Hye Dong Kim

Electronic transport properties of amorphous indium-gallium-zinc oxide semiconductor upon exposure to water

The effect of water exposure on amorphous indium-gallium-zinc oxide (a-IGZO) semiconductors was reported. It was found that water can diffuse in and out of the a-IGZO film, reversibly affecting the transistor properties. Two competing mechanisms depending on the thickness of the active channel were clarified. The electron donation effect caused by water adsorption dominated for the thicker a-IGZO films (⩾100nm), which was manifested in the large negative shift (>14V) of the threshold voltage. However, in the case of the thinner a-IGZO films (⩽70nm), the dominance of the water-induced acceptorlike trap behavior was observed. The direct evidence for this behavior was that the subthreshold swing was greatly deteriorated from 0.18V/decade (before water exposure) to 4.4V/decade (after water exposure) for the thinnest a-IGZO films (30nm). These results can be well explained by the screening effect of the intrinsic bulk traps of the a-IGZO semiconductor.

Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment

The effect of Ar plasma treatment on amorphous indium gallium zinc oxide (a-IGZO) thin films was investigated. The net electron carrier concentration (1020–1021cm−3) of the a-IGZO thin films dramatically increased upon their exposure to the Ar plasma compared to that (1014cm−3) of the as-deposited thin film. The authors attempted to reduce the contact resistance between the Pt∕Ti (source/drain electrode) and a-IGZO (channel) by using the Ar plasma treatment. Without the treatment, the a-IGZO thin film transistors (TFTs) with W∕L=50∕4μm exhibited a moderate field-effect mobility (μFE) of 3.3cm2∕Vs, subthreshold gate swing (S) of 0.25V∕decade, and Ion∕off ratio of 4×107. The device performance of the a-IGZO TFTs was significantly improved by the Ar plasma treatment. As a result, an excellent S value of 0.19V∕decade and high Ion∕off ratio of 1×108, as well as a high μFE of 9.1cm2∕Vs, were achieved for the treated a-IGZO TFTs.

3.1: Distinguished Paper: 12.1-Inch WXGA AMOLED Display Driven by Indium-Gallium-Zinc Oxide TFTs Array

The full color 12.1-inch WXGA active-matrix organic light emitting diode (AMOLED) display was, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) as an active-matrix back plane. It was found that the fabricated AMOLED display did not suffer from the well-known pixel non-uniformity of luminance, even though the simple structure consisting of 2 transistors and 1 capacitor was adopted as a unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductor. The n-channel a-IGZO TFTs exhibited the field-effect mobility of 8.2 cm2/Vs, threshold voltage of 1.1 V, on/off ratio of > 108, and subthreshold gate swing of 0.58 V/decade. The AMOLED display with a-IGZO TFT array would be promising for large size applications such as note PC and HDTV because a-IGZO semiconductor can be deposited on large glass substrate (> Gen. 7) using conventional sputtering system.

Origin of threshold voltage instability in indium-gallium-zinc oxide thin film transistors

We investigated the impact of the passivation layer on the stability of indium-gallium-zinc oxide (IGZO) thin film transistors. While the device without any passivation layer showed a huge threshold voltage (Vth) shift under positive gate voltage stress, the suitably passivated device did not exhibit any Vth shift. The charge trapping model, which has been believed to be a plausible mechanism, cannot by itself explain this behavior. Instead, the Vth instability was attributed to the interaction between the exposed IGZO backsurface and oxygen and/or water in the ambient atmosphere during the gate voltage stress.

High mobility bottom gate InGaZnO thin film transistors with SiOx etch stopper

The authors report on the fabrication of thin film transistors (TFTs), which use an amorphous indium gallium zinc oxide (a-IGZO) channel, by rf sputtering at room temperature and for which the channel length and width are patterned by photolithography and dry etching. To prevent plasma damage to the active channel, a 100-nm-thick SiOx layer deposited by plasma enhanced chemical vapor deposition was adopted as an etch stopper structure. The a-IGZO TFT (W∕L=10μm∕50μm) fabricated on glass exhibited a high field-effect mobility of 35.8cm2∕Vs, a subthreshold gate swing value of 0.59V∕decade, a thrseshold voltage of 5.9V, and an Ion∕off ratio of 4.9×106, which is acceptable for use as the switching transistor of an active-matrix TFT backplane.

