P.O. Hansson

Surface ripples, crosshatch pattern, and dislocation formation : Cooperating mechanisms in lattice mismatch relaxation

We study the interplay of elastic and plastic strain relaxation of SiGe/Si(001). We show that the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocations that results in ridge formation. These ridges then form a crosshatch pattern that relax strain elastically. (iv) Preferred nucleation and multiplication of dislocations in the troughs of the crosshatch pattern due to strain concentration. The preferred formation of dislocations again results in locally enhanced growth rates in the trough and thus leads to smoothing of the growth surface.

Reduced effective misfit in laterally limited structures such as epitaxial islands

Numerical finite element calculations have been reported to determine a correction function Φ that describes the reduction of the misfit that occurs when laterally limited structures such as faceted islands or mesa structures are grown on a substrate. The reduction of the average strain energy density is calculated in these three‐dimensional islands and compared to the constant strain energy density in a continuous layer. Ratios Φ are obtained from the calculation of different island geometries, i.e., different facet angles γ and different aspect ratios island width l to island height h. These discrete values are fitted by a function which can easily be applied to the full range of aspect ratios (l/h≳0) and facet angles (0°<γ<90°). Faceted Ge(Si) islands on Si(001) substrate, grown from the solution in the Stranski–Krastanov growth mode, serve as an example for the calculation. Experimental and theoretical values for the critical thickness of these islands agree well. This result demonstrates the drastic ...

Cathodoluminescence from relaxed GexSi1−x grown by heteroepitaxial lateral overgrowth

Abstract We present low temperature cathodoluminescence (CL) studies of relaxed Ge x Si 1− x , selectively grown on a patterned Si (111) substrate by liquid phase epitaxy, using heteroepitaxial lateral overgrowth. In the spectrally resolved CL of the resulting GeSi crystals, we observe an emission peak attributed to the recombination of free excitons (FEs). We further observe a peak of lower intensity at a lower energy position, caused by the unresolved phonon replicas of the FE peak. At an even lower energy position, we observe a comparatively broad luminescence band, attributed to CL from dislocations. Monochromatic images using the FE emission, or its phonon replica, reveal three types of contrast variations. On a 5 μm scale we observe dark line defects in the GeSi crystals, which we attribute to dislocations. In the lateral position of the seeding windows for the epitaxy, the GeSi crystals exhibit a dark, 20 μm wide band. On a 20 to 50 μm scale we observe small fluctuations in the Ge x Si 1− x composition, which manifests itself as local intensity variations with detection wavelength.

Dimensionality and critical sizes of GeSi on Si(100)

Abstract Pseudomorphic heteroepitaxy close to equilibrium is investigated by transmission electron microscopy for Ge x Si 1 − x , grown on Si(100) by liquid phase epitaxy. A transition from two-dimensional to island growth ( i.e. Stranski-Krastanov growth) at a thickness of h i = 1.2 nm (8 monolayers), is observed, which quantitatively validates theoretical assessments of Bauer [5] and Tersoff [8], based on assumptions of equilibrium. Results suggest that the true equilibrium phase of Stranski-Krastanov growth involves pseudomorphic islands, exclusively bound by {111} side facets. The critical island thickness h c,I = 30 nm exceeds predictions by a factor of two, supporting arguments of a kinetic barrier in the formation of the first misfit dislocation.

High quality GexSi1-x by heteroepitaxial lateral overgrowth

Abstract We present a method for non-pseudomorphic growth of high quality Ge x Si 1− x on thermally oxidized Si (111) substrates. Liquid phase epitaxy is utilized for the Ge x Si 1− x ( x = 0.90) deposition, which starts in oxide-free seeding windows and proceeds laterally over the oxide. Low temperature cathodoluminescence imaging and transmission electron microscopy do not reveal any lattice defects in the Ge x Si 1− x , laterally grown over the SiO 2 , for seeding window lengths up to 250 μm.

Early growth stages of Ge0.85Si0.15 on Si(001) from Bi solution

We investigate by atomic force microscopy the early growth stages of Ge 0.85 Si 0.15 grown by solution epitaxy on Si(001). The layers grow in the Stranski-Krastanov growth mode with facetted islands of pyramidal and, with further growth, truncated pyramidal shape. Finite element calculations of the strain fields within the islands at different growth stages yield an increasing strain energy density near the island basis edges. The effect of the increase in strain energy density is to limit the lateral growth, whereas the relaxed top regions enhance growth in height.

Solution growth of epitaxial semiconductor-on-insulator layers

Abstract We have grown defect-free semiconductor-on-insulator (SOI) layers by liquid phase epitaxy. Defect-free Si layers grow laterally over SiO 2 by starting from seeding windows or ridge seeds on selectively oxide-masked (111) Si substrates. Growth is terminated when {111} facets at the sidewalls develop. The thin SOI layers are slightly bent in relation to the substrate and adhere firmly to the oxide film. The surface of defect-free SOI layers is formed by a perfect (111) facet, whereas monoatomic steps are found at the interfacial bottom of the layers. Although the Si layers are slightly bent, layers grown from different seeds may coalesce defect-free with seams of up to 150 μm in length. In SiGe layers on patterned Si substrates misfit and threading dislocations promote vertical growth of relaxed layers; SiGe layers grown laterally over oxide-covered Si substrates contain only few dislocations.

Locally varying chemical potential and growth surface profile: a case study on solution grown Si(Ge)Si

Abstract We investigate by transmission electron microscopy and atomic force microscopy the development of the growth surface profile of Si 0.97 Ge 0.03 Si (001) grown from metallic solution near thermodynamic equilibrium. We show that sinusoidal surface undulations form to relax the mismatch strain elastically and result in a locally varying strain energy density at the growth surface. This leads to a locally varying difference in Gibbs free energy and thus to locally different growth rates. At sites where the strain energy is highest the layer dissolves. This dissolution in principle can be used to measure the local supersaturation of the solute at the growth surface.

Misfit dislocation formation and interaction in Ge(Si) on Si(001)

Abstract The misfit dislocation network that is present in the system of Ge on Si(001) after growth by liquid phase epitaxy from Bi solution is investigated by transmission electron microscopy (TEM). After a summary of the dislocation processes that occur in the growth islands during Stranski-Krastanov growth, the processes in the stage of coalescence to a continuous epitaxial layer are described.

Centrifugal techniques for solution growth of semiconductor layers

Two centrifugal techniques, I and II, have been applied to grow semiconductor layers in a rotating crucible. Technique I employs centrifugal forces to transport the solutions in the liquid phase epitaxial process. Layers of Si and SiGe grown on 100 mm diameter Si wafers and of GaAs on GaAs substrates outlined the capability and potentialities of technique I. Technique II requires higher rotational frequencies of the crucible. The higher centrifugal forces available in this technique influence the local distribution of the solute in the solvent. Locally increased solute concentration in the solution allows us to prepare multicrystalline self-supporting and inclusion-free films, or to prepare thick homo- and hetero-epitaxial LPE layers even on dissimilar substrates. We describe, in this paper, experimental results obtained by growing SiGe layers on 100 mm single-crystalline Si wafers using centrifugal technique I. We describe also the first results obtained by centrifugal technique II, and thereby focus on self-supporting multicrystalline sheets of Si, SiGe, and GaAs, and on multicrystalline Ge layers grown on sintered porous quartz substrates.