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Phosphor-free, white-light LED under alternating-current operation Yu-Feng Yao, Hao-Tsung Chen, Chia-Ying Su, Chieh Hsieh, Chun-Han Lin, Yean-Woei Kiang, and C. C. Yang* Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 10617, Taiwan *Corresponding author: [email protected] Received September 18, 2014; revised October 12, 2014; accepted October 12, 2014; posted October 14, 2014 (Doc. ID 223271); published November 3, 2014 A light-emitting diode structure, consisting of a p-GaN layer, a CdZnO/ZnO quantum-well (QW) structure, a hightemperature-grown ZnO layer, and a GaZnO layer, is fabricated. Under forward bias, the device effectively emits green–yellow light, from the QW structure, at the rim of device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer move into the conduction band of the GaZnO layer, through a QW-state-assisted tunneling process, to recombine with the injected holes in the GaZnO layer, for emitting yellow–red and shallow ultraviolet light over the entire mesa area. Also, carrier recombination in the p-GaN layer produces blue light. By properly designing the thickness of the high-temperature grown ZnO layer, the emission intensity under forward bias can be controlled such that, under alternating-current operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors, under forward and reverse biases, result in white light generation based on persistence of vision. © 2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (250.5590) Quantum-well, -wire and -dot devices. http://dx.doi.org/10.1364/OL.39.006371
Although a nitride-based light-emitting diode (LED) has been commercialized, it needs to be driven under the direct current (DC) condition. In this situation, a transformer is required for converting alternating current (AC) into DC, before LED application. Although implementations of AC-LED have been claimed, based on circuit designs for driving multiple LEDs [1,2], a single AC-driven LED device has not been reported. For the purpose of fabricating an AC-driven LED device, an LED structure, allowing current flow and carrier recombination under reverse bias, needs to be implemented first. Based on the electron tunneling mechanism under reverse bias, various device structures have been fabricated, to demonstrate emissions under both forward and reverse biases. Because of the large band offset between p-GaN and highly doped n-ZnO, the Fermi level in a p-GaN/n-ZnO junction can be close to the conduction band edge of n-ZnO. In this situation, under reverse bias, electrons can tunnel from the valence band of p-GaN, into the conduction band of n-ZnO, for carrier recombination and hence emission in the n-ZnO layer [3,4]. A similar phenomenon can also be observed in an n-ZnO/ n-GaN structure [5], and an n-ZnO/p-Si structure [6]. To slow down electrons under forward bias, for achieving effective carrier recombination on the n-type side, a thin layer of an insulating or high-band-gap material, such as SiO2 or MgO, was inserted at the junction [7–9]. Also, the n-type semiconductor material was replaced by graphene [8] or Au [9], to form a metal-insulator-semiconductor structure. Meanwhile, to control the emission wavelength under forward bias, an MgZnO heterostructure was added to the junction [9]. Although the emission wavelength can be changed, by switching device operation between forward and reverse biases, AC operation of such an LED has not been demonstrated. Because of the small lattice mismatch between GaN and ZnO, p-GaN has been used to replace p-ZnO for ZnObased device fabrication [10]. In this Letter, we report fabrication and characterization of an LED structure, 0146-9592/14/226371-04$15.00/0
consisting of a p-GaN layer, a CdZnO/ZnO quantum well (QW) structure, and a GaZnO layer, for implementing light emission of complementary colors, under forward and reverse biases. Under forward bias, carrier recombination in the QW structure provides us with green-yellow light, at the rim of device mesa. Under reverse bias, a QW-state-assisted tunneling process leads to carrier recombination in the GaZnO layer, for generating red, yellow, and shallow ultraviolet (UV) light over the entire mesa. The spectrally, spatially, and temporally mixed emission under AC operation results in phosphor-free, white-light generation. Figure 1 schematically shows the structure of the LED device. On a c-plane sapphire substrate, a template of a 2 μm un-doped GaN (u-GaN) layer (grown at 1000°C), and then a 500 nm p-type GaN (p-GaN) layer (grown at 930°C), is prepared with metal-organic chemical vapor deposition. In the p-GaN layer, the hole concentration is 2 × 1017 cm−3 . On this template, a structure of threeperiod CdZnO/ZnO QW is deposited, followed by a hightemperature (HT) grown un-doped ZnO (i.e., HT-ZnO) layer of 20 nm in thickness, and a 120 nm GaZnO layer, all with a molecular beam epitaxy reactor. The CdZnO/ ZnO QW structure is designed for controlling its emission wavelength around the green range, under forward bias
