Wide Band Gap Oxide Semiconductors Research Articles

(Invited) Ternary Oxide Semiconductors and Alloys for Photoelectrochemical Applications: Hope and Reality

This perspective talk focuses on the history and status of new materials with targeted application in solar energy conversion. Specifically, photoelectrochemical energy conversion/solar water splitting and wide bandgap oxide semiconductors and alloys for solar windows and displays are the two targeted application areas. The Cu-M-V-O family of oxide semiconductors will be discussed in this talk. Both synthetic aspects and chemical composition-property-performance correlations will be presented.Acknowledgements. This work was primarily supported by the National Science Foundation UTA/NU Partnership for Research and Education in Materials (NSF DMR-2122128).

Enhancing growth rate in homoepitaxial growth of β-Ga2O3 with flat surface via hydrochloric acid addition in mist CVD

Gallium oxide (Ga2O3) is a wide-bandgap oxide semiconductor, with a bandgap of ∼4.9 eV, making it a promising material for power device applications. This study focuses on the effect of hydrochloric acid addition on the growth rate in homoepitaxial growth of β-Ga2O3 using a mist chemical vapor deposition method. For homoepitaxial growth on a (001) β-Ga2O3 substrate, we introduced different concentrations of HCl into the source solution to assess its impact on the growth rate, crystal structures, and surface morphologies of the films. At a growth temperature of 900 °C, HCl addition linearly increased film thickness, enhancing the growth rate by 4.8 times with 9.09 vol. % HCl. No peaks associated with other phases were exhibited by each sample, indicating pure homoepitaxial growth. When comparing samples with similar film thicknesses, the root-mean-square (rms) roughness was enhanced by 1/7 with an increase in the HCl concentration. However, at 800 °C, an increasing solution concentration caused pronounced step bunching and elevated rms roughness, in contrast with the minimal effect observed at 900 °C. In experiments with hydrochloric acid addition at 900 °C, we observed a striped morphology, which maintained consistent rms roughness despite higher temperature.

Enhanced solar-driven photocatalytic hydrogen production, dye degradation, and supercapacitor functionality using MoS2–TiO2 nanocomposite

In our current study, we've devised a promising method to overcome the limitations of nanostructured titanium dioxide (TiO2) with molybdenum disulfide (MoS2) nanoflowers synthesized through a cost-effective and straightforward solvothermal process. The resulting heterostructures function as highly efficient photocatalysts and supercapacitors. The synthesized MoS2–TiO2 nanocomposite demonstrated exceptional performance in various applications, including efficient hydrogen (H2) evolution, dye degradation, and enhanced supercapacitor performance. The combination of MoS2, a two-dimensional transition metal dichalcogenide, and TiO2, a wide-bandgap metal oxide semiconductor, addresses critical challenges in clean energy, environmental remediation, and energy storage. Photocatalytic studies revealed that the MoS2–TiO2 catalysts exhibited the highest activity for hydrogen production, with a rate of 137.93 μmol h−1 g−1, which is three times greater than that of pure MoS2. Additionally, the MoS2–TiO2 nanocomposite demonstrated superior electrochemical performance, with a specific capacitance of 638 F/g. Electrochemical impedance spectroscopy (EIS) showed that the charge transfer kinetics were comparable, highlighting the positive impact of TiO2 without significantly increasing resistance. The MoS2–TiO2 nanocomposite emerges as a transformative material with multifaceted applications, showcasing its versatility in addressing global challenges related to clean energy and environmental sustainability.

Comprehensive effect on the development of multifunctional low emissive and transparent conductive amorphous-SIZO/Ag/amorphous-SIZO multilayer structure

Wide band gap oxide semiconductors based on transparent conducting electrodes (TCEs) are promising for the fabrication of a wide range of electronic and optoelectronic devices, which include flat panel displays, photovoltaics, transparent heaters, and smart windows. High optical transmittance in the visible spectral range and low electrical resistivity as obtained by intrinsic and/or extrinsic doping, lead to a high figure of merit (FoM) for oxide based TCEs. Recently, we have used wide band gap amorphous Si–In–Zn–O (SIZO) to realize a relatively new class of TCEs using SIZO/Ag/SIZO multilayer structures, which exhibit high FoM in addition to infrared reflection capability due to the embedded ultrathin Ag metal layer. These multifunctional TCEs are found to be highly useful for low emissive smart window applications, which minimize the outside infrared to coming inside and efficiently reflect inside heat to emit outside. In addition, amorphous OMO based TCEs show excellent mechanical endurance, unlike crystalline ITO based coatings. The optical and low emission characteristics of amorphous SIZO OMO were optimized to transmittance of 93.4 % at 550 nm and 58.6 % at 950 nm.

