Pentacene bottom-contact

A. Terfort, Influence of anthracene-2-thiol treatment on the device parameters of pentacene bottom-contact tran-... 

The reduced charge-carrier mobility (compared to typical pentacene bottom-contact TFTs) stemmed from the dielectric surface roughness (OTS was stamped on the dielectric surface). Using solution deposition methods to create the molecular templates yielded much smoother dielectric surfaces, and the performance of these... 

Bock C, Pham DV, Kunze U, Kafer D, Witte G, Terfort A (2007) Influence of anthiacene-2-thiol treatment on the device parameters of pentacene bottom-contact transistors. Appl Phys Lett91 052110... 

Fig. 2.4. The often dramatic effects bottom contacts can have on molecular ordering in organic semiconductors like pentacene. A. Schematic diagram of the type of disorder introduced in pentacene s lamellar structure as thin-film growth encounters a step, for example...

Fig. 15.9. Bottom-contact pentacene TFT characteristics before and after parylene passivation. The device has W/L = 400/80 pm. The passivation degrades the mobility whereas the threshold potential is relatively steady.

A 0.5-pm film of parylene-C was deposited on pentacene TFTs at room temperature. TFT characteristics were measured and compared before and after the deposition without patterning of the parylene. The bottom-contact TFT (W/L = 400/80 pm) characteristics before and after parylene passivation are compared in Fig. 15.9. Field-effect mobility decreased by 50% from 1.6 cm2 V-1 s-1 to 0.80 cm2 V 1 s 1 whereas the threshold voltage stays the same. 

The most simple pentaeene OTFT test structure used in many labs is based on a Si wafer piece covered with a thermal oxide. Here, the heavily doped Si wafer takes the role of the back gate electrode, and the Si02 takes the role of the gate dielectric. A pentacene thin film is deposited as the semiconducting layer. Source and drain electrodes are deposited either on the silicon oxide (bottom contact) or on top of the pentacene film (top contact). 

Figure 15.4 Characteristics of a bottom contact pentacene OTFT. Channel geometry i = 25 im, W= 1000 pm (see text and inset). Sweep rate ...

Figure 20.1 Sample geometries (a) Schematic contact layout for bottom-contacted pentacene OFETs, and (b) for top-contacted pentacene OFETs, with sample holder and electrical contacts.

Figure 20.4 (a) Results of a two-dimensional simulation of a pentacene OFET with geometric parameters close to the bottom-contacted device investigated with potentiometry, for // = 0.014 cm V s and an effective injection barrier of 0.42 eV, and (b) construction of an injection barrier of 0.73 eV out of the effective barrier of 0.42 eV and the electric field close to the source contact, as obtained in the simulation for a gate voltage of Uq = -30 V. 

In the following, we present the results of charge transient spectroscopy performed on the bottom contacted pentacene OFETs, a variant of DLTS where the current transient is integrated, yielding a charge transient [43, 44]. In combination with capacitance DLTS, this technique can also provide information on the depth profile of the trap distribution [45]. 

If the density of states (DOS) of the trap states were broadened, the charge transient Q(t) would not follow a single exponential rise as in Eq. (6), resulting in turn in broadened QTS traces with respect to the fit based on Eqs. (5), (6). Such a behaviour was observed for polymer-based diodes [20] and for phthalocyanines [46], but for our bottom-contacted pentacene OFETs, we found no evidence of a broadened DOS of the trap states with a corresponding distribution of de-trapping rates. 

Comparing the potentiometry measurements in Figure 20.8 obtained on top-contacted pentacene OFETs with the data for the bottom-contacted sample in Figure 20.2, the most striking difference is the absence of a substantial potential drop close to the source contact. The reduction of the contact resistance... 

Obviously contact resistance drops from DH4T to the larger molecules more than one order of magnitude and saturates between DH6T and DH7T at a value of 1 k 2 cm. In comparison with other organic semiconductors, this is a rather low value, especially for untreated gold contacts. Pentacene for example is reported to show a contact resistance of about 100 k 2 cm in comparable bottom contact devices [49]. 

In addition to the morphological features of the pentacene layer, the performance of an OTFT is influenced by the microscopic interface environment at the interface between the pentacene layer and a source and drain metallic contact. The electronic parameters of the interface may give rise to an increased contact resistance. Therefore it is important to understand the relationship between the chcrnical/structural characteristics of the OS/metal interface and charge carrier transport in OTFT. For example the difference in mobility between the top-contact and bottom-contact OTFT was associated to the different morphology of the pentacene layer near the metallic contacts [13],... 

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