Atomic force microscopy studies of ZnS films grown on (100) GaAs by the successive ionic layer adsorption and reaction m
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Atomic force microscopy studies of ZnS films grown on (100) GaAs by the successive ionic layer adsorption and reaction method Mika P. Valkonen, Seppo Lindroos, Tapio Kanniainen, and Markku Leskel¨a Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
Roland Resch, Gernot Friedbacher, and Manfred Grasserbauer Institute of Analytical Chemistry, Vienna University of Technology, Getreidemarkt 9/151, A-1060 Vienna, Austria (Received 12 May 1997; accepted 25 August 1997)
In this study zinc sulfide thin films were grown by the successive ionic layer adsorption and reaction (SILAR) technique on (100) GaAs substrates from aqueous precursor solutions. The atomic force microscopy (AFM) method was used to study the growth of the films up to a thickness of 180 nm. The ZnS thin films on (100) GaAs were smooth with an rms roughness of 0.2–1.9 nm depending on the film thickness. After the GaAs surface was covered with ZnS, the growth appeared to be nearly layerwise. In addition, in situ AFM studies were carried out to analyze the dissolution of (100) GaAs in water, which is a process competing with the thin film deposition by the SILAR.
I. INTRODUCTION
The successive ionic layer adsorption and reaction (SILAR) technique is a solution-based chemical thin film deposition method,1,2 based on growth cycles, which consist of four steps: adsorption of cations from aqueous solution, rinsing with purified water, adsorption of anions from aqueous solution followed by reaction, and lastly rinsing with water. The SILAR technique is analogous to the gas phase atomic layer epitaxy (ALE) method.3 In both methods the substrate is treated separately with each precursor followed by rinsing, but the SILAR growth is carried out in aqueous solutions at room temperature and ambient pressure, while in ALE reduced pressure and elevated temperature are normally applied. The SILAR method has been employed mainly for the deposition of II–VI compound thin films, like ZnS,1,2,4 ZnS : Mn,5 PbS,6 CdS,7,8 Cdx Zn12x S,8 and multilayer CdSyZnS thin films.7 These materials may be used in a variety of optoelectronic devices, for example, thin film electroluminescent devices (ZnS : Mn), infrared detectors (PbS), and solar cell window materials (CdS). The CdSyZnS superlattices and quantum wells emit blue luminescence. A ZnS buffer layer on the GaAs substrate is commonly used for the quantum well structures.9 Studies of ZnS thin film growth benefits the engineering of buffer layers for quantum well10 and multilayer structures.7 The morphology of thin film surfaces can be characterized by atomic force microscopy (AFM). AFM studies of ZnS thin film growth on glass and mica substrates have been described elsewhere.11,12 Both ex situ and in situ techniques have been utilized in these measurements. A method used in current work is tapping mode 1688
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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atomic force microscopy (TM-AFM). Here, the
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