- Email: [email protected]
n-GaN heterojunction
Journal of Luminescence 132 (2012) 1642–1645
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low-threshold pure UV electroluminescence from n-ZnO:Al/i-layer/n-GaN heterojunction Songzhan Li a,b, Guojia Fang a,n, Hao Long a, Haoning Wang a, Huihui Huang a, Xiaoming Mo a, Xingzhong Zhao a a
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, Department of Electronic Science and Technology, School of Physics and Technology, Wuhan University, Wuhan, Hubei 430072, PR China b School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan, Hubei 430073, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2011 Received in revised form 9 February 2012 Accepted 17 February 2012 Available online 28 February 2012
Ultraviolet (UV) electroluminescence (EL) of n-ZnO:Al (AZO)/i-layer/n-GaN heterojunctions with different intrinsic layers has been obtained. Rectifying behavior and EL spectra of the heterojunctions are investigated at room temperature. Under positive voltage, a dominant UV emission peak around 370 nm is observed for both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Nevertheless, the UV emission peak intensity of AZO/i-MgO/n-GaN heterojunction is much stronger than that of AZO/ i-ZnO/n-GaN heterojunction at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. The difference of EL spectra and the emission mechanism in these devices are discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Low-threshold UV Electroluminescence Magnetron sputtering
1. Introduction Short-wavelength semiconductor light-emitting devices are of great interest for a variety of applications such as microelectronic photolithography technology, backlight display, and biomedical analysis [1–6]. II–VI wide band gap semiconductor ZnO and III–V nitride semiconductor GaN are the most widely used materials for developing ultraviolet (UV) electroluminescence (EL). There are several reports on the EL of the heterojunctions and homojunctions based on ZnO or/and GaN, and the EL spectra cover the emission from visible to UV regions [7–12]. These reports are primarily focused on the p–n and p–i–n related structure junctions. Compared with the p–n junction, the n–n junction can generate the excitation level carriers from the interface localized states in the light or electric field, and it can easily form the closed carrier injection effect in virtue of the barrier spike. Moreover, the n–n junction can make the diffusion potential difference in the interface has the same polarity as that of the p–n junction, and then it has the majority carrier controlled rectifier effect [13,14]. Recently, some groups have reported the EL from ZnO-based n–n isotype heterojunctions, such as n-ZnO/n-Si and n-ZnO/n-GaAs [15–17]. However, there is only visible emission observed or the visible emission is predominant in the whole EL spectra from
n
Corresponding author. Tel.: þ86 27 6875 2147. E-mail address: [email protected] (G. Fang).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.02.038
these heterojunctions, while the UV emission is absent in their EL spectra. In this paper, we have fabricated n-ZnO:Al (AZO)/i-layer/ n-GaN (n–i–n) isotype heterostructures by radio frequency (RF) magnetron sputtering. The pure UV EL has been obtained at room temperature and the related mechanisms have been comparatively studied.
2. Experimental For the fabrication of the heterojunctions, commercially available GaN/sapphire templates were used. The GaN layer presents n-type conductivity with a carrier concentration 2 1018 cm 3, and the thickness is about 2.2 mm. An intrinsic layer (MgO or ZnO) with the thickness about 50 nm was subsequently grown on GaN/ Al2O3 (0 0 0 1) template by RF magnetron sputtering. The i-MgO layer is deposited at 300 1C from a Mg target, and i-ZnO layer was sputtered from a pure ZnO ceramics target at 300 1C with the pressure of 1.0 Pa. Thereafter, a 300 nm thick ZnO:Al layer was prepared by RF magnetron sputtering from Al doped ZnO ceramics target with the substrate temperature of 300 1C. The Hall measurement reveals that the electron concentration and mobility in AZO layer was 7 1018 cm 3 and 11 cm2 V 1 s 1, respectively, showing n-type characteristics. After the film growth, Ag electrodes were deposited on the surface of the AZO film by direct-current magnetron sputtering with the thickness about
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Fig. 2. I–V characteristics of AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Inset shows the I–V curves of In and Ag Ohmic contacts to n-GaN and n-ZnO:Al.
