Organic polymer based thin-film transistors (OP-TFTs) look very promising for flexible organic electronics. In this paper, we describe devices based on a gate-planarized structure and using spin-coated organic polymer. We have analyzed the role of the device source and drain contacts and we present data indicating Schottky behavior of the contacts in OP-TFTs. In addition, we describe a quantitative evaluation of the source drain series resistances and extract the OP-TFT intrinsic electrical parameters.

We have investigated the effects of white-light illumination on the electrical performance of organic polymer thin-film transistors (OP-TFTs). The OFF-state drain current is significantly increased, while the drain current in the strong accumulation regime is relatively unaffected. At the same time, the threshold voltage is decreased and the subthreshold slope is increased, while the field-effect mobility of the charge carriers is not affected. The observed effects are explained in terms of the photogeneration of free charge carriers in the channel region due to the absorbed photons.

We have identified longitudinal and transverse optical modes of Si-N bond in vibrational absorption spectrum of hydrogenated amorphous silicon nitride thin films. The locations of longitudinal optical resonances coincide with transverse mode of Si-O and closely neighbor bending modes of N-H, Si-H2 bond. Furthermore, the conventionally assigned asymmetric and symmetric stretching modes of Si-N bond are merely transverse modes of Si-N bond. Features of pure longitudinal vibrational modes can be revealed by subtracting appropriately scaled transverse mode components from the vibrational absorption spectrum. Analysis of the longitudinal spectrum indicates that, in addition to their association with Si-N bond, the density of the peak located at about 1215 cm−1 is also well correlated to hydrogen and nitride content in the film, while the density of the peak located at about 880 cm−1 is closed related to silicon density in the film. Peak located at about 1040 cm−1 is the longitudinal mode associated with Si-N bond.

UV-light effects on the polyimide films have been studied by characterizing the film property changes and by measuring the liquid crystal pretilt angle. Infrared, UV-visible and X-ray photoelectron spectroscopy measurements indicate that bond-breaking and oxidation occur during broadband UV-illumination of the film in the air. Free radicals are formed during UV-illumination as evidenced by the analysis of electron spin resonance spectra. Surface tension and polarity are increased after UV-illumination based on the contact angle measurements. While the pretilt angle on the rubbed polyimide film is decreased after nonpolarized UV-illumination, polarized UV-light generates a relatively small pretilt angle on a nonrubbed surface.

Aluminum metallization incorporating the anodic aluminum oxide has been optimized for high-performance amorphous silicon thin film transistors (a-Si:H TFTs). For such a device, the off-current is less than the picoampere and the ON-OFF current ratio is ∼2×107, the field effective mobility is about 1.25 cm /Vsec in the linear region and the subthreshold swing is about 0.38 V/decade. These electrical performances are acceptable for high-definition AMLCDs.

We have evaluated the electrical and the optical properties of a planarization polymer (benzocyclobutene; BCB) having a dielectric constant of 2.65 for the application to high aperture-ratio a-Si:H TFT-LCDs. An average optical transmittance of 96 % was found for the BCB film having a thickness of 3 μm in the visible-light region (400–700 nm) of the absorption spectrum. Experimental results showed that the back-channel etched a-Si:H TFT electrical performance was not significantly affected by the BCB passivation. In addition, the three-dimensional analysis of the coupling capacitances between ITO pixel electrode and metal bus lines have indicated that the low dielectric constant of planarization polymer is necessary for the application of this material to the high-aperture-ratio a-Si:H TFT-LCDs. Finally, we have established that for a given tolerance margin, a certain value of polymer thickness is needed to suppress the feed-through voltage and the vertical cross-talk between pixel electrode and metal bus lines.

A new LDD poly-Si TFT structure having symmetrical electrical characteristics independent of the process induced mis-alignment is described in this paper. Based on the experimental results, we have established there is no difference between the forward and reverse characteristics and a low leakage current, comparable to a conventional LDD poly-Si TFT, has been maintained for this new poly-Si TFTs. The maximum ON/OFF current ratio of about 1×108 is obtained for the LDD length of 1.0 μm. In addition, the kink effect in the output characteristics is remarkably improved in the new TFTs in comparison to the conventional non-LDD single- or dual-gate TFTs.

