Sputtering deposition of P-type SnO films with SnO₂ target in hydrogen-containing atmosphere

Tin/Tin Oxide Nanostructures: Formation, Application, and Atomic and Electronic Structure Peculiarities

For a very long period, tin was considered one of the most important metals for humans due to its easy access in nature and abundance of sources. In the past, tin was mainly used to make various utensils and weapons. Today, nanostructured tin and especially its oxide materials have been found to possess many characteristic physical and chemical properties that allow their use as functional materials in various fields such as energy storage, photocatalytic process, gas sensors, and solar cells. This review discusses current methods for the synthesis of Sn/SnO2 composite materials in form of powder or thin film, as well as the application of the most advanced characterization tools based on large-scale synchrotron radiation facilities to study their chemical composition and electronic features. In addition, the applications of Sn/SnO2 composites in various fields are presented in detail.

Facile synthesis of an organic/inorganic hybrid 2D structure tincone film by molecular layer deposition

Organic/inorganic hybrid tincone films were deposited by molecular layer deposition (MLD) using N,N'-tert-butyl-1,1-dimethylethylenediamine stannylene(II) as a precursor and hydroquinone (HQ) as an organic reactant. From previous studies it is known that SnO can be fabricated through a reaction with H₂O, which has low oxidizing power. Similarly, when combined with HQ having a bi-functional hydroxyl group, SnO-based 2D hybrid tincones can be produced. In most aromatic ring-based metalcones described in previous studies, graphitization by pyrolysis occurred during post-annealing. In this study of tincones fabricated with a divalent precursor after a vacuum post-annealing process, the structural rearrangement of the SnO and the benzene ring bonds proceeded to form a SnO-based hybrid 2D structure. The rearrangement of the resulting structure occurred through π-π stacking (without pyrolysis) of the benzene ring. To understand the mechanism of fabrication of 2D hybrid tincones by π-π stacking of the benzene ring and the increase of the crystallinity of SnO after the annealing process, the structural rearrangement was observed using X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), grazing-incidence wide-angle X-ray scattering (GIWAXS), and Raman spectroscopy. Thereafter, the design of the crystal structure was investigated.

Atomically Thin Tin Monoxide-Based p-Channel Thin-Film Transistor and a Low-Power Complementary Inverter

Atomically thin oxide semiconductors are significantly expected for next-generation cost-effective, energy-efficient electronics. A high-performance p-channel oxide thin-film transistor (TFT) was developed using an atomically thin p-type tin monoxide, SnO channel with a thickness of ∼1 nm, which was grown by a vacuum-free, solvent-free, metal-liquid printing process at low temperatures, as low as 250 °C in an ambient atmosphere. By performing oxygen-vacancy defect termination for the bulk-channel and back-channel surface of the ultrathin SnO channel, the presented p-channel SnO TFT exhibited good device performances with a reasonable TFT mobility of ∼0.47 cm2 V-1 s-1, a high on/off current ratio of ∼106, low off current of <10-12 A, and a subthreshold swing of ∼2.5 V decade-1, which was improved compared with the conventional p-channel SnO TFTs. We also fabricated metal-liquid printing-based n-channel oxide TFTs such as n-channel SnO2 and In2O3-TFTs and developed ultrathin-channel oxide-TFT-based low-power complementary inverter circuits with the developed p-channel SnO TFTs. The full swing of voltage-transfer characteristics with a voltage gain of ∼10 and a power dissipation of <4 nW for p-SnO/n-SnO2 and ∼120 and <2 nW for p-SnO/n-In2O3-CMOS inverters were successfully demonstrated.

Ultraviolet Light-Densified Oxide-Organic Self-Assembled Dielectrics: Processing Thin-Film Transistors at Room Temperature

Low-temperature, solution-processable, high-capacitance, and low-leakage gate dielectrics are of great interest for unconventional electronics. Here, we report a near room temperature ultraviolet densification (UVD) methodology for realizing high-performance organic-inorganic zirconia self-assembled nanodielectrics (UVD-ZrSANDs). These UVD-ZrSAND multilayers are grown from solution in ambient, densified by UV radiation, and characterized by X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and capacitance measurements. The resulting UVD-ZrSAND films exhibit large capacitances of >700 nF/cm² and low leakage current densities of <10^-7 A/cm², which rival or exceed those synthesized by traditional thermal methods. Both the p-type organic semiconductor pentacene and the n-type metal oxide semiconductor In₂O₃ were used to investigate UVD-ZrSANDs as the gate dielectric in thin-film transistors, affording mobilities of 0.58 and 26.21 cm²/(V s), respectively, at a low gate voltage of 2 V. These results represent a significant advance in fabricating ultra-thin high-performance dielectrics near room temperature and should facilitate their integration into diverse electronic technologies.

