Sapphire substrates are used to grow semiconductor compounds, most commonly GaN for LEDs and other applications such as high temperature superconductors. They are also used to grow silicon for manufacturing microwave ICs, high-speed ICs and pressure transducers.
Sapphire is the single crystal form of aluminum oxide (Al2O3), and single crystals are made of identical building blocks, known as unit cells, which repeat themselves continuously and unbroken from edge to edge. This makes the atom arrangement of a sample highly organized and predictable. The crystal structure of sapphire is hexagonal/rhombohedral.
To grow different material on sapphire with atom layer-to-layer precision requires atomic level agreement between the substrate and the material. These requirements are only satisfied along specific surfaces known as planes of the sapphire crystal. Optical properties, hardness and thermal conductivity of sapphire also vary significantly by the orientation.
Based on application specifications, sapphire single crystal structure is divided into multiple planes. The planes most frequently used — c, a, m and r — are shown in the figure with respect to sapphire crystal axis directions. The atomic arrangement in each plane is unique and shown in the figures below with grey atoms representing aluminum and red atoms representing oxygen.