- Email: [email protected]
Electrical and optical characteristics of organic thin films fabricated by laser ablation
applied
surface science ELSEVIER
Applied Surface Science 96-98 (1996) 625-629
Electrical and optical characteristics of organic thin films fabricated by laser ablation T. Fujii, H. Shima, N. Matsumoto, F. Kannari Department
*
ofElectrical Engineering, Faculty of Science and Technology, Keio Uniwrsit~, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223, Japan
Received 22 May 1995
Abstract Non-polymer organic thin films, copper-phthalocyanine (CuPc) and 4dialkylamino-&nitrostilbene (DANS), were fabricated by KrF laser ablation. Crystalline thin films with almost the same composition as the target material were deposited when reducing the laser ablation fluence close to the ablation threshold of N 20 mJ/cm’. Higher laser fluences characteristics of CuPc thin deplete some weakly bound atoms in the thin film, such as nitrogen in CuPc. Semiconductor film were examined in a Schottky diode. DANS-doped amorphous fluoropolymer was also fabricated by combining two ablation deposition processes. A crystalline feature of DANS disappears at low DANS concentrations in a fluoropolymer matrix.
1. Introduction Development of process technologies of various organic films, which are especially aimed for controlling their architecture in nanometer size, is growing interest. Although inorganic films such as hightemperature superconducting ceramic materials and ferroelectric ceramics have been successfully fabricated in laser ablation deposition schemes, there is less experimental result available on organic thin film deposition with laser ablation. Moreover non of organic thin films fabricated via laser ablation has been examined regarding their optical or electrical properties. Recently, we succeeded in fabrication of crystalline thin films of polytetrafluoroethylene (PTFE)
* Corresponding author. Tel.: +81-45 563 1141 (ext. 3301); fax: +81-45 563 2773: e-mail: [email protected]. 0169-4332/96/$15.00 ‘G 1996 Elsevier Science B.V. All rights reserved SSDI 0 169-4332(95)005641
by F, laser (157 nm> ablation [I]. X-ray photoemission spectra and FTIR absorption indicated that the composition of deposited films was similar to the source material. Since the laser ablation deposition technique holds a process controllability from shotto-shot, if this process technology can afford film qualities of organic materials at least comparable to other fabrication processes, e.g. vacuum evaporation, the laser ablation process is more suitable for fabricating new material structures consisting of different types of materials, such as organic and inorganic, with nanometer-size resolution. In this work. we experimentally studied the fabrication of non-polymer organic thin films with UV laser ablation method. Since most of organic molecules studied for opto-electronics applications have complex structures with relatively long chemical chains, reconstruction of such chemical structures on a substrate from decomposed fragments, which
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T. Fujii et al. /Applied
Surface Science 96-98 (1996) 625-629
was succeeded for simple molecular structures of PTFE polymers even with 10 times as high as the laser fluence at an ablation threshold ( N 90 mJ/cm*) [I], seems to be more difficult [2]. When decreasing the laser beam energy to minimum, the process is effectively equivalent to a molecular beam generation with keeping the original molecular structures, which is more like a laser desorption from solid surface, followed by layer formation on a substrate. Therefore, it is important to find a laser energy range allowed for organic thin film fabrications with accurate structure transfer from the target.
2. Experimental
methods
The thin film deposition was carried out in Ar ambient pressures of - 200 mTorr or N 10m5 Torr. The KrF laser beam with a pulse length of - 25 ns [a full width at half maximum @WI&I)] was incident through a focusing lens (f = 200 mm) on CuPc or DANS targets (Fig. l), which had been made by casting those powder to a block under high mechanical stress. The thin films were grown on a monocrystal silicon (100) substrate or a glass substrate located at 3 cm apart from the target under the substrate temperature of - 300 K. Film surface analyses were
performed by X-ray photoemission spectroscopy (XPS), scanning electron microscopy @EM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR). An electrical property of CuPc thin films was examined by fabricating a Schottky diode. Throughout the experiments, characteristics of thin films fabricated by the laser ablation were compared with those deposited with vacuum evaporation techniques. We did not introduce any temperature control for a substrate during the vacuum evaporation. Further, DANS was doped in an amorphous fluoropolymer matrix also via laser ablation method. In the first stage, an amorphous fluoropolymer, perflurinated cycle oxyaliphatic polymer (Cytop: Asahi Glass Co.), thin layer was produced by an F2 laser (157 nm) ablation in Ar ambient pressure of 200 mTorr. Details of the Cytop deposition with the Fz laser are similar to that performed for PTFE thin film fabrication [l]. Second, DANS was deposited by the KrF laser ablation on the Cytop layer. Again, a Cytop layer was formed over the DANS layer. We repeated this cycle by several times. The concentration of DANS in Cytop matrix was controlled by changing the number of ablation pulses at DANS and Cytop targets.
