Jae Kyeong Jeong

Origin of the improved mobility and photo-bias stability in a double-channel metal oxide transistor.

This study examined the performance and photo-bias stability of double-channel ZnSnO/InZnO (ZTO/IZO) thin-film transistors. The field-effect mobility (μFE) and photo-bias stability of the double-channel device were improved by increasing the thickness of the front IZO film (t(int)) compared to the single-ZTO-channel device. A high-mobility (approximately 32.3u2005cm(2)/Vs) ZTO/IZO transistor with excellent photo-bias stability was obtained from Sn doping of the front IZO layer. First-principles calculations revealed an increase in the formation energy of O vacancy defects in the Sn-doped IZO layer compared to the IZO layer. This observation suggests that the superior photo-bias stability of the double-channel device is due to the effect of Sn doping during thermal annealing. However, these improvements were observed only when t(int) was less than the critical thickness. The rationale for this observation is also discussed based on the oxygen vacancy defect model.

Photobias Instability of High Performance Solution Processed Amorphous Zinc Tin Oxide Transistors

The effects of the annealing temperature on the structural and chemical properties of soluble-processed zinc-tin-oxide (ZTO) films were examined by transmission electron microscopy, atomic force microscopy, high resolution X-ray reflectivity, and X-ray photoelectron spectroscopy. The density and purity of the resulting ZTO channel layer increased with increasing annealing temperature, whereas the oxygen vacancy defect density decreased. As a result, the device performance of soluble ZTO thin film transistors (TFTs) was improved at higher annealing temperature. Although the 300 °C-annealed ZTO TFT showed a marginal field-effect mobility (μFE) and high threshold voltage (Vth) of 0.1 cm(2)/(V s) and 7.3 V, respectively, the 500 °C-annealed device exhibited a reasonably high μFE, low subthreshold gate swing (SS), Vth, and Ion/off of 6.0 cm(2)/(V s), 0.28 V/decade, 0.58 V, and 4.0 × 10(7), respectively. The effects of dark negative bias stress (NBS) and negative bias illumination stress (NBIS) on the degradation of transfer characteristics of ZTO TFTs were also investigated. The instability of Vth values of the ZTO TFTs under NBS and NBIS conditions was suppressed with increasing annealing temperature. To better understand the charge trapping mechanism, the dynamics of Vth shift with NBS and NBIS time for all ZTO TFTs was analyzed on the basis of the stretched exponential relaxation. The negative Vth shift for each transistor was accelerated under NBIS conditions compared to NBS, which resulted in a higher dispersion parameter and smaller relaxation time for NBIS degradation. The relaxation time for NBS and NBIS instability increased with increasing annealing temperature, which is discussed on the basis of the transition mechanism of oxygen vacancy defects.

Anomalous behavior of negative bias illumination stress instability in an indium zinc oxide transistor: A cation combinatorial approach

This study examined the effects of the indium fraction in indium zinc oxide (IZO) on the performance and stability of IZO thin film transistors (TFTs). The field-effect mobility and sub-threshold swing were much improved with increasing In fraction; 41.0 cm2/Vs and 0.2u2009V/decade, respectively, at 85 at. % In, compared to 1.1 cm2/Vs and 2.4u2009V/decade of ZnO TFTs. In contrast, a local minimum negative bias illumination stress instability was observed near 73–77 at. % In. This behavior was explained by a poly-crystalline to amorphous phase transition in IZO thin films.

Improving the Morphological and Optical Properties of Sputtered Indium Tin Oxide Thin Films by Adopting Ultralow-Pressure Sputtering

cm were also achieved using a continuous two-step depositionprocess, in which the initial layer was deposited using ULPS and then the final layer was deposited with a SP of 6.7 −110 Pa,without the use of any other additional steps. Both the ULPS and continuous two-step deposition methods were found to beeffective for producing ITO thin films with enhanced morphologies that make them suitable for use in display devices.© 2008 The Electrochemical Society. DOI: 10.1149/1.3005562 All rights reserved.Manuscript submitted June 26, 2008; revised manuscript received September 3, 2008. Published November 5, 2008.

Impact of the cation composition on the electrical performance of solution-processed zinc tin oxide thin-film transistors.

This study examined the structural, chemical, and electrical properties of solution-processed (Zn,Sn)O3 (ZTO) films with various Sn/[Zn+Sn] ratios for potential applications to large-area flat panel displays. ZTO films with a Zn-rich composition had a polycrystalline wurtzite structure. On the other hand, the Sn-rich ZTO films exhibited a rutile structure, where the Zn atom was speculated to replace the Sn site, thereby acting as an acceptor. In the intermediate composition regions (Sn/[Zn+Sn] ratio from 0.28 to 0.48), the ZTO films had an amorphous structure, even after annealing at 450 °C. The electrical transport properties and photobias stability of ZTO thin film transistors (TFTs) were also examined according to the Sn/[Zn+Sn] ratio. The optimal transport property of ZTO TFT was observed for the device with an amorphous structure at a Sn/[Zn+Sn] ratio of 0.48. The mobility, threshold voltage, subthreshold swing, and on/off current ratio were 4.3 cm(2)/(V s), 0 V, 0.4 V/decade, and 4.1 × 10(7), respectively. In contrast, the device performance for the ZTO TFTs with either a higher or lower Sn concentration suffered from low mobility and a high off-state current, respectively. The photoelectrical stress measurements showed that the photobias stability of the ZTO TFTs was improved substantially when the ZTO semiconducting films had a lower oxygen vacancy concentration and an amorphous structure. The relevant rationale is discussed based on the phototransition and subsequent migration mechanism from neutral to positively charged oxygen vacancies.

