The authors investigate the structural properties of Bi2Se3 and Bi2Te3 topological insulator layers grown on sapphire (0001) substrates by molecular beam epitaxy, using various pregrowth optimization methods. Samples of Bi2Se3 grown on sapphire with a particular combination of pregrowth surface treatment steps showed evidence of greatly reduced twinning and significantly reduced surface roughness. Evidence of twinning, and its suppression by the appropriate choice of pregrowth steps, is obtained from x-ray diffraction Φ-scan measurements as well as atomic force microscopy (AFM) images. Improved surface roughness is also evident from the AFM images. Growth of Bi2Te3 on sapphire exhibited similar reduced twinning by the application of the pregrowth surface preparation steps, although the surface roughness was significantly greater than that of Bi2Se3. Additional improvements in surface roughness of the Bi2Te3 were achieved by growing the Bi2Te3 on a Bi2Se3 buffer layer.