AVS2009 Session TF1-MoM: Thin Films: Growth and Characterization I

Thin films of SrAl₂O₄:Eu²⁺,Dy³⁺ phosphor were grown on silicon substrates using a 248 nm KrF pulsed laser to evaluate the effects of different processing parameters on photoluminescence properties of the phosphor. The processing parameters which were varied during the films growth include temperature, pressure, and the number of pulses. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the structure, particle morphology and surface topography of the films. The chemical composition and thicknesses of the films were determined by Rutherford backscattering spectroscopy (RBS). Photoluminescence spectra of the films recorded by the Cary Eclipse spectrophotometer were characterized by major green phosphorescent emission with a maximum at ~520 nm and minor red emission with a maximum at 630 nm. The green and red photoluminescence at 520 and 630 nm are associated with the 4f⁶5d→4f⁷(⁸S_7/2) and ⁵D₀-⁷F₂ transitions of Eu²⁺ and residual Eu³⁺ ions respectively. The effects of processing parameters on the PL intensity and the possible mechanism of the green phosphorescence were discussed.

A double perovskite is a perovskite with the general formula of A₂BB’O₆, where A is a divalent alkaline earth cation and B and B’ are transition-metal ions. Depending on the relative size and oxidation state, the B and B’ ions can be crystallographically completely ordered, making up a rocksalt-type lattice. La₂NiMnO₆ is an ordered double perovskite that is a ferromagnetic semiconductor with a T_CM of 280 K. Recent studies of La₂NiMnO₆ in the bulk have revealed large magnetic-field induced changes in the resistivity and dielectric properties at temperatures as high as 280 K [1,2]. This is a much higher temperature than previously observed for such a coupling between the magnetic, electric, and dielectric properties in a ferromagnetic semiconductor. Substitution at the A site can also lead to multiferroic behavior in the double perovskites. Azuma et al. have succeeded in synthesizing the ‘designed’ compound Bi₂NiMnO₆ in the bulk under high pressure and established its multiferroic properties, with ferroelectric and ferromagnetic transition temperatures of 485 K and 140 K, respectively [3]. We have synthesized epitaxial thin films of La₂NiMnO₆ [4], Bi₂NiMnO₆ [5], and their heterostructures using the pulsed laser deposition (PLD) technique. A related ferromagnetic semiconductor, La₂CoMnO₆, has also been epitaxially stabilized [6]. High quality epitaxial films of these double perovskites are grown on lattice-matched substrates such as SrTiO₃, NdGaO₃ and LaAlO₃. We have structurally characterized the double perovskite films using a variety of techniques. Additionally, the magnetic, electrical and magnetodielectric properties of the thin films and heterostructures have been studied in detail.

[1] N. S. Rogardo, J. Li, A. W. Sleight, and M. A. Subramanian, Adv. Mater. 17, 2225 (2005).

[2] H. Das, U. V. Waghmare, T. Saha-Dasgupta, and D. D. Sarma, Phys. Rev. Lett. 100, 186402 (2008).

[3] M. Azuma, K. Tanaka, T. Saito, S. Ishiwata, Y. Shimakawa, and M. Takano, J. Am. Chem. Soc. 127, 8889 (2005).

[4] H. Guo, J. Burgess, S. Street, A. Gupta, T. G. Calvarese, and M. A. Subramanian, Appl. Phys. Lett. 89, 022509 (2006).

[5] P. Padhan, P. LeClair, A. Gupta, and G. Srinivasan, J. Phys.: Condens. Matter 20, 355003 (2008).

[6] H. Z. Guo, A. Gupta, T. G. Calvarese, and M. A. Subramanian, Appl. Phys. Lett. 89, 262503 (2006).

Ultrathin TiN films were grown by reactive dc magnetron sputtering on amorphous SiO₂ substrates and single-crystalline MgO substrates at various growth temperatures. The resistance of the films was monitored in-situ during growth to determine the coalescence and continuity thicknesses. TiN films grown on SiO₂ at 600°C are polycrystalline and have nominal coalescence and continuity thicknesses of 8 Å and 19 Å, respectively. TiN films grow epitaxially on the MgO substrates at 600°C. The nominal coalescence thickness is 2 Å and the thickness where the film becomes continuous cannot be resolved from the coalescence thickness. X-ray reflection measurements indicate a significantly higher density and lower roughness of the epitaxial TiN films.

Reactive magnetron sputter epitaxy was used to grow thin solid films of Sc_1-xAl_xN (0 ≤ x ≤ 1) onto ScN(111) and AlN(0001) seed layers at substrate temperatures of 600ºC and 800°C, respectively. The films were analyzed by Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), x-ray diffraction (XRD) and transmission electron microscopy (TEM). RBS and ERDA showed that stoichiometric films were obtained in the entire composition range (molar fractions x = 0, 0.14, 0.29, 0.51, 0.73, 0.90, 1.0) using elemental Sc and Al targets and N₂ as the reactive gas. TEM and XRD show that rocksalt structure (c) Sc_1-xAl_xN(111) solid solutions can be epitaxially grown onto the isostructural c-ScN(111) seed layers with AlN molar fractions up to x ~ 0.6 , whereafter the system phase separates into c- and wurtzite structure (w) Sc_1-xAl_xN. Upon phase separation, the w-domains are present in three different orientations relative to the seed layer, namely Sc_1-xAl_xN(0001) || ScN(111) with Sc_1-xAl_xN[-12-10] || ScN[1-10], Sc_1-xAl_xN(10-11) || ScN(111) with Sc_1-xAl_xN[-12-10] || ScN[1-10], and Sc_1-xAl_xN(10-11) || ScN(113). When growth was performed onto w-AlN(0001) seed layers, epitaxial w-Sc_1-xAl_xN(0001) with AlN molar fractions x in the range ~0.50 to 1.00 were obtained. For AlN molar fractions of 0.28 and less, the film formed an epitaxial c-Sc_1-xAl_xN(111) phase with double position domains.

The lattice parameter for the c-Sc_1-xAl_xN films closely followed the values predicted by first principles density functional theory calculations, only slightly deviating from Vegard’s law. On the contrary, the lattice parameters of the w-Sc_1-xAl_xN varied considerably less than predicted. Our calculated mixing enthalpies of c-, w-, and zinc blende Sc_0.50Al_0.50N solid solutions predict that the alloy is metastable with respect to phase separation for all temperatures below the melting points of AlN and ScN.