• 10.1021/nl101589x
  • Nano Letters
  • Volume 10
  • September 2010
  • pp 3467-3472

CN enabled VOFET potential to drive pixels in AMLED


In this manuscipt, a high current low voltage carbon nanotube (CN) enabled vertical organic field-effect transistor (VOFET) is reported that have the future potential to drive the pixels in active matrix organic light emitting diode (AMLED).

                         Usually, planar architecture based FETs using organic materials have low-speed, high operational voltage and poor performance due to their low mobility and high resistivity. For the basic operation of OFET please refer [1]. Alternate solutions to enhance the performance of OFET could be either to improve the electrical parameters of organic materials itself or change the device structure to produce efficient organic based devices. In this article, VOFET is employed as an alternate solution since in this architecture FET has a short channel with large effective area for a fast and large current operation. To have a comprehensive and clear understanding of VOFET operations please refer [2].  

                          The device structure consists of (1) thermally evaporated Al (60 nm) as gate electrode onto glass substrates followed by (2) the formation of Al2O3 as dielectric formed by O2 plasma oxidation on top of Al gate.  (3) A thin layer of benzocyclobutene (BCB) is formed by diluting it with trimethylbenzene and spin coated to achieve a thickness of ~ 5 nm (measured by AFM). BCB layer is hard baked at 2500 C for 1 h on a hot plate in Ar glovebox to crosslink and improve its impervious to solvents. In general, polymer dielectrics such as BCB has low dielectric constant and therefore in order to achieve high capacitance to operate FETs at low voltage (C = Q/V) very thin layer of BCB is deposited. CNT source is done by following the method desribed previously [3] and the contacts were made by depositing Cr/Au source contacts. The substrates were baked on a hot plate in a glove box at 2250 C for 1 h.(4) High hole mobility organic semiconductor dinaphtho-[2,3-b:2’3’f]thieno [3,2-b]-thiophene (DNTT) (received from Nippon Kayaku co.) is thermally evaporated to achieve a thickness of 480 nm. (5) Finally, Au (60 nm) is thermally evaporated to form drain electrode through a TEM grid shadow mask with a patterned size of 200 µm. All the depositions are done in Ar glovebox without exposure to ambient air except nanotube layer deposition part.

                            The article explores electrical measurements of current density vs output voltage (J-V) (measured by Keithley model 2612A) and gate dielectric capacitance vs frequency (C-f) (measured by HP 4284A precision LCR meter) of Al/Al2O3/Au (sample A) and Al/Al2O3+BCB/Au (sample B). These samples were fabricated in Metal-insulator-metal (MIM) sandwich structures to compare the quality and performance of dielectrics in both the samples. J-V curves revealed leakage current density (i.e. vertical current passing through the dielectric) of order of 1 µA/cm2 for sample A whereas for sample B it remains much lower (~ 1 order magnitude of J difference) below electric field 2 MV/cm with placing an upper limit of ± 2 V Al voltage. Further increase in electric field resulted in dielectric breakdown for both the samples. On the other hand, C-f characteristics showed little dependence of MIM capacitance on frequency indicating absence of mobile ion impurities. Sample A showed a capacitance of 1717 nF/cm2 whereas introduction of BCB layer as in sample B lead to the reduction of capacitance to 354 nF/cm2. From the above results, it is clear that authors had to trade-off between MIM capacitance and leakage current and eventually went with sample B with lower leakage current, but also with lower dielectric capacitance. In my opinion, a much better organic dielectric material could have been used for example fluorocarbon [3], which has an outstanding dielectric breakdown of about 9.8 MV/cm and a leakage current below than 6.5 X 10-11 A for an applied voltage of < 70 V.   

                          The transfer characteristics plotted with sweeping gate voltage (VG) from – 2 to 2 V and drain voltage (VD) as steps (- 0.1 V, -1 V, - 3 V) reveal on/off ratio of 104-105 with fully on-current reaching up to 110 mA/cm2. Also, the interesting part is the occurrence of very small and almost negligible part of current hysteresis indicating that the trap density for charge carriers at semiconductor-dielectric interface is very low. Hysteresis effects are often observed in organic transistors during sweeps of gate voltage and are known to affect the reproducibility of current-voltage behavior.  Output characteristics are also plotted with sweeping VD from 0 to – 3 V and VG as steps from 0 to – 2 V with – 0.5 V steps between them. The output characteristics revealed the existence of series resistance between the nanaotube source electrode and Au source contact to nanotubes resulting in the appearance of saturation in its characteristics. From the transfer characteristics, the authors have plotted on/off ratio (VG = - 2 V on and VG = 2 V off) vs on-current density as VD was swept from 0 to – 3 V and found on/off ratio stays above 105 to 50 mA/cm2 and claimed that luminance of OLED driven by this CN-VFET could be 4 to 5 times brighter than a typical computer screen. The electroluminescent efficiency (E) is given by  E = L/I  where L is luminance (in cd/m2) and I is current density (in A/cm2).

                           Finally, authors claimed since CN-VFET is a Schottky barrier device, threshold voltage (Vth )which is the minimum gate voltage needed to create a conducting channel between source and drain terminals could be extracted from linear Jdrain vs Vg plot (transfer characteristics) instead of conventional FET linear fit of Id1/2 vs Vg plot. The value of Vth obtained varies from – 0.98 to – 1.02 V. Does the same principle is applied to field-effect mobility is not very clear? In spite of moderate dielectric capacitance, CN-VFET operates at low voltage with highest output current density by factor of 3.9 when compared with other FETs (refer table 2 of this article) and employs minimum patterned size of 200 µm (drain electrode). Experimental reproducibility is very high since almost all of the fabrication steps were done in Ar glove box avoiding the exposure to ambient air and requires materials are easily available. It could be interesting to enhance the device performance further by using an optimum organic dielectric layer and calculate other parameters of the device like mobility for instance.  

[1] N. Tessler and Y. Roichman, Organic Field Effect Transistors, Microelectronic and Nanoelectronic centers, Technion, Isreal Institute of Technology.

[2] L. Pa and Y. Yang, Unique architecture and concept for high-performance organic transistors, Appl. Phys. Lett. vol. 85, p. 5084, 2004.

[3] W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen and B. Batlogg, High performance organic field-effect transistors with fluoropolymer gate dielectric, Proc. SPIE vol. 6658, Organic Field-Effect Transistors VI, p. 665807, 2007.

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