The injection current dependence of optical polarization of ultraviolet (UV) light-emitting diodes (LEDs) emitting at wavelength of 310 nm and 277 nm was investigated by electroluminescence (EL) measurements. For both diodes, it was found that the degree of polarization (DOP) decreased obviously as the injection current increased. We attribute the decrease in DOP to the different changing trend of the intensity of the light emission from transverse electric (TE) polarization (E⊥c) and transverse magnetic (TM) polarization (E∥c) as the injected carriers occupy higher states above k = 0 with increasing the injection current. For the 277 nm LED, even the polarization switching from TE to TM mode was observed.
© 2014 Optical Society of America
Wurzite AlGaN alloys have a very large direct energy gap ranging from 3.4 eV to 6.2 eV, which makes them become promising materials for UV LEDs. AlGaN-based UV LEDs have a wide range of potential applications, such as water purification, bio-agent detection, UV curing and data storage [1–3]. However, despite the great progress AlGaN-based UV LEDs have made recently [4–8], the reported external quantum efficiency (EQE) of them is still quite low. As reported in previous publications, light emitted by AlxGa1-xN epitaxial layers switches its polarization characteristic from TE to TM mode as Al composition increases [9–13]. Thus it is well understood that light emission from Al-rich AlGaN layers grown on c-plane sapphire is predominantly polarized along the c-axis and photons can’t be easily extracted from the surface of device , which results in a relatively poor light extraction efficiency (LEE). Hence, it’s very important to understand physical mechanisms about the polarization characteristics for designing the active region of UV LEDs. Some groups have also investigated the effect of strain and temperature on the optical polarization of AlGaN-based UV LEDs [14–17], and recently Park et al.  have reported the carrier density dependence of polarization characteristic of UV LEDs in theory. However, at present, there have been no reports about the effect of injection current on the DOP of AlGaN-based UV LEDs in experiments. As is well known, if the DOP of UV LEDs varies with the injection current, which means that the percentage of surface emission (or edge emission) changes, the LEE of UV LEDs will also be influenced. Therefore, in order to analyze and characterize the EQE of UV LEDs, the influence of injection current on DOP should be taken into account, which affects the LEE of UV LEDs directly.
In this paper, we investigated the injection current dependence of optical polarization of UV LEDs emitting at wavelength of 310 and 277 nm. Edge-emitting EL from UV LEDs was used to determine the DOP of LED as the injection current increased from 20 to 145 mA.
The UV LED structures in this study were grown by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. The 310 nm LED structure consisted of an AlN buffer, an AlN/AlGaN superlattice, a n-AlGaN cladding layer, a five periods of Al0.20Ga0.80N/Al0.35Ga0.65N multiple quantum wells (MQWs) active region (barriers: 10 nm and wells: 2.5 nm), an electron blocking layer (EBL), and a p-GaN contact layer. The 277 nm LED structure was similar to 310 nm LED except that its active region was composed of Al0.30Ga0.70N wells and Al0.50Ga0.50N barriers. These LEDs were flip-chip mounted and the sapphire substrates were left uncovered so that edge-emitting EL from UV LEDs can be collected by our experimental setup.
The optical characteristics of these UV LEDs were measured on a hot chuck under pulsed-current mode with a pulse frequency of 1 kHz and duty cycle of 0.1% at room temperature. The effect of temperature on the optical polarization of UV LEDs is negligible, because the pulse duration of the injection current is as short as 1μs. As shown in Fig. 1, edge-emitting EL from UV LEDs passed through a pinhole, Glan-Taylor prism, two lenses, and ultimately detected by a spectrometer. With rotating the Glan-Taylor prism along y axis, the lights of E∥x (light polarized perpendicular to c axis, defined as TE Mode) and E∥z (light polarized parallel to c axis, defined as TM Mode) polarizations can be resolved. An unpolarized light source was applied to calibrate the measurement results of polarization dependence of the equipments. The DOP is defined aswhereandare the integrated EL intensities for the polarization components of E∥x and E∥z, respectively. The DOP of edge-emitting EL from UV LEDs was measured with increasing the injection current from 20 to 145 mA.
