The third generation of solar cells aims at reducing the cost and at improving the efficiency. Thin film solar cells based on silicon nanostructures is one of the most researched system to achieve such a target 1 2 3 . Ever since the discovery of the visible luminescence of the porous Si by Canham 4 various research groups have exploited the room temperature photoluminescent nature of silicon by fabricating different kinds of Si-based nanostructures. The luminescence is attributed to the quantum confinement of carrier in Si-nanoclusters (Si-nc) 5 6 7 8 . Among the methods of obtaining the Si nanostructures we cite electrochemical etching 4 9 , fabrication of silicon dots by plasma sputtering technique 10 , and multilayer approach 8 11 12 .

The important part of the ongoing research involves Si-nc embedded in an amorphous matrix such as SiO₂, SiN _x , or amorphous silicon. Though Si-nc embedded in SiO₂ is the most common structure, the problem of carrier injection in this matrix comes as a major drawback owing to the large band gap of SiO₂. Hence the replacement of SiO₂ by other dielectric matrices with smaller bandgap turns out to be a solution. SiN _x matrix meets up these requirements and hence Si-nc embedded in SiN _x matrix has become a material of choice in the recent past. In this article, we develop a new multilayer composition silicon-rich silicon oxide (SRSO)/SiN _x to overcome the insulating nature of SiO₂ by taking advantage of the reduced bandgap in SiN _x . Using SiN _x as the host matrix favors the electrical conductivity of carriers while we still maintain the quantum confinement as done with the SiO₂ matrix. This study aims at fabricating and comparing the light emission properties of three different kinds of multilayer compositions: (a) SRSO/SiO₂, (b) SRSO/SiN _x , (c) SiN _x /SiO₂. Such a study is important to understand the influence of host matrices on the Si-nc and consequently to achieve an optimized solar cell device in the future.

Three kinds of multilayer structures were fabricated on 2" Si wafer by reactive magnetron sputtering comprising 50 patterns of SRSO/SiO₂, SRSO/SiN _x , and SiN _x /SiO₂. We define the gas flow rate as r _g = f _g/(f _g + f _Ar) where f _g represents the N or H₂ gas flow and f _Ar represents the Argon gas flow. The SiO₂ sublayer was fabricated by sputtering the SiO₂ cathode under pure Ar plasma. Reactive magnetron sputtering, an approach developed by our team, was used for the fabrication of SRSO sublayers. It takes advantage of the oxygen reducing capacity of hydrogen when introduced into the Ar plasma 8 . The hydrogen-rich plasma favors Si excess in the SiO₂ sublayer. Besides this in order to facilitate a higher incorporation of Si in the matrix, both SiO₂ and Si cathodes were used to fabricate the SRSO sublayer. The powers of SiO₂ and Si were maintained as 7.4 and 2.2 W/cm², respectively. The hydrogen rate r _H was maintained at 50% while the total flow f _g + f _Ar was fixed at 10 sccm. The pressure in the chamber was chosen as 3 mTorr. Thus the SRSO/SiO₂ multilayer structure was deposited by an alternative reactive sputtering under hydrogen-rich plasma for the SRSO layer and pure Ar plasma for the SiO₂ sublayer. The SiN _x layer was fabricated by sputtering the Si cathode and simultaneously introducing nitrogen into the Ar plasma. The nitrogen rate r _N was kept at 10% while the total flow rate was fixed at 10 sccm. The pressure in the chamber was chosen as 2 mTorr for SiN _x layers. The temperature of deposition was maintained at 500°C for all the cases. The thickness of the SRSO sublayer was fixed to be 3.5 nm in order to be within the quantum confinement regime. In order to understand the influence of SiN _x matrix, two different thicknesses of the SiN_x sublayer (3.5 and 5 nm) were chosen.

The FTIR spectra of these samples were recorded in absorption configuration using Nicolet Nexus spectrometer at Brewster's angle (65°). The photoluminescence (PL) spectra of the annealed samples were obtained in the visible range using Jobin Yvon monochromator in the wavelength range 550-1100 nm. The excitation wavelength of 488 nm (Ar laser) was used for measurements.

The PL spectra of the SRSO/SiN _x subjected to two different annealing treatments show that the quenching of the PL signal after an RTA can be attributed to the non-radiative defects either at the interface of Si-nc and the SiO₂ matrix or within the SiO₂ matrix itself which traps the photon arising from the recombination of the exciton within the Si-nc. On the contrary, it can be seen that the SiN _x sublayer favors luminescence even if this later could be attributed to the defects in the matrix. Noticing the shift in emission peak from 1.9 to 1.6 eV in the case of SiN _x /SiO₂ and SRSO/SiN _x , respectively, it can be said that the sandwiching of SRSO between SiN _x instead of SiO₂ sublayers not only favors luminescence but also exhibits luminescence in a region attributed to the emission from Si-nc. This implies that though at this temperature SiN _x shows a defect-related PL, when alternated with SRSO, the emission from Si-nc becomes dominant.

On the other hand, the quenching of PL in classically annealed SRSO/SiN _x is quite surprising as several authors have noticed an increase of the PL signal either from SRSO or SiN _x after such annealing. It also should be noted that the 'SRSO sublayer' fabricated under the same conditions and alternated with SiO₂ sublayer has a high emission. Hence one can conclude that the presence of the SiN _x sublayer quenches the PL. This can be attributed either to the coalescence of Si clusters at such an annealing treatment thereby overcoming the quantum confinement regime or to the non-radiative defects at the interface between SRSO and SiN _x or in SiN _x . The increase of the PL emission when increasing the number of layer could be the result of H diffusion during the deposition process which favors the recovering of the defects and the Si nanoparticles formation. Such a hypothesis is supported by the presence of N-H bonds revealed by FTIR experiments in the non-annealed multilayers and that can be attributed to the Si-nc formation 17 . Another explanation could be the increase of strain with the number of layer that favors the Si-np formation resulting in an increase of the Si-np density and hence in the PL emission. However, the comparison in the inset of Figure 3 of the two types of multilayers demonstrates the advantage to replace the SiO₂ sublayer by the SiN _x . HRTEM experiments are in progress to understand the optical behavior of these multilayers.

The multilayers were fabricated using the sputtering technique and the FTIR spectrum revealed its characteristic peaks. Although SiO₂ is the most sought host matrix, we evidenced the interest of replacing it with the SiN _x matrix. A higher intensity of PL emission was obtained for RTA when SiN _x matrix was used whereas from the SiO₂ matrix there was no considerable intensity at such an annealing treatment. We have achieved comparable intensity of emission within one minute of annealing and at a lesser temperature, in comparison to the classical annealing treatment that is done for longer time and slightly higher temperature. We also observe an increase in the PL emission with increase in the number of periods. High-resolution electron microscopy experiments are in progress to understand the effect of the annealing process on the achieved optical properties. This set of above-mentioned results paves the way for the fabrication of novel structures for solar cell device applications similar to the one recently reported by Di et al. 20 .

PL: photoluminescence; RTA: rapid thermal annealing; Si-nc: Si-nanoclusters; SRSO: silicon-rich silicon oxide.

The authors declare that they have no competing interests.

RPN fabricated the multilayers under investigation and carried out the characterization studies.CD and JC made significant contribution to the optical properties and interference effect. FG conceived of the study and participated in the coordination and writing of the manuscript. All authors read and approved the final manuscript.