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Electrochemical isotropic texturing of mc-Si wafers in KOH solution
Materials Chemistry and Physics 139 (2013) 756e764
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Electrochemical isotropic texturing of mc-Si wafers in KOH solution M. Abburi*, T. Boström, I. Olefjord Norut Narvik AS, P.O. Box 250, N-8504 Narvik, Norway
h i g h l i g h t s < A method to form isotropic textures on mc-Si wafers in KOH solution is presented. < The method is based on anodic polarization of silicon in KOH at high potentials. < Evolution of surface morphology is studied by varying the etch parameters. < Isotropic textures with lowest average reflectivity are obtained at 40 V. < A reaction model for texturing mechanism is discussed in the light of XPS data.
a r t i c l e i n f o
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Article history: Received 15 June 2012 Accepted 4 February 2013
Boron doped multicrystalline Si-wafers were anodically polarized in 2 M KOH and 4 M KOH at 40 C and 50 C. The applied potentials were 25 V, 30 V, 40 V and 50 V. The morphology of the textured surfaces, the surface products and the light reflectivity were analyzed by utilizing SEM, XPS and Lambda UV/Vis/ NIR spectrophotometer, respectively. Isotropic texturing was obtained. The lowest average reflectivity, 17%, was achieved after pre-etching for 10 min and polarization at 40 V for 10 min in 4 M KOH at 50 C. That reflection value is half of that measured on a chemical pre-etched surface, 34%. By increasing the voltage to 50 V the reflectivity rises to 28%. Polarizations to 25 V and 30 V at 50 C in both solutions give local pores in the mm-range. The etch attack initiation is located at protrusions on the surface. At 40 V and 50 V in both solutions the pores are extended onto the entire surface. The width of the pores is about 10 mm. Inside the micro-pores, nm-pores are formed; their lateral size is in the range 100 nme200 nm. A mechanism for the anodic dissolution reactions is discussed. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Etching Electrochemical techniques Electron microscopy Surface properties
1. Introduction The efficiency of solar cells is to a large extent dependent on their capability to absorb light radiation. In order to increase the absorption of light the silicon wafer surfaces are roughened by chemical or physical methods. A method for designing their surface morphology is wet chemical etching. The desired textured surface is obtained by chemical etching in either strong alkaline solutions or in acidic solutions containing a mixture of hydrofluoric acid and nitric acid. Strong alkaline solutions can be used because Si dissolves in alkaline solutions at pH > 10 [1]. Below that value Si is passivated due to formation of thermodynamically stable SiO2. Thereby, silicon is stable in acids except in hydrofluoric acid, which dissolves the oxide. The purpose of adding nitric acid to HF-solution is to oxidize silicon. The technique gives an isotropic etching, which is grain orientation independent [2e5].
* Corresponding author. Tel.: þ47 76965374; fax: þ47 76965351. E-mail addresses: [email protected], [email protected] (M. Abburi). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.027
Chemical etching of multicrystalline silicon (mc-Si) in alkaline solutions give a non-uniform (anisotropic) dissolution of Si and thereby an anisotropic texture, which gives a relatively high light reflection. The reason for the anisotropic texturing is that dissolution of mc-Si is due to the random orientation of grains, i.e., the crystallographic planes are etched at different etch rates [6e10]. It has been reported that etch rate of (111) surface is a few order of magnitudes lower than that of the fast etching planes, (100) and (110) [7,10,11]. Etching of Si (111) results in a stable shiny surface with high reflectivity [12]. The anisotropic alkaline etching technique is widely used for etching and texturing of monocrystalline (100) orientation silicon wafers [12e14]. Etching of Si (100) wafers in low concentration alkaline solutions (<2 M) at high temperatures (>50 C) forms pyramidal hillocks on the surface which reduce reflection of incoming sun light effectively. Although alkaline solutions are cheap, easy to handle during the process and environmentally friendly, this anisotropic etching technique is not effective enough for multicrystalline silicon wafers; the surface morphology becomes coarse and non-uniform. In addition, the etching can form
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unwanted steps at grain boundaries. The shiny steps at the grain boundaries not only increase the light reflection but also contain defects and act as recombination centers for charge carriers, thereby having a negative effect on the efficiency of a solar cell. Today, acidic etching in HF/HNO3 is used in order to achieve the desired textures on multicrystalline silicon wafers for effective reflection reduction. The acidic etching technique is well suitable for mc-Si wafers in forming isotropic textures [15e17]. However, the process is difficult to control. Due to its vigorous etching nature, grain boundaries and surface defects are attacked. The defect etching leads to pits, micro cracks and steps at boundaries [15,16,18,19]; the wafers are therefore prone to damage. Other significant drawbacks are that the environment is hazardous for humans and the disposal of the chemical waste containing fluorides is expensive. One possibility to overcome the problems with conventional alkaline and acid etchings is to anodically etch the wafers in an alkaline solution. The treatment has to be done at relatively high alkalinity where SiO2 is not chemically stable. In aqueous solutions like KOH and NaOH etc. the stable compounds are soluble silicates [1]. However, during polarization of Si in the solution metastable oxide products can be formed [20e23], which may possibly passivate the surface. The formation of the oxide products is dependent on the alkalinity and the temperature of the electrolyte [30]. In this paper it will be shown that it is possible to obtain homogenous textures on all crystallographic grains of mc-Si wafer surfaces by anodic polarization in alkaline solutions at relatively high temperatures and potentials. This electrochemical texturing method in alkaline solutions can effectively replace the isotropic texturing by acidic solutions for effective light trapping on mc-Si wafers and hence drastically reduce the usage of the hazardous hydrofluoric acid. 2. Experimental The test material was boron doped mc-Si wafers in its as-cut condition produced by REC Scan Wafer AS. The thickness of the wafers was 200 mm and their resistivity was in the range 1e1.5 U cm. The wafers were cut into 2.5 cm diameter samples by using a laser cutting process. The sample was mounted in a Teflon sample holder sealed with an O-ring. The sample was mounted on the metallic conductor of the cell by using a conducting copper tape. The exposed area was 1.32 cm2. The electrolyte solution was prepared from analytical grade KOH pellets and Millipore water of 18 MU cm resistivity. The sample holder was dipped into the cell in such a way that the sample surface becomes vertically oriented in the solution. The electrolyte was continuously stirred with a magnetic stirrer and the temperature was thermostatic controlled. In order to clarify eventually rise of the surface temperature during anodic polarization the temperature was measured by thermocouple mechanically fitted to the Si-surface. It was found that the current has to be increased to about 1 A cm2 before the surface temperature was raised by 1 C. Thus, the temperatures given below are within 0.5 C. Before applying the potential, the native surface oxide was removed by keeping the sample in the solution until hydrogen bubbles appear on the surface. The hydrogen evolution is an indication for chemical etching of the oxide-free silicon surface. The etching was extended at the open circuit potential (OCP) for a few minutes in order to remove the saw damaged layer. This process is termed ‘pre-etching’. The electrochemical experiments were carried out with a power source connected to a personal computer using a RS-232 interface and controlled by using in-house written Lab view software. A conventional two-electrode set up was used. The counter electrode was a Pt-sheet, 4 6 cm2. The large area gives a relatively low current density and thereby low polarization on the cathode. In this study, all the potentials were referred to the Pt-electrode.
