P. S. Peercy

Research opportunities on clusters and cluster-assembled materials —A Department of Energy, Council on Materials Science Panel Report

The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988. In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials. Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research. Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments. The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Technique for profiling 1H with 2.5‐MeV Van de Graaff accelerators

We describe an elastic recoil detection (ERD) analysis technique for profiling 1H in the near‐surface regions of solids using a 2.5‐MeV Van de Graaff accelerator commonly used for ion‐backscattering analysis. Energy analysis of 1H forward scattered by 2.4‐MeV 4He incident on the target tilted at an angle of ∼75° yields a depth resolution of ≲700 A and a sensitivity of better than 0.1 at.% for 1H to depths of ≲0.6 μm in solids.

Stability of strained quantum-well field-effect transistor structures

Conditions for stability of strained-layer structures and their implications for device fabrication are examined. Structures which have exhibited the best performance to date are found to be thermodynamically metastable (or at best marginally stable) structures, which will restrict the processing steps permissible in the integration of these devices to form complex circuits.<<ETX>>

Melt dynamics of silicon‐on‐sapphire during pulsed laser annealing

Transient electrical conductance measurements have been made on 0.45‐μm silicon‐on‐sapphire during pulsed laser annealing with 25‐ns ruby irradiation. The photoconductive contribution to the transient current was sufficiently small that the entire melt and resolidification process could be directly observed. The technique yields quantitative measures of melt depths, melting velocities (5–13 m/s), and solidification velocities (2.8–3.3 m/s). Combined with the complementary techniques of time‐resolved reflectivity, energy transmission, and calorimetric energy absorption, transient conductance provides a powerful new diagnostic for investigating melt dynamics.

Solidification kinetics of pulsed laser melted silicon based on thermodynamic considerations

The measured solidification velocities in silicon after pulsed laser melting are analyzed in terms of thermodynamic and kinetic considerations. Both interface and thermal transport limited growth regimes are observed. From the observed kinetics in the 1–6‐m/s regime, the undercooling at the liquid‐solid interface can be calculated. At velocities ≤6 m/s the undercooling increases with interface velocity at the rate of 15±5 deg/m/s.

The effects of ion‐implantation damage on the first‐order Raman spectra of GaP

We have analyzed the effects of ion‐implantation‐induced damage on the first‐order Raman spectra of GaP, and have correlated changes in the Raman spectra with direct measurements of implantation‐induced surface stresses. We find that, at low ion fluences, the effects of implantation damage are to produce point defects that remove free carriers from doped material and to produce stresses in the undamaged regions. In this regime, the stress‐induced shifts of the Raman lines can be predicted from the known elastic constants of undamaged GaP. At intermediate fluences, implantation damage alters the elastic properties of GaP so that the stress‐induced shifts of the Raman lines are less than those predicted from the properties of the undamaged crystal. At higher doses, implantation‐induced surface stresses exceed the elastic limit of the damaged material. This plastic deformation of the implanted surface is accompanied by a rapid broadening of the phonon lines in the first‐order Raman spectra.

Direct measurements of liquid/solid interface kinetics during pulsed-laser-induced melting of aluminum

We report time‐resolved electrical‐resistance measurements obtained during pulsed‐laser melting of a metal. Through heat‐flow calculations and solute‐diffusion measurements, the measured resistances are correlated with the thresholds for partial and full melting of a thin film of aluminum. Furthermore, simultaneous time‐resolved reflectance measurements establish that, in this geometry, melting and solidification proceed via the motion of a well‐defined, planar liquid/solid interface, whose position can be deduced from the resistance measurements. These measurements permit, for the first time, real‐time determinations of melt‐depth histories in fundamental studies of rapid solidification processing of metals.

Effect of nonequilibrium interface kinetics on cellular breakdown of planar interfaces during rapid solidification of Si-Sn

Abstract During rapid solidification, nonequilibrium interface kinetics alter the predictions of the Mullins-Sekerka theory for the stability of a planar interface against cellular breakdown. The velocity-dependence of the partition coefficient and of the Sn concentration at the onset of cellular breakdown have been measured during pulsed laser melting of Si-Sn alloys. The Mullins-Sekerka theory is modified by inserting a velocity-dependent partition coefficient and a velocity-dependent slope of the “kinetic liquids”, both of which are extracted from the continuous growth model for interface kinetics. These nonequilibrium interface kinetic effects increase the predicted critical concentration for cellular breakdown by two orders of magnitude for Sn in Si, and account fairly well for the experimental results.

RAMAN SCATTERING OF ION-IMPLANTED GaAs.

We report measurements of Raman scattering in ion‐implanted semiconductors. The broadening of the phonon modes produced by lattice strains on Xe ion implantation is measured for GaAs. The measurements show pronounced effects on the linewidth at moderate fluence levels and demonstrate that Raman scattering can provide a sensitive technique for probing stresses introduced by ion implantation. At higher‐fluence levels where the material begins to transform from crystalline to amorphous, the linewidth is found to increase rapidly with fluence.

Nucleation of amorphous germanium from supercooled melts

Thin germanium films on SiO2 completely melted by pulsed laser irradiation cool rapidly by thermal conduction to the substrate until they solidify. In situ measurements indicate that the liquid is supercooled by 420–530 K with respect to the crystalline phase prior to solidification. Cross‐sectional transmission electron microscopy reveals nucleation events at the Ge/SiO2 interface. The microstructure of these events is comprised of a very fine grained (5–15 nm) polycrystalline core with much larger grains extending laterally and toward the free surface. It is believed that nucleation of the amorphous phase, which was subsequently converted to the fine‐grained material, initiated solidification.