Fig. 1. The diagram of device fabrication.

Fig. 2. (a) A schematic diagram for ZnO/GaN LEDs. (b) The Gaussian decomposition of EL spectra for LED and the inset depicts the PL spectra of the GaN film as well as ZnO MWs. (c−f) The height of the step-like HfO₂ film with different sputtering time (c) 2 min, (d) 3 min, (e) 5 min and (f) 7 min. (g, h) AFM image for the GaN surface (g) before and (h) after bonding HfO₂ film with thickness of 5.03 nm.

Fig. 3. (a) Schematic diagram of ZnO/HfO₂/GaN for in-situ optical test. (b) Lasing emission intensity for ZnO MW versus the excitation power density on different substrates. (c−f) the corresponding PL spectra under different excitation power densities for ZnO MW with different thickness of HfO₂ films: (c) 0 nm, (d) 5.03 nm, (e) 8.79 nm and (f) 12.55 nm.

Fig. 4. The lifetime of ZnO MW on various substrates with excitation power of 8 μW.

Fig. 5. (a) I-V characteristics of ITO/ZnO/HfO₂/GaN LEDs. The inset is a schematic diagram for ITO/ZnO/HfO₂/GaN LEDs. (b) Normalized EL spectra for LEDs under an excitation current of 1 mA. (c−f) EL intensity of ZnO/HfO₂/GaN LEDs with HfO₂ films of different thickness and the inset is the light pictures: (c) 0 nm, (d) 5.03 nm, (e) 8.79 nm, (f) 12.55 nm. (g) Chromaticity coordinates of the spectra in (b). (h) EL peak intensities of the LEDs from (c) to (f).

Fig. 6. Gaussian conversion of Fig. 5(c)−5(f) at a current of 1 mA: (a) 0 nm, (b) 5.03 nm, (c) 8.79 nm, (d) 12.55 nm, and the inset images depict the contents of UV and visible lights. (e) The peak positions of emissions and (f) FWHM of emissions for these LEDs.

Fig. 7. (a) Energy band of ZnO/HfO₂/GaN LEDs based on theoretical simulation. (b) The distribution of electronical current density in ZnO/HfO₂/GaN LEDs. The inset picture displays a structure diagram of simulation.

Fig. 8. Schematic band structures of (a) ZnO/GaN, (b) ZnO /thin HfO₂/ GaN and (c) ZnO /thick HfO₂/ GaN.