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ChemInform Abstract: Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances
Advanced Materials
Abstract
Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.
... The creation of high-mobility amorphous n-type metal oxides, such as a-InGaZnO (ref. 1), and their integration into thin-film transistors (TFTs) have propelled advancements in modern large-area electronics and new-generation displays [2][3][4][5][6][7][8] . However, finding comparable p-type counterparts poses notable challenges, impeding the progress of complementary metal-oxide-semiconductor technology and integrated circuits [9][10][11] . ...
... Leveraging the insights gained from XANES and EXAFS results and the film density of 5.6 g per cm 3 obtained from X-ray reflectivity analysis, we conducted density functional theory (DFT) calculations to explore the energy band structure and electrical properties of amorphous Te-TeO x . For stoichiometric amorphous TeO 2 , the VBM primarily consists of localized O-2p states, indicating the poor p-type character (Extended Data Fig. 3). ...
... The X-ray photoelectron spectroscopy analysis was performed using a PHI 5000 VersaProbe instrument (Ulvac-PHI). The Se alloying percentages were characterized using high-resolution inductively coupled plasma mass spectrometry (Thermo Element XR) by dissolving the deposited films in a HNO 3 Supplementary Tables 1 and 2). ...
Compared to polycrystalline semiconductors, amorphous semiconductors offer inherent cost-effective, simple and uniform manufacturing. Traditional amorphous hydrogenated Si falls short in electrical properties, necessitating the exploration of new materials. The creation of high-mobility amorphous n-type metal oxides, such as a-InGaZnO (ref. ¹), and their integration into thin-film transistors (TFTs) have propelled advancements in modern large-area electronics and new-generation displays2–8. However, finding comparable p-type counterparts poses notable challenges, impeding the progress of complementary metal–oxide–semiconductor technology and integrated circuits9–11. Here we introduce a pioneering design strategy for amorphous p-type semiconductors, incorporating high-mobility tellurium within an amorphous tellurium suboxide matrix, and demonstrate its use in high-performance, stable p-channel TFTs and complementary circuits. Theoretical analysis unveils a delocalized valence band from tellurium 5p bands with shallow acceptor states, enabling excess hole doping and transport. Selenium alloying suppresses hole concentrations and facilitates the p-orbital connectivity, realizing high-performance p-channel TFTs with an average field-effect hole mobility of around 15 cm² V⁻¹ s⁻¹ and on/off current ratios of 10⁶–10⁷, along with wafer-scale uniformity and long-term stabilities under bias stress and ambient ageing. This study represents a crucial stride towards establishing commercially viable amorphous p-channel TFT technology and complementary electronics in a low-cost and industry-compatible manner.
... The main causes for their poor p-type performance include the localized valence band maximum (VBM), strong self-compensation effect, and poor material stability 8,11,12 . For instance, in p-type transition-metal oxides (halides) such as CuO x , SnO, and CuI, high concentrations of oxygen (halogen) vacancies typically function as compensating intrinsic defects that capture holes, thereby impeding their hole transport 13,14 . To bypass these ingrained issues, an alternative material design strategy is expected to explore better p-type semiconductors. ...
... In general, continuously tuning bandgaps and band-edge energies in conventional p-type semiconductors is difficult 13 . Apart from the low formation energy of the electron donor, incorporating foreign atoms could inevitably perturb the host lattice thermodynamic equilibrium, possibly counteracting the p-doping effect 14 . ...
Inorganic semiconductors typically have limited p-type behavior due to the scarcity of holes and the localized valence band maximum, hindering the progress of complementary devices and circuits. In this work, we propose an inorganic blending strategy to activate the hole-transporting character in an inorganic semiconductor compound, namely tellurium-selenium-oxygen (TeSeO). By rationally combining intrinsic p-type semimetal, semiconductor, and wide-bandgap semiconductor into a single compound, the TeSeO system displays tunable bandgaps ranging from 0.7 to 2.2 eV. Wafer-scale ultrathin TeSeO films, which can be deposited at room temperature, display high hole field-effect mobility of 48.5 cm²/(Vs) and robust hole transport properties, facilitated by Te-Te (Se) portions and O-Te-O portions, respectively. The nanosphere lithography process is employed to create nanopatterned honeycomb TeSeO broadband photodetectors, demonstrating a high responsibility of 603 A/W, an ultrafast response of 5 μs, and superior mechanical flexibility. The p-type TeSeO system is highly adaptable, scalable, and reliable, which can address emerging technological needs that current semiconductor solutions may not fulfill.
... The regularly arranged diffraction spots can be observed in both regions, indicating that both the Si layers have a high degree of crystallinity. 2 , where Ci, VT, W and L denote the gate capacitance, threshold gate voltage, channel width and length, respectively.) The ultrahigh mobility and steep SS should originate from the highly oriented large grains of BLDA poly-Si and the smooth channel/GI interface. ...
... The deposited and then annealed n + layer enables low resistance of source/drain regions. The fabricated top-gate BLDA-LTPS TFTs show high performance metrics, with μfe of 556.66 cm 2 /Vs and Ion/Ioff of 1.58×10 7 . Such BLDA-enabled LTPS technology noticeably strengthens the overall advantages of LTPS TFTs in the large-area electronics, especially by exploiting the cost effectiveness of high-generation display-panel production lines. ...
In this letter, a high performance and large area feasible top-gate low-temperature polysilicon thin film transistor (LTPS TFT) technology is reported. The poly-Si active layer was formed by crystallizing the plasma enhanced chemical vapor deposited (PECVD) amorphous silicon (a-Si) film using the blue laser diode anneal (BLDA) technique. The low resistance of source-drain (S/D) regions were formed from a heavily-doped PECVD a-Si layer. The fabricated top-gate LTPS TFTs exhibit excellent electrical performances, with the carrier mobility more than 556.66 cm2/V-s and on/off-current ratio over 1.58×107. This proposed technology is expected to promote the manufacturing lines to the higher generations.
... Since the report by Nomura et al. on thin-film transistors (TFTs) using InGaO 3 (ZnO) 5 , oxide semiconductors (OSs) have been extensively researched due to their promising properties, such as wide band gap, excellent electrical properties, simple preparation, and good compatibility with Si-based manufacture. OSs have emerged as exceptional candidates for TFTs in pixel switching/driving applications for flat-panel displays (FPDs) [1]. ...
... Among them, In 2 O 3 -based OSs are considered as the most promising materials to achieve high mobility. Firstly, the conduction band bottom of In 2 O 3 is composed of the 5s orbitals of the indium ion with a large orbital radius and a high overlap, which provides a highway for electron transportation [4,5]. Secondly, theoretical calculations show that In 2 O 3 has a small and isotropic effective mass of electrons (0.22 m 0 ) [6]. ...
In this article, this research demonstrates the influence of in-situ introduction of H2 into the working gas on the physical properties of post-annealed In2O3 thin films and the performance of associated devices. A gradual increase in the H2 ratio leads to improved film quality, as indicated by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscope analyses showing a reduction in defect states such as band-tail states and VO in the film, and a smoother surface morphology with the root mean square roughness approximately 0.446 nm. Furthermore, this hydrogen doping effect results in a distinct shift in the device’s threshold voltage toward the positive direction, and an improvement in the field-effect mobility and subthreshold swing. Consequently, a high-performance In2O3:H TFT is developed, exhibiting a field-effect mobility of 47.8 cm2/Vs, threshold voltage of −4.1 V and subthreshold swing of 0.25 V/dec. These findings highlight the potential of in-situ H doping as a promising approach to regulate In2O3-based TFTs.
... Maxwell's equation of electromagnetic (EM) waves demonstrated that electrical properties and optical properties are entirely contradictory to each other. When EM waves traveled through a semiconducting material (uncharged), the solution to Maxwell's equation determines the value of the refractive index in the form of real and imaginary parts, given below [10,11]: (2) components of the electric field vector is IT = 0 and EI = -ER. Hence, excellent conductors are opaque to EM radiations. ...
Transparent conducting oxides (TCOs) represent a remarkable class of materials that possess both excellent electrical conductivity and high optical transparency, which are typically considered mutually exclusive in traditional materials. In conventional materials, achieving both high electrical conductivity and optical transparency is difficult. Materials with a wide optical band gap are transparent in the visible region but lack electrical conductivity, while conductive metals are opaque. Hence, the only way to induce both properties in a single material is to create non-stoichiometry and/or defects. By introducing shallow defects near the conduction band for n-type materials and the valence band for p-type materials, it is possible to enhance the conductivity of the material at room temperature. However, developing efficient p-type TCOs has been particularly difficult due to the localized nature of the valence band derived from O 2p orbitals and the challenges associated with shallow acceptors, resulting in large effective mass of holes. While commercially available TCOs are predominantly n-type, such as Sn:In2O3, Al:ZnO, and F:SnO2, the development of efficient p-type TCOs lags. In this review, we have discussed the origin of p-type conductivity in TCOs, and the difficulties encountered in developing efficient p-type materials. We have also demonstrated the fundamental materials physics of p-type TCOs, including electronic structures, doping, defect properties, and optical properties. A range of deposition techniques has been adopted to prepare TCO films, and this review provides a detailed discussion of these techniques and their relative deposition parameters. Overall, we have presented an up-to-date and comprehensive review of different p-type transparent conducting oxide thin films, providing insights into ongoing research and potential future directions in this field.
... In 2004, amorphous indium-gallium-zinc-oxide (a-IGZO) was discovered by the group of Prof. Hosono, offering numerous outstanding characteristics such as reasonable field-effect mobility (μ FE ) > 10 cm 2 /Vs, extremely low off-current < 10 −24 A/μm, steep subthreshold swing (SS) of ~ 0.1 V/dec, and outstanding uniformity even when fabricated at low temperature [1][2][3][4][5] . Because of these merits, oxide semiconductor (OS) family has been studied intensively and has become a standard channel material of thin-film transistors (TFTs) in high-end active matrix organic light emitting diode (AMOLED) display backplanes. ...