Tunable Ga2O3 solar-blind photosensing performance via thermal reorder engineering and energy-band modulation

As an ultra-wide bandgap semiconductor, gallium oxide (Ga2O3) has been extensively applied in solar-blind photodetectors (PDs) owing to the absorbance cut-off wavelength of shorter than 280 nm, and the optimized technologies of detection performance is seriously essential for its further usages. Herein, a feasible thermal reorder engineering method was performed through annealing Ga2O3 films in vacuum, O2 and oxygen plasma atmospheres, realizing to tune solar-blind photosensing performance of Ga2O3 PDs. Thermal treatment, in fact a crystal reorder process, significantly suppressed the noise in Ga2O3-based PDs and enhanced the photo-sensitivity, with the dark current decreasing from 154.63 pA to 269 fA and photo-to-dark current ratio magically raising from 288 to 2.85 × 104. This achievement is dependent of energy-band modulation in Ga2O3 semiconductor, that is certified by first-principles calculation. Additionally, annealing in oxygen atmospheres notably reduces the concentration of oxygen vacancies in the surface of films, thereby improving the performance of the PDs; the oxygen vacancy is extremely concerned in oxide semiconductors in the view of physics of surface defects. In all, this work could display a promising guidance for modulating the performance of PDs based on wide bandgap oxide semiconductor, especially for hot Ga2O3 issue.

Low-temperature electron transport of rutile-type GexSn1−xO2

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K.

Ternary Oxide Semiconductors and Alloys: Hope and Reality

This perspective talk focuses on the history and status of new materials with targeted application in solar energy conversion. Specifically, photoelectrochemical energy conversion/solar water splitting and wide bandgap oxide semiconductors and alloys for solar windows and displays are the two targeted application areas. The Cu-M-V-O family of oxide semiconductors will be discussed in this poster. Both synthetic aspects and chemical composition-property-performance correlations will be presented.Acknowledgements. This work was primarily supported by the National Science Foundation UTA/NU Partnership for Research and Education in Materials (NSF DMR-2122128).

The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor

Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

Passivation of oxygen vacancy defects in conductive ZnO nanoparticles via low-temperature annealing in NF3

Oxygen vacancies (VO) often exist in wide-bandgap metal oxide semiconductors (MOSs) as deep-level defects and undermine the reliability of various optoelectronic devices based on MOSs. Conventional methods to passivate VO defects, such as high-temperature annealing or plasma treatment, can compromise device performance. This work demonstrates that passivation of VO defects in aluminum-doped ZnO (AZO) nanoparticles can be realized through low-temperature annealing (350 °C) in an NF3 atmosphere. After the NF3 annealing, the longitudinal optical phonon scattering mode in Raman spectroscopy, the visible light emission intensity in photoluminescence spectroscopy, and the oxygen deficiency-related peak in x-ray photoelectron spectroscopy decrease simultaneously, indicating the passivation of VO defects in AZO nanoparticles. As a result, AZO nanoparticles show higher visible light reflectance and better stability of electrical conductivity owing to the suppression of deep-level light absorption and gas molecule adsorption. This work also offers insights into the passivation mechanism of VO defects in MOSs.

