Growth of nanoscale Ge magic islands on Si(111)-7 × 7 substrate

doi:10.1016/S0039-6028(03)00753-2

Surface Science

Volume 539, Issues 1–3, 1 August 2003, Pages 113-119

https://doi.org/10.1016/S0039-6028(03)00753-2 Get rights and content

Abstract

We reported a formation of uniform nanoscale Ge islands, which shows rounded shape having a diameter of 3.8 nm, on a Si(1 1 1)-7 × 7 substrate as a template. The rearrangement process of stacking-fault layer in the 7 × 7 reconstructed structure is a rate determinant process, when the growing layer covers the substrate. The lateral growth of Ge island is hindered by the stacking-fault layer and the hindrance restricts the size distribution of Ge islands, so called “magic islands”. We observed the nanoscale Ge magic islands formed at the temperature of 300–360 °C. The some characteristic peaks in the size distribution suggest that the step energy for the Ge islands is smaller than that of Si island. From the temperature dependence of the size distribution, we estimate that the amount of the activation energy of the rearrangement is 2.1 eV. The value is slightly smaller than that for the homo-epitaxial case.

Introduction

In order to construct a building block of nano-structure, we tried to form the uniform nanoscale islands of Ge on the Si(1 1 1)-7 × 7 substrate as a template under various growth conditions (substrate temperature and annealing temperature). And we succeeded in the formation of uniform rounded Ge islands (R-island) with a diameter of 3.8 nm, when the Ge is deposited at 320 °C and annealed at 360 °C [1]. The R-island is composed of 162 atoms and very stable.

The idea of adoption of the Si(1 1 1)-7 × 7 substrate as a template is based on the results in the homo-epitaxial growth on the reconstructed surface [2], [3], [4], [5], [6], [7], [8]. The reconstructed structure on the substrate surface has a great influence on the nucleation and growth of growing layer on it. When the surface structure is very stable such as the 7 × 7 structure composed of dimer, adatom and stacking-fault layer (DAS) on the Si(1 1 1) surface [9], the surface structure hinders the lateral growth of epitaxial layer, because the stable surface structure needs a rearrangement to the normal bulk structure when the surface is covered with the epitaxial layer. This hindrance restricts the size of the two-dimensional islands, because the growth unit corresponds to the unit of the 7 × 7 structure at the initial growth stage of Si on the Si(1 1 1) [5], [6], [7], [8]. And the hindrance also influence on the size distribution of Si island after some relaxation processes [3], [4], [5], [6]. Especially, many nanoscale uniform Si islands showing round shape with a 3.8 nm diameter are self-assembled after annealing at 380 °C. Such rounded island has also been observed in the study of epitaxial growth by chemical vapor deposition [10].

Although we have succeeded in the formation of uniform Ge islands showing a rounded shape with a diameter of 3.8 nm [1], we have to obtain more detailed information about the growth of Ge islands to understand the formation mechanism of the nanoscale islands. In this paper, we will show some STM images observed from Ge films grown at various substrate temperatures (T_s) and annealed at same temperature (T_a=T_s) for 10 min, where some “magic islands” are formed. We will discuss the activation energy (E_a) of the rearrangement of the stacking-fault layer in DAS structure, which is a key factor in the formation of the uniform Ge rounded islands, from the size distribution of the Ge islands.

Section snippets

Experimental

The surface of a Si(1 1 1) substrate was heated up to about 1200 °C for a few hours by direct electron current flow in an ultra-high vacuum (UHV) chamber, which leads to a complete degassing of the sample and sample holder. After the treatment, the pressure can be kept 2 × 10⁻⁸ Pa or less, when the sample is heated at 1200 °C for a few minutes. We measured the T_s with an infrared radiation thermometer (T_s<800 °C) and an optical pyrometer (T_s>800 °C).

Ge films with thickness about 0.04 bilayer (BL: 1

The size distribution of Ge islands

Fig. 1 shows STM images (60 × 60 nm²) from Ge films deposited at (a) T_s=300 °C, (b) T_s=320 °C, (c) T_s=340 °C and (d) T_s=360 °C. The faulted half (FH) and the unfaulted half (UH) of the 7 × 7 structure on the substrate are indicated by a solid and an open triangle in the lower left corner of Fig. 1(b). At T_s=300 °C, some small triangular (ST) islands, which are surrounded by the FH of the substrate, are formed as indicated by ST. The ST-island covers only one FH and three UH under it. This island

Summary

We observed the nano-islands growth of Ge on the Si(1 1 1)-7 × 7 substrate, and we found out two characteristic features for the Ge islands by comparison with the growth of Si islands on the same surfaces: (1) The population of the star-like islands for the Ge is larger than that for the Si; and (2) the contour of the Ge nano-islands shows irregular line (R^′-islands) and the effective length of edge becomes long comparing to the R-islands in the Si cases. From these results, we conclude that the ε_s

Acknowledgements

This work was partly supported by “Takahashi Industrial and Economic Research Foundation” and “The Grants in Support of the Promotion of Research at Yokohama City University”.

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