Simultaneously Regulated Highly Polarized and Long-Lived Valley Excitons in WSe2/GaN Heterostructures.

Interlayer excitons, with prolonged lifetimes and tunability, hold potential for advanced optoelectronics. Previous research on the interlayer excitons has been dominated by two-dimensional heterostructures. Here, we construct WSe2/GaN composite heterostructures, in which the doping concentration of GaN and the twist angle of bilayer WSe2 are employed as two ingredients for the manipulation of exciton behaviors and polarizations. The exciton energies in monolayer WSe2/GaN can be regulated continuously by the doping levels of the GaN substrate, and a remarkable increase in the valley polarizations is achieved. Especially in a heterostructure with 4°-twisted bilayer WSe2, a maximum polarization of 38.9% with a long lifetime is achieved for the interlayer exciton. Theoretical calculations reveal that the large polarization and long lifetime are attributed to the high exciton binding energy and large spin flipping energy during depolarization in bilayer WSe2/GaN. This work introduces a distinctive member of the interlayer exciton with a high degree of polarization and a long lifetime.

Manipulations of Electronic and Spin States in Co-Quantum Dot/WS2 Heterostructure on a Metal-Dielectric Composite Substrate by Controlling Interfacial Carriers.

Charge and spin are two intrinsic attributes of carriers governing almost all of the physical processes and operation principles in materials. Here, we demonstrate the manipulation of electronic and spin states in designed Co-quantum dot/WS2 (Co-QDs/WS2) heterostructures by employing a metal-dielectric composite substrate and via scanning tunneling microscope. By repeatedly scanning under a unipolar bias, switching the bias polarity, or applying a pulse through nonmagnetic or magnetic tips, the Co-QDs morphologies exhibit a regular and reproducible transformation between bright and dark dots. First-principles calculations reveal that these tunable characters are attributed to the variation of density of states and the transition of magnetic anisotropy energy induced by carrier accumulation. It also suggests that the metal-dielectric composite substrate is successful in creating the interfacial potential for carrier accumulation and realizes the electrically controllable modulations. These results will promote the exploration of electron-matter interactions in quantum systems and provide an innovative way to facilitate the development of spintronics.

Chirality-Dependent Valley Polarization in Magnetic van der Waals Heterostructures via Spin-Selective Charge Transfer.

Magnetic proximity interaction provides a promising route to manipulate the spin and valley degrees of freedom in van der Waals heterostructures. Here, we report a control of valley pseudospin in the WS2/MoSe2 heterostructure by utilizing the magnetic proximity effect of few-layered CrBr3 and, for the first time, observe a substantial difference in valley polarization of intra/interlayer excitons under different circularly polarized laser excitations, referred to as chirality-dependent valley polarization. Theoretical and experimental results reveal that the spin-selective charge transfer between MoSe2 and CrBr3, as well as between MoSe2 and WS2, is mostly responsible for the chiral feature of valley polarization in comparison with the proximity exchange field. This means that a long-distance manipulation of exciton behaviors in multilayer heterostructures can be achieved through spin-selective charge transfer. This work marks a significant advancement in the control of spin and valley pseudospin in multilayer structures.

In-depth insights into polarization-dependent light extraction mechanisms of AlGaN-based deep ultraviolet light-emitting diodes.

The AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) dominated by transverse-magnetic (TM) polarized emission suffer from extremely poor light extraction efficiency (LEE) from their top surface, which severely limits the device performance. In this study, the underlying physics of polarization-dependent light extraction mechanisms of AlGaN-based DUV LEDs has been explored in depth via simple Monte Carlo ray-tracing simulations with Snell's law. It is especially worth noting that the structures of the p-type electron blocking layer (p-EBL) and multi-quantum wells (MQWs) have a significant impact on light extraction behavior, especially for TM-polarized emission. Thus, an artificial vertical escape channel (named GLRV) has been constructed to efficiently extract the TM-polarized light through the top surface, by adjusting the structures of the p-EBL, MQWs, sidewalls, and using the adverse total internal reflection in a positive manner. The results show that the enhancement times of the top-surface LEE is up to 18 for TM-polarized emission in the 300 × 300 µm2 chip comprising a single GLRV structure, and further increases to 25 by dividing this single GLRV structure into a 4 × 4 micro-GLRV array structure. This study provides a new perspective for understanding and modulating the extraction mechanisms of polarized light to overcome the inherently poor LEE for the TM-polarized light.

