Electrical and chemical stability engineering of solution-processed indium zinc oxide thin film transistors via a synergistic approach of annealing duration and self-combustion process

The electrical and chemical stability of solution-processed indium zinc oxide (IZO) channel thin-film transistors (TFTs) were engineered via a synergistic approach of annealing duration and self-combustion process. In particular, the amorphous IZO TFTs that were thermally treated at 400 °C for 3 h using the specific precursor combination to generate internal self-combustion energy showed the best electrical performance [high saturation mobility (μ_SAT)=2.7 cm²/V s] and stability [low threshold voltage shift (ΔV_TH) under positive bias stress of 10.5 V] owing to the formation of oxide films with excellent metal–oxide–metal (M–O–M) bonds, fewer impurities, and an amorphous phase compared to IZO TFTs using other precursor formulas and annealing times. Longer annealing times led to a saturated M–O bond ratio and crystallization via extreme thermal annealing, which induced electrical degradation (low μ_SAT and high ΔV_TH) of IZO TFTs. In the wet chemical patterning of electrodes, conventional acidic and basic wet etchants cause severe damage to the surfaces of the IZO channels; thus, insufficiently annealed IZO TFTs exhibited considerable degradation in terms of their on-current level and mobility. Alternatively, the TFTs subjected to an excessively long-term thermal annealing showed only a moderate decrease in mobility with the formation of small nanocrystals.     

Improvement in performance of indium gallium oxide thin film transistor via oxygen mediated crystallization at a low temperature of 200 °C

We report the fabrication of high-performance polycrystalline indium gallium oxide (IGO) thin film transistors (TFTs) at a low temperature of 200 °C. Growth of a highly aligned cubic phase with a bixbyite structure was accelerated at a certain proportion of oxygen plasma density during deposition of the IGO thin film, which leads to outstanding electrical characteristics. The resulting polycrystalline IGO TFT exhibited a high field-effect mobility of 56.0 cm²/V, a threshold voltage (V_TH) of 0.10 V, a low subthreshold gate swing of 0.10 V/decade, and a current modulation ratio of >10⁸. Moreover, the crystalline IGO TFTs have highly stable behaviors with a small V_TH shift of +0.8 and ?1.0 V against a positive bias stress (V_GS,ST ?V_TH = 20 V) and negative bias illumination stress (V_GS,ST ?V_TH = ?20 V) for 3,600 s, which is attributed to the high quality of the bixbyite crystalline structure.     

Rapid thermal annealing effect of transparent ITO source and drain electrode for transparent thin film transistors

We investigated the rapid thermal annealing (RTA) sequence effect on the electrical, optical, morphological, and structural properties of transparent thin film transistors (TTFTs) with an indium gallium zinc oxide (IGZO) channel and an indium tin oxide (ITO) source/drain. The electrical and optical properties of the IGZO channel and the ITO source/drain electrodes were compared as a function of RTA temperature in ambient air. The performance of a TTFT with only an RTA-processed IGZO channel was compared with that of a TTFT with an RTA-processed IGZO channel and ITO source/drain electrodes. Using the circular transmission line measurement (CTLM) method, we suggest a possible mechanism that explains the effect of the RTA process on the performance of the TTFT with only an annealed IGZO channel vs. that with an annealed IGZO/ITO multilayer. The TTFT with an RTA-processed IGZO/ITO multilayer showed a threshold voltage shift, an improved on/off ratio of 3.54 × 10¹¹, a subthreshold swing of 0.33 V/decade, and a high mobility of 8.69 cm²/V·s. This indicates that simultaneous RTA processing for an IGZO channel and an ITO electrode is beneficial for the fabrication of high-performance TTFTs.     

