Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications A. Shuaib, C. Levallois ⁎, J.P. Gauthier, C. Paranthoen, O. Durand, C. Cornet, N. Chevalier, A. Le Corre Université Européenne de Bretagne, France INSA, FOTON, UMR 6082, F-35708 Rennes, France abstract article info Article history: Received 9 November 2010 Received in revised form 6 April 2011 Accepted 18 April 2011 Available online 28 April 2011 Keywords: Multilayers Optical coatings Distributed Bragg reflectors Microcavity Optoelectronics devices Vertical cavity surface emitting lasers In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN x ) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm -1 with a-SiH layers, compared with 400 cm -1 loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN x periods have been included in an InP- based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Hydrogenated amorphous silicon (a-SiH) has attracted great interests during the last years, and it is considered as a promising material for electronic and optoelectronic devices. By the past, the development of a-SiH thin-film transistor used as a switching element, has contributed to the success of large area active matrix liquid crystal displays [1] and various image sensing devices for radiation detection [2]. Hydrogenated amorphous silicon has been also extensively studied as a photovoltaic material since it has acceptable electronic performance and great potential in low cost processing [3–5]. More recently, a-SiH has been used as optical waveguides which could be integrated at interconnection level in microelectronics [6–8]. For this potential application, the a-SiH is a promising candidate since it presents a high refractive index contrast with the air which allows a strong optical confinement. In addition, the waveguide transparency in the infrared wavelength range is improved since many dangling bonds in a-SiH are saturated by H. These optical properties, well-suited for optical waveguides on silicon, are also interesting for distributed Bragg mirrors (DBR) used in Vertical Cavity Surface Emitting Lasers (VCSEL) based on InP material and operating at the 1.55 μm telecom wavelength. For VCSEL, high reflectivity is required and can be obtained with DBR based on materials displaying high refractive index difference and low absorptions. However, high refractive index contrast is not available with lattice-matched materials on InP substrate and it requires an epitaxial growth of a great number of layers to achieve a high reflectivity [9]. Consequently, most of the VCSEL developments on InP substrates are using dielectric DBR or a combination of epitaxial grown and dielectric DBR to achieve efficient devices. Such DBR can be obtained with only few layer pairs by using a- SiH as a high refractive index material and SiO 2 , Si 3 N 4 , CaF 2 or MgO as a low refractive index material [10]. In the present work, we report on the characterizations of a-SiH thin films on glass substrate obtained by radiofrequency (RF) sputtering. The deposited layers with different hydrogen concentra- tions have been investigated and analyzed by optical transmission and atomic force microscopy (AFM) measurements. Optical indices and absorption coefficients of a-SiH have been optimized selecting the best hydrogen dilution ratio to apply during the deposition of a DBR comprising six a-SiH/a-SiN x periods. Consequently, a reflectivity of 99.2% has been estimated for a DBR deposited on a glass substrate, comprising four and a half a-SiH/a-SiN x periods only. Such optimized DBR have been introduced in a VCSEL device thanks to a technological procedure, detailed elsewhere [11]. A laser emission has been demonstrated at room-temperature (RT) under continuous-wave (CW) operation. Thin Solid Films 519 (2011) 6178–6182 ⁎ Corresponding author. Tel.: + 33 223238399; fax: + 33 223238618. E-mail address: christophe.levallois@insa-rennes.fr (C. Levallois). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.111 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf