لبا ب ةعماج / ةيساسلأا ةيبرتلا ةيلك ة لجم

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1 Photoluminescence from Etched Silicon Surface by High Power Laser Oday A. Abbass Department of Physics/College of Sciences/University of Kufa ABSTRACT Porous silicon layers (P-Si) has been prepared in this work via Laser-induced etching process (LIE) of n-type silicon wafer of 3 Ω.cm resistivity in hydrofluoric (HF) acid of 24.5 % concentration at different etching times (5 25min.). The irradiation has been achieved using laser beam of 2W power and 8 nm wavelength. The morphological and photoluminescence characteristics of these layers such as porosity, surface morphology and nanocrystallite size have been investigated using scanning electron microscope (SEM) and photoluminescence (PL) measurements and the gravimetric method. 1. Introduction During the past years there has been an increased interest in the luminescence properties of etched silicon (porous silicon (P-Si)). This material has efficient luminescence in the visible regime even at room temperature [1]. Canham was the first to ascribe the visible luminescence to quantum confinement of the excited electron-hole pair inside the small silicon structures, resulting in luminescence energy well above the bulk silicon band gap. It was further argued that the nonradiative recombination is much reduced due to the good surface passivation and the fact that the carriers are confined inside the crystallites and cannot diffuse far away to reach a nonradiative center [2]. Photochemical etching (PE) process or laser-induced etching (LIE) process of silicon is a technique known to create porous silicon [3,4]. The nature of nanocrystallites and their size distribution depend on the actual experimental conditions during the LIE process [5]. The nanocrystallite size and size distribution will depend on the experimental parameters, which include: nature of the substrate, penetration depth of the used laser, laser power density, laser irradiation time and HF concentration [6]. Moreover, illumination of n-type silicon wafers with infrared laser radiation could be used to modify the photoemission characteristics [7,8]. In our experiments we have shown that the visible light emission can be obtained from silicon surface at room temperature by laser-induced etching process, where we have kept all parameters constant, except etching time. 2. Experiment Crystalline wafer of n-type Silicon with resistivity of 3 Ω.cm, 508 µm thickness, and (111) orientation was used as starting substrate. The substrate was cut into rectangles with areas of 1 cm 2. The native oxide was cleaned in a mixture of HF and H 2 O (1:2). Laserinduced etching process then performed at room temperature in an ethanoic solution of 24.5% HF is obtained by 1 volume of ethanol and 1 volume of 49% wt. Hydrofluoric (HF) acid as shown in figure below. The HF concentration in the ethanoic solution is given by ((1 49%)/(1+1) = 24.5%)) [9]. The irradiation has been achieved using a commercially available CW diode laser with power (2W) and (8 nm) wavelength. 454

2 Figure (1-2): The laser-induced etching set-up. The morphological properties have been measured using scanning electron microscopy (SEM) (Leo-1550). The SEM measurements were carried out in the (Institute for Bio-and Nano-systems (IBN2)-Germany). The porosity has been measured using Gravimetric method and using the following equation []: M1 M M1 M 3 where M 1 (g) and M 2 (g) are the weights of silicon sample before and after etching process respectively. M 3 (g) is the weight of the silicon sample after removing of PS layer. The photoluminescence (PL) characteristics have been achieved in University of Technology. The spectroscopic instruments used for this purpose consist of the laser source, the monochromator and finally the detection system. Figure (2-2) shows the experimental setup of the photoluminescence spectra measurements. 1- Excitation Source A CW diode laser of 514 nm has been used to probe the P-Si layer as an excitation source with photon energy of 2.41 ev which could excite a wide range of nanocrystallite sizes. A laser power density of 5 mw/cm 2 was focused on the samples. 455

3 Lens Excitation laser Figure (2-2): The photoluminescence set up. 2- The Detection The analyzed scattered light was collimated by a very small focal length lens and detected by using power-meter which behaves as a detection device. 3- The Monochromator A single pass monochromator (Jobin-Yvon Co.) has been used to analyze the scattered light from the sample. The sample was mounted on a sample holder and placed in front of the monochromator entrance slit which was about (200 μm) while the exit slit was around (300 μm), where the optical system which consist of a set of different mirrors and a grating which was blazed for 6000 A 0 has a total length of 50 cm. 3. Results and Discussion 3.1. Surface morphology Figure (1-3) represents scanning electron microscopy images (top-view) of P-Si layers prepared at different etching times (5-15 min.) Wall a b Pore Cylindrical Pore Starful Triangular pore 456 c

