Gate-tunable subband degeneracy in semiconductor nanowires.

Degeneracy and symmetry have a profound relation in quantum systems. Here, we report gate-tunable subband degeneracy in PbTe nanowires with a nearly symmetric cross-sectional shape. The degeneracy is revealed in electron transport by the absence of a quantized plateau. Utilizing a dual gate design, we can apply an electric field to lift the degeneracy, reflected as emergence of the plateau. This degeneracy and its tunable lifting were challenging to observe in previous nanowire experiments, possibly due to disorder. Numerical simulations can qualitatively capture our observation, shedding light on device parameters for future applications.

Shewanella oneidensis MR-1 respires CdSe quantum dots for photocatalytic hydrogen evolution.

Living bio-nano systems for artificial photosynthesis are of growing interest. Typically, these systems use photoinduced charge transfer to provide electrons for microbial metabolic processes, yielding a biosynthetic solar fuel. Here, we demonstrate an entirely different approach to constructing a living bio-nano system, in which electrogenic bacteria respire semiconductor nanoparticles to support nanoparticle photocatalysis. Semiconductor nanocrystals are highly active and robust photocatalysts for hydrogen (H2) evolution, but their use is hindered by the oxidative side of the reaction. In this system, Shewanella oneidensis MR-1 provides electrons to a CdSe nanocrystalline photocatalyst, enabling visible light-driven H2 production. Unlike microbial electrolysis cells, this system requires no external potential. Illuminating this system at 530 nm yields continuous H2 generation for 168 h, which can be lengthened further by replenishing bacterial nutrients.

A Gauss's law analysis of redox active adsorbates on semiconductor electrodes: The charging and faradaic currents are not independent.

A detailed framework for modeling and interpreting the data in totality from a cyclic voltammetric measurement of adsorbed redox monolayers on semiconductor electrodes has been developed. A three-layer model consisting of the semiconductor space-charge layer, a surface layer, and an electrolyte layer is presented that articulates the interplay between electrostatic, thermodynamic, and kinetic factors in the electrochemistry of a redox adsorbate on a semiconductor. Expressions are derived that describe the charging and faradaic current densities individually, and an algorithm is demonstrated that allows for the calculation of the total current density in a cyclic voltammetry measurement as a function of changes in the physical properties of the system (e.g., surface recombination, dielectric property of the surface layer, and electrolyte concentration). The most profound point from this analysis is that the faradaic and charging current densities can be coupled. That is, the common assumption that these contributions to the total current are always independent is not accurate. Their interrelation can influence the interpretation of the charge-transfer kinetics under certain experimental conditions. More generally, this work not only fills a long-standing knowledge gap in electrochemistry but also aids practitioners advancing energy conversion/storage strategies based on redox adsorbates on semiconductor electrodes.

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Molecular Additives Improve the Selectivity of CO2 Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride.

Photoelectrochemical CO2 reduction (CO2R) is an appealing solution for converting carbon dioxide into higher-value products. However, CO2R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction that is dominant on semiconductor surfaces in aqueous electrolytes. We demonstrate that functionalizing gold/p-type gallium nitride devices with a film derived from diphenyliodonium triflate suppresses hydrogen generation from 90% to 18%. As a result, we observe increases in the Faradaic efficiency and partial current density for carbon monoxide of 50% and 3-fold, respectively. Furthermore, we demonstrate through optical absorption measurements that the molecular film employed herein, regardless of thickness, does not affect the photocathode's light absorption. Altogether, this study provides a rigorous platform for elucidating the catalytic structure-property relationships to enable engineering of active, stable, and selective materials for photoelectrochemical CO2R.

Resolving Multielement Semiconductor Nanocrystals at the Atomic Level: Complete Deciphering of Domains and Order in Complex CuαZnßSnγSeδ (CZTSe) Tetrapods.

