Tunable Emission Wavelength and Chromaticity Electroluminescence Realized in Ho Doped ZnO Microspheres Heterojunction

Tunable white light sources have huge potential applications in the scope of intelligent lighting or high-quality display system, but it is still a challenge to tune the emission wavelength and chromaticity in a simple structure. Herein, the Ho3+ doped ZnO microsphere was bonded onto the p-GaN substrate to form a heterojunctional light-emitting diode. Through a combination of the light confinement effect in the ZnO microsphere and multilevel emission system in rare earth materials, the device demonstrates tunable electroluminescence (EL) both in wavelength and chromaticity. The device shows broadband emission peak at 400 nm with full width at half maximum (FWHM) of 34 nm under forward pulsed driven bias. While it shows a series of sharp emissions from rare earth ions Ho3+ with peaks at 427 nm, 492 nm, 567 nm, 592 nm and 654 nm and FWHM less than 1.3 nm under reverse pulsed driven bias. More meaningful, the wavelength and chromaticity can be controlled with duty cycle or the rate of forward and reverse driven bias under intelligent programmable alternating current (AC) driven. Finally, the gradient from blueviolet light to white light can be realized.