Transparent Oxide Semiconductor Research Articles

Light Effect on Amorphous Tin Oxide Thin‐Film Transistors

Amorphous tin oxide (a‐SnOx) is a potential transparent oxide semiconductor candidate for future large‐area electronic applications. The thin‐film transistor (TFT) mobilities reach ≈100 cm2 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‐SnOx 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 × 1019 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 SnO2, 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 SnO2 and can explain the NBIS phenomenon observed in a‐SnOx TFT.

Cu-doped lithium oxide films with high mobility and bandgap prepared by pulsed direct-current sputtering

In this study, Cu-doped lithium carbonate (Cu:Li2O2) films are prepared by pulsed direct-current (DC) magnetron sputter using a lithium carbonate and copper oxide mixed target. The oxygen flow rate is varied during the sputtering process to systematically investigate its effect on structural, electrical and optical properties of the films. The experimental results show that all the Cu:Li2O2 films are mainly amorphous as confirmed by X-ray diffraction. Cu incorporation is able to reduce the aggregated clusters of the lithium oxides. At the oxygen flow rate of 20 sccm the film with a Cu content of ∼0.7 at.% has a low p-type resistivity of 0.82 Ω-cm, hole concentration of 1.1 × 1017 cm−3 and the highest mobility of 73 cm2V−1s−1, which is at least twice higher than those of other p-type metal oxides prepared at similarly low temperatures. The film also has a high optical band gap of 2.87 eV. Moreover, the Cu:Li2O2/ZnO heterojunction demonstrating a clear diode rectification supports the p-type nature of Cu:Li2O2 films. The pulsed DC Cu:Li2O2 films with these remarkable optoelectronic properties demonstrate great potential to be adopted as transparent oxide semiconductors in high-performance optoelectronics.

Facile diffusion of sulfur and fluorine into a-IGTO thin films for high-performance and reliability of transparent amorphous oxide semiconductor thin-film transistors

Facile diffusion of sulfur and fluorine into a-IGTO thin films for high-performance and reliability of transparent amorphous oxide semiconductor thin-film transistors

Fully Deep‐UV Transparent Thin Film Transistors Based on SrSnO3

AbstractUltra‐wide bandgap semiconductors are gaining attention for their promising properties for UV optoelectronics and UV transparent electronics as well as high‐power applications. Among them, La‐doped SrSnO3 exhibits excellent properties both for deep‐UV transparent oxide semiconductors and deep‐UV transparent conducting oxide. Here, the demonstration of thin film transistors (TFTs) with full deep‐UV transparency is reported, including electrodes, gate oxide, and substrate. The lightly La‐doped SrSnO3 for the channel layer is grown on MgO (100) substrates with buffer layers by pulsed laser deposition. TFTs with a metal—insulator–semiconductor structure are fabricated using high‐k perovskite dielectric LaScO3 as the gate oxide. A degenerately La‐doped SrSnO3 is used as the gate, the source, and the drain electrodes to obtain good ohmic contact with the channel layer as well as UV transparency. The resultant device shows a field effect mobility value of ≈24 cm2 V−1 s−1 and an on/off ratio >106. The optical transmittance of the entire device (including the substrate) is found to be >75% at 300 nm in wavelength. Furthermore, the electrical characteristics of the device exhibit excellent stability under visible irradiation. This research highlights the potential of SrSnO3 in advancing the field of UV optoelectronics and UV transparent electronics.

