Highly efficient AlGaN-based deep-ultraviolet light-emitting diodes: from bandgap engineering to device craft

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) spectral range (210–280 nm) have demonstrated potential applications in physical sterilization. However, the poor external quantum efficiency (EQE) hinders further advances in the emission performance of AlGaN-based DUV LEDs. Here, we demonstrate the performance of 270-nm AlGaN-based DUV LEDs beyond the state-of-the-art by exploiting the innovative combination of bandgap engineering and device craft. By adopting tailored multiple quantum wells (MQWs), a reflective Al reflector, a low-optical-loss tunneling junction (TJ) and a dielectric SiO2 insertion structure (IS-SiO2), outstanding light output powers (LOPs) of 140.1 mW are achieved in our DUV LEDs at 850 mA. The EQEs of our DUV LEDs are 4.5 times greater than those of their conventional counterparts. This comprehensive approach overcomes the major difficulties commonly faced in the pursuit of high-performance AlGaN-based DUV LEDs, such as strong quantum-confined Stark effect (QCSE), severe optical absorption in the p-electrode/ohmic contact layer and poor transverse magnetic (TM)-polarized light extraction. Furthermore, the on-wafer electroluminescence characterization validated the scalability of our DUV LEDs to larger production scales. Our work is promising for the development of highly efficient AlGaN-based DUV LEDs.


Introduction
Deep-ultraviolet (DUV) germicidal irradiation is a chemical-free, species-agnostic disinfection treatment because DUV light with a wavelength between 200 and 280 nm can achieve sterilization by damaging the deoxyribonucleic/ribonucleic acid of pathogens [1][2][3] .Compared with conventional mercury lamps, AlGaN-based DUV light-emitting diodes (LEDs) have a longer lifetime, greater stability, more environmentally friendly manufacturing and more flexible operation; thus, these LEDs have attracted much academic and commercial interest as the most competitive candidates for physical disinfection in recent years [4][5][6] .However, many technical challenges hinder the further development of AlGaN-based DUV LEDs and include poor external quantum efficiency (EQE), poor light extraction efficiency (LEE), strong quantum-confined Stark effect (QCSE) and large DUV light absorption due to the p-GaN ohmic contact layer and opaque p-electrode 7 .Among these issues, the course of achieving high-performance DUV LEDs is accompanied by strong QCSE and poor LEE.
In Al x Ga 1-x N/Al y Ga 1-y N multiple quantum wells (MQWs), lattice and thermal mismatches occur between the Al x Ga 1-x N quantum well (QW) and Al y Ga 1-y N quantum barrier (QB).Therefore, the Al x Ga 1-x N QWs suffer from in-plane compressive stress, leading to a large piezoelectric polarization field between the QW and QB and tilting of the energy band of the MQW.This phenomenon is called QCSE 8 .The QCSE can suppress carrier radiative recombination by separating holes and electrons and reducing carrier wavefunction overlap, thereby decreasing the internal quantum efficiency (IQE).In addition, the topmost valence sub-band near the Γ point is the crystal-field splitting band because of the spin-orbital and crystal-field splitting effects [9][10][11] , thus leading to laterally propagating transverse magnetic (TM)-polarized light dominating the high-Al-content Al x Ga 1-x N/Al y Ga 1-y N MQWs.To date, numerous studies have been performed to resolve these problems; these studies include employing SiO 2 -antireflection films 12 , modifying electrodes [13][14][15] , regulating polarization electric fields [16][17][18] , utilizing plasmonic structures or photonic crystals [19][20][21] , using high-quality templates [22][23][24][25][26] , and applying micro-/nano-LED geometries [27][28][29][30] .Nevertheless, a comprehensive solution to inhibit the QCSE and simultaneously increase the LEE of DUV LEDs is still lacking.
In this study, we merged bandgap engineering with device craft to systematically improve the emission performance of an AlGaN-based DUV LED at an emission wavelength of ~270 nm.First, a tailored QW structure is proposed to neutralize the QCSE and improve the electron-hole wavefunction overlap, thereby enhancing the IQE of the DUV LED.Then, the low-optical-loss structure, composed of an AlGaN tunneling junction (TJ) and a reflective p-electrode, is introduced into the DUV LED to suppress DUV light absorption.Furthermore, the SiO 2 insertion structure (IS-SiO 2 ), which not only increases the effective current injected into the active region but also manipulates light transmission to enhance the emission of the top facet, is embedded into the DUV LED to enhance the LEE.Our study establishes a universal strategy for efficient DUV LEDs on sapphire, with the aim to develop high-quality lighting sources.

