Morphological Evolution of Strained Films by Cooperative Nucleation

Abstract

We identify a new mechanism of stress driven surface morphological evolution in strained semiconductor films. Surface roughness forms by a cooperative mechanism involving the sequential nucleation of islands and pits, which is distinct from the conventional view of ripple formation as an Asaro-Tiller-Grinfeld (ATG) instability. This mechanism is operative both during annealing and growth and competes with the ATG instability as a kinetic pathway to ripple formation. [S0031-9007(96)00853-8] PACS numbers: 68.35.MdThe morphological stability of stressed solids is a subject of considerable scientific and technological importance. It is directly relevant to several key issues in materials science ranging from stress corrosion cracking through to phase transformations and strained layer epitaxy. Since the pioneering work of Asaro and Tiller [1] and Grinfeld (ATG) [2], it is generally argued that above a critical wavelength l c the planar surface of a stressed solid is unstable to the formation of undulations [3][4][5]. This is because the energy reduction associated with elastic relaxation of the undulations exceeds the increase in surface energy. In the case of thin-film deposition, it is therefore envisioned that an initially planar film surface will gradually roughen in the growth direction over extended regions with a characteristic lateral wavelength. The observation of continuous surface ripple patterns on strained semiconductor layers over large areas would appear to give direct confirmation of this view [6,7]. Indeed, the patterns observed in cross section often closely resemble the sinusoidal roughness profiles used as a basis for the instability theory [8,9].In this Letter we reveal an entirely new mechanism of surface ripple formation, which is linked to the activated nature of island and pit formation. The ripple forms by a cooperative mechanism involving the sequential nucleation of islands and pits. This is very different from the gradual strain induced roughening mechanism normally envisioned [3-9] and has important implications for our fundamental understanding of the 2D to 3D transition of strained systems.To determine the mechanism of surface ripple formation we have studied the stress induced 2D to 3D transition in the technologically important Si x Ge 12x system. Our method involves a novel two stage process in which a thin (5 nm) planar Si 0.5 Ge 0.5 alloy layer is first grown at relatively low temperatures to ensure a nominally planar surface. The ripple morphology is then formed by a gentle postdeposition anneal at around 590 ± C for 5 min. This approach emulates equilibrium surface conditions, at least locally, as closely as possible. A map of the surface evolution during growth was obtained from atomic force microscopy (AFM) measurements of the ripple geometry at different temperature regions of one sample wafer.To capture the mechanism of surface ripple formation, use was made of the natural temperature gradient across the sample [10]. At the center of the wafer, corresponding to a temperature...

High quality relaxed germanium layers grown on (110) and (111) silicon substrates with reduced stacking fault formation

“…23 This effect has also been seen in other systems when mass transport under strain from the growing layer resulted in 3D islanding. [24][25][26][27] The side facets of the island are at angles of 29.3 to the (111) growth plane, which corresponds to {113} surfaces. As the growth rate on {113} is higher than that for the (111) surface, 28 subsequent growth on {113} will lead to the HT Ge layer being deposited in-between the islands faster than on the top (111) surface and should eventually lead to a smoother layer.…”

Beyond the heteroepitaxial quantum dot: Self-assembling complex nanostructures controlled by strain and growth kinetics

“…With additional growth to 30 nm, Fig. 2͑d͒, the structures enlarge, and the material ejected from the pit nucleates cooperatively 13 around the edges, forming a quantum dot molecule ͑QDM͒-a fourfold symmetric grouping of islands bound to a central ͕105͖-faceted pit.…”

“…The structure contains a mix of highly anisotropic wirelike features, QDMs, and individual quantum dots. Clearly the wires and quantum dots form due to cooperative nucleation 13 due to the presence of the initial groove and ridge structure created during the anneal. Pits that did not elongate during the annealing step, or newly formed pits during deposition, serve as the sites for symmetric QDM formation.…”

Entropy-Driven Effects in Self-Organized Formation of Quantum Dots

“…There exist two basic classes of theoretical models of self-organized formation of QD arrays: thermodynamic models, which describe an array of strained islands as the equilibrium state of a heteroepitaxial system [2,3], and kinetic models, which emphasize strain-driven barriers for adatoms to attach to islands [4,5] or for a new atomic layer to nucleate on facets [6]. The barriers eventually lead to the self-limited growth of the islands.…”