W.M. Keck Microfabrication Facility

Atomically Flat Surfaces on Strontium Titanate and Sapphire Crystals

A.P. Li, C. Bednarski, Z. Dai, and B. Golding

Department of Physics and Astronomy
Center for Sensor Materials
Michigan State University

SrTiO3 and a-Al2O3 are attractive substrates for thin-film growth. However, as received mechanochemical polished crystals are not sufficiently smooth for atomic layer epitaxy. In this work, the SrTiO3 (100) and a-Al2O3 (0001) surface structures undergoing different treatments have been examined with atomic force microscopy. A well-controlled procedure has been developed that results in atomically flat surfaces on SrTiO3 (100) and a-Al2O3 (0001).

SrTiO3

Atomically flat surface appears as flat terraces separated by monatomic height steps. The terrace width is determined by the miscut angle of the crystal. The processing of SrTiO3 wafer consists of a careful solvent cleaning, a chemical etch followed by thermally annealing. In the above image (left), the step-and-terrace structure starts to form, and the irregularly-shaped steps are in unit cell height (0.4 nm) of SrTiO3 lattices along <100>. An appropriate treatment can result in straight step edges as shown in (right).

Step-and-terrace structures on SrTiO3 (100) surface (image size 1x1 um2). Left: irregularly-shaped steps. Right: straight steps

Al2O3

A step-and-terrace structure can also be obtained on a-Al2O3 surface after a thermal annealing process. The monatomic step on a-Al2O3 wafer surface, 0.22 nm, is one sixth of the unit cell height along [0001]. The terrace width in the left image is around 200 nm, and the estimated miscut angel of the wafer is 0.06 ° .

Step-and-terrace structures on a -Al2O3 (0001) surface (image (4 x 4 mm2)