Enhancing Deep Ultraviolet LED Performance via Locally Modulated NPN‑AlGaN Current‑Spreading Layers

Abstract

We present a novel strategy to improve current spreading in AlGaN‑based deep ultraviolet light‑emitting diodes (DUV LEDs) by inserting a locally doped n‑AlGaN/p‑AlGaN/n‑AlGaN (NPN‑AlGaN) structure into the electron‑supplier layer. The thin p‑AlGaN insertion creates a conduction‑band barrier that redistributes lateral current in the p‑type hole‑supplier layer, mitigating current crowding. Key parameters—Mg doping level, p‑layer thickness, AlN composition, and the number of NPN junctions—are shown to critically influence the spreading effect. Optimized NPN‑AlGaN layers enhance optical output, external quantum efficiency (EQE), and wall‑plug efficiency (WPE) for DUV LEDs.

Introduction

AlGaN‑based DUV LEDs are essential for disinfection, water purification, medical applications, and high‑density optical recording^[1‑8]. Recent advances in crystal quality—such as graphene‑assisted quasi‑Van der Waals epitaxy of AlN on nano‑patterned sapphire—have lowered dislocation densities and raised internal quantum efficiencies (IQE) to 80%^[9‑10]. However, DUV LEDs are driven electrically, where carrier injection and transport govern performance^[11‑13]. A prominent challenge is current crowding, especially under high bias, due to poor Mg‑doping efficiency in high‑Al‑content p‑AlGaN layers^[15‑16] and the lateral injection geometry of flip‑chip structures^[17‑18]. Current‑spreading schemes based on narrow‑strip p‑contacts^[19] or ITO/ZGO layers^[20] have shown limited success, often hampered by interface resistivity or insufficient lateral conductivity.

In this work, we shift the focus from the p‑side to the n‑type electron‑supplier layer. By engineering the doping profile in n‑AlGaN to form NPN‑AlGaN junctions, we generate controlled energy barriers that redistribute lateral current, thereby enhancing hole injection and overall device efficiency. This approach offers greater flexibility for epitaxial optimization while directly addressing current crowding.

Research Methods and Physics Models

The baseline DUV LED structure comprises a 4‑µm‑thick n‑Al_0.60Ga_0.40N layer doped with Si at 5×10¹⁸ cm⁻³, followed by five Al_0.45Ga_0.55N/Al_0.56Ga_0.44N MQWs (3 nm/12 nm). An 18‑nm Mg‑doped p‑Al_0.60Ga_0.40N electron‑blocking layer (EBL) caps the MQWs, succeeded by 50‑nm Mg‑doped p‑Al_0.40Ga_0.60N and p‑GaN layers, each with hole concentrations of 3×10¹⁷ cm⁻³. Devices feature a 350 × 350 µm² mesa.

Figure 1b illustrates the conduction‑band profile for a structure containing two NPN‑AlGaN junctions (NPNPN‑AlGaN). The depletion of the p‑AlGaN insertions creates vertical barriers (ϕ₁, ϕ₂) that tune lateral current distribution. The junction must be fully depleted to operate in reach‑through breakdown mode^[21]. All devices are compared to a reference LED lacking NPN junctions.

We model current flow with an equivalent circuit (Fig. 2a), separating vertical (I₁) and horizontal (I₂) components. The presence of NPN junctions introduces interfacial resistance R_npn per junction, reducing the I₁/I₂ ratio (Eq. 3). Increasing the number of junctions (N) or raising R_npn—by adjusting AlN content, thickness, or Mg doping—further promotes lateral spreading.

Device physics are simulated using Crosslight APSYS, with validated models for Auger recombination (1×10⁻³⁰ cm⁶s⁻¹), SRH lifetime (10 ns), light extraction (≈ 8 %), and a 50:50 band‑offset for AlGaN heterojunctions^[25]. Polarization charges are set at 40 % of the lattice mismatch^[29].

