Formation of Ge islands from a Ge layer on Si substrate during post-growth annealing (original) (raw)
Related papers
Applied Physics Letters, 1997
In this letter, we present an atomic-force-microscopy investigation of the Stranski-Krastanov growth of Ge on Si͑100͒. Upon increasing the base width of the islands, two morphology transitions are found. The first transition occurs at a base width of ϳ50-60 nm and marks the evolution from few-monolayer-thick terraces to square-base pyramidal islands. In the second transition, which takes place when the base width exceeds ϳ300 nm, the island shape changes from square base pyramids to tetragonal truncated pyramids. Both transitions are brought about by the need for the system to minimize the elastic energy. © 1997 American Institute of Physics. ͓S0003-6951͑97͒02104-9͔
Growth and characterization of Ge nanostructures selectively grown on patterned Si
Thin Solid Films, 2008
By utilizing different distribution of strain fields around the edges of oxide, which are dominated by a series of sizes of oxide-patterned windows, long-range ordered self-assembly Ge nanostructures, such as nano-rings, nano-disks and nano-dots, were selectively grown by ultra high vacuum chemical vapor deposition (UHV-CVD) on Si (001) substrates. High-resolution double-crystal symmetrical ω/2θ scans and two-dimensional reciprocal space mapping (2D-RSM) technologies employing the triple axis X-ray diffractometry have been used to evaluate the quality and strain status of as-deposited as well as in-situ annealed Ge nanostructures. Furthermore, we also compare the quality and strain status of Ge epilayers grown on planar unpatterned Si substrates. It was found that the quality of all Ge epitaxial structures is improved after in-situ annealing process and the quality of Ge nano-disk structures is better than that of Ge epilayers on planar unpatterned Si substrates, because oxide sidewalls are effective dislocation sinks. We also noted that the degree of relaxation for as-deposited Ge epilayers on planar unpatterned Si substrates is less than that for as-deposited Ge nano-disk structures. After in-situ annealing process, all Ge epitaxial structures are almost at full relaxation whatever Ge epitaxial structures grew on patterned or unpatterned Si substrates.
Influence of patterning on the nucleation of Ge islands on Si and SiO2 surfaces
Surface Science, 2007
Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO 2 surfaces were patterned by a nanolithographic process using a Focused Ion Beam (FIB). Ge was evaporated in Ultra High Vacuum at 873 K on these substrates, resulting in the formation of island arrays. Based on scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images, a statistical analysis was performed in order to highlight the effect of patterning on the size distribution of islands compared to a non patterned surface. We find that the selforganization mechanism on patterned substrates results in a very good arrangement and positioning of Ge nanostructures, depending on growth conditions and holes distance, both on Si and SiO 2 surfaces.
Growth of Ge islands on prepatterned Si (001) substrates
Physica E: Low-dimensional Systems and Nanostructures, 2004
We report on the growth and properties of Ge islands grown on (0 0 1) Si substrates with lithographically defined two-dimensionally periodic pits. After thermal desorption and a subsequent Si buffer layer growth these pits have an inverted truncated pyramid shape. We observe that on such prepatterned substrates lens-like Ge-rich islands grow at the pit bottoms with less Ge deposition than necessary for island formation on flat substrates. This is attributed to the aggregation of Ge at the bottom of the pits, due to Ge migration from the pit sidewalls. At the later stages of growth, dome-like islands with dominant f1; 1; 3g or f15; 3; 23g; or other high-index facets [i.e. f15; 3; 20g facets] are formed on the patterned substrates as shown by surface orientation maps using atomic force microscopy. Furthermore, larger coherent islands can be grown on patterned substrates as compared to Ge deposition on flat ones. r . Cross-sectional TEM image of 10 ML Ge deposition sample P 10 :
Evolution of Ge nanoislands on Si(110)-'16 × 2' surface under thermal annealing studied using STM
Nanotechnology, 2009
The initial nucleation of Ge nanoclusters on Si(110) at room temperature (RT), annealing-induced surface roughening and the evolution of three-dimensional Ge nanoislands have been investigated using scanning tunneling microscopy (STM). A few monolayers (ML) of Ge deposited at room temperature lead to the formation of Ge clusters which are homogeneously distributed across the surface. The stripe-like patterns, characteristic of the Si(110)-'16 × 2' surface reconstruction are also retained. Increasing annealing temperatures, however, lead to significant surface diffusion and thus, disruption of the underlying '16 × 2' reconstruction. The annealing-induced removal of the stripe structures (originated from '16 × 2' reconstruction) starts at approximately 300 °C, whereas the terrace structures of Si(110) are thermally stable up to 500 °C. At approximately 650 °C, shallow Ge islands of pyramidal shape with (15,17,1) side facets start to form. Annealing at even higher temperatures enhances Ge island formation. Our findings are explained in terms of partial dewetting of the metastable Ge wetting layer (WL) (formed at room temperature) as well as partial relaxation of lattice strain through three-dimensional (3D) island growth.