Effects of yttrium additions on the microstructure and charge transport properties of indium tin oxide semiconductors

The effects of yttrium (Y) additions (x=0, 0.05, 0.1, and 0.2) on the microstructure, chemical structure, and electrical properties of Y_xInSnO_y (YITO) thin films, prepared using a sol-gel process were examined. The transmission electron microscopy (TEM) observations showed that the undoped InSnO (ITO) film consisted of an amorphous structure with local crystalline domains on the film surface, whereas the Y additions (x=0.05, 0.1, and 0.2) to ITO suppressed the formation of the crystalline phase. X-ray photoelectron spectroscopy (XPS) analysis showed that the Y content decreased the concentration of oxygen vacancies owing to the strong incorporation of Y with oxygen. As a result of the Y incorporation, the carrier concentration of ITO films decreased. The saturation mobility (μ_sat), the on-off ratios (I_on/off), and the sub-threshold swing (S.S) of YITO films were 1.1 cm² V⁻¹ s⁻¹, ~10⁶, and ~0.5 V decade⁻¹, respectively, which are comparable with 1.7 cm² V⁻¹ s⁻¹, ~10⁵, and ~1.17 V decade⁻¹ of ITO film. Additionally, the initial threshold voltage (V_TH) was positive shift with increased of Y addition and V_TH shift (ΔV_TH) under the positive bias stress (PBS) results decreased by Y addition.     

High carrier mobility low-voltage ZnO thin film transistors fabricated at a low temperature via solution processing

Wide-bandgap ZnO TFTs have many potential applications in large-area, flexible electronics and transparent devices because of their low cost, high performance and excellent optical transmittance. High-performance ZnO TFTs fabricated via simple solution processing have been widely studied. However, the key issues of solution-processable ZnO TFTs are the relatively high processing temperature (> 300?°C) and the high operating voltage for achieving the desired electrical properties. Here, we successfully fabricated low-voltage ZnO TFTs at an annealing temperature of ≤?250?°C. The resulting ZnO transistors with high-k terpolymer P(VDF-TrFE-CFE) showed a mobility of up to 5.3?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. Furthermore, the influence of the dielectric constant on the carrier mobility was investigated. A lower k-value dielectric resulted in a high carrier mobility under the same carrier density. Therefore, with a low-k CYTOP dielectric applied to modify the interface between the ZnO semiconductor and the P(VDF-TrFE-CFE) layer, ZnO transistors annealed at 250?°C showed an electron mobility of 13.6?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. To the best of our knowledge, this mobility is the highest value reported to date among the low-voltage solution-processable undoped ZnO TFTs annealed at temperatures of ≤?300?°C.     

Superior optoelectrical properties of magnetron sputter-deposited cerium-doped indium oxide thin films for solar cell applications

Indium tin oxide (ITO) is the most commonly used front contact material for a variety of photovoltaic technologies. However, the presence of a high free carrier concentration in ITO thin films results in the well-known phenomenon of free carrier absorption in the near-infrared (NIR) region of the solar spectrum. This causes optical losses especially in those solar cells where the active layer is designed to preferentially absorb NIR photons. Therefore, a combination of high carrier mobility and high NIR transparency is desired for advanced transparent conductive oxides for substituting ITO in solar cells. In this work, cerium-doped indium oxide (ICeO) thin films are deposited by pulsed DC magnetron sputtering, giving a remarkable 137% improvement of the mobility (71 cm² V^?1 s^?1) compared to the previous record value of 30 cm² V^?1 s^?1 for DC magnetron sputtered cerium-doped ITO films on glass. When compared to conventional ITO films prepared in this work, the highest mobility of ICeO is found to be almost four times higher and also the NIR transmission is substantially enhanced. Theoretical modelling of the experimental results indicates that neutral impurity scattering limits the carrier mobility in our films. With the recent advancements in single and multi-junction organic and perovskite solar cells, the development of ICeO/glass substrates (as possible replacements for the commonly used ITO/glass substrates) demonstrates significant potential in minimizing optical losses in the NIR region.     

Rapid and facile low-temperature solution production of ZrO2 films as high-k dielectrics for flexible low-voltage thin-film transistors

A rapid lightwave (LW) irradiation method was presented for the low-temperature solution production of ZrO₂ films as high-k dielectrics for flexible high-performance thin-film transistors (TFTs). The LW irradiation process markedly decreased the required processing temperature and processing time. Microstructure characterizations confirmed the successful formation of ZrO₂ films with an ultrasmooth surface, large band gap (>5 eV) and low defect level. The ZrO₂ film produced via LW irradiation at ∼200 °C in only 8 min presented excellent dielectric properties, including a small leakage current of 3.3 × 10⁻⁸ A/cm² and a large capacitance of 296 nF/cm², significantly outperforming the films by the conventional high-temperature annealing process at 400 °C for 60 min. Furthermore, LW irradiation was extended to the channel layer. The rapid low-temperature solution-processed InZrO_x TFTs exhibited superior electrical characteristics, such as a high carrier mobility of 41.3 cm²V⁻¹s⁻¹ and a high on-off current ratio of 10⁵∼10⁶ at a low operation voltage of 3 V due to the employment of high-quality ZrO₂ dielectric films. Moreover, the flexible TFT on a polyimide (PI) plastic substrate achieved a high mobility of nearly 30 cm²V⁻¹s⁻¹, indicating that LW irradiation is highly promising for the rapid and low-temperature solution production of high-quality and flexible oxide electronic devices.     

