On the origin of the enhanced light extraction efficiency of DUV LED by using inclined sidewalls.

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) can be enhanced by using an inclined sidewall of mesa. However, the reported optimal inclined angles are different. In this work, to explore the origin for enhancing the LEE of DUV LED by using inclined sidewalls, we investigate the effect of an inclined sidewall angle on the LEE for AlGaN-based DUV LEDs with different mesa diameters by using ray tracing. It is found that when compared to large-size DUV LEDs with inclined sidewall, the LEE of small-size DUV LEDs with inclined sidewall is enhanced from both the bottom and side surfaces due to the reduced scattering length and material absorption. Additionally, the optimal inclined sidewall angle is recommended within the range of 25°-65°, and the optimal angle for DUV LEDs decreases as the chip size increases. It can be attributed to the fact that there are two scattering mechanisms for the inclined sidewall. For smaller chip sizes, most of the light is directly scattered into escape cones by the inclined sidewall, resulting in a larger optimal angle. For larger chip sizes, the light firstly experiences total internal reflections by the out-light plane and then is scattered into escape cones by the inclined sidewalls, leading to a smaller optimal angle.

Enhancing light extraction efficiency of the inclined-sidewall-shaped DUV micro-LED array by hybridizing a nanopatterned sapphire substrate and an air-cavity reflector.

In this work, we hybridize an air cavity reflector and a nanopatterned sapphire substrate (NPSS) for making an inclined-sidewall-shaped deep ultraviolet micro light-emitting diode (DUV micro-LED) array to enhance the light extraction efficiency (LEE). A cost-effective hybrid photolithography process involving positive and negative photoresist (PR) is explored to fabricate air-cavity reflectors. The experimental results demonstrate a 9.88% increase in the optical power for the DUV micro-LED array with a bottom air-cavity reflector when compared with the conventional DUV micro-LED array with only a sidewall metal reflector. The bottom air-cavity reflector significantly contributes to the reduction of the light absorption and provides more escape paths for light, which in turn increases the LEE. Our investigations also report that such a designed air-cavity reflector exhibits a more pronounced impact on small-size micro-LED arrays, because more photons can propagate into escape cones by experiencing fewer scattering events from the air-cavity structure. Furthermore, the NPSS can enlarge the escape cone and serve as scattering centers to eliminate the waveguiding effect, which further enables the improved LEE for the DUV micro-LED array with an air-cavity reflector.

Fabricating and investigating a beveled mesa with a specific inclination angle to improve electrical and optical performances for GaN-based micro-light-emitting diodes.

In this Letter, beveled mesas for 30 × 30 µm2 GaN-based micro-light-emitting diodes (µLEDs) with different inclination angles are designed, fabricated, and measured. We find that µLED with a mesa inclination angle of 28° has the lowest internal quantum efficiency (IQE) and the highest injection current density at which the peak IQE is obtained. This is due to the increased quantum confined Stark effect (QCSE) at the mesa edge. The increased QCSE results from the strong electric field coupling effect. Instead of radiative recombination, more nonradiative recombination and leakage current will be generated in the sidewall regions. Besides, the smallest angle (28°) also produces the lowest light extraction efficiency (LEE), which arises from the optical loss caused by the sidewall reflection at the beveled surface sides. Therefore, the inclination angle for the beveled mesa has to be increased to 52° and 61° by using Ni and SiO2 as hard masks, respectively. Experimental and numerical results show that the external quantum efficiency (EQE) and the optical power can be enhanced for the fabricated devices. Meanwhile, the reduced surface recombination rate also decreases the leakage current.

Is a thin p-GaN layer possible for making high-efficiency AlGaN-based deep-ultraviolet light-emitting diodes?

