Photoluminescence studies of ZnO films fabricated by using a combination of a hydrothermal method and plasma-assisted molecular beam epitaxy regrowth

Abstract

ZnO films were deposited on Si (100) substrates by using a two-step growth process. In the first step, ZnO nanorods were grown by using the hydrothermal method at 140 °C for 5 min. In the second step, a ZnO amorphous layer was deposited on the ZnO nanorods by spin-coating. After completion of the growth process, the films were annealed at 800 °C for 10 min, and a ZnO active layer was deposited on top of the amorphous layer by using plasma-assisted molecular beam epitaxy. Further, temperature-dependent photoluminescence (PL) measurement were conducted to study the optical properties of the prepared films. In the low-temperature PL spectra, emission peaks in the near-band-edge region were observed at 3.370, 3.362, 3.347, 3.329, 3.317, 3.288, 3.263, 3.219, 3.191, and 3.116 eV; these peaks were attributed to free excitons, neutral donor bound excitons, neutral acceptor donor excitons, two electron satellites, and donor acceptor pairs, respectively. These peaks were red-shifted, and their intensity decreased with increasing temperature. The binding energy of the donor was calculated as 43.1 meV by using the Haynes rule. Further, the value α and β, factors in the equation for the energy of localized excitons of donors and acceptors were obtained as 0.73 meV and 750 K, respectively, by fitting the free exciton (FX) peak according to Varshni’s equation. The full width at half-maximum of PL for the films was about 95.1 meV at room temperature; moreover, the following values were obtained for the films by using theoretical equations: the background impurity broadening, Γ₀ = 62 meV, the parameter describing exciton-LO phonon interaction, Γ _LO = 80 meV, LO phonon energy, ħω _LO = 72 meV, and, the coupling strength of an exciton-acoustic phonon interaction, γ _ph = 0.087 meV/K Furthermore, the activation energy was about 60.1 meV.