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Research
Highlights Record
High Lifetime Amorphous Si Thin-Film Transistors on Clear Plastic and Glass The
saturation current half-life of the a-Si TFT’s developed in this project
exceeds 10 years on high temperature clear plastic substrates (provided by
DuPont Company) and 100 years on glass, an improvement of more than 1000X
compared to conventional a-Si TFT’s used in active-matrix liquid
crystal displays and x-ray imagers (the TFT half-life is defined as the time
the TFT current drops to 50% under constant DC bias in saturation with an
initial current sufficient for driving high-quality green phosphorescent
OLED’s at 1000Cd/m 2 in a typical active-matrix OLED pixel design).
This lifetime improvement involved detailed study of the fundamental TFT
instability mechanisms and employing the findings to improve the quality of
the a-Si channel material and the gate nitride. The a-Si TFT’s are
grown by industry-standard plasma enhanced chemical vapor deposition allowing
the use of the existing industrial infrastructure for commercial production
of the TFT backplanes for OLED displays and other applications. Further
improvement of the TFT lifetime is in progress. Representative
publications: -
B. Hekmatshoar, et. al. , “Amorphous Silicon
Thin-Film Transistors with DC Saturation Current Half-Life of More than 100
Years”, Technical Digest – 2008 IEEE International Electron
Devices Meeting (IEDM 2008), pp. 89-92, December 2008 -
B. Hekmatshoar, et. al. , “Highly Stable
Amorphous-Silicon Thin-Film Transistors on Clear Plastic”, Applied
Physics Letters, vol. 93, no. 3, pp. 032103-1-3, July 2008 Amorphous-Si
Active-Matrix OLED Arrays on High-Temperature Flexible Clear Plastic
Substrates One of
the main obstacles to the realization of practical AMOLED displays on
flexible plastic substrates is the low working temperature of the
commercially available clear plastic substrates, limiting the reliability of
AMOLED pixel light-emission over time. In this project, prototype AMOLED
pixel arrays were fabricated on flexible clear plastic substrates at
temperatures close to 300°C, an improvement of about 150°C compared to
previous demonstrations. The electrical characterization of the fabricated
pixels shows the impact of high temperature processing of the pixel
reliability. The new experimental clear plastic substrates were developed by
DuPont Company, the plastic compatible TFT backplane fabrication and TFT/OLED
integration processes were developed in Princeton University and high quality
phosphorescent OLED’s were provided by Universal Display Co.
Controlling the mechanical stress of the deposited layers and adhesion of the
layers was essential to the development of the plastic-compatible process by
ensuring crack-free layers and a flat backplane surface. Representative
publication: -
B. Hekmatshoar, et. al. “Reliability of
Active-Matrix Organic Light-Emitting-Diode Arrays with Amorphous Silicon
Thin-Film Transistor Backplanes on Clear Plastic”, IEEE Electron
Device Letters, vol. 29, no. 1, pp. 63-66, January 2008 Novel TFT/OLED Integration Technique for Direct
Programming of AMOLED Pixels with Amorphous Si TFT backplanes and Standard
Bottom-Emission OLED’s The
direct voltage programming of active-matrix OLED pixels with n-channel
a-Si TFT’s requires a contact between the driver TFT and the
OLED cathode. Conventional TFT/OLED integration process constraints only
permit connecting the driver TFT to the OLED anode. The new
“inverted” integration technique makes the direct programming
possible by connecting the driver n-channel a-Si TFT to the OLED
cathode. This was achieved by using photoresist separators with
overhangs to selectively shadow-mask the evaporation of the organic layers
followed by the evaporation of cathode at an angle. As a result, the pixel
drive current increases by an order of magnitude for the same data
voltages and the pixel turn-on voltage 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. The new integration technique is important for
realizing active-matrix OLED displays with a-Si technology and conventional
bottom-emission OLED’s. Representative
publications: - B. Hekmatshoar, et.
al. “A Novel TFT-OLED Integration for OLED-Independent Pixel
Programming in Amorphous-Si AMOLED Pixels”, Journal of the Society
for Information Display, vol. 16, no. 1, pp. 183-188, January 2008 - B. Hekmatshoar, et.
al. “AMOLED Reliability with a-Si TFT’s in Normal vs. Inverted
TFT/OLED Integration Scheme”, 66th Annual Device Research
Conference (DRC 2008) Conference Digest, pp. 243-244, June 2008
Abstract (IEEE)
Full Text Low-Temperature Stress-Assisted Crystallization of
Germanium and Silicon-Germanium Alloys on Flexible Clear Plastic Substrates High
TFT mobility is desired for high drive currents in large area electronics and
displays. The process temperatures required for conventional growth of polycrystalline
materials is too high for flexible plastic substrates, and therefore
amorphous semiconductors with lower motilities must be used. We have
discovered that applying mechanical stress to the substrate during the
metal-induced crystallization (MIC) of amorphous semiconductors may reduce
the crystallization temperature significantly by lowering the activation
energy required for nucleation. In particular, the temperature required for
copper-induced crystallization of germanium (Ge) may be reduced from ~400°C
to as low as 130°C, allowing the use of PET substrates for growing
poly-Ge films and fabrication of p-channel TFT’s with mobilities as
high as 80cm2/Vs. This technique may also be applied to drive the
copper-induced lateral growth (MILC) of Ge which is not conventionally
possible. MILC films are preferred over those grown by MIC due to lower metal
contamination in the film. Another application of this technique is low
temperature Ge-seeded crystallization of Si-Ge alloys by applying mechanical
stress to the film during MIC of the Ge seed. Representative
publications: - B. Hekmatshoar, et.
al. “Low temperature crystallization of amorphous germanium on plastic,
by means of externally applied compressive stress” Journal of Vacuum
Science and Technology A, vol. 21, no. 3, pp. 752-755, May/June 2003 -
B. Hekmatshoar, et. al. “Low Temperature
Stress-Assisted Germanium-Induced Crystallization of Silicon-Germanium Alloys
on Flexible PET substrates”, Journal of Vacuum Science and
Technology A, vol. 22, no. 3, pp. 856-858, May/June 2004 -
B. Hekmatshoar, et. al. “Low
temperature copper-induced lateral growth of polycrystalline germanium
assisted by external compressive stress”, Journal of Applied Physics, vol. 97, no. 4, February 2005
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