Surface Texturing Behavior of Nano-copper Particles under Various Copper Salts System during Copper-assisted Chemical Etching


 In this work, the effects of different copper salts on the etching behavior of n-type monocrystalline silicon wafers were detailedly studied by Cu-assisted chemical etching method. Firstly, the inverted pyramid, inverted pyramid-like and oval pit texturing structures were obtained by HF/H2O2/Cu(NO3)2, HF/H2O2/CuSO4 and HF/H2O2/CuCl2 etching systems. Then, the evolution of copper particles deposition behavior was studied to reveal the influencing mechanism of different anion species, the textured wafer surfaces were characterized by scanning electron microscopy (SEM) and ultraviolet-visible (UV) spectrophotometer, the etching rate, silicon wafer thinning and the deposition amount of copper particle was systematically analyzed. We conclude that the binding force between anion and cation, the oxidation of anions and the formation of complex groups [CuCl2]− lead to great difference in the deposition behavior of copper, resulting in different etching morphology and etching rate. The moderate size copper particles deposited from HF/H2O2/Cu(NO3)2 system make that the etching process is mild and the anisotropic etching ability can fully demonstrated, and the regular inverted pyramid structures can be formed under low thinning of silicon wafers. This work will provide guidance for controllable preparation of inverted pyramid structure and future application in high efficiency solar cells.


Introduction
Solar energy has become one of the most promising renewable energy sources to replace traditional energy sources because of its clean and pollution-free reserves [1,2], and the installed capacity of photovoltaics has increased dramatically in recent years [3]. The solar cells, which can directly convert sunlight into electrical energy, are undoubtedly the core device of photovoltaic power generation, where the single crystal silicon (sc-Si) solar cell occupied more and more market share due to the continuous technological progress and cost reduction [4,5]. Moreover, the n type silicon show huge potential in developing low-cost and high-efficiency solar cells in the future due to its natural advantages of low light attenuation, high minority carrier lifetime [6,7]. Surface texture is an important optical management strategy for high-efficient silicon solar cell devices [8]. During sc-Si solar cell processing, KOH anisotropic etching is utilized to texture silicon wafer, and the upright pyramid light trapping structure can be formed, which reduces the reflectivity of silicon wafer to about 15% [9], however, the upright pyramid is not the most effective anti-reflection structure and it is not an optimal choice in prospective HIT high efficiency cell technology which needs a efficient interfaces passivation.
Metal assisted chemical etching is an effective wet texturing method which could reduce the surface reflectivity by forming highly light-trapping nanostructures [10][11][12]. In previous work, we have prepared vertical, inclined and zigzag nanowire structures on multi-crystalline silicon (mc-Si) substrates by Ag-assisted chemical etching [13][14][15]. Besides silver, the copper also show catalytic activity, and inverted pyramids (IPs) structure with ultra-low reflectivity can be obtained by anisotropic etching of copper [16][17][18][19][20]. The IPs structure owns superior light-trapping performance and attracted global attention since 2001, when the research team of the University of New South Wales applied IPs structure to Passivated Emitter and Rear Locally-diffused (PERL) solar cell, which created a efficiency record of 24.7% [21]. However, this structure has not been quickly applied to photovoltaic devices on a large scale, because it needs lithography and laser processes, which were complicated and expensive. Until 2015, Wang Yan et al obtained IPs structure with low reflectivity on the silicon surface by copper-assisted chemical etching using HF/H2O2/Cu(NO3)2 solution, which brought hope for the low-cost preparation of the IPs structure [22]. We also fabricated IPs structure with low reflectivity on mc-Si surface [23], moreover, we found that the etched morphologies in different copper salt etchants are quite different [24]. Studying the influence mechanism of anions in different copper salt etching systems will help us further understand the chemical etching mechanism of copper nanoparticle and the formation process of the inverted pyramid.
In this paper, we applied Cu-assisted chemical etching (Cu-ACE) on n-type monocrystalline silicon wafers, and the different copper salt etchant system of HF/H2O2/Cu(NO3)2, HF/H2O2/CuSO4 and HF/H2O2/CuCl2) was selected to explore the influence of anions species on the deposition behavior of Cu, etching rates and resulting surface morphologies. Finally, uniform IPs arrays with low reflectivity of 7.2% was prepared on silicon surface, and the etching rate was slow. Our work will contribute to a better understanding of the etching mechanism of Cu-ACE and the formation process of inverted pyramid structure.

