Black Silicon: Microfabrication Techniques and Characterization for Solar Cells Applications

Transcription

1 doi: /ijes Black Silicon: Microfabrication Techniques and Characterization for Solar Cells Applications A. Deraoui 1,2, A. Balhamri 3,4, M. Rattal 1, Y. Bahou 1, A. Tabyaoui 1, M. Harmouchi 1, Az. Mouhsen 1 and E. M. Oualim 1 1 Univ Hassan 1, Laboratoire Rayonnement et Matière, Settat, Maroc 2 Institute of Condensed Matter and Nanosciences Nanophysics (IMCN/NAPS), Université Catholique de Louvain, Chemin du Cyclotron 2, B 1348 Louvain la Neuve, Belgium 3 Univ Hassan 1, Ecole Supérieure de Technologie, 218 Berrechid, Maroc 4 Research Institute for Materials and Engineering, Chimie des Interactions Plasma Surface (ChIPIS), CIRMAP, Université de Mons, Place du Parc 20, B 7000, Belgium a.deraoui@gmail.com, adil.balhamri@gmail.com, mourad.rattal@gmail.com, ylouch2000@gmail.com, ylouch2000@gmail.com, mharmouchi@hotmail.com, az.mouhsen@gmail.com, oualim.elmostafa@gmail.com Abstract The properties of black silicon are well suited for use in photovoltaic applications. We plan to use the advantages of black silicon to build new types of solar cells that will be able to capture solar energy across a wider range, thereby realizing greater efficiency in future photovoltaic cells (J.S. Yoo 2006) (K. Nishioka 2009) (Y.M. Song 2010) (Y. Xia 2011). The method of production has brought the use of black silicon in photovoltaic arrays closer to commercial realisation, as it will reduce the cost of black silicon production, minimize reflectivity and maximize the absorbance (B.S. Kim 2011) (H. Sai 2006) (H. Sai 2007) (K. Tsujino 2006) (K. Nishioka 2008). The basic objective of this paper is to create black silicon by chemical wet etching techniques using Au nano particles directly formed on silicon substrates. We further decided to try to optimize this material using non heavily photolithographic methods, such as etching. The morphology of Black Si was characterized by Scanning Electron Microscopy (SEM) and the optical properties were given thanks to Cary Spectrometer. more absorbed and less reflected (figure 2). This is what gives black silicon its name. FIG. 1 SEM IMAGE OF A PYRAMID TEXTURED BLACK SILICON SURFACE Keywords Solar Cells; Black Silicon; Photolithographic; Thin films Introduction Black silicon is a material that is chemically equal to normal silicon. The only difference between these two is the surface treatment that changes the morphology. This special morphology has to represent a certain roughness of the surface and it is typically done by creating pyramid textures (figure 1) or trenches within the silicon. When these structures are in place, and when they are small enough, incident light will be FIG. 2 SCHEMATIC REPRESENTATION OF LIGHT INTERACTION WITH A NORMAL SILICON SURFACE (LEFT) AND A BLACK SILICON SURFACE (RIGHT) Due to its increased absorption this material shows a lot of promising of applicability in photoelectric devices, such as solar panels. The need for new energetic sources has stimulated considerably the drive for better renewable energy efficiency in which solar is included. Since black silicon can absorb more 403