Fig. 1. LED device structure. © 2014 Optical Society of America
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(a positive voltage applied to the p-GaN layer) [10–13]. The use of GaZnO of a high electron concentration, as the n-type material, can make the Fermi level close to its conduction band. In this situation, the offset between the valence band of p-GaN and the conduction band of GaZnO becomes smaller, such that electron tunneling can occur at a lower applied voltage under reverse bias (a positive voltage applied to the GaZnO layer) [14]. The oxygen-rich HT-ZnO layer is used to create a potential barrier, for slowing down injected electrons from the GaZnO layer, and hence more effective QW capture [15]. However, use of this layer leads to a larger turn-on voltage and device resistance, and hence weaker emission under forward bias. For growing a CdZnO well, a ZnO barrier, the HT-ZnO, and the GaZnO layers, the substrate temperatures (Zn effusion cell) are 175°C, 175°C, 555°C, and 350°C (280°C, 280°C, 285°C, and 320°C), respectively. The Cd effusion cell temperature for growing a CdZnO well layer is 250°C. The Ga effusion cell temperature for growing the GaZnO layer is 700°C. Under the previously mentioned GaZnO growth condition, the resistivity, mobility, and electron concentration of the GaZnO thin film are 1.9 × 10−4 Ω · cm, 37 cm2 ∕V · s, and 8.4× 1020 cm−3 , respectively. The GaZnO layer is an alloy, with Ga content varying spatially from 5% to 8%. The n-contact with Au/Ti is formed at the center of the device mesa. The p-contact is fabricated surrounding the mesa with Au/Ni. The mesa dimension is 300 × 300 μm. The inset of Fig. 2 shows the photoluminescence (PL) spectrum of the CdZnO/ZnO QWs, measured before growth of the GaZnO layer. Here, the spectral fluctuations are caused by Fabry–Perot oscillation. One can see that the PL spectral peak is ∼494 nm, after the Fabry–Perot oscillation is filtered. The small peak at 376 nm, indicated by the arrow, corresponds to the band-edge emission of ZnO. Figure 2 shows the relation between injection current and applied voltage (I–V curve) of the device. We can see that the turn-on voltage under forward bias is ∼8 V, although significantly larger electrical current is not observed until 32 V. The large current beyond 32 V can be due to the high-voltage induced impact ionization, and hence avalanche breakdown [9]. The resistance before avalanche breakdown can be reduced, by decreasing the thickness of the HT-ZnO layer. However, a thinner HT-ZnO layer may lead to less effective carrier capture of the QWs. Under reverse bias, electrical current starts
Fig. 2. I–V curve of the device. Inset shows the PL spectrum of the CdZnO/ZnO QWs, with the fluctuations caused by Fabry– Perot oscillation.
Fig. 3. Output spectra at different injection current levels, when the LED device is operated under forward bias. Inset shows a photograph of the lit LED device, under forward bias at 20 mA.
to become significant at approximately −6 V. Figure 3 shows the output spectra at different injection current levels, when the LED device is operated under forward bias. The emission peak wavelength is around 510 nm at high injection current, indicating that the emission mainly originates from the QWs. The inset of Fig. 3 shows the photograph of the lit LED device, under forward bias at 20 mA, in injection current. The emitted green-yellow light can be observed only at the rim of the device mesa. This spatial emission distribution is attributed to the high conductivity of the GaZnO layer, such that injected electrons combine with holes of low mobility, which come from the p-contact surrounding the mesa, around the rim of the mesa. Figure 4 shows the output spectra at various injection current levels, under reverse bias. We can see three spectral components, including a sharp peak around 380 nm, a small peak around 430 nm, and a broad hump around 620 nm. The intensities of the features at 380 and 620 nm increase with increasing injection current, under reverse bias. However, although the intensity at 430 nm first increases with increasing reverse-biased current, it decreases beyond −40 mA. Because of the broad spectrum of the 620 nm feature, the emission color looks yellowreddish, although there is a blue component in the spectrum, as shown in the photograph of the lit LED (at −20 mA in injection current) in the inset of Fig. 5. The emission is quite uniform over the entire mesa area. Figure 5 shows the LED output spectra at three applied root-mean square (rms) voltage levels (10, 12, and 14 V),
Fig. 4. Output spectra at different injection current levels, when the LED device is operated under reverse bias. Inset shows a photograph of the lit LED device, under reverse bias at −20 mA.
