Hardware Realization of the Pattern Recognition with an Artificial Neuromorphic Device Exhibiting a Short-Term Memory.

Materials exhibiting memory or those capable of implementing certain learning schemes are the basic building blocks used in hardware realizations of the neuromorphic computing. One of the common goals within this paradigm assumes the integration of hardware and software solutions, leading to a substantial efficiency enhancement in complex classification tasks. At the same time, the use of unconventional approaches towards signal processing based on information carriers other than electrical carriers seems to be an interesting trend in the design of modern electronics. In this context, the implementation of light-sensitive elements appears particularly attractive. In this work, we combine the abovementioned ideas by using a simple optoelectronic device exhibiting a short-term memory for a rudimentary classification performed on a handwritten digits set extracted from the Modified National Institute of Standards and Technology Database (MNIST)(being one of the standards used for benchmarking of such systems). The input data was encoded into light pulses corresponding to black (ON-state) and white (OFF-state) pixels constituting a digit and used in this form to irradiate a polycrystalline cadmium sulfide electrode. An appropriate selection of time intervals between pulses allows utilization of a complex kinetics of charge trapping/detrapping events, yielding a short-term synaptic-like plasticity which in turn leads to the improvement of data separability. To the best of our knowledge, this contribution presents the simplest hardware realization of a classification system capable of performing neural network tasks without any sophisticated data processing.

Ultraviolet Detectors Based on Wide Bandgap Semiconductor Nanowire: A Review.

Ultraviolet (UV) detectors have attracted considerable attention in the past decade due to their extensive applications in the civil and military fields. Wide bandgap semiconductor-based UV detectors can detect UV light effectively, and nanowire structures can greatly improve the sensitivity of sensors with many quantum effects. This review summarizes recent developments in the classification and principles of UV detectors, i.e., photoconductive type, Schottky barrier type, metal-semiconductor-metal (MSM) type, p-n junction type and p-i-n junction type. The current state of the art in wide bandgap semiconductor materials suitable for producing nanowires for use in UV detectors, i.e., metallic oxide, III-nitride and SiC, during the last five years is also summarized. Finally, novel types of UV detectors such as hybrid nanostructure detectors, self-powered detectors and flexible detectors are introduced.

Designing a Robust Kelvin Probe Setup Optimized for Long-Term Surface Photovoltage Acquisition.

We introduce a robust low-budget Kelvin probe design that is optimized for the long-term acquisition of surface photovoltage (SPV) data, especially developed for highly resistive systems, which exhibit-in contrast to conventional semiconductors-very slow photoinduced charge relaxation processes in the range of hours and days. The device provides convenient optical access to the sample, as well as high mechanical and electrical stability due to off-resonance operation, showing a noise band as narrow as 1 mV. Furthermore, the acquisition of temperature-dependent SPV transients necessary for SPV-based deep-level transient spectroscopy becomes easily possible. The performance of the instrument is demonstrated by recording long-term SPV transients of the ultra-slowly relaxing model oxide strontium titanate (SrTiO 3 ) over 20 h.

Transition Metal Oxide Nanomaterials: New Weapons to Boost Anti-Tumor Immunity Cycle.

Semiconductor nanomaterials have emerged as a significant factor in the advancement of tumor immunotherapy. This review discusses the potential of transition metal oxide (TMO) nanomaterials in the realm of anti-tumor immune modulation. These binary inorganic semiconductor compounds possess high electron mobility, extended ductility, and strong stability. Apart from being primary thermistor materials, they also serve as potent agents in enhancing the anti-tumor immunity cycle. The diverse metal oxidation states of TMOs result in a range of electronic properties, from metallicity to wide-bandgap insulating behavior. Notably, titanium oxide, manganese oxide, iron oxide, zinc oxide, and copper oxide have garnered interest due to their presence in tumor tissues and potential therapeutic implications. These nanoparticles (NPs) kickstart the tumor immunity cycle by inducing immunogenic cell death (ICD), prompting the release of ICD and tumor-associated antigens (TAAs) and working in conjunction with various therapies to trigger dendritic cell (DC) maturation, T cell response, and infiltration. Furthermore, they can alter the tumor microenvironment (TME) by reprogramming immunosuppressive tumor-associated macrophages into an inflammatory state, thereby impeding tumor growth. This review aims to bring attention to the research community regarding the diversity and significance of TMOs in the tumor immunity cycle, while also underscoring the potential and challenges associated with using TMOs in tumor immunotherapy.

