We study light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) using flip-chip (FC) devices with varied thickness in remaining sapphire substrate by experimental output power measurement and computational methods using 3-dimensional finite-difference time-domain (3D-FDTD) and Monte Carlo ray-tracing simulations. Light-output power of DUV-FCLEDs compared at a current of 20 mA increases with thicker sapphire, showing higher LEE for an LED with 250-μm-thick sapphire by ~39% than that with 100-μm-thick sapphire. In contrast, LEEs of visible FCLEDs show only marginal improvement with increasing sapphire thickness, that is, ~6% improvement for an LED with 250-μm-thick sapphire. 3D-FDTD simulation reveals a mechanism of enhanced light extraction with various sidewall roughness and thickness in sapphire substrates. Ray tracing simulation examines the light propagation behavior of DUV-FCLED structures. The enhanced output power and higher LEE strongly depends on the sidewall roughness of the sapphire substrate rather than thickness itself. The thickness starts playing a role only when the sapphire sidewalls become rough. The roughened surface of sapphire sidewall during chip-separation process is critical for TM-polarized photons from AlGaN quantum wells to escape in lateral directions before they are absorbed by p-GaN and Au-metal. Furthermore, the ray tracing results show a reasonably good agreement with the experimental result of the LEE.
© 2015 Optical Society of America
Deep-ultraviolet (DUV) light-emitting diodes (LEDs) based on AlxGaN1-xN wide-bandgap semiconductors with emission wavelengths shorter than λ = 285 nm offer a number of new applications as a compact, high-efficiency, and environment-friendly light source in air/water purification, sterilization, UV curing, and medical phototherapy [1–4]. Quantum efficiencies and optoelectronic performance characteristics of the DUV-LEDs, however, still have enough margins for further improvement, unlike the case of visible LEDs in which the improvement of internal and extraction quantum efficiencies are well studied with few exceptions including mitigation of efficiency droop possibly originating from Auger recombinations, electron spill-over, inhomogeneous hole distribution [5–10]. Low internal quantum efficiency (IQE) is related to crystalline defects and the nature of AlGaN heterostructures, including high density of threading dislocations in an AlN buffer layer and following heterostructures on a sapphire substrate, weak confinement of carriers, and the absence of defect-screening effect from quantum-dot-like localized states . In addition, carrier injection efficiency becomes lower with higher AlN mole fractions in the AlGaN layers.
External quantum efficiency (EQE) is also low due to low light-extraction efficiency (LEE). In general for visible LEDs, low LEE is due to total internal reflection (TIR) in the heterostructures with interfaces of GaN/sapphire and sapphire/air, having large differences in refractive indices of III-N (n ~2.46), sapphire (n = 1.78), and air (n = 1). Although refractive index of AlGaN is lower than that of GaN, which results in slightly less refractive index contrast to the surroundings, the LEEs of DUV-LEDs are still low. In DUV-LEDs, light propagating outside the escape cone that is reflected back to the III-N heterostructure is absorbed not only by active layers but also by p-type layers with narrower bandgaps and electrodes that is neither transparent nor reflective to DUV photons. A dominant origin of low LEE in DUV-LEDs has been believed to be UV photon absorption in a GaN p-contact layer. Although several UV-LED structures have been introduced to reduce light absorption in GaN materials [12–14], the LEEs of DUV-LEDs are still significantly lower than those of visible LEDs. Many LED structures have been developed to improve the LEEs using photonic crystals [15–19], patterned substrates [20,21], and integrated microlens arrays [22–25]. However, previous reports mostly focused on LEDs operating in either visible or near-UV spectral region.
Low LEEs of DUV-LEDs may be associated with emission from quantum wells (QWs) consisting of high AlN mole fractions. For such QWs, transverse magnetic (TM) polarized spontaneous emission // , is the most dominant emission, whereas the transverse-electric (TE) polarized spontaneous emission ⊥ is dominant for low-Al-content AlGaN QWs and InGaN QWs, where is electric field and is an axis normal to (0001) basal plane of hexagonal lattice. Photons with linear polarization parallel to the c-axis cannot be easily extracted within the escape cone to the surface .
