Randy Hoffman

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...

ZnO-channel thin-film transistors: Channel mobility

ZnO-channel thin-film transistor (TFT) test structures are fabricated using a bottom-gate structure on thermally oxidized Si; ZnO is deposited via RF sputtering from an oxide target, with an unheated substrate. Electrical characteristics are evaluated, with particular attention given to the extraction and interpretation of transistor channel mobility. ZnO-channel TFT mobility exhibits severe deviation from that assumed by ideal TFT models; mobility extraction methodology must accordingly be recast so as to provide useful insight into device operation. Two mobility metrics, μavg and μinc, are developed and proposed as relevant tools in the characterization of nonideal TFTs. These mobility metrics are employed to characterize the ZnO-channel TFTs reported herein; values for μinc as high as 25 cm2/V s are measured, comprising a substantial increase in ZnO-channel TFT mobility as compared to previously reported performance for such devices.

High-performance flexible zinc tin oxide field-effect transistors

Flexible transistors were fabricated by sputter deposition of zinc tin oxide (ZTO) onto plasma-enhanced chemical vapor deposition gate dielectrics formed on flexible polyimide substrates with a blanket aluminum gate electrode. The flexible transistors exhibited high on-currents of 1mA, on/off ratios of 106, subthreshold voltage slopes of 1.6V/decade, turn-on voltages of −17V, and mobilities of 14cm2V−1s−1. Capacitance measurements indicate that the threshold voltage and subthreshold slope are primarily influenced by residual doping in the ZTO rather than by defects at the semiconductor/dielectric interface, and are useful for assessing contact resistance.

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.

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.

Transparent p-type conducting BaCu2S2 films

p-type conducting films of α-BuCu2S2 have been deposited onto glass and KBr substrates, yielding a conductivity of 17 S/cm and a Hall mobility of 3.5 cm2/V s. For a 430-nm-thick film, the optical transparency approaches 90% in the visible portion of the spectrum at 650 nm, and a transparency of 40% extends throughout the infrared to the long-wavelength cutoff of the KBr substrate at 23 μm.

21.4: Zinc Indium Oxide Thin-Film Transistors for Active-Matrix Display Backplane

We report on the development of low-temperature gate dielectric materials for zinc indium oxide (ZIO) thin-film transistors (TFTs). Several films, including ALD HfO2 and PECVD SiNx (deposited at 175°C and 150°C, respectively), yield good TFT performance. Bias stress-induced threshold shift for HfO2 is quite small, however does not follow conventional trends associated with hydrogenated amorphous Si (a-Si:H) TFTs; PECVD SiNx conversely, shows bias stress characteristics that conform reasonably to a model appropriate for a-Si:H devices.

Passivation of Amorphous Oxide Semiconductors Utilizing a Zinc–Tin–Silicon–Oxide Barrier Layer

Sputter-deposited amorphous zinc-tin-silicon-oxide (ZTSO) is demonstrated to be a viable electronic passivation layer for bottom-gate thin-film transistors (TFTs) with amorphous zinc-tin-oxide and indium-gallium-zinc-oxide channels. ZTSO allows for successful passivation of these semiconductors without significant changes in turn-on voltage, hysteresis, or channel mobility that is commonly associated with unsuccessful passivation of amorphous oxide semiconductors (AOSs). Passivation of AOS TFTs using ZTSO significantly increases electrical stability under negative-bias illumination stress testing conditions compared with unpassivated AOS TFTs. ZTSO also acts as a barrier layer allowing for additional postprocessing (e.g., plasma-enhanced chemical vapor deposition processes) that in some cases can negatively effect an unprotected AOS layer.