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Non-destructive probing of the anisotropy of field-effect mobility in the rubrene single crystal
Synthetic Metals 157 (2007) 257–260
Non-destructive probing of the anisotropy of field-effect mobility in the rubrene single crystal Mang-Mang Ling, Colin Reese, Alejandro L. Briseno b , Zhenan Bao a,∗ a b
Department of Chemical Engineering, 381 North-South Mall, Stanford University, Stanford, CA 95305, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, CA 90095-1569, United States Received 6 October 2006; received in revised form 1 February 2007; accepted 28 February 2007 Available online 25 April 2007
Abstract Organic single crystals are valuable tools for the exploration of charge transport in organic materials. Here, we report two new methods for the non-destructive probing of anisotropic transport in molecular crystals, demonstrating an angular dependence of the field-effect mobility in the ab-plane of the rubrene single crystal. Clear minima and maxima are observed, corresponding to the a and b principle axes of the crystal, as determined by X-ray diffraction and visual inspection. While this phenomenon has been previously reported, the method presented here offers an angular resolution previously undemonstrated, with methods that eliminate the need to move the fragile crystal. The coincidence of this phenomenon between top- and bottom-contact geometries offers strong support for the performance correlation of mobility with specific molecular orientation, and an improved data set for comparison with transport theory. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic semiconductors; Single crystals; Rubrene; Anisotropic; Mobility; Transistors
1. Introduction Single crystals are ideal tools for the exploration of charge transport phenomena in organic materials [1–3]. The purity and long-range order possible in the molecular crystal allows the probing of intrinsic properties, avoiding many of the fabrication and morphological variations characteristic of thin-film devices. However, due to their fragility, it is challenging to fabricate and characterize single-crystal transistors without damaging the active layer material. In this letter, we report methods for the fabrication of organic single-crystal field-effect transistors designed to minimize damage to the molecular crystal, and employ these techniques to study the anisotropy of carrier transport in the rubrene single crystal. Specifically, we measure field-effect mobilities differing by several times from maxima and minima along the b- and a-axes, respectively. To date, anisotropic field-effect mobility has only been reported for the rubrene and pentacene single crystals [4–7] with lower angular resolution
∗
Corresponding author. E-mail address: [email protected] (Z. Bao).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.02.004
than that presented here. In order to effectively correlate transport properties with molecular orientation, a more defined data set is required for comparison with theoretical models. These methods therefore offer a facile, non-destructive technique for the study of intrinsic electronic properties of a large spectrum of organic single-crystal semiconductors. 2. Experimental procedure Rubrene single crystals were grown by horizontal physical vapor transport [8] in a stream of high purity argon carrier gas. The typical thickness of crystals used in this study was around 1 m. The rubrene crystals as grown were characterized via X-ray diffraction in order to ensure the quality of the measured materials. The narrow peak width (0.013◦ ) of the Omega-scan and the narrow (0 0 2) peak width (0.02◦ ) indicate that the rubrene single crystals were of high quality. Rocking curves were obtained from a Philips X’Pert PRO X-ray diffraction system, operated at 45 keV and 40 mA. Top-contact devices were fabricated as shown in Fig. 1a, using highly doped silicon wafers with 300 nm of thermally grown dry oxide as the dielectric. A divided, round electrode
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M.-M. Ling et al. / Synthetic Metals 157 (2007) 257–260
Fig. 1. Fabrication schematics and optical micrographs of (a, c) top-contact and (b, d) bottom-contact organic single-crystal field-effect transistors. An additional, overlaying crystal is present in the bottom-contact device, but does not affect the underlying crystal.
