Mobility enhancement for high stability tungsten-doped indium-zinc oxide thin film transistors with a channel passivation layer

This study investigates the electrical characteristics and physical analysis for an amorphous tungsten-doped indium-zinc oxide thin film transistor with different backchannel passivation layers (BPLs), which were deposited by an ion bombardment-free process. A 10 times increase in mobility was observed and attributed to the generation of donor-like oxygen vacancies at the backchannel, which is induced by the oxygen desorption and Gibbs free energy of the BPL material. The mechanism was well studied by XPS analysis. On the other hand, a HfO₂ gate insulator was applied for the InWZnO TFT device to control the extremely conductive channel and adjust the negative threshold voltage. With both a HfO₂ gate insulator and a suitable BPL, the InWZnO TFT device exhibits good electrical characteristics and a remarkable lifetime when exposed to the ambient air.

This study investigates the electrical characteristics and physical analysis for an amorphous tungsten-doped indium-zinc oxide thin film transistor with different backchannel passivation layers, which were deposited by an ion bombardment-free process.     

Dual role of Ag nanowires in ZnO quantum dot/Ag nanowire hybrid channel photo thin film transistors

High mobility and p-type thin film transistors (TFTs) are in urgent need for high-speed electronic devices. In this work, ZnO quantum dot (QD)/Ag nanowire (NW) channel TFTs were fabricated by a solution processed method. The Ag NWs play the dual role of dopant and providing the charge transfer route, which make the channel p-type and enhance its mobility, respectively. The best sample yields an on/off ratio (I_on/I_off) of 5.04 × 10⁵, a threshold voltage (V_T) of 0.73 V, a high field effect mobility (μ_FE) of 8.69 cm² V⁻¹ s⁻¹, and a subthreshold swing (SS) of 0.41 V dec⁻¹. Owing to the strong ultraviolet (UV) absorption and photo-induced carrier separation ability of ZnO QDs and the fast carrier transport of Ag NWs, the devices acquire a high external quantum efficiency (EQE) and ultra-fast response under 365 nm UV illumination. The UV-modulated ZnO QD/Ag NW hybrid channel photo TFTs have potential for future application in optoelectronic devices, such as photodetectors and photoswitches.

High mobility and p-type thin film transistors (TFTs) are in urgent need for high-speed electronic devices.     

Dual-gate low-voltage transparent electric-double-layer thin-film transistors with a top gate for threshold voltage modulation

Dual gate (DG) low-voltage transparent electric-double-layer (EDL) thin-film transistors (TFTs) with microporous-SiO₂ for both top and bottom dielectrics have been fabricated, both dielectrics were deposited by plasma-enhanced chemical vapor deposition (PECVD) at room temperature. The threshold voltage of such devices can be modulated from −0.13 to 0.5 V by the top gate (TG), which switches the device from depletion-mode to enhancement-mode. High performance with a current on/off ratio (∼2.1 × 10⁶), subthreshold swing (76 mV per decade), operating voltage (1.0 V), and field-effect mobility (∼2.6 cm² V⁻¹ s⁻¹) are obtained. Such DG TFTs are promising for ion-sensitive field-effect transistors sensor applications with low-power consumptions.

Dual gate (DG) low-voltage transparent electric-double-layer (EDL) thin-film transistors (TFTs) with microporous-SiO₂ for both top and bottom dielectrics have been fabricated, both dielectrics were deposited by plasma-enhanced chemical vapor deposition (PECVD).     

Effects of annealing temperature on properties of InSnZnO thin film transistors prepared by Co-sputtering

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work. The microstructure and optical properties of ITZO thin films at different annealing temperatures were analyzed. The impact of various annealing temperatures on the ITZO TFT performance characteristics was systematically investigated as well. It was found that ITZO TFT with annealing temperature of 300 °C exhibits excellent electrical performance with a high saturation field-effect mobility (μ_sat) of 27.4 cm² V⁻¹ s⁻¹, a low threshold voltage (V_th) of −0.64 V, a small subthreshold swing (SS) value of 0.23 V per decade, and the high on-off current ratio (I_on/I_off) of 1.8 × 10⁷. In addition, it also shows good output curves including gate control capabilities and good electrode contact as well as extreme atmospheric stability. As shown by photoluminescence (PL) analysis and X-ray photoelectron spectroscopy (XPS) analysis, the beneficial effects of various annealing temperatures on device performance are attributed to the reorganization of the amorphous network and the control of defect chemistry in the films. The correlation between the post-deposition thermal treatment and the characteristics of a transistor was investigated and excellent performance of the transistor was demonstrated.

