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Abstract
We report the performance enhancement of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) using AlGaN nanoporous template (NPT). The NPT was fabricated by the electrochemical etching method and served as the dislocation filtering layer and strain relieving layer. The n-AlGaN laterally regrown on NPT showed reduced dislocation density and mitigated compressive strain comparing with that on the as-grown template (AGT). A 23% improvement of internal quantum efficiency was achieved for the multiple quantum wells thereon. Moreover, the nanopores in the NPT transformed into elongated air voids during high temperature growth process, which could facilitate the escaping of photons by scattering and thus improve the light extraction efficiency. As a consequence, the DUV LED based on NPT demonstrated an increase of the light outpower by 50% at 20 mA than that on AGT.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
AlGaN-based deep ultraviolet light emitting diodes (DUV LEDs) have been the most promising UV light sources for water and air disinfection, chemical agent detection, secure communications, and lighting [1,2]. However, the mostly reported external quantum efficiency (EQE) of DUV LEDs with wavelength less than 280 nm is still in the single-digit percentage range [1], which postpones the further applications of DUV LEDs. The low EQE is closely associated with the properties of the heteroepitaxial Al-rich AlGaN LEDs including high dislocation density, polarization effect, low-density hole injection, serious light absorption of p-GaN contact layer and the total internal reflection (TIR) [3]. Extensive efforts have been developed to overcome these problems such as lateral overgrowth on the patterned substrate [4–6], quantum structure optimization [7,8], transparent p-AlGaN contact layer [9–11], substrate backside [12] and sidewall roughening [13,14], and various nanostructures [15–23].
Lateral overgrowth on patterned template is acknowledged as a critical technique to decrease the in-plane compressive strain, reduce the dislocation density and improve the light extraction efficiency (LEE) for DUV LEDs [4,24]. Recently, lateral overgrowth based on porous template has been proposed. Fareed et al. [25] fabricated the porous GaN structure by the ultraviolet radiation-enhanced electroless wet chemical etching. The 2.8-μm crack-free Al0.24Ga0.76N with higher material quality was successfully grown on the porous GaN due to the strain accommodation of the porous layer. Wei-Chih Lai et al. [26] reported GaN-based LEDs with embedded reshaped ellipsoidal voids using an electrochemically etched GaN template. After optimizing the size of the reshaped ellipsoidal voids, the light output power (LOP) was enhanced by 50%. Kwang Jae Lee et al. [27] also reported GaN-based blue LEDs on the porous GaN layer and found that the porous layer with reshaped ellipsoidal voids acted as a reflector to improve the LEE. Guan-Jhong Wang et al. [28] reported the GaN/AlGaN UVA LED with an embedded porous-AlGaN distributed Bragg reflector and the photoluminescence emission intensity of the LED was enhanced. Recently, we have studied the formation mechanism of the nanoporous ternary alloy AlGaN fabricated by the electrochemical etching (EC) and proven AlGaN regrown on the porous AlGaN with improved crystal quality [29]. However, few studies about AlGaN nanoporous DUV LEDs with wavelength in UVC have been reported and the extraction mechanism of the transverse-electric (TE) polarized and transverse-magnetic (TM) polarized photons in such LEDs is yet to be investigated.
In this work, we demonstrate DUV LEDs with improved performance on the AlGaN nanoporous template (NPT) prepared by EC. Compared to that on the as-grown template (AGT), the n-AlGaN layer on NPT showed better material quality and nearly free in-plane stress. The internal quantum efficiency (IQE) increased by 23% for the multiple quantum wells (MQWs) thereon. In addition, the air voids formed in the NPT during high temperature growth process could contribute to an enhanced LEE. The two-dimensional finite-different time-domain (2D-FDTD) simulation results revealed that the air-voids nanostructure redirected more DUV photons, especially TM-polarized photons, to the vertical direction. The improved IQE and LEE contributed to 50% higher LOP and EQE for the DUV LED based on NPT than that on AGT.