2 International Journal of Energy Science (IJES) Volume 3 Issue 6, December 2013 light due to its structure, it has the potential to be a major innovation in this domain. The biggest challenge for the moment resides in the making of black silicon, preferably using cheaper and simpler techniques. The fact that it is based on a well known material and that it shows real promise in enhancing photoelectric devices has made black silicon an interesting topic for researchers. The basic objective of this paper is to exploit different techniques of micro fabrication to create black silicon and then characterize it.; and further this material is determined to be built using non heavily photolithographic methods, such as etching (J. Zhao 1998). In this paper the wet etching process was developed to create black silicon. cleaning as follow: they were immersed in a sulfuric acid hydrogen peroxide mixture (SPM) for 10 min, rinsed with ultra pure water for 10 min and dipped in dilute hydrofluoric acid 2% HF for 15 seconds. Three different wafers have been prepared and they were all cleaned with the above method as shown in the figure below. Since now, we will use notations sample 1 for the one where oxidized layer was deposited, sample 2 for the one silicon nitride layer was deposited and sample 3 for the reference wafer where there was no insulation layer on it. Experimental Setup Wet etching methods have been developed to fabricate high aspect ratio holes in silicon. In this method, deep straight holes or trenches can be produced by wet chemical etching, which is suitable for mass production, in solutions containing hydrofluoric acid (HF). It is a simple and fast technique that creates these textures using wet chemical processes. The following mechanism is proposed: Cathode reaction (at metal): H2O2 + 2H + 2H2O + 2h + (2 1) 2H+ 2e H2 (2 2) Anode reaction (Si) : Si + 4h + + 4HF SiF4 + 4H + (2 3) SiF4 + 2HF H2SiF6 (2 4) Overall reaction: Si + H2O2 + 6HF 2H2O + H2SiF6 + H2 (2 5) Methods using thin metal films or particles loaded on silicon wafers for assisting wet chemical etching have recently been developed. These methods utilize the catalytic actions of metals for dissolution of silicon in HF based solutions. Since these methods do not need electrochemical equipment, they seem to be suitable for mass production. It has been found that cylindrical holes with diameters of tens nanometers were formed in silicon in the (100) direction by wet chemical etching in an aqueous solution containing HF and hydrogen peroxide (H2O2) when gold nanoparticles were loaded on the silicon surface before the etching process. As the holes were generated, gold particles sank into the bulk of silicon. Polished Si (100) wafers were used in the fabrication of black silicon. The Si samples were cleaned by standard FIG. 3 BASIC PROCESS FOR CREATING BLACK SILICON SURFACE WITH GOLD NANOPARTICLES On sample 1, 400 nm of oxide layer was deposited using oxidation wet technique and on sample 2, 200 nm silicon nitride (Si3N4) was deposited using Low pressure Chemical Vapor Deposition (LPCVD) technique. The temperature of deposition for this oxide layer is 1000 C ( K). Oxidation wet was used since it gives a better quality of oxide than the dry oxidation. On the third sample, no insulating layer was deposited since we want to study the effect of insulating layers on the performance of black silicon in relation with its optical and electrical properties by using this sample as a reference one. Then about 300 nm poly silicon (PSi) was deposited on sample 1 and sample 2 by LPCVD technique at 625 C. Then 1 nm, 3 nm and 7 nm of Au metal were deposited on samples 1, 2 and 3 by metal evaporation technique. This was performed by lift off set up on the front side of our samples. This could be achieved by controlling the rate of the evaporation. These samples were annealed at 500 C for 10 minutes. Although annealing is the basic step in making black silicon with wet etching, we have made sample without annealing to see the effect. Of course, this was done since it was not possible to make 404

3 annealing on our samples even though they could be helpful for comparison. The samples were etched in aqueous solutions containing HF and H2O2 after the samples were cleaned with H2O for deionization. The concentration of HF in solutions was 49%, and H2O2 was 30%. The importance of Gold in the wet etching was to use them as a catalyst. Unless we use catalysts to speed up the reaction, the whole process will be nothing except simple slow etching of the SiO2 layer, which doesn t have technological importance. First when the Si layer is exposed with H2O2, SiO2 and H2 gas will be removed from the product. Then the samples were etched with 49% HF. In this process there will be creation of SiF6 in the form of gas. As soon as this gas is evaporated from the sample, the gold metal particles will sink down, creating lots of high aspect ratio holes which will be the black silicon. 4(b) shows the same sample but after it was etched with HF and H2O2 for 120 seconds. From this figure we can see black dots on the brighter background. These black spots are supposedly the holes where the metal particles sink down. This could be further argued by the fact that the average size of these black spots (holes) is 122 nm which is in a good correlation with the average size of the Au particles. These two pictures are very critical in explaining the whole process of making the black silicon. The effect of the metal layer thickness on the penetration depth of the particles has been studied from their respective SEM images. The results are presented in the figures below: a b c Results and Discussion Morphologies and Cross sectional views of the samples were observed by scanning electron microscopy (SEM). With Scanning microscope, it is possible to compare the difference in morphology between the reference sample and those with different structures and layers. Even with naked eye, it is possible to see the difference between them. Figure below shows SEM images of black silicon sample where the thickness of the Au metal is 7 nm. The annealing temperature of these samples was 500ºC and the annealing time was 10 minutes as explained on the processing part. FIG. 5 SEM IMAGES OF SAMPLES AFTER ANNEALING 1nm Au LAYER (a), 3nm Au LAYER (b) AND 7nm Au LAYER (c) a b c a b FIG. 6 SEM CROSS SECTIONAL AREA IMAGES OF SAMPLES AFTER ANNEALING 1nm Au LAYER (a), 3nm Au LAYER (b) AND 7nm Au LAYER (c) FIG. 4 SEM IMAGE OF SAMPLE AFTER ANNEALING (a) AND AFTER ETCHING (b) From figure 4(a), we can see that when Au metal is annealed at high temperature, the metal atoms will diffuse and make gold particles. The temperature of this annealing is made high so as to increase the kinetics of diffusion, Au atoms will make nano sized particles. From this figure we can see clearly spaced gold particles with an average size of 131 nm. Figure The average size of the Au metal particles increases as the thickness of the metal layer increases. For the three images shown in figure 5, the average size of the grains is found to be 20, 40 nm and 120 nm respectively. This increase in the particle size is associated with the flux of particles diffusing when the temperature is high enough. When there is only 1 nm layer of Au atoms, there will be less number of atoms making the particles than the 7 nm layer even though the annealing temperature and the annealing time are kept the same. All the measurements were tested at the wavelength from 250 to 1100 nm at room temperature with a spectral resolution of 15 nm. The reflectance and the absorbance of each of the four samples were measured using a purpose built integrating sphere attachment of a high accuracy spectrometer. 405