Oxide semiconductors have gained significant attention in electronic device industry due to their high potential for emerging thin-film transistor (TFT) applications. However, electrical contact properties such as specific contact resistivity (ρC) and width-normalized contact resistance (RCW) are significantly inferior in oxide TFTs compared to conventional silicon metal oxide semiconductor field-effect transistors. In this study, a multi-stack interlayer (IL) consisting of titanium nitride (TiN) and indium-gallium-tin-oxide (IGTO) is inserted between source/drain electrodes and amorphous indium-gallium-zinc-oxide (IGZO). The TiN is introduced to increase conductivity of the underlying layer, while IGTO acts as an n⁺-layer. Our findings reveal IGTO thickness (tIGTO)-dependent electrical contact properties of IGZO TFT, where ρC and RCW decrease as tIGTO increases to 8 nm. However, at tIGTO > 8 nm, they increase mainly due to IGTO crystallization-induced contact interface aggravation. Consequently, the IGZO TFTs with a TiN/IGTO (3/8 nm) IL reveal the lowest ρC and RCW of 9.0 × 10⁻⁶ Ω·cm² and 0.7 Ω·cm, significantly lower than 8.0 × 10⁻⁴ Ω·cm² and 6.9 Ω·cm in the TFTs without the IL, respectively. This improved electrical contact properties increases field-effect mobility from 39.9 to 45.0 cm²/Vs. This study demonstrates the effectiveness of this multi-stack IL approach in oxide TFTs.
... Consequently, it has gained attention both the fields of scientific research and industry. However, the solution-based approach does present certain challenges, including issues of limited mobility and the intricate control of threshold voltage [2]. In response, numerous researchers have applied various post-treatment techniques to fine-tune the properties of TFTs, encompassing factors such as threshold voltage, on/off ratio, and mobility. ...
In this paper, a three-step annealing process was employed to treat IGZO thin-film field-effect transistors (TFTs). We observed that TFTs prepared by an initial high-temperature thermal annealing exhibited a low threshold voltage. By employing a second-step low-temperature annealing, the threshold voltage of the TFT was positively shifted, demonstrating improved turn-off characteristics. However, after this process, a significant decrease in TFT mobility and a change in threshold voltage were observed. By introducing a third 633 nm laser annealing technique, we managed to control the threshold voltage and improve mobility. To investigate the mechanism of the changes in TFT performances up to different post-annealing processes, a series of characterization techniques were employed to explain the TFT performance changes.
... Sub-threshold Swing (SS) is a FoM that measures the performance of FETs in the sub-threshold region. It is a measure of how much V G is required to increase I D by one decade, 78 and is given by ...
Colloidal quantum dots (CQDs) have been a hot research topic ever since they were successfully fabricated in 1993 via the hot injection method. The Nobel Prize in Chemistry 2023 was awarded to Moungi G. Bawendi, Louis E. Brus and Aleksey Yekimov for the discovery and synthesis of quantum dots. The Internet of Things (IoT) has also attracted a lot of attention due to the technological advancements and digitalisation of the world. This review first aims to give the basics behind QD physics. After, the history behind CQD synthesis and the different methods used to synthesize most widely researched CQD materials (CdSe, PbS and InP) are revisited. A brief introduction to what IoT is and how it works is also mentioned. Then, the most widely researched CQD devices that can be used for the main IoT components are reviewed, where the history, physics, the figure of merits (FoMs) and the state-of-the-art are discussed. Finally, the challenges and different methods for integrating CQDs into IoT devices are discussed, mentioning the future possibilities that await CQDs.
... Each time novel transistors or innovative fabrication techniques emerge, numerous applications that were previously inaccessible become achievable. This includes certain advancements, like ultrahigh-definition transparent displays and flexible electronic devices [10][11][12]. At present, light-emitting diodes (LEDs), relying on solid-state semiconductors, have emerged as the next evolution in lighting technology following incandescent and fluorescent lamps. ...
The pursuit of p-type semiconductors has garnered considerable attention in academia and industry. Among the potential candidates, copper iodide (CuI) stands out as a highly promising p-type material due to its conductivity, cost-effectiveness, and low environmental impact. CuI can be employed to create thin films with >80% transparency within the visible range (400–750 nm) and utilizing various low-temperature, scalable deposition techniques. This review summarizes the deposition techniques for CuI as a hole-transport material and their performance in perovskite solar cells, thin-film transistors, and light-emitting diodes using diverse processing methods. The preparation methods of making thin films are divided into two categories: wet and neat methods. The advancements in CuI as a hole-transporting material and interface engineering techniques hold promising implications for the continued development of such devices.
... Amorphous oxide thin-film transistors (TFTs), represented by amorphous indiumgallium-zinc-oxide (a-IGZO) TFTs, have attracted much attention for applications in largearea electronics such as active matrix displays due to their high mobility, low off-current, and low-cost production process [1][2][3][4]. Among the various device structures, the backchannel-etched (BCE) structure is widely used in a-IGZO TFTs due to its lower mask number and simple production process [5]. ...
This study reveals the pronounced density of oxygen vacancies (Vo) at the back channel of back-channel-etched (BCE) a-InGaZnO (a-IGZO) thin-film transistors (TFTs) results from the sputtered deposition rather than the wet etching process of the source/drain metal, and they are distributed within approximately 25 nm of the back surface. Furthermore, the existence and distribution depth of the high density of Vo defects are verified by means of XPS spectra analyses. Then, the mechanism through which the above Vo defects lead to the instability of BCE a-IGZO TFTs is elucidated. Lastly, it is demonstrated that the device instability under high-humidity conditions and negative bias temperature illumination stress can be effectively alleviated by etching and thus removing the surface layer of the back channel, which contains the high density of Vo defects. In addition, this etch method does not cause a significant deterioration in the uniformity of electrical characteristics and is quite convenient to implement in practical fabrication processes. Thus, a novel and effective solution to the device instability of BCE a-IGZO TFTs is provided.
... A prevalent constraint with both techniques is the uniform doping across the entire film. While this uniform doping approach is effective for certain applications, 5,6 it falls short when localized doping is crucial for specific device architectures. 7,8 As the demand for precision and tailored applications grows, refined control methods are imperative for realizing the potential of superconducting films. ...
In this study, we present a novel approach to localized superconductivity induction in BaFe2As2 films via targeted implantation of cobalt (Co) ions. Primarily, our study focuses on the systematic distribution of Co ions and the subsequent evolution of superconducting properties in Co-ion-implanted BaFe2As2 films. Our observations show that Co-ion distribution in the films is congruent with the results of analytical methodologies employed in the semiconductor industry, as confirmed via transmission electron microscopy imaging. The temperature-dependent resistivity curves reveal the concurrent presence of superconducting and non-superconducting regions. Moreover, the superconducting domain demonstrates the typical diamagnetic behavior intrinsic in superconductors. Importantly, Co-ion concentrations of ∼10²⁰ cm⁻³ can be achieved by finely tuning the beam energy and ion dose. This concentration is instrumental in establishing an effective superconducting percolation pathway within the films.
... High-performance SnO thin films typically require well-crystallized structures, necessitating crystallization annealing at high temperatures. 2,3,24 For instance, the highest recorded Hall mobility for SnO thin films is 21.0 cm 2 V À1 s À1 , achieved in epitaxial SnO using PLD on a (001)-preferred orientation yttria-stabilized zirconia substrate. 15 This process demands precise control of growth thermodynamics and kinetics by finetuning laser fluence and setting the substrate temperature to 350 1C. ...
The fabrication of p-type tin monoxide (SnO) thin films at room temperature poses significant challenges for conventional methods, primarily due to the electrically anisotropic nature and metastable phases of SnO. Because of this anisotropy, generating effective hole carriers with optimal mobility in SnO requires meticulous thermal annealing, which is nonetheless constrained by SnO's metastability. In this work, we employ ion-beam-assisted deposition (IBAD) to fabricate p-type SnO thin films at room temperature. These films, with their nanocrystalline structure, demonstrate promising electrical performance with a Hall mobility of 2.67 cm² V⁻¹ s⁻¹ and hole concentration of 5.94 × 10¹⁷ cm⁻³, notably without the need for annealing treatment. Our investigation has revealed a unique volcano-shaped trend in Hall mobility, and inversely, in carrier concentration in response to variations in the argon flow rate during the IBAD process. This relationship, when correlated with changes in the optical properties, structural phase, and chemical state of the films, is crucial for understanding the origin of p-type conductivity in room-temperature-fabricated SnO films—a topic that remains elusive in the current literature. We observed a direct correlation between enhanced mobility and reduced lattice disorder, as well as a strong association between increasing hole carrier concentration and the formation of oxygen interstitials. We also highlight that the intermediate phase composition plays a vital role in determining the degree of disorder in the SnO film, which is essential for creating transport pathways and the oxygen environment necessary for hole carrier formation. These insights are instrumental in guiding the design and characterization of room-temperature fabricated p-type SnO thin films, thus propelling advancements in the field of large-area, flexible electronics.
... Table I compares the turn-on voltage Von, the mobility μsat, the subthreshold swing S, and the on/off ratio Ion/I off in the three studied structures, extracted from the rising part of the transfer curve and averaged among six devices per structure. 26 The turn-on voltage is small, thus allowing low voltage operation. The leakage gate current is below 20 pA for all devices for a gate field of 0.8 MV/cm, indicating very good insulating properties of the dielectric structures, considering the low-temperature fabrication process. ...
