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 (0 0 0 1) 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/cm2), PL spectrum shows that one strong free exciton emission (FE) locates at 3.306 eV. As the excitation density increases up to 200 kW/cm2, a new emission peak (Pn) located at low-energy side of FE is attributed to the spontaneous emission due to an exciton-exciton (Ex-Ex) 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…∞. Under excitation density of 300 kW/cm2, the stimulated emission originating from Ex-Ex scattering is obtained. When the excitation density is above 580 kW/cm2, 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.