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The surfaces of the textured samples were studied by using a scanning electron microscope (SEM, Carl-Zeiss, Evo 60). The oxide formed during the polarization was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5500). The X-ray source was Mg Karadiation. Light reflectance from the surface was measured in 350e 1100 nm range by using a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer equipped with an integrating sphere. 3. Results 3.1. Surface morphology of as-cut and chemical etched surfaces Fig. 1 shows the surface morphology of an ‘as-cut’ mc-Si wafer surface. The surface morphology is characteristic for a wear and micro-cracked brittle material. A few microns of the top layer are damaged by the sawing process. The surface of the as-cut wafer is shiny and reflects a large portion of the radiation. The sawdamaged layer has to be removed before processing of the wafer because it is also electrically inactive. Surface morphologies of mc-Si wafers formed during chemical etching treatments are shown in Fig. 2. The etch patterns are characteristic for anisotropic etching of multicrystalline silicon in alkaline solutions. Etching for 10 min in 4 M KOH at 50 C is shown in Fig. 2a. It appears that the saw damaged layer is removed and the morphology depends on the grain orientation. Steps are developed at grain boundaries due to various etch rate of the grains. The surface is rough with protrusions and valleys. Etching for 10 min at lower concentration, 2 M KOH at 50 C is displayed in Fig. 2b. The etch pattern is similar to that obtained in the 4 M solution. The influence of etching time is demonstrated in Fig. 2c. The sample was etched for 15 min instead of 10 min in the same solution as above. It is clearly shown that close packed pyramidal hillocks are developed on the surface. The triangular etch patterns on the left side of the image contains terraces and steps, which are characteristic of alkaline etching of (111) grains [24]. The roof tile structures are due to anisotropic etching of (110) grains [12]. The influence of temperature is revealed in Fig. 2d. The etching was performed at 40 C for 10 min in 2 M KOH. At this temperature and short etching time the pyramids, terraces and steps are not fully developed. 3.2. Potentiostatic polarizations Fig. 3 shows the current density vs. time recorded during potentiostatic polarizations for 10 min at 25 V, 30 V, 40 V and 50 V in 4 M KOH at 50 C. The samples were pre-etched in the solution for 10 min before polarization (Fig. 2a). The potential is applied in one step from the OCP to the desired potential. It appears from the
Fig. 1. SEM micrograph of ‘as-cut’ mc-Si wafer surface.
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Fig. 2. SEM images of mc-Si wafer after chemical etching in: a) 4 M KOH at 50 C for 10 min, b) 2 M KOH at 50 C for 10 min, c) 2 M KOH at 50 C for 15 min and d) 2 M KOH at 40 C for 10 min.
figure that at all potentials the current goes through a minimum. In the first stage, the current corresponds to formation of oxide products and oxygen evolution due to oxidation of silicon and water, respectively. The drop in the current is due to formation of thick oxide products on the surface, which lowers the oxygen evolution reaction. For prolonged exposure the current goes through a minimum and then increases. It is suggested that the current increase is due to locally breakdown of oxide products, which allows oxidation of silicon and water. The anodic current (Fig. 3) is strongly potential dependent; at short polarization time the current is one order of magnitude higher at 50 V compared to 25 V. Exposure for 10 min gives a current difference of almost 100 times. Further, it can also be seen from the figure that the current minimum goes toward lower exposure times for increased potential.
Fig. 3. Current density vs. time for mc-Si wafers pre-etched for 10 min and polarized for 10 min in 4 M KOH solution at 50 C at 25 V, 30 V, 40 V and 50 V.
3.3. Surface morphology of electrochemically etched surfaces Fig. 4 shows surface morphology of mc-Si wafers after electrochemical treatments in 4 M KOH at 50 C for 10 min at various potentials. All the wafers were chemical pre-etched for 10 min in the solution before applying the potential (Fig. 2a). It appears from Fig. 4a and b that polarization at 25 V and 30 V causes formation of discrete micro-pores. The size of the pores is about 1 mm. The protrusions formed during pre-etching are not observable in Fig. 4a and b. That observation indicates that the pores are initiated on the top of the protrusions where the electric field strength is highest. By comparing the figures it is obvious that the pore coverage increases with the potential. Fig. 4c and e shows that the surfaces are completely covered with pores after increasing the potential to 40 V and 50 V. The micrographs at the low magnification, Fig. 4d and f, shows that the surface texture is independent of grain orientation; the photos show multi-grains. Thus, the desired isotropic texture is obtained. The steps at grain boundaries, which are formed during preetching, are effectively textured. The pores are well extended into the entire surface and concave cavities are formed. At 40 V, the concave cavities (Fig. 4c) are as large as 10 mm wide. Increasing the potential to 50 V, the isotropic nature of the etching is not changed, however the surface seems to be more etched and the concave cavities become smoother, Fig. 4e. Fig. 5a and b shows high magnification images (originally 100,000) of cavities displayed in Fig. 4b and c, respectively. The photos show a network of nano-pores formed inside the micro pores. The lateral widths of the nano-pores are in the range 100e 200 nm. It appears from the figures that the nano-pores are well developed at both potentials, 30 V, as at 40 V. The influence of lower KOH concentration on the surface morphology is shown in Fig. 6. The samples were pre-etched for
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Fig. 4. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 10 min in 4 M KOH at 50 C at: a) 25 V, b) 30 V, c) 40 V, d) 40 V at lower magnification, e) 50 V and f) 50 V at lower magnification.
10 min and polarized for 10 min in 2 M KOH at 50 C at 25 V, 30 V, 40 V and 50 V. The surface Fig. 6a shows that micro-pores are formed on protrusions and ridges during the anodic polarization at 25 V. The pore coverage increases as the potential is increased from 25 V to 30 V, Fig. 6b. This is the same picture as obtained at higher alkalinity. Polarization at 40 V and 50 V gives isotropic textures, Fig. 6c and d, respectively. Sponge like cavities features with nanopores inside are formed at 40 V. It seems that at 50 V the microcavities are smoother; probably due to enhanced dissolution. Fig. 7 shows the textures formed at 40 C in 2 M KOH at 30 V, 40 V and 50 V. The pre-etching and polarization exposure times were 10 min and 15 min, respectively. The surface morphology of the pre-etched surface is shown in Fig. 2d. The initial surface is relatively rough and homogenous due to etching at relatively low temperature; steps at grain boundaries are not formed. Polarization at 30 V results in formation of isolated micro-meter pores uniformly distributed over the entire surface, Fig. 7a and b. At 40 V, concave cavities features are formed homogeneously on the surface. All the grains are evenly etched as shown in Fig. 7c and d. At 50 V the
micro-texture seems to be the same as that formed at 40 V, Fig. 7e and f. However, it is obvious that the higher voltage influences on the nano-pore formation. The considerably higher current and thereby higher etch rate prevent formation of the fine structured pores; thereby the bottom of the cavities becomes flatter than at the lower potentials; this is clearly seen by comparing Fig. 7d and f. 3.4. XPS-analysis XPS-analysis was performed in order to determine the composition and thickness of oxide products formed during texturing. Before analysis the sample was pre-etched for 10 min and then polarized at 40 V for 15 min in 2 M KOH at 40 C. Fig. 8 shows Si 2p (Fig. 8a), and O 1s (Fig. 8b) signals after various steps of ion etching. The etch depths measured in nm are marked in the figure. The first analysis was done after removal of the contamination layer by 1 nm ion etching. Thereafter the sample was ion-etched and analyzed in steps down to 35 nm below the original surface.