In this report, we investigate the impact of a thin p-GaN layer on the efficiency for AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs). According to our results, the light extraction efficiency (LEE) becomes higher with the decrease of the p-GaN layer thickness, which can be ascribed to the decreased absorption of DUV emission by the thin p-GaN layer. Moreover, we also find that the variation trend of external quantum efficiency (EQE) is consistent with that of LEE. Therefore, we can speculate that high-efficiency DUV LEDs can be achieved by using thin p-GaN layer to increase the LEE. However, a thin p-GaN layer can also cause severe current crowding effect and the internal quantum efficiency (IQE) will be correspondingly reduced, which will restrict the improvement of EQE. In this work, we find that the adoption of a current spreading layer for such DUV LED with very thin p-GaN layer can facilitate the current spreading effect. For the purpose of demonstration, we then utilize a well-known p-AlGaN/n-AlGaN/p-AlGaN (PNP-AlGaN) structured current spreading layer. Our experimental and numerical results show that, as long as the current crowding effect can be suppressed, the DUV LED with thin p-GaN layer can significantly increase the EQE and the optical power thanks to the enhanced LEE.

Enhancing the light extraction efficiency for AlGaN-based DUV LEDs with a laterally over-etched p-GaN layer at the top of truncated cones.

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet light-emitting didoes (DUV LEDs) can be enhanced by using truncated cone arrays with inclined sidewalls. In this work, the air-cavity-shaped inclined sidewall is applied and the p-GaN layer at the top of the truncated cone is laterally over-etched so that more light escape paths are generated for AlGaN-based DUV LEDs. The experimental results manifest that when compared with DUV LEDs only having the air-cavity-shaped inclined sidewall, the optical power for the DUV LEDs with laterally over-etched p-GaN at the top of the truncated cone is enhanced by 30% without sacrificing the forward bias. It is because the over-etched p-GaN makes little effect on the carrier injection and does not affect the ohmic contact resistance. Moreover, the simulation results show that the truncated cone with laterally over-etched p-GaN layer can enhance the LEE because the reduced p-GaN area can suppress the optical absorption and supplies additional light paths for DUV photos. Then, more light will be reflected into escape cones at the sapphire side.

Step-type quantum wells with slightly varied InN composition for GaN-based yellow micro light-emitting diodes.

In this work, we propose adopting step-type quantum wells to improve the external quantum efficiency for GaN-based yellow micro light-emitting diodes. The step-type quantum well is separated into two parts with slightly different InN compositions. The proposed quantum well structure can partially reduce the polarization mismatch between quantum barriers and quantum wells, which increases the overlap for electron and hole wave functions without affecting the emission wavelength. Another advantage is that the slightly decreased InN composition in the quantum well helps to decrease the valence band barrier height for holes. For this reason, the hole injection capability is improved. More importantly, we also find that step-type quantum wells can make holes spread less to the mesa edges, thus suppressing the surface nonradiative recombination and decreasing the leakage current.

On the impact of a metal-insulator-semiconductor structured n-electrode for AlGaN-based DUV LEDs.

In this work, a 280 nm AlGaN-based deep ultraviolet light-emitting diode (DUV LED) with a metal-insulator-semiconductor (MIS) structured n-electrode is fabricated and studied. The SiO2 insulator layer is adopted to form the MIS structure by using an atomic layer deposition system. After adopting the MIS-structured n-electrode, the SiO2 intermediate layer enables electron affinity for the contact metal to be higher than the conduction band of the n-AlGaN layer, which favors the electrons to be injected into the n-AlGaN layer by intraband tunneling rather than thermionic emission. Moreover, the thin SiO2 insulator can share the applied bias, which makes the n-AlGaN layer surface less depleted and thus further facilitates the electron injection. The improved electron injection capability at the metal-semiconductor interface helps reduce the contact resistance and increase electron concentration in the active region, which then improves external quantum efficiency and wall-plug efficiency for the proposed DUV LED.

Alternative Strategy to Reduce Surface Recombination for InGaN/GaN Micro-light-Emitting Diodes-Thinning the Quantum Barriers to Manage the Current Spreading.