Metal oxide semiconductor nanowires enabled air-stable ultraviolet-driven synaptic transistors for artificial vision

The multi-terminal transistor can provide reliable platform to construct artificial synapses with various benefits. In particular, synaptic transistors based on wide-bandgap metal oxide semiconductors (MOSs) offer opportunities to realize ultraviolet (UV) artificial vision. In this work, via a low-cost electrospinning technique, we have fabricated bio-inspired synaptic transistors based on zinc tin oxide (ZnSnO) nanowires, which can be effectively tuned by UV laser to achieve air-stable synaptic characteristics, such as excitatory post-synaptic current, paired-pulse facilitation, and learning-forgetting process based on long-term potentiation. The adsorption/desorption of oxygen molecules governs the generation of free charge carriers in the ZnSnO nanowires. Via the combination of transmission electron microscopy and X-ray diffraction measurements, we found that the revealed SnO2/ZnO heterostructures can effectively mitigate the recombination of UV-induced free carriers in the ZnSnO nanowires. Furthermore, based on the synaptic characteristics obtained from ZnSnO nanowires, we present several applications relevant to the UV artificial vision. We believe this study can provide insights into importance of heterostructures in synaptic transistors based on low-dimension MOSs with superior environmental stability and various applications.

Low temperature synthesis of MoO3 nanoparticles by hydrothermal method: Investigation on their structural and optical properties

Molybdenum trioxide nanoparticles have recently achieved notable attention in optoelectronic and biomedical applications by virtue of their excellent structural, optical, electrical, and catalytic characteristics. The work presented here demonstrates the synthesis of orthorhombic MoO 3 through the facile hydrothermal method at low temperature. Structural and optical characterization of the prepared sample was examined. XRD studies and Raman spectroscopy were carried out to study the structural behavior of the sample. The XRD peaks were concordant with the standard peaks of MoO 3 , which corresponds to the orthorhombic structure of MoO 3 . Micro-strain effects were also verified by the W-H method using UDM, UDEDM, and USDM. Raman spectroscopic data ascertained the orthorhombic phase of MoO 3 . From Tauc plot, a wide bandgap value (4.9 eV) was evaluated. In photoluminescence spectroscopy, peaks are related to the transition among the sub-bands of Mo 5+ defects. Being a wide bandgap oxide semiconductor, MoO 3 is a promising and worthy material for luminescence applications.

Comparison and Analysis of the third generation of the solar cells

Since the current energy crisis, the efficiency of using solar cell energy has become increasingly important. In order to get a high efficiency, new kinds of solar cells would be needed, which called the third generation of solar cells. This paper review three kinds of solar cells, including the perovskite solar cells, the quantum dots solar cells and the organic solar cells. Firstly, the structure of perovskite solar cells is composed of a FTO conducting glass, a TiO2 blocking layer, a TiO2 mesoporous layer, a perovskite layer, a HTM (Hole Transport Material) layer, and mental electrode. Then, I sum up the manufacture and the development of the perovskite solar cells. Secondly, this paper analysis the structure, manufacture, and development of quantum dots solar cells. The structure of quantum dots solar cells includes the transparent conducting oxide, wide bandgap oxide semiconductor film, quantum dot sensitizer, electrolyte, and counter electrode. The third one is the organic solar cells. The structure of the organic solar cells includes layer, cathode, anode, and transparent substrate. Finally, this paper reviews the merits and defects of each solar cells.

ZnO nano-structured based devices for chemical and optical sensing applications

Zinc oxide (ZnO), a direct, wide band gap and n-type metal oxide semiconductor, is being anticipated as the next generation functional nano-material towards remarkably diversified sensing applications. ZnO and its composites have revealed a new era in the fabrication of sensors owing to their excellent opto-electronic, physicochemical, electrical properties such as low dielectric constant, plentiful Zn-O bonds, high luminous transmittance, good physicochemical stability, huge excitation binding energy, non-toxicity, biocompatibility, large surface area to volume ratio and so on. This comprehensive review summarizes the applications of ZnO nano-structures in the areas of environmental monitoring, biomedical, and optical sensing. Fundamental sensing mechanisms of the ZnO based sensors are examined to provide a better insight into the role of ZnO in each of these sensors. Wherever relevant, limitations of the existing approaches and future outlook have also been discussed.