consisting of a titanium (ca. 3 nm) adhesion layer followed by gold (ca. 40 nm) was then deposited onto the substrate through a shadow mask. A thin, flat organic crystal was then electrostatically bonded in the center of the round electrode. Separately, a small piece of poly(dimethyl siloxane) (PDMS) was prepared by casting and curing Sylgard 184 PDMS (Dow Corning; 1:10 (w/w) for cross linker and catalyst) at 65 ◦ C. Source and drain electrodes (channel width/length = 1500 m/100 m) were then formed on the PDMS in analogous fashion as those on the oxide layer. This PDMS piece containing source–drain electrodes was then flipped over and gently pressed onto the single crystal located in the center of the round electrodes to complete the top-contact device (Fig. 1a and c). The PDMS block was subsequently lifted, rotated and relaminated to probe along various orientations, allowing transistor characterization each 45◦ (four pairs of S–D electrodes). The key advantage of this approach is that the fragile organic single crystal does not have to be repeatedly lifted and bound to the dielectric, avoiding possible damage caused by this process. Bottom-contact devices were fabricated as shown in Fig. 1b. In this case, narrow electrodes (channel width/length = 100 m/1000 m) were deposited as those on top-contact substrates through a shadow mask. Rubrene single crystals were again carefully positioned and electrostatically bonded on the device in-place, contacting all electrodes. For each orientation, opposing pairs of electrodes were employed as source–drain pairs, allowing transistor characterization each 22.5◦ (eight pairs of S–D electrodes). This configuration eliminates the need for repositioning of the crystal or source–drain electrodes once the device is complete. Electrical characterization of the single-crystal devices was per-
formed using a Keithley 4200SCS semiconductor parameter analyzer. 3. Results and discussion Measured saturation regime charge carrier mobilities from top- and bottom-contact devices are plotted in Fig. 2 as a function of crystal orientation. Crystals in-place were measured via X-ray diffraction in order to determine the absolute orientation of the a- and b-axes relative to the electrodes and hence the direction of transport (the c-axis is perpendicular to the thin dimension of the crystal). This was performed by rotating the crystal in the ab-plane at a small angle of incidence and monitoring the detected intensity. For many crystals, it is additionally possible to determine the alignment by inspecting the facets and comparing the geometry to that of the crystal structure. It was determined by these methods that the mobility maximum occurs along the principle b-axis, with the minimum occurring along the a-axis. Repeated passes were carried out to ensure the reproducibility of measured mobility, which was in general very consistent. The periodicity of the mobility was approximately 180◦ , as would be expected for the symmetry of the crystal structure. For topcontact devices, the ratio of mobility maxima to minima was generally between 5 and 10, while for bottom-contact devices it varied between 3 and 5. These results are consistent with those previously reported [5]. These methods provide unambiguous evidence confirming the dependence of transport properties on the specific packing of molecules within the rubrene single crystal. The consistent coincidence of mobility maxima and minima with the principle axes of the rubrene crystal structure
M.-M. Ling et al. / Synthetic Metals 157 (2007) 257–260
259
Fig. 2. Measured effective field-effect mobility as a function of source–drain orientation and typical transfer characteristics for (a and c) top-contact and (b and e) bottom-contact devices at Vds = −100. The orientation of the top-contact devices was determined by X-ray diffraction, where the angle corresponds to degrees away from the principle b-axis. Bottom-contact crystal orientation was determined by visual inspection of the crystal facets relative to the source–drain electrode pairs. (d) Structure and lattice parameters of the rubrene single crystal.
