Photoacoustic spectra from copper phthalocyanine films on Si wafers
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Photoacoustic spectra from copper phthalocyanine films on Si wafers Masato OHMUKAI, Hitoshi KUBOTA and Yasuo TSUTSUMI Department of Electrical Engineering, Akashi College of Technology, Hyogo 674-8501, Japan ABSTRACT Copper phthalocyanine (CuPc) films were deposited on (100) silicon wafers by means of a vacuum evaporation method. We examined the films using photoacoustic (PA) spectroscopy to study their optical absorption properties. It was clarified that the PA signal from the silicon substrate can be excluded with a chopping frequency of 200 Hz when the film thickness is greater than 4.4 m. We detected an absorption band at 1.128 eV. INTRODUCTION Many researchers are devoted to study the application of organic materials to electronic and optical devices. The organic materials are highly attractive because of their innumerable derivatives. At a present stage, organic materials are powerless against inorganic semiconductors owing to their low conductivity. However, organic light emitting devices [1-5] are nowadays promising and leading application of organic materials. Copper phthalocyanin (CuPc) [6] is one of the well-known organic materials expectedly applied to electronic devices. CuPc is used as a hole-transporting layer in organic light emitting devices because the work function is relatively large [7,8]. CuPc is also possibly applied to a gas sensor [9,10] and an optical recording material [11]. It is necessary to form CuPc into films for an electronic device application. In this article, CuPc films were deposited on silicon wafers and investigated by means of photoacoustic (PA) spectroscopy [12]. This technique provides information of optical absorption spectra like a transmission measurement. The great advantage of PA spectroscopy is that the absorption only in the surface layer can be observed if experimental conditions are arranged adequately. It is why the absorption spectra of CuPc films can be obtained without removing an opaque substrate such as silicon. Another advantage is that PA spectroscopy provides absorption spectra in the region where the absorption coefficient is too large to examine with a transmission measurement. EXPERIMENTAL DETAILS Commercially available CuPc powder was put into a molybdenum boat with a small hole. The purity of CuPc powder was 98.8 %. A (100) p-type silicon wafer was put at a distance of 7 cm from the boat. The whole chamber for the vacuum evaporation was evacuated to the pressure of 2.0*10-5 Torr with a diffusion pump. The substrate was not heated or cooled. The thickness of the deposited CuPc films was determined with a surface profiler of a needle contact type. The structure of the films were checked by means of an X-ray diffractometer with a O3.3.1
Cu target at 40 kV and 30 mA. A schematic illustration of PA spectrometer is depicted in Fig. 1. Intense light generated in a 500 W xenon lamp was introduced into a monochromator. The selected light with a single wavelength was chopped by a mechanical chopper and then focused onto a sample that was installed in a PA sp
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