Wafer-scale vertical injection III-nitride deep-ultraviolet light emitters

doi:10.21203/rs.3.rs-4527364/v1

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Wafer-scale vertical injection III-nitride deep-ultraviolet light emitters

https://doi.org/10.21203/rs.3.rs-4527364/v1

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published 30 Oct, 2024

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A ground-breaking roadmap of III-nitride solid-state deep-ultraviolet (DUV) light emitters is demonstrated to realize the wafer-scale fabrication of devices in vertical injection configuration, from 2 to 4 inches, and expectably larger. The epitaxial device structure is stacked on a GaN template instead of conventionally adopted AlN, where the primary concernof the tensile strain for Al-rich AlGaN on GaN is addressed via an innovative decoupling strategy, making the device structure decoupled from the underlying GaN template. Moreover, the strategy provides a protection cushion against the stress mutation during the removal of substrates. As such, large-sized DUV light-emitting diode (LED) wafers can be obtained without surface cracks, even after the removal of the sapphire substrates by laser lifted-off. Wafer-scale fabrication of 280 nm vertical injection DUV-LEDs is eventually exhibited, where a light output power of 65.2 mW is achieved at a current of 200 mA, largely thanks to the significant improvement of light extraction. This work will definitely speed up the application of III-nitride solid-state DUV light emitters featuring high performance and scalability.

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Inorganic LEDs

Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Inorganic LEDs

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 Al_xGa_1-xN (3.4–6.1 eV) can cover almost the entire UV spectral range, making it being of great interests and considerable technological importance as the most promising candidate for solid-state UV light emitters, e.g. light-emitting diodes (LEDs) and laser diodes (LDs)^2-5. However, it is still a great challenge to achieve high-performance AlGaN-based UV emitters to date, both in terms of the improvement of crystalline quality and the optimization of device structure. In favor of transparency and strain control, the stacking of the device epilayers, especially in the deep-UV (DUV) case with wavelength shorter than 300 nm, generally stems from AlN/sapphire templates^2-6. Compared with GaN, AlN epitaxy is significantly more difficult since Al adatoms have a much weaker surface diffusion⁷, and then the typical growth temperature for low-defect-density AlN has to be 1200–1300℃ or even higher, ~200℃ higher than that for GaN^8-10. Hence appropriative high-temperature metal-organic chemical vapor deposition (HT-MOCVD) systems are required, 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 2023, while only fewer than 2% being HT-MOCVD for DUV-LEDs.

Besides, present available DUV-LEDs in flip-chip (FC) configuration are suffering from low efficiency as well as poor thermal management, both of which can be attributed to the retaining of the sapphire substrates. On the one hand, the refractive index differences between air, sapphire, and Al(Ga)N directly restrict the light extraction efficiency (LEE) 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 encapsulation^11,12 as well as roughening/patterning the backside of sapphire^13-15, the issue at the internal AlN/sapphire interface with a critical angle of 52° can hardly be solved. Meanwhile, on the other hand, the low thermal conductivity of sapphire does not meet the demand of heat dissipation, limiting the typical current density to be around several ten A/cm²(e.g. 100 mA for the die with a size of 20×20 mil²). Severe performance degradation and lifetime shortening at higher current operation have been revealed, owing to the rapidly increased junction temperature¹⁶.

Vertical injection configuration is hence considered as a thorough solution for devices with preferable performance, where the substrate is removed to fundamentally overcome the TIR and heat dissipation issues^3,16. Relevant studies are mainly focused on the efficient and scalable removal of substrates, and a series of approaches have been proposed. Wherein laser lifted-off (LLO) attracts more attention, which has shown great commercial values in InGaN-based visible LEDs as well as micro-LEDs based on GaN templates^17-19. 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. 193 nm ArF excimer laser²⁰, is required for AlN thermal decomposition, which is rare and costly; worse still, the precipitated Al from AlN decomposition is rigid, resulting in significant fracturing/cracking of the lifted-off epilayers²¹. Although AlGaN^16,22 or AlN/AlGaN superlattices²³ 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. Some alternative approaches, e.g. chemical lift-off²⁴ and chemical/mechanical thinning^25,26, are hence adopted to remove the substrate. However, in comparison with LLO, these approaches can hardly meet the demand of mass production, in terms of productivity, yield and cost. In a word, there is still no feasible solutions for the mass fabrication of wafer-scale vertical injection DUV-LEDs.