in top- and bottom-contact configurations illustrates the explicit dependence on molecular orientation and proximity. The use of complementary device structures in this study offers not only confidence in the reported anisotropic transport, but also insight into the physics particular to each configuration. The first obvious difference between the two geometries is the range of field-effect mobilities, which differ by more than an order of magnitude. The low W/L of the bottom-contact devices (W/L = 0.1) compared to the top-contact devices (W/L = 15) results in inflated effective mobility, due to the larger proportion of fringe current outside of the channel region, which is not accounted for in the FET model. An additional consideration is the thickness of the rubrene crystals used in the top-contact configuration. In thin-film transistors, the thickness of the active semiconductor layer is generally very thin, ∼50–100 nm, and in top-contact devices, carriers need only travel a distance on the order of the width of the conducting channel to reach semiconductor–dielectric interface. Similarly, in bottom-contact devices, the carriers are injected directly into the active channel region, a thin layer of the crystal at the dielectric interface. In the case of the top-contact single-crystal transistor, however, an injected carrier must travel through the entire thickness of
the crystal – which may range from hundreds of nanometers to several microns thick – to reach the conducting channel. This transport must occur in the c-direction of the crystal perpendicular to the substrate, which for linear acenes naphthalene, anthracene [9,10], and pentacene [11] has much diminished mobility compared to the ab-plane. This is anticipated to add a (large) resistance, and decrease the effective mobility commensurately. Another point of interest is the difference in apparent mobility ratios between maxima and minima along the principle axes. Top- and bottom-contact devices generally show ratios of 5–10 and 3–5, respectively. There are several obvious possibilities for the origin of such differences. First of all, while the mobility values have been inflated by the low channel aspect ratio and resulting fringe current, the bottom-contact configuration generally has a much larger contact resistance than that for top-contact. Because the contact resistance is not a function of orientation, this may have the effect of weakening the observed anisotropy. Contact-limited injection may also explain the slight differences in measured mobility when alternating source–drain pairs in the bottom-contact devices, as previously reported [12]. In addition, fringe current does not travel directly across the
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channel, and thus does not represent transport in exactly the observed orientation. This would have a similar diluting effect. In terms of overall performance, the top-contact devices generally showed poorer transfer characteristics, and device yield was substantially lower. Reported minimum mobilities for these devices – and therefore also the anisotropy ratios – should be given allowance for experimental error. Thus, each architecture has weaknesses that have been addressed in the next generation of devices, the subject of current study. We conclude from the current results that mobility ratio is at minimum that of the bottom-contact devices, which agrees with previously reported measurements [5–7].
sibility of an improved data set for comparison with transport theory in high-performance semiconductor materials.
4. Summary and conclusions
[1] R.W.I. de Boer, M.E. Gershenson, A.F. Morpurgo, et al., Physica Status Solidi a-Appl. Res. 201 (2004) 1302. [2] A.F. Stassen, R.W.I. de Boer, N.N. Iosad, et al., Appl. Phys. Lett. 85 (2004) 3899. [3] C. Reese, Z. Bao, J. Mater. Chem. 16 (2005) 329. [4] J.Y. Lee, S. Roth, Y.W. Park, Appl. Phys. Lett. 88 (2006). [5] V.C. Sundar, J. Zaumseil, V. Podzorov, et al., Science 303 (2004) 1644. [6] R. Zeis, C. Besnard, T. Siegrist, et al., Chem. Mater. 18 (2006) 244. [7] V. Podzorov, E. Menard, A. Borissov, et al., Phys. Rev. Lett. 93 (2004). [8] C. Kloc, P.G. Simpkins, T. Siegrist, et al., J. Cryst. Growth 182 (1997) 416. [9] W. Warta, R. Stehle, N. Karl, Appl. Phys. a-Mater. Sci. & Process. 36 (1985) 163. [10] N. Karl, J. Marktanner, R. Stehle, et al., Synth. Met. 42 (1991) 2473. [11] O.D. Jurchescu, J. Baas, T.T.M. Palstra, Appl. Phys. Lett. 84 (2004) 3061. [12] A.B. Chwang, C.D. Frisbie, J. Phys. Chem. B 104 (2000) 12202.
