Use of radio frequency power, silicon tetrafluoride and methane as parameters to tune structural properties of hydrogenated microcrystalline silicon carbon alloys

International audienceIn the search for a material with electrical properties similar to those of amorphous silicon or amorphous silicon germanium, but stable under light soaking, hydrogenated microcrystalline silicon–carbon alloy (µc-Si1 − xCx:H) thin films are a promising candidate. The interest in these materials lies in the possibility of varying the effective band gap by changing the amount of carbon in the alloy composition, while keeping a high crystalline fraction to maintain stability under light-soaking. In this study, µc-Si1 − xCx:H thin films were deposited by radio frequency (RF) plasma enhanced chemical vapour deposition using a silane and methane gas mixture highly diluted in hydrogen. Three deposition parameters were investigated as a means to control the film crystallinity: the RF power density, the methane flow rate and the presence of a small amount of silicon tetrafluoride in the gas mixture. Although all three parameters can result in materials with a Raman crystalline volume fraction above 50%, it is shown that they result in very different microstructures, as evidenced by x-ray diffraction and scanning transmission electron microscopy analyses. A growth model is proposed to explain the influence of each of these parameters on the final film structure

Explicit analytical modeling of the low frequency a-Si:H/c-Si heterojunction capacitance: Analysis and application to silicon heterojunction solar cells

International audienc

Influence of the undoped a Si H buffer layer on a Si H c Si heterojunctions from planar conductance and lifetime measurements

International audienceIn highly efficient amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si) solar cells, the c-Si wafer is passivated by a nanometer-thin buffer layer, which is undoped amorphous silicon. Here, we report on the systematic measurement of the passivation quality (minority carrier effective lifetime) by photo-conductance decay and of the band bending in c-Si using the planar conductance technique. The thickness of the buffer layers is varied. An analytical model to calculate the band bending in c-Si is presented; it aids in understanding the influence of the buffer layer on the band bending. We find that when the buffer layer thickness increases the passivation quality increases and the band bending decreases. Therefore, we suggest that an optimum has to be found to reach good interface defect passivation and a high band bending

Conductivity and surface passivation properties of boron-doped poly-silicon passivated contacts for crystalline silicon solar cells

International audienc

Structural properties of hydrogenated microcrystalline silicon–carbon alloys deposited by Radio Frequency Plasma Enhanced Chemical Vapor Deposition: Effect of microcrystalline silicon seed layer and methane flow rate

International audienceHydrogenated silicon–carbon thin films were deposited by Radio Frequency Plasma Enhanced Chemical Vapor Deposition using silane–methane gas mixtures highly diluted in hydrogen. The effects of the presence of a microcrystalline silicon seed layer and of the methane flow rate during deposition were investigated in order to find the optimal conditions for microcrystalline growth. The presence of a seed layer promotes the amorphous to microcrystalline transition at the interface with the substrate, reducing the incubation layer to a 5–10 nm thickness. At the same time, an excessive increase in the CH4 flow rate suppresses crystalline growth, leading to extremely flat amorphous samples (root-mean-square surface roughness ρRMS ~ 0.5 nm), even with a seed layer. The total carbon content was measured by Radio Frequency Glow Discharge Optical Emission Spectroscopy. It was found that carbon is only incorporated in the amorphous tissue, and not in the crystalline phase. Thus, the increase of the crystalline fraction, directly linked to the increase of the seed layer deposition time, leads to a decrease of the carbon incorporation in the subsequent layer