摘要:

The capability of producing electronically induced modifications in ultrathin (∼20 nm) light‐emitting porous silicon (PS) films by use of a scanning tunneling microscope (STM) operated in a high‐vacuum environment is demonstrated. Upon increasing the tunnel current to 2 nA and the tunnel voltage beyond a threshold value of ∼7 V, structures 20–50 nm in width can be created to any desired pattern. These nanopatterns are stable at least for four days at room temperature. Experiments with both voltage polarities but equal power densities reveal that these structures can only be induced by directing the intense electron beam provided by the STM tip towards the sample surface, excluding pure thermal effects for the layer modifying process. These observations can be well explained by a model which includes a local increase in the density of defect states in deep‐layer regions of the PS layer, which might be accompanied by a local quenching of the photo‐ or electroluminescence activity. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361291

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The free energy model previously developed for the prediction of the bonding in amorphous Si‐based alloys is extended here to amorphous carbon alloys,a‐CxH1−x, containing carbon atoms withsp3andsp2hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering atT=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. ForT≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for allT. Thesp3/sp2ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasingTdue to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)‐ and planar C(sp2)=C(sp2)‐centered units. It is found that essentially no aromatic or graphitic structures are present in typical alloys. Thea‐CxH1−xalloys have been proposed to consist of five amorphous components: diamondlike, graphitic, polymeric, olefinic, and mixed diamond–graphitic (d–g) components. It is predicted that the polymeric and mixed d–g components dominate in typical plasma‐deposited alloy films while the mixed d–g component dominates in hydrogen‐freea‐C films. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361292

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In the literature a relation is often used that correlates Debye temperature and bulk modulus by a square‐root law. It was recently shown that, for different cubic crystal structures, such a law is only fulfilled within relatively large error limits. If one takes, however, the average of the elastic constants of the transversal acoustic phonon modes as elastic modulus instead of the bulk modulus, the square‐root law is established with high precision. It is demonstrated that the same procedure may also be applied successfully to materials with hexagonal crystal symmetry such as hexagonal close‐packed metals and semiconducting compounds with the wurtzite structure, and to different structures of the tetragonal system. The adequate moduli areGh={c44[c44(c11−c12)/2]1/2}1/2andGt=[c44c66(c11−c12)/2]1/3for materials with hexagonal and tetragonal symmetry, respectively. The difference between the various structures of a crystal system is quantitatively described by the different number of atoms in the crystallographic unit cell. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361293

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The influence of low energy ball milling on the crystallite size, lattice strain, and storage of deformation energies of elemental metal powders is studied. The formation of nanosized grains (5–25 nm) and enhancement of lattice strain up to 0.4% is found. Excess enthalpies of up to 25% of the heat of fusion are reached. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361294

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Drops of molten silicon surrounded by a SiO2–BaO–CaO flux were undercooled at 350 K below their melting temperature. This undercooling is 75 K greater than the largest one reported so far for bulk silicon. To account for this result as well as the nucleation data from laser‐melted thin films, classical nucleation theory requires a crystal‐melt interfacial tension with a positive temperature coefficient. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361222

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A detailed mathematical analysis of isolated as well as multiple grain boundary diffusion through thin films of finite thickness has been performed. The thin film is bounded by two permeable and parallel surface planes, and a constant flux of diffusing particles through both surfaces is maintained during the entire diffusion process. The flux is kept small in order to guarantee the applicability of Fick’s second law of diffusion with constant and position independent diffusion coefficients. The boundary conditions considered can be found in symmetric electrochemical cells applied to the determination of chemical diffusion coefficients of mixed conductors. Both Fourier–Laplace transforms and Fourier analysis techniques are used to solve Fick’s second law of diffusion. Various concentration profiles for different parameter sets (grain boundary/volume diffusion coefficient ratio and grain boundary distance) have been calculated. The diffusion process in polycrystalline thin films is considerably enhanced by an increase of the grain boundary/volume diffusion coefficient ratio as well as a decrease of the grain boundary distance. The results for multiple grain boundaries are compared with those obtained from the isolated grain boundary model. A satisfactory coincidence between the concentration profiles of these two models is achieved when the grain boundary distance is sufficiently high or the diffusion time fairly small. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362651

