Improved carrier injection of AlGaN-based deep ultraviolet light emitting diodes with graded superlattice electron blocking layers

AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs), which emit at around 280 nm, have attracted a great deal of interest due to their wide range of potential applications such as water purification, sterilization, biological disinfections, and medical therapy. [1][2][3] Nevertheless, the efficiency of DUV-LEDs is much lower than that of the InGaN-based visible LEDs, which has become a bottleneck limiting their widespread commercial applications. The strong built-in polarization electric field and insufficient carrier confinement in the multiple-quantum-well (MQW) active region are two important issues limiting the output performance of the DUV-LEDs. To improve the carrier confinement, a simple method is to increase Al composition of the barrier layer. However, it brings some drawbacks including a more serious lattice mismatch between wells and barriers, which will produce the dislocations and severe quantum-confined Stark effects (QCSEs). The strengthened polarization electric field will further reduce the electronhole wave function overlap, and deteriorate the radiative recombination rate of the MQWs. In addition, the strong confining potential of quantum wells (QWs) will increase the ratio of Auger recombination. [4,5] The DUV-LEDs generally possess shallower wells to lower the lattice mismatch between the QWs and the quantum barriers (QBs), however, which will result in poor carrier confinement. Therefore, there is a trade-off between the carrier confinement and the overlap of the wave functions. [6] In particular, the insufficient carrier confinement in the MQWs will result in large amount of electrons concentrated in the last QW (LQW) close to the p-type electron blocking layer (p-EBL), and the electrons would be easily leaked into the p-type region to decrease the emission efficiency, especially at high current density injection. [7][8][9][10] To suppress electron leakage, many studies have focused on the p-EBL, [11][12][13][14] however, the p-EBL might also introduce a higher barrier in the valence band, hindering the injection efficiency of the holes. Recently, great efforts have been made to deal with the issues mentioned earlier. [15][16][17][18][19][20][21] The tailored Si-doping in the QBs is used by Zhu et al. to enhance the confinement of electrons in the QWs. [8] Li et al. designed triangular QBs to suppress the electron overflow and decrease the electrostatic field in the active region. [15] Lin et al. show a quaternary AlInGaN MQW design to reduce the polarization effect and suppress the electron leakage. [16] The compositional modulation schemes for the Al x Ga 1-x N MQWs have proposed by Kojima et al. to reduce the separation of the electron-hole wave functions. [17] Graded Al-composition in the QW layers shown by Yu et al. showed an improvement of the electron-hole wave function overlap due to the screening of the QCSE. [18]

ϕ_{normalC} = Δ E_{normalC} - k T ·ln (\frac{n}{N_{normalC}})

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Section: Resultsmentioning

confidence: 99%

Improved Wave Function Overlap and Carrier Confinement of AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with Graded Composition Multiple Quantum Wells

Xiao

Liu

et al. 2021

“…[ 23 ] To solve these problems and improve the optoelectronic performance of DUV‐LEDs, scientifically designed EBL and LQB epitaxial structures are necessary. To alleviate electron leakage without blocking hole injection, some unique epitaxial structures have been proposed, such as w‐shaped EBL, [ 24 ] InAlGaN/AlGaN EBL, [ 16 ] double‐peak superlattice EBL, [ 25 ] graded superlattice EBL, [ 26 ] composition‐graded AlGaN LQB, [ 27 ] and two‐step graded AlGaN LQB. [ 28 ] Except for the optimization of EBL and LQB structures, many researchers have focused on the polarization doping, [ 29 ] tunneling junctions, [ 30 ] and short‐period superlattice [ 31 ] hole injection layers (HIL) to enhance the hole injection efficiency of p‐type layers.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, it has been demonstrated that multibarrier EBL in DUV‐LEDs can effectively improve the carrier injection efficiency. [ 26 ] However, seeking for reasonable EBL and LQB to improve the quantum efficiency of DUV‐LEDs deserves further exploration.…”

Section: Introductionmentioning

confidence: 99%

Performance Improvement of AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with Multigradient Electron Blocking Layer and Triangular Last Quantum Barrier

Cao

et al. 2023

The performance improvements of AlGaN‐based deep ultraviolet light‐emitting diodes (DUV‐LEDs) with multigradient electron blocking layer (EBL) and triangular last quantum barrier (LQB) are investigated. The results show that the maximum internal quantum efficiency (IQE) is improved by 44.9% and light output power (LOP) is improved by 58.1% at 200 A cm−2, exhibiting a tremendous improvement compared to the conventional structure. These improvements are mainly attributed to the fact that both multigradient EBL and triangular LQB can generate negative polarization charges in the graded composition layer, which leads to a significant increase in hole concentration compared to the conventional EBL and LQB structures. Meanwhile, the multigradient EBL can transform the conventional triangular barrier into an arc‐shaped barrier, increasing the electron potential barrier height and decreasing the hole potential barrier height. This unique design suppresses electron leakage and enhances hole injection, leading to a significant enhancement of the IQE and alleviation of the efficiency droop.

“…To improve the hole activation and hole injection into the active region, different band‐engineered superlattice (SL) structures have been adopted; i.e., single‐/double‐step‐graded SL structure in the EBL [ 11,12 ] and SL hole injection layer (HIL). [ 13 ] A SL structure has been demonstrated for improved hole activation efficiency by reducing the Mg activation energy.…”

Section: Introductionmentioning

confidence: 99%

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

Bui

Dang

Doan

et al. 2021

Ultraviolet‐B (UVB) AlGaN light‐emitting diodes (LEDs) with a hybrid hole injection layer comprising a 5 nm thin p‐AlxGa(1−x)N linearly graded layer (LGL), x from 0.65 to 0.50, and a 15 nm conventional p‐AlGaN layer are proposed for ≈284 nm wavelength emission. The introduced p‐AlxGa(1−x)N LGL effectively improves the confinement of the electrons in the active region by effectively increasing the conduction band barrier height. Moreover, it enhances the hole injection capability into the active region by energizing the holes that minimize the effective valence band barrier height. As a result, the proposed LED structure exhibits an incredibly reduced electron leakage, ten times lower than that of the conventional structure. Moreover, the output power and electroluminescence intensity of the proposed structure are enhanced by approximately twice at 60 mA current injection. Thus, the LGL LED structure can be a potential candidate for high‐power UV light emitters.