Low-Leakage Current Core-Shell AlGaN Nanorod LED Device Operating in the Ultraviolet-B Band.

Despite the considerable potential of AlGaN-based ultraviolet-B light-emitting diodes (UV-B LEDs) in various applications such as phototherapy, UV curing, plant growth, and analytical technology, their development is still ongoing due to low luminescence efficiency. In this study, we introduced a novel epitaxial growth mechanism to effectively control the height and thickness of AlGaN multiple wells (MWs) on AlGaN nanorod structures using horizontal reactor-based metal-organic chemical vapor deposition (MOCVD). By adjusting the H2 carrier gas flow rate, we could control the growth boundary layer's thickness, successfully separating the AlGaN well and p-AlGaN layer from the substrate. Cathodoluminescence (CL) measurements confirmed the stability of the core-shell AlGaN quantum wells as a highly stable nonpolarized structure, with the wavelength peak remaining almost unchanged under various injection currents. Furthermore, transmission electron microscopy (TEM) provided clear evidence of differentiation, highlighting the distinct formation of the 275 nm AlGaN core and the 295 nm AlGaN shell structure. The developed AlGaN MW structure, characterized by these rectification features, not only demonstrated a significantly improved electroluminescence (EL) peak intensity but also exhibited a much lower leakage current compared to the conventional core-shell AlGaN structure. The newly proposed growth mechanism and advanced nonpolarized core-shell AlGaN structure are expected to serve as excellent alternatives for substantially enhancing the efficiency of the next generation of high-efficiency UV LEDs.

Photo-Charging of Li(Ni0.65Co0.15Mn0.20)O2 Lithium-Ion Battery Using Silicon Solar Cells.

This study reports an integrated device in which a lithium-ion battery (LIB) and Si solar cells are interconnected. The LIB is fabricated using the Li(Ni0.65Co0.15Mn0.20)O2 (NCM622) cathode and the Li4Ti5O12 (LTO) anode. The surface and shape morphologies of the NCM and LTO powders were investigated by field emission scanning electron microscopy (FE-SEM). In addition, the structural properties were thoroughly examined by X-ray diffraction (XRD). Further, their electrochemical characterization was carried out on a potentiostat. The specific discharge capacity of the NCM cathode (half-cell) was 188.09 mAh/g at 0.1 C current density. In further experiments, the NCM-LTO full-cell has also shown an excellent specific capacity of 160 mAh/g at a high current density of 1 C. Additionally, the capacity retention was outstanding, with 99.63% at 1 C after 50 cycles. Moreover, to meet the charging voltage requirements of the NCM-LTO full-cell, six Si solar cells were connected in series. The open-circuit voltage (VOC) and the short-circuit photocurrent density (JSC) for the Si solar cells were 3.37 V and 5.42 mA/cm2. The calculated fill factor (FF) and efficiency for the Si solar cells were 0.796 and 14.54%, respectively. Lastly, the integrated device has delivered a very high-power conversion-storage efficiency of 7.95%.

Synthesis of Superionic Conductive Li1+x+yAlxSiyTi2-xP3-yO12 Solid Electrolytes.

Commercial lithium-ion batteries using liquid electrolytes are still a safety hazard due to their poor chemical stability and other severe problems, such as electrolyte leakage and low thermal stability. To mitigate these critical issues, solid electrolytes are introduced. However, solid electrolytes have low ionic conductivity and inferior power density. This study reports the optimization of the synthesis of sodium superionic conductor-type Li1.5Al0.3Si0.2Ti1.7P2.8O12 (LASTP) solid electrolyte. The as-prepared powder was calcined at 650 °C, 700 °C, 750 °C, and 800 °C to optimize the synthesis conditions and yield high-quality LASTP powders. Later, LASTP was sintered at 950 °C, 1000 °C, 1050 °C, and 1100 °C to study the dependence of the relative density and ionic conductivity on the sintering temperature. Morphological changes were analyzed using field-emission scanning electron microscopy (FE-SEM), and structural changes were characterized using X-ray diffraction (XRD). Further, the ionic conductivity was measured using electrochemical impedance spectroscopy (EIS). Sintering at 1050 °C resulted in a high relative density and the highest ionic conductivity (9.455 × 10-4 S cm-1). These findings corroborate with the activation energies that are calculated using the Arrhenius plot. Therefore, the as-synthesized superionic LASTP solid electrolytes can be used to design high-performance and safe all-solid-state batteries.