Semiconductor nanocrystals (NCs) with high elemental and structural complexity can be engineered to tailor for electronic, photovoltaic, thermoelectric, and battery applications etc. However, this greater complexity causes ambiguity in the atomic structure understanding. This in turn hinders the mechanistic studies of nucleation and growth, the theoretical calculations of functional properties, and the capability to extend functional design across complementary semiconductor nanocrystals. Herein, we successfully deciphered the atomic arrangements of 4 different nanocrystal domains in CuαZnßSnγSeδ (CZTSe) nanocrystals using crucial zone axis analysis on multiple crystals in different orientations. The results show that the essence of crystallographic progression from binary to multielemental semiconductors is actually the change of theoretical periodicity. This transition is caused by decreased symmetry in the crystal instead of previously assumed crystal deformation. We further reveal that these highly complex crystalline entities have highly ordered element arrangements as opposed to the previous understanding that their elemental orderings are random.

Shadowed versus Etched Superconductor-Semiconductor Junctions in Al/InAs Nanowires.

Hybrid semiconductor-superconductor nanowires have emerged as a cornerstone in modern quantum devices. Integrating such nanowires into hybrid devices typically requires extensive postgrowth processing which may affect device performance unfavorably. Here, we present a technique for in situ shadowing superconductors on nanowires and compare the structural and electronic properties of Al junctions formed by shadowing versus etching. Based on transmission electron microscopy, we find that typical etching procedures lead to atomic-scale surface roughening. This surface perturbation may cause a reduction of the electron mobility as demonstrated in transport measurements. Further, we display advanced shadowing geometries aiding in the pursuit of bringing fabrication of hybrid devices in situ. Finally, we give examples of shadowed junctions exploited in various device geometries that exhibit high-quality quantum transport signatures.

Compositional Engineering of Cu-Doped SnO Film for Complementary Metal Oxide Semiconductor Technology.

Metal oxide semiconductor (MOS)-based complementary thin-film transistor (TFT) circuits have broad application prospects in large-scale flexible electronics. To simplify circuit design and increase integration density, basic complementary circuits require both p- and n-channel transistors based on an individual semiconductor. However, until now, no MOSs that can simultaneously show p- and n-type conduction behavior have been reported. Herein, we demonstrate for the first time that Cu-doped SnO (Cu:SnO) with HfO2 capping can be employed for high-performance p- and n-channel TFTs. The interstitial Cu+ can induce an n-doping effect while restraining electron-electron scatterings by removing conduction band minimum degeneracy. As a result, the Cu3 atom %:SnO TFTs exhibit a record high electron mobility of 43.8 cm2 V-1 s-1. Meanwhile, the p-channel devices show an ultrahigh hole mobility of 2.4 cm2 V-1 s-1. Flexible complementary logics are then established, including an inverter, NAND gates, and NOR gates. Impressively, the inverter exhibits an ultrahigh gain of 302.4 and excellent operational stability and bending reliability.

Observation of Ferromagnetism in Dilute Magnetic Halide Perovskite Semiconductors.

Dilute magnetic semiconductors (DMSs) have attracted much attention because of their potential use in spintronic devices. Here, we demonstrate the observation of robust ferromagnetism in a solution-processable halide perovskite semiconductor with dilute magnetic ions. By codoping of magnetic (Fe2+) and aliovalent (Bi3+) metal ions into CH3NH3PbCl3 (MAPbCl3) perovskite, ferromagnetism with well-saturated magnetic hysteresis loops and a maximum coercivity field of 1280 Oe was observed below 12 K. The ferromagnetic resonance measurements revealed that the incorporation of aliovalent ions modulates the carrier concentration and plays an essential role in realizing the ferromagnetism in dilute magnetic halide perovskites. Magnetic ions are proposed to interact through itinerant charge carriers to achieve ferromagnetic coupling. Our work provides a new avenue for the development of solution-processable magnetic semiconductors.

