On Wet Etching of n-Si (100) Coated with Sparse Ag-Particles in Aqueous NH₄F with the Aid of H₂O₂

摘要

Single crystalline n-Si (100) previously coated with sparse silver nano-particles were immersed invarious solutions of ammonium fluoride to investigate their wet etching. In the absence of H2O2, theopen circuit potential (OCP) of the silicon was more active in the solutions of 11.0 M than 1.0 MNH4F. In the present of H2O2, the OCP of the silicon increased with increasing the concentration ofH2O2 (from 1.0 to 5.0 M). The etching morphology of the specimens, examined through scanningelectron microscopy (SEM), revealed two different types. The first type of morphologies revealed anumber of deep pores produced on the n-Si (100) post its immersion in 1.0 M NH4F + 5.0 M H2O2 for1 h. These pores were 50 - 150nm in diameter and 200 - 300nm in depth. The second type ofmorphologies displayed few shallow pores on the Si (100) post its immersion in 11.0 M NH4F + 5.0 MH2O2 for 1 h. The study of electrochemical impedance spectroscopy (EIS) provided useful informationto understand the kinetics of this system. The experimental EIS data simulated with commercialsoftware (i.e., Z-view) were satisfactorily consistent with two distinct sets of proposed equivalentcircuit (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 of1.0 M NH4F + 5.0 M H2O2. The oxide capacitance (i.e., C1) present in EQB is absent in EQA andreplaced with an inductance (i.e., L1). EQA could be used to delineate the kinetics of n-Si (100) in twosingle solutions of 1.0 and 11.0 M NH4F and that in 11.0 M NH4F + 5.0 M H2O2. In the absence ofH2O2, the charge-transfer resistance (i.e., R2) in EQA is very high so that n-Si (100) is highly resistantto corrosion in both single 1.0 and 11.0 M NH4F. However, in the presence of H2O2, this chargetransfer (i.e., R2) is hugely reduced in the system of 1.0 M NH4F + 5.0 M H2O2 and 11.0 M NH4F + 5.0M H2O2. The contribution of hydrogen peroxide is not only to increase the open circuit potential butalso to facilitate the creation of holes in the catalytic process assisted by the sparse nano Ag-particleson the n-Si (100) surface. The mechanism could be confirmed by the plots of phase angle against theexerted frequencies.