Effect and Regulation Mechanism of Post-deposition Annealing on the Ferroelectric Properties of AlScN Thin Films.

Integrating ferroelectric AlScN with III-N semiconductors to enhance the performance and tunability of nitride devices requires high-quality AlScN films. This work focuses on the effect and regulation mechanism of post-annealing in pure N2 on the crystal quality and ferroelectric properties of AlScN films. It is found that the crystal quality improves with increasing annealing temperatures. Remarkably, the leakage current of AlScN films caused by grain boundaries could be reduced by four orders of magnitude after annealing at 400 °C. The crystal growth dynamics simulations and band structure calculations indicate that the energy supplied by the temperature facilitates the evolution of abnormally oriented grains to have a better c-axis orientation, resulting in the defect states at the Fermi-level disappearing, which is mainly the reason for the leakage current decrease. More interestingly, the reduction of leakage current leads to the previously leaking region exhibiting ferroelectric properties, which is of great significance to improve the ferroelectricity of AlScN and ensure the uniformity of devices. Furthermore, annealing enhances the tensile strain on the film, which flattens the energy landscape of ferroelectric switching and reduces the coercive field. However, the risk of incorporation of oxygen will also be increased if the annealing temperatures are higher than 400 °C, which will not only reduce the relative displacement of metal atoms and nitrogen atoms in AlScN but also enhance the ferroelectric depolarization field, leading to the remnant polarization decreasing dramatically. These discoveries facilitate a deeper understanding of the influencing mechanism on the ferroelectric properties of AlScN films and provide a direction for obtaining high-quality AlScN.

In Situ Growth of Wafer-Scale Patterned Graphene and Fabrication of Optoelectronic Artificial Synaptic Device Array Based on Graphene/n-AlGaN Heterojunction for Visual Learning.

The unique optical and electrical properties of graphene-based heterojunctions make them significant for artificial synaptic devices, promoting the advancement of biomimetic vision systems. However, mass production and integration of device arrays are necessary for visual imaging, which is still challenging due to the difficulty in direct growth of wafer-scale graphene patterns. Here, a novel strategy is proposed using photosensitive polymer as a solid carbon source for in situ growth of patterned graphene on diverse substrates. The growth mechanism during high-temperature annealing is elucidated, leading to wafer-scale graphene patterns with exceptional uniformity, ideal crystalline quality, and precise control over layer number by eliminating the release of volatile from oxygen-containing resin. The growth strategy enables the fabrication of two-inch optoelectronic artificial synaptic device array based on graphene/n-AlGaN heterojunction, which emulates key functionalities of biological synapses, including short-term plasticity, long-term plasticity, and spike-rate-dependent plasticity. Moreover, the mimicry of visual learning in the human brain is attributed to the regulation of excitatory and inhibitory post-synapse currents, following a learning rule that prioritizes initial recognition before memory formation. The duration of long-term memory reaches 10 min. The in situ growth strategy for patterned graphene represents the novelty for fabricating fundamental hardware of an artificial neuromorphic system.

The AlN lattice-polarity inversion in a high-temperature-annealed c-oriented AlN/sapphire originated from the diffusion of Al and O atoms from sapphire.

AlN films are widely used owing to their superior characteristics, including an ultra-wide bandgap, high breakdown field, and radiation resistance. High-temperature annealing (HTA) makes it easy to obtain high-quality AlN films, with the advantages of a simple process, good repeatability, and low cost. However, it is always found that there is a lattice-polarity inversion from a N-polarity near the sapphire to an Al-polarity in the HTA c-oriented AlN/sapphire. Currently, the formation mechanism is still unclear, which hinders its further wide applications. Therefore, the formation mechanism of the polarity inversion and its impacts on the quality and stress profile of the upper AlN in the HTA c-oriented AlN/sapphire were investigated. The results imply that the inversion originated from the diffusion of the Al and O atoms from the sapphire. Due to the presence of abundant Al vacancies (VAl) in the upper AlN, Al atoms in the sapphire diffuse into the upper AlN during the annealing to fill the VAl, resulting in the O-terminated sapphire, leading to the N-polar AlN. Meanwhile, O atoms in the sapphire also diffuse into the upper AlN during the annealing, forming an AlxOyNz layer and causing the inversion from N- to Al-polarity. The inversion has insignificant impacts on the quality and stress distribution of the upper AlN. Besides, this study predicts the presence of a two-dimensional electron gas at the inversion interface. However, the measured electron concentration is much lower than that predicted, which may be due to the defect compensation, low polarization level, and strong impurity scattering.

