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Morphology of quaterthiophene thin films in organic field effect transistors
ELSEVIER
Synthetic
Morphology
Metals
84 (1997)
583-584
of quaterthiophene thin films in organic field effect transistors
W.A. Schoonveld, R. W. Stok, J. W. Weijtntans, J. Vrijmoeth, J. Wildeman, TM. Klapwijk Department of Applied Physics and Materials Science Centre University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Abstract The morphology of vacuum evaporated unsubstituted quaterthiophene films is studied as a function of the evaporation parameters. X-ray diffraction and AFM studies show that the thin film has a layered structure. Upon increasing the substrate temperature an increase in size of the single crystallites forming the film is observed together with a more uniform orientation. Larger crystals can be obtained by evaporating at a non- constant increasing deposition rate. Field-effect transistor devices fabricated at room temperature and 100°C reveal an increase in the field-effect mobility by a factor of 100 to a value of unrr=4.8*10~ cm*V’s”. Keywords:y Introduction
Results
The potential for using organic semiconductor materials in field-effect transistors in large-area electronics has gamed a considerable scientific interest.’ Thiophene oligomers are intensively studied, mainly because of their high carrier mobilities and well defined length.z~3”~5These materials are generally known to form, either by casting or vacuum6 evaporation, a highly structured crystalline thin film in which the molecules stand almost Perpendicular to the substrate. In a field-effect transistor the field-induced conductivity is assumed to be confined to the interface between the active layer and the dielectric gate contact, similar to the case of Si MOSFFTs. The electric field is highest in the region close to the semiconductor-insulator interface and the field-induced charges are formed in the first few monolayers. The electrical characteristics are therefore expected to be a convolution of the intrinsic transport properties of the oligomer material, which is related to its molecular stacking with extrinsic influences; the extent of grain boundaries and the roughness of the interfaces with the source and drain contacts and the gate dielectric layer. In this paper we present the crystal structure of vacuum evaporated unsubstituted quaterthiophene thin films and the dependence of the film morphology on the evaporation parameters used. We observe on increasing the substrate temperature from room temperature to 100°C an increase in size of the crystal and a more uniform crystallinity. Alongside, an increase in field-effect mobility is observed.
The crystal structure of thiophene single crystals or thin films obtained by casting or vacuum evaporation has been investigated by X-ray diffraction experiments. Depending on the polymerisation length and the use of substituents the crystal structure ranges from a monoclinic cell with space group P2,/a7 to a orthorhombic unit cell with space group Pbca.6 Furthermore, many organic materials containing planar molecules are known to assume a ‘herringbone’ crystal structure, as is the case for thiophene oligomers. A simplified but illustrative way of describing the crystal structure of evaporated thin films of different oligomers6 is by a set of parallel layers with a spacing conesponding to the height of one molecule. The orientation of the molecules is nearly perpendicular to the substrate with a slight tilt angle between the long axis of the molecule and the substrate normal. We infer the tilt angle for c14-T films from 0-26 Xray diffraction data by analyzing the angles and relative intensities of the sharply defined diffraction peaks, which reflect the layering of the film. On the basis of our data, taken from a 100 nm thick film on SiOZ substrate, we fmd a layer spacing of 15.4 8, and a tilt angle of roughly 22”. The layered structure is also observed on ITO, glass and Muscovite mica. This independence of the substrate material suggests that mainly the strong van der Waals interactions between the molecules induce the crystal structure. The growth and morphology of vacuum evaporated a4-T is characterised with an Atomic Force Microscope and an optical microscope with cross polarisers. In analysing films with a submonolayer roverage deposited at room temperature we find a VolmerWeber type of growth mode. Islands nucleated in the initial stage of the growth process tend to grow in three dimensions without completing a closed first layer of c1.4-T. (Note that in the case of a5-T on SiO, a layer-by-layer growth is observed for the first two monolayers after which a simultaneous multilayer growth starts’). In a cross sectional view of a 3D island, terraces separated by monomolecular steps are found. The observed step height is close to the layer spacing (15.4 A), w h’rch is in agreement with the X-ray data. The morphology of thicker a4-T films is relatively smooth. The films consist of coalesced single crystal islands with a size in excess of 1 pm. As a consequence they can be analyzed with optical microscopy (see Fig. 1)
