Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin‐Film Transistors (2011)

To suppress undesirable short-channel effects in organic transistors with nanoscale lateral dimensions, aggressive gate-dielectric scaling (using an ultra-thin monolayer-based gate dielectric) and area-selective contact doping (using a strong organic dopant) are introduced into organic transistors with channel lengths and gate-to-contact overlaps of about 100 nm. These nanoscale organic transistors have off-state drain currents below 1 pA, on/off current ratios near 107, and clean linear and saturation characteristics. Organic thin-film transistors (TFTs) are of interest for electronic applications on flexible plastic substrates, such as rollable or foldable active-matrix displays,1 conformable sensor arrays,2 and flexible identification tags.3 Due to the relatively small intrinsic field-effect mobility in most conjugated organic semiconductors (<5 cm2 V−1 s−1), the maximum frequency at which organic TFTs can be operated is usually limited to about 1 MHz.3 For certain applications, such as the integration of the row and column drivers for high-resolution active-matrix displays or sensor arrays directly on the flexible backplane,4, 5 organic TFTs that can be operated at higher frequencies (>10 MHz) are highly desirable. Such high frequencies are indeed feasible, provided the lateral dimensions of the organic TFTs are sufficiently small (about 100 nm). However, TFTs with such small lateral dimensions will suffer from a variety of detrimental short-channel effects, unless a number of important scaling requirements are observed in the design and fabrication of the transistors. Here we report on the successful fabrication and detailed analysis of organic TFTs with channel lengths and gate overlaps of about 100 nm in which the short-channel effects are greatly suppressed by area-selective contact doping (using a strong organic dopant) and by aggressive gate-dielectric scaling (using a 5.7 nm-thick, low-temperature-processed gate insulator based on a mole­cular self-assembled monolayer). As a result, these nanoscale organic TFTs have off-state drain currents below 1 pA, on/off current ratios near 107, as well as clean linear and saturation characteristics. The transconductance of these transistors reaches 0.4 S m−1, which is the largest transconductance reported for organic TFTs with patterned gate electrodes. The gate electrodes and source/drain contacts of organic TFTs are usually defined by photolithography,1, 3, 4 shadow-masking,2, 5 or inkjet printing,6 and the minimum feature size that can be achieved with these methods is usually above 1 μm. The Cambridge University group has recently developed an innovative self-aligned inkjet-printing process that makes it possible to fabricate organic TFTs with a channel length of less than 200 nm and gate-to-source and gate-to-drain overlaps of less than 700 nm.7, 8 Organic TFTs with such small lateral dimensions can in principle reach frequencies above 10 MHz, despite the modest mobilities in organic semiconductors, and even if the TFTs are operated with low voltages of about 5 V or less (see Supporting Information, SI). The ability to manufacture organic TFTs with nanoscale lateral dimensions using large-area-compatible printing techniques, such as demonstrated by the Cambridge group, creates unique and exciting opportunities for organic TFTs in high-frequency electronic applications. However, when the channel length of a field-effect transistor is reduced, the thickness of its gate dielectric must also be reduced in order to keep the ratio between the channel length and the gate-dielectric thickness large, ideally at least about 20.9 Otherwise the electric potential along the carrier channel will be dominated by the lateral electrical field (determined by the drain–source voltage VDS and the channel length L), rather than by the transverse electric field (determined by the gate–source voltage VGS and the gate-dielectric thickness tdiel), which has undesirable consequences on the transistor characteristics, including large off-state currents, small on/off current ratios, and poor current saturation. This can be seen in most previous reports on organic TFTs with submicrometer lateral dimensions.7-17 A second important requirement for the realization of nanoscale organic TFTs is a substantial reduction of the contact resistance. If the contact resistance is not reduced along with the reduction in channel length, the drain current at small drain–source voltages will be greatly suppressed, which causes the well-documented nonline­arity in the output characteristics of the transistors.6-17 The contact resistance of organic TFTs