Low-Temperature Processed Complementary Inverter With Tin-Based Transparent Oxide Semiconductors

Low-Temperature Processed Complementary Inverter With Tin-Based Transparent Oxide Semiconductors

Transparent and highly conductive La-doped strontium stannate thin film growth by PI-MOCVD method on LaAlO3 and MgO substrates of (100), (110), (111) orientations

Wide band gap (4.6 eV) lanthanum doped SrSnO3 semiconductors of high electron mobility have potential applications in transparent oxide semiconductor technologies. However, there is a lack of data on the electrical properties of La:SrSnO3 both in single crystals and in thin films with an orientation other than (001). We show that after applying a modified metalorganic chemical vapor deposition method it is possible to deposit relaxed thick (001), (110) and (111) oriented La:SrSnO3 films on LaAlO3 and MgO substrates. Thin films on LaAlO3 substrates exhibit comparatively high charge carrier mobility values: µ100=71 cm2V−1s−1, µ110=72 cm2V−1s−1 and µ111=67 cm2V−1s−1. In this article, using the results of X-ray diffraction and crystallite orientation distribution modeling, as well as scanning and transmission electron microscopy data, the link between the obtained charge carrier mobility (µ100 ≈ µ110 > µ111) and film's microstructure and morphology is discussed.

Improved Electron Transport Properties of Zn-Rich In–Ga–Zn–O Thin-Film Transistors

Amorphous transparent oxide semiconductor InGaZnO4 (IGZO)-based thin-film transistors (TFTs) have been practically used as the backplane of flat panel displays. For future higher-definition displays, alternative active materials with a higher field effect mobility (μFE) are necessary. Although there are a few reports on InGaO3(ZnO)m with Zn-rich composition (IGZOm)-based TFTs, their electron transport properties have not been clarified. Here, we show that a Zn-rich composition enhances the electron transport properties of IGZOm-TFTs. The best TFT performance was obtained for m = 7 (μFE ∼12 cm2 V–1 s–1, subthreshold swing ∼0.1 V decade–1, and a negligibly small bias stress shift). The carrier effective mass (m*) of IGZOm films was found to be 0.16 m0, independent of the m-value. We found that μFE of IGZOm-TFT increased with the m-value for m ≤ 7, whereas it decreased for m > 7 due to the crystallization. The thermopower modulation analyses revealed that the effective channel thickness increased with m (m ≤ 7), which resulted in a longer carrier relaxation time. The present results provide an improving strategy toward new material design for next-generation TFTs with higher μFE values.

Post-annealing optimization of the heteroepitaxial La-doped SrSnO3 integrated on silicon via ALD.

Wide band gap (WBG) alkaline-earth stannate transparent oxide semiconductors (TOSs) have attracted increasing attention in recent years for their high carrier mobility and outstanding optoelectronic properties, and have been applied widely in various devices, such as flat-panel displays. Most alkaline-earth stannates are grown by molecular beam epitaxy (MBE); there are some intractable issues with the tin source including the volatility with SnO and Sn sources and the decomposition of the SnO2 source. In contrast, atomic layer deposition (ALD) serves as an ideal technique for the growth of complex stannate perovskites with precise stoichiometry control and tunable thickness at the atomic scale. Herein, we report the La-SrSnO3/BaTiO3 perovskite heterostructure heterogeneously integrated on Si (001), which uses ALD-grown La-doped SrSnO3 (LSSO) as a channel material and MBE-grown BaTiO3 (BTO) as a dielectric material. The reflective high-energy electron diffraction and X-ray diffraction results indicate the crystallinity of each epitaxial layer with a full width at half maximum (FWHM) of 0.62°. In situ X-ray photoelectron spectroscopy results confirm that there was no Sn0 state in ALD-deposited LSSO. Besides, we report a strategy for the post-treatment of LSSO/BTO perovskite heterostructures by controlling the oxygen annealing temperature and time, with a maximum oxide capacitance Cox of 0.31 μF cm-2 and a minimum low-frequency dispersion for the devices with 7 h oxygen annealing at 400 °C. The enhancement of capacitance properties is primarily attributed to a decrease of oxygen vacancies in the films and interface defects in the heterostructure interfaces during an additional ex situ excess oxygen annealing. This work expands current optimization methods for reducing defects in epitaxial LSSO/BTO perovskite heterostructures and shows that excess oxygen annealing is a powerful tool for enhancing the capacitance properties of LSSO/BTO heterostructures.