2. Experiment
The n-Al0.55Ga0.45N template (i.e. AGT) was grown on sapphire substrate by a home-made low-pressure metal-organic chemical vapor deposition (LP-MOCVD) system [30]. The schematic epitaxial structure is illustrated in Fig. 1(a). The AGT was 1-μm-thick and the electron density was estimated to be 1 × 1018 cm−3. Then the AGT was transformed into NPT by EC. The EC process was carried out under a 30 V constant direct voltage using 1wt% KOH solution as the electrolyte as reported in [29]. Figure 1(b) and 1(c) show the top-view and the cross-section scanning electron microscope (SEM) images of NPT, respectively. The pores are distributed nonuniformly caused by the nonuniformity of the defects [24]. The average pore diameter is about 35 nm and the density of pore is roughly estimated to be 5 × 1017 cm−3. Pores went through the whole n-AlGaN layer and stopped at the superlattices layers due to the selective etching determined by the material conductivity. The morphology exhibits near-vertical shape, decided by the distribution of the electric field in the n-AlGaN layer [29].
Fig. 1 (a) The schematic epitaxial structure of AGT. (b)Top-view and (c) cross-sectional SEM images of NPT.
After removing the surface oxide layer by hydrochloric acid solution (HCl: H2O = 1:10) immersion, the NPT was loaded back into MOCVD chamber for the subsequent n-AlGaN regrowth at 1000 °C. The reactor pressure was 50 mTorr and the V/III ratio was 2500. For comparison, a reference sample regrown on AGT was also implemented in the same batch. The surface morphology of samples was characterized by SEM and atomic force microscope (AFM). The high-resolution X-ray diffraction (HR-XRD) and Raman spectroscopy were used to reflect the material quality and the strain state, respectively.
The rest of DUV-LED structure was further regrown the n-AlGaN layer, consisting of five pairs of Al0.4Ga0.6N/Al0.5Ga0.5N (3 nm/12 nm) MQWs, a Mg-doped 30-nm-thick Al0.65Ga0.35N EBL, a 30-nm-thick p-AlGaN cladding layer and a 120-nm-thick highly doped p-GaN contact layer. A Ti/Al/Ti/Au metal was deposited on n-AlGaN as n-contact and annealed at 1000 °C in N2. The p-contact is Ni/Au metal stack and the annealing occurred at 700°C in air. The DUV LED wafers were fabricated into 500 μm × 500 μm chips. Finally, the diced LED chips were flip-chip bonded on submounts for the following measurements.
The temperature-dependent photoluminescence (TDPL) was performed to evaluate the IQE of DUV LEDs on NPT and AGT using a YAG:Nd laser (λ = 266 nm, pulse width = 7 ns, frequency = 5 kHz) to excite and a Princeton Instrument spectrometer (Acton SP2750) to record the signal. The electroluminescence (EL) spectra and the LOPs of DUV LEDs were measured using a calibrated integrating sphere and a calibrated high-resolution spectrometer system (HAAS 2000). The angle-resolved EL spectra were obtained by an angle-resolved spectrum system (R1, ideaoptics, China) equipped with highly sensitive spectrometer. The optical polarization characteristics were measured via the typical edge-emitting EL. The 2D-FDTD simulation was used to analyze the TE- and TM-polarized photons extraction mechanism in the air-void nanostructure.
