InGaN/AlGaN ultraviolet light-emitting diodes (UV LEDs) suffer from residual in-plane compressive stress and poor carrier injection efficiency. Here, we simultaneously reduce the stress and improve the carrier concentration in In0.018Ga0.982N/Al0.05Ga0.95N multiple quantum wells by optimizing the Al composition of the underlying InGaN/AlGaN superlattice strain release layer (SRL). On the one hand, higher Al composition of SRL can reduce the residual in-plane compressive stress of epilayers, which is beneficial for increasing radiative recombination rate. On the other hand, higher Al composition can enhance the effective barrier height in SRL, which can decelerate electrons and prevent holes from transporting into the n-region. As a result, the light output power of UV LEDs with In0.01Ga0.99N/Al0.06Ga0.94N SRL is 19.3% higher than that of UV LEDs with In0.01Ga0.99N/Al0.02Ga0.98N SRL at 100 mA.
The internal-roughed sapphire in a 275-nm AlGaN-based deep-ultraviolet (DUV) LED is fabricated using a laser stealth dicing technique to improve the high-angle extraction. Furthermore, the low-angle extraction is enhanced by depositing a SiO2-antireflection film on the internal-roughed sapphire surface. Compared with conventional DUV LEDs with a light output power (LOP) of 33.05 mW at 350 mA, the LOP of DUV LEDs with internal-roughed sapphire and SiO2-antireflection film increases by 20.85% to 39.94 mW. In addition, combined with finite-difference time-domain simulations, the effect of internal-roughed sapphire on the transmission and light extraction efficiency (LEE) of the DUV LEDs is revealed. The combination of the internal-roughed sapphire substrate and SiO2-antireflection film improves the LEEs of transverse electric (TE) and transverse magnetic (TM) polarized light by 1.6% and 108%, respectively. These results offer the potential for large-scale, low-cost industrial production of high-efficiency DUV LEDs.
The implementation of reflective p-type Ohmic contact is an effective way to solve current crowding and improve the optoelectronic performance of flip-chip light-emitting diodes (FCLEDs). Here, we investigate the effects of annealing temperature, annealing time, and N2 flow rate on the formation for Ag/p-GaN Ohmic contact and determine the optimal annealing process parameters. After inserting an indium-tin oxide layer between Ag and p-GaN, the specific contact resistance decreases from 6.66 × 10−3 to 1.86 × 10−3 Ω cm2. In addition, we discover the appearance of a “black line” around the edges of the chips after high-temperature annealing. Finite element analysis and experiments show that the “black line” is related to Ag agglomeration under high temperatures due to stress concentration at the edges of the chips. A strategy for manipulating the stress concentration by adjusting the thickness of the TiW diffusion barrier layer is proposed based on insight obtained by modeling the stress distribution at the edge of the chips. The electrical properties of the fabricated FCLEDs show that the proposed stress manipulation strategy solves the problem of “black line” effectively and maintains the performance of the chips well.
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