摘要:

The zinc concentration measured after organometallic vapor phase epitaxy (OMVPE) growth on (100)‐oriented GaAs at 700 °C has been compared to the zinc concentration measured after in‐diffusion under near‐equilibrium conditions. During diffusion, the concentration of Zn 20 nm below the surface was found to vary withP1/2Zn, as expected for bulk solid–vapor equilibrium. During growth, the concentration of Zn varied linearly withPZnup to a maximum value which was found to correspond to the solubility limit set by second phase formation, e.g., growth of Zn3As2. Although large differences were observed between the results of the two experiments when using nominally identical ambient conditions, all of the results are consistent with a thermodynamic model in which the Fermi level at the surface is pinned approximately 200 meV below the intrinsic Fermi level. Typical OMVPE growth conditions appear to give a bulk zinc concentration which is supersaturated relative to the ambient partial pressures used, and to enhance the diffusion of Zn into the substrate. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358821

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A rectangular strained epilayer with its length much larger than its heighthand width 2lis considered. The energy change on introducing a 60° dislocation either parallel to the long dimension (longitudinal) or parallel to the short dimension (transverse) into the epilayer is calculated. The effects of free surfaces on the mismatch stress and on the dislocation energy are taken into account approximately. For a given misfit, there is a critical height and critical width such that if the dimensions of the layer exceed their critical values, introduction of a longitudinal misfit dislocation is energetically favored. However we show that a transverse dislocation always sets in before a longitudinal one and as a result, the critical thickness is almost equal to that for a film of infinite extent even when the width is very small. The results are found to be different from an earlier calculation [S. Luryi and E. Suhir, Appl. Phys. Lett.49, 140 (1986)] and show that the relaxation caused by the free surfaces cannot explain the lower densities of dislocations observed experimentally in such structures. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358822

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The molecular dynamics and ordering of polymer‐dispersed liquid‐crystal displays are studied using the technique of paramagnetic resonance of spin probes. A complex spectrum is observed in which components are identified from probes dissolved in the droplet, binder, and interfacial regions of the display. In particular, the liquid‐crystal curing, phase separation, and structure of displays with both thermoplastic and epoxy binders are investigated. Observations are also performed as the liquid‐crystal droplet size is varied. Among the more interesting results is the demonstrated ability to observe the interfacial region. It appears to be quite fluid and has little or no average order. It is also observed that the compatibility between the binder and liquid crystal, seen from some of the formulations studied, correlates with the reported electro‐optical properties. These include the magnitude and hysteresis of switching voltages and the magnitudes of switching times. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358823

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray‐diffraction measurements were performed on smectic A octylcyanobiphenyl liquid‐crystal (LC) films deposited on glass and silicon gratings with periods between 664 nm and 200 &mgr;m. The film’s two interfaces, air‐LC and LC‐grating, imposed boundary conditions that required different alignments for the minimum energy configuration; the competition between these two forces was studied in thick (30+ &mgr;m) films. Consistent with earlier work, gratings with periods on the order of 10 &mgr;m and depths of 1.5–3 &mgr;m induced a portion of the film into homogeneous alignment in the direction of the grooves. As expected from theory, longer‐period 200 &mgr;m gratings with the same depth showed an inconsistent ability to produce an inplane alignment; however, contrary to theory, shallower 664 nm gratings also failed to consistently induce in‐plane alignment and did not produce an azimuthal preference. The results from the x‐ray scans were fit with a multilayer model to give estimates of the thicknesses, orientations, and mosaic spreads of the different regions in the films. Films having an in‐plane component were estimated to have a 5–10‐&mgr;m‐thick homeotropic skin at the surface. On 9 and 15 &mgr;m gratings, the in‐plane scattering arose from a single homogeneous region in which the molecules had a small tilt (up to 5°) with respect to the plane of the substrate. The director was not exactly parallel to the grooves but was offset by up to 20°, and the mosaic spread in the azimuthal direction was typically 20°. In some films there was an additional region that appeared to be disordered. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358824

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray‐diffraction measurements were done on octylcyanobiphenyl (8CB) films on a glass grating with a 9 &mgr;m period. The films were heated from the smectic to the isotropic phase using three different methods. It was found that the cooling rate, which determines the time the molecules are able to respond to the influence of the air and substrate interfaces, plays a critical role in the alignment of the liquid‐crystal molecules. In all cases, the surface tension was strong enough, and the energy cost of an interface with a second alignment was high enough, to produce a single homeotropic alignment in films as thick as 15 &mgr;m. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358825

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report on x‐ray diffraction measurements of the substitutional carbon contentyin pseudomorphic Si1−yCy(y<0.02) layers grown on (100)‐oriented silicon substrate. The samples were grown by molecular beam epitaxy and investigated during post growth annealinginsituby an x‐ray powder diffractometer. Despite the tensile strain in the 100‐nm‐thick layers and the high carbon supersaturation, the samples were stable up to 800 °C. Beyond this temperature range, the substitutional carbon content decreased exponentially during isothermal annealing. This effect can be explained by the precipitation of the carbon and the nucleation and diffusion limited growth of SiC nanocrystals. Since no strain relief by the introduction of misfit dislocations was detectible, we conclude that contrary to the mechanism of strain relief in Si1−xGex, in comparably strained Si1−yCyepilayers, the main high temperature process is precipitation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358826

