2.1.

Highly Reflective Electrodes

AlGaN-based UV LEDs usually adopt flip-chip configuration in consideration of the p-GaN light-absorption issue and the thermal management. If a semi- or transparent p-layer and highly reflective structures are introduced on the top side, the upward emission light and the reflected downward light can be then reflected back to the substrate side. In consequence, the escaping probability of photons through the substrate side would be improved.

Inazu et al.¹⁴ presented a 1.55-fold increase in LEE of a sub-300-nm DUV LED using aluminum reflective electrodes deposited to cover both n- and p-type mesh contact electrodes. Meanwhile, the p-GaN layer was partially removed to increase the reflective area. Fayisa et al.¹⁵ fabricated microring array 276-nm LEDs having a p-GaN-removed inner circle of the microrings to reduce the absorption loss in the p-GaN contact layer, together with ${\mathrm{MgF}}_2/\mathrm{Al}$ omnidirectional reflectors on the p-GaN etched circle region to redirect the DUV photons impinging on them toward the substrate side. The microring array DUV LED showed remarkably higher light output power (LOP) by 70% than the reference.

To fundamentally avoid the p-GaN absorption problem, Shatalov et al.^17,34 presented a normal incidence optical transmission at 275 nm increasing from 5% in DUV LED with p-GaN contact layer to above 60% in that with transparent p-SPSL. Correspondingly, the absorption coefficient of the p-layer is dramatically reduced to below $1000{\mathrm{cm}}^{-1}$. However, the vertical conductivity of p-SPSL and contact stability then become the crucial issues and need further optimization. Hirayama et al.^16,26,35 demonstrated 140% to 180% improvement in EQE of the 287/279-nm UV LEDs and 60% increase in the electroluminescence (EL) intensity of the 260-nm DUV LED,³⁶ when using transparent p-AlGaN contact layer and Ni/Al p-type electrode to replace Ni/Au electrode. Yet again, this LED structure without p-GaN contact layer also suffers from increased forward voltage due to higher p-type contact resistance than that in conventional LEDs. Recently, Zheng et al.³⁷ proposed a multidimensional Mg-doped SL to decrease the hole potential barrier along the $c$-axis and enhance the vertical hole conductivity, which is ascribed to the stronger ${\mathrm{p}}_{\mathrm{z}}$ hybridization between Mg and N. The grown p-type three-dimensional ${\mathrm{Al}}_{0.63}{\mathrm{Ga}}_{0.37}\mathrm{N}/{\mathrm{Al}}_{0.51}{\mathrm{Ga}}_{0.49}\mathrm{N}$ SLs have a hole concentration reaching a high value of $3.5\times {10}^{18}{\mathrm{cm}}^{-3}$ and a resistivity as low as $0.7\mathrm{\Omega }\mathrm{cm}$ at room temperature (RT), exhibiting 10 times improvement in conductivity compared with that of conventional SLs. Moreover, the resistivity is weakly dependent on temperature. The advances of multidimensional SLs provide a promising solution to reduce the forward voltage of UV LEDs.

Vertical emission can be obtained from the standing wave formed by lateral Bragg resonance in a two-dimensional photonic crystal (PhC) near the quantum-well emission layer.³⁸ Kashima et al.²⁷ introduced a highly reflective PhC (HR-PhC) into the surface of the p-AlGaN contact layer to induce the vertical light-propagation. According to their results, the effective reflectance of the HR-PhC p-AlGaN contact layer with the Ni/Mg electrode exceeded 90%. The EQE of a conventional DUV LED with emission around 283 nm was increased from 4.8% to 10% by using this highly reflective structure.

2.2.

Sidewall Reflection Enhancement Structure

Highly reflective structure at the slant mesa sidewalls can efficiently deflect the photons guided laterally along the $c$-plane into the vertical direction, which seems a tailored technique for the extraction of TM-polarized light.³⁹ Lee et al.²⁰ reported sidewall-emission-enhanced DUV LEDs with active mesa stripes or arrays of truncated cone-shaped active mesas³³ coated with ${\mathrm{MgF}}_2/\mathrm{Al}$ reflectors to effectively extract the in-plane TM-polarized emission.

We reported a sidewall reflection enhancement method for AlGaN-based 278-nm LEDs, consisting of mesa microarrays in a hexagonal shape and on-chip highly reflective metal-pads for UV light,²¹ as shown in Fig. 1. The total device area was $1{\mathrm{mm}}^2$. Each independent micromesa had a side length of $30\mu \mathrm{m}$ and a trapezoidal cross-sectional shape with a slant angle of $\sim 45\mathrm{deg}$ after the inductively coupled plasma dry etching. A Ti/Al/Ti/Au metal stack and a Ni/Au metal stack were deposited on the exposed n-AlGaN layer and p-GaN layer to form ohmic contacts, respectively. After the chip passivation with ${\mathrm{SiO}}_2$, chip-level metal-pads were deposited as anode/cathode electrodes and bond pads as well for the subsequent flip-chip process. Here, we compared the influence of two kinds of metal-pads on the LOP of AlGaN UV LEDs. Cr/Al/Ti/Au system is widely used in the standard 450-nm blue LED fabrication, but Cr metal is not a good UV reflector. In our new method, aluminum-based metal-pads, i.e., Al/Ti/Au, were evaporated, which not only acted as electrodes and bond pads but also covered the trapezoidal mesa sidewall to accommodate the light propagation path. Then, the separated LED chips were flip-chip bonded on silicon submounts with gold bumps for further device testing. It should be noted that the reflectivity of the on-chip metal-pads maintains consistently because of no high-temperature treatment involved.