Radiation dosimetry is crucial in many fields where the exposure to ionizing radiation must be precisely controlled to avoid health and environmental safety issues. Among solid state detectors, we recently demonstrated that Radiation sensitive OXide Field Effect Transistors (ROXFETs) are excellent candidates for personal dosimetry thanks to their fast response and high sensitivity to x rays. These transistors use indium–gallium–zinc oxide as a semiconductor, combined with a dielectric based on high-permittivity and high-atomic number materials. Here, we present a study on the ROXFET gate dielectric fabricated by atomic layer deposition, where we compare single- and multi-layer structures to determine the best-performing configuration. All the devices show stable operational parameters and high reproducibility among different detectors. We identified an optimized bi-layer dielectric structure made of tantalum oxide and aluminum oxide, which demonstrated a sensitivity of (63 ± 2) V/Gy, an order of magnitude larger than previously reported values. To explain our findings, we propose a model identifying the relevant charge accumulation and recombination processes leading to the large observed transistor threshold voltage shift under ionizing radiation, i.e., of the parameter that directly defines the sensitivity of the device.
A thin-film memristor using an amorphous metal–oxide semiconductor (AOS) with a gradient composition of conducting components has been developed, which is another way to achieve memristive properties. The advantages are that conductivity distribution is already obtained as fabricated without forming operation, and an analog characteristic is obtained by optimizing the component composition in AOS. As the binary characteristic, the set operation induces the transition from a low conductance state to a high conductance state, whereas the reset operation does vice-versa. The switching ratio (SR) is high, 448. As the analog characteristic, the SR becomes greater rapidly as the V set increases, namely, the dynamic range is outstanding.
Introducing an ultrathin MgO or AlO x interlayer between the IGZO semiconductor and polymer insulator in a top-gate, bottom-contact TFT significantly improves the performance and stability of the devices.
Tin monoxide (SnO) has been extensively studied due to its promising theoretical p-type performance. However, the fabrication of SnO thin films and SnO channel thin-film transistors (TFTs) faces hurdles that...
Oxygen transport mechanisms for two different Au/Ti/In2O3/Al2O3/p+-Si samples were experimentally evaluated by hard X-ray photoelectron spectroscopy (HAXPES). The deposition temperature for atomic layer deposition of In2O3, as well as the bias voltages applied on the entire stacked structures, were the main parameters used in the work. Chemical analyses of the In2O3 layers deposited at 150 and 200 °C for the samples named T_150 and T_200, respectively, revealed a decreased carbon impurity content in the host In2O3 used as a dopant. Ex situ interfacial analysis of In2O3/Al2O3 also indicated oxygen transport from Al2O3 to In2O3. Moreover, we observed that the Ti adhesion metal attracted oxygen and carbon from In2O3 to form TiO2 and TiC conductive interlayers. Furthermore, operando-HAXPES under an applied bias voltage also revealed that In2O3 underwent phase separation, likely due to variations in the space charge (carriers) around the In2O3/Al2O3 interface for the T_150 sample. Finally, our results emphasize the prominent roles of migration for the ionic oxygen/carbon species and the uncompensated interfacial charge formed by the bias voltage for the metal–semiconductor-oxide stacked structure.
The ability to fabricate an entire smart sensor patch with read‐out electronics using commercial printing techniques may have a wide range of potential applications. Although solution‐processed oxide thin film transistors (TFTs) are capable of providing high mobility electron transport, resulting in large ON‐state current and power output, there is hardly any literature report that uses the printed oxide TFTs at the sensor interfaces. Here, printed amorphous indium‐gallium‐zinc oxide (a‐IGZO)‐based deep‐subthreshold operated TFTs that comprise signal amplifiers and analog‐to‐digital converters (ADCs) that can successfully digitalize the analog sensor signals up to a frequency range of 1 kHz are reported. In addition, exploiting the high current oxide TFTs, a current drive circuit placed after the ADC unit has been found useful in producing easy‐to‐detect visual recognition of the sensor signal at a predefined threshold crossover. Notably, the entire smart sensor patch is demonstrated to operate at a low supply voltage of ≤2 V, thereby ensuring that it can be an on‐chip energy source compatible and standalone detection unit.
Flexible electronics offer a multitude of advantages, such as flexibility, lightweight property, portability, and high durability. These unique properties allow for seamless applications to curved and soft surfaces, leading to extensive utilization across a wide range of fields in consumer electronics. These applications, for example, span integrated circuits, solar cells, batteries, wearable devices, bio-implants, soft robotics, and biomimetic applications. Recently, flexible electronic devices have been developed using a variety of materials such as organic, carbon-based, and inorganic semiconducting materials. Silicon (Si) owing to its mature fabrication process, excellent electrical, optical, thermal properties, and cost efficiency, remains a compelling material choice for flexible electronics. Consequently, the research on ultra-thin Si in the context of flexible electronics is studied rigorously nowadays. The thinning of Si is crucially important for flexible electronics as it reduces its bending stiffness and the resultant bending strain, thereby enhancing flexibility while preserving its exceptional properties. This review provides a comprehensive overview of the recent efforts in the fabrication techniques for forming ultra-thin Si using top-down and bottom-up approaches and explores their utilization in flexible electronics and their applications.
Oxide semiconductors have a promising potential in future wearable, transparent, and flexible electronics, but the research to date is mostly limited to n-type thin-film transistors (TFTs) and circuits. Here, an entirely oxide-semiconductor-based complementary operational amplifier (op-amp) is developed, comprising n-type indium-gallium-zinc-oxide (IGZO) and p-type tin monoxide (SnO). The IGZO TFTs show a mobility of 21.5 cm
$^{\text{2}}$
/V
$\cdot$
s, a threshold voltage of 4.8 V, and a subthreshold swing of 0.2 V/decade. The SnO TFTs exhibit a mobility of 1.19 cm
$^{\text{2}}$
/V
$\cdot$
s and a threshold voltage of
$-$
3.5 V. The op-amp takes an area of 500
$\times$
250
$\mu$
m. Operating under a
$\pm$
12-V supply, the op-amp achieves a negative-feedback gain of 54 dB, the highest among the reported op-amps based on oxide semiconductor TFTs. The power consumption of the op-amp is 0.2 mW. The op-amp also exhibits a unit-gain frequency of 300 kHz, a bandwidth of 9 kHz, and a gain-bandwidth product (GBWP) of 4.5 MHz.
Among metal oxide material TFTs, IGZO TFTs are highly regarded for their exceptionally high mobility, exceeding 10 cm²/V·s, remarkable transparency of more than 80%, and their adaptable low-temperature fabrication techniques. High-performance displays operating at refresh rates of up to 144 Hz and undergoing millions of device switches demand IGZO TFTs with mobility exceeding 20 cm²/V·s and higher stability against impulse stress. The effect of IGZO material composition on device stability and recent strategies to promote the mobility and stability of IGZO TFT by modifying the transistor structure, preparation process, and post-processing techniques to reduce VO have been discussed. The paper describes the application of IGZO TFTs in flexible electronics.
We offer design guidelines with a top–down and bottom–up design approach for oxide semiconductor (OS) transistors, optimized for gain cell memory on a logic platform. With high-density, high-bandwidth on-chip gain cell memory, deep neural network (DNN) accelerator execution times can be shortened by 51–66%, by minimizing access to off-chip dynamic random access memory (DRAM). To balance retention time with memory bandwidth (top–down), atomic layer deposition (ALD) indium tin oxide (ITO) transistors are chosen (bottom–up). The experimentally optimized device exhibits low off-state current (
$2\times 10^{-{18}}$
A/
$\mu \text{m}$
at
${V}_{\text {GS}}$
= -0.5 V), good on-state current (26.8
$\mu \text{A} / \mu \text{m}$
for power supply < 2 V), low subthreshold swing (SS) (70 mV/dec), and good mobility (27 cm2V-1s-1). Using this optimized device, a gain cell memory macro with 64 rows (WL)
$\times256$
columns (BL) is simulated at the 28 nm node operating at
${V}_{\text {DD}}$
= 0.9 V. The simulation results show that hybrid OS-Si gain cell memory achieves
$0.98\times $
frequency and
$3\times $
density of static random access memory (SRAM), and the OS-OS gain cell memory is projected to operate at
$0.5\times $
frequency with
${N}$
times
$1.15\times $
density of SRAM with
${N}$
-layer of 3-D stacking.
The coexistence of high bias stress stability and high performance is the key problem of low-temperature application of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs). However, the main treatment strategy of regulating a-IGZO TFTs bias stress stability by specific gas is usually obtained at the expense of a certain degree of electrical performance. Here, we have innovatively designed a-IGZO framework (IGZO
$_{\text{I}}$
/IGZO
$_{\text{I}}$
:N/IGZO
$_{\text{II}}$
/IGZO
$_{\text{II}}$
:O) TFTs with simple structure and without extra processing to synergistically achieve outstanding bias stress stability and good performance. Therefore, the threshold voltage offset (4.) under negative bias stress (NBS) decreased significantly from
$-$
5.08 to
$-$
0.79 V. The field effect mobility (7.) has some-what optimized to 18.23 cm
$^{\text{2}}$
/Vs. The results showed that the strategy of fabricating a-IGZO framework (IGZO
$_{\text{I}}$
/ IGZO
$_{\text{I}}$
:N/IGZO
$_{\text{II}}$
/IGZO
$_{\text{II}}$
:O) TFTs may reduce the defects of bulk and the interface between the channel and the gate insulation layer, decrease the surface oxygen vacancy defect concentration, and avoid the interaction with extra oxygen or water vapor. This simple and ingenious framework provided a universality strategy for synergistically improving the bias stress stability and electrical properties of amorphous metal-oxide TFTs.
Top-gate coplanar-structure thin film transistor (TFT) combining advantages of both co-sputtered amorphous La-doped ZnSnO (a-La-ZTO) active layer and solution-based polymethylmethacrylate (PMMA) gate dielectric layer has been prepared under low temperature (100℃) with low cost for the first time. The results indicate that PMMA thin film demonstrates anti-reflection phenomenon when it combines with a- La-ZTO layer to form double-layer film, displaying the high transparency to visible light of ~90.3%. Moreover, it was found that the La target power during the deposition of a- La-ZTO film plays an important role in suppressing the formation of oxygen vacancies and adjusting the carrier concentration of a-La-ZTO active layer, thus impacting on a-La-ZTO TFTs performance. Overall, the optimum a-La-ZTO TFT with a La target power of 13.9W, working in a n-channel enhancement mode, possessed a large saturated mobility (＞10 cm2/Vs) and on/off drain current ratio over 105.
