n-Si(100)在氯化膽鹼有機溶液中之電化學蝕刻行為

Electrochemical Etching of n-Si(100) in Choline Chloride/ethylene Glycol Ionic Liquids

本文主要探討在不照光的室溫環境下，對於n-型矽（100）單晶，在含乙二醇（Ethylene glycol, EG）之氯化膽鹼（choline chloride）系統中，添加氟化銨（Ammonium fluoride）與二甲基亞石風（Dimethyl sulfoxide, DMSO）後的電化學蝕刻行為。蝕刻步驟先採用陽極動態極化法，在混合溶液中定義出適當之蝕刻電位，以利進行定電位蝕刻，進而研究蝕刻孔洞陣列的形貌差異。結果顯示：（1） n型矽的電化學蝕刻反應，隨著電壓增加其孔洞深度也隨之增加（蝕刻速率可達1.2μm/h）而蝕刻深度可達29μm。（2）施加偏壓有一較佳參數值（9 V），在此電壓下所形成的孔洞最深，且蝕刻孔洞的分佈較為均勻。（3）藉由Mott-Schottky的理論方程式可得到平坦電位，在氯化膽鹼有機溶液中的平坦電位較水溶液系統往正電位移動，此結果會造成能帶的彎曲幅度更大，使得空間電荷層變厚，激發更多的電子電洞對，進而可使更多的電洞到達半導體表面而產生蝕刻的效果。

The aim of this work was to prepare porous silicon (PS) by electrochemical etching of n-type silicon (100) in a choline chloride/ethylene glycol and dimethyl sulfoxide ionic liquid system containing NH4F without the aid of illumination. DC potentiodynamic polarization analyses were conducted, and the anodic polarization curves were analyzed in search of optimal potentials for preparing PS potentiostatically. It was found that the etching rate of the n-type silicon increased to 1.2 μm/h with increasing potential, and the depth of the pores could reach 29 μm. The optimal bias to obtain PS with a uniform distribution of pores that had the greatest depth was 9 V. The flat-band voltage of the system could be evaluated with the Mott-Schottky model. The flat-band voltage was found to be more positive in the choline chloride/ethylene glycol system than that in the aqueous solution. A higher flat-band voltage led to a greater bending in interfacial energy bands between the silicon and the electrolyte. The greater bending in the energy bands might have created a higher extent of charge separation, driven the electron holes to the silicon surface, and increased the thickness of the space charge. As a result, the etching rate of the n-type Si(100) was higher in the ionic liquid system than that in the aqueous system, as ammonium fluoride was used as the etchant instead of hydrogen fluoride.