Temperature dependence of electronic behaviors in quantum dimension junctionless thin-film transistor

The high temperature dependence of junctionless (JL) gate-all-around (GAA) poly-Si thin-film transistors (TFTs) with 2-nm-thick nanosheet channel is compared with that of JL planar TFTs. The variation of SS with temperature for JL GAA TFTs is close to the theoretical value (0.2 mV/dec/K), owing to the oxidation process to form a 2-nm-thick channel. The bandgap of 1.35 eV in JL GAA TFTs by fitting experimental data exhibits the quantum confinement effect, indicating greater suppression of I_off than that in JL planar TFTs. The measured ∂Vth∂T of −1.34 mV/°C in JL GAA nanosheet TFTs has smaller temperature dependence than that of −5.01 mV/°C in JL planar TFTs.     

Surface-channel MESFET with boron-doped contact layer

In this investigation an attempt has been made to incorporate a high temperature stable refractory metal ohmic contact deposited onto an oxygen terminated contact area into the surface channel field effect transistor concept based on a H-terminated surface in the channel area. First transistors were fabricated. A drain current density of 75 mA/mm and a threshold voltage of V_th = −1.5 V was obtained for 1 µm gate length.     

Fabrication of diamond p–i–p–i–p structures and their electrical and electroluminescence properties under high electric fields

Diamond p–i–p–i–p device structures with highly insulating (i) and p-type (p) regions were laterally fabricated using a microscopic etching process of high-pressure/high-temperature-synthesized (HPHT) Ib-type (N-included) diamond with focused ion beams and a homoepitaxial chemical-vapor-deposition (CVD) process. The i regions (at room temperature (RT)) were composed of thinned HPHT diamond with a thickness of 0.5 or 1 μm while the p regions were composed of boron-doped CVD diamond films with a thickness of 0.2 or 0.5 μm. Thus, high electric fields up to ≈ 1 × 10⁷ V/cm were successfully applied to the thinned i regions. When the electrical property of the p–i–p–i–p structures was measured at RT either in a vacuum (5 × 10^− 5 Pa) or in 1-atm ambient N₂ and Ar gases, dramatic current increases were observed for all the specimens examined at the voltages, V, slightly above the threshold voltages, V_th, depending on the i region thickness. For V > V_th, we observed electroluminescence whose spectra well corresponded to cathodoluminescence spectra taken from the devices. Furthermore, in the measurements in the gases, additional current increases appeared at V substantially higher than V_th due to gas discharges under high electric fields existing outside the diamond devices. These observed phenomena are discussed in relation to an avalanche current amplification process and the band structure of the p–i–p–i–p devices.     

Solution-processed ytterbium oxide dielectrics for low-voltage thin-film transistors and inverters

High permittivity (high k) metal-oxide thin films fabricated via solution processes have recently received much attention for the construction of low-operating voltage and high-performance thin-film transistors (TFTs). In this report, amorphous ytterbium oxide (Yb₂O₃) thin films were fabricated by spin coating and their applications in TFTs were explored. The physical properties of the solution-processed Yb₂O₃ thin films processed at different annealing temperatures were systematically investigated using various characterization techniques. To explore the feasibility of the Yb₂O₃ thin films as gate dielectrics for oxide TFTs, In₂O₃ TFTs based on Yb₂O₃ dielectrics were integrated. All the devices could be operated at 3 V, which is critical for the applications in portable, battery-driven, and low-power electronic devices. The optimized In₂O₃/Yb₂O₃ TFT exhibits high electrical performances, including field-effect mobility of 4.98 cm²/V s, on/off current ratio of ~ 10⁶, turn-on voltage around 0 V, and subthreshold swing of 70 mV/decade, respectively. To demonstrate the potential of In₂O₃/Yb₂O₃ TFT toward more complex logic application, the unipolar inverter was further constructed.     

