Photocurrent spectroscopy of ion-implanted organic thin film transistors

Photocurrent spectroscopy of ion-implanted organic thin film transistors

Synthetic Metals 161 (2012) 2585–2588 Contents lists available at SciVerse ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/...

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Synthetic Metals 161 (2012) 2585–2588

Contents lists available at SciVerse ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Photocurrent spectroscopy of ion-implanted organic thin film transistors B. Fraboni a,∗ , A. Scidà a , A. Cavallini a , S. Milita b , P. Cosseddu c,d , A. Bonfiglio c,d , Y. Wang e , M. Nastasi e a

Dipartimento di Fisica, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy CNR-IMM, Via P. Gobetti 101, 40129 Bologna, Italy c Dipartimento di Ingegneria Elettrica ed Elettronica, Università di Cagliari, piazza d’Armi, 09123 Cagliari, Italy d CNR-IMM S3 via Campi 213/a, 41100 Modena, Italy e Los Alamos National Laboratory, MS-K771, Los Alamos, NM 87545, USA b

a r t i c l e

i n f o

Article history: Received 13 May 2011 Received in revised form 22 August 2011 Accepted 2 September 2011 Available online 1 October 2011 Keywords: Organic thin film transistor Density of electronic states distribution Ion implantation

a b s t r a c t In this paper we investigate the distribution of the electrically available states near the band-edge in pentacene thin films of different thicknesses, aiming to the identification of the active thickness of pentacene layers in fully operational devices such as organic thin film transistors (OTFTs). The film structure has been studied by X-ray diffraction technique, while their relative electronic density of states distribution (DOS) around the band-edge has been investigated by photocurrent (PC) spectroscopy analyses. The effects of ion implantation on OTFTs have been investigated by PC analyses of OTFTs implanted with N+ ions of different energy and doses. We show how PC spectroscopy has the remarkable ability to detect modifications of the DOS distribution in a non invasive way, thus allowing the direct study of the active semiconductor film in fully operational OTFTs. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The determination and the understanding of the charge transport properties and the energy distribution of the electronic states in organic thin films are important for further improving the performance of organic electronic devices such as thin film transistors (OTFT). However, these issues are still not unequivocally resolved [1–4], and there is an unanimous agreement on the extreme importance of investigating the correlation between the molecular structure of the active layer and the carrier transport properties [5,6]. The experimental method employed to assess the performance of organic devices should allow one to characterize the thin film active layer structure without affecting or degrading the device functionality and, among them, photocurrent spectroscopy (PC) has proven to be a valuable and non-invasive tool for the study of the density of states distribution (DOS) of organic thin films [7–11]. Pentacene, widely used as the active layer of OTFTs thanks to its remarkable transport properties, is characterized by the coexistence of two phases: the “thin-film” phase, characterized by a d(0 0 1) spacing of 15.5 A˚ and the “bulk” phase, characterized ˚ the former being dominant for by d(0 0 1) a spacing of 14.5 A, thicknesses below 50 nm and the latter for films over 150 nm thick [12,13]. The Davydov splitting of the first absorption band,

∗ Corresponding author. Tel.: +39 0512095806; fax: +39 0512095113. E-mail address: [email protected] (B. Fraboni). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.09.017

correlated to modifications in the molecular structure and packing of the film, has been shown to increase when passing from the “thin film” to the “bulk” phase [5,9,14]. This phenomenon can be exploited to identify the range of thickness of pentacene thin films. Moreover, PC analyses have been successfully carried out on fully operational OTFTs, assessing how they provide a non-destructive and direct tool to study molecular packing modifications in the pentacene layer of OTFTs [9]. Previously reported results on ion irradiation of polymers, showed that a controlled damage induced by the implanted ions can determine strong chemical and structural modifications of the material, altering its mechanical and electrical properties [15,16]. However, the effects of ion implantation have never been investigated neither on small molecule organic semiconductors, such as pentacene, nor on fully operating organic devices. In this work, we report on the experimental characterization of the DOS distribution in pentacene thin films of different thicknesses (50 nm and 300 nm) by PC spectroscopy. Their structural properties, studied by X-ray diffraction analyses, allowed us to correlate the dominant film phase to the DOS distribution and to identify the effective range of thickness of the pentacene thin film. In addition, we characterized the effects of ion implantation on pentacene OTFTs by PC analyses of devices implanted with N ions of different energies and doses. Despite the strong molecular structure modifications, we have observed that a controlled damage depth distribution preserves the functionality of the device while reducing the effective thickness of the pentacene active layer.

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Fig. 1. (a) Schematic layout of the pentacene organic thin film transistor (OTFT), (b) layout of the on implantation process of pentacene thin films.

