Fabrication and characterization of ZnO-based light-emitting diodes

Abstract

Nowadays, solid-state lighting devices based on wide band gap semiconductor have attracted considerable attention. Efforts are being made in order to achieve light-emitting diodes (LEDs) with high efficiency, high colour gamut and low heating output. Along this direction, zinc oxide (ZnO) is one of the promising semiconductors because of its wide band gap (3.37 eV) and relatively large exciton binding energy (60 meV). In addition, ZnO possesses a number of intrinsic and extrinsic radiative defect levels which exhibit a wide emission range covered from ultraviolet to visible, leading to the potential application of white LEDs. In the present study, a series of undoped ZnO and aluminium (Al)-doped ZnO thin films were prepared by filtered cathodic vacuum arc (FCVA) technique. The electrical, optical and structural properties of the ZnO films were investigated as functions of substrate temperature and doping concentration. The ZnO films exhibited c-axis orientation. It is revealed that the resistivity decreased as the substrate temperature increased from room temperature to 400℃ due to enhanced crystallinity and larger grain size. In addition, all the ZnO films have an optical transmittance of over 80% in the visible spectrum. The blue-shifted photoluminescence (PL) peak of the ZnO with increasing Al doping concentration will be discussed. With the optimized growth parameters, n-ZnO:Al/i-ZnO/p-GaN:Mg heterojunction LEDs were fabricated. The electrical characteristics of the diodes were investigated. The ultraviolet (UV) electroluminescence (EL) from the device was detected at room temperature. The emission is attributed to the electron-hole radiative recombination in the ZnO region and is explained in detail by an energy band diagram. ZnO nanostructures are expected to have improved optical and electronic properties because of the quantum confinement effect. Using low-temperature aqueous chemical method, the ZnO nanorods arrays were grown on the buffer layers prepared at various temperatures. The nanorods were grown along [0001] direction. The PL measurement indicated that the emission spectrum covered a UV peak at ~ 380 nm and a broad visible band at ~ 560 nm. The PL spectra of the ZnO nanorods are independent on the growth temperature of the buffer layer. Moreover, the buffer-layer-thickness-dependent structural and optical properties were studied. The as-grown ZnO nanorods were utilized to fabricate hybrid LED with an organic semiconductor, N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4'-diamine (α-NPD), which is one of the most widely used hole transport and blue-emitting organic semiconductors. Current-voltage characteristics of the devices exhibited nonlinear rectifying behaviour. The EL spectra of the hybrid LEDs reveal a blue and broad yellowish green emission originated from the α-NPD layer and the defect levels of the ZnO respectively. The origin of the emission bands from the hybrid structures will be examined.