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etching for 1 nm. In the Si spectrum a low intensity peak appears at the position of Si0. The presence of this peak shows that the oxide products present on the surface are relatively thin at least in some part of the analyzed area. By using the ratio between Si4þ and Si0 the oxide thickness is estimated [25] to be 6 nm. However, the recorded spectra show that even after ion-etching to relatively deep depths, oxides remain on the surface. It is suggested that the ambiguous results is due to the presence of oxide particles of different sizes. Fig. 9 shows the recorded oxygen intensity vs. etch depth. The thickness of the oxide is normally determined as the etch depth at which the intensity of the oxygen signal has decreased to half of its maximum value. This method gives an average oxide thickness of 26 nm. This value is probably too high due to shadowing effect caused by the roughness of the surface. The oxide particles are probably burrowed deep into the pits. From the data presented in Figs. 8 and 9 it is concluded that porous oxide particles are formed during texturing. The fact that the zero valency state of silicon is registered shows that at least a part of the surface is covered with a very thin oxide layer, which might be formed during rinsing and handling of the sample after polarization. 3.5. Dissolution rate of silicon
Fig. 5. SEM micrographs at high magnification of nano-pores formed during polarization for 10 min in 4 M KOH at 50 C at: a) 30 V and b) 40 V.
The Si-signals show two peaks representing the four valency state, Si4þ, and the zero valency state, Si0, respectively. The oxygen signals, O2, correspond to oxygen bound to silicon in oxide products. The strongest Si4þ and O2 signals are achieved after ion-
The dissolution rate of mc-Si wafers was determined by weighting the sample before and after exposure to the solution. Table 1 shows measured average dissolution rates during chemical etching at OCP and during polarization at different voltages in 2 M and 4 M KOH at 40 C and 50 C, respectively. It appears from the table that the dissolution rates of the wafers at OCP in the two solutions are 0.6 0.1 mm min1. The dissolution rates at polarizations at 25 V and 30 V are 0.2 mm min1. At these potentials the dissolution is not uniform; the pits cover only partly
Fig. 6. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 10 min in 2 M KOH at 50 C at: a) 25 V, b) 30 V, c) 40 V and d) 50 V.
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Fig. 7. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 15 min in 2 M KOH at 40 C at: a) 30 V, b) 30 V at a higher magnification, c) 40 V, d) 40 V at a higher magnification, e) 50 V and f) 50 V at a higher magnification.
the surface. The etching rate becomes noticeably higher during polarization at 40 V and 50 V. The surfaces are uniformly textured. The dissolution rates in 2 M KOH at 40 C and 4 M KOH at 50 C are in the range 0.7 mm min1 to 2.0 mm min1. The highest dissolution rate is obtained at 4 M KOH at 50 C. The amounts of dissolved Si during the polarizations are orders of magnitudes larger than the corresponding amounts of Si bound in the surface oxide.
etched sample. The average reflectance from the pre-etched sample is 34%, which is about the same as the reflectance from polished surfaces. Polarization in the potential range 25 Ve40 V lowers the reflectance continuously with increasing potential. At 40 V the average reflectance is 17%, which is about half of the value obtained after pre-etching. After polarization to 50 V the average reflectance is increased to 28%, which is close to the value obtained after pre-etching.
3.6. Reflectance measurements The light reflectance from chemical etched and anodically textured wafers is shown in Fig. 10. The samples were pre-etched for 10 min in 4 M KOH at 50 C and then polarized for 10 min at 25 V, 30 V, 40 V and 50 V. The reflectance was measured within the wave length range 350 nme1100 nm. The average reflectance between 350 and 1100 nm is given in Table 2. It appears from the curves that the reflectance from the anodically polarized wafers is noticeably lower than from the chemically
4. Discussion This study demonstrates that it is possible to isotropically texture multicrystalline Si wafers by anodic polarization in strong alkaline solutions. For the future it is supposed that this method will be developed to a practical technique in order to form uniform textures on mc-Si wafers. The chemicals used in the process are biological friendly both for humans and the environment.
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M. Abburi et al. / Materials Chemistry and Physics 139 (2013) 756e764 Table 1 Dissolution rates of mc-Si wafers during electrochemical treatments. Dissolution rate
Solution parameters
Voltage (V) OCP
25
30
40
50
mm min1 mm min1
2 M KOH at 40 C 4 M KOH at 50 C
0.5 0.7
0.2 0.2
0.2 0.3
0.7 1.1
1.1 2.0
The aim of the texturing process is to minimize the light reflection in order to make the solar cells to absorb as much light as possible. This can be done by electrochemical treatment; by adjustment of the parameters it is possible to lower the average reflectivity to 17%, which is about half of the value obtained after chemical etching. The adjustable parameters are: pre-treatment of the wafer, the alkalinity of the solution, the temperature of the electrolyte and the applied voltage. So far the lowest reflectivity is obtained by pre-etching for 10 min and applying 40 V to the sample in 4 M KOH at 50 C for 10 min. The reflectance obtained is lower than the values from commercial acidic etching. Averaged reflectance values after standard industrial HF/HNO3 etching of multicrystalline silicon wafers are around 29% and solar spectrum averaged reflectance values are around 27% [26]. This treatment gives an isotropic texture covering 100% of the surface. It seems that the sensitivity for some of the parameters is not critical; 2 M KOH at 40 C and 50 C gives about the same result as that in 4 M KOH. It might be possible to find another set of parameters which gives even lower reflectivity. However, increasing the voltage from 40 V to 50 V gives a reflectivity which is close to the chemically pre-etched surface. It is obvious that the increased potential flatten the surface. This result agrees with the SEM micrographs, which show smoother surfaces at 50 V. At even higher voltages and higher temperatures, e.g. 70 C, the surface becomes more flattened and thereby the reflectance becomes higher than for the chemical etched surface [30]. The characteristic of the morphology of the fully textured surface (100% coverage) is that concave cavities with a lateral dimension in the range 1e10 mm are formed. Inside these cavities deep nm-size pits are created. The surface opening of the pits is between 100 and 200 nm. The depths of the pits were not analyzed in details, but preliminary AFM-measurements indicate that the depths are about the same as the size of the pit openings. Analyzes performed on samples prepared by polarization at lower voltage than 40 V keeping the other parameters constant show that cavities are initiated on protrusions, steps and other irregularities on the surface. At 25 V the surface coverage of the Fig. 8. XPS spectra of Si 2p (Fig. 8a) and O 1s (Fig. 8b) peaks recorded after ion etchings of a mc-Si wafer pre-etched for 10 min and anodically polarized for 15 min at 40 V in 2 M KOH at 40 C.
Fig. 9. Oxygen intensities vs. etch depth, corresponding to Fig. 8.
Fig. 10. Reflectance recordings of mc-Si wafers pre-etched for 10 min and polarized for 10 min in 4 M KOH solution at 50 C at 25 V, 30 V, 40 V and 50 V.
