Sublimation of Atomic Layers from a Chromium Surface S.-J. Tang, 1,2 S. Kodambaka, 1,3 W. Swiech, 1 I. Petrov, 1 C. P. Flynn, 1,2 and T.-C. Chiang 1,2 1 Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801-2902, USA 2 Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, USA 3 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 West Green Street, Urbana, Illinois 61801-2920, USA (Received 3 November 2005; published 30 March 2006) We employ low-energy electron microscopy to study the kinetics of thermal etching, or sublimation, of Cr(001) at 1100 K. Atomic layers are removed from the surface by spontaneous nucleation and growth of two-dimensional vacancy islands, by rotation of spiral steps, and by island decay. The growth rates of vacancy islands and the rotation frequencies of double spirals are measured as a function of temperature, and the results are correlated with activation barriers of surface processes. Mass transport between the surface and bulk is shown to be unimportant. DOI: 10.1103/PhysRevLett.96.126106 PACS numbers: 68.03.Fg, 68.35.Md, 68.37.Nq, 68.47.De A fundamental understanding of basic surface processes at nanoscales and microscales is important for the design, synthesis, and fabrication of materials, structures, and systems. Thin film growth and surface etching are pro- cesses central to various modern technologies [1– 4]. While data and theories abound, many questions remain. This Letter examines an especially simple case —the thermal etching, or sublimation, of an elemental metal surface, Cr(001), which has a simple (1  1) square-lattice struc- ture. There is actually very little known about surface sublimation of such metals. This process is evidently re- lated to homoepitaxial growth. However, contrary to what one might expect intuitively, the two processes are not directly connected by time-reversal symmetry, and details of the kinetics can be quite different. Issues of interest for the present study include the morphological evolution of the surface as atoms are removed, the kinetics in terms of atomic processes, and whether or not mass transport be- tween the surface and bulk plays a significant role in the observed surface evolution —the last being a topic under debate [5–7]. The method of our observation is low-energy electron microscopy (LEEM) [4]. The surface of Cr(001) begins to sublime at 1100 K at a slow but readily detectable rate. The surface is seen to erode mainly through three atomic- layer removal mechanisms: (A) spontaneous nucleation and growth of two-dimensional vacancy islands, (B) wind- ing motions of single- and double-spiral steps that are pinned by bulk dislocations terminating at the surface, and (C) island (or mound) decay. These processes speed up as the temperature of the Cr crystal rises, and the rates of morphological evolution are measured. The data are ana- lyzed in terms of the Burton-Cabrera-Frank model [8], from which important kinetic and energetic parameters are deduced. All three processes (A), (B), and (C) can be purely surface processes, but at sufficiently high tempera- tures, mass transport between the surface and bulk can also occur, especially in case (B), where the core of the bulk dislocation may function as an easy conduit for defects in the bulk moving to the surface. This mechanism has been demonstrated in the case of TiN [6]. Our measurements show, however, that this is not an important contribution in the present case. The experiment was carried out in a multichamber LEEM system [9]. A Cr(001) crystal was cleaned by sputtering and annealing. During LEEM measurements, the sample temperature was monitored by an infrared pyrometer. While the absolute temperature of the sample was uncertain by 50 K, the precision of the temperature measurement, determined by the pyrometer sensitivity, was 1K. Bright-field LEEM images were acquired at a rate of 30 frames per second with the beam energy typi- cally in the range of 8–11 eV. The data were analyzed using the software IMAGE SXM [10]. Figures 1(a)–1(d) are representative LEEM images, with a field of view of 3:4  1:7 m 2 , obtained at times t  0, 9, 47, and 90 s with sample temperature T  1186 K. Regions in Fig. 1 labeled A and A 0 show the spontaneous nucleation and growth of two-dimensional vacancy islands as expanding loops. Region B shows a spiral step. This type of feature is well known to arise as a result of a bulk dislocation terminating at the surface, and the vertex of the spiral is at the end point of the dislocation core [8]. The spiral rotates, or winds around its vertex, as a function of time. Each full rotation creates a new atomic step at the multistep boundaries of the terrace. Region C shows a mound, and the atomic layers making up the mound are seen to shrink and dissolve away as a function of time due to thermal evaporation. Figures 2(a)–2(d) present time-sequence images ac- quired at T  1146 K of a double spiral with its two vertices corresponding to the end points on the surface of a bulk dislocation loop. The two spirals rotate as discussed above but in opposite directions. A closed contour forms each time the two rotating spirals touch each other. The result is a loop of vacancy island step, which expands and PRL 96, 126106 (2006) PHYSICAL REVIEW LETTERS week ending 31 MARCH 2006 0031-9007= 06=96(12)=126106(4)$23.00 126106-1 © 2006 The American Physical Society