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Optically pumped wavelength-tunable vertical-cavity surface-emitting lasers (VCSELs) operating in the ultraviolet A (UVA) spectrum were demonstrated. The VCSELs feature double dielectric distributed brag reflectors and a wedge-shaped cavity fabricated using the substrate transfer technique and laser lift off, resulting in a graded cavity length in one device. A resonant period gain structure is used in the InGaN/GaN multi-quantum well active region to enhance the coupling between the cavity mode field and the active layers. The optical field inside the cavity is modulated by the cavity length; thus, tunable lasing at different wavelengths is realized at different points of a single VCSEL chip. The lasing wavelength extends from 376 to 409 nm, covering most of the UVA band below the band gap of GaN. The threshold pumping power density of the UVA VCSELs at different wavelengths ranges from 383 to 466 kW/cm2, which is among the lowest values for ultraviolet (UV) VCSELs. This study is promising for the development of small-footprint, power-efficient UV light sources.

Dual-wavelength switchable emission has been demonstrated in InGaN quantum dot (QD) micro-cavity light-emitting diodes (MCLEDs). By simply modulating the injected current levels, the output of the device can be dynamically tuned between the two distinct cavity modes at 498.5 and 541.7 nm, exhibiting deterministic mode switching in the green spectral range. Owing to the microcavity effect, high spectral purity with a narrow linewidth of 0.21 nm was obtained. According to the experimental and theoretical results, it can be concluded that the dual-wavelength switching for the investigated MCLEDs is ascribed to the broad and tunable gain of a thin InGaN QD active region, together with the mode selection and enhancement effect of the cavity. To provide additional guidelines for controllable dual-wavelength switchable operation in nitride-based light-emitting devices, detailed design and fabrication strategies are discussed. This work presents an effective method to achieve mode switching for practical applications such as multi-wavelength optical recording, frequency mixing, flip-flop and optical switches.

The anisotropy of X-ray diffraction scanning of (201) β-Ga2O3 bulk material has been investigated. Symmetric rocking curves (RCs) exhibit distinctly different broadening along different azimuths, with a maximum along [102] and a minimum along a direction rotated by 30° from [010]. Williamson–Hall analysis was applied to study possible factors causing the broadening in these RCs, including instrumental factors, mosaic tilt and coherent scattering. It was found that the RC broadening is determined by both isotropic mosaic tilt and anisotropy in the length over which the crystal structure is not disrupted by limiting factors such as grain boundaries or stacking faults, which we term the `lateral limited size'. In this case, the lateral limited size is governed by {200} stacking faults along the [102] direction and grain boundaries along the [010] direction. The result presents a new anisotropy characteristic of (201) β-Ga2O3.

Green micro-cavity (MC) with a quality factor (Q) exceeding 6000 was demonstrated under current injection. The MC consists of two dielectric distributed Bragg reflectors (DBRs) and InGaN/GaN quantum wells (QWs) in between. To form a lateral optical confinement (LOC) structure, a low-index thin AlN layer was used to replace part of the p-GaN. The device showed multi-longitudinal mode emission, from 460 to 540 nm, and higher-order lateral confined modes were clearly observed. By optimizing the cavity fabrication processes and lateral optical confinement, the linewidth of resonant mode is as narrow as 0.082 nm, indicating a high Q value of 6039 in green spectral region. This is the highest value in electrically injected GaN-based MC to the best of our knowledge. In addition, three-dimensional (3D) confined optical states were observed and studied via angle resolved measurement for the first time in an electrically injected GaN-based MC. The emission characteristics of devices as a function of cavity length, as well as the loss mechanism inside the cavity were also systematically analyzed.

An optically pumped AlGaN-based vertical-cavity surface-emitting laser (VCSEL) in the deep ultraviolet (DUV) range (< 280 nm) is demonstrated. The lasing wavelength is 275.91 nm with a threshold power density of 1.21 MW/cm2 and a linewidth of 0.78 nm. The lasing is believed to be benefited from high internal quantum efficiency (IQE) of the AlGaN-based multiple quantum wells (MQWs) and improved fabrication processes.

We report the temperature effects on the performance of ferroelectric field-effect transistor (FeFET)-based non-volatile memory (NVM) considering random grain phase variation in the ferroelectric layer through simulation. Based on the FE temperature effect model that accounts for both the transistor and ferroelectric degradation, we demonstrate that: (1) at a certain temperature, the memory window (MW) decreases with pronounced effect on low threshold voltage shift and its variation increases as the FE phase decreases; (2) with the temperature increases, the MW decreases with pronounced effect on high threshold voltage shift. The random grain phase variation further exacerbates the MW distribution, thus degrading the sensing margin. These results may provide insights for device design of high-performance FeFET-based NVMs.

