In-Situ Ellipsometry Study of Pulsed Laser Deposited ZnO Films
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IN-SITU ELLIPSOMETRY STUDY OF PULSED LASER DEPOSITED ZnO FILMS SHAKIL PITTAL, L.A. McCONVILLE, N.J. TANNO, AND P.G. SNYDER Department of Electrical Engineering, and Center for Microelectronic and Optical Materials Research, University of Nebraska, Lincoln, NE 68588-0511 ABSTRACT Ellipsometry is normally used to characterize layered structures that have uniform properties laterally across the area of the probe beam. In this study, pulsed laser deposition of ZnO films was found to have highly nonuniform growth properties in the lateral directions. We were able to account for the effects of the nonuniform growth on in-situ ellipsometric data, by considering two dynamic growth models. INTRODUCTION Zinc oxide has a large piezoelectric coefficient, and can be deposited in thin film form by a number of techniques. These include chemical vapor deposition, 1 ionized cluster deposition, 2 sputtering, 3 "5 and plasma sputtering. 7 Films can be deposited either as amorphous or as polycrystalline material, but a high degree of oriented polycrystalline growth, with the c-axis oriented along the growth direction, is generally desired for applications. In this paper, we describe the use of pulsed laser deposition (PLD) to deposit oriented polycrystalline ZnO films, and in-situ, real-time ellipsometry to characterize the film growth. Advantages of PLD include high deposition rates in low vacuum, and the ability to deposit a wide variety of materials as thin films. One of the potential difficulties of the technique, which is that the deposited film thickness may be highly nonuniform, is addressed in this paper, by the use of in-situ ellipsometry. Ellipsometry is a non-destructive optical technique, which is very sensitive to layer thicknesses, optical constants, interfacial quality, and other properties of interest. It is widely used in thin film, bulk, and surface analysis, and its value in in-situ characterization is rapidly gaining recognition. To date, it has been used only to characterize layered structures which are laterally uniform across the small area probed by the beam (typically on the order of 10 mm 2 ). This allowed the data to be analyzed assuming parallel interfaces and lateral homogeneity. In this work, because the film deposition was nonuniform over the area of the probe beam, the in-situ data analysis had to account for film nonuniformity throughout the deposition. EXPERIMENTAL The experimental setup consisted of a stainless steel vacuum chamber with three optical ports (Figure 1). One was for the KrF (248 nm, 10 nsec/pulse) excimer laser entry, and the other two were oriented to allow entry and exit of an ellipsometer beam, which reflected from the sample at a nominal incidence angle of 680. All ports contained quartz windows, and the ellipsometer windows were strain-free. A rotating feedthrough allowed the target to be rotated by a motor Mat. Res. Soc. Symp. Proc. Vol. 236. 01992 Materials Research Society
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Figure 1. Schematic of experiment. P= polarizer, A= analyzer, D= detector.
Target
Figure 2. The
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