Impact of Mg level on lattice relaxation in a p-AlGaN hole source layer and attempting excimer laser annealing on p-AlGaN HSL of UVB emitters

Abstract

Mg-doped p-type semiconducting aluminium-gallium-nitride hole source layer (p-AlGaN HSL) materials are quite promising as a source of hole ‘p’ carriers for the ultraviolet-B (UVB) light-emitting diodes (LEDs) and laser diodes (LDs). However, the p-AlGaN HSL has a central issue of low hole injection due to poor activation of Mg atoms, and the presence of unwanted impurity contamination and the existence of a localized coherent state. Therefore, first the impact of the Mg level on the crystallinity, Al composition and relaxation conditions in the p-AlGaN HSL were studied. An increasing trend in the lattice-relaxation ratios with increasing Mg concentrations in the p-AlGaN HSL were observed. Ultimately, a 40%–60% relaxed and 1.4 μm thick p-AlGaN HSL structure with total threading dislocation densities (total-TDDs) of approximately ∼8–9 × 108 cm−2 was achieved, which almost matches our previous design of a 4 μm thick and 50% relaxed n-AlGaN electron source layer (ESL) with total-TDDs of approximately ∼7–8 × 108 cm−2. Subsequently, structurally a symmetric p–n junction for UVB emitters was accomplished. Finally, the influence of excimer laser annealing (ELA) on the activation of Mg concentration and on suppression of unwanted impurities as well as on the annihilation of the localized energy state in the p-AlGaN HSL were thoroughly investigated. ELA treatment suggested a reduced Ga–N bonding ratio and increased Ga–O, as well as Ga–Ga bonding ratios in the p-AlGaN HSL. After ELA treatment the localized coherent state was suppressed and, ultimately, the photoluminescence emission efficiency as well as conductivity were drastically improved in the p-AlGaN HSL. By using lightly polarized p-AlGaN HSL assisted by ELA treatment, quite low resistivity in p-type AlGaN HSL at room temperature (hole concentration is ∼2.6 × 1016 cm−3, the hole mobility is ∼9.6 cm2 V1 s−1 and the resistivity is ∼24.39 Ω. cm) were reported. ELA treatment has great potential for localized activation of p-AlGaN HSL as well as n- and p-electrodes on n-AlGaN and p-AlGaN contact layers during the flip-chip (FC) process in low operating UVB emitters, including UVB lasers.

Achieving 9.6% efficiency in 304 nm p-AlGaN UVB LED via increasing the holes injection and light reflectance

“…In the earlier research studies, the optical transmittance (relative transmittance ≈ 90%) of UVB light through p-AlGaN HSL and p-AlGaN contact-layer with constant Al composition of AlGaN-based UVB LEDs, and the reflectance of UVB light from the p-electrode of UVB LED has been reported 8 , 23 , 26 . However, the transparency of the whole AlGaN-based UVB LEDs, including Al-graded p-MQB EBL and Al-graded p-AlGaN HSL 34 , has not been reported before. Highly transparent (relative transmittance ≈ 90%) AlGaN-based UVB LED, including Al-graded p-MQB EBL, Al-Grad p-AlGaN HSL and Al-Grad p-AlGaN contact-layer (sample–COR02), is realised for the first time which would ensure smooth transmission of the reflected UVB light (Fig.…”

“…The accepter energy level of Mg dopant in both p-AlGaN HSL and p-AlGaN contact-layer is quite deep (≈ 450 MeV), which is caused by the low activation energy of Mg atoms in both the layers 33 , 34 . Owing to a large difference in the electrons and hole transport behaviour (CIE) towards the active region in III–V materials, the highly energetic electrons from the shallow donor levels can easily overshoot from the active region towards the p-region of the UVB emitters.…”

“…We have attempted to enhance the hole generation and transportation across the highly transparent p-AlGaN UVB LEDs. Simon et al have reasonably improved the 3D hole generation level in the AlGaN/GaN UVC LED by polarisation effect accomplished upon distributing the Al composition profile from the p-AlGaN HSL to the p-GaN contact-layer 33 , 34 . Further, there is no report on better design for the highly transparent and Al-graded polarised p-AlGaN HSL for UVB emitters 4 , 8 , 23 , 26 .…”

Increasing the Carrier Injection Efficiency of GaN-Based Ultraviolet Light-Emitting Diodes by Double Al Composition Gradient Last Quantum Barrier and p-Type Hole Supply Layer

“…UV-A LEDs based on the GaN buffer layer are the mainstream, but UV light passing through the GaN layer will be absorbed. Growing AlGaN-based UV-A LEDs on AlN templates can solve this problem [25]- [27].…”

Enhancing Efficiency of AlGaN Ultraviolet‐B Light‐Emitting Diodes with Graded p‐AlGaN Hole Injection Layer

“…In addition, after the ELA treatment, the localized coherent state was suppressed, and ultimately, the photoluminescence, as well as conductivity, were improved drastically. [ 17 ]…”