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Tuning the threshold voltage in organic thin-film transistors by local channel doping using photoreactive interfacial layers.

Marchl M, Edler M, Haase A, Fian A, Trimmel G, Griesser T, Stadlober B, Zojer E - Adv. Mater. Weinheim (2010)

View Article: PubMed Central - PubMed

Affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria.

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Two of the most crucial device parameters are the charge carrier mobility and the threshold voltage (VTh)... Moreover, the VTh shifts for PHDBD interfacial layers are not stable for repeated measurement cycles, as shown in the right plot in Figure 2... For the PBHND-based OTFTs, even after more than 200 measurements the threshold voltage did not shift back considerably... They include an increase of the off current, a larger hysteresis, and a deterioration of the drain current at large negative gate bias... A detailed discussion can be found in the Supporting Information... The PHBND measurements were reproduced for two batches of freshly synthesized polymer and for various samples within one batch... To verify that the observed effects are indeed a consequence of interfacial acid doping, two test experiments were performed... The switch transistor is realized by a non-illuminated PBHND-containing device that has a negative threshold voltage of around VTh = –10 V... The load transistor (also a device containing a PBHND interfacial layer) was illuminated in steps of 1 s in an argon glove box after the inverter wiring was realized... After illuminating the load transistor for 3 s, an optimum value of VTh,load with respect to the threshold voltage of the switch-transistor is reached, resulting in a steep inverter transition with a maximum gain of about 40 (see bottom graph in Figure 3)... Therefore, further significant improvements can be expected by adapting the width-to-length ratio of the channel between the load and switch and by optimizing the performance of individual transistors with respect to mobility, gate leakage, etc... In conclusion, we have demonstrated an easy and reproducible way to switch OTFTs from enhancement to depletion mode by a photochemical reaction using photoacid generators as interfacial layers and demonstrated that this allows the fabrication of good quality depletion-load inverters with tunable characteristics... The first 5 nm were evaporated at a rate of 0.02 A s and the subsequent 30 nm at a rate of 0.1A s. 50-nm-thick Au source and drain electrodes were deposited through a shadow mask at a base pressure of 4 × 10 mbar... The resulting channel length and width were 50 μm and 7 mm, respectively... In this context it needs to be mentioned that in another series of experiments it was found that when illuminating PBHND prior to pentacene deposition and subsequently exposing the devices to air before growing the semiconductor, the observed VTh shifts were comparably small but the growth of the pentacene could be reproducibly tuned by the illumination time.

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a) Schematic OTFT setup with the photoreactive layer embedded between dielectric and active layer. b,c) Dominant photoreactions of PBHND and PHDBD under UV illumination. d) Transfer and e) output characteristics for a representative series of PBHND-OTFTs with illumination times of 0, 2, 3, 4, and 5 s; the arrows denote increasing illumination time. For the sake of clarity, only the off to on (forward) sweeps of the transfer characteristics are shown. The hysteresis is small and increases slightly with illumination time to ΔVG = 2 V at ID = 0.10 mA for the longest illumination time included in the plot. The full transfer characteristics including forward and backward gate voltage sweep and the characteristics for longer illumination times are included in the Supporting Information. As far as the output characteristics are concerned, forward and backward sweeps coincide.
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fig01: a) Schematic OTFT setup with the photoreactive layer embedded between dielectric and active layer. b,c) Dominant photoreactions of PBHND and PHDBD under UV illumination. d) Transfer and e) output characteristics for a representative series of PBHND-OTFTs with illumination times of 0, 2, 3, 4, and 5 s; the arrows denote increasing illumination time. For the sake of clarity, only the off to on (forward) sweeps of the transfer characteristics are shown. The hysteresis is small and increases slightly with illumination time to ΔVG = 2 V at ID = 0.10 mA for the longest illumination time included in the plot. The full transfer characteristics including forward and backward gate voltage sweep and the characteristics for longer illumination times are included in the Supporting Information. As far as the output characteristics are concerned, forward and backward sweeps coincide.

Mentions: Here, we significantly refine the concept of chemical channel doping by replacing the covalently bonded silane layers bearing sulfonic acid groups used in Refs. 25,26 with photoacid generator polymers.27 The goal hereby is to use them as an interface-modification layer (see Figure 1a), whose properties can be patterned photochemically, because, in contrast to the molecules used previously25,26, the acid group is formed only upon illumination. This paves the way for a photolithographic patterning of the interfacial doping and, thus, for controlling which transistors in a circuit operate in depletion or in enhancement mode. In other words, it enables an accurate local control of VTh through the illumination dose by a method that is fully compatible with lithographic techniques omnipresent in conventional semiconductor industry.