4 International Journal of Energy Science (IJES) Volume 3 Issue 6, December 2013 FIG. 7 NORMAL INCIDENCE SPECTRAL REFLECTANCE FROM BLACK SI SURFACE ETCHED WITH VARIOUS Au FILM THICKNESSES (1, 3 AND 7 nm) AND FROM UNTREATED SILICON SURFACE Figure 7 shows a typical normal incidence spectral reflectance of black Si surface. The reflectance of the untreated silicon is also plotted for comparison. The reflectance of the black Si sample using 7 nm Au film was the lowest. The reflectance was maintained at almost 0% for the visible region as well as the near IR region. The samples of Black Si using 1 nm and 3 nm Au film were also maintained at less than 3%. The reflection decreased drastically compared to untreated silicon surface as expected. The thickness of deposited thin film of gold on the surface played an important role and changed the value of the reflectance. It decreased when the thickness of Au film increased. We can conclude that our treatment changes significantly the optical properties of Black Si samples. The absorbance of black silicon is greater than 85%, in the wavelength range from 250 to 1100 nm, it reached 96% for the sample of black Si using 7 nm Au film and the absorbance of untreated silicon in the UV and NIR was much lower than the black silicon one and the absorbance bandwidth of black silicon was much wider than the untreated silicon one. This is mainly due to the micro structured surface of black silicon, such as column and pores, giving rise to the multireflection of the incident light and increasing the area of absorbing surface. The photon is reflected several times by the micro column and nanopores, which will increase its absorbing probability. Due to no introduction of impurity into the silicon, there is no extra absorbing energy level in the gap of black silicon. Consequently, the photons with the energy below band gap cannot be absorbed. Thus, the spectrum of absorbance of the black silicon achieved by wet etching is similar to the absorbing spectrum of the common untreated silicon at the wavelength larger than 1100 nm. Conclusions In this study, experiments were undertaken in order to determine the optimized process using various sizes of Au nano particle. The Au nano particles were fabricated via thermal annealing of Au thin films with 1, 3 and 7 nm thickness. It was found that the initial Au film thickness can affect the particle size and change the etching conditions and optimize optical properties of the surface. The absorption was significantly increased and the reflection decreased drastically in the wavelength range from 250 to 1100 nm after the formation of the antireflection, and if applied to a solar cell, it will greatly increase the conversion efficiency. REFERENCES FIG. 8 NORMAL INCIDENCE SPECTRAL ABSORBANCE FROM BLACK SI SURFACE ETCHED WITH VARIOUS Au FILM THICKNESSES (1, 3 AND 7 nm) AND FROM UNTREATED SILICON SURFACE B.S. Kim, J.H. Sung, J. Won Ki, M.W. Lee, C.H. Choi, S.G. Park, S.G. Lee, E.H. Lee, O. Beom Hoan, Microelectron. Eng. 88 (2011): H. Sai, H. Fujii, et al., Appl. Phys. Lett. 88 (2006): H. Sai, Y. Kanamori, K. Arafune, Y. Ohshita, M. Yamaguchi, Prog. Photovoltaic. Res. Appl. 15 (2007): J.S. Yoo, I.O. Parm, et al., Sol. Energy Mater. Sol. Cells 90 (2006): J. Zhao, A. Wang, M.A. Green, F. Ferrazza, Appl. Phys. Lett. 73 (1998): K. Nishioka, T. Sueto, et al., Appl. Surf. Sci. 255 (2009):

5 9507. K. Tsujino, M. Matsumura, Sol. Energy Mater. Sol. Cells 90 (2006): Y.M. Song, E.S. Choi, G..C. Park, C.Y. Park, S.J. Jang, Y.T. Lee, Appl. Phys. Lett. 97 (2010): K. Nishioka, S. Horita, et al., Sol. Energy Mater. Sol. Cells 92 (2008): Y. Xia, B. Liu, J. Liu, Z. Shen, C. Li, Sol. Energy 85 (2011): Abdessitir DERAOUI was born in Meknes Morocco in He is a teacher assistant at Catholic University of Louvain Belgium. He received his M.S in physics from Hassan 1st university of Settat Morocco. He is currently PhD student at the same University. His thesis topic is solar energy, it including developing and improving PV cells in several material systems, Characterizing PV cells, modules, and systems to improve performance and reliability and PV engineering. His scientific interests include design of Micro and Nanosystems, Microfacbrication techniques, biomimetic transfer of reverse engineered mechanisms to material science and vacuum physics and techniques. Adil BALHAMRI was born in Morocco in He received his Ph.D. degree in Physics in 2012 from the University of Hassan 1 er, Morocco. His scientific interests include solar cells and Renewable Energy, plasma diagnostics, thin film growth during ionized PVD processes in particular HPPMS. He is the author of several lectures and articles. After a Researcher as a PhD student at the University of Mons, Belgium, he currently holds a Professor position, at the High School of Technology, in particular at the Department of Industrial Engineering and Renewable Energy. 407