Polarization-Selective Enhancement of Telecom Wavelength Quantum Dot Transitions in an Elliptical Bullseye Resonator.

Semiconductor quantum dots are promising candidates for the generation of nonclassical light. Coupling a quantum dot to a device capable of providing polarization-selective enhancement of optical transitions is highly beneficial for advanced functionalities, such as efficient resonant driving schemes or applications based on optical cyclicity. Here, we demonstrate broadband polarization-selective enhancement by coupling a quantum dot emitting in the telecom O-band to an elliptical bullseye resonator. We report bright single-photon emission with a degree of linear polarization of 96%, Purcell factor of 3.9 ± 0.6, and count rates up to 3 MHz. Furthermore, we present a measurement of two-photon interference without any external polarization filtering. Finally, we demonstrate compatibility with compact Stirling cryocoolers by operating the device at temperatures up to 40 K. These results represent an important step toward practical integration of optimal quantum dot photon sources in deployment-ready setups.

Size Dependent Specific Heat Capacity of PbSe Nanocrystals.

Specific heat capacity is one of the most fundamental thermodynamic properties of materials. In this work, we measured the specific heat capacity of PbSe nanocrystals with diameters ranging from 5 to 23 nm, and its value increases significantly from 0.2 to 0.6 J g-1 °C-1. We propose a mass assignment model to describe the specific heat capacity of nanocrystals, which divides it into four parts: electron, inner, surface, and ligand. By eliminating the contribution of ligand and electron specific heat capacity, the specific heat capacity of the inorganic core is linearly proportional to its surface-to-volume ratio, showing the size dependence. Based on this linear relationship, surface specific heat capacity accounts for 40-60% of the specific heat capacity of nanocrystals with size decreasing. It can be attributed to the uncoordinated surface atoms, which is evidenced by the appearance of extra surface phonons in Raman spectra and ab initio molecular dynamics (AIMD) simulations.

High Performance Indium-Tin-Oxide Schottky Diodes for Terahertz Band Operation.

Schottky diode, capable of ultrahigh frequency operation, plays a critical role in modern communication systems. To develop cost-effective and widely applicable high-speed diodes, researchers have delved into thin-film semiconductors. However, a performance gap persists between thin-film diodes and conventional bulk semiconductor-based ones. Featuring high mobility and low permittivity, indium-tin-oxide has emerged to bridge this gap. Nevertheless, due to its high carrier concentration, indium-tin-oxide has predominantly been utilized as electrode rather than semiconductor. In this study, a remarkable quantum confinement induced dedoping phenomenon was discovered during the aggressive indium-tin-oxide thickness downscaling. By leveraging such a feature to change indium-tin-oxide from metal-like into semiconductor-like, in conjunction with a novel heterogeneous lateral design facilitated by an innovative digital etch, we demonstrated an indium-tin-oxide Schottky diode with a cutoff frequency reaching terahertz band. By pushing the boundaries of thin-film Schottky diodes, our research offers a potential enabler for future fifth-generation/sixth-generation networks, empowering diverse applications.

Wavelength-Dependent Multistate Programmability and Optoelectronic Logic-in-Memory Operation from the Narrow Bandgap pNDI-SVS Floating Gate.

This study introduces wavelength-dependent multistate programmable optoelectronic logic-in-memory (OLIM) operation using a broadband photoresponsive pNDI-SVS floating gate. The distinct optical absorption of the relatively large bandgap DNTT channel (2.6 eV) and the narrow bandgap pNDI-SVS floating gate (1.37 eV) lead to varying light-induced charge carrier accumulation across different wavelengths. In the proposed OLIM device comprising the p-type pNDI-SVS-based optoelectronic memory (POEM) transistor and an IGZO n-type transistor, we achieve controllable output voltage signals by modulating the pull-up performance through optical wavelength and applied bias manipulation. Real-time OLIM operation yields four discernible output values. The device's high mechanical flexibility and seamless surface integration among the paper substrate, pNDI-SVS, parylene gate dielectric, and DNTT region render it compatible for integration into paper-based optoelectronics. Our flexible POEM device on name card substrates demonstrates stable operational performance, with minimal variation (8%) after 100 cycles of repeated memory operation, remaining reliable across various angle measurements.