Fig. 1

The schematic structure of AlGaN-based UV LEDs using a sidewall reflection method. Inset: The top-view of the LED chip before the deposition of on-chip metal-pads.

The average LOP of the encapsulated LEDs with highly reflective metal-pads was increased by 30.1%, compared to that with traditional metal-pads. The improvement is attributed to the less absorption and the more efficient reflection of in-plane guided light arriving at the mesa sidewalls.³⁹ The LOP can be further increased via reducing the sidewall length of the mesa due to the larger sidewall area for DUV reflection. In addition, there were no significant differences between these two kinds of chips in the electrical properties and the yield, revealing a quite good compatibility of Al/Ti/Au metal-pads in the UV LED fabrication process.

Figure 2 shows the performance of one LED using the sidewall reflection method as a function of the injected CW current. At 100 mA, the LOP reaches 6.8 mW and the forward voltage is about 7 V. When driven to 500 mA, the LED presents a saturation LOP of around 25 mW due to the increased Auger recombination under high current density and heat. The device was stressed at 100 mA CW for $>6000\mathrm{h}$ at RT. The LOP decay shows the L80 (80% of the initial power retention) operation lifetime of $\sim 5000\mathrm{h}$ and extrapolated L50 lifetime to $>\mathrm{10,000}\mathrm{h}$.

Fig. 2

(a) The typical LOP- and forward voltage-injected CW current characteristics. Inset: the EL spectrum at 100 mA; (b) the reliability test of an encapsulated DUV LED with the sidewall reflection structure at 100-mA CW and RT.

3.1.

Substrate Surface Roughening

Substrate surface roughening is meant to provide numerous scattering events at the substrate/ambient interfaces for trapped photons to escape from the LED structure. Khizar et al.⁴⁰ reported on the fabrication of AlGaN-based 280-nm LEDs with monolithic integrated microlens array on the backside of sapphire substrates and proved a 55% enhancement in LOP at 20 mA compared with the same LED without miroclens. Pernot et al.⁴¹ demonstrated a 50% increase in on-wafer output power of 270-nm LEDs by fabricating a moth-eye structure on the sapphire substrate. Inoue et al.²⁹ designed hybrid structure nanostructure of PhCs and subwavelength nanostructures on HVPE-AlN substrates. A maximum light extraction enhancement of 196% was obtained in 265-nm LEDs with the hybrid structure in comparison to that of a flat surface device. To enable high-output and high-volume fabrication, they further presented 265-nm DUV LEDs with large-area nanoimprinted AlN nanophotonic structures,⁷ which showed wider near-field emitting areas, stronger far-field extracted light intensities, and an approximately 20-fold increase in LOP compared with a conventional flat-surface LED.

3.2.

Substrate Sidewall Roughening

As mentioned above, the TM-polarized light propagates mainly in the lateral direction and is extracted from the device sidewalls. Hence, it is also important to pay attention to the sidewall TIR mitigation such as roughening the device sidewalls, especially the substrate sidewalls that have lager area than the epitaxial layer sidewalls.

Substrate sidewall roughening can be realized by ultrashort pulse laser stealth dicing during the chip-separation process and has been proven to be helpful to enhance the LEE in visible and UV LEDs.^42–45 Lee at al.⁴⁵ demonstrated that thick sapphire substrates with roughened sidewalls are critical for TM-polarized photons from the AlGaN quantum wells to escape in lateral direction before they are absorbed by p-GaN and metal electrodes. We investigated the effect of sapphire substrate’s sidewall roughening on the light extraction behavior of AlGaN-based 275-nm UV LEDs by finite-difference time-domain (FDTD) simulation and experimental output power measurement.⁴⁶

Figure 3(a) shows the FDTD computational domain of the UV LED with a roughened sapphire substrate reference sidewall. For a given TE- or TM-mode dipole with a distance $d$ from the reference sidewall, its light extraction is enhanced only when the roughening position $z$ is within a specific region (not shown here), which is defined as the effective roughening region. As shown in Fig. 3(b), the effective roughening region is between the substrate’s backside and the lower boundary $\sim d\times \tan (\theta c)$, where $\theta c$ is the TIR critical angle. Roughening outside the effective roughening region will reduce the sidewall LEE due to the undesirable inward photon scattering at the substrate sidewall. The extracted light power of various dipoles with different positions was simulated individually and then combined to obtain the overall light extraction enhancement factor with respect to the roughening position. As shown in Fig. 3(c), the overall extraction efficiency of TE-mode light keeps improved regardless of the roughening position $z$ on the substrate sidewall. In contrast, the overall extraction of TM-mode light encounters a decrease when the roughening position $z$ is lower than a certain value, revealing that a roughening region for effective light extraction enhancement exists on the substrate’s sidewall. Moreover, the boundary of the overall effective roughening region of TM mode is found equal to that of the TM-polarized dipole in the center of the active region.⁴⁶ A simulation with multiple roughening layers in Fig. 3(d) further verified the effective roughening region assumption. Therefore, the effective roughening region of TM polarization in one LED with a side length of $L$ is deduced to from the sapphire substrate’s backside to the position $\sim L/2\times \tan (\theta c)$.

