Water permeation pathways in laminated organic single-crystal devices

Water permeation pathways in electronic devices should be eliminated for the suppression of operational instabilities. We investigated possible pathways in field-effect transistors based on a laminated single crystal (SC) of an organic semiconductor, rubrene. Water-induced instabilities were found to be more obvious with a thicker rubrene SC. Furthermore, under our simulation conditions, molecular dynamics calculations of water diffusion on a rubrene SC showed that no water molecules penetrated the SC. These findings indicate that a space at the SC/substrate interface is a dominant pathway. The present study clearly shows the importance of conformality of SC lamination onto the underlying substrate.


INTRODUCTION
Stable operation in air is an indispensable requirement for most electronic devices.
Instabilities are often induced by adsorption and diffusion of air molecules, such as oxygen and water. The effect of the former molecule is rather simple: it significantly degrades the device characteristics if easily oxidizable components are used in the device structure. The latter molecule possesses a permanent electric dipole, and thus, various mechanisms have been proposed to explain the effect of water molecules on the operation of electronic devices, such as field-effect transistors (FETs). The mechanisms include diffusion of water-related species into the gate insulator of FETs, 1-3 water-induced polarization of the gate insulator, 4,5 and charge carrier trapping at or near the channel formed at the semiconductor/insulator interface; 6-10 all of which are related to the gate insulator and the interface with the gate insulator. These gate-related phenomena are induced after diffusion and penetration of water into the semiconductor layer, and hence, they should be significant in loosely packed semiconductors.
Organic semiconductors are now regarded as a channel material that is promising for constructing large-area, flexible electronic devices. [11][12][13] However, inter-molecular interactions are mainly attributed to weak van der Waals forces, from which it is expected that penetration of small molecules from the air into the semiconductor layer occurs rather easily. Therefore, the effect of the water-related instabilities is expected to be significant in organic electronic devices. In the case of organic FETs (OFETs), various mechanisms of their operational instabilities have been extensively studied. The mechanisms have been mainly attributed to the gate insulator and the interface with the gate insulator, as exemplified above. 14 One of the authors (Nouchi) recently proposed a mechanism related to the source/drain contacts: 15 water molecules reach the source/drain electrode surfaces and 3 induce a change in electrode work function in response to the applied drain bias, which was evidenced by switching from symmetric to rectified current-voltage characteristics in twoterminal devices (even devices with symmetrically fabricated electrodes). This electroderelated mechanism also requires permeation of water molecules into the device structure.
In order to suppress such water-induced instabilities, water permeation pathways in OFETs should be determined for proper elimination of the instabilities. It is generally accepted that major pathways for water permeation through any substance are defects or pinholes within the substance. [16][17][18] Such defects are universally present in polycrystalline films as grain boundaries. Thus, it is natural to say that single crystalline films are a superior barrier against water permeation than their polycrystalline counterparts. Since grain boundaries act as a scattering/trapping center for charge carrier transport, [19][20][21] single crystals (SCs) are also desired from the viewpoint of OFET performance. In this study, we fabricated bottom-contact, bottom-gate FETs with a SC of an archetypal organic semiconductor, rubrene, as schematically shown in Fig. 1(a). Thus, among various permeation pathways depicted in Fig. 1(b), defects and grain boundaries can be excluded as the possible pathway.
In this device configuration, SCs are usually laminated by manually picking up a SC and placing it onto a target substrate with pre-defined electrodes, which can avoid formation of ill-defined contacts, as is problematic in top-contact devices. The interaction between the SC and the underlying substrate is categorized as a weak van der Waals interaction, which is the same as the inter-molecular interaction within the SC. Thus, two possible pathways for water permeation would be inter-molecular spacings in the SC and a space between the SC and the underlying substrate. We characterized SCFETs with different SC thicknesses and found that the SC/substrate interface is a dominant pathway for water permeation into laminated SC devices. Molecular dynamics (MD) simulations were also conducted on the diffusion of This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912 4 water molecules placed near a rubrene SC and water permeation through the inter-molecular spacings was found to be unlikely to occur. These findings indicate the importance of conformality of the SC lamination. This information should be general knowledge for analyses of the device characteristics based on semiconductor channels laminated by weak van der Waals forces.