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work.     

Solution-processable and photopolymerisable TiO2 nanorods as dielectric layers for thin film transistors

We report the fabrication of a solution-processed n-type Thin Film Transistor (TFT) with current on/off ratios of 10⁴, a turn-on voltage (V_ON) of 1.2 V and a threshold voltage (V_T) of 6.2 V. The TFT incorporates an insoluble and intractable dielectric layer (k = 7–9) prepared in situ from solution-processed and then photopolymerised ligand-stabilised, inorganic/organic TiO₂ nanorods. A solution processed zinc oxide (ZnO) layer acts as the semiconductor. The new surface-modified TiO₂ nanorods were synthesised using a ligand replacement process with a monolayer coating of photopolymerisable 10-undecynylphosphonic acid (10UCYPA) to render them both soluble in common organic solvents and be photopolymerisable using UV-illumination after having been deposited on substrate surfaces from solution and drying.

A prototype solution-processed n-type thin film transistor was fabricated. The film incorporates a dielectric layer prepared from solution-processed and photopolymerised inorganic/organic TiO₂ nanorods and zinc oxide as the semiconductor, also deposited from solution.     

Improvement of the long-term stability of ZnSnO thin film transistors by tungsten incorporation using a solution-process method

In this paper, W-doped ZnSnO (WZTO) thin films and TFT devices are successfully fabricated by a wet-solution technique. The impact of W doping on the film structure, surface morphology, optical properties and chemical compositions of ZTO thin films is analyzed by atomic force microscopy, X-ray diffraction, UV-visible spectroscopy and X-ray photoelectron spectroscopy. The results show that the WZTO thin films have a smooth surface, amorphous structure and fewer oxygen vacancies with increasing W levels. The oxygen vacancy concentration of WZTO thin films is reduced from 40% to 27% with W incorporation. Compared with films free of W doping, for example ZnSnO TFTs, the positive bias stress stability of WZTO TFTs and long-term stability in air are improved obviously and the shift of the threshold voltage (V_T) is restrained about six times. The critical reason for the improvement of the ZTO TFT properties is attributed to W-doping, wherein the suppression of oxygen vacancies by W ions plays a dominant role in changing the performance of ZTO thin films and the stability of TFTs.

In this paper, W-doped ZnSnO (WZTO) thin films and TFT devices are successfully fabricated by a wet-solution technique.     

Fully solution-induced high performance indium oxide thin film transistors with ZrOx high-k gate dielectrics

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics. In this context, research efforts have been increasingly focused on the development of suitable solution-processed materials for oxide based transistors. In this work, we report a fully solution synthesis route, using 2-methoxyethanol as solvent, for the preparation of In₂O₃ thin films and ZrO_x gate dielectrics, as well as the fabrication of In₂O₃-based TFTs. To verify the possible applications of ZrO_x thin films as the gate dielectric in complementary metal oxide semiconductor (CMOS) electronics, fully solution-induced In₂O₃ TFTs based on ZrO₂ dielectrics have been integrated and investigated. The devices, with an optimized annealing temperature of 300 °C, have demonstrated high electrical performance and operational stability at a low voltage of 2 V, including a high μ_sat of 4.42 cm² V⁻¹ s⁻¹, low threshold voltage of 0.31 V, threshold voltage shift of 0.15 V under positive bias stress for 7200 s, and large I_on/I_off of 7.5 × 10⁷, respectively. The as-fabricated In₂O₃/ZrO_x TFTs enable fully solution-derived oxide TFTs for potential application in portable and low-power consumption electronics.

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics.     

Facile synthesis of a semiconducting bithiophene-azine polymer and its application for organic thin film transistors and organic photovoltaics

A new azine polymer poly(4,4′-didodecyl-2,2′-bithiophene-azine) (PDDBTA) was synthesized in only three steps. PDDBTA showed hole mobilities of up to 4.1 × 10⁻² cm² V⁻¹ s⁻¹ in organic thin film transistors (OTFTs) as a p-channel material. As a donor in organic photovoltaics (OPVs), power conversion efficiencies (PCEs) of up to 2.18% were achieved, which is the first example of using an azine-based polymer for OPVs. These preliminary results demonstrate the potential of bithiophene-azine polymers as a new type of low-cost semiconductor material for OPVs and other organic electronics.