Nonradiative Dynamics Induced by Vacancies in Wide-Gap III-Nitrides: Ab Initio Time-Domain Analysis.

Insightful understanding of defect properties and prevention of defect damage are among the biggest issues in the development of photoelectronic devices based on wide-gap III-nitride semiconductors. Here, we have investigated the vacancy-induced carrier nonradiative dynamics in wide-gap III-nitrides (GaN, AlN, and AlxGa1-xN) by ab initio molecular dynamics and nonadiabatic (NA) quantum dynamics simulations since the considerable defect density in epitaxy samples. E-h recombination is hardly affected by Vcation, which created shallow states near the VBM. Our findings demonstrate that VN in AlN creates defect-assisted nonradiative recombination centers and shortens the recombination time (τ) as in the Shockley-Read-Hall (SRH) model. In GaN, VN improves the NA coupling between the CBM and the VBM. Additionally, increasing x in the AlxGa1-xN alloys accelerates nonradiative recombination, which may be an important issue in further improving the IQE of high Al-content AlxGa1-xN alloys. These findings have significant implications for the improvement of wide-gap III-nitrides-based photoelectronic devices.

Centimeter-Transferable III-Nitride Membrane Enabled by Interfacial Adhesion Control for a Flexible Photosensitive Device.

Flexible III-nitride-based optoelectronic devices are crucial for the next-generation foldable/wearable lighting sterilization and sensor working in the ultraviolet (UV) region. However, the strong bonding effect at the epitaxial interface of III-nitride and bare sapphire substrate makes it difficult for epilayer separation and flexible applications. Although the emerging van der Waals epitaxy (vdWE) with graphene insertion layer offers a feasible route for weakening the interfacial adhesion, the intact centimeter-transferable III-nitride membrane still remains challenging. The spontaneous delamination occurs due to the too weak interfacial adhesion of pure vdW force, and on the contrary, the structural damage of graphene by high-temperature hydrogen etching during the III-nitride growth might also cause separation failure. Up to now, the efficient control of vdWE interfacial adhesion is still an on-going research hotspot. Herein, we demonstrate the interfacial adhesion control of III-nitride vdWE by utilizing graded high-temperature nitridation treatment of the graphene insertion layer, which generates defects and N doping in different levels. The corresponding epitaxial modes of pure-vdWE, quasi-vdWE, and mixed epitaxy are achieved according to the interfacial adhesion difference. It reveals that the quasi-vdWE enabled by small graphene defects and proper N doping triggers the low formation energy for AlN nucleation; meanwhile, the proper interfacial adhesion ensures the growth integrality and intact separation of III-nitride membrane in the centimeter scale. The UV resin-assisted bonding technique is proposed for the successful transfer of III-nitride onto a flexible substrate. The flexible photodetector is fabricated by using a graphene monolayer as the photocarrier transport channel, and it achieves a high device yield of 90%, retaining ∼60% of its initial performance after 250 bending cycles. This work offers the promising strategy for controlling vdWE interfacial adhesion, and the separable and transferable III-nitride membrane lays the foundation for advances of future UV foldable and wearable devices.

Van der Waals Epitaxy of c-Oriented Wurtzite AlGaN on Polycrystalline Mo Substrates for Enhanced Heat Dissipation.

The epitaxy of III-nitrides on metallic substrates is competitive due to the advantages of vertical carrier injection, enhanced heat dissipation, and flexible application in various III-nitride-based devices. However, the serious lattice mismatch, atom diffusion, and interface reaction under the rigorous growth conditions have caused enormous obstacles. Based on the thermal and chemical stability of the graphene layer, we propose the van der Waals epitaxy of c-oriented wurtzite AlGaN on the polycrystalline Mo substrate by high-temperature metal-organic chemical vapor deposition. The insertion of a graphene layer interrupts the chaotic epitaxial relationship between the polycrystalline metal and epilayers, resulting in the single-crystalline orientation along the wurtzite (0002) plane and residual stress release in AlGaN because of the weak van der Waals interaction. We also demonstrate that the epitaxy of AlGaN on Mo metal possesses enhanced heat dissipation ability, in which the epilayer temperature is controlled at only 28.7 °C by the heating of a ∼54 °C hot plate. The heat dissipation enhancement for the present epitaxial structures provides a desirable strategy for the fabrication of efficient ultraviolet devices with excellent stability and lifetime.