Since the emerging development of CsPbBr3 perovskite, chemical vapor deposition (CVD) has become one of the most promising fabrication techniques by which to precisely deposit uniform perovskite thin films. However, there have been few reports on the growth dynamics and chemical reaction parameters (e.g., activation energy) for perovskite CVD. In this work, different deposition rates of CVD-grown CsPbBr3 thin films were obtained at different substrate temperatures. Dynamics equations were developed to relate the inflow rates, desorption coefficients and concentrations of reactants on the substrates. Only a small amount of reactant became activated at low temperature and a small amount of PbBr2 resided on the substrate at high temperature, and accordingly the maximal deposition rate was achieved at 250 °C. The Arrhenius activation energy of CVD-grown CsPbBr3 was also calculated, and found to be 31.64 kJ mol−1. We believe that our work provides a detailed picture of perovskite CVD growth.

With the simulation calibration for negative capacitor considering Landau model and multi-domain (MD) effect, MD MFIS negative capacitance field-effect transistor (NCFET) was thoroughly established for performing the simultaneous analysis of multi-variables (ferroelectric layer thickness ( ${T} _{\mathrm{ FE}}$ ), oxide layer thickness ( ${T} _{\mathrm{ OX}}$ ) and gate length ( ${L} _{\mathrm{ g}}$ )) effect on the device performance. In this study, subthreshold swing (SS) and hysteresis properties of MD-MFIS-NCFET were demonstrated by employing TCAD simulation tool. Compared with the previous reported study on single variable effect based on single-domain (SD) NCFET, the simultaneous analysis of multi-variables effect on MD-NCFET enabled to obtain better device performance and generate more comprehensive results. Convincing models were established based on the experimental data by calibration. Demonstration on the basic simulated results including the lowering SS mechanism and the multi-variables effect on MD-NCFET performance was completely presented based on the capacitance matching theory and short channel effect. With the optimal ${T} _{\mathrm{ FE}}$ and ${T} _{\mathrm{ OX}}$ , a trade-off mechanism between the SS and ${L} _{\mathrm{ g}}$ was shown with the consideration of ${L} _{\mathrm{ g}}$ scaling. Noticeable in-depth study in association with the simultaneous analysis of the multi-variables effect was carried out, indicating that the hysteresis-free SS obtained by simultaneous analysis of multi-variables was lower than that obtained by single-variable analysis. Final validation results demonstrate that the optimization proposed in this work by considering the multi-variable effect shows high compatibility with other NCFET devices, providing an instructive strategy for the high-performance NCFET optimization.

Double dielectric DBR based GaN-based micro cavity light emitting devices with two different structures were fabricated, and their thermal characteristics were investigated. To improve thermal dissipation, an AlN current confinement layer with a much higher thermal conductivity than SiO2 and an electroplated copper heat sink were utilized. The thermal resistance of the device decreased from 923 K/W to 457 K/W, half of that obtained with the typically used SiO2 current confinement layer and bonded substrate. This is the lowest reported value in GaN-based micro cavity light-emitting devices with double dielectric DBRs. Temperature distribution and heat flux inside of the device was simulated based on a steady state quasi three-dimensional cylindrical model. The results show that heat transport in vertical direction is efficiently bypassing the bottom DBR to the copper plate. This work provides an effective method to improve thermal characteristics of GaN-based micro cavity light-emitting devices with double dielectric DBR structure.

Two types of GaN vertical-cavity surface-emitting lasers (VCSELs), with and without lateral optical confinement (LOC) structure, were fabricated and their performances were compared. Compared with the VCSEL without LOC, the device with LOC showed a great improvement in threshold current, slope efficiency, output power and differential quantum efficiency, which was mainly due to the reduction of internal loss. Devices with LOC showed clear and multi-transversal mode structures. However, thermal dissipation became worse. The effects of such a design on thermal resistance and transverse modes were discussed.

