Flora M. Li

Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric

We report on the stability of nanocrystalline silicon (nc-Si) bottom-gate (BG) thin film transistors (TFTs) with various compositions of hydrogenated amorphous silicon nitride (a-SiNx:H) gate dielectric. TFTs with nitrogen-rich nitride exhibit higher output transconductance, threshold voltage stability, and effective field effect mobility (μFE) than the devices with silicon-rich gate dielectric. For example, μFE drops from 0.75to0.2cm2∕Vs when the gate dielectric composition [N]∕[Si] changes from 1.3 to 1. The corresponding threshold voltages (VT) are 4 and −2V. Following 5h electrical stress tests, the shift in threshold voltage (ΔVT) is larger for dielectrics with lower [N]∕[Si] content, regardless of the operating regime. Indeed, ΔVT in the saturation regime is considerably less and correlates with the charge concentration in the channel, i.e., ΔVT in saturation is about 2∕3 of that in the linear regime. Relaxation tests on the stressed TFTs show that the charge trapping is the instability mechanism in...

High performance nanocomposite thin film transistors with bilayer carbon nanotube-polythiophene active channel by ink-jet printing

Nanocomposite thin film transistors (TFTs) based on nonpercolating networks of single-walled carbon nanotubes (CNTs) and polythiophene semiconductor [poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12)] thin film hosts are demonstrated by ink-jet printing. A systematic study on the effect of CNT loading on the transistor performance and channel morphology is conducted. With an appropriate loading of CNTs into the active channel, ink-jet printed composite transistors show an effective hole mobility of 0.23 cm2 V−1 s−1, which is an enhancement of more than a factor of 7 over ink-jet printed pristine PQT-12 TFTs. In addition, these devices display reasonable on/off current ratio of 105–106, low off currents of the order of 10 pA, and a sharp subthreshold slope (<0.8 V dec−1). The work presented here furthers our understanding of the interaction between polythiophene polymers and nonpercolating CNTs, where the CNT density in the bilayer structure substantially influences the morphology and transisto...

High-k (k=30) amorphous hafnium oxide films from high rate room temperature deposition

Amorphous hafnium oxide (HfOx) is deposited by sputtering while achieving a very high k∼30. Structural characterization suggests that the high k is a consequence of a previously unreported cubiclike short range order in the amorphous HfOx (cubic k∼30). The films also possess a high electrical resistivity of 1014 Ω cm, a breakdown strength of 3 MV cm−1, and an optical gap of 6.0 eV. Deposition at room temperature and a high deposition rate (∼25 nm min−1) makes these high-k amorphous HfOx films highly advantageous for plastic electronics and high throughput manufacturing.

Zinc Oxide Nanostructures and High Electron Mobility Nanocomposite Thin Film Transistors

This paper reports on the synthesis of zinc oxide (ZnO) nanostructures and examines the performance of nanocomposite thin-film transistors (TFTs) fabricated using ZnO dispersed in both n- and p-type polymer host matrices. The ZnO nanostructures considered here comprise nanowires and tetrapods and were synthesized using vapor phase deposition techniques involving the carbothermal reduction of solid-phase zinc-containing compounds. Measurement results of nanocomposite TFTs based on dispersion of ZnO nanorods in an n-type organic semiconductor ([6, 6]-phenyl-C61-butyric acid methyl ester) show electron field-effect mobilities in the range 0.3-0.6 cm2 V-1s-1, representing an approximate enhancement by as much as a factor of 40 from the pristine state. The on/off current ratio of the nanocomposite TFTs approach 106 at saturation with off-currents on the order of 10 pA. The results presented here, although preliminary, show a highly promising enhancement for realization of high-performance solution-processable n-type organic TFTs.

Organic thin-film transistor integration using silicon nitride gate dielectric

The impact of film composition of silicon nitride (SiNx) gate dielectrics on the electrical performance of organic thin-film transistors (OTFTs) was investigated. Polythiophene OTFTs with SiNx dielectric, prepared using a series of interface modification processes, exhibited effective mobility of 0.09cm2∕Vs and on/off current ratio of 107. Overall improvement in mobility, on/off current ratio, and gate leakage current was observed as silicon content in SiNx increases. The results demonstrate the viability of using SiNx for OTFTs. The low temperature processing and large area deposition capabilities of SiNx hold great promise for integration of OTFT circuits for large area flexible electronic applications.