Experimental Thin films of quaterthiophene are obtained by thermal evaporation in a high vacuum environment at typical background pressures of 1* 10“ mbar. During evaporation the substrates are kept at a fixed temperature between room temperature and 150” C. Thermally oxidized (150 nm) p-doped Si wafers with a resistivity of IO-20 m&m were used as substrate. The deposition rate is monitored by an Intellemetrix IL400 quartz oscillator. Evaporation rates ranged from 0.3 to 0.6 A/s. Some samples were prepared with a non-constant deposition rate (NCR) in which the rate increased in time from 0 to 0.6 ,&Js. The morphology and the crystal structure of the films were studied by ambient and in situ UHV Atomic Force Microscopy and by means of an optical microscope with cross polarisers. 0379-5779/97/$17.00
Q 1997 Elsevier
PI7 SO379-6779(96)04061-1
Science S.k
All rights rserved
WA. Schoonveld
584
et al. /Synthetic
The orientations of the crystals are determined as follows. On the basis of the X-ray data, it is known that the individual crystallites share the same crystal axis in the direction normal to the surface. However, they have a random azimuthal direction within the surface plain, as can be seen with optical microscopy with a polarization filter (F&.1): Due to their azimuthals with respect to the polarizer, the individual islands
Fig.1: a4-Tfilm deposited at a substrate temperature of 120 “C. The arrows in the inset represent the different azimuthal angles of the individual ctystallites. show up with different intensities. Their azimuthal direction can therefore be determined by recording the intensity of the light reflected by the crystal as a function of the rotation angle of the polarizer: the reflected intensity will be at maximum if the polarisation direction is aligned with the azimuthal direction of the crystal. In the film deposited at room temperature, the average size of the crystallites is about 1.5 pm. Note that this value is roughly an order of magnitude larger than what is observed for @.6-T’. Moreover, we observe that the azimuthal direction of a single crystal shows a variation over the crystal area. This implies that the crystals must contain a vet-ylarge density of stacking faults to allow for the observed variation in reflected intensity. substrate temperature (“C)
crystal size with CR (pm2)
crystal size with NCR (pm*)
120 6 75 35 100 6 80 1.5 25 60 1 9 40 0.5 2.25 Table 1: Dependence of the crystallite size, measured directly from optical microscopy data, on substrate temperature and deposition rate for 150 nm thick a4-Tjilms. Upon depositing at an increased substrate temperature, we observe an increase in the crystallite size (with a more regular shape), together with an improved quality of the crystallites. Similar results have been obtained by Biscarini” et al in the case of c~6-T. The intensity reflected by one crystal is uniform over the crystal area and changes as a whole as the polarisation direction is varied. This indicates that the crystallite has the same azimuthal direction over the entire crystal, indicating a much lower density of stacking faults. We list the crystallites’ sizes obtained at different substrate temperatures in table 1. Both for constant and non-constant deposition rates the size of the crystallites increases with temperature. The increase of the crystallite size is more pronounced with a continuously increasing (NCR) deposition rate. We note that the film thicknesses obtained at high temperatures are smaller than the nominal amount of material deposited. Apparently significant desorption ties place for the higher substrate temperatures, resulting in a lower effective deposition rate. On the basis of these data we attribute the increase in crystal size with increasing substrate temperature to a larger diffusion mobility of the
Metals
84 (1997)
583-584
adsorbed molecules, combined with a lower effective deposition rate. This would favour the growth of the already existing islands as compared to the nucleation of a new island. The further increase in crystal size for a non-constant deposition rate can be understood by considering the variation of island density with the initial deposition rate. In the initial stage of growth, only a few islands are nucleated due to the extremely low effective deposition rates and the thereby increased diffusion length.” Upon further deposition, the adsorbed molecules tend to attach to already existing islands instead of nucleating new islands. This results in a film structure with large single crystal islands. The transport properties of quaterthiophene and their dependence on the morphology are studied by using a4-T as the active layer in a tieldeffect transistor device. The FETs are fabricated by lithographically defming gold contacts on the SiO? dielectric layer with channel lengths between 1 and 4 urn. The evaporation of a4-T forms the last step in the fabrication process of the FET. An important parameter describing the transport properties is the field effect mobility (um) of the charge carriers present in the channel. In the thiophene materials the majority charge carriers are believed to be positively charged polarons. ‘J FETs fabricated at substrate temperatures of 20°C and 100°C show a remarkable increase in fieldeffect mobilities by a factor of 100 (from um.,=5*10” cm2V’s.’ to i+a,=4.8*10”cm2V”s”). The highest value reported in literature” for unsubstituted a4-T is pw2.2* 1O’7cm*V”s.