After rapid thermal annealing of an ultra-thin germanium (Ge) layer in a tri-layer insulator structure at 1000degreesC in argon, the formation of Ge nanocrystals in the structure has been investigated by using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy. It was found that the Ge layer in the as prepared sample is a mixture of amorphous Ge, GeO2 and Ge suboxide. After 10 s annealing, Ge is distributed over a much wider amorphous layer than the as prepared case and a few Ge nanocrystals were observed. The density of Ge nanocrystals increases with increasing annealing time from 10 to 200 s, and then decreases as the time increases further to 400 s. A possible mechanism for the formation of Ge nanocrystals was discussed. Current-voltage and capacitance-voltage measurements show that the device with Ge nanocrystals has a pronounced charge storage capability. The electrical charge stored in the device reaches a maximum value when the annealing time increases from 10 to 200 s, and then decreases as the annealing time increases further to 400 s. (C) 2003 Elsevier B.V. All rights reserved.