Introduction

As of 2023, the total market for ultraviolet (UV) light source is expected to exceed US$ 2 B with a compound annual growth rate (CAGR) of 17.8%, which is mostly driven by UV curing, purification, and disinfection1. Wherein, solid-state UV emitters are the most attractive and fastest growing sector, largely thanks to the outstanding advantages of long lifetime, energy saving, and environmental friendly in comparison with the traditional mercury UV lamps. As a direct bandgap semiconductor, the bandgap of AlxGa1-xN (3.4–6.1 eV) covers almost the entire UV spectral range, making it the most promising candidate for solid-state UV light emitters, e.g., light-emitting diodes (LEDs) and laser diodes (LDs)2,3,4,5. Benefiting from the improvement of crystalline quality and the optimization of device structure, great progress of the device performance has been made in the past decade, where the peak wall-plug efficiency (WPE) and external quantum efficiency (EQE) have reached 15.3%6 and 20.3%7 for 275 nm deep-UV (DUV) LEDs, respectively. DUV-LEDs are typically grown from AlN, either on sapphire6,7,8, SiC9,10 or AlN substrates11,12,13; wherein sapphire is the most common one in consideration of cost and UV-transparency. Compared with GaN, AlN epitaxy is much more difficult since Al adatoms have a much weaker surface diffusion14, and then the typical growth temperature for low-defect-density AlN has to be 1200–1300 °C or even higher15,16,17, ~200 °C higher than that for GaN. Although the high-temperature process can be partially outsourced by alternative AlN template technologies (e.g., ex-situ annealing18), appropriative high-temperature metal-organic chemical vapor deposition (HT-MOCVD) systems are generally adopted19,20, which undoubtedly raises the bar and cost for the research and production of DUV emitters. Taking China as an example, statistics show that there are more than 2500 MOCVD systems in 202321, while only fewer than 2% being HT-MOCVD for UV-LEDs.

Besides, DUV-LEDs are suffering from low light extraction efficiency (LEE) with a typical value of single-digit percent (6–8%) if without addition improvement features3,22. Main challenges include the strong internal absorption and the narrow escape cone of DUV light. The former is attributed to the adoption of p-GaN as the p-cladding and contact layer, owing to the extremely inefficient p-doping in Al-rich AlGaN. The combination of DUV-transparent p-region (e.g., a thin p-GaN or even p-AlGaN contact layer) and highly-reflective p-electrode is hence proposed to improve the light extraction efficiency6,7. While in terms of the escape cone, the present available flip-chip (FC) configuration retains the sapphire substrates, where the refractive index differences between air, sapphire, and Al(Ga)N directly restrict the light extraction as a consequence of the total internal reflection (TIR). The bottom (sapphire side) escape cone of light is then limited to 26°, resulting in only 5% LEE for isotropic emission5. Although TIR at the sapphire/air interface can be effectively reduced by encapsulation23,24 as well as roughening/patterning the backside of sapphire25,26,27, the issue at the internal AlN/sapphire interface with a critical angle of 52° can hardly be solved. The voids during AlN epitaxial lateral overgrowth (ELO), e.g., AlN on nano-patterned sapphire substrates (NPSSs), are expected to redirect more light into the escape cone7,28, and further studies of a trade-off between AlN crystal quality and light extraction need to be performed. In general, the record LEE in DUV-LEDs is ~39% to date7, assuming the radiative recombination efficiency and carrier injection efficiency being 65% and 80%, respectively29. There is still enormous room for the improvement of LEE.