Fig. 1. (a) Schematic diagram of the fabricated n-AZO/i-layer/n-GaN heterojunction device, and (b) XRD pattern of sputtering ZnO film on n-GaN substrate.
100 nm, and In was deposited on GaN as another electrode. The schematic diagram of the fabricated n-AZO/i-layer/n-GaN heterojunction is shown in Fig. 1(a). The crystal structures of the films were characterized by X-ray diffraction (XRD, Burker Axs, D8 Advance) with Cu Ka radiation. The current–voltage (I–V) characteristics of the n–i–n heterojunction devices were measured with a Keithley 4200 semiconductor characterization system. The EL spectra for the n–i–n heterojunctions under different dc bias voltages were recorded using an Acton ˚ Note SpectraPro 2500i spectrometer with a scanning step size of 1 A. that all of the measurements were performed at room temperature.
3. Results and discussion Fig. 1(b) shows XRD pattern of sputtered ZnO film on n-GaN substrate. The high-quality n-GaN substrate has extremely strong (0 0 2) diffraction peak in XRD pattern. There is a pronounced peak, at around 34.41, indexed as the hexagonal ZnO (0 0 2), definitely indicating that ZnO film is hexagonal-wurtzite structured with highly c-axis preferred orientation [18,19]. The typical I–V characteristics of the AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions are shown in Fig. 2. Good Ohmic contacts are established and confirmed for Ag/AZO and In/GaN, as shown in the inset of Fig. 2. The I–V curves of both n–i– n heterojunctions, showing nonlinear at positive and negative bias, are reflecting the behavior of the back-to-back Schottky diode. In addition, it can be found that the current of AZO/i-ZnO/ n-GaN heterojunction is larger than that of AZO/i-MgO/n-GaN at the same voltage. This phenomenon is due to that the dielectric
constant of the i-MgO layer is larger than that of the i-ZnO layer, which attributes to the dielectric nature of i-MgO. The room temperature EL spectra of both AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterostructured devices with various positive voltages are shown in Fig. 3(a) and (b), respectively. Seen from Fig. 3, EL spectra are dominated by UV emission peak at around 370 nm under various voltages for both AZO/i-MgO/nGaN and AZO/i-ZnO/n-GaN heterostructured devices, while the visible emissions are greatly suppressed. The emission peak is situated at 366 nm at lower working voltage and shifts to 370 nm at higher applied voltage. Moreover, UV emission peaks become stronger and narrower with the increase of positive voltage. Comparing Fig. 3(a) with Fig. 3(b), it is worth noting that the UV emission peak intensity of AZO/i-MgO/n-GaN heterostructured device is much stronger than that of AZO/i-ZnO/n-GaN heterostructured devices at the same voltage. As shown in the inset of Fig. 3(a), the full-width at half maximum (FWHM) of UV emission peaks is about 8 nm. The redshift in UV emission EL spectra with increasing bias voltage can be attributed to the thermal effect of the device under larger injection current [20,21]. The photoluminescence (PL) data of n-GaN and ZnO:Al film was shown in the inset of Fig. 3(b). The PL spectrum of n-GaN film shows a dominant sharp near-band-edge emission at around 366 nm, and the deep-level emission is almost undetectable. However, PL of ZnO:Al film exhibits a weak emission band in visible region. As for the origin of the EL, the UV emission peak originates from the near-band edge emission of n-GaN. Since the carrier concentration of ZnO:Al is higher than that of GaN, it can be expected that the depletion region of the n–n heterojunction is mostly resided in the n-GaN region. Therefore, the radiative recombination occurs mainly at the n-GaN region. To investigate the UV EL mechanism of AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterostructured devices under bias voltage, the energy band diagrams have been built based on Anderson model [22], as shown in Fig. 4. The energy barrier for the electrons DEc in the interface of GaN/MgO is much higher than that for holes DEv in the interface of AZO/MgO for AZO/i-MgO/n-GaN heterojunction. It is expected that electrons in the n-GaN layer will be blocked by the MgO layer and accumulate at the interface of GaN/MgO (shown in Fig. 4(a)). The origin of the holes is the key to understand the mechanism of UV EL. Due to the quantum
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Fig. 4. Energy band diagrams of AZO/i-MgO/n-GaN (a) and AZO/i-ZnO/n-GaN (b) heterojunction under positive bias.
