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Zinc tin oxide (ZTO) thin films can be deposited by atomic layer deposition (ALD) with adjustable electrical, optical and structural properties. However, the ternary ALD processes usually suffer from low growth rate and difficulty in controlling film thickness and elemental composition, due to the interaction of ZnO and SnO2 processes. In this work, ZTO thin films with different Sn levels are prepared by ALD super cycles using diethylzinc, tetrakis(dimethylamido)tin, and water. It is observed that both the film growth rate and atom composition show nonlinear variation versus [Sn]/([Sn]+[Zn]) cycle ratio. The experimental thickness measured by spectroscopic ellipsometry and X-ray reflectivity are much lower than the expected thickness linearly interpolated from pure ZnO and SnOx films. The [Sn]/([Sn]+[Zn]) atom ratios estimated by X-ray photoelectron spectroscopy have higher values than that expected from the cycle ratios. Hence, to characterize the film growth behavior versus cycle ratio, a numerical method is proposed by simulating the effect of reduced density and reactivity of surface hydroxyls and surface etching reactions. The structure, electrical and optical properties of ZTO with different Sn levels are also examined by X-ray diffraction, atomic force microscope, Hall measurements and ultraviolet–visible–infrared transmittance spectroscopy. The ZTO turns out to be transparent nanocrystalline or amorphous films with smooth surface. With more Sn contents, the film resistivity gets higher (>1 Ω cm) and the optical bandgap rises from 3.47 to 3.83 eV.

The superior optical and electronic properties of the two-dimensional (2D) rhenium disulfide (ReS2) makes it suitable for nanoelectronic and optoelectronic applications. However, the internal defects coupled with with the low mobility and light-absorbing capability of ReS2 impede its utilization in high-performance photodetectors. Fabrication of mixed-dimensional heterojunctions is an alternative method for designing high-performance hybrid photodetectors. This study proposes a mixed-dimensional van der Waals (vdW) heterojunction photodetector, containing high-performance one-dimensional (1D) p-type tellurium (Te) and 2D n-type ReS2, developed by depositing Te nanowires on ReS2 nanoflake using the dry transfer method. It can improve the injection and separation efficiency of photoexcited electron-hole pairs due to the type II p-n heterojunction formed at the ReS2 and Te interface. The proposed heterojunction device is sensitive to visible-light sensitivity (632 nm) with an ultrafast photoresponse (5 ms), high responsivity (180 A/W), and specific detectivity (109), which is superior to the pristine Te and ReS2 photodetectors. As compared to the ReS2 device, the responsivity and response speed is better by an order of magnitude. These results demonstrate the fabrication and application potential of Te/ReS2 mixed-dimensional heterojunction for high-performance optoelectronic devices and sensors.

The effect of rapid thermal annealing treatments on the microstructure, surface morphology, and optical characteristics of zinc tin oxide (ZTO) films produced by plasma-enhanced atomic layer deposition was investigated. The ZTO films were annealed in oxygen atmosphere for 2 min at four selected temperatures from 500 to 800 °C. The X-ray diffraction showed that the annealing temperature has a great influence on the crystalline characteristics of ZTO films. The film shows complete amorphous structure for as-deposited ZTO film. Meanwhile, the spinel zinc stannate Zn2SnO4 was obtained for the samples annealed from 500 to 800 °C, which shows polycrystalline nature. The X-ray photoelectron spectroscopy proved that the annealing process in oxygen gas can effectively can reduce the oxygen vacancy defects in the films. In addition, the photoluminescence spectroscopy manifests an ultraviolet emission with a broad peak range from 345 to 385 nm. Moreover, the ultraviolet luminescence intensity increases continuously with the increase of annealing temperature. Spectroscopic ellipsometry analyses demonstrate that the refractive index of annealed films increases as the increase of annealing temperature, while the extinction coefficient decreases gradually with the increase of annealing temperature in the visible light range.