can in principle be reduced by area-selective impurity doping. If the energy of the lowest unoccupied molecular orbital (LUMO) of the dopant mole­cules is near (ideally below) the energy of the highest occupied molecular orbital (HOMO) of the host semiconductor, electrons can move from the host semiconductor to the dopant molecules, thereby creating excess holes in the semiconductor and thus increasing its electrical conductivity.18-22 This concept has previously been applied to organic p-channel TFTs with channel lengths down to 300 nm.23-28 In all these reports, however, the gate electrode of the TFTs was not patterned. Instead of a patterned gate electrode, a conducting silicon wafer served not only as the substrate, but also as the gate electrode for all the TFTs on the substrate. As a result, the overlap between the gate electrode and the source and drain contacts of the TFTs was very large (>100 μm). Such a large gate overlap has the distinct advantage that the charge injection from the contact into the semiconductor spreads across a large contact area,29, 30 but it has the distinct disadvantage of producing a large parasitic capacitance that limits the maximum frequency at which the transistors can be operated (see SI). Reducing the gate overlaps helps to reduce the parasitic capacitance, but will also create the problem of a severely reduced contact length and hence possibly much larger contact resistance.29, 30 Thus, a key question that has so far not been addressed is how useful the concept of contact doping is for organic TFTs with submicrometer channel length and with submicrometer gate overlap. To answer this question, electron-beam lithography and a proprietary organic dopant (Novaled NDP-9) have been employed in order to fabricate organic TFTs with submicrometer gate electrodes, precisely aligned, chemically doped source/drain contacts, and submicrometer channel length. Electron-beam lithography is obviously incompatible with large-area electronics, but it is helpful in understanding the material requirements for aggressive gate-dielectric scaling and controlled contact doping, until high-resolution printing techniques7, 8 become more routine. The organic TFTs developed here are based on the inverted staggered (bottom-gate, top-contact) device architecture. Each TFT has an aluminum gate electrode that is patterned by electron-beam lithography, a 5.7 nm-thick gate dielectric (composed of a thin AlOx layer created by surface oxidation and an organic self-assembled monolayer grown from solution31), a 20 nm-thick layer of the air-stable organic semiconductor dinaphtho-[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT),32-34 and 25 nm-thick gold source/drain contacts deposited on top of the organic semiconductor layer. To pattern nanoscale source/drain contacts on top of the semiconductor layer without damaging the organic semiconductor by exposure to organic solvents35 or elevated temperatures,36 an elegant process was employed that was developed by the RIKEN group, which uses a suspended resist bridge created by electron-beam lithography prior to the deposition of the organic semiconductor layer.15, 23-25 During the deposition of the organic semiconductor, the substrate is tilted, so that the semiconductor forms a continuous layer underneath the suspended resist bridge, while the deposition of the gold source/drain contacts is performed at an angle of 90°, using the resist bridge as a high-resolution shadow mask. Aside from the resist bake, the maximum process temperature is 60 °C, which is fully compatible with flexible polymeric substrates. The fabrication process is described in detail in the SI. Figure 1 shows a schematic cross-section and three electron microscopy images of the submicrometer organic TFTs. Top-contact organic TFTs with deep-submicrometer channel length and gate overlap and a 5.7 nm-thick low-temperature-processed gate dielectric. a) Schematic cross-section showing the patterned metal gate electrode, the gate dielectric (3.6 nm AlOx + 2.1 nm self-assembled monolayer), the organic semiconductor layer, the Au source/drain contacts, and the suspended resist bridge created by electron-beam lithography. b) Scanning electron microscope image of the suspended resist bridge and the Au source/drain contacts (sample tilted for imaging). c) Cross-sectional transmission electron microscope image of a TFT with a channel length of 100 nm and a gate overlap of 200 nm. The patterned Al gate electrode, the gate dielectric, and the source/drain contacts are clearly distinguished. The 20 nm-thick organic semiconductor layer, sandwiched between the gate dielectric and the source/drain contacts, is not resolved in this