In this study, the impact of interfacial charges including fixed charges and acceptor-type traps between the dielectric and the channel in negative capacitance ferroelectric FETs (FeFETs) is investigated by simulation based on the Landau–Khalatnikov model. The results reveal the separate impact of the fixed charges and acceptor-type traps on device performance degradation, respectively, including the subthreshold swing (SS), switching current (I on and I off) and threshold voltage (V T). On this basis, the combined impact of interfacial charges with the fixed charges and acceptor-type traps co-existing equally is further explored. Our findings indicate that the fixed charges play a key role in the V T and I off, while the acceptor-type traps predominate in the SS and I on. This study helps to understand the degradation mechanism of FeFETs and extend the device end-of-lifetime.

Exciton-photon interaction in strong coupling region is an interesting topic in exploring bosonic physics and devices. Until now, however, InGaN based exciton polariton was rarely reported since its large inhomogeneous broadening. In this study, the impact of inhomogeneous broadening Γ i n h of InGaN/GaN quantum wells (QWs) on the normal mode energy splitting Ω N M S , between the upper and lower polariton branches was theoretically analyzed by transfer matrix method and linear nonlocal dispersion model. Surprisingly, an optimal inhomogeneous broadening Γ o p t , corresponding to the maximum value of Ω N M S was deprived at low temperature, which has not been reported before. The effect of Γ i n h was divided into two regions for explaining the existence of Γ o p t . Meanwhile, the Γ o p t was found to be strongly correlated with the oscillator strength and homogeneous broadening.

Two-dimensional (2D) Ruddlesden-Popper perovskites with bulky organic cations have attracted extensive attention in light-emitting devices and photovoltaics due to their robust environment stability, tunable luminescent color, strong exciton binding and promising efficiency. A quantum well (QW) structure is spontaneously formed by sandwiching PbBr4 layers into bulky organic cations. However, some intrinsic excitonic mechanisms in these materials still need to be elucidated. In this study, the exciton–phonon interaction of quasi-2D (PEA)2(CsPbBr3)n−1PbBr4 with different PbBr4 layer numbers (n) was analyzed by temperature-varied photoluminescence (PL), scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD). The mechanism of bandgap shifting with temperature was found to be dominated by the thermal expansion effect in the large-n 2D and bulk perovskite, and gradually switched to exciton–phonon interaction in the n = 1 (PEA)2PbBr4 phase, indicating enhanced exciton–phonon interaction in the thinner quantum well structure. Further analysis showed that the enhanced exciton–phonon interaction originated from the longitudinal optical phonon-exciton Frohlich interaction rather than acoustic phonon-exciton coupling. We believe that our results will benefit the further optimization of light-emitting devices based on 2D perovskites.

To fabricate AlGaN-based ultraviolet (UV) vertical cavity surface-emitting laser diodes, a pair of distributed Bragg reflectors (DBRs) having a smooth surface is desired to have a high quality factor (Q). In this work, photoassisted chemical etching (PCE) was attempted to smoothen the - c ( 000 - 2 ) AlN surface after removing the sapphire substrate by means of the laser lift-off process. First, the conditions for PCE were optimized using an Xe lamp and KOH solution. The root-mean-square roughness of the - c ( 000 - 2 ) AlN surface was reduced from 30.5 to 5.6 nm, which enables us to fabricate an improved HfO2/SiO2 DBR on the AlN surface. Then, using the optimized PCE technique, improved UV (Al,Ga)N planar microcavities were fabricated and distinct cavity-mode-related emissions were observed using the photoluminescence (PL) technique. By comparing the PL spectra between the samples with and without PCE treatment, the Q value at 303 nm for the case of the improved DBR was determined to increase from 174 to 270. The increase in the Q value is mainly attributed to the reduction of scattering losses in optical cavities. Furthermore, the discussion on the mechanism of improved surface during the PCE treatment is given. Consequently, PCE is demonstrated to be a feasible approach to refine the quality of ultraviolet nitride microcavities.

In this work, crystalline anisotropy of heteroepitaxial ( 2 ˉ 01 ) β-Ga2O3 films on a c-plane sapphire substrate and a GaN template was investigated using x-ray diffraction. The ( 2 ˉ 01 ) ω-scan broadening of β-Ga2O3 on GaN exhibited six-fold rotational symmetric anisotropy along different azimuths, with maxima along the [010] direction and minima along the [102] direction, respectively. However, in β-Ga2O3 on sapphire, it was nearly isotropic. Smaller lattice mismatch between β-Ga2O3 and GaN were taken into account to explain the discrepancy, which also explained the better quality of β-Ga2O3 deposited on GaN. Our results present a new viewpoint of the crystallographic anisotropy of ( 2 ˉ 01 ) β-Ga2O3 thin films.

