Growth Of Organic Semiconductor Thin Films with Multi-Micron

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Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve Pavlo Fesenko,*,†,‡ Valentin Flauraud,§ Shenqi Xie,§ Enpu Kang,† Takafumi Uemura,∥ Jürgen Brugger,§ Jan Genoe,†,‡ Paul Heremans,†,‡ and Cédric Rolin† †

imec, Large Area Electronics, Kapeldreef 75, 3001 Leuven, Belgium KU Leuven, Department of Electrical Engineering, Kasteelpark Arenberg 10, 3001 Leuven, Belgium § EPFL, Microsystems Laboratory, CH-1015 Lausanne, Switzerland ∥ The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, 567-0047 Osaka, Japan ‡

ABSTRACT: To grow small molecule semiconductor thin films with domain size larger than modern-day device sizes, we evaporate the material through a dense array of small apertures, called a stencil nanosieve. The aperture size of 0.5 μm results in low nucleation density, whereas the aperture-to-aperture distance of 0.5 μm provides sufficient crosstalk between neighboring apertures through the diffusion of adsorbed molecules. By integrating the nanosieve in the channel area of a thin-film transistor mask, we show a route for patterning both the organic semiconductor and the metal contacts of thin-film transistors using one mask only and without mask realignment. KEYWORDS: organic semiconductors, C10-DNTT, stencil evaporation, diffusion, organic thin-film transistors hin films of organic semiconductors are intensively investigated for their potential application as the active component of organic thin-film transistors (OTFT).1−7 Because of the detrimental effect of grain boundaries on charge transport, the electrical performance of polycrystalline organic thin films increases with the crystalline domain size.8−13 For vacuum sublimated organic thin films, the domain size is directly linked to grain nucleation in the early stages of film growth.14 Therefore, strategies that limit nucleation density during vacuum film growth are desirable to obtain large crystalline domains. To this end, we have demonstrated the use of stencil masks with microfabricated apertures to reduce and localize nucleation sites on the substrate during organic film growth.15 This approach requires that the aperture size is close to the surface diffusion length of adsorbed molecules, that is, in the micrometer range. This poses a severe constraint on the lateral dimension of the film formed through a single aperture. In this work, we show how a continuous film of the technologically relevant organic semiconductor 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) can be grown through an array of apertures, called stencil nanosieve, thanks to surface diffusion of adsorbed molecules. The film has reduced nucleation density and presents a large domain size. Furthermore, as metal surface diffusion on top of the organic film is limited, we fabricate bottom gate top contact OTFTs using a single mask only, which integrates the stencil nanosieve. In this way both the active organic layer and metal contacts are vacuum evaporated through the same mask. The

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© XXXX American Chemical Society

mask position remains unchanged between the two steps so these can be performed directly one after the other. As substrates, we use highly doped Si wafers covered with 120 nm of thermally grown SiO2. In OTFTs the doped Si acts as a common bottom gate and the SiO2 as a gate dielectric. The substrates are sonicated in acetone for 10 min, rinsed in isopropyl alcohol (IPA) and then exposed to a UV/ozone treatment for 15 min. The self-assembled monolayer of octadecyltrichlorosilane (ODTS) is then applied by placing the substrate in a vacuum oven at 140 °C for 1 h in the presence of 50 μL of liquid ODTS. Next, the stencil mask is mounted on the surface of the substrate using Kapton tape and the assembly is placed in the vacuum chamber for organic semiconductor sublimation. A 30 nm-thick film of C10-DNTT is sublimated in high vacuum (1 × 10−8 Torr) at substrate temperature of 80 °C and deposition rate of 0.1 Å/s using the in-house molecular beam epitaxy (MBE) tool. Finally, for contact fabrication, 50 nm of Ag is evaporated on top of the C10-DNTT film through the same mask, without realignment of the substrate/mask assembly. The Ag deposition is done in high vacuum (1 × 10−7 Torr) with the substrate at room temperature and deposition rate of 1 Å/s using the Kurt J. Lesker Super Spectros tool. Regarding stencil mask fabrication, patterns are defined by lithography and reactive ion etching in a 200 nm-thick Si rich low stress SiN layer that is deposited by Received: May 10, 2017 Accepted: July 5, 2017 Published: July 5, 2017 A

