New Si-based multilayers for solar cell applications
© Nalini et al; licensee Springer. 2011
Received: 24 September 2010
Accepted: 18 February 2011
Published: 18 February 2011
In this article, we have fabricated and studied a new multilayer structure Si-SiO2/SiN x by reactive magnetron sputtering. The comparison between SiO2 and SiN x host matrices in the optical properties of the multilayers is detailed. Structural analysis was made on the multilayer structures using Fourier transform infrared spectroscopy. The effect of specific annealing treatments on the optical properties is studied and we report a higher visible luminescence with a control over the thermal budget when SiO2 is replaced by the SiN x matrix. The latter seems to be a potential candidate to replace the most sought SiO2 host matrix.
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 [1–3]. Ever since the discovery of the visible luminescence of the porous Si by Canham  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) [5–8]. Among the methods of obtaining the Si nanostructures we cite electrochemical etching [4, 9], fabrication of silicon dots by plasma sputtering technique , 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 . 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 SiNx 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.
Results and discussions
In SRSO/SiO2 around 1225 and 1080 cm-1 we notice the LO3 and TO3 peak from the Si-O stretching, the TO4-LO4 doublet between the 1100-1200 cm-1 and the TO2-LO2 asymmetric stretching of Si-O from SiO2 at 810 and 820 cm-1, respectively . The presence of Si-nc is attested by the intensity of the LO3 peak which is representative of the Si-O bond at the interface  between silicon and silica while the TO3 vibration mode at about 1080 cm-1 is the signature of the volumic silica.
The SiN x /SiO2 film has a broad peak in the 1250-950 cm-1 region which can be due to the contributions of both LO and TO modes from SiO2 and Si-N stretching mode [15–17]. The absorption band located around 860 cm-1 could be attributed to the Si-N asymmetric stretching mode.
In the case of SRSO/SiN x films, the shoulder around 1190 cm-1 may be due either to N-H bond [16, 18] or to a contribution of the LO3 mode of Si-O-Si bonds at 180° . Such a result is the signature of the Si nanoparticles formation within either the SiN x  and/or the SRSO sublayer . Between 1050 and 1070 cm-1 lies the LO peak of a-Si x N y H z from Si-N as it has been observed in the SiN x /SiO2 spectrum adding the contribution of the TO Si-O mode.
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 . 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. .
rapid thermal annealing
silicon-rich silicon oxide.
This study is supported by the DGA (Defense Procurement Agency) through the research program no. 2008.34.0031.
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