The impact of willemite Zn2SiO4 phase in B1 to B2 type structural phase transformation in CdxZn1-xO composite thin films

Abstract

This study explores the influence of willemite Zn₂SiO₄ phase upon the structural phase transformation from the B1 (NaCl) to B2 (CsCl) phase in Cd_xZn_1-xO (x = 0.20, and 0.60) composite thin films. X-ray diffraction and Raman spectroscopy analyses show that at an annealing temperature of 900 °C, the B2 phase of CdO nanoparticles becomes achievable with a relative atomic concentration of 60 % Cd, whereas it is absent with 20 % Cd concentration. The presence of the willemite Zn₂SiO₄ phase for 60 % Cd concentration at 900 °C annealing temperature, which plays an important role in this transformation, was confirmed by analyzing the Zn L_3,2 and O K edges X-ray absorption near edge spectroscopy, Si 2p edge X-ray photoelectron spectroscopy, and a broad emission in the green spectral region of the photoluminescence spectra. Electron microscopy indicates the out-diffusion of Zn₂SiO₄ nanoparticles from the film-substrate interface to the film surface, facilitated by the kinetic energy gained during high-temperature thermal annealing. The larger unit cell volume of the trigonal Zn₂SiO₄ phase was identified as inducing the necessary local pressure to trigger the B1 to B2 phase transformation in the CdO nanoparticles. Additionally, simulations using Zn K edge extended absorption fine structure suggest that the local environment around the Zn atoms remain unchanged during phase transition. Cross-sectional transmission electron microscopy images reveal the presence of an amorphous SiO_x layer at the film-substrate interface, facilitating atomic inter-diffusion and leading to the formation of the Zn₂SiO₄ nanoparticles. It is shown that a Cd concentration exceeding 40 % in the ZnO matrix enables the achievement of the experimentally challenging high-pressure B2 phase.