Controlling threshold voltage and leakage currents in vertical organic field effect transistors by inversion mode operation Controlling threshold voltage and leakage currents in vertical organic field[.]
Controlling threshold voltage and leakage currents in vertical organic field-effect transistors by inversion mode operation , Alrun A Günther , Christoph Hossbach, Michael Sawatzki, Daniel Kasemann, Johann W Bartha, and Karl Leo Citation: Appl Phys Lett 107, 233302 (2015); doi: 10.1063/1.4937439 View online: http://dx.doi.org/10.1063/1.4937439 View Table of Contents: http://aip.scitation.org/toc/apl/107/23 Published by the American Institute of Physics Articles you may be interested in Unique architecture and concept for high-performance organic transistors Appl Phys Lett 85, (2004); 10.1063/1.1821629 APPLIED PHYSICS LETTERS 107, 233302 (2015) Controlling threshold voltage and leakage currents in vertical organic field-effect transistors by inversion mode operation €nther,1,a) Christoph Hossbach,2 Michael Sawatzki,1 Daniel Kasemann,3 Alrun A Gu Johann W Bartha,2 and Karl Leo1,4 Institut f€ ur Angewandte Photophysik, Technische Universit€ at Dresden, 01062 Dresden, Germany Institut f€ ur Halbleiter- und Mikrosystemtechnik, Technische Universit€ at Dresden, 01062 Dresden, Germany CreaPhys GmbH, Niedersedlitzer Straße 75, 01257 Dresden, Germany Canadian Institute for Advanced Research (CIFAR), 180 Dundas Street West, Suite 1400, Toronto, Ontario M5G 1Z8, Canada (Received 26 August 2015; accepted 26 November 2015; published online 10 December 2015) The interest in vertical organic transistors as a means to overcome the limitations of conventional organic field-effect transistors (OFETs) has been growing steadily in recent years Current vertical architectures, however, often suffer from a lack of parameter control, as they are limited to certain materials and processing techniques, making a controlled shift of, e.g., the transistor threshold voltage difficult In this contribution, we present a vertical OFET (VOFET) operating in the inversion regime By varying the thickness or doping concentration of a p-doped layer in an otherwise n-type VOFET, we are able to shift the threshold voltage in a controlled manner from 1.61 V (for a normal n-type VOFET) to 4.83 V (for the highest doping concentration of 50 mol %) Furthermore, it is found that low doping concentrations of 20 mol % can improve the Off state of C 2015 Author(s) All article the VOFET through reduction of the source-drain leakage current V content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4937439] Organic field-effect transistors (OFETs) are the key devices for many future flexible electronics applications.1–3 They should be easy and cheap to fabricate, yet show high performance and adaptability to the specific needs of the application A simple way to meet these requirements is the vertical stacking of the transistor electrodes, resulting in a so-called vertical organic transistor.4 Many such devices have been reported recently,5–9 showing promising performance and easy fabrication procedures In many cases, however, a change in material system or geometry seems necessary in order to tune certain parameters, such as the threshold voltage or On/Off ratio, to the needs of a specific application In this letter, we report on a vertical OFET (VOFET)10,11 operating in inversion mode.12–15 Variation of the doping concentration or thickness of the inversion layer allows to control the threshold voltage and Off state current of the VOFET without altering the geometry or material system as a whole Previous efforts in this area have shown the good performance of VOFETs using C60 as the organic semiconductor.11 In order to test whether this VOFET geometry allows for inclusion of the inversion operation principle, it is implemented into VOFETs of the same type, with molybdenum trioxide (MoO3) as dopant16,17 for the p-doped inversion channel (see Fig 1) The VOFET geometry works much like a conventional OFET in the sense that charge carriers emitted from the source electrode first accumulate at the gate dielectric interface before entering the vertical channel to the drain electrode For an n-type VOFET, it is therefore expected that a p-doped layer, placed into the accumulation region, will lead to an inversion operation in the same way a) alrun.guenther@iapp.de 0003-6951/2015/107(23)/233302/4 as demonstrated previously for conventional p-type OFETs.13,14 To check for the formation of this inversion regime, capacitance-voltage (CV) measurements are performed on a series of metal-insulator-semiconductor (MIS) capacitors, resembling a material stack identical to the channel region underneath the source electrode of the VOFET (see Fig for the VOFET geometry) All devices are fabricated