Journal of Vacuum Science & Technology A publishes reports of original research, letters, and review articles that focus on fundamental scientific understanding of interfaces, surfaces, plasmas and thin films and on using this understanding to advance the state-of-the-art in various technological applications.
To re-register for Table of Content Alerts, please click here.
An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors contribute new experimental observations about the aging and oxidation process. The authors find that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a series of ∼3.2 Å high islands are observed by atomic force microscopy over approximately 10% of the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ∼120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d and Bi 4d peaks, are clearly present after ∼190 min of ambient exposure. Variable angle spectroscopic ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time dependent changes observed in the surface crystal structure by second harmonic generation. In addition to providing the ability to nondestructively measure oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify the thickness of Bi2Se3 flakes. With these methods, the authors have constructed a consistent, experimentally based model of aging process at the surface of Bi2Se3.
The generation of a two-dimensional electron gas (2DEG) with unprecedented high density at the interface between two complex oxides has spurred interest in the growth and characterization of these materials. Interfaces between SrTiO3 and the rare-earth titanates SmTiO3 and GdTiO3 exhibit 2DEG densities of 3 × 1014 cm−2. Band alignments are key descriptors of these interfaces, and the authors report a joint experimental/computational investigation. Photoemission spectroscopy was used to measure the band alignments at the SmTiO3/GdTiO3 (110)o interface. In parallel, hybrid density functional calculations were performed. The measured and calculated band alignments for both the top of the O 2p band and the Ti 3d lower Hubbard band agree to within 0.13 eV. Our results also shed light on the position of the lower Hubbard band with respect to the O 2p valence band.
The formation, atomic structure, and electronic properties of Tb silicide layers on the Si(111) surface were studied using scanning tunneling microscopy as well as core-level and angle-resolved photoelectron spectroscopy. For Tb exposures around one monolayer, the formation of a hexagonal TbSi2 monolayer was found, while higher coverages led to the formation of a hexagonal Tb3Si5 multilayer with a superstructure in the bulk layers. For the monolayer silicide, Si-2p core level spectra show a Fermi level position very close to the conduction band minimum of the silicon substrate, while the Fermi level shifts toward midgap in the multilayer case. The electronic structure of the monolayer is characterized by a Fermi surface consisting of electronlike ellipses around the points and a holelike state around the point. The effective masses of the band around the points are strongly anisotropic, with values around 1.45 m 0 in the long direction and 0.16 m 0 in the short direction of the ellipses. In the case of the multilayer, the ellipses around the points are less eccentric, and there are indications for Umklapp processes due to the superstructure in the silicide bulk layers. The overall behavior of Tb is found to be similar to that of other trivalent rare earths on Si(111).
Thermal atomic layer etching (ALE) of crystalline aluminum nitride (AlN) films was demonstrated using sequential, self-limiting reactions with hydrogen fluoride (HF) and tin(II) acetylacetonate [Sn(acac)2] as the reactants. Film thicknesses were monitored versus number of ALE reaction cycles at 275 °C using in situ spectroscopic ellipsometry (SE). A low etch rate of ∼0.07 Å/cycle was measured during etching of the first 40 Å of the film. This small etch rate corresponded with the AlOxNy layer on the AlN film. The etch rate then increased to ∼0.36 Å/cycle for the pure AlN films. In situ SE experiments established the HF and Sn(acac)2 exposures that were necessary for self-limiting surface reactions. In the proposed reaction mechanism for thermal AlN ALE, HF fluorinates the AlN film and produces an AlF3 layer on the surface. The metal precursor, Sn(acac)2, then accepts fluorine from the AlF3 layer and transfers an acac ligand to the AlF3 layer in a ligand-exchange reaction. The possible volatile etch products are SnF(acac) and either Al(acac)3 or AlF(acac)2. Adding a H2 plasma exposure after each Sn(acac)2 exposure dramatically increased the AlN etch rate from 0.36 to 1.96 Å/cycle. This enhanced etch rate is believed to result from the ability of the H2 plasma to remove acac surface species that may limit the AlN etch rate. The active agent from the H2 plasma is either hydrogen radicals or radiation. Adding an Ar plasma exposure after each Sn(acac)2 exposure increased the AlN etch rate from 0.36 to 0.66 Å/cycle. This enhanced etch rate is attributed to either ions or radiation from the Ar plasma that ma