We fabricated green vertical-cavity surface-emitting lasers by employing In x Ga1− x N/GaN quantum wells (QWs) active layers. The In fraction of the QW is 0.18 and the electroluminescence emission peak is around 445 nm, dominantly in the blue. With such QWs embedded in a microcavity, however, lasing was achieved at the emission edge (~493 nm), approaching to the green region. Such phenomenon is attributed to the cavity-enhanced recombination, where the cavity effect and photon–electron interactions or the gain enhancement factor play important roles.

Ultralow lasing threshold of $413~\mu \text{J}$ /cm2 in GaN-based vertical-cavity surface-emitting lasers (VCSELs) was observed by reducing the cavity lengths between double-side dielectric distributed Bragg reflectors to $6\lambda $ . To the best of our knowledge, the threshold is the lowest value ever reported in nitride optically pumped VCSEL. The spontaneous emission factor ( $\beta $ ) was 0.1, and the degree of polarization was 91%. Effects of short cavity on spontaneous emission factors, gain coefficient enhancement, and absorption reduction were analyzed. In addition, effects of coupled quantum wells on the confinement factor were also discussed. We believe that our results would be meaningful for refining nitride VCSEL structure in the future.

The optical properties of green InGaN/GaN multiple quantum wells (MQWs) can be greatly affected by the large lattice mismatch between the GaN barrier and the InGaN quantum well due to the introduction of stress. To improve the quality of green MQWs, the insertion of a pre-layer before the MQWs is generally adopted. However, there does not exist a clear theoretical model by which to explain the effects of the pre-layer. In the present study, a lattice evolution model is proposed to describe the epitaxial growth process, by which the stress-blocking effect of the InGaN pre-layer is demonstrated. In addition, optimal designs for the active region of green MQWs with pre-layer are discussed. These results provide a theoretical basis for the preparation of high-quality green InGaN/GaN MQWs.

We proposed a new approach of enhancing the quantum efficiency of InGaN/GaN multiple quantum well (MQW) solar cells using resonant cavity in this study. The resonant-cavity-enhanced (RCE) structure is designed with a top mirror and bottom distributed Bragg reflector (DBR). The origin of the enhancement in quantum efficiency was the resonance-induced increase of the optical field, which caused more photons to be absorbed in the InGaN absorption layers. We simulated the photovoltaic performance of InGaN/GaN MQW solar cells by comparing RCE-type and conventional solar cells. The best structural parameters are estimated to be 600 nm for cavity length, 10% for the reflectivity of top mirror and as high as possible for the reflectivity of bottom DBR (in simulation we used 99%). As a result, the RCE-type InGaN/GaN MQW solar cell shows an enhancement in short-circuit current density by 2.12 times and conversion efficiency by 2.13 times, as compared to those of a conventional InGaN/GaN MQW solar cell.

Laser lift off (LLO) was one of the most essential processes in fabrication of vertical GaN-based LEDs. However, traditional laser lift off of GaN on sapphire substrates needed high laser energy threshold, which deteriorated the GaN crystal. In this paper, it was found that inserting carbon nanotube between GaN and sapphire could effectively reduce the laser energy threshold in GaN LLO, from 1.5 J / cm 2 of conventional GaN/sapphire to 1.3 J / cm 2 of CNT inserted GaN/sapphire. The temperature distributions at the GaN/sapphire interfaces with and without CNTs were simulated by the finite elements calculation under laser irradiation. It was found that, due to the higher laser absorption coefficient of CNT, the CNT played as a powerful heating wire, sending out the thermal outside to elevate the GaN's temperature, and thus reduce the laser threshold for LLO. Raman and photoluminescence measurements indicated that residual stress of GaN membranes was as small as 0.3 GPa by the carbon nanotube assisted LLO. This work not only opens new application of CNTs, but also demonstrates the potential of high performance blue and green LEDs.

