XRD and RBS studies of quasi-amorphous zinc oxide layers produced by Atomic Layer Deposition

Highlights

• Thin ZnO films of low crystallographic quality were obtained by Atomic Layer Deposition at 60 °C.

• Nanopolycrystalline structure and atomically flat surface has been measured by X-ray diffraction.

• Stopping power measurements show a very good agreement with the calculated values.

Abstract

Although zinc oxide has been widely investigated for many important applications such as laser diodes, photovoltaics, and sensors, some basic properties of this material have not been established up to now. One of these are stopping power values which are crucial for the Rutherford Backscattering Spectrometry analysis. For this kind of measurements, amorphous materials should be used.

In this paper we show the results of stopping power measurements for ZnO films grown by Atomic Layer Deposition. The films were grown on a silicon (100) substrate and parameters of the growth were chosen in a way that prevents crystallization of ZnO films. A series of ZnO films with thickness between 20 and 160 nm have been investigated. Extended film characterization has proven that the obtained nanopolycrystalline ZnO films can be considered as truly amorphous with respect to ion beam applications.

ZnO films have been used for precise stopping power measurement of MeV He-ions in the energy range from 200 to 5000 keV. These results provide indispensable data for ion beam modification and analysis of ZnO.

Introduction

Zinc oxide has been widely investigated recently because of its attractive properties such as high band gap and related transparency, high exciton binding energy, a low thermal expansion coefficient and it is regarded as a promising candidate for many important applications such as laser diodes, photovoltaics, and sensors. Biological safety, abundant presence in the Earth's crust and possibility to grow ZnO films by relatively simple and inexpensive techniques contribute to the booming scientific interest in this material, which is observed in the last few years.

Rutherford Backscattering Spectrometry (RBS) is one of the frequently used technique for thin film analysis [1]. It allows depth profiling of a surface layer yielding its thickness and composition. If performed in the channeling conditions it can also provide structural defect depth profile. However, in order to ensure the depth sensitivity of the method the energy loss of analyzing ions has to be calculated. Stopping power of light ions can be measured either in transmission experiments using self-supporting thin foils or in backscattering geometry using thin layers deposited on a suitable substrate [2], [3]. Because of great technological problems involved with fabrication of self-supporting thin foils transmission method is almost exclusively used for stopping power measurements at high energies. Here we present the results of precise stopping power measurements for He ions of incident energy ranging from 200 to 5000 keV performed in the backscattering geometry using the method developed by Lantschner et al. [4]. To avoid channeling effects thin amorphous layers of this material should be used for such measurements.

In spite of the extensive investigation on this material appearing every year measurements of the stopping power of light ions in ZnO have not been performed yet. The only available data are based on the semiempirical approach of Ziegler et al. [5]. The updated data basing on such calculations can be found in the newest version of the SRIM code [6].