On Wet Etching of n-Si (100) Coated with Sparse Ag-Particles in Aqueous NH4F with the Aid of H2O2
Résumé
Single crystalline n-Si (100) previously coated with sparse silver nano-particles were immersed in various solutions of ammonium fluoride to investigate their wet etching. In the absence of H2O2, the open circuit potential (OCP) of the silicon was more active in the solutions of 11.0 M than 1.0 M NH4F. In the present of H2O2, the OCP of the silicon increased with increasing the concentration of H2O2 (from 1.0 to 5.0 M). The etching morphology of the specimens, examined through scanning electron microscopy (SEM), revealed two different types. The first type of morphologies revealed a number of deep pores produced on the n-Si (100) post its immersion in 1.0 M NH4F + 5.0 M H2O2 for 1 h. These pores were 50 - 150nm in diameter and 200 - 300nm in depth. The second type of morphologies displayed few shallow pores on the Si (100) post its immersion in 11.0 M NH4F + 5.0 M H2O2 for 1 h. The study of electrochemical impedance spectroscopy (EIS) provided useful information to understand the kinetics of this system. The experimental EIS data simulated with commercial software (i.e., Z-view) were satisfactorily consistent with two distinct sets of proposed equivalent circuit (i.e., EQA and EQB) in response to those two different etching morphologies. Based on EQB, we construct a schematic model to illustrate the formation of deep pores on n-Si (100) in the system of 1.0 M NH4F + 5.0 M H2O2. The oxide capacitance (i.e., C1) present in EQB is absent in EQA and replaced with an inductance (i.e., L1). EQA could be used to delineate the kinetics of n-Si (100) in two single solutions of 1.0 and 11.0 M NH4F and that in 11.0 M NH4F + 5.0 M H2O2. In the absence of H2O2, the charge-transfer resistance (i.e., R2) in EQA is very high so that n-Si (100) is highly resistant to corrosion in both single 1.0 and 11.0 M NH4F. However, in the presence of H2O2, this charge transfer (i.e., R2) is hugely reduced in the system of 1.0 M NH4F + 5.0 M H2O2 and 11.0 M NH4F + 5.0 M H2O2. The contribution of hydrogen peroxide is not only to increase the open circuit potential but also to facilitate the creation of holes in the catalytic process assisted by the sparse nano Ag-particles on the n-Si (100) surface. The mechanism could be confirmed by the plots of phase angle against the exerted frequencies.