Results and Discussions

Impact of NPN‑AlGaN on Current Spreading

Comparing reference LED A (no NPN) with LED B (one NPN junction, 20 nm p‑Al_0.60Ga_0.40N, Mg = 1×10¹⁸ cm⁻³) shows two conduction barriers in LED B (Fig. 3a). These barriers increase R_npn, lowering I₁/I₂ and enabling a more uniform lateral hole distribution in the last quantum well (LQW) (Fig. 3b). Consequently, hole concentrations and radiative recombination rates in the MQWs rise (Fig. 4a‑b), translating to higher EQE and optical power density (Fig. 5a). Although forward voltage increases slightly (Fig. 5b), the wall‑plug efficiency (WPE) surpasses the reference above ~56 A cm⁻² (Fig. 5c), confirming the benefit of NPN‑AlGaN engineering.

Effect of AlN Composition in the p‑AlGaN Layer

Five devices (C_i, i = 1‑5) were fabricated with AlN fractions of 0.60, 0.63, 0.66, 0.69, 0.72 in the 20‑nm p‑layer (Mg = 1.8×10¹⁸ cm⁻³). Higher AlN increases the conduction‑band barrier (Table 1), raising R_npn and reducing I₁/I₂ (Eq. 3). Lateral hole profiles (Fig. 6a) and MQW concentrations (Fig. 6b‑c) show progressively improved uniformity and higher carrier densities with larger AlN content. EQE and optical power density follow suit (Fig. 7a), while forward voltage exhibits a modest rise (Fig. 7b). WPE benefits are maximized at an intermediate AlN fraction (C₂), balancing barrier height against added voltage drop (Fig. 8).

Mg Doping Influence

Devices D_i (i = 1‑5) vary Mg doping in the 20‑nm p‑layer (3×10¹⁷ – 3×10¹⁸ cm⁻³) with AlN = 0.61. Increasing Mg widens the depletion region, raising the barrier and R_npn (Table 2). Lateral hole distributions become increasingly uniform (Fig. 9a), enhancing MQW hole concentrations and radiative recombination (Fig. 9b‑c). EQE and optical power density rise with doping (Fig. 10a), though forward voltage also increases, particularly for the highest Mg level (Fig. 10b). WPE improves up to Mg = 2×10¹⁸ cm⁻³ but declines at the highest doping due to excessive voltage drop (Fig. 11).

Thickness of the p‑AlGaN Insertion Layer

Series T_i (i = 1‑5) employ 18‑, 20‑, 24‑, 28‑, and 32‑nm p‑layers (AlN = 0.61, Mg = 1.5×10¹⁸ cm⁻³). Barrier heights increase with thickness (Table 3), further reducing I₁/I₂ and improving lateral spreading (Fig. 12a). MQW hole concentrations and recombination rates rise correspondingly (Fig. 12b‑c). Optical power and EQE improve (Fig. 13a), and forward voltage decreases significantly for 20‑, 24‑, and 28‑nm devices (Fig. 13b). However, the thickest layer (32 nm) shows a higher turn‑on voltage due to a parasitic N‑AlGaN/P‑AlGaN diode. WPE peaks at an optimal thickness, demonstrating the need for careful tuning (Fig. 14).

Number of NPN‑AlGaN Junctions

LEDs N_i (i = 1‑5) contain 1 to 5 NPN junctions, each with 20‑nm p‑layers (AlN = 0.61, Mg = 1.5×10¹⁸ cm⁻³). While individual barrier heights are similar (Table 4), the cumulative barrier increases with junction count, amplifying lateral spreading (Fig. 15a). MQW hole concentrations and recombination rates improve progressively (Fig. 15b‑c). EQE and optical power density rise with junction number, but the gains diminish after three junctions (Fig. 16a). Forward voltage decreases relative to the reference (Fig. 16b), and WPE benefits are evident up to five junctions, though the incremental advantage tapers (Fig. 17).

Conclusions

Embedding NPN‑AlGaN junctions within the n‑type electron‑supplier layer effectively mitigates current crowding in the p‑type hole‑supplier region of AlGaN‑based DUV LEDs. By tuning AlN composition, Mg doping, insertion‑layer thickness, and junction count, we can homogenize lateral current, enhance hole injection, and improve EQE and WPE. While EQE consistently benefits, WPE gains plateau when additional voltage drops from barriers outweigh the spreading advantage, underscoring the importance of balanced design. This study introduces a versatile, growth‑friendly current‑spreading strategy that advances the performance of high‑power DUV LEDs.