Strain relaxation by alloying effects in Ge islands grown on Si(001)

Abstract

Transmission electron microscopy is used to study the morphology and the composition profile of ''pure'' Ge islands grown at high temperature on Si͑001͒ by molecular beam epitaxy. An alloying process, involving mass transport from the substrate to the islands during the island growth, was identified. It was found that, as a result of Si mass transport to the Ge islands, the island/substrate interface moves towards the substrate, and trenches form on the substrate surface around the islands. Reduction of the misfit strain at the island/substrate interface is the driving force for this process. ͓S0163-1829͑99͒00748-1͔Semiconductor quantum dots ͑QD's͒ are attracting increasing interest because of their significant optoelectronic properties. 1 Although QD's can be produced in many ways, the method of coherent island formation is of great importance for materials with large lattice mismatch ͑such as Ge͑Si͒/Si 2,3 and In x Ga 1Ϫx As/GaAs 4 because of the possible combination of the QD growth and semiconductor integration techniques. Of crucial importance in determining the optoelectronic properties of the QD's are the structural parameters of the QD's including the shape, size and composition. 5 Uniformity in and control over these parameters are a prerequisite for many applications. To achieve this goal, a complete understanding of the mechanism of the QD growth is necessary. Although many investigations have concentrated on the shape and size 6,7 and evolution 8,9,10 during the QD growth, relatively less attention has been paid to the composition. 11,12,13 In the classical Stranski-Krastanow 14 ͑S-K͒ mode of coherent island formation, one material with a different lattice parameter and low interfacial energy is initially deposited on a substrate surface, layer by layer, forming a ''wetting layer''. When the wetting layer reaches a critical thickness ͓usually three to five monolayers for pure Ge on Si͑001͒, 3,15 island growth starts to partially release the mismatch strain energy between the epitaxial layer and the substrate. However, in the case of InAs/GaAs, recent investigations suggest that, for higher temperature growths, there is significant mass transport from both the wetting layer and the substrate to the islands. 16 Furthermore, the wetting layer in InAs/GaAs systems has been reported to be an ͑In,Ga͒As alloy with temperature dependent composition. 17 The composition of the alloy wetting layer will certainly affect any subsequent island growth procedure. These points suggest that the classical S-K growth mechanism needs to be modified to explain the details of island growth. In this paper, we demonstrate evidence for mass transport from the Si substrate to the Ge islands during high temperature molecular beam epitaxy ͑MBE͒ growth of ''pure'' Ge islands on the Si͑001͒ substrate. The mass transport changes the composition of the islands, moves the Ge/Si interface below the original substrate surface, and forms a trench around each island. It is proposed that the driving force for this mechanism is a reduction...

Composition and its impact on shape evolution in dislocated Ge(Si)/Si islands

“…The limiting base diameters for domes grown at 550 and 600°C are 50 and 80 nm, respectively, ͑measured from ͗110͒͘. In our previous investigation, 12 we showed that islands deposited at 700°C and with a deposition thickness of 0.8 nm are dome shaped and have an approximately uniform size distribution with limiting base diameter of about 100 nm. These results lead to the conclusion that the limiting size increases with increasing deposition temperature.…”

“…In our earlier work, 12 we reported that Ge͑Si͒/Si͑001͒ island growth at high temperatures is accompanied by an alloying process which results in the formation of a trench around each island and the island/substrate interface moving down to the substrate side. Based on our experimental observations, a modified S-K island growth mode for Ge͑Si͒/ Si͑001͒ at high temperatures was proposed.…”

“…The same conclusion is given by comparing the experimental results of Kamins et al 16 and our previous results. 12 Using atomic force microscopy and TEM, Kamins et al studied the growth process of Ge/Si͑001͒ at 550 and 600°C. They found that, with the increase of the deposition thickness, most of the islands transform to dome shaped with ͕311͖ facets and approach a limiting size.…”

Equilibrium Distributions and the Nanostructure Diagram for Epitaxial Quantum Dots

“…= n ν ν µ (27) where S ν is the contribution to the entropy coming from the ν-atom islands (and we have suppressed a term in the entropy of the form log ν n ν ! that is very weakly dependent on n ν ).…”

Исследование пространственного распределения фототока в плоскости Si-p-n-фотодиода с наноостровками GeSi методом сканирующей ближнепольной оптической микроскопии

“…Было установлено, что наноостровки состоят из твердого раствора Ge x Si 1−x , несмотря на то что на поверхность подложки осаждался чистый Ge. Наличие Si в материале наноостровков связано с диффузией Si из подложки в объем островков в процессе роста, что понижает общую упругую энергию системы " островки−подложка" [13]. Атомная доля Ge x в материале островков, выращенных при T g ≈ 700…”