摘要:

The effects of biaxial strain on the intervalence‐band absorption spectra ofp‐doped InGaAs/InP bulk layers are investigated. The study is performed by calculating and comparing the absorption coefficients corresponding to the direct transitions between the heavy and light hole bands, between the heavy hole and split‐off bands, and between the split‐off and light hole bands in both the lattice matched and the strained layers. The valence‐band structures of these layers are neither isotropic nor parabolic and hence thek⋅papproach is utilized to calculate the band structures and their corresponding wave functions. The quantities are then invoked in the calculation of the (joint) density of states, the Fermi energy, and the momentum matrix element, which are needed in the evaluation of the intervalence‐band absorption coefficients. These calculated results show that the intervalence‐band absorption coefficients depend on the strain in the layer. The dependence is determined by the bands involved in the intervalence transition, the polarization of the incident light, and the type of the strain (compressive or tensile). ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359064

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The nonlinear optical and transport properties of a nipi‐doped InxGa1−xAs/GaAs multiple‐quantum well sample (x=0.23) has been studied using a novel approach called electron‐beam‐induced absorption modulation (EBIA). The absorption in the sample is modulated as a result of screening of the built‐in electric field in the nipi structure due to excess carrier generation. The change in field causes a Stark shift of the first quantized optical transitions in QWs which are situated in the intrinsic layers. In EBIA, a scanning electron probe is used to locally generate an electron–hole plasma that is used to study the spatial distribution of defects that impede excess carrier transport and reduce the lifetime of spatially separated carriers. The Stark shift in the MQW structure is imaged with micrometer‐scale resolution and is compared with cathodoluminescence imaging results which show dark line defects resulting from strain‐induced misfit dislocations. Theoretical calculations using Airy functions in the transfer‐matrix method with a self‐consistent field approximation were used to determine the energy states, wave functions, and carrier recombination lifetimes of the MQW as a function of the built‐in field. A quantitative phenomenological analysis is employed to determine the built‐in field, excess carrier lifetime, and ambipolar diffusion coefficient as a function of the excitation density. The defects are found to create potential barriers and recombination centers which impede transport and markedly reduce the excess carrier lifetime. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359065

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Brillouin scattering is used to determine the refractive indexN=n+i&kgr; of Ga0.52In0.48P layers, lattice matched to GaAs, in a wavelength range never investigated before (400 nm<&lgr;<800 nm). It is shown that the variations ofnare given by two different analytical expressions, depending on whether the corresponding photon energiesEare lower or higher than the band gapEg. Optical absorption measurements are deduced from the Brillouin linewidth. The whole set of elastic constant is also determined through the Brillouin shifts. ForE∼Eg, i.e., in resonance conditions,ndisplays a small but sharp peak closely related to the abrupt increase of the optical absorption. Besides the Brillouin investigations, complementary results obtained by ellipsometry and Raman spectroscopy confirm our optical properties measurements in Ga0.52In0.48P and the electronic resonance behavior, respectively. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359066

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Quantitative emission spectroscopy has been used to study the plume formed following laser ablation of YBCO in an oxygen atmosphere. Excited state population distributions, determined from emission line intensity ratios, are used to investigate spatiotemporal variations in the local Boltzmann temperatures for both neutral and ionic species within the expanding plume. Temperatures, obtained from emission line intensity ratios of both Cu(I) and Y(I), decrease slowly and nonadiabatically in the range 1.0–0.5 eV during plume expansion. Higher initial Boltzmann temperatures of ∼3 eV are however obtained from the emission line intensity ratios of fast ions, which dominate the composition of the highly luminous expanding front of the plume. Quantitative comparison of emission intensities for the different neutral species present indicate that the neutral composition in the luminous region of the expanding plume is increasingly dominated by Cu(I) which has a faster expansion velocity and lower oxidation rate than Y(I) and Ba(I). ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359529

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Pulsed laser ablation of CdWO4at 266 nm is studied with a quadrupole mass spectrometric (QMS) time‐of‐flight method. Ablation threshold, energy distribution, and angular distribution of the ablated species as well as nonlinearity of the ablated species mass intensity are elucidated as a function of laser fluence. Ablated species of O+2, Cd+, Cd2+, W+, and WO+translate with energies strongly depending on the fragment mass, meaning that they are confined in a space with the same velocity distribution. Ablated species detected with the QMS filament off show a Gaussian distribution for their translation energy, which is interpreted by the Franck–Condon electron excitation mechanism. A simple model is proposed based on a photochemical bond breaking to explain the observed threshold and nonlinearity of the ablated species. Nonlinearity can be explained by photofragmentation of CdWO4cluster ions and the successively occurring volume expansion. The latter will be the main cause for the desorption of ion species by ablation and supports the narrow angular spreading of the ablated species. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359067

