Multifunctional properties of nanocrystalline porous silicon as a quantum-confined material

It is shown that the quantum properties of nanocrystalline silicon (nc-Si) appear in various ways. The most striking effect of quantum confinement in nc-Si is a significant bandgap widening. It causes efficient visible luminescence. To date, the external quantum and power efficiencies of red-band electroluminescence have been improved up to 1.1% and 0.4%, respectively, in porous silicon (PS) diodes. In the porosity-multilayer PS microcavity, nonlinear optical response due to a refractive index change of nc-Si was observed for photoexcitation and carrier injection. The PS diode also operates as a surface-emitting ballistic cold cathode. Under a high electric field, electrons can travel with an enlarged drift length, possibly via multiple tunneling through interfacial barriers between interconnected nc-Si. A strong carrier charging effect in nc-Si leads to a function as light-emissive nonvolatile information storage. An extremely lowered thermal conductivity due to complete carrier depletion makes it possible to use nc-Si as a thermally induced ultrasonic emitter without any mechanical vibrations. These are potentially applicable to integrated novel functional devices.