Morphology and carrier mobility of high-B-content BxAl1-xN ternary alloys from an ab initio global search.

The excellent properties of III-nitrides and their alloys have led to significant applications in optoelectronic devices. Boron, the lightest IIIA group element, makes it possible to extend the flexibility of III-nitride alloys. However, both BxAl1-xN and BxGa1-xN ternary alloys suffer from poor material quality during crystal growth, their B contents in experimental reports are no higher than 22%, and the underlying mechanism is still unclear. Herein, ab initio global calculation by particle swarm optimization combined with density functional theory is carried out to identify the ground structures of BxAl1-xN alloys with different B contents (x = 0.25, 0.5, and 0.75). Furthermore, the electronic properties and intrinsic carrier mobility are studied. For B0.25Al0.75N and B0.75Al0.25N, quasi-wurtzite and quasi-hexagonal structures are energetically favourable, respectively, indicating a wurtzite-to-hexagonal structural transition due to the three-coordinated B atoms being incorporated into the lattice. When the B content is 50%, B0.5Al0.5N shows a ten-membered ring structure with an indirect bandgap of 3.52 eV and strong anisotropy of mobility. Our results uncover the mechanism of the structural and electronic property evolution with B content and pave a route for the application of B-containing III-nitride alloys.

Point Defects in Monolayer h-AlN as Candidates for Single-Photon Emission.

A single-photon emission (SPE) system based on a solid state is one of the fundamental branches in quantum information and communication technologies. The traditional bulk semiconductors suffered limitations of difficult photon extraction and long radiative lifetime. Two-dimensional (2D) semiconductors with an entire open structure and low dielectric screening can overcome these shortcomings. In this work, we focus on monolayer h-AlN due to its wide band gap and the successful achievement of SPE compared to its bulk counterpart. We systematically investigate the properties of point defects, including vacancies, antisites, and impurities, in monolayer h-AlN by employing hybrid density functional theory calculations. The -1 charged Al vacancy (VAl-) and +1 charged nitrogen antisite (NAl+) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex CAlVN+, which is composed of vacancies and carbon substitutions, also can be used for SPE. Our results extend the avenue for realizing SPE in 2D semiconductors.

Cation Vacancy in Wide Bandgap III-Nitrides as Single-Photon Emitter: A First-Principles Investigation.

Single-photon sources based on solid-state material are desirable in quantum technologies. However, suitable platforms for single-photon emission are currently limited. Herein, a theoretical approach to design a single-photon emitter based on defects in solid-state material is proposed. Through group theory analysis and hybrid density functional theory calculation, the charge-neutral cation vacancy in III-V compounds is found to satisfy a unique 5-electron-8-orbital electronic configuration with Td symmetry, which is possible for single-photon emission. Furthermore, it is confirmed that this type of single-photon emitter only exists in wide bandgap III-nitrides among all the III-V compounds. The corresponding photon energy in GaN, AlN, and AlGaN lies within the optimal range for transfer in optical fiber, thereby render the charge-neutral cation vacancy in wide-bandgap III-nitrides as a promising single-photon emitter for quantum information applications.

Quantum engineering of non-equilibrium efficient p-doping in ultra-wide band-gap nitrides.

Ultra-wide band-gap nitrides have huge potential in micro- and optoelectronics due to their tunable wide band-gap, high breakdown field and energy density, excellent chemical and thermal stability. However, their application has been severely hindered by the low p-doping efficiency, which is ascribed to the ultrahigh acceptor activation energy originated from the low valance band maximum. Here, a valance band modulation mode is proposed and a quantum engineering doping method is conducted to achieve high-efficient p-type ultra-wide band-gap nitrides, in which GaN quantum-dots are buried in nitride matrix to produce a new band edge and thus to tune the dopant activation energy. By non-equilibrium doping techniques, quantum engineering doped AlGaN:Mg with Al content of 60% is successfully fabricated. The Mg activation energy has been reduced to about 21 meV, and the hole concentration reaches higher than 1018 cm-3 at room temperature. Also, similar activation energies are obtained in AlGaN with other Al contents such as 50% and 70%, indicating the universality of the quantum engineering doping method. Moreover, deep-ultraviolet light-emission diodes are fabricated and the improved performance further demonstrates the validity and merit of the method. With the quantum material growth techniques developing, this method would be prevalently available and tremendously stimulate the promotion of ultra-wide band-gap semiconductor-based devices.

