摘要:

It is shown that charge transport in SiC ceramics includes atomic mechanisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature‐dependent dc and ac measurements. The complex dielectric function (DF) of boron‐doped SiC ceramics with various additives has been measured at frequencies from 5 Hz to 2 GHz and at temperatures between 100 and 330 K. In addition, the dc conductivity was measured between 40 and 220 K. A transport mechanism on an atomic scale determines the temperature dependence of the dc conductivity. At low temperatures 3D variable range hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius‐like carrier activation becomes dominant. The ac behavior depends on the dc conductivity, but it reflects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 104. Two polarization processes have been identified. The low‐frequency process is related to a conduction current relaxation, i.e. to a partial interfacial polarization in conducting paths. The Barton‐Nakajima‐Namika relation holds, relating dc conductivity, relaxation time, and relaxator strength. On the other hand, the high‐frequency process is attributed to Maxwell‐Wagner‐Sillars interfacial polarization in crystalline SiC grains with a size of several &mgr;m. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363054

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The complex influence of recombination centers and potential fluctuations of the band gap on the scattering and recombination phenomena inn‐type semiinsulating liquid‐ encapsulated‐Czochralski‐grown GaAs were investigated by using the transient photoconductivity and photo‐Hall effects. The inhomogeneities cause a hyperbolic decrease of nonequilibrium carrier concentration and the saturation of Hall mobility, while the exponential parts of the decay appear due to the recharge of deep levels. The mean recombination barrier heights of potential fluctuations were evaluated. We propose a complex ‘‘island’’ model of scattering and recombination centers, consisting of defect clusters and their associations around dislocations, surrounded by potential barriers. At low light intensities and at the temperatures below 330 K they are insulating for majority charge carriers, thus reducing an effective crystal volume and causing percolation transport effects. At the temperature higher than 330–360 K the main barrier of the island can be recharged or screened by nonequilibrium carriers and its fine barrier structure appears as an effective scatterer, causing a sharp decrease of the nonequilibrium Hall mobility. It was demonstrated that although doping with Sb reduce dislocation density, it can intensify the effect of smaller defects on transport phenomena. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363055

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new technique for growing thin films of conducting polymer with arbitrary geometry on substrates compatible with the microelectronic technologies is proposed. The new procedure utilizes commercial photoresist patterns properly modified in a way to make them suitable for the oxidation of a large class of monomers. The technique was tested with pyrrole, obtaining thin films of a composite material (resist/polypyrrole) with high conductivity. The new material was optically, morphologically, and electrically characterized. Four probe resistors and Wheatstone microbridges were built in order to measure the resistance as a function of temperature and the resistance fluctuation spectral density, respectively. Analysis of the results on the basis of conduction models of disordered media is presented. The time stability of the electrical parameters was also investigated. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363056

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report the observation of the Wannier–Stark localization effect in short period GaAs/ InXAl1−XAs superlattices with strained barriers. The superlattices, each of which is contained in the intrinsic region of ap‐i‐ndiode structure, consist of GaAs (3.0 nm) wells and strained shallow InXAl1−XAs (0.9 nm) barriers (X=0, 0.1, 0.2, and 0.3) grown on GaAs by molecular beam epitaxy. In spite of the use of strained barriers, the Wannier–Stark localization effect is clearly observed for all samples at room temperature. Even the superlattice sample with the highest In content ofX=0.3 exhibits distinct photocurrent spectra showing several peaks associated with Wannier–Stark ladder transitions as well as Franz–Keldysh oscillations. It is found that the transition intensities are consistent with theoretically calculated oscillator strengths based on the simplified tight‐binding model. By increasing the In contentX, the miniband width increases and the absorption peak energy due to the zeroth order ladder (e1‐hh1 ande1‐lh1) transitions decreases because of the reduced barrier height. The transition energies are consistently explained by taking modulation effects into account on the valence subbands due to the compressively strained barriers. The above results show that the use of a large strain effect on the barriers is possible in Wannier–Stark localization effect type devices. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363057

