AlGaN has attracted considerable interest as a wide (direct)-band-gap semiconductor with high thermal and mechanical stability. Thus, it can be used to develop optoelectronic devices operating within the ultraviolet region at high power and under harsh environmental conditions. Despite their recognized prospective applications, Al-rich AlGaN optical devices suffer from low external quantum efficiency. To trace the origin of the said problem, a cathodoluminescence system combined with two scanning probes was set up to investigate the cross-section luminescence of the sample related to application bias. The luminescence from the quantum wells in a deep ultraviolet light-emitting device was identified by layer-resolved spectroscopy. Results show that the primary radiative emission at the band edge exhibits an abnormal behavior, which is different from the other emission that is dependent on external electric fields. First-principles simulations demonstrate that the dispersive crystal field split-off hole (CH) band caused by hole deconfinement is responsible for the abnormal radiative emissions. Analysis of the constituent orbitals of the hole bands reveals a strong head-over-head lobe structure in the barrier along the [0001] direction in the pz orbitals, contributing mainly to the CH band. Meanwhile, a weak side-by-side (0001) in-plane lobe structure is present in the px and py orbitals, contributing to the heavy and light hole bands. This study may serve as a basis for further investigations on quantum efficiency improvement in high-Al-content AlGaN optoelectronic devices.Keywords: AlGaN; III−V semiconductors; quantum structure; UV;

The light extraction from AlGaN deep ultraviolet light-emitting diodes (UV LEDs) is known to be limited by the fundamental valence band crossover issue. To study the effect of electrical injection on the complex structure of the valence bands, stress variations were characterized by the Raman shift of AlGaN deep UV LEDs under electrical injection. Results show that tensile stress builds up as the current increases. The first-principles simulations reveal that, as the tensile strain increases, the crystal-splitting hole band becomes more dominant at the top of valence bands and exhibits stronger dispersion along the Γ–A direction, leading to stronger TM polarized emission and a decrease of the total spontaneous emission rate. Furthermore, a promising way of controlling AlGaN MQWs under strain-free or compressive strain status is proposed to improve the transverse electric polarized emission and quantum efficiency in deep UV LEDs.

Oxygen-polar ZnO films are grown in step flow mode by molecular beam epitaxy. Driven by the step flow anisotropy, the growth leads to the occurrence of specific hexagonal pits in the surface. The specific pits are formed by interlacing steps of the {10} facets, thus quenching the macroscopic dipole moment along the c-axis and satisfying the stabilization principles. Nitrogen (N) doping trials are then performed on the basis of the stable surface. In doping, growth remains in step flow mode but the step flow anisotropy vanishes, resulting in an obvious change of the surface morphology. Besides, a distinct acceptor state appears by in situ scanning tunneling spectroscopy analysis. First-principles calculations reveal that N readily substitutes for step-edge Zn and acts as NO2 adsorbed at the step edge. Desorption of the NO2 facilitates the formation of NO–VZn shallow acceptor complexes, which contributes to the appearance of the acceptor state. According to the peculiarities of N dopants on the O-polar surface, vicinal O-polar substrates (e.g., {10} substrate) are promising in ZnO:N due to the easily achieved step flow growth and high density of step edges for N incorporation.

•Comparing the results of different etching temperature and time, the suitable etching temperature of 1500 °C and etching time of 10 min for 4H–SiC 4° off-axis 100 mm substrates were determined.•The results of different atmospheres indicated that the best etching environment for 4H–SiC 4° off-axis substrates is H2 + HCl.•With the modified in-situ etching process, an epi-layer of 12.3 μm thick 4H–SiC with excellent surface morphology with the defect density of 0.45 cm−2 has been obtained.The investigations of in-situ etching of 4HSiC epi-growth on 4° off-axis 100 mm diameter substrates under different conditions have been carried out in a commercial warm-wall multi-wafer planetary reactor. The surface morphologies of the as-etched substrates have been characterized by atomic force microscopy on 20 × 20 μm2. Based on the step height and roughness mean square, the best etching condition for 4HSiC 4° off-axis substrates was determined to be H2 + HCl at 1500 °C for 10 min. With the optimized in-situ etching process, high quality 4HSiC epitaxial layers with excellent surface morphology have been obtained, and the defect density is lowered to 0.45 cm−2 resulting in a projected 2 × 2 mm die yield of ∼98%.