Epitaxial growth of DUV LED structure
The DUV LEDs were all grown on a c-plane sapphire substrate by using a metal-organic chemical vapor deposition system (Promaxy UV).The group-III element sources used in the experiment included trimethylgallium (TMGa) and trimethylaluminum (TMAl).High-purity ammonia (NH 3 ) was used as the group-V element source.The dopants utilized to grow n-type and p-type layers are silane (SiH 4 ) and bis(cyclopentadienyl)magnesium (Cp 2 Mg), respectively.Hydrogen (H 2 ) served as a carrier gas to grow AlGaN, while H 2 and nitrogen (N 2 ) served as carrier gases to grow p-GaN.Figure 1a shows  The cross-sectional scanning electron microscopy (SEM) image of the epitaxial wafer and the transmission electron microscopy (TEM) image of the related MQW are shown in Fig. S1a, b (Supporting Information).The thicknesses of the QW and QB are 2.3 and 3.7 nm, respectively.Using an as-grown DUV wafer, we fabricated DUV LEDs via standard LED fabrication processes.The optical microscope image of a lighted DUV LED is shown in Fig. S1c (Supporting Information).The emission performance of the packaged DUV LED on the printed circuit board is shown in Fig. S1d (Supporting Information).

Results and discussion
Tailoring of the MQW structure and its application in DUV LEDs High-resolution X-ray diffraction rocking curve measurements were performed to characterize the crystalline quality of the DUV wafers, as shown in Fig. S2a, b (Supporting Information).The full width at half-maximums (FWHMs) of the (002) and (102) reflection peaks are related to the dislocation density 31 .The FWHMs of the (002)/(102) reflection peaks for Structures A, B and C are 260/460, 260/460 and 280/480 arcsec, respectively.The functional relationship between the dislocation density and FWHM can be plotted by using Equation S1 32 in Supplementary Note I.The estimated dislocation densities for Structures A, B and C are 4.36 × 10 9 , 4.36 × 10 9 and 5.19 × 10 9 cm −2 , respectively.Clearly, tailoring the QW structure by varying the Al content only exerts an inconsequential influence on the crystalline quality of the epitaxial wafer.
The strain states of Structures A, B and C were studied by room-temperature Raman scattering, and the results are shown in Fig. S3 (Supporting Information).For Structure A, the phonon modes near 599.8 cm −1 and 658.7 cm −1 correspond to the E H 2 (GaN-like) mode and the E H 2 (AlN-like) mode, respectively; this result confirms the presence of Al 0.6 Ga 0.4 N 33,34  To investigate the optical and electrical properties of DUV LEDs with different QW structures, electroluminescence (EL) and light output power (LOP)-current-voltage (L-I-V) characteristics for DUV LED with a size of 10 × 20 mil 2 were measured via an integrating sphere using a semiconductor parameter analyzer (Keithley 4200) at room temperature.The EL spectra of Structures A, B and C at various injection currents are shown in Fig. 2a-c in Supplementary Note I.The maximum EQE for Structure B and Structure C are 1.6% and 2.5%, respectively; these values are 1.9 times and 2.8 times greater than those of Structure A.
The underlying physical mechanism of the improved emission performance for Structures B and C was elucidated via SiLENSe 5.14 (STR Software Inc.); this software can be used to calculate the energy band structure and carrier wavefunctions in the active region.In the simulation, the offset ratio between the conduction band and the valence band is set to 7:3 35 .The electron and hole mobilities are 100 and 10 cm 2 V −1 s −1 , respectively 36 .The Poisson equation, drift-diffusion transport equations, Fermi-Dirac statistics, and Schrödinger equations are used during the simulation.The Auger recombination coefficient, radiative recombination coefficient and Shockley-Read-Hall (SRH) recombination coefficient are set as 2.3 × 10 30 cm 6 s −1 , 2×10 11 cm 3 s −1 , and 1.1 × 10 7 s −1 , respectively [37][38][39] .Figure 2g-i