on doped silicon wafers serving both as substrate and as the bottom contact for MIS samples/gate electrode of the transistors An aluminum oxide (Al2O3, e ¼ 7.8) dielectric of only 33 nm thickness is deposited on top of the wafers via thermal atomic layer deposition (ALD) (at IHM, for CV samples) or plasma-assisted ALD (at IAPP, for VOFET samples) The substrates are then immersed in acetone and isopropanol and cleaned in an FIG Device schematic of the VOFET with a p-doped layer for inversion operation in the Off and On state 107, 233302-1 C Author(s) 2015 V 233302-2 €nther et al Gu ultrasonic bath for each, followed by exposure to oxygen plasma for 10 In order to reduce interface traps, the cleaned substrates are dipped into hexamethyldisilazane (HMDS, Merck) for 30 and residuals of HMDS are removed by spin-rinsing with isopropanol All MIS capacitor samples for CV measurements are prepared in a single run by thermal deposition under high-vacuum conditions, using a single-chamber UHV tool (Kurt J Lesker Company) with a base pressure of 107 mbar Thin layers of C60 (Sensient, sublimated twice before use, ¼ 1.54 g/cm3, and thickness 0–10 nm), doped with varying concentrations of molybdenum oxide (MoO3, Sigma Aldrich, used as received, ¼ 4.50 g/cm3, and concentrations 0–50 mol %), are deposited on the Al2O3-coated substrates The layer thickness is monitored using individual quartz crystal micro-balances (QCMs) for both materials Intrinsic C60 is deposited by the same process to achieve a total thickness of 30 nm of the two layers 10 nm-thick Au contact pads are deposited through a shadow mask, followed by 40 nm of Al, which are deposited through the same shadow mask Layer thickness and doping concentration variations are achieved in this single run by use of the UHV system’s wedging tool, so as to provide the best comparability between samples Inversion VOFETs are prepared on the same type of substrates, but deposition is done in a multi-chamber UHV tool (base pressure also 107 mbar) in separate runs, so that the reference samples are never exposed to the MoO3 dopant Just as for MIS capacitors, thin layers of C60 doped with varying concentrations of MoO3 are deposited first, followed by intrinsic C60 to give a total layer thickness of 30 nm The source contacts of the VOFET are made of 50 nm Au and patterned using a bi-layer orthogonal photolithography process.18 120 nm of SiO2 is also deposited through this lithography mask via magnetron sputtering in order to reduce leakage currents between the source and drain electrodes The photolithography mask is subsequently lifted off, and the VOFETs are completed by depositing 30 nm of intrinsic C60 and a drain electrode, made of 40 nm Au, through a second photolithography mask, which is lifted off prior to electrical characterization All devices are annealed for h at 60 C under nitrogen atmosphere to recover the p-doping effect of MoO3 in C60 This step is necessary as the photolithography is performed under ambient conditions and the energy levels of MoO3 shift upon contact with air.17 All VOFETs have the same dimensions of 60 nm vertical channel length (L) and 600 lm channel width (W) Contact doping19–22 for either n- or p-type injection is not employed for the transistors as a sufficient injection of both carriers from the Au contacts into the C60 LUMO/ HOMO is expected without the aid of injection barrier lowering Electrical characterization (using an Autolab PGSTAT302N galvanostat for CV and a HP 4145B semiconductor parameter analyzer for VOFET transfer curves) is performed in a nitrogen glovebox to prevent additional pdoping by oxygen and the aforementioned energy level shift of MoO3 CV curves are obtained at a frequency of 100 kHz and an amplitude of 50 mV The DC voltage is applied to the injecting top contact, while the substrate, i.e., the back contact, is kept at ground potential The total capacitance per unit area of the MIS stack can be approximated as Appl Phys Lett 107, 233302 (2015) 1 1 dox VSG dtot d ỵ ẳ ỵ ỵ ẳ ỵ ; C Cox Cinv Csemi e0 eox qd e0 eC60 (1) where Cox, Cinv, and Csemi are the capacitances per unit area of the gate oxide, inversion layer, and intrinsic semiconductor layer q is the charge carrier density inside the inversion layer, which depends on the doping concentration, dox, d, and dtot are the oxide, inversion layer, and total semiconductor thicknesses and all other variables have their usual meanings As can be observed in Fig 2, the total capacitance varies noticeably between the individual samples We attribute this to a convolution of the doping effect and variations in dtot due to dopant addition, as well as small variations in device area due to processing conditions The application of a positive voltage to the injecting contact results in the