Excitonic properties of vertically aligned ZnO nanotubes under high-density excitation

In this paper, highly oriented and vertically arranged ZnO nanotubes are prepared on Al2O3 (0001) substrate without employing any metal catalysts by plasma-assisted molecular beam epitaxy. The photoluminescence (PL) spectra at room temperature are studied under high excitation densities. Under lower excitation density (< 60 kW/cm(2)), PL spectrum shows that one strong free exciton emission (FE) locates at 3.306 eV. As the excitation density increases up to 200 kW/cm(2), a new emission peak (P-n) located at low-energy side of FE is attributed to the spontaneous emission due to an exciton-exciton (E-x- E-x) scattering process from two ground state excitons, where one exciton is recombined by emitting a photon and the other is scattered into the excited states of n = 2, 3, 4...infinity. Under excitation density of 300 kW/cm(2), the stimulated emission originating from E-x-E-x scattering is obtained. When the excitation density is above 580 kW/cm(2), the emission from electron-hole plasma is observed in low-energy side of the P band and indicates a typical superradiation recombination processes with increasing excitation density. (c) 2006 Elsevier B.V. All rights reserved.