A new azine polymer poly(4,4′-didodecyl-2,2′-bithiophene-azine) (PDDBTA) is synthesized by a simple condensation reaction, which is a promising semiconductor for printed electronics.     

Effects of NaCl treatment on the performance and environmental stability of microporous SiO2-based thin film transistors

In this letter, we report the effects of NaCl treatment on the performance and environmental stability of microporous SiO₂-based thin film transistors (TFTs). It was found that appropriate amounts of NaCl treatment significantly improved the electric double layer (EDL) capacitance of such composite solid electrolytes from 1.9 to 4.7 μF cm⁻². A highest field effect mobility of 42.8 cm² V⁻¹ s⁻¹ was found for 1% NaCl treated microporous SiO₂-based TFTs. However, 10% and 26.5% NaCl treated microporous SiO₂-based TFTs showed good environmental stability of the I_on/I_off ratio with reasonable field effect mobility.

In this letter, we report the effects of NaCl treatment on the performance and environmental stability of microporous SiO₂-based thin film transistors (TFTs).     

Reliability enhancement in thin film transistors using Hf and Al co-incorporated ZnO active channels deposited by atomic-layer-deposition

Oxide thin films transistors (TFTs) with Hf and Al co-incorporated ZnO active channels prepared by atomic-layer deposition are presented. The Al concentration was fixed at 2.6 at% and the Hf concentration was varied from 3.3 to 6.3 at%. The HfAlZnO (HAZO) TFTs exhibited positive shifts in turn on voltages toward 0 V with a slight decrease in carrier mobility with increases in the incorporated Hf content and the post-annealing temperature. It was suggested that the carrier concentration and defect densities within the HAZO channels were reduced by incorporating Hf and performing the thermal annealing process. The TFT with HAZO channels with Hf content of 6.3 at% exhibited a turn-on operation at around 0 V and a low SS value of 0.3 V dec⁻¹ without a marked decrease in carrier mobility. Furthermore, the device stabilities under bias, illumination, and temperature stresses could be greatly enhanced by reducing the formation of additional carriers and defects caused by weak Zn–O bonds due to the high binding energy of Hf with oxygen.

Oxide thin films transistors (TFTs) with Hf and Al co-incorporated ZnO active channels prepared by atomic-layer deposition are presented.     

Rapid and facile method to prepare oxide precursor solution by using sonochemistry technology for WZTO thin film transistors

In this paper, a rapid and facile method of preparing metal-oxide semiconductor precursor solution using sonochemistry technology is proposed. Compared with the traditional method (water bath above 60 °C for several hours), the efficiency of preparing solution is improved, because sonochemical reaction is found to accelerate the dissolution of solutes and the agitation of solution. The color comparison and thermal gravimetric and differential scanning calorimetry of solution confirme the formation of W-doped zinc tin oxide (WZTO) precursor solution with good performance. The effects of sonochemical reactions on the film structure, surface morphology, optical properties and chemical composition of WZTO thin films are analyzed by atomic force microscopy, X-ray diffraction, UV visible spectrum and X-ray photoelectron spectroscopy. The results show that the film has a smooth surface, an amorphous structure, a high transmittance and more M–O bonding. Hence, a rapid process of preparing WZTO solution (sonochemical treatment for 10 min) and fabricate TFT with high electron mobility (2.7 cm² V⁻¹ s⁻¹) is established, while the corresponding mobility of the traditional method is 1.2 cm² V⁻¹ s⁻¹. The results show that the sonochemical reaction can improve the efficiency of preparing solution by 1800% and it is a fast and efficient method for preparing precursor solutions.

In this paper, a rapid and facile method of preparing metal-oxide semiconductor precursor solution using sonochemistry technology is proposed.     

Synaptic transistors based on a tyrosine-rich peptide for neuromorphic computing

In this article, we propose an artificial synaptic device based on a proton-conducting peptide material. By using the redox-active property of tyrosine, the Tyr–Tyr–Ala–Cys–Ala–Tyr–Tyr peptide film was utilized as a gate insulator that shows synaptic plasticity owing to the formation of proton electric double layers. The ion gating effects on the transfer characteristics and temporal current responses are shown. Further, timing-dependent responses, including paired-pulse facilitation, synaptic potentiation, and transition from short-term plasticity to long-term plasticity, have been demonstrated for the electrical emulation of biological synapses in the human brain. Herein, we provide a novel material platform that is bio-inspired and biocompatible for use in brain-mimetic electronic devices.

In this article, we propose an artificial synaptic device based on a proton-conducting peptide material.     

