Author:       2014-05-20

Bandgap engineering is an important method to improve the electrical and optical properties of semiconductors by tuning of multinary alloyed compounds. As an important wide bandgap material, zinc oxide (ZnO) is the most promising candidate as functional building blocks in efficient UV LEDs due to its large bandgap of 3.37 eV and high exciton binding energy of 60 meV. Many researches has reported the band-gap tuning of ZnO films by addition of dopants. Indeed, by alloying with Mg or Cd, the ZnO band-gap can be theoretically changed from ~7.8 eV (MgO) to ~2.3 eV (CdO), but limited by the crystal mismatch. On comparison with traditional thin-film structure, one-dimensional (1D) multinary alloyed nanowires (NWs) is an effective approach to bandgap tuning via the compositional effect. The NWs geometry offers a unique platform for epitaxial alloy growth because of their lattice strain-relieving properties, where the high-quality epitaxial nanowire heterojunctions with low interfacial strain and defect density provide the highly efficient electron–hole recombination luminescence. Furthermore, nanowire-based LEDs have drawn great interest because of their many advantages, such as, NWs can act as direct waveguide and favor light extraction without the use of lenses and reflectors. Therefore, n-type ZnO NW arrays grown on p-type GaN substrate LED devices have been intensely studied in the past several years. However, only rarely is research performed to tune the emission wavelength of ZnO NW arrays. In practical applications, e.g. green lighting without a phosphor, fullcolor LED panel and biomedical science, it is important to develop an efficient technology to widely tune the emission wavelength of ZnO nanostructures by bandgap engineering. Recently, Cu- and Cd-doped ZnO NWs resulted in an emission wavelength shift from UV to violet-blue spectral region, however, the EL emission peak redshift is only about 40 nm and 20 nm, respectively.

Addressing to this issue, Prof. Yihua Gao’s group demonstrated a wavelength-tunable light-emitting diodes (LEDs) of GaxZn1-xO nanowire arrays by a simple modified chemical vapor deposition heteroepitaxial growth on p-GaN substrate. As a gallium atom has similar electronegativity and ion radius to a zinc atom, high-level Ga-doped GaxZn1-xO nanowire arrays have been fabricated. As the x value gradually increases from 0 to 0.66, the near-band-edge emission peak of GaxZn1-xO nanowires shows a significant shift from 378 nm (3.28 eV) to 418 nm (2.96 eV) in room-temperature photoluminescence (PL) measurement. Importantly, the electroluminescence (EL) emission of GaxZn1-xO nanowire arrays LED continuously shifts with a wider range (��00 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The presented work demonstrates the possibility of bandgap engineering of low-dimensional ZnO nanowires by gallium doping and the potential application for wavelength-tunable LEDs.

This work was supported by the National Natural Science Foundation of China (Nos. 11074082, 11204093) etc. This research was just published in the top optics journal (IF: 7.976), Laser & Photonics Reviews 8 (3), 429-435 (2014).