Pentacene vacuum deposition

Aromatic hydrocarbon pentacene is important for application in OTFTs as it has superior field effect mobility, good semiconducting behavior, and stability [153], The chemical structure is shown in Fig. 6.10. Pentacene semiconductor films can be fabricated by sublimation in a vacuum deposition system. Optimization of the fabrication parameters, such as the substrate temperature and deposition rate, can yield a highly ordered pentacene film with improved device performance. Oriented films have optical and electrical anisotropies. 

FIGURE 3.1.1 Evolution of OFET performance with time for variousp-channel (pentacene, rubrene, other small molecules, and polymers) and -chaimel organic semiconductors, (v) Vacuum deposition (s) solution deposition (sc) single crystal. A range of mobilities for hydrogenated amorphous silicon (a-Si H) is shown for reference. Please refer to Table 3.1.1 for the respective molecule indices. 

The present device is schematically shown in Figure 6.3.15(a). A gate electrode consisting of a Cr adhesion layer and an Au layer is deposited through a shadow mask in a vacuum evaporator on a 13-pm thick polyimide film. Then, polyimide precursors are spin-coated to form 500-mn thick gate dielectric layers. A 50-mn thick pentacene is deposited, and then 50-mn thick Au drain and source electrodes are evaporated. The channel length L and width W of the FETs are 100 [im and... 

Fig. 22 Schematic setup and characteristics of a CMOS inverter consisting of OFETs both fabricated by vacuum-deposition of pentacene on a PMMA/Si02 dielectric. The transport character is achieved by proper choice of source- and drain contact metals...

To obtain the thin-film transistor array suitable to softening the panel, flexible 5 inch diagonal liquid crystal displays were also fabricated by lower-temperature vacuum deposition of the organic semiconductor (pentacene). The fabricated panel (QQVGA, 160 X 120 pixels) has confirmed the video display function [12]. 

The channel layer is 50-nm-thick pentacene deposited in the vacuum sublimation system at ambient substrate temperature with fine metal masks. After deposition of pentacene thin film, 30-nm-thick gold layers are deposited as source and drain though fine metal masks. 

Presently there exists a strong research interest in the understanding, development, and optimisation of organic field effect transistors (OFETs) [1, 2]. Two classes of semiconducting organic materials are considered, namely molecular materials which are processed into thin films by vacuum sublimation [1, 2], and polymers which are deposited onto substrates in the form of solutions, for instance by spin coating [3]. In this chapter we report on OFETs based on thin polycrystalline films of the molecular material pentacene (Pc) as the semiconducting material. 

Pentacene has all the qualities needed for an OTFT. The main challenge is the eontrol of the contact and dielectric interfaces to suppress the voltage drifts and hysteresis effects associated with trap states. Also the dependency of the injection efficiencies on contact preparation suggest that even if pentacene deposition technology can be applied under rather rough conditions, ultra high vacuum and clean environments may still be useful on the way of learning how to produce reliable devices. 

Despite this popularity, pentacene is not ideal. Pentacene is not soluble to an appreciable degree and can only be deposited using vacuum processes. Pentacene is relatively easily oxidized, especially at the 6,13 positions, which disrupts transport and crystallization in devices. Finally, pentacene can condense into two crystal phases which are closely related, but not perfectly matched. This often leads to polymorphic crystal growth, which can lead to mismatched grains and decreased OFET performance. Several approaches have been pursued to rectify these problems. 

---