Employment of IS-SiO 2 and its application in DUV LEDs
The p-GaN contact layer, together with the Ni/Au p-electrode, can strongly absorb the DUV photons emitted from the active region 40 , which can remarkably Figure 3c shows the measured I-V characteristics of the LED samples at room temperature.The tested DUV LEDs have almost the same turn-on voltage of ~5 V, indicating good consistency for the growth of DUV LEDs.As the voltage increases from 5 V, the tested DUV LEDs also show very similar I-V characteristics.Thus, IS-SiO 2 will not increase the sheet resistance of the DUV LEDs, and the introduction of IS-SiO 2 has a negligible influence on the electrical properties of the device.Figure 3d shows the extracted LOPs of the LED III and LED IV samples at 850 mA.The optimal sizes for both cuboid and columnar IS-SiO 2 are found.For columnar IS-SiO 2 , the optimal radius is 11 μm.For cuboid IS-SiO 2 , the optimal width is 11 μm. Figure 3e shows the dependence of the LOPs on the injection currents for the LED IV-R11, LED IV-W11, and LED III samples.The LED III samples are used as the reference devices.The maximum LOP of the LED IV-W11 sample reaches 140.1 mW at 850 mA; this value is 4.3 and 12.8 mW greater than that of the LED IV-R11 and LED III samples, respectively.The peak EQEs of LED IV-W11, LED IV-R11, and LED III are determined to be 6.9%, 6.5% and 6.0%, respectively, by using Equation S3 in Supplementary Note I. Figure 3f shows the relationship between the reverse leakage current and voltage of the three DUV LEDs.At −5 V, the reverse leakage current is measured as 2.2 × 10 −10 , 4.5 × 10 −11 and 2.3 × 10 −12 A for LED III, LED IV-R11, and LED IV-W11, respectively.The order of magnitude decrease in the reverse leakage current can be attributed to the screening of the channels for the leakage current originating from the introduction of the dielectric IS-SiO 2 .In addition, the screening effect of the continuous IS-SiO 2 cuboid is clearly much stronger than that of the discrete IS-SiO 2 cylinder.The LOP-I characteristics of LED IV with cuboid and columnar IS-SiO 2 are shown in Fig. S5 (Supporting Information).A comparison of the emission performance of our DUV LEDs with that of previously reported DUV LEDs is shown in Table S1 and Fig. S6 (Supporting Information).The wafer-scale emission performances of DUV LEDs with cuboid and columnar IS-SiO 2 are discussed in Supplementary Note IV.
The morphological characteristics of the DUV LEDs with a size of 30 × 30 mil 2 containing IS-SiO 2 are shown in Fig. 4. Figure 4a-c shows the top-view SEM images of LED III, LED IV-R11 and LED IV-W11.The LED samples with columnar and cuboid IS-SiO 2 were sprayed with gold for FIB-SEM measurements.The tilt angle is 49°for the FIB-SEM measurements.Figure 4d-f shows the FIB-SEM images of LED IV-R11.As shown in Fig. 4d, the columnar etching structure is partly filled with SiO 2 ; this represents a compromise between the design and fabrication of the LED device.The current IS-SiO 2 is fabricated by combining photolithography and ICP etching, followed by PECVD.If the columnar etching structure is completely filled with SiO 2 , the residual SiO 2 in Fig. 4e will be too thick to pose a problem for the deposition of the Ti/Ni p-pad.In addition, more etching is needed to remove the thick SiO 2 residue; this is not only detrimental to the fabrication cost but also detrimental to the surface of the epitaxial wafer.The sidewall of the columnar etching structure is almost vertical in Fig. 4f, which is not beneficial for the deposition of SiO 2 , the Al reflector and the Ti/Ni/Ti/Ni p-pad.Figure 4g-i shows the FIB-SEM images of LED IV-W11.Additional photolithography and exposure for LED IV-W11 are performed to ensure that no Ti/Ni/Ti/Ni metal stacks are deposited on the etched groove, as shown in Fig. 4g.The depth of the etched groove is calculated to be ~3.7 μm.Unlike that of LED IV-R11, the etching facet of LED IV-W11 is sloping with an inclination angle of 66.7°, as shown in Fig. 4i.The slant etching facet is more favorable for the complete deposition of the IS-SiO 2 and Al reflector, which strongly influences the current leakage and emission performance of the device.