Wiley‐SID Volume Delivers Latest Developments in Transparent Oxide Semiconductor Technology

AMORPHOUS OXIDE SEMIconductors (AOS) are a class of electronic materials that has seen significant rapid developments. Their success in the market is secured by the synergistic effects arising between favorable material properties and the ability to control them, a good understanding of the underlying properties that govern the electronic behavior, comparatively straightforward and economical processing, and a receptive audience in the information display community. These materials have helped satisfy the need for lower power consumption, higher brightness, faster refresh rates, and higher resolution. It is not surprising that they have rapidly established a strong position in the emerging field of high-performance portable and large-area displays. With the pace of this technical innovation showing no sign of wavering, pioneering researchers and editors Hideo Hosono and Hideya Kumomi have assembled contributions from more than 50 experts to produce a comprehensive reference text, Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory, in the Wiley-SID book series. The book's subtitle hints at the large family of highly versatile properties and focuses primarily on displays and memory. It emphasizes current leading applications, while also featuring a discussion of broader uses where it is most relevant or instructive. Structured in eight parts, the book begins with chapters that provide a valuable overview of the history, physical and electronic structures of oxide semiconductors, along with their properties in comparison to silicon and the need for applications. A major section follows on fundamentals covering material properties and structure, defects, doping, and conduction. This solid foundation provides an essential understanding for the sections that follow: informative chapters on processing, with a focus on synthesis, solution processing for flexible electronics, and recent advances in atomic layer deposition with applications for thin-film transistors (TFTs) using a variety of active materials. This segues into a handful of chapters on all aspects of transistor design and usage: treating high-performance bulk accumulation and elevated-metal TFT architectures, the origin and control of instabilities, modeling and simulation, and matters concerning oxide TFT applicability to flexible electronics and signal-processing arrays. The final two parts cover state-of-the-art applications and new materials. Applications focus mostly on displays, including those realized with printed techniques, imagers, and memory. These are complemented by chapters on backend line integration of oxide TFTs and neuromorphic chips using AOS devices, which introduce a wide range of future and emerging applications and techniques that lie outside of traditional areas. The section on materials covers light-emitting diodes, electroluminescent displays, and vertical-cavity surface-emitting lasers, as well as high-mobility AOS and progress in p-type materials for TFTs. The broad range of activity presented in both materials and applications is encouraging and confirms that where some applications have reached full technical maturity, there is significant room for development as well as an interest in broader uses. A careful examination of the sections revealed a measure of overlap in information among different chapters. However, given the breadth of applications covered, a degree of repetition on the basics of material and transport properties is unavoidable. For students or those less experienced in the field, it presents an opportunity to assimilate major themes through the viewpoints of several expert authors. In this regard, the book does an excellent job of reinforcing and subtly complementing the critical notions, repeatedly relating materials, process conditions, and structure to electrical performance. Most chapters have ample and relevant reference lists, including numerous milestone publications that will provide useful starting points for future literature searches as the field evolves. AOS is demonstrably successful and highly promising in diverse applications. Processing versatility allows for the realization of flexible electronic systems, and recent advances in controlling layers and interfaces are yielding highly efficient and low-operating-voltage devices. The editors demonstrate that the range of AOS applications likely will expand significantly, not only in the stated areas of displays and memories, but promisingly beyond. The book serves as a valuable reference for traditional applications and as a landmark in the development of these versatile electronic materials. The text is easy to navigate and rich in detailed information, which should satisfy a wide readership in both academia and industry at various levels of experience. Radu A. Sporea is a lecturer in power electronics and semiconductor devices at the Advanced Technology Institute, University of Surrey.

