Employing Ion-Beam Sputter Deposited TiO2 Buffer Layers for VO2-Related Devices

Vanadium dioxide (VO2) is considered to be the most promising material for thermochromic smart windows due to its insulator-to-metal transition causing an abrupt change of transmittance near ambient temperature. TiO2 buffer layers are expected to have a beneficial impact on the growth and performance of the functional VO2 thin film, as they may induce crystallinity at lower growth temperatures and act as a barrier layer for ion diffusion from glass substrates. However, a systematic evaluation of TiO2 buffer layers of different phases and thicknesses to assess potentially competing impacts is still lacking. Here, we grow TiO2 buffer layers by ion-beam sputter deposition (IBSD) at different substrate temperatures to obtain amorphous TiO2 as well as crystalline TiO2 in the anatase and rutile phase, respectively. We investigate the buffer layers themselves and shed light on how these buffer layers influence the performance of the functional VO2 layer grown on top by combining experimental and theoretical means. We find that the VO2||TiO2 interface is the most crucial part of the multilayer structure with an urgent need for minimum roughness. The two types of crystalline TiO2 buffers yield similar results in terms of optical performance; however, we find that only rutile buffer layers significantly relieve the growth conditions needed.