High-Performance Self-Powered Photoelectrochemical Ultraviolet Photodetectors Based on an In2O3 Nanocube Film.

Ultraviolet photodetectors (UV PDs) have attracted significant attention due to their wide range of applications, such as underwater communication, biological analysis, and early fire warning systems. Indium oxide (In2O3) is a candidate for developing high-performance photoelectrochemical (PEC)-type UV PDs owing to its high UV absorption and good stability. However, the self-powered photoresponse of the previously reported In2O3-based PEC UV PDs is unsatisfactory. In this work, high-performance self-powered PEC UV PDs were constructed by using an In2O3 nanocube film (NCF) as a photoanode. In2O3 NCF photoanodes were synthesized on FTO by using hydrothermal methods with a calcining process. The influence of the electrolyte concentration, bias potential, and irradiation light on the photoresponse properties was systematically studied. In2O3 NCF PEC UV PDs exhibit outstanding self-powered photoresponses to 365 nm UV light with a high responsivity of 44.43 mA/W and fast response speed (20/30 ms) under zero bias potential, these results are superior to those of previously reported In2O3-based PEC UV PDs. The improved self-powered photoresponse is attributed to the higher photogenerated carrier separation efficiency and faster charge transport of the in-situ grown In2O3 NCF. In addition, these PDs exhibit excellent multicycle stability, maintaining the photocurrent at 98.69% of the initial value after 700 optical switching cycles. Therefore, our results prove the great promise of In2O3 in self-powered PEC UV PDs.

Growth of Wide-Bandgap Monolayer Molybdenum Disulfide for a Highly Sensitive Micro-Displacement Sensor.

Two-dimensional (2D) piezoelectric semiconductor materials are garnering significant attention in applications such as intelligent sensing and energy harvesting due to their exceptional physical and chemical properties. Among these, molybdenum disulfide (MoS2), a 2D wide-bandgap semiconductor, exhibits piezoelectricity in odd-layered structures due to the absence of an inversion symmetry center. In this study, we present a straightforward chemical vapor deposition (CVD) technique to synthesize monolayer MoS2 on a Si/SiO2 substrate, achieving a lateral size of approximately 50 µm. Second-harmonic generation (SHG) characterization confirms the non-centrosymmetric crystal structure of the wide-bandgap MoS2, indicative of its piezoelectric properties. We successfully transferred the triangular MoS2 to a polyethylene terephthalate (PET) flexible substrate using a wet-transfer method and developed a wide-bandgap MoS2-based micro-displacement sensor employing maskless lithography and hot evaporation techniques. Our testing revealed a piezoelectric response current of 5.12 nA in the sensor under a strain of 0.003% along the armchair direction of the monolayer MoS2. Furthermore, the sensor exhibited a near-linear relationship between the piezoelectric response current and the strain within a displacement range of 40-100 µm, with a calculated response sensitivity of 1.154 µA/%. This research introduces a novel micro-displacement sensor, offering potential for advanced surface texture sensing in various applications.

Freestanding Crystalline ß-Ga2O3 Flexible Membrane Obtained via Lattice Epitaxy Engineering for High-Performance Optoelectronic Device.