In this work, we propose a ground-breaking roadmap of DUV-LEDs based on GaN/sapphire templates instead of AlN ones, leading to efficient LLO removal of sapphire as well as wafer-scale fabrication of vertical injection devices, from 2 to 4 inches, and expectably larger. The tensile strain and crystalline quality of the DUV-LED epitaxial structure are juggled via a matched decoupling strategy, which makes the DUV-LED structure decoupled from the underlying GaN template. Moreover, the decoupling structure provides a protection cushion against the thermal shock induced by laser irradiation, preventing the fracturing during LLO. 4-inch DUV-LED wafers are then obtained without surface cracks, even after the removal of sapphire by LLO (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 enhanced.

The primary concern in this roadmap 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 50–60% is adopted for n-AlGaN^27–30, suggesting ~ 1.2% in-plane lattice mismatch between AlGaN and GaN with a theoretical critical thickness of 30 nm for cracking³¹. Although the low-temperature Al(Ga)N interlayer allows the growth of crack-free AlGaN^32,33, it is seriously worried that the low-temperature-induced rough surface would deteriorate the quality of the subsequent DUV-LED structures, and hence be detrimental to the device performance^21,22. In order to juggle the strain and crystalline quality of DUV-LEDs, a decoupling strategy is proposed here, featuring the pre-crack and filling processes as shown in Fig. 1a (In-situ monitoring curve during MOCVD growth shown in Supplementary Fig. S1). Specifically, controlled pre-cracks are intentionally introduced through an AlGaN sacrificial layer on GaN templates, whose Al composition is as high as 80% to shallow down the 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 cracks, and thus recover the surface morphology. Meanwhile, as part of the optical propagation path, Al composition in the healing layer is reconciled to the emission wavelength, 65% here for 280 nm DUV-LEDs.

Surface morphology evolution of the decoupling structure, from pre-crack to filling, is characterized by atomic force microscopy (AFM) as shown in Figs. 1b and c. For the Al_0.8Ga_0.2N sacrificial layer (Fig. 1b), dense cracks are observed with a mean spacing of about 3 µm, which extend along the ${⟨\text{11}\stackrel{\text{-}}{\text{2}}\text{0}⟩}_{\text{AlGaN}}$ directions according to previous reports^34–36, consistent with the lower surface energy of $\left\{\text{1}\stackrel{\text{-}}{\text{1}}\text{00}\right\}$ cleavage planes against the $\left\{\text{11}\stackrel{\text{-}}{\text{2}}\text{0}\right\}$ ones. While after the growth of the Al_0.65Ga_0.35N 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 nearly strain-free as demonstrated by X-ray reciprocal space mapping (RSM) for the $\left(\stackrel{\text{-}}{\text{1}}\text{015}\right)$-plane reflection (Fig. 1d), mainly attributed to the plastic relaxation by pre-cracks. It is then convinced that the DUV-LED epitaxial structure in the present roadmap is almost fully decoupled from the underlying GaN template (Supplementary Fig. S3), by which visually 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 ${⟨\text{11}\stackrel{\text{-}}{\text{2}}\text{0}⟩}_{\text{AlGaN}}$), a “buried” V-shaped crack with clear outlines can be identified in Fig. 2a, where the actual critical thickness of Al_0.8Ga_0.2N on GaN is determined to be less than 80 nm (Supplementary Fig. S4). The pre-crack is then filled along with growth of the Al_0.65Ga_0.35N healing layer, and eventually a flat and sharp n-AlGaN/Al_0.65Ga_0.35N 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 approach³⁰, as enlarged in Figs. 2b and c, respectively. Specifically, the Al_0.65Ga_0.35N 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 (Figs. 2e and f, respectively). Evidently, higher Ga composition inside the pre-crack is observed in comparison with that in the surrounding Al_0.65Ga_0.35N healing layer, and the composition difference gradually decreases until the pre-crack is filled up at the thickness of 360 nm (Indicator II). In general, the healing of cracks is attributed to the atomic migration along the inclined sidewalls³⁷, it is hence convinced that there are more Ga atoms inside the filled pre-cracks since their diffusion length is much larger than Al ones. Further EDS line scanning across the pre-crack (Fig. 2g) demonstrates that the peak Ga composition inside the crack reaches 45%, transparent to the 280 nm emission light from the upper DUV-LED structure.

