Ion Beam Effects on the Formation of Semiconductor Nanoclusters
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ON
THE FORMATION
OF
SEMICONDUCTOR
S. SCHIESTEL, C.A. CAROSELLA, R. STROUD, S. GUHA, C.M. COTELL, K.S. GRABOWSKI, US Naval Research Laboratory, 4555 Overlook Ave., Washington, D.C. 20375 ABSTRACT Silicon rich silica films were deposited by coevaporation of silica and silicon with and without simultaneous ion bombardment. The Si-silica ratios are correlated to changes of index of refraction and shifts in an asymmetric stretching mode IR absorption. Photoluminescence between 550 and 600 nm is observed for all films which is attributed to Si0 2 defects. After annealing this photoluminescence peak shows a shift to 750 nm and an increase in intensity, indicating the formation of silicon nanoclusters. This effect is more pronounced for samples prepared without ion beam treatment. ERD measurements show a correlation of photoluminescence with the presence of hydrogen in the films. The microstructure of these films were investigated by TEM. Photoluminescence from the Si nanoclusters in the films is optimized when the arrival rate of Si/silica is slightly less than 0.4. INTRODUCTION The discovery of photoluminescence of porous silicon has stimulated enormous interest in the synthesis of nanocrystalline silicon for optoelectronic applications [1]. Many researchers are exploring techniques that would produce nanocrystals of Si with processes that are compatible with semiconductor technology. They have already demonstrated that silicon and other semiconductor nanoclusters are formed by ion implantation and subsequent annealing [2-4]. Ion implantation has obvious disadvantages imposed by the limitation of thicknesses of potential photoluminescent layers, the non uniform distribution of nanoclusters and the limited density of nanoclusters. We have shown that ion beam assisted deposition (IBAD) can overcome these difficulties for the production of noble metal clusters in various insulating matrices such as Si0 2 [5]. The energetics of the IBAD process resulted in a homogeneous, high density distribution of
metal clusters. The same behavior is expected for the synthesis of semiconductor clusters. This paper will explore preliminary work in this area. EXPERIMENT Silicon nanocluster containing films were deposited by coevaporation of silicon and silica from two e-beam hearths in an ultrahigh vacuum chamber (1.10-8 mbar). The system is both cryo-pumped and turbo-pumped during depositions. One set of samples was simultaneously bombarded with an argon ion beam from an 8 cm. ion gun. The energy of the argon ions was 100 eV; the current density was 35 ýtA/cm 2 . All films were deposited at room temperature. The deposition rate of silica was 0.2 nm/s; silicon deposition rate was varied from 0.04 to 0.16 nm/s. After deposition the samples were post-annealed in vacuum or nitrogen between 500 'C and 900 'C. The composition of the films was determined by Rutherford back scattering analysis (RBS) and elastic recoil detection of hydrogen (ERD), using He ions accelerated in a 3 MeV tandem Van de Graaff. The index of refraction was measured by
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