Key role of the wetting layer in revealing the hidden path of Ge/Si(001) Stranski-Krastanow growth onset

Abstract

The commonly accepted Stranski-Krastanow model, according to which island formation occurs on top of a wetting layer ͑WL͒ of a certain thickness, predicts for the morphological evolution an increasing island aspect ratio with volume. We report on an apparent violation of this thermodynamic understanding of island growth with deposition. In order to investigate the actual onset of three-dimensional islanding and the critical WL thickness in the Ge/Si͑001͒ system, a key issue is controlling the Ge deposition with extremely high resolution ͓0.025 monolayer ͑ML͔͒. Atomic force microscopy and photoluminescence measurements on samples covering the deposition range 1.75-6.1 ML, taken along a Ge deposition gradient on 4 in. Si substrates and at different growth temperatures ͑T g ͒, surprisingly reveal that for T g Ͼ 675°C steeper multifaceted domes apparently nucleate prior to shallow ͕105͖-faceted pyramids, in a narrow commonly overlooked deposition range. The puzzling experimental findings are explained by a quantitative modeling of the total energy with deposition. We accurately matched ab initio calculations of layer and surface energies to finite-element method simulations of the elastic energy in islands, in order to compare the thermodynamic stability of different island shapes with respect to an increasing WL thickness. Close agreement between modeling and experiments is found, pointing out that the sizeable progressive lowering of the surface energy in the first few MLs of the WL reverts the common understanding of the SK growth onset. Strong similarities between islanding in SiGe and III/V systems are highlighted.

Влияние шероховатости поверхности на смену режима роста с двумерного на трехмерный в напряженных SiGe-гетероструктурах

“…Как видно из рис. 2, b, образование неограненных возвы-шенностей (mounds) и мелких островков (pre-pyramids) в исследуемых решетках Ge/Si наблюдалось и при толщине Ge < 3 МС, которая для однослойных струк-тур считается термодинамически равновесной толщиной смачивающего слоя Ge на Si(001) (см., например, [7]). Данное наблюдение согласуется с ранее полученными результатами, свидетельствующими о возможности фор-мировании островков в многослойных SiGe-структурах, толщина отдельных слоев Ge в которых < 3 МС [9].…”

“…На-личие такого барьера затрудняет зарождение островков. В результате экспериментально определяемая критиче-ская толщина двумерного роста (h c ) Ge на Si(001) (4−5 МС) оказывается больше равновесной толщины смачивающего слоя (3 МС) [7]. Очевидно, что увеличе-ние (уменьшение) микрошероховатости поверхности ро-ста облегчит (затруднит) преодоление энергетического барьера и приведет к уменьшению (увеличению) крити-ческой толщины двумерного роста напряженных слоев Si 1−x Ge x .…”

Continuum modelling of semiconductor heteroepitaxy: an applied perspective

“…[166,210]. According to Equation (16), deposition of material at composition c d on a flat substrate (at initial composition c s ) leads to a continuous variation c(…”

“…Two alternative paths are available to release the excess elastic energy: plastic relaxation via the introduction of misfit dislocations or elastic relaxation via formation of 3D islands [11][12][13][14][15]. 3D growth can be preceded by the formation of a thin wetting layer (Stranski-Krastanow growth) depending both on the deposition conditions and on the (film-thickness dependent) difference between film and substrate surface energies [9,16,17].…”

Self-assembly of InAs quantum dots on GaAs(001) by molecular beam epitaxy

“…Although, like many other issues in the formation of InAs QDs, the physical nature of the InAs WL is still poorly understood [256], the basic physical picture of WL formation in the SK growth mode often seems to be rather clear and simple in many theoretical models as well as in the interpretation of experimental observations in the literature: A planar WL grows into a critical thickness via the layer-by-layer growth mode on the substrate before 3D epitaxial islands begin to form or the 2D-to-3D morphological transition occurs, and the WL thickness should be uniquely determined by either the energetics or the thermodynamics [257][258][259][260][261][262][263][264][265][266][267]. Therefore, the main task in a theoretical investigation of the mechanism of WL formation in the SK growth mode should be to identify various contributions to the formation energy of the WL and the QDs by a technical theory such as first-principles calculations.…”