R. J. Kennedy

Investigation of the cobalt distribution in TiO2:Co thin films

We report a detailed study of the morphology of TiO2:Co films grown on both LaAlO3 (001) and SrTiO3 (001) substrates by pulsed laser deposition. The films are optically transparent and ferromagnetic at room temperature with a magnetic moment of 1.7±0.4 μB/Co. Plan view transmission electron microscope studies show clear evidence of cobalt segregation with the clustering more pronounced for higher cobalt concentrations. Films grown on SrTiO3 substrates show the presence of a thin surface rutile (111) layer, into which the cobalt appears to migrate. These results are supported by x-ray structure determination and resistivity measurements.

Growth, disorder, and physical properties of ZnSnN2

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

A new laser ablation geometry for the production of smooth thin single-layer YBa2Cu3O7-x and multilayer YBa2Cu3O7-x/PrBa2Cu3O7-x films

Abstract The advantages and results of a new geometry for laser ablation are presented. The laser beam enters the ablation chamber via a computer controlled scanning mirror and is x−y rastered across the target surface. The laser beam hits the target at normal incidence through the ablation plume. The ablation products are allowed to thermalize in a 200 mTorr oxygen atmosphere before being deposited on a heated substrate (750 °C, SrTiO3, LaAlO3, yttria-stabilized zirconia) facing 180° to the initial plume direction. This overcomes the problem of molten rocks and blobs ejected from the target being deposited on the substrate and results in very smooth films (± 50 A for 5000 A thickness). The high temperature resistivities of YBa2Cu3O7−x were linear with temperature with their extensions passing through zero resistivity at 0 K. Typical Tc values of 90–92 K widths were regularly obtained. No oxygen anneal either in situ or post-deposition was required. X-ray analysis shows the films to be c axis oriented. The technique has been extended to the growth of very high quality YBa2Cu3O7−x/PrBa2Cu3O7−x multilayer films. Their X-ray spectra are presented for various thicknesses of the individual layers.

Investigation of the cobalt distribution in the room temperature ferromagnet TiO2:Co

Recent reports of room temperature ferromagnetism in thin films of TiO2 doped with small amounts of cobalt have excited a great deal of interest due to the possible applications of this ferromagnetic, transparent semiconductor to spintronics devices. Ferromagnetism has been reported for TiO2 grown in both the anatase and rutile phases, although a range of magnetic moments has been reported by different groups. We have grown epitaxial thin films of Ti1−xCoxO2, with x=0.01–0.07 by pulsed laser deposition in both the anatase and rutile forms on single crystal substrates of LaAlO3, SrTiO3, and Al2O3. These films are ferromagnetic at room temperature with a magnetic moment of ∼1.7 μB/Co in the anatase phase and ∼0.6 μB/Co in the rutile phase. Detailed transmission electron microscopy measurements show a variation in the cobalt distribution in these materials that corresponds to the different magnetic moments. The cobalt does not appear to enter the anatase phase of TiO2, but rather forms crystallites of cobalt...

films grown by laser ablation on Si(100) and GaAs(100) substrates with and without MgO buffer layers

films have been grown by laser ablation on (100) oriented Si and GaAs substrates. Optimal growth of was obtained at a substrate temperature of 450C in a molecular oxygen atmosphere of Torr. X-ray measurements show these films to be (111) oriented, with the growth direction random in the plane of the film. The introduction of (100) oriented, epitaxial MgO buffer layers as thin as 10 A on the substrates results in the growth of (100) oriented, epitaxial films. X-ray pole-figure measurements on these films indicate that both the and the MgO films are oriented cube on cube on the Si and GaAs substrates, namely . Room-temperature magnetization measurements of the (111) oriented films show that there is good agreement between the magnetic properties of the films grown on Si and on the GaAs substrates. Similar agreement is found for the magnetization measurements of the (100) oriented epitaxial films grown with a MgO buffer layer. Moreover, the hysteresis loops for the (111) and (100) oriented films are very similar, indicating that the epitaxy has not significantly improved the magnetic properties of the . In fact, a small decrease of the high-field magnetization is observed with increasing MgO film thickness.