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under AC operation at 60 Hz. The spectra cover a broad range, from shallow-UV through near-infrared. The inset of Fig. 5 shows the photograph of the lit LED driven at 12 V (rms), under AC operation. We can see the bright white light emission from the device. It is noted that the photograph in the inset is taken out of focus purposely, for visually mixing the green-yellow light from the mesa rim under forward bias, and the combined light of blue, yellow, and red colors from the mesa area under reverse bias. Such an out-of-focus image is similar to what is observed with human eyes. Figure 6 shows the band diagram of the LED device under reverse bias. In this situation, a strong electric field is built across the p-GaN layer, QW structure, and HTZnO layer. Hence, the electrons in the valence band of the p-GaN layer can move to the QW structure, and tunnel into its conduction band, through a QW-state-assisted tunneling process, as schematically shown in the inset of Fig. 6 [16]. During this process, because of the strong potential tilt in the QW structure, electrons cannot be effectively captured by the QW states for recombination. The tunneled electrons can move to the GaZnO layer, and fall into the deep-level states, for recombining with injected holes. Such recombination leads to the emission of the broad hump, covering the green through nearinfrared range in Fig. 4 [4,5,9]. Meanwhile, holes are available in the p-GaN layer, for recombining with injected electrons and emitting the spectral feature
around 430 nm [4,7]. As the applied voltage is increased, the even stronger electric field results in more tunneled electrons, for saturating the defect-level emission in the GaZnO layer. In this situation, the band-to-band emission of GaZnO emerges, to show the 380 nm peak in Fig. 4 [4,6,7]. Also, in increasing voltage, injected electrons (on the p-GaN side) are effectively pushed into the GaZnO layer for the band-to-band transition, such that the emission peak level at 380 nm keeps increasing, and that at 430 nm (emission of the p-GaN layer) is reduced, as shown in Fig. 4. When the injected electrons are pushed across the QW structure to reach the GaZnO layer, they cannot be effectively captured by the QWs, because of the strong potential tilt. The green-yellow LED output, in the forward-bias portion, makes an important contribution to white light generation under AC drive, even though its intensity is not strong enough for forming a peak in a spectrum shown in Fig. 5. The differences between the LED output spectra (Figs. 4 and 5) can be understood by evaluating their chromaticity coordinates. The results are shown in Fig. 7(a), which is a CIE 1931 chromaticity diagram, showing the coordinates of all the spectra in Figs. 3–5. The region with those spectral coordinates is magnified to give Fig. 7(b). The contours with the initial points labeled by A1, B1, and C1 correspond to the spectra in Figs. 3–5, respectively. The points of A1, B1, and C1 correspond to either the lowest injection current or the smallest applied AC voltage, under individual drive conditions. The chromaticity coordinates of LED output, when injection current or applied voltage increases, can be obtained by tracing along the individual contours. In the case of 12 V in rms applied voltage (∼17 V peak-topeak) under AC drive, the operation in the forward-bias portion remains in a small range of low injection current, leading to a small-intensity contribution to the overall emission. This contribution can be roughly represented by the chromaticity coordinate of point A1 at (0.350, 0.348); 5 mA in injection current or ∼18 V in applied voltage. Also, the contribution in the reverse-bias portion, under AC drive, can be roughly represented by the chromaticity coordinate of point B3 at (0.322, 0.272); 20 mA in injection current or ∼9 V in applied voltage. The combination of the forward-bias and reverse-bias emissions
Fig. 6. Band diagram of the LED device, under reverse bias. Inset schematically shows the QW-state-assisted tunneling process.
Fig. 7. (a) Contours of the coordinates of the LED output spectra in the CIE 1931 chromaticity diagram; (b) magnified chromaticity diagram of the concerned portion. Contours with the initial points of the lowest injection current, or applied voltage (labeled by A1, B1, and C1), correspond to the conditions under forward bias, reverse bias, and AC drive, respectively.
