Photoluminescence from Silicon Nanocrystals Formed by Pulsed-Laser Deposition
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Photoluminescence from Silicon Nanocrystals Formed by Pulsed-Laser Deposition X. Y. Chen, Y. F. Lu 1, Y. H. Wu, B. J. Cho, W. D. Song, H. Hu Laser Microprocessing Laboratory and Silicon Nano Device Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 1 Department of Electrical Engineering, University of Nebraska, Lincoln, NE 68588-0511
ABSTRACT Si nanocrystals (NCs) consisting of small crystals from 1 to 20 nm were formed by pulsedlaser deposition (PLD) in inert Ar gas and reactive O2 gas. The oxygen content of the Si NCs increases with increasing O2 ambient pressure and nearly SiO2 stoichiometry is obtained when O2 pressure is higher than 100 mTorr. The optical absorption of the Si NCs shows an indirect band transition. Broad PL spectra are observed from Si NCs. The peak position and intensity of the PL band at 1.8–2.1 eV are dependent on excitation laser intensity, while intensity changes and blue shifts are observed after oxidation and annealing. The PL band at 2.55 eV displays vibronic structures with periodic spacing of 97 ± 9 meV, while no peak shift is found before and after oxidation and annealing. The as-deposited Si NCs show a polycrystal structure and crystallinity improves after annealing. Combined with the PL of Si NCs obtained by crumbling electrochemical-etched porous Si layer, the results give strong evidence that the PL band at 1.8– 2.1 eV is due to the quantum confinement effect (QCE) in Si NC core while the PL band at 2.55 eV is related to the localized surface states at SiOx/Si interface. INTRODUCTION Ever since visible photoluminescence (PL) were observed in Si nanostructures, Si nanocrystals (NCs) have attracted great interests to the microelectronics and optoelectronics. In the metal-oxide-silicon (MOS) structure, memory based on NC floating gate has shown great promise for future applications in ultradense and ultralow power flash memory [1]. In optoelectronics, the development of Si nanostructured materials for light emitting devices has created a new opportunity for incorporating optoelectronic functions into Si integrated circuit technology. Si NCs have been synthesized by several techniques. Among them, PLD is one of the most flexible and promising techniques due to its ability to control size distribution of NCs and maintain crystal purity in a cold-wall processing ambient [2]. In this work, Si NCs were formed by PLD. The purpose of this work is to study the effects of deposition conditions and thermal treatment on the structures and PL properties of Si NCs.
EXPERIMENTAL DETAILS The PLD system utilized is shown in Fig. 1. A pulsed KrF excimer laser beam (λ = 248 nm, τ = 30 ns) was directed by a mirror and then focused by a lens onto the target at an incident angle
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