On Fast Optical Gain In Silicon Nanostructures

Abstract

During long time application of the well-developed silicon technology to optoelectronics had been limited by the extremely poor generation of light by bulk silicon. However, the properties of silicon were found to depend on its structure at the nanometer scale, and the bright photoluminescence (PL) from porous silicon (p-Si) was discovered by Canham in 1990 [1]. The band gap of silicon nanocrystals can be up to 3 eV depending on their size, and they can emit light with efficiency as high as 1%. Amorphous and crystalline Si nanostructures embedded in SiO2 constitute another emitting material of considerable interest [2]. The Si/SiO2 nanoscale materials can be prepared with a number of procedures, such as repeated growth of Si and SiO2 ultra-thin layers [so-called Si/SiO2 superlattice (SL)], co-deposition of Si and SiO2 (silicon rich silica SiOx, x > 2), implantation of Si into SiO2, etc. Thermal annealing can be used to promote the formation of Si nanocrystals. Unfortunately, the light emission decays in most cases relatively slowly, in microseconds, limiting the achievable repetition rate to 1 MHz, which is not sufficient for modern communication applications. An elegant way to proceed to short light pulses in silicon nanostructures is to speed up the emission kinetics by employing laser philosophy based on stimulated rather than spontaneous emission. In this situation, the silicon-laser pulse duration would be controlled by the existence of population inversion, and its lifetime can be efficiently shortened in the process of light amplification. The first experimental indications of light amplification (optical gain) in Si nanocrystals embedded in a SiO2 matrix in the 1.4 – 2.0 eV energy region were reported by Pavesi et al. [3]. They suggested that the population inversion occurred between states of the Si/SiO2 interface, and the gain occurred via a three-level scheme. Recently we have found that this optical gain can be very fast (∼ 1 ns), which is of essential importance for applications [4]. In this work, we describe the experimental data supporting the existence of fast optical gain in Si nanocrystals embedded in a SiO2 matrix. The possible sources of PL and optical gain are discussed.