In this study, we experimentally investigate output power change of DUV-LEDs by varying the thickness of sapphire substrates to study the improvement of LEEs. We also investigate a possible mechanism for enhanced LEE using numerical simulations by a three-dimensional (3D) finite-difference time-domain (FDTD) method and Monte Carlo ray-tracing to study the light propagation behaviors of DUV-LED structures in comparison to the case of visible LED structures.
2. Fabrication and structure
We used flip-chip LEDs (FCLEDs) for emitting DUV photons in this study. Epitaxial structures of AlGaN-based DUV-LEDs were grown on 2-inch-diameter AlN templates by metalorganic chemical vapor deposition (MOCVD). Trimethylgallium (TMGa, Ga(CH3)3), trimethylaluminum (TMAl, Al(CH3)3), bis-cyclopentadienilmagnesium (Cp2Mg, Mg(C5H5)2), and diluted silane (SiH4 balanced in H2) were used as precursors. The AlN template consists of a 2-μm-thick epitaxial AlN film on a (0001) sapphire substrate. The epitaxial layer structure consists of an 1.3-μm-thick silicon (Si)-doped n-type Al0.5Ga0.5N layer (n-Al0.5Ga0.5N:Si, 1.3 μm), five-period Al0.3Ga0.7N/Al0.5Ga0.5N (2.5/10 nm) multiple QWs (MQWs), a p-Al0.7Ga0.3N:Mg (35 nm) layer, a p-Al0.3Ga0.7N:Mg (30 nm) layer, and a p-GaN (80 nm) contact layer. The p-Al0.7Ga0.3N:Mg was employed as an electron-blocking layer (EBL) to effectively suppress the overflow of electrons out of active region.
Fabrication process of FCLEDs began with cleaning of the epitaxial wafers in H2SO4:H2O2 solution for 10 min. to remove metallic contaminants on the surface and then rinsed in running deionized water. The mesas for each 330 × 330 μm2 chip were formed. For n-type ohmic contact electrode, the n-Al0.5Ga0.5N layer was exposed using inductively-coupled plasma reactive-ion etching (ICP-RIE) with Cl2 gas during the mesa formation. For a part of an n-ohmic-contact electrode, a stack of Cr/Ni/Au layers was deposited by electron-beam (E-beam) evaporation, and then alloyed at 800 °C for 50 sec. in N2 to reduce a contact resistance. Ni/Au layers were deposited on top of the p-GaN layer for an ohmic-contact electrode by E-beam evaporation and then annealed at 560 °C for 80 sec. A SiO2 layer (1.0 μm) was deposited by plasma-enhanced chemical vapor deposition (PECVD) to passivate the side walls of the mesas.
The sapphire substrates were thinned to final thicknesses of ~100 μm, ~150 μm, ~200 μm, and ~250 μm, using backside grinding and lapping. Varied thickness of sapphire in FCLEDs is intended to investigate the influence on the output power by the sapphire substrates. The wafer was subsequently diced into separate chips by stealth laser dicing and breakage . Stealth laser dicing is a chip-separation method which forms a modified layer within the sapphire by focusing a laser inside the sapphire to induce localized thermal damage in the sapphire substrate. The laser beam was focused at a certain depth inside the sapphire substrate for scribing. The stealth laser dicing was carried out different times depending on the thickness of sapphire substrates, that is, 1, 2, 3, and 4 times for thicknesses of sapphire, 100 μm, 150 μm, 200 μm, and 250 μm, respectively. The DUV-LED chips were then separated using a tape expander. The DUV-LEDs were bonded with Au bumps onto a Si submount, and finally mounted on a ceramic package to complete the fabrication process of FCLEDs. Figure 1(a) is a cross-sectional schematic view of a fabricated DUV-FCLED. The electroluminescence (EL) spectra were taken from the backside of the LED chip through the remaining sapphire substrate. Current-voltage (I-V) characteristics of the DUV-FCLEDs were measured using a Keithley 2601A parameter analyzer. The optical output power characteristics were measured in UV integrated sphere with a calibrated power meter.
3. Optical properties
Figure 1(b) shows an EL spectrum of a DUV-FCLED at a current of I = 20mA. Effective bandgap of MQWs in the DUV-FCLED was controlled to target DUV emission in the range of λ = 270-285 nm. The EL peak of the LED in this study is positioned at λ ~283nm.
