Kinetic evolution and equilibrium morphology of strained islands

Self-assembled SiGe islands grown on Si(001) leave behind characteristic "footprints" that reveal that small islands shrink, losing material to nearby larger islands. The critical size, dividing shrinking from growing islands, corresponds to the pyramid-to-dome shape transition, consistent with "anomalous coarsening" While shrinking, {105}-faceted pyramids transform into truncated pyramids and ultimately into unfaceted mounds. The similarity to behavior during island growth indicates that island shape and facet formation are thermodynamically determined.     

Collective shape oscillations of SiGe islands on pit-patterned Si(001) substrates: a coherent-growth strategy enabled by self-regulated intermixing

The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower "dome" and steeper "barn" morphologies. Correspondingly, the average Ge content in the alloyed islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the islands to keep growing in size while avoiding plastic relaxation.     

Dendrochronology of strain-relaxed islands

We report on the observation and study of tree-ring structures below dislocated SiGe islands (superdomes) grown on Si(001) substrates. Analogous to the study of tree rings (dendrochronology), these footprints enable us to gain unambiguous information on the growth and evolution of superdomes and their neighboring islands. The temperature dependence of the critical volume for dislocation introduction is measured and related to the composition of the islands. We show clearly that island coalescence is the dominant pathway towards dislocation nucleation at low temperatures, while at higher temperatures anomalous coarsening is effective and leads to the formation of a depletion region around superdomes.     

Pathway for the strain-driven two-dimensional to three-dimensional transition during growth of Ge on Si(001)

The two-dimensional (2D) to three-dimensional (3D) morphological transition in strained Ge layers grown on Si(001) is investigated using scanning tunneling microscopy. The initial step takes place via the formation of 2D islands which evolve into small ( approximately 180 A) 3D islands with a height to base diameter ratio of approximately 0.04, much smaller than the 0.1 aspect ratio of 105-faceted pyramids which had previously been assumed to be the initial 3D islands. The "prepyramid" Ge islands have rounded bases with steps oriented along <110> and exist only over a narrow range of Ge coverages, 3.5-3.9 monolayers.     

Minute SiGe quantum dots on Si(001) by a kinetic 3D island mode

We investigated the initial growth stages of Si(x)Ge(1-x)/Si(001) by real time stress measurements and in situ scanning tunneling microscopy at deposition temperatures, where intermixing effects are still minute (< or =900 K). Whereas Ge/Si(001) is a well known Stranski-Krastanow system, the growth of SiGe alloy films switches to a 3D island mode at Si content above 20%. The obtained islands are small (a few nanometers), are uniform in shape, and exhibit a narrow size distribution, making them promising candidates for future quantum dot devices.     

Direct determination of strain and composition profiles in SiGe islands by anomalous x-Ray diffraction at high momentum transfer

Anomalous x-ray scattering is employed for quantitative measurements of the Ge composition profile in islands on Si(001). The anomalous effect in SiGe is enhanced exploiting the dependence of the complex atomic form factors on the momentum transfer. Comparing the intensity ratios for x-ray energies below and close to the K edge of Ge at various Bragg reflections in the grazing incidence diffraction setup, the sensitivity for the Ge profile is considerably enhanced. The method is demonstrated for SiGe dome-shaped islands grown on Si(001). It is found that the composition inside the island changes rather abruptly, whereas the lattice parameter relaxes continuously.     

Assessing the composition of hetero-epitaxial islands via morphological analysis: an analytical model matching GeSi/Si(001) data

A simple, but still three-dimensional, model describing the morphological stability of realistic SiGe islands on Si(001) is presented. The experimental evolution toward steeper islands with volume can be predicted for any average composition. Despite the use of elastic theory for stress relaxation under the assumption of a uniform SiGe distribution, and of a common mean surface energy of the faceted islands, the model seems to capture the essence of the energetic balance determining the morphological evolution with volume, with no fitting parameters. This is suggested by close comparison with molecular beam epitaxy data at three different temperatures (i.e.?compositions). The good agreement also allows for interpreting the minor scattering of experimental data with temperature and provides a reliable tool for extracting the average Ge content from standard atomic force microscopy analysis.     

