I.E. Graboy

Volatile Surfactant‐Assisted MOCVD: Application to LaAlO3 Thin‐Film Growth

A new approach to the CVD of oxides with kinetically hindered diffusion, called volatile surfactant-assisted (VSA) metal-organic chemical vapor deposition (MOCVD), consisting of film deposition in the presence of a volatile low melting point oxide (Bi 2 O 3 ) has been developed. The process was applied to the deposition of LaAlO 3 films, and a model of the process was proposed. Epitaxial and textured LaAlO 3 films on various substrates were obtained, both by thermal and VSA MOCVD. A marked improvement in crystalline quality and surface morphology was found for the films deposited by VSA MOCVD. LaAlO 3 films obtained in the presence of Bi 2 O 3 did not contain Bi. A significant increase (up to five times) of the deposition rate was observed for LaAlO 3 films deposited by VSA MOCVD compared with that for the films grown by thermal MOCVD.

Perovskite rare-earth nickelates in the thin-film epitaxial state

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, and Gd) under reduced oxygen pressure <0.02 bar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90° domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain.

Epitaxial phase stabilisation phenomena in rare earth manganites

Abstract Using injection, powder-flash and band-flash MOCVD techniques, epitaxial thin films of Nd, Ho, Y, Tm, Dy and Lu manganites were prepared. The formation of high-pressure phase NdMn 7 O 12 as a thin surface layer on Pnma Nd 1− x MnO 3+δ ( x ∼0.15) in an Nd-deficient system and HoMnO 3 , YMnO 3 , TmMnO 3 and LuMnO 3 as perovskite phases (known in the bulk only as high-pressure phases) on a LaAlO 3 substrate were observed. The remarkable reduction in the pressure needed for formation of the phases in epitaxial films in comparison with bulk samples is a manifestation of the epitaxial stabilisation discussed in the paper. The details of the crystal and domain structure of the films as observed by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques are described.

LuBa2Cu3O7–x thin films prepared using MOCVD

LuBa2Cu3O7–x thin films with Tc= 86–88 K were prepared by flash evaporation MOCVD on LaAlO3, SrTiO3, ZrO2(Y2O3) and NdGaO3 single-crystal substrates (deposition temperature 795 °C, partial oxygen pressure 1.35 Torr). Values for the critical current density jc(77 K., H= 100 Oe), of 9 × 105, 1.1 × 106 and 1.2 × 106 A cm–2 were measured for films on ZrO2(Y2O3), SrTiO3 and LaAlO3 respectively. The calculated magnitude of |ΔjcΔT| for LuBa2Cu3O7–x films on coherent substrates was found to be higher than that of YBa2Cu3O7–x films, indicating a more efficient magnetic flux pinning mechanism. The temperature dependence of conductivity fluctuations was considered in terms of a Lawrence–Donniach model and a 3D → 2D dimensional crossover was registered for the films on SrTiO3.

YBCO and BSCCO thin films prepared by wet MOCVD

The newly developed technique of low-pressure single aerosol source MOCVD (wet MOCVD) has been applied to prepare thin films of YBa2Cu3Ox and Bi2Sr2CaCU2Ox. The influence of the deposition rate, ρo2–T conditions, film stoichiometry on the phase composition, orientation and superconducting properties of the films was studied and compared for both systems. Thin films of YBa2Cu3Ox with high-Tc(end)= 92 K, jc(77 K)= 1.7 × 106 A cm–2; Bi2Sr2CaCu2Oxwith Tc(end)= 84 K, jc(77K)= 1 × 104A cm–2 were deposited. On the basis of the crystal chemistry approach the conditions for the various orientations were established.

