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I3.4.1

Time-Resolved Gain Dynamics in Silicon Nanocrystals L. Dal Negro1, M. Cazzanelli1, N. Daldosso1, L. Pavesi1, F. Priolo2, G. Franzò2, D. Pacifici2, and F. Iacona3 1

INFM-Dipartimento di Fisica, Università di Trento, via Sommarive 14, I-38050 Povo (Trento) INFM-Dipartimento di Fisica, Università di Catania, Corso Italia 57, I-95129 Catania, Italia 3 CNR-IMM, Sezione di Catania, Stradale Primosole 50, I-95121 Catania, Italia 2

ABSTRACT Time-resolved variable stripe length (VSL) experiments on a set of silicon nanocrystal waveguides obtained by plasma enhanced chemical vapor deposition (PECVD) have revealed a fast recombination dynamics (20 ns) related to population inversion under 6 ns optical pumping at 355 nm. Modal gain values about 10 cm-1 have been measured at 760 nm by VSL technique for the fast recombination component while optical losses about 15 cm-1 are measured for the integrated signal in the slow (lifetime of about 10 µs) recombination tail. Threshold behavior in the emission intensity together with a pumping length and pumping power dependence of both the intensity and the time duration of the fast recombination component has been observed. These results are explained within an effective four level model to describe the strong competition among different Auger processes and stimulated emission. INTRODUCTION One of the main future challenges for silicon microphotonics consists in the demonstration of a laser action in Si-based materials. Following the initial observations of optical gain in silicon nanocrystals (Si-nc) prepared by ion-implantation [1], other works have recently demonstrated the presence of stimulated emission in Si-nc [2,3,4]. As in other quantum dot based systems [5,6] Si-nc present a severe competition with efficient non-radiative processes, mainly non-radiative Auger processes, yielding a very fast dynamics in the optical gain. Although a clear understanding of the microscopic gain mechanism is still under debate, it has been suggested that interface radiative states associated with oxygen atoms can play a crucial role in determining the emission properties of Si-nc systems [7,8]. Here we report on light amplification dynamic studies in Si-nc and discuss a possible phenomenological gain model. We studied Si-nc samples produced by high temperature annealing of substoichiometric silicon oxide (SiOx) thin films grown by plasma enhanced chemical vapor deposition (PECVD). The structural and luminescence properties of such systems have been fully discussed in Ref. [9]. We focus on two different samples characterized by different total Si content: 42 at. % (named 3A, Si-nc mean radius 1.7 nm) and 39 at. % (named 5A, Si-nc mean radius 1.5 nm) both annealed at 1250 °C for one hour in nitrogen atmosphere. The oxide layer containing Si-nc was 250 nm thick and was embedded between two 100 nm thick stoichiometric SiO2 layers to form a waveguide. Planar waveguides were formed on a transparent quartz substrate and had an optical confinement factor of 0.74 and 0.62 for samples 3A and 5A, resp