In summary, new methods for the non-destructive probing of transport anistropy were demonstrated for the rubrene single crystal. Top- and bottom-contact geometries both showed consistent, marked anisotropy of field-effect mobility in the ab-plane with resolution previously undemonstrated, without necessitating movement of the fragile single crystal. X-ray diffraction allowed the determination of the absolute orientation of the crystal relative to the source–drain electrodes, and hence a direct correlation of minima and maxima to the principle a- and b-axes of the rubrene unit cell. Previous reports [5–7,11] have offered a glimpse into such dependencies, but with insufficient resolution to draw firm conclusions regarding these relationships. The methods presented here offer the pos-
Acknowledgements The authors thank Dr. Stefan Mannsfeld and Dr. Christian Kloc for helpful discussions. ZB acknowledges partial financial support from 3M Faculty Award, Finmeccanica Faculty Scholar Award, the Stanford Center for Integrated Systems and DuPont Science and Technology Grant. References
Non-destructive probing of the anisotropy of field-effect mobility in the rubrene single crystal Mang-Mang Ling, Colin Reese, Alejandro L. Briseno b , Zhenan Bao a,∗ a b
Department of Chemical Engineering, 381 North-South Mall, Stanford University, Stanford, CA 95305, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, CA 90095-1569, United States Received 6 October 2006; received in revised form 1 February 2007; accepted 28 February 2007 Available online 25 April 2007
Abstract Organic single crystals are valuable tools for the exploration of charge transport in organic materials. Here, we report two new methods for the non-destructive probing of anisotropic transport in molecular crystals, demonstrating an angular dependence of the field-effect mobility in the ab-plane of the rubrene single crystal. Clear minima and maxima are observed, corresponding to the a and b principle axes of the crystal, as determined by X-ray diffraction and visual inspection. While this phenomenon has been previously reported, the method presented here offers an angular resolution previously undemonstrated, with methods that eliminate the need to move the fragile crystal. The coincidence of this phenomenon between top- and bottom-contact geometries offers strong support for the performance correlation of mobility with specific molecular orientation, and an improved data set for comparison with transport theory. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic semiconductors; Single crystals; Rubrene; Anisotropic; Mobility; Transistors
1. Introduction Single crystals are ideal tools for the exploration of charge transport phenomena in organic materials [1–3]. The purity and long-range order possible in the molecular crystal allows the probing of intrinsic properties, avoiding many of the fabrication and morphological variations characteristic of thin-film devices. However, due to their fragility, it is challenging to fabricate and characterize single-crystal transistors without damaging the active layer material. In this letter, we report methods for the fabrication of organic single-crystal field-effect transistors designed to minimize damage to the molecular crystal, and employ these techniques to study the anisotropy of carrier transport in the rubrene single crystal. Specifically, we measure field-effect mobilities differing by several times from maxima and minima along the b- and a-axes, respectively. To date, anisotropic field-effect mobility has only been reported for the rubrene and pentacene single crystals [4–7] with lower angular resolution
∗
Corresponding author. E-mail address: [email protected] (Z. Bao).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.02.004
than that presented here. In order to effectively correlate transport properties with molecular orientation, a more defined data set is required for comparison with theoretical models. These methods therefore offer a facile, non-destructive technique for the study of intrinsic electronic properties of a large spectrum of organic single-crystal semiconductors. 2. Experimental procedure Rubrene single crystals were grown by horizontal physical vapor transport [8] in a stream of high purity argon carrier gas. The typical thickness of crystals used in this study was around 1 m. The rubrene crystals as grown were characterized via X-ray diffraction in order to ensure the quality of the measured materials. The narrow peak width (0.013◦ ) of the Omega-scan and the narrow (0 0 2) peak width (0.02◦ ) indicate that the rubrene single crystals were of high quality. Rocking curves were obtained from a Philips X’Pert PRO X-ray diffraction system, operated at 45 keV and 40 mA. Top-contact devices were fabricated as shown in Fig. 1a, using highly doped silicon wafers with 300 nm of thermally grown dry oxide as the dielectric. A divided, round electrode
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M.-M. Ling et al. / Synthetic Metals 157 (2007) 257–260
Fig. 1. Fabrication schematics and optical micrographs of (a, c) top-contact and (b, d) bottom-contact organic single-crystal field-effect transistors. An additional, overlaying crystal is present in the bottom-contact device, but does not affect the underlying crystal.