Enhanced light extraction efficiency of GaN-based green micro-LED modulating by a thickness-tunable SiO2 passivation structure.

Green micro-light emitting diodes (micro-LEDs) is one of the three primary color light sources as full-color display, which serves as a key research object in the field of micro-LED display. As the micro-LED size decreases, the surface-area-to-volume ratio of the device increases, leading to more serious damage on the sidewall by inductively coupled plasma (ICP) etching. The passivation process of SiO2 provides an effective method to reduce sidewall damage caused by ICP etching. In this work, green rectangular micro-LEDs with passivation layer thickness of 0∼600 nm was designed using the finite-difference time-domain (FDTD) simulation. In order to verify the simulation results, the micro-LED array was fabricated by parallel laser micro-lens array (MLA) lithography in high speed and large area. The effect of the SiO2 passivation layer thickness on the performance of the green micro-LED was analyzed, which shows that the passivation layer thickness-light extraction efficiency curve fluctuates periodically. For the sample with 90 nm thickness of SiO2 passivation layer, there exists a small leakage current and higher operating current density, and the maximum external quantum efficiency (EQE) is 2.8 times higher than micro-LED without SiO2 passivation layer.

Reply to the 'Comment on "Regulating the valence level arrangement of high-Al-content AlGaN quantum wells using additional potentials with Mg doping"' by W. Lambrecht, Phys. Chem. Chem. Phys., 2023, 25, DOI: 10.1039/D2CP01080A.

We are happy to receive the attention of Prof. Lambrecht regarding our paper and we appreciate his comments. However, it is hard to agree with his judgement about the "incorrect application", "incorrect interpretation", and the work being "misleading". Therefore, we would like to provide a defense and further discussion in this reply.

P-type nitrogen-doped ß-Ga2O3: the role of stable shallow acceptor NO-VGa complexes.

In-depth understanding of the acceptor states and origins of p-type conductivity is essential and critical to overcome the great challenge for the p-type doping of ultrawide-bandgap oxide semiconductors. In this study we find that stable NO-VGa complexes can be formed with ε(0/-) transition levels significantly smaller than those of the isolated NO and VGa defects using N2 as the dopant source. Due to the defect-induced crystal-field splitting of the p orbitals of Ga, O and N atoms, and the Coulomb binding between NO(II) and VGa(I), an a' doublet state at 1.43 eV and an a'' singlet state at 0.22 eV above the valence band maximum (VBM) are formed for the ß-Ga2O3:NO(II)-VGa(I) complexes with an activated hole concentration of 8.5 × 1017 cm-3 at the VBM, indicating the formation of a shallow acceptor level and the feasibility to obtain p-type conductivity in ß-Ga2O3 even when using N2 as the dopant source. Considering the transition from NO(II)-V0Ga(I) + e to NO(II)-V-Ga(I), an emission peak at 385 nm with a Franck-Condon shift of 1.08 eV is predicted. These findings are of general scientific significance as well as technological application significance for p-type doping of ultrawide-bandgap oxide semiconductors.

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper.

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W-1 @ 265-276 nm, and high photoconductive gain of ≈2 × 103 . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9 V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.

Simplified numerical modeling for Fano interference-induced asymmetric light reflectance effect using equivalent medium theory.

Localized surface plasmons exhibit promising capabilities in optoelectronic devices. In most cases, the metal nanoparticle arrays are located on interfaces or inside optical cavities. Fano interferences have been observed and explained via the interference between the waves generated by the localized surface plasmon and dielectric interfaces. Conventionally, these Fano interferences are modeled using the modified Fresnel equation. However, certain issues persist in the fundamental physics or in the numerical calculation process. Here, we adopt the equivalent medium theory (Maxwell-Garnett theory, MGT) to calculate and elucidate Fano interferences in different structures, in the region comprising nanoparticle arrays and dielectrics equivalent to a homogeneous layer of media via the mean field theory. Using this method, the Fano interference can be modeled by mixing different materials, i.e., metals and dielectrics in these cases. Furthermore, a multiple-layered equivalent medium theory is proposed to significantly improve the scalability of this simplified numerical method. In other words, this method can be easily extended to nanoparticles with different shapes, sizes, and materials; in addition, it exhibits robust practicability. Compared with the modified Fresnel equation and finite-difference time-domain methods, this MGT-based method can effectively minimize the calculation process, which is beneficial to the prospective application of plasmon photonics.

