GaN-based Green Light-emitting Diodes Research Articles

Spontaneous Emission Studies for Blue and Green InGaN-Based Light-Emitting Diodes and Laser Diodes

We investigated the efficiency droop phenomenon in blue and green GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), which poses a significant challenge in high-power LEDs and is characterized by a reduction in external quantum efficiency at higher injection currents. Utilizing identical epi-structures for blue and green LEDs and LDs, with variations only in indium composition, our experiments revealed a gradual blue shift in the emission wavelengths as the injection current increased. Notably, the blue LED demonstrated a smaller shift compared to the green LED. In addition, the full width at half maximum of emission spectra increased with increasing injection current density, indicative of efficiency droop. Significantly, LDs consistently exhibited lower junction temperatures despite operating at higher current densities. This is attributed to the enhanced heat dissipation capability of the ridge waveguide LD structure, which results in a narrower emission spectrum and reduced efficiency droop compared to mesa LED structures. These outcomes highlight the efficiency of the ridge waveguide LD structure in heat dissipation from the active layer, offering crucial insights for the advancement of high-power light-emitting devices.

GaN-based green resonant-cavity light-emitting diodes with Al mirror and copper plate.

In this Letter, GaN-based green resonant-cavity light-emitting diodes (RCLEDs) with a low-cost aluminum (Al) metal bottom mirror, a dielectric top mirror, and a copper (Cu) supporting plate were fabricated. The green-emitting epitaxial wafer was grown on a patterned sapphire substrate (PSS) to ensure high crystal quality (CQ). Laser lift-off (LLO) of the PSS and electrical plating of a Cu supporting plate were then carried out to realize the vertical device structure. The emission wavelength and full width at half maximum (FWHM) of the main emission peak of the device are ∼518 nm and 14 nm, respectively. Under the current density of 50 A/cm2, a relatively high light output power (LOP) of 11.1 mW can be obtained from the green RCLED. Moreover, when the current injection is 20 mA (8 A/cm2), the corresponding forward bias voltage is as low as ∼2.46 V. The reasons for the low operating voltage and high LOP can be attributed to the improvement of CQ, the release of residual compressive stress of the GaN-based epilayer due to the removal of PSS, and better heat dissipation properties of the Cu supporting plate.

Analysis of size dependence and the behavior under ultrahigh current density injection condition of GaN-based Micro-LEDs with pixel size down to 3 μm

In this paper, the GaN-based green micro light-emitting diodes (Micro-LEDs) with various sizes (from 3 to 100 μm) were fabricated and electro-optically characterized. Atom layer deposition (ALD) passivation and potassium hydroxide (KOH) treatment were applied to eliminate the sidewall damage. The size dependence of Micro-LED was systematically analyzed with current-versus-voltage and current density-versus-voltage relationship. According to the favorable ideality factor results (<1.5), the optimized sidewall treatment was achieved when the device size shrank down to <10 μm. In addition, the external quantum efficiency (EQE) droop phenomenon, luminance and output power density characteristics were depicted up to the highest current density injection condition to date (120 kA cm−2), and 6 μm device exhibited an improved EQE performance with the peak EQE value of 16.59% at 20 A cm−2 and over 600k and 6M cd cm−2 at 1 and 10 A cm−2, indicating a greater brightness quality for over 3000 PPI multiple display application. Lastly, the blue shift of 6 μm device with elevating current density was observed in electroluminescence spectra and converted to CIE 1931 color space. The whole shifting track and color variation from 1 A cm−2 to 120 kA cm−2 were demonstrated by color coordinates.

InGaN quantum well with gradually varying indium content for high-efficiency GaN-based green light-emitting diodes.

High-efficiency GaN-based green LEDs are of paramount importance to the development of the monolithic integration of multicolor emitters and full-color high-resolution displays. Here, the InGaN quantum well with gradually varying indium (In) content was proposed for improving the performance of GaN-based green LEDs. The InGaN quantum well with gradually varying In content not only alleviates the quantum-confined Stark effect (QCSE), but also yields a low Auger recombination rate. Consequently, the gradual In content green LEDs exhibited increased light output power (LOP) and reduced efficiency droop as compared to constant In content green LEDs. At 60 A/cm2, the LOPs of the constant In content green LEDs and the gradual In content green LEDs were 33.9 mW and 55.2 mW, respectively. At 150 A/cm2, the efficiency droops for the constant In content green LEDs and the gradual In content green LEDs were 61% and 37.6%, respectively. This work demonstrates the potential for the gradual In content InGaN to replace constant In content InGaN as quantum wells in LED devices in a technologically and commercially effective manner.