Stretchable Inorganic GaN-Nanowire Photosensor with High Photocurrent and Photoresponsivity.

To effectively implement wearable systems, their constituent components should be made stretchable. We successfully fabricated highly efficient stretchable photosensors made of inorganic GaN nanowires (NWs) as light-absorbing media and graphene as a carrier channel on polyurethane substrates using the pre-strain method. When a GaN-NW photosensor was stretched at a strain level of 50%, the photocurrent was measured to be 0.91 mA, corresponding to 87.5% of that (1.04 mA) obtained in the released state, and the photoresponsivity was calculated to be 11.38 A/W. These photosensors showed photocurrent and photoresponsivity levels much higher than those previously reported for any stretchable semiconductor-containing photosensor. To explain the superior performances of the stretchable GaN-NW photosensor, it was approximated as an equivalent circuit with resistances and capacitances, and in this way, we analyzed the behavior of the photogenerated carriers, particularly at the NW-graphene interface. In addition, the buckling phenomenon typically observed in organic-based stretchable devices fabricated using the pre-strain method was not observed in our photosensors. After a 1000-cycle stretching test with a strain level of 50%, the photocurrent and photoresponsivity of the GaN-NW photosensor were measured to be 0.96 mA and 11.96 A/W, respectively, comparable to those measured before the stretching test. To evaluate the potential of our stretchable devices in practical applications, the GaN-NW photosensors were attached to the proximal interphalangeal joint of the index finger and to the back of the wrist. Photocurrents of these photosensors were monitored during movements made about these joints.

Near-IR emission of InGaN quasi-quantum dots on non-polar GaN nanowire structures.

In group III-nitride based semiconductor structures, the incorporation of high-indium-composition InGaN has been severely limited by extremely inefficient strain-induced polarization fields and prohibitively large defect densities. So far, there is no clear approach to solve this issue. Here, we have shown a new approach to incorporate high concentrations of indium in the InGaN structure by using a non-polar quasi-quantum dot heterostructure. This unique epitaxial growth was achieved by integrating a 1-dimensional nanowire and a 0-dimensional quantum dot structure using an MOCVD system. The formation of a high-efficiency quantum-sliding heterostructure and high-quality nanowire structure was confirmed by FE-SEM and TEM measurements. Furthermore, it has been suggested that such a quantum-dot structure can dramatically improve radiative recombination through a new sliding bandgap mechanism. We also found that non-polar quantum dots can not only incorporate more indium than conventional multi-quantum well structures grown on the nanowire structure, but also significantly improve crystalline quality. The PL results verified that the wavelength of quantum dots fabricated on the nanowire structure can easily shift up to 913 nm. The first demonstration in the integration of nanowire and quantum dot structures will open a new avenue to break through the limitations of high indium incorporation in photonic semiconductor systems.

Highly Efficient Photoelectrochemical Water Splitting Using GaN-Nanowire Photoanode with Tungsten Sulfides.

In the present study, we have achieved high-performance photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) coated with tungsten sulfide (WxS1-x) (GaN-NW-WxS1-x) as a photoanode. The measured current density and applied-bias photon-to-current efficiency were 20.38 mA/cm2 and 13.76%, respectively. These values were much higher than those reported previously for photoanodes with any kind of III-nitride nanostructure. The amount of hydrogen gas formed was 1.01 mmol/cm2 from 7 h PEC-WS, which was also much higher than the previously reported values. The drastic improvement in the PEC-WS performance using the GaN-NW-WxS1-x photoanode was attributed to an increase in the number of photogenerated carriers due to the highly crystalline GaN NWs, and acceleration of separation of photogenerated carriers and consequent suppression of charge recombination because of nitrogen-terminated surfaces of NWs, sulfur vacancies in WxS1-x, and type-II band alignment between NW and WxS1-x. The degree of impedance matching, evaluated from Nyquist plots, was considered to analyze charge transfer characteristics at the interface between the GaN-NW-WxS1-x photoanode and 0.5-M H2SO4 electrolyte. Considering the material system and scheme for the PEC-WS, our approach provides an efficient way to improve hydrogen evolution reaction.

