Microstructure Analysis of Silicon Nanowalls: Insights from Positron Beam Doppler Broadening Measurements

Silicon nanowalls atop the (100) – oriented boron doped P-type (1–10 Ω-cm) single crystalline silicon wafers were prepared using a metal assisted chemical etching route for different durations 1, 5, 15, and 30 min. The scanning electron microscopy results revealed that the structure evolved up on etching is in the form of vertical silicon nanowalls with mean wall thickness of 70 nm. It can be observed that as the etching time increases, the height of the SiNWs increases linearly at an etching rate of ~ 301 nm per minute. The transmission electron microscopy results combined with FTIR spectroscopy results indicate that about one nanometer thick Si–O-Si- bonded amorphous layer formed at the surface of the grown silicon nanowalls. A defect sensitive Variable energy positron beam Doppler broadening technique used to study the etched silicon wafers confirms that the defect structures that are evolved in the silicon nanowalls with etching are different from that of planar Silicon. Analysis of the Doppler broadened line-shape profiles shows that the effective positron diffusion length and height of the silicon nanowalls are related logarithmically with a scaling exponent of—1/2, indicating that the implanted positrons that are thermalized in the silicon nanowalls diffuse back to the wall surfaces and are annihilated at the defects linked (Si–O-Si)/Si interface region. The present positron experimental results abound with literature reports suggest that understanding the microstructure of the surface layer in SiNWs is significantly important in determining their performance for producing efficient solar cells.