Narrow Results

Efficiency droop and green gap are terms that summarize performance limitations in GaInN/GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c-plane and non-polar m-plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V-defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

Low-resistance Ohmic contacts are essential for the fabrication of electrical devices. While low contact resistance has been achieved to p-type layers or n-type layers separately, contacts are likely to degrade when both types need to be integrated into a single fabrication process, in particular when prior mesa etching is required. We present a solution to the problem, resulting in low-resistance Ohmic contacts on n-type GaN layers without post-deposition thermal anneal, while maintaining the quality of typical p-type contacts. We implement an integrated process for both, n- and p-contacts, involving an oxygen pretreatment to fabricate light emitting diodes with lower series resistance in the contacts and lower voltage drop at high current when compared to separately optimized contacts.

We discuss physics, design, fabrication, performance, and selected applications of Deep Ultraviolet Light Emitting Diodes (DUV LEDs). Our analysis reveals the relative contributions of electrical injection, internal quantum efficiency, and light extraction efficiency to the overall DUV LED performance. Our calculations show that the reduction of the dislocation density at least below value of 2×10⁸ 1/cm³ is necessary for reaching high DUV LED efficiency. Better light extraction has been achieved using an innovative p-type transparent sub-contact layer and reflecting ohmic p-type contact resulting in nearly tripling DUV LED power. At high power dissipation, temperature rise might be significant, and we present data showing the power degradation with temperature increase and the results of the detailed 1D and 3D analysis of thermal impedance of DUV LEDs. As an example of DUV LED application, we report on microbial disinfection using 19 watt 275 nanometer DUV LED.

Tunability of correlated color temperature (CCT) and dimmability are the two major sought after features of functional solid-state lighting. This paper presents a low-cost, efficient, single-inductor-dual-output buck driver that facilitates CCT-control and dimming of a light emitting diode (LED) module composed of cool-white and warm-white chips. The proposed driver is designed based on two familiar integrated circuits (ICs), namely LM2596 and NE556. LM2596 serves as the main switching regulator and amplitude modulation (AM) controller whereas NE556 serves as the constant-frequency, dual-output pulse-width-modulation (PWM) controller. The exponential model of LED is exploited for improved estimation of the design parameters of the LED module and the driver circuit. The developed driver prototype is tested for its electrical, photometric, and colorimetric performances. The driver offers a full-load efficiency of about 90% for a 12-W test LED module. Under different operating modes, the LED system can produce warm, cool, or neutral white light and at the same time, the produced illuminance can be varied over a wide range. Moreover, the light flicker is imperceptible since the PWM frequency is set according to IEEE-1789 guideline.

Optical and electrical noises and correlation factor between optical and electrical fluctuations of nitride-based light emitting diodes (LEDs) have been investigated under forward bias. Their electrical, optical and noise characteristics were compared with ones of LEDs of other materials. LED noise characteristic changes during aging have been measured, too. It is found that optical and electrical noise spectra under forward bias for more reliable LEDs distinguish by lower l/f type fluctuations and Lorentzian type noise at higher frequencies. LEDs with intensive 1/f noise demonstrate shorter lifetime. It is shown that reason of LED degradation is related with defects presence in device structure. These defects can be formed during device fabrication or appear during operation. An analysis of LED current-voltage and electrical noise characteristics under forward and reverse bias has shown that LEDs with intensive 1/f electrical noise, large reverse current (low reverse breakdown voltage) and larger terminal voltage under forward bias distinguish by short lifetime.

In this paper, we studied portable blue and red light-emitting-diode (LED) light sources in phototherapy for mild to moderate acne vulgaris to evaluate the efficacy and tolerance of patients. Patients, randomly divided into blue and red groups, received either blue or red LED phototherapy twice a week for four weeks. After complete treatment, the number of lesions reduced by 71.4% in the blue group, in contrast to 19.5% in the red group. No obvious side effects were observed during and one month after the treatment, except for some mild dryness mentioned by several patients.

In perovskite-based light-emitting diodes (LEDs), employing an inorganic material for the hole-injection layer instead of an organic material is crucial for attaining optimal performance and device stability. This layer needs to exhibit $p$ -type conductivity with high carrier mobility while being fabricated with a simple and industrially compatible method. In this study, uniform and thin $p$ -type NiO films were deposited through spray pyrolysis in an air atmosphere. The as-deposited film showed poor crystallinity, and high resistance reduced hole-injection performance. Subsequent thermal treatment at 500^∘C enhanced the crystallinity of the layer, reducing resistance and improving LED performance. The film exhibited $p$ -type semiconductor characteristics, and its carrier mobility, carrier concentration and resistivity were 53.0 × ${10}^{15}$cm³, 116cm²⋅ ${\text{Vs}}^{-1}$ and 0.98$\mathrm{\Omega }$ ⋅ ${\text{cm}}^{-1}$, respectively. Moreover, a compact ${\mathrm{\text{CsPbBr}}}_3$ nanocrystal (NC) film as an emitting layer was deposited with a centrifugal coating technique. The resulting LED component featuring a fluoride-doped tin oxide (FTO)/NiO/${\mathrm{\text{CsPbBr}}}_3$ NCs/tris-(1-phenyl-1H-benzimidazole)/LiF/Au structure and demonstrating high luminance, current efficiency and external quantum efficiency (EQE) reached 12,000 cd⋅${\text{m}}^{-2}$, 8.8 cd ⋅ ${\text{A}}^{-1}$ and 3.56%, respectively, at 5.5 V. Our work offers a simple and unique approach for accelerating the development of advanced interfacial materials and circumventing major interfacial problems in solution-processed perovskite NC films for high-performance LEDs.

Efficiency droop and green gap are terms that summarize performance limitations in GaInN/GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c-plane and non-polar m-plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V-defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

We discuss physics, design, fabrication, performance, and selected applications of Deep Ultraviolet Light Emitting Diodes (DUV LEDs). Our analysis reveals the relative contributions of electrical injection, internal quantum efficiency, and light extraction efficiency to the overall DUV LED performance. Our calculations show that the reduction of the dislocation density at least below value of 2×10⁸ 1/cm³ is necessary for reaching high DUV LED efficiency. Better light extraction has been achieved using an innovative p-type transparent sub-contact layer and reflecting ohmic p-type contact resulting in nearly tripling DUV LED power. At high power dissipation, temperature rise might be significant, and we present data showing the power degradation with temperature increase and the results of the detailed 1D and 3D analysis of thermal impedance of DUV LEDs. As an example of DUV LED application, we report on microbial disinfection using 19 watt 275 nanometer DUV LED.