Abstract
In this work, we present new results on the plasma processing and structure of hydrogenated polymorphous silicon (pm-Si:H) thin films. pm-Si:H thin films consist of a low volume fraction of silicon nanocrystals embedded in a silicon matrix with medium range order, and they possess this morphology as a significant contribution to their growth comes from the impact on the substrate of silicon clusters and nanocrystals synthesized in the plasma. Quadrupole mass spectrometry, ion flux measurements, and material characterization by transmission electron microscopy (TEM) and atomic force microscopy all provide insight on the contribution to the growth by silicon nanocrystals during PECVD deposition. In particular, cross-section TEM measurements show for the first time that the silicon nanocrystals are uniformly distributed across the thickness of the pm-Si:H film. Moreover, parametric studies indicate that the best pm-Si:H material is obtained at the conditions after the transition between a pristine plasma and one containing nanocrystals, namely a total gas pressure around 2 Torr and a silane to hydrogen ratio between 0.05 to 0.1. From a practical point of view these conditions also correspond to the highest deposition rate achievable for a given RF power and silane flow rate.
Highlights
Nanostructure appears after an amorphous incubation layer
For both the a-Si:H and polymorphous silicon (pm-Si):H process conditions, there is a decrease in the SiHx signal, which is attributed to the fact that the silane is dissociated and consumed by the deposition
An interesting point is that poly-silane species, such as Si2Hx and Si3Hx, are significantly detected during pm-Si:H deposition while they are absent under a-Si:H deposition conditions
Summary
Nanostructure appears after an amorphous incubation layer. The mixed-phase nature of pm-Si:H has been assumed to be homogeneous through the film thickness, presumably because of the different deposition mechanism. This is an important point for practical applications because the nanostructure and physical properties of pc-Si:H mainly depend on both hydrogen dilution and film thickness while those of pm-Si:H are independent of thickness. We focus on this nanocrystal-based growth process and for the first time provide experimental evidence via Transmission Electron Microscopy (TEM) of the homogeneous distribution of silicon nanocrystals through the thickness of the layer. The correlation between the plasma conditions (in particular the ion flux) and the structure of the deposited material is carefully examined by real time plasma monitoring and characterization of thin film material properties
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