R. M. Walser

First phase nucleation in silicon–transition‐metal planar interfaces

What is the first compound that nucleates in planar solid silicon–transition‐metal binary couple reactions whose members form bulk equilibrium compounds? We propose, for couples annealed at low temperatures, the following simple rule: The first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest‐temperature eutectic on the bulk equilibrium phase diagram. The predictions of this rule are compared with experimental results.

FEMTOSECOND LASER EXCITATION OF THE SEMICONDUCTOR-METAL PHASE TRANSITION IN VO2

We have measured the subpicosecond optical response of a solid‐state, semiconductor‐to‐metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of VO2 thin films by making pump‐probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties of the high‐temperature metallic state being attained within 5 ps. The ultrafast change in complex refractive index enables ultrafast optical switching devices in VO2.

FEMTOSECOND LASER EXCITATION DYNAMICS OF THE SEMICONDUCTOR-METAL PHASE TRANSITION IN VO2

We have measured the subpicosecond optical response of a solid‐state, semiconductor‐to‐metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of polycrystalline VO2 thin‐films by making pump‐probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties very close to the high‐temperature metallic state being attained within about 5 ps. The equilibration of the metallic state after femtosecond excitation was modeled by non‐exponentially decaying perturbations in the metallic state electron density and collision frequency. The decay of both these plasma parameters was well fit by a 1/√t time dependence. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2.

Electromagnetic scattering of two-dimensional surface-relief dielectric gratings

We employed the rigorous vector coupled-wave theory [J. Opt. Soc. Am. 73, 1105 (1983)] to analyze the electromagnetic scattering from two dimensional (2-D) surface-relief dielectric gratings. A shoot-back method was developed for the numerical solution of the resulting coupled differential equations. This method allowed numerical solutions to be found for grating structures of arbitrary profiles and relatively deep grooves. It was most suitable where the grating medium refractive index was not too large and where only a small number of propagating orders existed. Experiments confirmed the numerically predicted reflectivities for 2-D surface-relief dielectric sinusoidal gratings. Reflectivity measurements were made on 2-D sinusoidal gratings fabricated on photoresist and on polycarbonate. The grating periodicities were of the order of 3000 lines/mm such that only the zero-order diffracted waves were propagating in the incident region, and possibly a few forward orders in the transmission region. The embossing technique that was used for replicating the grating patterns from photoresist onto polycarbonate proved to be a feasible method for the production of such gratings.

Polarity‐dependent memory switching in devices with SnSe and SnSe2 crystals

Polarity‐dependent memory switching has been observed in devices of crystalline SnSe and SnSe2 with aluminum contacts. Two types of low‐level switching (1 V/1 mA) with reversed polarity dependence can be obtained in suitably formed devices of either material. In addition, a high‐level (100 V/10 mA) polarized memory switching can be obtained in either device. The low‐level switching appears to involve an electronic process, while the high‐level switching is associated with an electrothermally driven mass transport.

Magnetic properties of RF diode sputtered Co/sub x/Fe/sub 100-x/ alloy thin films

The systematic variations in the magnetic properties of as-deposited (by RF diode sputtering) and annealed Co/sub x/Fe/sub 100-x/ (x=0, 15, 50, 67, 90, 100) alloy thin films were studied in this work. Maximum values of 4/spl pi/M/sub s/ (21.3 kGauss) and H/sub c/ (253 Oe) were observed in as-deposited films with, respectively, x=15 and x=50. Films with 0/spl les/x/spl les/15 had rotatable hysteresis loops with squareness S/spl ap/1. With increasing Co, S decreased and the films became isotropic. Large decreases in H/sub c/ and resistivity, and large increases in 4/spl pi/M/sub s/ were produced in films with x=50 and 67 by annealing in vacuum at 370/spl deg/C, in the presence of an in-plane field. The in-plane anisotropy of the annealed films had a complex biaxial symmetry. A similar biaxial anisotropy was observed in Co/sub 90/Fe/sub 10/ films deposited in an /spl ap/200 Oe planar field. This anisotropy was observed independent of whether the substrate was crystalline Si, glass or polyimide. Thus the biaxial anisotropy could not be ascribed to structural constraints at the film-substrate interface, but was probably induced in the film during deposition. The optimum binary CoFe alloy films for application at GHz frequencies require highly oriented uniaxial anisotropies. For reasons not fully understood, it was not possible to obtain well oriented as-deposited, or annealed, binary CoFe films with uniaxial anisotropy.