EXPERIMENTAL
A highly doped Si wafer with a 300 nm-thick thermal oxide layer was used as a substrate. The highly doped Si wafer and the thermal oxide layer serve as the gate electrode and the gate dielectric, respectively. The substrate was cleaned with acetone and 2-propanol using an ultrasonic bath, followed by an oxygen plasma treatment. The surface OH groups 22 on the cleaned substrate were then passivated by the treatment with 1,1,1,3,3,3-This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.  23 For the HMDS treatment, the cleaned Si substrate was immersed overnight in a hexane solution of HMDS, with the volume ratio of HMDS to hexane of 1/9. After removal from the solution, the substrate was cleaned by pure hexane in an ultrasonic bath in order to detach physisorbed multilayers of HMDS molecules. A surface profiler (Dektak150, Veeco) was used to determine the typical thickness of the fluoropolymer coating to be ca. 100 nm. After the passivation procedure, Au electrodes with a thickness of 15 nm were formed by thermal evaporation together with a Cr adhesion layer.

PLEASE CITE THIS ARTICLE AS
The electrodes were patterned by a metal shadow mask to form a 50 µm-long channel. For the semiconductor layer, a rubrene SC was synthesized by the physical vapor transport (PVT) of the source powder (≥ 98%, Sigma-Aldrich). The growth process was repeated three times in order to improve the purity of the crystal. Finally, the single crystal was manually laminated onto the pre-defined Au electrodes. 24,25 A molecularly flat surface of the rubrene crystals was confirmed by an atomic force microscope investigation (Hitachi High-Technologies, AFM5200S), from which we infer that the rubrene crystals are singlecrystalline (not polycrystalline). An optical micrograph of a FET fabricated in this study is shown in Figure 2(a). Electrical measurements were conducted using a semiconductor device analyzer (B1500A, Keysight) in ambient air at room temperature.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

RESULTS AND DISCUSSION
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912
7 instabilities that accompany an increase in |ID|. In our previous study based on short-channel two-terminal devices with an Au-rubrene-Au configuration, an electrical current was found to increase only in the presence of water when an external voltage was applied to the device. 15 The increase in electrical current was explained by an increase in the work function of the anode, which lowered the Fermi level and enhanced hole injection into the valence band derived by the highest occupied molecular orbital (HOMO) of rubrene. 15 Therefore, the counterclockwise-type hysteresis observed in Fig. 2(a) is ascribable to the electroderelated instability that can occur only in the presence of water.
The electrode-related instability discussed here has been understood as a change in the charge-injection barrier height at the electrode contact. 15 The barrier height is known to affect the shape of the low-voltage region in the output characteristics of FETs. A low barrier results in a linear ID-VD curve in the low-VD region, and a differential conductance (dID/dVD) of this region becomes constant in an ideal case. On the other hand, a high barrier leads to a super-linear curve that is generally explained by thermionic injection, and a differential conductance exhibits an increase as VD increases. Figure 2(d) shows the VD dependence of the differential conductance of the device shown in Figs. 2(a-c). As clearly shown in these figures, the differential conductance shows an increasing trend during the forward sweep of VD, and a linear portion is widened during the backward sweep. These results suggest a lower injection barrier during the backward sweep than the forward one, which supports a manifestation of the electrode-related instability in these devices.
To the contrary, the transfer characteristics in Figure 2(c) show a commonly observed clockwise-type hysteresis, where the magnitude of the ID in the backward sweep is lower than in the forward sweep of the bias voltage. This indicates that the SCFET has instabilities that accompany a decrease in |ID|, which is opposite to what was observed in the output This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. characteristics. This type of hysteresis is typical of most of gate-related instabilities. The discrepancy in the type of observed hysteresis between output and transfer characteristics has been reported in OFETs with source/drain electrodes whose surfaces were modified with low-density self-assembled monolayers of thiol molecules. 26 For the measurement of output characteristics, first VG is set and then VD is swept. The |ID| decrease caused by the gaterelated instabilities is expected to mostly finish during the forward sweep of VD. Thus, in the backward sweep, the |ID| increase by the electrode-related instabilities can be larger than the |ID| decrease by the gate-related ones. Since the observed hysteresis can be understood as a summation of the electrode and gate-related instabilities, in this case, the counterclockwisetype hysteresis is observed. Therefore, instabilities caused by application of VD, i.e., electrode-related instabilities, dominate the output characteristics, Fig. 2(b), and the opposite holds for the transfer characteristics, Fig. 2