This study investigated the effects of different Ga contents on the performance of single amorphous InGaZnO (a-IGZO) and bilayer InGa(0.5%)ZnO/InGa(1%)ZnO thin-film transistors (TFTs). Through rational design, a bilayer TFT exhibiting the best performance, including a
${V}_{\text {th}}$
of 1.2 V,
${I}_{\text {on}}/{I}_{\text {off}}$
of
$1\times 10^{{8}}$
, SS of 0.28 V/decade, and
$\mu _{\text {FE}}$
of 32.5 cm2/Vs, was obtained. This improved performance was attributed to the InGa(0.5%)ZnO front layer enhancing the
$\mu _{\text {FE}}$
with low surface defect and high
${N}_{\text {e}}$
and the InGa(1%)ZnO back layer controlling the
${V}_{\text {th}}$
with low
${V}_{\text {O}}$
and
${N}_{\text {e}}$
in the bilayer device. Owing to the formation of energy band bending, the electrons transferred from the InGa(0.5%)ZnO to InGa(1%)ZnO layer. This resulted in the accumulation of free electrons near the interface, thereby enhancing the
$\mu _{\text {FE}}$
of the bilayer device. Moreover, a minor shift in the
${V}_{\text {th}}$
(0.4 and −0.5 V) of InGa(0.5%)ZnO/InGa(1%)ZnO TFTs with HfO2/Al2O3 dual passivation layer (PVL) was observed under positive and negative gate bias light illumination stress with relative humidity 60% condition. This was attributed to the HfO2/Al2O3 PVL protecting the channel from oxygen adsorption/desorption and environmental influences. Thus, the designed bilayer InGa(0.5%)ZnO/InGa(1%)ZnO TFTs with HfO2/Al2O3 PVL have enabled new pathways for achieving high-performance and highly stable oxide TFTs.
Amorphous tin oxide (a‐SnO x ) is a potential transparent oxide semiconductor candidate for future large‐area electronic applications. The thin‐film transistor (TFT) mobilities reach ≈100 cm ² Vs ⁻¹ , a mobility higher than other multiple cation‐based oxide semiconductor TFTs. Few optical properties have been reported so far and therefore both the effect of visible light and negative bias illumination stress (NBIS) on a‐SnO x TFT performances, known to dramatically impact oxide semiconductor‐based TFTs, have been investigated. The variation of density of states (DOS) due to NBIS by device simulation is analyzed, and a fourfold increase of the donor‐like states and a decrease in the band edge DOS from 2.3 to 2.0 × 10 ¹⁹ cm ⁻³ eV ⁻¹ are showed. The evaluation of the effect of neutral, singly, and doubly ionized oxygen vacancies by density functional theory using 95 atoms reveals not only states in the gap of SnO 2 , but also variations in the electron density, and modifications in the crystal parameters compared to a structure without an oxygen vacancy. Material and device simulation analysis reveal that the oxygen vacancies have a dramatical impact on the DOS in the gap of SnO 2 and can explain the NBIS phenomenon observed in a‐SnO x TFT.
To meet the demand for next-generation electronic products with excellent driving ability and high switching speed, using InSnO electrode material as a semiconductor channel layer is one of the attempts to improve the mobility of thin-film transistors (TFTs) However, instabilities in the environment limit the development of high-mobility semiconductors. In this work, an ultrathin hafnium (Hf)-doped InSnO (HITO) layer is prepared with enhanced stability, which is achieved by radio frequency magnetron cosputtering at low temperature. The HITO layer is deposited onto InSnO (ITO) to reduce the oxygen vacancies concentration, and thus, the trapping and detrapping issues at the surface of ITO channel are suppressed, leading to a high mobility of 76.21 cm
$^{\text{2}}$
V
$^{{-\text{1}}}$
s
$^{-\text{1}}$
, a low threshold voltage of
$-$
0.86 V, and a steep subthreshold swing (SS) of 0.24 V/decade. The HITO layer is insensitive to water and oxygen and effectively blocks the permeability channel layer. The fabricated HITO/ITO TFTs obtain improved stability in positive bias stress (PBS) tests in humidity condition. The threshold voltage shifts under PBS at 85% relative humidity for 3600 s decreased from
$-$
8.2 to 0.97 V. This work provides a feasible method for low-cost, high-mobility oxide TFTs, and the whole process temperature control below 150
$^{\circ}$
C also expands its application in flexible electronic devices.
Solution-based memristors deposited by inkjet printing technique have a strong technological potential based on their scalability, low cost, environmentally friendlier processing by being an efficient technique with minimal material waste. Indium-gallium-zinc oxide (IGZO), an oxide semiconductor material, shows promising resistive switching properties. In this work, a printed Ag/IGZO/ITO memristor has been fabricated. The IGZO thickness influences both memory window and switching voltage of the devices. The devices show both volatile counter8wise (c8w) and non-volatile 8wise (8w) switching at low operating voltage. The 8w switching has a SET and RESET voltage lower than 2 V and − 5 V, respectively, a retention up to 10⁵ s and a memory window up to 100, whereas the c8w switching shows volatile characteristics with a low threshold voltage (Vth < − 0.65 V) and a characteristic time (τ) of 0.75 ± 0.12 ms when a single pulse of − 0.65 V with width of 0.1 ms is applied. The characteristic time alters depending on the number of pulses. These volatile characteristics allowed them to be tested on different 4-bit pulse sequences, as an initial proof of concept for temporal signal processing applications.
Direct Laser Writing (DLW) has been increasingly selected as a microfabrication route for efficient, cost‐effective, high‐resolution material synthesis and conversion. Concurrently, lasers participate in the patterning and assembly of functional geometries in several fields of application, of which electronics stand out. In this review, recent advances and strategies based on DLW for electronics microfabrication are surveyed and outlined, based on laser material growth strategies. First, the main DLW parameters influencing material synthesis and transformation mechanisms are summarized, aimed at selective, tailored writing of conductive and semiconducting materials. Additive and transformative DLW processing mechanisms are discussed, to open space to explore several categories of materials directly synthesized or transformed for electronics microfabrication. These include metallic conductors, metal oxides, transition metal chalcogenides and carbides, laser‐induced graphene, and their mixtures. By accessing a wide range of material types, DLW‐based electronic applications are explored, including processing components, energy harvesting and storage, sensing, and bioelectronics. The expanded capability of lasers to participate in multiple fabrication steps at different implementation levels, from material engineering to device processing, indicates their future applicability to next‐generation electronics, where more accessible, green microfabrication approaches integrate lasers as comprehensive tools.
Ultrathin films of In
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>
O
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>
have demonstrated high transistor mobility yet exhibit vulnerability to stability degradation under bias stress. Here, we engineered ultrathin In
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O
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>
films via atomic layer deposition, complemented with a cation-doped ZnSnO semiconductor layer for comprehensive passivation of intrinsic and surface imperfections. The resulting thin-film transistors with HfO
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>
dielectric layer show the improved mobility as 81.5 cm
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>
V
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>
s
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>
and the reversed subthreshold slope value down to 77.1 mV/decade. Under positive or negative gate bias stress (or with illumination), the threshold shift is improved to be -0.14 or -0.26 V (or -0.32 V), respectively. Combined electrical and photoelectrical spectral analysis indicates a robust protection of charge carriers by the multi-functional cation-doped back channel layer, which effectively diminishes defect-mediated scattering, shields against external environmental factors, and facilitates the recombination of excess photo-induced carriers.
The study emphasizes the benefits of buried gate IGZO transistor devices, showcasing enhanced electrical performance and reliability.
In this work, bottom-Schottky-structure InGaZnOx (IGZO) Schottky barrier diodes (SBDs) with sputtered PtOx anodes were fabricated and annealed in oxygen at different temperatures. Critical parameters and negative bias stress (NBS) stability of SBDs with different annealing temperatures are investigated. With the annealing temperature increases, the barrier height and rectification ratio of the SBDs exhibited a rising-then-declining trend, while the ideality factor slightly increased until 200 °C. The SBDs show up overall reliability except for a leakage current rising trend under light, which can be attributed to free electron generation from the ionized oxygen vacancy. Among all the SBDs, the 175 °C annealed ones exhibited the best overall performance, including a high barrier height of 0.89 eV, an ideality factor of 1.14, and a large rectification ratio of over 108. Compared to the initial SBDs, the annealed ones showed up great improvement in NBS stability except for the 200 °C annealed ones, which was permanently degraded and not able to recover to original states. According to experimental result analysis and IGZO material characteristics, a stability model based on the subgap trap transition from VO2+ to VO and new VO2+ creation was proposed, which applies to both the short-term and long-term NBS tests. The results above demonstrate that oxygen annealing at appropriate temperature is an effective method to improve both device performance and NBS stability for PtOx–IGZO SBDs.
The unique characteristics of an anti‐ambipolar switch (AAS) device exhibit Λ‐shaped transfer responses (namely delta conductance) and present unique opportunities to overcome the limit of silicon‐based, complementary metal‐oxide‐semiconductor (CMOS) logic circuits. It is crucial because a device that only turns on under a certain bias range can be utilized to simplify the logic circuit and reduce the device count and circuit area required to perform logic functions. In this study, a physically scalable AAS device is investigated using ZnO and dinaphtho[2,3‐ b :2′,3′‐ f ]thieno[3,2‐ b ]thiophene as heterojunction structures to reduce the operating voltage and enhance the peak current and peak‐to‐valley ratio of the AAS device. Moreover, novel logic circuits for AND, OR, XOR, DEMUX, and half‐adder functions are demonstrated using AAS devices. AAS device‐based logic circuits exhibit power‐efficiency characteristics (≈49 times lower than that of the 90‐nm silicon‐based CMOS inverter) and reduce the transistor count and the circuit area by ≈67% and ≈70%, respectively. These results indicate that the use of AAS device‐based logic circuits can be a promising approach to overcome the physical scaling limit of current CMOS technology.
