Epitaxial layers 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. 

The main issue involving GaN substrates for nitride epitaxy concerns obtaining optoelectronic devices without mismatch dislocations. The critical conditions for misfit-dislocation creation include lattice mismatch between the layer and the substrate, layer thickness, growth conditions and substrate quality. 

The state-of-the-art analysis methods for the evaluation of structural, chemical and electrical properties of thin layers in processed Si substrates are discussed. The properties of inclanted p-n junctions, Si-SiO interface, Ge inplant amorphization of Si aid misfit dislocation interface in epitaxial Si are exenplified to illustrate the features and limitations of the techniques. 

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. 

Misfit dislocation generation can be explained by the earlier-mentioned mechanisms when the growth mode is 2D and the epitaxial layer is flat. On the other hand, no dislocation generation mechanism has been reported when 3D islands are formed at the initial stage of growth and the lattice... 

Freund, L. B. (1990), A criterion for arrest of a threading dislocation in a strained epitaxial layer due to an interface misfit dislocation in its path. Journal of Applied Physics 68, 2073-2080. 

Figure 7.19 A transmission electron micrograph looking down upon the interface between a GaAs substrate and an InAs epitaxial layer. Both threading segments stretching from the heterojunction to the surface and reactions between dislocation segments are visible in the micrograph. The lines in the image are the misfit dislocations in the interface. [8]...

R. Hull, private communication. The reader is referred to the many excellent works by Hull and collaborators that describe the results of these observations. As an example of such work, see Hull R., and Bean J.C., New insights into the microscopic motion of dislocations in covalently bonded semiconductors by in-situ transmission electron microscope observations of misfit dislocations in strained epitaxial layers. Phys. Statu. SolidiA, 1993 138 533-46. 

The start of heteroepitaxy on porous sdieon was initiated by a Luryi and Suhir paper in 1986 (Luryi and Suhir 1986). They theoretieaUy predicted a possible approach for growing dislocation-fi-ee lattice-mismatched heteroepitaxial layers on small seed pads of lateral dimension L, having a uniform crystal orientation over the entire substrate wafer. It was proposed that when L is smaller than a specific length Lmm, which depends on the lattice misfit as well as the dislocation energy, the entire elastic stress in the epitaxial films will be accommodated without dislocations. Porous Si, studied at that time for dielectric isolation purposes (Imai 1981), was named as a suitable substrate. 

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