Semiconductor Thin-film Transistors Research Articles

Effects of praseodymium doping on the electrical properties and aging effect of InZnO thin-film transistor

Amorphous indium gallium zinc oxide is a popular semiconductor candidate for amorphous oxide semiconductor thin-film transistors in the field of flat-panel display. However, the existence of gallium component restricts the enhancement of mobility dramatically. In this study, we report a new praseodymium (Pr) dopant as a stabilizer in amorphous indium zinc oxide semiconductor (IZO) with high mobility and stability. Meanwhile, the PrIZO TFTs were fabricated to investigate the effects of Pr on electrical properties, stability and aging effect. The optimal PrIZO TFT exhibited a desired performance with a saturation mobility (μsat) of 25.8/32.6 cm2 V−1 s−1, an Ion/Ioff ratio of 3.5 × 107/5.4 × 107, a subthreshold swing value of 0.14/0.13 V dec−1 and a threshold voltage (Vth) of 2.9/2.1 V, respectively, before and after an air environment storage period of 90 days without passivation layer, which exhibits lower sensitivity of the channel region to oxygen/moisture from the atmosphere than IZO TFT. XRD analysis revealed that the Pr dopant had no effect on the amorphous state of IZO thin film with annealing up to 400 °C. XPS analyses suggested that the fraction of oxygen vacancy subpeak decreased significantly with Pr incorporated into IZO. The μ-PCD decay analyzation and the subgap density of states indicate that acceptor-like trap states induced by Pr ions lead to the suppression of ambient-induced excess carrier in conduction band. This work is anticipated to provide a kind of reliable stabilizer for amorphous oxide semiconductor without deteriorating mobility significantly.

Analysis of the hump phenomenon and needle defect states formed by driving stress in the oxide semiconductor

The reduction in current ability accompanied by the hump phenomenon in oxide semiconductor thin-film transistors to which high DC voltages and AC drive voltages are applied has not been studied extensively, although it is a significant bottleneck in the manufacture of integrated circuits. Here, we report on the origin of the hump and current drop in reliability tests caused by the degradation in the oxide semiconductor during a circuit driving test. The hump phenomenon and current drop according to two different driving stresses were verified. Through a numerical computational simulation, we confirmed that this issue can be caused by an additional “needle”, a shallow (~0.2 eV) and narrow (<0.1 eV), defect state near the conduction band minimum (CBM). This is also discussed in terms of the dual current path caused by leakage current in the channel edge.

A Site‐Specific Charge Carrier Control in Monolithic Integrated Amorphous Oxide Semiconductors and Circuits with Locally Induced Optical‐Doping Process

AbstractAmorphous oxide semiconductor (AOS) thin film transistors (TFTs) have found cutting‐edge applications in sensor technologies. To reduce manufacturing costs, sensors, analog front end, and digital signal processing circuits need to be integrated on the identical substrate. Unlike traditional silicon‐based devices, optimizations for locally controllable electrical parameters of the AOSs have rarely been investigated. Here, photoactivated combustion reduction is utilized as doping motivation for solution‐processed amorphous indium–gallium–zinc oxide (a‐IGZO) to tune their electrical performance. By controlling parameters of a‐IGZO TFTs, which can be partly doped with covering the desired area of the identical substrate, it is possible to match the particular threshold voltage for various circuits. For circuit optimization, automatic integrated circuit modeling spice is carried out to find the best match of the complementary metal–oxide semiconductor circuits. Finally, the site‐specific performance of switching TFTs, amplifiers, and ring oscillators implemented with low‐temperature solution‐processed a‐IGZO and p‐type single‐walled carbon nanotube TFTs is demonstrated. The optical‐doped a‐IGZO TFTs exhibiting a saturation mobility of &gt;9.15 cm2 V−1 s−1 with a locally tunable threshold voltage of −5 – 1.5 V are realized, enabling monolithic integration of functional devices. The resultant circuits demonstrate excellent amplification of 24 dB and an oscillation frequency of 12 kHz for 7‐stage ring oscillators.

