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

When silicon thin films are deposited by plasma enhanced chemical vapour deposition 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 present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800–1600mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency–mobility–lifetime product ημτ is increased by a factor of 200–700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mWcm−2 white light illumination and at a high temperature (100°C) in order to attain a final steady state, have been studied. pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit ημτ products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.