accumulation of holes at the interface between the p-doped C60 and the underlying Al2O3 It may be noted that this hole accumulation effect is observed not only for the samples with a p-doped layer, but, to a certain extent, also for the undoped reference sample, shown in black We attribute this to the processing conditions in the chamber: All samples were prepared in a single run, and even though the wedging tool employed during deposition covered the reference samples during MoO3 deposition, a slight MoO3 contamination of the reference samples is possible Applying a negative voltage to the injection contact instead accumulates electrons at the dielectric interface, i.e., FIG CV characteristics of (a) samples with a fixed doping layer thickness of nm and varying doping concentrations of MoO3 and (b) samples with a fixed MoO3 concentration of 30 mol % and varying doping layer thicknesses The turn-over point between majority carrier accumulation and depletion is marked as a dashed line, and a reference device without a p-doped C60 layer is shown as a black line 233302-3 €nther et al Gu Appl Phys Lett 107, 233302 (2015) the inversion regime is reached The turn-over point between these two regimes is marked in Fig as a dashed line for each sample As the doping concentration or doping layer thickness at the dielectric interface is increased in a controlled manner, we see a shift of the turn-over points from accumulation into depletion mode and then further into inversion, as typically observed for MOSFETs with a highfrequency test signal.15 We can only partly resolve the inversion regime in highly doped samples, as we are limited by sample processing and experimental set-up The characterization tool used for the CV measurements is limited to the voltage regime investigated here and low frequency measurements (which typically show a clear accumulation of minority carriers in inversion MOSFETs) could not produce reliable results due to the noise level in and around the glovebox and measurement set-up N-type contact doping would normally be employed in such a case to enhance the injection of minority charge carriers and thus make the inversion regime more visible This would necessitate an airstable n-dopant for C60, since we aim for the same material system in the MIS structures and VOFET samples and part of the fabrication process for VOFETs is performed in air (see above and Ref 18) As air-stable n-dopants are not readily available, we choose gold for the electrodes of both device types The work function of Au is approximately halfway between the LUMO and HOMO of C60 (Au ¼ 5.1 eV, C60 HOMO ¼ 6.4 eV, and C60 LUMO ¼ eV (Refs 23–25)), and the injection barriers for both carrier types are thus roughly comparable To investigate the effects of the inversion layer on the working VOFET, the transfer characteristic of each VOFET is measured in the saturation regime The transistor threshold voltage Vth, which can be extracted from these transfer characteristics, is expected to depend on the density of activated dopants NA(c) and on the doping layer thickness d according to Vth ẳ VFB ỵ eNA cịd ; Cox (2) where VFB is the transistor flatband voltage, c is the doping concentration, and e is the elementary charge.14 To obtain transfer characteristics, a constant potential is applied to the drain electrode, while keeping the source electrode at ground potential and sweeping the gate in the same way as for the CV measurement Figures 3(a) and 4(a) show a controlled threshold voltage shift with the increasing doping concentration in the inversion channel This same effect was previously demonstrated by L€ ussem and coworkers for a standard lateralchannel OFET.14 It may be noted that the Vth determined from the transfer characteristics is not identical to the turnover point marked in the CV curves This is attributed to differences in processing conditions and substrate quality (see methods section) It can be shown further that the threshold voltage is shifted by an increased thickness of the doped layer, as suggested in Eq (2) As can be observed from Figs and 4(b), a change in layer thickness does in fact produce a more systematic threshold voltage shift than a change in doping concentration, as layer thickness is more easily controlled in our setup than doping concentration FIG (a) Transfer curves of VOFETs with a fixed doping layer thickness of nm and varying doping concentrations of MoO3 The applied VD is ỵ6 V (b) Transfer characteristics of VOFETs with a fixed MoO3 concentration of 30 mol % and varying doping layer thicknesses The applied VD is ỵ8 V Reference devices without a p-doped C60 layer are shown as black lines in both cases These findings, together with the CV measurements, are further proof that the threshold voltage shift is due to an increased