Seeded Lateral Epitaxy of ALD Alumina Thin Films

Sapphire, a crystalline type of alumina (Al2O3), is a ceramic compound with several industrial applications. It is commonly used as a substrate wafer for compound semiconductor growth and LED manufacturing. In addition, it is often used as an optical window due to its high degree of optical transparency. The growth of large grain or single crystal sapphire however, is a time- and energy-intensive process, motivating new growth techniques. This investigation develops alternative pathways to produce large grained sapphire films on a variety of substrates. Atomic layer deposition (ALD) is an emerging growth technique capable of deposition of amorphous alumina with precise thickness control. Annealing of amorphous alumina films typically requires very high temperatures and results in nanoscopic crystals. We have demonstrated that by placing nanoparticle sapphire seeds on the amorphous ALD alumina, crystallization can be induced at far lower temperatures, resulting in large grains (several microns in diameter). Crystallization was observed at temperatures as low as 900 °C. By measuring the growth rates of the crystals at various temperatures, the growth temperature and time can be optimized to produce fully polycrystalline sapphire films on a variety of substrates.