Controlled Zr doping for inkjet-printed ZTO TFTs

This study examined the effects of controlled Zr doping on the electrical properties of inkjet-printed zinc-tin oxide (ZTO) thin-film transistors (TFTs). In contrast to previous reports, below a certain doping concentration, improved electrical properties were obtained due to the effectively suppressed oxygen vacancies, reduced trapped electrons, and controlled carrier concentrations. The 0.0025 M Zr-doped inkjet-printed ZTO TFTs showed higher mobility, higher on-to-off current ratio, lower threshold voltage, and lower subthreshold slope of 6.43 cm²/V s, 3.72×10⁸, 3.35 V, and 0.53 V/dec, respectively, compared to the un-doped TFTs. The bias stability of the Zr-doped inkjet-printed ZTO TFT was also improved.     

Preparation and effects of post-annealing temperature on the electrical characteristics of Li–N co-doped ZnSnO thin film transistors

In this study, transparent Li–N co-doped ZnSnO (ZTO: (Li, N)) thin film transistors (TFTs) with a staggered bottom-gate structure were fabricated by radio frequency magnetron sputtering at room temperature. Emphasis was placed on investigating the effects of post-annealing temperature on their physical and electrical properties. An appropriate post-annealing temperature contributes not only to achieving good quality thin films, but also to improving the electrical performance of the ZTO: (Li, N) TFTs. The ZTO: (Li, N) TFTs annealed at 675 °C showed the best electrical characteristics with a high saturation mobility of 26.8 cm²V⁻¹s⁻¹, a threshold voltage of 6.0 V and a large on/off current ratio of 4.5×10⁷.     

Fully solution-processed metal oxide thin-film transistors via a low-temperature aqueous route

We report a facile and low-temperature aqueous route for the fabrication of various oxide thin films (Al₂O₃, In₂O₃ and InZnO). A detail study is carried out to reveal the formation and properties of these sol-gel-derived thin films. The results show that the water-based oxide thin films undergo the decomposition of nitrate group as well as conversion of metal hydroxides to form metal oxide framework. High quality oxide thin film could be achieved at low temperature by this aqueous route. Furthermore, these oxide thin films are integrated to form thin-film transistors (TFTs) and the electrical performance is systematically studied. In particular, we successfully demonstrate In₂O₃/Al₂O₃ TFTs with high mobility of 30.88 cm² V⁻¹ s⁻¹ and low operation voltage of 4 V at a maximum processing temperature of 250 °C.     

Taguchi optimization of device parameters for fullerene and Poly (3-octylthiophene) based heterojunction photovoltaic devices

Photovoltaic devices were fabricated with the structure ITO/fullerene/Poly (3-octylthiophene)/Au and device parameters were optimized using Taguchi optimization technique. Optimized parameter such as fullerene and Poly (3-octylthiophene) film thickness, annealing temperature and annealing duration are found to be as 110 nm, 45 nm, 120° C and 15 min respectively. Fabricated device with optimized parameters shows short circuit current density (J_sc), open circuit voltage (V_oc) and fill factor (FF) as 2 × 10^− 4 mA/cm², 0.47 V and 0.25 respectively. Effect of solvent casting on C₆₀ layer was studied which shows formation of uneven surface providing large interfacial area.     

Synthesis of triarylamine-based alternating copolymers for polymeric solar cell

Two donor-acceptor alternating copolymers based on electron-rich triarylamine, di(1-(6-(2-ethylhexyl))naphthyl)phenylamine (DNPA), and electron-deficient benzothiadiazole and benzoselenadiazole derivatives were designed and synthesized via Suzuki coupling reaction. The resulting triarylamine-based alternating copolymers PDNPADTBT and PDNPADTBS showed good solubility in common organic solvents and good thermal stability. The optical band gaps determined from the onset absorption were 1.93 and 1.81 eV, respectively. By introducing the naphthalene ring into the triarylamine, copolymers had relatively deep HOMO energy levels of −5.48 and −5.45 eV, which led to a high open circuit voltage (V_oc) and good air stability for photovoltaic application. Bulk heterojunction solar cells were fabricated with a structure of ITO/PEDOT-PSS/copolymers-PC₇₀BM/LiF/Al by blending the copolymer with PC₇₀BM. Both blend systems showed remarkably high V_oc near 0.9 V, and the highest performance of 2.2% was obtained from PDNPADTBT, with V_oc = 0.88 V, J_sc = 7.4 mA/cm², and a fill factor of 34.4% under AM 1.5 G.     

