Molecular Beam Epitaxy of Rare Earth Telluride Thin Films—Crystal Growth and Physical Properties

The Rare Earth Tellurides (R-Te_x where x = 2, 2.5, 3 and R is a member of the lanthanide group) are a family of quasi two dimensional crystals which host a variety of low temperature phases including magnetic, charge-ordered and superconducting (under pressure) ground states. The crystal structure consists of corrugated R-Te insulating layers interspersed with conducting Te square-planar layers. Each additional Te in the chemical formula corresponds to an additional Te layer in the unit cell. Confinement of conduction electrons to these planes implies a strong 2D character reminiscent of the well-known high-Tc cuprate superconductors. A particularly well-studied feature of this family of materials is the formation charge density waves and their concomitant superlattice modulations.<br/><br/>Thus far, only studies on bulk crystals of these materials have been performed. In this talk we will present the first demonstration of epitaxial, single crystal thin films of DyTe₂ (Space group: P4/nmm, a = 4.29A, c = 8.91A). We will first describe our novel method for preparing atomically-flat MgO surfaces using laser annealing. We will then proceed to discuss the unique aspects of the layer-by-layer growth as monitored by <i>in-situ</i> reflection high energy electron diffraction. Oscillations in the specular intensity demonstrate a layer-by-layer growth mode and by comparing with thickness measured <i>ex-situ</i> , we conclude that each oscillation corresponds to the growth of a single unit cell. By observing the phase relationship between the temporal oscillations of spots of different diffraction orders, we are able to distinguish between the growth of corrugated layers and square-net sheets. We will show how changes to the diffraction pattern resulting from differing processing parameters can be used to optimize the growth conditions.<br/><br/>We will then discuss additional characteristics of films grown under differing growth temperatures and elemental flux ratios. Reciprocal space mapping using high resolution x-ray diffraction reveals films with lattice constants relaxed from the value of the substrate. Film quality is further demonstrated with scanning transmission electron microscopy (STEM) where minimal misoriented domains are observed. STEM measurements also confirm that the film grows unit cell-by-unit cell. A sharp interface with the MgO substrate formed by a single Te sheet was observed.<br/><br/>Similar to the bulk case, a wide range of stoichiometries due to Te deficiency were found to form stable, single crystal, single phase films. Using x-ray photoelectron spectroscopy we were able to both estimate relative stoichiometries between films and analyze the impact of off-stoichimetry on the photoelectron spectrum of Te and Dy. Additionally, <i>in-situ </i>low energy electron diffraction has revealed a variety of surface reconstructions for differing degrees of Te deficiency, evidence for the existence of charge density waves in the crystal.<br/><br/>Using electrical transport, we observe semiconducting behavior with a power-law temperature dependence, in line with previous reports on bulk crystals. This behavior, which deviates from the ideal Arrhenius-like activated transport behavior is reminiscent of a doped, small-gap semiconductor. While the formation of charge density waves is expected to open a gap in the fermi surface, this temperature dependence of resistance instead suggests an ordered lattice of Te defects may be responsible for the insulating behavior.<br/><br/>We conclude the talk with the prospects for growing other crystals in this family and also highlight some of the challenges for growth of crystals with larger numbers of Te sheets.