3. Results and discussion
Figure 2(a) shows the cross-sectional SEM image of 1-μm regrown n-AlGaN on NPT. The coalescence thickness is ultra-thin due to the small size of nanopores. Lots of nanopores in NPT transformed into elongated air voids during high temperature growth process [31]. The average diameter of air voids and space between air voids are about 140 nm and 120 nm, respectively. The density of air voids is approximately estimated to be 7 × 108 cm−2. And the porosity of porous layer is about 14%. These values are almost lower than the reported results [26–28]. Generally, the larger size and density of ellipsoidal air voids are desired to obtain higher reflectance to enhance the light extraction of upside of LEDs [26,27]. And the lateral pores fabricated via EC have the same purpose [28]. Here, the vertical elongated air voids can facilitate the escape of DUV photons from the substrate via their scattering effect and the high reflectance is bad for the LEE. The optimization of the air voids’ morphology is to be further developed. Figure 2(b) and 2(c) show the AFM images of n-AlGaN on AGT and NPT, respectively. The regrown n-AlGaN shows the atomically flat surface and a step-flow growth mode for both samples. The RMS roughness values are 0.21 nm 0.19 nm for n-AlGaN on AGT and NPT, respectively. By counting dark pits in the AFM images, the threading dislocation density (TDD) of n-AlGaN on NPT is about 1.2 × 109 cm−2, much less than that on AGT (2.1 × 109 cm−2).
Fig. 2 (a) Cross-sectional SEM image of regrown n-AlGaN on NPT. AFM images for regrowth of n-AlGaN on (b) AGT and (c) NPT.
Figure 3 shows the X-ray rocking curves (XRCs) of laterally regrown AlGaN films. The full width at half-maximum (FWHM) values of (002) and (102) reflections are 250 arcsec, 720 arcsec and 242 arcsec, 895 arcsec for the regrown AlGaN films on NPT and AGT, respectively. The reduced FWHM value of (102) reflection indicates the reduction of edge dislocations in n-AlGaN on NPT compared to that on AGT, which agrees well with the result of AFM measurement. The improved crystalline quality is mainly attributed to the reshaping of nanopores and the lateral growth process [32,33]. Some dislocations in nanoporous template would bend or remove when the nanopores transformed into elongated air voids. So only partial dislocations can reach the surface of NPT. Subsequently, the lateral growth of n-AlGaN made some dislocations adjacent to air voids bend to air voids sidewalls and annihilate. Therefore, fewer threading dislocations can reach the regrown n-AlGaN surface.
Figure 4 shows the Raman spectra of the regrown n-AlGaN on NPT and AGT. There are three strong phonon modes for two samples, corresponding to the mode of sapphire, E2H (GaN-like) of n-AlGaN and E2H of AlN from left to right, respectively. The modes positions of sapphire and the AlN layer for both samples are exactly identical, respectively. As for the E2H (GaN-like) modes, they are located at 590 and 593.6 cm−1 for n-AlGaN on NPT and AGT, respectively. This denotes that the biaxial compressive strain was relieved dramatically for n-AlGaN on NPT than that on AGT. V. Yu. Davydov et al. [34] have reported the nearly unstrained phonon frequency of Al0.55Ga0.45N at 590 cm−1, which is close to that of the n-AlGaN on NPT in this work. Hence, we believe that the n-AlGaN on NPT are almost stress-less and thus could relieve the strain-induced piezoelectric polarization in MQWs [35].
Figure 5(a) and 5(b) show the TDPL spectra of the two DUV LEDs with the temperature increasing form 10 K to 300 K, respectively. The peak wavelengths are both around 280 nm. Figure 5(c) shows the normalized integrated PL intensities as a function of inverse temperature in an Arrhenius plot. The IQEs are estimated to be about 37% and 30% for DUV LEDs based on NPT and AGT, respectively, demonstrating 23% enhancement for the former. As mentioned earlier, the compressive strain in regrown n-AlGaN on NPT was reduced dramatically, which would decrease the stress-induced piezoelectric polarization in MQWs thereon. The compressive strain induced piezoelectric polarization with opposite direction compared to the spontaneous polarization and the latter was much larger than the former. As a result, the total polarization field in MQWs was slightly increased. We also calculated the polarization intensity in the MQWs of two kinds of DUV LEDs, based on the parameters provided by [36]. With respect to the underlying AlN materials, the relaxation degrees of the n-AlGaN layers were 60% and 100% for DUV LEDs on AGT [30] and NPT, respectively. The total polarization intensities in the well were estimated to be 0.00823 C/m2 and 0.00846 C/m2 (2.8% increase) for DUV LEDs on AGT and NPT, respectively. Nevertheless, thanks to the large band offset and narrow well width in our samples, the energy level above the triangular potential wells is high [37]. Therefore, the increased polarization intensity has negligible adverse impact on the IQE. The improved IQE mainly benefits from the better crystal quality of the n-AlGaN on NPT.