DOI: 10.1021/acsami.7b06584 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Differential interference contrast (DIC) microscopy and atomic force microscopy (AFM) images of 30 nm thick C10-DNTT films deposited in one run on the same ODTS substrate through the apertures of (a, c) a = 0.5 μm and (b, d) a = 5 μm. The aperture-to-aperture distance is (a, b) d = 5 μm and (c, d) d = 0.5 μm. The deposition parameters of C10-DNTT are Tsub = 80 °C, rdep = 0.1 Å/s.

Figure 2. (a) DIC and (b) AFM images taken from the same area of the 30 nm thick C10-DNTT film deposited through the nanosieve with the apertures of a = 0.5 μm and the aperture-to-aperture distance of d = 0.5 μm. The deposition parameters of C10-DNTT are Tsub = 80 °C, rdep = 0.1 Å/ s. (c) Top view on the ab plane of C10-DNTT crystal and corresponding orientation of fast n1 and slow n2 axes. The alkyl chains of C10-DNTT are not displayed for clarity.

trigger nucleation and no film growth is observed.15 The few grains observed in Figure 1a may be the result of heterogeneous nucleation on surface defects. On the other hand, when considering large sparse apertures (a = 5 μm, d = 5 μm, Figure 1b), sufficient supersaturation is reached within the aperture, giving rise to multiple nucleation events. In this case impinging molecules are rapidly added to the growing crystallites and only molecules adsorbed along the perimeter of the aperture may diffuse underneath the mask.15 The Stranski−Krastanov film growth observed in the exposed regions is typical for C10DNTT.18 It consists of two-dimensional layered growth up to 5 monolayers (∼20 nm-thick) covered by several μm-long needles and ∼100 nm-thick. Polycrystalline square islands are clearly visible in Figure 1b and separated by regions with no nucleation. When the aperture array becomes dense and d is comparable to the surface diffusion length of adsorbed molecules, we observe that molecules can diffuse all the way to neighboring apertures. In case of deposition through a nanosieve with small dense apertures (a = 0.5 μm, d = 0.5 μm, Figure 1c), this crossaperture diffusion process permits a sufficient buildup of molecular concentration to reach supersaturation and trigger multiple nucleation events. After nucleation under one of the apertures, the diffusion flux from the neighboring apertures sustains film growth in the lateral direction. The obtained film is continuous with a two-dimensional layered growth at the bottom and long needles on top. As can be seen in the DIC image in Figure 1c, grains are large (up to 10 μm in diameter), encompass many apertures and contain long parallel needles on

low pressure chemical vapor deposition (LPCVD) on a prime grade double side polished Si wafer. The Si wafer is then backetched under the apertures to release the SiN stencil mask. This method can reliably produce square apertures with side length and interdistance down to 0.2 μm. More details about stencil fabrication can be found elsewhere.16 Because of the inherent curvature of the Si chip that contains the SiN stencil masks and of the SiO2 substrate used here, the placement of the mask on the substrate leaves a gap of possibly up to a few micrometers between the substrate surface and the mask. During sublimation of organic molecules through the apertures, this gap allows for the lateral diffusion of adsorbed molecules in the zone underneath the stencil mask. The gap size, however, is critical and should be kept as small as possible because larger gaps increase the feature size of deposited films due to geometrical blurring.17 We have previously described how below a certain aperture size this surface diffusion becomes a dominant factor in the growth of pentacene on SiO2.15 We observe a similar effect when evaporating C10-DNTT on ODTS-treated SiO2 through a stencil mask. Differential interference contrast (DIC) microscopy and atomic force microscopy (AFM) images show the evolution of the organic semiconductor film morphology as a function of the aperture size a as well as the aperture-to-aperture distance d (Figure 1). In the case of small sparse apertures (a = 0.5 μm, d = 5 μm, Figure 1a), C10-DNTT molecules readily diffuse away from the aperture upon adsorption. When neighboring apertures are situated farther than the surface diffusion length, the surface concentration of C10-DNTT molecules remains too low to B