Light-output powers of DUV-FCLEDs and visible FCLEDs with various sapphire thicknesses of 100 μm, 150 μm, 200 μm, and 250 μm were measured. DUV-FCLEDs and visible FCLEDs have Al0.3Ga0.7N MQWs and In0.2Ga0.8N MQWs, respectively. The FCLEDs with different sapphire thickness are labeled with a thickness value of sapphire, for example, a DUV-FCLED with a sapphire thickness of 100 μm is labeled as “DUV-FCLED: 100 μm”. Figure 2 compares increments in the light-output powers of FCLEDs having different sapphire thicknesses with respect to an FCLED with a sapphire thickness of 100 μm measured at I = 20mA. As the IQEs of FCLEDs having the same epitaxial and device structure at the same current density are expected to be the same, the increment in each FCLED of DUV and visible emission should be equivalent to the change in LEE. Enhancement of the light-output power with increasing thickness of the remaining sapphire substrate of the DUV-FCLEDs is significantly higher than that of the visible FCLEDs. The output power of the DUV-FCLEDs increases with the thickness of sapphire by 14.1% (DUV-FCLED: 150 μm), 22.2% (DUV-FCLED: 200 μm), and 38.6% (DUV-FCLED: 250 μm), as compared to that of a DUV-FCLED: 100 μm. On the other hand, the output power of the visible FCLEDs increases with relative small enhancements of 2.3% (visible FCLED: 150 μm), 3.9% (visible FCLED: 200 μm), and 6.1% (visible FCLED: 250 μm). These results may suggest that the optical output power of DUV-FCLEDs is significantly influenced by the sapphire substrate thickness, which may be a result of unique optical polarization properties with high-Al-content AlGaN QW active region. For low-Al-content AlGaN QWs, dominant transitions occur between a conduction band and heavy hole (HH)/light hole (LH) bands, which results in dominant TE polarized spontaneous emission. For high Al-content AlGaN QWs, dominants transitions are between a conduction band and a crystal-field split-off hole (CH) band with dominant TM polarized spontaneous emission .
In this study, we used 3D-FDTD simulation and measured radiation patterns of DUV-FCLED structures with different sapphire thicknesses to further understand the LEE and its improvement. Figure 3 shows electric- and magnetic-field intensity distributions of the TE- and TM-polarized light simulated by FDTD. Electric/magnetic-field intensity distribution of the light emitted from the dipole source positioned in the middle of the AlGaN-based MQW for TE/TM polarization modes can be estimated by FDTD simulation . The absorption coefficients of GaN, AlGaN MQW, and n-AlGaN layer are assumed to be 170,000, 1000, and 10 cm−1, respectively. In addition, the refractive indices of AlGaN, AlN, and sapphire material are set at 2.6, 2.16, and 1.8, respectively [30–32]. TE- and TM-modes propagate mainly in vertical and lateral directions, respectively. These results suggest that the control of sidewall emission is more important than surface emission for the improvement in the LEE of DUV-FCLEDs. Figure 4 compares the radiation patterns from a DUV-FCLED: 100 μm and a DUV-FCLED: 200 μm. The light output angle of a DUV-FCLED: 200 μm is larger than that of a DUV-FCLED: 100μm. Also the amount of side emission is enhanced for a DUV-FCLED: 200 μm. TM-polarized light in DUV-FCLEDs tends to propagate along lateral directions normal to the surface. The number of internal reflection in sapphire is expected to decrease in the cases of the thicker sapphires. Additionally, the emission pattern like heart-like shape is caused by the unique property of high-Al-content AlGaN because it has higher bond strength and ionicity, resulting in an anisotropic emission pattern.
4. Mechanism of light extraction enhancement
Measured light-output power and radiation patterns of DUV-FCLEDs with varied sapphire substrate thickness are further analyzed by 3D-FDTD simulation and Mote Carlo ray-tracing simulation to study the mechanisms of the light extraction in AlGaN-based DUV-FCLEDs in comparison to those in InGaN-based visible FCLEDs. Figure 5 shows the simulated results of relative LEE changes as a function of sapphire substrate thickness in different cases of TE mode of visible FCLEDs and TE and TM modes of DUV-FCLEDs. In each case, surface roughness of sidewalls is also changed, while that of sapphire backside-surface remains the same. The change in sidewall roughness is intended to simulate the situations of resulting surfaces by stealth laser dicing, which is to be further described later.
