摘要:

We have fabricated light‐emitting nanocrystallites embedded in ana‐Si:H matrix using a conventional plasma‐enhanced chemical‐vapor‐deposition system. It was found that the photoluminescence properties are directly related to the deposition parameters. The quantum size effect model is proposed to explain the photoluminescence. Two structural prerequisites are proposed for this kind of films to exhibit effective light emission: One is an upper limit for mean crystallite size of about 3.4 nm, the other is an upper limit for crystallinity of about 30%. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360778

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We attempt to deal with the physical processes involved in the temperature‐dependent photoreflectance of single AlGaAs/GaAs modulation‐doped heterojunction structures. Building on the assumption that photomodulation mechanism is due chiefly to modification of the band bending in the buffer layer, we apply the Franz–Keldysh theory to simulate the temperature‐dependent photoreflectance spectra of single AlGaAs/GaAs modulation‐doped heterojunction structures. In view of the nonuniformity of the electric field within the buffer layer, the field profile of which is calculated through the application of self‐consistent variational approach, the WKB method is used to approximate the effective change in the dielectric function. A comparison between the experimental and the simulated results attests the validity of our assumption. The effects of temperature on the electric‐field strengths, estimated from the extrema of the Franz–Keldysh oscillations, can be accounted for by considering the temperature dependence of the Fermi level. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360565

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Direct measurement of the deep defect density in thin amorphous silicon films with the help of the ‘‘absolute’’ constant photocurrent method is demonstrated here. We describe in detail how the optical (photocurrent) absorption spectrum can be measured directly in absolute units (cm−1) without additional calibration and undisturbed by interference fringes. Computer simulation was performed to demonstrate absolute precision of the measurement and to explain residual interferences which are sometimes observed. The residual interferences are shown to be direct fingerprints of an inhomogeneous defect distribution. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360566

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

dc sputtered indium‐tin‐oxide films have been excimer laser irradiated at subablation threshold fluences (<510 mJ/cm2). Optical characterization of irradiated products has been performed aiming at resolving the finer structure appearing in the IR–visible absorption spectra, as a function of laser fluence, and assigning such features to specific electronic defects which are produced upon irradiation. Four individual Gaussian‐like contributions to absorption spectra are identified at 0.7, 1.0, 1.6, and 2.6 eV, the intensity of which is observed to vary with fluence. Being absent in the original films and emerging in optical spectra at fluences exceeding 300 mJ/cm2, the 2.6 eV contribution is most characteristic to excimer laser processing and is responsible for the darkening of the film. Thermal model calculations reveal that such defects are produced only upon melting and fast resolidification of the film. The evolution of the chemistry actually taking place in the film upon irradiation is followed by x‐ray photoelectron spectroscopic analysis. A chemical approach to the production of such defects is proposed in which oxygen displacement in the atomic matrix leads to the formation of neutral ternary complexes of the type SnIn2O4. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360567

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Several possible contributions to the valley currents in InAs/AlSb/GaSb resonant interband tunneling diodes are analyzed. A theoretical model proposes that the field‐assisted thermionic hole emission is the main valley leakage mechanism at high temperature and the Fowler–Nordheim hole tunneling is the dominant leakage path at low temperature and high bias. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360568

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A theoretical model is presented which describes time dependent charging phenomena in insulators irradiated by an electron beam. This so called dynamic double layer model (DDM) is based on the simplified assumption of charge development in two axi‐symmetric cylindrical volumes at the insulator surface and at a certain depth below the surface, respectively. Important physical interaction mechanisms like the electrostatic interaction between the surface potential and emitted secondary electrons, the transport characteristics of internally generated secondary electrons in a positively charged insulator surface, as well as radiation induced electrical conductivity are included in the DDM model. Experimentally determined stationary and time dependent charging behaviour of Al2O3single crystals (sapphire) irradiated with an electron beam between 1 and 15 keV energy can be qualitatively explained by the DDM model. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360569

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin‐film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseconds to a few hundred picoseconds. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360570

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Epitaxial growth of erbium‐doped silicon films has been performed by plasma‐enhanced chemical vapor deposition using an electron‐cyclotron‐resonance source. The goal was to incorporate erbium as an optically active center (ErO6) through the use of metal‐organic dopant sources. The characteristic 1.5 &mgr;m emission was observed by photoluminescence. Chemical analysis of the film revealed, however, that the organic ligands were decomposing and contributing to the carbon contamination of the films. Analysis of the molecular flux to the substrate indicated that the metal‐organic compound used, tris(2,2,6,6‐tetramethyl‐3‐5‐heptanedionato)erbium(III), was most likely to decompose, and supply unbonded atomic erbium and not the optical active species, ErO6. Excessive carbon contamination lowered epitaxial quality and reduced the photoluminescent intensity. Photoluminescent intensity was improved by a 600 °C anneal but was strongly quenched by a 900 °C anneal. The low‐temperature anneal improved crystal quality, and the high‐temperature anneal resulted in silicide formation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360571

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The processes of crack growth and dislocation emission induced by the crack tip are investigated. A crystal with cubic lattice of atoms under plane strain conditions is considered. The main principles of the nanofracture mechanics approach employed in this study are outlined. Both ductile and brittle mechanisms of crack growth in the crystal are examined in nano‐ or interatomic scale. Only the fundamental constants of the classical theory of dislocations are used which include the interatomic spacing, elastic constants, the Schmid friction constant, and the true surface energy of crystal lattice. The efficient solution of the elastic problem for an arbitrary number of dislocations near the crack tip is obtained in terms of complex potential functions. The equilibrium of dislocation pairs near the crack tip during monotonic loading is investigated. It is shown that dislocation generation at the crack tip occurs at certain quantum levels of external load. The magnitude of external load corresponding to crack growth initiation and emission of the first pair of dislocations is calculated. The mathematical problem for an arbitraryNnumber of dislocation pairs near the crack tip is reduced to a parametric system ofNnonlinear equations, where the stress intensity factor of external loadKIplays the role of parameter andNthe role of discrete time. The minimum value ofKIat which the solution of this system of equations exists corresponds to the stress intensity factor at which theNth pair of dislocations is generated. The numerical method is presented to determine the minimum value ofKI. The approximate method of self‐consistent field is employed to reduce the order of the system of nonlinear equations. The approximate method is used to calculate the fracture curveKI(lc) relating the value ofKIwhich maintains the crack growth to the crack length incrementlc. The exact solution is also studied, and numerical results are given for a crack in an aluminum specimen and involve the quantum levels of external load corresponding to the moments of dislocation generation and the values of the superfine stress intensity factor up to 150 dislocations. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360572

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This work describes some investigations on the atomic and ionic beams generated by a CdS target under excimer laser irradiation at 308 nm, and on factors influencing the quality of the films obtained. The angular distribution of particles emitted in a vacuum was found to be highly directional, concentrated around a direction normal to the target surface. The angular emission pattern depends on laser fluence. The method used for studying the emission pattern helps in positioning substrates for optimum film quality and uniformity. Measurements of the energy of the emitted particles yielded high values (several tens of eV) for both ionized and neutral particles, suggesting a nonthermal mechanism. Some possible models are discussed. Finally, considerable improvement in both film uniformity and homogeneity was obtained through the use of a laser‐triggered plasma discharge in the source region. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360573

出版商:AIP    

年代:1995

数据来源: AIP