The effects of the atomic hydrogen treatment (H-treatment) of indium-tin oxide (ITO) and aluminum-doped zinc oxide (AZO) films have been investigated. The atomic hydrogen was generated by hot-wire chemical vapor deposition (HW-CVD) technique. Experimental results have shown that AZO films are chemically very stable under the H-treatment; almost no variation in the optical transmittance and electrical resistivity was observed. On the contrary, ITO films, either prepared by sputtering with ex-situ or in-situ thermal-annealing, have shown severe optical and electrical degradation and surface whitening after the H-treatment. SEM studies of the H-treated ITO surfaces have revealed that the surface whitening was due to the increase in surface roughness and the formation of granule-like metallic balls. Auger electron spectroscopy has indicated that the balls were mainly composed of indium atoms and the areas between balls were rich in oxygen atoms. These results were confirmed by X-ray diffraction and X-ray photoelectron spectroscopy measurements done on ITO before and after the H-treatment. Finally, we have demonstrated that a-SiO, deposited by PECVD will completely suppress the chemical reaction between ITO surfaces and atomic hydrogen generated by HW-CVD technique.

We have proposed a new two-dimensional simulation model, which takes into account the density of states of hydrogenated amorphous silicon (a-Si:H) and temperature-dependence of the source/drain series resistances (Rs), to explain the dependence of the activation energy (Eact) of drain-source current (IDs) on gate-source bias (VGs) in a-Si:H thin-film transistors (TFTs). We found that the influence of series resistance cannot be ignored, else an overestimated Eact will result. The results of our simulation are in agreement with experimentally observed saturation of the Eact at higher VGs.

Hydrogenated amorphous silicon nitride thin films, prepared in a large area plasma-enhanced chemical vapor (PECVD) deposition system utilizing high-rate deposition technique, have been characterized by various techniques. Experimental data obtained from this study are presented and compared to low-rate deposited PECVD films. Special attention has been devoted during this study to the difference between high- and low-rate deposited samples. The amorphous silicon (a-Si:H) thin-film transistors (TFTs) based on high-rate PECVD materials have been fabricated and characterized. The evaluation of a-Si:H TFTs indicates a good electrical performance which is comparable to its low-rate PECVD materials counterparts.

Polycrystalline silicon thin films have been selectively deposited at a deposition rate of 60–100Å/min and a substrate temperature of 300°C on molybdenum or silicon over silicon dioxide, silicon nitride or Coming 7059 glass substrate in a continuous hot-wire CVD process involving hydrogen and disilane. Excellent selectivity is achieved on features as small as one micrometer spaced molybdenum lines. The crystallinity of the selectively deposited polysilicon films is thickness dependent. The substrate selectivity is sensitive to both silicon content in the gas phase and substrate temperature. Selective deposition shifts to lower Si content in the gas phase when substrate temperature increases. At a certain substrate temperature, for a silicon content above a certain value in the gas phase, films with similar crystallinity are non-selectively deposited on both molybdenum and silicon dioxide.

In this study, we have deposited polycrystalline silicon (poly-Si) thin films by hot-wire Chemical Vapor Deposition (CVD) using hydrogen and disilane as the reactive gases. We selectively activate hydrogen and let disilane bypass the hot tungsten filament assembly and enter the reactor downstream from hydrogen. This may provide a better process chemistry, and by this approach, we have deposited poly-Si films at a substrate temperature as low as 310°C and at a growth rate as high as 100 Å/min. The substrate temperature is more than 2000C lower and the growth rate is more than twice higher compared to those of LPCVD poly-Si films. The effect of hydrogen flow rate, disilane flow rate and substrate temperature on the deposition rate and structural properties of the polysilicon films are investigated. The deposited films are characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and X-ray diffraction.

The electrical characteristics of n- and p-channel poly-Si thin film transistors having a double layer gate dielectric structure are reported. The gate dielectric consists of a silicon dioxide layer and a nitrogen-rich silicon nitride layer, both deposited by PECVD at low temperatures (≥400° C). When the silicon nitride was in contact with the poly-Si film, the effective carrier mobility (μeff), threshold voltage (Vth and subthreshold swing (St) for n-channel devices were 36 cm2/Vsec, -1.8 V and 1.65 V/decade, respectively, while for p-channel devices were 6 cm2/Vsec, -37 and 2.47 V/decade, respectively. These devices were not stable under negative gate bias stress, due to the injection of holes into the silicon nitride. When silicon dioxide was in contact with the poly-Si film, the μeff, Vth and St for n-channel devices were 26 cm2/Vsec, 3 V and 1.63 V/decade, respectively, while for p-channel devices were 10 cm2/Vsec, -22 V and 1.52 V/decade, respectively. These devices were stable under d.c. bias stress.

We report on the illumination time dependence of the generation of positive charge in gate-quality nitrogen-rich amorphous silicon nitride films subjected to sub-bandgap illumination at different temperature in vacuum. The influence of film thickness and gate bias applied during illumination on the generation of positive charge is also described. We have found that a stretched-exponential function, which characterizes dispersive charge transport in silicon nitride, gives the best description of our experimental results.