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The open‐tube diffusion of zinc in GaAs0.8P0.2from a zinc‐doped silica film was investigated in detail. Aluminum nitride (AlN) and silicon nitride (SiNx) films were used as the anneal caps. The dependence of diffusion depth on the thickness of an AlN‐cap was found to differ from its dependence on SiNx‐cap thickness. The selective masked diffusion of zinc using an AlN diffusion‐mask was also systematically studied. The diffusion depth in selective masked diffusion was found to depend on both AlN‐cap thickness and AlN‐diffusion‐mask thickness. The experimental results suggest that diffusion depth is not necessarily governed by either cap thickness or diffusion‐mask thickness. From this standpoint, the role of film stress on diffusion depth was then quantitatively investigated. It was found that diffusion depth can be scaled well with total film stress in the measured film‐thickness range. In this sense, it can be concluded that total film stress is the primary factor that determines the diffusion depth under the measured diffusion conditions. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362652

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Significant large‐scale modification of the surface of Al–Si conductors was observed, due to electromigration in wide lines and under low stress conditions. After electromigration stressing the Al layers showed local thickness variations, i.e., damage by thinning. The mechanism underlying this damage causes substantial metal transport. Nevertheless, damage by thinning has received little attention in the past. Thinning was observed: (1) in a number of different alloys (Al–Si, Al–Cu, Al–Si–V, and Al–Si–V–Pd), (2) with a number of different underlayers [SiO2, W–Ti (no vacuum break after Al deposition) and W–Ti (oxidized surface before Al deposition)], (3) over an extended temperature range, (4) over a range of current density, and (5) in structures with and without passivation. The results show that thinning is a general phenomenon. An activation energy of approximately 0.5 eV was determined for the temperature dependence of a combined mechanism of concurrent thinning plus voiding in Al99Si1. Several alternatives are examined to explain the observations, namely mass movement along dislocations, Al bulk diffusion, and diffusion at the interface between the Al and its oxide. It is shown that diffusion at the Al/Al oxide interface most probably plays an important role in the damage mechanism, even under stress conditions where grain boundary diffusion is traditionally thought to dominate. Results also showed that alloying of Al with Pd can reduce the effects of damage by thinning. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362653

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Double‐crystal x‐ray rocking curves from metamorphic epitaxial heterostructures are broadened by the presence of misfit dislocations and this vitiates the usual interpretation of rocking curves using simulation software. However, analysis of the moments of the rocking curve can be useful. Using single layers, we show that the first moment is equivalent to the peak splitting, so that the average strain and average composition of an inhomogeneous layer can be derived from its first moments. The usefulness of the method is demonstrated on complicated heterostructures including graded layers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361295

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We investigate the influence of carbon on the optical transitions ofSi1−yCyandSi1−x−yGexCylayers grown pseudomorphically on a Si(001) substrate also including full strain compensation. The layers were investigated by spectroscopic ellipsometry and electroreflectance spectroscopy for carbon fractionsy≤1.2 at. % and germanium fractions up tox=16 at. %. The spectra were analyzed by measuring and fitting electroreflectance spectra at 80 K and ellipsometry data at room temperature, resulting for both techniques in a weak and nearly linear dependence on the carbon fraction at all transitions. The results of both techniques are compared and discussed. The strong line broadening for increasing carbon fractions can be caused by a high scattering efficiency of the carbon. Our results indicate that the interpretation of optical spectra of carbon‐containing alloys cannot be performed straightforwardly by simple interpolating between the appropriate band structures of silicon, germanium, and carbon. An analysis based on strain‐induced contributions only also does not describe the experimental results correctly. For a complete description of the observed energy shifts detailed band structure calculations and further experimental data are necessary. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362705

出版商:AIP    

年代:1996

数据来源: AIP