Development of high-performance ammonia (NH3) sensor is imperative for monitoring NH3 in the living environment. In this work, to obtain a high performance NH3 gas sensor, structurally well-defined WO3@SnO2 core shell nanosheets with a controllable thickness of SnO2 shell layer have been employed as sensing materials. The prepared core shell nanosheets were used to obtain a miniaturized gas sensor based on micro-electro-mechanical system (MEMS). By tuning the thickness of SnO2 layer via atomic layer deposition, a series of WO3@SnO2 core-shell nanosheets with tunable sensing properties were realized. Particularly, the sensor base on the fabricated WO3@SnO2 nanosheets with 20-nm SnO2 shell layer demonstrated superior gas sensing performance with the highest response (1.55) and selectivity toward 15 ppm NH3 at 200 °C. This remarkable enhancement of NH3 sensing ability could be ascribed to the formation of unique WO3-SnO2 core-shell heterojunction structure. The detailed mechanism was elucidated by the heterojunction-depletion model with the help of specific band alignment.

For the first time, we report the successful fabrication of well-behaved field-effect transistors based on Nb-doped β-Ga2O3 nanobelts mechanically exfoliated from bulk single crystals. The exfoliated β-Ga2O3 nanobelts were transferred onto a purified surface of the 110 nm SiO2/Si substrate. These Nb-doped devices showed excellent electrical performance such as an ultrasmall cutoff current of ∼10 fA, a high current on/off ratio of >108, and a quite steep subthreshold swing (SS, ∼120 mV/decade). Furthermore, we investigated the temperature dependence down to 200 K, providing insightful information for its operation in a harsh environment. This work lays a foundation for wider application of Nb-doped β-Ga2O3 in nano-electronics.

Hierarchical heterostructured photoanodes have been considered as ideal candidates for oxidation in an effort to enhance the photoelectrochemical (PEC) performance. In this study, well-defined WO3@TiO2–Fe2O3 nanodendrite arrays composed of WO3@TiO2 core–shell nanosheets and Fe2O3 nanorods are synthesized by atomic layer deposition and a subsequent hydrothermal process. The growth of Fe2O3 nanorods on the surface of WO3@TiO2 is demonstrated to be an effective strategy to improve the active surface area and charge separation, which have been proved by the band structure analysis and electrochemical characterization. Particularly, the photocurrent density of the WO3@TiO2–Fe2O3 nanodendrite array with an 11 nm-thick TiO2 buffer layer (∼1.92 mA cm−2) is much higher than the photocurrent density of the WO3 photoanode (∼0.41 mA cm−2) at 1.23 V vs. RHE in 0.5 M Na2SO4 aqueous solutions (pH = 6.8), with outstanding long-term stability as well. This remarkable improvement that comes from rational compositional and structural design of the WO3@TiO2–Fe2O3 nanodendrite array may provide much inspiration for the development of heterostructured PEC electrodes and devices with novel properties.

Diethylzinc and H2O were used as the precursors for the thermal atomic layer deposition (TH-ALD) of ZnO deposition while the trimethylgallium and O2 plasma were used as a reactant for the plasma-enhanced atomic layer deposition (PE-ALD) of Ga2O3, respectively. The Zn-doped Ga2O3 (ZGO) films were fabricated by a combination of PE-ALD of Ga2O3 and TH-ALD of ZnO at a low temperature of 200 °C. The results show that as-deposited ZGO films were amorphous while ZnO with a crystalline structure. XPS results indicate that the Zn content in ZGO films increased from 9.70 to 24.65 at.% with the cycle ration of Ga2O3 with respect to ZnO decreasing from 7:1 to 3:1 while the oxygen vacancy increased from 27.65% to 37.93%. The rise in Zn doping contents is also accompanied by significant variations in the morphological, electrical, and optical properties of the ZGO films, including a decrease of film density and resistivity, an increase of RMS roughness, a strong transmittance in the ultraviolet-visible (UV–vis) area, and a widening of the band gap from 4.64 to 5.25 eV. These findings help deposit ZGO films with desired structure and properties for electronic device applications.

