Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Abstract

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane (B2H6) as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120to420K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range (300–120K), variable range hopping (VRH) was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hopping parameters, are calculated. A correlation between the hopping parameters for both models is deduced. A numerical factor that improves the value of each parameter is calculated.

On the Validity of Diffusional Model in Determination of Electric Transport Parameters of Semiconductor Compound

“…However, hopping activation energy calculated from percolation theory is notoriously much higher than that obtained by the diusion model. This was also observed in samples of microcrystalline silicon compensated with boron [7].…”

“…This fact characterizes the mechanism of VRH, where carriers tend to perform jumps over great distances in order to nd sites that remain energetically closer than its near neighbors. Table I shows that the calculated values for N (E F ) are consistent with those expected for materials that are used in the manufacture of photovoltaic devices [7,16,17]. Density of states acceptable for semiconductor materials must be established between a range of 10 15 to 10 20 cm 3 eV −1 , with defect states in the gap that enable the transport of carriers.…”

“…Using percolation theory it is possible to nd the values of the parameters W and R, which characterize the hopping transport, however, due to the discrepancy in the expected range for these parameters, we developed in a previous work [7] a model, called diusional model, which takes into account on the one hand, the relationship between the diusion coecient and the probability of hopping, and on the other hand, the Einstein relation between mobility and diusion coecient and conductivity. We have found a factor of dependence between hopping parameters obtained through percolation theory obtained from the diffusion model, which signicantly improved the value corresponding to the activation energies of variable range hopping (see Table I).…”

“…We have found a factor of dependence between hopping parameters obtained through percolation theory obtained from the diffusion model, which signicantly improved the value corresponding to the activation energies of variable range hopping (see Table I). [7].…”

Effect of hydrogen dilution on carrier transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloys

“…͑5͒, it is possible to estimate the g͑E F ͒ value of the p-nc-Si-SiC : H alloy films by assuming ␣ −1 to be 1 nm. 20,22 The calculated parameters are presented in Table I. The film at the onset of the nanocrystalline growth ͑H 2 / SiH 4 =15͒ has the highest DOS, which supports the assumption of many defects and structural disorder in the film.…”