Improvement in the negative bias temperature stability of ZnO based thin film transistors by Hf and Sn doping

We assessed the performance of ZnO TFTs using Si₃N₄ gate dielectrics after various treatments. A remarkable improvement in the transfer characteristics was obtained for the O₂ plasma treated ZnO TFT and SiO₂ interlayer deposited ZnO TFT. Also, we developed amorphous hafnium-zinc-tin oxide (HZTO) thin film transistors (TFTs) and investigated the influence of hafnium (Hf) doping on the electrical characteristics of the hafnium-zinc oxide (HZO) thin film transistors. Doping with Hf can decrease the carrier concentration, which may result from a decrease of the field effect mobility, and reduce oxygen vacancy related defects in the interfacial layer. Adding tin (Sn) can suppress the growth of a crystalline phase in the HZTO films. The HZTO TFTs exhibited good electrical properties with a field effect mobility of 14.33 cm²/Vs, a subthreshold swing of 0.97 V/decade, and a high I_ON/OFF ratio of over 10⁹.     

Copper phthalocyanine thin-film field-effect transistor with SiO2/Ta2O5/SiO2 multilayer insulator

Top-contact Copper phthalocyanine (CuPc) thin-film field-effect transistor (TFT) with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer as the dielectric was fabricated and investigated. With the multi-layer dielectric, drive voltage was remarkably reduced. A relatively large on-current of 1.1 × 10⁻⁷ A at a V_GS of −15 V was obtained due to the strong coupling capability provided by the STS multilayer gate insulator. The device shows a moderate performance: saturation mobility of μ_sat = 6.12 × 10⁻⁴ cm²/V s, on-current to off-current ratio of I_on/I_off = 1.1 × 10³, threshold voltage of V_TH = −3.2 V and sub-threshold swing SS = 1.6 V/dec. Atomic force microscope images show that the STS multilayer has a relative smooth surface. Experiment results indicate that STS multilayer is a promising insulator for the low drive voltage CuPc-based TFTs.     

Paraffin wax passivation layer improvements in electrical characteristics of bottom gate amorphous indium–gallium–zinc oxide thin-film transistors

In this research, paraffin wax is employed as the passivation layer of the bottom gate amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs), and it is formed by sol–gel process in the atmosphere. The high yield and low cost passivation layer of sol–gel process technology has attracted much attention for current flat-panel-display manufacturing. Comparing with passivation-free a-IGZO TFTs, passivated devices exhibit a superior stability against positive gate bias stress in different ambient gas, demonstrating that paraffin wax shows gas-resisting characteristics for a-IGZO TFTs application. Furthermore, light-induced stretch-out phenomenon for paraffin wax passivated device is suppressed. This superior stability of the passivated device was attributed to the reduced total density of states (DOS) including the interfacial and semiconductor bulk trap densities.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

Effect of N2O plasma treatment on the improvement of instability under light illumination for InGaZnO thin-film transistors

This paper investigates the impact of N₂O plasma treatment on the light-induced instability of InGaZnO thin film transistors with a SiO₂ passivation layer deposited by plasma-enhanced-chemical-vapor-deposition (PECVD). For the untreated device, because the deposition of the SiO₂ passivation layer by PECVD causes extra trap states, the anomalous subthreshold leakage current can be attributed to a lowering of the source side barrier due to trap-assisted photogenerated holes. In contrast, the N₂O plasma treatment applied to both the gate insulator and the active layer effectively suppresses the device instability under illumination. In order to clarify the influence of the N₂O plasma treatment, this study investigates a device with treatment of only the gate insulator. This device shows a slight decrease of light-induced subthreshold leakage current. This demonstrates that N₂O plasma treatment on IGZO active layer after its deposition is critical in preventing damage from the subsequent SiO₂ passivation deposition process. In addition, the instability of threshold voltage (V_T) under negative bias illumination stress (NBIS) is significantly improved by the N₂O plasma treatment. Furthermore, a different dark recovery rate follows NBIS for untreated and N₂O plasma-treated devices, indicating different hole-trapping levels exist in the energy band.     

Annealing effects of In2O3 thin films on electrical properties and application in thin film transistors

Considering practical applications in electronic devices, we studied the growth of In₂O₃ thin films on amorphous glasses by magnetron sputtering at room temperature and annealing effect on the structural and electrical properties. The vacuum annealed In₂O₃ thin films display a grain size enlargement and preferential orientation. Electrical characterization shows that the vacuum annealed In₂O₃ thin films exhibit a significant enhancement of both electron density and mobility, while air ambient annealing leads to a remarkable drop. The mechanism of the electrical characteristic changes in In₂O₃ thin films by annealing is explored by using different scattering mechanisms. Finally, a thin film transistor device using vacuum annealed In₂O₃ nano-meter thin films as active channel material is demonstrated.     

