OPTICAL PROPERTIES OF NANO-CRYSTALLINE SILICON FILMS PREPARED BY USING SOL-GEL SPIN COATING PROCESS

OPTICAL PROPERTIES OF NANO-CRYSTALLINE SILICON FILMS PREPARED BY USING SOL-GEL SPIN COATING PROCESS T. Fangsuwannarak,* and K. Khunchana School of Electrical Engineering Suranaree University of Technology 111 University Avenue, Muang, Nakhon Ratchasima, Thailand 3 thipwan@g.sut.ac.th Abstract - Nano-crystalline silicon (n) films consisting of Si nano-particles and phosphosilicate composition were synthesized by a sol-gel technique and subsequently heat-treated at different annealing temperatures. The n films were prepared by using the simple sol-suspension technique. The density of the n phase in the n films can be controlled with varying amount of Si nano powder in the solution. A comparative study of optical properties of the n films with varying n densities was conducted. The films were characterized by an atomic force microscopy (AFM), and an UV-visible spectrophotometer for their surface morphology, optical properties, respectively. It is found that the surface morphology changes in the n films were observed by AFM as a function of different n densities. The blue-shift of light absorption edge implied to the expansion of optical energy gap from 1.1 ev to 1.5 ev was presented because of the effect of the heat-treatment. The values of optical constant such as refractive index (n) and extinction coefficient (k) derived from the reflectance and transmittance (RT) measurements were evaluated. The film characterizations reveal the high absorption of sunlight and the minimized surface reflection at the wavelength of.6µm light. The refractive index values of the n thin film around.3-.7 show a good property of anti-reflection coating layer for the promising photovoltaic device. Keywords: Nano-crystalline silicon, Sol-gel preparation, Phosphosilicate glass, Photovoltaic. 1. Introduction Nanostructured silicon (Si) materials have been widely studied for further optoelectronic applications because of their interesting luminescent properties [1,]. The main focus of interest at the nanocrystalline silicon (nc- Si) is due to its discovered properties such as intense visible-light emission from its extension of the optical bandgap (E g ) [3]. These optical properties can open up various possibilities of a variety of uses of n film for photonic devices, such as optical amplifiers [4], waveguides [5], micro-disk resonators [6], lightemitting diodes [7], and tandem solar cells [8], etc. Nanostructured Si materials have been fabricated as usually thin film by using the various techniques. The traditional fabrications such as co-sputtering [], plasma enhanced chemical vapour deposition (PECVD) [9] and ion implantation [1] methods were accomplished with very high temperature annealing (more than 9 C) in order to achieve the high densities of Si atoms into thermal SiO. This way often leads to either destruction of the host oxide matrix or formation of the large clusters where the electrical and luminescence properties are weak. In addition, these techniques involve using such expensive equipment and complex processes. An alternative method for producing thin film is to use the spin coating in order to overcome these problems [11]. The spin coating technique is a low temperature process with an advantage in its low cost, using simple equipment and hence it is easy to be used in thin film fabrication for forming nanostructures. The preparation of nano-silicon clusters embedded in either SiO -based spin-on-glass (SOG) solutions or spin-on dopant (SOD) solutions would be one of the promising ways to handle sufficient nano-silicon concentration. V. Švrcek et al. [1] revealed that porous Si nano-clusters in doped SOD matrix provided the enhancement of optical emission properties as compared with pure SiO matrix. In term of all-si tandem solar cell application, the concept is the use of p-n junctions from several n structures stacked on top of each other. It has arrangement of the highest band-gap cell uppermost and lowest on the bottom [13]. Thus, the investigation of either boron or phosphorusdoped Si nanostructures is an important step toward realization of a nano-scaled p-n junction. In this work, the preparation of n films by using the simple sol-suspension technique was studied. The n films were deposited on quartz substrate by using the spin coating method. The nc-films were composed mainly of nano-si clusters and phosphorus doped 16 The Romanian Review Precision Mechanics, Optics & Mechatronics, 13, No. 43

