The Initial Phase of Photoelectrochemical Anodization of Si in Alkaline Media Investigated by Synchrotron Radiation Phot
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The Initial Phase of Photoelectrochemical Anodization of Si in Alkaline Media Investigated by Synchrotron Radiation Photoelectron Spectroscopy (SRPES) and Scanning Probe Microscopy (SPM) M. Letilly1,2, K. Skorupska1,3, M. Aggour4, M. Kanis1 and H.-J. Lewerenz2,1 1 Helmholtz-Zentrum Berlin, Solar Fuel and Energy Storage Materials, Hahn-Meitner Platz 1, 14109 Berlin, Germany 2 Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA 3 Brandenburg Technical University Cottbus, Platz der Deutschen Einheit 1, 03046 Cottbus, Germany 4 Faculty of Sciences, Ibn Tofail University, BP 133, 14000 Kenitra, Morocco ABSTRACT Anodic oxide formation and chemical and electrochemical etching of n-Si(111) have been investigated in alkaline media. Due to the complexity of the processes, the investigation has been restricted to the initial phase where a transitory anodic photocurrent peak is observed slightly positive from the open circuit potential (ocp). In-system photoelectron spectroscopy, performed at the U 49/2 beamline at Bessy, shows sub-monolayer silicon surface oxidation and remnant H-termination, indicating island-type oxide formation. Scanning probe microscopy shows the formation of macropores with 300-500 nm diameter and an average depth of 5-8 nm. The discussion comprises chemical and electrochemical dissolution mechanisms and routes to development of nanoemitter fuel generating devices. INTRODUCTION Wet processing of semiconductors is a means to produce scalable patterns at low (ambient) temperature. In addition, processes such as electrodissolution and electrodeposition are considerably less destructive than, for instance, sputtering or high temperature processing where chemical reactivity is substantially increased. The generation of well-defined, reproducible patterns, however, is a challenge in (photo)electrochemical sample conditioning. In the search for parameter fields where self-organized topographical ordering occurs, we have expanded the according space to alkaline media. Since the I-V characteristic is characterized by three main regions, we have restricted the investigation to the first potential regime, slightly anodic from ocp, where a transitory photocurrent peak is observed [1]. Besides the concomitant chemical dissolution of Si, two additional processes contribute to the structure formation: photoinduced oxidation and electrochemical dissolution. The system bears a certain resemblance to (photo)current oscillations of Si in acidic fluoride containing solution [2] where silicon oxidation and, in this case, oxide etching are competing reactions. Hitherto, however, the silicon surface conditions in the potential region before the first photocurrent maximum in alkaline solution has been assessed by ellipsometry [3], a method that depends heavily on assumptions made regarding the use of effective medium theory [4] and on the assumed optical properties of the considered media. Therefore, anodic oxides with contributions in the sub-monolaye
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