Defects misfit dislocations

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

Figure 6.11. Schematic diagram showing defects in a twinned particle. Here, m means misfit dislocation, and g.b. stands for grain boundary [102].

Cation vacancies and interstitials, (111) twins and stacking faults, grain boundaries, microstrains, misfit dislocation network at C03O4/C0O interface Dislocations and (100) stacking faults intergrowth of e and P phases. Cations vacancies and superstructure (110) stacking faults and twins Clusters of point defects (110) twins surface steps, dislocations, spinel microinclusions, planar defects stabilized by impurities. 

The mechanical interaction between the different epitaxial layers may result in the formation of misfit dislocations. Nucleation and propagation of cracks can ensue if the mismatch in thermal expansion coefficient is relatively large. The defects significantly influence the physical properties of the thin films. Examples from different material combinations and models of how to predict the numbers for critical thicknesses are provided in Section 14.4. 

II-VI semiconductor layers and bulk semiconductors like Si, GaAs, InP, etc. In particular, quantum wells are formed by thin epitaxial multilayered structures like (Zn, Cd)Se/ZnS. Nevertheless, the choice between bulk semiconductors and the layers deposited or between the multilayers is governed by the lattice mismatch between the two components as the lattice mismatch causes the formation of misfit dislocations. In the optical devices these defects are potential non-radiative centres and at worst they can cause the failure of injection lasers. Figure 29 is a map of energy gap versus lattice constants for a variety of semiconductors it can be used to select different heterostructures, not only for optoelectronics applications but also for photovoltaic cells. In the latter application the deposited films are generally polycrystalline and the growth of high-quality epitaxial layers has received little applications. 

Figures 2 and 3 indicate that very few defects are present at the interfaces and that the crystal growth of alternate layers proceeds without the generation of new misfit dislocations.

A vacancy is created in the metal phase, which can be aimihilated by operations of structural defects, disorientation, and misfit dislocations [116, 117]. Prolonged sulfidation also can cause void nucleation and cavity growth at the interface. In the case of sulfidation by liquid sulfur, the separation of sulfide and metal by cavities could be avoided by applying a pressure on the growing sulfide [13-18]. 

The parabolic rate law holds when the reaction layer is thick. When trying to be quantitative, there is the obvious question how to mark the location of the original interface. An additional complication arises if AO and AB2O4 are both cubic but not lattice matched then misfit dislocations must be present at the interface and these can move only if point defects on the O sublattice move. 

In epitaxial films on the substrate, which is usually much thicker than the film, the internal mechanical strains originate from mismatch between film and substrate lattice constants and their thermal expansion coefficients. The technological defects and imperfections can also be the sources of internal strains. The mechanical strains U can be either compressive or tensile, their values are around 1 GPa [6] and their relaxation occurs via misfit dislocations creation. However, there exists certain critical thickness he, such that these dislocations appear Sith>hc only. The calculations had shown [7] that he 1/1/ and for PbTiOs films on MgO or SrTiOs substrates he — 0.5 nm or 8.3 nm respectively. 

So far we only considered ideal heterojunctions. Often times in real heterojunction, the lattice constants of the contacting materials are never precisely the same. The lattice mismatch between the crystals gives rise to a network of so-called misfit dislocations, which can extend several nanometers into the bulk lattice. Although the details of such defects are beyond the scope of this book, the interface states introduced by dislocations and defects, similar to the surfaces states at the metal-semiconductor junction, induce additional band bending at the surface. 

Matthews, J. W. and Blakeslee, A. E. (1974), Defects in epitaxial multilayers I. Misfit dislocations, Journal of Crystal Growth 27, 118-125. 

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