Room‐Temperature Processed Lateral Trench‐Metal–Insulator–Semiconductor Schottky Barrier Diodes with Amorphous Gallium Oxide (a‐Ga2O3) Thin Films on Single‐Crystal Silicon <100>

This article proposes a novel design approach for the fabrication of lateral Schottky barrier diodes (SBD) using a wide bandgap oxide semiconductor on silicon. Structural and electrical properties of the fabricated device are reported and compared with published data. The metal–insulator–semiconductor (MIS) Schottky barrier is realized between the room‐temperature sputtered Al/a‐Ga2O3 on single‐crystal <100> boron‐doped silicon substrate (p‐type). The device fabrication process is implemented entirely at room temperature. The proposed trench diode yields an ideality factor of 1.65 and a rectification ratio of ≈1 × 106 at ±5.0 V. The extracted specific ON resistance is 78 mΩ cm2, and the breakdown voltage is not observed for 180 V for a 200‐nm‐thin layer of a‐Ga2O3. Due to such low‐temperature process, simple fabrication steps, and notably high Baliga's figure‐of‐merit (BFOM), the proposed diode is, therefore, promising for power electronics applications.

Semiconductor-to-metal transition in nanocomposites of wide bandgap oxide semiconductors

The semiconductor-to-metal transition (SMT) is reported in nanocomposite films of wide bandgap semiconducting oxides. This study offers insight into the existing models of temperature-dependent charge transport by tailoring the defects induced disorder into the lattice by swift heavy ions irradiation. The structural studies show the presence of both cadmium oxide and zinc oxides phases and confirm the nanocomposite nature of films. Temperature-dependent charge transport properties of the nanocomposite films were studied by carrying out four-probe resistivity measurements in the wide temperature range of 35–300 K. Here, the swift heavy ions irradiated films demonstrated the tuning of SMT phenomena and its transition temperature upon increasing fluence of irradiation. These irradiation effects are discussed in terms of irradiation-induced defects/disorders into the lattice. The change in the nature of temperature-dependent resistivity in pristine and irradiated films has been envisaged with a quantitative fit of experimental data by invoking quantum correction to conductivity (QCC). Our results are consistent with QCC model, which essentially depends on weak localization and renormalized electron-electron interactions. Thus, this work extends the validity of the weak localization model to composite systems.

Oxygen Adsorption and Photoconduction Models for Metal Oxide Semiconductors: A Review

Ultraviolet photodetectors have several critical applications in communications, early missile launch detection, pollution monitoring, and bacterial fluorescence detection. Wide-bandgap metal oxide semiconductors have proven to be high performance ultraviolet photodetectors with some photoresponsivities reaching several orders of magnitude greater than solid state commercial devices and even photomultiplier tubes. However, analysis and design of metal-oxide photodetectors is complicated by oxygen adsorption and photodesorption at the surface. A limited understanding of this adsorption phenomenon hinders the development of new UV photodetectors. This paper clarifies the adsorption and photodesorption process and makes analysis of metal-oxide photoconductors tractable. Several models of oxygen adsorption, oxygen desorption, and adsorption-controlled conduction from across disciplines are summarized. We discuss each model's use in their intended context, implications for design, fitting parameters, and limitations. We conclude with an argument that current models are insufficient for describing high-gain metal oxide ultraviolet photodetectors, and we outline the requirements for a new model suitable for high-gain sensors.

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices.

Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new materials and high-performing thin film transistor (TFT) devices in the context of fundamental understanding is presented. In particular, an in depth overview is first provided on current understanding of the electronic structures, defect and doping chemistry, optical and transport properties of oxide semiconductors, which provide essential guiding principles for new material design and device optimization. With these principles, recent advances in design of p-type oxide semiconductors, new approaches for achieving cost-effective transparent (flexible) electrodes, and the creation of high mobility 2D electron gas (2DEG) at oxide surfaces and interfaces with a wealth of fascinating physical properties of great potential for novel device design are then reviewed. Finally, recent progress and perspective of oxide TFT based on new oxide semiconductors, 2DEG, and low-temperature solution processed oxide semiconductor for flexible electronics will be reviewed.

High-mobility p-type semiconducting two-dimensional β-TeO2

Wide-bandgap oxide semiconductors are essential for the development of high-speed and energy-efficient transparent electronics. However, while many high-mobility n-type oxide semiconductors are known, wide-bandgap p-type oxides have carrier mobilities that are one to two orders of magnitude lower due to strong carrier localization near their valence band edge. Here, we report the growth of bilayer beta tellurium dioxide (β-TeO2), which has recently been proposed theoretically as a high-mobility p-type semiconductor, through the surface oxidation of a eutectic mixture of tellurium and selenium. The isolated β-TeO2 nanosheets are transparent and have a direct bandgap of 3.7 eV. Field-effect transistors based on the nanosheets exhibit p-type switching with an on/off ratio exceeding 106 and a field-effect hole mobility of up to 232 cm2 V−1 s−1 at room temperature. A low effective mass of 0.51 was observed for holes, and the carrier mobility reached 6,000 cm2 V−1 s−1 on cooling to −50 °C. Bilayer beta tellurium dioxide nanosheets with p-type characteristics can be formed through the surface oxidation of a mixture of tellurium and selenium, and used to create transistors with performance that matches their n-type oxide counterparts.

