Si-Rich Nitride Nanostructures: A Pathway to Si-Nanophotonics
Silicon-based nanophotonics has the potential to become a disruptive technology by enabling full-scale on-chip integration of optical functions within mainstream microelectronics [1,2]. However, progress towards this goal is curtailed by the lack of efficient light emission from silicon. In fact, photon emission from silicon originates from low-probability phonon-mediated transitions that compete unfavourably with fast non-radiative de-excitation paths, such as Auger and free carrier recombinations. In view of these limitations, several strategies have been recently developed to engineer Si into a more efficient light-emitting material.[3] In particular, the approach of quantum confinement has led to a dramatic improvement of light emission efficiency in Si nanocrystals (Si-nc) [4-8] leading to emission efficiencies as high as ∼ 60 % in the range 700 nm-800 nm for Si-nc embedded in Si-rich silicon oxide (SRO) films and 10-100 cm-1 optical gain. However, in the last few years, it has become progressively clearer that the optical emission and energy transfer in Si nanostructures can be strongly affected by the complex interplay of two main contributions: the system’s dimensionality (quantum confinement) and the structural/chemical properties of the Si-nc’s interfaces. In particular, it is now clear that the presence of Si-O double bonds at the surface of small Si crystallites embedded in SiO2 play a crucial role in determining the mechanism of light emission and optical gain.
Figure 1. a) Bright-field TEM cross-section micrograph of a typical Si-nc sample with 48% Si annealed at 700°C for 10 minutes. b) High resolution image of the crystalline Si ncs.
The demonstration of efficient light emission and optical gain from surface-localized exciton transitions highlights the importance to achieve a more robust understanding of the Si-nc’s interface structure and composition, which can be strongly affected by the nature of the embedding matrix. Recently, visible and near-infrared light-emission with nanosecond relaxation dynamics have been demonstrated [11-13] for Si-nc embedded in silicon nitride matrices fabricated by magnetron co-sputtering of Si-rich nitride (SRN). This novel nanostructured material system is suitable for efficient electrical pumping, which would enable the fabrication of stable Si-based electroluminescent devices.
Very recently, the our research group has demonstrated that, depending on the Si concentration, rapid thermal annealing (RTA) results in a high density (~5x1018 cm-3) of small (2 nm) Si-nc. Figure 1 (a) shows a TEM bright-field image of Si quantum dots obtained by co-sputtered SRN films and annealed at 700°C [14]. Figure 1 (b) shows a high resolution TEM image of the quantum dots. The lattice fringes in the image clearly indicate that the nano-dots are crystalline in nature, and have an average diameter of approximately 2 nm. These Si-nc samples lead to intense light emission in the 800 nm range at room temperature, and show nanosecond recombination dynamics [11-15]. Ab-initio theoretical calculations [11,13,15] show that nitrogen-related surface groups can introduce strongly-localized gap spates which play a crucial role, akin to double Si-oxygen bond, in the optical emission properties of Si-nc smaller than approximately 3 nm.
Figure 2. a) Bright-field TEM cross-section micrograph of SRN/Si multilayers. b) High-resolution image of the rectangular region marked in (a), showing amorphous Si nc in the SRN layers, some of which are marked by arrows.
It is important to understand the Si-nc nucleation and growth kinetics by varying the principal growth parameters as well as the annealing conditions and correlating the resulting microstructure to the optical emission properties of the Si-ncs. In addition, the structural changes induced by the introduction of Er ions in the nanocrystallites Si matrix, needs to be examined by TEM studies and the results need to be correlated with structural studies based on infrared absorption spectroscopy (FTIR).
In addition, the energy transfer phenomena and light emission from novel superlattice structures, recently demonstrated by our group at BU, needs to be understood. As shown in Figure 2, 5nm-layer superlattice structures shows amorphous Si clusters nucleating at the Si/SRN interfaces, opening a very attractive avenue for efficient electrical pumping.
