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Abbreviations DUV LEDs: Deep ultraviolet light-emitting diodes; EL: Electroluminescence; EQE: External quantum efficiency; MQW: Multiple quantum well; PL: Photoluminescence; QCSE: Quantum-confined Stark effect; QW: Quantum well; RT: Room temperature
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Abstract The influence of quantum-well (QW) width on electroluminescence properties of AlGaN deep ultraviolet light-emitting diodes (DUV LEDs) was studied at different temperatures. The maximum external quantum efficiency (EQE) ratios of LED with 3.5 nm QW to that with 2 nm increased from 6.8 at room temperature (RT) to 8.2 at 5 K. However, the rat...
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... Furthermore, blue shifts of 9 nm for blue micro-LEDs and 15 nm for green micro-LEDs occur with increasing current density from 1 A/cm 2 -1 kA/cm 2 . The emission wavelength is not constant depending on the chip size and varies within a deviation range of 5 nm [20,21]. Overall, one of the key points to improve the emission efficiency and wavelength stability in micro-LEDs is to suppress the nonradiative recombination centers in sidewalls and reduce the effect of QCSE from (0001) plane active structures. ...
To light emitting diodes (LEDs), solving the common non-uniform current injection and efficiency degradation issues in (0001) plane micro-LED is essential. Herein, we investigated the light emission characteristics of various mesa sizes and different p-electrode areas toward the realization of coaxial GaInN/GaN multi-quantum-shell (MQS) nanowires (NWs)-based micro-LEDs. As the mesa area was reduced, the current leakage decreases, and further reduction of the area showed a possibility of realizing micro-LED with less current leakage. The large leakage path is mainly associated with the defective MQS structure on the (0001) plane area of each NW. Therefore, more NWs involved in an LED chip will induce higher reverse leakage. The current density-light output density characteristics showed considerably increased electroluminescence (EL) intensity as the mesa area decreased, owing to the promoted current injection into the efficient NW sidewalls under high current density. The samples with a mesa area of 50 × 50 µm ² showed 1.68 times higher light output density than an area of 100 × 100 µm ² under a current density of 1000 A/cm ² . In particular, the emission from (1-101) and (10-10) planes did not exhibit an apparent peak shift caused by the quantum-confined Stark effect. Furthermore, by enlarging the p-electrode area, current can be uniformly injected into the entire chip with a trade-off of effective injection to the sidewall of each NW. High performance of the MQS NW-based micro-LED can be expected because of the mitigated efficiency degradation with a reducing mesa area and an optimal dimension of p-electrode.
... The increasing demand of materials with high electric and thermal properties in an electronic device has increased the development of new processes for such materials. Among semiconductor materials, aluminum nitride (AlN) is an important candidate for application in the optoelectronic devices such as photodetectors [1][2][3][4][5][6], laser diodes [7,8], and light-emitting diodes (LEDs) [9][10][11][12][13][14][15][16][17][18][19]. It offers a wide band gap tunable from 3.4 to 6.2 eV [20], a wide transparency window covering from ultraviolet (UV) to mid infrared (MIR) [21], an excellent piezoelectricity [22], a high electrical resistivity [23], a fast acoustic velocity [24], and a high thermal conductivity (248 W.m −1 .K −1 ) [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. ...
Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. It is one of the best materials when high thermal conductivity is required. In this work, it has been found that thermal conductivity, gap energy, and microstructural properties of AlN thin films are greatly influenced by plasma conditions during sputtering process. We have used the high sensitive photothermal deflection technique (PDT) to investigate the thermal conductivity of AlN thin films deposited at room temperature by reactive DC magnetron sputtering of Al target in pure nitrogen atmosphere. Thin films were deposited on silicon (Si) substrate at different DC power from 200 to 600 W. Transmittance, reflectance (R), absorbance (A), and absorption coefficient (α) spectra were analyzed using UV–Visible spectroscopy. It has been found from the typical reflectance spectrum of AlN single crystal that the more DC power decreases more the AlN sample becomes reflective. It was observed also from the transmittance spectra that by increasing the power, the film transparency decreased.
... Several studies have performed temperature-dependent EL measurements of AlGaN-based DUV LEDs. [15][16][17][18][19] However, some studies were carried out in the early stages of AlGaN-based DUV LED research. [15][16][17] In the past decade, the EQE has steadily improved. ...