The effect of gate dielectric on the electrical characteristics of n-channel polysilicon thin film transistors was investigated. The following insulators were studied: silicon dioxide grown by wet oxidation, silicon dioxide deposited by plasma enhanced chemical vapor deposition (PECVD) and nitrogen-rich silicon nitride deposited by PECVD. It was observed that the effective electron mobility in TFTs having a deposited dielectric, either silicon nitride or silicon dioxide was higher than that measured in devices with grown silicon dioxide. The TFT leakage current was found to be lowest in devices with PECVD silicon nitride. Devices with deposited dielectrics did not degrade after a positive gate bias stress. However, reduction of the threshold voltage was observed in devices with PECVD silicon nitride, when they were subjected to a negative gate bias stress.

Films of in-situ phosphorus-doped hydrogenated microcrystalline silicon (n--μc-Si:H) were deposited by plasma enhanced chemical vapor deposition (PECVD) on Si(100) and fused quartz substrates over a range of substrate temperatures (100 - 500°C) and reactant gas dilutions (I - 100% of 1% PH3/SiH4 in H2) while maintaining a constant RF power density (0.1 W-cm−2) and total gas pressure (1 Torr). Some of the films were subjected to a rapid thermal anneal (RTA) at temperatures between 600- 1000°C for a duration of 10 seconds. The μ-Mc-Si:H films were characterized, before and after RTA, in terms of their microstructure, optical band-gap, electrical conductivity, and hydrogen and phosphorus content. The deposition rate was determined to be insensitive to substrate temperature and to decrease with increasing H2 gas dilution indicating that deposition kinetics are dominated by plasma chemistry and are not thermally activated. For pre-annealed films, cross-sectional TEM confirmed the presence of a mixed phase material at all deposition temperatures with gas dilutions ≤10%. The surfaces of thick films (>0.15 μm) were rough, giving them a hazy appearance, while thin μic-Si:H films (<0.15 μm) were smooth and mirror-like. The rough surfaces were correlated with voids and microcracks in the μuc-Si:H films observed by TEM. The optical band-gap of all pre-annealed films was 11.8eV and the electrical conductivity varied between 1 and 20 (Δ-cm)−l. The H content was found to be independent of gas dilution but decreased with increasing substrate temperature; the P content depended on both the gas dilution and substrate temperature, decreasing at high deposition temperatures. RTA was observed to significantly alter film morphology and microstructure, increase electrical conductivity, and decrease the optical band-gap.

The contact properties between different metals and hydrogenated amorphous silicon, prepared by various deposition techniques in different laboratories, are reviewed. From these studies the appropriate metallizations have been established for the achievement of Schottky diode, quasi-ohmic or ohmic contact to undoped and doped films. The various characteristic parameters describing Schottky barrier interfaces such as ideality factor, current saturation, contact resistance and barrier height are discussed. The dependence of Schottky barrier height upon the metal work function, measuring and annealing temperature, and optical band-gap are also reported. The minority-carrier injection and series resistance effects on the contact properties of a-Si:H diodes are described. All the results are interpreted in terms of a self-consistent model that exhibits an electrode-limited to bulk-limited transition.

An extensive study of contacts properties to undoped and doped hydrogenated amorphous silicon, undertaken in our laboratory, has shown that ohmicity and contact quality are very dependent on the reactivity of the metal and the quality of the metal / a-Si:H interface. For example, metals such as Sc, Mg or Ti form exceptionally good ohmic (very low barrier height) contacts, while others like Al, Cu, Mo or V form very poor quasi-ohmic contacts (average barrier height) to undoped films. In addition, metals such as Y, Ho, Hf or Er create fair quasi-ohmic (low barrier height) contacts to undoped films, at room temperature. The barrier height and the magnitude of current density can be adjusted to some degree not only by the proper choice of metal work function but also by changing material bulk resistivity or/and interface quality. Consequently, specific attention is devoted to these parameters which not only determine the quality of ohmic contact but also the dominant conduction mechanism across the barrier.

Hydrogenated amorphous silicon nitride films, prepared in various commercially available plasma enhanced chemical vapor deposition systems, have been investigated in terms of different deposition conditions.The full characterization of these gate insulators has been carried out by different techniques.Experimental data and interesting findings obtained from this study are presented.Special attention has been devoted to the influence of hydrogen on optical and electrical properties.

Hydrogenated amorphous silicon nitride films, prepared in various commercially available plasma enhanced chemical vapor deposition systems, have been investigated in terms of different deposition conditions.The full characterization of these gate insulators has been carried out by different techniques.Experimental data and interesting findings obtained from this study are presented.Indeed, some valuable relationships between physico-chemical and mechanical properties have been established.Special attention has been devoted to the influence of hydrogen on these properties.