Ultrafast Photoinduced Interconnection of Metal-Polymer Composites for Fabrication of Transparent and Stretchable Electronic Skins.

The high interest sparked by the foldable smartphones recently released on the market is gradually shifting to the next generation of flexible electronic devices, such as electronic skins in the form of stretchable thin films. To develop such devices, good mechanical flexibility of all components (including the substrate, electrode, and encapsulant) is critical. Various technologies have been developed to enhance the flexibility of these components; however, progress in developing interconnection methods for flexible and stretchable devices has been limited. Here, we developed an ultrafast photoinduced interconnection method that does not require any adhesive or surface treatment. This method is based on heating metal nanostructures using intense pulsed light (IPL) and the reversible cross-linking of polymers. First, we synthesized a stretchable, transparent, and free-standing polymer substrate that can be reversibly cross-linked, and then Ag nanowire (AgNW) networks were formed on its surface. This electrode was irradiated with IPL, which locally heated the AgNWs, followed by decomposition of the polymer via the retro-Diels-Alder reaction and recross-linking. Independently fabricated AgNW/polymer films were layered and irradiated three times with IPL to form a bonded sample with excellent joint quality and no increase in electrical resistance compared to a single electrode. Furthermore, the interconnected electrodes were stretchable and optically transparent. Even when more than 200% strain was applied in a peel test, no breakage at the joint was observed. This allowed us to successfully produce a stretchable, transparent, and bending-insensitive pressure sensor for various applications such as motion detectors or pressure sensor arrays.

Pressure-Sensitive Adhesive with Controllable Adhesion for Fabrication of Ultrathin Soft Devices.

As the interest in foldable smartphones recently launched onto the market shifts toward the next generation of flexible electronics, the development of ultrathin devices is gaining considerable attention. The strain formed on the surfaces of film-based devices approximates the film thickness divided by twice the radius of curvature; therefore, the use of an ultrathin substrate is the key for the development of next generation foldable devices. However, the stiffness of ultrathin films is extremely low; thus, it cannot be easily used directly as a substrate for device fabrication. Therefore, these films generally undergo device manufacturing processes while being attached to a rigid substrate such as glass and are peeled from the rigid substrate after the process is finished. Thus, the initial adhesion of the adhesive used to fix the film to the temporary substrate should be strong, and after the process is completed, the adhesion must be lessened to enable soft peeling. In this study, we succeeded in developing a novel pressure-sensitive adhesive (PSA) whose adhesive strength can be severely reduced by water treatment. Accordingly, considering that amphiphilic oligomers promote water absorption through hydrogen bonding to water, amphiphilic oligomers were mixed with an acrylic polymer to prepare the water-responsive PSA (wr-PSA). The adhesion strength of the wr-PSA in the early stage, which reached 382(±22) N/m, dramatically dropped to 9(±2) N/m after a water immersion test. Using the wr-PSA, a 1.4 µm-thick polyethylene terephthalate film coated with Ag nanowires was softly peeled off from the glass after being immersed in warm water. In addition, the adhesion reduced by the immersion in water was recovered again when the water absorbed by the adhesive was dried. This implies that the developed adhesive can be reusable.

Core-Shell Tunnel Junction Nanowire White-Light-Emitting Diode.

We have demonstrated a new class of phosphor-free white LEDs with the use of tunnel junction structure in nonpolar core-shell InGaN nanowires. It is confirmed that the tunnel junction based nanowire LEDs can eliminate the use of the resistive p-GaN:Mg contact layer, leading to significantly enhanced hole injection and dramatically reduced voltage loss. The nonpolar core-shell nanowire heterostructure showed the enhanced carrier injection efficiency through the widened shell n-GaN contact area. The TEM analysis verified that the core-shell Al tunnel junction layers were uniformly grown on nonpolar surfaces of the GaN wurtzite crystal nanowire structure. We have also showed the monolithic integration of multiple-color emission on a single chip by using the multiple-stacked tunnel junction core-shell nanowire heterostructure. Compared to the conventional film based quantum well LEDs, the demonstrated nonpolar core-shell tunnel junction nanowire LEDs will be a very promising candidate for future solid-state lighting applications as well as phosphor-free white LEDs.

Self-Integratable, Healable, and Stretchable Electroluminescent Device Fabricated via Dynamic Urea Bonds Equipped in Polyurethane.

