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The authors report on the fabrication of p-Mg0.1Zn0.9O/n-GaN light emitting diodes (LEDs). Under forward bias, dominant ultraviolet (UV) electroluminescence is detected within 360-380 nm caused by near band edge (NBE) radiative recombination from both n-GaN and p-Mg0.1Zn0.9O. It is worth noting that the intensity ratio of UV-NBE/visible-DLE reaches up to 50, which indicates the potential applications of this structure in the short wavelength LEDs.
©2011 Optical Society of America
1. Introduction
Ultraviolet (UV) light emitting diode (LED) has great usage in lighting, displaying, communication areas, etc. Recently, the III-nitride material system is widely studied, and various approaches have been done on high efficiency visible nitride-based LEDs [1–8] and AlGaN/AlInN material systems based UV LEDs [9–14]. To overcome the polarizations fields in the structure, many methods are reported to reduce/remove the charge separation issues [15–18]. However, the growths of high-Al content III-nitride material are still challenging, thus the investigation of other alternative material system is of importance.
ZnO material has several advantages including higher quantum efficiency, greater resistance to high energy radiation and the possibility of wet chemical etching. It is recognized as one of the most potential candidate materials because of its direct band-gap structure and large exciton binding energy of 60 meV [19–23], which is much higher than the room temperature thermal ionization energy (25 meV). It indicates that exciton could exist at even higher temperature, and the ZnO based light emitting device would have high extraction efficiency. However, the current development of ZnO material is still less matures. Attributing to the low formation energy of intrinsic donor defects, the p-ZnO alloy is difficult to realize. However, there have been some promising results reported recently for p-ZnO based LEDs [24,25]. The basic properties of the MgZnO are very similar to that of the ZnO when the Mg content is relatively low. Kazuto Koike et al. [26] have reported that the band-gap of wurtzite MgZnO could be widened up to 4.45 eV without phase separation by increasing the Mg content. Thus, MgZnO films were previously used as the barrier layers in ZnO/MgZnO multiple quantum well structures [27,28]. More importantly, it could be also used to prepare the LEDs with shorter wavelength emission than that of ZnO based devices. This superior characteristic has caused much attention in recent years. The as-grown MgZnO film usually shows n-type conductivity. Naturally, researchers try to employ other p-type semiconductors to form n-MgZnO based p-n heterojunction LEDs. Considering the crystal and band structures, the p-type GaN material could be one of the best choices. H. Zhu et al. [29] have successfully prepared the n-MgZnO/MgO/p-GaN heterojunction LED, and 374 nm light emission was detected from n-MgZnO layer under forward bias. In this structure, the MgO layer is very important for realizing n-MgZnO light emitting; it could confine electrons in n-MgZnO side, while let holes tunnel through. It shows a common phenomenon that the light emitting area is usually at p-type side in n-MgZnO/p-semiconductor structures. However, inserting an insulating electron barrier layer will obviously increase the operating voltage, which could deteriorate the device performance.
According to the above discussion, with the purpose of improving the luminous efficiency, one of the best ways is to fabricate p-MgZnO/n-GaN heterojuction structures. As known, the preparation of n-GaN is very mature, so the n-GaN films could easily show good crystalline qualities with high electron concentration and mobility. This is very helpful to fabricate high performance heterojunction devices. Thus the key point is the growth of p-MgZnO layer. Recently, several research groups have made a lot of efforts on realizing p-type doping in MgZnO using group V elements (N, P, As, Sb) [30–33], and fabricated many kinds of p-MgZnO related LEDs. But in the electroluminescence (EL) spectra of these LEDs, the intensity of the defects related visible emission is usually stronger than that of the UV emission. It indicates that the device is still far from application. And greater efforts should be paid to grow high quality p-MgZnO films by different doping methods. In this paper, we have grown stable arsenic doped p-type MgZnO films on n-GaN/Al2O3 substrate by metal-organic chemical vapor deposition (MOCVD). Subsequently, the p-MgZnO/n-GaN heterojunction LEDs were fabricated and measured. Under forward bias, the dominant UV (360-380 nm) emission was realized from these diodes at room temperature.