number of holes near the gate dielectric interface, resulting from the p-doping effect of MoO3 in C60 Once the holes are depleted from the p-doped layer, it is possible to accumulate electrons at the gate dielectric interface in the same way as in the reference VOFET The precise voltage at which this turn-over happens, i.e., the transistor threshold voltage, is determined by the amount of holes in the p-doped layer The presented data suggest another benefit of the inversion operation, which has not been demonstrated before: The p-doping layer at the gate dielectric interface allows for control of the Off state current As visible in Fig 3, a comparably small MoO3 concentration of 20 mol % does not yet affect the threshold voltage much but results in the presence of small amounts of holes near the gate dielectric interface This leads to carrier recombination with the electrons leaking out of the source electrode in the transistor’s Off state This effect reduces the Off state current while having only a slight effect on the On state current If combined with an efficient n-dopant interlayer as injection booster at the source, this effect could provide another useful tool to enhance the On/Off ratio in n-type VOFETs As higher MoO3 concentrations introduce more holes into the p-doped layer, however, there are no longer sufficient amounts of electrons from the source-drain leakage current to recombine with; thus, the 233302-4 €nther et al Gu Appl Phys Lett 107, 233302 (2015) devices Being able to control them with simple fabrication processes can give VOFETs the boost required to make them truly successful high-performance control devices for flexible electronics applications This work has received funding from the Dr IsoldeDietrich-Stiftung and the European Community’s 7th Framework Programme under Project NUDEV (FP7-267995) H Klauk, Chem Soc Rev 39, 2643 (2010) H Sirringhaus, Adv Mater 26, 1319 (2014) M L Hammock, A Chortos, B C.-K Tee, J B.-H Tok, and Z Bao, Adv Mater 25, 5997 (2013) B L€ ussem, A G€ unther, A Fischer, D Kasemann, and K Leo, J Phys.: Condens Matter 27, 443003 (2015) N Stutzmann, R H Friend, and H Sirringhaus, Science 299, 1881 (2003) M A McCarthy, B Liu, E P Donoghue, I Kravchenko, D Y Kim, F So, and A G Rinzler, Science 332, 570 (2011) A J Ben-Sasson and N Tessler, J Appl Phys 110, 044501 (2011) A Fischer, R Scholz, K Leo, and B L€ ussem, Appl Phys Lett 101, 213303 (2012) M Uno, B.-S Cha, Y Kanaoka, and J Takeya, Org Electron 20, 119 (2015) 10 K Nakamura, T Hata, A Yoshizawa, K Obata, H Endo, and K Kudo, Appl Phys Lett 89, 103525 (2006) 11 H Kleemann, A G€ unther, K Leo, and B L€ ussem, Small 9, 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U Zschieschang, T W Canzler, A Werner, K Takimiya, M Ikeda, T Sekitani, T Someya, and H Klauk, Small 7, 1186 (2011) 23 C.-P Cheng, Y.-W Chan, C.-F Hsueh, and T.-W Pi, J Appl Phys 112, 023711 (2012) 24 W Zhao and A Kahn, J Appl Phys 105, 123711 (2009) 25 O V Molodtsova and M Knupfer, J Appl Phys 99, 053704 (2006) FIG Threshold voltage (a) and On/Off ratio (b) as a function of doping concentration and doping layer thickness for at least three devices per sample Box areas represent the interval of 25% to 75% of the data distribution, and whiskers denote the maximum and minimum values of the distribution The horizontal black lines within the boxes represent the mean value of the distribution holes themselves now form a considerable leakage current in the Off state of the transistor, as can be seen particularly well for an MoO3 concentration of 50 mol % or a doped layer thickness of 10 nm Our results show that vertical organic field-effect transistors can be improved by using the inversion operation concept Variations in the doping concentration or layer thickness of a p-doped layer in an n-type VOFET can control the threshold voltage and Off state of the device, with the layer thickness variation producing a particularly controlled shift of these parameters Indeed, the reduction of leakage currents, and thus the potential for improving the On/Off ratio, is an especially interesting feature of this operation, since leakage currents are an issue often faced in short-channel ...APPLIED PHYSICS LETTERS 107, 233302 (2015) Controlling threshold voltage and leakage currents in vertical organic field- effect transistors by inversion mode operation €nther,1,a) Christoph Hossbach,2... show that vertical organic field- effect transistors can be improved by using the inversion operation concept Variations in the doping concentration or layer thickness of a p-doped layer in an n-type... report on a vertical OFET (VOFET)10,11 operating in inversion mode. 12–15 Variation of the doping concentration or thickness of the inversion layer allows to control the threshold voltage and Off