(Invited, Digital Presentation) Nondegenerate Hydrogen-Doped Polycrystalline Indium Oxide (InOx:H) Thin Films for High-Mobility Thin Film Transistors

Transparent metal oxide semiconductors (OSs) have been extensively investigated for use as the active channel layer of thin film transistors (TFTs) for next-generation flat-panel displays, nonvolatile memories, image sensors, and pH sensors, to name a few. Among OSs, the amorphous In–Ga–Zn–O (IGZO) has attracted particular attention for TFT applications owing to its high field effect mobility (μFE) of more than 10 cm2V−1s−1, steep subthreshold swing (S.S.), extremely low off-state current, large-area uniformity, and good bias stress stability. Although the μFE of an IGZO TFT is approximately one order of magnitude higher than that of an amorphous Si TFT, further improvement of the μFE of OS TFTs is required to expand their range of applications as an alternative to polycrystalline Si TFT.Single-crystalline In2O3 has a Hall mobility as high as 160 cm2V−1s−1, which makes amorphous (a-) or polycrystalline (poly-) InOx a potential material for enhancing the μFE of OS TFTs. However, undoped InOx thin films is known as a degenerate semiconductor with high background electron density of over 1020 cm-3, which is attributed to the presence of native defects, such as oxygen vacancies, making them unsuitable for a channel material of OS TFTs. In this presentation, nondegenerate hydrogen-doped polycrystalline InOx (poly-InOx:H) thin films were successfully prepared by low-temperature solid phase crystallization (SPC). A degenerate amorphous InOx:H thin film was deposited by sputtering in Ar, O2, and H2 gases, and an amorphous to polycrystalline phase transition (SPC) of the film was achieved after PDA at more than 175 °C. By PDA at 250 °C in air, a nondegenerate poly-InOx:H film could be obtained with a carrier density as low as 2.4 × 1017 cm−3, which is approximately three orders of magnitude lower than that of the initial a-InOx:H film. The TFTs with a 50 nm thick nondegenerate poly-InOx:H channel could be fully depleted by a gate electric field. A maximum μFE of 125.7 cm2V−1s−1 was exhibited by the TFT with the poly-InOx:H channel. The use of a nondegenerate poly-InOx:H film is a promising approach to boost the μFE of OS TFTs.

(Digital Presentation) Multimodal Encapsulation of p-SnOx to Engineer the Carrier Density for Thin Film Transistor Applications

It has been challenging to synthesize p-type SnOx (1≤x<2) and engineer the electrical properties such as carrier density and mobility due to the narrow processing window and the localized oxygen 2p orbitals near the valence band.We recently reported on the processing of p-type SnOx and an oxide-based p-n heterostructures, demonstrating high on/off rectification ratio (>103), small turn-on voltage (<0.5 V), and low saturation current (~1×10-10 A)1. In order to further understand the p-type oxide and engineer the properties for various electronic device applications, it is important to identify (or establish) the dominating doping and transport mechanisms. The low dopability in p-type SnOx, of which the causation is also closely related to the narrow processing window, needs to be mitigated so that the electrical properties of the material are to be adequately engineered 2, 3.Herein, we report on the multifunctional encapsulation of p-SnOx to limit the surface adsorption of oxygen and selectively permeate hydrogen into the p-SnOx channel for thin film transistor (TFT) applications. Time-of-flight secondary ion mass spectrometry measurements identified that ultra-thin SiO2 as a multifunctional encapsulation layer effectively suppressed the oxygen adsorption on the back channel surface of p-SnOx and augmented hydrogen density across the entire thickness of the channel. Encapsulated p-SnOx-based TFTs demonstrated much-enhanced channel conductance modulation in response to the gate bias applied, featuring higher on-state current and lower off-state current. The relevance between the TFT performance and the effects of oxygen suppression and hydrogen permeation is discussed in regard to the intrinsic and extrinsic doping mechanisms. These results are supported by density-functional-theory calculations. Acknowledgement This work was supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088. S.L. and H.W.S. acknowledge the support from the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS. K.N. was supported by Basic Science Research Program (NRF-2021R11A1A01051246) through the NRF Korea funded by the Ministry of Education. References Lee, D. H.; Park, H.; Clevenger, M.; Kim, H.; Kim, C. S.; Liu, M.; Kim, G.; Song, H. W.; No, K.; Kim, S. Y.; Ko, D.-K.; Lucietto, A.; Park, H.; Lee, S., High-Performance Oxide-Based p–n Heterojunctions Integrating p-SnOx and n-InGaZnO. ACS Applied Materials & Interfaces 2021, 13 (46), 55676-55686.Hautier, G.; Miglio, A.; Ceder, G.; Rignanese, G.-M.; Gonze, X., Identification and design principles of low hole effective mass p-type transparent conducting oxides. Nat Commun 2013, 4.Yim, K.; Youn, Y.; Lee, M.; Yoo, D.; Lee, J.; Cho, S. H.; Han, S., Computational discovery of p-type transparent oxide semiconductors using hydrogen descriptor. npj Computational Materials 2018, 4 (1), 17. Figure 1

