Ballistic transport mode detected by picosecond time-of-flight measurements for nanocrystalline porous silicon layer

Abstract

The electron transport mechanism in nanocrystalline porous silicon (nc-PS) with a controlled structure has been studied for a self-supporting sample by time-of-flight (TOF) measurements at room and low temperatures using a picosecond-width UV laser pulse. In contrast to both single-crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H), the TOF signals of nc-PS show characteristic behavior that involves a ballistic component. The drift velocity vd determined from observed TOF signals shows no signs of saturation with increasing field strength F. At F~3× 104 Vcm, the vd value in nc-PS at room temperature reaches 2.2× 108 cms. The corresponding electron mean free path is 1.6 μm. These values are considerably larger than those in c-Si. The ballistic transport mode becomes clear at low temperatures. The results support the model that electrons can travel ballistically with little scattering losses in a nanocrystalline silicon dot chain interconnected via thin silicon dioxide films. © 2005 American Institute of Physics.