Experiment process
The diamond wire sawed (100)-oriented n type crystalline silicon wafers with thickness of 156 ± 5 μm and resistivity of 1~3 Ω·cm were used. Before the experiment, the wafers were cleaned in acetone, ethanol and deionized water for 10 mins under sonication, and then immersed in 10% HF solution for 10 mins in order to remove native SiO2. Subsequently, the wafers were divided into three groups different mixture. First group (3 pieces) were placed in HF/H2O2/Cu(NO3)2 (labeled NO3 -system), HF/H2O2/CuSO4 (labeled SO4 2-system) and HF/H2O2/CuCl2 (labeled Clsystem) solution for 7 mins, so as to analyze the difference of etching morphology, etching rate and the resulting reflectivity. Second group (12 pieces) was placed in HF/Cu(NO3)2, HF/CuSO4 and HF/CuCl2 deposition system for 5 s, 10 s, 30 s and 60 s in every system, so as to observe the evolution of copper deposition state on wafers in the absence of H2O2. Third group (12 pieces) was etched by the same three etching systems as the first group, and etching times were 1, 3, 5 and 7 mins in every system, in order to observe the morphology evolution of etching structures and deposited Cu nanoparticles (Cu-NPs). The detailed etching parameters were shown in tables 1 and 2. After etching, samples of group 1 were immersed into HNO3 solution to eliminate residual Cu-NPs. Finally, the all samples were thoroughly rinsed with distilled water and dried in vacuum drying chamber. Moreover, the mass change of silicon wafer before and after etching were recorded to calculate the etching rate (Ra) based on Eq. (1), the relative amounts of deposited Cu (Q) were obtained by Eq. (2). The morphologies were characterized by scanning electron microscopy (SEM), the surface reflectivity of wafers was measured by the Ocean Optic USB-4100 spectrometer. Table 1 Etching conditions of n-type monocrystalline silicon wafers by different copper salt system Table 2 Deposition conditions of n-type monocrystalline silicon wafers by different copper salt system Where ∆ is the mass loss of etched wafers (g), is the mass density of crystalline silicon (g/cm 2 ), S is the silicon surface area (cm 2 ), and t is the etching time (min), 1 is the mass of the as raw silicon wafer, 2 is the mass of silicon wafer after deposition where the copper is cleaned by nitric acid, and 3 is the mass of the wafer plus the deposited copper after deposition formed under NO3 -system, and variable IP-like structures were obtained under SO4 2-system. In addition, numerous oval pits with 5 μm length and 1 μm width were formed when wafers were etched in Clsystem, this structure is similar to that of HF/HNO3 texturing method. In terms of reaction rate (see Fig. 1d), the etching rate of NO3 -system is the lowest, followed by Clsystem, and etching rate of the SO4 2-system is the fastest. The IPs and IP-like structure owns low reflectance of 7.2% and 11.7%, the oval pits obtained by Clsystem etching presents the highest reflectivity of 18.8% (see Figs. 1e, f), which may be due to the large size of the pit which is not conducive to the refraction of light. The results indicated that anions would greatly effect the etching behavior of Cu-NPs and leading to different morphologies and etching rate as well as reflectivity. current results indicate that Cu 2+ ions can uniformly deposition with small size and form a dense copper film in the HF/CuSO4 system. In HF/CuCl2 system (see Figs. 2i, j, k, l), it is obvious that heavily agglomerated rod-shaped nano-particles appear along the saw marks regions. Figure 2p shows the relative amount of the deposited Cu-NPs under three deposition systems, it can be seen that the amount of deposited Cu-NPs in the HF/CuSO4 system is slightly more than that in the HF/Cu(NO3)2 system at the beginning stage of 30 s, as the reaction proceeds, the amount of Cu-NPs deposited in HF/CuSO4 system becomes much higher than that in HF/Cu(NO3)2 system, by contrast, the amount of Cu-NPs in HF/CuCl2 system is the least. The deposition behavior of Cu is related to anion concentration and anion properties [24][25][26][27][28].

