Xifeng Li

Characterization of amorphous Si-Zn-Sn-O thin films and applications in thin-film transistors

Amorphous silicon-zinc-tin-oxide (α-SZTO) thin films were prepared, and their properties were investigated physically and electrically, with an emphasis on the Si effects. An appropriate Si content in the matrix can not only achieve stable and dense films, but also suppress the formation of oxygen vacancies efficiently, due to its high oxygen bonding ability. Thin film transistors (TFTs) with α-SZTO active channel layers exhibited a field-effect mobility of around 1 cm2V−1s−1, an on/off current ratio of 107, and a subthreshold swing of 0.863 V/decade with a good long-term stability. The α-SZTO TFT is a potential candidate for electronic applications.

Performance and stability of amorphous InGaZnO thin film transistors with a designed device structure

We propose a specifically designed structure to fabricate thin-film transistors using amorphous indium-gallium-zinc-oxide (a-IGZO) films as the active channel layers. The I-shaped gate electrode is employed to define the channel width, reducing overlaps between the gate and source/drain electrodes. The devices with such a structure exhibit acceptable electrical performance and stability after annealing treatment. The XPS data show that the as-deposited a-IGZO film has not a very dense structure that may induce shallow traps. A shallow trap model is proposed to explain the large threshold voltage shifts of the as-deposited device. Annealing treatment can eliminate these shallow traps and improve the device stability.

Amorphous ZnAlSnO thin-film transistors by a combustion solution process for future displays

A combustion solution method was developed to fabricate amorphous ZnAlSnO (a-ZATO) for thin-film transistors (TFTs). The properties of a-ZATO films and behaviors of a-ZATO TFTs were studied in detail. An appropriate Al content in the matrix could suppress the formation of oxygen vacancies efficiently and achieve densely amorphous films. The a-ZATO TFTs exhibited acceptable performances, with an on/off current ratio of ∼106, field-effect mobility of 2.33 cm2·V−1·S−1, threshold voltage of 2.39 V, and subthreshold swing of 0.52 V/decade at an optimal Al content (0.5). The relation between on- and off-resistance of the ZATO TFT was also within the range expected for fast switching devices. More importantly, the introduced Al with an appropriate content had the ability to evidently enhance the device long-term stability under working bias stress and storage durations. The obtained indium- and gallium-free a-ZATO TFTs are very promising for the next-generation displays.

Instability Induced by Ultraviolet Light in ZnO Thin-Film Transistors

We investigate the electrical properties of ZnO thin-film transistors (TFTs) under the ultraviolet (UV) light illumination to clarify the device reliability. The ON/OFF current ratio of ZnO TFTs shows an evident reduction due to the creation of UV photo-induced current. This behavior degradation becomes severe for devices exposed to a short-wavelength UV light. The persistent photoconductivity is observed in ZnO TFTs under the UV illumination with a much slower decay for the ON-state than the OFF-state, which is attributed to the reduced recombination under a positive gate bias. The UV light instabilities of ZnO TFTs can be explained on the basis of UV photoexcited oxygen vacancies. A feasible mechanism model is proposed in this paper. These observations and analytical model not only provide useful physical insight into the device behaviors, but also offer basic design guideline for oxide TFTs.

Combustion-process derived comparable performances of Zn-(In:Sn)-O thin-film transistors with a complete miscibility

Amorphous zinc-indium-tin oxide (a-ZITO) thin-film transistors (TFTs) have been prepared using a low-temperature combustion process, with an emphasis on complete miscibility of In and Sn contents. The a-ZITO TFTs were comparatively studied in detail, especially for the working stability. The a-ZITO TFTs all exhibited acceptable and excellent behaviors from Sn-free TFTs to In-free TFTs. The obtained a-ZTO TFTs presented a field-effect mobility of 1.20 cm2 V−1 s−1, an on/off current ratio of 4.89 × 106, and a long-term stability under positive bias stress, which are comparable with those of the a-ZIO TFTs. The In-free a-ZTO TFTs are very potential for electrical applications with a low cost.

Water assisted oxygen absorption on the instability of amorphous InAlZnO thin-film transistors

Stability of amorphous InAlZnO thin-film transistors was investigated under vacuum conditions and an oxygen atmosphere with different rates of relative humidity. The mechanism of water assisted oxygen absorption on the back channel is proposed to explain the phenomenon that wet oxygen leads to a larger positive threshold voltage (Vth) shift than dry oxygen.

Effect of etching stop layer on characteristics of amorphous IGZO thin film transistor fabricated at low temperature

Transparent bottom-gate amorphous Indium-Gallium-Zinc Oxide (a-IGZO) thin-film transistors (TFTs) had been successfully fabricated at relative low temperature. The influence of reaction gas ratio of N2O and SiH4 during the growth of etching stop layer (SiOx) on the characteristics of a-IGZO TFTs was investigated. The transfer characteristics of the TFTs were changed markedly because active layer of a-IGZO films was modified by plasma in the growth process of SiOx. By optimizing the deposition parameters of etching stop layer process, a-IGZO TFTs were manufactured and exhibited good performance with a field-effect mobility of 8.5 cm2V-1s-1, a threshold voltage of 1.3 V, and good stability under gate bias stress of 20 V for 10000 s.

Formation mechanism of shear bands in ultrafine lamellar Ti–Fe eutectics

Shear bands with alternating lamellar and serrated structures were observed on the fracture surface of an Ti–Fe ultrafine eutectic. The shear bands are composed of coalesced subgrains. Hence, a subgrain rotational mechanism is proposed to explain their formation. The work is a direct experimental evidence of the hypothetical rotational mechanism.

Effects of external magnetic field on the morphology and magnetic property of BiFeO3 particles prepared by hydrothermal synthesis

The perovskite multiferroic bismuth ferrite (BiFeO3) particles were synthesized by hydrothermal method under external magnetic fields with respective intensities of 0 T, 4 T, 8 T, and 12 T. High magnetic field greatly affected the growth behavior and magnetic property of BiFeO3. The morphology of BiFeO3 changed from regular cube to irregular sphere, and finally became a chain-like structure. Meanwhile, the magnetization of the as-prepared BiFeO3 particles measured under a 1.8 T magnetic field was enhanced from 0.1003 emu/g (0 T) to 0.2067 emu/g (12 T). The possible mechanisms of crystal growth and magnetic property were also discussed.

Solution-Processed Low-Operating-Voltage Thin-Film Transistors With Bottom-Gate Top-Contact Structure

It was demonstrated that low-operating-voltage bottom-gate top-contact (BGTC) structure thin-film transistors (TFTs) were fabricated using solution-processed amorphous zinc indium tin oxide (ZITO) and hafnium aluminum oxide (HAO) films. Due to the excellent chemical stability of ZITO film and high etching selectivity between ZITO and indium tin oxide (or between ZITO and Mo), the manufacture of BGTC structure requires only traditional lithography and wet etching. The usage of high-k HAO films lowered the operating voltage (below 2 V) of the TFT devices. TFT devices based on ZITO and HAO films annealing at 500 °C showed a saturated field-effect mobility of 13.5 cm2V-1s-1, a small subthreshold swing of 87 mV/decade, a large ON-to-OFF current ratio of 7.2 × 106, and a threshold voltage of -0.6 V. The hysteresis (0.14 V) of TFT devices confirmed further that the ZITO and HAO films could be the promising candidates for TFT devices.