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Epitaxial growth using graphene (GR), weakly bonded by van der Waals force, is a subject of interest for fabricating technologically important semiconductor membranes. Such membranes can potentially offer effective cooling and dimensional scale-down for high voltage power devices and deep ultraviolet optoelectronics at a fraction of the bulk-device cost. Here, we report on a large-area β-Ga2O3 nanomembrane spontaneous-exfoliation (1 cm × 1 cm) from layers of compressive-strained epitaxial graphene (EG) grown on SiC, and demonstrated high-responsivity flexible solar-blind photodetectors. The EG was favorably influenced by lattice arrangement of SiC, and thus enabled β-Ga2O3 direct-epitaxy on the EG. The β-Ga2O3 layer was spontaneously exfoliated at the interface of GR owing to its low interfacial toughness by controlling the energy release rate through electroplated Ni layers. The use of GR templates contributes to the seamless exfoliation of the nanomembranes, and the technique is relevant to eventual nanomembrane-based integrated device technology.

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

The effect of a graphene oxide (GO) intermediate layer on the performance of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells was studied. The formation of dielectric MoSe2, which can be induced by a reaction between CZTSSe and Mo, was successfully suppressed. The GO layer prevented the decomposition of CZTSSe along with the formation of binary chalcogenides. Furthermore, the interference effect generated between the incident light and the light reflected by Mo was effectively alleviated, thereby reducing photon loss to effectively absorb light. The incorporation of the GO layer resulted in a stronger electric field in the absorber, as determined by the transfer matrix method, and accordingly, a higher absorption spectrum was observed in absorption measurements. Consequently, the series resistance was reduced because MoSe2 formation was inhibited, and a significant improvement in the Jsc value was confirmed via the mitigation of the interference effect and an improved series resistance. The CZTSSe solar cell exhibited a notable power conversion efficiency of 11.2%, which represented an improvement over a conventional reference cell that was fabricated with a GO layer.

The unique degradation behavior of a tunneling field-effect transistor (TFET) under hot-carrier injection (HCI) stress has been previously investigated. However, while the source side (p+/p junction) of degradation (due to HCI stress) has been extensively studied, the drain side (p/n+ junction) has not been investigated yet. Our study revealed that both bulk oxide and interfacial layer degradation occurred at the drain side, while an interfacial layer degradation was dominant at the source side at 300 K. This evidences a unique degradation mechanism of the tunneling FET.

The doping of Mg and Ga into ZnO is a method for obtaining excellent optical and electrical properties for a window layer in an inorganic solar cell. Because a tradeoff exists between the electrical and optical properties in the window layer, balancing them is important for enhancing the performance of the solar cells. From this viewpoint, the thickness change of the window layer affects the transmittance and the conductivity. In particular, it affects the transmittance in the NIR-IR region significantly, which can enhance the current collection but lead to poor conductivity when the transmittance of the window layer is increased. We examine the thickness effect of MGZO films for CZTSSe solar cells in order to determine the optimal conversion efficiency. The enhanced transmittance in the NIR-IR region with a thin MGZO layer contributed to a high current density, although the electrical resistivity was relatively high. An enhanced conversion efficiency of 7.54% was eventually recorded for an optimal thickness of the MGZO layer with an improved current density over 7 mA/cm2.

Light-emitting diodes (LEDs), which emit efficient and bright light ranging from infrared to ultraviolet, have been adopted in various fields to improve quality of life. In particular, micro-LEDs, which are III–V inorganic-based micro-miniaturized light sources, which can form clear and vivid displays for smart portable devices and can be used with artificial intelligence, are drawing great attention. The micro-LEDs can be used in applications where it is difficult to use existing inorganic LEDs, such as in medicine, clothing, and miniature electronics. Research on developing micro-LEDs is underway around the world as the demand for micro-miniaturized light sources increases, especially for display applications. However, problems are also emerging that hinder the mass production of micro-LED displays. Herein, we introduce the current research trends and issues involved in overcoming various problems associated with using micro-LEDs as full-color light sources in display applications. We also briefly review newly presented research and development strategies and technologies for micro-LEDs by research institutes and companies, including their patents and products.