Green vertical-cavity surface-emitting lasers (VCSELs) were fabricated with two different kinds of gain medium, a green-emitting InGaN quantum dot (QD) active region and a normally blue-emitting quantum well (QW) active region. The VCSELs have dual dielectric DBRs and room temperature (RT) continuous wave (CW) lasing was observed in both type VCSELs. For the QD VCSELs, lasing at different wavelengths from 491.8 to 565.7 nm was obtained, covering most of the “green gap”. The lasing wavelength could be controlled by adjusting the cavity length, and the devices were featured with low threshold current of less than 1 kA/cm2. For the QW VCSELs, the emission peak of active layer is around 445nm, dominantly in the blue. However, lasing was observed at around 493 nm, locating at the emission edge and approaching to the green region. The green emission comes from the fluctuation-induced localization centers. And the cavity-enhanced recombination played an important role in realization of lasing action. These results open up opportunities to design and fabricate semiconductor green VCSELs that are useful for wide-gamut, low consumption power and compact displays and projectors.

The relative optical gains at three near infrared wavelengths (1550 nm, 1064 nm, and 980 nm) were achieved in NaYF4: Er3+, Yb3+ nanoparticle-doped SU-8 waveguides when using two low-cost light-emitting diodes (LEDs) instead of traditional 980 nm semiconductor laser as pump source. The polymer waveguides were fabricated by one-step photolithography process. The fluorescence bands around 1550 nm and 1000 nm wavelengths due to the 4I13/2 → 4I15/2 transition of Er3+ ions and 2F5/2 → 2F7/2 transition of Yb3+ ions were observed under the excitation of 405 nm and 520 nm LEDs. By using the vertical top pumping mode of LEDs, the relative gains of 4.2 dB, 1.7 dB, and 2.1 dB at 1550 nm, 1064 nm, and 980 nm wavelengths were achieved, respectively, on a 10-mm-long waveguide.

A 980 nm semiconductor laser is always selected as the pump source for erbium-ytterbium co-doped optical waveguide amplifiers. In this work, two low-cost blue-violet LEDs, rather than an expensive 980 nm laser, were used to pump an Er3+-Yb3+ co-doped phosphate planar waveguide. When the signal power was 0.4 mW at a 1550 nm wavelength, internal optical gains of about 4.1 and 4.5 dB/cm were respectively obtained under the excitations of a 32 mW/cm2, 275 nm LED and a 914 mW/cm2, 405 nm LED. It was found that 51.17% of the total Er3+ ions in the 2H9/2 state contributed to the luminescence at 1550 nm, and a theoretical model of gain simulation was established under the excitation of a 405 nm LED. The calculated gain of about 4.1 dB/cm was found to be in accordance with the experimental optical gain results.

Photoluminescence (PL) characteristics of the structure consisting of green InGaN/GaN multiple quantum wells (MQWs) and low indium content InGaN/GaN pre-wells are investigated. Several PL peaks from pre-wells and green InGaN/GaN MQWs are observed. The peak energy values for both pre-wells and green InGaN/GaN MQWs display an S-shaped variation with temperature. In addition, the differences in the carrier localization effect, defect density, and phonon–exciton interaction between the pre-wells and green InGaN/GaN MQWs, and the internal quantum efficiency of the sample are studied. The obtained results elucidate the mechanism of the luminescence characteristics of the sample and demonstrate the significant stress blocking effect of pre-wells.

Optical losses in GaN-based resonant planar cavity are discussed and experimentally examined in respects of indium tin oxide (ITO) inner-cavity current spreading layer, the defect-rich GaN layer, and multi-quantum well (MQW) active region. Devices with different structures are adopted to study the optical loss. It is demonstrated that a thin ITO layer and a coupled MQW active region are essential to achieve a low loss and high Q-value cavity. The results show that low-loss MQW design is particularly important in fabricating high-Q value GaN-based planar cavities.

Patterned sapphire substrate has been used extensively in the growth of gallium nitride (GaN) material and related light emitting devices (LEDs). Recently, carbon nanotube patterned sapphire (CNTPS) was utilized to improve the GaN material and LED devices. In this article, intrinsic analyzation of LEDs on CNTPS were studied. LEDs grown on three layers of CNTs showed highest radiative quantum efficiency and internal quantum efficiency, while LEDs on double layers of CNTs exhibited the best light output power and external quantum efficiency. The physics of carriers’ injection, radiative, non-radiative, Auger recombination and light extraction in CNT patterned LEDs were unraveled by the ‘ABC’ modelsimulation.

We report a method of using neodymium complexes as active waveguide core materials to achieve optical gain at the 1060 nm wavelength when using an LED instead of an 808 nm semiconductor laser as the pump source. Through the intramolecular energy transfer mechanism between ligands and Nd3+ ions, the photoluminescence spectrum could be obtained on a 100µm thick film of neodymium complex doped PMMA polymer under excitation of a 380 nm-450 nm LED. We also present calculations showing that the pump power required to generate optical gain (turn-on power) for a LED and 808 nm laser is 3.3 mW and 40 mW, respectively. An optical gain of about 6 dB can be obtained on a 20-mm-long waveguide when pumped by a 25 mW LED compared with that of about 1.4 dB excited by a 60 mW 808 nm semiconductor laser.