Wearable and flexible ß-Ga2O3-based semiconductor devices have attracted considerable attention, due to their outstanding performance and potential application in real-time optoelectronic monitoring and sensing. However, the unavailability of high-quality crystalline and flexible ß-Ga2O3 membranes limits the fabrication of relevant devices. Here, through lattice epitaxy engineering together with the freestanding method, we demonstrate the preparation of a robust bending-resistant and crystalline ß-Ga2O3 (-201) membrane. Based on this, we fabricate a flexible ß-Ga2O3 photodetector device that shows comparable performance in photocurrent responsivity and spectral selectivity to conventional rigid ß-Ga2O3 film-based devices. Moreover, based on the transferred ß-Ga2O3 membrane on a silicon wafer, the PEDOT:PSS/ß-Ga2O3 p-n heterojunction device with self-powered characteristic was constructed, further demonstrating its superior heterogeneous integration ability with other functional materials. Our results not only demonstrate the feasibility of obtaining a high-quality crystalline and flexible ß-Ga2O3 membrane for an integrated device but also provide a pathway to realize flexible optical and electronic applications for other semiconducting materials.

Growth of bulk ß-Ga2O3 crystals from melt without precious-metal crucible by pulling from a cold container.

We report the growth of bulk ß-Ga2O3 crystals based on crystal pulling from a melt using a cold container without employing a precious-metal crucible. Our approach, named oxide crystal growth from cold crucible (OCCC), is a fusion between the skull-melting and Czochralski methods. The absence of an expensive precious-metal crucible makes this a cost-effective crystal growth method, which is a critical factor in the semiconductor industry. An original construction 0.4-0.5 MHz SiC MOSFET transistor generator with power up to 35 kW was used to successfully grow bulk ß-Ga2O3 crystals with diameters up to 46 mm. Also, an original diameter control system by generator frequency change was applied. In this preliminary study, the full width at half maximum of the X-ray rocking curve from the obtained ß-Ga2O3 crystals with diameters ≤ 46 mm was comparable to those of ß-Ga2O3 produced by edge-defined film fed growth. Moreover, as expected, the purity of the obtained crystals was high because only raw material-derived impurities were detected, and contamination from the process, such as insulation and noble metals, was below the detection limit. Our results indicate that the OCCC technique can be used to produce high-purity bulk ß-Ga2O3 single crystalline substrate.

Effect of zinc addition and vacuum annealing time on the properties of spin-coated low-cost transparent conducting 1 at% Ga-ZnO thin films.

Pure and 1 at% gallium (Ga)-doped zinc oxide (ZnO) thin films have been prepared with a low-cost spin coating technique on quartz substrates and annealed at 500 °C in vacuum ∼10-3 mbar to create anion vacancies and generate charge carriers for photovoltaic application. Also, 0.5-1.5 at% extra zinc species were added in the precursor sol to investigate changes in film growth, morphology, optical absorption, electrical properties and photoluminescence. It is shown that 1 at% Ga-ZnO thin films with 0.5 at% extra zinc content after vacuum annealing for 60 min correspond to wurtzite-type hexagonal structure with (0001) preferred orientation, electrical resistivity of ∼9 × 10-3 Ω cm and optical transparency of ∼65-90% in the visible range. Evidence has been advanced for the presence of defect levels within bandgap such as zinc vacancy (VZn), zinc interstitial (Zni), oxygen vacancy (Vo) and oxygen interstitial (Oi). Further, variation in ZnO optical bandgap occurring with Ga doping and insertion of additional zinc species has been explained by invoking two competing phenomena, namely bandgap widening and renormalization, usually observed in semiconductors with increasing carrier concentration.

Surface Engineering in SnO2/Si for High-Performance Broadband Photodetectors.