Flexible high dielectric thin films based on cellulose nanofibrils and acid oxidized multi-walled carbon nanotubes

Flexible high dielectric materials are of prime importance for advanced portable, foldable and wearable devices. A series of flexible high dielectric thin films based on cellulose nanofibrils (CNF) and acid oxidized multi-walled carbon nanotubes (o-MWCNT) was prepared in aqueous solution. Though no organic solvent was involved during the preparation, the SEM images showed that o-MWCNTs have good distribution within the CNF matrix. The dielectric constant of CNF/o-MWCNT (6.2 wt%) composite films was greatly increased from 25.24 for pure CNF to 73.88, while the loss tangent slightly decreased from 0.70 to 0.68, and the AC conductivity decreased from 3.15 × 10⁻⁷ S cm⁻¹ for CNF to 1.77 × 10⁻⁷ S cm⁻¹ (at 1 kHz). The abnormal decrease of loss tangent and AC conductivity were attributed to the introduction of oxide-containing groups on the surface of MWCNTs. The nanocomposite films showed excellent flexibility such that they could be bent a thousand times without visible damage. The presence of MWCNTs also helped to improve the thermal stability of the composite films. The excellent dielectric and mechanical properties of the CNF/o-MWCNT composite film demonstrate its great potential to be utilized in the field of energy storage.

The acid oxidized MWCNTs have excellent dispersity in CNF, resulting in outstanding dielectric properties of the flexible CNF/o-MWCNT nanocomposite films.     

Highly transparent copper iodide thin film thermoelectric generator on a flexible substrate

Simultaneously transparent and flexible conductive materials are in demand to follow the current trend in flexible technology. The search for materials with compliant optoelectronic properties, while simultaneously retaining their electric conductivity at high strain deformation, comprises promising opportunities in modern nanotechnology. Copper iodide (CuI) is not only the most transparent and highly conductive p-type material, but its optimization has contributed to improved ZT values in planar thin-film thermoelectrics. In this work, the readiness of CuI thin films to transparent, flexible technology is evidenced. A maximum ZT value of 0.29 for single CuI thin films of ca. 300 nm in thickness is reported. Values of open-circuit voltage V_oc, short circuit current I_sc and power output of p–n thermoelectric modules of Gallium-doped zinc oxide (GZO) and CuI thin films deposited on a transparent flexible Kapton® (type CS) substrate are reported, and a prototype of a flexible transparent thermoelectric generator based on 17 p–n modules was constructed. Bending analysis of CuI thin films reveals interesting, distinct results when submitted to compression and tension analysis – a behaviour not seen in conventional semiconducting thin films under equivalent strain conditions. A plausible account for such diversity is also included.

Simultaneously transparent and flexible conductive materials are in demand to follow the current trend in flexible technology. A highly transparent and flexible thermoelectric generator of 17 p–n modules was constructed based on copper iodide thin films.     

Correction: Extremely low coercivity in Fe3O4 thin film grown on Mg2TiO4 (001)

Correction for ‘Extremely low coercivity in Fe₃O₄ thin film grown on Mg₂TiO₄ (001)’ by X. H. Liu et al., RSC Adv., 2017, 7, 43648–43654.

The authors regret that two co-authors were not included in the author list for the original article. The corrected author list, to which A. C. Komarek and C. F. Chang have been added, is presented herein. The corresponding correction to the first sentence of the Acknowledgements is presented below:“We thank L. H. Tjeng from the MPI CPfS for stimulating discussions”.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Ellipsometry study on gold-nanoparticle-coated gold thin film for biosensing application

The amplified plasmonic response from various distributions of gold nanoparticles (AuNPs) coated on top of gold thin film was studied via ellipsometry under total internal reflection mode. The surface plasmon resonance dip can be tuned from the visible to near infrared by simply varying the AuNP concentration. Theoretical modeling based on effective medium theory with a multi-slice model has been employed to fit the experimental results. Additionally, this experimental tool has been further extended to study bio-molecular interactions with metal surfaces as well as in studying protein-protein interaction without any labeling. Hence, this technique could provide a non-destructive way of designing tunable label-free optical biosensors with very high sensitivity.     