The superior optical and electronic properties of the two-dimensional (2D) rhenium disulfide (ReS

The growth of hetero-epitaxial ZnO-AlN core–shell nanowires (NWs) and single crystalline AlN films on non-polar ZnO substrate at temperature of 380 °C by atomic layer deposition (ALD) was investigated. Structural characterization shows that the AlN shells have excellent single-crystal properties. The epitaxial relationship of [0002]ZnO//[0002]AlN, and [10−10]ZnO//[10−10]AlN between ZnO core and AlN shell has been obtained. The ZnO NW templates were subsequently removed by annealing treatment in forming gas, resulting in ordered arrays of AlN single-crystal nanotubes. The impact factors on the epitaxial growth of AlN films are thoroughly investigated. It turned out that the growth parameters including lattice mismatch between substrate and AlN, growth temperature, and the polarity of ZnO substrate play important roles on the growth of single-crystal AlN films by ALD. Finally, non-polar AlN films with single-crystalline structure have been successfully grown on m-plane ZnO (10−10) single-crystal substrates. The as-grown hollow AlN nanotubes arrays and non-polar AlN films with single-crystalline structures are suggested to be highly promising for applications in nanoscale devices. Our research has developed a potential method to obtain other inorganic nanostructures and films with single-crystalline structure at fairly low temperature.

As a well-known semiconductor that can catalyse the oxygen evolution reaction, TiO2 has been extensively investigated for its solar photoelectrochemical water properties. Unmodified TiO2 shows some issues, particularly with respect to its photoelectrochemical performance. In this paper, we present a strategy for the controlled deposition of controlled amounts of GaOxNy cocatalysts on TiO2 1D nanowires (TiO2@GaOxNy core-shell) using atomic layer deposition. We show that this modification significantly enhances the photoelectrochemical performance compared to pure TiO2 NW photoanodes. For our most active TiO2@GaOxNy core-shell nanowires with a GaOxNy thickness of 20 nm, a photocurrent density up to 1.10 mA cm-2 (at 1.23 V vs. RHE) under AM 1.5 G irradiation (100 mW cm-2) has been achieved, which is 14 times higher than that of unmodified TiO2 NWs. Furthermore, the band gap matching with TiO2 enhances the absorption of visible light over unmodified TiO2 and the facile oxygen vacancy formation after the deposition of GaOxNy also provides active sites for water activation. Density functional theory studies of model systems of GaOxNy-modified TiO2 confirm the band gap reduction, high reducibility and ability to activate water. The highly efficient and stable systems of TiO2@GaOxNy core-shell nanowires with ALD deposited GaOxNy demonstrate a good strategy for the fabrication of core-shell structures that enhance the photoelectrochemical performance of readily available photoanodes.

Ultrathin Ga2O3 films nanomixed with few atom-thick SiO2 interlayer were deposited on silicon and quartz substrates through plasma-enhanced atomic layer deposition. Upon adjusting the number of SiO2 atomic layers, different Si doping levels in Ga2O3 films were obtained. Herein, we report for the first time the use of this method to investigate the doping effect and realize the precise control of the microstructural, optical, and electrical properties of the Ga2O3 films. The experimental results indicated that the microstructural properties such as density, roughness, and chemical composition of the films are all affected by the number of doped SiO2 atomic layers. As the Si doping concentration increased in the films, the refractive index of the Ga2O3 films decreased monotonically, the optical transparency improved, and the average transmittances were >95% from ultraviolet to visual wavelengths for all the Si-doped Ga2O3 films. Moreover, the energy band-gap increased with the increase in Si composition, increasing from ∼4.8 eV for the undoped film to ∼5.1 eV for the film with a 20 at% Si doping level. Moreover, the breakdown field of the Ga2O3 film linearly improved from 6.8 MV cm−1 to 10.7 MV cm−1 as a function of the Si content. More importantly, this result enlightened us that the breakdown field of the Ga2O3 film can be further improved by increasing the Si doping concentration. This study provides a means to expand the properties of the new doped films and develop Ga2O3 film-based devices, such as transparent electrodes, photodetectors, thin film transistors, or high-power high-voltage devices.