Light extraction analysis
To reveal the underlying mechanisms for the superior LOPs resulting from IS-SiO 2 in the DUV LEDs, we further performed simulation experiments by using the SimuLED commercial software package and finite-difference timedomain (FDTD) method to investigate the influence of IS-SiO 2 on the current distribution and light extraction of DUV LEDs.In all simulations, the refractive indices and absorption coefficients of AlGaN, MQWs, sapphire, and SiO 2 were taken from the literature 44 .
We first simulated the current identity distribution in LED III, LED IV-R11 and LED IV-W11.The results from the simulated emission power densities of LED III, LED IV-R11, and LED IV-W11 at 350 mA and 850 mA are shown in Fig. S7 (Supporting Information).The calculated root mean square (RMS) values of the current density for LED III, LED IV-R11 and LED IV-W11 are 192.079,198.315, and 202.172A cm −2 at 350 mA, respectively.The RMS values of the current density for LED III, LED IV-R11, and LED IV-W11 are 500.731,514.699, and 521.392A cm −2 at 850 mA, respectively.Thus, insulated IS-SiO 2 can enhance the effective current injected into the MQW and is anticipated to improve the LOP.However, the calculated output powers at 350/850 mA for LED III, LED IV-R11, and LED IV-W11 are 96.281/188.274,95.553/186.774,and 95.102/186.143mW, respectively.These paradoxical results can be ascribed to the dual character of IS-SiO 2 .On the one hand, IS-SiO 2 can enhance the effective current injection to increase the emission power.On the other hand, the introduction of IS-SiO 2 leads to an area loss of the MQW structure.The positive and negative effects of IS-SiO 2 provide a balance and produce in similar output powers for the three LED samples.Since the current distribution simulation is not sufficient to elucidate the improvement in LOP resulting from IS-SiO 2 , we employed FDTD simulation to investigate the light extraction behavior inside the devices.To simplify the computations, the sizes of all LED simulation models need to be scaled down.The sizes of the LED simulation models are shown in Fig. S8 (Supporting Information).First, we simulated the electrical field intensity distribution of LED III and LED IV-W11, as shown in Fig. S9 (Supporting Information).The total LEE of LED IV-W11 (32.0%) is greater than that of LED III (30.9%).The top LEE of LED IV-W11 (9.5%) is also greater than that of LED III (8.4%).We speculate that IS-SiO 2 can break the TM-polarized waveguide of DUV light and simultaneously enhance the transverse-electric (TE)-polarized waveguide, thereby increasing the LEE of the top facet.However, the electrical field intensity distribution of LED IV-W11 did not sufficiently improve the LEE of the top facet.Thus, the sapphire substrate is potentially too thick to show the effect of IS-SiO 2 on the light extraction of the top facet.
To verify the abovementioned deduction regarding IS-SiO 2 , we simplified the simulated models of LED III, LED IV-R11 and LED IV-W11 by removing the sapphire substrate, as shown in Fig. 5a.The normalized electrical field intensity distributions of LED III, LED IV-W11 and LED IV-R11 are shown in Fig. 5b, d, f.For LED IV-R11 and LED IV-W11, the electrical intensity field distributions on the surface of the simulation models are both much stronger than those of LED III.In addition, the electrical intensity field distributions on the surfaces of LED IV-R11 and LED IV-W11 are discrete and continuous; this results are in agreement with the arrangements of the corresponding IS-SiO 2 .The Monte Carlo   To understand the physical mechanism of the improved LEE at the top facet of the DUV LED with IS-SiO 2 , the light trajectories at different interfaces are analyzed using Snell's law. Figure 6d shows the typical light trajectory inside the IS-SiO 2 of LED IV-W11.We only focus on the light rays that travel in the IS-SiO 2 before leaving the SiO 2 .Figure 6e shows the light trajectory analysis in Region I.According to the refractive indices of AlGaN and SiO 2 , the critical angle of total internal reflection at the AlGaN/SiO 2 interface is 36.6°,such as light ray β.When the incident angle is θ < 36.6°, the light rays radiating on the AlGaN/SiO 2 interface are refracted into IS-SiO 2 , such as light ray α.If 36.6°<θ < 90°, the light rays