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The dynamics of the species ejected by excimer laser ablation of a BiSrCaCuO target in different gas environments is studied by spatially resolved, real‐time optical emission spectroscopy. The evolution of the velocity and the emission intensity of the excited species versus the distance and the pressure of the foreign gas present a similar behavior both in reactive (oxygen) and inert (argon) environments. Furthermore, the results show that the plume expansion process is dominated by the interaction of the ejected species and the gas background atoms or molecules through collisional interactions rather than by reactions in the gas phase. The dynamics of the plume expansion is analyzed in the frame of the shock wave and drag models, the latter leading to a very good agreement with the experimental results and the dependence of the slowing coefficient with the gas pressure is established. Finally, the influence of the excitation energy of the considered transition on the observed emission features is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359068

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A synchrotron‐radiation photoemission study of K evaporated on rf‐sputtereda‐Si:H,a‐Si, andc‐Si(001) semiconductor surfaces at room temperature is reported. Thea‐Si substrate was obtained from sputtering of thec‐Si(001) surface. It is found that the change in work function upon K adsorption is smallest on the most disordered surface,a‐Si:H. Astrongcovalent K–Si interface state at about 5.9 eV binding energy appears only in disordered surfaces. Surface structure plays a major role in its occurrence. Further, the amorphous surfaces stay semiconducting despite large doses of K atoms, while the K/c‐Si(001) surface becomes metallic at the instance of a work‐function minimum. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359069

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Excimer laser ablation has been used to deposit epitaxial films of &bgr;‐SiC on single‐crystal Si wafers, in a vacuum, at substrate temperatures between 1050 and 1250 °C. Such films can be grown by ablating ceramic SiC, carbon, or alternating silicon and carbon targets at a range of growth rates. X‐ray &thgr;‐2&thgr; diffraction shows the presence of strong, sharp reflections from crystal planes parallel to the substrate, 200 and 400 for [100] substrates and 111 and 222 for [111] oriented substrates. Wrong reflections, such as 111 for [100] substrates, are extremely weak or absent, indicating alignment with the substrates. The characterization of these films by a number of techniques is discussed. In all cases the film‐substrate interface shows a characteristic microstructure of cavities in the Si substrate, similar to that observed for the carbonization layer initially formed as a precursor for chemical‐vapor deposition of SiC films on Si. This implies that the initial film growth, for all cases, involves chemical reaction of the Si substrate with the carbon in the plume as well as transport through the growing film. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359070

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The particle reflection coefficient is determined experimentally and by computer simulation for the bombardment of two different kinds of carbon with deuterium at normal and oblique incidence in the energy range from 1 keV down to 33 eV. Highly oriented pyrolytic graphite (HOPG) and EK98 graphites served as targets, the former as an example with a relatively flat surface, the latter with a rough surface topography. The experimental technique is based on the measurement of the trapped amount by nuclear reaction analysis using the reactiond(3He,p)&agr;; protons are detected with a surface barrier detector. It is found that the usual assumption of complete trapping at low fluences is not fulfilled at low energies (≤100 eV). This is demonstrated by measuring the decrease of the implanted amount of deuterium with further bombardment of protons at the same energy. This loss of implanted atoms can be described by an exponential function which can be used to determine the correct trapping coefficient and from this the correct particle reflection coefficient. The experimentally determined particle reflection coefficients for HOPG agree reasonably well with data calculated with the Monte Carlo programTRIM.SP(versionTRVMC); only at the lowest energy of 50 eV the experimental values are somewhat higher at intermediate angles of incidence than the calculated ones. The rough surface of EK98 is investigated with a scanning tunneling microscope. It is found that this surface can be described by a fractal surface of dimension 2.05. For this surface agreement of the experimental values with those calculated with the programVFTRIM(based onTRIM.SP, assuming a fractal surface) is found. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359071

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Secondary ion mass spectrometry and refractive index measurements have been carried out on silicon oxy‐nitride produced by plasma‐enhanced chemical vapor deposition (PECVD). Nitrous oxide and ammonia were added to a constant flow of 2% silane in nitrogen, to produce oxy‐nitride films with atomic nitrogen concentrations between 2 and 10 at. %. A simple atomic valence model is found to describe both the measured atomic concentrations and published material compositions for silicon oxy‐nitride produced by PECVD. A relation between the Si–N bond concentration and the refractive index is found. This relation suggest that the refractive index of oxy‐nitride with a low nitrogen concentration is determined by the material density. It is suggested that the relative oxygen concentration in the gas flow is the major deposition characterization parameter, and that water vapor is the predominant reaction by‐product. A model, that combine the chemical net reaction and the stoichiometric rules, is found to agree with measured deposition rates for given material compositions. Effects of annealing in a nitrogen atmosphere has been investigated for the 400 °C– 1100 °C temperature range. It is observed that PECVD oxy‐nitrides release nitrogen and hydrogen in the form of NH for annealing temperatures in the 500 °C–700 °C range. The relaxation process during annealing is found to be governed by a viscoelastic relaxation process similar to the relaxation process observed for thermally grown SiO2. Upon nitrogen release, the PECVD material is in a state of internal tension. The viscoelastic relaxation process for temperatures above 700 °C is dominated by the relaxation of this internal tension. A linear relation between the refractive index and material density is determined for silicon oxy‐nitride with a nitrogen concentration below 30 at. %. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359072

出版商:AIP    

年代:1995

数据来源: AIP