Optical and structural characterization of inhomogeneities in a-Si:H TO mc-Si transition L. Pru  sáková a, * , V. Vavru  nková a , M. Netrvalová a , J. Müllerová b , P.  Sutta a a University of West Bohemia, New Technology Research Centre, Univerzitni 8, 306 14 Plzen, Czech Republic b Department of Engineering Fundamentals, Faculty of Electrical Engineering, University of  Zilina, 031 01 Liptovský Mikulá s, Slovakia Keywords: A-Si:H film Phase transition PE-CVD X-ray diffraction Raman spectroscopy abstract The a-Si:H films with different thickness and microstructure have been deposited with rf-PECVD using a plasma of silane diluted with hydrogen. The structure and optical analysis were carried out by X-ray diffraction, UVeVIS and Raman spectroscopy. Spectral refractive indices, optical energy band gaps, extinction coefficients, phases ratio and grain size were determined as a function of the hydrogen dilution (R ¼ H 2 /SiH 4 ). Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to micro-crystalline hydrogenated silicon (mc-Si:H). XRD analysis indicated that films with R ¼ 0 and R ¼ 20 were amorphous and homogeneous, while films with R ¼ 40 and higher were micro-crystalline consisting medium range ordered silicon hydride (Si 4 H) and mc-Si phases with different size of crystallites, which was confirmed also by Raman spectroscopy. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Amorphous silicon (a-Si:H) and micro-crystalline silicon (mc-Si:H) thin films have been the subject of interest of many research groups for more than 30 years. Many studies demonstrate the important role of hydrogen in the formation of the a-Si:H and mc-Si:H network. It is known that hydrogen assists in improving medium range order and crystallisation [1]. Understanding the influence of deposition parameters on film growth may clarify some effects in a-Si:H and mc-Si:H applications. PECVD is the most often used technique to deposit a-Si:H thin films with tailored structure and suitable physical properties for photovoltaics and optoelectronics. Due to high optical absorption they are suitable for optically-based devices such as solar cells, photosensors and thin film LEDs. The use of mc-Si:H as the absorber layer in a-Si:H/mc-Si:H tandem solar cells especially promise significantly higher and stable efficiencies [2]. In this paper we focused our attention on analysis of the absorber material layers convenient for the second generation photovoltaic cells based on amorphous silicon and explain the amorphous to micro-crystalline phase transition using X-ray diffraction, UVeVis and Raman spectroscopy. 2. Experimental details The a-Si:H films under study (Table 1) were prepared by the 13.56 MHz rf excited parallel plate PECVD SAMCO 220N deposition system onto clean Corning glass substrates. A change in the amount of H 2 incorporated in the films was obtained by varying the deposition conditions, especially by using different flow rate ratios (FRRs) of hydrogen and a mixture of silane in argon gas (10% SiH 4 ). The dilution ratio R ¼ H 2 /SiH 4 was kept at the values of 0, 20, 40, 50. A summary of the deposition conditions is listed in Table 1 . Several samples for each hydrogen dilution were deposited and investigated to obtain more accurate information about the microstructure prop- erties as a function of the deposition conditions and on the other hand also to obtain information about the reproducibility of the deposition processes. The thickness of the films varied from 140 to 570 nm. The structural properties of the films were studied by X-ray diffraction analysis (XRD) using an automatic Philips X-ray powder diffractometer X’pert Pro equipped with a fast semiconductor detector Pixcel as a point detector. Copper Ka characteristic radia- tion (l ¼ 0.154 nm) was used. The XRD patterns were recorded using asymmetric geometry where the incident angle of the X-ray beam (0.5  ) was kept constant during the measurements and the diffraction angle 2q was varied from 15 to 65  . In order to find the real structure of the films (phase analysis and crystallite size eval- uation) line profile analysis was used. This procedure, proposed by * Corresponding author. Tel.: þ420 3770634 733; fax: þ420 377 634 702. E-mail address: lprusak@ntc.zcu.cz (L. Pru sáková). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.01.021 Vacuum 85 (2010) 502e505