Results and discussion
Although the concentration of Cu 2+ is the same in the three deposition systems, the anion concentration is different. Compared with NO3 -and Cl -, the concentration of SO4 2-is the lowest, which leads to the weakest electrostatic attraction between SO4 2-and Cu 2+ , thus the Cu 2+ ions are easier to get electrons and evenly deposit on silicon surface as shown in Fig. 3a. Moreover, the difference in the surface energy of the silicon wafer has no great influence on the Cu deposition in HF/CuSO4 system, which leads to the Cu-NPs is dispersed and form a dense film. In HF/Cu(NO3)2 system, the higher concentration of anion concentration enhances the attraction between NO3 -and Cu 2+ and thus limits the copper ions deposition (see Fig. 3b), which causes the Cu-NPs tend to deposit at the saw marks and defect sites, slightly agglomerated and grow into bigger round particle.
With the reaction proceeds, the NO3 -also oxidize Cu-NPs into Cu 2+ ions, which controls the size of the copper particles not to be too large, and the deposition rate is slower.
As for HF/CuCl2 system, the concentration of Clis the same as that of NO3 -in HF/CuCl2 system. But due to the more stable covalent bond of CuCl2, the Cu 2+ tends to be more difficult to break free and deposition. Moreover, many Cu 2+ and the deposited Cu-NPs are prone to form [CuCl2] -complexes in rich Clsolution, and further form CuCl precipitate [29], as shown in Fig. 3c [CuCl2] -→ CuCl↓+ Cl - (6) Besides, due to the large number of [CuCl2] -complexes, the migration of ions in the solution is greatly hindered, these factors make the Cu 2+ really difficult to get electronic and deposit on silicon surface, so that the Cu particles and CuCl precipitate preferentially deposit along the saw marks and defect sites and heavily agglomerated into a rod-shaped structure. It can be seen that the chemical properties of anions greatly affect the deposition of Cu-NPs, which affects the etching behavior during the Cu-ACE. on, more IP-like structures with various size can be seen, which indicates that the anisotropic etching ability of copper in SO4 2-system is not obvious as that in NO3 -system. As for Clsystem (see Figs. 4i, j, k, l), crescent-shaped grooves appear at the beginning of the reaction, and the crescent-shaped pits gradually widening and deepening, eventually change into elliptical pits with various lengths, as seen in Fig. 4l. NPs are deposited on the sidewalls of the pits which is similar to the situation of NO3 -system, but the size of Cu-NPs is smaller. As the reaction proceeds, the Cu particles become less and smaller.
The small pits on the sidewall (see Fig. 5g) can infer that the copper-assisted chemical etching intensely happens. As for Clsystem (see Figs. 5i, j, k, l), a large amount of Cu and CuCl particles were stacked in the crescent-shaped groove at 1 min, and the size is much bigger than that in NO3and SO4 2-system. These deposits (CuCl particles) becomes bigger as the reaction goes on, in which is contrary to the situation of NO3 -and SO4 2-system.
Different copper etching behaviors naturally lead to different etching rates (see Fig. 6). it is obvious that at the beginning of reaction, the etching rate of SO4 2-system is much higher than the others, and rapidly decreased to 1.52 μm/min. While the etching rate of NO3 -system and Clsystem are very close and both stable at about 1 μm/min. tiny Cu-NPs with high surface activity, lead to intense reaction, but they are quickly oxidized by H2O2 (see Fig. 7a), therefore, only a large number of corrosion pits and a few small Cu-NPs lied on the side wall as the reaction goes on, and the corresponding reaction rate rapidly decrease, even so the average etching rate at 7 mins is still much higher than the other two. Due to the intense reaction, the anisotropic etching ability of Cu-NPs cannot be completely demonstrated, so the regular IP structures cannot be formed but many IP-like structures instead. In NO3 -system, H2O2 and NO3make the Cu-NPs the proper size, which can fully demonstrate an anisotropic etching ability, and eventually form a regular IP structure as shown in Fig. 7b. Moreover, the proper size of Cu-NPs make the reaction mild and the reaction rate is always keep about 1μm/min. For the Clsystem (see Fig. 7c), Cu-NPs and CuCl-NPs will both deposit on the silicon surface, the H2O2 can oxidize Cu-NPs, but it has no effect on the CuCl-NPs. Therefore, as the reaction proceeds, a large amount of [CuCl2] -complex ions changed into CuCl precipitate and agglomerate in the corrosion pits.
Moreover, due to the uneven deposition, the resulting structure is not uniform, leading to numerous oval pits with different size, and the cutting lines are still obvious after etching, the slow etching rate of Clsystem is mainly due to the slow deposition rate of Cu 2+ .

4.Conclusion
The effects of NO3 -, SO4 2-and Clon the etching morphology of n-type single crystal silicon in the copper-assisted etching process are firstly systematically analyzed, the different anions affects the copper deposition process, result in different etching rates and behaviors.
The weak attraction between SO4 2-species and Cu 2+ ions leads to that Cu 2+ deposition is quick and easily form tiny Cu-NPs, which causes intense reaction process and result in lots of grooves and inverted pyramid-like structure. In the Clspecies etching system, the covalent bond of CuCl2 and formation of [CuCl2] -complex ions lead to that the Cu 2+ ions preferentially deposit in defects sites and agglomerate into rod-shaped particles, resulting in oval corrosion pits, which is not conductive to saw marks' removal.
In comparison, the NO3 -species show stronger oxidation and which can relieve the Cu-NPs aggregation, these Cu-NPs with proper size show better anisotropic etching ability, thus forming regular inverted pyramid structure which can almost completely disappeared saw marks and lower reflection to 7.2%. It can be seen that the type of anion does affect the etching behavior of Cu-NP to form different morphologies, through this work we can further understands the formation mechanism of inverted pyramid by HF/H2O2/Cu(NO3)2 system, moreover, it provides guidance for controllable preparation of inverted pyramid structure and future high efficiency single crystal solar cells.      Diagram of average etching rate at different etching times for different system Figure 7