Crystallographically orientated fcc Co nanocrystals in rutile TiO2 thin films

Cobalt nanocrystals dispersed in rutile thin films on (0001) α-Al2O3 substrates are grown by pulsed-laser deposition. Their microstructure is investigated by transmission electron microscopy and their magnetic properties measured at temperatures from 5to350K. The Co nanocrystals have fcc structure with no multi-twin defects and are crystallographically orientated with their (111) planes parallel to the substrate surface having an orientation relationship to TiO2 of (111)Co‖(100)rutile, and in-plane orientations of [112¯]Co‖[010]rutile and [1¯10]Co‖[001]rutile. The diameter of the Co nanocrystals is 4.4±0.15nm. The samples show superparamagnetic behavior at low temperatures. The zero-field-cooled magnetization versus temperature curve has a peak at about 105K. There is strong magnetic dipolar interaction between the particles which might have resulted in the observed hysteresis at room temperature.

Hopping transport in TiO2:Co: A signature of multiphase behavior

TiO2:Co films have been grown in the rutile phase under varying oxygen growth pressures. Detailed microstructural analysis using transmission electron microscopy shows that reduced growth pressure increases the presence of cobalt clusters. The resistivity is found to follow a log(ρ)∼T−1/2 dependence over a wide temperature range. This behavior is characteristic of hopping transport in multiphase systems and implies the presence of cobalt clustering. Thus, transport measurements are shown to be sensitive to the appearance of metallic clusters in these systems and suggest a rapid method for determing the presence of clustering in these and other magnetically doped semiconductor systems.

Growth of MgO thin films on M-, A-, C- and R-plane sapphire by laser ablation

Epitaxial MgO thin films have been grown on single crystal substrates of M-plane (100), A-plane (20), C-plane (0001) and R-plane (102) sapphire by laser ablation of a Mg metal target in a molecular oxygen atmosphere using 1064 nm radiation from a Nd:YAG laser. X-ray measurements indicate that the MgO grows epitaxially on all substrates, with its orientation dependent on the cut of the sapphire substrate. (110)-oriented MgO grows on M-plane sapphire, while (111)-oriented MgO films are found on both the A-plane and the C-plane sapphire. The orientation of MgO found on R-plane sapphire appears to be surface and temperature dependent. The principal growth orientation obtained on the R-plane is (100)-oriented MgO although (100) growth tilted 30° from the [11] Al2O3 direction can result. X-ray area mapping has been performed to determine the mosaicity, d value spread and strain present in the films. These data are compared with the in-plane orientation and the mismatch of the MgO and Al2O3 lattices in an attempt to relate the preferred orientation to the plane of the sapphire on which it is grown.

The growth of iron oxide, nickel oxide and cobalt oxide thin films by laser ablation from metal targets

Thin films of iron oxide, nickel oxide and cobalt oxide have been grown by the technique of laser ablation. The films have been deposited on [100] LaAlO/sub 3/, SrTiO/sub 3/, MgO and cubic zirconia (YSZ); on sapphire, mica, glass slides and Kapton. High quality c axis [100] oriented growth occurs for Fe/sub 3/O/sub 4/ and NiO on the first three substrates; on YSZ the films grow [111] oriented, Co/sub 3/O/sub 4/ grows [100] oriented on SrTiO/sub 3/ and MgO, [111] oriented on YSZ and randomly oriented on LaAlO/sub 3/. All three materials grow twinned on YSZ. Growth of the materials on glass slides gives both [111] and [100] oriented growth depending on the growth rate. However the films are randomly oriented in the plane of the glass slide. Amorphous films result for growth on sapphire, mica and Kapton substrates. The conditions for the optimum growth of highly oriented films are presented along with the X-ray texture plots showing the film quality.

Strain effects in thin films of CrO2 on rutile and sapphire substrates

Detailed x-ray diffraction characterizations were made of chromium dioxide films fabricated by chemical vapor deposition onto (100) and (110) oriented TiO2 and (0001) Al2O3 substrates. Pole figures were used to examine the epitaxy of these systems, and the lattice parameters were calculated using x-ray area maps. The film on (100) TiO2 exhibits the best epitaxy of the three; however, it is significantly strained relative to bulk CrO2. The film on (110) TiO2 is distorted from the ideal tetragonal structure by 0.17° in the angle between the a and b lattice directions, and also exhibits significant mosaicity. The film on sapphire contains crystallites that have grown in three in-plane orientations, but exhibit the best rocking curve widths and the least degree of strain of the films studied. Magnetization and magnetotransport measurements are shown to demonstrate effects of strain and crystalline structure on the physical properties of these films.