Structural and electrical characterization of pentacene films on SiO2 grown by molecular beam deposition
Introduction
In recent years, organic thin film transistors (OTFTs) based on either polymer or small molecules have attracted much attention. Their field-effect carrier mobility has been dramatically improved in this a few years and been surpassing that of amorphous silicon. OTFTs are promising for applications such as driving circuits for displays requiring low cost, large area coverage and relatively high mobility. Pentacene is one of the candidates of OTFT materials having practical performance, because high mobility of >1 cm2/Vs has been reported for vacuum-evaporated films [1], [2], [3]. Active layer of OTFTs is generally used in polycrystalline state and transistor characteristics are strongly influenced by the film morphology. Therefore, investigation of the correlation between film morphology and electronic properties is important to improve the device performance.
There have been several reports on the film morphology and electrical properties of pentacene-evaporated films. Flatness of gate insulator surface is reported to be important to enlarge grain size and increase mobility [4]. By X-ray diffraction (XRD), it is revealed that vacuum-evaporated pentacene films are composed of two crystalline phases, namely ‘thin film phase’ and ‘bulk phase’, and the ratio of the two phases varies by the growth conditions [5]. A paper mentions that the films consisting of thin film phase have higher mobility than those with mixture of the two phases [6]. There are, however, few researches with a viewpoint of the correlation between film morphology and electronic properties for wide range of growth conditions.
In this study, we have formed pentacene thin films on SiO2 substrates by molecular beam deposition (MBD), which can control the growth conditions, i.e. growth rate and temperature, broadly and precisely. Surface morphology and crystal structure of MBD-grown pentacene films were studied with atomic force microscopy (AFM) and XRD. Top contact TFTs are fabricated with the films and the correlation between electrical characteristics and film morphology has been investigated.
Section snippets
Experimental
Heavily doped n-type silicon was used as substrates, which was also used as a gate electrode of a TFT. Thermally grown SiO2 of 200 nm thickness was formed on each substrate. The RMS roughness of the SiO2 surface was 0.6 nm. The substrate surface was cleaned with UV/O3 treatment at the temperature of 200 °C just before the film growth. Pentacene with 97% purity was purchased from Aldrich Chemical, and degassed at 150 °C for 3 h and then 180 °C for 3 h in vacuum to eliminate low molecular weight
Results and discussion
Morphology of pentacene crystalline grains was observed with AFM, and was categorized into five groups, as shown in Fig. 1, according to their characteristic shapes. Fig. 1a shows ‘lamellar grains’ marked with dotted ovals, of which height tends to be higher than surrounding other grains. Fig. 1b–d shows typical morphologies of ‘pyramidal grains’ which have square pyramidal shape with relatively high symmetry, ‘inclined grains’ which are similar to but more asymmetric than the pyramidal grains,
Summary
Surface morphology, crystal structure and transistor characteristics of MBD-grown pentacene films on SiO2 were investigated. Five types of grain morphology were observed with AFM, which was summarized in a morphodrom. Above the growth temperature of 40 °C, recessed region were observed on the film surfaces. Origin of the recess was assumed to be the phase transition from thin film to bulk phase due to the tensile stress during the cooling-down process. Among many features of grown films, the
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