Gas Phase Surface Passivation of Silicon Nanocrystals

Richard Liptak (ECE)
Advisors: Steve Campbell (ECE) and Uwe Kortshagen (ME)

There is ongoing research into creating both photoluminescent (PL) and electroluminescent (EL) devices (based on Si nanocrystals (Si-NC)) which will “hopefully” replace existing optical displays. However, in order create efficient EL and PL devices a robust passivation layer is needed. Silicon oxide is not a viable option. Its bandgap is not suitable for charge injection and the formation of an Si=O (Silicon-Oxygen double bond at the surface of the nanoparticle pins the PL in the red. A robust passivation layer leads to a high quantum efficiency (QE), thermal stability, and a stable PL spectrum.

Our work focuses on in-flight gas phase passivation of Si-NC. We have investigated three methods thus far. First, we investigated the feasibility of using nitridation (thermal or LPCVD) to passivate the surface of the Si-NC with silicon nitride. Secondly, we have studied using plasma etching (Silicon Dry Etch using CF₄) to passivate the surface. By using the CF₄ process we are able to passivate the surface with a Teflon like coating and etch the Si-NC to smaller sizes – thus allowing the creation of Si-NC which emit in the entire visible light spectrum. Using the nitridation process we are able to passivate the surface of the Si-NC for up to 7 days before we observe PL, which indicates that an oxide has formed on the surface of our Si-NC.

We are also interested in Si-NC based devices such as nanoparticle LEDs. In order to fabricate viable LED's, we need to meet several criteria. First of all, the Si-NP must be made with a controlled size and must be free of defects. Nonradiative defects (such as surface states) must be passivated. Then the particles must be embedded into a light emitting film or a multilayer structure. Finally, these films are incorporated into a device and then characterized.