Size Distribution Of Embedded Nano-Crystallites In Polymorphous Silicon Studied By Raman Spectroscopy And Photoluminesce
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A19.7.1
Size Distribution Of Embedded Nano-Crystallites In Polymorphous Silicon Studied By Raman Spectroscopy And Photoluminescence V. Tripathi, Y. N. Mohapatra Department of Physics, IIT, and Kanpur-208016, INDIA E-mail: [email protected] Md. N. Islam QAED/SRG, Space Application Centre (ISRO), Ahemdabad-380015 (India) V. Suendo, P. Roca i Cabarrocas Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 du CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex, France
Abstract. Polymorphous Silicon (pm-Si:H) deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) has emerged as an alternative material to amorphous silicon (a-Si:H). Deposition parameters of pm-Si:H are such that small crystallites get embedded in a relaxed amorphous silicon matrix, thus improving the optical and electrical properties. We study the size of crystallites and degree of order in pm-Si:H using Raman and photoluminescence (PL) spectra of pm-Si:H and a-Si:H. Raman Spectra of a variety of hydrogenated nanostructured silicon (pmSi:H) and amorphous Silicon (a-Si:H) samples grown at different pressures were analyzed. Deconvolution of observed multiple peaks in photoluminescence spectra and fitting to Gaussian size distribution also yields particle size to be in the range of 2.3 to 3.5nm in agreement with Transmission Electron Microscopy and Raman results. INTRODUCTION Hydrogenated amorphous Silicon (a-Si: H) is important to large area electronics in applications such as thin film transistors for flat panel displays, solar cells and photocopiers etc. Recently, it has been shown that optical and transport properties of such material can be improved by embedding small nanocrystallites in a-Si:H matrix. This material has been termed as hydrogenated polymorphous silicon (pm-Si: H). It is deposited by PECVD under conditions close to dust formation in the vacuum chamber. Presence of nanocrystallites in amorphous medium changes the properties of the material. pm-Si:H has been shown to have much lower density of states [2] and devices made by pm-Si:H have exhibited superior characteristics [3]. Understanding the role of crystallites in altering the properties of this material is crucial for its further improvement and application. Quantum confinement influences both phonon and electronic states. The influence of size effect on phonons can be traced to changes in lineshape of Raman lines corresponding to a particular mode. The size effect in nanoparticles of silicon can induce band to band transitions resulting in radiative recombination in an otherwise indirect band gap semiconductor. Photoluminescence due to nanostrucure of silicon has been widely studied in varied systems including isolated clusters, porous silicon, embedded nanocrystallites. Hence a large variety of information is already available. It has also been observed in PECVD grown aSi:H which has nanocrystallites incorporated into it [4,5]. We seek to use this wealth of information to deduce particle size and their distribution in polymorphous silicon. The size dependence
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