Heteroepitaxy

From ArticleWorld

Heteroepitaxy refers to a specialized thin-film technique of deposition in which different materials are involved. It is used for the growth of semiconductor crystals of a certain material on the crystal face of another material. A thin-film whose material lattice orientation is the same as that of the substrate on which it is deposited is used. The thin-film will be a single crystal if the substrate happens to be a single crystal. The technique of heteroepitaxy is applied to growing crystalline films of materials of which individual crystals cannot be obtained. Integrated crystalline structures of different layers, used in semiconductor technology are fabricated using heteroepitaxy.

Applications

The technique of heteroepitaxy is widely used in the manufacture of semiconductor and photonic devices. Nanotechnology also requires the use of such a technique. Heteroepitaxy is the most affordable method of growing high crystalline quality structures for some of the most important semiconductor materials like gallium arsenide and indium phosphide, which are used in a wide variety of electrical engineering applications.

Steps in making semiconductor devices

The first step in making semiconductor devices is to start with a semiconductor wafer. Wafers are obtained by slicing large crystals. Epitaxial growth is initiated over the substrate material by a number of means, prominent among them being molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD). The principle of operation in MBE is that gases are bound to cold surfaces by means of a phenomenon called cryocondensation. Epitaxial growth starts with the heating of the compound required for the addition of a certain atomic species into the vacuum chamber. In an advanced form of this method, room temperature can be used to carry out the appropriate reactions. Inside the ultra-high vacuum, the free atoms have a long mean-free path and collisions with other atoms are rare. Everything is remotely controlled by a computer, thus facilitating the beams of atoms from the so-called “effusion cells” to attach to the substrate material and facilitate an epitaxial layer to form. The MOCVD is another widely used method of preparing epitaxial layers on a substrate by carefully controlled deposition of atoms. The substrate wafer to be layered is kept on a susceptor made of graphite, inside a reaction chamber which is heated by a RF induction heater. The temperature maintained may be between 500°C and 700°C depending on the compounds used. A medium pressure metal-organic gas is kept in the reaction chamber. The growth precursors like arsine (AsH3¬¬¬ for group V), trimethyl gallium Ga(CH3)3 or (TMG for group III) decompose on contact with the heated substrate, resulting in epitaxial layers being formed. Wafer bonding can be done to fuse two semiconductor materials at the atomic level. This method is useful when bonding materials have different lattice constants. Quartz reactors are used for this purpose. The wafers should necessarily be flat and free of any contaminants on them.