(c).
Among various gate-related instabilities, slow polarization of water molecules embedded in the gate dielectric can also cause a counterclockwise-type hysteresis. 27 Although the slow polarization mechanism is one of the gate-related instabilities, a counterclockwise-type hysteresis was observed only in output characteristics. Therefore, we concluded that the slow polarization mechanism was not significant in our devices.
We discuss here which instabilities, the |ID| increase or decrease, should be employed as a probe of water permeation into the device. The electrode-related |ID| increase in twoterminal Au-rubrene-Au devices has been confirmed to be detected only in the presence of water in the surrounding environment. 15 On the other hand, a variety of mechanisms have been developed for the gate-related |ID| decrease, some of which are indeed known to be enhanced by water molecules. However, mechanisms that are not related to water (e.g., trap formation by the bias stress, etc.) 28 should also contribute to the |ID| decrease. Furthermore, This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912
9 the hydrophobic nature of the passivated surfaces significantly reduces the water-related instabilities at the interface between the gate insulator and the semiconductor layer. From the considerations above, we concluded that the |ID|-increasing hysteresis caused by the electrode-related instabilities could be used as a probe of the water permeation into the device structure.
A water permeation rate via inter-molecular spacings is dependent on the thickness of SCs. Thus, a SC much thinner than that used in Fig. 2(a)  them. It should be noted that the hysteresis window in Fig. 2(g) is significantly wider than that in Fig. 2(c). Since the observed hysteresis is a summation of the electrode and gaterelated instabilities, even the clockwise-type hysteresis, which is dominated by the gaterelated instabilities, is contributed to by the electrode-induced instabilities. Therefore, the wider window of the |ID|-decreasing (clockwise-type) hysteresis in Fig. 2(g) is understood by a smaller contribution from the |ID|-increasing instabilities caused by the electrode-related mechanism. These experimental facts indicate that water permeation becomes more inefficient in a thinner SC. The lower permeation efficiency with the thinner SC indicates that inter-molecular spacings are not a dominant pathway for water permeation.
It should be noted that the FET characteristics shown in Fig. 2