Optical Properties of GaN-Based Green Light-Emitting Diodes Influenced by Low-Temperature p-GaN Layer.

GaN-based green light-emitting diodes (LEDs) with different thicknesses of the low-temperature (LT) p-GaN layer between the last GaN barriers and p-AlGaN electron blocking layer were characterized by photoluminescence (PL) and electroluminescence (EL) spectroscopic methods in the temperature range of 6–300 K and injection current range of 0.01–350 mA. Based on the results, we suggest that a 20 nm-thick LT p-GaN layer can effectively prevent indium (In) re-evaporation, improve the quantum-confined Stark effect in the last quantum well (QW) of the active region, and finally reduce the efficiency droop by about 7%.

Effect of in situ degradation on the atomic structure and optical properties of GaN-based green light-emitting diodes

The structure at the atomic scale and optical properties of GaN-based green light-emitting diodes (LEDs) before and after in situ degradation were investigated by spherical aberration corrected scanning transmission electron microscopy and temperature-dependent micro-photoluminescence. Indium (In) interstitial atoms existed in the degraded sample, due to the small-bond-energy In atoms deviating from their normal lattice position, caused by the relaxation of the InGaN well. Both the peak wavelengths of the original and degraded green LEDs had similar temperature-dependent behaviors, due to the localization states in the InGaN well. These wavelengths indicate that the degradation had little influence on the localization states. However, the emission peak of the degraded green LED redshifted by 1.6 nm at 300 K, and the integrated intensity decreased by 36.8%, compared to the peak and intensity of the original sample, respectively. Based on first-principles calculations, the calculated bandgap for the relaxation of the InGaN well was small. Therefore, the wavelength redshifted, and the luminous efficiency of the green LED decreased after degradation. These features are attributed to a decreased bandgap due to the relaxation of the InGaN well; increased defect density, resulting from In interstitial atoms; and an increase in the InGaN well thickness.

Nonpolar GaN-based nanopillar green light-emitting diode (LED) fabricated by using self-aligned In3Sn nanodots

We fabricate nonpolar InGaN/GaN single quantum-well based a-plane nanopillar green light-emitting diode (LED). The top-down fabrication method was used for fabricating nanopillar LED from the planar LED, where self-aligned In3Sn nanodots were used as an etching mask. Fabricated nanopillars have high yield with large height-to-diameter aspect ratio found by scanning-electron-microscopy characteristics. The size of nanopillars depends on the size of In3Sn nanodots which are fabricated from the ITO (indium tin oxide) thin film by 3% HCl solution. It is systematically investigated that the size of nanodots depends on the ITO thickness rather than the etching time. In order to achieve uniform current injection into nanopillars, a p-type transparent ITO contact is selectively deposited on the top of each nanopillar. The ITO contact layer consists of a slanted oblique-angle (85○/−85○) layer followed by a thick blanket layer for uniform current distribution. The macroscopic optoelectronic characteristics show the uniform current distribution over the chip area, which is also interactively explained.

Effect of V-Shaped pits on optical properties of GaN-Based green light-emitting diodes

Abstract The optical properties of GaN-based green light-emitting diodes (LED) are investigated by transmission electron microscopy (TEM) and temperature-dependent micro-photoluminescence (μ-TDPL) in this study. Compared with the emission of normal c-plane multiple quantum wells (C-MQWs), the emission of C-MQWs close to V-shaped pits (V-pits) exhibits a shoulder peak phenomenon at cryogenic temperature, and the shoulder peak gradually disappears as the temperature increases. At room temperature, the peak of C-MQWs close to V-pits (C2) exhibits a red-shift. The formation of V-pits would induce strain relaxation. Strain relaxation can result in Indium (In) incorporation and localization, and reduced quantum-confined stark effects (QCSE). In incorporation and localization can lead to the red-shift of C2, reduced QCSE can cause the blue-shift of C2. Therefore, the red-shift of C2 arises from further In incorporation and localization of C-MQWs. It is worth noting that In incorporation and localization is dominant in the formation of V-pits for green InGaN/GaN C-MQWs. Additionally, the carriers limited in sidewall MQWs (S-MQWs) of V-pits escape into C-MQWs close to V-pits can mitigate the common phenomenon in which the emission intensity of C-MQWs decreases as the temperature increases, which helps improve the emission intensity of GaN-based green LEDs.