Enhanced Faraday effects of magneto-plasmonic crystals with plasmonic hexagonal hole arrays.

Magneto-optical (MO) properties of the bilayed Au/BIG and trilayered Au/BIG/Au magneto-plasmonic crystals (MPCs) were analyzed by the finite-difference time-domain method. In contrast to the low deflection angle and transmission of the smooth thin film, all the heterostructures with perforated holes in the top Au film displayed a similar trend with two strong resonant bands in Faraday rotation and transmittance in the near infrared wavelength range. The bands and electric distribution relative to the component and hole structure were revealed. The MPC with plasmonic hexagonal holes was found to own superior Faraday effects with distinctive anisotropy. The evolution of the resonant bands with the size and period of hexagonal holes, the thickness of different layers, and the incident light polarization was illustrated. The Faraday rotation of the optimized bilayed and trilayered hexagonal MPCs was improved 15.3 and 17.5 times, and the transmittance was enhanced 12.1 and 11.1 folds respectively at the resonant wavelength in comparison to the continuous Au/BIG film, indicating that the systems might find potential application in MO devices.

Regulating the valence level arrangement of high-Al-content AlGaN quantum wells using additional potentials with Mg doping.

Quantum states and arrangement of valence levels determine most of the electronic and optical properties of semiconductors. Since the crystal field split-off hole (CH) band is the top valence band in high-Al-content AlGaN, TM-polarized optical anisotropy has become the limiting factor for efficient deep-ultraviolet (DUV) light emission. Additional potentials, including on-site Coulomb interaction and orbital state coupling induced by magnesium (Mg) doping, are proposed in this work to regulate the valence level arrangement of AlN/Al0.75Ga0.25N quantum wells (QWs). Diverse responses of valence quantum states |pi〉 (i = x, y, or z) of AlGaN to additional potentials due to different configurations and interactions of orbitals revealed by first-principles simulations are understood in terms of the linear combination of atomic orbital states. A positive charge and large Mg dopant in QWs introduce an additional Coulomb potential and modulate the orbital coupling distance. For the CH band (pz orbital), the Mg-induced Coulomb potential compensates the orbital coupling energy. Meanwhile, the heavy/light hole (HH/LH) bands (px and py orbitals) are elevated by the Mg-induced Coulomb potential. Consequently, HH/LH energy levels are relatively shifted upward and replace the CH level to be the top of the valence band. The inversion of optical anisotropy and enhancement of TE-polarized emission are further confirmed experimentally via spectroscopic ellipsometry.

Electronic structure variations of polar and nonpolar ZnO lattices with nitrogen-ion bombardment using synchrotron-based in situ photoemission and X-ray absorption spectroscopy.

Surface polarity with different crystal orientations has been demonstrated as a crucial parameter in determining the physical properties and device applications in many transition metal oxide and semiconductor compound systems. The influences of surface polarity on electronic structures in nitrogen-incorporated ZnO lattices have been investigated in the present work. The successful doping of nitrogen atoms in ZnO lattices is suggested by the existence of N-related chemical bonds obtained from X-ray photoelectron spectroscopy analysis where a pronounced N-Zn peak intensity has been observed in the (000\bar 1)-terminated polar ZnO compound compared with the (10\bar 10)-terminated nonpolar ZnO compound. An energy shift of the valence band maximum towards the Fermi level has been resolved for both polar and nonpolar ZnO lattices, whereas a charge redistribution of the O 2p hybridized states is only resolved for o-plane ZnO with a polar surface. Angular-dependent X-ray absorption analyses at the O K-edge reveal enhanced surface-state contributions and asymmetric O 2p orbital occupations in the (000\bar 1)-terminated o-plane ZnO compound. The results shed light on the efficient nitrogen doping in ZnO lattices with polar surfaces. The comprehensive electronic structure investigations of correlations between impurity doping and surface polarity in ZnO lattices may also offer guidance for the material design in other transition metal oxide and semiconductor systems.

Localized surface plasmon enhanced Ga2O3 solar blind photodetectors.