Recently, microLED-based displays have been steadily developing and one of the technologies, the “RGB LED array monolithic integration” technology, has considerable prospects. This chapter introduces device manufacturing methods using only semiconductor patterning processes after bonding of red, green, and blue LEDs. These technologies are suitable for high-resolution applications such as AR/VR where it is hard to use a pick-and-place method. Here, various bonding technologies for color integration are reported: substrate bonding for two colors by wafer stacking, a method that realizes full color by combining a selective area growth method and a bonding technology, and a method that realizes full color using only a bonding technology. We also propose integration technologies using a monolithic 3D(M3D)-like process for high-resolution displays. We demonstrate ultra-high-resolution stacked LEDs exceeding 60,000 PPI estimated from their pattern size. Furthermore, we demonstrate one transistor-one photodiode M3D integration that has a clear readout operation without any degradation of the bottom Si devices after the integration process.

It is well‐known that the alkali doping of polycrystalline Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)(Se,S)2 has a beneficial influence on the device performance and there are various hypotheses about the principles of performance improvement. This work clearly explains the effect of Na doping on the fill factor (FF) rather than on all of the solar cell parameters (open‐circuit voltage, FF, and sometimes short circuit current) for overall performance improvement. When doping is optimized, the fabricated device shows sufficient built‐in potential and selects a better carrier transport path by the high potential difference between the intragrains and the grain boundaries. On the other hand, when doping is excessive, the device shows low contact potential difference and FF and selects a worse carrier transport path even though the built‐in potential becomes stronger. The fabricated CZTSSe solar cell on a flexible metal foil optimized with a 25 nm thick NaF doping layer achieves an FF of 62.63%, thereby clearly showing the enhancing effect of Na doping.

In this study, we proposed a strategy to fabricate vertically stacked subpixel (VSS) micro-light-emitting diodes (μ-LEDs) for future ultrahigh-resolution microdisplays. At first, to vertically stack the LED with different colors, we successfully adopted a bonding-interface-engineered monolithic integration method using SiO2/SiNx distributed Bragg reflectors (DBRs). It was found that an intermediate DBR structure could be used as the bonding layer and color filter, which could reflect and transmit desired wavelengths through the bonding interface. Furthermore, the optically pumped μ-LED array with a pitch of 0.4 μm corresponding to the ultrahigh-resolution of 63 500 PPI could be successfully fabricated using a typical semiconductor process, including electron-beam lithography. Compared with the pick-and-place strategy (limited by machine alignment accuracy), the proposed strategy leads to the fabrication of significantly improved high-density μ-LEDs. Finally, we systematically investigated the effects of surface traps using time-resolved photoluminescence (TRPL) and two-dimensional simulations. The obtained results clearly demonstrated that performance improvements could be possible by employing optimal passivation techniques by diminishing the pixel size for fabricating low-power and highly efficient microdisplays.

Recently, researches on the micro-displays using inorganic LEDs have stirred much attentions due to its various advantages such as reduced power consumption, long life time and short response time. In this study, we achieved a full-color integration of LEDs for display applications by combining growth and bonding technologies with two different approaches. First method was integrating inorganic red, green, and blue LEDs on a single substrate using metalorganic chemical vapor deposition and thin film bonding techniques. Second method was fabricating inorganic red, green, and blue LEDs by using GaN-on-Si-based blue LEDs and adhesive bonding.

Deep ultraviolet light-emitting diodes (DUV LEDs) (

In recent years, research into implementing microdisplays for use in virtual reality and augmented reality has been actively conducted worldwide. Specifically, inorganic light-emitting diodes (LED)...

Monolithic 3-D integration has emerged as a promising technological solution for traditional transistor scaling limitations and interconnection bottleneck. The challenge we must overcome is a processing temperature limit for top side devices in order to ensure proper performance of bottom side devices. To solve this problem, we developed a low temperature III–V and Ge layer stacking process using wafer bonding and epitaxial lift-off, since these materials can be processed at a low temperature and provide extended opportunity/functionality (sensor, display, analog, RF, etc.) via heterogeneous integration. In this paper, we discuss technology for integrating III–V and Ge materials and its applicability to CMOS, thin film photodiodes, mid-infrared photonics platforms, and MicroLED display integration for creating the ultimate 3-D chip of the future.

