Molecular beam epitaxy (MBE) takes place in high vacuum or ultra-high vacuum (10-8 Pa). The most important aspect of MBE is the deposition rate (typically less than 3000 nm per hour) allowing the films to grow epitaxially. These deposition rates require proportionally better vacuum to achieve the same impurity levels as other deposition techniques. The absence of carrier gases as well as the ultra-high vacuum environment result in the highest achievable purity of the grown films.
In solid-source MBE, elements such as gallium and arsenic, in ultra-pure form, are heated in separate quasi-Knudsen effusion cells until they begin to slowly sublime. The gaseous elements then condense on the wafer, where they may react with each other. In the example of gallium and arsenic, single-crystal gallium arsenide is formed. The term “beam” means that evaporated atoms do not interact with each other or vacuum chamber gases until they reach the wafer, due to the long mean free paths of the atoms.
During operation, reflection high energy electron diffraction (RHEED) is often used for monitoring the growth of the crystal layers. A computer controls shutters in front of each furnace, allowing precise control of the thickness of each layer, down to a single layer of atoms. Such control has allowed the development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots. Such layers are now a critical part of many modern semiconductor devices, including semiconductor lasers and light-emitting diodes.
Our lab is equipped with two MBE systems. The first consists of five ultra-high vacuum (UHV) chambers interconnected by a sample transfer stage (R2P2, Vacuum Generator, Inc.) through gate valves, as shown in the picture to the right. The R2P2 is a multi-exit radial distribution system with six radial ports for sample transfer with a single drive giving both linear and rotational movements. The reactive MBE is equipped with a 10 keV RHEED and mass spectrometer, and five solid sources (e.g., Ga) and an electron cyclotron resonance (ECR) plasma source (MPDR 620i, Wavemat Inc.). The Bi2Se3 MBE is equipped with Bi, Sb, Se, and Te solid sources, and an e-beam source for Fe.
The second is a VG MBE system consisting of two VG 4″ MBE (VH-80) chambers, one sample preparation chamber, and one loadlock chamber. One MBE chamber is equipped with another Wavemat ECR plasma source, and is dedicated to the growth of magnetic III-nitrides. The other MBE chamber is used for the growth of As-, P- and Sb- compounds.