the polarization was studied between ambient temperature and the glass‐transition temperature of 175 °C. The mean polarization was found to be proportional to the poling field up to the maximum field used of 42 MV/m and to be very stable up to 150 °C. The polarization profiles obtained after poling were found to be highly inhomogeneous, especially for short poling times, indicating positive charge injection from the positive electrode during poling. The polarization distribution continues to evolve after the mean polarization has reached a steady state indicating separate time scales for space‐charge migration and dipole reorientation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363639

出版商:AIP    

年代:1996

数据来源: AIP