High internal quantum efficiency of green GaN-based light-emitting diodes by thickness-graded last well/last barrier and composition-graded electron blocking layer

To enhance the performance of green light-emitting diodes (LEDs), numerical analysis of structural engineering of conventional LED has been presented. Our proposed structure with composition-graded electron blocking layer as well as thickness-graded last quantum barrier and last quantum well shows improved radiative recombination rate, lower efficiency droop and higher light output power at high current density in comparison to other devices. The observed improvements are because of better electron and hole confinement as well as improved hole transportation inside the active region.

Increasing the internal quantum efficiency of green GaN-based light-emitting diodes by employing graded quantum well and electron blocking layer

By numerical investigation of various structures, we have proposed a promising single quantum well device for efficient green InGaN-based light-emitting diodes. The peak internal quantum efficiency of the proposed structure, at lower current density, is enhanced and shows significant droop reduction at high current density. The efficiency droop ratio of the proposed structure is ~ 31% at 100 A cm−2. Similarly, the light output power is also doubled as compared to conventional light-emitting diode. In the proposed structure, the simultaneously graded quantum well and electron blocking layer are employed. Therefore, the injection of holes into the active region becomes easier due to graded electron blocking layer and radiative recombination is boosted due to graded quantum well. By implement the proposed structure, both the problem of injection of holes and separation of electron–hole wavefunctions are greatly reduced.

Reduction in efficiency droop/decline of green GaN-based light-emitting diodes by employing heterostructure cap layer

In this study, we theoretically studied the enhancement of carrier injection by employing heterostructure layers before p-GaN in green light-emitting diodes. It is evident from the simulation results that improved carrier injection results in high internal quantum efficiency and light output power in the structures using heterostructures on the p-side. In comparison to conventional structure, reduced efficiency droop is observed suggesting promising high-performance green light-emitting diodes.

Augmenting the internal quantum efficiency of GaN-based green light-emitting diodes by sandwiching active region with p-AlGaN layers

In this work, InGaN-based green light-emitting diodes (LEDs) are studied theoretically. We have minimized the problem of asymmetrical carrier distribution in the active region. We have sandwiched the active region between p-AlGaN layers and observed improvement in the device performance. The hole injection, as well as the distribution, is improved remarkably in all the QWs. The asymmetry between the concentrations of electrons and holes in the emission region is reduced in contrast to the conventional structure. The radiative recombination is also enhanced because all the QWs contribute greatly in radiative recombination.

Analysis of various electron blocking layers to improve efficiency in green light-emitting diodes

Efficiency of GaN-based green light-emitting diodes with various electron blocking layers has been studied numerically. By employing conventional AlGaN, graded AlGaN, quaternary AlGaInN and ternary AlInN electron blocking layers, performance of each device has been evaluated. The device characteristics presented include carrier transport, radiative recombination rate, electrostatic field, emission spectra and efficiency.

Effect of AlGaN interlayer on luminous efficiency and reliability of GaN-based green LEDs on silicon substrate**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0400600 and 2016YFB0400601) and the National Natural Science Foundation of China (Grant Nos. 61704069 and 51705230).

The effect of AlGaN interlayer in quantum barrier on the electroluminescence characteristics of GaN-based green light emitting diodes (LEDs) grown on silicon substrate was investigated. The results show that AlGaN interlayer is beneficial to improve the luminous efficiency of LED devices and restrain the phase separation of InGaN. The former is ascribed to the inserted AlGaN layers can play a key role in determining the carrier distribution and screening dislocations in the active region, and the latter is attributed to the increased compressive stress in the quantum well. However, when the electrical stress aging tests were performed at a current density of 100 A/cm2, LED devices with AlGaN interlayers are more likely to induce the generation/proliferation of defects in the active region under the effect of electrical stress, resulting in the reduced light output power at low current density.

