Temperature Dependent 4-, 5- and 6-Fold Coordination of Aluminum in MOCVD-Grown Amorphous Alumina Films: From Local Coordination to Material Properties

Abstract

Aluminum 5-fold coordination coexisting with 4-and 6-fold coordination structurally characterizes amorphous aluminum oxide. For nearly 30 years now, 27Al MAS NMR has enabled to detect and later on to quantify this feature thanks to advances in high-resolution instrumentation. The Introduction shortly reviews the results of investigations of 5-coordinate aluminum in amorphous alumina through NMR analysis. Aluminum oxide is not a glass-forming oxide. A convenient way to obtain the amorphous state is by thin film deposition. We present here 27Al NMR analysis of a series of thin films of aluminum oxide prepared by metalorganic chemical vapor deposition (MOCVD) in the temperature range 360 ≤ Td ≤ 720 °C. In this range, low Td yield OH-containing films, while high Td yield nanocrystallites-containing films. The variation of the [4]Al, [5]Al and [6]Al content with Td is presented and discussed. It is correlated with the Td dependence of mechanical and corrosion protection properties. These properties are optimal when the structural disorder is to the utmost. Al coordination dependence on film thickness and the formation of metallic aluminum during the deposition process are also presented.