Instabilities in crystal growth by atomic or molecular beams

When growing a crystal, a planar front is desired for most of the applicati ons. This plane shape is often destroyed by instabilities of various types. In the case of growth from a condensed phase, the most frequent instabilit ies are diffusion instabilities, which have been studied in detail by many authors but will be briefly discussed in simple terms in Section 2. The pre sent review is mainly devoted to instabilities which arise in ballistic gro wth, especially molecular beam epitaxy (MBE). The reasons of the instabilit ies can be geometric, but they are mostly kinetic (when the desired state c annot be reached because of a lack of time) or thermodynamic (when the desi red state is unstable). The kinetic instabilities which will be studied in detail in Sections 4 and 5 result from the fact that adatoms diffusing on a surface do not easily cross steps (Ehrlich-Schwoebel or ES effect). When t he growth front is a high symmetry surface, the ES effect produces mounds w hich often coarsen in time according to power laws. When the growth front i s a stepped surface, the ES effect initially produces a meandering of the s teps, which eventually may also give rise to mounds. Kinetic instabilities can usually be avoided by raising the temperature, but this favours thermod ynamic instabilities of the thermodynamically unstable materials (quantum w ells, multilayers...) which are usually prepared by MBE or similar techniqu es. The attention will be focussed on thermodynamic instabilities which res ult from slightly different lattice constants a and a + delta a of the subs trate and the adsorbate. They can take the following forms. (i) Formation o f misfit dislocations, whose geometry, mechanics and kinetics are analysed in detail in Section 8. (ii) Formation of isolated epitaxial clusters which , at least in their earliest form, are 'coherent' with the substrate, i.e. dislocation-free (Section 10). (iii) Wavy deformation of the surface, which is presumably the incipient stage of (ii) (Section 9). The theories and th e experiments are critically reviewed and their comparison is qualitatively satisfactory although some important questions have not yet received a com plete answer. Short chapters are devoted to shadowing instabilities, twinni ng and stacking faults, as well as the effect of surfactants. (C) 2000 Else vier Science B.V. All rights reserved.