Reversible bonding between polymer chains has been used primarily to induce self-healing of damaged polymers. Inspired by the dynamic nature of such bonding, we have developed a polyurethane equipped with dynamic urea bonds (PEDUB) that has high strength sufficient to make it be freestanding and have a healing capability and self-bonding property. This allowed subsequent heterogeneous multicomponent device integration of electrodes/substrate and light-emitting pixels into a light-emitting device. We first used the PEDUB to individually fabricate a highly stretchable electrode containing Ag nanowires and stretchable composites with ZnS-based particles. They were successfully assembled into a stretchable, waterproof electroluminescent (EL) device even under mild conditions (60 °C for 10 min) owing to the reversible exchange of urea bonds and low glass transition temperature of PEDUB. The assembled device with an AC-driven EL architecture retained excellent EL characteristics even after stretching, submersion in water, and cutting owing to the robust solid-state bonding interfaces induced by the dynamic urea bonds. Consequently, various shapes of the illuminating elastomer and an illuminated picture were realized for the first time using the mosaic-like assembly method. This first demonstration of multicomponent assembly paves the way for future stretchable multifunctional devices.

Fabrication of a Bending-Insensitive In-Plane Strain Sensor from a Reversible Cross-Linker-Functionalized Silicone Polymer.

A reversibly cross-linkable and transparent polymer featuring stretchability and thermal healability is prepared by introducing Diels-Alder (DA)-reactive moieties into polydimethylsiloxane (PDMS), namely, a healable PDMS (h-PDMS). Inspired by the fact that retro-DA reactions occur even at low temperatures (albeit at a low rate), we maximize the effectiveness of small reactant products, demonstrating that self-healing and self-integration realized by 1-3 min exposure of cured h-PDMS to methyl ethyl ketone (MEK) vapor is more efficient than that achieved by direct sample heating at high temperatures. This technology is first used to uniformly transfer Ag nanowires (Ag NWs) formed on a temporary substrate to the h-PDMS surface, and further MEK vapor treatment allows the transferred NWs to be impregnated below the h-PDMS surface to afford an in-plane strain sensor. Most importantly, the developed method is used to perfectly integrate two identical Ag NW/h-PDMS films and thus place NWs on a neutral plane. Consequently, because of the unique structure in which a percolated network of AgNWs is formed on the interface where the two identical h-PDMS films are chemically integrated, the fabricated sensor is transparent, self-healable, stretchable, and insensitive to bending but sensitively responds to in-plane strain induced by lateral deformation.

Fast Response Characteristics of Flexible Ultraviolet Photosensors with GaN Nanowires and Graphene.

We report the fast response characteristics of flexible ultraviolet photosensors with GaN nanowires (NWs) and a graphene channel. The GaN NWs used as light-absorbing media are horizontally and randomly embedded in a graphene sandwich structure in which the number of bottom graphene layers is varied from zero to three and the top is a fixed single layer of graphene. In the response curve of the photosensor with a double-layer bottom graphene, as obtained under pulsed illumination with a pulse width of 50 ms and a duty cycle of 50%, the rise and decay times were measured as 24.1 ± 0.1 and 28.2 ± 0.1 ms, respectively. The eye-crossing percentage was evaluated as 52.1%, indicating no substantial distortion of the duty cycle and no pulse symmetry problem. The rise and decay times estimated from an equivalent circuit analysis represented by resistances and capacitances agree well with the measured values. When the device was under the bending condition, the rise and decay times of the photosensor were comparable to those in the unbent state.

Application of Hexagonal Boron Nitride to a Heat-Transfer Medium of an InGaN/GaN Quantum-Well Green LED.