Liquid-Metal-Based Spin-Coating Exfoliation for Atomically Thin Metal Oxide Synthesis.

Two-dimensional (2D) semiconductors possess exceptional electronic, optical, and magnetic properties, making them highly desirable for widespread applications. However, conventional mechanical exfoliation and epitaxial growth methods are insufficient in meeting the demand for atomically thin films covering large areas while maintaining high quality. Herein, leveraging liquid metal oxidation reaction, we propose a motorized spin-coating exfoliation strategy to efficiently produce large-area 2D metal oxide (2DMO) semiconductors with high crystallinity, atomically thin thickness, and flat surfaces on diverse substrates. Moreover, we realized a 2D gallium oxide-based deep ultraviolet solar-blind photodetector featuring a metal-semiconductor-metal structure, showcasing high responsivity (8.24 A W-1) at 254 nm and excellent sensitivity (4.3 × 1012 cm Hz1/2 W-1). This novel liquid-metal-based spin-coating exfoliation strategy offers great potential for synthesizing atomically thin 2D semiconductors, opening new avenues for future functional electronic and optical applications.

Near-Field Probing of Microwave Oscillators with Josephson Microscopy.

Voltage-controlled oscillators, serving as fundamental components in semiconductor chips, find extensive applications in diverse modules such as phase-locked loops, clock generators, and frequency synthesizers within high-frequency integrated circuits. This study marks the first implementation of superconducting Josephson probe microscopy for near-field microwave detection on multiple voltage-controlled oscillators. Focusing on spectrum tracking, various phenomena, such as stray spectra and frequency drifts, were found under nonsteady operating states. Parasitic electromagnetic fields, originating from power supply lines and frequency divider circuits, were identified as sources of interference between units. The investigation further determined optimal working states by analyzing features of the microwave distributions. Our research not only provides insights into the optimization of circuit design and performance enhancement in oscillators but also emphasizes the significance of nondestructive near-field microwave microscopy as a pivotal tool in characterizing integrated millimeter-wave chips.

Atomic Layer Deposition of WO3-Doped In2O3 for Reliable and Scalable BEOL-Compatible Transistors.

Tungsten oxide (WO3) doped indium oxide (IWO) field-effect transistors (FET), synthesized using atomic layer deposition (ALD) for three-dimensional integration and back-end-of-line (BEOL) compatibility, are demonstrated. Low-concentration (1∼4 W atom %) WO3-doping in In2O3 films is achieved by adjusting cycle ratios of the indium and tungsten precursors with the oxidant coreactant. Such doping suppresses oxygen deficiency from In2O2.5 to In2O3 stoichiometry with only 1 atom % W, allowing devices to turn off stably and enhancing threshold voltage stability. The ALD IWO FETs exhibit superior performance, including a low subthreshold slope of 67 mV/decade and negligible hysteresis. Strong tunability of the threshold voltage (Vth) is achieved through W concentration tuning, with 2 atom % IWO FETs showing an optimized Vth for enhancement-mode and a high drain current. ALD IWO FETs have remarkable stability under bias stress and nearly ideal performance extending to sub-100 nm channel lengths, making them promising candidates for high-performance monolithic 3D integrated devices.

Heat Dissipation Mechanisms in Hybrid Superconductor-Semiconductor Devices Revealed by Joule Spectroscopy.