Cu2ZnSn(S,Se)4 (CZTSSe) has attracted much attention as the absorption layer of photovoltaic devices because of its appropriate bandgap energy and low cost. However, CZTSSe solar cells have so far shown low power conversion efficiency compared to other solar cells and studies are being conducted to improve it. In this study, we achieved an improvement in the shunt resistance and power conversion efficiency (PCE) of CZTSSe solar cells with quantification of the effective thermal conductivity. A CZTSSe solar cell fabricated using a graphite box designed with high thermal conductivity exhibited a high shunt resistance of 413 Ω cm2 and a fill factor of 63.8%, and additionally, secondary phases such as Cu2S/Se and CuS/Se were not detected. On the other hand, a CZTSSe solar cell fabricated using a graphite box with low thermal conductivity showed a rougher surface and low shunt resistance mainly related to secondary phase formation. As a result, a PCE of 9.72% was achieved using a graphite box designed with high thermal conductivity.

The effect of magnetic fields on the optical output power of flip-chip light-emitting diodes (LEDs) with InGaN/GaN multiple quantum wells (MQWs) was investigated. Films and circular disks comprising ferromagnetic cobalt/platinum (Co/Pt) multilayers were deposited on a p-ohmic reflector to apply magnetic fields in the direction perpendicular to the MQWs of the LEDs. At an injection current of 20 mA, the ferromagnetic Co/Pt multilayer film increased the optical output power of the LED by 20% compared to an LED without a ferromagnetic Co/Pt multilayer. Furthermore, the optical output power of the LED with circular disks was 40% higher at 20 mA than the output of the LED with a film. The increase of the optical output power of the LEDs featuring ferromagnetic Co/Pt multilayers is attributed to the magnetic field gradient in the MQWs, which increases the carrier path in the MQWs. The time-resolved photoluminescence measurement indicates that the improvement of optical output power is owing to an enhanced radiative recombination rate of the carriers in the MQWs as a result of the magnetic field gradient from the ferromagnetic Co/Pt multilayer.

Highly crystallized Cu 2 SnS 3 (CTS) thin films are synthesized by sputtering based deposition route. Further, Influence of different annealing temperatures on crystallographic, electrical and optical properties of CTS thin films have been investigated. The synthesized CTS thin films are grown as a monoclinic structure observed from X-ray diffraction (XRD) of (112), (220), and (312) plane and it is further confirmed using Raman spectroscopy by the presence of Raman peaks at 295 and 354 cm −1 . The direct band gap energy of CTS thin film annealed at 580 °C is found to be 1.03 eV by extrapolation from external quantum efficiency (EQE) measurement. An X-ray photoelectron spectroscopy (XPS) study suggests that the ionic states of Cu, Sn, and S are Cu + ,Sn 4+ , and S 2− , respectively. The CTS based thin film solar cells (TFSCs) are fabricated with a structure of Mo/CTS/CdS/i-ZnO/AZO/Al. The preliminary power conversion efficiency of 1.35% with a short circuit current density of 28.3 mA/cm 2 , an open circuit voltage of 147.5 mV, and a fill factor of 32% were obtained, although the processing parameters have not yet been optimized.

We have demonstrated the effect of p-GaN hole concentration on a graphene current spreading layer (CSL) for stabilization and improved performance of a near-ultraviolet light-emitting diode (NUV LED). While NUV LEDs with a more lightly-doped p-GaN showed poor electrical and optical properties and unstable performance, NUV LEDs with more heavily-doped p-GaN (∼2 × 1017 cm−3) showed very stable, outstanding performance. The main factor of the improvement was the enhanced contact property between the graphene CSLs and the p-GaN that resulted from the increase of the hole concentration, which led to a thinner barrier and an enhanced current injection. From our results, we were able to determine that hole concentration as heavy as 2 × 1017 cm−3 in p-GaN layers is a primary condition in NUV LEDs with graphene-based CSLs.

Hybrid concentrator photovoltaic (CPV) architectures that combine CPV modules with low-cost solar cells have the advantage of functioning well in modest direct normal irradiance (DNI) regions as well as high-DNI regions, where these architectures allow for higher performance in a limited space. For higher performance of a hybrid CPV module, we optimized the secondary optical element (SOE) using raytracing software and conducted experimental measurements that consider the effective wavelength range. Our experiments with the optimized SOE (&theta, = 30&deg, h = 15 mm) demonstrated a maximum output power on the triple-junction cell and polycrystalline silicon cell of 212.8 W/m2 and 5.14 W/m2, respectively.

