Enhanced Current Spreading in AlGaN‑Based Deep‑UV LEDs via PNP‑AlGaN Structures

Introduction

Since their first demonstration in 2003, AlGaN‑based DUV LEDs have emerged as key components for water sterilization, air purification, and other UV applications [1–7]. Nonetheless, when emission wavelengths fall below 300 nm, EQE typically remains under 10 % [8], primarily due to limited internal quantum efficiency (IQE) governed by carrier injection and dislocation‑mediated recombination [8]. Many DUV devices are fabricated on sapphire using a flip‑chip architecture to enhance light extraction, but this requires both n‑ and p‑electrodes on the same side, which induces non‑uniform lateral current density and the notorious current‑crowding effect [9]. Current crowding can generate local Joule heating and uneven light output [10–12], shortening device lifetime. Poor Mg‑doping efficiency in Al‑rich p‑AlGaN further hampers electrical conductivity [13], underscoring the need for improved current spreading.

While various strategies—selective ion implantation, current‑blocking layers, nitrogen‑vacancy engineering, and optimized ohmic contacts—have successfully mitigated current crowding in GaN‑based blue LEDs [15–21], analogous solutions for DUV LEDs are scarce. Here we propose a p‑AlGaN/n‑AlGaN/p‑AlGaN (PNP‑AlGaN) current‑spreading layer that generates a valence‑band barrier, modulating the resistivity of the p‑type hole injection layer and steering lateral current flow. By optimizing the PNP‑AlGaN design, we demonstrate significant gains in EQE, WPE, and reduced forward voltage. We also conduct a comprehensive parametric study on the impact of the PNP loop number, Si doping, layer thickness, and AlN composition on device performance.

Research Methods and Physics Models

All simulated devices consist of a 4‑µm‑thick n‑Al_0.60Ga_0.40N base (Si 5 × 10¹⁸ cm⁻³), followed by five periods of 3‑nm Al_0.45Ga_0.55N/12‑nm Al_0.56Ga_0.44N multiple quantum wells (MQWs). The MQWs are capped with an 18‑nm p‑Al_0.60Ga_0.40N electron‑blocking layer (EBL), a 198‑nm p‑Al_0.40Ga_0.60N layer, and a 50‑nm p‑GaN cap. Hole concentrations are set to 3 × 10¹⁷ cm⁻³ throughout. For the PNP‑AlGaN devices, the conventional p‑Al_0.40Ga_0.60N layer is replaced by a p‑Al_0.40Ga_0.60N/n‑Al_xGa_1‑xN/p‑Al_0.40Ga_0.60N stack, as shown in Fig. 1b. The n‑Al_xGa_1‑xN insert is Si‑doped (5.3 × 10¹⁷ cm⁻³) and 20‑nm thick, creating a valence‑band barrier that shapes lateral current flow (Fig. 1c).

a Schematic of the studied devices (reference LED A and PNPNP‑AlGaN LED), b PNP‑AlGaN stack, c valence‑band diagram illustrating the barrier heights φ₁, φ₂, and φ_n.

To illustrate the mechanism, we model the DUV LED on sapphire as an equivalent circuit (Fig. 2a). With a thin current‑spreading layer (≈200 nm) and a thick n‑AlGaN injection layer (≈4 µm), the lateral current density decreases from the p‑electrode outward, producing current crowding. Introducing PNP‑AlGaN splits the total current into a vertical component (J₁) and a horizontal component (J₂) as depicted in Fig. 2b. The relationship follows

$$\frac{J_1}{J_2}\approx\frac{l}{\frac{\rho_p}{\rho_{\mathrm{CL}}}t_p+\frac{N\cdot\rho_{\mathrm{PNP}}}{\rho_{\mathrm{CL}}}}$$

where ρ_p is the p‑layer resistivity, ρ_CL the spreading‑layer resistivity, ρ_PNP the interfacial resistivity of each PNP junction, and N the junction count. This expression shows that reducing ρ_CL or increasing ρ_PNP (via thicker or more heavily doped n‑AlGaN) enhances the horizontal current J₂, improving spreading.

a Equivalent circuit with current crowding (J₁ > J₂ > …), b simplified circuit for the LED with PNP‑AlGaN, indicating J₁ and J₂.

Device physics are simulated with APSYS, employing a 50:50 conduction/valence band offset ratio for AlGaN/AlGaN heterojunctions, 40 % polarization charge, Auger coefficient 1.0 × 10⁻³⁰ cm⁶/s, SRH lifetime 10 ns, and a 8 % light extraction efficiency for DUV LEDs [33].

Results and Discussions

Effectiveness of the PNP‑AlGaN Junction

Comparing the reference LED A (no PNP‑AlGaN) with LED B (two PNP‑AlGaN loops), we observe that holes encounter two valence‑band barriers before reaching the MQWs, which promotes lateral homogenization (Fig. 3a). The lateral hole distribution in the last quantum well (LQW) confirms a more uniform profile for LED B (Fig. 3b), aligning with the energy‑band analysis.

a Energy band diagram for LED B at 170 A/cm²; b lateral hole distribution in the LQW for LEDs A and B.

