**5. Conclusion and outlook**

EL spectra of the LED matched well with the PL spectrum of the p-type ZnO film, indicating

**Figure 21.** EL spectra of p–n homojunction ZnO LED operated at forward currents of 20 and 40 mA; PL spectrum of p-

**Figure 22.** *I*-*V* characteristics of *p*-*p*, *p*-*n*, *n*-*n*, and *n*-FTO regions in a single ZnO rod. The upper left and lower right insets show the schematic diagram and a SEM image of probing by Zyvex nanomanipulator, respectively. From Ref.

Sun et al reported on UV emission from a ZnO rod homojunction LED [97]. Vertically aligned ZnO rods (Diameters: 200–500nm; Length:3.5μm) were uniformly grown on fluo‐

type ZnO obtained at room temperature. From Ref. [98].

48 Optoelectronics - Advanced Materials and Devices

[97].

that the recombination of electrons and holes occurred mostly in p-type ZnO layer.

With a large direct band gap of 3.37eV and a large exciton binding energy of 60meV, ZnO has attracted much attention for its application in optoelectronics applications, such as LEDs, photodetector and laser diodes. In the paper, based on the introduction of the bandgap engineering and doping in ZnO, we presented a comprehensive review of ZnO-based LEDs. Band-gap engineering in ZnO can be achieved by alloying with MgO, CdO or BeO. Theoretically, the energy band gap of AxZn1-xO can be continuously modulated from 0.9eV (CdO) to 10.6eV (BeO) by changing the A concentration. As a n-type semiconductor, high quality n-type ZnO materials can be obtained by doping doping III elements (Al, Ga and In). Although the fabrication of p-type ZnO remain great challenges due to the self-compensa‐ tion, p-type ZnO have been prepared by doping different elements (N, P, As, Sb, Li, Na and K) with various techniques. ZnO based heterojunction and homojunction LEDs have been achieved, which makes ZnO as a strong candidates for solid-state light. Although the effi‐ ciency of ZnO-based LEDs is not high, improvements are rapid and the emitters have the potential to outperform their GaN rivals.

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ZnO-based LEDs show great promise for the future, however, there are some severe issues that are in need of further investigation to transition ZnO-based LEDs to commercial use from the current stage. One problem is that the usable, reproducible p-type ZnO are not easy to fabricate, although some researchers have been successful. Another is the achieve‐ ment of high quality p-n junction based ZnO. The p-n junction with good threshold and breakdown voltages is necessary for the LEDs. In addition, diode-like behavior and light emission have been observed, however, the mechanism of the properties remain unclear.