Thin-film configuration is then considered as a thorough solution for DUV-LEDs with preferable performance, where the substrate is removed to fundamentally overcome the TIR issue3. Relevant studies are mainly focused on the efficient and scalable removal of substrates, and a series of approaches have been proposed, including laser lift-off (LLO)30,31,32,33,34, chemical lift-off35 and chemical/mechanical thinning10,36. Besides, the mechanical transfer based on the h-BN37,38 or graphene39 release layer shows great potential in the fabrication of thin-film devices, and the quality improvement of remote (or van der Waals) epitaxial AlN and AlGaN for DUV-LEDs is ongoing40,41. Among these approaches, LLO can comprehensively meet the demands of mass production, in terms of productivity, yield and cost, making it the most promising candidate in the fabrication of LEDs, e.g., InGaN-based visible LEDs as well as micro-LEDs42,43,44. However, it is fairly hard to apply LLO in DUV-LEDs based on AlN/sapphire templates. First, a high power and short wavelength laser, e.g., a 193 nm ArF excimer laser30, is required for AlN thermal decomposition, which is rare and costly; worse still, the precipitated Al from AlN decomposition is rigid, resulting in severely fracturing/cracking of the lifted-off epilayers31. Although AlGaN32,33 or AlN/AlGaN superlattices34 can serve as a sacrificial layer for LLO with longer wavelength lasers (248 nm KrF excimer laser, etc.), the issue of fracturing/cracking is insoluble since the precipitation of Al is unavoidable within these strategies.

In this work, we propose a ground-breaking strategy of DUV-LEDs based on GaN/sapphire templates instead of AlN ones, leading to efficient LLO removal of sapphire as well as wafer-scale (2 and 4 inches) fabrication of vertical injection devices. The tensile strain and crystalline quality of the DUV-LED epitaxial structure are juggled via a matched decoupling technology, which makes the DUV-LED structure decoupled from the underlying GaN template. Moreover, the decoupling provides a protection cushion against the thermal shock induced by laser irradiation, preventing the fracturing during LLO. DUV-LED wafers are then obtained without surface cracks, even after the removal of sapphire by LLO with a 355 nm frequency-tripled Nd:YAG laser. As such, 280 nm vertical injection devices are mass fabricated, and the performance is demonstrated to be significantly improved.

Results

The primary concern in this strategy of vertical injection DUV-LEDs is the tensile strain-induced cracks for Al-rich AlGaN on GaN templates, which severely disrupt the device fabrication. Taking 280 nm DUV-LEDs as an example, a typical Al composition range of 55%–70% is adopted for n-AlGaN7,45,46, suggesting greater than 1.2% in-plane lattice mismatch between AlGaN and GaN with a theoretical critical thickness of 30 nm for cracking47. Although the low-temperature Al(Ga)N interlayer allows the preparation of crack-free AlGaN48,49, a serious concern is that the generation of threading dislocations (TDs) during the low-temperature process will be detrimental to the device performance. Besides, an approach of AlGaN hetero-ELO on facet-controlled GaN templates is demonstrated to obtain crack-free wafers with Al composition up to 30%50,51, while further studies are still needed to meet the severe demand of DUV-LEDs with higher Al composition.

In order to juggle the strain and crystalline quality of DUV-LEDs on GaN templates, a novel decoupling technology is proposed, featuring the controlled inducing and filling processes of cracks as shown in Fig. 1a (In situ monitoring curve during MOCVD growth shown in Supplementary Fig. S1). In order to distinguish with the potential surface cracks in the DUV-LED wafer, these cracks are hereinafter referred as pre-cracks, since they are intentionally induced and buried inside the epitaxial structure. Here an AlGaN pre-crack layer is grown on the GaN template, whose Al composition is as high as 80% to shallow down the pre-cracks within a depth of ~100 nm (details shown in Fig. 2), laying a solid foundation for the following filling process (Supplementary Fig. S2). Subsequently, an AlGaN healing layer is employed to fill up the pre-cracks, and thus recover a smooth surface morphology. Considering that this layer should be transparent for light extraction, an Al composition of 65% is adopted here for 280 nm DUV-LEDs.

Fig. 1: DUV-LEDs based on GaN templates.
Fig. 1: DUV-LEDs based on GaN templates.
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a Schematic illustration of DUV-LEDs on GaN/sapphire templates. b, c Surface morphology evolution (AFM) of the decoupling structure, from pre-crack to filling-up. d X-ray reciprocal space mapping for the \(\left(\bar{1}015\right)\) reflection of the Al0.8Ga0.2N pre-crack and Al0.65Ga0.35N healing layer. e Photograph of the 4 inch surface crack-free DUV-LED wafer based on GaN template. f Surface crack mapping of the 4 inch DUV-LED wafer. EE edge exclusion.

