Alternative Oxide Substrates for GaN Heteroepitaxy

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. 

TABLE 1 Properties of important alternative substrates for nitride semiconductor epitaxy. 

Material Space group Misfit to Thermal expansion x 10-6 K1 

An enormous range of properties is found in oxides. The most successful and most widely used substrate for GaN to date is sapphire, AI2O3. Except for wurtzite materials, few of the unit cells in these materials match with GaN, but it is usually useful to think of these systems as having a close-packed nitrogen lattice in GaN matching to a near close-packed oxygen lattice in the oxide. 

The spinel structure and spinel related structures are found in a large number of oxides. The (111) face is usually the lowest surface energy face, so it is typically easy to prepare. The oxygen lattice is face-centred cubic, which presents a similar stacking layer along [1 1 1] as found in the wurtzite structure along [0 0 0 1], 

---

B2.3 Epitaxy of III-N layers on GaN substrates B2.4 Alternative oxide substrates for GaN heteroepitaxy B2.5 Cubic substrates for growth of GaN and related compounds... 

---