Low-temperature thin-film deposition and crystallization.

Crystalline oxide films are important components in a wide array of electronic and optical devices, and their study and manufacture involve major aspects of current science and technology. A plethora of methods, such as sputtering, chemical vapor deposition, pulsed laser deposition, and sol-gel, are commonly used to deposit these films, and many new techniques are being developed (1). In each of these methods, however, the deposited film is amorphous. For those applications requiring a crystalline film, an additional high-temperature processing step is required. This high-temperature step can lead to considerable constraints in combining the desirable characteristics of a crystalline oxide film with those of thermally unstable substrates and other device components. High-temperature processing also adds considerable costs to manufacturing. We describe a simple method that provides a general means for both low-temperature deposition and crystallization of oxide films. To demonstrate the generality of the method, we describe here the production of Zn2SiO4, ZrO2, and MnO2 films, which are of interest for applications in displays, electronics, and energy storage, respectively. The present method derives from our report on the preparation of oxide powders by a method of precipitation and hydrothermal dehydration (2). In this process, a hydroxo precipitate is dehydrated under hydrothermal conditions to yield an anhydrous, crystalline oxide at temperatures as low as 400 K. We have now combined this dehydration and crystallization process with the successive-ionic-layer-adsorptionand-reaction (SILAR) deposition method (3) to produce fully crystalline oxide films at low temperatures. In the SILAR process, the cation constituent is first adsorbed onto the substrate surface, followed by a rinsing step in water to produce an approximate monolayer of coverage. The substrate is then transferred to a solution containing the anionic constituent, wherein a precipitation reaction occurs at the surface of the substrate; the process is completed with an additional water rinse. This cycle of coating and rinsing is repeated many times under robotic control to achieve a desired film thickness. For the examples given here, substrates were immersed in 0.1 M solutions and rinse baths for 10 s each, and 700 cycles were used to develop a film thickness near 250 nm. Dehydration was performed overnight ( 12 hours), although crystallinity in some systems has been observed after a 30-min treatment. Films of Zn2SiO4 were produced on glass and nitrided silicon (Si3N4/Si) substrates by using the SILAR process with Zn (aq) and