M. Abburi et al. / Materials Chemistry and Physics 139 (2013) 756e764 Table 2 Average reflectivity of mc-Si wafers after surface treatments shown in Fig. 10. Surface treatment
Pre-etch
25 V
30 V
40 V
50 V
Average reflectance (%)
34
25
21
17
28
cavities is about 60%. Increasing the voltage to 30 V gives an increased coverage of the cavities. At 40 V and higher voltages the coverage of the cavities are 100%. Pre-etching is in fact one of the most critical parameters for obtaining successful texturing. It has been found that it is not possible to form cavities and nano-pits on an as-cut surface even if all other electrochemical parameters are optimized; only a few relatively large pits are formed with low coverage. Further, preetching for more than 15 min gives also a non-uniform surface morphology. The reason is that over etching gives a heterogenous surface; large flat (111) planes appear and on (100) planes close packed pyramid hillocks are formed. The electrochemical reactions occurring on the surface is anodic dissolution of silicon and oxidation of water to oxygen gas and protons (Si þ 2H2O þ 4hþ / SiO2 þ 4Hþ and 2H2O / O2 þ 4Hþ þ 4e). The dissolution of Si was measured by weighting the samples before and after polarizations. The results show as expected that the dissolution increases with the voltage. In 2 M KOH at 40 C at 25 V and at 40 V the average dissolution rate is found to be 0.2 and 0.7 mm min1, respectively. At higher concentrations and temperatures the dissolution rate is increasing. Utilizing Faradays law gives that at 40 V where the texturing is 100% about 80% of the current is used for dissociation of water to oxygen and the rest for anodic dissolution of Si. XPS analysis after polarization at 40 V for 15 min in 2 M KOH at 40 C shows that oxide products are formed on the surface. Determination of the oxide thickness on a rough surface is not straight forward due to shadowing effects both at analysis and ion etching. Therefore, the two methods (the intensity ratio Si4þ/Si0 and ion etching) used to determine the oxide thickness give disparate results, 6 nm and 26 nm. From this data it is concluded that the surface oxide is not uniform in thickness. The surface is partly covered with a thin oxide layer and also of relatively coarse oxide particles. The dissolution of Si during the exposure for 15 min at 40 V is about 15 mm. Thus, the silicon content in the oxide is only a small fraction of the amount of silicon dissolved. Thus, most of the anodically oxidized silicon goes into the solution and form silicates. As stated above, it is expected that during the texturing process relatively coarse discrete oxide particles are formed [27e29]. The thin oxide layer might be formed during rinsing and handling of the sample before analysis. For comparison, a corresponding analysis of a Si (100) sample treated at the OCP in the same solution and temperature gives an oxide thickness less than 1 nm. That surface was covered with pyramids, which is expected to have the same influence on the determination of the oxide thickness as the textured surface. At the OCP treatment no oxide is formed on the surface; the detected oxide is formed during rinsing and handling. Thus, it is concluded that the oxide detected after polarization at 40 V is formed on the surface during the texturing process. A further indirect evidence for formation of nonuniform oxide particles during the anodization is the strong oxygen evolution, which takes place on the surface. The oxidation reaction of water to oxygen can only take place on an oxide free surface or on a surface covered with thin oxide in order to allow transport of charges through the layer. It is suggested that the formation of cavities and pits are driven by dynamic reactions occurring on the surface during texturing. The reactions take place in microcells on the surface. It has been shown that the process starts and proceeds at protrusions and other defects where the electric field is strong and the surface
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becomes oxide free so that anodic dissolution of Si and the oxygen reaction can take place. The oxidation of water gives hydrogen ions besides oxygen gas. The increase of hydrogen ion concentration on the surface lowers the pH locally. Hereby, SiO2 becomes stable [1] and oxide grows on the surface in the low pH area. The size of these areas corresponds to the nm-pits. It is assumed that the oxide is porous because the oxidation of silicon and the oxygen gas emission takes place in the same pit even if the reactions are considered to occur separated on different nm-cells [27,28]. For prolonged exposure the growth of the oxide particles lowers the oxidation rate of Si and water. Thereby, the growth of the pits ends. On the nearby inactivated areas where no oxygen reaction takes place the surface oxide is dissolved due to the high alkalinity of the solution; water soluble silicates are the only stable Si-compounds. Next step in the process is activation of these previously inactivated areas. The process creates a micro pitted surface. The novel texturization process discussed in this paper could potentially have a significant impact on mc-Si solar cell processing. The effect of the process on the carrier lifetime and the surface recombination velocity has not been considered. In order to obtain an optimized textured wafer the size and distribution of both macro- and nano-pores have to be adjusted by finding the most approving parameters: temperature, concentration of the solution, exposure time and electrochemical parameters. 5. Conclusions This work demonstrates that it is possible to achieve an isotropic texture by anodic polarization in strong alkaline solution on multicrystalline B-doped Si-wafers. The SEM-micrographs and the reflectance measurements reveal that electrochemical treatment in environmentally friendly alkaline solutions is a promising alternative to chemical etching in acidic solutions. The optimized structure formed gives a light average reflection, which is half of the value obtained after chemical pre-etching in a corresponding solution. The parameters used and their values, which gave the isotropic structure and low light reflection, were found at: The composition of the electrolyte: 2 M and 4 M KOH. The temperature of the bath: 40 C and 50 C. The applied voltage 40 V; at lower voltage the pits do not cover the entire surface, at higher voltage nano-pits are less pronounced. Exposure time 10 min and 15 min; depends on concentration, temperature and potential. Pre-etching for 10 mine15 min depending on the temperature and the alkalinity. The mechanism for nano-pit formation is local change of the pH close to the surface. The pH change is due to the severe water oxidation reaction, which forms protons on the surface, and thereby stabilizes oxide products until the local oxidation reaction is blocked due to passivation. Thereafter the inhibiting oxide is dissolved. The presence of oxide particles was discovered by XPS. Acknowledgment The authors would like to express sincere thanks to professor Kemal Nisancioglu at the Department of Material Science, NTNU, Trondheim, Norway for fruitful discussion and to engineer Urban Jelvestam at the Department of Material and Production Technology, Chalmers, Gothenburg, Sweden for doing the XPS-analysis. M.Sc. Clara Good at Norut Narvik AS is acknowledged for writing the Labview program. Norwegian Research Council, REC Solar AS and Norut Narvik AS are acknowledged for financing the work.