Fig. 5. Output spectra at three applied rms voltage levels (10, 12, and 14 V), under AC operation at 60 Hz. Inset shows the photograph of the lit LED driven at 12 V (rms), under AC operation.
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leads to the chromaticity coordinate of point C2 at (0.340, 0.319). Although the emission intensity from the forwardbias portion is small, it is important to shift the chromaticity coordinate, from the blue-violet-reddish region into the white area. In summary, we fabricated and characterized an ACoperated LED. Under forward bias, assisted by the HTZnO layer, the device emitted effectively green-yellow light from the QW structure, at the rim of the device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer moved into the conduction band of the GaZnO layer, through a QW-state-assisted tunneling process, to recombine with the injected holes in the GaZnO layer, for emitting yellow-red and shallow UV lights over the whole mesa area. The emission intensity under forward bias could be controlled such that under AC operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors, under forward and reverse biases, resulted in white light generation based on persistence of vision. This Letter was supported by Ministry of Science and Technology, Taiwan, Republic of China, under Grants NSC 102-2120-M-002-006, NSC 102-2221-E-002-204-MY3, and NSC 102-2221-E-002-199, by the Excellent Research Projects of National Taiwan University. References 1. J. Cho, J. Jung, J. H. Chae, H. Kim, J. W. Lee, S. Yoon, C. Sone, T. Jang, Y. Park, and E. Yoon, Jpn. J. Appl. Phys. 46, L1194 (2007). 2. G. A. Onushkin, Y. J. Lee, J. J. Yang, H. K. Kim, J. K. Son, G. H. Park, and Y. Park, IEEE Photon. Technol. Lett. 21, 33 (2009).
3. X. Y. Chen, A. M. C. Ng, F. Fang, A. B. Djurišić, W. K. Chan, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, J. Electrochem. Soc. 157, H308 (2010). 4. X. Y. Chen, A. M. C. Ng, F. Fang, Y. H. Ng, A. B. Djurišić, H. L. Tam, K. W. Cheah, S. Gwo, W. K. Chan, P. W. K. Fong, H. F. Lui, and C. Surya, J. Appl. Phys. 110, 094513 (2011). 5. H. Huang, G. Fang, S. Li, H. Long, X. Mo, H. Wang, Y. Li, Q. Jiang, D. L. Carroll, J. Wang, M. Wang, and X. Zhao, Appl. Phys. Lett. 99, 263502 (2011). 6. P. Chen, Xi. Ma, and D. Yang, J. Appl. Phys. 101, 053103 (2007). 7. M. K. Wu, Y. T. Shih, W. C. Li, H. C. Chen, M. J. Chen, H. Kuan, J. R. Yang, and M. Shiojiri, IEEE Photon. Technol. Lett. 20, 1772 (2008). 8. C. W. Chang, W. C. Tan, M. L. Lu, T. C. Pan, Y. J. Yang, and Y. F. Chen, Adv. Funct. Mater. 23, 4043 (2013). 9. P. N. Ni, C. X. Shan, B. H. Li, S. P. Wang, and D. Z. Shen, ACS Appl. Mater. Interfaces 6, 8257 (2014). 10. H. S. Chen, S. Y. Ting, C. H. Liao, C. Y. Chen, C. Hsieh, Y. F. Yao, H. T. Chen, Y. W. Kiang, and C. C. Yang, IEEE Photon. Technol. Lett. 25, 317 (2013). 11. S. Sadofev, S. Kalusniak, J. Puls, P. Schäfer, S. Blumstengel, and F. Henneberger, Appl. Phys. Lett. 91, 231103 (2007). 12. S. Blumstengel, S. Sadofev, H. Kirmse, and F. Henneberger, Appl. Phys. Lett. 98, 031907 (2011). 13. S. Y. Ting, Y. F. Yao, W. L. Chung, W. M. Chang, C. Y. Chen, H. T. Chen, C. H. Liao, H. S. Chen, C. Hsieh, and C. C. Yang, Opt. Express 20, 21860 (2012). 14. W. I. Park and G. C. Yi, Adv. Mater. 16, 87 (2004). 15. H. Long, S. Li, X. Mo, H. Wang, H. Huang, Z. Chen, Y. Liu, and G. Fang, Appl. Phys. Lett. 103, 123504 (2013). 16. J. Singh, Physics of Semiconductors and Their Heterostructures (McGraw-Hill, 1993).