2. Experiments
The n-GaN film was deposited on sapphire substrate using Thomas Swan CCS 3 × 2″MOCVD system. The substrates were baked at 1090 °C for 5 minutes, and 20 nm GaN buffer was grown at 550 °C, then 2 μm u-GaN and 2 μm n-GaN were prepared at 1040 °C, successively. The n-type GaN epitaxial layer is (0001) orientation, with an electron concentration and mobility of about 5 × 1018 cm−3 and 280 cm2⋅V−1⋅s−1, respectively. For fabricating p-MgZnO film, about 20 nm GaAs interlayer was pre-coated on the c-Al2O3 and n-GaN/Al2O3 substrate by sputtering method. The GaAs interlayer could act as the arsenic doping source. After that the MgZnO film (~430 nm) was grown by MOCVD method. Bubbled diethylzinc and bis (methylcyclopentadienyl) magnesium were used as the Zn and Mg sources with high purity O2 as the oxygen source. The growth process was done under 500 °C for half an hour. Thereafter, the sample was annealed in situ for 20 minutes at the same temperature under oxygen atmosphere to promote the doping quantity. For fabricating the device, part of the MgZnO layer was removed by dilute hydrochloric acid. Indium electrode was made on the n-GaN side. The thickness and diameter of Indium electrode are about 0.1 mm and 1.5 mm, respectively. And dot Au electrodes were deposited on the surface of MgZnO side by thermal evaporation method. Then, the device was annealed at 350 °C for 2 min under high purity N2 protective atmosphere to reduce the contact resistance. And the EL spectra of this LED were recorded using photodiode detector.
3. Results and discussions
The Mg content x in the MgxZn1-xO film is estimated to be 0.1 obtained from the XPS results. The inset of Fig. 1 shows the arsenic XPS spectrum. The As3d peak is located at around 45 eV [34], which can be ascribed to As-O bond, indicating that most of arsenic atoms are combined with oxygen atoms in the Mg0.1Zn0.9O film. This is consistent with our previous research on the fabrication of p-ZnO:As film [35]. And according to the Hall measurement, the arsenic doped Mg0.1Zn0.9O film grown on sapphire substrate is confirmed to present p-type conductivity. The hole concentration and mobility are 2.5 × 1017 cm−3 and 0.7 cm2V−1s−1, respectively. Figure 1 shows XRD 2θ-scan of the p-Mg0.1Zn0.9O film gown on sapphire substrate, two peaks are observed at 34.6° and 72.8° corresponding to MgZnO (002) and (004) diffraction. It reveals that the p-Mg0.1Zn0.9O film is single c-axis orientation with good crystalline quality.
Figure 2 shows the PL spectra of the p-Mg0.1Zn0.9O and n-GaN measured at room temperature. The NBE emission of the n-GaN is located at 363 nm with DLE at around 540 nm. We think the DLE is caused by the VGa-SiGa defect in n-GaN [36]. Meanwhile, for p-Mg0.1Zn0.9O film, the NBE emission is at 370 nm, and the DLE is too weak to be distinguished from the instrument noise. This indicates that the p-Mg0.1Zn0.9O film also has good optical quality.
Figure 3 is the schematic diagram and I-V curve of the p-Mg0.1Zn0.9O/n-GaN LED. As seen, the I-V curve demonstrates a typical rectifying behavior of p-n junction. The turn-on voltage is about 3 V and the reverse break-down voltage is higher than −3 V. For comparison, we also fabricate the n-MgZnO/n-GaN structure. The n-MgZnO is prepared at the same condition of growing p-Mg0.1Zn0.9O without arsenic doping. As seen in the inset of Fig. 3, the linear I-V curve shows the Ohmic contact characteristics. This result indicates the rectifying behavior mentioned above is caused by the p-Mg0.1Zn0.9O/n-GaN heterojunction.
