Photoinduced Temporal Change of Surface-Potential Undulation on Alq3 Thin Films Observed by Kelvin Probe Force Microscop
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Photoinduced Temporal Change of Surface-Potential Undulation on Alq3 Thin Films Observed by Kelvin Probe Force Microscopy Kazunari Ozasa, Hiromi Ito, Mizuo Maeda, and Masahiko Hara Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
ABSTRACT The surface potential (SP) undulation on the surfaces of tris(8-hydroxyquinolinato) aluminum (III) (Alq3) films has been investigated with Kelvin probe force microscopy (KFM) and scanning near-field optical microscope (SNOM)-KFM. The SP undulation observed on the amorphous Alq3 films with thicknesses of up to 300 nm showed a cloud-like morphology of 200–300 nm in lateral size. The temporal change of SP undulation was traced through cyclic measurement with KFM observation with intermittent photoexposure, as well as in situ localized photoexcitation with SNOM-KFM. We concluded that the origin of the SP undulation is the nonuniform distribution of charged traps and drift mobility in the Alq3 films.
INTRODUCTION Tris(8-hydroxyquinolinato) aluminum (III) (Alq3) is one of the major organic semiconductors for light emitting devices. When an Alq3 film is prepared by vacuum deposition with light shielding, surface potential (SP) proportional to the film thickness is spontaneously formed [1]. Photoexposure reduces the SP of Alq3 [1-4]. We have previously reported the dependence and mechanism of the photoinduced SP reduction, and concluded that the mechanism is the drift of photoexcited carriers according to electric fields in the films and subsequent capturing the carriers by deep traps [3,4]. Recently, we found that the spatial undulation of the SP grows once and then decreases with photoexposure. We traced the temporal change of SP undulation through cyclic measurement with Kelvin probe force microscopy (KFM) with intermittent photoexposure, as well as in situ localized photoexcitation with a scanning near field optical microscope (SNOM)KFM [5].
EXPERIMENT Amorphous Alq3 films with various thicknesses up to 300 nm were formed on n-Si(001) by vacuum deposition at room temperature with light shielding at a deposition rate of 0.15-0.2 nm/s. The topographic and SP- morphology were measured with KFM with lift mode.
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Intermittent photoexcitation was carried out with a white lamp light for probe positioning in KFM apparatus. Localized photoexcitation was examined with a blue laser through fiber probe of SNOM-KFM [5].
DISCUSSION For 100-nm thick Alq3 film, which showed an average SP of 4.2 V, the undulation of SP was 13 mV (histogram width) with spatial size 200–400 nm, while the topographic morphology has spatial size 20-30 nm and height variation
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