Clifford H. Griffiths

Influence of stoichiometry on the metal‐semiconductor transition in vanadium dioxide

The structure and resistivity of vanadium dioxide films close to the metal‐semiconductor transition have been studied as a function of stoichiometry. Unit‐cell volume is shown to go through a minimum and the transition temperature and resistivity ratio through a maximum at the apparent stoichiometric oxygen level. The observed resistivities can be accounted for by an increased electron concentration due to vanadium interstitials in the low‐oxygen films and by a second phase in the high‐oxygen films. The variation of resistivity and transition temperature with electron concentration provides experimental support for the previous suggestion that the driving force of the transition is mainly an antiferroelectric distortion.

The structure, magnetic characterization, and oxidation of colloidal iron dispersions

Colloidal iron dispersions have been prepared by the thermolysis of Fe(CO)5 in solutions of functional polymers. The structure of the particles was very disordered at sizes ≲100 A but changed to become single crystal with a disordered core as the size increased to 100–200 A. Particles ≲100 A were superparamagnetic, and particles in the 100–200‐A range had a time‐dependent hysteresis. On exposure to the atmosphere an ∼30‐A‐thick γ‐Fe2O3 oxide film was produced on the surface of the particles. This is the ’’passive oxide film’’ detected previously by a number of techniques but never before imaged directly in situ. As water was absorbed from the atmosphere the chlorinated solvent‐based dispersions reacted further to give β‐FeOOH. This reaction was promoted by chloride‐ion impurity. The magnetic moment decayed with oxidation roughly in proportion to the quantities of Fe, γ‐Fe2O3, and β‐FeOOH present. Disorder in the structure of ≲100‐A particles formed in nonchlorinated solvent‐based dispersions produced an i...

Thin film crystal growth of Si on fused silica: Effects of growth front dynamics on crystallography

Reflective high speed video microphotography was used to study the advance of the resolidification front during CO2 laser melting of encapsulated polycrystalline silicon thin film structures on bulk fused silica substrates. Prepatterned silicon films were laser recrystallized, removed from the substrate, and then examined by transmission electron microscopy. The crystal structure which results from seeded and unseeded growth and from various liquid‐solid growth front shapes was characterized. Supercooling of the melt and subsequent dendritic polycrystalline regrowth is observed in cases where nucleation occurs in isolated molten regions. Transmission electron micrographs demonstrate the necessity of providing controlled seeding conditions, such as regrowth through a narrow neck of silicon. A flat liquid‐solid interface supports a [111] growth direction whereas concave thermal profiles cause twinning or low angle grain boundaries in the films.