... Hence, the huge EL thermal quenching (that is, low IQE × CIE at RT) observed in the previous studies does not reflect the performance of current AlGaN-based DUV LEDs. Although other studies have recently been reported, 18,19 the CIE, IQE, and LEE have not been estimated separately. Herein, we perform temperature-dependent EL measurements for highly crystalline 265-nm AlGaN-based DUV LEDs grown on AlN substrates. ...
Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and temperature dependences of the product (IQE × CIE) are derived. The temperature dependence of the EQE cannot be simply explained by the Auger recombination processes. This observation enables the CIE and IQE to be separately extracted by rate equation analysis. The room-temperature EQE of the AlGaN-based DUV LEDs is limited by the CIE and not the IQE. We propose that the relatively low CIE may originate from the nonradiative recombination process outside quantum-well layers.
... The high-energy emission peak approximately locates at 364 nm, and does not change with QW thickness. On the other hand, the low-energy peak red shifts as increasing QW thickness, in good agreement with the reduced quantum confinement and thus lower emission energy [26]. However, the red shift is only less than 7 nm, which is far less than that observed in the experiment. ...
The non-centrosymmetricity of III-nitride wurtzite crystals enables metal or nitrogen polarity with dramatically different surface energies and optical properties. In this work, III-polar and N-polar nanostructured ultraviolet multiple quantum wells (UV-MQWs) were fabricated by nanosphere lithography and reactive ion etching. The influence of KOH etching and rapid thermal annealing treatments on the luminescence behaviors were carefully investigated, showing a maximum enhancement factor of 2.4 in emission intensity for III-polar nanopillars, but no significant improvement for N-polar nanopillars. The discrepancy in optical behaviors between III- and N-polar nanopillar MQWs stems from carrier localization in III-polar surface, as indium compositional inhomogeneity is discovered by cathodoluminescence mapping, and a defect-insensitive emission property is observed. Therefore, non-radiative recombination centers such as threading dislocations or point defects are unlikely to influence the optical property even after post-fabrication surface treatment. This work lays solid foundation for future study on the effects of surface treatment on III- and N-polar nanostructured light-emitting-diodes and provides a promising route for the design of nanostructure photonic devices.
... AlN-based materials are promising materials to fabricate deep ultraviolet (DUV) optoelectronic devices such as light-emitting diodes (LEDs) [1][2][3][4][5], laser diodes [6][7][8], and photodetectors [9,10] due to the direct bandgap tunable from 3.4 to 6.2 eV [11]. The refractive index of AlN has effects on the performance of the optoelectronic devices directly. ...
The refractive index of AlN has a direct influence on AlGaN-based deep ultraviolet optoelectronic devices, such as the external quantum efficiency of light-emitting devices. Revealing the dependence of the refractive index of AlN on the threading dislocations is meaningful since high-density threading dislocations usually exist in AlN. In this paper, the effect of different dislocation densities on the refractive index of AlN is investigated. With the increase of dislocation densities from 4.24 × 108 to 3.48 × 109 cm- 2, the refractive index of AlN decreases from 2.2508 to 2.2102 at 280 nm. Further study demonstrates that the nanoscale strain field around dislocations changes the propagation of light and thus decreases the refractive index of AlN. This study will be beneficial to the design of optoelectronic devices and thus realizing high-performance deep ultraviolet optoelectronic devices.
... However, the device performance was still limited by many factors, such as the crystal quality, electrode contacts, and conductivity. In order to improve the performance of AlN based deep UVLEDs, many works have been done to improve the crystal quality [12][13][14][15][16], the p-type doping efficiency and the device structures [17][18][19][20][21][22]. It was demonstrated that the annealing process at high temperature resulted in the improvement of AlN quality [14]. ...