Fig. 3

(a) Schematic diagram of the FDTD computational domain of the UV LED structure with the roughened sapphire sidewall. Inset: Scanning electron microscopy (SEM) image of sidewall morphology of one roughening layer.⁴⁶ (b) Schematic diagram of the effective roughening region for one dipole on the reference sidewall. The dependence of overall sidewall light extraction enhancement factor on the position of (c) a single roughening layer and (d) multiple roughening layers.

We designed four sidewall roughening schemes for TM-mode-dominant UV LEDs. As shown in Fig. 4, the LEDs with three roughening layers have 13.2% higher LOP at 20 mA than those with two roughening layers, due to the larger roughening area in the effective roughening region. However, when further applying additional roughening layers beyond the effective roughening region, the LOP decreases, which is in consistent with the simulation result. The current–voltage characteristics of these LEDs were similar, indicating that the LOP variation mainly comes from different roughening schemes. Therefore, depth-controlled substrate’s sidewall roughening is beneficial to maximize the LOP of AlGaN-based UV LEDs.

Fig. 4

The normalized average LOP of LEDs at 20 mA. Inset: The EL spectra of the LED wafer for TE- and TM-modes at 20 mA.

3.3.

Substrate Shaping

Modifying the geometry of the LED chip can also help outcouple the light trapping in the structure. It could be regarded as the upscaling of the surface texturing.⁴⁷ Krames et al.⁴⁸ fabricated truncated-inverted-pyramid AlGaInP-based LEDs sawn by a beveled dicing blade, exhibiting 40% improvement in EQE compared to the conventional devices. Lee et al.⁴⁹ reported 55% light output enhancement in flip-chip GaN-based LEDs with oblique sapphire sidewalls on the bottom of the substrate using a wet etching technique.

Laser micromachining has been introduced to shape the sapphire substrate, such as nanosecond laser dicing either with a laser-beam turning mirror⁵⁰ or with an oblique sample stage⁵¹ to bevel the substrate sidewalls and shifted picosecond (ps) laser stealth dicing to form wavy substrate sidewalls.⁵² We proposed a convenient ps laser multiple scribing method to shape the sapphire substrates into an oblique sidewall geometry for enhanced LEE.⁵³ In general, the scribing lines beneath the same street are distributed in the same vertical plane inside the substrate; thus, the LEDs have vertical sidewalls after splitting. In our study, the applied multiple scribing lines beneath the same street are intentionally aligned along an oblique plane with designed angles to guide the chip splitting direction. The LED chips are thereby shaped into an oblique sidewall geometry. Compared with the convention chip with vertical sidewalls, the 276-nm LED with two opposite sidewalls partially shaped with 60 deg inclination shows 13.8% higher LOP at 50 mA. The FDTD simulation reveals that this oblique substrate sidewall geometry is effective for TM-polarized light extraction.

3.4.

Interface Modification

According to the simulations^12,21,54 and experimental results, a large amount of TE- and TM-polarized light encounter TIR at the interfaces among the substrate, AlN layer, n-AlGaN layer, multiple-quantum-well region, electron blocking layer, and p-AlGaN cladding layer, resulting in light confinement and absorption. A careful consideration should be given to the interface modification, or in other words, effective refraction index adjustment around the interfaces.

The AlN epitaxial lateral overgrowth on pattered substrates or templates can not only improve the crystal quality of the overgrown epitaxial layers but also form embedded air voids in the AlN layer (as shown in Fig. 5).^{11,35,55–57} The effective refraction index around the interface is thereby between the AlN layer and the substrates (or the templates). The light scattering at the interface reduces the TIR and increases the photons’ escape opportunity. We reported AlGaN-based 282-nm LEDs grown on nanopatterned sapphire substrates (NPSS), exhibiting 98% better performance relative to those grown flat sapphire substrate.¹¹ The internal quantum efficiency (IQE) enhancement is estimated to be 60% enhancement, so the LEE enhancement would be more than 20%. Lee et al.⁵⁶ fabricated DUV LEDs on NP-AlN/sapphire templates, with air surrounding the AlN nanorods. Light emitted from the multiple quantum wells can propagate vertically by passing through the embedded nanostructures and thus the TIR is avoided.

Fig. 5

A cross-sectional SEM image of overgrown AlN on NPSS.