Elsevier

Microelectronics Journal

Volume 40, Issues 4–5, April–May 2009, Pages 759-761
Microelectronics Journal

PECVD grown Ge nanocrystals embedded in SiO2: From disordered to templated self-organization

https://doi.org/10.1016/j.mejo.2008.11.008Get rights and content

Abstract

We present a new “templated self-organization” method for the preparation of Ge nanocrystals in SiO2 that combines a bottom-up with a top-down approach for nanostructuring. Ge nanocrystals are formed by self-organization induced by thermal annealing of thin Ge films embedded in SiO2 whose areas are predefined by nanoimprint patterning. Thus much smaller structure sizes can be achieved than by pure nanostructuring and much more regular structures can be prepared than by pure self-organization. In particular, the method enables the generation of Ge nanocrystals of equal size at predefined vertical and lateral positions thus facilitating the fabrication of nanoscaled devices due to the suppression of structural fluctuations.

Introduction

In the last decade, Ge nanocrystals (NCs) embedded in SiO2 have attracted considerable research interest due to possible applications in optoelectronic [1] and non-volatile memory devices [2]. Different types of self-organization processes have been investigated for the preparation of the Ge NCs in order to achieve high NC areal densities and small average NC diameters. The application of NCs in progressively downscaled electronic devices require a defined number of equal-sized NCs per device to avoid large fluctuations in the device characteristics and an exact alignment of the position of NCs with respect to neighboring structures as, e.g., electrical contacts. Self-organization, however, is usually accompanied by random vertical and/or lateral positions of the NCs, by local fluctuations of the NC areal density, by significant size variations of the NCs, and by a high sensitivity to process parameters. Up to now, only a few reports have addressed this important issue [3], [4].

In this work, we used self-organization during rapid thermal annealing to transform a continuous ultra-thin (2–3 nm) layer of amorphous Ge sandwiched between a bottom and a top SiO2 layer into isolated NCs [5]. Driving mechanisms of this self-organization process are homogeneous crystal nucleation and the endeavor to minimize the Ge/SiO2 interface [6]. By a combination of this bottom-up approach with the top-down approach of nanostructuring, we succeeded in achieving an exact definition of the vertical and lateral NC position and of the NC size. For nanostructuring, nanoimprint patterning [7], [8] was used since this method combines the ability to create sub 25 nm structures with the possibility of rapid patterning of large areas [9].

Section snippets

Experimental

Continuous Ge layers with a thickness of 2.3 nm were deposited by plasma enhanced chemical vapor deposition (PECVD) at 200 °C onto 4 nm SiO2 layers, thermally grown on Si (1 0 0) substrates. They were subsequently capped in situ with 12 nm SiO2 PECVD films deposited at 400 °C. Therefore, the vertical NC position was determined and any exposure of the Ge to detrimental ambient influences was avoided. Fig. 1(a) shows a cross section TEM image of the as-prepared layer stack. Annealing these unstructured

Results and discussion

The PDMS mold was equipped with arrays of circular pits predefining the Ge reservoirs. Different reservoir areas with diameters dr=50, 60, and 80 nm were investigated which were arranged in a simple cubic pattern with periods of 150,180, and 240 nm, respectively. Fig. 3 shows the successful transfer of the structures after nanoimprint patterning and removal of the excessive areas of the layer stack by RIE. The inset shows a schematic of the cross section along AA.

In order to ensure the

Conclusion

A promising new “templated self-organization” method has been presented which combines a bottom-up and a top-down approach for the preparation of equal-sized Ge NCs in SiO2 with well-defined vertical and lateral positions. In order to predefine Ge reservoirs for each NC, a layer stack consisting of an amorphous Ge layer embedded between a bottom and a top SiO2 was structured by nanoimprint patterning. This method combines high resolution and rapid patterning. By thermal annealing, each Ge

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Current address: Department of Physics, Harvard University, Cambridge, MA 02138, USA.

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