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Silicon nitride is a polyvalent coating for silicon solar cells: Electrical passivation, chemical passivation and anti-reflection properties on silicon. In addition to the low temperature budget of the Plasma Enhanced Chemical Vapour Deposition it is a method which allows us to tune very easily the parameters of deposition and thus the parameters of the layers deposited. The study of the modifications of every parameter of deposition is long and tedious work. In this work, in order to investigate two important properties of the SiNx (optical and electrical passivation properties), only the modifications induced by variation of the ratio of the precursor gases and the temperature of deposition were studied. Comparison with silicon oxide and amorphous silicon were performed to investigate the charge-carrier kinetics at the different interfaces in the case of the inverted a-Si:H/c-Si heterocontact solar cell.

Concerning the deposition parameters, the ratio of the silicon and nitrogen content inside the layers was found to be proportional to the ratio R of the precursor gases in the plasma (SiH4/NH3). The depositions are homogeneous in the plane of the layer concerning the refractive index and the thickness. The increase of the ratio R was studied. It had the effect of an increased deposition rate, an increased refractive index and a higher number of Si-H bonds in the layers. On the other hand, the increase in the temperature of deposition decreases the deposition rate but increases the refractive index. Structural characterizations performed with ERDA show that the hydrogen content stays the same independently of the ratio R.

The optical properties of silicon nitride were confirmed to be appropriate for an anti- reflection front coating for the solar spectrums maximum. The SiNx has a really low absorption for a large range of the ratio of SiH4/NH3 of the precursor gases {0.16 until 1}. For a ratio R lower than 1 the absorption at 520 nm is lower than 0.7%. However, for high R (R=5), a formation of clusters of silicon in the amorphous SiNx matrix was found. These clusters lead to a strong increase of the absorption of the light.

0.03%. This value is very small but now further work has to be performed in order to reduce the overall reflection, with multiple layer systems. These two optical properties show the advantage of using a SiNx layer as front coating to maximize the light coupling.

The electrical passivation induced at the surface by a coating of silicon nitride was also observed to be excellent and appropriate for silicon solar cells. This effect is especially efficient when the SiNx is deposited on n-type silicon because of the accumulation layer created at the silicon surface. On p-type silicon it was found that the mobility of the electrons at the interface c-Si/SiNx is reduced. This effect does not appear with silicon oxide. The electrical passivation is not strongly influenced by the silicon content in the SiNx, the best result was found for low silicon content (R=0.16 at 350°C). Very low surface recombination velocities (< 20 cm.s-1) were obtained showing the excellent electrical passivation. The temperature of deposition has more influence on the passivation effect. The surface recombination velocity increases to 60 cm.s-1 for deposition at 300°C. This electrical passivation effect will increase the excess charge-carriers lifetime and will therefore increase the photocurrent (and thus the efficiency of a solar cell).

During this work a problem appeared. The possibility to reproduce a layer with the same electrical passivation properties was found to be very difficult. A variation of ± 47% for the lifetime was observed for samples deposited under the same conditions. The reasons for this variation were not clearly determinable. However, it was observed that a reduction of this variation is possible by the use of a thin silicon oxide layer between the silicon substrate and the SiNx coating. The variation of the lifetime for samples with the thin oxide layer is reduced to ±15%.

Some remarkable effects were also observed. The formation of silicon clusters at high silicon content was observed with ellipsometrical measurements. This effect can be reasonably attributed to the increase of the silicon contents inside the layer. Such observations have been reported in the past but the mechanisms [VI.1] of formation need to be clarified. Another phenomenon observed was the formation of well-like holes with a diameter between 30 μm and 400 μm in the silicon nitride layer during strong annealing. Even if this process

was proven to destroy the passivation effect of the layer, it would be interesting to understand and explain the formation of these wells (maybe for other applications)

The properties of the silicon nitride as a front coating were proven to be appropriate for silicon solar cells. This work has also shown that these properties are sensitive to the deposition parameters and further studies of the effect of the deposition parameters are needed for two reasons:

- The mechanisms behind these effects must be understood more clearly.

- The properties of the SiNx layer have to be optimized in order to fit as best as possible with our needs and optimize the efficiency of our cells.