Owing to high surface-to-volume ratio, InGaN-based micro-light-emitting diodes (µLEDs) strongly suffer from surface recombination that is induced by sidewall defects. Moreover, as the chip size decreases, the current spreading will be correspondingly enhanced, which therefore further limits the carrier injection and the external quantum efficiency (EQE). In this work, we suggest reducing the nonradiative recombination rate at sidewall defects by managing the current spreading effect. For that purpose, we properly reduce the vertical resistivity by decreasing the quantum barrier thickness so that the current is less horizontally spreaded to sidewall defects. As a result, much fewer carriers are consumed in the way of surface nonradiative recombination. Our calculated results demonstrate that the suppressed surface nonradiative recombination can better favor the hole injection efficiency. We also fabricate the µLEDs that are grown on Si substrates, and the measured results are consistent with the numerical calculations, such that the EQE for the proposed µLEDs with properly thin quantum barriers can be enhanced, thanks to the less current spreading effect and the decreased surface nonradiative recombination.

Enhancing the lateral current injection by modulating the doping type in the p-type hole injection layer for InGaN/GaN vertical cavity surface emitting lasers.

In this report, we propose GaN-based vertical cavity surface emitting lasers with a p-GaN/n-GaN/p-GaN (PNP-GaN) structured current spreading layer. The PNP-GaN current spreading layer can generate the energy band barrier in the valence band because of the modulated doping type, which is able to favor the current spreading into the aperture. By using the PNP-GaN current spreading layer, the thickness for the optically absorptive ITO current spreading layer can be reduced to decrease internal loss and then enhance the lasing power. Furthermore, we investigate the impact of the doping concentration, the thickness and the position for the inserted n-GaN layer on the lateral hole confinement capability, the lasing power, and the optimization strategy. Our investigations also report that the optimized PNP-GaN structure will suppress the thermal droop of the lasing power for our proposed VCSELs.

Integrating remote reflector and air cavity into inclined sidewalls to enhance the light extraction efficiency for AlGaN-based DUV LEDs.

In this work, we propose and demonstrate the concept of remote reflections, which help to multiply the photon propagations for increasing the light extraction efficiency (LEE) for both transverse magnetic (TM)- and transverse electric (TE)-polarized light. The remote reflection is enabled by using a remote-metal-reflector-based air cavity extractor. According to our study, the remote reflections can significantly avoid the optical absorption when compared with the conventional inclined-sidewall-shaped deep-ultraviolet light-emitting diodes with the metal Al reflector on the inclined sidewalls. As a result, the optical power for our proposed devices has been significantly enhanced by 55% experimentally. Numerical simulations further reveal that the remote metal reflector not only favors more total internal refection on the inclined sidewalls but also supports additional light escaped channels for enhancing the LEE.

On the origin for the hole confinement into apertures for GaN-based VCSELs with buried dielectric insulators.

A better lateral current confinement is essentially important for GaN-based vertical-cavity-surface-emitting lasers (VCSELs) to achieve lasing condition. Therefore, a buried insulator aperture is adopted. However, according to our results, we find that the current cannot be effectively laterally confined if the insulator layer is not properly selected, and this is because of the unique feature for GaN-based VCSELs grown on insulating substrates with both p-electrode and n-electrode on the same side. Our results indicate that the origin for the current confinement arises from lateral energy band bending in the p-GaN layer rather than the electrical resistivity for the buried insulator. The lateral energy band in the p-GaN layer can be more flattened by using a buried insulator with a properly larger dielectric constant. Thus, less bias can be consumed by the buried insulator, enabling better lateral current confinement. On the other hand, the bias consumption by the buried insulator is also affected by the insulator thickness, and we propose to properly decrease the insulator layer thickness for reducing the bias consumption therein and achieving better lateral current confinement. The improved lateral current confinement will correspondingly enhance the lasing power. Thanks to the enhanced lateral current confinement, the 3dB frequency will also be increased if proper buried insulators are adopted.

Improving the Current Spreading by Locally Modulating the Doping Type in the n-AlGaN Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

In this report, we locally modulate the doping type in the n-AlGaN layer by proposing n-AlGaN/p-AlGaN/n-AlGaN (NPN-AlGaN)-structured current spreading layer for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). After inserting a thin p-AlGaN layer into the n-AlGaN electron supplier layer, a conduction band barrier can be generated in the n-type electron supplier layer, which enables the modulation of the lateral current distribution in the p-type hole supplier layer for DUV LEDs. Additionally, according to our studies, the Mg doping concentration, the thickness, the AlN composition for the p-AlGaN insertion layer and the NPN-AlGaN junction number are found to have a great influence on the current spreading effect. A properly designed NPN-AlGaN current spreading layer can improve the optical output power, external quantum efficiency (EQE), and the wall-plug efficiency (WPE) for DUV LEDs.