Fig. 3. (a) Room temperature EL spectra of AZO/i-MgO/n-GaN heterostructured device under various positive voltages, the inset shows the EL spectra in the UV band of AZO/i-MgO/n-GaN heterostructured device. (b) EL spectra of AZO/i-ZnO/ n-GaN heterostructured device with various positive voltages, inset is the PL spectrum of n-GaN and ZnO:Al films.
confinement effect, an inversion layer will be gradually induced in n-ZnO:Al layer, resulting in 2-D holes gas near the interface of MgO/AZO. On the other hand, under high electric field, most of the voltage will be applied on the MgO layer due to its dielectric nature. Although the carriers in the MgO layer are few, the carriers will gain much high energy under such a high electric field and they will impact with the lattice of the MgO layer [23], thus the holes in the valence band will be excited. In such a way, holes are generated by the inversion layer and the impactionization process. Under the forward bias, the generated holes can be injected into the GaN layer, and recombine radiatively with the electrons accumulated at the MgO/GaN interface, resulting in UV emission from AZO/i-MgO/n-GaN heterojunction. As we all know, the band gap of i-ZnO is much smaller than that of MgO, the effective energy barrier for the electrons DEc in the conduction band is greatly reduced, as shown in Fig. 4(b). Therefore, there are no accumulated electrons that will be involved in the effective recombination of electron-hole pairs. As a result, the intensity of UV EL emission from AZO/i-ZnO/n-GaN heterojunction is much weaker than that of AZO/i-MgO/n-GaN at the same voltage.
Fig. 5. Integrated intensity of the UV emission of AZO/i-MgO/n-GaN heterostructured device as a function of positive voltage.
Fig. 5 shows the integrated intensity of UV emission for AZO/iMgO/n-GaN heterostructured device as a function of positive voltage. The UV emission intensity enhances very much when the bias voltage is about 2.3 V, which is the threshold voltage of AZO/i-MgO/n-GaN heterostructured device. Such a low threshold voltage has not been reported in similar structured heteojunction devices [24,25]. The electrical conductivity of AZO layer is 8.18 10 2 O cm, and then the low threshold voltage is due to the AZO layer with superior electrical conductivity. The AZO layer has double functions in this structure: (i) AZO layer is regarded as an improved n type layer, which increases the generation
S. Li et al. / Journal of Luminescence 132 (2012) 1642–1645
efficiency of carriers, (ii) it acts as a transparent current spreading layer underneath the Ag electrode, reducing current crowding effect of the device. Moreover, the smaller series resistance also causes the smaller heating effect, which is beneficial to the operation of the device.
4. Conclusion In summary, we have demonstrated AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterojunctions. Under various voltages, the EL spectra of both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions are dominated by UV emission peaks, while the visible emissions are greatly suppressed. UV emission of AZO/i-MgO/nGaN heterojunction is stronger than that of AZO/i-ZnO/n-GaN at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. This study demonstrates the possibility of fabrication of AZO/i-MgO/n-GaN UV EL devices which can be driven by two ordinary dry batteries. Such AZO/i-MgO/n-GaN isotype heterojunction is promising for the development of short-wavelength and high performance optoelectronic devices.
Acknowledgments This work was supported in part by the Natural Science Foundation of China (11074194, 11104209), by the National High Technology Research and Development Program of China (2009AA03Z219), by the National Basic Research Program of China (2011CB933300), by the Natural Science Foundation of Hubei Province (2010CDA016), by the Education Department of Hubei Province (Q20101608), and by the Fundamental Research Funds for the Central Universities.