3. Results and discussion
DANS CUPC
(CFz)x \ / \ CFz-YF FF-(CFz)y 0
:
CFz \ / (CFz)z
t In x=0.1 y=o.1 LZO,l
CYtoP Fig. 1. Molecular stmc~ures of organic materials used for thin film deposition via laser ablation.
The first step of our experiments was aimed to clarify whether a process, in which only inter-molecular bonds are dissociated by the laser ablation with minimum decomposition of molecules, is possible when reducing the laser fluence to the laser ablation threshold as close as possible. The ablation threshold of CuPc was estimated by measuring etched depths to - 20 mJ/cm*. XPS spectra obtained for CuPc thin films deposited under - 10m5 Torr showed that when a large laser fluence of 1.5 J/cm2 was used, the relative concentration of nitrogen to carbon (N/C) extremely decreases. However, there is no clear energy shift in the spectra compared with those of CuPc target. Similar reduction in N has been observed for polyimide polymer film surfaces when irradiated with KrF lasers above the laser ablation threshold [3]. Therefore, it is speculated that nitrogen
T. Fujii et al. /Applied Surface Science 96-98 (1996) 625-629
atoms are easily removed during the laser ablation and never recontained again in the reconstructed film. On the other hand, the XPS spectra obtained for the CuPc film with a low laser fluence of 30 ml/cm2 showed only a slight reduction in the N/C ratio compared with that of the target. Fig. 2 shows FTIR spectra for CuPc thin films deposited with different KrF laser fluences under N 10m5 Torr. In the l!TIR spectrum obtained by the laser ablation at 1.5 J/cm2 (Fig. 2(c)), the peaks that can be observed below 1400 cm-’ in a vacuum evaporated CuPc film (Fig. 2(a)) completely disappear. Therefore, at higher laser fluences, CuPc molecules are significantly decomposed to small fragments. In contrast, the thin film obtained at 30 mJ/cm* (Fig. 2(b)) shows the similar IR spectra to that of the vacuum evaporated film. Therefore, from the XPS and FTIR spectra the main architecture of CuPc seems to be kept in the thin films at low laser fluences near the ablation threshold. Crystallinity of CuPc films was evaluated with XRD. Although the CuPc target exhibited many peaks with a highest peak at 20 = 7”, the film deposited with the vacuum evaporation showed only a single peak at 7”, which characterize an a-type molecular orientation. The film deposited at 1.5 J/cm2 did not show any crystalline peak, whereas the film deposited at 30 ml/cm* exhibited a single peak at 7” indicating that an a-type molecular orientation is also dominant in the film deposited by the laser ablation. The SEM image of CuPc films deposited at room
I
1600
1
1400
1200
I
1000
800
I 600
Wavenumber (cm -1) Fig. 2. FTIR spectra of CuPc thin films via (a) vacuum evaporation; (b) KrF laser ablation at 30 mJ/cm*; and (c) KrF laser ablation at 1500 mJ/cn-?
I 1700
1,
I
1400
621
I
I1
1200
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/
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/,
600
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Wavenumber (cm -1) Fig. 3. FTIR spectra of DANS: (a) DANS cast target from the powder; (b) DANS thin film fabricated via KrF laser ablation at 30 rnJ/cm2: (c) DANS thin films fabricated with vacuum evaporation.
temperature by the laser ablation showed many particles and porous structures on the film. The films fabricated by the vacuum evaporation have a more smooth surface. However, as demonstrated in PTFE thin film deposition with an F, laser ablation, a slight increase in substrate temperature will drastically improve the surface morphology 111. A Schottky diode was constructed between an Al electrode and the thin films deposited by the laser ablation. Since the electrical field is parallel to the thin film surface in the diode we fabricated, the surface resistance also affected on the V-Z characteristics. However, a clear unidirectional V-I characteristics, which was also observed in a diode fabricated with a CuPc film deposited by the vacuum evaporation, was obtained. Therefore, CuPc thin films deposited by laser ablation still keep the electrical characteristics of CuPc. Similar thin film depositions were repeated for DANS with the KrF laser ablation. DANS film fabricated at a laser fluence of 30 mJ/cm2 showed similar XPS spectra to those of films fabricated by the vacuum evaporation. Fig. 3 shows FTIR spectra of DANS. Since the DANS target was also fabricated by casting DANS powder under high pressure, the FTIR spectrum shows many broadened structures indicating a random molecule orientations. The DANS thin film deposited by the laser ablation showed narrower spectra that correspond to the spectra of target, whereas the film fabricated by the
628
T. Fujii et al. /Applied
I
296
,
Surjke
I
292 288 284 280 Binding Energy (eV)
Fig. 4. XPS spectra of (a) pure DANS thin film; (b) DANS-doped Cytop thin film; and (c) pure Cytop thin film.