Enhancement in the light interaction between plasmonic nanoparticles (NPs) and semiconductors is a promising way to enhance the performance of optoelectronic devices beyond the conventional limit. In this work, we demonstrated improved performance of Ga2O3 solar-blind photodetectors (PDs) by the decoration of Rh metal nanoparticles (NPs). Integrated with Rh NPs on oxidized Ga2O3 surface, the resultant device exhibits a reduced dark current of about 10 pA, an obvious enhancement in peak responsivity of 2.76 A/W at around 255 nm, relatively fast response and recovery decay times of 1.76 ms/0.80 ms and thus a high detectivity of ∼1013 Jones. Simultaneously, the photoresponsivity above 290 nm wavelength decreases significantly with improved rejection ratio between ultraviolet A (UVA) and ultraviolet B (UVB) regions, indicative of enhanced wavelength detecting selectivity. The plasmonic resonance features observed in transmittance spectra are consistent with the finite difference time-domain (FDTD) calculations. This agreement indicates that the enhanced electric field strength induced by the localized surface plasmon resonance is responsible for the enhanced absorption and photoresponsivity. The formed localized Schottky barrier at the interface of Rh/Ga2O3 will deplete the carriers at the Ga2O3 surface and lead to the remarkable reduced dark current and thus improve the detectivity. These findings provide direct evidence for Rh plasmonic enhancement in solar-blind spectral region, offering an alternative pathway for the rational design of high-performance solar-blind PDs.

Nanowire-Seeded Growth of Single-Crystalline (010) ß-Ga2 O3 Nanosheets with High Field-Effect Electron Mobility and On/Off Current Ratio.

2D ß-Ga2 O3 nanosheets, as fundamental materials, have great potential in next generations of ultraviolet transparent electrodes, high-temperature gas sensors, solar-blind photodetectors, and power devices, while their synthesis and growth with high crystalline quality and well-controlled orientation have not been reported yet. The present study demonstrates how to grow single-crystalline ultrathin quasi-hexagonal ß-Ga2 O3 nanosheets with nanowire seeds and proposes a hierarchy-oriented growth mechanism. The hierarchy-oriented growth is initiated by epitaxial growth of a single-crystalline ( 2 - 01 ) ß-Ga2 O3 nanowire on a GaN nanocrystal and followed by homoepitaxial growth of quasi-hexagonal (010) ß-Ga2 O3 nanosheets. The undoped 2D (010) ß-Ga2 O3 nanosheet field effect transistor has a field-effect electron mobility of 38 cm2 V-1 s-1 and an on/off current ratio of 107 with an average subthreshold swing of 150 mV dec-1 . The from-nanowires-to-nanosheets technique paves a novel way to fabricate nanosheets, which has great impact on the field of nanomaterial synthesis and growth and the area of nanoelectronics as well.

In-plane Anisotropy of Quantum Transport in Artificial Two-dimensional Au Lattices.

We report an experimental observation and direct control of quantum transport in artificial two-dimensional Au lattices. Combining the advanced techniques of low-temperature deposition and newly developed double-probe scanning tunneling spectroscopy, we display a two-dimensional carrier transport and demonstrate a strong in-plane transport modulation in the two-dimensional Au lattices. In well-ordered Au lattices, we observe the carrier transport behavior manifesting as a band-like feature with an energy gap. Furthermore, controlled structural modification performed by constructing coupled "stadiums" enables a transition of system dynamics in the lattices, which in turn establishes tunable resonant transport throughout a wide energy range. Our findings open the possibility of the construction and transport engineering of artificial lattices by the geometrical arrangement of scatterers and quantum chaotic dynamics.

Effect of Surface Morphology and Magnetic Impurities on the Electronic Structure in Cobalt-Doped BaFe2As2 Superconductors.

Combined scanning tunneling microscopy, spectroscopy, and local barrier height (LBH) studies show that low-temperature-cleaved optimally doped Ba(Fe1-xCox)2As2 crystals with x = 0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba- and As-dominated surface terminations, respectively. Magnetic impurities, possibly due to Co or Fe atoms, are believed to create local in-gap state and, in addition, suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors and its relation with the electronic structures.

Structural and optical characteristics of the hexagonal ZnO films grown on cubic MgO (001) substrates.