In addition to the tensile strain, potential quality degradation of DUV-LED structure is another essential issue in the novel roadmap, in particular the possible massive generation of threading dislocations (TDs) during the healing of pre-cracks. Cross-sectional TEM measurement is hence carried out as shown in Fig. 3a, where the TD density in the Al_0.65Ga_0.35N healing layer is roughly equal to that in the GaN template (details in Supplementary Fig. S5). Special attention should be paid to the pre-crack marked by the arrow (enlarged STEM image in Fig. 3b), where TDs nucleate at the sidewalls and extend up into the DUV-LED structure. Herein, a screw-type TD (labelled S) and an edge-type (labelled E) ones are identified according to the TEM measurements under two-beam conditions with $\text{g = }\left[\text{0002}\right]$and $\text{g =}\text{ }\left[\text{11}\stackrel{\text{-}}{\text{2}}\text{0}\right]\text{ }$ (Fig. 3c). 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 $\left(\text{0002}\right)$- and $\left(\text{1}\stackrel{\text{-}}{\text{1}}\text{02}\right)$-planes are measured here for the GaN template and Al_0.65Ga_0.35N healing layer, respectively, while only slight broadening is observed for Al_0.65Ga_0.35N. 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 Al_0.65Ga_0.35N (294/387 arcsec)³⁸, laying a solid foundation for the subsequent DUV-LED structure. 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 Al_0.65Ga_0.35N 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.

The TD density in the active region would directly determine the radiative recombination efficiency³⁹, which is a key factor in assessing device performance, and can be evaluated via the photoluminescence (PL) measurements. Figure 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 Al_0.37Ga_0.63N wells and 8 nm-thick Al_0.5Ga_0.5N barriers. The temperature-dependent PL is then performed from 10 K 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 internal quantum efficiency (IQE) of 70.9%, at the same level with those on AlN templates^39–42. In addition, the dependence of IQE on excitation power is investigated and shown in Fig. 3h. It is found that the IQE monotonically increases with excitation power from 37.2% (5 mW) to 70.9% (37 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) model^43,44. Since the IQE value doesn’t saturate here, even greater IQE can be expected under higher excitation power in PL measurements, or under higher injection current in DUV-LEDs⁴⁴.

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 (4-inch wafer in Fig. 4a, 2-inch one in Supplementary Fig. S6). It is worth mentioning here that there are two benefits by adopting the GaN templates instead of AlN: (i) the frequency-tripled Nd:YAG laser (355 nm) meets the requirement of removing the sapphire in this roadmap, 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 the 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 Al_0.8Ga_0.2N sacrificial 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. 4c^16,18,20,25, which is expected to improve the light extraction efficiency by favorable scattering geometries⁴⁵. The n-electrode is deposited as the final process, after windowing to the n-Al_0.55Ga_0.45N layer (Fig. 4d). It should be mentioned that it is still quite difficult to obtain Ohmic contact on the etched $\left[\text{000}\stackrel{\text{-}}{\text{1}}\right]$-plane of n-Al_0.55Ga_0.45N, leading to increased operating voltage as a consequence (Supplementary Fig. S9).

