Influence of patterning on the nucleation of Ge islands on Si and SiO2 surfaces
Introduction
Quantum dots (QDs) grown on semiconductor surfaces represent an important area of focus for applications in nanotechnology. The heteroepitaxial self-assembling of 3-D islands holds the promise of an easy route to fabricate QDs for future electronic applications [1]. The production of nanomemories based on Ge islands on Si substrates requires the growth of nanocrystalline islands embedded in a well characterized SiO2 layer [2], [3]. In fact the quality of the oxide tunnel barrier affects greatly the working characteristics of the memory device such as the retention time, dot charging and stacking capability [4].
Moreover, to improve the performance of electronic devices, considerable effort has to be devoted to control the size uniformity, density and positioning of the self-assembled nanostructures [5], [6], [7], [8], [9], [10]. Different techniques have been developed to achieve long-range ordering of islands with a very narrow size distribution in the case of Ge/Si(0 0 1) systems, such as growth of stacked multilayers of heteroepitaxial islands [11], [12] or deposition of thin relaxed films of SiGe [13], [14]. Recently very interesting results have been obtained by using a combination of self-assembling QDs growth and pre-patterning of substrates [15], [16], [17], [18], [19] on a length scale where conventional lithographic techniques [20], [21], [22] are no longer applicable. Two different routes have been taken towards nanopatterning: one, artificial, based on the development of new patterning techniques with nanometer resolution, such as high resolution electron lithography [6], [7], [9], [20], [21], [22] or STM nanolithography [23], [24], [25] and the other one which takes advantage of ordered patterns occurring spontaneously [26], [27], [28] in certain conditions of temperature or substrate stress. We have recently, proposed two natural approaches to control Ge island positioning, one based on step bunching instabilities which spontaneously occur on Si(1 1 1) during direct current annealing [29] and the other one in which nanostructuring is induced by Ge deposition on misoriented Si(0 0 1) surfaces [30]. However, these methods are still unreliable for the applications, since either uniformity or sizes of the islands do not match the high standards required in nanoelectronics. Also, in nanolithography the shape and size of the single hole play an important role to stabilize the quantum dots that are nucleating in the nearby region or at its edge [21]. For this reason a detailed study of island nucleation and growth on substrates with a dense patterning is required.
Section snippets
Experimental
In the present paper, we report new results on growth and arrangement of Ge islands on Si surfaces patterned by using focused ion beam (FIB) [31], [32] at different densities. Two kinds of patterned surfaces were studied: a bare Si(0 0 1) surface and a 5 nm thick SiO2 layer grown on Si(0 0 1) substrate. In the first case, we followed, in real time, the growth of Ge nanostructures using a scanning tunnelling microscope (STM). We established the influence of patterning on Ge growth i.e. the
Patterned Si(0 0 1) substrates
Two different arrays of pits with depth of 30 nm, diameter of 150 nm and a periodicity of 780 ± 30 nm and 500 ± 30 nm, respectively, have been produced on the Si(0 0 1) bare substrates. On these surfaces, we have followed in real-time the Ge growth at a temperature of 873 K. As displayed in Fig. 1 (2.5 ML), the nucleation starts nearby a hole and develops into a Ge island covering the entire pit. This occurs in most cases, producing a nicely ordered pattern at 8 ML coverage. In Fig. 2, Si(0 0 1) surfaces with
Patterned SiO2 substrates
To study the patterning effect on Ge growth on SiO2, dense holes arrays (4 × 1010 holes cm−2) were produced by FIB on oxidized Si. In this case, a double procedure of oxidation and chemical etching has been undertaken, in order to develop a 5 nm clean SiO2 layer presenting a patterned surface.
A first experiment consists in depositing 5.2 ML of Ge at 873 K. The surface (Fig. 5A) displays randomly nucleated islands on a patterned SiO2 surface. Some islands seem nucleate inside the holes, other nearby.
Acknowledgements
This work was supported by the European Community (EC) through FORUM-FIB Contract (IST-2000-29573) at Roma Tor Vergata University.
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