Fig. 2: Characteristics of the decoupling structure.
Fig. 2: Characteristics of the decoupling structure.
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a Cross-sectional bright-field STEM image of the filled-up pre-crack. b, c Enlargement of the indicator lines at the filling thickness of 360 nm and 180 nm, respectively. d Dependence of the filling-up thickness on Al composition in the healing layer. e, f Distribution of Al and Ga atoms (EDS mapping) around the pre-crack in (a). g Compositional variation along the EDS scanning line (inset) across the filled-up pre-crack.

Surface morphology evolution of the decoupling structure, from pre-crack to filling, is characterized by atomic force microscopy (AFM) as shown in Fig. 1b, c. For the Al0.8Ga0.2N pre-crack layer (Fig. 1b), dense cracks are observed with a mean spacing of about 3 μm, which extend along the \({\left\langle 11\bar{2}0\right\rangle }_{{{\rm{AlGaN}}}}\) directions according to previous reports52,53,54, consistent with the lower surface energy of \(\left\{1\bar{1}00\right\}\) cleavage planes against the \(\left\{11\bar{2}0\right\}\) ones. While after the growth of the Al0.65Ga0.35N healing layer (Fig. 1c), no cracks can be identified any more, and the surface is recovered to the typical step-terrace morphology with a root-mean-square (RMS) roughness of 0.51 nm (10 × 10 μm2). Worthy of note is that the healing layer is pseudomorphically grown on the Al0.8Ga0.2N pre-crack layer, meanwhile achieves an approximately complete strain relaxation as demonstrated by X-ray reciprocal space mapping (RSM) for the \(\left(\bar{1}015\right)\)-plane reflection (Fig. 1d). It is then convinced that the DUV-LED epitaxial structure in the present strategy is almost fully decoupled from the underlying GaN template (Supplementary Fig. S3), by which visually surface crack-free DUV-LED wafers can be obtained, typically as the 4 inch one shown in Fig. 1e. Further optical measurements (Fig. 1f) demonstrate that no surface cracks are identified except the edge exclusion region (EE, 3 mm), successfully driving the fabrication of DUV-LEDs into high-production 4 inch era.

Moreover, cross-sectional scanning transmission electron microscopy (STEM) is employed to reveal the healing of pre-cracks. Benefiting from the parallel direction between the incident-electron and crack extension (both along \({\left\langle 11\bar{2}0\right\rangle }_{{{\rm{AlGaN}}}}\)), a buried V-shaped pre-crack with clear outlines can be identified in Fig. 2a, where the actual critical thickness of Al0.8Ga0.2N on GaN is determined to be less than 80 nm (Supplementary Fig. S4). The pre-crack is then filled along with growth of the Al0.65Ga0.35N healing layer, and eventually a flat and sharp n-AlGaN/Al0.65Ga0.35N interface is presented, consistent with the crack-free surface morphology in Fig. 1c. It is worth mentioning that two indicator lines are buried into the healing layer through the in situ desorption-tailoring approach46, as enlarged in Fig. 2b, c, respectively. Specifically, the Al0.65Ga0.35N growth is intentionally suspended every 180 nm by stopping the precursor (TMAl and TMGa) supply, and then the desorption difference between Al and Ga atoms on the surface as well as on the inclined sidewalls of cracks (if exist) leads to higher Al composition with shallow contrast in the bright-field STEM image, directly indicating the filling degree of pre-cracks. At the filling thickness of 180 nm, there is an obvious bending (white arrows) in Indicator I (Fig. 2c), which corresponds to the crack sidewalls and hence demonstrates the existence of cracks. While increasing the thickness to 360 nm, Indicator II becomes straight and coherent, suggesting that the crack has almost been filled up.

As characterized by the indicator lines (STEM) and AFM, the thickness needed to fill the pre-cracks is found to be directly related to the Al composition of the healing layer. The higher the Al composition, the greater the thickness is required as shown in Fig. 2c, suggesting that Ga atoms play a key role in the filling process. Energy-dispersive X-ray spectroscopy (EDS) mapping is then adopted to reveal the atomic behavior of Al and Ga (Fig. 2e, f, respectively). Evidently, higher Ga composition inside the pre-crack is observed in comparison with that in the surrounding Al0.65Ga0.35N healing layer, and the composition difference gradually decreases until the pre-crack is filled up at the thickness of 360 nm (Indicator II). The healing of cracks is generally attributed to the atomic migration along the inclined sidewalls55, as a consequence, there are more Ga atoms inside the filled pre-cracks since their diffusion length is much longer than Al ones. Further EDS line scanning across the pre-crack (Fig. 2g) demonstrates that the peak Ga composition inside the crack reaches 45% (Al composition 55%), transparent to the 280 nm emission light from the upper DUV-LED structure.

