Conference-ISSS-4-Organic Static Induction Transistors with Nano-Hole Arrays Fabricated by Colloidal Lithography

Organic static induction transistors (SITs) with nano-hole arrays in a lateral direction was fabricated using a ’colloidal lithography’ technique. Positively charged polystyrene particles of 200 nm diameter as a deposition mask were adsorbed on a glass surface via electrostatic self assembly. A source electrode, a bottom CuPc layer, and a Schottky gate electrode were deposited succeedingly on the particle-adsorbed glass substrate. After the particles were removed, a large number of nano-holes were formed without destroying the films. A top CuPc layer and a top drain electrode were deposited on the nano-hole layers to complete the SIT, where the nano-holes work in parallel as active conducting channels. The obtained SIT characteristics showed that the source-drain current is modulated by a positive gate bias. [DOI: 10.1380/ejssnt.2005.327]


I. INTRODUCTION
Organic thin-film transistors (OTFTs) have been attracting many research efforts owing to their applicability to flexible electronic devices [1][2][3].Organic vertical-type transistors such as static induction transistors (SITs) are suitable for high-speed switching because of their short channel length and wide channel width [4,5].To improve the electrical characteristics of organic SITs, fabrication of nano-scale structures in the horizontal direction is strongly desired without introducing an electrical damage to functional organic materials.The size of the fine lateral structure must be in the same length scale as the thickness of an organic semiconductor film which corresponds to channel length of the vertical-type transistors.However, for organic vertical-type devices, it is difficult to fabricate microstructures in a lateral direction, because conventional microfabrication processes, such as lithography, plasma etching and ion implantation, cannot be used for organic materials.Due to such a limitation, channel width or density can not be effectively increased to enable the large current flow over entire device area, although FIG.1: Schematic illustration of an organic SIT with higherorder nanostructures.
this is one of the advantages of vertical-type transistors.
We have been studying the vertical-type devices using a fabrication technique based of a new concept, 'Spontaneous Patterning of Higher Order Structures' (SPHOS) [6][7][8].Using these techniques, we could fabricate a microstructure in the lateral direction within a largescale organic device using non-photolithographic processes.One of the techniques adopted in the SPHOS is 'colloidal lithography' [9] which forms porous films by removing particles after the film deposition.
In this study, we fabricated organic SITs with higherorder nanostructures using colloidal lithography.Figure 1 schematically shows the device structure.We studied the optimization of the particle adsorption process and the device characteristics.

II. EXPERIMENTAL
Figure 2 schematically shows the fabrication process of organic SITs using colloidal lithography.We used a glass substrate which was cleaned by ultrasonic agitation in ultra pure water, acetone respectively, and exposed to UV/O 3 for 20 min at room temperature.Positively charged polystyrene particles (200 nm, tetramethylammonium latex) were adsorbed onto the substrates from dispersion by electrostatic interactions.A particle concentration was 0.01 wt% and the dispersion was dialyzed overnight against ultra pure water.Immersion time was 30 min to allow the adsorption to reach saturation.Excess particles were rinsed off in a beaker with ultra pure water at 98 • C for 30 s.The samples were then rinsed in cold water and blown dry with clean air.After adsorbing the particles, Cr was deposited by RF sputtering, and Au, Copper(II) phthalocyanine (CuPc), and Al were deposited successively on the sample by vacuum evaporation.After the deposition of Al/CuPc/Au/Cr, the particles were selectively removed by an adhesive tape.After the particles were removed, a large number of nano-holes were formed without destroying the films.Then a top CuPc layer and a Au electrode were deposited on the nano-hole layers to complete a SIT.The active area of the SIT was 4.0 cm 2 .