vacuum evaporation shows only selected peaks. Therefore, the vacuum evaporation process holds the selectivity in DANS molecular orientation. Fig. 4 shows C 1s XPS spectra from three films deposited by the laser ablation: (a) a pure DANS thin film; (b) a DANS + Cytop thin film; and (c) a pure Cytop thin film. For a pure Cytop, a peak at 292 eV is identified as a shift induced by bonding with (F-C-F) fluorine atoms. For pure DANS thin film, only one broad peak is observed. The XPS spectrum of DANS + Cytop is a linear combination of the DANS and Cytop, which indicates the DANS and Cytop are not chemically reacted at this DANS concentration.
(a)
28 (Degrees) Fig. 5. X-ray diffraction (XRD) scans of (a) pure DANS thin film; (b) DANS-doped Cytop thin film at DANS concentration of - 50% relative to Cytop; and (c) DANS-doped Cytop thin film at DANS concentration of - 25% relative to Cytop. Total amount of DANS deposited was kept constant in (a)-(c).
Science 96-98 (1996) 625-629
A vacuum evaporated pure DANS thin film showed large peaks at 6 and 12” in 28 XRD scans. However, a DANS thin film fabricated by the laser ablation showed a strong peak at 20” with weak and broader peaks at 12, 23 and 25”, which correspond to the XRD peaks observed for the DANS cast target. Therefore, XRD measurements indicated the absence of selectivity of crystal orientation in the present laser ablation method. Fig. 5 shows 28 XRD scans of DANS + Cytop thin film at different DANS concentrations. Although the total ablation pulses for DANS were kept constant, a XRD peak at 20”, which characterize the crystalline DANS, becomes weak with reducing the relative DANS concentration in Cytop matrix. Therefore, at low DANS concentration, DANS thin films are in an amorphous state. Since DANS crystal has a centrosymmetric structure, it makes no contribution to the electro-optic effect. The DANS in amorphous state in Cytop will allow the effect of their large molecular hyperpolarizability to be detected after proper poling treatment at a glass transition temperature of Cytop - 108°C.
4. Conclusion Non-polymer organic thin films of CuPc and DANS were deposited by the KrF laser ablation. Crystalline thin films with accurate composition transfer from the target material were fabricated when reducing the laser ablation fluence close to the ablation threshold fluence. Higher laser fluences cause the decomposition of molecules to smaller size fragments, which can not be reconstructed on the substrate. Electrical contact between the Al and CuPc films fabricated at a laser fluence of 30 mJ/cm’ under lo-’ Torr showed V-I characteristics of Schottky diode, indicating that the film still maintains electrical property of semiconductor. Similarly, DANS films with no significant modification in the molecular structure were fabricated with the laser ablation. However, both FTIR and XRD spectra indicated that the thin films deposited at a substrate temperature at 300 K have no molecular orientation selectivity for DANS. By combining the DANS and amorphous fluoropolymer thin film deposition processes alternately, DANS were doped in amorphous
T. Fujii et al./Applied Surface Science 96-98 (1996) 625429
state in the fluoropolymer matrix, which is expected to exhibit EO effect after some poling treatments. In order to use the organic thin films fabricated with laser ablation scheme for novel functional materials, the control of molecular orientations and uniform surfaces and boundaries between different layers have to be achieved. Epitaxial film growth of organic materials with laser ablations, which is already an established technique in inorganic materials, is growing interest.