Silicon-based photodetectors are important optoelectronic devices in many fields. Many investigations have been conducted to improve the performance of silicon-based photodetectors, such as spectral responsivity and sensitivity in the ultraviolet band. In this study, we combine the surface structure engineering of silicon with wide-bandgap semiconductor SnO2 films to realize textured Si-based heterojunction photodetectors. The obtained SnO2/T-Si photodetectors exhibit high responsivity ranging from ultraviolet to near-infrared light. Under a bias voltage of 1 V, SnO2/T-Si photodetectors (PDs) with an inverted pyramid texture show the best performance, and the typical responsivities to ultraviolet, visible, and near-infrared light are 0.512, 0.538, 1.88 (800 nm, 67.7 µW/cm2) A/W@1 V, respectively. The photodetectors exhibit short rise and decay times of 18.07 and 29.16 ms, respectively. Our results demonstrate that SnO2/T-Si can serve as a high-performance broadband photodetector.

Directional Carrier Transport in Micrometer-Thick Gallium Oxide Films for High-Performance Deep-Ultraviolet Photodetection.

Incorporating emerging ultrawide bandgap semiconductors with a metal-semiconductor-metal (MSM) architecture is highly desired for deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in semiconductors complicate the rational design of MSM DUV photodetectors due to their dual role as carrier donors and trap centers, leading to a commonly observed trade-off between responsivity and response time. Here, we demonstrate a simultaneous improvement of these two parameters in ε-Ga2O3 MSM photodetectors by establishing a low-defect diffusion barrier for directional carrier transport. Specifically, using a micrometer thickness far exceeding its effective light absorption depth, the ε-Ga2O3 MSM photodetector achieves over 18-fold enhancement of responsivity and simultaneous reduction of the response time, which exhibits a state-of-the-art photo-to-dark current ratio near 108, a superior responsivity of >1300 A/W, an ultrahigh detectivity of >1016 Jones, and a decay time of 123 ms. Combined depth-profile spectroscopic and microscopic analysis reveals the existence of a broad defective region near the lattice-mismatched interface followed by a more defect-free dark region, while the latter one serves as a diffusion barrier to assist frontward carrier transport for substantially enhancing the photodetector performance. This work reveals the critical role of the semiconductor defect profile in tuning carrier transport for fabricating high-performance MSM DUV photodetectors.

Realization of Robust and Ambient-Stable Room-Temperature Ferromagnetism in Wide Bandgap Semiconductor 2D Carbon Nitride Sheets.

Due to their weak intrinsic spin-orbit coupling and a distinct bandgap of 3.06 eV, 2D carbon nitride (CN) flakes are promising materials for next-generation spintronic devices. However, achieving strong room-temperature (RT) and ambient-stable ferromagnetism (FM) remains a huge challenge. Here, we demonstrate that the strong RT FM with a high Curie temperature (TC) up to ∼400 K and saturation magnetization (Ms) of 2.91 emu/g can be achieved. Besides, the RT FM exhibits excellent air stability, with Ms remaining stable for over 6 months. Through the magneto-optic Kerr effect, Hall device, X-ray magnetic circular dichroism, and magnetic force microscopy measurements, we acquired clear evidence of magnetic behavior and magnetic domain evolutions at room temperature. Electrical and optical measurements confirm that the Co-doped CN retains its semiconductor properties. Detailed structural characterizations confirm that the single-atom Co coordination and nitrogen defects as well as C-C covalent bonds are simultaneously introduced into CN. Density functional theory calculations reveal that introducing C-C bonds causes carrier spin polarization, and spin-polarized carrier-mediated magnetic exchange between adjacent Co atoms leads to long-range magnetic ordering in CN. We believe that our findings provide a strong experimental foundation for the enormous potential of 2D wide bandgap semiconductor spintronic devices.

Progress in Gallium Oxide Field-Effect Transistors for High-Power and RF Applications.