In addition to the tensile strain, the potential quality degradation of DUV-LED structure is another essential issue in the novel strategy, in particular the possible massive generation of TDs during the healing of pre-cracks. Cross-sectional dark-field TEM is hence carried out under two-beam conditions with g = [0002] and g = [11\(\bar{2}\)0], as shown in Fig. 3a, b, respectively. The TD density in the Al0.65Ga0.35N healing layer (as well as n-AlGaN) is found to be slightly greater than that in the GaN template. Special attention should be paid to the pre-crack marked by the arrow (enlarged images in Fig. 3c), where a screw-type TD (labeled S) and an edge-type (labeled E) one are identified, attributed to the filling process. In consideration of the quite small surface coverage of the pre-cracks (Fig. 1b), it is convinced that the filling-induced TDs have less effect on the total TD density, as further verified by the X-ray rocking curves (XRCs) in Fig. 3d. XRCs of the \(\left(0002\right)\)- and \(\left(1\bar{1}02\right)\)-planes are measured here for the GaN template and Al0.65Ga0.35N healing layer, respectively, while only slight broadening is observed for Al0.65Ga0.35N. The full width at half maximum (FWHM) values are then extracted, and the corresponding TD density is calculated to be 1.35 × 109 cm−2 in Al0.65Ga0.35N (294/387 arcsec)56, comparable with the reported values of AlGaN on AlN/sapphire templates57,58,59, whereas even lower values were demonstrated as well. Furthermore, the TD density in the active region of DUV-LED is estimated as 1.18 × 109 cm−2 in the plan-view STEM image (Fig. 3e), being approximate to that in the Al0.65Ga0.35N healing layer. As a feature, some of TDs are distributed along the straight dashed lines in Fig. 3e, suggesting that they originate from the filling process of pre-cracks.

Fig. 3: Crystalline quality and radiative recombination efficiency of DUV-LED structure on GaN templates.
Fig. 3: Crystalline quality and radiative recombination efficiency of DUV-LED structure on GaN templates.
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a, b Cross-sectional dark-field TEM images under two-beam conditions with g = [0002] and \({{{\rm{g}}}}=[11\bar{2}0]\), respectively. c Enlarged dark-field TEM images for the pre-crack. The label S and E correspond to the screw- and edge-type TD, respectively. d (0002) and \(\left(1\bar{1}02\right)\)-plane X-ray rocking curves of GaN template and AlGaN healing layer, respectively. e Plan-view bright-field STEM image of the active region (MQWs) in the DUV-LED structure. f Cross-sectional HAADF-STEM image of the active region (MQWs) in the DUV-LED structure. g Temperature-dependent PL spectra of MQWs in (f). h Dependence of the radiative recombination efficiency on the excitation power.

The TD density in the active region would directly determine the radiative recombination efficiency (RRE)60, which is a key factor in assessing device performance, and can be evaluated via the photoluminescence (PL) measurements. Fig. 3f shows the high-angle annular dark-field (HAADF) STEM image for the multiple quantum wells (MQWs) active region assembled by 1.8 nm-thick Al0.37Ga0.63N wells and 8 nm-thick Al0.5Ga0.5N barriers. The temperature-dependent PL (a 213 nm laser as the excitation source) is then performed from 10 to 300 K (Fig. 3g), where the emission peak red-shifts with rising temperature and reaches 280 nm at room temperature. Assuming the non-radiative recombination centers frozen at 10 K, the MQWs exhibit a room-temperature RRE of 70.9%, at the same level with those on AlN templates20,61,62. In addition, temperature-dependent PL measurements under resonant excitation63 were carried out utilizing a 266 nm laser, where a similar RRE value is obtained (Supplementary Fig. S5). The dependence of RRE on excitation power (213 nm laser) is additionally investigated and shown in Fig. 3h. It is found that the RRE monotonically increases with excitation power from 37.2% (1.6 mW) to 70.9% (11 mW), suggesting that the dominant recombination process gradually changes from the non-radiative recombination to the radiative one according to the Shockley-Read-Hall (SRH) model64,65. Since the RRE value does not saturate here, even greater radiative efficiency can be expected under higher excitation power in PL measurements, or under higher injection current in DUV-LEDs.

