摘要:

Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3&ohgr; method.SiO2and SiNxfilms are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films>100nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVDSiO2films is only∼10&percent; smaller than the conductivity ofSiO2grown by thermal oxidation. The thermal conductivity of PECVD SiNxfilms is approximately a factor of 2 smaller than SiNxdeposited by atmospheric pressure CVD at 900 °C. For films<50nm thick, the apparent thermal conductivity of bothSiO2and SiNxfilms decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistanceRI.At room temperature,RI∼2×10−8K m2 W−1and is equivalent to the thermal resistance of a∼20nm thick layer ofSiO2 . ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363923

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Dislocations are used as “atomic saws’’ to cut a 5 nm GaAs quantum well into a multiple quantum wire structure. The direct observation of these cuts in the volume, was performed by transmission electron microscopy, using cross section specimens, thinned perpendicular to the quantum wire axis by a highly localized preparation technique. This special thinning procedure, involving electron-beam lithography and reactive ion etching, allowed us to realize a statistical analysis of the distances between neighboring cuts and the heights of cuts. This dimensional analysis revealed the formation of coupled quantum wires with a width of 18±9 nm and free from any lateral roughness on 100 nm lengths. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364304

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The depth profile of the chemical composition inInxGa1−xAssingle quantum wells (SQWs), epitaxially grown onto a GaAs[001] substrate and covered by a GaAs cap layer, has been determined by use of grazing incidence diffraction (GID). This method allows the scattering signal from the SQW to be enhanced and the scattering depth to be tailored. The coherently illuminated area is large, due to the small incident angle&agr;i;this makes GID a unique technique for investigating buried thin layers over a lateral length scale of several microns. In the case of very thin SQWs the measurements could be described assuming a Gaussian-like distribution of the In content with depth. The broad In profile seen using this method is in contrast with the sharp monolayer signal achieved by photoluminescence measurements. This can be explained by the assumption of a terracelike In distribution and the very different lateral integration length of both experiments. For thicker SQWs we could verify that at least one of the two interfaces is not sharp but shows a gradient in the chemical composition. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363924

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

InGaP/GaAs heterointerfaces grown by metalorganic chemical vapor deposition have been characterized by a high-resolution x-ray diffraction analysis of multiple quantum well structures. The flow of AsH3to InGaP surface produces an InGaAs-like interfacial layer at the GaAs-on-InGaP interface, indicating P atoms of the InGaP surface are easily replaced by As atoms. The flow of PH3to GaAs surface, on the other hand, does not make any detectable interfacial layer, indicating that almost no As atoms of the GaAs surface are replaced by P atoms. It is also found that the flow of trimethylgallium (TMG) to the InGaP surface produces a GaP-like interfacial layer. This interfacial layer is probably formed by the reaction between TMG and excessive P atoms on the InGaP surface or residual PH3in the growth chamber. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364297

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have used a 90° double reflection high-energy electron diffraction (RHEED) setup to perform a comprehensive real-time study of the morphology of vicinal GaAs(001) surfaces during molecular beam epitaxy. The technique allows to record RHEED intensities simultaneously in the [1¯10] and [110] azimuths and thus enables a detailed study of anisotropy effects. Comparative measurements on surfaces with 2° misorientation towards (111)Ga (A surface) and towards (11¯1)As (B surface), respectively, show that independent on the step type and reconstruction anisotropy, recordings of the specular beam intensity in the azimuth perpendicular to the steps are clearly dominated by the evolution of the staircase order whereas intensity recordings in the azimuth parallel to the steps reveal the evolution of the step edge roughness. Measurements over a wide range of substrate temperatures give insight in the competition between kinetic processes and thermodynamic equilibrium on a length scale accessible to RHEED. For the A surface the transition between two-dimensional (2D) growth and step-flow growth occurs not only at higher temperature than on the B surface, but the disappearance of the intensity oscillations occurs also at different substrate temperatures in different azimuths. The ∼20 °C higher disappearance temperature in the [1¯10] azimuth is explained with a model based on previous scanning tunneling microscopy results which revealed an increasing elongation of the islands in [1¯10] direction with increasing substrate temperature. The B surface is more isotropic and hence no difference in the transition temperature in the two azimuths could be detected. During growth in the transition range between 2D and step-flow growth we observe increased terrace width fluctuations on the B surface, whereas the A surface becomes more uniformly stepped. This demonstrates that in the kinetically controlled regime the anisotropic barrier height for downward diffusion of adatoms over step edges plays an important role for the evolution of the surface morphology. At elevated temperature the barrier height allows downward jumps of the adatoms over B-type steps but not over A-type steps. At conditions close to the thermodynamic equilibrium a kinetic smoothing is observed on the A as well as on the B surface indicating another mechanism to be effective with a change of the energetics due to ordering of the steps in combination with a disordering of the reconstruction on the terraces. This surface is, however, metastable and recovers after growth interruption rapidly (at substrate temperatures >580 °C within less than 1s) to the equilibrium bunched surface. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363925