2. Experimental Pentacene films of thickness of 50 nm and of 300 nm, were deposited on SiO2 /Si substrates to study by means of photocurrent and X-ray diffraction (XRD) analyses the relative weight of the “thin-film” and “bulk” phase. Pentacene OTFTs were fabricated by thermally depositing pentacene in vacuo on highly conductive silicon substrate (acting as gate electrode) with a 500 nm thermally grown silicon dioxide layer, which acts as gate insulator (Fig. 1a). The film thickness was monitored by a quartz crystal microbalance. All devices have been realized using a bottom contact configuration with gold source and drain electrodes (W = 5 mm, L = 50 ␮m, being W and L channel width and length, respectively). For all devices, both mobility and threshold voltage were determined from the transfer characteristics in the saturation regime. Since the hysteresis was negligible for all devices, the device parameters are reported for the forward gate voltage sweep. OTFTs with a 300 nm thick pentacene layer have been implanted with N+ ions with energies varying from 25 keV to 55 keV. The beam energy values have been chosen, following TRIM simulations [17], in order to limit the damage to the top portion of the active layer (Fig. 1b). Ion implantation was performed at room temperature with N ions in various fluencies on Varian CF-3000 ion implanter at Ion Beam Materials Laboratory in Los Alamos national Laboratory. Monte Carlo SRIM code was used to estimate ion beam parameters that would produce the required damage thickness and ion profile in the pentacene. For example, under 25 keV N ion bombardment, the damaged layer thickness in the pentacene is below 100 nm. PC spectroscopy analyses were carried out in the common mode planar configuration both on pentacene thin films and OTFTs, in air and at room temperature with a 150 W QTH lamp coupled to a SPEX monochromator and mechanically chopped at low frequency (<20 Hz), using a current amplifier connected to a digital lock-in amplifier (Stanford Research 850). The energy resolution was 0.01–2.40 eV. The photon flux was measured with a calibrated Si photodiode. The analyses were carried out under low-injection conditions (1 × 1013 photons/cm2 at  = 450 nm) and no variations were induced in the PC spectra by the incident photon beam, as assessed by comparing consecutively acquired spectra. During the PC spectra acquisition, the OTFT gate was grounded while a V = 5 V bias was applied between the source and drain contacts. Symmetrical XRD measurements (–2 scans) were carried out with a diffractometer equipped by a rotating anode source

(SmartLab-Rigaku). A focus line X-ray beam (Cu K␣) was collimated by a parabolic graded multilayer mirror placed in front of the sample and a double slits were mounted before the detector to achieve the required angular resolution. 3. Results and discussion The typical electrical transport parameters, namely charge carrier mobility and threshold voltage of typical pentacene OTFTs with 50 nm and 300 nm thick pentacene layer were 300 = 2 × 10−2 V/cm2 s and VT300 = −12 V and 50 = 3 × 10−2 V/cm2 s and VT50 = −16 V, respectively. The PC spectra of pentacene layers of different thicknesses (50 nm and 300 nm) are quite different, as shown in Fig. 2. The differences in the shape of the two PC spectra can be associated to the different structural phases present in the film: in the 50 nm thick film, the “thin-film” phase is supposed to dominate, while the “bulk” phase should prevail in the 300 nm thick one. To better assess such statement we have carried out XRD analyses, reported in Fig. 3, that clearly support this hypothesis. PC spectra of 300 nm (50 nm) show several major bands, located at about 1.81 eV (1.88 eV), 1.96 eV (1.97 eV), 2.06 eV (2.12 eV) and 2.26 eV, that have been associated to inter- and intra-molecular excitons,

Fig. 2. Photocurrent spectra of pentacene thin films of two different thicknesses: 50 nm (solid line) and 300 nm (dashed line).

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Fig. 3. X-ray diffraction curves of 50 nm (solid line) and 300 nm (dashed line) thick pentacene thin films.

even if their attribution is still widely debated [7,10,13–16]. We will focus our attention on the first two lower energy peaks (marked by dotted lines in Fig. 2), that constitute the first absorption band. While amorphous pentacene shows a thickness-independent single optical absorption band at about 1.87 eV, in crystalline samples this band split into two peaks, located at about 1.84 eV and 1.96 eV, respectively [13,14]. Such splitting, associated to the changes in the internal film structure induced by its growing thickness, i.e. by the shift from the dominant role of the “thin-film” phase to the “bulk” phase, was explained as Davydov splitting [14] and was confirmed by micro-Raman spectroscopy analyses of pentacene OTFTs that undergo a structural modification after a long-term bias stress [5]. We have assessed that PC spectra of the here studied pentacene OTFTs provide the same information as optical absorption spectroscopy, the method usually employed to study DOS distributions [9]. Our PC results (Fig. 2) clearly show how the Davydov splitting occurring in the first absorption band is enhanced with increasing thickness of the pentacene layer, from 50 nm to 300 nm. The energy separation between the first two peaks varies from 0.10 eV to 0.14 eV, respectively, in very good quantitative agreement with data previously reported by optical absorption analyses [9,14]. We use this result to investigate the effect of ion implantation on OTFTs. Ion-implantation, a technological process conventionally used to dope semiconductor materials, induces a strong structural modification in pentacene thin films in terms of relevant hydrogen loss and modifications of the C–H and C–C molecular bonds, forming a hydrogen-depleted carbon-rich matrix [18]. This damage may be limited to the top portion of the pentacene film if its penetration depth is determined and controlled by choosing an appropriate