This study presents the high-voltage (HV) amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) with an offset region modulated by the indium–tin oxide (ITO) capping layer (ICL) near the drain side. The breakdown voltage (BV) is elevated because the offset region lowers the electric field between the gate and the drain. For the device without an ICL, a BV of 465 V and a large ON-resistance (
${R} _{\text {on}}$
) of 1000
$\text{M}\Omega $
are obtained at the offset region length (
${L} _{\text {offset}}$
) of
$5 ~\mu \text{m}$
. The ICL effectively improves the ON-state characteristics of the HV device by reducing the resistance of the offset region. The output current of the TFTs increases with increasing ICL thickness (
${T} _{\text {ICL}}$
) due to the elevated electron density in the ITO film. The a-IGZO TFT achieves a BV of 326 V and a low
${R} _{\text {on}}$
of 207
$\text{k}\Omega $
with an offset region length (
${L} _{\text {offset}}$
) of
$5 ~\mu \text{m}$
and a
${T} _{\text {ICL}}$
of 11.4 nm. The gate dielectric near the gate corner is found to be a breakdown weak point due to the electric field crowding, as confirmed by the TCAD simulations.
In comparison to reports on n ‐type semiconducting oxides, p ‐type oxide semiconducting materials are still rare. Scarcely reported p ‐type oxide transistors demonstrated unsatisfactory environmental stability which still hinders their implementation for all oxide transistors and circuit applications. In this study, for the first time on α ‐TeO 2 as an active channel material with p ‐type characteristics accessible by direct evaporation technique. Notably, the fabricated 5 nm α ‐TeO 2 thin film in connection with an equally thin passivation layer exhibits a remarkable low processing temperature of 50 °C generating a hole mobility of 3.8 cm ² V ⁻¹ s ⁻¹ , an on‐state current of 966 µA, and an on/off ratio of 3.8 × 10 ³ . Additionally, the reproducibility of these devices confirmed a narrow variation in the TFT metrics, yielding an average hole mobility, on‐current, and on/off ratio of 3.59 cm ² V ⁻¹ s ⁻¹ , 914 µA, and 3.3 × 10 ³ , respectively. Furthermore, the devices are subjected to extensive stability testing under ambient atmospheric conditions that exhibits a marginal mobility reduction while maintaining a stable on/off ratio over 125‐day period, highlighting their robust environmental stability. Notably, the low processing temperatures with both exceptional transistor performance and environmental endurance makes them suitable for the integration onto flexible substrates, particularly bendable/stretchable displays.
Flexible wearable electronics have been developing rapidly in recent years, and one of its core devices, thin‐film transistor (TFT), is also attracting attention. Current TFT preparation processes usually require high annealing temperatures (>350 °C), which is not conducive to their application on most flexible substrates (PET, PEN, nanopaper, etc.). In this article, a strategy for the room temperature preparation of Nd:AIZO/Al2O3 dual‐layer TFT devices is proposed, which has appreciable electrical properties without additional annealing treatment. Taguchi orthogonal experimental methods are used to investigate the effects of three essential process parameters on the performance of the films and devices. As Nd:AIZO deposition time decreases and Al2O3 oxygen percentage and time during deposition increase, the μsat and SS of TFT devices are improved. With the preferred combination of parameters applied to the device preparation, the device exhibits a saturation mobility μsat of 24.3 cm² V⁻¹ s⁻¹, a threshold voltage Vth of −1.4 V, a subthreshold swing SS of 0.21 V decade⁻¹ and an Ion/Ioff ratio of 4.26 × 10⁸. The ultra‐thin Al2O3 layer act as defect modification and form high‐speed electron transport in channel layer. The room temperature preparation of the dual‐layer TFT process proposed in this article is compatible with large‐size low‐cost flexible substrates. It has great potential for application in green flexible wearable electronics.
Solution processing has emerged as a promising technique for the fabrication of oxide thin‐film transistors (TFTs), offering advantages such as low cost, high throughput, and exceptional compositional control. However, achieving reasonable electrical properties typically demands high annealing temperatures in the fabrication process. In addressing this challenge, a novel combination strategy is proposed that involves integrating the H 2 O 2 inducement technique with infrared (IR) irradiation annealing. The study investigates the effects of precursors and IR irradiation annealing temperatures on the electrical properties of In 2 O 3 TFTs. It is found that H 2 O 2 can help accelerate the decomposition of organic residues, while IR irradiation annealing could enhance the film densification. By employing the proposed strategy, metal oxide TFTs consisting of a Zr‐Al‐O dielectric fabricated at 230 °C and an In 2 O 3 channel layer fabricated at 185 °C demonstrated high performance with field‐effect mobility = 31.7 cm ² V ⁻¹ ·s ⁻¹ , threshold voltage = 1.3 V, subthreshold swing = 0.13 V per decade, and on‐to‐off current ratio = 1.1 × 10 ⁵ . This work demonstrates the proposed combinational strategy is a general method to fabricate not only metal oxide semiconductors but also dielectrics.
The amorphous IGZO (a-IGZO) transparent TFT with optimized AZO/Ag/AZO S/D electrodes exhibited high transparency (79.17%) in the 380-600 nm wavelength region despite the use of a metallic Ag layer due to effective antireflection. In addition, the TTFT with AZO/Ag/AZO S/D electrodes showed higher field effect mobility (mu(FE): 12.17 cm(2)/V-s) and on to off current ratio (I(on/off): 9.37 x 10(8)) than the TTFT with single AZO S/D electrodes due to a reduction in resistivity caused by insertion of the metallic Ag layer. Comparable TTFT performance of the a-IGZO based TTFT to Ti S/D electrodes indicates that the AZO/Ag/AZO multilayer electrode is a promising transparent S/D scheme for high performance TTFTs.
To investigate the eect of oxygen on the optical and the electrical properties of amorphous InGaZnO (a-IGZO), we prepared thin lms by RF magnetron sputtering in various oxygen atmo-spheres at room temperature and the thin-lm transistors (TFTs) were evaluated. The oxygen concentration during the deposition process aected both the optical band-gap and the mobility of a-IGZO-based devices. As the oxygen concentration in the processing chamber during deposition was increased, the optical band-gap and the saturation mobility decreased concurrently. The high-est optical band-gap and the best device performance were obtained from the a-IGZO lm deposited in an atmosphere of 10 % oxygen. The a-IGZO lm deposited at this condition exhibited an optical band-gap of 3.29 eV and the transistors fabricated with this lm revealed a saturation mobility of 2.6 cm 2 /V¡s, a subthreshold swing of 0.93 V/decade, an on-o current ratio of 10 7 and a threshold voltage of 13.9 V.
Through first-principles calculations, the electronic structures and the microscopic properties of the O vacancies (VO) in the transparent semiconductor oxide InGaZnO4 are investigated. The results are compared to those for vacancies in In2O3, Ga2O3, and ZnO. Electronic structure calcu-lations indicate that the VO make deep-donor levels in binary oxides while the VO make shallow donor-like levels in the quaternary InGaZnO4. The vacancy induced a tensile stress in all the tested oxides. The total energy calculations indicate that the VO are energetically favored to be located between the In-O and the Ga-Zn-O layers in InGaZnO4.
Through first-principles calculations, we investigate the electronic structures of single crystals of InGaZnO4 (sc-InGaZnO4), which have a complex structure consisting of InO2 layers inter-stacked with GaZnO2 double layers. Within the GaZnO2 layers, Zn and Ga atoms alternatively occupy the cation sites. We find that the conduction band of the sc-InGaZnO4 is composed of the hybridization of In-s, Ga-s, Zn-s and O-p orbitals while the valence bands are characterized dominantly by O-p orbitals. At the conduction band, the energy of In-s band are lower than those of Ga and Zn atoms the Ga-driven states are slightly lower than Zn-driven states, and the band widths of the Zn-and Ga-driven bands are wider than that of the In-driven band.
We report the results of the characterization of Cu oxide thin films deposited by radio frequency (r.f.) magnetron sputtering at different annealing temperatures. The deposited Cu oxide thin films were investigated by scanning electron microscopy, spectroscopic ellipsometry, X-ray diffraction, atomic force microscopy, Xray photoelectron spectroscopy, and contact angle measurements. The thickness of the films was about 180 nm and the monoclinic CuO phase was detected. The and phases were grown as amorphous phase and the ratio of the three phases were independent on the annealing temperature. The surface of Cu oxide films changed from hydrophilic to hydrophobic as the annealing temperature increased. This phenomenon is due to the increase of the surface roughness. The direct optical band gap was also obtained and laid in the range between 2.36 and 3.06 eV.
A comparative study is made of the low-frequency noise (LFN) in amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with Al2O3 and Al2O3/SiNx gate dielectrics. The LFN is proportional to 1/fgamma, with gamma ~ 1 for both devices, but the normalized noise for the Al2O3/SiNx device is two to three orders of magnitude lower than that for the Al2O3 device. The mobility fluctuation is the dominant LFN mechanism in both devices, but the noise from the source/drain contacts becomes comparable to the intrinsic channel noise as the gate overdrive voltage increases in Al2O3/SiNx devices. The SiNx interfacial layer is considered to be very effective in reducing LFN by suppressing the remote phonon scattering from the Al2O3 dielectric. Hooge's parameter is extracted to ~6.0 times 10-3 in Al2O3/SiNx devices.
Degradation of
Ga2O3-In2O3-ZnO (GIZO)
thin-film transistors (TFTs), which are promising for driving circuits
of next-generation displays, was studied. We found a degradation mode
that was not observed in silicon TFTs. A parallel shift without any
change of the transfer curve was observed under gate voltage stress.
Judging from the bias voltage dependences we confirmed that the mode was
mainly dominated by a vertical electric field. Thermal distribution was
measured to analysis the degradation mechanism. Joule heating caused by
drain current was observed; however, a marked acceleration of
degradation by drain bias was not found. Therefore, we concluded that
Joule heating did not accelerate degradation. Recovery of electrical
properties independent of stress voltage were observed.
Copper oxide, Cu2O and CuO, thin films have been synthesized on Si (100) substrates using pulsed laser deposition method. The influences of substrate temperature and oxygen pressure on the structural properties of copper oxide films were discussed. The X-ray diffraction results show that the structure of the films changes from Cu2O to CuO phase with the increasing of the oxygen pressure. It is also found that the (200) and (111) preferred Cu2O films can be modified by changing substrate temperature. The formation of Cu2O and CuO films are further identified by Fourier transform infrared spectroscopy. For the Cu2O films, X-ray photoelectron spectroscopic studies indicate the presence of CuO on the surface. In addition, the optical gaps of Cu2O and CuO films have been determined by measuring the transmittance and reflectance spectra.
Transparent conducting materials are key elements in a wide variety of current technologies including flat panel displays, photovoltaics, organic, low-e windows and electrochromics. The needs for new and improved materials is pressing, because the existing materials do not have the performance levels to meet the ever- increasing demand, and because some of the current materials used may not be viable in the future. In addition, the field of transparent conductors has gone through dramatic changes in the last 5-7 years with new materials being identified, new applications and new people in the field. “Handbook of Transparent Conductors” presents transparent conductors in a historical perspective, provides current applications as well as insights into the future of the devices. It is a comprehensive reference, and represents the most current resource on the subject.
Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.
Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.
To investigate the effect of RF power on the structural, optical and electrical properties of amorphous InGaZnO (a-IGZO), its thin films and TFTs were prepared by RF magnetron sputtering method with different RF power conditions of 40, 80 and 120 W at room temperature. In this study, as RF power during the deposition process increases, the RMS roughness of a-IGZO films increased from 0.26 mn to 1.09 nm, while the optical band-gap decreased from 3.28 eV to 3.04 eV. In the case of the electrical characteristics of a-IGZO TFTs, the saturation mobility increased from 7.3 cm2/Vs to 17.0 cm2/Vs, but the threshold voltage decreased from 5.9 V to 3.9 V with increasing RF power. It is regarded that the increment of RF power increases the carrier concentration of the a-IGZO semiconductor layer due to the higher generation of oxygen vacancies.
Transparent thin-film transistors (TTFTs) with amorphous InGaZnO (a-IGZO) channels have been prepared by using a combinatorial thin-film synthesis approach. The In composition ratio, In(%) [= In×100/In+Zn+Ga(%)]' of the channel of the TTFTs varied with position from 50 % to 85 %. This allowed us to study TTFTs characteristics as a function of channel composition. The mobility, the on-off ratio and the threshold voltage of the TTFTs varied from 0.2 cm/V-s to 13 cm/V-s, 3 × 10 0 to 8 × 10 7 and -24 to 20 V, respectively. The TTFT with In(%) = 60 % showed a mobility as high as 13 cm/V-s with an on-off ratio of 5.8 × 10 7 and a threshold voltage of 14 V.
This chapter gives an overview about GIZO TFTs, comprising an introductory section about generic TFT structure and operation, different semiconductor technologies for TFTs – with special emphasis on AOSs and particularly on GIZO – and then some experimental results obtained for GIZO TFTs fabricated in CENIMAT. Thin-film transistors (TFTs) are important electronic devices which are predominantly used as On/Off switches in active matrix backplanes of flat panel displays (FPDs), namely liquid crystal displays (LCDs) and organic light emitting device (OLED) displays. Even if a-Si:H is still dominating the TFT market in terms of semiconductor technology, oxide semiconductors are emerging as one of the most promising alternatives for the next generation of TFTs, bringing the possibility of having fully transparent devices, low processing temperature, low cost, high performance and electrically stable properties [1, 2]. Amorphous oxide semiconductors (AOS) such as Gallium–Indium–Zinc oxide (GIZO) [3, 4], even if fabricated at temperatures below 150∘C, are currently capable of providing transistors with field-effect mobility (μFE) exceeding \(20\,{\mathrm{cm}}^{2}\,{\mathrm{V}}^{-1}\,{\mathrm{s}}^{-1}\), threshold voltage (V
T) close to 0 V, On/Off ratios above 108, subthreshold swing (S) around 0. 20 V dec−1 and fully recoverable V
T shift (ΔV
T) lower than 0.5 V after 24 h stress with constant drain current of 10 μA.
The microstructure of nanometer-scale SnO anode materials (100 nm) used in lithium rechargeable batteries has been investigated using high-resolution transmission electron microscopy (HRTEM) at deep discharge states of a Li/SnO cell. A passivating film formed by an irreversible electrolyte decomposition reaction on the surfaces of SnO particles was observed. The composition of the film was analyzed by Fourier transform infrared spectroscopy and determined to consist of Li2CO3 and ROCO2Li. In the core regions of the particles, many 1-3 nm crystallites dispersed in an amorphous matrix, were observed and identified to be a mixture of Li-Sn alloy and lithium metal by HRTEM. This provides direct evidence for the so-called two-step reaction mechanism for tin oxide anodes in lithium rechargeable batteries. © 1998 The Electrochemical Society. S1099-0062(98)06-031-3. All rights reserved.
We have successfully fabricated a full-bit shift register with low temperature amorphous indium gallium zinc oxide (a-IGZO) technology. Bottom-gate, staggered a-IGZO thin film transistors (TFTs) are fabricated with radio frequency-sputtered IGZO thin films with plasma-enhanced chemical vapor deposition deposited SiO2 as a gate dielectric and sputtered Mo as source, drain, and gate electrodes. The a-IGZO TFTs provide a good scalability and present field effect mobility in the range of 9-11 cm(2)/Vs. Integrated a-IGZO full-bit shift register consists of only 12 transistors per stage and operates at rail to rail voltage of about 18V. Stable full-bit shifting operation at a maximum operating frequency of about 25 kHz is confirmed.
Instead of the conventional furnace annealing process with a temperature higher than 300 degrees C, two low temperature annealing methods are successfully demonstrated to suppress the instability problem of amorphous indium gallium zinc oxide (IGZO) thin film transistors (TFTs). With adequate Nd:yttrium aluminum garnet laser (266 nm) annealing energy density or Xe excimer UV lamp (172 nm) irradiation time, the on voltage shift is greatly suppressed from over 10 to 0.1 V. The influence of laser energy density and UV lamp irradiation time on the performance of IGZO TFTs is also investigated and explained. The proposed methods are promising for the development of amorphous IGZO TFTs on flexible substrates.
Amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs) with methyl-siloxane based gate dielectric were fabricated on glossy paper substrate at low processing temperature (≤150°C). Glossy paper planarized by acrylate polymer was introduced as a new low cost flexible substrate and its rms surface roughness of 1 nm was comparable to commercial glass substrate. Compared to low temperature (≤150°C) silicon dioxide dielectric, a-IGZO TFTs using a methyl-siloxane based dielectric on the paper substrate demonstrated excellent performances with field effect mobility of ∼20 cm 2 V -1 s -1, onoff current ratio of ∼10 6, and low leakage current, which show the enormous potential for flexible electronics application.
Copper oxide thin films produced by rf magnetron sputtering on glass substrates have been characterised by XRD, SEM, AFM, spectrophotometer and four-point probe measurements. It is shown that process parameters can be varied to produce films of varying compositions, and optical and electrical properties. Depositions at 200 W power and an oxygen flowrate of 2 sccm in particular are found to favour the formation of single phase cuprous oxide. The optical and electrical properties of the films prepared under the stated conditions are of potential interest for application in semiconducting materials.
Thin-film transistors (TFTs) with amorphous indium gallium zinc oxide (a-IGZO) active layers were fabricated using radio-frequency sputter deposition with variable dc substrate bias. High substrate bias sputtered a-IGZO TFTs produce more carriers in an a-IGZO layer and result in a negative gate threshold voltage (VT) shift, an increase in the field-effect mobility (μFE), and an increase in the off-state current (Ioff). The subthreshold gate swing (S) is degraded with an increased substrate bias. The different TFT characteristics as a function of substrate bias can be attributed to energetic ion bombardment during a-IGZO growth.
This paper investigates the environmental effects and related adsorbent species reactions on sol-gel derived amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs). The discrepancy between device characteristics measured in atmospheric and vacuum conditions was clarified through experiments with thermal annealing and different gas partial pressures. The measurement of a-IGZO TFTs in simulated water vapor environment was also utilized. We verified that the adsorbed water originating from the surrounding atmosphere can cause an increase in off-current and also enhance more oxygen molecule adsorption on the exposed back-channel surface, leading to more serious degradation in on-current.
The calculation of the molecular flux of sputtered indium gallium zinc oxide (IGZO) and oxygen during IGZO growth is used to generalize the optimum synthesis conditions for thin film transistors (TFTs). TFTs fabricated with an O(2) and IGZO incorporation ratio approximate to 1 demonstrated the best transfer curve with the resistivity of similar to 5 x 10(3) Omega cm at similar to 1 mTorr of oxygen partial pressure. We demonstrate that the resistivity can be systematically varied almost 7 orders of magnitude by varying the oxygen partial pressure during sputtering. The relative Fermi level position was estimated as a function of the O(2)/IGZO incorporation ratio. It is demonstrated that the resistivity curve with O(2)/IGZO incorporation ratio can be used as a powerful tool to determine the IGZO sputtering conditions for optimum TFT fabrication. Furthermore, the proposed oxygen incorporation calculation method is applied to subsequent passivation layer sputtering; when the oxygen incorporation rate during SiO(2) passivation sputtering is increased to a point that is higher than the amorphous IGZO incorporation rate during IGZO sputtering, the leakage current via the back channel is noticeably reduced.
The original influence of water on the back-channel of sol-gel derived amorphous indium-gallium-zinc-oxide thin film transistors was studied in various relative humidity environments. As humidity increased from 0 to 80%, the mobility increased from 0.22 to 0.24 cm(2)/(V s), threshold voltage decreased from 6.6 to 4.4 V, and subthreshold swing changed from 0.77 to 1.27 V/dec. The conflicting phenomenon among the three parameters was suggested to be due to a division of the gate voltage by the water molecules which adsorbed on the thin film transistor back-channel and acted as dipoles.
We have developed a high performance, transparent bottom gate inkjet printed zinc tin oxide (ZTO) thin-film transistor (TFT) at the maximum process temperature of 300°C. The transparent ZTO TFT inkjet printed at the substrate temperature of 90°C exhibited a field-effect mobility of 1.8 cm 2 V-1 s-1, on/off ratio of ∼ 107, and gate swing of 288 mV/dec. The evolution of the threshold voltage with bias-stress time follows a stretched exponential behavior, indicating that the shift is mainly due to the carrier trapping at the interface region. The high performance and good stability appear to be due to the presence of Cl in the ZTO remained from the precursors.
This paper presents an RFID chip for 13.56-MHz band communication fabricated on a glass substrate by using amorphous In-Ga-Zn-O thin-film transistors. Low driving-voltage logic circuits were achieved with a small Vth, a high field effect mobility of 15cm2/Vs and “active load” inverters that had small consumption currents. The RFID tag was successively driven by 13.56-MHz wireless input.
Hole current was directly observed in oxide semiconductors under illumination. Two stacks were fabricated, with In-Ga-Zn-O (IGZO) as the channel and SiO(2) or SiN(x) as the gate insulator. X-ray photoelectron spectroscopy confirmed the IGZO/SiN(x) interface has no hole barrier while the IGZO/SiO(2) interface has a significant one. This was used to analyze hole generation in IGZO by illuminating light at various wavelengths. As a result, a threshold wavelength, where holes start to emerge in the channel, was found to exist. Negative bias-illumination-temperature stress experiments showed that no additional threshold voltage shift exists at wavelengths longer than this threshold wavelength.
Thin-film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin-film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.
— Zinc oxide (ZnO) and indium gallium zinc oxide (IGZO) thin films subjected to laser irradiation were investigated. The structural, optical, and electrical properties of the as-deposited and laser-irradiated films at different laser dosages were studied. The crystallinity of the structure increased after laser treatment. The transmittances without/with laser irradiation had a net rise of 85–92% and 80–95% (@550 nm) for 250-nm ZnO and IGZO films, respectively. Thin-film transistors (TFTs) with ZnO and IGZO as the active layer were fabricated. The as-deposited ZnO/IGZO TFT devices had a field-effect mobility of 0.19 and 1.3 cm2/V-sec, respectively. The electrical characteristics increased by more than 2.8 times for ZnO and by 5.8 times for IGZO with laser treatment. The field-effect mobility of ZnO and IGZO are 0.5 and 7.65 cm2/V-sec.
— Positive-current-bias (PB) instability and negative-bias—light-illumination (NBL) instability in amorphous-In—Ga—Zn—O (a-IGZO) thin-film transistors (TFTs) have been examined. The channel- thickness dependence indicated that the Vth instability caused by the PB stress is primarily attributed to defects in the bulk a-IGZO region for unannealed TFTs and to those in the channel—gate-insulator interface for wet-annealed TFTs. The interface and bulk defect densities (Dit and Nss, respectively) are Dit = 4.8 × 1011 cm−2/eV and Nss = 7.0×1016 cm−3/eV for the unannealed TFT, which increased to 5.2×1011 cm−2/eV and 9.8×1016 cm−3/eV, respectively, by the PB stress test. These are reduced significantly to Dit = 0.82×1011 cm−2/eV and Nss = 3.2×1016 cm−3/eV for the wet-annealed TFTs and are unchanged by the PB stress test. It was also found that the photo-response of a-IGZO TFTs begins at 2.3 eV of photon excitation, which corresponds to subgap states observed by photoemission spectroscopy. The origin of the NBL instability for the wet-annealed TFTs is attributed to interface effects and considered to be a trap of holes at the channel-gate—insulator interface where migration of the holes is enhanced by the electric field formed by the negative gate bias.
High-performance and excellent-uniformity thin-film transistors (TFTs) having bottom-gate structures are fabricated using an amorphous indium-gallium-zinc-oxide (IGZO) film and an amorphous-silicon dioxide film as the channel layer and the gate insulator layer, respectively. All of the 94 TFTs fabricated with an area 1 cm(2) show almost identical transfer characteristics: the average saturation mobility is 14.6 cm(2)/(V-sec) with a small standard deviation of 0.11 cm(2)/(V-sec). A five-stage ring-oscillator composed of these TFTs operates at 410 kHz at an input voltage of 18 V. Pixel-driving circuits based on these TFTs are also fabricated with organic light-emitting diodes (OLED) which are monolithically integrated on the same substrate. It is demonstrated that light emission from the OLED cells can be switched and modulated by a 120-Hz ac signal input. Amorphous-IGZO-based TFTs are prominent candidates for building blocks of large-area OLED-display electronics.
We review the features of amorphous In-Ga-Zn-O (a-IGZO) film transistors (TFTs), as well as circuit operation based on thin-these TFTs. We also report a novel TFT structure which improves environmental stability of the TFT operation by taking full advantage of the a-IGZO properties, where a conventional PECVD a-SiNx:H films serve not only as an effective barrier layer but also as a hydrogen source to form the coplanar source and drain.
Both zinc-oxide (ZnO) and gallium-indium-ZnO (IGZO) are attractive as semiconductors to replace hydrogenated amorphous silicon in flexible thin-film transistors (TFTs) due to their high charge carrier mobility and low deposition temperature. However, the electrical performance of flexible TFTs needs to be insensitive to mechanical bending. We have fabricated TFTs using ZnO and IGZO semiconducting layers on polyimide substrates and exposed TFTs to tensile bending radii down to 10 mm. While the mobility, threshold voltage, and subthreshold slope of IGZO TFTs remained essentially unchanged over the entire bending range, the electrical performance parameters of ZnO TFTs were strongly degraded by bending. For ZnO TFTs bent to a radius of 10 mm, the mobility decreased by more than two orders of magnitude, the threshold voltage increased by a factor of ~ 5, and the subthreshold slope increased by a factor of ~ 2. Our results show that IGZO should be the material of choice for robust flexible thin-film transistors. Experimental evidence points toward the formation of microcracks as the cause of ZnO sensitivity to bending.
— Amorphous-oxide thin-film-transistor (TFT) arrays have been developed as TFT backplanes for large-sized active-matrix organic light-emitting-diode (AMOLED) displays. An amorphous-IGZO (indium gallium zinc oxide) bottom-gate TFT with an etch-stop layer (ESL) delivered excel lent electrical performance with a field-effect mobility of 21 cm2/V-sec, an on/off ratio of >108, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous-oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold-voltage (VTH) compensation pixel circuits, especially in the negative state. A full-color 19-in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19-in. AMOLED display. The AMOLED display with a-IGZO TFT array is promising for large-sized TV because a-IGZO TFTs can provide a large-sized backplane with excellent uniformity and device reliability.
— Amorphous-oxide-semiconductor thin-film transistors (TFTs) have gained wide attention in recent years due to their many merits. In this paper, a series of top-gate transparent thin-film transistors (TFTs) based on amorphous-indium—gallium—zinc—oxide (a-IGZO) semiconductors have been fabricated and investigated. Specifically, low-temperature SiNx and SiOx were used as the gate insulator and different Ar/O2 gas-flow ratios were used for a-IGZO channel deposition to study the influences of gate insulators and channel-deposition conditions. In addition to the investigation of device performance, the stability of these TFTs was also examined by applying constant-current stressing. It was found that a high mobility of 30-45 cm2/V-sec and small threshold-voltage shift in constant-current stressing can be achieved using SiNx with suitable hydrogen-content stoichiometry as the gate insulator and the carefully adjusted Ar/O2 flow ratio for channel deposition. These results may be associated with hydrogen incorporation into the channel, the lower defect trap density, and the better water/oxygen barrier properties (impermeability) of the low-temperature SiNx.
Inorganic solids with wide bandgaps are usually classified as electrical insulators and are used in industry as insulators, dielectrics, and optical materials. Many metallic oxides have wide bandgaps because of the significant contribution of ionic character to the chemical bonds between metallic cations and oxide ions. Their ionic nature simultaneously suppresses the formation of easily ionizable shallow donors or acceptors and enhances the localization of electrons and positive holes. Thus it is understandable that interest in these wide-gap oxides as conductive materials has not been strong.
Identifying candidate materials to replace SiO2 as the gate dielectric for complementary metal oxide semiconductor (CMOS) applications is a difficult task. Proper assessment of the critical materials requirements is essential, and it is important to devise an approach to predict materials properties without having to make many unnecessary measurements on high-ĸ materials. Such an approach helps to eliminate unlikely candidates and focus on the most promising ones. Clearly, this type of modeling approach requires an understanding of several physical and chemical characteristics, including the bonding and electronic structure, band alignment with Si, and the nature of the dielectric constant and interface properties. We present a critical assessment of some existing methods and models of materials properties, as well as a comparison of the present modeling approach with some experimentally determined values.
Complementary use of p-type organic and n-type oxide semiconductors is
presented. First, we demonstrated complementary circuits using
low-voltage operating high performance pentacene and amorphous InGaZnO
(a-IGZO) thin-film transistors (TFTs). The field-effect mobilities of
the pentacene and a-IGZO transistors are 0.6 and 17.1 cm2/V
s, respectively at an operating voltage of 10 V. A complementary
inverter composed of these transistors exhibits good voltage transfer
characteristics with a high gain of ~56. A five-stage ring oscillator
with the inverters yields an output frequency of 200 Hz at 10 V,
corresponding to a propagation delay of 1 ms. Second, together with the
electrical device, we demonstrated an optoelectronic device,
light-emitting diodes (LEDs), using organic/oxide hybrid junctions. The
hybrid p-n junction LEDs are composed of
N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine
(α-NPD) and sputtered ZnO. Similar with conventional p-n junction
diodes, the hybrid junction shows a good current rectification and
electroluminescence (EL) under forward bias. We found that the EL bands
from the device agree well with the photoluminescence peaks from
α-NPD and ZnO, implying the radiative recombination of injected
charges occurs in both components of the junction.