Improvement in light output intensity of InGaN/GaN multiple-quantum-well blue light-emitting diode by SiO2/Si3N4 distributed Bragg reflectors and silver back mirror

In this article, the light output intensity of InGaN/GaN multiple-quantum-well (MQW) light emitting diodes (LEDs) is improved by using SiO₂/Si₃N₄ distributed Bragg reflectors (DBRs) as window layer and Ag back mirror. The SiO₂/Si₃N₄ DBRs can take several advantages, such as high reflectance with less number of DBR, passive characteristics, and high reliability due to growth in one pump down growth system. The experimental results indicated that InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs show a maximum light output intensity of 64 mcd under 20 mA driving current and an improvement of 42% as compared to that of InGaN/GaN LEDs without SiO₂/Si₃N₄ DBRs. In addition, the turn-on voltage, forward resistance, and full width at half maximum (FWHM) of the emission spectra for InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs and Ag back mirror are 3.23 V, 16 Ω, and 22.4 nm under 20 mA forward current.     

Illumination interface stability of aging-diffusion-modulated high performance InZnO/DyOx transistors and exploration in digital circuits

In current study,the rare-reported solution-driven DyOx films have been prepared to act as the dielectric layer of high performance InZnO/DyOx thin film transistors (TFTs).Annealing temperature depen-dent thermal decomposition,morphology,crystallization behavior,and chemical compositions of DyOx and InZnO films have been investigated respectively.Results have demonstrated that air-annealed InZnO/DyOx TFTs possess the improved electrical performance,including ultrahigh on/offcurrent ratio of 1 × 109,larger saturation mobility of 12.6 cm2 V-1 s-1 and negligible hysteresis after 10 d aging diffusion in the relative humidity (RH) of 40 ％ air ambient,which has been explored by the variable range-hopping(VRH) percolation model and energy band theory.The distinct illumination bias stability can be attributed to the generated various interface defects and concluded that the white light illuminated TFT behaves the higher stability with the smaller threshold voltage shift of 0.25 V.To confirm its feasible application in digital circuit,a resistor-loaded inverter based on InZnO/DyOx TFTs has been constructed.A high gain of 10.1 and good dynamic response behavior have been detected at a low operating voltage of 2 V.As a result,it can be inferred that diffusion-induced enhanced carrier transporting mechanism is an economical and effective method to optimize the electrical performance of solution-derived lnZnO/DyOx TFTs,indicating its potential application prospects in flexible transparent electronics with low power consumption.     

Preparation of SiO2 thin films using the Cat-CVD method

Using the catalytic chemical vapor deposition (Cat-CVD) method, a-Si and SiN_x films have been the main focus of studies. SiO₂ films have not been studied because of the limited life of catalysts such as tungsten or molybdenum in an oxidative atmosphere. In this report, we describe oxide film preparation using an iridium catalyst. We determined the most appropriate catalyst material for the oxide film process by exposing heated materials in tetraethoxysilane (TEOS) or O₂ gas. As the result, it was confirmed that the Ir catalyst works in a slow oxidative atmosphere. Using the Ir catalyst, SiO₂ films were deposited in two gas combinations: TEOS and N₂O, and SiH₄ and N₂O. Although the SiO₂ film processed with the combination of TEOS and N₂O was stoichiometric, its breakdown voltage is not sufficient. The SiO₂ film processed with the combination of SiH₄ and N₂O showed good electrical property.     

Investigation of self-assembled monolayer treatment on SiO2 gate insulator of poly(3-hexylthiophene) thin-film transistors

We have investigated self-assembled monolayer (SAM) treatment on SiO₂ gate insulator of poly(3-hexylthiophene) (P3HT) thin-film transistor (TFT), and demonstrated a correlation between mobility and surface free energy of the insulator. The device with lower surface free energy shows higher mobility. The docosyltrichlorosilane (DCTS)-treated device exhibits the best performance among the various SAM-treated devices examined. Field-effect mobility, on/off ratio and threshold voltage of the DCTS-treated P3HT TFT were 0.015 cm²/Vs, >10⁵ and −14 V, respectively.     

Improvement in etch selectivity of SiO2 to CVD amorphous carbon mask in dual-frequency capacitively coupled C4F8/CH2F2/O2/Ar plasmas

Highly selective etching of a SiO₂ layer using a chemical vapor deposited (CVD) amorphous carbon (a-C) mask pattern was investigated in a dual-frequency superimposed capacitively coupled plasma etcher. The following process parameters of the C₄F₈/CH₂F₂/O₂/Ar plasmas were varied: the CH₂F₂/(CH₂F₂ + O₂) flow ratio (Q(CH₂F₂)), the high frequency power (P_HF), and the low frequency power (P_LF). It was found a process window exists to obtain infinitely high etch selectivity of the SiO₂ layer to the CVD a-C. The process parameters of Q(CH₂F₂), P_HF, and P_LF played critical roles in determining the process window for oxide/CVD a-C etch selectivity, presumably due to the disproportionate degree of polymerization on the SiO₂ and CVD a-C surfaces.     

Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si₃N₄ particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si₃N₄ powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si₃N₄ powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.     

Effect of NH3 plasma treatment on the device performance of ZnO based thin film transistors

We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH₃ plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH₃ plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm²/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 10⁻¹¹ A and on to off ratio higher than 10⁵. These results demonstrate that NH₃ plasma treatment could effectively enhance the performance of the ZnO based TFT device.     

