Abstract
The performance of AlGaN-based light-emitting diodes (LEDs) emitting at UVA–UVC regions can be severely compromised due to the polarization difference (ΔP) between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this work, the different situations of the bandgap difference (ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) and ΔP of InAlN/AlGaN and AlGaN/AlGaN heterojunctions fully strained on GaN and AlN substrates are discussed. It shows that the InAlN/AlGaN heterojunctions could produce positive or negative sheet charges at the heterointerface under ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> >0, which could not be realized by the conventional AlGaN/AlGaN heterojunctions. To demonstrate and utilize the feature, the polarization-modulated InAlN LQBs with 0.14–0.16 indium compositions of 320 nm UVB LEDs are designed and investigated. It is observed that the InAlN LQBs could replace the conventional AlGaN LQB to improve electron confinement and hole injection by affecting effective barrier heights. By modulating the LQB/EBL polarization using InAlN, the proposed UV LED has a 32% enhancement in internal quantum efficiency and lower efficiency droop (from 16.9% to 0.7%) compared with the conventional one without modulation. The operation voltage at the same current also significantly decreases. The improvement of optical output power and wall plug efficiency at 60 mA in proposed structures are near 90% and 100%, respectively. This study provides a novel and highly effective methodology for development of high efficiency UV LEDs.
Highlights
RECENTLY, AlGaN-based UV light-emitting diodes (LEDs) have undergone accelerating development due to important applications such as UV curing, phototherapy, sensing, and disinfection
UV LEDs still suffer from low internal quantum efficiency (IQE) due to Auger recombination, Shockley–Read–Hall (SRH) recombination, electron overflow, and constrained hole injection, etc. [2]–[7]
Researchers have utilized various approaches, including nonpolar and semi-polar orientations [10]; and introduced Si doping in quantum barriers (QBs) to screen the internal polarization induced by the electric field [11]; and polarization-matched and lattice-matched multiple quantum wells (MQWs) have been designed [12], [13]
Summary
RECENTLY, AlGaN-based UV light-emitting diodes (LEDs) have undergone accelerating development due to important applications such as UV curing, phototherapy, sensing, and disinfection. GaN and AlN substrates corresponding to the largest and smallest lattice constant in the AlGaN material system are chosen as two extreme cases to calculate the piezoelectric polarization and total polarization It could reveal the general properties and trends of InyAl1-yN/AlxGa1-xN heterojunctions on n-AlGaN substrates instead of focusing on one specific operation wavelength. For the AlyGa1-yN/AlxGa1-xN heterojunctions in Fig. 2 (a) and (b), there are only two types of situations (C and G), which are shown in Fig. 3 (c) and (e) and widely found in AlGaN-based devices such as the QB on the QW and the EBL on the LQB in UV LEDs. The dash lines with ∆P=0 correspond to the AlyGa1yN/AlxGa1-xN (x=y) homojunction. For Type A (shaded region in Fig. 2), the AlxGa1-xN layer has a wider bandgap compared with the InyAl1yN layer and the sheet charges at the heterointerface are negative, which is the polarization-reversed structure mentioned above and could be applied to the LQB/EBL heterojunction. This method could be extended to LEDs and LDs in other UV regions
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