Charge injection into porous silicon electroluminescent devices

Light emitting porous silicon has been studied in a high resolution transmission electron microscope. These studies show that the microstructure is highly dependent on the doping level of the silicon substrate, and on the formation conditions. We have shown that the most efficient light emitting structures are quantum dot like, and that the microstructure consists of silicon dots of size 3–4 nm surrounded by a thin oxide layer. Based on this we have developed a new type of contact in order to inject holes into an electroluminescent device based on an n-type porous silicon layer. Poly(9-vinyl carbazole) is spin coated on a fresh porous silicon layer, and then either indium tin oxide or NiO is used to inject holes. The polymer layer increases the luminescence output by two orders of magnitude, while the use of NiO as a replacement for ITO reduces the switch-on voltage from 55–60 to 10–15 V. These results allow us to propose an energy band model of the device, which is discussed in terms of both the quantum wire and quantum dot models of porous silicon.