摘要:

In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The case of uniform field, obtained with this specialized structure as well as that of the nonuniform field, are addressed. The various experimental techniques are also briefly reviewed. We then summarize the various experimental results obtained in the last few years using these special structures and FKOs and find that, even in this short period, good use has been made of the technique and the structure. This is followed by a brief review of the work on nonuniform fields. In this case, the work on actual device structures has significant technological implications. Important issues such as metallization and processing, the effects of surface treatment and thermal annealing, Schottky barrier heights of different metals, piezoelectric fields in (111) grown strained InGaAs/GaAs quantum wells, and Fermi level in low‐temperature grown GaAs have been studied using this structure. This structure has also been used to study the dynamics of photomodulation, revealing the nature of the cw photoreflectance. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360131

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present a numerical technique for open‐boundary quantum transmission problems which yields, as the direct solutions of appropriate eigenvalue problems, the energies of (i) quasi‐bound states and transmission poles, (ii) transmission ones, and (iii) transmission zeros. The eigenvalue problem results from reducing the inhomogeneous transmission problem to a homogeneous problem by forcing the in‐coming source term to zero. This homogeneous problem can be transformed to a standard linear eigenvalue problem. By treating either the transmission amplitudet(E) or the reflection amplituder(E) as the known source term, this method also can be used to calculate the positions of transmission zeros and ones. We demonstrate the utility of this technique with several examples, such as single‐ and double‐barrier resonant tunneling and quantum waveguide systems, includingt‐stubs and loops. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360132

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Optical excitation of half‐leaky guided modes has been used to determine the refractive indices of a ferroelectric liquid crystal (Merck‐SCE13) in a homeotropically aligned state. The uniform homeotropic alignment is realized, with no surface treatment, by the application of an in‐plane dc electric field. This applied field, of the order of 5×105V m−1is sufficient to fully unwind theS*chelix giving a uniformly tilted homeotropic monodomain for optical characterization. Analytic and numerical modeling results indicate that, for a slab with its optic axis tilted in a plane orthogonal to the plane of incidence, two distinct critical angles appear in the half‐leaky guided mode response. These independently relate simply to the ordinary and extraordinary refractive indices of theS*cmaterial. By fitting theoretical angle dependent reflectivities to those recorded experimentally the two refractive indices have been obtained for a range of temperatures in both theS*candSAphases. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360133

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A method to fabricate chromium‐doped lithium niobate substrates in which the active ions are introduced by thermal diffusion from a film is reported. Chromium concentration depth profiles have been obtained by secondary‐ion‐mass spectrometry and the relevant diffusion parameters have been derived. Fluorescence spectrum and upper laser level lifetime of chromium diffused proton‐exchanged and chromium/titanium‐diffused lithium niobate waveguides have been measured. A simple model has been used to estimate the performance of such structures as waveguide optical amplifiers and lasers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360134

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The Raman effect in semiconductor waveguides below half‐gap is studied both experimentally and numerically. We report the depolarized Raman gain spectra up to 300 cm−1in Al0.24Ga0.76As at pump wavelengths of 0.515 and 1.55 &mgr;m from the measurement of the absolute Raman scattering cross sections using GaAs as a reference scatterer. In addition, the coupled propagation equations for the AlGaAs waveguides are modified to include the Raman effect. By solving the coupled propagation equations numerically, we verify that the energy transfer between two orthogonally polarized pulses demonstrated in previous pump‐probe experiments [M. N. Islametal., J. Appl. Phys.71, 1927 (1992)] is caused by Raman effect. We also show numerically that the Raman effect induces spectral distortions on the pulses, and the energy transfer is inversely proportional to the pulse widths. The energy transfer results in a severe cross‐talk problem for sub‐picosecond pulses in AlGaAs waveguides. For example, the energy exchange is about 30% for 300 fs pulses under &pgr; phase shift conditions. Therefore, the Raman effect limits the performance of semiconductor waveguides in optical switching applications for sub‐picosecond pulses. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360135

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We study by photoacoustic spectroscopy the band‐gap shift effect of CdS films. The CdS films were grown by chemical bath deposition and exposed to different annealing atmospheres over a range of temperature in which the sample structure changes. We show the band‐gap evolution and resistivity as a function of temperature of thermal annealing and determine the process that produces the best combination of high band‐gap energy and low resistivity. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360136

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Theoretical and experimental possibilities are presented of a modulated photothermal method, laser‐induced photoreflectance, for inspecting thermal diffusivities and quality of interfaces in composite materials with micron‐scale spatial resolutions. The models are established for semi‐infinite materials containing interfaces parallel or perpendicular to the sample surface. The applications concern thermal diffusivity measurements of anisotropic polycrystals and detection of thermal resistance in damaged materials and at interfaces between reinforcements and matrix in composites. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360137

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We describe methods for automating the control and tracking of states within or near a chaotic attractor. The methods are applied in a simulation using a recently developed model of thermal pulse combustion as the dynamical system. The controlled state is automatically tracked while a parameter is slowly changed well beyond the usual flame‐out point where the chaotic attractor ceases to exist because of boundary crisis. A learning strategy based on simple neural networks is applied to map‐based proportional feedback control algorithms both with and without a recursive term. Adaptive recursive proportional feedback is found to track farther beyond the crisis (flame‐out) boundary than does the adaptive non‐recursive map‐based control. We also found that a continuous‐time feedback proportional to the derivative of a system variable will stabilize and track an unstable fixed point near the chaotic attractor. The positive results suggest that a pulse combustor, and other nonlinear systems, may be suitably controlled to reduce undesirable cyclic variability and extend their useful operating range. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360138

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A self‐consistent hybrid Monte Carlo‐fluid model for a direct current glow discharge is presented. The Monte Carlo part simulates the fast electrons while the fluid part describes the ions and slow electrons. Typical results of the model include collision rates of the fast electrons, energy distributions of these electrons, fluxes and densities of the different plasma species, the electric field and the potential distribution, all as a function of position from the cathode. The influence of the negative glow on the calculations in the cathode dark space is studied. Moreover the influence of three‐dimensional scattering instead of forward scattering and the incorporation of side wall effects is investigated. Calculations are carried out for a range of voltages and pressures in order to study their influence on the calculated quantities. Comparison was made between total electrical currents calculated in the model and experimentally measured ones to check the validity of the model. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360139

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Nanometer‐thick hydrocarbon films were plasma polymerized in a rf pulse discharge in an acetylene/argon mixture and were mechanically patterned by the AFM (atomic force microscope). In addition a dc bias voltage was applied to the gold coated Si3N4AFM tip. Depending on the experimental conditions, different patterns have been observed: mechanical indentation, electric charge, and material deposition. The viscous properties of the plasma deposited film affects the movement of the AFM tip while it is scanning the surface in a contact mode, and also affects the size and shape of the patterned area. Spikes of about 25–72 nm height and 60–200 nm width were formed from gold transferred from the newly mounted gold coated tips. The mechanism of gold deposition could be assigned to the Joule heating of the tip, resulting from the electric breakdown of underlying dielectric layers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360140

出版商:AIP    

年代:1995

数据来源: AIP