Power electronics are becoming increasingly more important, as electrical energy constitutes 40% of the total primary energy usage in the USA and is expected to grow rapidly with the emergence of electric vehicles, renewable energy generation, and energy storage. New materials that are better suited for high-power applications are needed as the Si material limit is reached. Beta-phase gallium oxide (ß-Ga2O3) is a promising ultra-wide-bandgap (UWBG) semiconductor for high-power and RF electronics due to its bandgap of 4.9 eV, large theoretical breakdown electric field of 8 MV cm-1, and Baliga figure of merit of 3300, 3-10 times larger than that of SiC and GaN. Moreover, ß-Ga2O3 is the only WBG material that can be grown from melt, making large, high-quality, dopable substrates at low costs feasible. Significant efforts in the high-quality epitaxial growth of ß-Ga2O3 and ß-(AlxGa1-x)2O3 heterostructures has led to high-performance devices for high-power and RF applications. In this report, we provide a comprehensive summary of the progress in ß-Ga2O3 field-effect transistors (FETs) including a variety of transistor designs, channel materials, ohmic contact formations and improvements, gate dielectrics, and fabrication processes. Additionally, novel structures proposed through simulations and not yet realized in ß-Ga2O3 are presented. Main issues such as defect characterization methods and relevant material preparation, thermal studies and management, and the lack of p-type doping with investigated alternatives are also discussed. Finally, major strategies and outlooks for commercial use will be outlined.

Enhanced Photoresponsivity UV-C Photodetectors Using a p-n Junction Based on Ultra-Wide-Band Gap Sn-Doped ß-Ga2O3 Microflake/MnO Quantum Dots.

Solar-blind self-powered UV-C photodetectors suffer from low performance, while heterostructure-based devices require complex fabrication and lack p-type wide band gap semiconductors (WBGSs) operating in the UV-C region (<290 nm). In this work, we mitigate the aforementioned issues by demonstrating a facile fabrication process for a high-responsivity solar-blind self-powered UV-C photodetector based on a p-n WBGS heterojunction structure, operating under ambient conditions. Here, heterojunction structures based on p-type and n-type ultra-wide band gap WBGSs (i.e. both are characterized by energy gap ≥4.5 eV) are demonstrated for the first time; mainly p-type solution-processed manganese oxide quantum dots (MnO QDs) and n-type Sn-doped ß-Ga2O3 microflakes. Highly crystalline p-type MnO QDs are synthesized using cost-effective and facile pulsed femtosecond laser ablation in ethanol (FLAL), while the n-type Ga2O3 microflakes are prepared by exfoliation. The solution-processed QDs are uniformly dropcasted on the exfoliated Sn-doped ß-Ga2O3 microflakes to fabricate a p-n heterojunction photodetector, resulting in excellent solar-blind UV-C photoresponse characteristics (with a cutoff at ∼265 nm) being demonstrated. Further analyses using XPS demonstrate the good band alignment between p-type MnO QDs and n-type ß-Ga2O3 microflakes with a type-II heterojunction. Superior photoresponsivity (922 A/W) is obtained under bias, while the self-powered responsivity is ∼86.9 mA/W. The fabrication strategy adopted in this study will provide a cost-effective means for the development of flexible and highly efficient UV-C devices suitable for energy-saving large-scale fixable applications.

MOF-Derived In2O3 Microrods for High-Performance Photoelectrochemical Ultraviolet Photodetectors.

Ultraviolet photodetectors (UV PDs) have attracted extensive attention owing to their wide applications, such as optical communication, missile tracking, and fire warning. Wide-bandgap metal-oxide semiconductor materials have become the focus of high-performance UV PD development owing to their unique photoelectric properties and good stability. Compared with other wide-bandgap materials, studies on indium oxide (In2O3)-based photoelectrochemical (PEC) UV PDs are rare. In this work, we explore the photoresponse of In2O3-based PEC UV PDs for the first time. In2O3 microrods (MRs) were synthesized by a hydrothermal method with subsequent annealing. In2O3 MR PEC PDs have good UV photoresponse, showing a high responsivity of 21.19 mA/W and high specific detectivity of 2.03 × 1010 Jones, which surpass most aqueous-type PEC UV PDs. Moreover, In2O3 MR PEC PDs have good multicycle and long-term stability irradiated by 365 nm. Our results prove that In2O3 holds great promise in high-performance PEC UV PDs.

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a "top-down" inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter.

