## 1 Introduction  
SiGe thin film research is largely motivated by diverse applications in various semiconductor devices [1; 2], such as solar cells, thin film transistors [3], Schottky diodes [4], temperature sensors [5], detectors and bolometers [6; 7]. However, the effect of the full-scale composition on the properties of thin SiGe films has not been investigated thoroughly; in most of the reports, high film deposition rates have been chosen as a rule instead. The majority of these publications deal with a relatively low Ge concentration [8] and only these films have provided opto-electronic properties suitable for application in device structures. Beyond the special applications, the structure and electrical properties of SiGe alloys have been investigated [9], also for a-Si\({}_{1-x}\)Ge\({}_{x}\) films over the entire range of \(0\leq x\leq 1\)[10]. The actuality of our work is given by the fact that mid-infrared (IR) photonic circuits (mid-IR PCs) have recently gained significance due to the huge number of applications projected over the 2-20 \(\upmu\)m wavelength range [6; 11]. The Si-based technologies have shown a great potential for the implementation of new mid-IR PCs, following a real trend motivated by the large supply of devices already demonstrated at telecom wavelengths [12]. The implementations of SiGe-on-Si is particularly interesting since it allows the fine control of material properties such as the bandgap or the refractive index by balancing the Ge concentration in the alloy, while extending the operation range up to at least 8 \(\upmu\)m (wavelength of Si absorption onset)--with the possibility of further extending it up to 14 \(\upmu\)m (Ge transparency cutoff wavelength) for devices with low Si concentration [12; 13]. Moreover, Ge presents higher nonlinear properties than Si, hence, preparing Ge-rich structures is a convenient choice to develop nonlinear optical devices in the mid-IR spectral range [14]. Besides chemical vapour deposition (CVD), numerous preparation methods have been used to elaborate this alloy to make a competitive material, however, physical vapor deposition including direct current (DC) magnetron sputtering method is not extensively used. Two parameters are of primary importance: the low temperature and the high rate of deposition. This technique has the advantages of simplicity, possibility of separating material components, and of relatively high deposition rates. Materials libraries are deposited mostly by space resolved combinatory using DC magnetron sputtering from two to four targets arranged in a static set-up. The targets are arranged at certain distances from each other above a large substrate or a number of distinct substrates covering the exposed area. As a result of the static arrangements, composition spread alloy films are obtained in which the gradient is not constant and the concentration range is limited; usually, does not cover 0-100% of the components [15; 16; 17]. In this study, we present the preparation of non-hydrogenated gradient SiGe films by DC magnetron sputtering over a surface length of 2 cm. This is a second stage in the development of a combinatorial method [18; 19] utilizing the \"one sample concept\" demonstrated for transmission electron microscopy (TEM) characterizations in smaller samples like TEM grids [20]. The thin film samples have been deposited with different, gradually varying compositions in the whole range and at preparation conditions identical for Si and Ge. Silicon-based SiGe thin films have been demonstrated for microelectronics, the electronic properties of which are well known. The optical properties of crystalline alloys have also been determined for certain compositions [21; 22]. Our objective is to establish the relation between the refractive index and the composition of the layers deposited by the DC magnetron technique.