Domain Matching Epitaxy: A Unified Standard Model for Thin Film Growth

摘要

We present a unified standard model for thin film epitaxy where single crystal films with small and large lattice misfits are grown by domain matching epitaxy (DME). We define epitaxy as having a fixed orientation rather than the same orientation with respect to the substrate. The DME involves matching of lattice planes between the film and the substrate, and this matching could be different in different directions. In this framework, the conventional lattice matching epitaxy (LME) becomes a special case where a matching of lattice constants or the same planes is involved with a small misfit of less than 7-8%. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of major lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100)with 4/5 matching, and the ZnO/a-Al2O3(0001) with 6/7 matching of major planes across the film/substrate interface. By varying the domain-size, which is equal to integral multiple of lattice planes, in a periodic fashion, it is possible to accommodate additional misfit beyond the perfect domain matching. Thus, we can potentially design epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces. In addition to atomically clean surfaces, interfacial energy and interatomic potentials play an important role in producing epitaxial thin films. In-situ X-ray diffraction studies on initial stages of growth of ZnO films on sapphire correctly identify a compressive stress and a rapid relaxation within one to two monolayers, consistent with the DME framework and the fact that the critical thickness is less than a monolayer. DME examples ranging from the Ge-Si/Si(100) system with 49/50 matching (2% strain) to Metal/Si systems with 1/2 matching (50% strain) are tabulated, strategies for growing strain-free films by engineering the misfit to be confined near the interface are presented, and the potential for epitaxial growth of films with any lattice misfit on a given substrate with atomically clean surfaces is discussed.