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Using the asymptotic transfer‐matrix method, we investigate the resonant tunneling of holes in double‐barrier semiconductor structures in the presence of an in‐plane magnetic field. The transmission coefficients including ll (light to light hole), hl (light to heavy hole), hh (heavy to heavy hole), and lh (heavy to light hole) are calculated as a function of energy. As in the case of nonzero parallel wave vectors, the mixing of hole tunneling can also occur due to the in‐plane magnetic field. Moreover, as has been observed by resonant magnetotunneling spectroscopy, we also find that the different resonances have quite different magnetic‐field dependences. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363058

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The activation efficiencies of implanted Si, Be, and C in GaAs0.93P0.07have been measured in the annealing range 650–950 °C. Be provides much higher sheet hole densities than C, even when the latter is coimplanted with Ar to enhance the electrical activity. The maximum activation efficiency of Be is ∼60% at a dose of 5×1014cm2whereas that of C is an order of magnitude lower. Si produces donor activation percentages up to ∼20% under optimized annealing conditions. Capless proximity annealing is adequate for surface preservation up to ∼950 °C, as measured by scanning electron microscopy and atomic force microscopy. Photoluminescence measurements provide evidence that nonradiative, damage‐related point defects remain in the GaAsP even after annealing of 950 °C. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363059

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Highly carbon‐doped Ga0.47In0.53As layers grown by chemical beam epitaxy have been exposed to a deuterium plasma. Deuterium diffusion profiles reflect very strong C–D interactions. Concerning electronic transport properties, fromp‐type when as‐grown, these GaInAs samples turn ton‐type after plasma exposure. Annealings of deuterated layers have also been performed. They show that temperatures as high as 450 °C must be reached beforep‐type conductivity is fully restored in the material. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363060

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effective capture rates of electrons and holes by the defects of undopeda‐Si:H are measured using a technique based on the optical bias dependence of the modulated photocurrent. The evolution of the phase shift and modulated photocurrent spectra with the optical bias intensity is used to study the capture coefficient of the defects at different trap depths in the energy gap. The modulated photocurrent spectra are recorded using uniformly, or strongly absorbed light in order to obtain information about the trapping and recombination processes in the surface, the interface, and the bulk of the films. The calculated capture rates of carriers are studied in a series ofa‐Si:H films and information about the defect structure is obtained. It is found that the effective capture rates of the carriers by the defects are not constant in the studied films and may vary by up to two orders of magnitude from sample to sample. Finally, the importance of the atomic environment and the local strains of the defects ofa‐Si:H for the capture process of the carriers is discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363061

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Changes in the energy dissipation of the tape form Ag‐(Bi,Pb)2Sr2Ca2Cu3Oxconductor have been interpreted in terms of magnetic flux movement from, or into, the intergrain regions of the highly textured plate like grain structure. The time dependent increase or decrease of the transport critical current (Ic), originates from time dependent angular superposition of the current induced self field (HSF), and external magnetic field [field cooling (HFC), and zero‐field cooling (HZFC)] at the intergrain regions. The existence of time dependent changes ofIccan be explained in terms of the time dependent flux redistribution in grains and grain boundaries caused by thermal relaxation processes. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363062

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Structural and electrical properties of YBa2−xSrxCu3Oyprepared under ambient oxygen pressure were systematically investigated for 0≤x≤2.0. Samples with high Sr contents,x≥1.7, showed semiconductive properties with an activation energy of ∼150 meV at high temperature,T≥80 K. At low temperatures,T≤80 K, activation energy dropped to ∼3.00 and ∼0.85 meV forx=1.7–1.8 andx=1.9–2.0, respectively. It is argued that YSr2Cu3Oyis likely to have a tetragonal structure and that the disorder introduced by Sr substitution affects electrical conduction, which involves charge hopping between the CuO chains at high Sr contents. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363063

出版商:AIP    

年代:1996

数据来源: AIP