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Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low-threshold pure UV electroluminescence from n-ZnO:Al/i-layer/n-GaN heterojunction Songzhan Li a,b, Guojia Fang a,n, Hao Long a, Haoning Wang a, Huihui Huang a, Xiaoming Mo a, Xingzhong Zhao a a
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, Department of Electronic Science and Technology, School of Physics and Technology, Wuhan University, Wuhan, Hubei 430072, PR China b School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan, Hubei 430073, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2011 Received in revised form 9 February 2012 Accepted 17 February 2012 Available online 28 February 2012
Ultraviolet (UV) electroluminescence (EL) of n-ZnO:Al (AZO)/i-layer/n-GaN heterojunctions with different intrinsic layers has been obtained. Rectifying behavior and EL spectra of the heterojunctions are investigated at room temperature. Under positive voltage, a dominant UV emission peak around 370 nm is observed for both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Nevertheless, the UV emission peak intensity of AZO/i-MgO/n-GaN heterojunction is much stronger than that of AZO/ i-ZnO/n-GaN heterojunction at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. The difference of EL spectra and the emission mechanism in these devices are discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Low-threshold UV Electroluminescence Magnetron sputtering
1. Introduction Short-wavelength semiconductor light-emitting devices are of great interest for a variety of applications such as microelectronic photolithography technology, backlight display, and biomedical analysis [1–6]. II–VI wide band gap semiconductor ZnO and III–V nitride semiconductor GaN are the most widely used materials for developing ultraviolet (UV) electroluminescence (EL). There are several reports on the EL of the heterojunctions and homojunctions based on ZnO or/and GaN, and the EL spectra cover the emission from visible to UV regions [7–12]. These reports are primarily focused on the p–n and p–i–n related structure junctions. Compared with the p–n junction, the n–n junction can generate the excitation level carriers from the interface localized states in the light or electric field, and it can easily form the closed carrier injection effect in virtue of the barrier spike. Moreover, the n–n junction can make the diffusion potential difference in the interface has the same polarity as that of the p–n junction, and then it has the majority carrier controlled rectifier effect [13,14]. Recently, some groups have reported the EL from ZnO-based n–n isotype heterojunctions, such as n-ZnO/n-Si and n-ZnO/n-GaAs [15–17]. However, there is only visible emission observed or the visible emission is predominant in the whole EL spectra from
n
Corresponding author. Tel.: þ86 27 6875 2147. E-mail address: [email protected] (G. Fang).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.02.038
these heterojunctions, while the UV emission is absent in their EL spectra. In this paper, we have fabricated n-ZnO:Al (AZO)/i-layer/ n-GaN (n–i–n) isotype heterostructures by radio frequency (RF) magnetron sputtering. The pure UV EL has been obtained at room temperature and the related mechanisms have been comparatively studied.
2. Experimental For the fabrication of the heterojunctions, commercially available GaN/sapphire templates were used. The GaN layer presents n-type conductivity with a carrier concentration 2 1018 cm 3, and the thickness is about 2.2 mm. An intrinsic layer (MgO or ZnO) with the thickness about 50 nm was subsequently grown on GaN/ Al2O3 (0 0 0 1) template by RF magnetron sputtering. The i-MgO layer is deposited at 300 1C from a Mg target, and i-ZnO layer was sputtered from a pure ZnO ceramics target at 300 1C with the pressure of 1.0 Pa. Thereafter, a 300 nm thick ZnO:Al layer was prepared by RF magnetron sputtering from Al doped ZnO ceramics target with the substrate temperature of 300 1C. The Hall measurement reveals that the electron concentration and mobility in AZO layer was 7 1018 cm 3 and 11 cm2 V 1 s 1, respectively, showing n-type characteristics. After the film growth, Ag electrodes were deposited on the surface of the AZO film by direct-current magnetron sputtering with the thickness about
S. Li et al. / Journal of Luminescence 132 (2012) 1642–1645
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Fig. 2. I–V characteristics of AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Inset shows the I–V curves of In and Ag Ohmic contacts to n-GaN and n-ZnO:Al.