With the above two issues being solved, wafer-scale vertical injection DUV-LEDs with a wavelength of 280 nm are eventually fabricated as shown in Fig. 4. After preparation of the p-electrode, the epitaxial structure is crack-freely transferred from the sapphire substrate to a Si submount by means of wafer bonding and subsequent LLO (a 4 inch wafer in Fig. 4a, 2 inch one in Supplementary Fig. S6). It is worth mentioning that there are two benefits by adopting the GaN templates instead of AlN: (i) a 355 nm frequency-tripled Nd:YAG laser meets the requirement of removing the sapphire substrate in this strategy, which is low-cost and widely used; (ii) the absence of Al metal during LLO effectively avoids the occurrence of fracturing/cracking, ensuring the high yield of dies. Moreover, the decoupling strategy is supposed to provide a protection cushion against the thermal shock induced by laser irradiation, equally preventing fracturing during LLO. Following the removal of sapphire, the GaN template must be sufficiently thinned by chlorine-based inductively coupled plasma (ICP) etching till the Al0.8Ga0.2N pre-crack layer is exposed, considering that GaN strongly absorbs the DUV emission light from the active region (Fig. 4b, details in Supplementary Fig. S8). Subsequently, KOH roughening is carried out to obtain the surface morphology of random hexagonal pyramids texture as shown in Fig. 4c10,30,32,43, which is expected to improve the light extraction efficiency by favorable scattering geometries66. The n-electrode is deposited as the final process, after windowing to the n-Al0.55Ga0.45N layer (Fig. 4d). Typical I-V curve of the vertical injection die is shown in Supplementary Fig. S9, and further studies are still needed to reduce the operating voltage, in particular realizing Ohmic contacts with low specific contact resistivity on the etched \(\left(000\bar{1}\right)\)-plane of n-AlGaN.

Fig. 4: Fabrication and performance of 280 nm vertical injection DUV-LEDs based on GaN templates.
Fig. 4: Fabrication and performance of 280 nm vertical injection DUV-LEDs based on GaN templates.
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a Photograph of a 4 inch surface crack-free DUV-LED wafer after removal of the sapphire substrate by laser lift-off. b, c Surface morphology (SEM) after GaN etching and surface roughening, respectively. d Top-view optical microscopy image of the vertical injection DUV-LED die. e EL spectrum of DUV-LEDs operated at 100 mA. f Dependence of the light output power and external quantum efficiency on the injection current for DUV-LEDs. g Full far-field radiation pattern of vertical injection DUV-LEDs.

Wafer-scale fabrication of vertical injection DUV-LEDs is herein realized in both 2 and 4 inch wafers, largely thanks to the decoupling structure. The 280 nm DUV-LED die with a size of 0.5 × 0.5 mm2 presents a light output power of 38.4 and 65.2 mW at 100 and 200 mA (Fig. 4f), respectively, much higher than the conventional flip-chip device with the same DUV-LED structure on AlN/sapphire template (23.6 and 42.1 mW at 100 and 200 mA, respectively, Supplementary Fig. S11). The improvement mainly benefits from the enhancement of the light extraction efficiency, owing to the surface roughening as well as the elimination of the total internal reflection at the epi/substrate interface. This leads to a peak external quantum efficiency of 9.63% at 20 mA, one of the highest values reported to date3. Moreover, the full far-field radiation pattern (Fig. 4g) is measured at variable emission angle θ and azimuthal angle φ, where θ = 0° and 90° correspond to the vertical and horizontal emission, respectively. A Lambertian radiation pattern is observed and the on-axis intensity is significantly enhanced in comparison with that in the flip-chip configuration devices (Supplementary Fig. S12), consistent with the result of the light output power.

