Ultra‑Efficient AlGaN Deep‑UV LEDs with Superlattice p‑Electron Blocking Layer for Near‑Zero Efficiency Droop

Background

Deep‑ultraviolet (DUV) light in the 200–280 nm range is crucial for applications such as water‑purification systems. AlGaN‑based DUV LEDs are attractive because of their low driving voltage and compatibility with existing purification architectures. However, achieving high EQE at wavelengths below 280 nm remains challenging. Threading dislocation densities (TDD) in Al‑rich quantum wells are typically on the order of 10⁹ cm⁻², which severely limits the internal quantum efficiency (IQE). Even when TDD is reduced to 10⁸ cm⁻², efficiency droop at injection currents above 80 A cm⁻² can reduce EQE below 5 %. Light‑extraction efficiency (LEE) for bare DUV chips is only about 10 % as estimated by FDTD simulations. A major contributor to droop in III‑nitride LEDs is electron spillover into the p‑type hole‑injection layer, where the free‑hole concentration can be as low as 10¹⁷ cm⁻³. Various strategies—such as inserting spike layers, grading Al composition, or adding low‑Al AlGaN layers—have been explored to improve electron capture, but the hole‑blocking effect of Al‑rich p‑EBLs remains a bottleneck for DUV LEDs. A superlattice p‑EBL, previously demonstrated for blue LEDs, offers a promising route to enhance hole transport and reduce electron leakage, yet experimental evidence for DUV devices is scarce. This work experimentally demonstrates that a specifically engineered AlGaN/AlGaN superlattice p‑EBL can simultaneously improve hole injection and suppress efficiency droop, achieving near‑zero droop and a 100 % increase in EQE.

Methods/Experimental

The two LED architectures (LED A and LED B, shown in Fig. 1) were grown on 4‑µm‑thick AlN templates fabricated on c‑plane sapphire via hydride vapor phase epitaxy (HVPE). A 20‑period AlN/Al_0.50Ga_0.50N superlattice served as a strain‑relief buffer. A 2‑µm‑thick n‑Al_0.60Ga_0.40N layer with an electron concentration of 1 × 10¹⁸ cm⁻³ provided the n‑contact. The active region comprised five periods of Al_0.45Ga_0.55N quantum wells (3 nm) and Al_0.56Ga_0.44N barriers (12 nm). A 10‑nm‑thick AlGaN p‑EBL capped the MQWs. LED A employed a bulk Al_0.60Ga_0.40N p‑EBL, whereas LED B incorporated a five‑period superlattice consisting of 1‑nm Al_0.45Ga_0.55N / 1‑nm Al_0.60Ga_0.40N layers. This design begins the superlattice immediately after the last barrier, ensuring a negative polarization‑induced sheet charge at the barrier/EBL interface, which depletes electrons and reduces leakage. Subsequent layers included 50 nm of p‑Al_0.40Ga_0.60N and 50 nm of p‑GaN, followed by a 10‑nm heavily Mg‑doped p⁺‑GaN cap. The wafers were annealed at 800 °C in N₂ for 15 min to activate Mg donors, yielding hole concentrations of ~1 × 10¹⁷ cm⁻³ (p‑AlGaN) and ~3 × 10¹⁷ cm⁻³ (p‑GaN).

Schematic architectural structures for the studied LEDs. The sketched energy‑band diagrams for the two p‑EBLs are also provided: LED A has a bulk Al_0.60Ga_0.40N‑based EBL, whereas LED B incorporates a superlattice Al_0.45Ga_0.55N/Al_0.60Ga_0.40N EBL. The superlattice starts with the thin Al_0.45Ga_0.55N layer to induce a negative polarization charge at the last barrier/EBL interface.

Devices were processed into flip‑chip LEDs with 650 × 320 µm² mesas. Ti/Al contacts were deposited on the n‑side and annealed at 900 °C (1 min). Ni/Au current‑spreading layers were applied and annealed at 550 °C (5 min). The final n‑ and p‑electrodes were formed by Ti/Al/Ni/Au stacks. Light extraction was measured with an integrating sphere on the sapphire side.

Numerical simulations were performed using the APSYS package, incorporating SRH lifetime (10 ns), Auger coefficient (1 × 10⁻³⁰ cm⁶ s⁻¹), 50:50 band‑offset ratio, and 40 % polarization for the [0001] orientation. LEE was set to 10 % for bare DUV chips.

Results and Discussions

Measured electroluminescence (EL) spectra (Fig. 2a) show a stable peak at ~270 nm for both LEDs across the tested current range, confirming minimal self‑heating. LED B exhibits stronger EL intensity. Optical power and EQE versus current density (Fig. 2b) reveal a ~90 % EQE enhancement for LED B, with efficiency droop reduced from ~24 % (LED A) to ~4 % (LED B) at 110 A cm⁻². Numerical results (Fig. 2c) closely match the experimental data, validating the simulation models.

Figure 3a shows that hole concentrations in the MQWs are higher for LED B. The superlattice p‑EBL increases the hole density by two orders of magnitude (Fig. 3b), reducing the valence‑band barrier height and facilitating thermionic hole transport. Consequently, electron leakage into the p‑type layer is suppressed, as evidenced by lower electron concentrations in the hole‑injection layers (Fig. 3d).

Numerically calculated hole concentration profiles in the MQWs (a) and in the p‑type layers (b); measured EL spectra (c); and electron concentration in the hole‑injection layers (d) for LEDs A and B at 50 A cm⁻².

Radiative recombination rates (Fig. 4) are higher in LED B, confirming that improved hole injection directly boosts photon generation while mitigating droop.

Computed radiative recombination rates for LEDs A and B at 50 A cm⁻².

Energy‑band simulations (Fig. 5) illustrate that the superlattice p‑EBL raises the conduction‑band barrier (from ~295 meV to ~391 meV) and lowers the valence‑band barrier (from ~324 meV to ~281 meV), thereby suppressing electron escape and enhancing hole thermionic emission. The reduced barriers also lower the forward voltage (Fig. 5c).

Energy‑band profiles near the p‑EBL for LED A (a) and LED B (b); measured current‑density versus bias (c) at 50 A cm⁻².

Conclusions

This work demonstrates that a carefully engineered AlGaN/AlGaN superlattice p‑EBL enables simultaneous enhancement of hole injection and suppression of electron leakage in 270 nm DUV LEDs. The result is a 100 % EQE boost and a dramatic drop in efficiency droop, bringing the devices close to the theoretical limit. The findings provide a clear pathway for next‑generation high‑efficiency DUV emitters and deepen the understanding of carrier dynamics in III‑nitride optoelectronics.

Abbreviations

APSYS

Advanced Physical Models of Semiconductor Devices

DUV

Deep ultraviolet light‑emitting diodes

EL

Electroluminescence

EQE

External quantum efficiency

HVPE

Hydride Vapor Phase Epitaxy

ICP

Inductively Coupled Plasma

IQE

Internal quantum efficiency

LEE

Light extraction efficiency

MOCVD

Metal‑organic chemical vapor deposition

MQWs

Multiple quantum wells

p-EBL

p‑type electron blocking layer

TDD

Threading dislocation density