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The structure and properties of adsorbates on GaAs surfaces are investigated. The vibrational states of adsorbed hydrogen and oxygen on GaAs surfaces are calculated. Specific attention is focused on the vibrational energies of surface species and their reported variation with surface stoichiometry. The symmetry and localization of the vibrational states are also discussed. Comparisons are made with experimental high resolution electron energy loss spectroscopy spectra. Our results here agree well with a variety of experimental data. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363926

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition,sp3bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. &percent; N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363927

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Epitaxial strained growth ofSi1−yCyalloys with rather high C concentrations(y∼1.5&percent;)has been performed on Si(111) and Si(001) using molecular beam epitaxy (MBE) Si evaporation and thermal interaction of the growth surface with a lowC2H4pressure at 500 °C. Carbon contents, determined by secondary ion mass spectrometry, infrared (ir) spectrometry,in situC 1sandSi 2px-ray photoelectron spectroscopy measurements and x-ray diffraction (XRD), are being compared. Monocrystalline quality of the epilayers is checked by low energy electron diffraction and x-ray photoelectron diffraction (XPD). As indirectly ascertained by the ir local vibration mode (LVM) and a shifted partially strain induced epilayer diffraction line in the &thgr;-2&thgr; XRD analysis, carbon is accommodated in substitutional sites(Csub)whose local atomic order is investigated for the first time by XPD,C 1spolar angle distributions being measured in different azimuthal directions. As the data reveal, for a C emitter, next nearest neighbor bond orientations identical to those for Si atoms in a Si matrix, XPD readily provides direct evidence in favor ofCsubpositions. Up to now, our limited angular resolution does not allow observation of possible bond orientation changes due to local strain-induced lattice distortions around C atoms. Nevertheless, by increasing growth temperature (600–650 °C) and promoting formation of more C-rich phases(SinCor SiC), the effects of substitution of second or higher nearest Si neighbors by C atoms can be clearly evidenced. By the way, a significantC 1sbinding energy difference betweenCsuband C in C-rich phases is observed and may be used as a signature of the C dilution in the grown epilayer: a characteristic value of 283.8 eV is obtained for theCsubsite giving rise to the LVM. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363968

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electrical resistivity measurements have been carried out between 20 and 300 K onCe2Fe16.8, on theCe2Fe17−xAlxsolid solutions, withx=0.4, 1, 2, 5, 6, 8, and 9, and on theCe2Fe17−xSixsolid solutions, withx=0.2, 0.4, 1, and 2, with the four probe method. The temperature dependence of the resistivity ofCe2Fe16.8shows an inflection at 110 K, a feature which is also observed in the temperature dependence of the magnetization and is related to a magnetic phase transition from a helical structure, above 110 K, to a fan structure, below 110 K. The temperature dependence of the resistivity ofCe2Fe16.8is characteristic over the investigated temperature range of weak antiferromagnetic behavior with incommensurate periodicity as has been observed in the earlier neutron diffraction study. The resistivity ofCe2Fe17−xAlxincreases with temperature in agreement with a density of states at the Fermi level dominated by thedband. For a given temperature, the resistivity increases substantially and regularly withxfor both series of solid solutions because of an increase in the number of conduction electron scattering potentials. This increase in resistivity also corresponds to a decrease in the number of conduction electrons due to their increasing transfer into the localized cerium4forbitals as the cerium valence state changes from a mixture of trivalent and tetravalent inCe2Fe17to predominantly trivalent inCe2Fe17−xAlxandCe2Fe17−xSixwith increasingx. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363928

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Composite materials containing conducting polypyrrole and insulating poly (vinyl methyl ether) (PVME) have been synthesized by oxidative polymerization of pyrrole in ethanol using FeCl3oxidant in the presence of PVME. The ac conductivity measurements have been carried out in the frequency range of 100 Hz to 10 MHz and in the temperature range of 110 to 350 K. The frequency dependent conductivity has been explained on the basis of a small polaron tunnelling mechanism. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363929

出版商:AIP    

年代:1997

数据来源: AIP