Light Absorption and Emission by Defects in Doped Nickel Oxide

Nickel oxide is a versatile p-type semiconducting oxide with many applications in optoelectronic devices, but high doping concentrations are often required to achieve necessary electrical conductivity. In contrast to many other transparent oxide semiconductors, even moderate doping levels in NiO can lead to significant optical absorption in the visible spectral range, limiting the application range of the material. This correlation has been reported extensively in the literature, but its origin has been unknown until now. This work combines experimental data on optical properties from a variety of NiO samples with results from hybrid density functional theory calculations. It shows that strong electron–phonon interaction leads to a significant blueshift (0.6–1 eV) of electronic transitions from the valence band maximum to defect states by light absorption with respect to the thermodynamic charge transition levels. This essentially renders NiO a narrow-gap semiconductor by defect band formation already at moderate doping levels, with strong light absorption for photon energies of approximately 1 eV. The calculations are also shown to be fully consistent with experimental data on defect-related light emission in NiO.

Hydrogen Behavior at Crystalline/Amorphous Interface of Transparent Oxide Semiconductor and Its Effects on Carrier Transport and Crystallization.

The role of disorder and particularly of the interfacial region between crystalline and amorphous phases of indium oxide in the formation of hydrogen defects with covalent (In-OH) or ionic (In-H-In) bonding are investigated using ab initio molecular dynamics and hybrid density-functional approaches. The results reveal that disorder stabilizes In-H-In defects even in the stoichiometric amorphous oxide and also promotes the formation of deep electron traps adjacent to In-OH defects. Furthermore, below-room-temperature fluctuations help switch interfacial In-H-In into In-OH, creating a new deep state in the process. This H-defect transformation limits not only the number of free carriers but also the grain size, as observed experimentally in heavily H-doped sputtered In2Ox. On the other hand, the presence of In-OH helps break O2 defects, abundant in the disordered indium oxide, and thus contributes to faster crystallization rates. The divergent electronic properties of the ionic vs covalent H defects─passivation of undercoordinated In atoms vs creation of new deep electron traps, respectively─and the different behavior of the two types of H defects during crystallization suggest that the resulting macroscopic properties of H-doped indium oxide are governed by the relative concentrations of the In-H-In and In-OH defects. The microscopic understanding of the H defect formation and properties developed in this work serves as a foundation for future research efforts to find ways to control H species during deposition.

Solution processed thin-film transistors with bilayer channel consisting of ZnO nanoparticles and SnO2