‘. Our combined results on structure and electrical behaviour suggests that deposition at elevated temperatures results in an improved electrical behaviour as a consequence of better film quality. Indeed, inspection of the devices prepared at elevated temperatures shows that their active area contains only a few (4-5) monocrystals with good azimuthal uniformity. It remains to be addressed whether or not these devices attain the intrinsic mobility of charge carriers in a4-T, as was claimed for a6-T transistors by Torsi et al.” Conclusions Using X-ray diffraction, AFM and optical microscopy with cross p&risers, we find that the islands of a4-T consists of a layered structure in which the molecules stand almost perpendicular to the substrate, with a tilt angle of 22”. Upon increasing the substrate temperature we observe an increase in the size of the crystallite together with a more uniform orientation throughout the crystal.The crystallite size can be increased even more by evaporating at a non-constant increasing deposition rate. In analysing the FET data, fabricated at substrate temperatures of 20” and lOO”C, we find an increase in the field-effect mobility of a factor of 100 at higher substrate temperatures giving ue4.8* 10’ cmzV”s”. These results suggest that the electrical performance of an organic field-effect transistor can be substantially improved by obtaining an enhanced film morphology. References [l] F. Gamier et al., Science 265, 1684, 1994. [2] K. Waragai et al., Phys. Rev. B 52(3), 1786, 1995. [3] B. Servet et al., Chem. Mater. 6, 1809, 1994. [4] A. Dobadalapur et al., Science 268,270, 1995. [5] P.Ostoja et al, Adv. Mater. Opt. Electron. 1, 127, 1992 [6] S. Hotta et al., Adv. Mater., 5(12), 896, 1993. [7] W. Porzio et al., Acta Polym. 44,266, 1993. [S] G.Horowitz et al, Chem. Mater. 7, 1337, 1995 [9] 0. Bijhme et al., Synth. Met. 67, 87, 1994. [lo] M.C. Bartelt et al., Phys.Rev.B 46(19), 12675, 1992. [ 1 I] F.Biscarini et al. Phys. Rev. B 52(20), 14868, 1995 [ 121 H. Akamichi et al., Appl.Phys.Lett 58(14), 1500,199l. [13] L. Torsi et al., Science 272, 1462, 1996.
Synthetic
Morphology
Metals
84 (1997)
583-584
of quaterthiophene thin films in organic field effect transistors
W.A. Schoonveld, R. W. Stok, J. W. Weijtntans, J. Vrijmoeth, J. Wildeman, TM. Klapwijk Department of Applied Physics and Materials Science Centre University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Abstract The morphology of vacuum evaporated unsubstituted quaterthiophene films is studied as a function of the evaporation parameters. X-ray diffraction and AFM studies show that the thin film has a layered structure. Upon increasing the substrate temperature an increase in size of the single crystallites forming the film is observed together with a more uniform orientation. Larger crystals can be obtained by evaporating at a non- constant increasing deposition rate. Field-effect transistor devices fabricated at room temperature and 100°C reveal an increase in the field-effect mobility by a factor of 100 to a value of unrr=4.8*10~ cm*V’s”. Keywords:y Introduction
Results
The potential for using organic semiconductor materials in field-effect transistors in large-area electronics has gamed a considerable scientific interest.’ Thiophene oligomers are intensively studied, mainly because of their high carrier mobilities and well defined length.z~3”~5These materials are generally known to form, either by casting or vacuum6 evaporation, a highly structured crystalline thin film in which the molecules stand almost Perpendicular to the substrate. In a field-effect transistor the field-induced conductivity is assumed to be confined to the interface between the active layer and the dielectric gate contact, similar to the case of Si MOSFFTs. The electric field is highest in the region close to the semiconductor-insulator interface and the field-induced charges are formed in the first few monolayers. The electrical characteristics are therefore expected to be a convolution of the intrinsic transport properties of the oligomer material, which is related to its molecular stacking with extrinsic influences; the extent of grain boundaries and the roughness of the interfaces with the source and drain contacts and the gate dielectric layer. In this paper we present the crystal structure of vacuum evaporated unsubstituted quaterthiophene thin films and the dependence of the film morphology on the evaporation parameters used. We observe on increasing the substrate temperature from room temperature to 100°C an increase in size of the crystal and a more uniform crystallinity. Alongside, an increase in field-effect mobility is observed.
