S. E. Ready

All jet-printed polymer thin-film transistor active-matrix backplanes

Thin-film transistor (TFT) backplanes fabricated by using jet printing as the only patterning method are reported. Additive and subtractive printing processes are combined to make 128×128 pixel active matrix arrays with 340μm pixel size. The semiconductor used, a regioregular polythiophene, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]; (PQT-12) is deposited by inkjet printing and exhibits average TFT mobility of 0.06cm2∕Vs, on/off ratios of 106, and minimal bias stress. The printed TFTs have high yield with a narrow performance distribution. The pixel design benefits from the registration accuracy of jet printing and it is shown that the electrical performance is suitable for addressing capacitive media displays.

Jet printing flexible displays

Jet printing is an interesting patterning technique for electronic devices because it requires no physical mask, has digital control of ejection, and provides good layer-to-layer registration. It also has the potential to reduce display manufacturing costs and enable roll-to-roll processing. The technique is illustrated with examples of prototype printed displays using amorphous silicon and polymer semiconductors.

Structural, optical, and spin properties of hydrogenated amorphous silicon‐germanium alloys

We report on a detailed study of structural and electronic properties of hydrogenated amorphous silicon‐germanium alloys deposited by rf glow discharge from SiH4 and GeH4 in a diode reactor. The chemical composition of the alloys is related to the deposition conditions, with special emphasis on preferential incorporation of Ge into the solid phase and on the role of inert dilutant gases. Hydrogen bonding in the alloys is investigated with nuclear magnetic resonance and vibrational (Raman and infrared) spectroscopy. The optical properties of a‐SiGe:H samples deposited under optimal conditions are analyzed with the help of subgap absorption measurements and band‐tail luminescence for the entire range of alloy composi‐tions. A large part of the article describes an investigation of the electron‐spin‐resonance response of undoped alloys. The spin density associated with dangling bond defects localized on Si and Ge atoms has been measured as a function of alloy composition for optimized material. In addition, ...

Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors

The factors determining the x-ray sensitivity of HgI2 and PbI2 as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with 512×512 pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of HgI2 can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. PbI2 films exhibit lower sensitivity, ...

LASER DEHYDROGENATION/CRYSTALLIZATION OF PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITED AMORPHOUS SILICON FOR HYBRID THIN FILM TRANSISTORS

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon (a‐Si:H) has been developed. This process removes hydrogen by laser irradiations at three energy steps. Studies of hydrogen out‐diffusion and microstructure show that hydrogen out‐diffusion depends strongly on film structure and the laser energy density. Both high quality and low leakage bottom gate polycrystalline silicon and a‐Si:H thin film transistors were monolithically fabricated on the same Corning 7059 glass substrate with a maximum process temperature of only 350 °C.

Low temperature crystallization of amorphous silicon using an excimer laser

Low temperature processing is a prerequisite for compatible technologies involving combined a-Si and poly-silicon devices or for fabricating these devices on glass substrates. This paper describes excimer-laser-induced crystallization of thin amorphous silicon films deposited by plasma CVD (a-Si:H) and LPCVD (a-Si). The intense, pulsed UV produced by the laser is highly absorbed by the thin amorphous material, but the average temperature is compatible with low temperature processing. The process produces crystallites whose structure and electrical characteristics vary according to starting material and laser scan parameters. The crystallized films have been principally characterized using x-ray diffraction, TEM, and transport measurements. The results indicate that crystallites nucleate in the surface region and are randomly oriented. The degree of crystallization near the surface increases as the doping level and/or deposited laser energy density is increased. The crystallite size increases with a power law dependence on deposited energy, while the conductivity increases exponentially above threshold for unintentionally doped PECVD films. The magnitude of the Hall mobility of the highly crystallized samples is increased by two orders of magnitude over that of the amorphous starting material.

Two Dimensional Amorphous Silicon Image Sensor Arrays

Large two dimensional amorphous silicon image sensor arrays offer a new approach to electronic document input and x-ray imaging. The sensor array technology is now capable of image capture at greater than 10 frames/sec and with resolution of 200–400 spi. We describe our new high resolution imaging system, comprising a page-sized sensor array with nearly 3 million pixels, and the accompanying high speed read out and processing electronics. The key technological issues of pixel resolution, sensor fill factor, leakage currents and noise are reviewed. Measurements of a new array architecture are described, in which the sensor is formed as a single continuous film on top of the matrix addressing components.

Thick films of X-ray polycrystalline mercuric iodide detectors

Polycrystalline HgI 2 thick film detectors are among the leading semiconductor materials to be used as direct converters in X-ray digital radiography. Their properties along with a survey of the properties of alternative materials, such as PbI 2 or A-Se, will be given. The preparation of HgI 2 detector plates, both by direct evaporation (Physical vapor deposition, (PVD)) and by binding the individual crystallites with polymeric glue, forming screen-printed (SP) detector plates, will be described. The microstructure of the PVD thick films showing a columnar morphology, as determined by SEM measurements, will be shown. The X-ray response to radiological X-ray generator of 85 kVp using the current integration mode will be reported for both PVD and SP films. Finally, some X-ray images taken at Xerox-Parc using HgI 2 polycrystalline detectors will be shown.

Excimer‐laser‐induced crystallization of hydrogenated amorphous silicon

The electronic transport properties and structural morphology of fast‐pulse excimer‐laser‐ crystallized hydrogenated amorphous silicon (a‐Si:H) thin films have been measured. The room‐temperature dark dc conductivities and Hall mobilities increase by several orders of magnitude at well‐defined laser energy density thresholds which decrease as the impurity concentration in the films increases. The structural morphology of the films suggests an impurity‐induced reduction of the a‐Si:H melt temperature as the origin of this behavior.

Amorphous silicon thin-film transistors and arrays fabricated by jet printing

Phase-change wax-based printed masks, in place of conventional photolithography, were used to fabricate hydrogenated amorphous silicon thin-film transistors (TFTs). Wax-mask features with a minimum feature size of ∼20 μm were achieved using an acoustic-ink-printing process. Both discrete and matrix addressing structured bottom-gate TFTs with source–drain contacts overlapping the channel were created using a four-mask process. The TFTs had current–voltage characteristics comparable to photolithographically patterned devices, with mobility of 0.6–0.9 cm2/V s, threshold voltage of 2–3 V, and on/off ratios exceeding 107 for devices with channel lengths below 50 μm.