N-Heteroacenes and N-Heteroarenes as N-Nanocarbon Segments.

N-Heteroacenes and N-heteroarenes are the heterocyclic congeners of the acenes and arenes, in which one or several perimeter C-H bonds have been substituted by pyridine-type nitrogen atoms. They are formally segments out of N-doped nanographenes. Position and number of the nitrogens vary greatly, making N-heteroacenes and N-heteroarenes define a vast class of N-nanographene segments; they display modular electronic and structural properties. The nitrogen atoms in the perimeter lead to finely tunable frontier molecular orbital positions and therefore improved electron affinity and higher oxidative stability but conversely also require and allow different synthetic approaches than those reported for the synthesis of their hydrocarbon and nanographene analogues. The chemistry of N-heteroarenes, despite being known for more than a century, has made significant progress in the last years and established these materials both as powerful n-channel semiconductors in thin film transistors and as useful emitters in organic light emitting diodes (OLEDs) and in photovoltaic devices. The electronegative nitrogen atoms impart a deep LUMO into the azaacenes and azaarenes, improve electron injection, and enable powerful electron transport but also charge separation in bulk-heterojunction type organic photovoltaic (OPV) devices. At the same time, azaacenes and azaarenes are fundamentally exciting materials that push the limits of structure and stability, constantly displaying novel topologies and structures as variations of a simple leitmotif; we expect a bright future for esthetically pleasing yet highly functional N-heterocyclic species. Firstly, we discuss novel structures and structural elements that have evolved during the lastyears in N-heteroacene and N-heteroarene chemistry and delineate their properties. An important aspect is the oligomerization or better multimerization of azaacene and azaarene units into novel and surprising topologies, in which multiple azaarenes or azaacenes are stitched together. Examples are tetrahedral assemblies of tetraazapentacenes but also cyclic tetramers of different types of azaacenes and linearly bent, S-shaped, formally dimeric species. An exciting aspect of the exploration of the structural manifold of azaacenes is their electronic interaction in such assemblies and their solid-state microstructure. A further aspect of this work is the increase in size of the azaacenes and concepts that allow stabilization of the larger congeners. The attachment of four benzo units to the azaacene core is a powerful concept that stabilizes tetraazaheptacenes and should also be useful to achieve persistent tetraazanonacenes. Secondly, we describe the success of N-heteroacenes and N-heteroarenes in organic electronic devices; specifically, the use of symmetrical halogenated tetraazapentacenes as superb n-channel transistor materials with air stable and persistent radical anions as charge carriers; we discuss the structural reason for their success. Use of azaacenes and azaarenes is not restricted to transistors, but they are also applied in bulk heterojunction photovoltaic devices and in brightly emitting OLEDs. Azaacenes and azaarenes are attractive segments out of hetero-nanographenes and objects of study, starting from fundamental structural and topological questions, ranging to powerful applications in organic electronics. The general interest in azaacenes is witnessed by the constantly increasing number of groups who discover and work on these materials as novel functional and flexible species.

Fabrication and characterization of transparent nanocrystalline ZnO thin film transistors by a sol–gel technique

A nanocrystalline zinc oxide (ZnO) thin film-based metal–insulator–semiconductor thin film transistor (MIS TFT) was fabricated by a facile sol–gel technique onto silicon di-oxide/indium tin oxide-coated glass substrates. The microstructural study of the ZnO thin films indicated uniform crystalline growth with typical (002) X-ray diffraction peaks for h-ZnO with a wurtzite structure. The optical transmittance of the ZnO thin films was \({>}80\%\) in the visible region of the electromagnetic spectrum. The field effect transistor (FET) aluminium top contacts were fabricated using suitable shadow masking. The transfer characteristics of a typical ZnO MIS FET revealed nonlinearity in a linear plot. From the slope and crossover, we obtained a first estimate of field effect mobility (\(\mu \)) and threshold voltage (\(V_{\mathrm{T}}\)) of \(0.13~\hbox {cm}^{2}\) \(\hbox {V}^{-1}~\hbox {s}^{-1}\) and 1.03 eV, respectively. The ZnO TFT operated in enhanced mode with n-channel characteristics and the drain current on–off ratio was \(10^{5}\). The deposition parameter needs to be optimized to obtain TFTs with a higher modulation ratio and larger field-effect mobility.