X-ray spectroscopy study on the electronic structure of Sn-added p-type SnO films

The electronic structure of the Sn-added p-type SnO thin film was examined using x-ray absorption spectroscopy (XAS). Sn was intentionally added to a pristine SnO film, and the film was annealed to form p-type SnO. Sn L₁- and L₃-edge XAS was used to examine the oxidation states of the Sn-added p-type SnO. Compared to the case of the reference SnO, the spectrum of the Sn-added SnO (after annealing) partly contained the lineshape for SnO₂, suggesting that the oxidation of Sn  +  SnO was progressed such that the film became preferably SnO₂  +  SnO rather than Sn  +  SnO₂. O K-edge XAS, x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) were also used to scrutinize the electronic structure. The direct bandgap of the annealed film was estimated to be ~3.6 eV, consistent with the reported SnO₂ bandgap, while that of the as-deposited Sn-added SnO was  <2.5 eV. The large bandgap after annealing suggests that the metallic Sn was no longer in existence and manifested the functionality of the annealed Sn  +  SnO as a p-type semiconductor.

Highly sensitive thermoelectric touch sensor based on p-type SnO x thin film

Here, the ability of using p-type tin oxide (SnO _x ) thin films as a thermal sensor has been investigated. Firstly, the thermoelectric performance was optimized by controlling the thickness of the SnO _x film from 60 up to 160 nm. A high Seebeck coefficient of +263 μV K^-1 and electrical conductivity of 4.1 × 10² (S m^-1) were achieved in a 60 nm thick SnO _x film, due to a compact nanostructured film and the absence of the Sn metallic phase, which was observed for the thicker SnO _x film leading to a typical thermoelectric transport properties of a n-type Sn film. Moreover, x-ray photoelectron spectroscopy revealed the co-existence of SnO (79.7%) and SnO₂ (20.3%) phases in the 60 nm thick SnO _x film, while the optical measurements revealed an indirect gap of 1.8 eV and a direct gap of 2.7 eV, respectively. The 60 nm-SnO _x thin film have been tested as a thermoelectric touch sensor, achieving a V_signal /V_noise  ≈ 20, with a rise time <1 s. Therefore, this work provides an efficient way for developing highly efficient thermal sensors with potential use in display technologies.

Inorganic "Conductive Glass" Approach to Rendering Mesoporous Metal-Organic Frameworks Electronically Conductive and Chemically Responsive

A representative mesoporous metal-organic-framework (MOF) material, NU-1000, has been rendered electronically conductive via a robust inorganic approach that permits retention of MOF crystallinity and porosity. The approach is based on condensed-phase grafting of molecular tin species onto the MOF nodes via irreversible reaction with hydroxyl and aqua ligands presented at the node surface, a self-limiting process termed solvothermal installation (of metal ions) in MOFs (SIM, a solution-phase analog of atomic layer deposition in MOFs). Treatment of the modified MOF with aerated steam at 120 °C converts the grafted tin molecules to tetratin(IV)oxy clusters, with the clusters being sited between insulating pairs of zirconia-like nodes (the zirconium component being key to endowing the parent material with requisite chemical and thermal stability). By introducing new O-H presenting ligands on the modified-MOF node, the high-temperature steam step additionally serves to reset the material to reactive form, thus enabling a second self-limiting tin-grafting step to be run (and after further steam treatment, enabling a third). Difference-envelop-density (DED) analyses of synchrotron-derived X-ray scattering data, with and without installed tin species, show that the clusters formed after one cycle are spatially isolated, but that repetitive SIM cycling adds metal and oxygen ions in a way that enshrouds nodes, links clusters, and yields continuous one-dimensional strands of oxy-tin(IV), oriented exclusively along the c axis of the MOF. Two-probe conductivity measurements show that the parent MOF and the version containing isolated oxy-tin(IV) clusters are electrically insulating, but that the versions featuring continuous strands show an electrical conductivity of 1.8 × 10^-7 S/cm after three Sn-SIM cycles. When combined with interdigitated microelectrodes, the solvent-free and conductive-glass-modified material (three Sn-SIM cycles) displays a substantial and persistent increase in electrical conductivity during exposure to 5% H₂, indicating a role for dissociated H₂ as an electronic dopant. The increase can be repetitively reversed by alternating H₂ with air, illustrating the ability of the conductive MOF to function as a resistive sensor for H₂ and suggesting further potential applications that may capitalize on the combination of high volumetric surface area, high mesoporosity, high chemical and thermal stability, and significant electrical conductivity.