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The structure of polycrystalline Si (poly‐Si) films, prepared by annealing amorphous Si (a‐Si) films deposited using Si2H6, has been investigated by x‐ray diffraction (XRD), Raman scattering, transmission electron microscopy (TEM), and electron spin resonance, as functions of deposition conditions, such as deposition temperatureTd(450–580 °C) of thea‐Si and annealing time under a fixed temperature of 600 °C. A dominant texture of the poly‐Si films changed from a ⟨100⟩ texture forTdbelow 530 °C to a ⟨111⟩ forTdabove 530 °C, independent of the deposition rate of thea‐Si films and of the film thickness. Although the XRD grain size was independent ofTd, the TEM grain size increased from 1.0 to 2.5 &mgr;m with decreasingTd. It is suggested that the increase in this TEM size is caused by enhanced lateral growth of ⟨100⟩ grains due to the presence of strain. The spin densityNsand the factorgwere found to first increase with the annealing time, and rapidly decreased after the films were crystallized. It is also found that the value ofgfor the poly‐Si films decreased from 2.0051 to 2.0048 with decreasingTd. Furthermore, effects of post‐hydrogenation on the poly‐Si films with different textures were investigated, and a structural change of the boundary regions in the poly‐Si films is discussed in connection with a change in the dominant texture, through the corresponding change ing. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358827

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Si self‐interstitial diffusivities can be extracted from the diffusive behavior of certain metals (e.g., Au) in an inert annealing ambient or from the diffusion of dopant markers (typically B) under oxidizing conditions. Each type of experiment yields fairly consistent results; however, interstitial diffusivities obtained in these two ways differ greatly. The marker layer experiments rely on the assumption that the presence of the dopant does not disturb the diffusion of the interstitials, and the validity of this assumption is explored. A model of interstitial diffusivity in the presence of B is developed, two extreme cases of the B‐atom–interstitial interaction strength are considered, and the predictions of the model are compared with experiments of oxidation‐enhanced diffusion in B doping‐superlattices. From this comparison it is concluded that trapping of interstitials by B atoms in the markers cannot be responsible for the different values of the Si interstitial diffusivity reported in the literature. Further, it is shown that the presence of the dopant does not perturb the behavior of the Si self‐interstitials in the doping‐superlattices, i.e., the markers are ‘‘unobtrusive’’ probes of interstitial behavior. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358828

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Metalorganic molecular beam epitaxy (MOMBE) growth characteristics of GaAs using triethylgallium (TEGa) and trisdimethylaminoarsenic (TDMAAs) are studied in detail by using reflection high energy electron diffraction intensity oscillations. It is found that the GaAs growth rate variation with substrate temperature (Tsub) is similar to that in the MOMBE growth of GaAs with TEGa and elemental As (As4) except in the highTsubregion, despite the use of uncracked TDMAAs instead of As4. GaAs growth starts at about 350 °C and shows a maximum growth rate at about 500 °C. However, the absolute growth rate is about 15% lower than that using As4with the same TEGa flow rate in the mass‐transport limited growth region. Above 600 °C the growth rate shows a rapid decrease with increasingTsub. Further, it is found that etching of GaAs occurs when only TDMAAs is supplied to the GaAs surface atTsubabove 500 °C. Unintentionally doped GaAs films shown‐type conduction in the wholeTsubrange investigated with the highest 77 K electron mobility of 22 000 cm2/V s and a low carrier concentration of 2×1015cm−3for the 650 °C grown samples. 4.2 K photoluminescence spectra show an exciton‐bound‐to‐impurity emission peak at the wavelength of 819.3 nm with a full width at half‐maximum of less than 3.5 meV indicating good optical qualities. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358829

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

SiGe/Si quantum well layers are selectively grown by low pressure chemical vapor deposition on patterned Si substrates. Transmission electron microscopy (TEM) shows that the growth rate of SiGe in convex corners between different surface planes is at least ten times higher than the growth rate observed on (001) planes. This high growth rate leads to the formation of quantum wires and dots between such facets. Photoluminescence (PL) spectra of square and rectangular patterns, bounded by quantum wires, ranging in size from 300&mgr;m down to 500nm are taken. The observed energy shifts of the (001) quantum well PL–peaks are explained by surface diffusion of Ge adatoms into the quantum wires. A surface diffusion model is used to obtain a Ge diffusion length of &lgr;=2.5±0.6 &mgr;m at 700°C. Thus, a method for the determination of surface diffusion lengths in strained layer epitaxy is introduced. For SiGe layers grown above the Stranski–Krastanow critical thickness for three dimensional (3D) growth, a competition between the SiGe wires in the interfacet corners and the SK islands on the (001) planes is observed. In squares as large as 2×2 &mgr;m2the SiGe wires lead to a suppression of 3D growth on the (001) plane altogether, as observed by TEM and PL. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358830

出版商:AIP    

年代:1995

数据来源: AIP