(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

In this invited talk, we will report our latest advances in the field of nanostructured silicon-based light emitters with special emphasis in those that are rare earth doped. We have recently shown that it is possible to push external quantum efficiencies over the 10% by fine tuning the material composition and by using a new device concept that we have called hot electron engineering.

Most of the rare earth amount introduced in the host material must be optically active. This can be accomplished by using a deposition method that introduces the rare earth during film growth and by a suitable passivating annealing procedure that: i) does not promote rare earth precipitation and clustering; ii) brings the rare earth to the 3+ oxidation state and iii) reduces non radiative defects. Additionally, for electroluminescent devices the matrix composition must be engineered so that it allows conduction-band transport in contrast with defective systems in which transport is hopping-like.

The hot electron engineering concept relies on the fact that the rare earth excitation process in Si-based dielectric matrices proceeds mainly via impact excitation by hot electrons. Thus, devising suitable structures for which electrons can be accelerated to average energies that are resonant to the rare earth excitation energies, will improve both efficiency and maximum power emission. Additionally, driving the device with pulsed polarization and short duty cycle will improve device efficiency and lifetime. Finally, the structures must allow at the same time significant light extraction efficiency. This can be accomplished by improving the antireflective properties of the whole stack and/or by nanostructuring the ITO or polysilicon electrode.

We will be show silicon oxides, nitrides and oxynitrides as hosts for the rare-earth species and, additionally, we will display the advantages and drawbacks of introducing small percentages of carbon and aluminum. The rare-earth species incorporated in those matrices are Er, Ce, Tb, Eu and Nd and their light emission efficiency depend in a complex way on matrix composition, material processing, device structure and device polarization. The simplest hot electron engineering structures are bilayers and trilayers with silicon oxide as accelerator. More complex structures like multilayers and superlattices will be also introduced and optimal designs will be proposed. Some reliability issues and potential solutions for them will be also presented.

We acknowledge existing collaborations in this field with teams of the McMaster University at Hamilton (Canada), Institut de Microelectrònica de Barcelona (Spain), Nanophotonics Technology Center of Valencia (Spain), Instituto de Óptica of Madrid (Spain), Ion Beam Institute at Rossendorf (Germany), IMTEK at the University of Freiburg (Germany) and CIMAP-CNRS at Caen (France).