image. d) Oxygen map recorded by electron energy loss spectroscopy in a transmission electron microscope. The oxygen signals from the SiO2 (top layer of the substrate) and from the AlOx (part of the gate dielectric) are clearly seen. All electrical measurements were performed in ambient air at room temperature. The electrical characteristics of a TFT with a channel length of 90 nm, a channel width of 500 nm, and a gate overlap of 200 nm are shown in Figure 2. The ratio between the channel length (90 nm) and the gate-dielectric thickness (5.7 nm) is sufficiently large to facilitate strong gate coupling, so the off-state drain current is very small (about 10−13 A) and the on/off current ratio is very large (107), essentially identical to TFTs with a channel length of 30 μm.32 This on/off current ratio is the largest reported for a submicrometer organic TFT. The field-effect mobility extracted from the current–voltage characteristics is 0.05 cm2 V−1 s−1. The TFT has a maximum transconductance of 0.2 μS (also extracted from the current–voltage characteristics) and a gate capacitance of 1.7 fF (calculated from the device geometry and materials parameters), so the maximum frequency of operation predicted by Equation S1, SI, is 20 MHz. The transconductance normalized to the channel width (500 nm) is 0.4 S m−1, which is the largest width-normalized transconductance that has so far been reported for an organic TFT with a patterned gate electrode. (A transconductance of 0.7 S m−1 has recently been obtained for organic TFTs fabricated on a conducting silicon wafer serving both as the substrate and as a global gate electrode.25) Electrical characteristics of a submicrometer TFT without contact doping (L = 90 nm, ΔL = 200 nm). a) Transfer characteristics. The off-state drain current (at VGS = 0 V) and the gate current are near the detection limit (<10−13 A), the on/off current ratio is 107, the subthreshold swing is 160 mV dec−1, and the field-effect mobility extracted from the transfer characteristics is 0.05 cm2 V−1 s−1. The hysteresis in the current–voltage curve is negligible. The molecular structure of the organic semiconductor DNTT is shown in the inset. b) Output characteristics of the same device. For large drain–source voltages (VDS > VGS-Vth) the drain current shows good saturation, despite the small channel length. For small drain–source voltages (VDS < VGS-Vth), the drain current is greatly suppressed due to the large contribution of the contact resistance to the total device resistance. c) Transconductance (gm = ∂ID/∂VGS) per channel width as a function of gate-source voltage for the same device. The width-normalized transconductance reaches a maximum of 0.4 S m−1, which is the largest width-normalized transconductance reported for an organic transistor with a patterned gate electrode. However, due to the large contribution of the contact resistance in these nanoscale TFTs, the output characteristics in Figure 2 show the familiar non-linearity of the drain current at small drain–source voltages (-1 V ≤ VDS ≤ 0 V). In order to reduce the contact resistance and improve the drain-current linearity, we have also fabricated TFTs with a nominally 1 nm thick, vacuum-deposited layer of the organic molecular dopant NDP-9 inserted between the semiconductor layer and the gold source/drain contacts. Atomic force microscope (AFM) images of DNTT layers without NDP-9 and with various amounts of NDP-9 deposited on top of a DNTT layer are shown in SI, Figure S2. The images indicate that depositing a 1 nm thick NDP-9 layer onto DNTT leads to isolated clusters that coalesce into a continuous layer when more than 1 nm of NDP-9 is deposited. To confirm that doping with NDP-9 indeed increases the electrical conductivity of DNTT, we first fabricated TFTs with the dopant molecules deposited along the entire channel and in the contact regions. Comparing the transfer characteristics of TFTs without doping (Figure 3a) and with contact and channel doping (Figure 3b) shows that the channel doping greatly increases the off-state drain current. To rule out that the observed current increase is due to charge flow through the NDP-9, rather than through the DNTT, devices with 30 nm thick NDP-9 instead of DNTT were also made. SI, Figure S3, shows the current–voltage characteristics of such an NDP-9-only device. As can be seen, the current through the 30 nm-thick NDP-9 layer is below the leakage level, confirming that there is essentially no charge flow through the NDP-9 in this device. This confirms that charge flow in the DNTT TFTs with contact and channel doping (Figure 3b) occurs only through the DNTT and not through the NDP-9. Impact of channel doping and contact doping on the