Crystalline to amorphous phase transformation of Ta2O5 quantum dots driven by residual stress

Abstract

Simple vacuum thermal evaporation technique was used to grow Ta2O5 quantum dots. The small dots with large voids between them in ultra-thin films were found to gradually grow in size with decreasing voids with increase in film thickness. The development of residual stress was observed with increasing dot size or film thickness. Residual stress effect on band gap of orthorhombic Ta2O5 quantum dots is studied. Both SAED and XRD confirm that the films thicker than 45 nm become amorphous. Significantly varying optical band gap with film thickness was observed by studying optical absorption of the films. The variation of residual stress with film thickness scales linearly very well with the varying band gap with film thickness. Stress free band gap (3.94 eV) and pressure coefficient of band gap (∂Eg/∂P)T = −26.4 meV/GPa) were determined from this study. Transformation from crystalline to amorphous phase in orthorhombic form of Ta2O5 has been determined to be at about 18 GPa. Both crystalline to amorphous transition and pressure coefficient of band gap for orthorhombic Ta2O5 driven by residual stress have been determined for the first time in the literature.