High performance thin film transistors with cosputtered amorphous indium gallium zinc oxide channel

The authors report the fabrication of high performance thin film transistors (TFTs) with an amorphous indium gallium zinc oxide (a-IGZO) channel, which was deposited by cosputtering using a dual IGZO and indium zinc oxide (IZO) target. The effect of the indium content on the device performance of the a-IGZO TFTs was investigated. At a relatively low IZO power of 400W, the field-effect mobility (μFE) and subthreshold gate swing (S) of the a-IGZO TFTs were dramatically improved to 19.3cm2∕Vs and 0.35V/decade, respectively, compared to those (11.2cm2∕Vs and 1.11V/decade) for the TFTs with the a-IGZO channel (reference sample) prepared using only the IGZO target. The enhancement in the subthreshold IDS-VGS characteristics at an IZO power of 400W compared to those of the reference sample was attributed to the reduction of the interface trap density rather than the reduction of the bulk defects of the a-IGZO channel.

Origin of Subthreshold Swing Improvement in Amorphous Indium Gallium Zinc Oxide Transistors

The effect of the channel deposition pressure on the device performance of amorphous indium-gallium-zinc oxide (a-IGZO) transistors was investigated in detail. The performance of the fabricated transistors improved monotonously with decreasing chamber pressure: at a pressure of 1 mTorr, the field-effect mobility (μ FE ) and subthreshold gate swing (S) of the a-IGZO thin-film transistors were dramatically improved to 21.8 cm 2 /Vs and 0.17 V/decade, respectively, compared to those (11.4 cm 2 /Vs and 0.87 V/decade) of the reference transistors prepared at 5 mTorr. This enhancement in the subthreshold characteristics was attributed to the reduction of the bulk defects of the a-IGZO channel, which might result from the greater densification of the a-IGZO films at the lower deposition pressure.

Control of threshold voltage in ZnO-based oxide thin film transistors

We investigated the feasibility of controlling the threshold voltage (Vth) by adjusting the thickness of the active layer (tactive) rather than by conventional chemical doping in indium-gallium-zinc oxide (IGZO) transistors with an inverted staggered structure. The value of Vth of the IGZO transistor was linearly modulated from −15.3±1.6to−0.1±0.21V by reducing tactive without any significant change in the field-effect mobility (μFE), subthreshold gate swing, or Ion∕off ratio. The free electron density extracted from the relationship between tactive and Vth was 1.9×1017cm−3, which was consistent with the value of 1.5×1017cm−3 obtained from the C-V measurement for the 30-nm-thick IGZO films. The slight increase in the μFE with increasing tactive, which was in contradiction with the behavior of the corresponding amorphous Si transistor, was explained by the anomalous behavior of the source/drain contact resistance.

Carrier trapping and efficient recombination of electrophosphorescent device with stepwise doping profile

We have presented a physical concept for enhancing efficiency and lifetime of doped electrophosphorescent organic light-emitting devices. In order to provide a control parameter for higher device performance, a stepwise doping concentration profile at the emission layer was prepared. A more than 30% improvement of power efficiency was obtained for green electrophosphorescent device with a higher doping ratio at the emission layer-hole transport layer interface. We explained the carrier trapping and transport mechanism with direct recombination of an exciton in an iridium-based dopant system. When compared to green device, phosphorescent red devices showed a more significant charge trapping effect at low doping concentration, which is responsible for shifting the recombination zone far from the emission layer-hole transport layer interface. Therefore, charge trapping by doping control in an emission layer could be utilized for a charge-balancing technique for the confinement of a triplet exciton.

Amorphous-oxide TFT backplane for large-sized AMOLED TVs

— Amorphous-oxide thin-film-transistor (TFT) arrays have been developed as TFT backplanes for large-sized active-matrix organic light-emitting-diode (AMOLED) displays. An amorphous-IGZO (indium gallium zinc oxide) bottom-gate TFT with an etch-stop layer (ESL) delivered excel lent electrical performance with a field-effect mobility of 21 cm2/V-sec, an on/off ratio of >108, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous-oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold-voltage (VTH) compensation pixel circuits, especially in the negative state. A full-color 19-in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19-in. AMOLED display. The AMOLED display with a-IGZO TFT array is promising for large-sized TV because a-IGZO TFTs can provide a large-sized backplane with excellent uniformity and device reliability.