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ACS Applied Materials & Interfaces

Figure 3. (a) Scanning electron microscopy (SEM) image of the OTFT stencil mask. The inset shows the stencil nanosieve with square apertures of a = 0.5 μm and the aperture-to-aperture distance of d = 0.5 μm that is integrated in the beam of the channel area. (b) DIC image of the 30 nm-thick C10-DNTT film deposited through the stencil nanosieve. The deposition parameters of C10-DNTT are Tsub = 80 °C, rdep = 0.1 Å/s. (c) The DIC image of the 50 nm thick Ag film deposited on top of C10-DNTT without removing the stencil mask. The deposition parameters of Ag are Tsub = RT, rdep = 1 Å/s. (d) Layer schematics of the fabricated OTFT. (e) Transfer (saturation) and (f) output characteristics of the OTFT. The OTFT dimensions are W = 200 μm, L = 10 μm and the SiO2 thickness is tox = 123 nm.

fabrication. In the mask design, two open square areas of 200 × 200 μm define the OTFT contacts and a 10 μm-wide beam defines the transistor channel separating the two contacts (see Figure 3a). The beam area will in essence be shadowed for metal evaporation when source and drain contacts are evaporated. The beam is perforated with a stencil nanosieve. Now when evaporating a C10-DNTT film, the material grows not only in the contact area, through the large contact openings, but also in the channel area, through the nanosieve. The film is thinner in the channel area than in the contacts because of the difference in net flux per unit area. Therefore, it appears less bright in the DIC image in Figure 3b. Then Ag is evaporated without realignment of the shadow mask. It is observed that in the channel area, the Ag evaporated through the stencil nanosieve does not diffuse on top of the rough C10DNTT. Therefore, Ag forms disconnected islands in the transistor channel. In contrast, a continuous Ag film is formed in the contact region (see Figure 3c). Despite the Ag islands in the channel area (see Figure 3d), the obtained device works as a regular OTFT. The transfer curve in the saturation regime (see Figure 3e) reveals an onset close to 0 V, a low off current in the fA range, a high on/off ratio of 107 and a limited hysteresis. An apparent mobility of μsat,app = 0.46 cm2/(V s) and a threshold voltage of VT = −4.4 V are conventionally extracted from a linear fit of the evolution of the square root of the channel current IDS with gate voltage VGS in Figure 3e.23 The output curve in Figure 3f shows a good saturation but a nonlinearity in the low VDS range that is a sign of contact resistance24 provoked by the important mismatch between Ag work function of 4.35 eV25 and C10-DNTT HOMO level of −5.38 eV.3 High contact resistance Rc strongly affects short channel devices up to a point where the gradual channel approximation model is no longer applicable.26 We therefore analyzed the output curves in Figure 3f using a model proposed by Torricelli et al. that decouples contact and channel characteristics.27 This analysis confirms the high contact resistance (RcW = 25 kΩ cm at VGS = −25 V) and yields μsat