We achieved an improvement in shunt resistance and power conversion efficiency (PCE) of CZTSSe solar cell by optimization and quantification of the annealing process. A CZTSSe solar cell fabricated using a graphite box designed with high thermal conductivity exhibited a high shunt resistance of $413~\Omega cm^{2}$and a fill factor of 63.8%. On the other hand, a CZTSSe solar cell fabricated using a graphite box with low thermal conductivity showed rougher surface and low shunt resistance related mostly with secondary phase formation. Finally, a PCE of 9.72% was achieved using a graphite box designed with high thermal conductivity.

Efforts to detect patient deterioration early have led to the development of early warning score (EWS) models. However, these models are disease-nonspecific and have shown variable accuracy in predicting unexpected critical events. Here, we propose a simpler and more accurate method for predicting risk in respiratory ward patients. This retrospective study analyzed adult patients who were admitted to the respiratory ward and detected using the rapid response system (RRS). Study outcomes included transfer to the intensive care unit (ICU) within 24 hours after RRS activation and in-hospital mortality. Prediction power of existing EWS models including Modified EWS (MEWS), National EWS (NEWS), and VitalPAC EWS (ViEWS) and SpO2/FiO2 (SF) ratio were compared to each other using the area under the receiver operating characteristic curve (AUROC). Overall, 456 patients were included; median age was 75 years (interquartile range: 65-80) and 344 (75.4%) were male. Seventy-three (16.0%) and 79 (17.3%) patients were transferred to the ICU and died. The SF ratio displayed better or comparable predictive accuracy for unexpected ICU transfer (AUROC: 0.744) compared to MEWS (0.744 vs. 0.653, P = 0.03), NEWS (0.744 vs. 0.667, P = 0.04), and ViEWS (0.744 vs. 0.675, P = 0.06). For in-hospital mortality, although there was no statistical difference, the AUROC of the SF ratio (0.660) was higher than that of each of the preexisting EWS models. In comparison with the preexisting EWS models, the SF ratio showed better or comparable predictive accuracy for unexpected ICU transfers in the respiratory wards.

We have designed and fabricated Ag-mesh-combined graphene current spreading layers (CSLs) for GaN-based near-ultraviolet light-emitting diodes (NUV LEDs). Three different types of the CSLs were fabricated having 300 μm, 150 μm, and 75 μm gaps between the mesh lines. Optical and electrical properties were investigated and the performance of the LEDs having the CSLs and were compared with those of LEDs having indium tin oxide and graphene CSLs. The light output power of the LEDs with the Ag-mesh-combined graphene CSL adopting 150 μm-gap was highest at both the same current injection and at the same input power. This was possible due to the lowering of the sheet resistance via the adoption of an Ag mesh and the networking of the Ag mesh using a graphene sheet with very little transmittance loss.

Highly ordered, position-controlled gallium nitride (GaN) nanowire based multiple-quantum-wells (MQWs) core–shell architecture arrays are synthesized by metalorganic chemical vapor deposition (MOCVD). We investigate the possibility of using GaN nanowire arrays as a basal template for the growth of In x Ga 1− x N/GaN MQWs. The MQWs on three different crystal facets ( c -, m -, and semipolar -plane) of GaN nanowire exhibit dissimilar structural properties. The structural characteristics of InGaN/GaN core–shell arrays are inspected by cross-sectional high-resolution transmission electron microscopy (HR-TEM). We also investigate the optical properties of MQW core–shell structure nanoarrays. The luminescent characteristics of InGaN/GaN core–shell structure arrays are determined by photoluminescence (PL) and cathodoluminescence (CL) measurements. The monochromatic CL images clearly show the light emission behavior of InGaN/GaN MQW coaxial structure. Two distinguishable light emission peaks were observed in the GaN nanowire based core–shell structure. The characteristic of light emission mainly depends on the properties of MQWs, which are generated from different crystal facets of GaN. In addition, the light emission intensity shows different behaviors depending on the area of the GaN nanowire m -plane. The results of this study suggest that GaN nanowire arrays can be used as a good alternative basal template for next-generation light-emitting diodes (LEDs).