Back-End of Line Compatible Transistors for Hybrid CMOS Applications

The low-temperature back-end of line (BEOL) compatible transparent amorphous oxide semiconductor (TAOS) TFTs and poly-Si TFTs are the suitable technology platform for three-dimensional (3D) integration hybrid CMOS technologies. The n-channel amorphous indium tungsten oxide (a-IWO) ultra-thin-film transistors (UTFTs) have been successfully fabricated and demonstrated in the category of indium oxide based thin film transistors (TFTs). We have scaled down thickness of a-IWO channel to 4nm. The proposed a-IWO UTFTs with low operation voltages exhibit good electrical characteristics: near ideal subthreshold swing (S.S.) ~ 63mV/dec., high field-effect mobility (μFE) ~ 25.3 cm2/V-s. In addition, we also have fabricated the p-channel poly-Si TFTs with high-κ gate insulator (GI). The matched electrical characteristics of n-channel and p-channel devices with low operation voltage and low IOFF are exhibiting the promising candidate for future hybrid CMOS applications.

Superconcentrated aqueous electrolyte and UV curable polymer composite as gate dielectric for high-performance oxide semiconductor thin-film transistors

Due to the increasing miniaturization, lightweight, wearable, and portable electronic devices, there is an increasing need to reduce power consumption through the lower operating voltages of thin-film transistors (TFTs). In this study, we proposed a “water-in-bisalt” (WiBS) and an ultraviolet (UV)-curable polymer composite material, which is an aqueous electrolyte with a high capacitance, and realized a dielectric material for low power consumption and a stable voltage range of TFTs that can be handled in the ambient condition. We fabricated a free-standing WiBS/polymer dielectric film with a high capacitance value of 6.72 uF/cm2 at 0.5 Hz and high electrochemical stability by embedding a WiBS electrolyte into the UV crosslinked polymer network by photocuring. The devices of InOx TFTs using WiBS/polymer dielectric films show excellent electrical characteristics: an on/off current ratio of over 103, a subthreshold swing of 110 mV/dec at a drain voltage of 0.1 V, and a linear field-effect mobility of 30.7 cm2/Vs at a drain voltage of 0.1 V and a gate voltage of 1 V.

Effect of strontium doping on indium zinc oxide thin film transistors fabricated by low-temperature solution process

Solution-processed metal oxide semiconductors for thin film transistors (TFTs) require high-temperature annealing to improve the electrical performance. On the other hand, a high-temperature process cannot be applied to flexible substrates, which is an obstacle preventing the application of MOS films to flexible devices. This paper reports a simple method for improving the electrical characteristics of low-temperature solution-processed indium zinc oxide (IZO) TFTs with strontium (Sr)-doping and vacuum annealing. The addition of Sr to the IZO thin film decreased the density of oxygen vacancies due to the strong bonding energy of Sr with oxygen ions and improved the electrical stability of the IZO TFTs. On the other hand, vacuum annealing increased the number of oxygen vacancies in the IZO film resulting from the low oxygen partial pressure, which led to an increase in carrier concentration. By combining these two methods, Sr-doped IZO TFTs with a high field-effect mobility of 8.75 cm2 V−1 s−1 at 2 mol. % of Sr doping were fabricated.

