385 nm AlGaN Near-Ultraviolet Micro Light-Emitting Diode Arrays with WPE 30.18% Realized Using an AlN-Inserted Hole Spreading Enhancement S Electron Blocking Layer

Abstract

In this work, we demonstrate high-efficiency 385 nm AlGaN-based near-ultraviolet micro light emitting diode (NUV-Micro LED) arrays. The epi structure is prepared using a novel AlN-inserted superlattice electrical blocking layer which enhances hole spreading in the p-type region significantly. The NUV-Micro LED arrays in this work comprise 228 chips in parallel with wavelengths at 385 nm, and each single chip size is 15 × 30 μm². Compared with conventional bulk AlGaN-based EBL structures, the NUV-Micro LED arrays that implemented the new hole spreading enhanced superlattice electrical blocking layer (HSESL-EBL) structure proposed in this work had a remarkable increase in light output power (LOP) at current density, increasing the range down from 0.02 A/cm² to as high as 97 A/cm². The array’s light output power is increased up to 1540% at the lowest current density 0.02 A/cm², and up to 58% at the highest current density 97 A/cm², measured under room temperature (RT); consequently, the WPE is increased from 13.4% to a maximum of 30.18%. This AlN-inserted HESEL-EBL design significantly enhances both the lateral expansion efficiency and the hole injection efficiency into the multi quantum well (MQW) in the arrays, improving the concentration distribution of the holes in MQW while maintaining good suppression of electron leakage. The array’s efficiency droop has also been greatly reduced.

1. Introduction

Ultraviolet light-emitting diodes (UV LEDs) are extensively researched for applications including water/air disinfection, medical esthetics, and non-line-of-sight communication. Their wavelength tunability, low power consumption, and environmental sustainability position them as a leading alternative to mercury lamps [1]. Notably, high-efficiency near-ultraviolet (NUV) LEDs (360–385 nm) show significant potential in niche markets such as UV curing, medical devices, 3D printing resin curing, and quantum dot excitation [2,3]. Unlike conventional high-power UV systems, these applications prioritize photoelectric conversion efficiency at micro-scale sizes and ultra-low current densities [4]. AlGaN-based semiconductors, with tunable bandgaps enabling precise UV wavelength control (220–405 nm), are pivotal for ensuring high-performance UV optoelectronics [5,6]. However, AlGaN-based NUV-Micro LEDs still face challenges in terms of light output power and efficiency compared to commercial InGaN-based visible LEDs, primarily due to low external quantum efficiency (EQE) [7,8]. Since EQE correlates directly with wall-plug efficiency (WPE), this study evaluates device performance using WPE as a proxy for EQE [9].

The low internal quantum efficiency (IQE) and light extraction efficiency (LEE) of ultraviolet light-emitting diodes (UV-LEDs) are key contributors to their poor external quantum efficiency (EQE) and wall-plug efficiency (WPE) [10]. While AlGaN/InAlGaN quantum wells (QWs) with emission wavelengths of 360–405 nm (near-ultraviolet, NUV) theoretically achieve IQE values of 70%–80% [11], practical NUV-LED devices exhibit WPE values below 15%, significantly lower than those of commercial InGaN blue LEDs [12]. This gap stems from the following:

• Low carrier injection efficiency (CIE): Electron leakage from QWs, exacerbated by insufficient hole concentrations and poor lateral spreading in p-type AlGaN layers [13,14].
• Material limitations: High Al composition in AlGaN barriers and electron-blocking layers (EBLs) improves CIE but introduces spontaneous/piezoelectric polarization effects in p-AlGaN. These effects increase Mg-H activation energy, hindering hole concentration enhancement compared to p-GaN [6,15,16,17].
• Imbalanced carrier distribution: Reduced wavefunction overlap due to uneven electron–hole injection into the active region [18,19], coupled with severe electron leakage into the p-type region from low hole concentrations [20,21,22].

Collectively, these factors—electron overflow, inefficient hole transport, and AlGaN’s wide bandgap-induced high activation energy—restrict the NUV-LED WPE to sub-15% levels [23,24].