4 Figure (1-3): Scanning electron microscopy images (top-view) of P-Si layers prepared at different etching times (a) 5 min., (b) min., (c) 15 min. These included SEM images in figure (1-3), are prove growth of P- Si layers on the surface of silicon, and growth process of these P-Si layers can be clear according to Lehmann model as follows: If photogenerated charge carriers (hole (h + )) reach the surface, nucleophilic attack on Si-H bonds by fluoride ions (F ) can occur and a Si-F bond is established. Due to the polarizing influence of the bonded F, another F ion can attack and bond under generation of an H 2 molecule and injection of one electron into the bulk silicon. Because of the polarization induced by the Si-F groups, the electron density of the Si-Si back bonds is lowered and these weakened-bonds will now be attacked by HF or H 2 O in a way that the silicon surface atoms remain bonded to hydrogen When a silicon atom becomes removed from an atomically flat surface by this reaction, an atomic size dip remains. This change in surface geometry will change the inert electric field distribution in such a way that holes transfer occurs at this location preferentially. Therefore pores of about 1 µm will establish in a few minutes on polished n-type silicon surface by this process [9,11]. We can note from figure (1-3) that the pore width increases with increasing of etching time. This largeness in pore width may be attributed to increasing of holes number on surface of silicon electrode with etching time which leads to preferential dissolution between nearestneighbor pores, thereby promoting the pore-pore overlap. However, the etching rates may be different and then leads to nonuniformity in values of the pores width as summarized in table (1-3). The cause of this variation may be due to the nonuniform of the power density distribution of illumination, which leads to nonuniform photocurrent densities consequently, resulting in different pore width. The nonuniform of the power density distribution of illumination is attributed to fact that the laser beam intensity will decrease gradually from its center to its periphery (Gaussian-laser beam). Figure (1-3,a) shows the P-Si layer possesses pores with cylindrical shape, while the P-Si layer in figure (1-3,b) has nearly structure with cylindrical shape. From figure (1-3,c) we can observe P-Si layer possessing approximate construction composed of pores with starful and rectangular shapes. Table (1-3); Wall thickness, pore width and pore shape of prepared P-Si layers under Etching time (min) Pore width (μm) Pore shape Wall thickness (μm) Cylindrical Cylindrical Starful & Rectangular different etching times (5-15 min.). 457

5 Porosity (%) 3.2. Porosity Figure (2-3) shows the relationship between porosity of P-Si layers prepared under different etching times (5-25 min.) and etching time Etching time (min.) Figure (2-3); Porosity of P-Si layers as a function of etching time. From previous figure, we can see the values of porosity are increasing with increasing of etching time. These results are ascribed to increasing the number and width of the pores with increasing of etching time. These consequences are consistent with SEM images, where the roughness of surface is a function of etching time and porosity as shown in figure (1-3). Our results agree with outcomes of [12,13] Photoluminescence Characteristics and Nanocrystallite Size A quantum confinement model has been used by Canham [2] and modified by Yorikawa et al [14] to study the nanocrystallite size distributions in P-Si layer from their PL spectra. According to this model, each nanocrystallite in a P-Si sample contributes a characteristic sharp PL spectrum so that the total PL intensity S(E) from an assembly of crystallites including size distribution is given by [15]: 1 RE S( E) c. ( Eexc E). D( RE ).... (2) 0 n E E where E g g is the gap energy of bulk silicon and R E is the crystallite radius defined by R E = {δ/e - E g } 1/n, δ= 14.8 (ev. A n ) and n=1.25. PL spectra of three P-Si samples prepared under different etching times (5,, 15 min.). As shown in figure (3-3), left column represents the experimental and the theoretical calculation obtained by the equation (2). For the sample prepared with a 5 minutes etching time, it is found that the crystallite size was 32 A and the standard deviation was 0.09 A as shown in figure (3-3 a). With increasing the time up to minutes, as shown in figure (3-3 b), the obtained nanocrystallite size decreases to 31 A with a standard deviation of 0.18 Aº. While the nanocrystallite size decreases to 30.3 A for 15 minutes, as shown in figure (3-3 c), the standard deviation was found to increase up to 0.23 A. 458