Ternary ZnxCd1−xSe alloys are potential materials for sensitized photovoltaic devices because of their tunable bandgaps and band structures. In this work, random deposition of Cd and Zn was realized by the co-evaporated chemical vapor deposition method to tune the distribution of the lattice constant and the mixing of the structural phase to the utmost, and thus release the misfit stress by the self-adaption mechanism. As a result, the coaxial nanowire with an approximate composition x of 0.5 exhibited the high crystal quality and the long nonequilibrium carrier lifetime because of the alloy disorder effect. The corresponding photo-electrochemical cell acquired a power conversion efficiency of 3.68%. This work paves the way for the design and development of high-efficiency coaxial solar cells based on ternary or multi-component alloys.

AlGaN has attracted growing interest for applications in electro-optic devices that generate and process optical signals. To enhance the electric-optic effect with polarity, we designed and grew GaN/AlxGa1-xN quantum structures with x of about 0.5 by metal–organic vapor-phase epitaxy. Spectroscopic ellipsometry measurement and ab initio calculation demonstrated that the stronger polarization fields induced by higher Al contents in the barrier result in larger electro-optic effects. By applying external biases directly on both sides of these quantum structures, a high intensity of external field was achieved, and extraordinary and ordinary refractive indices were characterized by fitting the variable-angle spectroscopic ellipsometry. The values of deduced electric-optic coefficients were significantly enhanced by combining the resonance effect with the internal and external fields. Given the comparable electric-optic coefficients with conventional electro-optic crystals, AlGaN is an attractive candidate for nonlinear optical material, which provides a basis for large-scale integrations of ultraviolet electro-optic devices based on nitride semiconductors.Keywords: AlGaN; electro-optic material; III−V semiconductors; quantum structure; UV

A tiny number of Zn atoms were deposited on Si(111)-(7×7) surface to study the evolution process of Zn-induced nanoclusters. After the deposition, three types (type I, II, and III) of Zn-induced nanoclusters were observed to occupy preferably in the faulted half-unit cells. These Zn-induced nanoclusters are found to be related to one, two, and three displaced Si edge adatoms, and simultaneously cause the depression of one, two, and three closest Si edge adatoms in the neighboring unfaulted half-unit cells at negative voltages, respectively. First-principles adsorption energy calculations show that the observed type I, II, and III nanoclusters can reasonably be assigned as the Zn3Si1, Zn5Si2, and Zn7Si3 clusters, respectively. And Zn3Si1, Zn5Si2, and Zn7Si3 clusters are, respectively, the most stable structures in cases of one, two, and three displaced Si edge adatoms. Based on the above energy-preferred models, the simulated bias-dependent STM images are all well consistent with the experimental observations. Therefore, the most stable Zn7Si3 nanoclusters adsorbed on the Si(111)-(7×7) surface should grow up on the base of Zn3Si1 and Zn5Si2 clusters. A novel evolution process from Zn3Si1 to Zn5Si2, and finally to Zn7Si3 nanocluster is unveiled.

Large-scale 2D Au lattices with honeycomb-like structure are fabricated on Si(111)-7 × 7 surface at room temperature. The growth pattern investigated by reflection high-energy electron diffraction and in situ scanning tunneling microscopy indicates that the 2D Au lattices are composed of two interfacial distinct layers that are completely formed one after another with a close-packed structure. A unique wide forbidden gap of 4.1 eV is measured around the Fermi level of the 2D Au lattices by scanning tunneling spectroscopy. Bias-dependent STM images and theoretical simulations suggest that the in-plane quantum coupling and carrier transport behavior are responsible for the novel electronic properties. In addition to local electronic states, the electronic structures of 2D Au lattices are further modulated by the carrier transport preference that is determined by carrier energy and symmetry of 2D lattices. These findings will provide some references for the controlled fabrication and for routing the carrier transport behavior of low-dimensional metal structures.