To enhance the WPE of near-ultraviolet light-emitting diodes (NUV-LEDs), researchers have proposed innovative electron-blocking layer (EBL) designs, band structure engineering, and quantum barrier (QB) configurations to improve carrier injection efficiency (CIE) [25,26,27]. Simulations demonstrate that these approaches—including graded superlattice (SL) structures, asymmetric step quantum barriers, and Si-doped quantum barriers—effectively suppress electron overflow from quantum wells to the p-type region [28,29,30,31]. For instance, advanced EBL architectures such as linear Al-gradient EBLs, step-graded superlattices, and inverted-V-shaped Al composition profiles enhance electron–hole wavefunction overlap and device performance [32,33,34,35,36,37,38,39,40]. However, existing studies predominantly focus on theoretical models, with limited experimental validation of fine-tuned EBL variations in NUV epitaxial wafers or micro-scale chips. Systematic experimental validation and in-depth mechanistic analysis of EBL structures remain critical for confirming their practical efficacy in NUV-LEDs. C.K. Wang’s simulation study demonstrated that introducing a high-Al-composition interlayer between the MQW region and the EBL can effectively mitigate efficiency degradation while enhancing optical output power and IQE [41]. However, incorporating such high-Al-content layers significantly increases the complexity of epitaxial growth and may compromise crystal quality due to lattice mismatch and growth challenges [42]. Zhiyuan Liu et al. proposed a polarization-modulated InAlN last quantum barrier (LQB) design with Indium compositions ranging from 0.14 to 0.16 for 320 nm UVB LEDs. By tailoring the polarization at the LQB/EBL interface using InAlN, their simulation achieved a 32% improvement in IQE and WPE of 7.52% [43]. Despite these promising results, the experimental realization of high-Al-composition AlGaN or AlN thin layers remains technically challenging due to the difficulties in growing high-quality films at relatively low temperatures (<1000 °C). Mohd Ann et al. developed a 388 nm NUV-LED using an InGaN/GaN/Al_xGa_1−xN/GaN MQW structure, achieving a WPE of up to 14% [44]. However, to accommodate for the low growth temperatures required by the quantum well, the Al content in the AlGaN layer was limited to less than 5%, which constrains the broader applicability of this approach. Chaozhi Xu et al. reported that inserting an undoped AlGaN layer of appropriate thickness between the LQB and EBL in a 398 nm device led to significant improvements in both light output power and quantum efficiency, with a remarkable WPE of 50% achieved while keeping the Al content below 15% [45]. Nonetheless, the performance impact of inserting a high-quality AlN layer within the EBL structure at elevated temperatures (e.g., ~950 °C), especially for shorter-wavelength NUV-LEDs, remains largely unexplored and warrants further investigation [46].

In this work, we propose a new AlN-inserted Hole Spreading Enhanced Superlattice Electron Blocking Layer structure design, using an AlN insertion transition layer combined with an ${\mathrm{A}\mathrm{l}}_x{\mathrm{G}\mathrm{a}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ superlattice structure. The photoelectric performance parameters under low and high current densities are compared with traditional SL-EBL structures and graded superlattice EBLs. By performing a detailed analysis of light output power and photoelectric conversion efficiency at micro sizes and extremely low current densities, the mechanisms of hole carrier lateral spreading efficiency and transport efficiency can be thoroughly investigated. The EBL structure designed in this work consists of a graded superlattice ${\mathrm{A}\mathrm{l}}_{0.15-0.12}\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ layer, with an AlN interval-inserted expansion layer, and two groups of nested heavy Mg-doping p-GaN. We also investigated the effects of using HSESL-EBL on the WPE/EQE values of the NUV-Micro LED arrays. The AlN-inserted HSESL-EBL creates an effective barrier height, significantly suppressing electron leakage and improving hole transport efficiency in NUV-Micro LED arrays, particularly at low current densities. The experimental results show that this AlN-inserted HSESL-EBL can provide the “effective barrier height” and thus significantly reduces electron leakage in NUV-Micro LED arrays, thereby enhancing hole transport efficiency, especially under low current density operation mode. NUV-Micro LED arrays of 385 nm using this structure successfully achieved record high WPE and light output power (LOP) and show a significant improvement in the efficiency droop effect.

2. Device Structures and Parameters

In this study, four near-ultraviolet micro-light-emitting diode chip array samples with identical epitaxial layers (except EBL variations) were fabricated on 4-inch patterned sapphire substrates (PSSs) to isolate the EBL effects. The growth process was as follows: Initially, a 15 nm thick aluminum nitride (AlN) nucleation layer was deposited on a C-plane 4-inch PSS using physical vapor deposition (PVD). Subsequently, the following layers were sequentially grown on the AlN template via K465i Veeco metal–organic chemical vapor deposition (MOCVD): trimethyl–gallium (TMGa), trimethyl–aluminum (TMAl), trimethylindium (TMIn), bis-cyclopentadienyl magnesium (Cp2Mg), disilane (Si2H6), and ammonia (NH3), used as the Ga, Al, In, Mg, Si, and N sources, respectively.

