engleski

The structure of amorphous-nano-crystalline Si thin films by Raman spectroscopy, HRTEM, GISAXS

The amorphous-nano-crystalline silicon thin films, with various crystal to amorphous phase volume ratio were deposited by plasma enhanced chemical vapour deposition method using radio frequency discharge in silane gas diluted by hydrogen. The variation of crystal fraction and individual crystal size distribution were obtained by variation of dilution. The samples were analysed by Raman spectroscopy, high resolution Transmission electron microscopy (HRTEM) and grazing incidence small angle x-ray scattering (GISAXS). The Raman crystallinity, obtained as the ratio of areas under corresponding TO phonon peaks in Raman, was between 0 and 50%. The crystalline TO peak positions varied between 500 and 521 cm-1 which corresponds to average crystal sizes between 2 and 20 nm, considering only phonon confinement due to nano-size of crystals. However, the TO line was asymmetric and line-width was broad suggesting also the existence of smaller and larger crystals. HRTEM micrographs showed that up to highest degree of crystallinity used here, the structure corresponds to individual nano-crystals embedded in amorphous matrix. At low crystal volume contribution the peak of individual size distribution was close to 1.5 nm. For higher crystal fraction, peak shifts slightly towards 2 nm and number of lager crystals with sizes 5-20 nm increase. The comparison of size distributions of nano-crystals estimated from HRTEM and Raman showed excellent match in average individual sizes when results of both methods were fitted as two size distributions. However, HRTEM distributions were slightly different than those obtained by Raman and suggested larger contribution of smaller crystals. Furthermore, HRTEM micrographs suggested presence of certain strain in the nano-crystals. GISAXS of deposited samples indicated particles with Gyro radii between 2 and 6 nm. Larger particles were present in the samples with higher crystal fraction and larger crystals. However, The samples were porous to certain degree (as concluded from optical measurements) and direct correlation between particles and crystals were not easy to establish. GISAXS obtained under variation of incidence angle allowed estimating in depth distribution of particles. For growing conditions that favour smaller nano-crystals, the individual sizes of particles were uniform going from surface towards depth of the sample and shape was spherically symmetric, for growth that resulted in larger crystals the particles were elongated parallel to the surface and smaller in the "bulk" of the samples. The differences in results obtained by applied methods were discussed as a consequence of oversimplified models that neglects influence of strain and shape of nano-crystals (Raman) and possible in-homogeneity of samples that makes Raman and GISAXS not straight forward comparable to microscopic methods (HRTEM).

amorphous-nano-crystalline Si; thin film; structure

nije evidentirano