Heterostructured Electroluminescent Devices Based On Low-dimensional ZnO

ZnO is a group II-VI binary compound semiconductor that possesses direct wide bandgap (Eg=3.37eV) and superior optoelectronic characteristics. One of the most important features of ZnO is its large exciton binding energy of60meV, which is much higher than that of the GaN (25meV), suggesting that it is possible to realize more efficient excitonic emission in ZnO at room temperature or higher temperatures than that of the GaN. Besides, one can obtain a variety of low-dimensional (LD) ZnO nanostructures through various simple and low-cost approaches. Thus, it is highly possible to dramatically lower the cost of the ZnO-based light-emitting devices (LEDs) by using LD ZnO nanostructures. However, stable and reliable p-ZnO with high hole concentration is still lacking due to the strong self-compensation effect. As a result, this study mainly focuses on the heterostructures and metal-insulator-semiconductor (MIS) structures to realize the ZnO-based LEDs. In this work, low-cost ZnO-based LEDs have been obtained by using LD ZnO nanostructures, including zero-dimensional (0D) ZnO quantum dots (QDs), one-dimensional (1D) ZnO nanowires (NWs) and two-dimensional (2D) ZnO quantum wells (QWs). Different innovative LED structures are best designed to fit the features of different LD ZnO nanostructures and the primary achievements are described and listed in the following paragraphs.First of all, uniform0D ZnO QDs with an average diameter of～5.9nm are produced via zinc acetate dihydrate and NaOH through low-temperature solution method. As compared with other ZnO film preparation technique, it is much cheaper by using ZnO QDs. High-k dielectric HfO2is rationally introduced as the electron blocking layer (EBL) for n-ZnO QDs/p-GaN LEDs to block electrons in the ZnO so as to improve the recombination in ZnO, thus enhancing the ZnO emission. As compared with the LEDs without the HfO2EBL, ZnO related390and414nm emissions are greatly improved in the n-ZnO QDs/HfO2/p-GaN LEDs whereas the interfacial defects related477nm emission is suppressed effectively. Furthermore, a spectral narrowing is observed as the thickness of the HfO2EBL is increased. The great enhancement of the ZnO related390and414nm emissions but deeply suppression of the interfacial defect related477nm emission could be the reason for this spectral narrowing. Besides, valence band offset (△Ev) of the sputtered HfO2/p-GaN is determined for the first time to be0.16±0.2eV by using X-ray photoemission spectra (XPS) measurement and the energy band alignment of the n-ZnO/HfO2/p-GaN heterojunction under thermal equilibrium condition is deduced. It is found that the conduction band offset (AEc) of the n-ZnO/HfO2interface is large enough to block the electrons in ZnO while the AEv for the HfO2/p-GaN is small enough to rarely affect the hole injection, indicating that HfO2can serve as a good EBL for the n-ZnO/p-GaN LEDs.Secondly, vertical1D ZnO NWs are produced on the ITO glass substrates through low-temperature hydrothermal method. A novel direct-bonding structure is designed to avoid the adverse influence of the ZnO seed layer on the electroluminescence (EL) and make the best use of the single crystal features of the ZnO NWs. High-quality n-ZnO/p-GaN heterojunctions are formed between the n-ZnO nanorod tips and the p-GaN via a simple thermal treatment. The EL spectra measured in the UV-visible range show that there is only a pure near-UV emission centered at400nm and no defect related green or orange light is observed. The emission threshold is as low as0.64mA and the energy conversion efficiency is1.2%. The EL spectrum at2.30mA, which is fitted with the Gaussian functions, shows that the EL emission is actually the superposition of the emissions from the p-GaN, the n-ZnO nanorod and possibly, the interface. Thirdly, vertical1D ZnO NWs are fabricated in glass vials through low-temperature hydrothermal method. The purpose of selecting glass vials as the reaction containers instead of the conventional heavy Teflon-lined autoclaves is to provide a soft and benign NW growth environment, which is beneficial to improve the quality of the ZnO crystals. A MgO insulator is rationally designed as the shells for the ZnO NWs to modify/passivate the surface defects. Since the refractive index of i-MgO (n=1.72) is located between the n-ZnO (n=2.45) and air (n=1.0), the optically pumped380nm near-band-edge emission of the bare ZnO NWs is enhanced nearly four times after modifying/passivating the i-MgO shells whereas the surface deep-level related visible emission is suppressed. An all-inorganic heterostructured n-ZnO@i-MgO core-shell NW/p-NiO LED is then fabricated. Interestingly, the EL spectra measured in the whole UV-visible range reveal that light emission can only be detected when the LEDs are applied with reverse bias. The emission color can also be tuned from orange to bright white with increasing reverse bias. We explor these interesting results tentatively in terms of the energy-band diagram of the heterojunction and it is found that the interfacial i-MgO shells not only act as an insulator to prevent a short circuit between the two electrodes, but also offer a potential energy difference so that electron tunneling is energetically possible, both of which are essential to generate the reverse-bias EL. On the other hand, since the extremely low hole mobility in p-NiO might result in the incapability for holes to inject into n-ZnO due to blocking of the depletion layer and the i-MgO barrier even under high forward bias, holes are apt to dominantly recombine with the tunneling electrons from n-ZnO in the p-NiO side. No light can be emitted even under high forward bias because the d-d transitions in NiO are dipole-forbidden by the Laporte selection rule.At last, ZnO/MgZnO2D QWs are fabricated via a radio-frequency magnetron sputtering system. n-GaN is chosen as the substrate due to the low lattice mismatch between ZnO and GaN. Since stable and reliable p-ZnO is still a challenge, a metal-insulator-semiconductor (MIS) structure is designed to realize ZnO LEDs. To avoid the drawbacks of the conventional ZnO MIS LEDs (high emission threshold voltage and low quantum efficiency), ZnO/MgZnO QWs is introduced to the conventional ZnO MIS LEDs to increase carrier confinement, thus improving the carrier recombinations. The EL intensity and the output power of the conventional Au/MgO/ZnO LEDs are improved nearly100%by using ZnO/MgZnO QWs under the same voltage. The energy conversion efficiency is improved from0.23%to0.51%but the emission threshold is as low as～2.5V, indicative of being driven by two ordinary batteries. Besides, the junction temperature of the ZnO/MgZnO QW MIS LEDs with and without post-annealing is investigated for the first time. It is found that the junction temperature of the annealed LEDs is much higher than that of the unannealed LEDs under the same injection current density. This phenomenon is possibly attributed to the fact of stress relaxation and the diffusion of the atoms in the constituent ZnO/MgZnO layer during the post-annealing process, which can in deed produce more interfacial defects. These interfacial defects may increase the Auger recombination and aggravate the leakage current and heating effect, thus making the junction temperature of the annealed LEDs increase.