Abstract
In this paper, highly oriented and vertically arranged ZnO nanotubes are prepared on Al 2O 3 (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/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 … ∞ . 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.
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