Hydrogen Passivation of Si Nanocrystals in Silica
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Hydrogen Passivation of Si Nanocrystals in Silica Stephanie Cheylan and Robert G. Elliman Electronic Materials Engineering Department, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
ABSTRACT This paper explores the effect of hydrogen on the luminescence properties of silicon nanocrystals formed in silica by high-dose ion-implantation and thermal annealing. For samples implanted to low fluence (small nanocrystals), passivation is shown to result in a uniform enhancement of the PL emission for all wavelengths. However, for samples implanted to high fluence, preferential enhancement of the emission from larger nanocrystals is evident, resulting in a red-shift of emission spectra. Both the intensity enhancement and the red-shift are shown to be reversible, with spectra returning to their pre-passivation form when H is removed from the samples by annealing. The luminescence lifetime is also shown to increase after passivation, confirming that defect-containing nanocrystals luminesce.
INTRODUCTION Unlike bulk-silicon, nanocrystalline silicon produces strong room-temperature luminescence [1,2] and when embedded in a dielectric host, such as silica, offers a robust and stable material for integrated-optics applications. The incorporation of silicon nanocrystals in silica is readily achieved by the precipitation of excess silicon, introduced into a film either during deposition [3,4] or subsequently by ion implantation [2]. However, a significant fraction of the silicon nanocrystals produced by this, and related methods, contain defects that reduce their luminescence efficiency [5]. The effect of hydrogen on the optical and electrical properties of semiconductors and its interactions with defects in silicon has been extensively studied [6,7,8,9]. It has also clearly been demonstrated that a significant increase in the photoluminescence emission intensity of porous and nanocrystalline silicon can be achieved by hydrogen passivation of non-radiative defects [10,11]. This paper extends early studies by showing that passivation can change the spectral distribution of the luminescence emission, and that it increases the luminescence lifetime, consistent with a model in which defective nanocrystals continue to luminesce.
EXPERIMENTAL DETAILS Fused silica (infrasil) substrates, 1mm thick, were implanted at room temperature with 400 keV Si ions to fluences in the range from 6x1016 Si.cm-2 to 4x1017 Si.cm-2. Si nanocrystals were subsequently formed by annealing the implanted samples at 1100oC in a flowing nitrogen ambient for 1 hr. Hydrogen passivation was achieved by annealing at 500oC in forming gas (95% N2+ 5% H2) for 1 hr. Photoluminescence (PL) measurements were performed at room temperature using the 488 nm line of an Ar+ ion laser as the excitation source. Time resolved PL R3.38.1
measurements were performed using a frequency-doubled yttrium-aluminum-garnet (YAG) laser (355 nm). The pulse length was ~ 100 ns and the repetition rate 45 Hz. The emitted PL
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