Monolithic 3D (M3D) integration has attracted lots of attentions to continue equivalent scaling by vertically stacking transistors [1]. It allows the reduction of the interconnect delay, resulting in reduction of the power consumption of the chip, which is the attractive driving force for M3D integration. Moreover, with M3D, many other components such as digital, analog, MEMS, sensors, etc. can be heterogeneously integrated together in a single chip to provide enhanced functionality (Fig. 1). In this context, M3D concept can have additional benefit by integrating two or more different materials [2]. Heterogeneous integration of different materials combined with M3D is more powerful, because each processes with different materials don't have to share same process step and sequence, allowing us more flexible process design and it naturally has many benefits from their advantageous physical properties.

In general, to realize full color, inorganic light-emitting diodes (LEDs) are diced from respective red-green-blue (RGB) wafers consisting of inorganic crystalline semiconductors. Although this conventional method can realize full color, it is limited when applied to microdisplays requiring high resolution. Designing a structure emitting various colors by integrating both AlGaInP-based and InGaN-based LEDs onto one substrate could be a solution to achieve full color with high resolution. Herein, we introduce adhesive bonding and a chemical wet etching process to monolithically integrate two materials with different bandgap energies for green and red light emission. We successfully transferred AlGaInP-based red LED film onto InGaN-based green LEDs without any cracks or void areas and then separated the green and red subpixel LEDs in a lateral direction; the dual color LEDs integrated by the bonding technique were tunable from the green to red color regions (530–630 nm) as intended. In addition, we studied vertically stacked subpixel LEDs by deeply analyzing their light absorption and the interaction between the top and bottom pixels to achieve ultra-high resolution.

We introduce high-performance metal mesh/graphene hybrid transparent conductive layers (TCLs) using prime-location and metal-doped graphene in near-ultraviolet light-emitting diodes (NUV LEDs). Despite the transparency and sheet resistance values being similar for hybrid TCLs, there were huge differences in the NUV LEDs’ electrical and optical properties depending on the location of the graphene layer. We achieved better physical stability and current spreading when the graphene layer was located beneath the metal mesh, in direct contact with the p-GaN layer. We further improved the contact properties by adding a very thin Au mesh between the thick Ag mesh and the graphene layer to produce a dual-layered metal mesh. The Au mesh effectively doped the graphene layer to create a p-type electrode. Using Raman spectra, work function variations, and the transfer length method (TLM), we verified the effect of doping the graphene layer after depositing a very thin metal layer on the graphene layers. From our results, we suggest that the nature of the contact is an important criterion for improving the electrical and optical performance of hybrid TCLs, and the method of doping graphene layers provides new opportunities for solving contact issues in other semiconductor devices.

We investigate the effect of a magnetic field on red, green, and blue CdSe/ZnS quantum dot light-emitting diodes (QDLEDs). Circular multilayer ferromagnetic cobalt/platinum (Co/Pt) disks are deposited on a MgF2 layer covering an Al electrode, and a perpendicular magnetic field is applied to the QDs in the active layer. Carriers injected into the active layer are then trapped and efficiently recombined inside the QDs because of strong carrier localization caused by the perpendicular magnetic field. The luminescence of the QDLEDs in the multilayer increases by 33.31% at 7.5 V, 22.34% at 7.5 V, and 16.73% at 7.0 V compared with that of QDLEDs without the multilayer. The time-resolved photoluminescence of all the QDLEDs also indicates that their increased luminescence results from improved radiative recombination through the stronger carrier localization in the QDs.