Full capacitance model, considering the specifics of amorphous oxide semiconductor thin film transistors structures

A full capacitance model for Amorphous Oxide Semiconductor Thin Film Transistors (AOSTFTs), considering the effect of the drain contact overlap in bottom gate passivated structures is presented. It is shown that this drain overlap, on top of the passivation layer, serves as a second gate with an applied voltage equal to VDS. When VDS > VT the semiconductor-passivation (S-P) interface will be in accumulation and the behavior of the different capacitance is affected. An expression to represent this effect is included in the present model. The overlap capacitance between gate and drain/source, as well as the effect of reducing the channel capacitance as the drain is increased, are also considered by the model. The calculated capacitance is a function of the threshold voltage, (VT), the mobility and saturation parameters (γa ,αs), and the sharpness of the knee region m, which are extracted using the Unified Model and Extraction Method (UMEM) for AOSTFTs. Results are compared with simulated and experimental data.

Effect of the Active Channel Thickness Variation in Amorphous In-Zn-Sn-O Thin Film Transistor.

Ternary oxide thin films in the In₂O₃-ZnO-SnO₂ system were studied for potential applications in oxide semiconductor thin film transistors (TFTs). An amorphous In-Zn-Sn-O (a-IZTO) active channel layer was deposited by RF magnetron sputtering at room temperature on an n++ Si substrate. Films from a sintered ceramic target with a nominal chemical composition of In:Zn:Sn = 40:50:10 at.% were prepared with thicknesses ranging from 15 nm to 150 nm, which was followed by annealing at 350 °C for 30 minutes in air. Subsequently, a bilayer Cu/Ti metal contact was deposited as the source/drain electrodes on the top surface through a shadow mask using an e-beam evaporator. The thickness of the active channel layer greatly influenced the characteristics of the oxide thin film transistors. The best transistor characteristics were observed from the test device with a channel thickness of 30 nm and a high on/off current ratio of approximately 108, high field effect mobility of 25 cm²/Vs, low threshold voltage of -0.1 V, and very small subthreshold swing of 0.14 V/dec.

Surface-Functionalized Interfacial Self-Assembled Monolayers as Copper Electrode Diffusion Barriers for Oxide Semiconductor Thin-Film Transistor

The stability and control of copper (Cu) electrode application in oxide semiconductor indium gallium zinc oxide (IGZO) can improve the resistance–capacitance (RC) delay, which is greatly critical f...

Photoconductive laser spectroscopy as a method to enhance defect spectral signatures in amorphous oxide semiconductor thin-film transistors

Defects in semiconductor thin-films often leave optical spectral signatures that can be used for their identification. In this letter, we report on spectrally resolved photoconductivity measurements of amorphous oxide semiconductor thin-film transistors. In contrast to previously reported photoconductive spectroscopy measurements recorded using spectrally filtered broadband light sources, we used a wavelength tunable picosecond laser to illuminate the thin-film. We extracted the absorption coefficient as a function of wavelength from the photocurrent measurement and showed that it followed the typical characteristic behaviour previously reported for amorphous oxide semiconductor thin-films. However, in addition, we observed several sharp spectral peaks in the photoconductivity spectrum which can be associated with sub-bandgap defects. These enhanced peaks are not normally visible in previously reported photoconductivity spectra. Furthermore, we show that we can control the sensitivity of our measurement by changing the applied gate bias voltage when the thin-films were fabricated into transistors. The enhancement achieved by using the wavelength tunable laser makes this a particularly sensitive characterisation tool and can additionally be used to discriminate between defects which have been incorporated after device fabrication.

Electric field modified Arrhenius description of charge transport in amorphous oxide semiconductor thin film transistors

While it is known that the charge-carrier mobility in amorphous metal oxide semiconductor thin film transistors (TFT) deviates from Arrhenius temperature dependence, we found that the Hall mobility measured in amorphous In-Ga-Zn-O (a-IGZO) follows an Arrhenius relation surprisingly well. We explain these observations by the effect of strong vertical electric field created by the gate voltage, which facilitates direct tunneling of trapped carriers into the conductive band and leads to virtually temperature independent mobility. We present a generalized Arrhenius model based on the effective temperature concept. We show that our model allows quantitative description of the temperature dependence of the mobility in a-IGZO TFTs over a broad temperature range.

