Formation of Si wells and pyramids on (1 0 0) surface as a result of Zn–Si interaction

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Abstract

Interaction between metallic zinc and Si wafer at moderately high temperatures was found to generate either concave wells or hollow pyramid-shaped protrusions on the (1 0 0) wafer surface. The formation of the wells is attributed to the dissolution of silicon at temperatures much lower than its melting point in the form of Zn–Si eutectic. When the temperature was raised, Zn atoms in the eutectic droplets evaporated and Si atoms in them precipitated accordingly. If a droplet contained plenty of Si atoms, these Si atoms could encounter each other during precipitation at the surface of the droplet and condense into a hollow pyramid-shaped protrusion.

Introduction

Study of the interactions between silicon and other substances is of vital importance for Si wafer processing. One branch of this field is the study of a variety of metal–Si eutectics, which can occur at temperatures below both the melting point of silicon and those of the constituent metals.

In the microelectronic industry, the formation of Au–Si eutectic permits low temperature welding of Au contact wires to Si devices [1]. Moreover, eutectic phenomena have also been found to play an important role in nanotechnology. For example, both the vapor–liquid–solid (VLS) and the vapor–solid–solid (VSS) growth of Si nanowires are actually triggered by the formation of metal–Si eutectics [2], [3].

This communication reports some novel structures that occurred during the interaction between metallic zinc and Si wafers at moderately elevated temperatures. As opposed to metals typically used (i.e. gold, aluminum and titanium) to form ohmic contacts on silicon, zinc forms a eutectic with silicon that is nearly pure zinc. Thus the Zn–Si interaction leads to significantly different solid-state structures than the interaction of Si with gold, aluminum or titanium. As discovered by the authors, the formation of the Zn–Si eutectic produces both concaves and convex structures on the wafer surfaces. Interestingly, the convex structures assumed a pyramid-like shape and were hollow inside.

Section snippets

Experimental section

The experiment was conducted in a horizontal tube furnace that resembled those respectively described in Refs. [4], [5]. It mainly consisted of a horizontal furnace, an alumina tube, a rotary vane vacuum pump and a gas supply system. The Si wafers used in this experiment were 10 mm×10 mm in dimensions.

A quartz boat that contained the source materials, i.e. Si wafers covered with Zn powder was pushed to the middle of the tube. After the tube cavity was evacuated, argon was introduced into the tube

Results and discussion

Fig. 1 gives the morphologies of the structures fabricated on the Si (1 0 0) surface. Their dimension ranged from ∼102 nm to ∼101 μm. The structures were either convex or concave from the surface and are named as pyramids and wells, respectively. All the edges of these pyramids and wells were along the 〈1 1 0〉 directions. They had either vertexes, as shown in Fig. 1(b) and (e), or edges, as shown in Fig. 1(c) and (f), as their highest or lowest terminals. It merits noticing that the pyramids and the

Conclusion

The formation of the Zn–Si eutectic produced {1 1 1} plane-terminated wells on the Si (1 0 0) surface at temperatures lower than the melting point of silicon. The whole process presumably fell into a number of stages that occurred consecutively with rising temperature. They were liquefaction of metallic zinc, entrance of Si atoms into the eutectic droplets, simultaneous emergence of the beneath wells and evaporation of Zn atoms. Furthermore, if the eutectic droplets were commensurate in volume with

Acknowledgements

The authors benefited greatly from the discussion with Prof. Qiyun Zhang. This work was supported by the Ministry of Science and Technology of the People's Republic of China (no. 2010CB934203) and the National Natural Science Foundation of China (no. 61076057).

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