Background Although scoring and machine learning methods have been developed to predict patient deterioration, bedside assessment by nurses should not be overlooked. This study aimed to evaluate the performance of subjective bedside assessment of the patient by the rapid response team (RRT) nurses in predicting short-term patient deterioration. Methods Patients noticed by RRT nurses based on the vital sign instability, abnormal laboratory results, and direct contact via phone between November 1, 2016, and December 12, 2017, were included. Five RRT nurses visited the patients according to their shifts and assessed the possibility of patient deterioration. Patient acuity rating (PAR), a scale of 1–7, was used as the tool of bedside assessment. Other scores, including the modified early warning score, VitalPAC early warning score, standardised early warning score, and cardiac arrest risk triage, were calculated afterwards. The performance of these scores in predicting mortality and/or intensive care unit admission within 1 day was compared by calculating the area under the receiver operating curve. Results A total of 1,426 patients were included in the study, of which 258 (18.1%) died or were admitted to the intensive care unit within 1 day. The area under the receiver operating curve of PAR was 0.87 (95% confidence interval [CI] 0.84–0.89), which was higher than those of modified early warning score (0.66, 95% CI 0.62–0.70), VitalPAC early warning score (0.69, 95% CI 0.66–0.73), standardised early warning score (0.67, 95% CI 0.63–0.70) and cardiac arrest risk triage (0.63, 95% CI 0.59–0.66) (P

We report the enhanced optical and electrical properties of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with strain-relaxing Ga-doped ZnO transparent conducting layers (TCLs). Ga-doped ZnO was epitaxially grown on p-GaN by metal-organic chemical vapor deposition. The optical output power of a LED with a 500-nm- thick-Ga-doped ZnO TCL increased by 30.9% at 100 mA, compared with that of an LED with an indium tin oxide (ITO) TCL. Raman spectroscopy measurement and the simulation of wavefunction overlap of electron and hole in MQWs revealed that the enhanced optical output power was attributed to the increased internal quantum efficiency due to the decreased compressive strain in the active region. The increase of optical output was also attributed to the increased optical transmittance of the Ga-doped ZnO TCL owing to its higher refractive index compared to that of ITO TCL. Furthermore, the forward voltage of LED with a Ga-doped ZnO TCL was lower than that of LED with an ITO TCL because of the increased carrier concentration and mobility in the Ga-doped ZnO TCL.

Thick InGaN alloys with high In content are essential for emerging InGaN photovoltaic applications. By applying a compositionally graded structure, various thicknesses of InGaN layers were grown using metal organic chemical vapor deposition. We could obtain pseudomorphic and highly strained layer even upto 100 nm of InGaN film as confirmed by X-ray reciprocal space mapping and Raman spectra. To probe the validity of those InGaN layers for solar cell applications, optical transmittance measurements were carried out and absorption properties were compared.

We report hybrid n-ZnO/i-InGaN/p-GaN solar cells in which the n-GaN layer of n-GaN/i-InGaN/p-GaN solar cell was replaced with n-type ZnO. In this study, inverted structures were used for hybrid ZnO/nitride solar cells where p-type GaN was first grown on sapphire substrate, followed by i-InGaN and n-ZnO layers. The as-fabricated device showed high series resistance and low energy conversion efficiency due to the formation of damaged p-GaN region during dry etching. On the other hand, formation of microrods on the p-GaN eased the removal of the damaged p-GaN resulting in significantly lowered series resistance and enhanced energy conversion efficiency. (a) n-ZnO/i-InGaN/p-GaN solar cell structure without microrods, (b) n-ZnO/i-InGaN/p-GaN solar cell structure with microrods, and (c) magnified image of the dashed box on (b).

We fabricated nanowire-based ultraviolet (UV) light emitting diodes (LEDs) consisting of ITO/p-NiO/n-ZnO nanowires/AZO/n-Si heterostructure by using a dimensional transition technique of 1D to 2D. The p-type NiO layer was directly deposited on the ZnO nanowire arrays, followed by the growth of ITO electrode, resulting in the full coverage of the top surface of the ZnO nanowires and their electrical interconnection. The current–voltage curve of the LED showed obvious rectifying characteristics with a threshold voltage of about 8 V and low leakage current. Under forward bias, this device exhibited UV electroluminescence located at around 380 nm coming from band edge transition of the ZnO and the broad visible emission peak around 450–650 nm was attributed to the n-ZnO defect level emission. The origin of the UV and visible emission were discussed in term of the energy band diagram of the p-NiO/n-ZnO nanowires heterjunctions.

