Structural and emitting properties of Zinc Oxide nanocrystals synthesized by high-fluence ion implantation

Abstract

Samples of SiO2 (600 nm)/Si have been implanted with Zn ions (140 keV, 5×10 16 cm-2) and O ions (50 keV, 5×10 16 cm-2) at room temperature and afterwards annealed at 750ºС for 120 min in air ambient. The Zn depth distribution profiles of the implanted and annealed samples were analyzed by Rutherford backscattering spectroscopy (RBS). The structural and light emitting properties of the annealed samples were studied using TEM in “plan-view” geometry as well as electron micro-diffraction and photoluminescence (PL) at low (4 K) and room temperatures. The RBS analysis has revealed the bimodal shape of the Zn concentration profile. TEM analysis shows a non-uniform Zn-based cluster size distribution after annealing. The near-surface (0-60 nm) layer contains small clusters (2-11 nm). The large clusters (up to 35 nm) and clusters covered with oxide shells as well as and the secondary defects have been formed in the depth of 60-120 nm. The deepest implanted layer (120 - 200 nm) layer contains small nanoclusters with sizes of 5-10 nm. The crystal structure of the synthesized ZnO clusters is confirmed by the presence of narrow rings in diffraction patterns. Room-temperature PL spectrum exhibits the intensive deep level emission (DLE) in visible spectral range with dominant orange band (2.1 eV) and sharp weak near band edge (NBE) emission at 3.244 eV. At low-temperature (4.2 K) UV band dominates in PL spectrum which consists of two intense peaks at 3.324 and 3.274 eV and some narrow bands in blue-violet range attributed to phonon replicas of these bands. The crucial role of oxygen interstitials in radiative recombination is discussed.