Molecular beam epitaxy

Molecular beam epitaxy (MBE), a technique used to form an epitaxial growth, is very often used in semiconductor industry as deposition method of thin films on wafer substrates due to its efficiency, controlled doping characteristic, repeatability, and uniformity. But before immersing to MBE, let’s take a quick look on its mother form the epitaxial deposition.

Epitaxial Growth Deposition

In 1960s, Bell Laboratories developed a technique to deposit materials in an orderly fashion at a very low speed; and therefore, allowing an accurate control on the deposit’s thickness and dopant concentration. This technique is now known as epitaxial growth deposition.

Epitaxial growth deposition grows a single crystal film over a substrate by arranging the atoms on top of the substrate.  The same structure of atom arrangement were grown layer-by-layer; hence, giving the film a well-ordered crystal formation that matches the crystalline structure of the substrate

Types of Epitaxial Growths

Epitaxial growth can be categorized as either homoepitaxial or heteroepitaxial depending on the type of material grown on the substrate.

A homoepitaxial growth has a film of the same material as the substrate (i.e. Si on Si growth).  This forms a film purer than the substrate and provides the ability to dope the layers independently from the substrate.

A heteroepitaxial growth, on the other hand, has a film of different material than the substrate (i.e. AlAs on GaAs or GaAs on Si growth). The difference on the materials used leads to unmatched lattice formation which will either stress or relax the growth. This will affect some properties of the film such as electrical, optical, thermal and mechanical. Though may be a misfit, this property of heteroepitaxy is used as an advantage on optoelectronic and band gap engineering applications.

Lattice formation

Figure 1. Lattice formation: (a) homoepitaxial; (b) heteroepitaxial

Processes to Form Epitaxial Growths

Deployment of epitaxy can be performed in a number of ways two of which are physical and chemical vapor deposition.

Chemical vapor deposition (CVD) decomposes the gaseous chemicals through heat at temperature lower than the melting point of the substrate. Since most of the candidates as wafer film deposits are difficult to transport through gas (because of low vapor pressure), CVD chemically attaches a compound with a high vapor pressure to the material. The attached compound will aid the surface mobility of the material. At raised temperature the material-compound bond will be easily broken, leaving the material on the substrate, while the compound pressure will be pumped away by vacuum.

Physical vapor deposition (PVD), on the other hand, uses the vapor formation of the material in deployment of thin film into the substrate. The material is either heated to its vaporization point like in thermal evaporation or its ions are knocked-off from it like in sputtering. Another PVD technique wherein the material’s atom diffuses to the growing film in an ultra high vacuum (UHV) environment is the Molecular Beam Epitaxy (MBE).

Chemical vapor deposition

Figure 2. Chemical vapor deposition: (1) material element is bonded with an organic compound; (2) composition is transported to the substrate; (3) heat breaks apart their bond, leaving the element free to bond to the growing film; (4) organic compound evaporates.

Physical Vapor Deposition Techniques

Figure 3. Physical vapor deposition techniques: thermal evaporation (a) and sputtering (b)

Molecular Beam Epitaxy (MBE)

Molecular Beam Epitaxy

Figure 4. Molecular Beam Epitaxy

The choice of epitaxial growth technique depends on the kind of epitaxial structure required for the device application and the needs of production. Like in mass production, MBE may not be the main option because compared to other techniques, MBE has lower growth rate. However, for cases where material purity and the accuracy on control of the film thickness and doping profile are more important than production yield MBE is usually the choice in manufacturing.

MBE is an ultra high vacuum (UHV) technique used in growing high quality epitaxy. Its process is somewhat very simple: the material is heated by the effusion cell, the flux of atoms are then transported to the UHV environment where they will travel directly towards the heated substrate. Once on the substrate, the atoms will diffuse on the growing film and bonds with it. Each element is transported separately by a controlled beam, providing a more controlled doping concentration for every element. The beam flux can be turned on or off through a shutter or valve.

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