Critical shape and size for dislocation nucleation in Si1-xGex islands on Si(001)

The critical volume for the onset of plastic strain relaxation in SiGe islands on Si(001) is computed for different Ge contents and realistic shapes by using a three-dimensional model, with position-dependent dislocation energy. It turns out that the critical bases for dome- and barnlike islands are different for any composition. By comparison to extensive atomic force microscopy measurements of the footprints left on the Si substrates by islands grown at different temperatures (and compositions), we conclude that, in contrast with planar films, dislocation nucleation in 3D islands is fully thermodynamic.     

Selective vapor phase etching of SiGe by HCl in a RPCVD reactor

Chemical vapor phase etching of epitaxial SiGe by HCl was investigated using a single wafer reduced pressure CVD (RPCVD) system. For the sample preparation, patterning and dry etching were performed on the Si substrate with SiGe buried layers to open the sidewalls of the buried SiGe layers. The etchrate of the lateral direction was measured. The etchrate of SiGe is increasing with increasing SiGe thickness saturating at SiGe thicknesses higher than ∼25 nm. This result could be caused by diffusion effects of molecules in the narrow trenches during the etching. At the same SiGe thickness, the etchrate of SiGe is increasing with increasing etching temperature. B doping does not affect the etchrate of SiGe. P doping is increasing and C doping is decreasing the etchrate. Facet formation of the etchfront of Si and SiGe depends on initial surface orientation. These results enable improved process controllability of the etching process using doped layers for different applications.     

X-ray diffraction study of the composition and strain fields in buried SiGe islands

We report on studies of strain and composition of two-dimensionally ordered SiGe islands grown by molecular beam epitaxy using high resolution x-ray diffraction. To ensure a small size distribution of the islands, pit-patterned (001) Si wafers were used as substrates. The Si wafers were patterned by optical lithography and reactive ion etching. The pits for island growth are ordered in regular 2D arrays with periods ranging from 500 to 1000 nm along two orthogonal 〈110〉 directions. After the growth of a Si buffer layer, 5 to 9 monolayers of Ge are deposited, leading to the formation of islands with either dome- or barn shape, depending on the number of monolayers deposited. The Si capping of the islands is performed at low temperatures (300^○C) to avoid intermixing and thus strain relaxation. Information on the surface morphology obtained by atomic force microscopy (AFM) was used to set up models for three-dimensional Finite Element Method (FEM) simulations of the islands including the patterned Si substrate. In the model, special attention was given to the non uniform distribution of the Ge content within the islands. The FEM results served as an input for calculating the diffracted x-ray intensities using kinematical scattering theory. Reciprocal space maps around the vicinity of symmetric (004) and asymmetric (113) and (224) Bragg peaks were recorded in coplanar geometry. Simulating different germanium gradients leads to altered scattered intensity distribution and consequently information on this quantity is obtained for both dome- and barn-shaped islands as well as on the strain fields.     

Highly-ordered SiGe-islands grown by dislocation patterning using ion-assisted MBE

Fabrication of device structures based on laterally self-ordered systems without the use of expensive and time-consuming nanolithography could have undoubted advantages. For such applications, it is proposed to use misfit dislocation networks from partially relaxed SiGe layers on (1 0 0) Si substrate as a template for the growth of highly ordered SiGe islands. Ion bombardment during molecular beam epitaxy of metastable SiGe layers leads to such a partial relaxation by misfit dislocation networks. The ions are generated by the interaction of the evaporated Si flux with the electrons in an electron beam evaporator, which causes a partial ionization of Si atoms in the molecular beam. We demonstrate by atomic force microscopy that subsequent growth of SiGe on such relaxed SiGe (25-50% Ge) layers leads to the formation of uniform three-dimensional islands highly ordered in 〈1 1 0〉 directions.     

Surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands

Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface the diffusivity of Ge is 10(2)-10(3) times higher than that of Si in the temperature range of 300 to 900 K, while on a tensile surface, the two diffusivities are comparable. Consequently, the growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.     

