Wantae Lim

Room temperature deposited indium zinc oxide thin film transistors

Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5to−6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5cm2V−1s−1, lower than the Hall mobility of ∼17cm2V−1s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.

High performance indium gallium zinc oxide thin film transistors fabricated on polyethylene terephthalate substrates

High-performance amorphous (α−) InGaZnO-based thin film transistors (TFTs) were fabricated on flexible polyethylene terephthalate substrates coated with indium oxide (In2O3) films. The InGaZnO films were deposited by rf magnetron sputtering with the presence of O2 at room temperature. The n-type carrier concentration of InGaZnO film was ∼2×1017 cm−3. The bottom-gate-type TFTs with SiO2 or SiNx gate dielectric operated in enhancement mode with good electrical characteristics: saturation mobility 11.5 cm2 V−1 s−1 for SiO2 and 12.1 cm2 V−1 s−1 for SiNx gate dielectrics and drain current on-to-off ratio >105. TFTs with SiNx gate dielectric exhibited better performance than those with SiO2. This is attributed to the relatively high dielectric constant (i.e., high-k material) of SiNx. After more than 500 h aging time at room temperature, the saturation mobility of the TFTs with SiO2 gate dielectric was comparable to the as-fabricated value and the threshold voltage shift was 150 mV.

Effects of ambient atmosphere on the transfer characteristics and gate-bias stress stability of amorphous indium-gallium-zinc oxide thin-film transistors

We investigated the transfer characteristics and the gate-bias stability of amorphous indium-gallium-zinc oxide thin-film transistors when the channel layer was exposed to hydrogen, oxygen, air, or vacuum at room temperature during measurements. The threshold voltage and the drain current were changed by the ambient atmospheres. The threshold voltage shift (ΔVth) under gate-bias stress was faster in hydrogen than in oxygen and vacuum. It is suggested that hydrogen exposure degrades the gate-bias stress stability due to surface accumulation layer creation. The characteristic trapping times, τ, in H2, O2, air, and vacuum were 5×103, 1.5×104, 2×104, and 6.3×104 s, respectively.

High-Performance Indium Gallium Zinc Oxide Transparent Thin-Film Transistors Fabricated by Radio-Frequency Sputtering

Thin-film transistors (TFTs) based on amorphous indium gallium zinc oxide (a-IGZO) were fabricated by radio-frequency magnetron sputtering on glass substrates. The TFT device structure was a bottom-gate type, consisting of indium zinc oxide and HfO 2 as electrodes (gate, source, and drain) and gate dielectric, respectively. The resistivity of the a-IGZO channel layer was ∼1 Ω cm. TFTs with a 6 μm gate length and 100 μm gate width displayed a saturation mobility of ∼7.2 cm 2 V -1 s -1 , a threshold voltage of 0.44 V, a drain current on-off ratio of ∼10 5 , and subthreshold gate-voltage swing of ∼ 0.25 V decade -1 . After 1000 h aging time at room temperature, the saturation mobility remained almost constant while the threshold voltage shift was as small as 460 mV. The IGZO TFTs based on HfO 2 gate dielectrics sputtered near room temperature were found to be good candidates for applications on organic flexible substrates.

Room temperature hydrogen detection using Pd-coated GaN nanowires

Multiple GaN nanowires produced by thermal chemical vapor deposition were employed as gas sensors for detection of hydrogen at concentrations from 200–1500 ppm in N2 at 300 K. Palladium coating of the wires improved the sensitivity by a factor of up to 11 at low ppm concentrations relative to uncoated controls. The GaN nanowires showed relative responses of ∼7.4% at 200 ppm and ∼9.1% at 1500 ppm H2 in N2 after a 10 min exposure. Upon removal of hydrogen from the measurement ambient, ∼90% of the initial GaN conductance was recovered within 2 min. Temperature dependent measurements showed a larger relative response and shorter response time at elevated temperature. The adsorption activation energy of the sensor was 2.2 kcal mol−1 at 3000 ppm H2 in N2. These sensors exhibit low power consumption (<0.6 mW) at 300 K.