3. Results and discussion
Generally, the EL intensity of light emission increases as carrier density (or injection current) increases. In order to clearly investigate the changing trend of the intensity of the light emission from TE and TM mode, the experimental data of EL spectra of 310 nm LED has been normalized as shown in Fig. 2. The injection current is 20 mA, 70 mA, and 145 mA with a pulse frequency of 1 kHz and duty cycle of 0.1%. It is clear that the EL intensity of TM polarized emission gradually approaches that of TE polarized emission with increasing the injection current. The calculated DOP decreased from 0.317 to 0.204 as the injection current changed from 20 to 145 mA. It shows that the increasing rate of TM polarized emission is larger than that of TE polarized emission. This phenomenon is closely related to the change in spontaneous emission rate of TM and TE mode due to the band filling effect induced by increasing injection current. When the injection current is low, most injected carriers occupy the states in vicinity of Γ point of the Brillouin zone and light emission is dominated by TE mode. As the injection current increases, more and more carriers occupy higher states above k = 0 and the ratio of TE to TM polarized emission changes. Park et al.  have discussed the effect of carrier density on polarization characteristics of light emission in AlGaN/AlN quantum well, and they reported that the matrix elements for TM mode are much larger than those for TE mode at higher states above k = 0, which causes that the spontaneous emission rate of TM mode becomes larger than that of TE mode in case of high carrier densities. Hence, the decrease in DOP of our 310 nm LED with increasing the injection current is consistent with their theoretical results.
As discussed above, the light emission from 310 nm LED is dominated by TE mode, but the polarization switching from TE to TM mode can be achieved for UV LEDs with a shorter emission wavelength (higher Al composition). Nam et al.  have reported that the polarization switching for AlxGa1-xN epilayers grown on c-plane sapphire was observed when the Al composition (x) was larger than 0.25. For AlGaN MQWs grown on AlN/SiC templates, the critical Al composition for polarization switching was about 0.36-0.41, reported by Kawanishi et al. . According to their results, the Al composition of our 277 nm LED is close to the “switching point”, namely, the intensity of TM polarized emission is close to that of TE polarized emission. Besides the Al composition, the optical polarization of light emitted from UV LEDs is also affected, to a large extent, by the strain state of the active region. For our two diodes grown on c-plane sapphire substrate, the MQWs in the active region are compressively strained. Several groups have reported that the QW structure with compressive residual strain can enhance TE polarized emission [12,15], which further confirms that our 277 nm LED may not achieve the polarization switching. The polarization characteristics of 277 nm LED, whose DOP is near zero, were also investigated by edge-emitting EL measurements.
The EL spectra of 277 nm LED for TE and TM polarizations are shown in Fig. 3. The experimental data has also been normalized. Figure 3(a) shows that the EL intensity of TM polarized emission is quite close to that of TE polarized emission, which is consistent with what we have discussed in previous part of this article. With increasing the injection current, EL intensity of TM mode is even larger than that of TE mode, as shown in Fig. 3(b). The DOP of 277 nm LED decreased from 0.008 to −0.017 as the injection current increased from 20 to 145 mA. As is well known, the recombination between electrons in the conduction band and holes in heavy hole (HH) or light hole (LH) band at the Γ point is TE mode, while the recombination between electrons in the conduction band and holes in split-off hole (CH) band is TM mode . From Fig. 3, it can be seen that the peak wavelength of TM mode is always shorter than that of TE mode in the process of increasing current, which suggests that the lower valance bands (CH) should not overlap with the upper valance bands (HH/LH). Therefore, we think that the polarization switching from TE to TM mode in our 277 nm LED originates from the transition of optical-transition matrix elements from TE to TM mode as the injected carriers occupy higher states above k = 0 when the injection current is high.