Figure 5(a) compares the LEE changes of FCLEDs with mirror-like surfaces on the sidewalls of sapphire. In the case of visible FCLEDs, patterning between a GaN buffer and the front surface of sapphire was used, as patterned sapphire substrate (PSS) is a dominantly used technique in visible LEDs . For the cases of mirror-like sidewalls of sapphire substrates, the LEE for the TE mode of visible FCLEDs increases linearly to 6.1% with increasing sapphire thickness from 100 μm to 250 μm, whereas those for TE and TM modes of DUV-FCLEDs hardly increases with changes less than ~1%. The result suggests that the LEE of DUV-FCLEDs is not enhanced just by increasing the thickness of sapphire. Other origins for the enhancement of the light-output power in DUV-FCLEDs, as shown in Fig. 2, are to be investigated.
We applied modified structures with rough surfaces on the sidewalls of sapphire substrates in the simulation. The process of separating each chip by the stealth laser technique is expected to yield surface roughening on the sidewalls of the sapphire substrates. Stealth dicing is a dicing method that forms a modified layer in the bulk of the transparent material (sapphire substrate in this case) by focusing a laser. This modified zone creates a line of mechanically weak spots along the material which guides following breakage. A periodic pattern of rough region is created after the breakage. Figures 5(b) and 5(c) compare the LEE changes in FCLEDs with rough surfaces on the sidewalls with different surface roughness values. Sidewall roughness in this simulation is defined by a height of conical forms in a periodic array. The relative LEEs for TE mode of visible FCLEDs with rough sidewalls of sapphire increase to 6.9% and 7.1% with increasing sapphire in the case of surface roughness of 100 nm and 200 nm, respectively, which are similar to the case of mirror-like surfaces (6.1%). Roughening of sidewalls of sapphire substrates in visible FCLEDs do not result in significant improvement in the LEE. In general, the output power of InGaN-QW-based visible FCLEDs slightly increases with increasing sapphire thickness regardless of sidewall roughness of sapphire substrate, which matches well with simulation results.
The relative LEE change of DUV-LEDs, however, strongly depends on roughness of sidewalls. The LEEs for the TE and TM modes of DUV-FCLEDs with a sidewall roughness of 100 nm increase to 3.5% and 3.8%, respectively, with increasing sapphire thickness from 100 μm to 250 μm, as shown in Fig. 5(b). These values are slightly higher than those of DUV-FCLEDs with mirror-like sidewalls (less than ~1%). The LEEs for both modes of DUV-FCLEDs with a sidewall roughness of 100 nm are nearly the same and still lower than that of the TE mode of visible FCLEDs (6.9%). When the surface roughness for sidewalls of sapphire is increased to 200 nm, the LEE improvement with increasing sapphire thickness becomes significant, as shown in Fig. 5(c). The LEEs for the TE and TM modes in DUV-FCLEDs increase to 15.5% and 18.8% for a sapphire thickness of 250 μm. Also, the LEE for the TM mode of DUV LEDs becomes higher than TE modes of DUV-LEDs and visible LEDs. If the roughness of sidewall of sapphire substrate for DUV-FCLED becomes significant, photons can be escaped more easily through the lateral directions (TM mode) as the sapphire thickness increases. The higher LEE can be attributed to the increase of photons scattering from the roughened sidewalls formed by stealth laser dicing of DUV-FCLEDs and from the thicker sapphire thickness.