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Phosphor-free, white-light LED under alternating-current operation Yu-Feng Yao, Hao-Tsung Chen, Chia-Ying Su, Chieh Hsieh, Chun-Han Lin, Yean-Woei Kiang, and C. C. Yang* Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei 10617, Taiwan *Corresponding author: [email protected] Received September 18, 2014; revised October 12, 2014; accepted October 12, 2014; posted October 14, 2014 (Doc. ID 223271); published November 3, 2014 A light-emitting diode structure, consisting of a p-GaN layer, a CdZnO/ZnO quantum-well (QW) structure, a hightemperature-grown ZnO layer, and a GaZnO layer, is fabricated. Under forward bias, the device effectively emits green–yellow light, from the QW structure, at the rim of device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer move into the conduction band of the GaZnO layer, through a QW-state-assisted tunneling process, to recombine with the injected holes in the GaZnO layer, for emitting yellow–red and shallow ultraviolet light over the entire mesa area. Also, carrier recombination in the p-GaN layer produces blue light. By properly designing the thickness of the high-temperature grown ZnO layer, the emission intensity under forward bias can be controlled such that, under alternating-current operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors, under forward and reverse biases, result in white light generation based on persistence of vision. © 2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (250.5590) Quantum-well, -wire and -dot devices. http://dx.doi.org/10.1364/OL.39.006371
Although a nitride-based light-emitting diode (LED) has been commercialized, it needs to be driven under the direct current (DC) condition. In this situation, a transformer is required for converting alternating current (AC) into DC, before LED application. Although implementations of AC-LED have been claimed, based on circuit designs for driving multiple LEDs [1,2], a single AC-driven LED device has not been reported. For the purpose of fabricating an AC-driven LED device, an LED structure, allowing current flow and carrier recombination under reverse bias, needs to be implemented first. Based on the electron tunneling mechanism under reverse bias, various device structures have been fabricated, to demonstrate emissions under both forward and reverse biases. Because of the large band offset between p-GaN and highly doped n-ZnO, the Fermi level in a p-GaN/n-ZnO junction can be close to the conduction band edge of n-ZnO. In this situation, under reverse bias, electrons can tunnel from the valence band of p-GaN, into the conduction band of n-ZnO, for carrier recombination and hence emission in the n-ZnO layer [3,4]. A similar phenomenon can also be observed in an n-ZnO/ n-GaN structure [5], and an n-ZnO/p-Si structure [6]. To slow down electrons under forward bias, for achieving effective carrier recombination on the n-type side, a thin layer of an insulating or high-band-gap material, such as SiO2 or MgO, was inserted at the junction [7–9]. Also, the n-type semiconductor material was replaced by graphene [8] or Au [9], to form a metal-insulator-semiconductor structure. Meanwhile, to control the emission wavelength under forward bias, an MgZnO heterostructure was added to the junction [9]. Although the emission wavelength can be changed, by switching device operation between forward and reverse biases, AC operation of such an LED has not been demonstrated. Because of the small lattice mismatch between GaN and ZnO, p-GaN has been used to replace p-ZnO for ZnObased device fabrication [10]. In this Letter, we report fabrication and characterization of an LED structure, 0146-9592/14/226371-04$15.00/0
consisting of a p-GaN layer, a CdZnO/ZnO quantum well (QW) structure, and a GaZnO layer, for implementing light emission of complementary colors, under forward and reverse biases. Under forward bias, carrier recombination in the QW structure provides us with green-yellow light, at the rim of device mesa. Under reverse bias, a QW-state-assisted tunneling process leads to carrier recombination in the GaZnO layer, for generating red, yellow, and shallow ultraviolet (UV) light over the entire mesa. The spectrally, spatially, and temporally mixed emission under AC operation results in phosphor-free, white-light generation. Figure 1 schematically shows the structure of the LED device. On a c-plane sapphire substrate, a template of a 2 μm un-doped GaN (u-GaN) layer (grown at 1000°C), and then a 500 nm p-type GaN (p-GaN) layer (grown at 930°C), is prepared with metal-organic chemical vapor deposition. In the p-GaN layer, the hole concentration is 2 × 1017 cm−3 . On this template, a structure of threeperiod CdZnO/ZnO QW is deposited, followed by a hightemperature (HT) grown un-doped ZnO (i.e., HT-ZnO) layer of 20 nm in thickness, and a 120 nm GaZnO layer, all with a molecular beam epitaxy reactor. The CdZnO/ ZnO QW structure is designed for controlling its emission wavelength around the green range, under forward bias