Improvement of photo-induced negative bias stability of oxide thin film transistors by reducing the density of sub-gap states related to oxygen vacancies

The optical absorption in the sub-gap region of amorphous indium zinc oxide films and the photo-induced negative bias stability of the resulting thin film transistors were studied. As the indium ratio increases, optical absorption via sub-gap states increases, and the threshold voltage degradation under negative bias temperature stress (NBTS) with light illumination becomes more severe. By applying high pressure anneal treatments in oxygen ambient, the density of sub-gap states is reduced by an order of magnitude compared to air-annealed devices. Consequently, significant improvements are observed in the threshold voltage shifts and the stretched exponential parameters under NBTS with light illumination.

Li-Assisted Low-Temperature Phase Transitions in Solution-Processed Indium Oxide Films for High-Performance Thin Film Transistor

Lithium (Li)-assisted indium oxide (In2O3) thin films with ordered structures were prepared on solution-processed zirconium oxide (ZrO2) gate dielectrics by spin-casting and thermally annealing hydrated indium nitrate solutions with different Li nitrate loadings. It was found that the Li-assisted In precursor films on ZrO2 dielectrics could form crystalline structures even at processing temperatures (T) below 200u2009°C. Different In oxidation states were observed in the Li-doped films, and the development of such states was significantly affected by both temperature and the mol% of Li cations, [Li+]/([In3+]u2009+u2009[Li+]), in the precursor solutions. Upon annealing the Li-assisted precursor films below 200u2009°C, metastable indium hydroxide and/or indium oxyhydroxide phases were formed. These phases were subsequently transformed into crystalline In2O3 nanostructures after thermal dehydration and oxidation. Finally, an In2O3 film doped with 13.5u2009mol% Li+ and annealed at 250u2009°C for 1u2009h exhibited the highest electron mobility of 60u2009cm2 V−1 s−1 and an on/off current ratio above 108 when utilized in a thin film transistor.

High Performance Metal Oxide Field-Effect Transistors with a Reverse Offset Printed Cu Source/Drain Electrode

Nonvacuum and photolithography-free copper (Cu) films were prepared by reverse offset printing. The mechanical, morphological, structural, and chemical properties of the Cu films annealed at different temperatures were examined in detail. The Ostwald ripening-induced coalescence and grain growth in the printing Cu films were enhanced with increasing annealing temperature in N2 ambient up to 400 °C. Simultaneously, unwanted chemical impurities such as oxygen, hydrogen, and carbon in the Cu films decreased as the annealing temperature increased. The high electrical conductivity (∼6.2 μΩ·cm) of the printing Cu films annealed at 400 °C is attributed to the enlargement of the grain size and reduction of the incorporation of impurities. A printing Cu film was adopted as a source/drain (S/D) electrode in solution processable zinc tin oxide (ZTO) field-effect transistors (FETs), where the ZTO film was prepared by simple spin-coating. The ZTO FETs fabricated at a contact annealing temperature of 250 °C exhibited a promising field-effect mobility of 2.6 cm(2)/(V s), a threshold voltage of 7.0 V, and an ION/OFF modulation ratio of 2 × 10(5).

A solution-processed silicon oxide gate dielectric prepared at a low temperature via ultraviolet irradiation for metal oxide transistors

A facile route for the preparation of a solution-processed silicon oxide dielectric from perhydropolysilazane (PHPS) at a low temperature (≤150 °C) is proposed. Deep ultraviolet (DUV) irradiation on the PHPS-derived silicon oxide film, where the coupling agent of vinyltriethoxysilane (VTES) was introduced to assist the formation of the Si–O lattice network at a low temperature, allowed the prepared silicon oxide film to have device-quality insulating properties. The metal/insulator/metal capacitor with DUV-annealed silicon oxide exhibited a low gate leakage current density of 7.0 × 10−8 A cm−2 at 1 MV cm−1, which was attributed to the photon-assisted purification and densification of the silicon oxide film. The suitability of this silicon oxide film as a gate insulator was evaluated in all-solution-processed indium zinc oxide (IZO) thin-film transistors (TFTs). The IZO TFTs that were fabricated at a contact annealing temperature of 150 °C exhibited a high field-effect mobility of 17.3 cm2 V−1 s−1, a threshold voltage of 2.7 V, and an ION/OFF modulation ratio of 1 × 105. Therefore, DUV-assisted IZO TFTs with a PHPS-derived silicon oxide insulator are promising candidates for low-temperature, large-area, and flexible electronics for use on inexpensive plastic substrates.

Effect of sputter power on the photobias stability of zinc-tin-oxide field-effect transistors

This study examined the effect of sputtering power on the performance of zinc-tin-oxide field-effect transistors and the stability under photobias stress. Large improvements in the saturation mobility and subthreshold swing were found in devices fabricated at higher sputtering powers; 13.80u2009cm2/V·s, 0.33u2009V/decade at a power of 400u2009W compared with 2.70 cm2/V·s, 1.19u2009V/decade at a power of 50u2009W. The threshold voltage shift under negative bias illumination stress (NBIS) for the device fabricated at a power of 400u2009W shows superior properties (−2.41u2009V) compared with that (−5.56u2009V) of the device fabricated at 50u2009W. The improvements in electrical performance and NBIS stability were attributed to the formation of a denser film and the reduced dielectric/channel interfacial trap densities due to the more energetic bombardment used under high power sputtering conditions.