Characterization of Thin ZnO Films by Vacuum Ultra-Violet Reflectometry
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Characterization of Thin ZnO Films by Vacuum Ultra-Violet Reflectometry T. Gumprecht1,2, P. Petrik1,3, G. Roeder1, M. Schellenberger1, L. Pfitzner1, B. Pollakowski4, B. Beckhoff4 1
Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Schottkystrasse 10, 91058 Erlangen, Germany 2
Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Strasse 9, 91052 Erlangen, Germany
3 Institute for Technical Physics & Materials Science (MFA), Research Centre for Natural Sciences, Konkoly Thege u. 29-33, 1121 Budapest, Hungary 4
Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, 10587 Berlin, Germany
ABSTRACT ZnO has a huge potential and is already a crucial material in a range of key technologies from photovoltaics to opto and printed electronics. ZnO is being characterized by versatile metrologies to reveal electrical, optical, structural and other parameters with the aim of process optimization for best device performance. The aim of the present work is to reveal the capabilities of vacuum ultra-violet (VUV) reflectometry for the characterization of ZnO films of nominally 50 nm, doped by Ga and In. Optical metrologies have already shown to be able to sensitively measure the gap energy, the exciton strength, the density, the surface nanoroughness and a range of technologically important structural and material parameters. It has also been shown that these optical properties closely correlate with the most important electrical properties like the carrier density and hence the specific resistance of the film. We show that VUV reflectometry is a highly sensitive optical method that is capable of the characterization of crucial film properties. Our results have been cross-checked by reference methods such as ellipsometry and X-ray fluorescence. Key words: Zink oxide, vacuum ultra-violet reflectometry, spectroscopic ellipsometry, atomic layer deposition INTRODUCTION ZnO is a key material in a range of optoelectronic applications, and has promising properties for numerous other applications. Therefore, it continues to be intensively studied in the recent years [1]. ZnO layers can be prepared by different methods including sputtering [2], atomic layer deposition (ALD) [3], pulsed laser deposition [4, 5, 6], spin coating from nanoparticulates [7], or spray pyrolysis [8]. Highly sensitive optical methods such as ellipsometry are frequently applied for the quick and non-destructive determination of a range of material and structural parameters such as thickness, surface nanoroughness, interface quality, density, homogeneity, band gap and exciton strength. These methods make indirectly also possible the determination of electrical properties.
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The aim of the present work is the investigation of optical modeling of ZnO layers using a commercial vacuum ultra-violet (VUV) reflectometer, and the cross-checking of the result using reference metrologies. This study is part of a European project, intended to be a step towards the establishment of validated reference methodologies for a
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