Dynamics Studies of Nitrogen Interstitial in GaN from Ab Initio Calculations.

Understanding the properties of defects is crucial to design higher performance semiconductor materials because they influence the electronic and optical properties significantly. Using ab initio calculations, the dynamics properties of nitrogen interstitial in GaN material, including the configuration, migration, and interaction with vacancy were systematically investigated in the present work. By introducing different sites of foreign nitrogen atom, the most stable configuration of nitrogen interstitial was calculated to show a threefold symmetry in each layer and different charge states were characterized, respectively. In the researches of migration, two migration paths, in-plane and out-of-plane, were considered. With regards to the in-plane migration, an intermediated rotation process was observed first time. Due to this rotation behavior, two different barriers were demonstrated to reveal that the migration is an anisotropic behavior. Additionally, charged nitrogen Frenkel pair was found to be a relatively stable defect complex and its well separation distance was about 3.9 Å. Part of our results are in good agreement with the experimental results, and our work provides underlying insights of the identification and dynamics of nitrogen interstitial in GaN material. This study of defects in GaN material is useful to establish a more complete theory and improve the performance of GaN-based devices.

Physical understanding of the bending of nanostructures caused by cellular force.

The bending of nanostructures (NSs), such as nanopillars and nanowires, caused by cell adhesion is an interesting phenomenon and is important for the measurements of cellular forces, understanding the biological behavior of cells, and disease diagnosis. However, which factors are related to the bending of NSs and how the factors affect bending displacement are still not well understood. Here, we establish an analytic thermodynamic theory to study the bending mechanism of NSs caused by cellular force during the cell adhesion process, and analyze the factors affecting bending displacement. It is found that the bending of NSs is determined by the competition between the stretching energy of the membrane and the strain energy of the NSs. The bending displacement can be evaluated based on our model.

An analytical model for the bending of radial nanowire heterostructures.

Extremely thin nanowires (NWs) would bend during the heteroepitaxial growth process. This phenomenon can increase the emission intensity due to the strain fields within bent NWs. Although the growth mechanism of NW heterostructures has been widely studied in theory, the theoretical studies are centered on growth on the surface of straight NWs, and the bending mechanism on extremely thin NWs has not been clearly explored. In this contribution, we have established an analytical thermodynamic theory to study the mechanism of bending induced by heteroepitaxial growth on the surface of thin NWs. It is found that the balance between surface energy and elastic strain energy plays a crucial role in the determination of the bending of NWs. The strain relaxation energy induces bending of NWs with small radii and high deposited amounts, while the size-dependent surface energy becomes more significant and restrains the bending of NWs with large radii and low deposited amounts. The established theoretical model not only explained the bending mechanism of NWs but also provided useful information to design the epitaxial growth on the surface with a nanoscale curvature.

A Noninvasive Accurate Measurement of Blood Glucose Levels with Raman Spectroscopy of Blood in Microvessels.

Raman spectra of human skin obtained by laser excitation have been used to non-invasively detect blood glucose. In previous reports, however, Raman spectra thus obtained were mainly derived from the epidermis and interstitial fluid as a result of the shallow penetration depth of lasers in skin. The physiological process by which glucose in microvessels penetrates into the interstitial fluid introduces a time delay, which inevitably introduces errors in transcutaneous measurements of blood glucose. We focused the laser directly on the microvessels in the superficial layer of the human nailfold, and acquired Raman spectra with multiple characteristic peaks of blood, which indicated that the spectra obtained predominantly originated from blood. Incorporating a multivariate approach combining principal component analysis (PCA) and back propagation artificial neural network (BP-ANN), we performed noninvasive blood glucose measurements on 12 randomly selected volunteers, respectively. The mean prediction performance of the 12 volunteers was obtained as an RMSEP of 0.45 mmol/L and R2 of 0.95. It was no time lag between the predicted blood glucose and the actual blood glucose in the oral glucose tolerance test (OGTT). We also applied the procedure to data from all 12 volunteers regarded as one set, and the total predicted performance was obtained with an RMSEP of 0.27 mmol/L and an R2 of 0.98, which is better than that of the individual model for each volunteer. This suggested that anatomical differences between volunteer fingernails do not reduce the prediction accuracy and 100% of the predicted glucose concentrations fall within Region A and B of the Clarke error grid, allowing acceptable predictions in a clinically relevant range. The Raman spectroscopy detection of blood glucose from microvessels is of great significance of non-invasive blood glucose detection of Raman spectroscopy. This innovative method may also facilitate non-invasive detection of other blood components.