Low-Temperature As-Grown Crystalline β-Ga2O3 Films via Plasma-Enhanced Atomic Layer Deposition.

We report on the low-temperature growth of crystalline Ga2O3 films on Si, sapphire, and glass substrates using plasma-enhanced atomic layer deposition (PEALD) featuring a hollow-cathode plasma source. Films were deposited by using triethylgallium (TEG) and Ar/O2 plasma as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed within 150-240 °C substrate temperature and 30-300 W radio-frequency (rf) plasma power ranges. Additionally, each unit AB-type ALD cycle was followed by an in situ Ar plasma annealing treatment, which consisted of an extra (50-300 W) Ar plasma exposure for 20 s ending just before the next TEG pulse. The growth per cycle (GPC) of the films without Ar plasma annealing step ranged between 0.69 and 1.31 Å/cycle, and as-grown refractive indices were between 1.67 and 1.75 within the scanned plasma power range. X-ray diffraction (XRD) measurements showed that Ga2O3 films grown without in situ Ar plasma annealing exhibited amorphous character irrespective of substrate temperature and rf power values. With the incorporation of the in situ Ar plasma annealing process, the GPC of Ga2O3 films ranged between 0.76 and 1.03 Å/cycle along with higher refractive index values of 1.75-1.79. The increased refractive index (1.79) and slightly reduced GPC (1.03 Å/cycle) at 250 W plasma annealing indicated possible densification and crystallization of the films. Indeed, X-ray measurements confirmed that in situ plasma annealed films grow in a monoclinic β-Ga2O3 crystal phase. The film crystallinity and density further enhance (from 5.11 to 5.60 g/cm3) by increasing the rf power value used during in situ Ar plasma annealing process. X-ray photoelectron spectroscopy (XPS) measurement of the β-Ga2O3 sample grown under optimal in situ plasma annealing power (250 W) revealed near-ideal film stoichiometry (O/Ga of ∼1.44) with relatively low carbon content (∼5 at. %), whereas 50 W rf power treated film was highly non-stoichiometric (O/Ga of ∼2.31) with considerably elevated carbon content. Our results demonstrate the effectiveness of in situ Ar plasma annealing process to transform amorphous wide bandgap oxide semiconductors into crystalline films without needing high-temperature post-deposition annealing treatment.

Energy-driven multi-step structural phase transition mechanism to achieve high-quality p-type nitrogen-doped β-Ga2O3 films

It is a big challenge to grow β-Ga2O3 films of good p-type conductivity and stability due to the large acceptor ionization energy, strong hole-trapping effect, low hole mobility, and self-compensation effects. In this study, we demonstrate for the first time the success in the growth of high-quality p-type N-doped β-Ga2O3 films with an acceptor ionization energy of 0.165 eV, Hall resistivity of 17.0 Ωcm, Hall hole mobility of 23.6 cm2V−1s−1, hole concentration of 1.56 × 1016 cm−3 and good stability, and in the fabrication of high-performance p-type β-Ga2O3 films-based field effect transistors. The success paves the way to fabricate full β-Ga2O3 films based p-n diodes and will advance the applications of next-generation high-performance β-Ga2O3-based optoelectronic and electronic devices. In addition, we discover the growth mechanism of multi-step structural phase transitions from hexagonal P63mc GaN to rhombohedral R3¯c α-GaNxO3(1-x)/2 and finally to monolithic C2/m N-doped β-Ga2O3, which improves the crystalline quality, facilitates the acceptor doping, increases the acceptor activation efficiency, and thus enhances the p-type conductivity and stability of the N-doped β-Ga2O3 films. The novel p-type oxide film growth technique and mechanism open a new horizon to the other wide bandgap oxide semiconductors.