In this Letter, we report on the structural and optical characteristics of ZnO films with a wurtzite structure grown on MgO (001) substrates with cubic structures. The ZnO films were prepared through the molecular beam epitaxy method, and growth orientation transformation from [0001] to [10-10] direction was observed with the change of growth temperature and thickness. The x-ray diffraction pole figures and in situ RHEED patterns demonstrated that the rotational relationship among grains within the ZnO films appeared in a typical two-fold rotation of about 30° for the [0001] growth orientation and four-fold rotation of about 30° or 60° for the [10-10] growth orientation, respectively. Last, we investigated their optical properties through measuring the transmission and photoluminescence spectra of the ZnO films, which showed the bulk-like bandgap feature of the ZnO films in spite of the existing growth orientation transformation.

Material Design of Ultra-Thin InN/GaN Superlattices for a Long-Wavelength Light Emission.

GaN heterostructure is a promising material for next-generation optoelectronic devices, and Indium gallium nitride (InGaN) has been widely used in ultraviolet and blue light emission. However, its applied potential for longer wavelengths still requires exploration. In this work, the ultra-thin InN/GaN superlattices (SL) were designed for long-wavelength light emission and investigated by first-principles simulations. The crystallographic and electronic properties of SL were comprehensively studied, especially the strain state of InN well layers in SL. Different strain states of InN layers were applied to modulate the bandgap of the SL, and the designed InN/GaN heterostructure could theoretically achieve photon emission of at least 650 nm. Additionally, we found the SL had different quantum confinement effects on electrons and holes, but an efficient capture of electron-hole pairs could be realized. Meanwhile, external forces were also considered. The orbital compositions of the valence band maximum (VBM) were changed with the increase in tensile stress. The transverse electric (TE) mode was found to play a leading role in light emission in normal working conditions, and it was advantageous for light extraction. The capacity of ultra-thin InN/GaN SL on long-wavelength light emission was theoretically investigated.

Phase-Dependent Magnetic Proximity Modulations on Valley Polarization and Splitting.

Proximate-induced magnetic interactions present a promising strategy for precise manipulation of valley degrees of freedom. Taking advantage of the splendid valleytronic platform of transition metal dichalcogenides, magnetic two-dimensional VSe2 with different phases are introduced to intervene in the spin of electrons and modulate their valleytronic properties. When constructing the heterostructures, 1T-VSe2/WX2 (X = S and Se) showcases significant improvement in the valley polarizations at room temperature, while 2H-VSe2/WX2 exhibits superior performance at low temperatures and demonstrates heightened sensitivity to the external magnetic field. Simultaneously, considerable valley splitting with a large geff factor up to -29.0 is observed in 2H-VSe2/WS2, while it is negligible in 1T-VSe2/WX2. First-principles calculations reveal a phase-dependent magnetic proximity mechanism on the valleytronic modulations, which is dominated by interfacial charge transfer in 1T-VSe2/WX2 and the proximity exchange field in 2H-VSe2/WX2 heterostructures. The effective control over valley degrees of freedom will bridge the valleytronic physics and devices, rendering enormous potential in the field of valley quantum applications.

Ferromagnetic Insulating Ground-State Resolved in Mixed Protons and Oxygen Vacancies-Doped La0.67Sr0.33CoO3 Thin Films via Ionic Liquid Gating.

The realization of ferromagnetic insulating ground state is a critical prerequisite for spintronic applications. By applying electric field-controlled ionic liquid gating (ILG) to stoichiometry La0.67Sr0.33CoO3 thin films, the doping of protons (H+) has been achieved for the first time. Furthermore, a hitherto-unreported ferromagnetic insulating phase with a remarkably high Tc up to 180 K has been observed which can be attributed to the doping of H+ and the formation of oxygen vacancies (VO). The chemical formula of the dual-ion migrated film has been identified as La2/3Sr1/3CoO8/3H2/3 based on combined Co L23-edge absorption spectra and configuration interaction cluster calculations, from which we are able to explain the ferromagnetic ground state in terms of the distinct magnetic moment contributions from Co ions with octahedral (Oh) and tetrahedral (Td) symmetries following antiparallel spin alignments. Further density functional theory calculations have been performed to verify the functionality of H+ as the transfer ion and the origin of the novel ferromagnetic insulating ground state. Our results provide a fundamental understanding of the ILG regulation mechanism and shed light on the manipulating of more functionalities in other correlated compounds through dual-ion manipulation.