Ultraviolet (UV), 365-nm light-emitting diodes (LEDs) were grown on an AlGaN template on patterned sapphire substrates (PSS) and then compared with those grown on a conventional GaN template. Complete coalescence was achieved by using AlGaN up to 4.9% Al composition, although the AlGaN template displayed more dislocations than the GaN template. The 365-nm UV LEDs on the AlGaN template showed no emission absorption in the template, as seen in the GaN template, because AlGaN is sufficiently far from the absorption edge of GaN. As a result, a total emission enhancement about 4 times stronger than that obtained w hat using the GaN template was obtained at 80 mA.

We report the usage of ZnO material as an alternative for n-GaN for realizing III-nitride based solar cell. The fabricated solar cell shows large turn-on voltage of around 8 volts and a rapid decrease of photocurrent at low bias voltage under darkness and 1-sun illumination conditions, respectively. This phenomenon can be attributed to the formation of high-resistive ultra-thin layers at the ZnO/ p-GaN junction interface during high temperature deposition. Transmission electron microscopy (TEM) studies carried out on the grown samples reveals that the ultra-thin layer consists of ZnGa2O4. It is found that the presence of insulating ZnGa2O4 film is detrimental in the performance of proposed heterostructure for solar cells.

We introduce ITO on graphene as a current-spreading layer for separated InGaN/GaN nanorod LEDs for the purpose of passivation-free and high light-extraction efficiency. Transferred graphene on InGaN/GaN nanorods effectively blocks the diffusion of ITO atoms to nanorods, facilitating the production of transparent ITO/graphene contact on parallel-nanorod LEDs, without filling the air gaps, like a bridge structure. The ITO/graphene layer sufficiently spreads current in a lateral direction, resulting in uniform and reliable light emission observed from the whole area of the top surface. Using KOH treatment, we reduce series resistance and reverse leakage current in nanorod LEDs by recovering the plasma-damaged region. We also control the size of the nanorods by varying the KOH treatment time and observe strain relaxation via blueshift in electroluminescence. As a result, bridge-structured LEDs with 8 min of KOH treatment show 15 times higher light-emitting efficiency than with 2 min of KOH treatment.

In this study, we have fabricated a blue-green color-tunable monolithic InGaN/GaN LED having a multi-junction structure with three terminals. The device has an n-p-n structure consisting of a green and a blue active region, i.e., an n-GaN / blue-MQW / p-GaN / green-MQW / n-GaN / Alsub2/subOsub3/substructure with three terminals for independently controlling the two active regions. To realize this LED structure, a typical LED consisting of layers of n-GaN, blue MQW, and p-GaN is regrown on a conventional green LED by using a metal organic chemical vapor deposition (MOCVD) method. We explain detailed mechanisms of three operation modes which are the green, blue, and cyan mode. Moreover, we discuss optical properties of the device.

We report on the improvement in the performance of InGaN/GaN multi-quantum well-based solar cells by the introduction of a Cu-doped indium oxide (CIO) layer at the interface between indium tin oxide (ITO) p-electrode and p-GaN. The solar cell fabricated with the 3 nm-sample exhibits an external quantum efficiency of 29.8% (at a peak wavelength of 376 nm) higher than those (25.2%) of the cell with the ITO-only sample. The use of the 3-nm-thick CIO layer gives higher short circuit current density (0.72 mA/cm2) and fill factor (78.85%) as compared to those (0.65 mA/cm2 and 74.08%) of the ITO only sample. Measurements show that the conversion efficiency of the solar cells with the ITO-only sample and the 3 nm-sample is 1.12% and 1.30%, respectively. Based on their electrical and optical properties, the dependence of the CIO interlayer thickness on the efficiency of solar cells is discussed.

( 11 2 ¯ 2 ) semipolar GaN thin films were grown on intentionally miscut m-plane sapphire substrates using metal organic chemical vapor deposition. We investigated the material and electrical characteristics by changing the miscut angle from −1° to +1°. While the coexistence of ( 11 2 ¯ 2 ) surface and inclined { 10 1 ¯ 1 } surfaces was observed on GaN films on the on-axis m-plane sapphire substrates, { 10 1 ¯ 1 } surfaces were dominant on the GaN films on the +1° miscut sapphire substrates. As the miscut angle was changed from −1° to +1°, the crystallinity of the GaN films and the electroluminescence intensity of the LEDs were significantly improved.

