Semiconductor-dielectric interfaces

Electronic States at the Dielectric/Semiconductor Interface in Organic Field-Effect Transistors... 

As a matter of fact, the observed degradation in the device parameters occurs with the first indication of a metallic fraction in the Ca adsorbate. Even more, the device performance is significantly degraded as soon as the Ca characteristic Ca2p3/2 and Ca2pi/2 emission lines are clearly developed. This implies that the availability of metallic Ca at the dielectric interface, even in small quantities, has a negative effect on the electron transport along the dielectric-semiconductor interface. It is likely that the metallic fraction in the oxidized Ca layer disturbs or even fully screens the electric field in the transistor channel. 

Controlling the chemistry and physics of the dielectric-semiconductor interface is essential to controlling and optimizing the performance of organic electronic devices. There are two main issues associated with the dielectric-semiconductor interface (1) the electrical properties of this interface, which are essential to the reliability, stability, and threshold voltage and (2) the chemical/mechanical properties of the interface, which relate to the ability to orgaiuze the semiconductor layer for efficient charge transport. 

Low fixed charge density and interface states have been reported for this material. A recent report using BCB as a dielectric states -type conduction has been observed for semiconducting polymers such as polyfluorene and poly(p-phenyle-nevinylene), which have otherwise been considered exclusively p-type semiconductors [24], It is proposed that n-type conduction in these polymeric semiconductors has not been observed previously due to trapping of electrons at the dielectric-semiconductor interface due to hydroxyl groups. This study highlights the critical nature of the dielectric-semiconductor interface and its role in modifying the performance of the semiconductor. 

Efforts at loading titanium oxide nanoparticles in PVA (commercially available from Nanophase) have been reported [65], In this report, titanium nanoparticles are dispersed in an aqueous solution of PVA with poly(melamine-co-formaldehyde). The solution is spun onto substrate and heated to generate a cross-Unked polymer-nanoparticle dielectric. A modest enhancement of dielectric constant is achieved for 600-nm thick films. Thin-film transistors using this composite show excellent pen-tacene mobility (> 0.2 cm V s ) and reasonable on/off ratios 10. Vj- —TV is reasonably high, suggesting static charge at the dielectric-semiconductor interface. 

The actual device structure and materials can play a significant role in the OFET performance due to the position of the source and drain contacts, energy level offsets between metal and semiconductor molecular orbitals, and the nature of the dielectric-semiconductor interface. This can be especially true in the case of blended semiconductors where phase separation can lead to different concentrations of the components at the different interfaces. By changing the contact positions, four device architectures can be achieved (Figure 8.2), which are either staggered or coplanar depending on whether the dielectric is separated from the... 

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