Wafer-scale fabrication of vertical injection DUV-LEDs is herein realized in both 2- and 4-inch wafers, and those with larger sizes are expectable as well, largely thanks to the decoupling structure. The 280 nm DUV-LED die with a size of 508×508 µm² 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. S10 and S11). It is convinced that 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. Meanwhile, the temperature distribution in vertical injection DUV-LEDs by infrared thermography is shown in the inset of Fig. 4f, where the die temperature is around 57.8℃ after an operation time of 5 mins at 100 mA, demonstrating better thermal management in the vertical injection devices than that in the flip-chip ones (59.3℃, Supplementary Fig. S11d). It should be noted that the poor n-contact as mentioned above inevitably results in more heat generation during operation, in other words, the die temperature could be further reduced with the issue of n-contact addressed in the vertical injection configuration. 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 roadmap of AlGaN-based DUV-LEDs stacked on GaN templates is demonstrated to realize the wafer-scale fabrication of devices in vertical injection configuration, from 2 to 4 inches, and even expectably larger. The primary concern of the tensile strain-induced cracks in Al-rich AlGaN on GaN is addressed via the decoupling structure consisting of the strain sacrificial layer and healing layer, making the DUV-LED structure decoupled from the underlying GaN. Moreover, the decoupling structure provides a protection cushion against the thermal shock induced by laser irradiation, preventing the fracturing during LLO. 2- and 4-inch DUV-LED wafers are thus obtained without surface cracks, even after the removal of the sapphire substrates by LLO (355 nm frequency-tripled Nd:YAG laser). In terms of the device performance, the DUV-LED structure is demonstrated to roughly inherit the crystalline quality from the GaN template, leading to an IQE of 70.9% in the active region. It is more important that the 280 nm vertical injection DUV-LEDs in this roadmap 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.

MOCVD Growth of DUV-LEDs. All samples in this study were grown by an Aixtron 1×4 in. (or 3×2 in.) close-coupled showerhead MOCVD system, and repeated by an AMEC Prismo HiT3 (4×4 or 19×2 in.) MOCVD system. A 4 µm-thick GaN template was first grown on the 4-inch sapphire substrate by the two-step method, followed by a 120 nm-thick Al_0.8Ga_0.2N sacrificial layer and a 540 nm-thick Al_0.65Ga_0.35N healing layer grown at 1075°C and 1095°C, respectively. Then, the DUV-LED structure is grown, including 1.1 µm-thick n-Al_0.55Ga_0.45N, 5-period Al_0.5Ga_0.5N/Al_0.37Ga_0.63N MQWs, a 10 nm-thick p-Al_0.8Ga_0.2N electron blocking layer (EBL), p-Al_0.63Ga_0.37N/Al_0.46Ga_0.54N 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-Al_0.55Ga_0.45N approximate to that on GaN.

Fabrication of vertical injection DUV-LEDs. Vertical injection DUV-LED devices were fabricated with a die size of 20×20 mil². 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 frequency-tripled Nd:YAG laser with the wavelength of 355 nm. 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 Al_0.8Ga_0.2N sacrificial layer by ICP, after which a heated KOH solution was used to roughen the exposed AlGaN surface. Eventually, the epilayer is partially etched to the n-Al_0.55Ga_0.45N 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) 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). The temperature distribution of the DUV-LED die was measured by a microscopic infrared thermography system (GMARG-A4, Gold Medal Analytical & Testing Group, China).

Competing interests

The authors declare no competing interests.

Author contributions

J.W., and F.X. conceived the experiments. J.W., C.J., J.L. and L.Z. grew the samples and performed relevant measurements. F.X., X.Y., N.T., X.W., W.G. and B.S. gave support in the measurements and analyses. C.J. and J.L. performed device fabrication under X.K. and Z.Q. supervision. J.W. wrote the manuscript with the assistance of F.X., W.G. and B.S. All authors discussed the results and commented on the manuscript.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2023YFB3609700 to F.X.), the National Natural Science Foundation of China (62234001 and 61927806 to B.S.; 62135013 to F.X.; 62374007 to J.W.; 62204005 to J.L.).

Data availability

All data are available in the main text or the Supplementary Information. Data are available from the corresponding author upon reasonable request.

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Wafer-scale vertical injection III-nitride deep-ultraviolet light emitters

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