At the heart of every LED, laser diode, solar cell and transistor is a stack of semiconductor layers, each with a carefully controlled composition, thickness and electrical conductivity. To ensure optimal device performance, these layers must be deposited in a carefully controlled manner, atom by atom, to form a virtually perfect crystal.
MBE is the ideal growth technology for depositing films in this manner. This growth process begins by loading all of the materials that are to be incorporated into the epitaxial film into separate, inert containers, known as crucibles. Every crucible is heated up to a temperature that causes its source to evaporate at a stable, desired rate. When a shutter is opened, a beam of atoms or molecules travels under an ultra-high vacuum from its cell to a heated substrate. Heating ensures that every molecule ‘cracks’ into atoms when hitting the surface of the substrate.
Compound semiconductors form when two or more sources are directed at the substrate simultaneously. For example, a film of gallium arsenide grows when gallium and arsenic atoms impinge on the wafer at the same time. Their freedom to move around the surface before settling down holds the key to forming a perfect crystal with alternating atoms of gallium and arsenic.