Investigation of the Electroluminescence Mechanism of GaN-Based Blue and Green Light-Emitting Diodes with Junction Temperature Range of 120–373 K

Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120–373 K and in a current range of 80–240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).

Amelioration of internal quantum efficiency of green GaN-based light-emitting diodes by employing variable active region

Abstract We have designed and investigated green InGaN-based light-emitting diodes numerically. We have proposed an efficient device structure with improved carrier transport as well as distribution of holes in the active region. In our proposed structure, we employ a variable active region to improve the device performance. The variable active region comprises of quantum wells with increasing thickness and quantum barriers with decreasing thickness. Both quantum wells and quantum barriers are varied from n-side to p-side of the device. The transport and distribution of holes have significantly improved in our proposed structure exhibiting excellent performance in comparison with other structures.

Origins of Inhomogeneous Light Emission From GaN-Based Flip-Chip Green Micro-LEDs

Spatially resolved electroluminescence emitted through the sapphire substrate of GaN-based flip-chip green microscale light-emitting diodes with $20\times 20\,\,\mu \text{m}^{\text {2}}$ mesas has been collected via microscopic hyperspectral imaging. The current crowding is identified in the central region of the mesa, corresponding to the position of the p-pad. The bright-spot emissions at the edges of the mesa originate from localized potential energy valleys. This letter suggests that the comprehensive information about current crowding and the distribution of the light emission across the mesa of microscale light-emitting diode can be obtained by capturing spatially resolved light emission from the side of the transparent substrate.

Zigzag-shaped quantum well engineering of green light-emitting diode

We report the influence of zigzag-shaped quantum well in green GaN-based light-emitting diode. Our analysis shows that the droop onset, efficiency droop, internal quantum efficiency, light output power and radiative recombination rate is significantly improved due to strong hole confinement in the indium regions in the proposed quantum well. • Utilizing bandgap-engineering to improve the internal quantum efficiency of GaN-based LEDs is of keen interest to the community. • In this work, we propose a zig-zag-shaped engineered quantum well to improve the device performance of GaN-based green light-emitting diodes. • Detailed comparison of optoelectronic properties of the reference and our proposed GaN-based light-emitting diodes has been presented. • We believe that our proposed design may substantially improve the device performance, especially in the green emission range.

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values.

Intense red electroluminescence from ZnO: Sm3+/Tb3+ LED by efficient energy transfer from Tb3+ to Sm3+

Nowadays, though blue and green GaN-based light-emitting diodes (LEDs) are already successfully commercialized, full-color displays via GaN-based LEDs are difficult to realize due to the low output power of red GaN-based LEDs. Hence, highly efficient red LEDs should be studied. Herein, ZnO:Sm LED was prepared via magnetron co-sputtering. Orange-red electroluminescence originated from f-f transitions of Sm3+ is obtained from the device by applying a DC voltage. To enhance the emission of Sm3+, Tb was introduced in the ZnO:Sm film via co-sputtering. With a proper Tb concentration, the red emission from the ITO/ZnO: Sm, Tb/n-Si LED can be enhanced dramatically. Notably, the electroluminescence intensity is greatly dependent on the annealing temperature of ZnO:Sm/Tb layer. Therefore, influence of the annealing temperature on the morphology and crystalline structure of the ZnO:Sm/Tb film was investigated. Under a smaller input power, the electroluminescence intensity from the ITO/ZnO:2.1 mol% Sm, 3.4 mol% Tb/n-Si device is one order of magnitude higher than that of the ITO/ZnO:2.1 mol% Sm/n-Si device due to the energy transfer from Tb3+ to Sm3+. This results demonstrate the EL intensity from trivalent rare earth ions doped-inorganic semiconductor are expected to be greatly enhanced via an efficient ET from another co-doping RE ions as we recently reported. Besides, color of the EL emission move to the red direction after Tb doping. These results offering new ideas for highly efficient red LED used in full-color displays.