Growth mode of alumina atomic layer deposition on nanopowders

Abstract

Alumina films were grown by atomic layer deposition in a rotary reactor on tungsten nanopowder with an average particle size of 54 nm. Films of various thicknesses were formed using trimethylaluminum and water at a reaction temperature of 110 °C by varying the number of deposition cycles from 2 to 78. The sharp contrast between the deposited alumina shell and the tungsten core in transmission electron microscopy allows for easy film thickness measurements and determination of the film thickness as a function of the deposition cycle. The growth curve shows that the rate of film thickness increase does not follow a single linear response but instead consists of three characteristically different growth phases. These phases occur in different deposition cycle regimes: phase I occurs in ≤5 cycles, phase II between 5 and ∼15 cycles, and phase III begins in ∼15 cycles and continues until at least 78 cycles. The average growth per cycle for phases I, II, and III is ∼4.5, <1, and 2 Å/cycle, respectively. The average growth per cycle for phase III is in excellent agreement with literature values for alumina growth on nanopowder and corresponds to ∼1 monolayer per cycle. The distinctly high/low average growth per cycle in phase I/phase II can be explained by the preferential contribution of added alumina in perpendicular/lateral growth of islands. Films grown from two deposition cycles include tall islands and regions which are ≤2 monolayer thick, similar to the dimpled surface of a golf ball. The amount of ≤2 monolayer thick regions decreases in areal density with an increase in the number of deposition cycles, resulting in an almost plateau followed by a linear increase in the growth curve and suggesting a transition of the growth mode. Such evolutions in growth are best described as the Stranski–Krastanov and Frank Vander Merwe type.