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Molecular beam epitaxy (MBE)

Preparing well-defined heterostructures on a nanometer scale has not only led to fundamentally new phenomena such as the quantum Hall effect [1] and the fractional quantum Hall effect [2] but has also revealed new technologies (e.g. double heterojunction laser [3], higher electronic device speeds [4], and ''band gap'' engineering of materials [5]). Molecular Beam Epitaxy (MBE) [6,7] is a well established technique to grow such heterostructures under very clean and controlled conditions. The term epitaxy is derived from the Greek words epi (meaning ''on'') and taxis (meaning ''arrangement'') [8] and describes the crystalline growth of one material on the same (homoepitaxy) or on a different material (heteroepitaxy).

Figure 1: Basic principle of molecular beam epitaxy showing coevaporation of atoms or molecules from two different sources onto a heated substrate (picture from C. Kreis, Ph.D. thesis).

Figure 1 displays a schematic illustration of the MBE process. Under ultra-high vacuum (UHV) conditions the various constituents are coevaporated onto a clean crystalline substrate surface which serves as seed crystal and is usually heated to provide high mobility of the impinging particles for crystalline growth. In conventional MBE especially group IV semiconductors (Si, Ge), group III/V (III = Al, Ga, In; V = P, As, Sb) and group II/VI (II=Pb,Cd,Hg; VI=S, Se, Te) compounds are widely used [9]. These crystals exhibit covalent bonding in all three space directions (3D materials) and dangling bonds occur at their surfaces which have to be matched at these 3D/3D heterojunctions [Fig. 2]. Since length and angle of the covalent bonds at the interface can not easily be changed [10], the lattice mismatch Da which is defined as

Da = |af - as|

(af and as denote the in-plane lattice parameter of film and substrate, respectively) is of crucial importance for the quality of the junction. For a lattice mismatch exceeding only 1%, the covalent interaction at the interface usually causes strong disturbances at the interface like strain, structural imperfections and dangling bonds (edge dislocations) introducing a considerable defect density. This drastically limits the possible choice of material combinations.

Figure 2: Heterojunction of lattice mismatched three-dimensional crystals with dangling bonds at the interface (picture from C. Kreis, Ph.D. thesis).


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