Group III-nitride light-emitting diodes (LEDs) fabricated on sapphire substrates typically suffer from insufficient heat dissipation, largely due to the low thermal conductivities (TCs) of their epitaxial layers and substrates. In the current work, we significantly improved the heat-dissipation characteristics of an InGaN/GaN quantum-well (QW) green LED by using hexagonal boron nitride (hBN) as a heat-transfer medium. Multiple-layer hBN with an average thickness of 11 nm was attached to the back of an InGaN/GaN-QW LED (hBN-LED). As a reference, an LED without the hBN (Ref-LED) was also prepared. After injecting current, heat-transfer characteristics inside each LED were analyzed by measuring temperature distribution throughout the LED as a function of time. For both LED chips, the maximum temperature was measured on the edge n-type electrode brightly shining fabricated on an n-type GaN cladding layer and the minimum temperature was measured at the relatively dark-contrast top surface between the p-type electrodes. The hBN-LED took 6 s to reach its maximum temperature (136.1 °C), whereas the Ref-LED took considerably longer, specifically 11 s. After being switched off, the hBN-LED took 35 s to cool down to 37.5 °C and the Ref-LED took much longer, specifically 265 s. These results confirmed the considerable contribution of the attached hBN to the transfer and dissipation of heat in the LED. The spatial heat-transfer and distribution characteristics along the vertical direction of each LED were theoretically analyzed by carrying out simulations based on the TCs, thicknesses, and thermal resistances of the materials used in the chips. The results of these simulations agreed well with the experimental results.

Fabrication and Characterization of a Capacitive Photodetector Comprising a ZnS/Cu Particle/Poly(vinyl butyral) Composite.

Most photodetectors developed to date essentially measure photocurrents induced by the generation and separation of electron-hole pairs in semiconductors during irradiation. Although the above light detection method is well established, highly sensitive, and applicable to a broad range of semiconductor materials, it requires the presence of a stable and direct contact between the semiconductor and the electrode for accurate photocurrent measurements. In turn, this prerequisite necessitates the use of various costly processes for device fabrication (e.g., photolithography and vacuum deposition of semiconductors/metals) and complicates the development of flexible devices. Herein, inspired by the fact that the dielectric properties of certain materials can be changed by light irradiation, we dispersed ZnS/Cu semiconducting particles in poly(vinyl butyral) to prepare a free-standing composite film and formed two layers of Ag nanowire electrodes on both sides of the cured composite to fabricate a photodetector of a completely new type. The developed device exhibited a capacitance very sensitive to irradiation with light of a specific wavelength and additionally featured the advantages of simple structure/operation mechanism, mechanical flexibility, and transparency, not showing any signs of performance deterioration even after severe damage.

Impact of Al Pre-Deposition Layer on Crystalline Quality of GaN Grown on Si(111) Substrates.

The effects of Al metal pre-deposition under different conditions on GaN grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated. Al pre-deposition improves surface morphology and crystal quality of GaN grown on Si. The surface morphology of Al pre-deposition layer, AlN, and GaN vary depending on Al pre-deposition temperature. With the increase of Al pre-deposition temperature, Al cluster size is observed to increase in the Al predeposition layer due to increased lateral mobility of Al atoms. The Al pre-deposition carried out at about 750 °C enables to grow pit-free GaN layer on Si(111) substrate.

Highly Efficient and Flexible Photosensors with GaN Nanowires Horizontally Embedded in a Graphene Sandwich Channel.

In this study, we report highly efficient and flexible photosensors with GaN nanowires (NWs) horizontally embedded in a graphene sandwich structure fabricated on polyethylene terephthalate. GaN NWs and the graphene sandwich structure are used as light-absorbing media and the channel for carrier movement, respectively. To form uniform high-quality crystalline GaN NWs on Si(111) substrates, the initial nucleation behavior of the NWs was manipulated by applying the new growth technique of Ga predeposition. High-resolution transmission electron microscopic images obtained along the vertical direction of GaN NWs showed that stacking faults, typically observed in Si-based (In,Ga)As NWs, were rare. Consequently, narrow and strong optical emission was observed from the GaN NWs at wavelengths of 365.12 nm at 300 K. The photocurrent and photoresponsivity of the flexible photosensor with 802 nm long GaN NWs horizontally embedded in the graphene sandwich channel were measured as 9.17 mA and 91.70 A/W, respectively, at the light intensity of 100 mW/cm2, which are much higher than those previously reported. The high optical-to-electrical conversion characteristics of our flexible photosensors are attributed to the increase in the effective interface between the light-absorbing media and the carrier channel by the horizontal distribution of the GaN NWs within the graphene sandwich structure. After 200 cyclic-bending test of the GaN NW photosensor at the strain of 3%, the photoresponsivity under strain was measured as 89.04 A/W at 100 mW/cm2, corresponding to 97.1% of the photoresponsivity obtained before bending. The photosensor proposed in this study is relatively simple in device design and fabrication, and it requires no sophisticated nanostructural design to minimize the resistance to metal contacts.

