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Molecular Beam Epitaxy

Details of the MBE facility at UCL
MBE Machine

Molecular Beam Epitaxy (MBE) is an advanced ultra-high-vacuum facility (basic pressure 10-13 bar) to make compound semiconductor materials with great precision (< 0.01 nanometer) and purity (>99.99999%). These materials are layered one on top of the other to form semiconductor devices such as transistors and lasers, which are devices being used in such applications as fiber-optics, cellular phones, satellites, radar systems, solar cells, and display devices.

Semiconductor devices consist of various materials with differing electronic and optical properties stacked up much like a many-layered cake. How electrons move through the various layers determines the behaviours and function of the device. For example, a transistor acts like a switch by only allowing electrons to pass through when a voltage is applied. Other structures, used for light-emitting diodes and lasers, produce light as electrons pass through the layers. To make each layer, ultra-pure elements (>99.99999%) are requested to be delivered to a surface with growth rate between 0.001 monolayer/second and 2 monolayer/second (The thickness of 1 monolayer GaAs is about 0.28 nm). Within MBE system, the atoms or molecules in the gas deposit on the substrate to form the growing solid layer. Typically, each element is delivered in a separately controlled beam, so the choice of elements and their relative concentrations can be adjusted for any given layer, thereby defining the precise composition and electrical and optical characteristics of that layer. Each gas beam can be turned on and off rapidly with a shutter or a valve within 0.2 second. MBE offers tremendous control over layer thickness, composition and purity. The growth rate is sufficiently slow that layers only a few atoms thick can be produced reliably. Thicker layers are obtained with longer deposition times.

Example of MBE Growth

MBE growth produces most complex structures of varying layers which are further processed to produce a range of electronic and optoelectronic devices, including high speed transistors, light-emitting diodes, high-efficiency solar cells, and solid state lasers. MBE is a powerful technique both for research into new materials and layer structures, and for producing high-performance semiconductor devices. MBE is one of the fundamental facilities for Digital Economy, Energy, and Semiconductor Nanotechnology research fields.

The supplying of high-quality low-dimensional semiconductor materials is one of the key limiting factors of semiconductor materials and devices in UCL. The new MBE system is the first new MBE system in UCL, and addresses this key weakness of UCL, and hence will play an important role to further enhance the international reputation in nanotechnology and photonics for UCL and London Centre for Nanotechology. This new MBE system will act to enhance the UCL global competitiveness in optoelectronic and semiconductor science, and generate new research direction in UCL. Furthermore, new UCL MBE facility has very special source arrangement with the combination of Arsenic, Antimony and Phosphorus crackers with Gallium, Aluminium, and Indium solid sources, which is unique in the UK and in Europe and enables to produce to grow very complex III-V epitaxial structures.