Effects of an Applied Electric Field on Silicon Nanocrystal Photoluminescence
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1145-MM01-07
Effects of an Applied Electric Field on Silicon Nanocrystal Photoluminescence Alexandre Lacombe1, David Barba1, Félix Beaudoin1, François Martin1, Guy G. Ross1 1 INRS-EMT, 1650 Boulevard Lionel-Boulet, Varennes, Québec, Canada J3X 1S2
ABSTRACT The photoluminescence (PL) of silicon nanocrystals (Si-nc) obtained by ion implantation in the oxide layer of a MOS structure was measured during the application of a slowly varying electric field generated by biasing the gate electrode. As a result, both PL intensity enhancement and quenching have been observed. These reproducible intensity modulations exhibit a hysteresis effect when the applied electric field is varied and persist even after it is removed. The behavior of the current density and the absence of wavelength shift in the PL spectra during gate voltage sweeps suggest that these modulations are related to the motion of charge carriers rather than to field-induced mechanisms such as quantum-confined Stark effect (QCSE).
INTRODUCTION Since the discovery of PL from porous Si in 1990 [1], extensive efforts have been devoted to the development of an efficient silicon-based light emitter. In particular, Si-nc embedded in SiO2, which can be produced with various techniques compatible with VLSI technology such as ion implantation [2], sputtering [3] or PECVD [4], exhibit both PL and EL, although EL intensity is typically 2 or 3 orders of magnitude lower than PL intensity (see e.g. [5]). Resistivity of the surrounding SiO2 matrix and the effect of the high electric field applied are suspected to be responsible for the weak EL. In the present work, these hypotheses are being investigated by applying variable bias voltages (i.e. electric fields) to a MOS device containing Si-nc in the oxide layer while measuring the resulting electric current and PL spectrum.
EXPERIMENT An amorphous SiO2 layer with a thickness of 57 nm (as determined by ellipsometry) has been thermally grown on a n-type silicon substrate (3 Ω·cm), followed by two successive Si+ ion implantations with energies and fluences of 25 keV, 2.5 x 1016 cm-2 for the first implantation, and 12 keV, 1.0 x 1016 cm-2 for the second implantation. The implantation profiles, according to SRIM simulations and previously published for similar samples [6], indicate a maximal excess Si concentration of 9 x 1021 cm-3 (28%) with a majority of implanted ions lying in the lower half of the oxide, near the Si/SiO2 interface. The sample has been annealed at 1050ºC for 1 hour under N2 ambient, and then passivated in a forming gas of H2 (5%) and N2 (95%) at 500ºC for 30 min. PVD was used to deposit a circular 15 nm-thick semitransparent gold gate electrode with a 2 mm diameter on top of the oxide, and an Ag electrode on the back of the Si substrate, previously etched with a 5% buffered HF solution. A sketch of the final structure of the MOS device is shown in Fig. 1a.
Photoexcitation was generated by a 405 nm laser diode directed at 45° on the top electrode of the samples, while the PL signal was collected normal to
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