On the origin of enhanced hole injection for AlGaN-based deep ultraviolet light-emitting diodes with AlN insertion layer in p-electron blocking layer.

For the [0001] oriented AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the holes in the p-type electron blocking layer (p-EBL) are depleted due to the polarization induced positive sheet charges at the last quantum barrier (LQB)/p-EBL interface. The hole depletion effect significantly reduces the hole injection capability across the p-EBL. In this work, we propose inserting a thin AlN layer between the LQB and the p-EBL, which can generate the hole accumulation at the AlN/p-EBL interface. Meanwhile, the holes can obtain the energy when traveling from the p-EBL into the multiple quantum wells (MQWs) by intraband tunneling through the thin AlN layer. As a result, the hole injection and the external quantum efficiency (EQE) have been remarkably enhanced. Moreover, we point out that the thick AlN insertion layer can further generate the hole accumulation in the p-EBL and increase the hole energy which helps to increase the hole injection. We also prove that the intraband tunneling for holes across the thick AlN insertion layer is facilitated by using the optimized structure.

Impact of the surface recombination on InGaN/GaN-based blue micro-light emitting diodes.

In this work, the size-dependent effect for InGaN/GaN-based blue micro-light emitting diodes (µLEDs) is numerically investigated. Our results show that the external quantum efficiency (EQE) and the optical power density drop drastically as the device size decreases when sidewall defects are induced. The observations are owing to the higher surface-to-volume ratio for small µLEDs, which makes the Shockley-Read-Hall (SRH) non-radiative recombination at the sidewall defects not negligible. The sidewall defects also severely affect the injection capability for electrons and holes, such that the electrons and holes are captured by sidewall defects for the SRH recombination. Thus, the poor carrier injection shall be deemed as a challenge for achieving high-brightness µLEDs. Our studies also indicate that the sidewall defects form current leakage channels, and this is reflected by the current density-voltage characteristics. However, the improved current spreading effect can be obtained when the chip size decreases. The better current spreading effect takes account for the reduced forward voltage.

Effects of Meshed p-type Contact Structure on the Light Extraction Effect for Deep Ultraviolet Flip-Chip Light-Emitting Diodes.

In this work, flip-chip AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with various meshed contact structures are systematically investigated via three-dimensional finite-difference time-domain (3D FDTD) method. It is observed that both transverse electric (TE)- and transverse magnetic (TM)-polarized light extraction efficiencies (LEEs) are sensitive to the spacing and inclined angle for the meshed structure. We also find that the LEE will not be increased when a large filling factor is adopted for the meshed structures, which is because of the competition among the p-GaN layer absorption, the Al metal plasmon resonant absorption, and the scattering effect by meshed structures. The very strong scattering effect occurring in the hybrid p-GaN nanorod/p-AlGaN truncated nanocone contacts can enormously enhance the LEE for both TE- and TM-polarized light, e.g., when the inclined angle is 30°, the LEE for the TE- and TM-polarized light can be increased by ~ 5 times and ~ 24 times at the emission wavelength of 280 nm, respectively.

White light-emitting diodes based on carbon dots and Mn-doped CsPbMnCl3 nanocrystals.

CsPbX3 perovskite nanocrystals (NCs) are becoming a promising material for optoelectronic devices that possess an optically tunable bandgap, and bright photoluminescence. However, the toxic Pb is not environmentally friendly and the quantum yield (QY) of blue emitting NCs is relatively low. In addition, the red emitting perovskite containing iodine is not stable under light illumination. In this paper, high QY, blue emitting, non-toxic fluorescent nanomaterial carbon dots and orange-emitting CsPb0.81Mn0.19Cl3 NCs with partial Pb replacement are combined to fabricate white light-emitting diodes (WLEDs). A WLED with color coordinates of (0.337, 0.324) and a correlated color temperature of 4804 K is fabricated. Compared to red emitting perovskite containing iodine, the CsPb0.81Mn0.19Cl3 NCs are stable no matter whether they are stored in the air or exposed under ultraviolet light. Therefore, the as-fabricated WLED shows good color stability against increasing currents and long-term working stability.