Hole concentration and radiative recombination profiles at 230 µm from the mesa edge reveal that LED B benefits from higher hole injection into the MQWs, leading to a 22.2 % EQE increase (3.38 % → 4.13 %) and a slight rise in forward voltage (Fig. 5b). Despite this, the wall‑plug efficiency (WPE) improves from 3.56 % to 4.27 % (Fig. 5c). A more pronounced voltage drop across the PNP‑AlGaN junction explains the observed WPE droop at high current densities (Fig. 8).

a Output power density and EQE vs. current; b I‑V characteristics; c WPE vs. current for LEDs A and B.

Optimizing n‑AlGaN Thickness

By varying the 20‑nm n‑AlGaN insertion from 6 to 34 nm, we systematically increase the valence‑band barrier height (Table 1). Enhanced barriers raise the interfacial resistivity (ρ_PNP), driving more lateral current and uniformly distributing holes across the LQW (Fig. 6a). Correspondingly, hole concentration and radiative recombination in the MQWs rise, boosting EQE and optical power (Fig. 7a). However, the forward voltage climbs with barrier height, and WPE peaks at a 20‑nm insertion before declining for thicker layers (Fig. 8). These results highlight the trade‑off between spreading benefit and additional voltage drop, guiding the optimal insertion thickness.

a Lateral hole distribution in the LQW; b hole concentration and c radiative recombination in the MQWs for LEDs A, T1–T5.

a Optical power density; b I‑V characteristics; inset: zoomed I‑V.

WPE vs. current for LEDs A, T1–T5; inset: WPE and EQE at 170 A/cm².

Influence of n‑AlGaN Doping

Increasing Si doping from 1.3 × 10¹⁷ to 1.73 × 10¹⁸ cm⁻³ in the 20‑nm n‑AlGaN layer raises the valence‑band barrier (Table 2), which enhances horizontal current and homogenizes hole distribution (Fig. 9a). The resulting higher hole concentration in the MQWs leads to improved radiative recombination and EQE (Fig. 10a). However, the forward voltage rises sharply beyond 1.33 × 10¹⁸ cm⁻³, indicating a parasitic diode effect, and the WPE declines as voltage consumption dominates (Fig. 11).

a Lateral hole distribution; b hole concentration and c radiative recombination in the MQWs.

a Output power density and EQE vs. current; b I‑V characteristics; inset: zoomed I‑V.

WPE vs. current for LEDs A, D1–D5; inset: WPE and EQE at 170 A/cm².

Effect of PNP‑AlGaN Junction Count

With fixed 20‑nm, 5.3 × 10¹⁷ cm⁻³ n‑AlGaN and AlN = 0.40, we compare devices with 1 to 4 PNP‑AlGaN loops (LEDs N1–N4). More loops increase the cumulative barrier (Table 3), further flattening the lateral hole distribution (Fig. 12a) and boosting hole concentration and radiative recombination (Fig. 12b,c). EQE and output power rise with loop count, while the forward voltage increases modestly (Fig. 13b). WPE remains stable, and both EQE and WPE saturate beyond four loops, indicating diminishing returns when the p‑layer becomes too thin to supply sufficient holes.

a Lateral hole distribution; b hole concentration; c radiative recombination for LEDs A, N1–N4.

a Output power and EQE vs. current; b I‑V characteristics; inset: zoomed I‑V.

WPE vs. current for LEDs A, N1–N4; inset: WPE and EQE at 170 A/cm².

AlN Composition in n‑AlGaN

Varying the AlN fraction in the 20‑nm n‑AlGaN from 0.40 to 0.51 raises the valence‑band barrier (Table 4). A moderate increase to 0.43 optimizes current spreading without excessively blocking hole injection. At higher compositions, the barrier becomes too large, reducing hole penetration (Fig. 15a,b). EQE improves with composition up to 0.43, while the forward voltage climbs with higher AlN fractions (Fig. 16b). WPE reaches a maximum at 0.51 for currents above 89 A/cm², illustrating the need to balance spreading against voltage penalty (Fig. 17).

a Lateral hole distribution; b hole concentration; c radiative recombination for LEDs A and C1–C5.

a Output power and EQE vs. current; b I‑V characteristics for LEDs A–C5.

WPE vs. current for LEDs A, C1–C5; inset: WPE and EQE at 170 A/cm².

Conclusions

The study demonstrates that a p‑AlGaN/n‑AlGaN/p‑AlGaN (PNP‑AlGaN) junction effectively improves current spreading in AlGaN‑based DUV LEDs. By tailoring the insertion thickness, Si doping, AlN composition, and junction count, the vertical resistance is increased while the horizontal current is enhanced, leading to higher EQE and WPE. However, excessive barrier height or too many junctions raise forward voltage and can reduce WPE. Therefore, a balanced design—optimizing all four parameters—is essential for maximizing device efficiency.