Rene A. Lujan

Microstructure of plasma‐deposited a‐Si : H films

Using transmission and scanning electron microscopy, it is shown that plasma deposition of amorphous silicon hydrogen films from silane or silane/argon mixtures proceeds via nucleation and growth of islands of average lateral dimensions ∼100 A. If these islands do not coalesce into a homogeneous film, subsequent growth produces columnar morphology with low‐density interstitial regions. There is a strong correlation between the columnar structure and the presence of nonradiative recombination centers.

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.

Effects of inert gas dilution of silane on plasma‐deposited a‐Si:H films

Electrical, optical, and structural characterization of hydrogenated amorphous silicon films plasma‐deposited from mixtures of SiH4 with different inert‐gas diluents reveals substantial differences in a number of properties. A general trend of increasing defect density with atomic weight of the inert gas is observed. Of specific interest to device applications is the observation that high deposition rates can be achieved concurrently with low defect densities when helium is used as a deluent.

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.

Electrical stability of inkjet-patterned organic complementary inverters measured in ambient conditions

Complementary organic inverters were fabricated by inkjet patterning of both the metal contacts and the semiconductors. Bottom-gate, bottom-contact organic thin-film transistors with Ta2O5-polymer bilayer dielectrics, inkjet-printed silver electrodes, and inkjet-printed organic semiconductors exhibit hole and electron mobilities as high as ∼10−2 cm2/V s. Complementary inverters based on these transistors operate in ambient and exhibit a gain of −4.4 with supply voltage VDD=+20 V and −3 dB cutoff at 100 kHz with a load of 0.02 pF. The electrical stability of the inverters was evaluated for analog and digital operation, and a noise margin ≥1.1 V at VDD=+15 V was measured with bias-stress effects included.

Effects of the surface roughness of plastic-compatible inorganic dielectrics on polymeric thin film transistors

The effects of the roughness of the gate dielectric on the performance of thin film transistors (TFTs) fabricated with poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b] thiophene) (PBTTT-C16) were examined. The field effect mobility of coplanar TFTs made with PBTTT-C16 and plasma-enhanced chemical vapor deposited SiO2∕SiNx dielectrics decreased nearly exponentially with surface roughness. Films of PBTTT-C16 have similar crystalline structure on smooth and rough surfaces, but the domain size decreases with increasing roughness. Surface roughness was found to have less of an impact on field effect mobility than the chemical treatment of the dielectric.

Sol-gel solution-deposited InGaZnO thin film transistors.

Thin film transistors (TFTs) fabricated by solution processing of sol-gel oxide semiconductor precursors in the group In-Ga-Zn are described. The TFT mobility varies over a wide range depending on the precursor materials, the composition, and the processing variables, with the highest mobility being about 30 cm(2)/(V s) for IZO and 20 cm(2)/(V s) for IGZO. The positive dark bias stress effect decreases markedly as the mobility increases and the high mobility devices are quite stable. The negative bias illumination stress effect is also weaker in the higher mobility TFTs, and some different characteristic properties are observed. The TFT mobility, threshold voltage, and bias stress properties are discussed in terms of the formation of self-compensated donor and acceptor states, based on the chemistry and thermodynamics of the sol-gel process.

Printing Methods and Materials for Large-Area Electronic Devices

Two digital printing methods for the fabrication of active matrix thin-film transistor (AM-TFT) backplanes for displays are described. A process using printed resists layers, referred to as digital lithography, was used to fabricate arrays of hydrogenated amorphous silicon TFTs. TFTs were also fabricated using a combination of digital lithography to pattern metals and inkjet printing to pattern and deposit a polymeric semiconducting layer. The relative performance of amorphous silicon and polymer TFTs were evaluated. The utility of digital lithographic processing was demonstrated by the fabrication of prototype reflective displays using electrophoretic media.

Reflectivity of disordered silicon nanowires

Disordered silicon nanowire mats have high diffuse optical reflectivity, in striking contrast to the low reflectivity of orientationally ordered nanowires. The reflectivity decreases with increasing photon energy across the visible and near infrared spectrum due to absorption in the nanowires. A simple model is used to estimate that the incident photon interacts with up to 20 nanowires before being reflected. A thin coating of hydrogenated amorphous silicon enhances the nanowire absorption.