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[17] J.H. Zhao, A.H. Wang, M.A. Green, F. Ferrazza, 19.8% efficient "honeycomb’ textured multicrystalline and 24.4% monocrystalline silicon solar cells, Appl. Phys. Lett. 73 (1998) 1991e1993. [18] R. Einhaus, E. Vazsonyi, J. Szlufcik, J. Nijs, R. Mertens, Isotropic texturing of multicrystalline silicon wafers with acidic texturing solutions, in: Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference e 1997, IEEE, New York, 1997, pp. 167e170. [19] K.J. Weber, A.W. Blakers, A novel silicon texturization method based on etching through a silicon nitride mask, Prog. Photovoltaics 13 (2005) 691e695. [20] J.W. Faust, E.D. Palik, Study of the orientation dependent etching and initial anodization of Si in aqueous KOH, J. Electrochem. Soc. 130 (1983) 1413e1420. [21] R.M. Hurd, N. Hackerman, Passivity phenomena at the silicon/electrolyte interface, Electrochim. Acta 9 (1964) 1633e1643. [22] E.D. Palik, O.J. Glembocki, I. Heard, Study of bias-dependent etching of Si in aqueous KOH, J. Electrochem. Soc. 134 (1987) 404e409. [23] X.H. Xia, C.M.A. Ashruf, P.J. French, J. Rappich, J.J. Kelly, Etching and passivation of silicon in alkaline solution: a coupled chemical/electrochemical system, J. Phys. Chem. B 105 (2001) 5722e5729. [24] P. Allongue, V. Costakieling, H. Gerischer, Etching of silicon in NaOH solutions e 1. In situ scanning tunneling microscopic investigation of n-Si(111), J. Electrochem. Soc. 140 (1993) 1009e1018. [25] I. Olefjord, X-ray photoelectron spectroscopy, in: D. Brune (Ed.), Surface Characterization: a User’s Sourcebook, Scandinavian Science Publisher, Oslo, 1997. [26] J. Nievendick, J. Specht, L. Zahner, M. Zimmer, D. Biro, J. Rentsch, An industrially applicable honeycomb texture, in: 26th European Photovoltaic Solar Energy Conference and Exhibition, Hamburg, Germany, 2011, pp. 1722e1725. [27] M.A. Amin, S. Frey, F. Ozanam, J.N. Chazalviel, Macromorphologies in electrochemically formed porous silica, Electrochim. Acta 53 (2008) 4485e4494. [28] S. Frey, B. Gresillon, F. Ozanam, J.N. Chazalviel, J. Carstensen, H. Foll, R.B. Wehrspohn, Self-organized macrostructures in anodically formed mesoporous silica, Electrochem. Solid-State Lett. 8 (2005) B25eB29. [29] S. Frey, S. Keipert, J.N. Chazalviel, F. Ozanam, J. Carstensen, H. Föll, Electrochemical formation of porous silica: toward an understanding of the mechanisms, Phys. Status Solidi (a) 204 (2007) 1250e1254. [30] M. Abburi, Electrochemical Passivation and Texturing of Silicon Wafers in Alkaline Solutions, PhD Thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2012.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical isotropic texturing of mc-Si wafers in KOH solution M. Abburi*, T. Boström, I. Olefjord Norut Narvik AS, P.O. Box 250, N-8504 Narvik, Norway
h i g h l i g h t s < A method to form isotropic textures on mc-Si wafers in KOH solution is presented. < The method is based on anodic polarization of silicon in KOH at high potentials. < Evolution of surface morphology is studied by varying the etch parameters. < Isotropic textures with lowest average reflectivity are obtained at 40 V. < A reaction model for texturing mechanism is discussed in the light of XPS data.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2012 Accepted 4 February 2013
Boron doped multicrystalline Si-wafers were anodically polarized in 2 M KOH and 4 M KOH at 40 C and 50 C. The applied potentials were 25 V, 30 V, 40 V and 50 V. The morphology of the textured surfaces, the surface products and the light reflectivity were analyzed by utilizing SEM, XPS and Lambda UV/Vis/ NIR spectrophotometer, respectively. Isotropic texturing was obtained. The lowest average reflectivity, 17%, was achieved after pre-etching for 10 min and polarization at 40 V for 10 min in 4 M KOH at 50 C. That reflection value is half of that measured on a chemical pre-etched surface, 34%. By increasing the voltage to 50 V the reflectivity rises to 28%. Polarizations to 25 V and 30 V at 50 C in both solutions give local pores in the mm-range. The etch attack initiation is located at protrusions on the surface. At 40 V and 50 V in both solutions the pores are extended onto the entire surface. The width of the pores is about 10 mm. Inside the micro-pores, nm-pores are formed; their lateral size is in the range 100 nme200 nm. A mechanism for the anodic dissolution reactions is discussed. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Etching Electrochemical techniques Electron microscopy Surface properties
1. Introduction The efficiency of solar cells is to a large extent dependent on their capability to absorb light radiation. In order to increase the absorption of light the silicon wafer surfaces are roughened by chemical or physical methods. A method for designing their surface morphology is wet chemical etching. The desired textured surface is obtained by chemical etching in either strong alkaline solutions or in acidic solutions containing a mixture of hydrofluoric acid and nitric acid. Strong alkaline solutions can be used because Si dissolves in alkaline solutions at pH > 10 [1]. Below that value Si is passivated due to formation of thermodynamically stable SiO2. Thereby, silicon is stable in acids except in hydrofluoric acid, which dissolves the oxide. The purpose of adding nitric acid to HF-solution is to oxidize silicon. The technique gives an isotropic etching, which is grain orientation independent [2e5].
* Corresponding author. Tel.: þ47 76965374; fax: þ47 76965351. E-mail addresses: [email protected], [email protected] (M. Abburi). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.027
Chemical etching of multicrystalline silicon (mc-Si) in alkaline solutions give a non-uniform (anisotropic) dissolution of Si and thereby an anisotropic texture, which gives a relatively high light reflection. The reason for the anisotropic texturing is that dissolution of mc-Si is due to the random orientation of grains, i.e., the crystallographic planes are etched at different etch rates [6e10]. It has been reported that etch rate of (111) surface is a few order of magnitudes lower than that of the fast etching planes, (100) and (110) [7,10,11]. Etching of Si (111) results in a stable shiny surface with high reflectivity [12]. The anisotropic alkaline etching technique is widely used for etching and texturing of monocrystalline (100) orientation silicon wafers [12e14]. Etching of Si (100) wafers in low concentration alkaline solutions (<2 M) at high temperatures (>50 C) forms pyramidal hillocks on the surface which reduce reflection of incoming sun light effectively. Although alkaline solutions are cheap, easy to handle during the process and environmentally friendly, this anisotropic etching technique is not effective enough for multicrystalline silicon wafers; the surface morphology becomes coarse and non-uniform. In addition, the etching can form
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unwanted steps at grain boundaries. The shiny steps at the grain boundaries not only increase the light reflection but also contain defects and act as recombination centers for charge carriers, thereby having a negative effect on the efficiency of a solar cell. Today, acidic etching in HF/HNO3 is used in order to achieve the desired textures on multicrystalline silicon wafers for effective reflection reduction. The acidic etching technique is well suitable for mc-Si wafers in forming isotropic textures [15e17]. However, the process is difficult to control. Due to its vigorous etching nature, grain boundaries and surface defects are attacked. The defect etching leads to pits, micro cracks and steps at boundaries [15,16,18,19]; the wafers are therefore prone to damage. Other significant drawbacks are that the environment is hazardous for humans and the disposal of the chemical waste containing fluorides is expensive. One possibility to overcome the problems with conventional alkaline and acid etchings is to anodically etch the wafers in an alkaline solution. The treatment has to be done at relatively high alkalinity where SiO2 is not chemically stable. In aqueous solutions like KOH and NaOH etc. the stable compounds are soluble silicates [1]. However, during polarization of Si in the solution metastable oxide products can be formed [20e23], which may possibly passivate the surface. The formation of the oxide products is dependent on the alkalinity and the temperature of the electrolyte [30]. In this paper it will be shown that it is possible to obtain homogenous textures on all crystallographic grains of mc-Si wafer surfaces by anodic polarization in alkaline solutions at relatively high temperatures and potentials. This electrochemical texturing method in alkaline solutions can effectively replace the isotropic texturing by acidic solutions for effective light trapping on mc-Si wafers and hence drastically reduce the usage of the hazardous hydrofluoric acid. 2. Experimental The test material was boron doped mc-Si wafers in its as-cut condition produced by REC Scan Wafer AS. The thickness of the wafers was 200 mm and their resistivity was in the range 1e1.5 U cm. The wafers were cut into 2.5 cm diameter samples by using a laser cutting process. The sample was mounted in a Teflon sample holder sealed with an O-ring. The sample was mounted on the metallic conductor of the cell by using a conducting copper tape. The exposed area was 1.32 cm2. The electrolyte solution was prepared from analytical grade KOH pellets and Millipore water of 18 MU cm resistivity. The sample holder was dipped into the cell in such a way that the sample surface becomes vertically oriented in the solution. The electrolyte was continuously stirred with a magnetic stirrer and the temperature was thermostatic controlled. In order to clarify eventually rise of the surface temperature during anodic polarization the temperature was measured by thermocouple mechanically fitted to the Si-surface. It was found that the current has to be increased to about 1 A cm2 before the surface temperature was raised by 1 C. Thus, the temperatures given below are within 0.5 C. Before applying the potential, the native surface oxide was removed by keeping the sample in the solution until hydrogen bubbles appear on the surface. The hydrogen evolution is an indication for chemical etching of the oxide-free silicon surface. The etching was extended at the open circuit potential (OCP) for a few minutes in order to remove the saw damaged layer. This process is termed ‘pre-etching’. The electrochemical experiments were carried out with a power source connected to a personal computer using a RS-232 interface and controlled by using in-house written Lab view software. A conventional two-electrode set up was used. The counter electrode was a Pt-sheet, 4 6 cm2. The large area gives a relatively low current density and thereby low polarization on the cathode. In this study, all the potentials were referred to the Pt-electrode.
