High efficiency light-emitting devices based on energy band gap engineering of Si nanocrystals
Laboratorio di Nanoscienze, Dipartimento di Fisica, Università di Trento, Via Sommarive 14, I-38050 Povo (Trento), Italy
Multilayer approach to nanocrystalline silicon composite material allows us to control independently nanocrystals size and density. After a high-temperature annealing silicon-rich silicon oxide layer thickness in the multilayer structure determines the nanocrystals size, while an excess silicon content of the silicon-rich silicon oxide determines the nanocrystal density. In addition, a tight control over the silicon oxide layer quality and thickness that separates silicon nanocrystals in the multilayer structure is possible. This is a must for nanocrystalline Si-based light emitting devices, since it controls the tunneling or injecting electrical current. The nanocrystalline silicon light-emitting device (LED) having a multilayer structure show high power efficiency and low operating voltage. The high efficiency and low voltages are provided by direct tunneling of both electrons and holes among the nanocrystals in the multilayer. Indeed the power or wall-plug efficiency of our multilayer LED is 0.2% which is comparable with the best Si-based LEDs reported so far. More important, in contrast to these reports, our LEDs are grown by a standard plasma-enhanced CVD, a CMOS compatible technique, which makes possible their monolithic integration with CMOS photonic circuits.
In addition, the multilayer approach provides a versatile tool for energy band-gap engineering of silicon nanocrystals for high-efficiency LED and solar cell applications. We proposed a new scheme of high-efficiency nanocrystalline Si-based LED which is based on a graded energy gap structure where the sizes of the nanocrystals increase from the active region towards the electrodes. In this way the conductivity of the layer is increased due to the average large silicon nanocrystal sizes, while the luminescence is high due to the thin small nanocrystal region. Also the contact potential is decreased by the tuning of the effective barrier height which progressively increases to the active layer region.