A thin-film growth mode in which, after layer-by-layer growth of a few atomic layers, three-dimensional islands grow to relax the strain from lattice mismatch. It is used to make quantum dots.
Japanese name: ストランスキー・クラスタノフ成長
Stranski–Krastanov growth appears in 1 practice question across 1 topic, including Thin films, surfaces and nanomaterials.
Thin-film growth modes are roughly classified by the substrate surface energy γs, the film surface energy γf and the interface energy γi. If γs ≥ γf + γi, the film wets the substrate and grows one layer at a time (layer-by-layer, or Frank–van der Merwe, growth); in the opposite case islands form from the start (island, or Volmer–Weber, growth). Stranski–Krastanov (SK) growth lies between the two: the film first wets the substrate and grows in layers, but switches to islands beyond a certain thickness. The characteristic feature is this switch from layers to islands partway through growth.
A film whose lattice parameter differs from the substrate's initially grows stretched or compressed in-plane to match the substrate (coherently), so strain energy builds up in proportion to film thickness. Beyond a critical thickness, the energy becomes lower if the film forms islands whose upper parts can relax toward their natural lattice spacing, even at the cost of extra surface area. Three-dimensional islands then sit on a thin layer called the wetting layer. The lattice mismatch is about 7% for InAs/GaAs and about 4% for Ge/Si, and both are classic examples of SK growth.
Besides forming islands, strain can be relaxed by introducing misfit dislocations at the film–substrate interface. In combinations with small mismatch, the film stays flat and dislocations appear once the critical thickness is exceeded, so islands are less likely to form. The early islands produced by SK growth are coherent and free of dislocations, which is why they can be used as quantum dots. If growth continues and the islands become larger, dislocations also enter the islands, and the light-emission efficiency drops sharply. SK growth can be seen as a special form of relaxation that relieves strain elastically.
Because SK growth makes nanometer-scale islands arrange themselves without lithographic patterning, it is widely used to make self-assembled quantum dots. For InAs/GaAs grown by molecular beam epitaxy (MBE), islands are said to start forming beyond about 1.5–1.7 monolayers; buried in GaAs, they become the active layer of quantum-dot lasers and single-photon sources. The switch from two-dimensional to three-dimensional growth can be identified in situ when the reflection high-energy electron diffraction (RHEED) pattern changes from streaky to spotty. Island size and density are adjusted through the substrate temperature and the amount deposited.
Answer: B. After layer-by-layer growth of a few atomic layers, three-dimensional islands grow, driven by accumulated strain and other factors
In SK growth, a few atomic layers called the wetting layer first grow layer by layer; then, as strain energy from lattice mismatch accumulates, three-dimensional islands grow to relax it. The self-assembly of quantum dots of InAs on GaAs (lattice mismatch of about 7%) is the classic example. Growth as three-dimensional islands from the start is Volmer–Weber (VW) growth, which occurs when the bonds among film atoms are stronger than the bonds between film and substrate, and is often seen for metal films on oxides. Layer-by-layer growth to the end is Frank–van der Merwe (FM) growth, which occurs ideally in lattice-matched epitaxial growth.
In VW growth the film does not wet the substrate, and islands form from the very beginning. In SK growth the first few atomic layers grow as layers and form a wetting layer, and islands then form on top of it to relax strain.
It is the flat film, a few atomic layers thick, that remains beneath the islands in SK growth. It still connects the islands after they form, and because it acts as a thin quantum well itself, it affects light emission and current flow in quantum-dot devices.
Because nanoscale islands without dislocations form all at once at high density without any patterning. When buried, they become quantum dots surrounded by interfaces with few defects, and they emit light efficiently.
In SK growth, a few atomic layers called the wetting layer first grow layer by layer; then, as strain energy from lattice mismatch accumulates, three-dimensional islands grow to relax it.
Among thin-film growth modes, which correctly describes Str…Quantum dot Critical nucleus radius Ostwald ripening Quantum size effect Thermal stress