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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.

Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells are showing great promise due to using earth-abundant and nontoxic materials and tuning the band gap through the amount of S and Se. Flexible high-effi...

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.

ZnO is of great interest for large-area optoelectronic devices, e.g., flat panel displays, light-emitting diodes, transparent semiconductors, and transparent conductive oxides (TCOs) etc., owing to its good optical properties. In the present study, quaternary ZnO thin films are deposited on a soda lime glass substrate using Mg and group III elements, such as Al, Ga, and In, to enhance the optical and electrical properties. The structural, optical, and electrical properties of quaternary ZnO TCO materials are investigated for improved thin film solar cell performance. All of the films show a uniform microstructure without any void and crack and have a transmittance over 75% in the visible region. They possess comparable band gap differences. Especially, as for the optical properties, a Mg and Ga co-doped ZnO, MgGaZnO (MGZO) thin film shows a high optical band gap of 3.87 eV with a transmittance of about 90% in the visible region. In addition, the MGZO thin film shows improved electrical properties with the lowest resistivity of 3.97 × 10−4 Ωcm, higher carrier concentration of 1.11 × 1021cm−3, high mobility of 14.07 cm2V−1s−1 and lower sheet resistance of 7.39Ω/sq. Moreover, a wide band gap of 3.87 eV is obtained for MGZO thin films, and the performance of a Cu2ZnSn(S, Se)4 (CZTSSe) photovoltaic device fabricated with a MGZO TCO layer is improved owing to higher band gap and outstanding electronic properties. The improved short circuit current results in a device with a MGZO thin film with a power conversion efficiency of 8.79%.

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.

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.

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)...

Ge alloyed Cu 2 SnS 3 (CTGS) thin films were prepared by annealing the sputtered deposited Cu-Ge-Sn precursor films under sulfur atmosphere at different annealing temperatures. The influence of different annealing temperatures on morphological, compositional, crystal structure of CTGS thin films were investigated. It was found that the annealing temperature of 550 °C provides a favorable sulfurization environment to promote grain growth leading to a compact thin film formation. Improved performance is ascribed to high Ge contents as evidenced from X-ray fluorescence (XRF) studies. Well incorporated Ge atoms into CTS thin film can be confirmed by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) study provides an evidence of existence of Ge atoms where its binding energy located at 25.78 and 26.78 eV, respectively. However, the decreased performance was found at unsuitable annealing temperatures such as 500 °C, 520 °C, 580 °C and 600 °C. Finally, with annealing temperatures of 550 °C, the best power conversion efficiency (PCE) of 2.14% was attained with an open circuit voltage ( V oc ) of 220 mV, a short circuit current density ( J sc ) of 23.74 mA/cm 2 and a fill factor ( FF ) of 41%.

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.

We report the effects of precursor uniformity on the efficiency of Cu 2 ZnSnS x Se 4 − x (CZTSSe) solar cells using sputtered precursors. The reduction of Sn cluster agglomeration in the Cu/Sn/Zn stacked precursor created synthesis of more uniformly distributed CZTSSe absorbers during the annealing process. The secondary phase difference between the 80 W and 40 W sputtered CZTSSe absorbers was confirmed with Raman analysis at 178 cm − 1 and 259 cm − 1 . The 40 W sputtered CZTSSe cell produced blue region enhancement of external quantum efficiency results due to improvement of the pn-junction properties. The CZTSSe 40 W cells achieved a maximum power conversion efficiency of 5.7% and short-circuit current of 32.4 mA/cm 2 .

Severe melt-back etching has forced the epitaxy of GaN on Si to use an AlN buffer layer for growing high-quality two-dimensional layers, despite its high resistivity. Herein, we report a metal-based pre-orienting layer (POL) for growing GaN nanostructures (NSs) to replace the traditional AlN buffer layer. Two metals, titanium (Ti) and hafnium (Hf), were evaluated as POLs. We succeeded in fabricating arrays of GaN NSs with highly preferred orientation using selective-area growth. The crystallographic phase of the POLs critically affected the evolved orientation of the crystals. Photoluminescence measurements revealed that GaN NSs with Hf-based POLs were of reasonably high quality. We believe that this result will facilitate broader III-V semiconductor applications using alternative substrates moving beyond conventional Si-based optoelectronics.