Ultra-short-period (AlN)m/(GaN)n superlattices with tunable well and barrier atomic layer numbers were grown by metal–organic vapour phase epitaxy, and employed to demonstrate narrowband deep ultraviolet photodetection. High-resolution transmission electron microscopy and X-ray reciprocal space mapping confirm that superlattices containing well-defined, coherently strained GaN and AlN layers as thin as two atomic layers (∼0.5 nm) were grown. Theoretical and experimental results demonstrate that an optical absorption band as narrow as 9 nm (210 meV) at deep-ultraviolet wavelengths can be produced, and is attributable to interband transitions between quantum states along the [0001] direction in ultrathin GaN atomic layers isolated by AlN barriers. The absorption wavelength can be precisely engineered by adjusting the thickness of the GaN atomic layers because of the quantum confinement effect. These results represent a major advance towards the realization of wavelength selectable and narrowband photodetectors in the deep-ultraviolet region without any additional optical filters.

Wide-bandgap semiconductors are potential materials for photovoltaic (PV) devices in solution-based applications because of their higher stability compared with narrow-bandgap semiconductors. However, these materials are poor absorbers of photons in the solar spectrum and yield modest conversion efficiencies. Here we show how this problem can be solved by the growth and control of pseudomorphic crystals in type-II wide-bandgap ZnO/ZnSe coaxial nanowires. Light absorption of coaxial nanowires under critical strains significantly extended beyond the near-infrared region to cover up to 94% of the solar power. The photocurrent response of the nanowire solar cell was markedly enhanced in visible and infrared regions with a threshold of approximately 0.9 eV, accounting for more than 35% of the conversion efficiency. This work paves the way for stable and efficient PV devices based on wide-bandgap semiconductors.

Two different asymmetric Ag/ZnO composite nanoarrays were fabricated. These nanoarrays are proposed as highly sensitive and uniform surface-enhanced Raman scattering (SERS) substrates with plasmonic-enhanced UV-visible photocatalytic properties for self-cleaning. The asymmetric nanostructures are composed of Ag nanoparticles hanging inside or capping on the top of ZnO hollow nanospheres, which allows the generation of a strong local electric field near the contact area owing to the asymmetric dielectric environment. Experimental and simulation results showed that these asymmetric structures are favorable for achieving high photocatalytic activity under UV and visible light irradiation, in addition to improving the SERS performance. The electron transfer model based on band gap alignment was employed to further illustrate the mechanisms of the photocatalytic activity, which was dependent on the wavelength of the irradiation. Given the dramatically improved photocatalytic performance, together with the reproducible and uniform SERS signals verified by the Raman mapping results, the large area ordered asymmetric metal/semiconductor nanoarrays have been demonstrated to be suitable for further applications in multifunctional photoelectrochemical chips.

Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices.

Growing AlN layers remains a significant challenge because it is subject to a large volume fraction of grain boundaries. In this study, the nature and formation of the AlN growth mechanism is examined by ab initio simulations and experimental demonstration. The calculated formation enthalpies of the constituent elements, including the Al/N atom, Al–N molecule, and Al–N3 cluster, vary with growth conditions in N-rich and Al-rich environments. Using the calculation results as bases, we develop a three-step metalorganic vapor-phase epitaxy, which involves the periodic growth sequence of (i) trimethylaluminum (TMAl), (ii) ammonia (NH3), and (iii) TMAl+NH3 supply, bringing in hierarchical growth units to improve AlN layer compactness. A series of AlN samples were grown, and their morphological and luminescent evolutions were evaluated by atomic force microscopy and cathodoluminescence, respectively. The proposed technique is advantageous because the boundaries and defect-related luminescence derived are highly depressed, serving as a productive platform from which to further optimize the properties of AlGaN semiconductors.

