John F. Wager

ZnO-based transparent thin-film transistors

Highly transparent ZnO-based thin-film transistors (TFTs) are fabricated with optical transmission (including substrate) of ∼75% in the visible portion of the electromagnetic spectrum. Current–voltage measurements indicate n-channel, enhancement-mode TFT operation with excellent drain current saturation and a drain current on-to-off ratio of ∼107. Threshold voltages and channel mobilities of devices fabricated to date range from ∼10 to 20 V and ∼0.3 to 2.5 cm2/V s, respectively. Exposure to ambient light has little to no observable effect on the drain current. In contrast, exposure to intense ultraviolet radiation results in persistent photoconductivity, associated with the creation of electron-hole pairs by ultraviolet photons with energies greater than the ZnO band gap. Light sensitivity is reduced by decreasing the ZnO channel layer thickness. One attractive application for transparent TFTs involves their use as select-transistors in each pixel of an active-matrix liquid-crystal display.

High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layer

Transparent thin-film transistors (TTFTs) with an amorphous zinc tin oxide channel layer formed via rf magnetron sputter deposition are demonstrated. Field-effect mobilities of 5–15 and 20–50cm2V−1s−1 are obtained for devices post-deposition annealed at 300 and 600°C, respectively. TTFTs processed at 300 and 600°C yield devices with turn-on voltage of 0–15 and −5–5V, respectively. Under both processing conditions, a drain current on-to-off ratio greater than 107 is obtained. Zinc tin oxide is one example of a new class of high performance TTFT channel materials involving amorphous oxides composed of heavy-metal cations with (n−1)d10ns0 (n⩾4) electronic configurations.

Transparent thin-film transistors with zinc indium oxide channel layer

High mobility, n-type transparent thin-film transistors (TTFTs) with a zinc indium oxide (ZIO) channel layer are reported. Such devices are highly transparent with ∼85% optical transmission in the visible portion of the electromagnetic spectrum. ZIO TTFTs annealed at 600 °C operate in depletion-mode with threshold voltages −20 to −10V and turn-on voltages ∼3V less than the threshold voltage. These devices have excellent drain current saturation, peak incremental channel mobilities of 45–55cm2V−1s−1, drain current on-to-off ratios of ∼106, and inverse subthreshold slopes of ∼0.8V∕decade. In contrast, ZIO TTFTs annealed at 300 °C typically operate in enhancement-mode with threshold voltages of 0–10V and turn-on voltages 1–2V less than the threshold voltage. These 300 °C devices exhibit excellent drain–current saturation, peak incremental channel mobilities of 10–30cm2V−1s−1, drain current on-to-off ratios of ∼106, and inverse subthreshold slopes of ∼0.3V∕decade. ZIO TTFTs with the channel layer deposited ne...

Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs

A simple, low-cost, and nontoxic aqueous ink chemistry is described for digital printing of ZnO films. Selective design through controlled precipitation, purification, and dissolution affords an aqueous Zn(OH)(x)(NH(3))(y)((2-x)+) solution that is stable in storage, yet promptly decomposes at temperatures below 150 degrees C to form wurtzite ZnO. Dense, high-quality, polycrystalline ZnO films are deposited by ink-jet printing and spin-coating, and film structure is elucidated via X-ray diffraction and electron microscopy. Semiconductor film functionality and quality are examined through integration in bottom-gate thin-film transistors. Enhancement-mode TFTs with ink-jet printed ZnO channels annealed at 300 degrees C are found to exhibit strong field effect and excellent current saturation in tandem with incremental mobilities from 4-6 cm(2) V(-1) s(-1). Spin-coated ZnO semiconductors processed at 150 degrees C are integrated with solution-deposited aluminum oxide phosphate dielectrics in functional transistors, demonstrating both high performance, i.e., mobilities up to 1.8 cm(2) V(-1) s(-1), and the potential for low-temperature solution processing of all-oxide electronics.

Spin-coated zinc oxide transparent transistors

A ZnO transparent thin-film transistor (TTFT) with a channel layer formed via spin-coating deposition is demonstrated. The TTFT is highly transparent and exhibits n-channel, enhancement-mode behaviour with a channel mobility as large as 0.20 cm2 V−1 s−1 and a drain current on-to-off ratio of nearly 107.