As an alternative to Cu(In,Ga)(S,Se)2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTS)-based thin film solar cells (TFSCs), the ternary chalcogenide semiconductor Cu2SnS3 (CTS) is an emerging material with suitable optical band gap energy ranging from 0.93 to 1.77 eV and high absorption coefficients (>104 cm−1). In this study, we report the preparation of high-quality CTS thin films by the annealing of the sputtered Cu-Sn metallic precursor under S vapor atmosphere in a graphite box using the RTA process. Furthermore, the influence of different S vapor partial pressures in the graphite box during annealing on the properties of the CTS thin films has been investigated systematically. It is observed that the properties and photovoltaic performance of the CTS thin films are strongly dependent on the S vapor partial pressure during annealing. The monoclinic crystal structure is observed from X-ray diffraction (XRD) of the (1 1 2), (2 2 0), and (3 1 2) planes, 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 is found to be 0.91 eV by extrapolation from external quantum efficiency (EQE) measurement of a CTS thin film annealed using 60 mg of S powder. The preliminary best power conversion efficiency (PCE) of 2.21% with a short circuit current density of 29.7 mA/cm2, an open circuit voltage of 179.5 mV, and a fill factor of 41% have been obtained for a CTS thin film annealed using 60 mg of S powder, although the processing parameters have not been optimized.

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.

Transparent electrodes are often regarded as a trade-off between transmittance and sheet resistance, although the effect of thin-film interference on the performance of an optoelectronic device is significant. In this paper, we report the effect of optical thin-film interference of an Al-doped ZnO (AZO) transparent electrode on the performance of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. First, we distinguished the transmittance in the overall wavelength region (450–800 nm) of the AZO transparent electrode from the transmittance in the maximum absorption region for CZTSSe solar cells (550–700 nm). From the results, a thicker AZO transparent electrode showed higher transmittance (87.1%) than that of a thinner AZO transparent electrode (82.9%) in the maximum absorption region. These electrodes were applied in CZTSSe solar cells, which showed a difference of up to 26% in power conversion efficiency. In addition, the colors generated by the optical interference between the AZO transparent electrodes. We anticipate that our results will be useful for improving the performance and/or color tuning of thin-film solar cells for application in building-integrated photovoltaics.

We report on a method for controlling the polarity of gallium nitride (GaN) using an E-beam evaporated aluminum (Al) layer on a sapphire substrate. A high-temperature nitridation process was designed to enable the amorphous Al layer to serve as a nucleation layer for single-crystal Ga-polar GaN growth. The Al layer also acts as a mask that prevents N-polar GaN growth. As a result, Ga-polar and N-polar GaN can be grown on the Al layer and sapphire surface, respectively. This method is not only advantageous for the selective polarity control but also to simplify the fabrication process of lateral polarity structures.

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.

In recent years, the method of forming Cu2ZnSn(S,Se)4 (CZTSSe) by heating precursors has been widely used, but the chalcogen vapor conditions and the placement of the sample have not been disclosed completely. Here, we demonstrate a novel inverted annealing process that enables the reaction between the chalcogen vapor and precursors to remain constant. As a result, we obtained CZTSSe absorber films with an improved morphology and a uniformly distributed elemental composition. The inverted annealed samples showed less variation in the S and Se content as well as in the statistical analysis of the device performance compared with those prepared via the original annealing process. These variations in the S and Se content were confirmed several times through X-ray diffraction, energy-dispersive X-ray, and external quantum efficiency analysis. A pn-junction analysis using a Mott-Schottky plot was also conducted. We anticipate that the inverted annealing process will become widely used as it is easily applicable and effective in improving the morphology of CZTSSe absorber films.

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.

Metal nanowires have been gaining increasing attention as the most promising stretchable transparent electrodes for emerging field of stretchable optoelectronic devices. Nanowelding technology is a major challenge in the fabrication of metal nanowire networks because the optoelectronic performances of metal nanowire networks are mostly limited by the high junction resistance between nanowires. We demonstrate the spontaneous and selective welding of Ag nanowires (AgNWs) by Ag solders via an electrochemical Ostwald ripening process and high electrostatic potential at the junctions of AgNWs. The AgNWs were welded by depositing Ag nanoparticles (AgNPs) on the conducting substrate and then exposing them to water at room temperature. The AgNPs were spontaneously dissolved in water to form Ag+ ions, which were then reduced to single-crystal Ag solders selectively at the junctions of the AgNWs. Hence, the welded AgNWs showed higher optoelectronic and stretchable performance compared to that of as-formed AgNWs. Th...

To extend the availability of nanostructure-based optoelectronic applications, vertically elongated nanorods with precisely controlled morphology are required. For group III nitrides, pulsed-mode growth has recently been reported as an effective method for growing nanorod arrays with geometric precision. Here, we demonstrated the growth of arrays of highly elongated nanorods on Si substrates by metal–organic chemical vapor deposition using a pulsed-mode approach. Unlike the thick and high (or middle)-quality GaN templates normally used, nanorod growth was performed on an ultrathin and low-quality AlN/Si platform. Using kinetically controlled growth conditions and a patterning process, exceptionally long GaN nanorods were achieved with high geometric precision. The grown nanorods showed considerably improved optical and structural properties while remaining in uniform arrays. This approach can be used with a variety of materials to obtain nanorods with high quality, high uniformity, and high aspect ratio, and it can also serve as an effective fabrication method for InAlGaN-alloyed core/shell nanostructures for optoelectronic nanodevices with ultrahigh efficiency.