The crystal structure of thiophene single crystals or thin films obtained by casting or vacuum evaporation has been investigated by X-ray diffraction experiments. Depending on the polymerisation length and the use of substituents the crystal structure ranges from a monoclinic cell with space group P2,/a7 to a orthorhombic unit cell with space group Pbca.6 Furthermore, many organic materials containing planar molecules are known to assume a ‘herringbone’ crystal structure, as is the case for thiophene oligomers. A simplified but illustrative way of describing the crystal structure of evaporated thin films of different oligomers6 is by a set of parallel layers with a spacing conesponding to the height of one molecule. The orientation of the molecules is nearly perpendicular to the substrate with a slight tilt angle between the long axis of the molecule and the substrate normal. We infer the tilt angle for c14-T films from 0-26 Xray diffraction data by analyzing the angles and relative intensities of the sharply defined diffraction peaks, which reflect the layering of the film. On the basis of our data, taken from a 100 nm thick film on SiOZ substrate, we fmd a layer spacing of 15.4 8, and a tilt angle of roughly 22”. The layered structure is also observed on ITO, glass and Muscovite mica. This independence of the substrate material suggests that mainly the strong van der Waals interactions between the molecules induce the crystal structure. The growth and morphology of vacuum evaporated a4-T is characterised with an Atomic Force Microscope and an optical microscope with cross polarisers. In analysing films with a submonolayer roverage deposited at room temperature we find a VolmerWeber type of growth mode. Islands nucleated in the initial stage of the growth process tend to grow in three dimensions without completing a closed first layer of c1.4-T. (Note that in the case of a5-T on SiO, a layer-by-layer growth is observed for the first two monolayers after which a simultaneous multilayer growth starts’). In a cross sectional view of a 3D island, terraces separated by monomolecular steps are found. The observed step height is close to the layer spacing (15.4 A), w h’rch is in agreement with the X-ray data. The morphology of thicker a4-T films is relatively smooth. The films consist of coalesced single crystal islands with a size in excess of 1 pm. As a consequence they can be analyzed with optical microscopy (see Fig. 1)
Experimental Thin films of quaterthiophene are obtained by thermal evaporation in a high vacuum environment at typical background pressures of 1* 10“ mbar. During evaporation the substrates are kept at a fixed temperature between room temperature and 150” C. Thermally oxidized (150 nm) p-doped Si
Q 1997 Elsevier
PI7 SO379-6779(96)04061-1
Science S.k
All rights rserved
WA. Schoonveld
584
et al. /Synthetic
The orientations of the crystals are determined as follows. On the basis of the X-ray data, it is known that the individual crystallites share the same crystal axis in the direction normal to the surface. However, they have a random azimuthal direction within the surface plain, as can be seen with optical microscopy with a polarization filter (F&.1): Due to their azimuthals with respect to the polarizer, the individual islands
Fig.1: a4-Tfilm deposited at a substrate temperature of 120 “C. The arrows in the inset represent the different azimuthal angles of the individual ctystallites. show up with different intensities. Their azimuthal direction can therefore be determined by recording the intensity of the light reflected by the crystal as a function of the rotation angle of the polarizer: the reflected intensity will be at maximum if the polarisation direction is aligned with the azimuthal direction of the crystal. In the film deposited at room temperature, the average size of the crystallites is about 1.5 pm. Note that this value is roughly an order of magnitude larger than what is observed for @.6-T’. Moreover, we observe that the azimuthal direction of a single crystal shows a variation over the crystal area. This implies that the crystals must contain a vet-ylarge density of stacking faults to allow for the observed variation in reflected intensity. substrate temperature (“C)
crystal size with CR (pm2)
crystal size with NCR (pm*)