Strontium doping effects on the characteristics of solution-processed aluminum oxide dielectric layer and its application to low-voltage-operated indium-gallium-zinc-oxide thin-film transistors

In this paper, we investigated the strontium doping effects on the electrical and physical characteristics of solution-processed aluminum oxide dielectric layer and its application to low-voltage-operated indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs). With an optimized doping concentration of strontium (5 at%) in aluminum oxide (Al₂O₃), an oxide gate dielectric layer having a dielectric constant of ~7 and low leakage current characteristics (~4 × 10⁻⁷ A/cm² at 3 MV/cm) could be achieved by a solution process, which are comparably better than those of pristine Al₂O₃ film. The enhanced dielectric properties from strontium doping can be attributed to the change in the physical properties of Al₂O₃ film incorporated with strontium, providing charge relaxation of defect states in Al₂O₃ film. Also, since the strontium is highly reactive with oxygen, the strontium substitution through a doping leads to more strongly bound structure in an Al₂O₃ film without considerable lattice distortion. Using the strontium-doped aluminum oxide film as a gate dielectric layer, having a thickness less than 10 nm, solution-processed IGZO TFTs operating at ≤ 1 V were demonstrated showing a field-effect mobility of 1.74 ± 1.10 cm²/V s and an on-current level of ~10⁻⁵ A.     

Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated polymers: Synthesis, redox, and photovoltaic device

Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2 and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM (1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (V_oc) of 0.52 V, short-circuit current density (J_sc) of 3.24 mA/cm² and power conversion efficiency (PCE) of 0.60% for P1, and showed a V_oc of 0.33 V, J_sc of 2.11 mA/cm², PCE of 0.39% for P2.     

Low-bandgap copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges

Two donor–acceptor-type alternating copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges were synthesized by Suzuki cross-coupling polymerization, and their optical, electrochemical, and photovoltaic properties were compared. The selenophene π-bridged copolymer (PCz-DSeBSe) exhibited a smaller band-gap (1.82 eV) than the thiophene-bridged polymer (PCz-DTBSe; 1.89 eV). PCz-DSeBSe also showed a deeper highest occupied molecular orbital energy level (−5.36 eV) than PCz-DTBSe (−5.20 eV). Moreover, the PCz-DSeBSe thin film showed higher crystallinity and hole mobility than the PCz-DTBSe thin film. Organic photovoltaic devices were fabricated using the polymers as the donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. The device using PCz-DSeBSe showed a higher open circuit voltage (V_oc), short circuit current density (J_sc), and power conversion efficiency (PCE) than that using PCz-DTBSe. The fabricated indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PCz-DSeBSe:PC₇₁BM/LiF/Al device showed the maximum PCE of 2.88% with a J_sc of 7.87 mA/cm², an V_oc of 0.80 V, and a fill factor of 0.50 under AM 1.5G irradiation (100 mW/cm²).     

Towards single-gate field effect transistor utilizing dual-doped bilayer graphene

In this work, based on experimental possibilities, our first-principles calculations predict a sizeable bandgap opening in bilayer graphene (BLG) by n-doping from decamethylcobaltocene (DMC) and p-doping from functionalized amorphous SiO₂ (a-SiO₂) gate dielectric. With DMC monolayer on BLG and the maximum O₂⁻ on the surface of a-SiO₂ gate dielectric, the dual-doped BLG presents a bandgap of 390–394 meV and a Dirac level shift of −59 to −52 meV. The former is very close to the technical requirement of 400 meV, while the latter properly lies in the accessible range of the gate voltage of 300 meV. The high carrier mobility largely remains with the on/off current ratio satisfying the technical requirement of 10⁴−10⁷. The external electric field is not needed in this technique, which avoids a complex fabrication step for preparing a dual-gate structure and a substantial reduction in carrier mobility and on/off current ratio induced by adding an extra gate.