silicon oxide phase as called phosphorus silicate glass (PSG) primarily formed from sol-gel solution. The density of the n phase embedded in PSG phase can be controlled with varying amount of Si nano powders in the solution. The aim of this study is to investigate the influence of various n densities of the thin films on the surface morphology and the optical properties such as absorption coefficient and optical constant values. Micro-Raman spectroscopy for estimation of crystallites size and crystal fraction, while for optical characterization the main method was to use an UV- Visible spectrophotometer will be discussed in detail.. Experimental Procedure Synthesis of sol-suspension The n thin film layer containing commercial nano- Si powders dispersed in PSG matrix was deposited by a spin-coating solution technique. Phosphorus-doped SOD solution was prepared by blending Tetraethylorthosilicate (TEOS, 98% Fluka), Ethanol absolute (EtOH, 99% BDH) and H O at room temperature. The mixture was stirred for 1 minutes to ensure homogeneity. Subsequently, phosphoric acid (H 3, 85% Ajax) was added in the starting solution and stirred for 1 minutes. Then dilute HCL as a catalyst with.1m was stirred for 1 minutes. The reaction mixture was stirred for 6 minutes to allow the gel ph value of.8 ph with a volume (ml) ratio of TEOS:EtOH:H O:H 3 :HCl of 9.98:8.7:1.66:.7:.5 ml. Then the nano-si powders ( 1 nm diameter size) with different amounts (.3g,.6g, and.1g) were mixed in the P-doped SOD solution and then the resulting solutions were denoted as Sample.3g, Sample.6g, and Sample.1g, respectively. In order to minimize agglomeration of the substances, remove the air bubbles, and improve Si powder distribution in the P- doped SOD solution, the mixture was treated in an ultrasonic stirrer for 6 minutes. Three different mixture solutions were immediately spun on quartz substrates and baked at the different temperatures (5 C, 15 C, and C) for 3 minutes. By varying the contents of n particles, the prepared Si-nc films on the quartz substrates were denoted as Sample xxg (xxg =.3g-.1g). Characterization procedures of n film The nanostructure and crystallite quality of Si powders are essential to be verified before forming a n film. Thus, a micro-raman spectroscopy (NT-MDT, Ntegra Spectra) was used with a. cm -1 spectral resolution to investigate the nanocrystal quality and to approximately determine the average size of Si powders. After spinning the prepared Si-nc suspension, the thickness of all Si-nc films was measured by an optical profiler (Veeco, WYKO NT11). The surface morphology of the samples was investigated in air by non-contact mode Atomic force microscopy (AFM, SII SPA 4). A double-beam UV-Visible spectrophotometer (AJUK, SPECORD 5-P133) with an integrating sphere mode was used to measure light transmittance and reflectance of the samples in the wavelength range of 3-11 nm with a resolution of 1 nm. The obtained spectra were used to derive the light absorbance and the photon absorption coefficient for the samples. 3. Results and discussions Figure 1 shows the peak position of the broader Raman spectrum of Si powders is at 511 cm -1 as compared with the sharp spectrum of a bulk at 51 cm -1. It was noticed that the n powders provided high crystalline quality due to the confirmation of the symmetrical- and the few broad-spectrum. The various amounts of n powders of.3g,.6g and.1g provided the different thicknesses of Si-nc films of.915μm, 1.348μm and 1.553 μm, respectively. Normalized intensity (a.u.) 1..8.6.4 n by American elements.. Bulk Si 4 45 45 475 5 55 55 575 6 Wavenumber (cm -1 ) Figure 1: Comparison of the Raman spectra of the commercial n powder and single crystalline silicon wafer AMF images of the samples treated at 5 C under the different n densities are shown in Figure (a)-(c). For the Sample.3g a large mumber of small pores occuring on the P-doped SOD phase are observed in Fig. (a). The existence of small pores (5-7 nm size) in SiO phase was reported in the work of Fadad [14] due to the acid catalysts as HCl and H 3 resulting in the hydrolysis rate faster. The features of Sample.6g and Sample.1g having the high Si cluster density illustrate the removal of the small porosity as shown in Figure (b) and (c), respectively. The disapperance of such small pores may be due to a change of the densification of the P-doped SOD dielectric structure, which is similar to the previous work [15]. In addition, it was found that the spherical parks ( 5 nm) were formed at the surface. The textured surface is important for optical application because light scattering at the film surface is minimized. By controlling the amount of Si nano-powders, the samples with various n contents were obtained. The different thickness values of samples were of.915μm, 1.348μm and 1.553μm for Sample.3g, Sample.6g and Sample.1g, respectively. The Romanian Review Precision Mechanics, Optics & Mechatronics, 13, No. 43 17

