Surface mound formation during epitaxial growth of CrN(001)

Single crystal CrN(001) layers, 10 to 160 nm thick, were grown on MgO(001) by reactive magnetron sputtering at growth temperatures T-s = 600 and 800 degrees C. In situ scanning tunneling microscopy shows that all layer surfaces exhibit mounds with atomically smooth terraces that are separated by monolayer-high step edges aligned along (110) directions, indicating N-rich surface islands. For T-s = 600 degrees C, the root mean square surface roughness sigma initially increases sharply from 0.7 +/- 0.2 for a thickness t = 10 nm to 2.4 +/- 0.5 nm for t = 20 am, but then remains constant at sigma=2.43 +/- 0.13 nm for t=40,80 and 160 nm. The mounds exhibit square shapes with edges along (110) directions for t <= 40 nm, but develop dendritic shapes at t = 80 nm which revert back to squares at t = 160 nm. This is associated with a lateral mound growth that is followed by coarsening, yielding a decrease in the mound density from 5700 to 700 mu m(-2) and an initial increase in the lateral coherence length xi from 7.2 +/- 0.6 to 16.3 +/- 0.8 to 24 +/- 3 nm for t = 10, 20, and 40 nm, respectively, followed by a drop in xi to 22 +/- 2 and 16 +/- 2 nm for t = 80 and 160 nm, respectively. Growth at T-s = 800 degrees C results in opposite trends: sigma and xi decrease by a factor of 2, from 2.0 +/- 0.4 and 20 +/- 4 nm for t = 10 nm to 0.92 +/- 0.07 and 10.3 +/- 0.4 nm for t = 20 nm, respectively, while the mound density remains approximately constant at 900 mu m(-2). This unexpected trend is associated with mounds that elongate and join along (100) directions, yielding long chains of interconnected square mounds for t = 40 nm. However, coalescence during continued growth to t = 160 nm reduces the mound density to 100 mu m(-2) and increases sigma and xi to 2.5 +/- 0.1 and 40 +/- 2 nm, respectively. (C) 2009 Elsevier B.V. All rights reserved.