Fig. 5 Temperature-dependent PL spectra for DUV LEDs based on AGT (a) and NPT (b) and their Arrhenius plot of integrated PL spectra (c).
Figure 6(a) shows the EL spectra of DUV LEDs based on NPT and AGT at the typically driving current of 20 mA. The peak wavelengths are both ~280 nm arising from MQWs. A weak shoulder locating at about 330 nm attributes to the carrier recombination in p-AlGaN cladding layer [38], which signifies that the electron-blocking layer needs to be further optimized. Figure 6(b) shows the I-V characteristics for both kinds of DUV LEDs plotted in linear and semilogarithmic scale. The turn-on voltages are both about 6.5 V. It is worth noting that the reverse leakage currents are 1.2 × 10−5 and 7 × 10−6 A at −10 V for DUV LEDs based on AGT and NPT, respectively. Since the higher leakage current comes from higher defect density in devices [4], it symbolizes higher crystal quality of LED epitaxial layers on NPT than that on AGT. As shown in Fig. 6(c), the relative LOP and the EQE of the DUV LED based on NPT are about 50% higher than those of the DUV LED based on AGT throughout the whole injection current. It mainly derives from the improvement of the IQE and LEE. As mentioned earlier, the regrown n-AlGaN layer on NPT with higher crystalline quality contributes to the enhancement of IQE. Besides, the light scattering effect of air voids existing in nanoporous layer would increase the LEE. Figure 6(d) shows the normalized angle-resolved EL spectra of DUV LED based on NPT and AGT at 20 mA. The emission pattern for DUV LED based on AGT exhibits heart-like caused by the photon polarization in the Al-rich AlGaN, while that of the DUV LED on NPT behaves more like Lambertian pattern. The divergence angles (defined as the angle between the positions where the emission intensity is half the maximum [39]) are 150° and 135° for former and latter, respectively. It represents that more photons are redirected to the substrate side.
Fig. 6 The EL spectra (a), I-V characteristics (b) plotted in linear and semilogarithmic scale, relative LOPs and EQEs (c), angle-resolved EL spectra (d) of the DUV LEDs on NPT and AGT.
The optical polarization characteristics were performed via the typical edge-emitting EL [40]. Figure 7(a) and 7(b) shows EL spectra of TE and TM polarized light emitted from DUV LED based on AGT and NPT at 80 mA, respectively. The TM-mode polarization is the dominant emission mode for the DUV LED on AGT, while TM-mode and TE-mode polarization are almost equal with each other for the DUV LED on NPT. That is attributed to the more significant improvement of LEE for TM-mode than TE-mode via air voids in the DUV LED on NPT. As a result, the detected TM-mode photons are less from the edge of LED chips, which leads to the reduced proportion of TM-mode. The degree of polarization (DOP) is defined as (ITE-ITM)/(ITE + ITM), where ITE and ITM are the integrated EL intensities for the polarization components of TE and TM. The DOP of our two samples as a function of injection current are depicted in Fig. 7(c). We can observe that the DOP of the DUV LED on AGT decreased with the increasing injection current, which is ascribed to the larger optical-transition matrix elements for TM mode than TE mode when the increasing injected carriers occupy higher states [40]. That will lead to a significant reduction of the percentage of surface light emission of c-plane AlGaN-based DVU ELDs. While the DOP of the DUV LED on NPT exhibited inverse tendency. We believe that the case stems from the more significant enhancement of LEE for TM-mode than TE-mode via air voids in the DUV LED on NPT. TM-mode photons are more easily extracted from the substrate. So, the percentage of TM-mode photons extracted from the lateral edge of LED chip will decrease compared the TE-mode. As a result, the DOP of the DUV LED on NPT increases with increasing injection current. Subsequently, the 2D-FDTD simulation was demonstrated to verify our inference.