Study on the Effect of Oxygen Defects on the Electrical and Optical Properties of Thin Films

SnO₂ thin films grown directly on the Si substrate had larger average grain sizes as the power intensity increased, but the average grain size of the SnO₂ thin films grown in oxygen atmosphere decreased as the power intensity increased. Hall measurement of pure SnO₂ thin films showed that the carrier density increased with increasing power. However, upon annealing the SnO₂ thin films, the carrier density decreased with increasing power owing to the formation of oxygen vacancies and the SiO₂ layer between the Si substrate and SnO₂ thin films. The photoluminescence (PL) of the SnO₂ thin film grown in the oxygen atmosphere changed, and it was affected by the oxygen defects at the surface and interfaces of the thin film.

Composition, Microstructure, and Electrical Performance of Sputtered SnO Thin Films for p-Type Oxide Semiconductor

p-Type SnO thin films were deposited on a Si substrate by a cosputtering process using ceramic SnO and metal Sn targets at room temperature without adding oxygen. By varying the dc sputtering power applied to the Sn target while maintaining a constant radio frequency power to the SnO target, the Sn/O ratio varied from 56:44 to 74:26 at the as-deposited state. After thermal annealing at 180 °C for 25 min under air atmosphere using a microwave annealing system, the films were crystallized into tetragonal SnO when the Sn/O ratio increased from 44:56 to 57:43. Notably, the metallic Sn remained when the Sn/O ratio was higher than 55:45 at an annealed state. When the ratio was lower than 55:45 at the annealed state, the incorporated Sn fully oxidized to SnO, making the films useful p-type semiconductors, whereas the films became metallic conductors at higher Sn/O ratios. At the Sn/O ratio of 55:45 at the annealed state, the film showed the highest Hall mobility of 8.8 cm² V^-1 s^-1 and a hole concentration of 5.4 × 10¹⁸ cm^-3. Interestingly, the electrical conduction behavior showed trap-mediated hopping when the Sn metal was cosputtered, whereas the single SnO film showed regular band conduction behavior. The residual stress effect could interpret such property variation originated from the sputtering power and postoxidation-induced volumetric effects. This report makes a critical contribution to the in-depth understanding of the composition-structure-property relationship of this technically important thin film material.

Extremely Sensitive Dependence of SnOx Film Properties on Sputtering Power

An extremely sensitive dependence of the electronic properties of SnO_x film on sputtering deposition power is discovered experimentally. The carrier transport sharply switches from n-type to p-type when the sputtering power increases by less than 2%. The best n-type carrier transport behavior is observed in thin-film transistors (TFTs) produced at a sputtering power just below a critical value (120 W). In contrast, at just above the critical sputtering power, the p-type behavior is found to be the best with the TFTs showing the highest on/off ratio of 1.79 × 10⁴ and the best subthreshold swing among all the sputtering powers that we have tested. A further increase in the sputtering power by only a few percent results in a drastic drop in on/off ratio by more than one order of magnitude. Scanning electron micrographs, x-ray diffraction spectra, x-ray photoelectron spectroscopy, as well as TFT output and transfer characteristics are analyzed. Our studies suggest that the sputtering power critically affects the stoichiometry of the SnO_x film.

Recent Developments in p-Type Oxide Semiconductor Materials and Devices

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

Highly sensitive and selective CO gas sensor based on a hydrophobic SnO2/CuO bilayer

CO gas sensing mechanism of SnO₂/CuO bilayer sensor.

Composition-dependent structural and electrical properties of p-type SnOx thin films prepared by reactive DC magnetron sputtering: effects of oxygen pressure and heat treatment

The composition-dependent structural and electrical properties of p-type SnOx films prepared by reactive DC sputtering at various oxygen partial pressures (P_O) and post-heat treatment temperatures (T_A) were investigated.

Interface Bonds Determined Gas-Sensing of SnO2-SnS2 Hybrids to Ammonia at Room Temperature

Unique gas-sensing properties of semiconducting hybrids that are mainly related to the heterogeneous interfaces have been considerably reported. However, the effect of heterogeneous interfaces on the gas-sensing properties is still unclear, which hinders the development of semiconducting hybrids in gas-sensing applications. In this work, SnO2-SnS2 hybrids were synthesized by the oxidation of SnS2 at 300 °C with different times and exhibited high response to NH3 at room temperature. With the increasing oxidation time, the relative concentration of interfacial Sn bonds, O-Sn-S, among the total Sn species of the SnO2-SnS2 hybrids increased first and then decreased. Interestingly, it can be found that the response of SnO2-SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3. Our results revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and the importance of the interfacial bonds, which offers guidance for the material design to develop hybrid-based sensors.

Metal oxides and metal salt nanostructures for hydrogen sulfide sensing: mechanism and sensing performance

Metal oxides and metal salt nanostructures for hydrogen sulfide sensing based on conductivity response.