Anisotropic Thermal Interface Materials: Directional Heat Transfer in Uniaxially Oriented Liquid Crystal Networks.

For the development of anisotropic thermal interface materials (TIMs), a rod-shaped reactive monomer PNP-6MA is newly designed and successfully synthesized. PNP-6MA reveals a smectic A (SmA) mesophase between crystalline (K) and isotropic (I) phases. PNP-6MA can be oriented under a magnetic field ( B = 2 T), and its macroscopic orientation can be robustly stabilized by in situ polymerization. Even without macroscopic orientations, the fabricated thermal conducting liquid crystal (TCLC) films show the outstanding thermal conductivity of 1.21 W/m K, which is higher than conventional organic materials. The thermal conductivity of uniaxially and macroscopically oriented TCLC films can be 2.5 W/m K along the long axis of mesogenic core. The newly developed TCLC film can be used as a TIM between a high-power light-emitting diode and a heat sink.

Formation of spherical-shaped GaN and InN quantum dots on curved SiN/Si surface.

This paper reports the formation of GaN and InN quantum dots (QDs) with symmetric spherical shapes, grown on SiN/Si(111). Spherical QDs are grown by modulating initial growth behavior via gallium and indium droplets functioning as nucleation sites for QDs. Field-emission scanning electron microscope (FE-SEM) images show that GaN and InN QDs are formed on curved SiN/Si(111) instead of on a flat surface similar to balls on a latex mattress. This is considerably different from the structural properties of In(Ga)As QDs grown on GaAs or InP. In addition, considering the shape of the other III-V semiconductor QDs, the QDs in this study are very close to the ideal shape of zero-dimensional nanostructures. Transmission-electron microscope images show the formation of symmetric GaN and InN QDs with a round shape, agreeing well with the FE-SEM results. Compared to other III-V semiconductor QDs, the unique structural properties of Si-based GaN and InN QDs are strongly related to the modulation in the initial nucleation characteristics due to the presence of droplets, the degree of lattice mismatch between GaN or InN and SiN/Si(111), and the melt-back etching phenomenon.

Interfacial Engineering for the Synergistic Enhancement of Thermal Conductivity of Discotic Liquid Crystal Composites.

To develop an advanced heat transfer composite, a deeper understanding of the interfacial correlation between matrix and filler is of paramount importance. To verify the effect of interfacial correlations on the thermal conductivity, the conductive fillers such as expanded graphite (EG) and boron nitride (BN) are introduced in the discotic liquid crystal (DLC)-based polymeric matrix. The DLC matrix exhibits better interfacial affinity with EG compared to BN because of the strong π-π interactions between EG and DLC. Thanks to its excellent interfacial affinity, the EG-DLC composites show a synergistic increment in thermal conducting performance.

A III-nitride nanowire solar cell fabricated using a hybrid coaxial and uniaxial InGaN/GaN multi quantum well nanostructure.

III-Nitride nanowires are currently considered as next generation photovoltaic materials due to their excellent physical properties together with reduced dislocation densities, increased surface area and thus enhanced light absorption and direct path for carrier transport. Here, we investigate the photovoltaic characteristics of a solar cell fabricated from a novel hybrid nanostructure comprising uniaxial and coaxial InGaN/GaN multi-quantum wells (MQWs) along with an InGaN nano-cap layer. Various characterization methods were employed to study the optical and structural properties of the hybrid nanostructure. Transmission electron microscopy images revealed the hybrid nanostructure consists of distinct uniaxial and coaxial InGaN/GaN MQWs along with the InGaN nano-cap layer. The InGaN/GaN MQW architectures have a significant effect on the performance of the photovoltaic device. The solar cell fabricated with the hybrid nanostructure exhibits superior photovoltaic performance compared to the uniaxial as well as the coaxial InGaN/GaN nanowire MQW structures. The improved photovoltaic characteristic is primarily attributed to the considerably larger InGaN active area grown in the hybrid nanostructure. A conversion efficiency of 1.16% along with a fill factor of 70% was obtained for the device fabricated with the hybrid nanostructure. This study provides an experimental demonstration of the improvement of III-nitride nanowire based solar cells incorporating uniaxial and coaxial InGaN/GaN MQWs.