Epitaxial films, lattice mismatch

Chapters 5-8 discuss uses of porous SiC, and porous intermediate layers of other materials, as substrates for GaN epitaxy. The lattice mismatch between GaN and SiC leads to dislocations in GaN films, and use of a porous template offers a mechanism for reducing the dislocation density. 

There are many deposit-substrate combinations where the basic lattice mismatch is very large, such as when a compound is formed on an elemental substrate, but where excessive strain does not necessarily result. Frequently a non one-to-one lattice match can be formed. If a material can match up every two or three substrate surface unit cells, it may still form a reasonable film [16]. In many cases the depositing lattices are rotated from the substrate unit cells, as well. In a strict definition of epitaxy, these may not be considered, however, it is not clear why high quality devices and materials could not be formed. 

The literature in this field is almost unduly preoccupied with lattice mismatch, so it seems appropriate to make a comment before proceeding. In this Datareview, I use the Matthew s convention [1] to compute the misfit. This convention gives a misfit which is simply related to the number of dislocations the film needs to accommodate the misfit. In practice, coherent epitaxy is never achieved for misfits larger than about 2%, so computation of a misfit may not be terribly meaningful. Lattice misfits to the nitride semiconductors for the materials discussed in this paper are given in TABLE 1. 

The growth of 3C-SiC on porous Si using the cold-wall LPCVD method resulted in a slightly better film quality compared with that on standard Si, as determined by LTPL. Further improvement in film quality was obtained by growing on a stabilized porous Si substrate. The epitaxial films did contain TBs and APBs at the interface, which is common due to the 20 % lattice mismatch between Si and SiC. 

Heteroepitaxial growth of GaN is usually performed on sapphire or SiC (13 % and 3.4% lattice mismatch, respectively, with GaN). Such a lattice mismatch between GaN and these substrates results in a high dislocation density in the epitaxial films. A variety of techniques have been employed in the past to reduce this high dislocation density and one of the common methods has been to engineer the substrate surface to control, and thus inhibit, the formation of threading dislocations. 

The increase in the TD density in the films grown on relatively thick (6-8 pm) PSC is most probably caused by a specific plastic relaxation process, occurring as a reaction to a particular state of strain that appears in these epitaxial films. This can be stated on the basis of strain inversion in the films grown on PSC, as well as on the increase in compressive stress with the thickness of the PSC layer increasing. These effects show that apart from the stress caused by the GaN/SiC lattice mismatches, an additional built-in stress arises in the films. Obviously, the additional stress is caused by the presence of (0001) PDs, because one can expect that a part of GaN film within the faulted region may have altered its mechanical properties as compared with unfaulted material [72]. Then the increase in dislocation density in GaN grown on relatively thick PSC can be explained by a plastic relaxation process, which relieves the built-in stress and occurs because this internal stress/(0001) PD density reaches a certain critical value. 

Epitaxial growth, with one orientation relationship, in substrate/film systems with relatively large lattice mismatch can be promoted by use of graphoepitaxy. 

An epitaxial film is strained in the initial stages of film growth. The strain energy increases with film thickness and may eventually be relaxed by the introduction of misfit dislocations [14.32-14.35], see Fig. 14.7, or by formation of (110) twins in the YBCO [14.36]. The critical thickness at which the misfit dislocations form depends on the lattice mismatch and the elastic properties of the film. The misfit in epitaxial c-axis-oriented YBCO films is accommodated by the formation of twins and edge dislocations with Burgers vectors [100]ybco and [010]ybco [14.37],... 

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