Silver-doped tin oxide for electrical property enhancement in p-type channel thin film transistor

Though tin oxide (SnO) is one of the strong candidates for p-type oxide semiconductor thin film transistors (TFTs), poor properties and a narrow window of deposition condition are obstacles for implementation. An Ag-doped Sn thin film was studied for the improvement of electrical performance of TFTs. SnO and Ag-doped SnO thin films were deposited by direct current magnetron sputtering as the channel for thin film transistors. The concentration of Ag doping was controlled by the amount of Ag pellets in the sputtering target preparation. A change in oxide states was observed in an Ag-doped SnO film, which can lead to the reduction of the Sn4+ oxide phase. Ag-doped SnO showed improvement of carrier mobility and concentration and resulted in improved electrical characteristics.

Review Article: Atomic layer deposition for oxide semiconductor thin film transistors: Advances in research and development

This article is a review of recent research and development advances in oxide thin film transistors (TFTs) fabricated by atomic layer deposition (ALD) processes. The ALD process is remarkable as it offers accurate control of film thickness and composition as well as the ability to achieve excellent uniformity over large areas at relatively low temperatures. Firstly, an introduction to n-type oxide TFTs is provided with a focus on the development of active-layer material combinations from binary oxide active layers, like zinc oxide and indium oxide, to ternary and quaternary oxide active layers formed by doping with elements such as gallium or tin to achieve high mobility and high device stability for TFTs. Secondly, ALD p-type channel oxide TFTs are also introduced, which are required for the realization of many types of low-power circuits, such as complementary metal oxide semiconductor devices.

Boosting Modulation of Oxide Semiconductors via Voltage-Based Ambi-Ionic Migration.

In recent years, high-performance amorphous oxide semiconductor thin-film transistor (AOS TFT) technology is required to meet the increasing demand for novel displays, such as rollable, transparent, or augmented reality head-up displays. It has been demonstrated that voltage-based modulation techniques for AOS-based active layers can achieve high-performance AOS TFTs. The voltage-based modulation technique allows specific ions to migrate into the active layer depending on the polarity of the applied voltage, thus easily modulating the active layer. Additionally, potassium superoxide (KO2) solution is employed in AOS TFTs as a source of potassium (K+) and highly reactive superoxide radical (O2•-) ions. The K+ and O2•- ions in the KO2 solution are controlled by an applied voltage bias and rapidly migrate into the active layer, directly changing its chemical composition and electrical properties. AOS TFTs that use this technique exhibit better electrical performance than conventional AOS TFTs: the field-effect mobility improved from 10.05 to 15.31 cm2/V·s; the subthreshold swing decreased from 0.44 to 0.33 V/dec; the Ion/off ratio increased from 1.24 × 107 to 3.17 × 108; and the threshold voltage shift decreased from 5.2 to 3.4 V under a positive bias stress test conducted over 10 000 s. Ultimately, this approach to modulating the internal ion distribution in oxide semiconductors could provide opportunities for various AOS devices to attain desirable electrical characteristics.

Application of Single-Pulse Charge Pumping Method on Evaluation of Indium Gallium Zinc Oxide Thin-Film Transistors

A novel single-pulse charge pumping (SPCP) method was proposed and implemented to profile the density of states (DOSs) in metal–oxide–semiconductor thin-film transistors (TFTs). The proposed SPCP method was demonstrated in characterizing the DOS below the conduction band of indium gallium zinc oxide TFTs. The DOSs, including oxygen vacancy donorlike states and band tail states, were characterized successfully with high resolution. This method has all the advantages of the conventional CP method and single-pulse characterization.