Permalloy nanoparticles generated by laser ablation

We describe a new method for producing ultrafine magnetic particles that employs pulsed-laser particle ablation. Pulsed-laser radiation with wavelength of 248 nm was used to ablate 1 to 10 /spl mu/m diameter Permalloy microparticles under normal atmospheric conditions. The ejected nanoparticles were collected on silicon substrates for further analysis. Scanning electron micrographs of the samples were analyzed by computer aided image processing to determine the effect of laser fluence on the particle size distribution. Results showed that mean particle diameters were in the range from 60 to 90 nm, and that the particle size distributions were closely log-normal.

Electrical performance of laser damaged silicon photodiodes.

Laser induced electrical parameter degradation and morphological damage have been observed in silicon photodiodes. The samples were RCA reach-through avalanche photodiodes and EG&G PIN photodiodes. The laser source was a 1064-nm Q-switched Nd: YAG laser (10-Hz, 10-ns pulses with a 300-microm spot radius). Reverse saturation current, noise current, breakdown voltage, junction capacitance, and surface morphology were monitored for permanent laser induced change. The current characteristics were clearly the most sensitive electrical parameters; however, the electrical performance was generally insensitive to severe surface morphological damage. The damage behavior indicated that the electrical degradation in photodiodes may be modeled by the introduction of defects into the depletion region by deep melting transients.

Precision measurements of the effect of implanted boron on silicon solid phase epitaxial regrowth

The epitaxial recrystallization rates of self-ion amorphitized layers in silicon wafers with 〈100〉 substrate orientation were measured by in situ , high precision, isothermal cw laser interferometry. With this one-sample technique the changes produced by implanted boron impurity concentrations ( N B ) in the activation energy E a and preexponential V 0 of solid phase epitaxy were measured for concentrations in the range 5 × 10 18 cm −3 N B 20 cm −3 and for temperatures from 450 to 550°C. The differential changes in E a produced were measured to within ± 23 meV when systematic errors were eliminated. Changes in activation energy and entropy [ E a and log ( V 0 )] were found to be linearly correlated for all concentrations. This observation is consistent with the idea that electronically active impurities alter regrowth velocities by reducing the critical temperature for disordering at some of the interfacial sites at which elementary reconstructive processes are driven by thermal fluctuations. For small N n , the critical temperature of the impurity-modified reconstruction is estimated at 1200K, approximately 200 K below the melting temperature of amorphous silicon. The E a decreased exponentially with N B to a concentration N infl , larger than the estimated equilibrium solubility limit, where there was an inflection point in the V vs N B curve. The E a increased for values of N B larger than N infl , showing that the differential increase in V for higher concentrations was due to a differential increase in the activation entropy. A change in the correlation between E a and log ( V 0 ) at N infl indicated that larger N B produced an additional reduction of the critical temperature of the reconstruction. For small N B , the data support a simple Fermi level shifting model for the “electronic effect” of impurities on SPE (solid phase epitaxial) regrowth.

Ionization‐enhanced solid phase epitaxy of amorphous silicon with boron impurities

We have made the first high‐resolution, laser interferometric measurements of the functional dependence of the velocity of (100) silicon solid phase epitaxy on the concentration of implanted boron impurities NB. At all measurement temperatures (450–550 °C), and for all NB<5×1019 cm−3 the fractional increase in the velocity was identically equal to the dimensionless ratio NB/Ni, where Ni is a thermally activated factor. This result disagrees with the predictions of all models for the effect of impurities on solid phase epitaxial growth.