The use of silicon on sapphire (SOS) as a substrate for YBa2Cu3O7−&dgr;allows the growth of thick (∼4000 A˚) films without the thermally induced cracking characteristic of epitaxial films on bulk Si substrate. Epitaxy is sustained and reaction is prevented by an intermediate buffer layer of yttria‐stabilized (YSZ). The transport critical current density is as high as 4.6×106A/cm2at 77 K, and surface resistance measurements at 4.2 K are reported. Microtwin propagation from Si into YSZ is shown not to occur.