Tuning the threshold voltage in organic thin-film transistors by local channel doping using photoreactive interfacial layers.

Marchl M, Edler M, Haase A, Fian A, Trimmel G, Griesser T, Stadlober B, Zojer E - Adv. Mater. Weinheim (2010)

a) Schematic OTFT setup with the photoreactive layer embedded between dielectric and active layer. b,c) Dominant photoreactions of PBHND and PHDBD under UV illumination. d) Transfer and e) output characteristics for a representative series of PBHND-OTFTs with illumination times of 0, 2, 3, 4, and 5 s; the arrows denote increasing illumination time. For the sake of clarity, only the off to on (forward) sweeps of the transfer characteristics are shown. The hysteresis is small and increases slightly with illumination time to ΔVG = 2 V at ID = 0.10 mA for the longest illumination time included in the plot. The full transfer characteristics including forward and backward gate voltage sweep and the characteristics for longer illumination times are included in the Supporting Information. As far as the output characteristics are concerned, forward and backward sweeps coincide.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3021719&req=5

fig01: a) Schematic OTFT setup with the photoreactive layer embedded between dielectric and active layer. b,c) Dominant photoreactions of PBHND and PHDBD under UV illumination. d) Transfer and e) output characteristics for a representative series of PBHND-OTFTs with illumination times of 0, 2, 3, 4, and 5 s; the arrows denote increasing illumination time. For the sake of clarity, only the off to on (forward) sweeps of the transfer characteristics are shown. The hysteresis is small and increases slightly with illumination time to ΔVG = 2 V at ID = 0.10 mA for the longest illumination time included in the plot. The full transfer characteristics including forward and backward gate voltage sweep and the characteristics for longer illumination times are included in the Supporting Information. As far as the output characteristics are concerned, forward and backward sweeps coincide.
Mentions: Here, we significantly refine the concept of chemical channel doping by replacing the covalently bonded silane layers bearing sulfonic acid groups used in Refs. 25,26 with photoacid generator polymers.27 The goal hereby is to use them as an interface-modification layer (see Figure 1a), whose properties can be patterned photochemically, because, in contrast to the molecules used previously25,26, the acid group is formed only upon illumination. This paves the way for a photolithographic patterning of the interfacial doping and, thus, for controlling which transistors in a circuit operate in depletion or in enhancement mode. In other words, it enables an accurate local control of VTh through the illumination dose by a method that is fully compatible with lithographic techniques omnipresent in conventional semiconductor industry.

View Article: PubMed Central - PubMed

Affiliation: Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

Two of the most crucial device parameters are the charge carrier mobility and the threshold voltage (VTh)... Moreover, the VTh shifts for PHDBD interfacial layers are not stable for repeated measurement cycles, as shown in the right plot in Figure 2... For the PBHND-based OTFTs, even after more than 200 measurements the threshold voltage did not shift back considerably... They include an increase of the off current, a larger hysteresis, and a deterioration of the drain current at large negative gate bias... A detailed discussion can be found in the Supporting Information... The PHBND measurements were reproduced for two batches of freshly synthesized polymer and for various samples within one batch... To verify that the observed effects are indeed a consequence of interfacial acid doping, two test experiments were performed... The switch transistor is realized by a non-illuminated PBHND-containing device that has a negative threshold voltage of around VTh = –10 V... The load transistor (also a device containing a PBHND interfacial layer) was illuminated in steps of 1 s in an argon glove box after the inverter wiring was realized... After illuminating the load transistor for 3 s, an optimum value of VTh,load with respect to the threshold voltage of the switch-transistor is reached, resulting in a steep inverter transition with a maximum gain of about 40 (see bottom graph in Figure 3)... Therefore, further significant improvements can be expected by adapting the width-to-length ratio of the channel between the load and switch and by optimizing the performance of individual transistors with respect to mobility, gate leakage, etc... In conclusion, we have demonstrated an easy and reproducible way to switch OTFTs from enhancement to depletion mode by a photochemical reaction using photoacid generators as interfacial layers and demonstrated that this allows the fabrication of good quality depletion-load inverters with tunable characteristics... The first 5 nm were evaporated at a rate of 0.02 A s and the subsequent 30 nm at a rate of 0.1A s. 50-nm-thick Au source and drain electrodes were deposited through a shadow mask at a base pressure of 4 × 10 mbar... The resulting channel length and width were 50 μm and 7 mm, respectively... In this context it needs to be mentioned that in another series of experiments it was found that when illuminating PBHND prior to pentacene deposition and subsequently exposing the devices to air before growing the semiconductor, the observed VTh shifts were comparably small but the growth of the pentacene could be reproducibly tuned by the illumination time.

Show MeSH