consisting of a titanium (ca. 3 nm) adhesion layer followed by gold (ca. 40 nm) was then deposited onto the substrate through a shadow mask. A thin, flat organic crystal was then electrostatically bonded in the center of the round electrode. Separately, a small piece of poly(dimethyl siloxane) (PDMS) was prepared by casting and curing Sylgard 184 PDMS (Dow Corning; 1:10 (w/w) for cross linker and catalyst) at 65 ◦ C. Source and drain electrodes (channel width/length = 1500 m/100 m) were then formed on the PDMS in analogous fashion as those on the oxide layer. This PDMS piece containing source–drain electrodes was then flipped over and gently pressed onto the single crystal located in the center of the round electrodes to complete the top-contact device (Fig. 1a and c). The PDMS block was subsequently lifted, rotated and relaminated to probe along various orientations, allowing transistor characterization each 45◦ (four pairs of S–D electrodes). The key advantage of this approach is that the fragile organic single crystal does not have to be repeatedly lifted and bound to the dielectric, avoiding possible damage caused by this process. Bottom-contact devices were fabricated as shown in Fig. 1b. In this case, narrow electrodes (channel width/length = 100 m/1000 m) were deposited as those on top-contact substrates through a shadow mask. Rubrene single crystals were again carefully positioned and electrostatically bonded on the device in-place, contacting all electrodes. For each orientation, opposing pairs of electrodes were employed as source–drain pairs, allowing transistor characterization each 22.5◦ (eight pairs of S–D electrodes). This configuration eliminates the need for repositioning of the crystal or source–drain electrodes once the device is complete. Electrical characterization of the single-crystal devices was per-
formed using a Keithley 4200SCS semiconductor parameter analyzer. 3. Results and discussion Measured saturation regime charge carrier mobilities from top- and bottom-contact devices are plotted in Fig. 2 as a function of crystal orientation. Crystals in-place were measured via X-ray diffraction in order to determine the absolute orientation of the a- and b-axes relative to the electrodes and hence the direction of transport (the c-axis is perpendicular to the thin dimension of the crystal). This was performed by rotating the crystal in the ab-plane at a small angle of incidence and monitoring the detected intensity. For many crystals, it is additionally possible to determine the alignment by inspecting the facets and comparing the geometry to that of the crystal structure. It was determined by these methods that the mobility maximum occurs along the principle b-axis, with the minimum occurring along the a-axis. Repeated passes were carried out to ensure the reproducibility of measured mobility, which was in general very consistent. The periodicity of the mobility was approximately 180◦ , as would be expected for the symmetry of the crystal structure. For topcontact devices, the ratio of mobility maxima to minima was generally between 5 and 10, while for bottom-contact devices it varied between 3 and 5. These results are consistent with those previously reported [5]. These methods provide unambiguous evidence confirming the dependence of transport properties on the specific packing of molecules within the rubrene single crystal. The consistent coincidence of mobility maxima and minima with the principle axes of the rubrene crystal structure
M.-M. Ling et al. / Synthetic Metals 157 (2007) 257–260
259
Fig. 2. Measured effective field-effect mobility as a function of source–drain orientation and typical transfer characteristics for (a and c) top-contact and (b and e) bottom-contact devices at Vds = −100. The orientation of the top-contact devices was determined by X-ray diffraction, where the angle corresponds to degrees away from the principle b-axis. Bottom-contact crystal orientation was determined by visual inspection of the crystal facets relative to the source–drain electrode pairs. (d) Structure and lattice parameters of the rubrene single crystal.