Alloying in Ge(Si)∕Si(001) self-assembled islands during their growth and capping: XPS and AFM study
Physical Review B, 2008
In this paper, we present a study on the Ge composition and shape evolution of self-assembled Ge/ Si͑001͒ islands during the island growth and the subsequent Si capping at 750°C. By combining atomic force microscope images and x-ray photoemission spectroscopy data, we quantitatively determine the Ge distribution in the wetting layer and in the islands, separately. We found that in as-grown sample, the wetting layer is substantially Si-richer than the islands, its average composition being independent of the growth rate. Upon capping, the islands proceed to a reverse Stranski-Krastanov shape evolution, with a progressive Si enrichment of both the wetting layer and the islands. We demonstrate that this evolution occurs at constant island volume. The observed behavior indicates the suppression of the lateral diffusion of both Ge and Si atoms from the wetting layer to the surface of the enlarging islands, and vice versa.
Nanotechnology
The effect of Ge deposition rate on the morphology and structural properties of self-assembled Ge/Si(001) islands was studied. Ge/Si(001) layers were grown by solid-source molecular-beam epitaxy at 500 °C. We adjusted the Ge coverage, 6 monolayers (ML), and varied the Ge growth rate by a factor of 100, R = 0.02-2 ML s(-1), to produce films consisting of hut-shaped Ge islands. The samples were characterized by scanning tunnelling microscopy, Raman spectroscopy, and Rutherford backscattering measurements. The mean lateral size of Ge nanoclusters decreases from 14.1 nm at R = 0.02 ML s(-1) to 9.8 nm at R = 2 ML s(-1). The normalized width of the size distribution shows non-monotonic behaviour as a function of R and has a minimum value of 19% at R = 2 ML s(-1). Ge nanoclusters fabricated at the highest deposition rate demonstrate the best structural quality and the highest Ge content (∼0.9).
2011
Growth of narrow-neck, epitaxial as well as non-epitaxial and nearly spherical Ge islands on air-exposed Si(111)-(7\times7) surfaces has been investigated by in-situ scanning tunnelling microscopy (STM) and ex-situ high resolution cross-sectional transmission electron microscopy (HRXTEM). A thin oxide is formed on Si(111)-(7\times7) surfaces via air exposure. Ge islands are grown on this oxide. STM measurements reveal the growth of very small (~2 nm diameter) Ge islands with a high number density of about 1.8\times10^12 cm-2. The island size has been found to depend on the amount of deposited Ge as well as the substrate temperature during Ge deposition. HRXTEM micrographs reveal that the islands are nearly spherical in shape, making narrow-neck contact with the substrate surface. At 520{\deg}C growth temperature both epitaxial and non-epitaxial islands grow. However, at 550{\deg}C, Ge islands predominantly grow epitaxially by a narrow-contact with Si via voids in the oxide layer. Growth of vertically elongated Ge islands is also observed in HRXTEM measurements with a very small diameter-to-height aspect ratio (~0.5-1), a hitherto unreported feature of epitaxial Ge growth on Si surfaces. In addition, stacking fault and faceting are observed in islands as small as 5 nm diameter. Ge islands, not even in contact with the Si substrate, appear to be in epitaxial alignment with the Si substrate. The island size distribution is essentially monomodal. As the contact area of Ge islands with Si through the voids in the oxide layer can be controlled via growth temperature, the results indicate that tunability of the potential barrier at the interface and consequently the tunability of electronic levels and optical properties can be achieved by the control of growth temperature.