III. RESULTS AND DISCUSSION
In fabricating the organic SIT with higher order nanostructures, controllability of the adsorption of polystyrene particles is important because the area of adsorbed particles is finally used as an active region of the SIT.Two methods to control the film structure were evaluated.In the first case, polystyrene particles were adsorbed by spincoating.Figure 3  microscopy (SEM) image of the substrate surface with spreading particles by spincoating (2000 rpm for 60 sec, dispersion concentration 0.1 wt%).Surface coverage was measured as area of particles divided by total area.The coverage of the sample with particles by spincoating was only 0.013.The second approach used immersion of a substrate to particles dispersion in order to utilize electrostatic interaction between surfaces of particles and the substrate.As schematically shown in Fig. 3(c), a surface of clean glass substrate have a lot of anionic groups, and hence polystyrene particles having N + Me 4 as cationic groups were used to be adsorbed on the substrate.The coverage of the sample prepared by immersion method was 0.33 as shown in Fig. 3(b).
In the organic SIT with higher order nanostructures, a high surface coverage while maintaining dispersive adsorption is required to obtain high current densities.In this structure, the total circumference of all the particles (and its aggregates) edges corresponds to the channel width.The immersion method increased the particle coverage, but unfortunately defective aggregation was introduced in the dried particle film.The aggregation of particles on the surface is normally ascribed to capillary forces created by the menisci between particles as the water evaporates [10].The capillary interactions increase sharply as the spacing between particles decreases [11].Therefore, to suppress the particle aggregation and retain control of the particle film structure, the adsorbed particle film was heated while still in solution.This method was designed to overcome the capillary forces and keep the particles in place.As shown in Fig. 4 of adsorbed particles (and its aggregates) density at various rinsing temperature in the same coverage (0.33) show that the density of the sample heated at 98 • C is approximately 10 times higher than that of the unheated sample.T g of bulk polystyrene is in between 100 and 110 • C, and the particles were presumably deformed slightly on the surface increasing the contact area and thus the adhesion force became ineffective to move the particles [12].Utilizing the combination of immersion and heating methods, various particle diameters and coverages could be controlled in the range of 200 to 600 nm and 0.10 to 0.33, respectively, as shown in Fig. 5.
After Al/CuPc/Au/Cr layers were deposited on the particles adsorbed substrate, two methods to remove particles were attempted.One is a sonication method which is utilized when the total thickness of the layers was below half the particle diameter.As the total thickness of the layers exceeded half the particle diameter, sonication caused a mechanical damage to the Al/CuPc/Au/Cr layers (Fig 6(a)).However, to prevent short-circuiting between the electrodes and to provide enough conduction through the electrodes, sufficient thicknesses of the electrodes and semiconductor layers were needed.In the organic SIT, using larger thickness of the layers with larger particles causes increase of off-state current, because the spreading of depletion region could not cover the whole nano-hole.Therefore, adhesive-tape-peeling method was adopted instead.As the results, the particles (200 nm diameter) in the Al/CuPc/Au/Cr layers (200 nm thick) were perfectly removed without causing a mechanical damage to the layers (Fig. 6(b)).
After removal of the particles, CuPc and top Au layers were evaporated on the nano-hole arrays to complete the SIT structure.Figure 7(a) schematically shows the device structure with the thickness of each layer and the circuit diagram for SIT measurement.In this SIT structure, the thickness of the top CuPc layer corresponds to the channel length (300 nm), and the total circumference of all the pores corresponds to the channel width (approximately 29 m).In an organic SIT, a buried Al gate electrode supplies the modulating potential barrier to limit the drain current.Therefore, the property of the Al Schottky gate/CuPc semiconductor interface is important matter to fabricate high performance organic SITs.The I-V characteristics between the bottommost Au (source) and the Al (gate) electrodes of the nanostructured SIT clearly indicated a good Schottky behavior.Figure 7(b) shows the SIT characteristics of an obtained device.Drainsource current is modulated by gate bias.A maximum I D is obtained at V GS = 0, and I D decreases with increasing V GS .This normally-on type character is typical in a SIT.However, the problems in this SIT are a large leak current between source and gate, and a small onstate current in spite of the large channel width.It can be seen from SEM image of top Au surface that the nanoholes were filled with large CuPc grains of which size was from 50 to 100 nm.This can be attributed to the possibility that not all nano-holes are working as conducting channels because part of the CuPc layer may not be in contact with the bottom electrode.It should also be considered that the top Au electrode may be less conductive by discontinuity of the electrode.Accordingly, optimiza- tion of the channel fabrication process needs to be further studied to obtain better properties of the organic SIT.

IV. CONCLUSIONS
To fabricate nanostructures in the lateral direction, we employed colloidal lithography.High density adsorption of masking particles was enabled by immersing method utilizing electrostatic interaction between surfaces of the particles and the substrate.In addition, we could reduce the particle aggregation and retain control of the particle film structure by heating method.We also fabricated nano-hole arrays using adhesive tape peeling without causing mechanical damage to the arrays.Using these techniques, nano-porous films with a regular hole diameter were formed, and organic SITs with higher-order nanostructures were fabricated.Drain-source current of the SIT is modulated by gate bias.The device properties will be improved by optimizing the fabrication process of the top CuPc films.

FIG. 2 :
FIG. 2: Schematic illustrations showing the fabrication process of organic SITs using colloidal lithography: (a) adsorption of polystyrene particles on a glass substrate, (b) deposition of Al/CuPc/Au/Cr layers using the particles as evaporation masks, (c) removal of the particles by an adhesive tape, and (d) deposition of CuPc and Au layers.

FIG. 3 :
FIG. 3: SEM image of the glass surface with polystyrene particles adsorbed by (a) spincoating and (b) immersion methods.The surface coverages of them were 0.013 and 0.33, respectively.(c) Schematic illustration of the electrostatic interaction between surfaces of particles and a substrate.
(a)  shows a scanning electron

FIG. 4 :
FIG. 4: SEM image of the glass surface with adsorbed particles (a) without heating and (b) using heating method.These surface coverages were equally 0.33.(c) Particle density against rinsing temperature.
FIG. 5: SEM images of the glass surfaces having various adsorbed particles diameters and coverages.

FIG. 6 :
FIG. 6: SEM images of Al surface after removal of particles using (a) sonication and (b) adhesive-tape-peeling methods.