There is a widespread interest in silicon nanocrystals as an optoelectronic material system. However, the insulating matrix that defines a nanocrystal makes efficient electrical carrier injection challenging. The development of efficient electrical pumping methods is known to be a critical issue for the improvement of silicon nanocrystal-based optoelectronic devices.

Field-effect luminescence in silicon nanocrystals was firstly described by Walters et al. (Nature Materials 4 143, 2005). In this work, the authors reported on electrical pumping of silicon nanocrystals via a novel sequential programming technique termed field-effect electroluminescence. They attributed the observed electroluminescence to exciton formation by sequentially programming the nanocrystals with charge carriers of each sign, resulting in electroluminescence at transitions in the gate bias. This approach is a departure from the previous carrier injection schemes in which nanocrystals were excited by a constant electrical current.

It was said that FELEDs could offer significant advantages over diode-based designs for nanocrystal light sources by enabling precise control over carrier injection processes. For example, durability could be maintained by exciting nanocrystals without resorting to impact ionization processes, in which excess hot carrier energy could result in oxide wear-out and eventual device failure.

New perspectives

Josep Carreras and his colleagues at the University of Barcelona and IMM-CNR have developed a compact model that is able to predict the charge transport among quantum dots as well as the emitted light through a suitable rate equation, which opens up new perspectives for the applicability to electrically excited light-emitting devices, nanocrystal-based solar cells or in the optimization of memory cells that rely on quantum dots as storage nodes.

The physical compact model also challenges the origin of the recently reported field-effect luminescence; it is shown that sequential exciton formation would imply a ratio between luminescence peaks obtained by injection of holes into electron-charged nanocrystals and injection of electrons into hole-charged nanocrystals not supported by experimentation. In addition, the presented model nicely reproduces empirical data by assuming that nanocrystals can be electrically excited by impact ionization of electrons/holes, injected through quantum tunneling from the same substrate at the gate voltage discontinuities. This approach does not require any assumption on the sequential formation of quantum-confined excitons and subsequent recombination, revealing impact excitation of electrons/holes as being the main physical origin of the field effect luminescence in silicon nanocrystals.

The researchers presented their work in Nanotechnology.