Vertical-cavity surface-emitting lasers (VCSELs) are promising in various applications including full-color mobile projectors, laser precision processing, display, armarium and high-speed air-water optical wireless communication system with a unique combination of advantages. However, GaN-based VCSELs with emission wavelengths in green are challenging because of the low emission efficiency of green emitting InGaN QWs. This is well known as the "Green Gap", which is mainly caused by Quantum-Confined Stark effect (QCSE) and the high density of defects and dislocations. In this paper, we would like to discuss the origin of "green gap" and possible approaches to overcome it, and then review our recent progress in green VCSELs: (1) Lasing from 479.6 nm to 565.7 nm by using QD active area ; (2) Lasing at 545 nm by using QD-in- QW active structure; (3) Lasing at 493 nm by utilizing blue emitting InGaN QWs with the combination of cavity effect.

We fabricated GaN-based blue resonant-cavity light-emitting diodes (RCLEDs) by inserting InGaN quantum well (QW) active region between two dielectric distributed Bragg reflectors (DBRs). Due to the different gain enhancement factors in a single device, multi-longitudinal modes were observed and tuned with changing the injection current density: pure-blue (PB) at low current density, violet-blue (VB) at intermediate current density, and PB again at high current density. The variation of emission spectra is explained by the competition between band-filling effect and self-heating effect.

In this paper, we report the exciton polaritons in both positive and negative detuning micro cavities based on InGaN multi-quantum wells (MQWs) and the first polariton lasing in InGaN/GaN MQWs at room temperature by utilizing a 4.5λ Fabry-Perot (F-P) cavity with double dielectric distributed Bragg reflectors (DBRs). Double thresholds corresponding respectively to polariton lasing and photonic lasing are observed along with half-width narrowing and peak blue-shifts. The threshold of polariton lasing is about half of the threshold of photonic lasing. Our results paved a substantial way for ultra-low threshold lasers and room temperature Bose-Einstein Condensate (BEC) in nitride semiconductors.

National Natural Science Foundation of China [61106044, 61274052]; Doctoral Program Foundation of Institutions of Higher Education of China [20110121110029]

Recently, CsPbX3 (X: halogen) materials have been considered as promising light-emitting materials, because of their good environmental durability and outstanding optoelectronic properties. Understanding the exciton and phonon behaviors in CsPbX3 is crucial in developing advanced optoelectronic devices. In this work, distinct acoustic phonon–exciton interaction, which was first observed in perovskite, was discovered in our established highly luminescent and durable CsPbBr3 film. The acoustic phonon–exciton coupling was induced by strong exciton confinement (~70 meV exciton binding energy) and accompanied by large phonon energy ascribed to local vibration modes. We believe that our results will serve to in improve performance of the corresponding optoelectronic devices.

Low threshold lasing at room temperature was achieved in optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with sub-nanometer roughness polishing. The cavity region sandwiched by two dielectric distributed Bragg reflectors, incorporating InGaN/GaN multiquantum wells, a p-type AlGaN layer, and n- and p-type GaN layers, is a typical structure for electrically driven VCSELs. We observed lasing at a wavelength of 431.0 nm with a low threshold pumping energy density of ~ 3.2 mJ/cm2 and a high spontaneous emission coupling factor of ~ 0.09. These results were attributed to the significant reduction of the internal cavity loss by the removal of the high-dislocation GaN region, the reduction of cavity length, and the achievement of sub-nanometer level surface roughness (root mean square roughness of 0.3 nm) via inductively coupled plasma etching and chemical mechanical polishing. The loss mechanism is discussed and loss is quantitatively calculated in this letter.

In0.135Ga0.865N/GaN p–i–n solar cells are fabricated and investigated with three types of transparent current spreading layers (TCSLs): Ni/Au layer type (Ni/Au-L), Ni/Au grid type (Ni/Au-G), and ITO layer type (ITO-L). The performance of solar cell is improved with optimized TCSL and the best result is achieved in device with ITO-L. These results indicate that TCSL can strongly affect the device property, and its optimization is necessary in order to obtain better photovoltaic performance.