Tin oxide transparent thin-film transistors

A SnO2 transparent thin-film transistor (TTFT) is demonstrated. The SnO2 channel layer is deposited by RF magnetron sputtering and then rapid thermal annealed in O2 at 600°C. The TTFT is highly transparent, and enhancement-mode behaviour is achieved by employing a very thin channel layer (10–20 nm). Maximum field-effect mobilities of 0.8 cm2 V−1 s−1 and 2.0 cm2 V−1 s−1 are obtained for enhancement- and depletion-mode devices, respectively. The transparent nature and the large drain current on-to-off ratio of 105 associated with the enhancement-mode behaviour of these devices may prove useful for novel gas-sensor applications.

p-Type oxides for use in transparent diodes

Several p-type oxides of the delafossite structure have been investigated in the hope that the conductivity and transparency will be high enough to render them useful in the manufacture of transparent p–n junction diodes and other transparent devices. The highest conductivity achieved to date has been 220 S/cm in CuCr1−xMgxO2 thin films. Oxygen intercalation in CuSc1−xMgxO2+y films improves the conductivity at the expense of optical transparency. We have improved the conductivity of CuGaO2-based films from 0.02 to 1 S/cm by substitution of Fe for Ga. p-Type conductivity has been demonstrated in an Ag-based delafossite film. A sputter-deposited AgCoO2 film has a conductivity of 0.2 S/cm, a Seebeck coefficient of 230 μV/K and a band gap of 4.1 eV at room temperature. CuNi2/3Sb1/3O2 films have been produced that are p-type conductors when doped with Sn.

Constant-Voltage-Bias Stress Testing of a-IGZO Thin-Film Transistors

Constant-voltage-bias (V_DS = V_GS = 30 V) stress measurements are performed for a period of 10⁵ s on thin-film transistors (TFTs) with amorphous indium-gallium-zinc-oxide (IGZO) channel layers fabricated via RF sputtering using a postdeposition annealing temperature of 200degC, 250degC, or 300degC. Thermal silicon dioxide is employed as a TFT bottom-gate insulator. All SiO₂/IGZO TFTs tested exhibit the following: 1) a positive rigid log(I_D)- V_GS transfer curve shift; 2) a continuous drain-current decrease over the entire stress duration; and 3) recovery of the log(I_D)-V_GS transfer curve toward the prestressed state when the stressed TFT is left unbiased in the dark at room temperature for an extended period of time. The SiO₂/IGZO TFTs subjected to a higher postdeposition annealing temperature are more stable. A small (and typically negligible) amount of clockwise hysteresis is present in the log(I_D) -V_GS transfer curves of IGZO TFTs. These instability and hysteresis observations are consistent with a SiO₂/ IGZO TFT instability mechanism involving electron trapping within the IGZO channel layer.

Thin-film transistors with transparent amorphous zinc indium tin oxide channel layer

Thin-film transistors (TFTs) with transparent amorphous zinc indium tin oxide (ZITO) channel layer are demonstrated. Optical transmission of the channel layer is approximately 85% in the visible portion of the electromagnetic spectrum. The channel layer is formed via rf magnetron sputter deposition and then furnace annealed in air. Peak incremental mobilities of 5–19 cm2 V−1 s−1 and turn-on voltages of −4 to −17 V are obtained for devices annealed post-deposition at 100–300 °C, respectively. Current–voltage measurements indicate n-channel, depletion-mode transistor operation with excellent drain current saturation and a drain current on-to-off ratio greater than 106. ZITO is one example of an emerging class of high performance TFT channel materials involving transparent amorphous multicomponent oxides composed of heavy-metal cations with (n − 1)d10ns0 (n ≥ 4) electronic configuration.

Electrical characterization of transparent p - i - n heterojunction diodes

Transparent p–i–n heterojunction diodes are fabricated using heavily doped, p-type CuYO2 and semi-insulating i-ZnO thin films deposited onto a glass substrate coated with n-type indium tin oxide. Rectification is observed, with a ratio of forward-to-reverse current as high as 60 in the range −4–4 V. The forward-bias current–voltage characteristics are dominated by the flow of space-charge-limited current, which is ascribed to single-carrier injection into the i-ZnO layer. Capacitance measurements show strong frequency dispersion, which is attributed to i-ZnO traps. The diode structure has a total thickness of 0.75 μm and an optical transmission of ∼35%–65% in the visible region.