HREM characterisation of interfaces in YBa2Cu3O7−δ/CeO2/R-Al2O3 structures

Abstract Interfaces between a sapphire ( R -Al 2 O 3 ) substrate, a CeO 2 buffer layer, and a YBa 2 Cu 3 O 7− δ (YBCO) layer — both layers obtained by MOCVD — have been studied by high-resolution electron microscopy. Lattice parameter mismatch between the sapphire substrate and the CeO 2 layer was observed to be accommodated within the first one or two atomic planes of the CeO 2 layer. The density of misfit dislocations correlated well with the mismatch between the CeO 2 and sapphire unit cell parameters. Small imperfections in the sapphire surface were observed not to lead to a change in orientation of the overlying CeO 2 layer, nor to an increased roughness of the CeO 2 surface. The roughness of the CeO 2 surface was found to be considerably less than that of the sapphire surface. The YBCO/CeO 2 interface contained only a small number of dislocations, located in the YBCO layer. Steps in the CeO 2 surface were observed to be accommodated by antiphase boundaries, or planar defects close to the interface.

Colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial thin film on (001)ZrO2(Y2O3) substrate over a wide temperature range

Colossal negative magnetoresistance is found over a wide range of temperatures below the Curie point TC≈240 K in an epitaxial La0.35Nd0.35Sr0.3MnO3 film on a single-crystal (001)ZrO2(Y2O3) wafer substrate. Isotherms of the magnetoresistance of this film reveal that its absolute value increases with the field, abruptly in the technical magnetization range and almost linearly in stronger fields. For three epitaxial films of the same composition on (001)LaAlO3, (001)SrTiO3, and (001)MgO substrates, colossal magnetoresistance only occurred near TC≈240 K and at T<TC it increased weakly, almost linearly with the field. In the film on ZrO2(Y2O3) substrate the electrical resistivity was almost 1.5 orders of magnitude higher than that in the other three films. It is shown that this increase is attributable to the electrical resistance of the interfaces between microregions having four types of crystallographic orientation, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunnelling of polarized carriers across these interfaces which coincide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for all four samples, which is only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thickness. The colossal magnetoresistance near TC and the low-temperature magnetoresistance in fields exceeding the technical saturation level can be attributed to the existence of strong s-d exchange which is responsible for a steep drop in the mobility of the carriers (holes) and their partial localization at levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from these levels.

In situ growth of superconducting (Pb,Cu)2(Ba,Pb)2YCu2Oy epitaxial thin films by MOCVD

Abstract Superconducting ( T c =38 K) thin films of (Pb,Cu) 2 (Ba,Pb) 2 YCu 2 O y have been grown by MetalOrganic Chemical Vapor Deposition (MOCVD) on single crystal (001)LaAlO 3 substrates in a single deposition run. The critical parameters for stabilizing lead-containing compounds in thin films are considered. It was found that a high vapor pressure of lead-containing species in the gas phase during the film growth determines the stability of formed compounds. The results of high-resolution transmission electron microscopy (HREM) and X-ray diffraction analysis (XRD) analyses of (Pb,Cu) 2 (Ba,Pb) 2 YCu 2 O y thin films are presented.

Lattice strain in the epitaxial thin films of perovskites

The epitaxial films of perovskite manganites (La1−xPrx)0.7Ca0.3MnO3 (x=0–1) and nickelates RNiO3 (R=Pr, Nd, Sm, Gd) were grown by single source MOCVD on perovskite substrates producing anisotropic lattice strain in the film. We applied the data on the metal–insulator transition in the perovskite films as a probe for the microscopic strain mechanism. As a result, the anisotropic tilting of the rigid octahedra was deduced for the manganites and octahedra deformation for the nickelates.

Epitaxial phase stabilization in thin films of complex oxides

Abstract A survey of experimental results is given presenting the successful growth of oxide epitaxial thin films in thermodynamic conditions ( T , pressure, p (O 2 ), composition), which are far from those necessary for the stable existence of these phases in polycrystalline state. The unstable-in-bulk BaCu 3 O 4 , NdMn 7 O 12 , rare earth nickelates RNiO 3 and unusual phase relations in R–Ba–Cu–O systems are the examples. The stabilizing effect was observed in thin films only if the epitaxial growth was induced. The effect is thought to be of thermodynamic origin, rather than of kinetic one, and it is supposed to be the result of the low surface energy of coherent interfaces formed at the epitaxy.