Conclusion
In summary, the integration of the tailored MQWs, a low-optical-loss p-electrode/ohmic contact layer and insulating IS-SiO 2 is achieved in our DUV LEDs; these devices deliver an LOP of 140.1 mW at 850 mA.More importantly, the EQEs of our DUV LEDs are enhanced by 4.5 times in contrast to those of their conventional counterparts.Moreover, to validate the experimental results, the band structure, current distribution and FDTD simulations were used to investigate the carrier radiative recombination and light extraction behavior of our DUV LEDs.Furthermore, the on-wafer electroluminescence properties of our DUV LEDs confirm that our method can be expanded to mass manufacturing scales.The understanding and exploitation of bandgap engineering and device craft presented in this study provide a widely applicable strategy for fabricating highpower AlGaN-based DUV emitters for use in biomedical testing, water/air purification, and other relevant fields.
a schematic illustration of the AlGaN-based DUV LED.The epitaxial films of the DUV LED include the following layers from the bottom up: a 3-μm-thick undoped AlN buffer layer, a 410-nm-thick AlN/Al 0.7 Ga 0.3 N superlattice insertion layer, a 650-nm-thick n-type Al 0.7 Ga 0.3 N transition layer, an ~1-μm-thick n-type Al 0.6 Ga 0.4 N electron source layer, five pairs of Al x Ga 1-x N/Al 0.68 Ga 0.32 N MQW, a 2-nm-thick Al 0.68 Ga 0.32 N last quantum barrier, a 40-nm-thick p-type Al 0.65 Ga 0.35 N electron blocking layer (EBL), a 44-nmthick p-type Al m Ga 1-m N (m ~0.65 → 0) hole source layer and an ~40-nm-thick p-type GaN ohmic contact layer.DUV LEDs with conventional, gradient and staggered QWs are labeled as Structures A, B and C, respectively, as shown in Fig. 1b.Compared to the conventional QWs in Structure A, the tailored QWs in Structures B and C are anticipated to enhance the radiative recombination rate of electron-hole pairs by alleviating the QCSE, thereby improving the emission performance of the device.The in situ temperature transients are shown in Fig. 1c.The growth temperatures of the MQWs in Structures A, B and C are all maintained at 1150 °C, which confirms that the growth temperature has a negligible impact on the change in the QW structure.Figure 1d illustrates the growth conditions of the QW in Structures A, B and C. We trim the QW structures only by adjusting the flux of TMAl.