Homoepitaxy ZnO monolayer growth was investigated from dynamics to kinetics taking ZnO molecules and Zn–O cluster monomers into account in the atomistic growth by first-principles calculations and Monte Carlo simulations and compared with experimental growth by molecular beam epitaxy. Theoretically, the ZnO molecules were found to scatter equivalently on both the wurtzite sites (WSs) and zincblende sites (ZSs) and stick even at high temperatures. The Zn3O1 monomers resulted in a larger island size and a higher compact degree and the growth approached to the two-dimensional mode at high temperature; the film structure finalized in the single wurtzite phase structure with more vacancies, which agreed with the in situ scanning tunnel microscopy observation for the growth in Zn-rich conditions to form Zn3O1 monomers. For the Zn1O3 monomers, the transformation from ZSs to WSs was more difficult even with temperature increase and they could locate at both WSs and ZSs, consistent with the in situ reflection high energy electron diffraction for the growth in O-rich conditions to form Zn1O3 monomers. Combining the advantages of both cluster monomers, a two step-growth technique was developed by alternatively supplying Zn and O. The resultant ZnO films exhibited flat texture and uniform phase structure as indicated by the atomic steps in the STM images and the streaky RHEED patterns.

Nanowire-based photovoltaic devices have the advantages over planar devices in light absorption and charge transport and collection. Recently, a new strategy relying on type-II band alignment has been proposed to facilitate efficient charge separation in core/shell nanowire solar cells. This paper reviews the type-II heterojunction solar cells based on core/shell nanowire arrays, and specifically focuses on the progress of theoretical design and fabrication of type-II ZnO/ZnSe core/shell nanowire-based solar cells. A strong photoresponse associated with the type-II interfacial transition exhibits a threshold of 1.6 eV, which demonstrates the feasibility and great potential for exploring all-inorganic versions of type-II heterojunction solar cells using wide bandgap semiconductors. Future prospects in this area are also outlooked.

Well-aligned ZnO/ZnSe core/shell nanowire arrays with type-II energy alignment are synthesized via a two-step chemical vapor deposition method. Morphology and structure studies reveal a transition layer of wurtzite ZnSe between the wurtzite ZnO core and the cubic ZnSe shell. Type-II interfacial transitions are observed in the spectral region from visible to near infrared in transmission and photoluminescence. More significantly, for the first time, the interfacial transition is shown to extend the photoresponse of the prototype photovoltaic device based on the coaxial nanowire array to a threshold much below the bandgap of either component (3.3 and 2.7 eV, respectively) at 1.6 eV, with an external quantum efficiency of ∼4% at 1.9 eV and 9.5% at 3 eV. These results represent a major advance towards the realization of all-inorganic type-II heterojunction photovoltaic devices in an optimal device architecture.

ZnO based nanostructures including ZnO and erbium (Er) doped ZnO nanocrystals (NCs) were synthesized by a one-step solvothermal method. Compared with the pure ZnO NCs, photoluminescence (PL) of the Er-doped ZnO NCs made an improvement in the visible emissions, which was attributed to the defects or vacancies resulted from Er-dopants. The enhanced defects or vacancies resulted in high activity surfaces, which benefit sensing properties. Furthermore, sensing experiments confirmed that the sensing performance of Er-doped ZnO NCs was improved almost doubled than the pure ZnO NCs. Therefore, the high sensibility gas sensors based on Er-doped ZnO NCs were successfully fabricated.Graphical abstractSensing properties of ZnO based nanostructures including ZnO and Er-doped ZnO nanocrystals were investigated, and high-sensibility gas sensors based on Er-doped ZnO nanocrystals were fabricated.Highlights► ZnO based nanostructures including ZnO and erbium doped ZnO nanocrystals were successfully synthesized by a one-step solvothermal method. ► Sensing properties of ZnO based nanostructures were investigated. ► High-sensibility gas sensors based on Er-doped ZnO nanocrystals were fabricated by a simple route.

A wurtzite AlN epilayer grown on (0 0 0 1) sapphire substrate was characterized by variable-angle spectroscopic ellipsometry. The asymmetries of the ellipsometric spectra caused by optical anisotropy were observed below and above the Brewster angle. Tanguy’s dispersion model is employed in order to determine the extraordinary and ordinary refractive indices and extinction coefficients in the spectral range of 1.5–6.5 eV. In addition, the birefringence and dichroism were derived exhibiting near the band gap a maximum and a minimum, respectively. The anisotropy is attributed to the valence band ordering at the center of the Brillouin zone, in particular to the large negative crystal-field splitting, and the corresponding optical selection rules.