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The surfaces of the textured samples were studied by using a scanning electron microscope (SEM, Carl-Zeiss, Evo 60). The oxide formed during the polarization was analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5500). The X-ray source was Mg Karadiation. Light reflectance from the surface was measured in 350e 1100 nm range by using a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer equipped with an integrating sphere. 3. Results 3.1. Surface morphology of as-cut and chemical etched surfaces Fig. 1 shows the surface morphology of an ‘as-cut’ mc-Si wafer surface. The surface morphology is characteristic for a wear and micro-cracked brittle material. A few microns of the top layer are damaged by the sawing process. The surface of the as-cut wafer is shiny and reflects a large portion of the radiation. The sawdamaged layer has to be removed before processing of the wafer because it is also electrically inactive. Surface morphologies of mc-Si wafers formed during chemical etching treatments are shown in Fig. 2. The etch patterns are characteristic for anisotropic etching of multicrystalline silicon in alkaline solutions. Etching for 10 min in 4 M KOH at 50 C is shown in Fig. 2a. It appears that the saw damaged layer is removed and the morphology depends on the grain orientation. Steps are developed at grain boundaries due to various etch rate of the grains. The surface is rough with protrusions and valleys. Etching for 10 min at lower concentration, 2 M KOH at 50 C is displayed in Fig. 2b. The etch pattern is similar to that obtained in the 4 M solution. The influence of etching time is demonstrated in Fig. 2c. The sample was etched for 15 min instead of 10 min in the same solution as above. It is clearly shown that close packed pyramidal hillocks are developed on the surface. The triangular etch patterns on the left side of the image contains terraces and steps, which are characteristic of alkaline etching of (111) grains [24]. The roof tile structures are due to anisotropic etching of (110) grains [12]. The influence of temperature is revealed in Fig. 2d. The etching was performed at 40 C for 10 min in 2 M KOH. At this temperature and short etching time the pyramids, terraces and steps are not fully developed. 3.2. Potentiostatic polarizations Fig. 3 shows the current density vs. time recorded during potentiostatic polarizations for 10 min at 25 V, 30 V, 40 V and 50 V in 4 M KOH at 50 C. The samples were pre-etched in the solution for 10 min before polarization (Fig. 2a). The potential is applied in one step from the OCP to the desired potential. It appears from the
Fig. 1. SEM micrograph of ‘as-cut’ mc-Si wafer surface.
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Fig. 2. SEM images of mc-Si wafer after chemical etching in: a) 4 M KOH at 50 C for 10 min, b) 2 M KOH at 50 C for 10 min, c) 2 M KOH at 50 C for 15 min and d) 2 M KOH at 40 C for 10 min.
figure that at all potentials the current goes through a minimum. In the first stage, the current corresponds to formation of oxide products and oxygen evolution due to oxidation of silicon and water, respectively. The drop in the current is due to formation of thick oxide products on the surface, which lowers the oxygen evolution reaction. For prolonged exposure the current goes through a minimum and then increases. It is suggested that the current increase is due to locally breakdown of oxide products, which allows oxidation of silicon and water. The anodic current (Fig. 3) is strongly potential dependent; at short polarization time the current is one order of magnitude higher at 50 V compared to 25 V. Exposure for 10 min gives a current difference of almost 100 times. Further, it can also be seen from the figure that the current minimum goes toward lower exposure times for increased potential.
Fig. 3. Current density vs. time for mc-Si wafers pre-etched for 10 min and polarized for 10 min in 4 M KOH solution at 50 C at 25 V, 30 V, 40 V and 50 V.
3.3. Surface morphology of electrochemically etched surfaces Fig. 4 shows surface morphology of mc-Si wafers after electrochemical treatments in 4 M KOH at 50 C for 10 min at various potentials. All the wafers were chemical pre-etched for 10 min in the solution before applying the potential (Fig. 2a). It appears from Fig. 4a and b that polarization at 25 V and 30 V causes formation of discrete micro-pores. The size of the pores is about 1 mm. The protrusions formed during pre-etching are not observable in Fig. 4a and b. That observation indicates that the pores are initiated on the top of the protrusions where the electric field strength is highest. By comparing the figures it is obvious that the pore coverage increases with the potential. Fig. 4c and e shows that the surfaces are completely covered with pores after increasing the potential to 40 V and 50 V. The micrographs at the low magnification, Fig. 4d and f, shows that the surface texture is independent of grain orientation; the photos show multi-grains. Thus, the desired isotropic texture is obtained. The steps at grain boundaries, which are formed during preetching, are effectively textured. The pores are well extended into the entire surface and concave cavities are formed. At 40 V, the concave cavities (Fig. 4c) are as large as 10 mm wide. Increasing the potential to 50 V, the isotropic nature of the etching is not changed, however the surface seems to be more etched and the concave cavities become smoother, Fig. 4e. Fig. 5a and b shows high magnification images (originally 100,000) of cavities displayed in Fig. 4b and c, respectively. The photos show a network of nano-pores formed inside the micro pores. The lateral widths of the nano-pores are in the range 100e 200 nm. It appears from the figures that the nano-pores are well developed at both potentials, 30 V, as at 40 V. The influence of lower KOH concentration on the surface morphology is shown in Fig. 6. The samples were pre-etched for
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Fig. 4. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 10 min in 4 M KOH at 50 C at: a) 25 V, b) 30 V, c) 40 V, d) 40 V at lower magnification, e) 50 V and f) 50 V at lower magnification.