Fig. 1. LED device structure. © 2014 Optical Society of America
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(a positive voltage applied to the p-GaN layer) [10–13]. The use of GaZnO of a high electron concentration, as the n-type material, can make the Fermi level close to its conduction band. In this situation, the offset between the valence band of p-GaN and the conduction band of GaZnO becomes smaller, such that electron tunneling can occur at a lower applied voltage under reverse bias (a positive voltage applied to the GaZnO layer) [14]. The oxygen-rich HT-ZnO layer is used to create a potential barrier, for slowing down injected electrons from the GaZnO layer, and hence more effective QW capture [15]. However, use of this layer leads to a larger turn-on voltage and device resistance, and hence weaker emission under forward bias. For growing a CdZnO well, a ZnO barrier, the HT-ZnO, and the GaZnO layers, the substrate temperatures (Zn effusion cell) are 175°C, 175°C, 555°C, and 350°C (280°C, 280°C, 285°C, and 320°C), respectively. The Cd effusion cell temperature for growing a CdZnO well layer is 250°C. The Ga effusion cell temperature for growing the GaZnO layer is 700°C. Under the previously mentioned GaZnO growth condition, the resistivity, mobility, and electron concentration of the GaZnO thin film are 1.9 × 10−4 Ω · cm, 37 cm2 ∕V · s, and 8.4× 1020 cm−3 , respectively. The GaZnO layer is an alloy, with Ga content varying spatially from 5% to 8%. The n-contact with Au/Ti is formed at the center of the device mesa. The p-contact is fabricated surrounding the mesa with Au/Ni. The mesa dimension is 300 × 300 μm. The inset of Fig. 2 shows the photoluminescence (PL) spectrum of the CdZnO/ZnO QWs, measured before growth of the GaZnO layer. Here, the spectral fluctuations are caused by Fabry–Perot oscillation. One can see that the PL spectral peak is ∼494 nm, after the Fabry–Perot oscillation is filtered. The small peak at 376 nm, indicated by the arrow, corresponds to the band-edge emission of ZnO. Figure 2 shows the relation between injection current and applied voltage (I–V curve) of the device. We can see that the turn-on voltage under forward bias is ∼8 V, although significantly larger electrical current is not observed until 32 V. The large current beyond 32 V can be due to the high-voltage induced impact ionization, and hence avalanche breakdown [9]. The resistance before avalanche breakdown can be reduced, by decreasing the thickness of the HT-ZnO layer. However, a thinner HT-ZnO layer may lead to less effective carrier capture of the QWs. Under reverse bias, electrical current starts
Fig. 2. I–V curve of the device. Inset shows the PL spectrum of the CdZnO/ZnO QWs, with the fluctuations caused by Fabry– Perot oscillation.
Fig. 3. Output spectra at different injection current levels, when the LED device is operated under forward bias. Inset shows a photograph of the lit LED device, under forward bias at 20 mA.
to become significant at approximately −6 V. Figure 3 shows the output spectra at different injection current levels, when the LED device is operated under forward bias. The emission peak wavelength is around 510 nm at high injection current, indicating that the emission mainly originates from the QWs. The inset of Fig. 3 shows the photograph of the lit LED device, under forward bias at 20 mA, in injection current. The emitted green-yellow light can be observed only at the rim of the device mesa. This spatial emission distribution is attributed to the high conductivity of the GaZnO layer, such that injected electrons combine with holes of low mobility, which come from the p-contact surrounding the mesa, around the rim of the mesa. Figure 4 shows the output spectra at various injection current levels, under reverse bias. We can see three spectral components, including a sharp peak around 380 nm, a small peak around 430 nm, and a broad hump around 620 nm. The intensities of the features at 380 and 620 nm increase with increasing injection current, under reverse bias. However, although the intensity at 430 nm first increases with increasing reverse-biased current, it decreases beyond −40 mA. Because of the broad spectrum of the 620 nm feature, the emission color looks yellowreddish, although there is a blue component in the spectrum, as shown in the photograph of the lit LED (at −20 mA in injection current) in the inset of Fig. 5. The emission is quite uniform over the entire mesa area. Figure 5 shows the LED output spectra at three applied root-mean square (rms) voltage levels (10, 12, and 14 V),
Fig. 4. Output spectra at different injection current levels, when the LED device is operated under reverse bias. Inset shows a photograph of the lit LED device, under reverse bias at −20 mA.