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 report the fabrication of near-vertically elongated GaN nanorods on quartz substrates. To control the preferred orientation and length of individual GaN nanorods, we combined molecular beam epitaxy (MBE) with pulsed-mode metal–organic chemical vapor deposition (MOCVD). The MBE-grown buffer layer was composed of GaN nanograins exhibiting an ordered surface and preferred orientation along the surface normal direction. Position-controlled growth of the GaN nanorods was achieved by selective-area growth using MOCVD. Simultaneously, the GaN nanorods were elongated by the pulsed-mode growth. The microstructural and optical properties of both GaN nanorods and InGaN/GaN core–shell nanorods were then investigated. The nanorods were highly crystalline and the core–shell structures exhibited optical emission properties, indicating the feasibility of fabricating III-nitride nano-optoelectronic devices on amorphous substrates.

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.

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.

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.

We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the $c$ plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for $c$-GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.

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.

High-performance GaAs tunnel diodes (TDs) are fabricated by using Si delta-doping technique. The GaAs TDs exhibited a high peak tunnel-current density of 2,735 A/cm2 and low specific resistivity of 1.46 × 10−4 Ωcm2. However, the performance of the GaAs TDs deteriorated once the amount of Si delta doping exceeded a certain limit, which has been rarely reported elsewhere. Detailed analyses and numerical simulations of GaAs TDs with various amounts of Si delta doping prove that Si amphoteric behavior governs the performance limit. GaAs TDs with precisely controlled Si delta doping are suitable for cutting-edge tandem solar cell applications.

Injecting current with a uniform carrier concentration is important for applications with three-dimensional architectures such as vertical power devices or displays. In III-nitride nanostructures, dopants not only incorporate differently depending on the surface orientation but can also seriously affect the kinetic equilibrium shapes of the nanorods. Herein, we report selective-area growth of doped GaN nanorods grown by pulsed-mode metalorganic chemical vapor deposition. Two dopants, Si and Mg, were employed as donor and acceptor atoms, respectively, for a mono-doping approach. Furthermore, a mixed flow of Si and Mg was supplied for a co-doping approach. We compared the morphological effects and growth rates of each doped GaN nanorod array. Then, we proposed appropriate growth mechanisms for the doped GaN nanorods on the basis of our structural characterizations. These results might extend the morphological functionality of GaN nanorods by including doping and may also provide an appropriate foundation for the design of nanostructure-based electronic or photonic devices.

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.

In this paper, we introduce very thin Indium tin oxide (ITO) layers (5, 10, and 15 nm) hybridized with a metal mesh to produce high-performance transparent conductive layers (TCLs) in near-ultraviolet light-emitting diodes (NUV LEDs). Using UV–vis–IR spectrometry, Hall measurement, and atomic force microscopy, we found that 10 nm was the optimal thickness for the very thin ITO layers in terms of outstanding transmittance and sheet resistance values as well as stable contact properties when hybridized with the metal mesh. The proposed layers showed a value of 4.56 Ω/□ for sheet resistance and a value of 89.1% for transmittance. Moreover, the NUV LEDs fabricated with the hybrid TCLs achieved ∼140% enhanced light output power compared to that of 150 nm thick ITO layers. Finally, to verify the practical usage of the TCLs for industrial applications, we packaged the NUV LED chips and obtained improved turn-on voltage (3.48 V) and light output power (∼116%) performance.

GaN-based blue LEDs were fabricated and studied with porous, dense, and dual-layer indium tin oxide (ITO) structures as transparent top electrodes to enhance light extraction. The electroluminescence intensity of the LED with a thickness-optimized and refractive-index-tuned ITO dual layer at I = 20 mA was higher by 19.7% than that of the conventional LED with a 200 nm planar ITO. This study confirmed that an ITO dual layer can be made with a single material by optimizing the thickness and tuning the refractive index, which improves the power output without any electrical property degradation.

We report on the material and optical properties of core–shell InGaN layers grown on GaN nanorod arrays. The core–shell InGaN layers were well grown on polarization-reduced surfaces such as semipolar pyramids and nonpolar sidewalls. In addition, to compensate the biaxial strain between GaN and InGaN layers, we grew interlayers underneath a thick InGaN layer. Here, the interlayers were composed of multiple superlattice structures. We could observe that the indium composition of core–shell InGaN structures increased with the number of interlayers. This indicates that the absorption energy band of InGaN alloys can be better matched to the spectral irradiance of the solar spectrum in nature. We also implemented a simulation of Ga-polar and nonpolar InGaN-based solar cells based on the indium composition obtained from the experiments. The result showed that nonpolar InGaN solar cells had a much higher efficiency than Ga-polar InGaN solar cells with the same thickness of the absorption layer.

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.