A. Sambri

Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry

By combining structural and chemical thin film analysis with detailed plume diagnostics and modeling of the laser plume dynamics, we are able to elucidate the different physical mechanisms determining the stoichiometry of the complex oxides model material SrTiO3 during pulsed laser deposition. Deviations between thin film and target stoichiometry are basically a result of two effects, namely, incongruent ablation and preferential scattering of lighter ablated species during their motion towards the substrate in the O2 background gas. On the one hand, a progressive preferential ablation of the Ti species with increasing laser fluence leads to a regime of Ti-rich thin film growth at larger fluences. On the other hand, in the low laser fluence regime, a more effective scattering of the lighter Ti plume species results in Sr rich films.

Propagation dynamics of a LaMnO3 laser ablation plume in an oxygen atmosphere

The effect of ambient gas on the expansion dynamics of the plasma plume generated by excimer laser ablation of a LaMnO3 target is investigated by using fast photography. The plume propagation in an oxygen environment is examined with pressure ranging from vacuum to a few millibars. Imaging analysis performed over a complete set of images of the plume emission, for each pressure, has allowed following the changes in the plume structure, the plume front dynamics and stopping length, as well as the variation of plume angular distribution as a function of time and pressure. Our experimental findings evidence a number of interesting effects occurring at various times of the expansion and at different pressure levels. At low pressure (<10−2mbar) the plume expands freely resembling the propagation in high vacuum condition. Then, at intermediate pressure levels (10−2–1mbar) spatial splitting of the plume, plume front oscillations, plume broadening and sharpening, and, finally, plume stopping are observed as a fun...

Plume propagation dynamics of complex oxides in oxygen

The influence of background gas pressure and deposition temperature on plume propagation dynamics of La0.7Sr0.3MnO3 in oxygen is investigated and discussed. Fast imaging analysis is exploited to obtain a complete set of images of plume emission at the typical oxygen pressures used in pulsed laser deposition of this material, and for deposition temperatures ranging from room temperature up to 900 °C. We have observed that the deposition temperature can induce a remarkable change on both plume emission characteristics, in the early stage of the expansion, and on the velocity of the species impacting the substrate during film growth. The results are interpreted in the frame of a simple model which yields consistent and quantitative agreement with the experimental data. Our findings point to an interesting cross-correlation among important deposition parameters, such as background gas pressure and deposition temperature, also suggesting that the optimization of processing parameters, such as background gas pr...

Substrate heating influence on plume propagation during pulsed laser deposition of complex oxides

We investigate the effects of the substrate-heater temperature on the expansion dynamics of laser plumes of complex oxides in oxygen atmosphere. We observed a considerable reduction of the background gas resistance to plume propagation as the substrate temperature was increased, leading to a remarkable change in the velocity of the species impacting the substrate during film growth. The deposition temperature thus influences film growth not only through its direct thermal effect on surface kinetics of adatoms, but also by affecting the energetic properties of the precursors in the gas phase. We interpret the results with a simplified model of plume front propagation, accounting for the change in the background gas density induced by the substrate temperature.

Impact of the interplay between nonstoichiometry and kinetic energy of the plume species on the growth mode of SrTiO3thin films

We studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. The kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. We could show that although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. We attribute this to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length.

Plasma plume effects on the conductivity of amorphous LaAlO3/SrTiO3 interfaces grown by pulsed laser deposition in O2 and Ar

Amorphous-LaAlO3/SrTiO3 interfaces exhibit metallic conductivity similar to those found for the extensively studied crystalline-LaAlO3/SrTiO3 interfaces. Here, we investigate the conductivity of the amorphous-LaAlO3/SrTiO3 interfaces grown in different pressures of O2 and Ar background gases. During the deposition, the LaAlO3 ablation plume is also studied, in situ, by fast photography and space-resolved optical emission spectroscopy. An interesting correlation between interfacial conductivity and kinetic energy of the Al atoms in the plume is observed: to assure conducting interfaces of amorphous-LaAlO3/SrTiO3, the kinetic energy of Al should be higher than 1 eV. Our findings add further insights on mechanisms leading to interfacial conductivity in SrTiO3-based oxide heterostructures.

Critical influence of target-to-substrate distance on conductive properties of LaGaO3/SrTiO3 interfaces deposited at 10−1 mbar oxygen pressure

We investigate pulsed laser deposition of LaGaO3/SrTiO3 at 10−1 mbar oxygen background pressure, demonstrating the critical effect of the target-to-substrate distance, dTS, on the interface sheet resistance, Rs. The interface turns from insulating to metallic by progressively decreasing dTS. The analysis of the LaGaO3 plume evidences the important role of the plume propagation dynamics on the interface properties. These results demonstrate the growth of conducting interfaces at an oxygen pressure of 10−1 mbar, an experimental condition where a well-oxygenated heterostructures with a reduced content of oxygen defects is expected.

Time-resolved photoluminescence of n-doped SrTiO3

Following the recent surge of interest in n-doped strontium titanate as a possible blue light emitter, a time-resolved photoluminescence analysis was performed on nominally pure, Nb-doped and oxygen-deficient single-crystal SrTiO3 samples. The doping effects on both the electronic states involved in the transition and the decay mechanism are respectively analyzed by comparing the spectral and dynamic features and the yields of the emission. Our time-resolved analysis, besides shedding some light on the basic recombination mechanisms acting in these materials, sets the intrinsic bandwidth limit of the proposed blue light emitting optoelectronic devices made of Ti-based perovskites heterostructures in the gigahertz range.

Effects of oxygen background pressure on the stoichiometry of a LaGaO3 laser ablation plume investigated by time and spectrally resolved two-dimensional imaging

The plume expansion dynamics strongly affects the growth and the chemistry of pulsed laser deposited thin films. The interaction with the background gas determines the kinetic energy of the species impinging on the substrate, their angular broadening, the plasma chemistry, and eventually the cations stoichiometric ratio in oxide films. Here, we exploit two-dimensional, spectrally resolved plume imaging to characterize the diverse effects of the oxygen background pressure on the expansion dynamics of La, Ga, and LaO species during pulsed laser deposition of LaGaO3. The propagation of the ablated species towards the substrate is studied for background oxygen pressures ranging from high vacuum up to ≈10−1 mbar. Our experimental results show specie-dependent effects of the background gas on the angular distribution of the precursors within the plume. These findings suggest that even in the presence of a stoichiometric ablation and of a globally stoichiometric plume, cations off-stoichiometry can take place in...

Ultrafast dynamical response of strongly correlated oxides: role of coherent optical and acoustic oscillations

We describe some general features in the transient behaviour of strongly correlated transition metal oxides, following ultrafast excitation by a femtosecond laser pulse. Our analysis is based on time-resolved reflectivity measurements on (V1−x Cr x )2O3, a prototype Mott–Hubbard material, and the manganite compound (La0.67Sr0.33)MnO3, performed over a time window of several tenths of ps. We point out the contribution of coherent lattice oscillations–both optical and acoustic–to the overall signal, and show how they can depend on the crystallographic orientation of the material with respect to the laser beam wavevector and polarisation. In particular, an acoustic wave is always found to be present in our measurements, and we show that its oscillating behaviour is superposed to the average time evolution of the material, which can be instead related in different ways to its electronic properties and thermodynamic phase. All these effects are to be taken into account in any further analysis leading to the estimation of relevant physical parameters for correlated materials, and they appear to play a significant role not only for reflectivity techniques, but also in other kinds of ultrafast pump–probe experiments.