Influence of patterning on the nucleation of Ge islands on Si and SiO2 surfaces

doi:10.1016/j.susc.2006.12.090

Surface Science

Volume 601, Issue 13, 1 July 2007, Pages 2778-2782

https://doi.org/10.1016/j.susc.2006.12.090 Get rights and content

Abstract

Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO₂ 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 and atomic force microscopy 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 self-organization 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 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[15], [16], [17], [18], [19] on a length scale where conventional lithographic techniques [20], [21], [22] are no longer applicable.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.

References (33)

N. Deng et al.
J. Cryst. Growth
(2004)
M. Goryll et al.
Mater. Sci. Eng. B
(2003)
L. Vescan et al.
Mater. Sci. Eng. B
(2000)
C. Teichert et al.
Thin Solid Film
(2000)
J.J. Metois et al.
Surf. Sci.
(1999)
G. Jin et al.
Thin Solid Film
(2000)
H. Omi et al.
Thin Solid Film
(2000)
I. Berbezier et al.
Surf. Sci.
(2003)
J. Stangl et al.
Rev. Mod. Phys.
(2004)
P.W. Li et al.
Appl. Phys. Lett.
(2006)

I. Berbezier, A. Karmous, A. Ronda, A. Sgarlata, A. Balzarotti, P. Castrucci, M. Scarselli, M. De Crescenzi, Appl....

Y. Liu et al.

Appl. Phys. Lett.

(2006)

J.L. Gray et al.

Phys. Rev. Lett.

(2004)

Zhenyang Zhong et al.

Appl. Phys. Lett.

(2003)

L. Vescan et al.

J. Appl. Phys.

(2002)

O.G. Schmidt et al.

Appl. Phys. Lett.

(2000)

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View full text

Influence of patterning on the nucleation of Ge islands on Si and SiO2 surfaces

Abstract

Introduction

Section snippets

Experimental

Patterned Si(0 0 1) substrates

Patterned SiO2 substrates

Acknowledgements

J. Cryst. Growth

Mater. Sci. Eng. B

Mater. Sci. Eng. B

Thin Solid Film

Surf. Sci.

Thin Solid Film

Thin Solid Film

Surf. Sci.

Rev. Mod. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. Lett.

Appl. Phys. Lett.

J. Appl. Phys.

Appl. Phys. Lett.

Influence of patterning on the nucleation of Ge islands on Si and SiO₂ surfaces

Patterned SiO₂ substrates