Electrochemical Passivation and Texturing of Silicon Wafers in Alkaline Solutions
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Texturing of silicon wafers is an important process step in fabrication of solar cells in order to reduce the reflection of incoming light radiation on the solar cell surfaces. The conventional wet chemical etching methods have clear limitations in etching and texturing of multicrystalline silicon (mc-Si) wafers. Chemical etching of mc-Si wafers in alkaline solutions results in a coarse and non uniform surface morphology since the dissolution rate depends on the grain orientation. In contrast to alkaline etching, textures formed by acidic etchants (HF+HNO3) are uniform. However, acidic etching is difficult to control, the disposal of the waste is expensive and the method is in general environmentally hazardous. The aim of this thesis is to study an environmentally friendly etching method, which is supposed to be an alternative to acidic texturing of both single and multicrystalline B-doped silicon wafers. The method is based on anodic polarization of Si in alkaline solutions. In spite of the fact that silicon is one of the most studied elements due to its applicability in electronic devices, its electrochemistry in alkaline solutions at increased temperatures and at high potentials has been scarcely investigated. In the first part of the thesis monocrystalline (100) and (111) silicon samples were polarized in a strong alkaline solution at different temperatures and potentials in order to study anodic passivation mechanism. In the second part single- and multicrystalline Si-wafers were textured by utilizing electrochemical treatment in hot alkaline solutions. Anodic dissolution and passivation of Si (100) and Si (111) were studied by potentiodynamic and potentiostatic polarizations in 2 M KOH at temperatures ranging from 23 oC to 70 oC and at potentials from the open circuit potential (OCP) to 14.5 V (with respect to Ag/AgCl reference electrode). Potentiostatic polarization experiments were performed on as-received (mechanically sawed or polished) and pre-etched (at OCP in the alkaline test solution) samples. The composition and thickness of the reaction products formed on the surfaces were analysed by X-ray photoelectron spectroscopy (XPS). The morphology of the surfaces was studied by scanning electron microscopy (SEM). The oxide products formed during passivation was analyzed after polarization to the Flade potential at about 1 V. A thin (about 1 nm) homogeneous layer of Sioxyhydroxide (H2SiO3) is formed at this potential. It is formed on both crystal orientations independent of temperature. The hydrated silica layer is gradually transformed into the stoichiometric oxide SiO2 by polarizing Si (100) above the Flade potential at 23 oC or 30 oC. The phase change is indicated by an oxidation peak at around 4 V during potentiodynamic polarizations. Further, XPS analysis shows that both hydrated and dehydrated silica coexist at this potential. The oxide thickness formed during potentiodynamic polarization to 14.5 V is 22 nm. Oxygen evolution is observed at about 4 V. At higher potentials the oxygen evolution is inhibited due to growth of thick oxide. The stoichiometric oxide SiO2 forms and grows with increasing potential and exposure time on Si (111) at all temperatures irrespective of pre-etching. The thicknesses of the oxide are in the range 3 nm to 40 nm depending on the polarization method, potential and exposure time. The formation and growth of SiO2 on Si (100) at 40 oC and higher temperatures depends on surface pre-etching. Polarization of pre-etched Si (100) at 40 oC and higher temperatures does not form SiO2 on the surface. The compound present after polarization at all potentials was the thin, 1 nm thick hydrated silica layer. It was formed irrespectively of the polarization method or potential. Thus, pre-etching is an important parameter in the oxidation mechanism of Si (100). Oxide growth on non-pre-etched Si (100) at the high temperatures was similar to that of Si (111). However, pre-etching of Si (100) for more than 10 min at the high temperatures results in formation of pyramidal hillocks on the surface. During polarization at the high temperatures, the pyramidal hillocks are dissolved and the surface becomes flattened. Hydrated silica layer is the only detected oxidation product at all potentials. In the second part of the thesis a method is developed for electrochemical texturing of as-cut multicrystalline and single crystals, Si (111) and Si (100), in alkaline solutions. The wafers are potentiostatically polarized in 2 M KOH and 4 M KOH at temperatures in the range 30 oC to 70 oC. The applied potentials are varied from 20 V to 50 V (referred to Pt-counter electrode). Before the polarization, the samples are pre-etched in the test solution to remove the deformed zone on the as-cut surfaces. The morphology of the textured surfaces, the composition and thickness of the surface products and the light reflectivity are analyzed by utilizing SEM, XPS and Lambda UV/Vis/NIR spectrophotometer, respectively. Pre-etching of multicrystalline silicon (mc-Si) wafers results in anisotropic surface morphology with protrusions and valleys due to etching of different grains at different etch rates in alkaline solutions. During anodic polarization at potentials above 20 V and at temperatures at 40 oC and above, all the grains are uniformly textured. At 25 V and 30 V, micro-pits are formed on the protrusions. With increasing the potential to 40 V and 50 V, concave cavities are formed on the entire surface. Isotropic texturing is achieved. Isotropic texturing on mc-Si wafers is characterized by formation of the concave cavities of 1 to 10 μm. Inside the micropits and cavities, nano-pits are formed. Their lateral size is in the range 100 nm to 200 nm. At 50 V, the nano-pits are etched away and the cavities become shallower due to enhanced dissolution. The lowest average reflectivity, 17%, is achieved on electrochemically textured surface after polarization at 40 V for 10 min in 4 M KOH at 50 oC, which is 50% lower than the reflectivity on the pre-etched surface. A model for the isotropic texturing mechanism is proposed based on XPS analysis and electrochemical measurements. It is suggested that the formation of pits and concave cavities is due to local pH variations caused by oxygen evolution reaction, which control formation and dissolution of the surface oxide. Protons formed during oxygen evolution reduce the pH on the surface locally. Hereby, SiO2 becomes stable and grows on the surface in the low pH area. For prolonged exposure the growth of the oxide passivates the surface and lowers the oxidation rate of Si and water. Due to high alkalinity of the electrolyte, oxide dissolves at weak points thus creating pits. Oxidation of silicon and oxygen evolution occurs at the oxide breakdown sites. Continuous formation and local dissolution of oxide causes the formation of micro-pitted surface. Formation of pits and concave cavities are not affected by changes in the electrolyte concentration and temperature. However, pre-etching is an important factor in obtaining the uniform textures on all the grains of the mc-Si wafers. This is due to the fact that pre-etching influences the formation of SiO2 on (100) grains at high temperatures. Isotropic textures are not obtained on mc-Si wafers containing significant number of (100) grains when pre-etched for more than 10 min. On Si (111) wafer surfaces, pre-etching results in formation of steps and terraces. The terraces contain true (111) planes are stable and steps contain fast etching planes such as (100) and (110) are more reactive. During polarization, terrace planes are passivated by formation of a stable oxide, and pits are initiated at the step edges. With increasing potential, temperature and exposure time, pitting spreads over the terrace planes. The surface reflectance is decreased with increasing pit coverage. It is demonstrated that the electrochemical texturing method can effectively replace the present isotropic etching by acidic etchants. Both single crystal and multicrystalline materials have successfully been textured.