In summary, a ground-breaking strategy of AlGaN-based DUV-LEDs stacked on GaN templates is demonstrated to realize the wafer-scale fabrication of devices in vertical injection configuration. The primary concern of the tensile strain-induced cracks in Al-rich AlGaN on GaN is addressed via the decoupling structure consisting of the pre-crack and healing layer, making the DUV-LED structure decoupled from the underlying GaN, and thus bringing about 2 and 4 inch DUV-LED wafers without surface cracks, even after the removal of the sapphire substrates by LLO with a 355 nm frequency-tripled Nd:YAG laser. Meanwhile, the DUV-LED structure is demonstrated to roughly inherit the crystalline quality from the GaN template, leading to a radiative recombination efficiency of 70.9% in the active region. It is more important that the 280 nm vertical injection DUV-LEDs in this strategy exhibit a significant performance improvement, whose LOP reaches 65.2 mW at a current of 200 mA, largely thanks to the essential improvement of light extraction. This work will definitely speed up the application of DUV-LEDs featuring high performance and scalability. What’s more, beneficial from the substitution of AlN templates by GaN ones, ordinary MOCVD systems as well as mature LLO process for InGaN-based visible LEDs can be conveniently employed in the fabrication of DUV-LEDs, greatly promoting the development of this field.

Methods

MOCVD growth of DUV-LEDs

All samples in this study were grown by an Aixtron 1 × 4 inch (or 3 × 2 inch) close-coupled showerhead MOCVD system, and repeated by an AMEC Prismo HiT3 (4 × 4 or 19 × 2 inch) MOCVD system. A 4 μm-thick GaN template was first grown on the sapphire substrate by the two-step method, followed by a 120 nm-thick Al0.8Ga0.2N pre-crack layer and a 540 nm-thick Al0.65Ga0.35N healing layer grown at 1075 °C and 1095 °C, respectively. Then, the DUV-LED structure is grown, including 1.1 μm-thick n-Al0.55Ga0.45N, 5-period Al0.5Ga0.5N/Al0.37Ga0.63N MQWs, a 10 nm-thick p-Al0.8Ga0.2N electron blocking layer (EBL), p-Al0.63Ga0.37N/Al0.46Ga0.54N superlattices, and a 6 nm-thick p-GaN contact layer in sequence.

As a reference device, the same DUV-LED structure was grown on the AlN template for the flip-chip configuration, where the TD density in the AlN template was estimated as 9.8 × 108 cm−2, making the crystalline quality of upper n-Al0.55Ga0.45N approximate to that on GaN.

Fabrication of vertical injection DUV-LEDs

Vertical injection DUV-LED devices were fabricated with a die size of 0.5 × 0.5 mm2. Ni/Au/Rh metal stack was first deposited as the p-electrode. Then, the wafer was bonded to a Si submount using metallization bonding technology, followed by the laser lift-off process by employing a 355 nm frequency-tripled Nd:YAG laser. After the removal of sapphire, the exposed N-face of GaN was cleaned in the HCl solution for 1 min to remove the Ga metal from GaN decomposition. The residual GaN layer was then etched down to the Al0.8Ga0.2N pre-crack layer by ICP, after which a heated KOH solution was used to roughen the exposed AlGaN surface. Eventually, the epilayer is etched to the n-Al0.55Ga0.45N layer, where Ti/Al/Ni/Au was deposited as the n-electrode.

Characterization

The cross-sectional/plan-view TEM, STEM, and EDS were imaged with a Thermo Scientific Themis Z STEM operated at 200 kV, while the corresponding specimens were prepared by FIB (Thermo Scientific Helios G4 HX Dual Beam). AFM (Bruker Dimension Icon), XRD (Panalytical X’Pert3 MRD), SEM (Nova NanoSEM 430), and surface crack mappings (AK Optics E1000) were carried out. Temperature-dependent and excitation-dependent PL were characterized by the homemade system at Peking University, where a 213 nm laser (Xiton Photonics Impress 213) was employed as the excitation source. Temperature-dependent measurements were performed by employing a closed-cycle helium cryostat (JANIS SVT-400) attached to the temperature controller (Scientific Instruments 9700). The LOP and far-field radiation pattern of DUV-LEDs were measured by an integrating sphere for UV light (Everfine Haas-2000-UV).