Preparation of Inverted Pyramids by Electrochemical and Chemical Etching of n-Type Silicon under Thermodynamic Equilibrium State

Abstract

Inverted pyramids on silicon surfaces are usually produced by the combination of lithographic techniques and anisotropic wet etching. However, lithographic techniques are expensive and complicated. In this report, inverted pyramids were prepared successfully by electrochemical etching of moderately doped n-type Si in 8% HF ethanol solution at 15 mA cm−2 under back side illumination. The surface and interface morphologies of porous silicon (PSi) films and inverted pyramids were characterized by FE-SEM. The results show that inverted pyramids with thermodynamic stable {111} planes form at the PSi-Si interfaces and a thermodynamic equilibrium establishes between the dynamic-competitive electrochemical and chemical etching behaviors during the same etching process. They compete not only for the limited photocarriers in silicon but also for the limited reactive ions in HF solution. The anisotropic electrochemical etching in multiple <100> directions, the isotropic chemical etching in hemispheric-shaped directions and the local microenvironment facilitated by macroprores are also essential to the formation of inverted pyramids. Contrasted to lithographic techniques, this method in terms of electrochemical and chemical etching at thermodynamic equilibrium state is a simple, low-cost, but efficient method. We believe such a method will be beneficial to the crystalline silicon solar cells or MEMS in the future.