DoITPoMS


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Introduction

Epitaxy by MBE involves expensive vacuum deposition equipment such as that shown in the figure.

Figure 1 : Epitaxial growth equipment at the University of Liverpool

It is used to deposit thin layers (usually less than a micrometre thick) intended to form the active layers in optoelectronic devices. Such layers must be flat, of precise composition with the appropriate concentration of dopant atoms, and may involve abrupt changes of composition, for instance in order to form quantum wells. The details of the arrangement of the deposited atoms is therefore of great importance: Not only must they adopt the same crystal structure and orientation as their substrate, but the surface of the growing crystal must either be flat or , if not , then rough in a controlled and predictable way.

When the deposited material is of identical composition to the substrate we refer to homoepitaxy, while if the deposit is different from the substrate we refer to heteroepitaxy.

The images in figures 2 and 3 show high resolution TEM images of typical layers, in which the columns of atoms can be seen.

TEM micrograph of quantum wells in AlGaAs/GaAs imaged so that there is strong contrast between the layers of different composition

Figure 2 . TEM micrograph of quantum wells in AlGaAs/GaAs imaged so that there is strong contrast between the layers of different composition.

The following image (Figure 3) shows the columns of atoms in alternating epitaxial layers of GaAs and AlAs. The interfaces can be seen to be flat to about one monolayer. The appearance of the AlAs is different from that of the GaAs despite their identical crystal structure because different planes are imaged in the two phases.

Figure 3 : A high resolution TEM image of alternating GaAs and AlGaAs layers (Simone Montanari PhD thesis (2005))

Figure 4 illustrates the epitaxial growth of a complex compound. The substrate plane is (001) and the heavy atoms show as white dots in a square array. The interface is clearly very flat.

Bi0.5Mn0.5FeO3 film (left hand side, lighter contrast), epitaxially grown on a strontium titanate (SrTiO3 ) single crystal substrate

Figure 4: Bi0.5Mn0.5FeO3 film (left hand side, lighter contrast), epitaxially grown on a strontium titanate (SrTiO3 ) single crystal substrate.


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