Investigation On Na-doped ZnO Nanorod Arrays: Synthesis And Optoelectronic Properties

Zn O, one of II-VI group semiconductors, has great promising applications in the field of LEDs, UV detectors and lasers due to its wide bandgap of 3.37 e V and high exciton binding energy of 60 me V at room temperature. It is well know that to realize the high quality p-type Zn O is difficult because of the intrinsic defects in Zn O. As the development of fabrication technologies on semiconductor materials and nanotechnology, great efforts have been made to achieve p-type Zn O. Group V(N, P, As, and Sb) and IA(Li, Na and K) elements have been considered as suitable p-type dopants of Zn O. In this dissertation, the fabrication of the quasi one-dimensional(1D) Zn O nanostructures and p-type Na-doped Zn O nannorods, as well as their optoelectronic properties have been studied systematically:(1) Quasi 1D Zn O nanorod arrays have been prepared on S i substrates assisted by buffer- layer technique. The influences of buffer layer thickness on the growth of Zn O nanorod arrays, photoluminescence and field emission properties have been studied. The optimum thickness of buffer layer is ~50 nm in current work. Based on the TEM analysis, two steps vapor-solid(VS) growth mechansim has been proposed as that a Zn O transition layer forms on buffer layer first, followed by nanorods growth along the transition layer.(2) A two-step surface modification, which involves Ar+ plasma etching treatment followed by coating with a very thin Al N layer, has been employed to enhance the field emission(FE) properties of Zn O nanorods. Compared with the FE properties of as-grown Zn O nanorods, the turn on field of the two-step surface modified Zn O nanorods decreased to 42% from 16.0 to 6.8 V μm-1 at the current density of 10 m A cm2, and the FE current density increased by about 40 times, reaching as high as 4.1 m A cm2 at an electric field of 17.4 V μm-1. It has been proposed that the enhancement in the field emission properties is due to the reduction of the effective work function and the low electron affinity of the thin Al N coating layer, as well as the enhanced local field near the reduced tips of the ZnO nanorods.(3) P-type Na-doped Zn O(p-Zn O :Na) nanorods have been grown on the Si substrates by acatalyst- free chemical vapor deposition method. Element analysis reveals that doping concentration of Na is in the range of 0.4 – 3.7 at. %. Morphological and electrical properties of the Zn O nanorods are highly dependent on the Na concentration. Hall measurement indicates the realization of p-Zn O :Na nanorods with hole concentration in the order of 1015 cm-3. Temperature dependent photoluminescence measurement down to 10 K confirms the shallow acceptor level, which is about ~132 me V. Desirable rectifying behavior has also been observed from I-V characteristic of p-Zn O :Na nanorods/Zn O/n-Si heterostructure and the turn on voltage is ~ 3.0 V. The effect of thermal annealing on Na-doped Zn O nanorods have been investigated. The hole concentration of Na-doped Zn O nanorods increased from 3.4 × 1015 cm-3 to 1.2 × 1016 cm-3 at heating rate of 50 oC/s under N2 atmosphere due to activating of interstitial Na. And the hole concentration reaches up to 4.1×1016 cm-3 with heating rate of 10 oC/min under O2 atmosphere due to conbination of activating of interstitial and reduced oxygen vacancy. This study provides an effective approach for enhancing hole concentration of p-type Na-doped Zn O nanorods.(4) P-type Na-doped Zn O nanorods was also fabricated by thermal diffusion. Spin coat the precursor with Na on the surface of Zn O nanorods first, then heat the sample at 850-950 oC. It has been found that the Na ions have been successfully incorporated into Zn O lattice according to XRD, TEM and XPS analyses. However, the maximum doping concentration of Na is only ~ 0.9 at. % and the Na-doped Zn O nanorods exhibit weak p-type conductivity with hole concentration of ~ 1014 cm-3.(5) P-Zn O :Na nanorods/n-AZO homojunction has been fabricated and investigated. Desirable rectifying behavior has been observed from I-V characteristic of the device and the turn on voltage is ~0.5 V.