A Classic Molecular Semiconductor Revisited: New Aspects of Growth Mode and Conduction Characteristics in Thin Films of
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0965-S06-25
A Classic Molecular Semiconductor Revisited: New Aspects of Growth Mode and Conduction Characteristics in Thin Films of Phthalocyaninatozinc (PcZn) Harry Brinkmann and Derck Schlettwein Institute of Applied Physics, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, Giessen, D- 35392, Germany
ABSTRACT PcZn as a well known organic semiconductor received new examination of its growth mode in thin films and reactions to oxygen exposure. PcZn thin films were grown on glass and polyimide substrates by physical vapor deposition. Ag electrodes had been deposited by a sputter process to allow electrical contact to the films. Conductivity measurements were performed insitu during the growth of films under high vacuum conditions. Characteristic current peaks were observed and explained by a Stranski-Krastanov growth mode. Current voltage characteristics revealing space charge limitation could be obtained beginning in the monolayer range. For pristine films n-type conduction was observed. Following deposition an increase of the conductivity was measured, caused by changes of the film structure. Ripening under high vacuum conditions could be separately discussed from doping effects caused by oxygen exposure which led to the well- known p-type characteristics of PcZn. INTRODUCTION Thin films of molecular materials such as phthalocyanines are of great interest to study their electrical and optical properties from a fundamental as well as applied perspective. In order to realize and optimize, e. g., organic field effect transistors (O-FET`s) the control of growth and structure of molecular assemblies is essential [1]. Physical vapor deposition is an attractive method of film preparation because of the possible control of crystallization by substrate characteristics and deposition conditions. Different molecular adhesion on different substrates leads to changes in crystallinity of the produced thin film. This interaction between substrate and molecules provides a mechanism to order the first layers of organic semiconductors. Under the appropriate growth conditions long range ordered monolayers can therefore be obtained [2-5]. A change of molecular order in a transition from the monolayer to multilayer range has recently been observed [4]. Phthalocyanines are an attractive group of materials in this respect because of their chemical versatility, chemical and thermal stability. PcZn serves as an example for p-type conduction [6], which is of interest in this context, and also because of its photoactivity [6].
Conduction measurements have been established earlier to allow insight into the characteristics of film growth for such materials [7]. The conductivity in thin films of organic semiconductors is not only related to the structure of the thin films but also widely affected by doping effects [6]. The electrical properties of nominally undoped thin films are therefore often governed by unknown impurities or by gas molecules such as oxygen [8-10]. Most organic semiconductors show p-type conduction because of th
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