Shelby Forrester Nelson

Stacked pentacene layer organic thin-film transistors with improved characteristics

Using two layers of pentacene deposited at different substrate temperatures as the active material, we have fabricated photolithographically defined organic thin-film transistors (OTFTs) with improved field-effect mobility and subthreshold slope. These devices use photolithographically defined gold source and drain electrodes and octadecyltrichlorosilane-treated silicon dioxide gate dielectric. The devices have field-effect mobility as large as 1.5 cm/sup 2//V-s, on/off current ratio larger than 10/sup 8/, near zero threshold voltage, and subthreshold slope less than 1.6 V per decade. To our knowledge, this is the largest field-effect mobility and smallest subthreshold slope yet reported for any organic transistor, and the first time both of these important characteristics have been obtained for a single device.

Temperature-independent transport in high-mobility pentacene transistors

The charge-carrier transport mechanism in the organic semiconductor pentacene is explored using thin-film transistor structures. The variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior. This result excludes thermally activated hopping as the fundamental transport mechanism in pentacene thin films, and suggests that traps and/or contact effects may strongly influence the observed characteristics. These results also indicate that field-effect transistors may not be appropriate vehicles for illuminating basic transport mechanisms in organic materials.

Pentacene organic thin-film transistors-molecular ordering and mobility

Pentacene-based organic thin-film transistors (TFTs) with field-effect mobility as large as 0.7 cm/sup 2//V/spl middot/s and on/off current ratio larger than 10/sup 8/ have been fabricated. Pentacene films deposited by evaporation at elevated temperature at low-to-moderate deposition rates have a high degree of molecular ordering with micrometer-sized and larger dendritic grains. Such films yield TFTs with large mobility. Films deposited at low temperature or by flash evaporation have small grains and poor molecular ordering and yield TFTs with low mobility.

Pentacene-based organic thin-film transistors

Organic thin-film transistors using the fused-ring polycyclic aromatic hydrocarbon pentacene as the active electronic material have shown mobility as large as 0.7 cm/sup 2//V-s and on/off current ratio larger than 10/sup 8/; both values are comparable to hydrogenated amorphous silicon devices. On the other hand, these and most other organic TFTs have an undesirably large subthreshold slope. We show here that the large subthreshold slope typically observed is not an intrinsic property of the organic semiconducting material and that devices with subthreshold slope similar to amorphous silicon devices are possible.

Solvent-induced phase transition in thermally evaporated pentacene films

We report a solvent-induced phase transition in pentacene thin films, from a “thin film” phase to a bulk-like phase. X-ray diffraction indicates that as-deposited thermally evaporated pentacene films consist mainly of (001)-oriented pentacene with an elongated (001) plane spacing of 15.5±0.1 A, and a minor amount with a (001) plane spacing of 14.5±0.1 A. When such films are exposed to solvents such as acetone, isopropanol, or ethanol, the plane spacing of the entire layer shifts abruptly from the elongated (001) plane spacing to the bulk value and this shift is accompanied by a macroscopic change in film morphology. While molecular ordering is maintained as indicated by x-ray diffraction, thin film transistor performance is severely degraded, most likely as a result of the morphological changes in the film.

Stable ZnO thin film transistors by fast open air atomic layer deposition

We report stable, high performance zinc oxide thin film transistors grown by an atmospheric pressure atomic layer deposition system. With all deposition and processing steps kept at or below 200°C, the alumina gate dielectric shows low leakage (below 10−8A∕cm2) and high breakdown fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 108, near zero turn-on voltage, little or no hysteresis, and mobility greater than 10cm2∕Vs. With alumina passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in the literature.We report stable, high performance zinc oxide thin film transistors grown by an atmospheric pressure atomic layer deposition system. With all deposition and processing steps kept at or below 200°C, the alumina gate dielectric shows low leakage (below 10−8A∕cm2) and high breakdown fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 108, near zero turn-on voltage, little or no hysteresis, and mobility greater than 10cm2∕Vs. With alumina passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in the literature.

Oxide Electronics by Spatial Atomic Layer Deposition

We report on zinc oxide (ZnO)-based devices produced by a fast, open-air atomic layer deposition (ALD) process relying upon the spatial isolation of reactive gases. At deposition rates of greater than 100 Aring per minute, ZnO-based thin-film transistors by spatial atomic layer deposition (S-ALD) show mobility above 15 cm2/Vs and excellent stability. Measurement and modeling of the gas isolation in the deposition head is discussed. Saturation curves obtained for aluminum oxide (Al2O3) growth using trimethylaluminum and water are shown to be consistent with chamber ALD systems. Finally, the ability of this new ALD process to leverage patterning by using poly(methyl methacrylate) (PMMA) as a growth inhibitor for selective area deposition is discussed. Relatively thin films of PMMA (~ 40 Aring) are shown to be capable of inhibiting the growth of ZnO for at least 1200 ALD cycles.

Pentacene TFT driven AM OLED displays

Pentacene organic thin-film transistors (TFTs)-driven active matrix organic light-emitting diode (OLED) displays has been investigated. This letter addresses several process issues unique to this type of display which are important in achieving bright and uniform displays. A bottom contact structure was used to fabricate the pentacene TFT backplane. Polyvinyl alcohol and parylene were used to isolate the pentacene active layer and passivate the backplane. The low processing temperature may allow the use of polymeric substrates and lower cost processing. Uniform TFT performance is achieved with reasonably good mobility and on/off ratio on the backplane. The initial OLED display performance is also presented.

Thermal stability of undoped strained Si channel SiGe heterostructures

We have investigated the thermal stability of Si/SiGe n‐channel heterostructures. To eliminate the complication of dopant diffusion, we have fabricated undoped Si/SiGe heterostructure Hall effect devices. With no modulation doping used in our structures, the electron concentration in the strained Si channel is controlled from a back gate. Our devices show high electron mobility of up to 19 500 cm2/V s at 77 K and are stable with negligible 77 K mobility reduction after anneals at 800 °C for 30 min and at 950 °C for 3 min. These results conform well with a simulation of the diffusion of Ge into the Si channel.

ZnO Thin-Film Transistor Ring Oscillators with 31-ns Propagation Delay

We have fabricated ring oscillators (ROs) using ZnO thin films deposited by using a spatial atomic layer deposition process at atmospheric pressure and low temperature (200degC). Bottom-gate thin-film transistors with aluminum source and drain contacts were fabricated with a field-effect mobility of > 15 cm2/V ldr s. Seven-stage ROs operated at a frequency as high as 2.3 MHz for a supply voltage of 25 V, corresponding to a propagation delay of 31 ns/stage. These circuits also had propagation delays of ~100 ns/stage at a supply voltage of 15 V. To the best of our knowledge, these are the fastest ZnO circuits reported to date.