Solid-phase crystallization kinetics in dopeda-Si chemical-vapor-deposition films

Solid-phase crystallization kinetics of undoped, phosphorus-doped (0<[${\mathrm{PH}}_3$]/[${\mathrm{SiH}}_4$]<2×${10}^{\mathrm{-}3}$), and boron-doped (0<[${\mathrm{B}}_2$ ${\mathrm{H}}_6$]/[${\mathrm{SiH}}_4$ ]<2×${10}^{\mathrm{-}3}$) amorphous silicon films prepared by chemical-vapor deposition (CVD) of silane have been studied with use of conductivity measurements. The crystallization growth rates ($V_g$) and their activation energies ($E_v$) are obtained during isothermal annealings in a large temperature range (510<$T_A$ <650 °C). The growth rate is found to be enhanced as the doping content increases, whereas $E_v$ remains almost constant [$E_v$=(2.9±0.1) eV] in the entire doping range. The most striking point is that the increase of $V_g$ is observed for relatively low doping concentrations and that there exists a simple correspondence between the increase of $V_g$ and the decrease of the density of neutral dangling bonds measured by EPR. For boron doping the growth rate increases by a factor of 4 in the range (0–7)×${10}^{\mathrm{-}6}$ of diborane. In the same range, the neutral dangling bonds become positively charged and nonparamagnetic by electronic compensation with the acceptor atoms, and the EPR signal decreases from ${10}^{19}$ to ${10}^{17}$ spins/${\mathrm{cm}}^3$. At higher doping concentrations , $V_g$ remains constant except for the heavily doped sample, where an additional increase of $V_g$ is observed. A similar behavior is obtained for phosphorus doping with a relatively small increase of $V_g$. We believe that these results indicate that dangling bonds play an important role in crystallization processes and the effect is dependent on their charge state. Possible models to account for this behavior will be discussed. X-ray-diffraction and Raman-spectroscopy measurements have also been carried out in order to control the crystallinity of the films.