1 Fig. 1
Fig. 1 Epitaxial growth of the DUV LEDs with tailored QWs.Schematic illustration of the (a) AlGaN-based DUV LED and (b) corresponding energy band diagram of the tailored MQWs.c In situ temperature transients during the epitaxial growth of Structures A, B, and C. d Schematic illustration of the growth conditions of QW in Structures A, B, and C

Fig. 2
Fig. 2 Electroluminescence properties of the DUV LEDs with tailored QWs.Normalized EL intensity of (a) Structure A, (b) Structure B, and (c) Structure C at various injection currents.d I-V, e LOP-I, and f EQE-I characteristics at injection currents from 0 to 200 mA.Calculated wavefunction overlap in the first QW region of (g) Structure A, (h) Structure B, and (i) Structure C at 5 V

Fig. 3 L
Fig. 3 L-I-V characteristics of the DUV LEDs with IS-SiO 2 .Schematic illustration of (a) the DUV LED with IS-SiO 2 and (b) the mesas with cuboid and columnar etching structures.c I-V curves of LED III and LED IV. d Extractive LOP of LED III and LED IV at 850 mA. e LOP-I and EQE-I curves of LED III, LED IV-R11, and LED IV-W11.f Logarithmic I-V characteristics of LED III, LED IV-R11 and LED IV-W11

Fig. 4
Fig. 4 Morphology characteristics of the DUV LEDs with IS-SiO 2 .Top-view SEM images of (a) LED III, (b) LED IV-R11 and (c) LED IV-W11.d FIB-SEM image of LED IV-R11.Enlarged FIB-SEM images of Regions (e) A and (f) B in LED IV-R11.g FIB-SEM image of LED IV-W11.Enlarged FIB-SEM images of Regions (h) C and (i) D in LED IV-W11

Fig. 5
Fig. 5 Simulations for the light extraction behaviors of the DUV LEDs with IS-SiO 2 based on FDTD and Monte Carlo ray-tracing methods.a Schematic illustration of the simulation models for LED IV-W11 and LED IV-R11.The radius of the columnar IS-SiO 2 is set as 1.1 μm.The x plane of the normalized electrical field intensity distribution of (b) LED III, (d) LED IV-W11, and (f) LED IV-R11.Cross-sectional ray-tracing images of (c) LED III, (e) LED IV-W11, and (g) LED IV-R11 φ where φ r, sides and φ r, top are the relative LEE of the side and top facets for the LED sample, respectively, φ TE and φ TM are the LEE of the TE-and TM-polarized light, respectively, φ TE, sides and φ TM, sides are the LEE of the TE-and TM-polarized light emitted from the side facets of the LED chip, respectively, and φ TE, top and φ TM, top are the LEE of the TE-and TM-polarized light emitted from the top facet of the LED chip, respectively.Based on Equations (1) and (2), the calculated φ r, sides and φ r, top of LED III, LED IV-R11 and LED IV-W11 are shown in Fig. 6b.Clearly, reduced sidewall emission and enhanced top emission are achieved by the IS-SiO 2 in the LED IV. Figure 6c shows the simulated far-field radiation patterns of LED III, LED IV-R11 and LED IV-W11.The estimated LEEs for LED III, LED IV-R11, and LED IV-W11 are 4.47%, 4.97% and 5.45%, respectively.These results are consistent with those of other simulations and experiments and further confirm the superior light extraction ability of IS-SiO 2 .

Fig. 6
Fig. 6 Analyses of the light extraction behaviors of the DUV LEDs with IS-SiO 2 .a TE-/TM-polarized LEEs, b relative LEEs at the top/side facets, and c simulated far-field radiation patterns of LED III, LED IV-R11 and LED IV-W11.d Schematic illustration of light trajectories inside the IS-SiO 2 of LED IV-W11.Analyses of light trajectories in Regions (e) I, (f) II and (g) III