NiCo2O4 thin film prepared by electrochemical deposition as a hole-transport layer for efficient inverted perovskite solar cells

Spinel NiCo₂O₄ is a promising p-type semiconductor for optoelectronic devices; however, it is difficult to prepare uniform and large-area NiCo₂O₄ films, which hinders its application as a hole transport material for perovskite solar cells (PSCs). In this study, a novel, mild, and low-cost KCl-assisted electrochemical deposition (ECD) approach was developed to directly prepare a uniform NiCo₂O₄ film on a fluorine-doped tin oxide (FTO) substrate. A uniform NiCo₂O₄ film prepared through an ECD approach was used as a hole-transport layer (HTL) in inverted PSCs. The resulting NiCo₂O₄ HTL-based device achieved a power conversion efficiency (PCE) of 19.24% with negligible hysteresis and excellent reproducibility. Additionally, it outperformed a NiO_x-based device (PCE = 18.68%). The unsealed devices retained 90.7% of their initial efficiency when subjected to stability measurements for 360 h in an ambient atmosphere. This study shows the great potential of ECD-prepared NiCo₂O₄ HTLs for large-area PSCs in the future.

An electrochemical deposition approach was developed to prepare a NiCo₂O₄ hole transport layer for inverted perovskite solar cells.     

Phase transition and dewetting of a 5CB liquid crystal thin film on a topographically patterned substrate

We report thermally induced nematic to isotropic (N–I) phase transition as well as dewetting of 5CB Liquid Crystal (LC) thin films coated on flat and topographically patterned substrates with grating geometry of different line width (l_P) and periodicity (λ_P). On a flat substrate, the nematic to isotropic (N–I) phase transition, which takes place within a temperature range between 31.1 °C and 34.4 °C is fully reversible, with re-appearance of identical Schlieren texture when the sample is cooled down during isotropic to nematic (I–N) transition. Upon further heating beyond N–I transition and annealing at T ≈ 65 °C, the film undergoes nucleated dewetting with formation and growth of holes, which eventually merge to form isolated droplets. The morphology of the dewetted structures remains unaltered when the film is cooled to room temperature from this stage, though the features undergo phase transition to the nematic state. In contrast on a topographically patterned substrate, the phase transition cycle is associated with a change of the texture of the film during cooling to the nematic stage. Interestingly the molecules exhibit homeotropic anchoring when λ_P ≈ 1.5 μm and planar anchoring when λ_P large (≈10 μm). When heated further to T ≈ 65 °C, the film dewets on topographically patterned substrates resulting in a collection of droplets, which are aligned to the substrate patterns when λ_P is large (≈10 μm). In contrast the dewetted droplets are random and not correlated to the patterns when λ_P is lower (≈1.5 μm).

Thermally induced nematic to isotropic (N–I) phase transition and dewetting of 5CB liquid crystal thin films on flat and topographically patterned substrates.     

Flexible pH sensor based on a conductive PANI membrane for pH monitoring

pH is a critical parameter used to specify the acidity or alkalinity of an aqueous solution in chemistry, food processing, and medical care. In this study, a conductimetric-type micro pH sensor has been achieved using PANI membrane fabricated on a flexible substrate film aiming to monitor wound healing. The sensor is based on the incorporation of a polyaniline (PANI) membrane, interdigital electrode, and polyimide (PI) substrate. PANI was doped with dodecyl benzene sulfonic acid (DBSA) to obtain good conductivity. The electrodes were patterned on the PI film by etching. The contact area between the PANI and interdigital electrodes improves the responsiveness of the pH sensor. A sensitivity of 58.57 mV per pH over the entire pH range from 5.45 to 8.62 was obtained experimentally, along with a superior repeatability of 8% FS (full scale) and a temperature drift of 6.8% FS. This micro flexible pH sensor aims to monitor the pH value of wound healing, which also facilitates the realization of online monitoring of the pH for telemedicine, food safety, and home health care.

A conductimetric flexible film pH sensor working in sensing materials of PANI membrane was developed for clinic wound monitoring.     

Investigations on the bias temperature stabilities of oxide thin film transistors using In–Ga–Zn–O channels prepared by atomic layer deposition

Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm). Even when the IGZO channel thickness was reduced to 6 nm, the device exhibited good characteristics with a high saturation mobility of 15.1 cm² V⁻¹ s⁻¹ and low sub-threshold swing of 0.12 V dec⁻¹. Excellent positive and negative bias stress stabilities were also obtained. When positive bias temperature stress (PBTS) stability was tested from 40 to 80 °C for 10⁴ s, the threshold voltages (V_TH) of the device using the 6 nm-thick IGZO channel shifted negatively, and the V_TH shifts increased from −0.5 to −6.9 V with the increasing temperature. Time-dependent PBTS instabilities could be explained by a stretched-exponential equation, representing a charge-trapping mechanism.

Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm).