P-type Oxide Thin-film Transistors Research Articles

One-volt oxide based complementary circuit

In low-power electronics, there is a substantial demand for high-performance p-type oxide thin-film transistors (TFTs) that are capable of efficient operation at low voltages. In this study, we employ anodization to form an aluminum oxide gate dielectric layer, enabling the fabrication of p-type tin oxide (SnO) TFTs that effectively operate at a low voltage of 1 V. Under optimal device fabrication conditions, the SnO TFT demonstrates an on/off current ratio exceeding 103 and a saturation mobility of 1.94 cm2 V−1 s−1 at 1 V operation. The optimal SnO TFT fabrication conditions are subsequently used to fabricate a complementary inverter, comprising a SnO TFT and an n-type indium gallium zinc oxide TFT, achieving a gain of up to 38 at a 1 V supply voltage. Notably, the inverter’s switching point voltage is finely tuned to the ideal value, precisely half of the supply voltage. This oxide-based complementary inverter showcases promising potential in low-power electronics.

Recent progress of the p-type oxide thin films for transistor applications: Nickel oxide, Tin oxide, and Copper oxide

Over the past decade, many research groups have been striving to develop high-performance p-type switching oxide materials for implementing complementary metal–oxide–semiconductor (CMOS) thin film devices. However, realizing p-type oxide thin film transistors (TFTs) whose electrical properties are comparable to n-type oxide TFTs has been challenging. This is because of inherent characteristics of p-type oxide materials such as the high formation energy of native acceptors and high hole effective mass caused by localized hole transport path. Developing a p-type oxide with a delocalized hole transport pathway and low hole formation energy is crucial for the production of CMOS circuits utilizing oxide thin films. NiO, SnO, and CuO<sub>x</sub> are being actively studied as candidate materials that satisfy these requirements. This review discusses the latest advances in the synthesis method of p-type binary oxide thin films and the approach for electrical performance enhancement.

Effects of Post-UV/Ozone Treatment on Electrical Characteristics of Solution-Processed Copper Oxide Thin-Film Transistors.

To realize oxide semiconductor-based complementary circuits and better transparent display applications, the electrical properties of p-type oxide semiconductors and the performance improvement of p-type oxide thin-film transistors (TFTs) are required. In this study, we report the effects of post-UV/ozone (O3) treatment on the structural and electrical characteristics of copper oxide (CuO) semiconductor films and the TFT performance. The CuO semiconductor films were fabricated using copper (II) acetate hydrate as a precursor material to solution processing and the UV/O3 treatment was performed as a post-treatment after the CuO film was fabricated. During the post-UV/O3 treatment for up to 13 min, the solution-processed CuO films exhibited no meaningful change in the surface morphology. On the other hand, analysis of the Raman and X-ray photoemission spectra of solution-processed CuO films revealed that the post-UV/O3 treatment induced compressive stress in the film and increased the composition concentration of Cu-O lattice bonding. In the post-UV/O3-treated CuO semiconductor layer, the Hall mobility increased significantly to approximately 280 cm2 V-1 s-1, and the conductivity increased to approximately 4.57 × 10-2 Ω-1 cm-1. Post-UV/O3-treated CuO TFTs also showed improved electrical properties compared to those of untreated CuO TFTs. The field-effect mobility of the post-UV/O3-treated CuO TFT increased to approximately 6.61 × 10-3 cm-2 V-1 s-1, and the on-off current ratio increased to approximately 3.51 × 103. These improvements in the electrical characteristics of CuO films and CuO TFTs can be understood through the suppression of weak bonding and structural defects between Cu and O bonds after post-UV/O3 treatment. The result demonstrates that the post-UV/O3 treatment can be a viable method to improve the performance of p-type oxide TFTs.

Low-temperature solution-processed nanoparticle-doped nickel oxide thin-film transistor

Developing p-type oxide thin-film transistors (TFTs) is an essential path for further application in complementary metal oxide semiconductor (CMOS) components. However, the inferior electrical performance of p-type MO TFTs compared to n-type TFTs remains an ongoing challenge. Herein, for the first time, a low temperature, facile material engineering approach by incorporating n-type nanoparticles (NPs) was proposed for preparing p-type transparent NiOx TFTs. The characteristics of thin films blending NPs and the electrical performances of TFTs were investigated. The field effect mobility of TFTs with doping was nearly 20 times higher than pristine TFTs without doping, which was mainly beneficial from the suitable band alignment between NPs and p-type oxide, the increasing Ni3+ oxidation state in NiOx, as well as the improved dielectric/semiconductor interface quality. Electrons from drain electrode injected into metal oxide turn accepted in NPs rather than being trapped in the dielectric/semiconductor interface due to a strong surface electron depletion effect of NPs. NPs with small particle size and appropriate concentration would promote continuous hole transport by electrons transferring and reducing the interface trap state. The facile material engineering strategy is a promising technique for preparing p-type transparent MO-TFTs at low temperature, which showed great potential to be applicable in CMOS circuits on flexible substrates.

Modulation of the Al/Cu2O Schottky Barrier Height for p-Type Oxide TFTs Using a Polyethylenimine Interlayer.

We introduced an organic interlayer into the Schottky contact interface to control the contact property. After inserting an 11-nm-thick polyethylenimine (PEI) interlayer between the aluminum (Al) source/drain electrode and the cuprous oxide (Cu2O) channel layer, the Cu2O thin-film transistors (TFTs) exhibited improved electrical characteristics compared with Cu2O TFTs without a PEI interlayer; the field-effect mobility improved from 0.02 to 0.12 cm2/V s, the subthreshold swing decreased from 14.82 to 7.34 V/dec, and the on/off current ratio increased from 2.43 × 102 to 1.47 × 103, respectively. Careful investigation of the contact interface between the source/drain electrode and the channel layer established that the performance improvements were caused by the formation of electric dipoles in the PEI interlayer. These electric dipoles reduced the Schottky barrier height by neutralizing the charges at the metal/oxide semiconductor interface, and the holes passed the reduced Schottky barrier by means of tunneling or thermionic injection. In this way, p-type oxide TFTs, which generally need a noble metal having a high work function as an electrode, were demonstrated with a low-work-function metal. As a basic application for logic circuits, a complementary inverter based on n-type indium-gallium-zinc oxide and p-type Cu2O TFTs was fabricated using only Al source/drain electrodes. This research achieved advances in low-cost circuit design by broadening the electrode metals available for the manufacture of p-type oxide semiconductor-based electronics.

Role of ALD Al2O3 Surface Passivation on the Performance of p-Type Cu2O Thin Film Transistors.

High-performance p-type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low hole mobility and high off-state currents. We fabricated p-type TFTs with a phase-pure polycrystalline Cu2O semiconductor channel grown by atomic layer deposition (ALD). The TFT switching characteristics were improved by applying a thin ALD Al2O3 passivation layer on the Cu2O channel, followed by vacuum annealing at 300 °C. Detailed characterization by transmission electron microscopy-energy dispersive X-ray analysis and X-ray photoelectron spectroscopy shows that the surface of Cu2O is reduced following Al2O3 deposition and indicates the formation of a 1-2 nm thick CuAlO2 interfacial layer. This, together with field-effect passivation caused by the high negative fixed charge of the ALD Al2O3, leads to an improvement in the TFT performance by reducing the density of deep trap states as well as by reducing the accumulation of electrons in the semiconducting layer in the device off-state.

(Invited) P-Type Thin Film Oxide Semiconductors for Thin Film Transistor-Based CMOS Logic

The development of products, and perhaps most notably displays, which rely on thin film transistors (TFTs) has been enabled by the availability of high quality, thin film semiconducting materials. Hydrogenated amorphous silicon (a-Si:H) was the first such material to meet the demands of the display industry. Other materials have since also gained importance including polycrystalline silicon, organic semiconductors and, most recently, oxide semiconductors. In this latter case, indium gallium zinc oxide (IGZO) has been successfully commercialised at scale. However, TFT circuits using any of these materials are based on NMOS or PMOS logic rather than CMOS preventing their use where low power consumption is essential such as display driving circuits, memories or logic as it has not proved to be possible to produce n-channel and p-channel TFTs with sufficiently similar performance characteristics in one material system. In the case of the thin film silicon-based materials, this is because the hole mobility is much lower than the electron mobility, whilst in organic semiconductors it is the p-type materials that are superior to the n-type. In this paper, the situation in oxide semiconductors is considered in detail.There are several oxide semiconductors which have been used to make excellent TFTs, perhaps most notably amorphous (a-)IGZO and amorphous zinc tin oxide (a-ZTO). Both of these materials are naturally n-type as their charge neutrality level is close to the conduction band edge. As a result, it has not proved to be possible to successfully introduce sufficient acceptor-type impurities in order to stably produce a p-type behaviour in these materials; completely different p-type thin film oxide semiconductors are therefore required. Cuprous oxide (Cu2O) is one such material, and this will be used as an exemplar in this work, although alternatives such as tin oxide (SnO) and nickel oxide (NiO) have also been reported.This move to alternative material systems presents other challenges. The conduction band minimum in n-type oxides is dominated by spatially extensive and spherically symmetric metal s-shell electron orbitals, and this allows an amorphous phase to be induced by stoichiometry control of oxide semiconductor which has no grain boundaries and is highly uniform but which retains a high electron mobility. However, the valence band maximum in the p-type materials is dominated by oxygen 2p-orbitals (sometimes with metal d-orbital hybridisation is found in Cu2O) which are highly directional. This means that any attempt to introduce an amorphous microstructure would result in a significant reduction in hole mobility compared with the crystalline state (it is this effect that is responsible for a-Si:H having an electron mobility that is 1,000 times lower than its crystalline counterpart).The types of deposition techniques (most commonly sputtering) that are used to produce thin film oxide semiconductors do not permit the production of single-crystalline thin film materials; nanocrystalline microstructures are created instead. This leads to the presence of grain boundaries, which are highly problematic as they contain a high density of defect states. As a result, they act as trap sites for holes and significantly reduce carrier mobility compared with the single crystal state. Some degree of mitigation is possible by controlling grain orientation during growth, and for Cu2O it is possible to select either the (100) or (111) orientation [1]. Grain size may also be increased and boundary defects decreased after growth by annealing [2]. The result is TFTs with a significantly lower field effect mobility (typically less than 1 cm2 V–1 s–1) than is found in the n-type oxide TFTs (typically over 10 cm2 V–1 s–1). Furthermore, these defective grain boundaries can allow the accumulation of electrons in the off-state leading to much higher off-state currents than would be expected [3]. As a result the switching ratio of p-type oxide TFTs is much lower (typically less than 103) than in equivalent n-type devices (>107).Despite these limitations, it is still possible to combine p-type and n-type oxide TFTs to make CMOS logic gates, but it is important to consider carefully the application requirements and the associated compromises that must be made in circuit design.The funding for this work by the Engineering and Physical Sciences Research Council through grant numbers EP/M013650/1, EP/P027032/1 and EP/L015455/1 is gratefully acknowledged.[1] Han, S. and Flewitt, A.J., Thin Solid Films, 704, 138000 (2020)[2] Han, S., et al., Appl. Phys. Lett., 109, 173502 (2016)[3] Han, S. and Flewitt, A.J., IEEE Electron Dev. Lett., 28, 1394 (2017)

Effects of P-Type SnOx Thin-Film Transistors with N₂ and O₂ Ambient Furnace Annealing.

Recently oxide-based thin-film transistors (TFTs) are investigated for emerging applications of the next generation display devices and other electronic circuits (Fortunato, E., et al., 2012. Oxide semiconductor thin-film transistors: A review of recent advances. Advanced Materials, 24, pp.2945-2986). Despite of the great success in n-type oxide semiconductors with high transparency and high field-effect mobility, high performance P-type oxide TFTs are so highly desired that complementary circuits can be realized with low power and high performance (Ou, W.C., et al., 2008. Anomalous P-channel amorphous oxide transistors based on tin oxide and their complementary circuits. Applied Physics Letters, 92, p.122113). There are some oxides such as SnO, CuO, Cu₂O and NiO are regarded as promising P-type semiconductor materials. In this investigation, tin oxide SnOx is fabricated to be active layer for TFTs device, and furnace annealing with several combinations of nitrogen and oxygen ambient is compared to enhance the electrical characteristics of P-type SnOx TFTs (Park, K.S., et al., 2009. High performance solution-processed and lithographically patterned zinc-tin oxide thin-film transistors with good operational stability. Electrochemical and Solid-State Lett., 12, pp.H256-H258). The results show that with N₂+O₂ ambient, 30 minutes furnace annealing, the P-type SnOx TFTs device shows better performance with mobility (μFE) 0.883 cm²/V · S, threshold voltage (VT) -4.63 V, subthreshold swing (SS) 1.15 V/decade, and Ion/Ioff ratio 1.01×10₃.

Progress and challenges in p-type oxide-based thin film transistors

AbstractTransparent electronics has attracted much attention and been widely studied for next-generation high-performance flat-panel display application in the past few years, because of its excellent electrical properties. In display application, thin film transistors (TFTs) play an important role as the basic units by controlling the pixels. Among them, oxide-based TFTs have become promising candidates and gradually replaced the conventional amorphous and polycrystalline silicon TFTs, due to high mobility, good transparency, excellent uniformity and low processing temperature. Even though n-type oxide TFTs have shown high device performance and been used in commercial display application, p-type oxide TFTs with the equal performance have been rarely reported. Hence, in this paper, recent progress and challenges in p-type oxide-based TFTs are reviewed. After a short introduction, the TFT device structure and operation are presented. Then, recent developments in p-type oxide TFTs are discussed in detail, with the emphasis on the potential p-type oxide candidates as copper oxide, tin oxide and nickel oxide. Moreover, miscellaneous applications of p-type oxide TFTs are also presented. Despite this, the performance of p-type oxide TFTs still lags behind, as compared with that of n-type counterparts. Thus, the current issues and challenges of p-type oxide TFTs are briefly discussed.

Polyol Reduction: A Low-Temperature Eco-Friendly Solution Process for p-Channel Copper Oxide-Based Transistors and Inverter Circuits.

An optimized polyol reduction method was proposed for p-type CuxO deposition using a spin-coating method at low temperatures. The film characterizations and the electrical properties of integrated thin-film transistors (TFTs) were investigated as a function of annealing temperature and polyol type. The CuxO TFTs derived from propylene glycol showed the optimal performance with an average field-effect hole mobility of 0.15 cm2 V-1 s-1, an on/off current ratio of ∼104, and a threshold voltage of -7 V at a low temperature of 220 °C. Considering few investigations on the operational/air stability of solution-processed p-type oxide TFTs, we carried out systematical studies and revealed different roles of O2 and H2O on the device performance. The low activation energy (0.16 eV) for hole transport and high voltage gain (37) of the composed complementary inverter highlight great potential for the construction of all oxide-based transparent flexible electronics and circuits.

Radiation-Tolerant p-Type SnO Thin-Film Transistors

This present work investigated the effects of proton-beam irradiation on SnO-based p-type oxide thin-film transistors (TFTs) for the first time. The experiments were performed using a 5-MeV proton beam with doses ranging from $10^{12}$ to $10^{14}\,\,\textsf {p}\cdot \textsf {cm}^{-2}$ . The experimental results showed that the transfer curves of the p-type SnO TFT rarely changed following the proton irradiation at every irradiation condition, indicating that the p-type SnO TFT could be potentially useful in implementing complementary-logic-based oxide TFT circuits operating in harsh environments. The insensitivity of current conduction paths to SnO lattice disorder was considered as a possible mechanism for the observed radiation tolerance of the p-type SnO TFTs.

Transparent Inorganic Copper Bromide (CuBr) p-Channel Transistors Synthesized From Solution at Room Temperature

Transparent polycrystalline p-channel copper bromide (CuBr) thin-film transistors (TFTs) are fabricated using a simple one-step spin coating at room temperature. Optimized CuBr TFTs operate in depletion mode with an on-voltage of 35 V and exhibit a hole mobility of 0.15 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> , high on/off current ratio ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> and cycling bias sweep/air stabilities. The performance is superior to that of most solution-processed p-type oxide TFTs, highlighting CuBr as a promising p-type candidate for nextgeneration printable transparent electronics.

Off-current reduction in p-type SnO thin film transistors

SnO is one of the few candidates for p-type oxide thin film transistors (TFTs) because it retains a reasonable high hole mobility in a nanocrystalline film. However, the high off-current of SnO TFT limits its usefulness. In this work, SnO TFTs were fabricated using thermal evaporation under ultra-high vacuum. In order to decrease the off-current in p-type SnO thin film transistors (TFTs), we used yttrium to reduce n-type minority charges in the channel. The on/off ratio of the TFT increases from 102 to 5 × 104 and the mobility of the TFT in the saturated regime reduces from 1.6 to 1.4 cm2/V s when the SnO channel is doped with 1 wt. % of Y. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy reveal that the reduction of SnO2 in the Y-doped SnO TFT channel is the main reason for the improvement in the TFT characteristics.