top. Cross-aperture diffusion also affects the case of large dense apertures (a = 5 μm, d = 0.5 μm, Figure 1d) but to a lesser degree. In this case, since most adsorbed molecules are consumed by nucleation and growth within the aperture, only peripheral molecules may diffuse outwards. Multiple bridges form between neighboring islands but some deep ridges are also apparent in the frontier regions that break the continuity of the film. From the four cases analyzed in Figure 1, the film with the largest crystalline domain size is obtained from the stencil nanosieve with the dense array of small apertures. To further assess the crystallinity of this film, we compare the DIC and AFM images of the same area (see Figure 2a, b, respectively). We have shown previously how fast and slow axes can be deduced from DIC images.19 We apply this analysis to Figure 2a where two domains with different crystal orientations are marked with cyan and magenta dashed lines. Because the image was taken in the negative DIC bias, the slow axis n2 of the bright domain is parallel to the shear axis of the Nomarski prism while the slow axis n2 of the dark domain is perpendicular to it. The slow n2 axis of C10-DNTT is also aligned with its crystal axis a because the transition dipole moment for the lowest optical transition (S0 → S1) has the largest projection along this direction in the herringbone crystal structure (see Figure 3c).15,19−21 Strikingly, the AFM image of the same area shows that both cyan and magenta grains display a group of parallel needles that are aligned along the crystal direction [100], extend over many apertures and stop precisely at the grain boundaries (see Figure 3b). We speculate that the correlation between needles and crystal orientation originates from an epitaxial relationship.22 In consequence, these needles, that are easily observable in AFM images, act as markers for grain size and in-plane crystal orientation. They show, for example, that the islands obtained in the large apertures (a = 5 μm) are polycrystalline (see the AFM images in Figure 1b, d). With its large grains of 5 × 5 μm, the continuous C10-DNTT film grown through the stencil nanosieve is the most interesting for charge transport. Therefore, we proceed with OTFT C


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ACS Applied Materials & Interfaces = 0.56 cm2/(V s) and VT = 2.5 V that are characteristics of the C10-DNTT thin film in the channel. The mobility of our C10-DNTT films is much lower than the μsat ≈ 10 cm2/(V s) previously reported in thin films of the same material.26 The transfer characteristics of the short channel device measured here (L = 10 μm) are heavily affected by high contact resistance throughout the whole VDS range. In this case, the analysis method used here cannot efficiently decouple channel from contact effects and yields lower estimates for both μsat and RcW. Better decoupling would be obtained with the transfer length method, but it requires several devices with different channel lengths which were not accessible to us. Furthermore, using a better contact material would improve the device characteristics. Au is usually a preferred contact metal for C10-DNTT as its work function of 5.38 eV25 matches the C10-DNTT HOMO level. Experimentally, however, Au evaporation (at Tevap ≈ 1400 °C) yields a heat load on the substrate that is heavier than for Ag evaporation (at Tevap ≈ 1100 °C). As the samples with stencil masks could not be actively cooled in our evaporation chamber, the heat load during Au evaporation led to cracks in C10-DNTT films that destroyed devices. Throughout this analysis, we have ignored the presence of the Ag islands in the channel. The transfer characteristics display very limited hysteresis, an onset close to 0 V and a reasonable subthreshold slope. In consequence, the devices do not suffer from charge trapping,28 showing that Ag island either fully participate or do not participate at all in current flow. Considering the large energy level mismatch between Ag and the semiconductor, we speculate the latter: that Ag essentially remains isolated during current flow. Determination of the exact role of Ag islands is left to further work. In conclusion, micropatterns of continuous thin films of the organic semiconductor C10-DNTT can be grown through a stencil nanosieve, that is a shadow mask with a dense array of small apertures. The use of the nanosieve limits nucleation density, which augments crystalline domain size up to 5 μm. This growth is enabled when the stencil aperture size and aperture interdistance become smaller than the surface diffusion length of adsorbed organic molecules. Furthermore, we fabricated functional OTFTs using a single mask process that integrates a nanosieve in the transistor channel. Because of suppressed diffusion of the metal atoms on the C10-DNTT film, its deposition results in the isolated islands in the channel area and can be performed right after the deposition of the organic layer without mask realignment.

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the staff of the Center of Micro/Nanotechnology (CMI) EPFL for the valuable discussions and support. Part of this research was funded by the European Commission (FP7-ICT-2011-7, NANO-VISTA, under Grant Agreement 288263).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +32 16 28 33 53. ORCID

Pavlo Fesenko: 0000-0002-6917-6173 Valentin Flauraud: 0000-0002-1393-3198 Jürgen Brugger: 0000-0002-7710-5930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n°320680 (EPOS CRYSTALLI). The authors thank D


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Growth Of Organic Semiconductor Thin Films with Multi-Micron

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