Ion bombardment effects on microcrystalline silicon growth mechanisms and on the film properties

Abstract

The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest that ion bombardment, and particularly the implantation of hydrogen ions, favors the formation of a porous layer where the nucleation of crystallites takes place. These results are further supported by in situ spectroscopic ellipsometry measurements of the film morphology as a function of the ion energy.

Surface passivation of c-Si for silicon heterojunction solar cells using high-pressure hydrogen diluted plasmas

“…18 At the same time it has been shown that such attempts to increase the hydrogen content and improve the passivation quality can significantly alter the nanostructure of a-Si:H leading to void-rich material and in some cases cause irreversible damage at the interface. In this work we demonstrate PECVD conditions using high hydrogen dilution (200 sccm H 2 compared to 2.5 to 10 sccm SiH 4 ), low power density (∼0.05 W/cm 2 ), and relatively high deposition pressure (8 mbar) 20 that result in material close to the a-Si:H-to-µc-Si:H transition regime, passivating c-Si and achieving effective lifetimes (τ eff ) > 10 ms. As a result of the low power density and high deposition pressure the ion bombardment energy is decreased, 21 thereby minimizing possible damage on and just below the surface of the c-Si substrate while shifting the a-Si:H-to-µc-Si:H transition to higher dilutions and allowing to deposit amorphous silicon at extremely high dilution conditions. We show how the τ eff correlates to changes in the nanostructure of the material, which in turn is varied by changing the hydrogen dilution and substrate temperature.…”

Ultra-thin crystalline silicon films produced by plasma assisted epitaxial growth on silicon wafers and their transfer to foreign substrates

“…A further increase in H 2 dilution results in a decrease of the layer thickness. The reason for such behavior may result from a trade off between deposition rate and the hydrogen desorption from the growing film [21]. In Figure 2b also is shown the structural composition of the interface layer as a function of H 2 flow rate.…”

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

“…42 In general, the improvement of the electrical properties of the cells may be due to the presence of a denser microstructure and larger grains as a result of lower ion bombardment energy, as previously observed. 19,28,43 Local amorphization of the grains through ion bombardment is likely caused by heavy ions through ion-induced Si bulk displacement mechanism. 41 Also, when working in capacitively coupled VHF SiH 4 / H 2 discharges in this range of pressure, previous studies 42,44 would tend to suggest that ions, the most likely responsible for this amorphization process, are the monosilicon hydride ion group SiH m + and polysilicon hydride ion groups Si nՆ2 H m + .…”

“…[12][13][14] Second, process parameters, easily adjustable for a given reactor: power density, pressure, substrate temperature, and input gas flows. [15][16][17][18][19] Substrate morphology has been shown to be important as well. 20,21 Thus, much of the research effort is directed towards the understanding of how these externally adjustable parameters affect the internal physical plasma characteristics upon which the material quality of c-Si: H ultimately depends.…”