Toward a Better Understanding of Hemiwicking: A Simple Model to Comprehensive Prediction.

The hemiwicking state has attracted much interest because of numerous important potential applications in inking, printing, boiling heat transfer, and condensation. However, the mechanism of the emergence of hemiwicking has not been well understood, especially the effects of geometry of patterned surfaces on the hemiwicking state has not been systematically investigated. Here, we presented a new method to study the critical conditions for hemiwicking on patterned surfaces. By minimizing the variation of the free energy, we obtain the corresponding stable height of the hemiwicking film and find that it is easier for a droplet to be in the hemiwicking state if the pillar surface has small spacing, large radius and height, and a small intrinsic contact angle. Our established model is applied to a flat-topped cylindrical pillar-patterned surface, and the modeling results are in well agreement with experiments and other existing theories. Besides, our model is also applied to other kinds of patterned surfaces including hemispherical-topped cylindrical and conical pillars, about which the other existing theories are deficient. Our theoretical results not only are in well agreement with the experimental observations but also provide some important predictions, which implies that the established model could be applicable to understanding the basic physical mechanism of the hemiwicking state and be useful in guiding the design and fabrication of hemiwicking surfaces.

Quantum Interference in Singlet Fission: J- and H-Aggregate Behavior.

The quantum interference in singlet fission (SF) among the multiple pathways from singlet excited states to correlated triplet pair states is comprehensively investigated. The analytical analysis reveals that this interference is strongly affected by the exciton-exciton coupling and is closely related to the property of J- and H-type of aggregates. Different from the interference in the spectra of aggregates, which depends only on the sign of exciton-exciton coupling, the interference in SF is additionally related to the signs of couplings between singlet excited states and triplet pair states. The interference dynamics is further demonstrated numerically by a time-dependent wavepacket diffusion method with electron-phonon interactions incorporated. Finally, we take a pentacene dimer as a concrete example to show how to adjust the constructive and destructive interferences in SF dynamics in terms of J-/H-aggregate behaviors. The results presented here may provide guiding principles for designing efficient SF materials through directly tuning quantum interference via morphology engineering.

Irradiation Induced Microstructure Evolution in Nanostructured Materials: A Review.

Nanostructured (NS) materials may have different irradiation resistance from their coarse-grained (CG) counterparts. In this review, we focus on the effect of grain boundaries (GBs)/interfaces on irradiation induced microstructure evolution and the irradiation tolerance of NS materials under irradiation. The features of void denuded zones (VDZs) and the unusual behavior of void formation near GBs/interfaces in metals due to the interactions between GBs/interfaces and irradiation-produced point defects are systematically reviewed. Some experimental results and calculation results show that NS materials have enhanced irradiation resistance, due to their extremely small grain sizes and large volume fractions of GBs/interfaces, which could absorb and annihilate the mobile defects produced during irradiation. However, there is also literature reporting reduced irradiation resistance or even amorphization of NS materials at a lower irradiation dose compared with their bulk counterparts, since the GBs are also characterized by excess energy (compared to that of single crystal materials) which could provide a shift in the total free energy that will lead to the amorphization process. The competition of these two effects leads to the different irradiation tolerance of NS materials. The irradiation-induced grain growth is dominated by irradiation temperature, dose, ion flux, character of GBs/interface and nanoprecipitates, although the decrease of grain sizes under irradiation is also observed in some experiments.

Mechanisms of large Stokes shift and aggregation-enhanced emission of osmapentalyne cations in solution: combined MD simulations and QM/MM calculations.

Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations. The results demonstrate that the large Stokes shift in the gas phase comes from a photoexcitation-induced deformation of the osmium plane, whereas in solution it corresponds to the variation of osmium ring symmetry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 join the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution functions of their mass-central distances display several peaks immersed in a broad envelope due to different aggregation pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation structures do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.