Well-aligned ZnO nanowires were grown on Si substrate by chemical vapor deposition. The experimental results showed that the density of nanowires was related to the heating process and growth temperature. High-density ZnO nanowires were obtained under optimal conditions. The growth mechanism of the ZnO nanowires was presented as well.

We studied growth kinetic processes of AlN molecules on the Al-polar surface of AlN using ab initio and Monte Carlo simulations. Molecular processes were presented and analyzed during the nucleation, ripening, and coalescence stages. The results show that the nucleus number decreases as temperature rises due to the increasing diffusion of the molecules. By analyzing the growth time dependence of average cluster size, interface-limited Ostwald ripening is found to be the main ripening mechanism when the temperature is lower than 1773 K. As cluster-corner crossing diffusion is limited, the growth is fractal-like extension, and the coalescence is achieved through adhesion of clusters, leading to a generally continuous morphology with some vacancies and closure failures, which is in good agreement with our experimental results. Moreover, coverage/temperature kinetic phase diagrams under different deposition rates are presented (from 0.025 to 0.1 ML/s). Our finding suggests that a temperature higher than 1800 K is crucial for growth of an ideal atomic-scale Al-polar AlN surface.

The strains of bond lengths and lattice in GaN/ultrathin InN/GaN quantum wells are modified by varying the well and barrier thickness. Inspection of position-dependent densities of state (DOS) calculated by ab initio simulations shows that this tunable strain significantly affects the electronic structures. The band gap and the band bending of the well decrease as the well width increases, whereas they are enhanced with an increase of the barrier thickness. The coherence and strain variation are realized by metalorganic chemical vapor deposition, according to the results of high resolution transmission electron microscopy and X-ray diffraction. Remarkable emission in the short wavelength region from the quantum wells is observed by cathodoluminescence to shift in the trend of the simulated band gap variation. The results show that the strained quantum wells have advantages for phase separation suppression and band structures engineering in the short wavelength region.

Self-assembled growth of Au nanoclusters on the Si(111)-7 × 7 surface has been studied using kinetic Monte Carlo simulations. A model considering various atomic processes of deposition, adsorption, diffusion, nucleation, and aggregation is introduced, and the main energetic parameters are optimized based on the experimental results. The evolution of surface morphology during Au growth is simulated in real time, from which the atomic behaviors of Au could be really captured. Most of Au atoms diffuse on the substrate in the very early stage of growth, and Au clusters nucleate and grow with the increasing coverage. The competition among various atomic processes results in the distinct distribution of Au clusters under different coverages. The growth conditions are further optimized, showing that the higher uniformity of Au clusters would be obtained at a low deposition rate and an optimal substrate temperature of about 380 K.

On the basis of the first-principles total-energy calculations within the density-functional theory, the surface formation energies and the charge density differences of AlN (0001) and (000−1) polar surfaces with an In-, Al-, or N-adlayer have been studied. The results show that In presents surfactant effect on AlN (0001) but does not behave as such on AlN (000−1), which is the same as the case of GaN. The surfactant effect of In is attributable to the much weaker In−Al bond than the Al−N bond on the Al-polar AlN surface and the bigger average diffusion coefficient for Al atoms on the Al-polar AlN surface with In-adlayer according to the calculated charge density differences and diffusion barriers, respectively.

We combined scanning tunneling microscope/spectroscopy studies and first-principles calculations to investigate the evolution of the electronic structures associated with the adsorption of Au on the Si(1 1 1)-7 × 7 surface. The charge transfer between the deposited Au and its adjacent Si atoms was clearly revealed by the local density of states and charge density difference. The Au-5d states are hybridized with the Si-3p states and the dangling bonds of Si atoms are saturated, which induced a reduction of empty states at about +0.4 eV and an increase of filled states with additional peaks at about −0.6 eV and −1.5 eV.

The relation of the phase shifts between the 7×7 domains surrounding a triangular defect region has been revealed by introducing a phase-shift vector on the Si(1 1 1) reconstructed surface. I–V measurement is of great benefit to the investigation on the local electronic properties. To get further information about the influence of the experimental conditions, different thermal annealing treatments on the Si(1 1 1) surface have been carried out. In situ STM data show that the density of the reconstruction domains varies with the quenching current and cooling rate.