Self-Heating Suppressed Structure of a-IGZO Thin-Film Transistor

Self-heating degradation in semiconductor devices generated by driving voltage is a serious problem in the development of highly reliable products. This letter focuses on oxide semiconductors, as a representative of flexible devices on which thermal accumulation on the substrate is critical. We propose a heat-suppressing thin-film transistor (TFT) structure to improve the reliability of transparent amorphous oxide semiconductor TFTs. Findings from this letter indicate that a structure with a curved line, such as a Corbino disk, is effective for suppressing self-heating. Based on these results, we suggest a U-shaped TFT as a heat-suppressing structure and demonstrate its device simulation and thermal analysis.

Interfacial Reaction between Metal and Oxide

Transparent oxide semiconductor thin film transistor (TFT) is being used in display such as OLED (organic light emitting diode) display and LCD (liquid crystal display). The active oxide layer in oxide TFT is transparent, comprising band gap higher than 3.0 eV. In TFT fabrication metal layer contacts to the active oxide layer and the reaction between active oxide layer and metal degrade TFT characteristics due to thin metal oxide formation. In this paper the reaction between the amorphous indium- gallium -oxide (a-IGZO) and metal was studied. Metal oxide formation was reported with interfacial micro-structures between a-IGZO and the metal electrode. TiOx is formed at the interfacial region between Ti and IGZO layers by oxygen out-diffusion from IGZO. The reaction depends on process parameters such as temperature, annealing time and the environment of annealing. For several metals such as Al, Ti, and Mo, the interfacial reaction by thermal anneal was compared for various annealing conditions. The a-IGZO was deposited on ITO electrode layer and the 0.8 mm diameter metal electrodes are formed on a-IGZO layer. After annealing at various conditions, formation of metal oxide was verified by electrical characteristics. The leakage current and breakdown voltages were measured. Figure 1 shows typical I-V characteristics between metal and ITO electrode. Figure 1

(Invited) New p-Type Oxide Semiconductor with High Hole Mobility over 7 cm2/Vs Formed By Solution Process

We report new p-type metal oxide semiconductor thin-film transistor with high field-effect hole mobility fabricated in solution process in this work. Metal oxide semiconductors have long attracted large attention as large-area electronic devices by virtue of high electrical and optical performances such as transparency, large electron mobility, good uniformity and low-temperature processability since the pioneering work of Prof. Hosono group in 2004, and their mass production has now been successfully realized in the active matrix backplane of flat-panel or flexible display. Few p-type metal oxide semiconductors with high performance, however, have been reported compared with n-type metal oxide semiconductors such as InGaZnO, InZnSnO, etc. This is mostly ascribed as the localization of valence band maximum which mainly consists of oxygen p orbitals. To enhance the dispersion of valence band, it has been theoretically proposed that the filled diffuse metal d or s orbital to hybridize with oxygen 2p orbital can be utilized and indeed it was experimentally demonstrated that Sn 5s orbital and Cu 3d orbital played such a role in SnO and CuO, respectively. But, SnO phase is very vulnerable to disproportion reaction to metallic Sn and SnO2 which behaves as an n-type semiconductor so that the process window to realize it in electron devices was very narrow and the electrical performance in the devices such as thin-film transistor was very limited, e.g low on/off ratio. Inspired by the theoretical work of high throughput materials screening performed by the first principles calculation, we fabricated the alkali metal doped SnO thin-film with solution process and low-temperature annealing less than 400 oC. We found that the oxidation state of Sn in thin-films changed from +4 state (SnO2 phase) to +2 state (SnO phase) depending on the content of alkali metal ions from x-ray photoelectron spectroscopy (XPS). Furthermore, Hall-effect measurement indicated the type conversion of the majority carrier from n-type to p-type as the content of alkali metal ion increases, in consistence with XPS results. The high hall mobility of 51 cm2/Vs was obtained at the optimal alkali-metal ion doped SnO thin film and we successfully fabricated thin-film transistors with alkali-metal ion doped SnO with a field-effect hole mobility over 7 cm2/Vs. We hope that this work will contribute to open a new era for transparent large area electronics based on oxide semiconductors with high functionalities such as complementary logic circuit.