The physics and technological aspects of the transition from amorphous to microcrystalline and polycrystalline silicon

Abstract

Silicon thin films grown near the boundary between the amorphous/microcrystalline growth offer superior properties for industrial applications. Series of silicon samples, in which crossing of this transition region was achieved by changing a single technological parameter (dilution of silane in hydrogen, deposition temperature, sample thickness) were used to test our model of transport, connecting the macroscopically observed transport properties and the crystallinity, hydrogen content, grain size and grain boundaries. Microscopic study by AFM led to the formulation of the geometrical model of growth of mixed phase Si. The demand for research of microcrystalline or polycrystalline silicon prepared at low substrate temperatures is stimulated by the use of cheap plastic substrates. In addition to a direct deposition an alternative technology, such as metal-induced crystallization supported by the electric field is discussed. Possible future application of thin silicon films, for example in a "nanolithography", is also shown.

Some controversial points related to transport in microcrystalline silicon

“…In the following section we shall show that there is no conflict between our macroscopic conductivity [10][11][12][13][14] and microscopic C-AFM results [2], which was broadly discussed in [6].…”

“…We have studied the transition from a-Si:H to µc-Si:H on many series of thin silicon films [10][11][12][13][14], ranging from pure a-Si:H through the mixed phase samples at relatively sharp a-Si:H / µc-Si:H transition region to highly crystalline samples, called a "single phase" µc-Si:H by the authors of [6]. In this point, we agree with the authors of [6] that what is generally called µc-Si:H is actually a wide class of materials.…”

“…1). Indeed, we have observed on many series [10][11][12][13][14] that the most pronounced changes of the conductivity during the a-Si:H / µc-Si:H transition occur near X c ~ 60% and the transition is completed at X c ~70-80%.…”

The statistical shift of the chemical potential causing anomalous conductivity in hydrogenated microcrystalline silicon

“…The corresponding Meyer-Neldel energy from the calculation is found to be 40 meV, which is consistent with values reported from experiments listed by Ram et al 30 The flat region above 0.5 eV in the calculated data is not very recognizable in the experimental data but is in fact found experimentally. The data of Kočka 2 show this most clearly. The reason for this plateau in the calculation of 0 is that is so far away from the conduction band of c-Si that at RT the statistical shift does not occur.…”

“…Eventually the layer grows thick enough for the cones to merge and form large grain aggregates, which seem to be separated by thin walls of amorphous, or disordered tissue. 2 Formation of mid gap states, in the form of silicon dangling bonds, is hard to control during growth and at increasingly high X c , the probability of incorporating oxygen and other contaminants increases. 11,12 If extended voids are present, also after deposition the material can easily become contaminated with moisture or oxygen.…”