Characterization of hydrogenated nanocrystalline silicon thin films prepared with various negative direct current biases
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El-Hang Lee School of Information and Communication Engineering, Inha University, Incheon 402-751, South Korea
Han-Sup Lee Department of Textile Engineering, Inha University, Incheon 402-751, South Korea (Received 13 August 2007; accepted 26 November 2007)
Hydrogenated nanocrystalline Si (nc-Si:H) thin films were prepared by plasmaenhanced chemical vapor deposition (PECVD). The films were deposited with a radio-frequency power of 100 W, while substrates were exposed to direct current (dc) biases in the range from 0 to −400 V. The effects of dc bias on the formation of nanoscale Si crystallites in the films and on their optical characteristics were investigated. The size of the Si crystallites in the films ranges from ∼1.9 to ∼4.1 nm. The relative fraction of the crystallites in the films reached up to ∼56.5% when a dc bias of −400 V was applied. Based on the variation in the structural, chemical, and optical features of the films with dc bias voltages, a model for the formation of nanostructures of the nc-Si:H films prepared by PECVD was suggested. This model can be utilized to understand the evolution in the size and relative fraction of the nanocrystallites as well as the amorphous matrix in the nc-Si:H films.
I. INTRODUCTION
Silicon has been one of the most dominant materials in semiconductor electronics, and Si-related electronic devices and process technologies have been developed rapidly over the past several decades. Despite this progress, Si technologies have not allowed any integration of Sibased electronics with Si-based optical components; this is mainly because Si is a poor light emitter at room temperature due to its indirect band-gap structure. However, when the size of Si crystallites is less than ∼5 nm, nanocrystalline Si (nc-Si) becomes free from direct- and indirect-transition regimes due to strong quantum confinement effects (QCE).1–3 As nanotechnology has advanced over the past decade, nc-Si has received greater attention for application in optoelectronic nanodevices such as light-emitting diodes,4 optical memories,5 solar cells,6 thin film transistors,7 and single-electron transistors.8 Much effort has been made to explore the possibility that Si can play a crucial role as a light-emitting source; this includes porous Si,1–3 erbium- and terbium-doped a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0092 790 J. Mater. Res., Vol. 23, No. 3, Mar 2008 http://journals.cambridge.org Downloaded: 14 Mar 2015
nc-Si,4 and nc-Si in thin films prepared by various methods.5 In particular, much research has been performed to obtain hydrogenated nanocrystalline Si (nc-Si:H) films because of their optoelectronic properties suitable for device operation. In conventional radio-frequency (rf) glow-discharge systems, Si substrate mounted on the grounded electrode is exposed to a flux of ions with typical current densities of ∼1 A/cm2 and kinetic energies of 10–20 eV,6 although a wide range of ion energy and flux can be utilized by varying the reactor geometry or t
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