Owing to their many intriguing properties, transparent metal oxide semiconductors have become the most attractive alternatives for silicon-based semiconductors widely used today and been widely investigated by researchers. However, most of high performance metal oxide thin film transistor (TFT) relied on vacuum processing equipment and expensive rare element indium in the manufacturing process, the need for developing low cost indium-free metal oxide TFT has become urgent. In this work, a bilayer TFT composed of ZnO nanoparticles (NPs)/SnO2 was prepared by solution method. In this device structure, SnO2 film can provide high conductivity and carrier mobility, while the ZnO NPs in the upper layer can modulate and control channel conductance and further protect SnO2 film from air environment. The obtained device exhibits the excellent performances including μFE of 22.9 cm2V−1s−1 and Ion/Ioff about 1 × 107, subthreshold swing (SS) value about 0.36 V dec−1, and excellent environmental stability have been effectively achieved. Furthermore, the performance enhancement mechanism for the bilayer transistor with the help of simulation and related experiments was further explored. The bilayer ZnO NPs/SnO2 TFT has great cost advantage compared with indium based metal oxide TFTs and exhibits excellent potential in low cost thin-film electronic devices.

Water-Processed Ultrathin Crystalline Indium-Boron-Oxide Channel for High-Performance Thin-Film Transistor Applications.

Thin-film transistors (TFTs) made of solution-processable transparent metal oxide semiconductors show great potential for use in emerging large-scale optoelectronics. However, current solution-processed metal oxide TFTs still suffer from relatively poor device performance, hindering their further advancement. In this work, we create a novel ultrathin crystalline indium–boron–oxide (In-B-O) channel layer for high-performance TFTs. We show that high-quality ultrathin (~10 nm) crystalline In-B-O with an atomically smooth nature (RMS: ~0.15 nm) could be grown from an aqueous solution via facile one-step spin-coating. The impacts of B doping on the physical, chemical and electrical properties of the In2O3 film are systematically investigated. The results show that B has large metal–oxide bond dissociation energy and high Lewis acid strength, which can suppress oxygen vacancy-/hydroxyl-related defects and alleviate dopant-induced carrier scattering, resulting in electrical performance improvement. The optimized In-B-O (10% B) TFTs based on SiO2/Si substrate demonstrate a mobility of ~8 cm2/(V s), an on/off current ratio of ~106 and a subthreshold swing of 0.86 V/dec. Furthermore, by introducing the water-processed high-K ZrO2 dielectric, the fully aqueous solution-grown In-B-O/ZrO2 TFTs exhibit excellent device performance, with a mobility of ~11 cm2/(V s), an on/off current of ~105, a subthreshold swing of 0.19 V/dec, a low operating voltage of 5 V and superior bias stress stability. Our research opens up new avenues for low-cost, large-area green oxide electronic devices with superior performance.

Optoelectronic properties of transparent oxide semiconductor ASnO3 (A = Ba, Sr, and Ca) epitaxial films and thin film transistors

The importance of transparent oxide semiconductors is growing immensely due to their unprecedented dual properties: high electrical conductivity and optical transparency. They have been widely used in many transparent electronics devices due to their excellent electronic properties. In this Review, we discuss our recent research progresses on transparent ASnO3 (A = Ba, Sr, and Ca)-based thin films and thin film transistors (TFT). Here, we have explored the underlying materials physics through the investigation of fundamental properties such as effective mass, effective channel thickness, carrier mobility, electrical characteristics, and optical properties. High Hall mobility and wide bandgap are the key deciding parameters to consider when choosing ASnO3 for transparent electronic applications. Thus, carrier mobility improvisation was also carried out via modifying thin film preparation conditions such as using the highly oxidative atmosphere, vacuum annealing, and increasing the film thickness. Furthermore, we clarified the operating mechanism of BaSnO3-SrSnO3 solid solution-based TFTs and succeeded in fabricating the deep-UV La-doped SrSnO3 TFTs, which has great potential in biological applications. We have also demonstrated that the optoelectronic properties ASnO3 may be controlled by changing the A-site, which is consistent with expectations from the lattice parameter changes. This Review provides better options for designing ASnO3-based transparent devices in future.

