Growth modes of W and Mo thin epitaxial (110)films on (11(2)over-bar0)sapphire

Growth modes of W(110) and Mo(110) epitaxial thin films grown with pulsed laser deposition (PLD) on (11(2)over-bar0) sapphire were studied. The films properties were studied both in reciprocal space using reflection high energy electron diffraction (RHEED) and X-ray diffraction and in real space using atomic force microscope (AFM). The conventionally used high temperature growth process, usually considered to be the best process - according to reciprocal space characterization - to minimize in-plane crystallites twinning and mosaicity,is proved to yield three-dimensional growth and rough surfaces. A sample continuously wedged up to a thickness of 600 A was studied and only partial islands coalescence was observed. An alternative optimized growth process is proposed, based on 200 degrees C deposition, subsequent annealing, and further high temperature deposition, that enables the elimination of in-plane twinned-crystallites while yielding to nearly perfect W(110) or Mo(110) surfaces, from both crystallographically and geometrically points of view. Throughout the paper we discuss information gained respectively from RHEED and AFM characterization, and analyse their relationships and complementarity. The comparison with MBE grown films suggests that what we report on is not a PLD-specific behavior. Eventually we report on a R(15x6)Mo surface reconstruction, which had never been observed before to our knowledge. (C) 1998 Elsevier Science B.V. All rights reserved.