(a) (b) (c) Figure : AFM images of the n films by varying the amounts n powder : (a).3g (b).6g and (c).1g of nano Si powder Figure 3 shows the light transmittance and reflectance spectra of the reference quartz and the P-doped SOD film, the Sample.3g, Sample.6g and Sample.1g, and the Sample.1g with the different heat-treatment temperatures. As indicated at Figure 3(a), there is an increasing trend of the light optical transmittance intensity in the longer wavelength range. It is observed that the films with low n density show the increase of the transmittance intensity due to the effect of a reduction of film thickness and n content. As can be seen in figure 3(b), the reflectance intensity of films decreases when the n density decreases. However, it was noticed that the low reflectance intensity region is observed for the n films in the low wavelength range below 5 nm as seen in Figure 3(b). Meanwhile, the influence of an increase in optical transmittance intensity of Sample.1g with higher heat-treatment is observed. For Sample.1g heated at C, the optical transmittance intensity increases in the wavelength range above 5 nm as compared with Sample.1g heated at 15 C possibly due to nanocrystal cluster oxidized at their surfaces [16]. Reflectance (R%) 7 6 5.1g_ O C 4.1g_15 O C 3.1g_5 O C.6g_5 O C.3g_5 O C 1 Quartz TEOS_H 3 3 4 5 6 7 8 9 1 11 1 8 Quartz 6 4.3g_5 O C.6g_5 O C.1g_15 O C.1g_ O C.1g_5 O C 3 4 5 6 7 8 9 1 11 (a) (b) Figure 3: Transmittance (a) and reflectance spectra (b) of n films with the different densities of n powders Transmittance (T%) The spectra of light absorption coefficient (α) of the n films can be derived by the transmittance and reflectance measurement for a sample with the use of equation (1) [17]. Meanwhile, the absorption coefficient as a significant factor for the optoelectronic devices can be estimated by an extrapolation of the linear curve portion as a function of incident of photon energy (hν) as presented in equation (). Since the photon absorption considered as an indirect band-toband transition, the equation () is referred to a Tauc s formula [18]. 1 α ln t 4 ( 1 R) + 4T R ( 1 R) = TR (1) where t is the thickness of the film, R and T are the reflectance and transmittance, respectively. 18 The Romanian Review Precision Mechanics, Optics & Mechatronics, 13, No. 43

( h ) α hν = ν () E g where E g is the optical band gap, h is Planck s constant, and ν is the frequency of the radiation The optical absorption coefficient of n films is shown in Figure 4. In addition, the edge of absorption coefficient can be ascribed as an indirect allowed band-to-band transition dominates over the optical absorption and implied to optical band-gap of the materials [19]. As can be seen in Figure 4, the absorption spectra of the samples are as high as that of in the photon energy range between.5-1.1 ev. Meanwhile, the effect of the increase of heat-treatment temperature on the shift of absorption edge toward the higher photon energy is observed. The optical absorption edge expanded from 1.1 ev to 1.5 ev which results from an increase in the heattreatment temperature. The absorbance curves of the films with containing n clusters show an absorbance peak in the wavelength range between 3 and 5 nm, while the SOD film without n clusters expresses almost zero absorbance value. It was noticed that the light absorbance in the wavelength range below 5 nm increased with the increase of the n density. It seems to be higher intensity than a reference sample. When the n film is heated up, the reduction of the absorbance is clearly observed. The absorption intensity of n particles decreased with increasing heat-treatment temperature, which was possibly caused from the n particles oxidized []. This cause may lead to the shrink of n size and then the shift of absorption edge toward the higher photon energy. For the samples heated at 15 C and C, no much distinguishing absorbance beyond is observed. Absorption coefficient (cm -1 ) 1 5 1 3.1g_ O C.1g_15 O C 1 1.6g_5 O C.1g_5 O C.3g_5 O C 1-1 1. 1.5..5 3. 3.5 4. Photon Energy (ev) Figure 4: Absorption spectra of Si-nc films with the different amounts of nano-si powders as compared with a Si reference [19] In Figure 5, the light absorbance data (A) is described as the fraction of incident solar irradiation that is absorbed by the n films. The data was calculated by using the expression as Absorbance 1 8 6 4 A = 1 R T (3).1g_5 O C.6g_5 O C.3g_5 O C.1g_ O C.1g_15 O C 4 6 8 1 1 Figure 5: Light absorbance of Si-nc films with the different amounts of nano-si powders and as compared with a Si reference [19] n k K 6 4 4 Sample.1_5 O C Sample.1_ O C Sample.1_15 O C 3 4 5 6 7 8 9 1 11.8.6.4.. -. Sample.1_5 O C Sample.1_ O C TEOS_H3PO4 Sample.1_15 O C 4 6 8 1 1 Figure 6: Refractive index (n) and extinction coefficient (k) versus photon energy for Si-nc films with the different amounts of nano-si powders as compared with material. Fig. 6 presents the refractive indices (n) and extinction coefficient (k) of the n films are related to the reflectance by [19]; The Romanian Review Precision Mechanics, Optics & Mechatronics, 13, No. 43 19