The authors observed the photoluminescence (PL) spectra of amorphous InGaZnO (a-IGZO) for the first time. At liquid nitrogen temperature, even weak near-band-edge emission was clearly observed at a wavelength of ∼400 nm (3.1 eV) accompanied by a much stronger broad deep emission peaking at around 700 nm (1.77 eV) for 1-μm-thick samples deposited by sputtering on sapphire substrates at room temperature. The PL intensity of each emission strongly depends on the electron concentration of a-IGZO ranging from 1016 to 1018 cm-3. As the carrier concentration increased, the PL intensity of the broad deep emission decreased. The near-band-edge emission energy of 3.0 eV (413 nm) was in good agreement with the estimated absorption energy of 3.03–3.08 eV (403–409 nm) at 77 K. The depth profile of the carrier concentration of the a-IGZO layer was estimated using step-etching Hall measurements and was found to be uniform. The width of the depletion layer was determined by the film-thickness dependence of the sheet carrier concentration. If the substrate-side depletion layer is negligible, they estimated the upper limit of Vbi as ∼1.9 eV (653 nm), in the middle of the bandgap, when assuming an 11.5 value for permittivity.
The effect of heat treatment on the electrical properties of an
amorphous In-Ga-Zn-O (a-IGZO) film deposited by radio frequency (rf)
magnetron sputtering at room temperature (RT) was investigated as a
function of the oxygen flow rate. All of the films deposited with
increasing O2 concentration in the Ar and Ar + O2
atmospheres exhibited insulating behavior in terms of their electrical
properties, but their carrier concentration and resistivity were
markedly affected after annealing at 400 °C in either a vacuum
furnace or an rapid thermal annealing (RTA) system. However, the carrier
concentration and resistivity of the films annealed in a normal furnace
were not changed, compared with those of the as-deposited films annealed
in the vacuum furnace. The difference in the electrical properties as a
function of the annealing conditions was attributed to the number of
oxygen vacancies. From X-ray photoemission spectroscopy (XPS), we
confirmed that the formation of an O-rich surface was reduced by heat
treatment under vacuum conditions. The output and transfer
characteristics of the thin-film transistors (TFTs) exhibited excellent
performance as depletion-mode n-channel field-effect TFTs after being
annealed irrespective of the annealing system.
Transparent electronics is an emerging technology that employs wide band-gap semiconductors for the realization of invisible circuits. This monograph provides the first roadmap for transparent electronics, identifying where the field is, where it is going, and what needs to happen to move it forward. Although the central focus of this monograph involves transparent electronics, many of the materials, devices, circuits, and process-integration strategies discussed herein will be of great interest to researchers working in other emerging fields of optoelectronics and electronics involving printing, large areas, low cost, flexibility, wearability, and fashion and design. © 2008 Springer Science+Business Media, LLC. All rights reserved.
The present status and recent research results on amorphous oxide semiconductors (AOSs) and their thin-film transistors (TFTs) are reviewed. AOSs represented by amorphous In–Ga–Zn–O (a-IGZO) are expected to be the channel material of TFTs in next-generation flat-panel displays because a-IGZO TFTs satisfy almost all the requirements for organic light-emitting-diode displays, large and fast liquid crystal and three-dimensional (3D) displays, which cannot be satisfied using conventional silicon and organic TFTs. The major insights of this review are summarized as follows. (i) Most device issues, such as uniformity, long-term stability against bias stress and TFT performance, are solved for a-IGZO TFTs. (ii) A sixth-generation (6G) process is demonstrated for 32'' and 37'' displays. (iii) An 8G sputtering apparatus and a sputtering target have been developed. (iv) The important effect of deep subgap states on illumination instability is revealed. (v) Illumination instability under negative bias has been intensively studied, and some mechanisms are proposed. (vi) Degradation mechanisms are classified into back-channel effects, the creation of traps at an interface and in the gate insulator, and the creation of donor states in annealed a-IGZO TFTs by the Joule heating; the creation of bulk defects should also be considered in the case of unannealed a-IGZO TFTs. (vii) Dense passivation layers improve the stability and photoresponse and are necessary for practical applications. (viii) Sufficient knowledge of electronic structures and electron transport in a-IGZO has been accumulated to construct device simulation models.
A ZnO transparent thin-film transistor (TTFT) with a channel layer formed via spin-coating deposition is demonstrated. The TTFT is highly transparent and exhibits n-channel, enhancement-mode behaviour with a channel mobility as large as 0.20 cm2 V−1 s−1 and a drain current on-to-off ratio of nearly 107.
It is shown that the conductivity in the ohmic part of the cuprous oxide layer can be explained with the usual band picture of semiconductors only by assuming the presence of some donor-type impurities in addition to the usual acceptor type. The energy difference between the acceptors and the filled band is 0.3 electron volt, and the total number of impurity atoms is about 1014 to 1016 per cm3, the number of donors being less than but of the same order as the number of acceptors. Applying the Schottky theory of the space charge exhaustion layer, one finds from the dependence of capacity of the rectifier on bias voltage that the density of ion charge in the rectifying layer is of the same order of magnitude as the difference between the donors and acceptors found from the conductivity, thus furnishing a check for the theory. The field at the copper-cuprous oxide interface calculated from the space charge is about 2×104 volts/cm; the height of the potential at the surface as compared with the oxide interior is about 0.5 volt; and the thickness of the space charge layer about 5.0×10-5 cm. The diffusion equation for flow of current through this space charge region can be integrated to give the current in terms of the field at the interface and the applied potential across the space charge layer. Two currents are involved, one from the semiconductor to the metal (Is) and one from the metal to the semiconductor (Im) which is similar to a thermionic emission current into the semiconductor. The net current is, of course, I=Im-Is. One can get this "emission" current (Im) by dividing the true current by the factor 1-exp(-eVa/kT), where Va is the applied potential. This emission current depends on the absolute temperature and on the field at the copper-cuprous oxide interface. At high fields the logarithm of the current is proportional to the square root of the field, and at low fields the current decreases more rapidly indicating a patchy surface having small areas of low potential maximum from which all the emission comes when the field is large. This effective potential maximum measured from the Fermi level in the copper is about 0.5 ev, and the fraction of the total area effective ranges from 10-2 to 10-5 depending on how the rectifier was made. This last factor—the fraction of the area having this low potential maximum—is by far the most important variable, resulting in low reverse currents when the fraction is small and large reverse currents when the fraction is large.
Cited By (since 1996):27, Export Date: 7 October 2013, Source: Scopus
Cited By (since 1996):32, Export Date: 7 October 2013, Source: Scopus
We present a method to pattern solution-processed oxide semiconductor thin films by all laser process. A metal thin film is first photoetched by a spatially-modulated pulsed Nd–YAG laser beam and this layer is then covered with a semiconductor film. Uniform irradiation by the same laser generates a thermo-elastic force on the underlying metal layer and this force serves to detach it from the substrate, leaving only a patterned semiconductor structure. Sharp-edged zinc–tin oxide (ZTO) patterns at the micrometer scales could be fabricated over a few square centimeters by a single pulse of 850 mJ. A mobility of 7.6 × 10−2 cm2 V−1 s−1, an on/off ratio higher than 106, and an off-current of 1.91 × 10−11 A were achieved from a thin film transistor (TFT) with the patterned ZTO channel. These values were similar to those from a reference TFT, demonstrating the feasibility of this patterning process for electronic devices.
This study examined the characteristics of Ga:In2O3 (IGO) co-sputtered Zn:In2O3 (IZO) films prepared by dual target direct current (DC) magnetron sputtering at room temperature in a pure Ar atmosphere for transparent electrodes in IGZO-based TFTs. Electrical, optical, structural and surface properties of Ga and Zn co-doped In2O3 (IGZO) electrodes were investigated as a function of IGO and IZO target DC power during the co-sputtering process. Unlike semiconducting InGaZnO4 films, which were widely used as a channel layer in the oxide TFTs, the co-sputtered IGZO films showed a high transmittance (91.84%) and low resistivity (4.1 × 10− 4 Ω cm) at optimized DC power of the IGO and IZO targets, due to low atomic percent of Ga and Zn elements. Furthermore, the IGO co-sputtered IZO films showed a very smooth and featureless surface and an amorphous structure regardless of the IGO and IZO DC power due to the room temperature sputtering process. This indicates that co-sputtered IGZO films are a promising S/D electrode in the IGZO-based TFTs due to their low resistivity, high transmittance and same elements with channel InGaZnO4 layer.
A series of 0.2–0.6 μm thick SnOx films were deposited onto borosilicate and sodalime silica glass substrates by atmospheric plasma discharge chemical vapor deposition at 80 °C. SnOx films deposited from monobutyltin trichloride contained a large percentage of SnCl2:2H2O, and therefore were partially soluble in water. SnOx coatings deposited from tetrabutyltin were not soluble in water or organic solvents, had good adhesion even at growth rates as high as 2.3 nm/s, had high transparency of ∼ 90% and electrical resistivity of 107 Ω cm. As-grown tin oxide coatings were amorphous with a small concentration of SnO2, SnO and Sn crystalline phases as determined by grazing angle X-ray diffraction and X-ray photoelectron spectroscopy measurements. Upon annealing in air at 600 °C the resistivity of SnOx films decreased to 5–7 Ω cm. Furthermore, optical and X-ray measurements indicated that SnOx was converted into SnO2 (cassiterite) with a direct band gap of 3.66 eV. Annealing of as-grown SnOx films in vacuum at 340 °C led to formation of the p-type conductor SnO/SnOx. The indirect band gap of SnO was calculated from the optical spectra to be 0.3 eV.