electrical characteristics of submicrometer TFTs (L = 150 nm, ΔL = 200 nm). a) Transfer and output characteristics of submicrometer TFTs without doping. b) Transfer and output characteristics of submicrometer TFTs with contact and channel doping. The transfer characteristics show that the channel doping greatly increases the off-state drain current (from <10−13 to about 10−9 A at VGS = 0 V), confirming that the dopant is electrically active and increases the excess carrier concentration in the semiconductor. The output characteristics show that the contact doping greatly increases the drain current at small drain–source voltages (VDS < VGS-Vth), which confirms that the doping reduces the width of the Schottky barrier at the contact/semiconductor interfaces, rendering the contacts essentially Ohmic. c) Transfer and output characteristics of submicrometer TFTs with contact doping (no channel doping). The transfer characteristics show a small off-state drain current and a large on/off ratio (similar to the TFTs without doping), while the output characteristics indicate Ohmic contact characteristics. Figure 3c shows the transfer characteristics (measured within 3 h after fabrication) of TFTs in which the dopant NDP-9 was deposited only in the contact regions, but not in the channel region. In this case the off-state drain current and the subthreshold swing are identical to those of TFTs without doping (Figure 3a), suggesting that the dopants do not drift or diffuse from the contact regions into the channel region during or shortly after transistor fabrication. Comparing the output characteristics of TFTs without doping (Figure 3a) and with contact doping (Figure 3c) reveals that contact doping with NDP-9 greatly increases the drain current at small drain–source voltages. For example, the drain current at VGS = -3 V and VDS = -0.5 V increases from -5 nA in the TFT without doping to -30 nA in the TFT with contact doping. This 6-fold increase in drain current is not due to a shift in threshold voltage, but due to a reduction in the width of the Schottky barrier at the contact/semiconductor interfaces, which means that a larger portion of the drain–source voltage drops along the channel, rather than across the contacts. Comparing the output characteristics in Figure 3a,c shows that contact doping also changes the shape of the measured ID–VDS curves closer to the ‘ideal’ linear shape. The above results confirm that NDP-9 acts as a p-type dopant in DNTT, which in turn confirms that electrons are transferred from the HOMO of DNTT to the LUMO of NDP-9, thereby creating excess holes in the DNTT and increasing the conductivity of the semiconductor. This suggests that at the interface between the evaporated DNTT and the evaporated NDP-9, the LUMO energy of NDP-9 is near or below the HOMO energy of DNTT. Since we cannot measure the orbital energies at the DNTT/NDP-9 interface, we have to rely on conclusions from other measurements to elucidate the energy lineup at the DNTT/NDP-9 interface. SI, Figure S4, summarizes the results of several measurements we have carried out to compare the doping strength of NDP-9 to that of an organic dopant that has been previously employed in organic TFTs, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ).26, 27 We found that the sheet resistance of a 30 nm-thick DNTT layer doped with NDP-9 (nominally 1 nm thick) is a factor of 7 smaller than the sheet resistance of a 30 nm-thick DNTT layer doped with F4-TCNQ (also nominally 1 nm thick; SI, Figure S4a). In addition, our cyclic voltammetry measurements on NDP-9 and F4-TCNQ indicate that the LUMO energy of NDP-9 is more negative by 0.1 eV than the LUMO energy of F4-TCNQ (SI, Figure S4d). Since calculations and measurements by several other groups have shown that the LUMO energy of F4-TCNQ is approximately -5.2 eV,21, 22, 37, 38 this indicates that the LUMO energy of NDP-9 is about -5.3 eV and thus sufficiently low to allow charge transfer between the LUMO of NDP-9 and the HOMO of DNTT (for which an energy of -5.3 ± 0.1 eV has been determined by cyclic voltammetry and density functional theory;32 see SI, Figure S3c). The quantitative analysis of the contact resistance is often performed using the transmission For analysis it is that the total resistance of a transistor the resistance measured at the of the is the of the channel resistance and the contact resistance. the channel resistance the contact resistance are directly but both of can be extracted by analysis (see SI). However, analysis useful results only if the the transistors are the our submicrometer TFTs are large to analysis (see SI, Figure the contact resistance cannot be larger than the total an limit of the contact