Evaporation OFETs

In addition to the potential cost advantage due to easier processing via printing or evaporation, OFETs potentially offer reduced bias stress in current drive applications over a-Si transistors fabricated at less than 200°C. At these temperatures, transistors can be fabricated on a range of transparent flexible substrates and are particularly applicable to flexible OLED displays. There are also circuit and architecture advantages to using PFETS for bottom emission OLED displays [136]. 

An altemative means to realize well-organized thin films of small molecules is the Langmuir-Blodgett (LB) technique, which allows a fine control of both the stmcture and thickness of the film. We note, however, that this technique is in principle restricted to amphiphilic molecules, composed of a hydrophobic chain and a hydrophobic head-group, which is not the case for most molecules used in OFETs. Nevertheless, LB-grown OFETs have been reported with mixed layers of quinquethiophene (5T) and arachidic acid, which gave well-behaved devices [56]. However, the necessary mixing of the electrically active compound with an inactive compound leads to a substantial decrease in the mobility, as compared to that of a vacuum-evaporated film. 

Source and drain contacts were deposited on top of the Pc films by thermal evaporation of Au (purity 99.99%) through shadow masks. The layout of the shadow mask yielded 8 independent OFETs on one sample. Samples are noted as A to F in the following, and distinct OFETs are noted as Al, A2, etc. 

The OFETs using inorganie gate dieleetries were prepared on a heavily doped silieon substrate, whieh also serves as the eommon gate eleetrode for all fabricated devices. The semiconducting film consists of evaporated pentacene, which is commercially available. In Figure 18.1 the transistor architecture with bottom gate and bottom drain and source contacts is shown. 

Figure 18.3 Output characteristic of an OFET with a channel length of 1 pm and a channel width of 1000 pm, using a 100 nm thick sihcon dioxide layer as gate dielectric. Before evaporating the pentaeene in high vacuum the sample was pretreated in an oxygen plasma for 30 s at a plasma power of 100 W.

Therefore, further experiments have been done to differentiate between the particular gases. A chip containing an OFET of 1000 pm channel width and of 1 pm channel length was bonded, connected to the measurement setup and put into a vacuum chamber. Then, an approximately 30 nm thick layer was thermally evaporated at low deposition rate and a process pressure of 7x10 mbar. Next, the sample was characterised in high vacuum before the chamber was flooded by pure technical oxygen so that the OFET did not come into contact with the ambient atmosphere. The device was measured a second time before the chamber was evacuated again. 

The source and drain contacts of the examined OFETs were deposited by thermal evaporation of Au as described above for deposition of the radiotracers. Deposition of the contacts was not performed in the same chamber as the radio-tracer deposition in order to avoid contamination of the sample with radioactive isotopes. Patterning of the contact stractures was obtained using a stainless steel shadow mask. By deposition of Au an array of nine contacts was formed. The contact area of the Au was 50 x 50 pm and the distance between the contacts varied from 300 pm to 3290 pm. Three Au contact arrays with a thickness of 50 nm were deposited onto a 40 nm Pc film at a substrate temperature of 75 °C. The first contact array (Array 1 in the following) was deposited at a rate of 0.8 nm/min. For the second set of contacts (Array 2 in the following) first a submonolayer of Au was deposited very slowly (< 1 ML/h) on top of the Pc film in order to allow strong diffusion. Afterwards, the contacts were deposited at the same rate of 0.8 nm/min as the first set. The third array (Array 3 in the following) was deposited at 0.8 nm/min with the substrate at room temperature. 

Figure 21.11 Schematic of the ferroelectric OFET. We use P3HT as active layer and thermal evaporated Au electrodes for source and drain. The channel length is 40 pm, the channel width is 1 cm.

When shrinking the ehannel length of the transistor deviee, the ehannel resis-tanee is redueed. Beneath a eertain threshold, contaet properties beeome visible in the present materials. To obtain optimal eharaeteristies for the sub-mierometer transistors, the ehoiee of the electrode material is erueial. Figure 22.7 shows mobility values ju(L) obtained from three series of OFETs with different eleetrode materials and DH4T as semiconduetor. The thiekness of the semieonduetor was 10 nm ( 4 monolayers), deposited at an evaporation speed of 2 pm/s. The eleetrode materials were Ti/Au (thiekness 5/20 nm), Ti/Pt (5/20 nm), and Pd (25 nm). Samples were produced by optieal lithography and... 

DCNDBQT organic field effect transistors (OFETs) were fabricated on a highly doped n-Si wafer with 30 nm silicon dioxide. Firstly, the silicon surface was rinsed with Dl-water, acetone and iso-propanol in order to remove small particles and organic impurities. Secondly, the substrate was treated with oxygen plasma and silanised for 26 hours at 60 °C by hexamethyldisilazane (HMDS) in order to improve the OFET performance [21]. As source-drain contacts of the bottom contact transistors (BOC) gold was used, which was evaporated through a shadow mask on the silieon dioxide (see Figure 5.2). 

Rubrene thin films were evaporated by HWE on mica. After the formation of an amorphous matrix, spherulite structures start to grow, which finally cover the whole surface. Due to their polycrystalline property the spherulites are resistant against oxidization while the amorphous matrix becomes transparent upon exposure to air, which is a clear sign for oxidation. The mbrene spherulites provide therefore a promising material for the fabrication of OFETs. 

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