display accidental ID
jumps. This anomaly is a sign of the presence of an incompletely laminated portion within the SC channel. Accidental attachment/detachment of the SC to/from the substrate might be induced by the gate electric field, leading to the current jumps. We frequently experienced that the lamination process became difficult after passivating the SiO2 surface with HMDS.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. The lamination of a SC is known to become more conformal to the underlying surface as the SC becomes thinner, which is ascribable to the enhanced flexibility of thinner crystals. 29 Thus, the SC with a thickness of 4 μm in Figs. 2(a-d) is more rigid than that with a thickness of 0.35 μm in Figs. 2(e-g). The incompleteness of the lamination provides another permeation pathway, i.e., a space between the SC and the underlying substrate. The importance of the SC/substrate interface is further corroborated by comparing FET characteristics with and without bubbles unintentionally formed during the SC lamination process. Figure 3(a) shows an optical micrograph of a SCFET with such bubbles. Its output and transfer characteristics are shown in Figs. 3(b) and 3(c), which display a counterclockwise and clockwise-type hysteresis, respectively. A SCFET with a similar SC thickness but without such bubbles, Fig. 3(d), was also tested. Its output and transfer characteristics in Figs. 3(e) and 3(f) display the same hysteresis type as those in Figs. 3(b) and 3(c), respectively. However, the SCFET without the bubbles show a smaller counterclockwise-type hysteresis in the output characteristics and a larger clockwise-type hysteresis in the transfer characteristics than that with the bubbles. Thus, the |ID|-increasing type hysteresis caused by the electrode-related instabilities is more significant in the SCFET with the bubbles than in that without the bubbles. This fact also indicates the importance of the SC/substrate interface as a water permeation pathway.   Fig. 4(a) is the as-measured ID; that of Fig. 4(b) is the ratio of ID during the backward (IDb) and forward sweep (IDf) of VD. The ID-VD characteristics show a clockwise-type hysteresis, leading to IDb/IDf being less This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912
than 1. This result is attributable to a significant gate-bias stress exerted by a large amount of hydroxyl groups on the bare SiO2 surface. Nevertheless, although its behavior is complicated and not monotonic in the low-VD region, the general trend is that a higher RH value led to a higher IDb/IDf ratio. Since the observed hysteresis is a summation of the currentdecreasing and current-increasing hystereses, the fact that the IDb/IDf ratio becomes higher for the higher RH is the confirmation that the current-increasing hysteresis becomes more dominant for higher RH. In order to provide further confirmation, we conducted MD simulations on the diffusion of water molecules placed near a rubrene SC. To mimic water permeation towards the electrode/SC interface, water molecules were placed on a rubrene/Au(100)/amorphous-SiO2 stack. Prior to the MD simulations, the partial charge of rubrene and water molecules was determined by Gaussian 09 with the Hartree-Fock method using 6-31G(d) basis sets. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912
13 The same system was utilized for the structural optimization of a water molecule. For the structure of a rubrene molecule, we employed a low-temperature structure reported with a rubrene SC grown by the PVT method, 30   water molecules were found to show a (partially) wetting behavior on rubrene. Importantly, no penetration of water molecules into the rubrene SC was observed within the MD simulations, which suggests that inter-molecular spacings are not an efficient pathway for water permeation. It should be noted here that we employed the rigid model and thus the This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI:10.1063/5.0009912
14 intra-molecular degree of freedom was not allowed during the MD simulation. This was because inclusion of the intra-molecular motion resulted in disruption of the rubrene SC structure. There remains a possibility that proper inclusion of the intra-molecular motion leads to water permeation into the rubrene SC to some extent. However, considering the relatively wide dispersion of the HOMO-derived band of a rubrene SC, 34 it is reasonable to conclude that the inter-molecular interaction is strong enough to hinder the water permeation. In the present study, the total thickness of the electrodes was as small as 15 nm. The percolation threshold thickness for Au films on insulating substrates was determined to be around 7 nm. 37 Considering the evolution of the film structure from island, to mesh, to a continuous film, the thickness required to obtain a continuous electrode film can be estimated to be ~10 nm. Since the thickness of the Au electrodes in the present study is only ~5 nm above the required thickness for continuous films, the degree of the protrusion was minimized in the present devices. Even with such thin electrodes, formation of the space was found to be significant for SCFETs with a thick SC, as shown in Figs. 2(a-c). Ultrathin conductors 38 and embedded electrode structures 39 would be efficient for elimination of the space around the electrode edges.

CONCLUSION
In conclusion, we have investigated a water permeation pathway in laminated organic SC devices by utilizing the electrode-related instabilities as a probe of water permeation. By employing a rubrene SC as a channel of organic FETs, defects and grain boundaries were excluded as a possible pathway. The electrode-related instabilities, which are observable only in the presence of water in the surrounding environment, were found to be suppressed with thinner SCs. Thus, inter-molecular spacings can also be excluded as a possible pathway because the permeation via inter-molecular spacings should be more efficient with a thinner This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

DATA AVAILAVILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.