( n 1) + k R = (4) ( n + 1) + k The extinction coefficient (k) and the absorption coefficient (α) are related by αc k = (5) 4πν where c is the velocity of light in vacuum. With increasing n density, the n curves of the n films shift up and approach to the n curve of material at the near IR wavelength range. The refractive index values of the n thin films around.3-.7 provided a good property of antireflection coating layer for the promising photovoltaic device. The k values of the n films are very small as compared with material. It was found that their k peaks belong to the visible spectral region. Therefore, the extinction caused by scattering is primarily in the UV and visible spectral region. 4. Conclusion Silicon nano-crystals dispersed in P-doped silica films were prepared by sol-gel spin coating method for the study of the optical properties. It was found that the absorption spectra are sufficiently high in the visible spectral region that is useful for an absorber layer property of photovoltaic device. These results suggest that the obtained absorption edge that implied to the optical band gap of the n films shifted toward the higher photon energy. In addition, the optical absorption exhibited a blue-shift from 1.1 ev to 1.5 ev when the heat-treatment temperature was increased from 5 C to C. This blue-shift was due to the reduction in the n particle size. 5. Acknowledgments This work has been supported under the 1 grant from Suranaree University of Technology, Thailand. The authors would like to express gratitude to the researchers from National Electronics and Computer Technology Center (NECTEC), Thailand for measurement supports. 6. References [1] M. Cazzanelli, et al.: Optical gain in monodispersed silicon nanocrystals. J. Appl. Phys. 4; 96: 3164 3171. [] M. Zacharias, et al.: Size-controlled highly luminescent silicon nanocrystals: A SiO/SiO superlattices approach. Appl. Phys.Lett. ; 8: 661 [3] L.T. Canham: Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl. Phys. Lett. 199; 57: 146 148. [4] G.R. Lin, et al.: Gain analysis of opticallypumped Si nanocrystal waveguide amplifiers on silicon substrate: Optics Express. 1; 18: 913 919. [5] J. Ruan, et al.: Stimulated emission in nanocrystalline silicon superlattices. Appl. Phys. Lett. 3; 83: 54795481. [6] R.J. Zhang, et al.: Visible range whisperinggallery mode in microdisk array based on size-controlled Si nanocrystals. Appl. Phys. Lett. 6; 88: 1531. [7] M. Peralvarez, et al.: Si-nanocrystal-based LEDs fabricated by ion implantation and plasma-enhanced chemical vapour deposition. Nanotechnology. 9; : 451. [8] M.A. Green, et al.: All-silicon tandem cells based on artificial semiconductor synthesised using Si quantum dots in a dielectric matrix. Proceedings of the th European Photovoltaic Solar Energy Conference, Barcelona, Spain, 5: 3 6. [9] A. Le Donne, et al.: Structural characterization of n films grown by low-energy PECVD on different substrates. Appl. Surf. Sci. 8; 54: 84 88. [1] L. Nikolova, et al.: Si nanoparticle formation in SiO by Si ion implantation: Effect of energy and influence on size distribution and on SiO composition. Surface & Coatings Tech. 9; 3: 51 55. [11] C. Wen-Tse, et al.: High-quality nano spinoxide for possible applications in metal oxide semiconductor field-effect transistor., Micro & Nano Letters 6 (11) 686-688. [1] V. Švrček, et al.: Photoluminescence studies from silicon nanocrystals embedded in spin on glass thin films. J. Lumines. 3; 11: 69 74. [13] G. Conibeer, et al.: Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films. 8; 516: 6748 6756. [14] M. A. Fardad, : Catalysts and the structure of SiO sol-gel films. J. Mat. Sci. ; 35: 1835-1841. [15] I. Vasiliu, et al.: SiO x P O films promising components in photonic, Structure, Opt Quant Electron. 7; 39:511 51. [16] W. Li, et al.: Physical and optical properties of sol-gel nano-silver doped silica film on glass substrate as a function of heat-treatment temperature, J. Appl. Phys. 3; 93 (1): 9553-9561. [17] D. K. Schroder, Semiconductor material and device characterization, John wiley and Son, (1998) 594-597. [18] J. Tauc, et al.: Optical properties and electronic structure of amorphous germanium., Phys. Status Solidi 15 (1966) 67. [19] J. I. Pankove, Optical processes in semiconductors, New York, International Standard Book Number : -486-675-3, 1971 [] W. Li, et al.: Physical and optical properties of sol-gel nano-silver doped silica film on glass substrate as a function of heat-treatment temperature. J. Appl. Phys. 3; 93 (1): 9553-9561. 11 The Romanian Review Precision Mechanics, Optics & Mechatronics, 13, No. 43