Wafers with normal light-emitting diode structure were grown by metal organic chemical vapor deposition system. The pressure and temperature were varied during growth of buffer layer in order to grow different types of epilayers. The cathodoluminescence results show that the interface distortion of quantum well plays an important role in radiant efficiency. The electroluminescence detections indicate that the dislocations also influence the external quantum efficiency by lowering the electron injection efficiency.

Dislocation semi-loops in the lateral epitaxial region of GaN have been observed and characterized using transmission electron microscopy. The loops are not coplanar in a (0 0 0 1) close-packed plane and the motion of vertical glide is found. The Burgers vectors are determined to be parallel to [0 0 0 1] direction. These evidences demonstrate that a c-axial stress field exists in the lateral region transited from the in-plane stress of the window region. The value and distribution of this special stress field have been measured via Auger electron spectroscopy.

Phase separation in wurtzite InxGa1−x  N is investigated theoretically by ab initio calculations. The calculated mixing free energies are positive with a maximum at composition x=0.4375x=0.4375 and the spinodal decomposition will occur while the lattice is in equilibrium and uniformity. The mixing free energies turn into negative and have three retuse minima at compositions of 0.3125, 0.5000, and 0.6875 if strain is introduced by replacing the GaN or InN lattice constant with the equilibrium lattice constants. Detailed comparisons among the clustered, phase-separated, and uniform atomic configurations show that InN cluster is favourable in energy if the lattice is further fully relaxed, but uniform distribution is kept on under the strained lattices. The results indicate that the phase separation can be suppressed under either the compressive or tensile stress field.

The electronic structures of the substitutional O on N site and C on Ga site in wurtzite GaN have been studied by employing ab initio ‘mixed-basis + norm conserving non-local pseudo-potential’ method and a 32-atom wurtzite supercell with and without lattice relaxations. Present calculations indicate that the host Ga atoms bonding to O impurity relax outward slightly while one of them draws along the c-axis toward another. The charge density distribution appears distinctly lower with lattice relaxations near the host Ga atoms bonding to the O impurity. These results suggest that the substitutional O with cation–cation-bond configuration is likely to act as the DX center in wurtzite GaN with heavy O dopants. On the other hand, the host N atoms bonding to the substitutional C relax inward largely which is accompanied by one of them turning toward another. The charge density distributions around the substitutional C are distinctly higher with lattice relaxations. The results of the energy band structure suggest that the substitutional C acts as a deep electron trap that is expected to offer electrons under light excitation.

The precipitates were investigated in undoped AlGaN epilayers grown by metal organic vapor phase epitaxy. Surface morphologies above the precipitates studied via atomic force microscopy are characterized by tiny protuberances. The precipitates were detected to contain C impurities by a scanning electron microscope with energy dispersive X-ray spectroscopy. In bright-field transmission electron microscope images, the precipitate appeared as a dark contrast region surrounded by a number of punched out dislocations, differing from that of nanopipes. The difference in the images suggests that the precipitates in the present samples have a different structure due to a different formation mechanism.

Photoluminescence (PL) was investigated in undoped GaN from 4.8 K to room temperature. The 4.8 K spectra exhibited recombinations of free exciton, donor–acceptor pair (DAP), blue and yellow bands (Ybs). The blue band (BB) was also identified to be a DAP recombination. The YB was assigned to a recombination from deep levels. The energy-dispersive X-ray spectroscopy show that C and O are the main residual impurities in undoped GaN and that C concentration is lower in the epilayers with the stronger BB. The electronic structures of native defects, C and O impurities, and their complexes were calculated using ab initio local-density-functional (LDF) methods with linear muffin-tin-orbital and 72-atomic supercell. The theoretical analyses suggest that the electron transitions from ON states to CN and to VGa states are responsible for DAP and the BB, respectively, and the electron transitions between the inner levels of the CN–ON complex may be responsible for the YB in our samples.