Fig. 1. (a) Schematic diagram of the fabricated n-AZO/i-layer/n-GaN heterojunction device, and (b) XRD pattern of sputtering ZnO film on n-GaN substrate.
100 nm, and In was deposited on GaN as another electrode. The schematic diagram of the fabricated n-AZO/i-layer/n-GaN heterojunction is shown in Fig. 1(a). The crystal structures of the films were characterized by X-ray diffraction (XRD, Burker Axs, D8 Advance) with Cu Ka radiation. The current–voltage (I–V) characteristics of the n–i–n heterojunction devices were measured with a Keithley 4200 semiconductor characterization system. The EL spectra for the n–i–n heterojunctions under different dc bias voltages were recorded using an Acton ˚ Note SpectraPro 2500i spectrometer with a scanning step size of 1 A. that all of the measurements were performed at room temperature.
3. Results and discussion Fig. 1(b) shows XRD pattern of sputtered ZnO film on n-GaN substrate. The high-quality n-GaN substrate has extremely strong (0 0 2) diffraction peak in XRD pattern. There is a pronounced peak, at around 34.41, indexed as the hexagonal ZnO (0 0 2), definitely indicating that ZnO film is hexagonal-wurtzite structured with highly c-axis preferred orientation [18,19]. The typical I–V characteristics of the AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions are shown in Fig. 2. Good Ohmic contacts are established and confirmed for Ag/AZO and In/GaN, as shown in the inset of Fig. 2. The I–V curves of both n–i– n heterojunctions, showing nonlinear at positive and negative bias, are reflecting the behavior of the back-to-back Schottky diode. In addition, it can be found that the current of AZO/i-ZnO/ n-GaN heterojunction is larger than that of AZO/i-MgO/n-GaN at the same voltage. This phenomenon is due to that the dielectric
constant of the i-MgO layer is larger than that of the i-ZnO layer, which attributes to the dielectric nature of i-MgO. The room temperature EL spectra of both AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterostructured devices with various positive voltages are shown in Fig. 3(a) and (b), respectively. Seen from Fig. 3, EL spectra are dominated by UV emission peak at around 370 nm under various voltages for both AZO/i-MgO/nGaN and AZO/i-ZnO/n-GaN heterostructured devices, while the visible emissions are greatly suppressed. The emission peak is situated at 366 nm at lower working voltage and shifts to 370 nm at higher applied voltage. Moreover, UV emission peaks become stronger and narrower with the increase of positive voltage. Comparing Fig. 3(a) with Fig. 3(b), it is worth noting that the UV emission peak intensity of AZO/i-MgO/n-GaN heterostructured device is much stronger than that of AZO/i-ZnO/n-GaN heterostructured devices at the same voltage. As shown in the inset of Fig. 3(a), the full-width at half maximum (FWHM) of UV emission peaks is about 8 nm. The redshift in UV emission EL spectra with increasing bias voltage can be attributed to the thermal effect of the device under larger injection current [20,21]. The photoluminescence (PL) data of n-GaN and ZnO:Al film was shown in the inset of Fig. 3(b). The PL spectrum of n-GaN film shows a dominant sharp near-band-edge emission at around 366 nm, and the deep-level emission is almost undetectable. However, PL of ZnO:Al film exhibits a weak emission band in visible region. As for the origin of the EL, the UV emission peak originates from the near-band edge emission of n-GaN. Since the carrier concentration of ZnO:Al is higher than that of GaN, it can be expected that the depletion region of the n–n heterojunction is mostly resided in the n-GaN region. Therefore, the radiative recombination occurs mainly at the n-GaN region. To investigate the UV EL mechanism of AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterostructured devices under bias voltage, the energy band diagrams have been built based on Anderson model [22], as shown in Fig. 4. The energy barrier for the electrons DEc in the interface of GaN/MgO is much higher than that for holes DEv in the interface of AZO/MgO for AZO/i-MgO/n-GaN heterojunction. It is expected that electrons in the n-GaN layer will be blocked by the MgO layer and accumulate at the interface of GaN/MgO (shown in Fig. 4(a)). The origin of the holes is the key to understand the mechanism of UV EL. Due to the quantum
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Fig. 4. Energy band diagrams of AZO/i-MgO/n-GaN (a) and AZO/i-ZnO/n-GaN (b) heterojunction under positive bias.