The influence of surface modification of undoped AlN by oxygen plasma on the Pd/Al/Au Ohmic contacts was studied. Pd/Al/Au alloys were deposited on undoped AlN films by electron beam evaporation and annealed from 750 °C to 900 °C for 30 s to form metal–semiconductor contacts. All the samples without oxygen plasma treatment were Schottky contacts. After oxygen plasma treatment, the contacts below annealing temperature of 850 °C were Schottky type. However, the contacts changed to be Ohmic type with specific contact resistivity of 3.01 Ω · cm² when the annealing temperature was 900 °C. The samples were characterized by scanning electron microscopy and x-ray diffraction, which indicated that the formation of AlPd alloy was believed to play a key role for the formation of low Ohmic contacts. The first-principle calculation indicated that the surface modification by oxygen plasma could increase the work function of AlN and therefore benefit the formation of Ohmic contacts. This result can promote the application prospects of AlN in deep ultraviolet optoelectronic devices and high frequency and high power RF devices.
Ultraviolet band‐C (UV‐C) micro‐light‐emitting diodes (Micro‐LEDs) with high optical power density are increasingly demanded in the utilization of sterilization, solar‐blind communications, and neuroscience for the robust structure and adjustable emission wavelength. In this work, AlGaN UV‐C Micro‐LEDs are fabricated and characterized in 5×5, 10×10, 20×20, 30×30, 50×50, 80×80, and 100×100 μm2. With pixel size scaling down, the smaller devices have the potential to emit more considerable light output power (LOP) density at the same injected current density. This LOP density sizing effect implies higher luminescence efficiencies on small‐sized UV‐C Micro‐LEDs, which could be widely adopted by the industry.
Ultraviolet band‐C (UV‐C) micro‐light‐emitting diodes (Micro‐LEDs) with high optical power density are increasingly demanded in the utilization of sterilization, solar‐blind communications, and neuroscience for the robust structure and adjustable emission wavelength. In this work, AlGaN UV‐C Micro‐LEDs are fabricated and characterized in 5×5, 10×10, 20×20, 30×30, 50×50, 80×80, and 100×100 µm ² . With pixel size scaling down, the smaller devices have the potential to emit more considerable light output power (LOP) density at the same injected current density. This LOP density sizing effect implies higher luminescence efficiencies on small‐sized UV‐C Micro‐LEDs, which could be widely adopted by the industry.
Despite a wide array of applications, deep ultra-violet light emitting diodes offer relatively poor efficiencies compared to their optical counterparts. A contributing factor is the lower light extraction efficiency due to both highly absorbing p-contacts and total internal reflection. Here, we propose a structure consisting of a hexagonal periodic array of cylindrical nanoholes in the multi-layered p-contact which are filled with platinum. This nanostructure reduces the absorption of the p-contact layer, leading to a higher emission into the n-contact compared to a planar reference. An optimum geometry of the nanostructure allows a light extraction efficiency of 15.0%, much higher than the typical 4.6% of a planar reference. While the nanostructure strongly decreases the light absorption in the p-contact, it is still not able to considerably reduce the total internal reflection. Consequently, the nanostructured p-contact should be combined with other optical strategies, such as nanopatterned sapphire substrates to increase the efficiency even further. Despite this, the nanostructure described in this work provides a readily realizable path to enhancing the light extraction efficiency of state-of-the-art deep ultra-violet light emitting diodes.
Gallium nitride (GaN) light emitting diode (LED) has been used as highly efficient solid-state light sources to radiate a wide range of wavelengths in the visible spectrum. Larger currents drive bright LEDs, causes reduced efficiency which known as LED droop. Insertion of strained-layer superlattice (SLS) in GaN LED gives uniform electron distribution, lead to high emission and efficiency. The primary aim of this work is to investigate the operation of LED with the inclusion of SLS by simulation. The GaN LED was designed in cylindrically symmetrical configuration with the insertion of the InGaN/GaN SLS of 3 nm thick. This work investigates the effect of different number of SLS layers, which is represented by 1, 2 and 3 periods of SLS, which represent the total SLS thicknesses of 3 nm, 6 nm and 9 nm respectively. A GaN cylindrical chip of 700 nm diameters was designed with p- and n-type doped active region. The results has shown that the efficient operation of the LED can occur within the lowest current of 0.1 mA for the 2 periods of SLS with the total emission rate displayed linearly increased. A sharp decline of internal quantum efficiency (IQE) droops close to zero despite the increasing current was significantly observed for the 2 periods of SLS, which has also resulted to operate at a low turn-on voltage of 2.5 V. However, it was found that higher than 3 periods with SLS total thickness of higher than 6 nm cause the pn junction failure with no emission.