10 min and polarized for 10 min in 2 M KOH at 50 C at 25 V, 30 V, 40 V and 50 V. The surface Fig. 6a shows that micro-pores are formed on protrusions and ridges during the anodic polarization at 25 V. The pore coverage increases as the potential is increased from 25 V to 30 V, Fig. 6b. This is the same picture as obtained at higher alkalinity. Polarization at 40 V and 50 V gives isotropic textures, Fig. 6c and d, respectively. Sponge like cavities features with nanopores inside are formed at 40 V. It seems that at 50 V the microcavities are smoother; probably due to enhanced dissolution. Fig. 7 shows the textures formed at 40 C in 2 M KOH at 30 V, 40 V and 50 V. The pre-etching and polarization exposure times were 10 min and 15 min, respectively. The surface morphology of the pre-etched surface is shown in Fig. 2d. The initial surface is relatively rough and homogenous due to etching at relatively low temperature; steps at grain boundaries are not formed. Polarization at 30 V results in formation of isolated micro-meter pores uniformly distributed over the entire surface, Fig. 7a and b. At 40 V, concave cavities features are formed homogeneously on the surface. All the grains are evenly etched as shown in Fig. 7c and d. At 50 V the
micro-texture seems to be the same as that formed at 40 V, Fig. 7e and f. However, it is obvious that the higher voltage influences on the nano-pore formation. The considerably higher current and thereby higher etch rate prevent formation of the fine structured pores; thereby the bottom of the cavities becomes flatter than at the lower potentials; this is clearly seen by comparing Fig. 7d and f. 3.4. XPS-analysis XPS-analysis was performed in order to determine the composition and thickness of oxide products formed during texturing. Before analysis the sample was pre-etched for 10 min and then polarized at 40 V for 15 min in 2 M KOH at 40 C. Fig. 8 shows Si 2p (Fig. 8a), and O 1s (Fig. 8b) signals after various steps of ion etching. The etch depths measured in nm are marked in the figure. The first analysis was done after removal of the contamination layer by 1 nm ion etching. Thereafter the sample was ion-etched and analyzed in steps down to 35 nm below the original surface.
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etching for 1 nm. In the Si spectrum a low intensity peak appears at the position of Si0. The presence of this peak shows that the oxide products present on the surface are relatively thin at least in some part of the analyzed area. By using the ratio between Si4þ and Si0 the oxide thickness is estimated [25] to be 6 nm. However, the recorded spectra show that even after ion-etching to relatively deep depths, oxides remain on the surface. It is suggested that the ambiguous results is due to the presence of oxide particles of different sizes. Fig. 9 shows the recorded oxygen intensity vs. etch depth. The thickness of the oxide is normally determined as the etch depth at which the intensity of the oxygen signal has decreased to half of its maximum value. This method gives an average oxide thickness of 26 nm. This value is probably too high due to shadowing effect caused by the roughness of the surface. The oxide particles are probably burrowed deep into the pits. From the data presented in Figs. 8 and 9 it is concluded that porous oxide particles are formed during texturing. The fact that the zero valency state of silicon is registered shows that at least a part of the surface is covered with a very thin oxide layer, which might be formed during rinsing and handling of the sample after polarization. 3.5. Dissolution rate of silicon
Fig. 5. SEM micrographs at high magnification of nano-pores formed during polarization for 10 min in 4 M KOH at 50 C at: a) 30 V and b) 40 V.
The Si-signals show two peaks representing the four valency state, Si4þ, and the zero valency state, Si0, respectively. The oxygen signals, O2, correspond to oxygen bound to silicon in oxide products. The strongest Si4þ and O2 signals are achieved after ion-
The dissolution rate of mc-Si wafers was determined by weighting the sample before and after exposure to the solution. Table 1 shows measured average dissolution rates during chemical etching at OCP and during polarization at different voltages in 2 M and 4 M KOH at 40 C and 50 C, respectively. It appears from the table that the dissolution rates of the wafers at OCP in the two solutions are 0.6 0.1 mm min1. The dissolution rates at polarizations at 25 V and 30 V are 0.2 mm min1. At these potentials the dissolution is not uniform; the pits cover only partly
Fig. 6. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 10 min in 2 M KOH at 50 C at: a) 25 V, b) 30 V, c) 40 V and d) 50 V.
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Fig. 7. SEM micrographs of mc-Si surfaces after pre-etching for 10 min and anodic polarization for 15 min in 2 M KOH at 40 C at: a) 30 V, b) 30 V at a higher magnification, c) 40 V, d) 40 V at a higher magnification, e) 50 V and f) 50 V at a higher magnification.
the surface. The etching rate becomes noticeably higher during polarization at 40 V and 50 V. The surfaces are uniformly textured. The dissolution rates in 2 M KOH at 40 C and 4 M KOH at 50 C are in the range 0.7 mm min1 to 2.0 mm min1. The highest dissolution rate is obtained at 4 M KOH at 50 C. The amounts of dissolved Si during the polarizations are orders of magnitudes larger than the corresponding amounts of Si bound in the surface oxide.
etched sample. The average reflectance from the pre-etched sample is 34%, which is about the same as the reflectance from polished surfaces. Polarization in the potential range 25 Ve40 V lowers the reflectance continuously with increasing potential. At 40 V the average reflectance is 17%, which is about half of the value obtained after pre-etching. After polarization to 50 V the average reflectance is increased to 28%, which is close to the value obtained after pre-etching.
3.6. Reflectance measurements The light reflectance from chemical etched and anodically textured wafers is shown in Fig. 10. The samples were pre-etched for 10 min in 4 M KOH at 50 C and then polarized for 10 min at 25 V, 30 V, 40 V and 50 V. The reflectance was measured within the wave length range 350 nme1100 nm. The average reflectance between 350 and 1100 nm is given in Table 2. It appears from the curves that the reflectance from the anodically polarized wafers is noticeably lower than from the chemically
4. Discussion This study demonstrates that it is possible to isotropically texture multicrystalline Si wafers by anodic polarization in strong alkaline solutions. For the future it is supposed that this method will be developed to a practical technique in order to form uniform textures on mc-Si wafers. The chemicals used in the process are biological friendly both for humans and the environment.
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M. Abburi et al. / Materials Chemistry and Physics 139 (2013) 756e764 Table 1 Dissolution rates of mc-Si wafers during electrochemical treatments. Dissolution rate
Solution parameters
Voltage (V) OCP
25
30
40
50
mm min1 mm min1
2 M KOH at 40 C 4 M KOH at 50 C
0.5 0.7
0.2 0.2
0.2 0.3
0.7 1.1
1.1 2.0
The aim of the texturing process is to minimize the light reflection in order to make the solar cells to absorb as much light as possible. This can be done by electrochemical treatment; by adjustment of the parameters it is possible to lower the average reflectivity to 17%, which is about half of the value obtained after chemical etching. The adjustable parameters are: pre-treatment of the wafer, the alkalinity of the solution, the temperature of the electrolyte and the applied voltage. So far the lowest reflectivity is obtained by pre-etching for 10 min and applying 40 V to the sample in 4 M KOH at 50 C for 10 min. The reflectance obtained is lower than the values from commercial acidic etching. Averaged reflectance values after standard industrial HF/HNO3 etching of multicrystalline silicon wafers are around 29% and solar spectrum averaged reflectance values are around 27% [26]. This treatment gives an isotropic texture covering 100% of the surface. It seems that the sensitivity for some of the parameters is not critical; 2 M KOH at 40 C and 50 C gives about the same result as that in 4 M KOH. It might be possible to find another set of parameters which gives even lower reflectivity. However, increasing the voltage from 40 V to 50 V gives a reflectivity which is close to the chemically pre-etched surface. It is obvious that the increased potential flatten the surface. This result agrees with the SEM micrographs, which show smoother surfaces at 50 V. At even higher voltages and higher temperatures, e.g. 70 C, the surface becomes more flattened and thereby the reflectance becomes higher than for the chemical etched surface [30]. The characteristic of the morphology of the fully textured surface (100% coverage) is that concave cavities with a lateral dimension in the range 1e10 mm are formed. Inside these cavities deep nm-size pits are created. The surface opening of the pits is between 100 and 200 nm. The depths of the pits were not analyzed in details, but preliminary AFM-measurements indicate that the depths are about the same as the size of the pit openings. Analyzes performed on samples prepared by polarization at lower voltage than 40 V keeping the other parameters constant show that cavities are initiated on protrusions, steps and other irregularities on the surface. At 25 V the surface coverage of the Fig. 8. XPS spectra of Si 2p (Fig. 8a) and O 1s (Fig. 8b) peaks recorded after ion etchings of a mc-Si wafer pre-etched for 10 min and anodically polarized for 15 min at 40 V in 2 M KOH at 40 C.