High mobility InGaZnO4 thin-film transistors on paper

We report on the fabrication and the electrical properties of amorphous (α-)InGaZnO4 thin-film transistors deposited on cellulose paper by sputtering at room temperature. The 150-μm-thick paper was used as both a gate dielectric and a substrate for device structural support. The transistors on paper were patterned by lithography and operated in enhancement mode with a threshold voltage of 3.75 V, and exhibited saturation mobility, subthreshold gate-voltage swing, and drain current on-to-off ratio of ∼35 cm2 V−1 s−1, 2.4 V decade−1, and ∼104, respectively. These results verify that simple cellulose paper is a good gate dielectric as well as a low-cost substrate for flexible electronic devices such as paper-based displays.

Stable room temperature deposited amorphous InGaZnO4 thin film transistors

Enhancement-mode amorphous indium gallium zinc oxide (α-IGZO) channel thin film transistors (TFTs) with a 6μm gate length and a 100μm gate width were fabricated on glass substrates by rf magnetron sputtering near room temperature. The resistivities of the α-IGZO films were controlled from 10−1to103Ωcm by varying the deposition power of 75–300W. The n-type carrier concentration in the channel was 6.5×1017cm−3. The gate oxide was 90-nm-thick SiNx, deposited by plasma enhanced chemical vapor deposition at 70°C. The bottom-gate TFTs had saturation mobility of ∼17cm2V−1s−1 and the drain current on-to-off ratio of ∼>105, a subthreshold gate-voltage swing of ∼0.5Vdecade−1, and a threshold voltage of 2.1V. In the TFT with a gate length of 6μm and a gate width of 100μm, the relative change of saturation mobility and threshold voltage was less than ±1.5% after 500h aging time at room temperature. This demonstrates that α-IGZO films are promising semiconductor materials for long-term-stable transparent TFT applications.

Low-voltage indium gallium zinc oxide thin film transistors on paper substrates

We have fabricated bottom-gate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100 °C). As a water and solvent barrier layer, cyclotene (BCB 3022–35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2 V−1 s−1, threshold voltage (VTH) of 1.9 V, subthreshold gate-voltage swing (S) of 0.65 V decade−1, and drain current on-to-off ratio (ION/IOFF) of ∼104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat∼2.1 cm2 V−1 s−1, VTH∼0 V, S∼0.74 V decade−1, and ION/IOFF=105–106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. The ability to achieve InGaZnO TFTs on cyclotene-coated paper substrates demonstrates the enormous potential for applications such as low-cost and large area electronics.

Nitride and oxide semiconductor nanostructured hydrogen gas sensors

In this paper, we discuss the progress of nitride and oxide semiconductor nanostructures for hydrogen gas sensing. The use of catalyst metal coatings on GaN, InN and ZnO nanowires is found to greatly enhance the detection sensitivity. Pt- and Pd-coated GaN nanowires biased at small voltages show large changes in currents upon exposure to H2 gas at concentrations in the ppm range. Improvements in growth techniques for InN nanostructures have produced nanobelts and nanorods capable of hydrogen detection down to 20 ppm after catalyst coating. Functionalized ZnO nanorods were also investigated for hydrogen detection, but did not generate a relative response as high as that for the nitride-based sensors. All sensors tested exhibited no response at room temperature upon exposure to various other gases including O2, C2H5, N2O and CO2. The high surface-to-volume ratio of nanowires and the ability to use simple contact fabrication schemes make them attractive for hydrogen sensing applications.

Temperature dependence of current-voltage characteristics of Ni–AlGaN/GaN Schottky diodes

Ni–AlGaN/GaN Schottky barrier diodes (SBDs) with lateral geometry were fabricated on sapphire substrates. At 300 K, devices with 500-μm-diameter Schottky contacts exhibited breakdown voltage (VB) of 765 V, forward current (IF) of 0.065 A at 1.5 V, and specific on-resistance (Ron) of 81.3 mΩ cm2, producing a figure-of-merit (VB2/Ron) of ∼7.2 MW cm−2. Measured in multifinger patterns, the same parameters were 420 V, 3.2 A, 4.6 mΩ cm2, and 38.4 MW cm−2, respectively, at 300 K. With the increase in measurement temperature from 300 to 450 K, SBDs with dimensions of 3000×3000 μm2 showed larger effective barrier heights (0.8 eV at 300 K and 1.27 eV at 475 K) and a slightly negative temperature coefficient (−0.48 V K−1) for reverse breakdown voltage, while there was a little change in reverse leakage current. These results show the strong influence of barrier height inhomogeneity on the temperature dependence of apparent barrier heights obtained through current-voltage measurements.