Development of high-performance ammonia (NH

As a promising oxygen evolution reaction semiconductor, TiO2 has been extensively investigated for solar photoelectrochemical water splitting. Here, a highly efficient and stable strategy for rationally preparing GaON cocatalysts on TiO2 by atomic layer deposition is demonstrated, which we show significantly enhances thephotoelectrochemical performance compared to TiO2-based photoanodes. For TiO2@20 nm-GaON core-shell nanowires a photocurrent density up to 1.10 mA cm-2 (1.23 V vs RHE) under AM 1.5 G irradiation (100 mW cm-2) has been achieved, which is 14 times higher than that of TiO2 NWs. Furthermore, the oxygen vacancy formation on GaON as well as the band gap matching with TiO2 not only provides more active sites for water oxidation but also enhances light absorption to promote interfacial charge separation and migration. Density functional theory studies of model systems of GaON-modified TiO2 confirm the band gap reduction, high reducibility and ability to activate water. The highly efficient and stable systems of TiO2@GaON core-shell nanowires provide a deeper understanding and universal strategy for enhancing photoelectrochemical performance of photoanodes now available.

The α-Fe2O3/ZnO core-shell nanowires (NWs) on micro-electro-mechanical system structure were prepared for hydrogen sulfide (H2S) gas sensing. Core α-Fe2O3 NWs were prepared by thermal oxidation and the ZnO shell layer was coated by atomic layer deposition, respectively. The diameter of the obtained α-Fe2O3/ZnO core-shell NWs increases uniformly with the increase of ZnO shell thickness. When the operating temperature is 250 °C, the maximum response of the α-Fe2O3/ZnO core-shell NWs gas sensor to 5 ppm H2S is 5.98, which is about five times of pure α-Fe2O3 NWs gas sensor (∼1.1). Moreover, α-Fe2O3/ZnO core-shell NWs-based gas sensors showed good selectivity, response, and recovery properties to H2S. The improvement of gas response of core-shell NWs is due to the slender α-Fe2O3/ZnO core-shell NWs and the appropriate energy band structure between ZnO and α-Fe2O3. Thus, the proposed α-Fe2O3/ZnO core-shell NWs will be one of ideal sensitive materials for H2S high-performance sensor.

Gallium oxynitride (GaON) is an emerging material suitable as a key component to build efficient heterostructures for photoelectrochemical (PEC) water splitting. However, the great difficulty in controlled growth of GaON films limits their applications. This study developed a novel method for depositing highly uniform GaON films by a one-step co-deposition process via plasma-enhanced atomic layer deposition (ALD). Importantly, this material presents high-quality epitaxial growth behavior on ZnO nanowires (NWs) only at 200 °C to construct ZnO-GaON core-shell NWs with different shell thickness (5–60 nm). Benefiting from the precisely controlled ALD technique, thickness-dependent PEC performance and its mechanism were studied deeply. It was found the ZnO-GaON NWs with an optimum shell thickness (~40 nm) presented largest electric field enhancement and light-trapping ability, thus greatly improved the photocurrent from ~0.24 (pristine ZnO) to 2.25 mA/cm2 at 1.23 V versus reversible hydrogen electrode. Meanwhile, this structure presents an ultrahigh incident photon-to-current conversion efficiency of ~90% in the UV region. A comparative study assesses the ultrahigh carrier density (~1021 cm−3) and suitable bandgap of GaON relative to GaN and Ga2O3, revealing a higher photocurrent for the ZnO-GaON core-shell NWs. These encouraging results indicated that higher PEC performance is worthy expected upon optimization of the nitrogen and oxygen concentrations and by combining with narrow bandgap materials in further studies.

The energy band alignment of AlN/β-Ga2O3 heterostructures was investigated by X-ray photoelectron spectroscopy. The valence band offsets were estimated to be −0.09 ± 0.1 eV (type II alignment) for AlN grown by plasma enhanced atomic layer deposition (PEALD) on β-Ga2O3 and 0.72 ± 0.1 eV (type I alignment) for AlN prepared by thermal atomic layer deposition (T-ALD) on β-Ga2O3, which gives the conduction band offsets of 1.39 ± 0.1 eV for PEALD AlN and 0.58 ± 0.1 eV for T-ALD AlN. The large difference in the band alignment for the AlN/β-Ga2O3 heterostructures is dominated by different levels of oxygen incorporation into the AlN films as a result of different deposition techniques. The determination of the band alignment of the AlN/β-Ga2O3 heterostructure has significant implications for the design of electronic and optical devices based on AlN/β-Ga2O3 heterojunctions.