Optical and electrical properties of 2 wt.% Al2O3-doped ZnO films and characteristics of Al-doped ZnO thin-film transistors with ultra-thin gate insulators

Al-doped ZnO (AZO) thin films have been prepared on the c-Si oriented direction of (100) and glass substrates, by radio frequency magnetron sputtering from ZnO-2 wt.% Al₂O₃ ceramic targets. The effects of the working pressure on the optical and electrical properties of the films have been studied. The optical properties, measured by the ultraviolet-visible system, show that the transmittance and optical bandgap energy are influenced by the working pressure. The Hall resistivity, mobility, and carrier concentration were obtained by a Hall measurement system and these parameters were also influenced by the working pressure. The AZO thin-film transistors (TFTs) were fabricated on highly doped c-Si substrates. The TFT structures were made up AZO as the active layer and SiO_xN_y/SiN_x/SiO_x as the gate layer with 20 nm and 35 nm thickness, respectively. The ultra-thin TFTs had an on/off current ratio of 10⁴ and a field-effect mobility of 0.17 cm²/V·s. These results show that it is possible to fabricate an AZO TFT that can be operated with an ultra-thin gate dielectric.     

Evaluation of Y2O3 gate insulators for a-IGZO thin film transistors

In this work, Y₂O₃ was evaluated as a gate insulator for thin film transistors fabricated using an amorphous InGaZnO₄ (a-IGZO) active layer. The properties of Y₂O₃ were examined as a function of various processing parameters including plasma power, chamber gas conditions, and working pressure. The leakage current density for the Y₂O₃ film prepared under the optimum conditions was observed to be ~ 3.5 × 10^− 9 A/cm² at an electric field of 1 MV/cm. The RMS roughness of the Y₂O₃ film was improved from 1.6 nm to 0.8 nm by employing an ALD (Atomic Layer Deposition) HfO₂ underlayer. Using the optimized Y₂O₃ deposition conditions, thin film transistors (TFTs) were fabricated on a glass substrate. The important TFT device parameters of the on/off current ratio, sub-threshold swing, threshold voltage, and electric field mobility were measured to be 7.0 × 10⁷, 0.18 V/dec, 1.1 V, and 3.3 cm²/Vs, respectively. The stacked insulator consisting of Y₂O₃/HfO₂ was highly effective in enhancing the device properties.     

The effects of gas flow rates on the etch characteristics of silicon nitride with an extreme ultra-violet resist pattern in CH2F2/N2/Ar capacitively coupled plasmas

The effects of CH₂F₂ and N₂ gas flow rates on the etch selectivity of silicon nitride (Si₃N₄) layers to extreme ultra-violet (EUV) resist and the variation of the line edge roughness (LER) of the EUV resist and Si₃N₄ pattern were investigated during etching of a Si₃N₄/EUV resist structure in dual-frequency superimposed CH₂F₂/N₂/Ar capacitive coupled plasmas (DFS-CCP). The flow rates of CH₂F₂ and N₂ gases played a critical role in determining the process window for ultra-high etch selectivity of Si₃N₄/EUV resist due to disproportionate changes in the degree of polymerization on the Si₃N₄ and EUV resist surfaces. Increasing the CH₂F₂ flow rate resulted in a smaller steady state CH_xF_y thickness on the Si₃N₄ and, in turn, enhanced the Si₃N₄ etch rate due to enhanced SiF₄ formation, while a CH_xF_y layer was deposited on the EUV resist surface protecting the resist under certain N₂ flow conditions. The LER values of the etched resist tended to increase at higher CH₂F₂ flow rates compared to the lower CH₂F₂ flow rates that resulted from the increased degree of polymerization.     

Texture development of Ca3Co4O9 thermoelectric oxide by high temperature plastic deformation and its contribution to the improvement in electric conductivity

High temperature uniaxial compression is conducted on Ca₃Co₄O₉ layered cobaltite, in order to achieve a thermoelectric oxide with low resistivity by the development of (0 0 1) texture. It is found that flow stress varies depending on deformation temperature and strain rate. Development of a sharp texture having the maximum (0 0 1) pole density of about 33 times as high as the random level is achieved. It is found that the high temperature compression process is quite effective for the simultaneous achievement of densification and (0 0 1) texture development. It is experimentally confirmed that resistivity decreases drastically by the construction of a sharp (0 0 1) texture.     

Morphological evolution of the poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester, oxidation of the silver electrode, and their influences on the performance of inverted polymer solar cells with a sol-gel derived zinc oxide electron selective layer

The inverted polymer solar cell (PSC) based on a sol-gel derived zinc oxide (ZnO) thin film as an electron selective layer is investigated. The device performance is improved after the fabricated device is placed in air for a few days. The improvement is attributed to the self-organization of the poly(3-hexylthiophene)/[6,6]-phenyl-C₆₁-butyric acid methyl ester layer and oxidation of the silver electrode with time, resulting in a significant enhancement in the short circuit current, fill factor and open circuit voltage. The investigation shows that the inverted PSC based on ZnO thin film exhibits a high efficiency of 3.8% on the 6th day after fabrication without the use of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) and encapsulation.