The DOP of our two samples as a function of injection current are depicted in Fig. 4, and two notable features can be observed. First, it can be seen that the DOP of 277 nm LED decreases drastically compared with 310 nm LED at the same injection current. This has been investigated by many groups, and the polarization switching can be achieved by increasing the Al composition, which means that the topmost valance bands of the MQWs change from Γ9 similar to that in GaN to Γ7 like that in AlN . Second, it shows that the absolute value of slope of experimental data in Fig. 4 for 310 nm LED is obviously larger than that of 277 nm LED, which implies that the changing speed of DOP of 310 nm LED is larger than that of 277 nm LED. For a two-dimensional system (such as QW), the density of states (DOS) is proportional to the effective mass. This difference in slope may be due to the fact that the DOS of AlGaN in active region becomes larger caused by the increase of the effective mass when emitting wavelength changes from 310 to 277 nm. Because the effective mass of AlGaN can be expressed as the Eq. (1):21]. Hence, band filling level caused by injection current for 310 and 277 nm LED will be different at the same injection current, which may lead to the different changing speed of DOP. Besides, the decrease in DOP with increasing the injection current, as shown in Fig. 4, will result in a significant reduction of the percentage of surface emission in total light emission from c-plane AlGaN-based UV LEDs. Thus, the LEE of UV LEDs will be obviously influenced by the injection current level. Therefore, the influence of injection current on DOP of UV LEDs should be taken into account when analyzing the EQE of UV LEDs by EL.
In summary, we have investigated the injection current dependence of optical polarization of UV LEDs emitting at wavelength of 310 nm and 277 nm by edge-emitting EL. The DOP of both 310 and 277nm LED decreased with increasing the injection current, and especially the polarization switching from TE to TM mode was observed in 277 nm LED. This polarization switching can be attributed to the larger optical-transition matrix elements for TM mode compared with TE mode when the injected carriers occupy higher states above k = 0 in case of high injection current. Most importantly, when we analyze the EQE of UV LEDs by EL, the decrease in DOP of UV LEDs due to the increase of injection current should be taken into consideration.
This work was supported by National Basic Research Program of China (Nos: 2012CB619301 and 2012CB619306) and National High Technology Research and Development Program of China (2014AA032608).
References and links
1. K. Davitt, Y.-K. Song, W. Patterson Iii, A. V. Nurmikko, M. Gherasimova, J. Han, Y.-L. Pan, and R. K. Chang, “290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles,” Opt. Express 13(23), 9548–9555 (2005). [CrossRef] [PubMed]
2. C. J. Collins, A. V. Sampath, G. A. Garrett, W. L. Sarney, H. Shen, M. Wraback, A. Y. Nikiforov, G. S. Cargill, and V. Dierolf, “Enhanced room-temperature luminescence efficiency through carrier localization in AlxGa1−xN alloys,” Appl. Phys. Lett. 86(3), 031916 (2005). [CrossRef]
3. M. A. Würtele, T. Kolbe, M. Lipsz, A. Külberg, M. Weyers, M. Kneissl, and M. Jekel, “Application of GaN-based ultraviolet-C light emitting diodes - UV LEDs - for water disinfection,” Water Res. 45(3), 1481–1489 (2011). [CrossRef] [PubMed]
4. H. Tsuzuki, F. Mori, K. Takeda, T. Ichikawa, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Yoshida, M. Kuwabara, Y. Yamashita, and H. Kan, “High-performance UV emitter grown on high-crystalline-quality AlGaN underlying layer,” Phys. Status Solidi 206(6), 1199–1204 (2009). [CrossRef]