Fig. 3. (a) Room temperature EL spectra of AZO/i-MgO/n-GaN heterostructured device under various positive voltages, the inset shows the EL spectra in the UV band of AZO/i-MgO/n-GaN heterostructured device. (b) EL spectra of AZO/i-ZnO/ n-GaN heterostructured device with various positive voltages, inset is the PL spectrum of n-GaN and ZnO:Al films.
confinement effect, an inversion layer will be gradually induced in n-ZnO:Al layer, resulting in 2-D holes gas near the interface of MgO/AZO. On the other hand, under high electric field, most of the voltage will be applied on the MgO layer due to its dielectric nature. Although the carriers in the MgO layer are few, the carriers will gain much high energy under such a high electric field and they will impact with the lattice of the MgO layer [23], thus the holes in the valence band will be excited. In such a way, holes are generated by the inversion layer and the impactionization process. Under the forward bias, the generated holes can be injected into the GaN layer, and recombine radiatively with the electrons accumulated at the MgO/GaN interface, resulting in UV emission from AZO/i-MgO/n-GaN heterojunction. As we all know, the band gap of i-ZnO is much smaller than that of MgO, the effective energy barrier for the electrons DEc in the conduction band is greatly reduced, as shown in Fig. 4(b). Therefore, there are no accumulated electrons that will be involved in the effective recombination of electron-hole pairs. As a result, the intensity of UV EL emission from AZO/i-ZnO/n-GaN heterojunction is much weaker than that of AZO/i-MgO/n-GaN at the same voltage.
Fig. 5. Integrated intensity of the UV emission of AZO/i-MgO/n-GaN heterostructured device as a function of positive voltage.
Fig. 5 shows the integrated intensity of UV emission for AZO/iMgO/n-GaN heterostructured device as a function of positive voltage. The UV emission intensity enhances very much when the bias voltage is about 2.3 V, which is the threshold voltage of AZO/i-MgO/n-GaN heterostructured device. Such a low threshold voltage has not been reported in similar structured heteojunction devices [24,25]. The electrical conductivity of AZO layer is 8.18 10 2 O cm, and then the low threshold voltage is due to the AZO layer with superior electrical conductivity. The AZO layer has double functions in this structure: (i) AZO layer is regarded as an improved n type layer, which increases the generation
S. Li et al. / Journal of Luminescence 132 (2012) 1642–1645
efficiency of carriers, (ii) it acts as a transparent current spreading layer underneath the Ag electrode, reducing current crowding effect of the device. Moreover, the smaller series resistance also causes the smaller heating effect, which is beneficial to the operation of the device.
4. Conclusion In summary, we have demonstrated AZO/i-MgO/n-GaN and AZO/i-ZnO/n-GaN heterojunctions. Under various voltages, the EL spectra of both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions are dominated by UV emission peaks, while the visible emissions are greatly suppressed. UV emission of AZO/i-MgO/nGaN heterojunction is stronger than that of AZO/i-ZnO/n-GaN at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. This study demonstrates the possibility of fabrication of AZO/i-MgO/n-GaN UV EL devices which can be driven by two ordinary dry batteries. Such AZO/i-MgO/n-GaN isotype heterojunction is promising for the development of short-wavelength and high performance optoelectronic devices.
Acknowledgments This work was supported in part by the Natural Science Foundation of China (11074194, 11104209), by the National High Technology Research and Development Program of China (2009AA03Z219), by the National Basic Research Program of China (2011CB933300), by the Natural Science Foundation of Hubei Province (2010CDA016), by the Education Department of Hubei Province (Q20101608), and by the Fundamental Research Funds for the Central Universities.
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