摘要:

We have measured current–voltage curves and the temperature dependence of the zero bias conductance for ap-type Be-doped GaInP/GaAs heterojunction grown by the molecular beam epitaxy method. We have determined the valence band offset&Dgr;E&ngr;from both measurements and find it to be 310 meV within 5&percent; of accuracy. Similarly, we find for ann-type Si-doped sample that the conduction band offset&Dgr;ECis 95 meV. First-principles calculations have been carried out for the atomic and electronic structures of the interfaces. For the thermodynamically favored interfaces, the valence band offset is found not to be sensitive to atomic relaxations at the interface. The calculated values are in good agreement with the experiments. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365650

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Current–voltage characteristics of 30 period 390 Å GaAs/110 ÅAl0,3Ga0,7Assuperlattices are studied. Features revealed in multistable current branches of the hysteresis loops made it possible to estimate gradual changes in the size and shape of the domain boundary and reconstruct the final shape of the domain boundary. It was concluded that a considerable current bistability observed in the investigated superlattices is due to the domain boundary expansion. Switching between bistable current states in each hysteresis loop is detected. The switching time depends on the direction of the current jump. The switching time from a high- to low-current state is about10−7 s;the time of the reverse switching to the high-current state is about10−6 s.The observed switching is attributed to the shrinkage and expansion of the domain boundary size. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365651

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

InGaAs/AlAs quantum wells on GaAs substrates were studied for near-infrared intersubband transitions. The relationships between the properties of the intersubband absorption and the structural/growth conditions of the quantum wells were quantitatively investigated by using theoretical calculations based on ak-pperturbation method. The total intersubband absorption magnitude is shown to be closely related to the leakage of free carriers from the &Ggr; minimum of the well to theXminimum of the barrier. This work provides important information for design of devices utilizing short wavelength intersubband transitions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365652

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Nonequilibrium (hot) optical phonon effects on electron runaway from GaAs quantum wires embedded in AlGaAs have been investigated by Monte Carlo technique. We have simulated the carrier runaway kinetics in the0<E<1000 V/cmelectric-field range for a lattice temperature of 30 K. Due to optical phonon mode confinement by GaAs/AlGaAs heterointerfaces, the buildup of generated hot phonons is strongly pronounced in the quantum wires. Even at moderate electron concentrations and electric fields, the accumulation of these phonons may become significant and substantially affect all transport properties in the structure. As a result of reduced hot electron cooling rates in the presence of nonequilibrium optical phonons, the high-energy tail of the carrier distribution function extends above the potential barriers at the quantum wire boundaries. This may eventually lead to significant electron escape from the potential well, even at relatively low electric fields, what significantly affects the performance of such nanoscale systems. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365653

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated the midinfrared absorption between confined levels of undoped InAs/GaAs quantum dots obtained by self-organized growth. The infrared absorption is measured by a photoinduced infrared spectroscopy. Quantum dots with different sizes are analyzed as a function of temperature, interband pump photon energy, intensity, and infrared polarization. We show that in the 90–250 meV energy range the quantum dots exhibit intraband absorption between confined levels, which are polarized along the growth axis as for usual conduction intersubband transitions in quantum wells. Intraband absorption is observed for either selective excitation of the dots or excitation via absorption in the wetting and GaAs layers. Based on the energy position and the temperature dependence, the infrared resonances are attributed to intraband transitions between confined holes and to bound-to-continuum transitions of electrons, which, respectively, shift to high and low energy as the dot size is decreased. The reported features are found in qualitative agreement with the theoretical predictions of Grundmann &etal; [Phys. Rev. B52, 11 969 (1995)]. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365654

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have studied the electronic states in N-doped ZnSe/ZnTe type-II superlattice (SL) by deep level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy (ICTS). The capture and emission processes of holes between a miniband of the SL and the valence band of ZnSe barrier were investigated. From the analysis of DLTS and ICTS spectra, the activation energy for the hole emission from the miniband energy level was determined to be0.28±0.03 eV,which is consistent with a theoretical value (0.25 eV) of the band offset between the ZnSe/ZnTe SL calculated based on the Kronig–Penney model. A deep level with an activation energy of0.48±0.03 eVwas observed and has been assumed to originate from an interface defect in the SL region. A deep level located at0.54±0.03 eVabove the valence band of ZnSe was also observed in the ZnSe capping layer. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365655