in top- and bottom-contact configurations illustrates the explicit dependence on molecular orientation and proximity. The use of complementary device structures in this study offers not only confidence in the reported anisotropic transport, but also insight into the physics particular to each configuration. The first obvious difference between the two geometries is the range of field-effect mobilities, which differ by more than an order of magnitude. The low W/L of the bottom-contact devices (W/L = 0.1) compared to the top-contact devices (W/L = 15) results in inflated effective mobility, due to the larger proportion of fringe current outside of the channel region, which is not accounted for in the FET model. An additional consideration is the thickness of the rubrene crystals used in the top-contact configuration. In thin-film transistors, the thickness of the active semiconductor layer is generally very thin, ∼50–100 nm, and in top-contact devices, carriers need only travel a distance on the order of the width of the conducting channel to reach semiconductor–dielectric interface. Similarly, in bottom-contact devices, the carriers are injected directly into the active channel region, a thin layer of the crystal at the dielectric interface. In the case of the top-contact single-crystal transistor, however, an injected carrier must travel through the entire thickness of
the crystal – which may range from hundreds of nanometers to several microns thick – to reach the conducting channel. This transport must occur in the c-direction of the crystal perpendicular to the substrate, which for linear acenes naphthalene, anthracene [9,10], and pentacene [11] has much diminished mobility compared to the ab-plane. This is anticipated to add a (large) resistance, and decrease the effective mobility commensurately. Another point of interest is the difference in apparent mobility ratios between maxima and minima along the principle axes. Top- and bottom-contact devices generally show ratios of 5–10 and 3–5, respectively. There are several obvious possibilities for the origin of such differences. First of all, while the mobility values have been inflated by the low channel aspect ratio and resulting fringe current, the bottom-contact configuration generally has a much larger contact resistance than that for top-contact. Because the contact resistance is not a function of orientation, this may have the effect of weakening the observed anisotropy. Contact-limited injection may also explain the slight differences in measured mobility when alternating source–drain pairs in the bottom-contact devices, as previously reported [12]. In addition, fringe current does not travel directly across the
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M.-M. Ling et al. / Synthetic Metals 157 (2007) 257–260
channel, and thus does not represent transport in exactly the observed orientation. This would have a similar diluting effect. In terms of overall performance, the top-contact devices generally showed poorer transfer characteristics, and device yield was substantially lower. Reported minimum mobilities for these devices – and therefore also the anisotropy ratios – should be given allowance for experimental error. Thus, each architecture has weaknesses that have been addressed in the next generation of devices, the subject of current study. We conclude from the current results that mobility ratio is at minimum that of the bottom-contact devices, which agrees with previously reported measurements [5–7].
sibility of an improved data set for comparison with transport theory in high-performance semiconductor materials.
4. Summary and conclusions
[1] R.W.I. de Boer, M.E. Gershenson, A.F. Morpurgo, et al., Physica Status Solidi a-Appl. Res. 201 (2004) 1302. [2] A.F. Stassen, R.W.I. de Boer, N.N. Iosad, et al., Appl. Phys. Lett. 85 (2004) 3899. [3] C. Reese, Z. Bao, J. Mater. Chem. 16 (2005) 329. [4] J.Y. Lee, S. Roth, Y.W. Park, Appl. Phys. Lett. 88 (2006). [5] V.C. Sundar, J. Zaumseil, V. Podzorov, et al., Science 303 (2004) 1644. [6] R. Zeis, C. Besnard, T. Siegrist, et al., Chem. Mater. 18 (2006) 244. [7] V. Podzorov, E. Menard, A. Borissov, et al., Phys. Rev. Lett. 93 (2004). [8] C. Kloc, P.G. Simpkins, T. Siegrist, et al., J. Cryst. Growth 182 (1997) 416. [9] W. Warta, R. Stehle, N. Karl, Appl. Phys. a-Mater. Sci. & Process. 36 (1985) 163. [10] N. Karl, J. Marktanner, R. Stehle, et al., Synth. Met. 42 (1991) 2473. [11] O.D. Jurchescu, J. Baas, T.T.M. Palstra, Appl. Phys. Lett. 84 (2004) 3061. [12] A.B. Chwang, C.D. Frisbie, J. Phys. Chem. B 104 (2000) 12202.
In summary, new methods for the non-destructive probing of transport anistropy were demonstrated for the rubrene single crystal. Top- and bottom-contact geometries both showed consistent, marked anisotropy of field-effect mobility in the ab-plane with resolution previously undemonstrated, without necessitating movement of the fragile single crystal. X-ray diffraction allowed the determination of the absolute orientation of the crystal relative to the source–drain electrodes, and hence a direct correlation of minima and maxima to the principle a- and b-axes of the rubrene unit cell. Previous reports [5–7,11] have offered a glimpse into such dependencies, but with insufficient resolution to draw firm conclusions regarding these relationships. The methods presented here offer the pos-
Acknowledgements The authors thank Dr. Stefan Mannsfeld and Dr. Christian Kloc for helpful discussions. ZB acknowledges partial financial support from 3M Faculty Award, Finmeccanica Faculty Scholar Award, the Stanford Center for Integrated Systems and DuPont Science and Technology Grant. References