Fig. 9. Oxygen intensities vs. etch depth, corresponding to Fig. 8.
Fig. 10. Reflectance recordings of mc-Si wafers pre-etched for 10 min and polarized for 10 min in 4 M KOH solution at 50 C at 25 V, 30 V, 40 V and 50 V.
M. Abburi et al. / Materials Chemistry and Physics 139 (2013) 756e764 Table 2 Average reflectivity of mc-Si wafers after surface treatments shown in Fig. 10. Surface treatment
Pre-etch
25 V
30 V
40 V
50 V
Average reflectance (%)
34
25
21
17
28
cavities is about 60%. Increasing the voltage to 30 V gives an increased coverage of the cavities. At 40 V and higher voltages the coverage of the cavities are 100%. Pre-etching is in fact one of the most critical parameters for obtaining successful texturing. It has been found that it is not possible to form cavities and nano-pits on an as-cut surface even if all other electrochemical parameters are optimized; only a few relatively large pits are formed with low coverage. Further, preetching for more than 15 min gives also a non-uniform surface morphology. The reason is that over etching gives a heterogenous surface; large flat (111) planes appear and on (100) planes close packed pyramid hillocks are formed. The electrochemical reactions occurring on the surface is anodic dissolution of silicon and oxidation of water to oxygen gas and protons (Si þ 2H2O þ 4hþ / SiO2 þ 4Hþ and 2H2O / O2 þ 4Hþ þ 4e). The dissolution of Si was measured by weighting the samples before and after polarizations. The results show as expected that the dissolution increases with the voltage. In 2 M KOH at 40 C at 25 V and at 40 V the average dissolution rate is found to be 0.2 and 0.7 mm min1, respectively. At higher concentrations and temperatures the dissolution rate is increasing. Utilizing Faradays law gives that at 40 V where the texturing is 100% about 80% of the current is used for dissociation of water to oxygen and the rest for anodic dissolution of Si. XPS analysis after polarization at 40 V for 15 min in 2 M KOH at 40 C shows that oxide products are formed on the surface. Determination of the oxide thickness on a rough surface is not straight forward due to shadowing effects both at analysis and ion etching. Therefore, the two methods (the intensity ratio Si4þ/Si0 and ion etching) used to determine the oxide thickness give disparate results, 6 nm and 26 nm. From this data it is concluded that the surface oxide is not uniform in thickness. The surface is partly covered with a thin oxide layer and also of relatively coarse oxide particles. The dissolution of Si during the exposure for 15 min at 40 V is about 15 mm. Thus, the silicon content in the oxide is only a small fraction of the amount of silicon dissolved. Thus, most of the anodically oxidized silicon goes into the solution and form silicates. As stated above, it is expected that during the texturing process relatively coarse discrete oxide particles are formed [27e29]. The thin oxide layer might be formed during rinsing and handling of the sample before analysis. For comparison, a corresponding analysis of a Si (100) sample treated at the OCP in the same solution and temperature gives an oxide thickness less than 1 nm. That surface was covered with pyramids, which is expected to have the same influence on the determination of the oxide thickness as the textured surface. At the OCP treatment no oxide is formed on the surface; the detected oxide is formed during rinsing and handling. Thus, it is concluded that the oxide detected after polarization at 40 V is formed on the surface during the texturing process. A further indirect evidence for formation of nonuniform oxide particles during the anodization is the strong oxygen evolution, which takes place on the surface. The oxidation reaction of water to oxygen can only take place on an oxide free surface or on a surface covered with thin oxide in order to allow transport of charges through the layer. It is suggested that the formation of cavities and pits are driven by dynamic reactions occurring on the surface during texturing. The reactions take place in microcells on the surface. It has been shown that the process starts and proceeds at protrusions and other defects where the electric field is strong and the surface
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becomes oxide free so that anodic dissolution of Si and the oxygen reaction can take place. The oxidation of water gives hydrogen ions besides oxygen gas. The increase of hydrogen ion concentration on the surface lowers the pH locally. Hereby, SiO2 becomes stable [1] and oxide grows on the surface in the low pH area. The size of these areas corresponds to the nm-pits. It is assumed that the oxide is porous because the oxidation of silicon and the oxygen gas emission takes place in the same pit even if the reactions are considered to occur separated on different nm-cells [27,28]. For prolonged exposure the growth of the oxide particles lowers the oxidation rate of Si and water. Thereby, the growth of the pits ends. On the nearby inactivated areas where no oxygen reaction takes place the surface oxide is dissolved due to the high alkalinity of the solution; water soluble silicates are the only stable Si-compounds. Next step in the process is activation of these previously inactivated areas. The process creates a micro pitted surface. The novel texturization process discussed in this paper could potentially have a significant impact on mc-Si solar cell processing. The effect of the process on the carrier lifetime and the surface recombination velocity has not been considered. In order to obtain an optimized textured wafer the size and distribution of both macro- and nano-pores have to be adjusted by finding the most approving parameters: temperature, concentration of the solution, exposure time and electrochemical parameters. 5. Conclusions This work demonstrates that it is possible to achieve an isotropic texture by anodic polarization in strong alkaline solution on multicrystalline B-doped Si-wafers. The SEM-micrographs and the reflectance measurements reveal that electrochemical treatment in environmentally friendly alkaline solutions is a promising alternative to chemical etching in acidic solutions. The optimized structure formed gives a light average reflection, which is half of the value obtained after chemical pre-etching in a corresponding solution. The parameters used and their values, which gave the isotropic structure and low light reflection, were found at: The composition of the electrolyte: 2 M and 4 M KOH. The temperature of the bath: 40 C and 50 C. The applied voltage 40 V; at lower voltage the pits do not cover the entire surface, at higher voltage nano-pits are less pronounced. Exposure time 10 min and 15 min; depends on concentration, temperature and potential. Pre-etching for 10 mine15 min depending on the temperature and the alkalinity. The mechanism for nano-pit formation is local change of the pH close to the surface. The pH change is due to the severe water oxidation reaction, which forms protons on the surface, and thereby stabilizes oxide products until the local oxidation reaction is blocked due to passivation. Thereafter the inhibiting oxide is dissolved. The presence of oxide particles was discovered by XPS. Acknowledgment The authors would like to express sincere thanks to professor Kemal Nisancioglu at the Department of Material Science, NTNU, Trondheim, Norway for fruitful discussion and to engineer Urban Jelvestam at the Department of Material and Production Technology, Chalmers, Gothenburg, Sweden for doing the XPS-analysis. M.Sc. Clara Good at Norut Narvik AS is acknowledged for writing the Labview program. Norwegian Research Council, REC Solar AS and Norut Narvik AS are acknowledged for financing the work.
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