5. J. R. Grandusky, S. R. Gibb, M. C. Mendrick, and L. J. Schowalter, “Properties of Mid-Ultraviolet Light Emitting Diodes Fabricated from Pseudomorphic Layers on Bulk Aluminum Nitride Substrates,” Appl. Phys. Express 3(7), 072103 (2010). [CrossRef]
6. W. Sun, M. Shatalov, J. Deng, X. Hu, J. Yang, A. Lunev, Y. Bilenko, M. Shur, and R. Gaska, “Efficiency droop in 245–247 nm AlGaN light-emitting diodes with continuous wave 2 mW output power,” Appl. Phys. Lett. 96(6), 061102 (2010). [CrossRef]
7. H. Hirayama, Y. Tsukada, T. Maeda, and N. Kamata, “Marked Enhancement in the Efficiency of Deep-Ultraviolet AlGaN Light-Emitting Diodes by Using a Multiquantum-Barrier Electron Blocking Layer,” Appl. Phys. Express 3(3), 031002 (2010). [CrossRef]
8. 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]
9. K. B. Nam, J. Li, M. L. Nakarmi, 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]
10. H. Kawanishi, M. Senuma, and T. Nukui, “Anisotropic polarization characteristics of lasing and spontaneous surface and edge emissions from deep-ultraviolet (λ≈240 nm) AlGaN multiple-quantum-well lasers,” Appl. Phys. Lett. 89(4), 041126 (2006). [CrossRef]
11. T. Kolbe, A. Knauer, C. Chua, Z. Yang, S. Einfeldt, P. Vogt, N. M. Johnson, M. Weyers, and M. Kneissl, “Optical polarization characteristics of ultraviolet (In)(Al)GaN multiple quantum well light emitting diodes,” Appl. Phys. Lett. 97(17), 171105 (2010). [CrossRef]
12. J. E. Northrup, C. L. Chua, Z. Yang, T. Wunderer, M. Kneissl, N. M. Johnson, and T. Kolbe, “Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells,” Appl. Phys. Lett. 100(2), 021101 (2012). [CrossRef]
13. T. M. Altahtamouni, J. Y. Lin, and H. X. Jiang, “Optical polarization in c-plane Al-rich AlN/AlxGa1-xN single quantum wells,” Appl. Phys. Lett. 101(4), 042103 (2012). [CrossRef]
14. T. Kolbe, A. Knauer, C. Chua, Z. Yang, V. Kueller, S. Einfeldt, P. Vogt, N. M. Johnson, M. Weyers, and M. Kneissl, “Effect of temperature and strain on the optical polarization of (In)(Al)GaN ultraviolet light emitting diodes,” Appl. Phys. Lett. 99(26), 261105 (2011). [CrossRef]
15. T. K. Sharma, D. Naveh, and E. Towe, “Strain-driven light-polarization switching in deep ultraviolet nitride emitters,” Phys. Rev. B 84(3), 035305 (2011). [CrossRef]
16. C. Netzel, A. Knauer, and M. Weyers, “Impact of light polarization on photoluminescence intensity and quantum efficiency in AlGaN and AlInGaN layers,” Appl. Phys. Lett. 101(24), 242102 (2012). [CrossRef]
17. S. Fan, Z. Qin, C. He, X. Wang, B. Shen, and G. Zhang, “Strain effect on the optical polarization properties of c-plane Al₀.₂₆Ga₀.₇₄N/GaN superlattices,” Opt. Express 22(6), 6322–6328 (2014). [CrossRef] [PubMed]
18. S.-H. Park and J.-I. Shim, “Carrier density dependence of polarization switching characteristics of light emission in deep-ultraviolet AlGaN/AlN quantum well structures,” Appl. Phys. Lett. 102(22), 221109 (2013). [CrossRef]
19. Y. Taniyasu, M. Kasu, and T. Makimoto, “Radiation and polarization properties of free-exciton emission from AlN (0001) surface,” Appl. Phys. Lett. 90(26), 261911 (2007). [CrossRef]
20. R. G. Banal, M. Funato, and Y. Kawakami, “Optical anisotropy in -oriented AlxGa1−xN/AlN quantum wells (x>0.69),” Phys. Rev. B 79(12), 121308 (2009). [CrossRef]
21. P. Rinke, M. Winkelnkemper, A. Qteish, D. Bimberg, J. Neugebauer, and M. Scheffler, “Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN,” Phys. Rev. B 77(7), 075202 (2008). [CrossRef]