resistance can be by the total resistance of a large number of transistors. For our TFTs in the linear (VDS = V) we a total resistance of for TFTs without doping and for TFTs with contact doping (see SI, Figure To facilitate a more analysis of the contact we also fabricated TFTs with channel lengths from 10 to 60 and with gate overlaps from 5 to 200 μm. These TFTs were fabricated using shadow In these TFTs the contribution of the contact resistance is much smaller than in the submicrometer TFTs, so the mobility extracted from the current–voltage characteristics of the TFTs is much larger cm2 V−1 SI, Figure than that of the submicrometer TFTs cm2 V−1 Figure the in the TFTs are sufficiently small to allow analysis (see SI, Figure For TFTs without contact doping, we a contact resistance of doping reduces the contact resistance to The that the contact resistance of the submicrometer TFTs is larger than that of the TFTs is by the in gate overlap. that charge flow between the source/drain contacts and the channel occurs only in those the gate and the source and drain contacts the contact length is identical to the gate overlap = The current density across the contact/semiconductor interface is not but has a maximum at the contact and in the from the (see SI, Figure The transfer length is defined as the contact length which of the charge flow between contact and semiconductor 30 analysis indicates a transfer length of for the DNTT TFTs without contact doping (see SI, Figure and measurements show that when is reduced below the contact resistance increases due to the loss of for charge flow (see SI, Figure This TFTs with our submicrometer have larger contact resistance than TFTs with our also shows that the of the gate overlap is a between the requirement for a small contact resistance a large so that ideally > and the requirement for a small parasitic capacitance for the see SI, Equation the show that contact doping reduces the transfer length (from to 5 see SI, Figure doping is a to the and allow the reduction of contact resistance and overlap capacitance SI, Figure this the potential of contact doping for submicrometer TFTs with patterned gate electrodes in high-frequency electronic applications. A with area-selective doping in organic TFTs is the of the If the dopants were to drift or diffuse into the channel, the channel doping large off-state Figure 4 shows results from a performed on a submicrometer TFT length 150 nm) with contact doping. The TFT was for 1 h in air with the maximum possible gate–source voltage = -3 to a electric field of and with the maximum possible drain–source voltage (VDS = -3 to a lateral electric field of 0.2 The results indicate that not to an increase of the off-state drain current to a loss of the in the output characteristics, of which be by dopants the transistor These results confirm the of the organic dopant NDP-9 in the contact regions of the DNTT transistors. on submicrometer TFTs (L = 150 nm) with contact doping. a) Transfer characteristics of a submicrometer TFT with contact doping and after During a gate–source voltage of -3 V to a transverse field of and a drain–source voltage of -3 V to a lateral field of 0.2 were applied for 1 The results indicate that the dopant molecules into the contact regions of the TFTs do not drift or diffuse into the transistor channel, as this to an increase in the off-state drain current or a loss of the in the output characteristics. b) current during Output characteristics of the same device and after In we have employed aggressive gate-dielectric scaling and area-selective contact doping in order to suppress short-channel effects in organic thin-film transistors that have channel lengths and gate overlaps of about 100 nm. We have shown that small off-state drain currents (about 10−13 A) and large on/off current ratios can be achieved in submicrometer organic TFTs, provided that the thickness of the gate dielectric is along with the channel length. A transconductance of 0.4 S m−1 has been obtained for a TFT with a channel length of 90 nm and a gate overlap of 200 nm. We have also shown that contact doping with a strong molecular dopant reduces the transfer length of the TFTs and is thus a to reduce the contact resistance and the parasitic overlap capacitance in organic TFTs. doping has been found to reduce the contact resistance from to Supporting is from the or from the on the fabrication process of the transistors and are The at the for for the transmission electron microscopy and at the for for for and the at for shadow and at the for and the for The provided by the and of of to are as Such are but not or are as by the The is not for the or of by the than be to the for the