629
References [l] Y. Ueno, T. Fujii and F. Kannari, Appl. Phys. Lett. 65 (1994) 1370. [2] K. Higaki, C. Nagai, 0. Murata and H. Itoh, J. Photopolym. Sci. Technol. 6 (1993) 429. [3] M. Schumann, MC Smayling and R. Sauerbrey, Appl. Phys. Lett. 58 (1991) 428.
surface science ELSEVIER
Applied Surface Science 96-98 (1996) 625-629
Electrical and optical characteristics of organic thin films fabricated by laser ablation T. Fujii, H. Shima, N. Matsumoto, F. Kannari Department
*
ofElectrical Engineering, Faculty of Science and Technology, Keio Uniwrsit~, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223, Japan
Received 22 May 1995
Abstract Non-polymer organic thin films, copper-phthalocyanine (CuPc) and 4dialkylamino-&nitrostilbene (DANS), were fabricated by KrF laser ablation. Crystalline thin films with almost the same composition as the target material were deposited when reducing the laser ablation fluence close to the ablation threshold of N 20 mJ/cm’. Higher laser fluences characteristics of CuPc thin deplete some weakly bound atoms in the thin film, such as nitrogen in CuPc. Semiconductor film were examined in a Schottky diode. DANS-doped amorphous fluoropolymer was also fabricated by combining two ablation deposition processes. A crystalline feature of DANS disappears at low DANS concentrations in a fluoropolymer matrix.
1. Introduction Development of process technologies of various organic films, which are especially aimed for controlling their architecture in nanometer size, is growing interest. Although inorganic films such as hightemperature superconducting ceramic materials and ferroelectric ceramics have been successfully fabricated in laser ablation deposition schemes, there is less experimental result available on organic thin film deposition with laser ablation. Moreover non of organic thin films fabricated via laser ablation has been examined regarding their optical or electrical properties. Recently, we succeeded in fabrication of crystalline thin films of polytetrafluoroethylene (PTFE)
* Corresponding author. Tel.: +81-45 563 1141 (ext. 3301); fax: +81-45 563 2773: e-mail: [email protected]. 0169-4332/96/$15.00 ‘G 1996 Elsevier Science B.V. All rights reserved SSDI 0 169-4332(95)005641
by F, laser (157 nm> ablation [I]. X-ray photoemission spectra and FTIR absorption indicated that the composition of deposited films was similar to the source material. Since the laser ablation deposition technique holds a process controllability from shotto-shot, if this process technology can afford film qualities of organic materials at least comparable to other fabrication processes, e.g. vacuum evaporation, the laser ablation process is more suitable for fabricating new material structures consisting of different types of materials, such as organic and inorganic, with nanometer-size resolution. In this work. we experimentally studied the fabrication of non-polymer organic thin films with UV laser ablation method. Since most of organic molecules studied for opto-electronics applications have complex structures with relatively long chemical chains, reconstruction of such chemical structures on a substrate from decomposed fragments, which
626
T. Fujii et al. /Applied
Surface Science 96-98 (1996) 625-629
was succeeded for simple molecular structures of PTFE polymers even with 10 times as high as the laser fluence at an ablation threshold ( N 90 mJ/cm*) [I], seems to be more difficult [2]. When decreasing the laser beam energy to minimum, the process is effectively equivalent to a molecular beam generation with keeping the original molecular structures, which is more like a laser desorption from solid surface, followed by layer formation on a substrate. Therefore, it is important to find a laser energy range allowed for organic thin film fabrications with accurate structure transfer from the target.
2. Experimental
methods
The thin film deposition was carried out in Ar ambient pressures of - 200 mTorr or N 10m5 Torr. The KrF laser beam with a pulse length of - 25 ns [a full width at half maximum @WI&I)] was incident through a focusing lens (f = 200 mm) on CuPc or DANS targets (Fig. l), which had been made by casting those powder to a block under high mechanical stress. The thin films were grown on a monocrystal silicon (100) substrate or a glass substrate located at 3 cm apart from the target under the substrate temperature of - 300 K. Film surface analyses were
performed by X-ray photoemission spectroscopy (XPS), scanning electron microscopy @EM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR). An electrical property of CuPc thin films was examined by fabricating a Schottky diode. Throughout the experiments, characteristics of thin films fabricated by the laser ablation were compared with those deposited with vacuum evaporation techniques. We did not introduce any temperature control for a substrate during the vacuum evaporation. Further, DANS was doped in an amorphous fluoropolymer matrix also via laser ablation method. In the first stage, an amorphous fluoropolymer, perflurinated cycle oxyaliphatic polymer (Cytop: Asahi Glass Co.), thin layer was produced by an F2 laser (157 nm) ablation in Ar ambient pressure of 200 mTorr. Details of the Cytop deposition with the Fz laser are similar to that performed for PTFE thin film fabrication [l]. Second, DANS was deposited by the KrF laser ablation on the Cytop layer. Again, a Cytop layer was formed over the DANS layer. We repeated this cycle by several times. The concentration of DANS in Cytop matrix was controlled by changing the number of ablation pulses at DANS and Cytop targets.