Optimization Strategy of 4H-SiC Separated Absorption Charge and Multiplication Avalanche Photodiode Structure for High Ultraviolet Detection Efficiency.

In this work, parametric investigations on structural optimization are systematically made for 4H-SiC-based separated absorption charge and multiplication (SACM) avalanche ultraviolet photodiode (UV APD). According to our results, the breakdown voltage can be strongly affected by the thickness for the multiplication layer and the doping concentration for the charge control layer. The thickness for the n-type ohmic contact layer, the absorption layer, and the charge control layer can remarkably affect the light penetration depth, which correspondingly influences the number of photo-generated electron-hole pairs, and therefore the aforementioned layer thickness has a strong impact on the responsivity for SACM APD. For enhancing the responsivity of the APD, we require a reduced energy band barrier height at the interface of the optical absorption layer and the charge control layer, so that the promoted carrier transport into the multiplication layer can be favored. In addition, we investigate positive beveled mesas with smaller angles so as to reduce the electric field at the mesa edge. Thus, the dark current is correspondingly suppressed.

On the p-AlGaN/n-AlGaN/p-AlGaN Current Spreading Layer for AlGaN-based Deep Ultraviolet Light-Emitting Diodes.

In this report, AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with different p-AlGaN/n-AlGaN/p-AlGaN (PNP-AlGaN) structured current spreading layers have been described and investigated. According to our results, the adopted PNP-AlGaN structure can induce an energy barrier in the hole injection layer that can modulate the lateral current distribution. We also find that the current spreading effect can be strongly affected by the thickness, the doping concentration, the PNP loop, and the AlN composition for the inserted n-AlGaN layer. Therefore, if the PNP-AlGaN structure is properly designed, the forward voltage, the external quantum efficiency, the optical power, and the wall-plug efficiency for the proposed DUV LEDs can be significantly improved as compared with the conventional DUV LED without the PNP-AlGaN structure.

Establishment of the relationship between the electron energy and the electron injection for AlGaN based ultraviolet light-emitting diodes.

This work establishes the relationship between the electron energy and the electron concentration within the multiple quantum wells (MQWs) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The electron energy of different values can be obtained by modulating the Si doping concentration in the n-AlGaN layer and/or engineering the polarization induced interface charges. The modulated Si doping concentration in the n-AlGaN layer will cause the interface depletion region within which the electric field can be generated and then tunes the electron energy. The polarization induced charges and the polarization induced electric field can be obtained by stepwisely reducing the AlN composition for the n-AlGaN layer along the [0001] orientation. We find that the electron concentration in the MQWs can be increased once the electron energy is reduced to a proper level, which correspondingly improves the external quantum efficiency (EQE) for DUV LEDs. According to our investigations, it is more advisable to adopt the n-AlGaN layer with the stepwise AlN composition, which can make both the EQE and the wall plug efficiency high.

Nearly Efficiency-Droop-Free AlGaN-Based Ultraviolet Light-Emitting Diodes with a Specifically Designed Superlattice p-Type Electron Blocking Layer for High Mg Doping Efficiency.

This work reports a nearly efficiency-droop-free AlGaN-based deep ultraviolet light-emitting diode (DUV LED) emitting in the peak wavelength of 270 nm. The DUV LED utilizes a specifically designed superlattice p-type electron blocking layer (p-EBL). The superlattice p-EBL enables a high hole concentration in the p-EBL which correspondingly increases the hole injection efficiency into the multiple quantum wells (MQWs). The enhanced hole concentration within the MQW region can more efficiently recombine with electrons in the way of favoring the radiative recombination, leading to a reduced electron leakage current level. As a result, the external quantum efficiency for the proposed DUV LED structure is increased by 100% and the nearly efficiency-droop-free DUV LED structure is obtained experimentally.