Understanding heating and cooling mechanisms in mesoscopic superconductor-semiconductor devices is crucial for their application in quantum technologies. Owing to their poor thermal conductivity, heating effects can drive superconducting-to-normal transitions even at low bias, observed as sharp conductance dips through the loss of Andreev excess currents. Tracking such dips across magnetic field, cryostat temperature, and applied microwave power allows us to uncover cooling bottlenecks in different parts of a device. By applying this "Joule spectroscopy" technique, we analyze heat dissipation in devices based on InAs-Al nanowires and reveal that cooling of superconducting islands is limited by the rather inefficient electron-phonon coupling, as opposed to grounded superconductors that primarily cool by quasiparticle diffusion. We show that powers as low as 50-150 pW are able to suppress superconductivity on the islands. Applied microwaves lead to similar heating effects but are affected by the interplay of the microwave frequency and the effective electron-phonon relaxation time.

Spectrum-Dependent Image Convolutional Processing via a Two-Dimensional Polarization-Sensitive Photodetector.

Currently, the improvement in the processing capacity of traditional processors considerably lags behind the demands of real-time image processing caused by the advancement of photodetectors and the widespread deployment of high-definition image sensors. Therefore, achieving real-time image processing at the sensor level has become a prominent research domain in the field of photodetector technology. This goal underscores the need for photodetectors with enhanced multifunctional integration capabilities than can perform real-time computations using optical or electrical signals. In this study, we employ an innovative p-type semiconductor GaTe0.5Se0.5 to construct a polarization-sensitive wide-spectral photodetector. Leveraging the wide-spectral photoresponse, we realize three-band imaging within a wavelength range of 390-810 nm. Furthermore, real-time image convolutional processing is enabled by configuring appropriate convolution kernels based on the polarization-sensitive photocurrents. The innovative design of the polarization-sensitive wide-spectral GaTe0.5Se0.5-based photodetector represents a notable contribution to the domain of real-time image perception and processing.

Unveiling the 3D Morphology of Epitaxial GaAs/AlGaAs Quantum Dots.

Strain-free GaAs/AlGaAs semiconductor quantum dots (QDs) grown by droplet etching and nanohole infilling (DENI) are highly promising candidates for the on-demand generation of indistinguishable and entangled photon sources. The spectroscopic fingerprint and quantum optical properties of QDs are significantly influenced by their morphology. The effects of nanohole geometry and infilled material on the exciton binding energies and fine structure splitting are well-understood. However, a comprehensive understanding of GaAs/AlGaAs QD morphology remains elusive. To address this, we employ high-resolution scanning transmission electron microscopy (STEM) and reverse engineering through selective chemical etching and atomic force microscopy (AFM). Cross-sectional STEM of uncapped QDs reveals an inverted conical nanohole with Al-rich sidewalls and defect-free interfaces. Subsequent selective chemical etching and AFM measurements further reveal asymmetries in element distribution. This study enhances the understanding of DENI QD morphology and provides a fundamental three-dimensional structural model for simulating and optimizing their optoelectronic properties.

Metal-Semiconductor Heterojunction with Ohmic Contact Realizes Efficient Infrared-Light-Driven Photocatalysis.

Efficient utilization of solar energy for photocatalytic applications, particularly in the infrared spectrum, is crucial for addressing environmental challenges and energy scarcity. Herein we present a general strategy for constructing efficient infrared-driven photocatalysts in a metal/semiconductor heterojunction with Ohmic contact, where metals with low work function as the infrared-light absorber and semiconductors with electron storage ability can overcome the unfavorable electron flowback. Taking the NixB/MO2 (M = Ce, Ti, Sn, Ge, Zr, etc.) heterojunction as an example, both experimental and theoretical investigations reveal that the formation of an Ohmic contact facilitates the transfer of hot electrons from NixB to MO2, which are stored by the ion redox pairs for the variable valence character of M. As expected, the heterojunction exhibits remarkable photocatalytic activity under infrared light (λ ≥ 800 nm), as evidenced by the efficient photofixation of CO2 to high-value-added cyclic carbonates. This study offers a general platform for designing infrared-light-driven photocatalysts.