We measured the roughness of sapphire substrate sidewalls and performed ray-tracing simulations to further study the light extraction improvement in DUV-FCLEDs. Monte Carlo ray-tracing method, which is a representative way of simulating light propagation in LEDs, was employed to examine the light propagation and extraction behavior of DUV-FCLED structures. Figure 6(a) shows a rough surface of a sapphire side wall with a thickness of 250 μm, containing multiple scribe lines and disruptive modified zones. A periodic stripe pattern with a spacing of ~40 μm is observed. The scan length of each line is ~330 μm, as measured by a confocal laser scanning technique on a sidewall of the sapphire substrate. A space of 3 μm between the grooves was determined by the laser-scribing speed. An average roughness value (Ra) of each line is estimated to be ~1.6 μm, ~1.5 μm, ~1.1 μm, and ~1.5 μm. Ra is an arithmetic average of absolute values in the roughness profile ordinate and is an effective surface roughness measure commonly adopted in general engineering practice. We used an average of four Ra values (~1.43 μm) in the simulation to define roughness of sapphire sidewalls. We also included an absorption rate of 80% to include the effect of absorption in the p-GaN and Au metal layers.
Figure 7 shows the results of ray-tracing simulation for DUV-FCLEDs without and with sidewall roughness of sapphire substrates. A significant amount of photons is extracted through the rough surfaces of sidewalls by scattering. However, top emission rarely increases regardless of sidewall roughness. DUV photons propagating and reflected in vertical direction will have higher chance to be absorbed inside an LED chip during internal reflection process due to high absorption rates in narrower-bandgap p-type and metal layers. Also, TM-polarized photons which are mainly emitted from high Al-content AlGaN QWs tend to propagate in lateral directions and are extracted more easily by sidewall roughness. In contrast, the LEE of visible FCLEDs is less affected by the rough surface of sidewalls, because photons emitted from QWs is sufficiently extracted by scattering effects from PSS substrates.
Ray tracing simulation is now applied to estimate the change in LEE as a function of the thickness of sapphire substrate with rough sidewalls. As the sapphire substrate thickness increases, the relative LEE increases almost linearly up to the 250 μm and then starts saturating with further increase. As shown in Fig. 8, comparing it to experimental results for DUV-FCLEDs with a sidewall roughness of ~1.43 μm (measured average roughness of lines), simulations and experiments show a close match in improvement of the LEE.
Light-output power of AlGaN-based DUV-FCLEDs increased with increasing thickness in remaining sapphire substrate more significantly than that of InGaN-based visible FCLEDs. Especially for DUV-LEDs with a sapphire substrate having a thickness of 250 μm separated by stealth laser dicing, the LEE was enhanced by 38.6%, compared to that with a 100-μm-thick sapphire substrate. The optical output power of DUV-FCLEDs seems to be strongly influenced by sapphire substrate thickness. However, the LEE is not enhanced by thickness of sapphire itself but by a synergistic effect with roughened sidewalls of sapphire induced by laser dicing. The sidewall roughness of the sapphire substrate plays an important role in LEE. The LEE of DUV-LEDs increases with increasing sapphire substrate thickness only when the sidewalls become rough. Roughened sidewalls of thick sapphire substrate increase the extraction probability of photons outside chip in lateral directions before the photons are absorbed by p-GaN and Au-metal. Furthermore, the simulation results showed reasonable agreement with the experimental results of the LEE for DUV-FCLEDs. In summary, the LEE of DUV-FCLEDs is mainly affected by the roughened sidewall, which can be escaped more easily through the lateral directions (TM mode), whereas the LEE of visible FCLEDs is less affected by the rough surface of sidewalls, because photons emitted from QWs (TE mode) is sufficiently extracted by scattering effects from PSS substrates.
J.-H. Ryou acknowledges partial financial support from Texas Center for Superconductivity at the University of Houston (TcSUH). The authors at SCNU acknowledge the support from Basic Science Research Program through the NRF of Korea fund by the Ministry of Education (NRF-2015R1D1A3A01019050 and NRF-2014R1A6A1030419).
