We demonstrate that ZnO/metal junctions, produced by a commonly used electrochemical oxidation procedure, are prone to lateral (two-dimensional) heterogeneities. These heterogeneities are not apparent in bulk structural measurements (such as X-ray diffraction data), but are evident in the electrodeposit's electrical (current-voltage) and optical (luminescence) properties. The spatial variations in the ZnO films are related to incomplete oxidation during the final stage of their multi-step electrochemical formation process. Support for this explanation comes from a surprisingly simple equivalent circuit that accurately models the current-voltage response as a combination of resistive (Ohmic) and rectifying (Schottky) junction contacts at the ZnO/substrate interface. © 2012 The Electrochemical Society. [DOI: 10.1149/2.002202ssl] All rights reserved.Manuscript submitted February 22, 2012; revised manuscript received April 30, 2012. Published July 20, 2012 Electrochemical synthesis of metal oxide materials is an expedient way to produce thin films of many functional semiconductors for potential device applications, and it has received increasing attention during the last two decades.1,2 In the case of ZnO formation, the most widely used method of electrochemical synthesis involves using an applied potential to produce an interfacial pH change in the immediate vicinity of the working electrode surface that triggers ZnO formation. This complex process has been studied and utilized extensively, and is seemingly well understood. 3,4 Control over ZnO electrodeposit epitaxy, texture, morphology, carrier concentration, and orientation has been demonstrated, 2,4 as has selectivity between purely resistive (Ohmic) or rectifying (Schottky) electrical responses. 5,6 Here, we show that electrodeposited rectifying ZnO junctions, fabricated using a well-characterized nitrate reduction route, 3 have spatial (lateral) inhomogeneities that are not apparent using common characterization techniques such as X-ray diffraction or scanning electron microscopy. The origin of this effect is related to incomplete conversion of Zn 2+ to ZnO on some parts of the working electrode during the multi-step electrochemical synthesis process. These findings have two important implications. First, for the specific case of ZnO electrosynthesis, it means that the lateral heterogeneities inherently coincide with the presence of the rectifying electrical responses, thereby limiting the usefulness of this film production method unless the oxidation process can be controlled more completely. Second, for the general case of oxide formation by electrochemical methods, it shows the importance of using spatially resolved measurements in conjunction with bulk measurements to assess film quality for potential device applications.
ExperimentalFor this study, more than 60 ZnO electrodeposits (thicknesses ∼1 μm) were analyzed to span a range of deposition potentials and pH conditions that yield rectifying junctions. All samples were prepared at a constant deposition...