3. Results and discussion
DANS CUPC
(CFz)x \ / \ CFz-YF FF-(CFz)y 0
:
CFz \ / (CFz)z
t In x=0.1 y=o.1 LZO,l
CYtoP Fig. 1. Molecular stmc~ures of organic materials used for thin film deposition via laser ablation.
The first step of our experiments was aimed to clarify whether a process, in which only inter-molecular bonds are dissociated by the laser ablation with minimum decomposition of molecules, is possible when reducing the laser fluence to the laser ablation threshold as close as possible. The ablation threshold of CuPc was estimated by measuring etched depths to - 20 mJ/cm*. XPS spectra obtained for CuPc thin films deposited under - 10m5 Torr showed that when a large laser fluence of 1.5 J/cm2 was used, the relative concentration of nitrogen to carbon (N/C) extremely decreases. However, there is no clear energy shift in the spectra compared with those of CuPc target. Similar reduction in N has been observed for polyimide polymer film surfaces when irradiated with KrF lasers above the laser ablation threshold [3]. Therefore, it is speculated that nitrogen
T. Fujii et al. /Applied Surface Science 96-98 (1996) 625-629
atoms are easily removed during the laser ablation and never recontained again in the reconstructed film. On the other hand, the XPS spectra obtained for the CuPc film with a low laser fluence of 30 ml/cm2 showed only a slight reduction in the N/C ratio compared with that of the target. Fig. 2 shows FTIR spectra for CuPc thin films deposited with different KrF laser fluences under N 10m5 Torr. In the l!TIR spectrum obtained by the laser ablation at 1.5 J/cm2 (Fig. 2(c)), the peaks that can be observed below 1400 cm-’ in a vacuum evaporated CuPc film (Fig. 2(a)) completely disappear. Therefore, at higher laser fluences, CuPc molecules are significantly decomposed to small fragments. In contrast, the thin film obtained at 30 mJ/cm* (Fig. 2(b)) shows the similar IR spectra to that of the vacuum evaporated film. Therefore, from the XPS and FTIR spectra the main architecture of CuPc seems to be kept in the thin films at low laser fluences near the ablation threshold. Crystallinity of CuPc films was evaluated with XRD. Although the CuPc target exhibited many peaks with a highest peak at 20 = 7”, the film deposited with the vacuum evaporation showed only a single peak at 7”, which characterize an a-type molecular orientation. The film deposited at 1.5 J/cm2 did not show any crystalline peak, whereas the film deposited at 30 ml/cm* exhibited a single peak at 7” indicating that an a-type molecular orientation is also dominant in the film deposited by the laser ablation. The SEM image of CuPc films deposited at room
I
1600
1
1400
1200
I
1000
800
I 600
Wavenumber (cm -1) Fig. 2. FTIR spectra of CuPc thin films via (a) vacuum evaporation; (b) KrF laser ablation at 30 mJ/cm*; and (c) KrF laser ablation at 1500 mJ/cn-?
I 1700
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I
I1
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/
1000
/,
600
I 600
Wavenumber (cm -1) Fig. 3. FTIR spectra of DANS: (a) DANS cast target from the powder; (b) DANS thin film fabricated via KrF laser ablation at 30 rnJ/cm2: (c) DANS thin films fabricated with vacuum evaporation.
temperature by the laser ablation showed many particles and porous structures on the film. The films fabricated by the vacuum evaporation have a more smooth surface. However, as demonstrated in PTFE thin film deposition with an F, laser ablation, a slight increase in substrate temperature will drastically improve the surface morphology 111. A Schottky diode was constructed between an Al electrode and the thin films deposited by the laser ablation. Since the electrical field is parallel to the thin film surface in the diode we fabricated, the surface resistance also affected on the V-Z characteristics. However, a clear unidirectional V-I characteristics, which was also observed in a diode fabricated with a CuPc film deposited by the vacuum evaporation, was obtained. Therefore, CuPc thin films deposited by laser ablation still keep the electrical characteristics of CuPc. Similar thin film depositions were repeated for DANS with the KrF laser ablation. DANS film fabricated at a laser fluence of 30 mJ/cm2 showed similar XPS spectra to those of films fabricated by the vacuum evaporation. Fig. 3 shows FTIR spectra of DANS. Since the DANS target was also fabricated by casting DANS powder under high pressure, the FTIR spectrum shows many broadened structures indicating a random molecule orientations. The DANS thin film deposited by the laser ablation showed narrower spectra that correspond to the spectra of target, whereas the film fabricated by the
628
T. Fujii et al. /Applied
I
296
,
Surjke
I
292 288 284 280 Binding Energy (eV)
Fig. 4. XPS spectra of (a) pure DANS thin film; (b) DANS-doped Cytop thin film; and (c) pure Cytop thin film.
