Thermal Stability and Electronic Structure of Atomically Uniform Pb Films on Si(111) M. H. Upton, 1,2 C. M. Wei, 3,4 M.Y. Chou, 4 T. Miller, 1,2 and T.-C. Chiang 1,2 1 Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana–Champaign, 104 S. Goodwin Avenue, Urbana, Illinois 61801-2902, USA 2 Department of Physics, University of Illinois at Urbana–Champaign, 1110 W. Green Street, Urbana, Illinois 61801-3080, USA 3 Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan 11529, Republic of China 4 School of Physics, Georgia Institute of Technology, USA, Atlanta, Georgia 30332-0430, USA (Received 13 October 2003; published 7 July 2004) Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N  5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N  6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties. DOI: 10.1103/PhysRevLett.93.026802 PACS numbers: 73.21.Fg, 68.55.Jk, 79.60.Dp Miniaturization of materials architecture has led to feature length scales approaching the nanoscale. Two im- portant issues arise in this regime: one is the emergence of quantum mechanical effects associated with confinement and interference; the other is an increased sensitivity of properties to atomic-scale variations in size. Thin films are widely employed in device designs. A change in film thickness by just one atomic layer can result in property variations on the order of 1=N, where N is the thickness of the film in terms of monolayers (ML). This can lead to interesting and important properties, and the present work is a demonstration of this effect in a simple model system, Pb films on Si(111). We report on the thermal stability and the electronic structure as well as their relationship. Angle-resolved photoemission measurements show sharp and intense quantum well states [1–3] derived from Pb valence elec- trons confined in the film for odd film thicknesses N  5, 7, and 9 ML, but not for the intervening even film thick- nesses N  6 and 8. The measured thermal stability is well correlated with these even-odd variations, with the N  6 and 8 films being much more stable than the others. These observations, augmented by theoretical re- sults, shed light on Friedel-like quantum oscillations in properties which are expected to be prevalent at the nanoscale. The success of this experiment relies on the preparation of atomically uniform films. Despite the large lattice mismatch between Pb and Si, smooth growth is achieved by a special growth process. While previous studies of Pb on Si showed preferred island heights [4], thus hinting at stability variations, the present work goes further by providing quantitative stability data as a function of both thickness and temperature. The experiment was performed at the Synchrotron Radiation Center, University of Wisconsin–Madison. Angle-resolved photoemission data were taken with a Scienta analyzer equipped with a two-dimensional detec- tor. The Si(111) substrates were prepared from n-type wafers with a resistivity of 1–60  cm. As shown by previous studies and by our own experimentation [4–9], direct deposition of Pb on the (7  7) reconstructed Si(111) at various temperatures always resulted in rough films. To prepare atomically uniform films, we first de- posited approximately 2 ML of Pb on Si111-77. The sample was then annealed at above 400  C to desorb the Pb. The desorption curve, as measured by photoemission intensities of the Pb 5d and Si 2p core levels, exhibited breaks corresponding to the formation of distinct phases [9–11]. From previous studies, it is known that the   3 p   3 p R30   phase forms at a residual Pb coverage of 1.1 ML (equivalent to 4=3 ML of Si coverage). The   3 p   3 p R30   phase forms at a residual Pb coverage of 0.28 ML (equivalent to 1=3 ML of Si coverage). Our experiment showed that atomically uniform films of Pb were obtained by depositing Pb on the  phase, the  phase, or any intermediate phases at 100 K. The final results were the same for the same total amounts of Pb deposition including the initial Pb coverages. The photo- emission spectra shown below were obtained with the sample at 100 K, and the film thicknesses indicated are the total number of Pb monolayers in the film, excluding the initial  phase Pb coverages. The three-dimensional plot in Fig. 1(a) shows the normal-emission intensity as a function of binding en- ergy and film thickness. Three major peaks, at binding energies of 0.4, 0.26, and 0.15 eV below the Fermi level, attain their maximum intensities at film thicknesses N  5, 7, and 9, respectively, while no such peaks are observed at the layer thicknesses N  6 and 8. These peaks are quantum well states formed by con- finement of the Pb p-band electrons by the Si band gap [12,13]. No such quantum well states exist for N  6 and 8. PHYSICAL REVIEW LETTERS week ending 9 JULY 2004 VOLUME 93, NUMBER 2 026802-1 0031-9007= 04=93(2)=026802(4)$22.50  2004 The American Physical Society 026802-1