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

By using the ultrahigh vacuum plasma enhanced chemical vapor deposition system to prepare nc-Si:H films with high conductivity, the experimental results show that the conductivity of nc-Si:H films increases with decreasing the mean grain size of films. Hence, there exists a small size effect on the conduction process. Based on the experimental data, we used the effective-medium theory to calculate the partial conductivity&sgr;cof crystallites and&sgr;iof the interface conductivity, respectively. Otherwise, we found that there existed two structure phase change point results from the effective-medium theory calculated for the materials of silicon films. The results suggest that the high conductivity of nc-Si:H films results mainly from the crystallites, and moreover, the interface region may serve as insulator layers. Thus, we may consider that the crystallites in nc-Si:H films act as quantum dots. In this paper, we present a heteroquantum dot tunneling model to discuss the transport process for the nc-Si:H films. Our calculated results agree very well with the experimental conductivity data for nc-Si:H films. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365656

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd1−xMnxTe system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365657

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Diamond metal-insulator-semiconductor diodes and field-effect transistors (MISFETs) have been prepared usingCaF2gate insulator and nondoped (in some cases, boron was doped) diamond homoepitaxial films. The resultant capacitance-voltage(C-V)curves and drain current-drain voltage(ID−VD)curves strongly depended on the amount of oxygen contamination of diamond surface. From analyses ofC-VandID−VDcurves, it was found that the oxygen contamination induced the surface states with two distribution peaks locating very near the valence band edge and at the energy of∼1 eVfrom the valence band edge. Although fluorination of oxygen-terminated diamond surface proceeded to a certain extent duringCaF2deposition at the elevated temperatures in vacuum, it still allowed surface state formation of about∼1014/cm2 eVnear the valence band edge due to uncompleted exchange of oxygen by fluorine atoms and easy penetration of residual oxygen in the chamber through theCaF2insulator at elevated temperatures. Reduced-oxygen process by diamond surface passivation with hydrogen (hydrogenation) and room temperature deposition ofCaF2greatly improved the surface stability, and consequently, the surface state density near the valence band edge was reduced to∼1012/cm2 eV.In this manner, the effective hole mobility of∼10 cm2/V swas obtained from the diamond MISFET, which can be well compared with the surface Hall mobility of35 cm2/V s.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365658

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Quantifying the relationship between crystallographic texture and magnetic properties is highly desirable for engineering high-energy-product magnets. Such an undertaking is a challenge for melt-quenched magnets formed fromNd2Fe14B-based alloys because they possess a nanoscale microstructure complicated by the presence of multiple phases, both ferromagnetic and paramagnetic. Procedures for the evaluation of texture in permanent magnets often rely upon magnetic remanence measurements; however, such determinations are based on the use of the Stoner–Wohlfarth model, and are applicable only to assemblies of noninteracting particles, which nullifies their use in texture determinations of “exchange-spring” magnets. To overcome these inherent experimental difficulties, alternative techniques for the measurement of the bulk texture of permanent magnets are explored in this work. Crystallographic alignment was investigated in both neutron and hard x-ray diffraction experiments. Transmission synchrotron x-ray diffraction measurements of the rocking curve width as a function of both the vertical and horizontal positions were performed on a series of thermomechanically processed, melt-quenched magnets based on theNd2Fe14Bcomposition. The results of the hard x-ray diffraction experiments were verified by neutron diffraction work. A simple model based on a squared-Lorentzian distribution is used to obtain a quantitative estimation of the population of misoriented grains for a given rocking curve half-width. It is deduced that even the most deformed magnets still possess a significant degree of particle misalignment; over 50&percent; of the constituent grains have a misorientation of at least 15° from the overall axial direction at the center of the magnet. The spatial distribution of the rocking curve widths indicates that the development of thec-axis texture is initiated in the center region of the magnet and progresses to the perimeter with increasing deformation. Samples with lower deformation levels possess more uneven texture distributions than do those with higher deformation levels, and the degree of orientation seems to develop rapidly near a deformation level of 50&percent;. The results of this work are utilized to produce a phenomenological model, consistent with the findings of past researchers, of deformation based on mass transport under applied stress via a liquid grain-boundary phase. It is proposed that the micromagnetic structure consists primarily of relatively large clusters of exchange-coupled grains that result in lower-coercivity areas within the magnet. Recommendations are included for improving the production and processing of thermomechanically deformed melt-quenched magnets. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365659

出版商:AIP    

年代:1997

数据来源: AIP