6 PL intensity (arb. We found evolution of the etching process with time. When the silicon wafer is illuminated with the 8 nm wavelength where the photon energy is 1.53 ev, the energy gaps of the produced crystallites are smaller than the excited energy leading to generate electron-hole pairs within the illuminated area to form the P-Si layer. Therefore, larger irradiation time duration's leads to further etching and size reduction. However, as the etching proceeds, the small crystallites in the porous layer increase the band gap and the porous layer becomes transparent to the laser light. At that time, an optical absorption would take place at the P-Si /bulk Si interface and leads to increase the P-Si layer thickness a 5 min. a b min. b 8 6 D(R c 15 min. c Photon Energy (ev) Crystallite Size (A ) Figure (3-3): The left column represents the experimental data (discrete plot) fitted on the quantum confinement model. While the right column represents the Gaussian distribution of crystallites sizes contribute the PL emission. 459

7 4. Conclusion We can conclude from previous results and facts that the laserinduced etching with 8 nm wavelength of the silicon can be produce a luminescent silicon material. The photoluminescence characteristics of etched silicon are attributed to possess this material to crystals with nano-size. 5. Acknowledgement We are grateful to Dr. Hans Bohn for its helpful to me in achieving of SEM images. References 1- Canham L. T., Houlton M. R., Leong W. Y., Pickering C., and Keen J. M., Atmospheric impregnation of porous silicon at room temperature, J. Appl. Phys, Vol. 70, No. 1, Canham L. T., Silicon Quantum Wire Array Fabrication by Electrochemical and Chemical Dissolution of Wafers, Appl. Phys. Lett. Vol. 57, No., Kolasinski K. W., Barnard C. J., Ganguly S., Koker L., Wellner A., Aindow M., Palmer R. E., Field C. N., Hamley P.A., Poliakoff M., On the role of the pore filling medium in photoluminescence from Photochemically etched porous silicon, J. Appl. Phys. Vol. 88, No. 2472, Beale M. I. J., Benjamin J. D., Uren M. J., Chew N. G. and Cullis A. G., An Experiment and Theoretical Study of the Formation and Microstructure of Porous Silicon, Journal of Crystal Growth, Vo. 73, pp , Lim P., Brock J. R. and Trachtenbreg I., Laser-induced etching of Silicon in hydrofluoric acid, Appl. Phys. Lett. Vol. 60, No. 4, Thonissen M., Berger M. G., Arens-Fischer R., Gluck O., Kruger M., Luth H., Illumination-assisted formation of Porous Silicon, Thin solid films, Vol. 276, pp , Noguchi N. and Suemune I., production of porous silicon by laser-induce etching, Appl. Phys. Lett. Vol. 26, No. 12, Choy C. H. and Cheah K. W., Laser-induced etching of Silicon, Appl. Phys. A, Vol. 61, pp , Canham L. T., Properties of porous silicon, Inspec, England, (1998). - Z. C. Feng and R. Tsu, Porous silicon, World scientific, London (1994). 11- Lehmann V. and Gosele U., Porous Silicon Formation: A quantum Wire Effect, Appl. Phys. Lett. Vol. 58, No. 8, ( Lee S. and Kim D. Y., Effect of Incoherent Light Illumination During Electrochemical Anodization on the Formation and the Properties of Porous Si, Journal of the Korean Physical Society, Vol. 47, No. 5, Matthias S., Muller F. and Gosele U., Controlled nouniformity in macroporous silicon pore growth, Appl. Phys. Lett. Vol. 87, No. 22, H. Yorikawa and S. Muramatsu, Silicon Nano-Structure, Appl. Phys. Lett. Vol. 197, No. 644,