ZnO/Al 2O 3 nanolaminates fabricated by atomic layer deposition: growth and surface roughness measurements

Abstract

Nanolaminates are unique nanocomposites that allow various thin film properties to be tuned by changing the composition and interfacial density. ZnO/Al 2O 3 nanolaminates allow the surface roughness to be controlled because ZnO is crystalline and Al 2O 3 is amorphous at low deposition temperatures. ZnO/Al 2O 3 nanolaminates were grown using atomic layer deposition (ALD) methods. The ZnO and Al 2O 3 films were deposited at 450 K using alternating diethyl zinc/H 2O exposures and trimethyl aluminum/H 2O exposures, respectively. The growth rate and surface topography of the pure oxide films were examined using ex situ ellipsometry, stylus profilometry and atomic force microscopy (AFM) techniques. The ZnO ALD films grew at 2.01 Å/cycle and roughened significantly versus film thickness because of the presence of ZnO nanocrystals. In contrast, the Al 2O 3 ALD films grew at 1.29 Å/cycle and remained remarkably smooth versus film thickness because of their amorphous structure. In situ quartz crystal microbalance measurements were employed to study the ZnO/Al 2O 3 nanolaminate growth. The individual ZnO and Al 2O 3 nanolayers nucleated and grew easily on each other with minimal interfacial effects. ZnO/Al 2O 3 nanolaminate films were prepared where the number of ZnO/Al 2O 3 interfaces was varied but the total thickness remained constant at ∼1250 Å. Using AFM techniques, the RMS roughness of the nanolaminates was observed to decrease substantially with increasing ZnO/Al 2O 3 interfacial density. The Al 2O 3 nanolayer interrupts the ZnO crystal growth and forces the ZnO nanolayer to renucleate on the Al 2O 3 surface. AFM measurements revealed that a single trimethyl aluminum/H 2O reaction cycle was sufficient to reduce markedly the surface roughness of the ZnO/Al 2O 3 nanolaminate films. ZnO/Al 2O 3 nanolaminates should be useful to fabricate a surface topography with a controlled surface roughness.