Influence of the Si cap layer on the SiGe islands morphology

Transmission electron microscopy (TEM), atomic force microscopy (AFM), and EDX methods were used to study morphology and chemical composition of SiGe/Si(001) islands grown at 700 degrees C and covered at 550 degrees C and 700 degrees C by Si layers of different thickness. The samples were grown in ultra high vacuum chemical vapor deposition process (UHVCVD) controlled with in situ reflection of high-energy electron diffraction (RHEED). The islands transformed from initial pyramid and dome shapes to lens shape for 1.4 nm and 3.7 nm cap layer thickness at 550 degrees C and 700 degrees C, respectively. An increase of lateral to vertical ratio was observed during the transformation. For the higher depositing temperature the ratio was bigger and was increasing continuously with cap layer thickness. Also, with increasing Si cap layer thickness, the Ge concentration was decreasing, which was more observable for higher capping temperature.     

Evolution of stress and strain relaxation of Ge and SiGe alloy films on Si(0 0 1)

The growth of Ge and SiGe alloy films on Si substrates has attracted considerable interest in the last years because of their importance for optoelectronic devices as well as Si-based high speed transistors. Here we give a short overview on our recent real time stress measurements of Ge and SiGe alloy films on Si(0 0 1) performed with a sensitive cantilever beam technique and accompanied by structural investigations with atomic force microscopy. Characteristic features in the stress curves provide detailed insight into the development and relief of the misfit strain. For the Stranski–Krastanow system Ge/Si(0 0 1) as well as for SiGe films with Si contents below 20%, the strain relaxation proceeds mainly into two steps: (i) by the formation of 3D islands on top of the Ge wetting layer; (ii) via misfit dislocations in larger 3D islands and upon their percolation.     

Formation mechanism of one-dimensional Si islands on a H/Si(001) surface

The formation mechanism of one-dimensional Si islands on a H/Si(001)-(2x1) surface is studied using scanning tunneling microscopy/spectroscopy and first-principles calculations. We observed that one-dimensional islands that are made from dimer chains are formed at the initial growth stages similar to the bare Si(001) surface. It is found that the number of odd-numbered dimer chains is larger than that of even-numbered dimer chains. We propose the growth processes of the two types of growth edges to explain the observation.     

Ion-beam-assisted MBE growth of semi-spherical SiGe/Si nano-structures

Semi-spherical SiGe/Si nano-structures of a new type are presented. Epitaxial islands of 30–40 nm in base diameter and 11 nm in height and having a density of about 6×10¹⁰ cm^-2 were produced on (001) Si by molecular beam epitaxial growth of Si/Si_0.5Ge_0.5 layers with in situ implantation of 1-keV As⁺ ions. It was found by cross-section transmission electron microscopy that the islands have a complicated inner structure and consist of a micro-twin nucleus and semi-spherical nano-layers of various SiGe compositions. The nature of the surface patterning is interpreted by stress relaxation through implantation-induced defects. Received: 12 July 2001 / Accepted: 4 September 2001 / Published online: 2 October 2001     

Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si_0.8Ge_0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si_0.8Ge_0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10⁵ cm⁻² and a strain relaxation of 89%.     

Photoluminescence of Ge nanostructures grown by gas source molecular beam epitaxy on silicon (1 1 8) surface

In this paper, we present a photoluminescence (PL) study of Si/Ge/SiGe/Si structures grown by gas source molecular beam epitaxy on an (1 1 8) undulated surface with various Ge coverage. Nucleation and growth of Ge films is obtained by the Stranski–Krastanov mechanism. The influence of the substrate orientation on the changeover 2D–3D growth mode is investigated. Furthermore, we show the use of growing an SiGe wetting layer to control the uniformity of the Ge island size. The PL signal related to the Ge islands is found to be highly dependent of the power excitation and is observed up to room temperature.     

Lateral motion of SiGe islands driven by surface-mediated alloying

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.     

Effect of a Low-Temperature Si Underlayer on the Photoluminescence of Misfit Dislocations in Si(001)Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heterostructures with Ge-Rich Alloy Layers

The effect a low-temperature Si underlayer has on the photoluminescence of misfit dislocations in Si(001)Si_{1 – x}Ge _x heterostructures with high Ge contents in a SiGe alloy layer is investigated. It is found that the introduction of this layer prevents the formation of dislocations in the alloy layer.