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ion beam energy and dose. In order to assess such a hypothesis we have measured the charge carrier mobility of OTFTs implanted with N+ ions of different energies and doses. Fig. 4a reports the relative variation of the mobility values measured after implantation with N+ ions of fixed energy (25 keV) and different increasing dose. The mobility modifications induced by ions of different energy but identical dose (5 × 1015 cm−2 ) are shown in Fig. 4b. The interesting result is that the OTFT mobility is affected only for doses higher than about 1015 ions cm−2 (at a fixed energy of 25 keV). Since carrier transport in an OTFT is limited to few nanometers at the semiconductor/dielectric interface, the lack of a mobility degradation effect indicates that the ion implantation process does not affect the device transport channel. Higher energies, on the other hand severely affect the device performance, confirming that the ioninduced damage reaches a greater depth into the pentacene film (see Fig. 1b). The molecular electronic density of states distribution of the pentacene layer in OTFTs has been investigated by photocurrent (PC) spectroscopy analyses both before and after ion implantation, to assess the modifications induced at the molecular level on the electronic transport properties [9–11]. PC spectra of OTFTs implanted with 25 keV N+ ions at different doses are shown in Fig. 5a together with 2 non-implanted reference OTFTs 50 nm and 300 nm thick, respectively. These results provide a clear indication of the presence of an undamaged active pentacene film, despite the damage induced in the top portion of the original 300 nm thick layer by the implanted ions. Fig. 5a shows the gradual decrease in the Davydov splitting with increasing implantation dose starting from the 300 nm thick reference non-implanted OTFT. Interestingly, the PC spectra of OTFTs implanted at the two highest tested doses are better compared with a non-implanted, 50 nm thick, reference sample. This observation clearly indicates that the effective thickness of the active pentacene layer decreases with increasing ion-induced damage, until the DOS shape of the film more closely resembles that of a 50 nm thick reference OTFT. In fact, the thin film phase is located in the first layers of the pentacene film at the interface with dielectric, i.e. it dominates in the portion of the film that is left unaffected by the implantation process, that only damages the top portion of the layer (bulk phase). It is noteworthy that the PC spectrum of the highest dose implanted sample (1 × 1016 ions cm−2 ) has an extremely poor quality and, correspondingly, the OTFT device functionality is severely compromised (Fig. 4a). This effect is also confirmed by PC spectra of samples implanted at higher energies (Fig. 5b), that dramatically affect not only the OTFT carrier mobility (Fig. 4b), but also the DOS shape of the pentacene active layer, that completely looses its typical spectral features, i.e. it is irreversibly deteriorated. These PC results well correlate with the electrical ones and indicate that a thin

Fig. 4. (a) Relative modification in the charge carrier mobility () of OTFTs implanted with 25 keV N+ ions as a function of the ion dose. (b) Relative modification in the charge carrier mobility () of OTFTs implanted with a dose of 5 × 1015 N+ ions/cm2 as a function of the ion energy.

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Fig. 5. (a) Photocurrent spectra of OTFTs implanted at different N+ ion doses compared with the spectra from non-implanted OTFTs of different thicknesses: 50 nm (solid line) and 300 nm (dashed line). The ion energy is 25 keV. The reduction in the Davydov splitting as the pentacene layer thickness decreases is highlighted. (b) Photocurrent spectra of OTFTs implanted at different N+ ion energies compared with the spectrum of a 50 nm thick non-implanted OTFT (solid line). The ion dose is 5 × 1015 ions/cm2 .

undamaged layer of pentacene can still be preserved under the damaged top layer after ion implantation with selected doses and energies, and this layer grants the full functionality of the implanted OTFTs. It is noteworthy that PC spectroscopy, that provides a “fingerprint” of the DOS distribution on the pentacene film, allows one to identify the actual thickness range of the electrically active layer left undamaged after ion implantation, without affecting the operation of the device. Other techniques usually applied to obtain information on pentacene phases, such as XRD or optical absorption spectroscopy, could not be easily applied to fully operational OTFTs. 4. Conclusions We have investigated the distribution of the electrically available states near the band-edge in pentacene thin films and OTFTs of different thickness (50 nm and 300 nm) by PC spectroscopy analyses. The differences observed in the DOS distribution of the films with different thickness have been correlated to their molecular structure and morphology characterized by XRD analyses, allowing to identify the range of thickness of the pentacene films. The effects of ion implantation on OTFTs have been investigated by electrical measurements and PC analyses on OTFTs implanted with N+ ions of different energy and doses. Our results indicate that it is possible to preserve the full functionality of the OTFT device by properly choosing ion implantation conditions. We have applied PC spectroscopy to operational pentacene OTFTs (before and after ion implantation) to investigate the effects on the electrically active DOS distribution induced by structural

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