We report on the fabrication of highly integrated semipolar {101} GaN micro-pyramid light-emitting diode (LED) arrays on a c-plane sapphire substrate obtained via confined selective area growth (SAG). The cathodoluminescence (CL) revealed that the micro-pyramid arrays were under a severe local strain and potential fluctuations were observed depending on the location of the pyramid facets. The emissive region in the micro-pyramid facets was also found to be different in the monochromatically captured CL images. Furthermore, the electroluminescence (EL) of the micro-pyramid LED arrays had a palpable emissive spectra for high indium composition compared to c-plane LEDs, and optical polarization switching was also observed.

The recent development of dielectric microsphere lithography has been able to open up new means of performing simple and easy patterning on the semiconductor surfaces. Here, we report uniform and high-density arrays of microspheres using a solution-based spin-coating method. The arrays of microspheres were used for etching mask to form the arrays of III-nitride microrods. By regrowing GaN layer on the microrod structures, high-quality GaN layer was achieved in terms of surface morphology as well as XRD characterization. To apply the advantages such as improved crystal quality and light extraction enhancement, light-emitting diodes (LEDs) were grown and then fabricated. The regrown LEDs with microspheres showed much improved optical output power and forward voltage characteristics in the same current injection. Therefore, we believe that this approach is quite useful for the development of high efficiency LEDs for future lighting.

Blue to green tunable GaN-based LEDs having a dual-junction structure have been fabricated. We observed that the color of the dual-junction LEDs can be tuned by controlling both current path and density through three terminals.

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

InGaN/GaN p-i-n solar cells were fabricated either without a current spreading layer or with ITO or Ni/Au spreading layers. A 10.8% indium composition was confirmed within an i-InGaN layer using X-ray diffraction. I-V characteristics were measured at AM1.5 conditions, with solar cell parameters being obtained based on I-V curves in all cases. Current spreading layers produced strong effects on efficiency. The solar cell with the ITO current spreading layer showed the best results, i.e., a short circuit current density of 0.644 mA/cm2, an open circuit voltage of 2.0 V, a fill factor of 79.5%, a peak external quantum efficiency of 74.1%, and a conversion efficiency of 1.0%.

GaN micro- (or nano-) column structures have been studied in an attempt to enhance the LED performance by improving the light extraction and reducing the strain due to the large lattice mismatch between GaN and InGaN. Nowadays columnar LEDs are drawing attention as a candidate for a multicolor emission source as illustrated in Fig. 1. Though numerous studies have focused on selective area growth, the top-down approach has often been the subject of study since it can realize a columnar structure more easily than when using the bottom-up method. However, predominant issue in the top-down approach is dry-etch damage resulting from the inductively coupled plasma (ICP) and reactive ion etching (RIE) systems. According to the previous reports the damage usually results in the increase in the sheet-resistance of GaN, along with the decrease in the reverse breakdown voltage and the reductions in the Schottky barrier height in the diodes formed on GaN. The roughened sidewall after etching also exhibited higher series resistance in the device and deteriorates the electrical characteristics. KOH treatment has been found to improve the electrical characteristics through the removal of the damaged region and providing the vertical profile in the sidewall with a smooth surface. Moreover, the size control of GaN micro- (or nano-) columns also becomes possible by adopting the KOH treatment.

We report a color tunable display consisting of two passive-matrix micro-LED array chips. The device has combined vertically stacked blue and green passive-matrix LED array chips sandwiched by a transparent bonding material. We demonstrate that vertically stacked blue and green micro-pixels are independently controllable with operation of four color modes. Moreover, the color of each pixel is tunable in the entire wavelength from the blue to green region (450 nm - 540 nm) by applying pulse-width-modulation bias voltage. This study is meaningful in that a dual color micro-LED array with a vertically stacked subpixel structure is realized. (C) 2017 Optical Society of America.

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

We demonstrate a cost-effective top-down approach for fabricating InGaN/GaN nanorod arrays using a wet treatment process in a KOH solution. The average diameter of the as-etched nanorods was effectively reduced from 420 nm to 180 nm. The spatial strain distribution was then investigated by measuring the high-resolution cathodoluminescence directly on top of the nanorods. The smaller nanorods showed a higher internal quantum efficiency and lower potential fluctuation, which can subsequently be exploited for high-efficiency photonic devices. (C) 2013 Optical Society of America