• The substrate was cleaned in H₂ ambient at a temperature of 1080 °C for 8 min to remove any impurities and oxygen.
• A 3 μm thick n-type GaN layer with a silicon doping concentration of 2 × 10¹⁹ cm⁻³; grown at 1070 °C with a V/III ratio of 4000.
• There was a 9 Loops period active region, and each pair was composed of 3 nm In_0.02GaN quantum wells (QWs) + 12 nm Al_0.15GaN barrier layer (N-type doping 2 × 10¹⁷ cm⁻³) with Al composition equal to 15%, grown at 900 °C with an identical V/II ratio of 33,000 to prevent fluctuations of the temperature during the growth of active layers.
• A 20 nm p-type Al_0.15GaN EBL (Mg: 2 × 10¹⁸ cm⁻³) grown at 930 °C with a V/III ratio of 800 and four different EBL structures, namely sample 1, 2, 3, and 4, to investigate the effect of EBL on the structure of NUV-LEDs.
• A 20 nm p-type GaN HSESL (Mg: 6 × 10¹⁹ cm⁻³) followed with a V/III ratio of 5000 at 950 °C.
• A 2 nm p-type GaN contact layer (Mg: 2.5 × 10²⁰ cm⁻³) with the same growth condition of the p-type GaN HSESL layer.

The four samples exhibited distinct Al composition profiles: conventional EBL structures or HSEL-EBL composite architectures embedded within AlN key design. The specific designs were as follows:

• Sample 1: Graded Al EBL (3%→15%, linear) + dual Mg doping peaks.
• Sample 2: Uniform Al EBL (15%) + dual Mg peaks.
• Sample 3: Graded Al EBL + single Mg peak with polarization doping for enhanced hole concentration.
• Sample 4: AlN-inserted HSESL-EBL composite (graded Al + dual Mg peaks).

The Al composition profiles and the whole structure of the samples are illustrated in Figure 1a,b.

Following epitaxy, flip-chip technology was employed to fabricate the NUV-Micro LED arrays as shown in Figure 2a,b [47]. Both p-type and n-type electrodes were fabricated using Ni/Au metallization. The arrays consisted of 228 individual NUV-Micro LED pixels connected in parallel, with each pixel measuring 15 × 30 μm². The operational current densities range of the arrays spanned from 0.02 A/cm² to 97 A/cm², corresponding to a single current range of 0.1 μA to 438 μA for a single pixel. Finally, the optoelectronic performance of the samples was systematically characterized under bare-chip Chip on Board (COB) conditions, including measurements of the current density–voltage (I–V) characteristics and WPE, among other critical parameters.

3. Results and Discussion

Figure 3 illustrates the light output power (a) (b) and current voltage (IV) characteristics (c) (d) of 385 nm AlGaN NUV-Micro LED array chips, designed with different composite EBL structures, under varying injection currents. All measurements were conducted under room temperature (RT cw) continuous-wave conditions. The relationship between the current density and light output power reveals that sample 4 exhibits a significant advantage in light output power at both low and high current densities. At low current densities, sample 4 maintains a relatively high light output power, which is an order of magnitude greater than that of the other three traditional EBL samples, as shown in Figure 3b.

Specifically, whether the EBL is composed of a graded Al composition (Graded-EBL), a constant Al composition (Constant-EBL), or a Mg-doped design with polarization doping, as the driving current density increases from 0.02 A/cm² to 97 A/cm², the total light output power of the device array significantly increases at low current densities, from 1.0 × 10⁻⁵ mW (at 3.08 V) for sample 1 to 1.5 × 10⁻³ mW (at 2.84 V) for sample 4. At high current densities, the total light output power increases from 58.76 mW (at 4.37 V) for sample 1 to 93.26 mW (at 3.8 V) for sample 4, as shown in Figure 3c.

The significant increase in the light output power of the device array is accompanied by a decrease in operating voltage, which directly leads to an increase in device WPE from 13.4% to a maximum of 30.18%, 180% higher than that of the other three samples, as shown in Figure 4a,b. The significant improvements in the photoelectric output power of the device array at both low and high current densities are attributed to the improvements in the NUV-Micro LED chip’s carrier concentration distribution in the UV quantum wells and the increased recombination probability of electron–hole pairs, leading to a more balanced carrier concentration distribution in the quantum well region of the device. This is particularly evident in the improvement in the hole concentration distribution and lateral spreading efficiency, while the changes in the electron concentration distribution are not significant.