References and links
1. H. Hirayama, T. Yatabe, N. Noguchi, T. Ohashi, and N. Kamata, “231–261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire,” Appl. Phys. Lett. 91(7), 071901 (2007). [CrossRef]
2. M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodriguez, S. Einfeldt, Z. Yang, N. M. Johnson, and M. Weyers, “Advances in group III-nitride-based deep UV light-emitting diode technology,” Semicond. Sci. Technol. 26(1), 014036 (2011). [CrossRef]
3. K. Davitt, Y. K. Song, W. Patterson Iii, A. Nurmikko, M. Gherasimova, J. Han, Y. L. Pan, and R. Chang, “290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles,” Opt. Express 13(23), 9548–9555 (2005). [CrossRef] [PubMed]
4. H. Hirayama, S. Fujikawa, N. Noguchi, J. Norimatsu, T. Takano, K. Tsubaki, and N. Kamata, “222-282 nm AlGaN and InAlGaN-based deep-UV LEDs fabricated on high-quality AlN on sapphire,” Phys. Status Solidi 206(6), 1176–1182 (2009). [CrossRef]
5. J. Iveland, L. Martinelli, J. Peretti, J. S. Speck, and C. Weisbuch, “Direct measurement of Auger electrons emitted from a semiconductor light-emitting diode under electrical injection: Identification of the dominant mechanism for efficiency droop,” Phys. Rev. Lett. 110(17), 177406 (2013). [CrossRef] [PubMed]
6. Z.-H. Zhang, W. Liu, Z. Ju, S. T. Tan, Y. Ji, Z. Kyaw, X. Zhang, L. Wang, X. W. Sun, and H. V. Demir, “InGaN/GaN multiple-quantum-well light-emitting diodes with a grading InN composition suppressing the Auger recombination,” Appl. Phys. Lett. 105(3), 033506 (2014). [CrossRef]
7. M.-H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. Park, “Origin of efficiency droop in GaN-based light-emitting diodes,” Appl. Phys. Lett. 91(18), 183507 (2007). [CrossRef]
8. S. Choi, H. J. Kim, S.-S. Kim, J. P. Liu, J. Kim, J.-H. Ryou, R. D. Dupuis, A. M. Fischer, and F. A. Ponce, “Improvement of peak quantum efficiency and efficiency droop in III-nitride visible light-emitting diodes with an InAlN electron blocking layer,” Appl. Phys. Lett. 96(22), 221105 (2010). [CrossRef]
9. J. P. Liu, J.-H. Ryou, R. D. Dupuis, J. Han, G. D. Shen, and H. B. Wang, “Barrier effect on hole transport and carrier distribution in InGaN/GaN multiple quantum well light-emitting diodes,” Appl. Phys. Lett. 93(2), 021102 (2008). [CrossRef]
10. Z.-H. Zhang, W. Liu, S. T. Tan, Y. Ji, L. Wang, B. Zhu, Y. Zhang, S. Lu, X. Zhang, N. Hasanov, X. W. Sun, and H. V. Demir, “A hole accelerator for InGaN/GaN light-emitting diodes,” Appl. Phys. Lett. 105(15), 153503 (2014). [CrossRef]
12. N. Lobo, H. Rodriguez, A. Knauer, M. Hoppe, S. Einfeldt, P. Vogt, M. Weyers, and M. Kneissl, “Enhancement of light extraction in ultraviolet light-emitting diodes using nanopixel contact design with Al reflector,” Appl. Phys. Lett. 96(8), 081109 (2010). [CrossRef]
13. S. Hwang, M. Islam, B. Zhang, M. Lachab, J. Dion, A. Heidari, H. Nazir, V. Adivarahan, and A. Khan, “A hybrid micro-pixel based deep ultraviolet light-emitting diode lamp,” Appl. Phys. Express 4(1), 012102 (2011). [CrossRef]
14. T. Inazu, S. Fukahori, C. Pernot, M. H. Kim, T. Fujita, Y. Nagasawa, A. Hirano, M. Ippommatsu, M. Iwaya, T. Takeuchi, S. Kamiyama, M. Yamaguchi, Y. Honda, H. Amano, and I. Akasaki, “Improvement of light extraction efficiency for AlGaN-based deep ultraviolet light-emitting diodes,” Jpn. J. Appl. Phys. 50(12R), 122101 (2011). [CrossRef]
15. T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, “III-nitride blue and ultraviolet photonic crystal light emitting diodes,” Appl. Phys. Lett. 84(4), 466–468 (2004). [CrossRef]
