Combinatorial synthesis of ZnTe nanocrystals in SiO2 on silicon by ion implantation

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Abstract

In the field of organic chemistry combinatorial synthesis and screening of a large number of compounds is a common method for discovering and optimizing specific chemical and physical properties. Furthermore this approach is well suited when there are no reliable theories for the predictability of the chemical reaction paths and the resulting basic properties. Here we describe the application of the combinatorial synthesis technique on the synthesis of optically active ZnTe nanocrystals buried in SiO2 on a Si(1 0 0) surface. This and similar materials are of great technical interest, because they enable the combination of quantum dots of direct band gap semiconductor materials with silicon. The redistribution of the implanted constituents before and after annealing is measured by Rutherford-backscattering spectroscopy, transmission electron microscopy and shows a complex concentration distribution pattern strongly influenced by the annealing process gas.

Introduction

Research interest has focused on the synthesis, structural analysis and properties of nanocrystalline materials, due to their unique size-dependent electronic, optical and mechanical properties [1], [2], [3], [4]. These open a wide range of potential applications, which initiated the development of synthesis techniques for nanocrystals (NCs), each with its particular advantages. Most of these techniques produce nanocrystalline powders, which have to be embedded into host materials for further applications in a separate procedure. In contrast ion implantation, which is well established in semiconductor processing technology, allows direct synthesis of buried NCs. With this technique, energetic ions of the elemental constituents of the NCs are implanted into the surface-near region of an appropriate host material. The formation and growth of the NCs is controlled by an annealing procedure. There is a wide variety of NCs accessible by ion implantation, ranging from metal NCs, single component semiconductors like Si and Ge to compound semiconductors and alloys in different host materials (e.g. Si, A12O3, Si3N4, SiO2) [5], [6], [7]. In this article, the synthesis of ZnTe in 300 nm thick thermally grown SiO2 on Si(1 0 0) by ion beam implantation of the elements will be described. High dose ion beam implantation into a thin layer is a great challenge, since diffusion is influenced by the interfaces which settle the boundary conditions.

The combinatorial approach is already widely used in the field of chemical synthesis and screening of organic and inorganic compounds [8]. Here we describe the application of the combinatorial ion beam synthesis method to the formation of ZnTe NCs, which consists of the ion beam implantation step, in which the single elements are implanted sequentially with different dose and different stoichiometry by the use of a specially designed target station. After implantation the samples are annealed in a rapid thermal processor in different atmospheres. The resulting elemental concentration depth distributions are analyzed by Rutherford-backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). From the RBS-spectra information on the concentration depth distribution were extracted and compared with the corresponding TEM images.

Section snippets

Experimental

The application of the combinatorial method on ion beam synthesis of buried compound semiconductors NCs, exemplified by ZnTe in the following, consists of three steps: (i) ion beam implantation of 68Zn with doses of 1×1016, 2×1016, 4×1016, 8×1016 ions/cm−2 and an implantation energy of 130 keV; (ii) implantation of the element 130Te with doses identical to those of 68Zn, but an implantation energy of 190 keV which ensures nearly overlapping concentration profiles of Zn and Te material inside

Results and discussion

The as-implanted samples were analyzed by RBS and the spectra are shown in Fig. 1. Due to the relatively large difference between their atomic masses the peaks are well separated in the RBS spectra. The position and shape of the concentration profiles correspond well with the TRIM calculations and both elements have their concentration maximum at about 90 nm depth and a width of approximately 70 nm. Since high doses are implanted, the matrix material is diluted, leading to a corresponding dip

Conclusion

The formation of ZnTe NCs in thermally grown 300 nm thick SiO2 on Si(1 0 0) was achieved by sequential high dose ion implantation of the elements and subsequent annealing. The resulting spatial distributions of the NCs were investigated using RBS, TEM and XRD. Depending on annealing conditions concentration oscillations of Zn and Te material can be observed. TEM images confirm, that there are three bands of NCs formed in the SiO2 layer in pure Ar atmosphere annealed samples. In addition some of

Acknowledgements

We acknowledge support from the Deutsche Forschungsgemeinschaft (DFG).

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