120 6 75 35 100 6 80 1.5 25 60 1 9 40 0.5 2.25 Table 1: Dependence of the crystallite size, measured directly from optical microscopy data, on substrate temperature and deposition rate for 150 nm thick a4-Tjilms. Upon depositing at an increased substrate temperature, we observe an increase in the crystallite size (with a more regular shape), together with an improved quality of the crystallites. Similar results have been obtained by Biscarini” et al in the case of c~6-T. The intensity reflected by one crystal is uniform over the crystal area and changes as a whole as the polarisation direction is varied. This indicates that the crystallite has the same azimuthal direction over the entire crystal, indicating a much lower density of stacking faults. We list the crystallites’ sizes obtained at different substrate temperatures in table 1. Both for constant and non-constant deposition rates the size of the crystallites increases with temperature. The increase of the crystallite size is more pronounced with a continuously increasing (NCR) deposition rate. We note that the film thicknesses obtained at high temperatures are smaller than the nominal amount of material deposited. Apparently significant desorption ties place for the higher substrate temperatures, resulting in a lower effective deposition rate. On the basis of these data we attribute the increase in crystal size with increasing substrate temperature to a larger diffusion mobility of the
Metals
84 (1997)
583-584
adsorbed molecules, combined with a lower effective deposition rate. This would favour the growth of the already existing islands as compared to the nucleation of a new island. The further increase in crystal size for a non-constant deposition rate can be understood by considering the variation of island density with the initial deposition rate. In the initial stage of growth, only a few islands are nucleated due to the extremely low effective deposition rates and the thereby increased diffusion length.” Upon further deposition, the adsorbed molecules tend to attach to already existing islands instead of nucleating new islands. This results in a film structure with large single crystal islands. The transport properties of quaterthiophene and their dependence on the morphology are studied by using a4-T as the active layer in a tieldeffect transistor device. The FETs are fabricated by lithographically defming gold contacts on the SiO? dielectric layer with channel lengths between 1 and 4 urn. The evaporation of a4-T forms the last step in the fabrication process of the FET. An important parameter describing the transport properties is the field effect mobility (um) of the charge carriers present in the channel. In the thiophene materials the majority charge carriers are believed to be positively charged polarons. ‘J FETs fabricated at substrate temperatures of 20°C and 100°C show a remarkable increase in fieldeffect mobilities by a factor of 100 (from um.,=5*10” cm2V’s.’ to i+a,=4.8*10”cm2V”s”). The highest value reported in literature” for unsubstituted a4-T is pw2.2* 1O’7cm*V”s.‘. Our combined results on structure and electrical behaviour suggests that deposition at elevated temperatures results in an improved electrical behaviour as a consequence of better film quality. Indeed, inspection of the devices prepared at elevated temperatures shows that their active area contains only a few (4-5) monocrystals with good azimuthal uniformity. It remains to be addressed whether or not these devices attain the intrinsic mobility of charge carriers in a4-T, as was claimed for a6-T transistors by Torsi et al.” Conclusions Using X-ray diffraction, AFM and optical microscopy with cross p&risers, we find that the islands of a4-T consists of a layered structure in which the molecules stand almost perpendicular to the substrate, with a tilt angle of 22”. Upon increasing the substrate temperature we observe an increase in the size of the crystallite together with a more uniform orientation throughout the crystal.The crystallite size can be increased even more by evaporating at a non-constant increasing deposition rate. In analysing the FET data, fabricated at substrate temperatures of 20” and lOO”C, we find an increase in the field-effect mobility of a factor of 100 at higher substrate temperatures giving ue4.8* 10’ cmzV”s”. These results suggest that the electrical performance of an organic field-effect transistor can be substantially improved by obtaining an enhanced film morphology. References [l] F. Gamier et al., Science 265, 1684, 1994. [2] K. Waragai et al., Phys. Rev. B 52(3), 1786, 1995. [3] B. Servet et al., Chem. Mater. 6, 1809, 1994. [4] A. Dobadalapur et al., Science 268,270, 1995. [5] P.Ostoja et al, Adv. Mater. Opt. Electron. 1, 127, 1992 [6] S. Hotta et al., Adv. Mater., 5(12), 896, 1993. [7] W. Porzio et al., Acta Polym. 44,266, 1993. [S] G.Horowitz et al, Chem. Mater. 7, 1337, 1995 [9] 0. Bijhme et al., Synth. Met. 67, 87, 1994. [lo] M.C. Bartelt et al., Phys.Rev.B 46(19), 12675, 1992. [ 1 I] F.Biscarini et al. Phys. Rev. B 52(20), 14868, 1995 [ 121 H. Akamichi et al., Appl.Phys.Lett 58(14), 1500,199l. [13] L. Torsi et al., Science 272, 1462, 1996.