vacuum evaporation shows only selected peaks. Therefore, the vacuum evaporation process holds the selectivity in DANS molecular orientation. Fig. 4 shows C 1s XPS spectra from three films deposited by the laser ablation: (a) a pure DANS thin film; (b) a DANS + Cytop thin film; and (c) a pure Cytop thin film. For a pure Cytop, a peak at 292 eV is identified as a shift induced by bonding with (F-C-F) fluorine atoms. For pure DANS thin film, only one broad peak is observed. The XPS spectrum of DANS + Cytop is a linear combination of the DANS and Cytop, which indicates the DANS and Cytop are not chemically reacted at this DANS concentration.
(a)
28 (Degrees) Fig. 5. X-ray diffraction (XRD) scans of (a) pure DANS thin film; (b) DANS-doped Cytop thin film at DANS concentration of - 50% relative to Cytop; and (c) DANS-doped Cytop thin film at DANS concentration of - 25% relative to Cytop. Total amount of DANS deposited was kept constant in (a)-(c).
Science 96-98 (1996) 625-629
A vacuum evaporated pure DANS thin film showed large peaks at 6 and 12” in 28 XRD scans. However, a DANS thin film fabricated by the laser ablation showed a strong peak at 20” with weak and broader peaks at 12, 23 and 25”, which correspond to the XRD peaks observed for the DANS cast target. Therefore, XRD measurements indicated the absence of selectivity of crystal orientation in the present laser ablation method. Fig. 5 shows 28 XRD scans of DANS + Cytop thin film at different DANS concentrations. Although the total ablation pulses for DANS were kept constant, a XRD peak at 20”, which characterize the crystalline DANS, becomes weak with reducing the relative DANS concentration in Cytop matrix. Therefore, at low DANS concentration, DANS thin films are in an amorphous state. Since DANS crystal has a centrosymmetric structure, it makes no contribution to the electro-optic effect. The DANS in amorphous state in Cytop will allow the effect of their large molecular hyperpolarizability to be detected after proper poling treatment at a glass transition temperature of Cytop - 108°C.
4. Conclusion Non-polymer organic thin films of CuPc and DANS were deposited by the KrF laser ablation. Crystalline thin films with accurate composition transfer from the target material were fabricated when reducing the laser ablation fluence close to the ablation threshold fluence. Higher laser fluences cause the decomposition of molecules to smaller size fragments, which can not be reconstructed on the substrate. Electrical contact between the Al and CuPc films fabricated at a laser fluence of 30 mJ/cm’ under lo-’ Torr showed V-I characteristics of Schottky diode, indicating that the film still maintains electrical property of semiconductor. Similarly, DANS films with no significant modification in the molecular structure were fabricated with the laser ablation. However, both FTIR and XRD spectra indicated that the thin films deposited at a substrate temperature at 300 K have no molecular orientation selectivity for DANS. By combining the DANS and amorphous fluoropolymer thin film deposition processes alternately, DANS were doped in amorphous
T. Fujii et al./Applied Surface Science 96-98 (1996) 625429
state in the fluoropolymer matrix, which is expected to exhibit EO effect after some poling treatments. In order to use the organic thin films fabricated with laser ablation scheme for novel functional materials, the control of molecular orientations and uniform surfaces and boundaries between different layers have to be achieved. Epitaxial film growth of organic materials with laser ablations, which is already an established technique in inorganic materials, is growing interest.
629
References [l] Y. Ueno, T. Fujii and F. Kannari, Appl. Phys. Lett. 65 (1994) 1370. [2] K. Higaki, C. Nagai, 0. Murata and H. Itoh, J. Photopolym. Sci. Technol. 6 (1993) 429. [3] M. Schumann, MC Smayling and R. Sauerbrey, Appl. Phys. Lett. 58 (1991) 428.