In this work, we investigate how the film properties of silicon nitride (SiNx) depend on its deposition conditions when formed by plasma enhanced chemical vapour deposition (PECVD). The examination is conducted with a Roth & Rau AK400 PECVD reactor, where the varied parameters are deposition temperature, pressure, gas flow ratio, total gas flow, microwave plasma power and radio-frequency bias voltage. The films are evaluated by Fourier transform infrared spectroscopy to determine structural properties, by spectrophotometry to determine optical properties, and by capacitance–voltage and photoconductance measurements to determine electronic properties. After reporting on the dependence of SiNx properties on deposition parameters, we determine the optimized deposition conditions that attain low absorption and low recombination. On the basis of SiNx growth models proposed in the literature and of our experimental results, we discuss how each process parameter affects the deposition rate and chemical bond density. We then focus on the effective surface recombination velocity Seff, which is of primary importance to solar cells. We find that for the SiNx prepared in this work, 1) Seff does not correlate universally with the bulk structural and optical properties such as chemical bond densities and refractive index, and 2) Seff depends primarily on the defect density at the SiNx-Si interface rather than the insulator charge. Finally, employing the optimized deposition condition, we achieve a relatively constant and low Seff,UL on low-resistivity (≤1.1 Ωcm) p- and n-type c-Si substrates over a broad range of n = 1.85–4.07. The results of this study demonstrate that the trade-off between optical transmission and surface passivation can be circumvented. Although we focus on photovoltaic applications, this study may be useful for any device for which it is desirable to maximize light transmission and surface passivation

Titanium dioxide is shown to afford good passivation to non-diffused silicon surfaces and boron-diffused surfaces after a low-temperature anneal. The passivation most likely owes to the significant levels of negative charge instilled in the films, and passivation is enhanced by illumination-advantageous for solar cells-indicating that a titanium dioxide photoreaction is at least partly responsible for the low surface recombination. We demonstrate a surface recombination velocity of less than 30 cm/s, on a 5-Ω cm n-type silicon, and an emitter saturation current density of 90 fA/cm 2 on a 200-Ω/sq boron diffusion. If these titanium dioxide passivated boron-diffused surfaces were employed in a crystalline silicon solar cell, an open-circuit voltage as high as 685 mV could be achieved. Given that TiO 2 has a high refractive index and was deposited with atmospheric pressure chemical vapour deposition, an inexpensive technique, it has the potential as a passivating antireflection coating for industrial boron-diffused silicon solar cells.

We report the fabrication and characterization of TiO2 sol–gel diffraction gratings on silicon substrates by using nanoimprint lithography. The gratings are homogeneous and free of defects and cover an area of 25 cm2. Minority carrier lifetimes of up to 900 µs for imprinted samples under illumination are reported, which Kelvin probe measurements indicate is due to light‐generated negative charge in the films. The structures reported here are very promising as light‐trapping, passivating coatings for solar cells. Copyright © 2011 John Wiley & Sons, Ltd.

No abstract

The charge density of a TiO2 film deposited on a SiO2 -passivated silicon wafer is determined. The TiO2 is deposited by atmospheric pressure chemical vapor deposition at 400°C , and the SiO2 is grown thermally at 950°C . This TiO2–SiO2 stack is a useful coating for the front surface of a silicon solar cell, as it has a high optical transmission and a low density of interface states Dit(E) at the SiO2–Si interface. While these properties are beneficial to high efficiency solar cells, so too is a large charge density, as what occurs in Si3normalN4–SiO2 (+1012cm−2) and Al2normalO3–SiO2 (−1013cm−2) stacks. The Dit(E) and charge density of TiO2 -coated and SiO2 -passivated silicon are evaluated by capacitance–voltage and Kelvin probe measurements. The charge density of the TiO2 is within the conservative limits of −8.5 and −1×1011cm−2 after deposition and of −10 and +1×1011cm−2 after a subsequent 800°C oxygen anneal. Photoconductance measurements suggest that the dangling-bond defects at the SiO2–Si interface are predominantly donorlike and, hence, that the change density in the TiO2 is closer to the upper limits (less negative); this charge is too small to benefit solar cells.