Inkjet-printed p-type nickel oxide thin-film transistor

High-performance inkjet-printed nickel oxide thin-film transistors (TFTs) with Al2O3 high-k dielectric have been fabricated using a sol-gel precursor ink. The “coffee ring” effect during the printing process was facilely restrained by modifying the viscosity of the ink to control the outward capillary flow. The impacts on the device performance was studied in detail in consideration of annealing temperature of the nickel oxide film and the properties of dielectric layer. The optimized switching ability of the device were achieved at an annealing temperature of 280 °C on a 50-nm-thick Al2O3 dielectric layer, with a hole mobility of 0.78 cm2/V·s, threshold voltage of −0.6 V and on/off current ratio of 5.3 × 104. The as-printed p-type oxide TFTs show potential application in low-cost, large-area complementary electronic devices.

Demonstration of high-performance p-type tin oxide thin-film transistors using argon-plasma surface treatments

In this work, we investigated the effects of low-temperature argon (Ar)-plasma surface treatments on the physical and chemical structures of p-type tin oxide thin-films and the electrical performance of p-type tin oxide thin-film transistors (TFTs). From the x-ray photoelectron spectroscopy measurement, we found that SnO was the dominant phase in the deposited tin oxide thin-film, and the Ar-plasma treatment partially transformed the tin oxide phase from SnO to SnO2 by oxidation. The resistivity of the tin oxide thin-film increased with the plasma-treatment time because of the reduced hole concentration. In addition, the root-mean-square roughness of the tin oxide thin-film decreased as the plasma-treatment time increased. The p-type oxide TFT with an Ar-plasma-treated tin oxide thin-film exhibited excellent electrical performance with a high current on–off ratio (5.2 × 106) and a low off-current (1.2 × 10−12 A), which demonstrates that the low-temperature Ar-plasma treatment is a simple and effective method for improving the electrical performance of p-type tin oxide TFTs.

Hole mobility modulation of solution-processed nickel oxide thin-film transistor based on high-k dielectric

Solution-processed p-type oxide semiconductors have recently attracted increasing interests for the applications in low-cost optoelectronic devices and low-power consumption complementary metal-oxide-semiconductor circuits. In this work, p-type nickel oxide (NiOx) thin films were prepared using low-temperature solution process and integrated as the channel layer in thin-film transistors (TFTs). The electrical properties of NiOx TFTs, together with the characteristics of NiOx thin films, were systematically investigated as a function of annealing temperature. By introducing aqueous high-k aluminum oxide (Al2O3) gate dielectric, the electrical performance of NiOx TFT was improved significantly compared with those based on SiO2 dielectric. Particularly, the hole mobility was found to be 60 times enhancement, quantitatively from 0.07 to 4.4 cm2/V s, which is mainly beneficial from the high areal capacitance of the Al2O3 dielectric and high-quality NiOx/Al2O3 interface. This simple solution-based method for producing p-type oxide TFTs is promising for next-generation oxide-based electronic applications.

ChemInform Abstract: Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

AbstractReview: 500 refs.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

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.

Oxide semiconductors: Order within the disorder

The effect is considered of order and disorder on the electrical and optical performance of ionic oxide semiconductors used to produce optoelectronic devices such as p–n heterojunction solar cells and thin-film transistors (TFTs). The results obtained show that p-type c-Si/a-IZO/poly-ZGO solar cells exhibit efficiencies above 14% in device areas of about 2.34 cm2, whereas amorphous oxide TFTs based on the Ga–Zn–Sn–O system demonstrate superior performance to the polycrystalline ZnO TFTs, with I ON/I OFF ratio exceeding 107, turn-on voltage below 1–2 V and saturation mobility above 25 cm2 V−1 s−1. In addition, preliminary data on a p-type oxide TFT based on the Zn–Cu–O system are presented.

P -channel thin-film transistor using p-type oxide semiconductor, SnO

This paper reports that among known p-type oxide semiconductors, tin monoxide (SnO) has a high hole mobility and produces good p-type oxide thin-film transistors (TFTs). Device-quality SnO films were grown epitaxially on (001) yttria-stabilized zirconia substrates at 575°C by pulsed laser deposition. These exhibited a Hall mobility of 2.4cm2V−1s−1 at room temperature. Top-gated TFTs, using epitaxial SnO channels, exhibited field-effect mobilities of 1.3cm2V−1s−1, on/off current ratios of ∼102, and threshold voltages of 4.8V.