Intrinsic photoluminescence from low temperature deposited zinc oxide thin films as a function of laser and thermal annealing

An investigation into the modification of low temperature deposited ZnO thin films by different annealing processes has been undertaken using laser, thermal and rapid thermal annealing of 60 nm ZnO films deposited by high-target-utilization sputtering. Single-pulse laser annealing using a KrF excimer laser (λ = 248 nm) over a range of fluences up to 318 mJ cm−2 demonstrates controlled in-depth modification of internal film microstructure and luminescence properties without the film degradation produced by high temperature thermal and RTA processes. Photoluminescence (PL) properties show that the ratio of defect related deep level emission (DLE, 450–750 nm, 2.76–1.65 eV) to excitonic near band-edge emission (NBE at 381 nm, 3.26 eV) is directly correlated to processing parameters. Thermal and rapid thermal processing results in the evolution of a strong visible orange/red DLE PL (with peaks at 590 nm, 2.10 eV and 670 nm, 1.85 eV) dominated by defects related to excess oxygen. At higher temperatures, the appearance of a green/yellow emission (530 nm, 2.34 eV) indicates a transition of the dominant radiative transfer mechanism. In contrast, laser processing removes defect related DLE and produces films with intense NBE luminescence, correlated to the observed formation of large grains (25–40 nm).

Degradation behavior and damage mechanisms of CCD image sensor with deep-UV laser radiation

As the deep-ultraviolet (DUV) laser technology continues to mature, an increasing number of industrial applications is shifting to intense DUV radiation sources. This trend necessitates the development of DUV sensitive charge-coupled device (CCD) cameras to provide imaging capability for process control and inspection purposes. In this paper, we examine the effects of DUV laser radiation on CCD image sensor characteristics and the mechanisms responsible for DUV laser damage in CCDs. When samples of thinned front-illuminated linescan CCD sensors are exposed to F/sub 2/(/spl lambda/=157 nm) excimer laser radiation, fluctuation in the extrinsic quantum efficiency (QE) and a substantial upsurge in the dark current density are observed as a function of exposure dose. The visible QE, dark current, and charge conversion efficiency (CCE) are also permanently altered by the DUV irradiation. These instabilities can be attributed to a variety of UV-induced effects that modify the optical and electrical properties of the SiO/sub 2/ layer and Si-SiO/sub 2/ interface, resulting in temporary and permanent shifts in CCD performance. Optimization of the overlying oxide thickness and the Si-SiO/sub 2/ interface quality are necessary in order to realize CCD sensors with the desired performance, radiation tolerance and stability at DUV wavelengths.

Convergence of interactive displays with smart mobile devices for effective advertising: A survey

The trend of replacing public static signages with digital displays creates opportunities for interactive display systems, which can be used in collaborative workspaces, social gaming platforms and advertising. Based on marketing communication concepts and existing models for consumer behavior, three stages, namely attraction, interaction and conation, are defined in this article to analyze the effectiveness of interactive display advertising. By reviewing various methods and strategies employed by existing systems with attraction, interaction and conation stages, this article concludes that smart mobile devices should be integrated as a component to increase the effectiveness of interactive displays as advertising tools. Future research challenges related to this topic are also discussed.

Non-ohmic contact resistance and field-effect mobility in nanocrystalline silicon thin film transistors

Contact resistance has a significant impact on the electrical characteristics of thin film transistors. It limits their maximum on-current and affects their subsequent behavior with bias. This distorts the extracted device parameters, in particular, the field-effect mobility. This letter presents a method capable of accounting for both the non-ohmic (nonlinear) and ohmic (linear) contact resistance effects solely based upon terminal I-V measurements. Applying our analysis to a nanocrystalline silicon thin film transistor, we demonstrate that contact resistance effects can lead to a twofold underestimation of the field-effect mobility.

Photolithographically defined polythiophene organic thin-film transistors

A photolithography process for the fabrication of organic thin-film transistor (OTFT) and integrated circuits will be presented. Fully encapsulated polythiophene OTFTs in the top-gate, bottom-gate, and dual-gate configurations have been demonstrated using this approach. Photolithography steps are incorporated for the definition of gate electrodes and source/drain contacts, isolation of transistors, and formation of vias and interconnects. These steps are vital to achieve device integration and to realize organic integrated circuits for applications such as flat panel displays and radio-frequency identification tags. The fabrication process is compatible with various substrates (e.g., glass and plastic), and enables the realization of discrete transistors and OTFT-based circuits through consecutive photolithographic steps and a tailored etch recipe for patterning of the polymer film. This article will provide a detailed examination of the fabrication approach, analyze the electrical performance of various ...