摘要:

Photon‐induced local departures from charge neutrality are the origin of the photorefractive effect. When a periodic photorefractive grating is stored in the material, one can associate with it quasi‐Fermi levels having corresponding periodic variations. The hopping band structure is modified across the local variation in charge distribution. A model is presented for the shift of the Fermi level with photon‐induced trap occupation in Bi12SiO20. The shift of the Fermi level is dependent on the density of trap sites in the band gap, the radiation‐induced occupation density, and the energy levels of the traps. Shifts in the Fermi levels with trap occupation have been calculated, using a simple model. Estimations of the change of density of sites in the hopping band with Fermi level shift for typical photoexcited charge concentration are shown to be consistent with experimental data.

ISSN:0021-8979    

DOI:10.1063/1.351316

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Luminescence and related characteristics have been investigated for Er3+‐doped silica glass fibers and for fibers and bulk material of calcium metaphosphate glass. Special interest has been directed toward elucidating relaxation processes leading to 1.5‐&mgr;m light emission. Several emissions from higher excited states were investigated and excited‐state lifetimes were also measured. Approximate radiative and nonradiative transition probabilities have been obtained for the excited levels. These results have confirmed that dominant transitions populating the lowest excited level are step‐by‐step nonradiative relaxations and that only a limited amount of energy is supplied via intermediate radiative transitions. It has also been shown that the 1.5‐&mgr;m emission decay time constant elongates with increasing fiber length.

ISSN:0021-8979    

DOI:10.1063/1.351317

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Photoluminescence time behavior has been studied with a photon counting technique by using long pulse excitation (1–10 &mgr;s). The presence of a long luminescence decay at &lgr;=632 nm in Corning CS‐261 glass has been revealed.

ISSN:0021-8979    

DOI:10.1063/1.351318

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The effect of polarization of ions in an electric field on their trajectories is considered with respect to the mass transfer from the tip. The effect is in focusing of the ion beam due to the dipole momenta of the ions induced by the electric field between the tip and the surface. The dependence of the focusing effect on the tip shape, distance and potential difference between the tip and the surface as well as on material the tip is made of and the kind of the ions, is investigated. The results show the possibility of utilizing the focusing effect in installations for producing a molecule‐size structure on the surface, e.g., in nanoelectronics.

ISSN:0021-8979    

DOI:10.1063/1.351319

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The possibility of local elastic instabilities is considered in a first‐order structural phase transition, typically a thermoelastic martensitic transformation, with associated interfacial and volumic strain energy. They appear, for instance, as the result of shape change accommodation by simultaneous growth of different crystallographic variants. The treatment is phenomenological and deals with growth in both thermoelastic equilibrium and in nonequilibrium conditions produced by the elastic instability. Scaling of the transformed fraction curves against temperature is predicted only in the case of purely thermoelastic growth. The role of the transformation latent heat on the relaxation kinetics is also considered, and it is shown that it tends to increase the characteristic relaxation times as adiabatic conditions are approached, by keeping the system closer to a constant temperature. The analysis also reveals that the energy dissipated in the relaxation process has a double origin: release of elastic energyWiand entropy productionSi. The latter is shown to depend on both temperature rate and thermal conduction in the system.

ISSN:0021-8979    

DOI:10.1063/1.351320

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The influence of underlying metals on hydrogen evolution from plasma‐enhanced chemical vapor deposited silicon nitride (P‐SiN) films was investigated. Titanium (Ti), tungsten (W), and tungsten silicide (WSix:x=2.3) cap layers were deposited onto pure‐Al, Al‐1%Si, Al‐0.5%Cu and Al‐1%Si‐0.5%Cu films on oxidized silicon wafers. The bilayers and Al‐alloys were covered withP‐SiN films and heated. It was found that the hydrogen evolution rates for theP‐SiN films were influenced by the underlying bilayers as well as Al‐alloys. Copper (Cu) addition to underlying Al films raised the hydrogen evolution rate peak temperature for the coverP‐SiN film, due to the suppression of coverP‐SiN film blistering and cracking as a result of Al‐Cu hardening. Titanium thin‐film capping on the Al‐alloy also raised the hydrogen evolution rate peak temperature for theP‐SiN film, as results of Ti adhesive layer and thermally stable intermetallic compound formation, TiAl3.

ISSN:0021-8979    

DOI:10.1063/1.351321

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report here enhancement of nucleation and adhesion of diamond films on nondiamond substrates such as copper, stainless steel, and silicon substrates. The enhancement of nucleation is accomplished by pulsed laser irradiation which converts some of the amorphous carbon on the surface into the diamond phase or forms a reaction product that facilitates nucleation of diamond phase. The laser can also be used to evaporate carbon preferentially, leaving behind diamond particles unaffected. By pulsed laser irradiation it is possible to melt the substrate and embed the diamond particles into it, thus improving the adhesion of the diamond film.

ISSN:0021-8979    

DOI:10.1063/1.351322

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Multilayer sol‐gel optical high reflectors with greater than 99% reflection have been prepared on substrates up to 20 cm in diameter by spin coating silica/alumina colloidal suspensions. These coatings are radially nonuniform, owing to the extensive shear‐thinning rheology of the high‐index alumina suspension. To a large degree the film thickness nonuniformity can be compensated for by the reflection bandwidth. The rheological properties of the alumina suspension under steady shear have been measured. The low‐shear reduced viscosity and the shear‐thinning time constant are shown to vary exponentially with &fgr;2, where &fgr; is the solids volume fraction. At &fgr;=0.1 the sol has effectively gelled. A model for spincoating with a non‐Newtonian fluid has been developed that uses the Carreau rheology model to fit the measured viscometric data. Modeling and experimental results show that as long as these non‐Newtonian effects are sufficiently large (as in this case) the radial film uniformity is determined only by these parameters and cannot be significantly influenced by spin rate, initial solids fraction, or any other parameters under the control of the operator. However, most of the film thickness variation occurs in the first 1–2 cm from the substrate center, leaving the remainder almost uniform. Therefore the degree of nonuniformity does not appreciably increase with increasing substrate size.

ISSN:0021-8979    

DOI:10.1063/1.351323

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

During the study of the oxidation of titanium nitride, we previously identified an initiation period during which oxidation proceeded much more slowly than after the initiation period. In the present work, we measure the kinetics of oxidation during these initial stages. The film growth during this period is roughly an order of magnitude slower than during the ‘‘normal’’ oxidation. No correlation was observed between the amount of oxygen in the bulk of the TiN and the oxidation time/temperature. The film growth kinetics are parabolic with an activation energy of 38.6 kcal/mol.

ISSN:0021-8979    

DOI:10.1063/1.351324

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The epitaxial growth of GaAs and (Al,Ga)As doped with boron (from diborane) and silicon was investigated to examine the effect of boron on (DX) centers in silicon‐doped material. The addition of diborane to the growth of GaAs and (Al,Ga)As results in the superlinear incorporation of boron into the solid with a concurrent reduction in the growth rate. Boron incorporation also decreases as the growth temperature is increased. Additionally, the AlAs mole fraction increases with increasing diborane during (Al,Ga)As growth. The DX center was not eliminated in (Al,Ga)As by the addition of boron. The thermal stability of these materials was also investigated.

ISSN:0021-8979    

DOI:10.1063/1.351295

出版商:AIP    

年代:1992

数据来源: AIP