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under AC operation at 60 Hz. The spectra cover a broad range, from shallow-UV through near-infrared. The inset of Fig. 5 shows the photograph of the lit LED driven at 12 V (rms), under AC operation. We can see the bright white light emission from the device. It is noted that the photograph in the inset is taken out of focus purposely, for visually mixing the green-yellow light from the mesa rim under forward bias, and the combined light of blue, yellow, and red colors from the mesa area under reverse bias. Such an out-of-focus image is similar to what is observed with human eyes. Figure 6 shows the band diagram of the LED device under reverse bias. In this situation, a strong electric field is built across the p-GaN layer, QW structure, and HTZnO layer. Hence, the electrons in the valence band of the p-GaN layer can move to the QW structure, and tunnel into its conduction band, through a QW-state-assisted tunneling process, as schematically shown in the inset of Fig. 6 [16]. During this process, because of the strong potential tilt in the QW structure, electrons cannot be effectively captured by the QW states for recombination. The tunneled electrons can move to the GaZnO layer, and fall into the deep-level states, for recombining with injected holes. Such recombination leads to the emission of the broad hump, covering the green through nearinfrared range in Fig. 4 [4,5,9]. Meanwhile, holes are available in the p-GaN layer, for recombining with injected electrons and emitting the spectral feature
around 430 nm [4,7]. As the applied voltage is increased, the even stronger electric field results in more tunneled electrons, for saturating the defect-level emission in the GaZnO layer. In this situation, the band-to-band emission of GaZnO emerges, to show the 380 nm peak in Fig. 4 [4,6,7]. Also, in increasing voltage, injected electrons (on the p-GaN side) are effectively pushed into the GaZnO layer for the band-to-band transition, such that the emission peak level at 380 nm keeps increasing, and that at 430 nm (emission of the p-GaN layer) is reduced, as shown in Fig. 4. When the injected electrons are pushed across the QW structure to reach the GaZnO layer, they cannot be effectively captured by the QWs, because of the strong potential tilt. The green-yellow LED output, in the forward-bias portion, makes an important contribution to white light generation under AC drive, even though its intensity is not strong enough for forming a peak in a spectrum shown in Fig. 5. The differences between the LED output spectra (Figs. 4 and 5) can be understood by evaluating their chromaticity coordinates. The results are shown in Fig. 7(a), which is a CIE 1931 chromaticity diagram, showing the coordinates of all the spectra in Figs. 3–5. The region with those spectral coordinates is magnified to give Fig. 7(b). The contours with the initial points labeled by A1, B1, and C1 correspond to the spectra in Figs. 3–5, respectively. The points of A1, B1, and C1 correspond to either the lowest injection current or the smallest applied AC voltage, under individual drive conditions. The chromaticity coordinates of LED output, when injection current or applied voltage increases, can be obtained by tracing along the individual contours. In the case of 12 V in rms applied voltage (∼17 V peak-topeak) under AC drive, the operation in the forward-bias portion remains in a small range of low injection current, leading to a small-intensity contribution to the overall emission. This contribution can be roughly represented by the chromaticity coordinate of point A1 at (0.350, 0.348); 5 mA in injection current or ∼18 V in applied voltage. Also, the contribution in the reverse-bias portion, under AC drive, can be roughly represented by the chromaticity coordinate of point B3 at (0.322, 0.272); 20 mA in injection current or ∼9 V in applied voltage. The combination of the forward-bias and reverse-bias emissions
Fig. 6. Band diagram of the LED device, under reverse bias. Inset schematically shows the QW-state-assisted tunneling process.
Fig. 7. (a) Contours of the coordinates of the LED output spectra in the CIE 1931 chromaticity diagram; (b) magnified chromaticity diagram of the concerned portion. Contours with the initial points of the lowest injection current, or applied voltage (labeled by A1, B1, and C1), correspond to the conditions under forward bias, reverse bias, and AC drive, respectively.
Fig. 5. Output spectra at three applied rms voltage levels (10, 12, and 14 V), under AC operation at 60 Hz. Inset shows the photograph of the lit LED driven at 12 V (rms), under AC operation.
