Molecular beam epitaxy (MBE)

Molecular beam epitaxy (MBE) is a widely used commercial technique for the fabrication of specialised semiconductor devices, particularly those based on III-V materials and used in optoelectronics and high frequency applications. However, it is also a conceptually very simple method of crystal growth. Crystal growth by MBE requires ultra-high vacuum conditions and so MBE is naturally compatible with many surface science techniques. Of particular importance are those techniques which can be operated in situ as growth proceeds - principally reflection high energy electron diffraction (RHEED) and reflection anisotropy spectroscopy (RAS). Other techniques can be employed (e.g. ion scattering, EDX and STM) and practically any surface science tool can be used ex situ by a process of sample transfer under vacuum from the growth chamber.

Briefly, in MBE one directs beams of atoms or molecules at a heated target crystal. Under UHV conditions, the beams do not interact and can be easily controlled. Under the correct conditions of beam fluxes and substrate temperature, the beam constituents can react to form new crystalline material at the surface. If the beam constituents are the same as the substrate crystal (e.g. Ga and As directed at a GaAs crystal) we have 'homoepitaxy' and the crystal is simply extended in the growth direction. We are primarily interested in 'heteroepitaxy' - for example, the growth of InAs on GaAs or NiMnSb on InP.

Different elements require different vacuum deposition techniques: here is a useful table and another.

PDF files available:

A one-page cartoon of the MBE process.
A very short description of MBE, in situ surface analysis and ferromagnet-semiconductor hybrid structures.
A brief summary of nanostructure formation by self-organised growth.
The PhD thesis of Dr. Stuart Hatfield (my first research student) on MBE growth of MnSb on III-V substrates.
The PhD thesis of Dr. James Aldous (PhD student #2) on MBE growth and surface properties of NiSb and MnSb.