Fig. 7 The EL spectra of TE and TM polarized light emitted from DUV LED based on (a) AGT and (b) NPT at 80 mA, respectively. And their DOP as a function of injection current (c).
The 2D-FDTD simulation is basically built on the epitaxial structure depicted earlier to analyze the mechanism of light extraction for both kinds of DUV LEDs. But the superlattice layer was removed and the thick of sapphire was set to 2 μm for simplification [41,42]. The refraction indices AlN, GaN and sapphire were set to be 2.16, 2.9 and 1.8 [41,42], respectively. The refractive indices of AlGaN were obtained by linear combinations of the components of GaN and AlN [42]. The absorption coefficients of n-AlGaN, MQWs and p-GaN were set to be 10, 1000, 17000 cm−1 [41,42]. A dipole source emitting at 280 nm with a full width at half maximum of 10 nm was placed at the middle of the active region. The TE- and TM-modes were considered for the simulation. Figure 8 shows the light propagation at 7 fs intervals in DUV LEDs on NPT [TE (a)-(c), TM (g)-(i)] and AGT [TE (d)-(f), TM (j)-(l)]. In Fig. 8(e) and 8(j), the DUV light with incident angle encounters total internal reflection at the flat n-AlGaN/AlN interface, resulting in the internal absorption and huge light loss. In Fig. 8(a) and 8(g), the air void nanostructure is embedded around the n-AlGaN/AlN interface. When the photons enter into the air-voids or the regions among the air-voids, the propagation directions of light are possible to be changed to the vertical direction via the scattering effect. Hence, more DUV photons can be extracted from the substrate side, especially for TM-mode photons that originally transfer mainly in the lateral direction. That can be explain confirm the different behaviors of DOPs and the change of radiation pattern for DUV LEDs on AGT and NPT. Moreover, the light absorption also has a chance to be reduced because of the shortened photon escaping path length. Therefore, the LEE of the DUV LED on NPT increased remarkably.
Fig. 8 FDTD simulation of light propagation at 7 fs intervals in DUV LEDs on NPT [TE (a)-(c), TM (g)-(i)] and AGT [TE (d)-(f), TM (j)-(l)].
4. Conclusion
In summary, we demonstrate performance improvement of AlGaN-based DUV LEDs on NPT prepared by EC technique. The NPT could play as the dislocation filtering layer and strain relieving layer, resulting in the better crystal quality and mitigated compressive strain for the n-AlGaN grown on NPT. The IQE of DUV LED on NPT increased by 23% than that of DUV LEDs on AGT. Significantly, the transformed air voids in NPT increased the LEE of the DUV LED via the scattering effect. The 2D-FDTD simulation intuitively manifests that more photons, especially TM-polarized photons, are directed into vertical direction by the air-voids nanostructure and extracted from the substrate for the DUV LED on NPT. The EQE and LOP of the DUV LED on NPT increased by 50% than that of the DUV LED on AGT. Therefore, the approach to boost the internal quantum efficiency as well as the light extraction efficiency using NPT provides a promising way to realize efficient UV-emitters.
Funding
National Key R&D Program of China (Nos. 2016YFB0400803 and 2016YFB0400802), the National Natural Sciences Foundation of China (Grant Nos. 61527814, 61674147, U1505253), Beijing Nova Program Z181100006218007 and Youth Innovation Promotion Association CAS 2017157.
Acknowledgments
The authors thank Ideaoptics Inc. for the support on the angle-resolved spectrum measurements.
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