The improved structure maintains the sufficient electron barrier height of the traditional EBL structure, effectively blocking excessive electron leakage into the quantum well layer and the p-type layer of the device at a micro size.

Based on the I–V characteristic curves in Figure 3, the NUV-Micro LED arrays exhibit typical diode-like behavior, with sample 4 demonstrating enhanced performance. Under identical injection current conditions, sample 4 exhibits a significantly lower voltage compared to the other samples, likely due to the gradient Al composition in the EBL. Specifically, the gradient Al’s composition induces a gradual tilt in the valence band from the high-Al end to the low-Al end within the EBL, forming a “slope” structure that facilitates hole transport and effectively reduces the hole injection barrier. This structural modification facilitates the activation of Mg acceptors, increases the hole concentration, and significantly reduces the p-type contact resistance. Furthermore, the gradient design increases the spatial distribution depth of the holes within the quantum well, thereby enhancing the radiative recombination efficiency and further reducing the overall resistance. As illustrated in the inset figure, sample 4 maintains exceptional electrical performance even at low injection currents. Moreover, at a current density of 1 A/cm², the increase in resistance for sample 4 is notably slower compared to the other samples. These findings provide solid evidence that the proposed gradient EBL design significantly improves the electrical performance of the NUV-Micro LED arrays. This can be attributed to several factors, particularly the optimization of the carrier concentration distribution within the UV quantum wells and the increased electron–hole recombination probability, resulting in a more balanced carrier distribution within the quantum well region. Notably, the improvement in the hole concentration distribution and lateral spreading efficiency plays a critical role, leading to more uniform carrier injection into the quantum wells. While the improvement in the electron concentration distribution is relatively modest, it still contributes to the overall optimization of device performance. The modified EBL structure maintains an adequate electron potential barrier and effectively suppresses excessive electron leakage into the quantum well and p-type layers at the micron scale. This results in a significant increase in light output power, as well as a reduction in operating voltage, which together enhance the device’s optoelectronic performance.

It needs to be emphasized that for this study, the chip fabrication process and the main quantum well region’s structural design for all samples are identical, confirming that the IQE and LEE (light extraction efficiency) of the comparison group devices should remain unchanged. Therefore, the effective wall barrier of the optimized AlN-inserted HSESL-EBL structure design in this work is the primary reason for the great improvement in the light output power and WPE in the NUV-Micro LED array.

4. Conclusions

In summary, we proposed an AlN-inserted HSESL-EBL structure for use in NUV LEDs and successfully fabricated NUV-Micro LED arrays consisting of 228 NUV-Micro LED chips with a central wavelength of 385 nm, each with a size of 15 × 30 μm², connected in parallel. The measured results show that the optimized composite EBL structure in this work results in lower voltage (by 0.2 V) and 150 times higher LOP as compared to NUV-Micro LEDs with conventional EBL structures under a low current density operation mode of 0.02 A/cm², and 2 times higher LOP under 97 A/cm² current density. Since the NUV-Micro LED arrays operate in this ultra-low injection current mode, the device’s photoelectric performance test results can effectively avoid the influence of electron leakage. With a consistent MQW main structure among the experimental group chips, the epi dislocation density and IQE performance of the active region are kept consistent. The significant improvement in photoelectric performance is attributed to the AlN-inserted HSESL-EBL structure design, which greatly enhances the hole lateral transport efficiency and carrier injection efficiency in the main quantum well region. This effectively proves that the optimization through the band engineering of electron and hole carriers through the EBL improves the hole injection efficiency, which optimizes the carrier concentration distribution in the main quantum well region. The optimized hole injection efficiency leads to a desired change in the carrier concentration distribution, resulting in much improved photoelectric efficiency. This enhancement of the band regulation results in a noticeable improvement in the results, whether under ultra-low current density drive or high current density drive. When the arrays operate at a current of 100 mA, the output ultraviolet power reaches 93 mW and WPE could reach 30.18%, respectively. This is currently the highest reported value in the 385 nm NUV-LED reports. However, practical constraints prevented the inclusion of band diagrams to explain the enhanced electron blocking efficiency, representing a limitation of this study. Future work will address this through band structure simulations for further theoretical validation.