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leads to the chromaticity coordinate of point C2 at (0.340, 0.319). Although the emission intensity from the forwardbias portion is small, it is important to shift the chromaticity coordinate, from the blue-violet-reddish region into the white area. In summary, we fabricated and characterized an ACoperated LED. Under forward bias, assisted by the HTZnO layer, the device emitted effectively green-yellow light from the QW structure, at the rim of the device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer moved into the conduction band of the GaZnO layer, through a QW-state-assisted tunneling process, to recombine with the injected holes in the GaZnO layer, for emitting yellow-red and shallow UV lights over the whole mesa area. The emission intensity under forward bias could be controlled such that under AC operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors, under forward and reverse biases, resulted in white light generation based on persistence of vision. This Letter was supported by Ministry of Science and Technology, Taiwan, Republic of China, under Grants NSC 102-2120-M-002-006, NSC 102-2221-E-002-204-MY3, and NSC 102-2221-E-002-199, by the Excellent Research Projects of National Taiwan University. References 1. J. Cho, J. Jung, J. H. Chae, H. Kim, J. W. Lee, S. Yoon, C. Sone, T. Jang, Y. Park, and E. Yoon, Jpn. J. Appl. Phys. 46, L1194 (2007). 2. G. A. Onushkin, Y. J. Lee, J. J. Yang, H. K. Kim, J. K. Son, G. H. Park, and Y. Park, IEEE Photon. Technol. Lett. 21, 33 (2009).
3. X. Y. Chen, A. M. C. Ng, F. Fang, A. B. Djurišić, W. K. Chan, H. L. Tam, K. W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, J. Electrochem. Soc. 157, H308 (2010). 4. X. Y. Chen, A. M. C. Ng, F. Fang, Y. H. Ng, A. B. Djurišić, H. L. Tam, K. W. Cheah, S. Gwo, W. K. Chan, P. W. K. Fong, H. F. Lui, and C. Surya, J. Appl. Phys. 110, 094513 (2011). 5. H. Huang, G. Fang, S. Li, H. Long, X. Mo, H. Wang, Y. Li, Q. Jiang, D. L. Carroll, J. Wang, M. Wang, and X. Zhao, Appl. Phys. Lett. 99, 263502 (2011). 6. P. Chen, Xi. Ma, and D. Yang, J. Appl. Phys. 101, 053103 (2007). 7. M. K. Wu, Y. T. Shih, W. C. Li, H. C. Chen, M. J. Chen, H. Kuan, J. R. Yang, and M. Shiojiri, IEEE Photon. Technol. Lett. 20, 1772 (2008). 8. C. W. Chang, W. C. Tan, M. L. Lu, T. C. Pan, Y. J. Yang, and Y. F. Chen, Adv. Funct. Mater. 23, 4043 (2013). 9. P. N. Ni, C. X. Shan, B. H. Li, S. P. Wang, and D. Z. Shen, ACS Appl. Mater. Interfaces 6, 8257 (2014). 10. H. S. Chen, S. Y. Ting, C. H. Liao, C. Y. Chen, C. Hsieh, Y. F. Yao, H. T. Chen, Y. W. Kiang, and C. C. Yang, IEEE Photon. Technol. Lett. 25, 317 (2013). 11. S. Sadofev, S. Kalusniak, J. Puls, P. Schäfer, S. Blumstengel, and F. Henneberger, Appl. Phys. Lett. 91, 231103 (2007). 12. S. Blumstengel, S. Sadofev, H. Kirmse, and F. Henneberger, Appl. Phys. Lett. 98, 031907 (2011). 13. S. Y. Ting, Y. F. Yao, W. L. Chung, W. M. Chang, C. Y. Chen, H. T. Chen, C. H. Liao, H. S. Chen, C. Hsieh, and C. C. Yang, Opt. Express 20, 21860 (2012). 14. W. I. Park and G. C. Yi, Adv. Mater. 16, 87 (2004). 15. H. Long, S. Li, X. Mo, H. Wang, H. Huang, Z. Chen, Y. Liu, and G. Fang, Appl. Phys. Lett. 103, 123504 (2013). 16. J. Singh, Physics of Semiconductors and Their Heterostructures (McGraw-Hill, 1993).
