单晶硅碱性和铜碱酸溶液各向异性蚀刻特性的差异

Silicon (Si) surface texturing is an indispensable step for the fabrication of Si solar cells due to the high refection  of planar Si wafers. Tus, surface texture and antirefection coating such as SiNx are necessary to reduce the surface refectance of solar cells. Currently, industrialized techniques for texturing Si wafers are generally based on  alkaline solutions for single-crystalline Si (c-Si) by the anisotropic etching or acid solutions for multi-crystalline  Si (mc-Si) by the isotropic etching . Usually, upright random sized pyramidal structures for c-Si and ‘worm  like’ structures for mc-Si will be obtained, respectively. More specifcally, the texturization of c-Si is usually  based on alkali/isopropyl alcohol (IPA) mixture required processing at high temperatures (75°C–85°C) for about  20minutes , which had already been commercialized on (100) oriented Si wafers decades ago. It is well-known  that each atom of Si {100} surface has two dangling bonds and two back bonds in contrast to one dangling and  three back ones for each atom of {111} surface. Terefore, the activation energy to remove an atom from {100}  surface is smaller than that from {111} surface because it only needs breaking two back bonds rather than three  ones in the case of {111} surface. Tus, in alkaline etchant, the etching rate is faster along the directions of [100]  than that along other directions due to the lower activation energy of the atoms on {100} surface . Tereby, the  etching mainly occurs along the [100] directions and stops at the {111} planes, leading to the formation of upright  pyramidal structures.

In this paper, more detailed analysis will be proposed for the anisotropic etching of Si based on a HF/H2O2 process with the assistance of Cu nanoparticles by taking advantage of the anisotropic electrochemical behavior  of a Si crystal. Te etching mechanism of diferent oriented Si surfaces is systematically studied. Te Si etching  rate and (100)/(111) etching ratio are investigated as well. Additionally, surface morphologies and refectivity afer  etching are characterized.

Figure 3 shows SEM images of c-Si (100) wafers etched in Cu based acid solutions for diferent time and a  schematic of the simulated inverted pyramid. Generally, metal-assisted chemical etching method is accepted as  a localized electrochemical process in which the oxidation and dissolution of Si only occur under the metal particles. Unlike the point structures obtained in alkaline etchant starting with the entire Si surface, the etching of Si  only happened under metal particles by CACE. Small pits are formed in the early time because of the sinking of  metal particles. As can be observed from Fig. 3(a), Cu nanoparticles were anisotropically deposited on Si (100)  surface which leading to the formation of inverted pyramid. Obviously, only several pits were formed on the  polished c-Si (100) surface. For metal assisted chemical etching, the defective sites of the Si surface function as the starting points. Terefore, less inverted pyramids were obtained in comparison with those on the surfaces  of raw Si wafers used in industry due to the less defects on polished Si surface at the early time. With the prolongation of reaction time, the region without defects could also be etched, and the size of inverted pyramid became  larger, as shown in Fig. 3(b), though there are still some unetched regions. With the etching time increasing to  15 minutes, inverted pyramid became deeper and more standard across the whole surface (Fig. 3(c)). Similar  geometry of cavities had been obtained by alkaline solution with opening masks previously, employed more  complex techniques involving lithography, laser processes,. Te opening masks and the laser induced  pits were used to localize the alkali etching inside the masks or pits, just like the localized electrochemical anisotropic etching of CACE. However, as the alkali etching time increased, the regular inverted pyramidal structure  collapsed and tiny upright pyramids began to appear on the surface. Because the laser induced holes became  shallow and the opening masks were destroyed, leading to the etching process was no longer localized inside the  holes or masks. Hence, the formation of inverted pyramids not only need the anisotropic etching, but also the  localized etching process.

Te refectance spectra of the three structures obtained via Cu assisted anisotropic etching for 15 min on Si  (100), (110) and (111) substrate and the upright pyramids obtained by alkaline etchant are shown in Fig. 7. Te  average refectance (R) of inverted pyramid obtained on Si (100) substrates is lower than 5%, while the R of  upright pyramid is 12%. Te superior light-trapping characteristic is original from the inverted pyramidal structure that about 37% of the incoming light undergoes a triple bounce before being refected away While, the R of  Si (110) and (111) substrates were larger than 20%, due to the poor light management efect. As can been seen  from the photos, the appearance of Si (100) substrate is black and the appearance of Si (110) substrate is gray,  while the appearance of Si (111) substrate is a bit bright because most of the incoming light was refected away  afer the frst bounce. By thorough understanding the etching mechanism of CACE, the morphology, size and  surface roughness of the etched inverted pyramids are under control. Te low-cost micron-sized inverted pyramid texture technique results in surfaces with excellent optical and electronic properties, and is therefore well  suited to high-efciency silicon solar cells.

Boron-doped (1–3 Ω·cm), 500 μm thick, (100), (110) and (111) oriented, double polished Si wafers were thoroughly rinsed in acetone to remove any organic contaminants and then rinsed with deionized water before  etching. Te upright pyramid texture was obtained by etching in alkaline solutions containing 2wt% potassium  hydroxide (KOH) and 10 vol% IPA. Meanwhile, we got the inverted pyramid texture by using Cu based acid  solutions containing 5 mM Cu(NO3)2, 4.6M HF, and 0.55M H2O2 at 50 °C. Te residual Cu nanoparticles were  removed away using concentrated nitric acid in a sonication bath. Te morphologies and structures of the wafers  were characterized by a Hitachi S-4800 scanning electron microscope (SEM). Te hemispheric total refectance  for normal incidence in wavelength from 300 nm to 1000 nm was measured using a Varian Cary 5000 spectrophotometer with an integrating sphere.