Some electronic and metastability properties of a new nanostructured material: hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapour de position in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We pre sent here the optoelectronic and stability properties of pm-Si films deposi ted from a mixture of silane diluted with hydrogen at total gas pressures i n the range 800-1600 mTorr. A comparison with the properties of standard hy drogenated amorphous silicon (a-Si:H) is made. While some properties of bot h materials are similar, many others differ in a striking manner. Character izations of as-deposited pm-Si films show that the best samples exhibit enh anced transport properties, such as the fact that the quantum efficiency-mo bility-lifetime product eta mu tau is increased by a factor of 200-700 comp ared with that measured on a-Si:H under the same conditions. This correlate s with a lower density of deep states. The kinetics of creation of defects, performed under 670 mW cm(-2) white light illumination and at a high tempe rature (100 degrees C) in order to attain a final steady state, have been s tudied, pm-Si samples exhibit faster kinetics of creation as well as of ann ealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit eta mu tau products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhan ced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embe dded in an amorphous matrix.