Kunigunde H. Cherenack

Highly stable amorphous-silicon thin-film transistors on clear plastic

Hydrogenated amorphous-silicon (a-Si:H) thin-film transistors (TFTs) have been fabricated on clear plastic with highly stable threshold voltages. When operated at a gate field of 2.5×105V∕cm, the threshold voltage shift extrapolated to only ∼1.2V after ten years. This stability is achieved by a high deposition temperature for the gate silicon nitride insulator which reduces charge trapping and high hydrogen dilution during a-Si:H growth to reduce defect creation in a-Si:H. This gate field of 2.5×105V∕cm is sufficient to drive phosphorescent organic light emitting diodes (OLEDs) at a brightness of 1000Cd∕m2. The half-life of the TFT current is over ten years, slightly longer than the luminescence half-life of high quality green OLEDs.

Amorphous-Silicon Thin-Film Transistors Fabricated at 300

We have made hydrogenated amorphous-silicon thin-film transistors (TFTs) at a process temperature of 300degC on free-standing clear-plastic foil substrates. The key to the achievement of flat and smooth samples was to design the mechanical stresses in the substrate passivation and transistor layers, allowing us to obtain functional transistors over the entire active surface. Back-channel-passivated TFTs made at 300 degC on glass substrates and plastic substrates have identical electrical characteristics and gate-bias-stress stability. These results suggest that free-standing clear-plastic foil can replace display glass as a substrate from the points of process temperature, substrate and device integrity, and TFT performance and stability.

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Both zinc-oxide (ZnO) and gallium-indium-ZnO (IGZO) are attractive as semiconductors to replace hydrogenated amorphous silicon in flexible thin-film transistors (TFTs) due to their high charge carrier mobility and low deposition temperature. However, the electrical performance of flexible TFTs needs to be insensitive to mechanical bending. We have fabricated TFTs using ZnO and IGZO semiconducting layers on polyimide substrates and exposed TFTs to tensile bending radii down to 10 mm. While the mobility, threshold voltage, and subthreshold slope of IGZO TFTs remained essentially unchanged over the entire bending range, the electrical performance parameters of ZnO TFTs were strongly degraded by bending. For ZnO TFTs bent to a radius of 10 mm, the mobility decreased by more than two orders of magnitude, the threshold voltage increased by a factor of ~ 5, and the subthreshold slope increased by a factor of ~ 2. Our results show that IGZO should be the material of choice for robust flexible thin-film transistors. Experimental evidence points toward the formation of microcracks as the cause of ZnO sensitivity to bending.

on a Free-Standing Foil Substrate of Clear Plastic

We have fabricated active-matrix organic light emitting diode (AMOLED) test arrays on an optically clear high-temperature flexible plastic substrate at process temperatures as high as 285 degC using amorphous silicon thin-film transistors (a-Si TFTs). The substrate transparency allows for the operation of AMOLED pixels as bottom-emission devices, and the improved stability of the a-Si TFTs processed at higher temperatures significantly improves the reliability of the light emission over time.

Impact of Mechanical Bending on ZnO and IGZO Thin-Film Transistors

A new gate dielectric material is used to fabricate hydrogenated amorphous-silicon (a-Si:H) thin-film transistors (TFTs) with high field-effect mobilities. The dielectric is a homogeneous SiO2-silicone hybrid, which is deposited by plasma-enhanced chemical vapor deposition system at nominal room temperature. This new dielectric results in a-Si:H TFTs with measured field-effect mobilities of ∼2 cm2/V s for electrons and ∼0.1 cm2/V s for holes.

Reliability of Active-Matrix Organic Light-Emitting-Diode Arrays With Amorphous Silicon Thin-Film Transistor Backplanes on Clear Plastic

The stability of thin-film transistors (TFTs) of hydrogenated amorphous-silicon (a-Si:H) against gate-bias stress is improved by raising the deposition power and temperature of the silicon nitride gate dielectric. We studied the effects of power density between 22 and 110 mW/cm2 and temperature between 150degC and 300degC . The time needed to shift the threshold voltage by 2 V varies by a factor of 12 between low power and low temperature, and high power and high temperature. These results highlight the importance of fabricating a-Si:H TFTs on flexible plastic with the SiNx gate dielectric deposited at the highest possible power and temperature.

Amorphous silicon thin-film transistors with field-effect mobilities of 2 cm2/V s for electrons and 0.1 cm2/V s for holes

We fabricated back-channel-cut and back-channel-passivated hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) on clear-plastic (CP) foil substrates using a silicon nitride (SiN∞) deposition temperature of 300°C. The TFTs were fabricated on CP and are as stable under high gate bias as TFTs made on glass substrates. A self-alignment technique was developed to align the channel passivation, the a-Si:H island, and the source/drain (S/D) terminals to the gate. Self-alignment allowed us to fabricate discrete TFTs across 7 7 × cm2 of a free-standing sheet of CP foil to reduce the TFT channel length L to 3 m and reduce the S/D overlap with the gate LSD to ~1. To test the self-alignment techniques, we fabricated ring oscillators on the CP substrates. These results show that it is possible to fabricate state-of-the-art self-aligned a-Si:H TFTs and TFT circuits on plastic substrates.

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The transition of thin-film transistor (TFT) backplanes from rigid plate glass to flexible substrates requires the development of a generic TFT backplane technology on a clear plastic substrate. To be sufficiently stable under bias stress, amorphous-silicon (a-Si:H) TFTs must be deposited at elevated temperatures, therefore the substrate must withstand high temperatures. We fabricated a-Si:H TFT backplanes on a clear plastic substrate at 200degC. The measured stability of the TFTs under gate bias stress was superior to TFTs fabricated at 150degC. The substrate was dimensionally stable within the measurement resolution of 1, allowing for well-aligned 8 times 8 and 32 times 32 arrays of pixels. The operation of the backplane is demonstrated with an electrophoretic display. This result is a step toward the drop-in replacement of glass substrates by plastic foil.

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— The direct voltage programming of active-matrix organic light-emitting-diode (AMOLED) pixels with n-channel amorphous-Si (a-Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n-channel a-Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn-on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active-matrix OLED displays with a-Si technology and conventional bottom-anode OLEDs.

Gate Dielectric Deposition Power and Temperature on a-Si:H TFT Stability

We report amorphous silicon thin film transistors (a-Si TFTs) with an extrapolated DC saturation current half-life of more than 100 years, an improvement of over 1000 times compared to the previous art (1-4). This TFT half-life is higher than the luminance half-life of high-quality green phosphorescent OLEDs, showing that the TFTs are promising for driving OLEDs in active-matrix OLED displays.