16. J. Shakya, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes,” Appl. Phys. Lett. 85(1), 142–144 (2004). [CrossRef]
17. T. N. Oder, J. Shakya, J. Y. Lin, and H. X. Jiang, “III-nitride photonic crystals,” Appl. Phys. Lett. 83(6), 1231–1233 (2003). [CrossRef]
18. E. Matioli, S. Brinkley, K. M. Kelchner, S. Nakamura, S. DenBaars, J. Speck, and C. Weisbuch, “Polarized light extraction in m-plane GaN light-emitting diodes by embedded photonic-crystals,” Appl. Phys. Lett. 98(25), 251112 (2011). [CrossRef]
19. J. J. Wierer Jr, A. David, and M. M. Megens, “III-nitride photonic-crystal light-emitting diodes with high extraction efficiency,” Nat. Photonics 3(3), 163–169 (2009). [CrossRef]
20. A. Bell, R. Liu, F. A. Ponce, H. Amano, I. Akasaki, and D. Cherns, “Light emission and microstructure of Mg-doped AlGaN grown on patterned sapphire,” Appl. Phys. Lett. 82(3), 349–351 (2003). [CrossRef]
21. D. S. Wuu, W. K. Wang, W. C. Shih, R. H. Horng, C. E. Lee, W. Y. Lin, and J. S. Fang, “Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates,” IEEE Photonics Technol. Lett. 17(2), 288–290 (2005). [CrossRef]
22. M. Khizar, Z. Y. Fan, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Nitride deep-ultraviolet light-emitting diodes with microlens array,” Appl. Phys. Lett. 86(17), 173504 (2005). [CrossRef]
23. M.-K. Lee and K.-K. Kuo, “Single-step fabrication of Fresnel microlens array on sapphire substrate of flip-chip gallium nitride light emitting diode by focused ion beam,” Appl. Phys. Lett. 91(5), 051111 (2007). [CrossRef]
24. X.-H. Li, R. Song, Y.-K. Ee, P. Kumnorkaew, J. F. Gilchrist, and N. Tansu, “Light extraction efficiency and radiation patterns of III-nitride light-emitting diodes with colloidal microlens arrays with various aspect ratios,” IEEE Photonics J. 3(3), 489–499 (2011). [CrossRef]
25. Y.-K. Ee, R. A. Arif, N. Tansu, P. Kumnorkaew, and J. F. Gilchrist, “Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays,” Appl. Phys. Lett. 91(22), 221107 (2007). [CrossRef]
26. K. B. Nam, J. Li, M. L. Nakami, J. Y. Lin, and H. X. Jiang, “Unique optical properties of AlGaN alloys and related ultraviolet emitters,” Appl. Phys. Lett. 84(25), 5264–5266 (2004). [CrossRef]
27. M. Kumagai, N. Uchiyama, E. Ohmura, R. Sugiura, K. Atsumi, and K. Fukumitsu, “Advanced dicing technology for semiconductor wafer-stealth dicing,” IEEE Trans. Semicond. Manuf. 20(3), 259–265 (2007). [CrossRef]
28. J. Zhang, H. Zhao, and N. Tansu, “Effect of crystal-field split-off hole and heavy-hole bands crossover on gain characteristics of high Al-content AlGaN quantum well lasers,” Appl. Phys. Lett. 97(11), 111105 (2010). [CrossRef]
29. H.-Y. Ryu, I.-G. Choi, U.-S. Choi, and J.-I. Shim, “Investigation of light extraction in AlGaN deep-ultraviolet light-emitting diodes,” Appl. Phys. Express 6(6), 062101 (2013). [CrossRef]
30. G. Yu, G. Wang, H. Ishikawa, M. Umeno, T. Egawa, J. Watanabe, and T. Jimbo, “Optical properties of wurtzite structure GaN on sapphire around fundamental absorption edge (0.78–4.77 eV) by spectroscopic ellipsometry and the optical transmission method,” Appl. Phys. Lett. 70(24), 3209–3211 (1997). [CrossRef]
32. K. Takeuchi, S. Adachi, and K. Ohtsuka, “Optical Properties of AlxGa1-xN alloy,” J. Appl. Phys. 107(2), 023306 (2010). [CrossRef]
33. M. Yamada, T. Mitani, W. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys. 41(Part 2, No. 12B), L1431–L1433 (2002). [CrossRef]