Digital Electrochemical Etching of Compound Semiconductors
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DIGITAL ELECTROCHEMICAL ETCHING OF COMPOUND SEMICONDUCTORS
Q. Paula Lei and John L. Stickney Department of Chemistry, University of Georgia, Athens, Georgia 30602
Abstract The principles for an electrochemical digital etching method for compound semiconductors are described and initial results reported. The method is designed to allow atomic level control over the etching process, resulting in the removal of a bilayer of the compound for each cycle. An atomic layer of one element is removed at one potential and then an atomic layer of the second element is removed at a second potential to complete one cycle. The results reported here are for the etching of CdTe. For CdTe, Te is stripped by reduction to Te2 " while Cd is stripped by oxidation to Cd2". Underpotentials are chosen so that only the top atomic layer of an element is removed. Potentials sufficient to strip the element from the bulk of the CdTe substrate are avoided. Application of the method should involve the use of a simple electrochemical cell, with solution convection. The substrate is placed in the cell and a square wave applied, where each cycle results in the dissolution of a bilayer of the compound. The two potentials of the square wave correspond to underpotential stripping potentials for Cd and Te respectively. Directions for the future development of this etching method are discussed.
Introduction The trend towards low dimensional materials in the field of electro-optics has resulted in the need for atomic level control of deposit structure and dimensions. The family of techniques referred to as Atomic Layer Epitaxy (ALE) has arisen to help address that need (1-3). The principle of ALE is that atomic layers of individual elements are deposited sequentially. In this way, layer-by-layer growth (Frank-van der Merwe) is maintained and deposit thickness is controlled by the number of deposition cycles, not by the reactant flux. Our group has been working on the electrochemical analogue of ALE, which we refer to as Electrochemical Atomic Layer Epitaxy (ECALE) (4-6). ECALE is a method for the formation of compound semiconductors where individual elements are alternately electrodeposited at underpotential from separate solutions. The term underpotential (7) refers to the fact that compound formation is generally energetically favorable, compared with formation of bulk deposits of the individual elements. Thus, electrodeposition of one element on a second, forming a surface compound, will frequently occur at a potential prior to (under) that required to deposit the bulk element. Deposition is surface limited: limited to an atomic layer, as diffusion of the depositing element into the substrate is generally a slow process at room temperature. In addition to atomic control over deposition, atomic control over material removal is desirable. Digital etching is just beginning to emerge as an active area of research (i). Present attempts at digital etching involve cycles where a reactant gas is first adsorbed on the substrate surface and then a reaction is
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