CAMPBELL

Now, researchers at the University of Washington, Seattle, report that a microcalorimetric procedure can be used to monitor heats of adsorption as silver atoms are deposited on a silicon surface [Phys. Rev. Lett., 87, 106102 (2001)]. The study uncovers details of the driving forces and kinetics of metal film growth on semiconductors. Film growth processes affect electronic device performance by controlling electrical contacts and interface properties.

The study was conducted by chemistry professor Charles T. Campbell, graduate students David E. Starr and Jana E. Musgrove, and postdocs Jane H. Larsen and Jeffery T. Ranney.

To measure heats of adsorption, the Seattle group pulses beams of silver atoms at a thin silicon specimen under vacuum. The clouds of metal atoms impinge upon silicon's (100) crystal face and bond to the semiconductor, causing transient temperature rises in the specimen. The small bursts of heat are measured by a pyroelectric ribbon sensor that contacts the backside of the specimen.

Campbell notes that other researchers, using scanning tunneling microscopy (STM) at room temperature, found that silver forms a uniform film on silicon as the metal film grows to nearly a full monolayer (ML) of coverage. But just beyond the 1-ML coverage level, much of the silver leaves the uniform coating (wetting) layer--preferring to ball up into 3-D nanoparticles.

Based on their recent measurements, the Seattle group explains the STM results, noting that at coverages greater than approximately 0.5 ML, the silver film is metastable yet continues to grow because of kinetics.

INTERACTING Using a sensitive detector to measure heats of metal atom adsorption during film growth on silicon (above), University of Washington researchers can differentiate between scenarios in which metal atoms wet a surface (below middle) or ball up (below right).

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Copyright © 2001 American Chemical Society