Fig. 3 The schematic diagram and I-V curve of p-Mg0.1Zn0.9O/n-GaN LED, the inset shows the I-V curve of n-MgZnO/n-GaN structure.
Figure 4 shows the EL spectra of the p-Mg0.1Zn0.9O/n-GaN LED under different injection currents. All of the EL spectra are collected from the top of the device at room temperature. Note that the intensities of visible emissions are all very weak, even at higher injection level. From 140 mA to 160 mA, more and more carriers were injected into the device. The intensity of UV emission was obviously enhanced with the increase of injection current. The intensity ratio of NBE/DLE is about 50 at 160 mA. More importantly, the light emission here is concentrated in the UV region, which is different from most of the p-MgZnO based LEDs existing a dominant visible emission. This improvement is caused by the reduction of the deep level defects in the p-MgZnO film. It is consistent with the PL result that the deep level defect related visible emission is much weaker than UV emission. The p-MgZnO/n-GaN heterojunction structure has an advantage in fabricating UV LEDs.
Fig. 4 EL spectra of the UV emission as a function of the injection current, the inset shows the Gaussian fitting analysis of the UV emission peak obtained at 160 mA.
The inset of Fig. 4 shows the Gaussian fitting analysis of the UV peak obtained at 160 mA. The broad spectrum consists of two distinct bands (A and B) centered in 376 nm and 365 nm, respectively, and each emission band corresponds to a particular recombination process. According to the PL results shown in Fig. 2, the peak A and B can be attributed to the NBE emission of p-Mg0.1Zn0.9O and n-GaN, respectively.
For further analyzing, the band gap diagrams under thermal equilibrium and forward bias cases are illustrated in Figs. 5(a) and 5(b). There are two band discontinuities across the interface of p-Mg0.1Zn0.9O/n-GaN heterojunction shown in Fig. 5(a). The affinity of GaN is 4.2 eV [37], and the calculated affinity of p-Mg0.1Zn0.9O is 4.0 eV [29]. Thus, the ΔEC can be calculated using Eq. (1):
where χGaN and χMgZnO are the electron affinity potential of n-GaN and p-Mg0.1Zn0.9O. To investigate the valence band offset of our samples precisely, we measured the absorption spectra to determine the optical band gaps of both n-GaN and p-Mg0.1Zn0.9O, and found that the optical band gap difference is about 60 meV. Assuming the value as the band gap difference, therefore, the valence band offset can be calculated by Eq. (2):where Eg GaN and Eg MgZnO are the bandgap of n-GaN and p-Mg0.1Zn0.9O.Fig. 5 Energy band diagrams of the p-MgZnO/n-GaN heterojunction device under (a) thermal equilibrium and (b) forward bias cases.
Seeing from Fig. 5(b), part of the electrons can be confined in n-GaN side at MgZnO/GaN interface due to the energy barrier ΔEC (0.2 eV), and some electrons could cross this barrier and inject into p-Mg0.1Zn0.9O side. Similarly, because of the energy barrier ΔEv (0.26 eV) in valence band, part of the holes are confined in p-Mg0.1Zn0.9O side, and some of them can inject into n-GaN side. Thus, the NBE radiative recombination between electrons and holes occur at both n-GaN and p-Mg0.1Zn0.9O sides.
4. Conclusion
In conclusion, dominant UV electroluminescence is realized in p-Mg0.1Zn0.9O /n-GaN LED. The NBE emission of p-Mg0.1Zn0.9O is detected around 376 nm, and the 365 nm emission peak is caused by the radiative recombination in n-GaN. The intensity ratio of NBE/DLE obtained here is 50. Our result strongly shows great potential applications of the p-MgZnO material in fabricating high performance UV LEDs.
Acknowledgments
This work is supported by the “973” program No. 2011CB302005; the Key Project of NNSFC No. 61076045, 11004020, 60877020 and 60976010; the Fundamental Research Funds for the Central Universities dut11rc(3)45; the “863” program No. 2009AA03Z401 and No. 2011AA03A102; the China Postdoctoral Science Foundation under Grant No. 20110491539.
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