In order to efficiently facilitate various research works related to power converter design and testing for solar photovoltaic (PV) generation systems, it is a great merit to use advanced power-converter-based and digitally controlled PV emulators in place of actual PV modules to reduce the space, cost, and time to obtain the required scenarios of solar irradiances for various functional tests. This paper presents a flexible PV emulator based on gallium nitride (GaN), a wide-bandgap (WBG) semiconductor, and a based synchronous buck converter and controlled with a digital signal processor (DSP). With the help of GaN-based switching devices, the proposed emulator can accurately mimic the dynamic voltage-current characteristics of any PV module under normal irradiance and partial shading conditions. With the proposed PV emulator, it is possible to closely emulate any PV module characteristic both theoretically, based on manufacturer's datasheets, and experimentally, based on measured data from practical PV modules. A curve fitting algorithm is used to handle the real-time generation of control signals for the digital controller. Both simulation with computer software and implementation on 1 kW GaN-based experimental hardware using Texas Instruments DSP as the controller have been carried out. Results show that the proposed emulator achieves efficiency as high as 99.05% and exhibits multifaceted application features in tracking various PV voltage and current parameters, demonstrating the feasibility and excellent performance of the proposed PV emulator.

Electrostatic Control of Insulator-Metal Transition in La-doped SrSnO3 Films.

We investigate the ion gel gating of wide bandgap oxide, La-doped SrSnO3 films grown using radical-based molecular beam epitaxy. An applied positive bias resulted in a reversible electrostatic control of sheet resistance over 3 orders of magnitude at low temperature driving sample from Mott variable range hopping to a weakly localized transport. Analysis of low temperature transport behavior revealed electron-electron interaction and weak localization effects to be the dominant scattering mechanisms. A large voltage window (-4 V ≤ Vg ≤ +4 V) was obtained for reversible electrostatic doping of SrSnO3 films showing robustness of stannate with regards to redox chemistry with electrolyte gating irrespective of the bias type.

Influence of Carrier Gases on the Quality of Epitaxial Corundum-Structured α-Ga2O3 Films Grown by Mist Chemical Vapor Deposition Method.

This report systematically investigates the influence of different carrier gases (O2, N2, and air) on the growth of gallium oxide (Ga2O3) thin films on c-plane sapphire substrates by using the mist-CVD method. Although XRD and Raman measurements show that the pure corundum-structured α-Ga2O3 with single (0006) plane orientation was successfully obtained for all three different carrier gases, the crystal quality could be greatly affected by the carrier gas. When O2 is used as the carrier gas, the smallest full-width at half maximum (FWHM), the very sharp absorption cutoff edge, the perfect lattice structure, the highest growth rate, and the smooth surface can be obtained for the epitaxial α-Ga2O3 film as demonstrated by XRD, UV-VIS, TEM, AFM (Atomic Force Microscope), and SEM measurements. It is proposed that the oxygen content in carrier gas should be responsible for all of these results. XPS (X-ray photoelectron spectroscopy) analysis also confirms that more oxygen elements can be included in epitaxial film when O2 is used as the carrier gas and thus help improve the crystal quality. The proper carrier gas is essential for the high quality α-Ga2O3 growth.

A Novel GaN Metal-Insulator-Semiconductor High Electron Mobility Transistor Featuring Vertical Gate Structure.

A novel structure scheme by transposing the gate channel orientation from a long horizontal one to a short vertical one is proposed and verified by technology computer-aided design (TCAD) simulations to achieve GaN-based normally-off high electron mobility transistors (HEMTs) with reduced on-resistance and improved threshold voltage. The proposed devices exhibit high threshold voltage of 3.1 V, high peak transconductance of 213 mS, and much lower on-resistance of 0.53 mΩ·cm2 while displaying better off-state characteristics owing to more uniform electric field distribution around the recessed gate edge in comparison to the conventional lateral HEMTs. The proposed scheme provides a new technical approach to realize high-performance normally-off HEMTs.