We designed Mg-doped InGaN/GaN quantum well (QW), strained ultrathin InN/GaN QWs, and Mg and Si δ-doped AlGaN/GaN superlattices (SLs) by using first-principles simulations. The designed structures were grown by metal–organic vapor phase epitaxy (MOVPE) on high-quality thick GaN using an interruption technique. The injection-current-dependent electroluminescence characteristics of Mg-doped QW exhibit smaller injection-current-induced blueshift and energy separation, which indicate that modification of quantized levels has occurred in Mg-doped QW due to the reduction of polarization field. The cathodoluminescence intensity of the strained ultrathin InN/GaN QWs is enhanced with increasing barrier thickness due to the suppression of interwell coupling, while the emission wavelength redshifts significantly as the well width increases due to reduction of strain in the QW. Higher hole concentration and mobility are achieved in Mg and Si δ-doped p-type SLs compared to those in modulation-doped SL. These results are in excellent agreement with those of theoretical designs.

Wide-bandgap semiconductors are potential materials for photovoltaic (PV) devices in solution-based applications because of their higher stability compared with narrow-bandgap semiconductors. However, these materials are poor absorbers of photons in the solar spectrum and yield modest conversion efficiencies. Here we show how this problem can be solved by the growth and control of pseudomorphic crystals in type-II wide-bandgap ZnO/ZnSe coaxial nanowires. Light absorption of coaxial nanowires under critical strains significantly extended beyond the near-infrared region to cover up to 94% of the solar power. The photocurrent response of the nanowire solar cell was markedly enhanced in visible and infrared regions with a threshold of approximately 0.9 eV, accounting for more than 35% of the conversion efficiency. This work paves the way for stable and efficient PV devices based on wide-bandgap semiconductors.

Well-aligned ZnO/ZnSe core/shell nanowire arrays with type-II energy alignment are synthesized via a two-step chemical vapor deposition method. Morphology and structure studies reveal a transition layer of wurtzite ZnSe between the wurtzite ZnO core and the cubic ZnSe shell. Type-II interfacial transitions are observed in the spectral region from visible to near infrared in transmission and photoluminescence. More significantly, for the first time, the interfacial transition is shown to extend the photoresponse of the prototype photovoltaic device based on the coaxial nanowire array to a threshold much below the bandgap of either component (3.3 and 2.7 eV, respectively) at 1.6 eV, with an external quantum efficiency of ∼4% at 1.9 eV and 9.5% at 3 eV. These results represent a major advance towards the realization of all-inorganic type-II heterojunction photovoltaic devices in an optimal device architecture.

Ternary ZnxCd1−xSe alloys are potential materials for sensitized photovoltaic devices because of their tunable bandgaps and band structures. In this work, random deposition of Cd and Zn was realized by the co-evaporated chemical vapor deposition method to tune the distribution of the lattice constant and the mixing of the structural phase to the utmost, and thus release the misfit stress by the self-adaption mechanism. As a result, the coaxial nanowire with an approximate composition x of 0.5 exhibited the high crystal quality and the long nonequilibrium carrier lifetime because of the alloy disorder effect. The corresponding photo-electrochemical cell acquired a power conversion efficiency of 3.68%. This work paves the way for the design and development of high-efficiency coaxial solar cells based on ternary or multi-component alloys.

Two different asymmetric Ag/ZnO composite nanoarrays were fabricated. These nanoarrays are proposed as highly sensitive and uniform surface-enhanced Raman scattering (SERS) substrates with plasmonic-enhanced UV-visible photocatalytic properties for self-cleaning. The asymmetric nanostructures are composed of Ag nanoparticles hanging inside or capping on the top of ZnO hollow nanospheres, which allows the generation of a strong local electric field near the contact area owing to the asymmetric dielectric environment. Experimental and simulation results showed that these asymmetric structures are favorable for achieving high photocatalytic activity under UV and visible light irradiation, in addition to improving the SERS performance. The electron transfer model based on band gap alignment was employed to further illustrate the mechanisms of the photocatalytic activity, which was dependent on the wavelength of the irradiation. Given the dramatically improved photocatalytic performance, together with the reproducible and uniform SERS signals verified by the Raman mapping results, the large area ordered asymmetric metal/semiconductor nanoarrays have been demonstrated to be suitable for further applications in multifunctional photoelectrochemical chips.