Controlling the preferred orientation in silicalite-1 films synthesized by seeding

Single crystal silicon (100) wafers were seeded with colloidal silicalite-1 crystals and hydrothermally treated in a precursor solution to grow thin silicalite-1 films. A total of 28 experiments in eight series were investigated with SEM and XRD to evaluate the preferred orientation of the crystals constituting the films. The investigated parameters in the film formation process were seed crystal size, amount of adsorbed seed crystals and film thickness after hydrothermal treatment of the seeded substrates. In thin films, most of the crystalline material is oriented with the b-axis perpendicular to the substrate surface. In thick films, most of the crystalline material is oriented with the a-axis perpendicular to the substrate surface. The change in preferred orientation with film thickness is faster when small seeds are used. The amount of adsorbed seeds has a larger influence on the preferred orientation when large seeds are used. A mechanism explaining these trends is proposed. The choice of size and coverage of seeds can be used to control the preferred orientation of the crystals in a film of given thickness within certain limitations. (C) 1999 Elsevier Science B.V. All rights reserved.