Effects of Microwave and Furnace Annealing for P-Type SnO Thin Film Material in Oxygen Ambient

Transparent conductive oxide (TCO) semiconductors are attracted considerable attention due to a wide range of applications, such as flat panel display (FPD), touch panels, solar cells, and other optoelectronic devices. Owing to the different carrier conduction paths between n-type and P-type TCOs, the n-type TCO used in TFTs usually have high Ion/Ioff current ratio (>107) and high electron mobility (>10 cm²/V·s), P-type TCO TFTs are both lower than that of n-type one. For complementary circuits design and applications, however, both P-type and n-type semiconductor materials are equally important. For SnO thin films, it is important to adjust the ratio of Sn2+ (SnO P-type) and Sn4+ (SnO₂ n-type) in order to modulate the electrical characteristics. In this investigation of post treatment for SnO thin films, both microwave annealing (MWA) and furnace annealing process with 0₂ ambient are studied. The results show that SnO thin films are optimized at 300 °C, 30 minutes furnace annealing, the P-type SnO/SnO₂ thin film shows surface mean roughness 0.168 nm, [Sn2+]/[Sn4+] ratio as 0.838, at least 80% transmittance between 380 nm-700 nm visible light. Withthe results, SnO can be even used to fabricate high performance P-type thin film transistors (TFTs) device for future applications.

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators.Graphic abstract

Heavy-ion irradiation assisted modification of p-type transparent CuAlO2 films: Electrical, optical and thermoelectric properties

Heavy-ion (HI) irradiation is a unique approach to induce defects in semiconductor films. The ion irradiation leaves continuous columnar tracks to create various defects and reduce film density, modulating the thermoelectric properties of semiconductor films. As a p-type transparent oxide semiconductor (TOS), the as-synthesized CuAlO2 films were irradiated with different doses of Fe10+ ions at room temperature using the 320 kV High Voltage Platform, and then post-annealed to improve the microstructure and thermoelectric properties. The irradiated CuAlO2 films, with 15 × 1013 ions cm−2, can achieve an electrical conductivity of 0.14 S cm−1, which is about 200 times larger than the unirradiated counterparts. The Seebeck coefficient reaches up to 189.80 and 208.58 μV K−1 at room temperature and 600 K, respectively. Therefore, a high power factor (PF) of 9.15 μW m−1K−2 is rendered by the irradiated CuAlO2 films, whereas the unirradiated films are not detected in the given measurement range. The improved thermoelectric performance is mainly due to the increased copper vacancies in irradiated films without significantly reducing the crystallinity. The current work presents a promising strategy to improve the thermoelectric performance of semiconductor films.

One-step epitaxy of high-mobility La-doped BaSnO3 films by high-pressure magnetron sputtering

As unique perovskite transparent oxide semiconductors, high-mobility La-doped BaSnO3 films have been successfully synthesized by molecular beam epitaxy and pulsed laser deposition. However, it remains a big challenge for magnetron sputtering, a widely applied technique suitable for large-scale fabrication, to grow high-mobility La-doped BaSnO3 films. In this work, we developed a method to synthesize high-mobility epitaxial La-doped BaSnO3 films (with mobility up to 121 cm2 V−1 s−1 at the carrier density of ∼4.0 × 1020 cm−3 at room temperature) directly on SrTiO3 single crystal substrates using high-pressure magnetron sputtering. The structural and electrical properties of La-doped BaSnO3 films were characterized by combined high-resolution x-ray diffraction, x-ray photoemission spectroscopy, and temperature-dependent electrical transport measurements. The room-temperature electron mobility of La-doped BaSnO3 films achieved in this work is two to four times higher than the reported values of the films grown by magnetron sputtering. Moreover, in the high carrier density range (n &amp;gt; 3 × 1020 cm−3), the electron mobility value of 121 cm2 V−1 s−1 achieved in our work is among the highest values for all reported doped BaSnO3 films. It is revealed that high argon pressure during sputtering plays a vital role in stabilizing the fully relaxed films and inducing oxygen vacancies, which facilitates high mobility at room temperature. Our work provides an easy and economical way to massively synthesize high-mobility transparent conducting films for transparent electronics.