Atmosphere Optics, Spectroscopy and Lasers Laboratory  LOA-SL

Influence of deposition conditions on the structural and functional properties of oxide and chalcogenide thin films obtained by laser ablation  

The study of thin film as well as of the techniques used for their deposition presents an important aspect in high-tech applications. The demand for smaller devices with increased response speed requires the development of new materials and methods for nano-sized structure production. A deposition technique that has achieved outstanding results for different types of materials is laser ablation. Pulsed laser deposition (PLD) is a technique based on the irradiation of a solid material using high-intensity pulsed lasers in order to remove from the target surface atoms and molecules. The removed particles form a plume that departs from the target surface with speeds of km/s and then deposits on a substrate, thereby forming a thin layer. Considered as a two-dimensional system, the advantage of using thin film is that it needs a much smaller amount of material (and, implicitly, low cost) and presents the same functions as the solid material and in some cases, with a more promising response. Thus, by choosing appropriately the experimental conditions and type of material properties we can obtain thin layer with controlled properties to satisfy a particular application. An indicative of the advantages of pulsed laser deposition technique PLD is the large number of articles published in the last 20 years, associated with a significant increase in the number of citations. 

• In this context, we believe that the theme of the project presents an interesting topic at international level. The study takes into account both fundamental aspects (processes involved in the interaction of laser radiation-solid material, characterization of generated plasma and thin film growth) and applications (detailed characterization of the deposited layers and their ability to be used in various applications). Two classes of materials are considered: a) magnetic and magnetostrictive materials of cobalt ferrite (undoped and doped with rare earth elements) and b) chalcogenide glasses based on Ge, Sb and Te.  
• Cobalt ferrite materials have attracted a great interest in fundamental and applied research due to their thermal stability, mechanical hardness, large coercive field, high magnetostriction coefficient and anisotropy constant. All these characteristics encourage their use in a wide range of applications from medicine (e.g.: MRI contrast agents, DNA isolation, magnetically activated drug delivery) to electronics (e.g.: magnetostrictive and gas sensors, optoelectronics, microwave frequency devices, storage media). Relating to all these possible applications, different research groups performed various studies on the influence rare earths cations on the properties of CoFe₂O₄ in bulk form, thin films or nanoparticles. 
•  The interest in rare earth elements and their influence on the microstructure and magnetic properties of substituted ferrite is related to the occupancy of the 4f electron shell (from 0 (La) to 14 (Lu)) and magnetic moments (from 0 (La) to 10.6 µ_B (Dy)). Due to their moderate elastic constants and the large orbital component in their moments, the lanthanide metals display the largest known magnetostrictions. The RE elements present large ionic radii which, when substituting cations with smaller ionic radii in other types of structures, can determine a change in cell symmetry and thus generate internal stress. As a consequence, not only the structural properties of the material are changed (e.g. increased cell parameter, decreased average crystallite and grain dimensions) but also the dielectric, magnetic and magnetostrictive properties of substituted materials. While in bulk materials the presence of RE elements leads to residual phase formation and decreased magnetic response, in thin films and nanocrystals only the spinel lattice is observed with an increased magnetization for the RE elements with higher magnetic moment than Fe.  
•  Due to their atomic bonding structure, chalcogenide materials are considered to be more rigid than polymers but with a higher flexibility than oxides, thus their transition temperatures and elastic properties being between those found for the two mentioned types of materials. Although chalcogenide glasses present low mechanical hardness and thermal stability, other characteristics such as high thermal expansion and refractive index, optical non-linearity and larger range of infrared transparency make them suitable for various applications from civil to medical and military areas.  Chalcogenide glasses are low-phonon energy materials which increases their potential for active devices related to photoluminescence. Considering the large variety of elements that can form the chalcogenide glasses, several characteristics (e.g. refractive index, optical band gap energy) can be modified to fit ones purpose.  

The main objective of this study is to obtain a correlation between the experimental parameters and properties of thin films in order to control the characteristics of the nano-structures for their later use in specific applications in various technological fields. Achieving this objective requires a systematic study on:  

a) spectral analysis of laser induced plasma;  

b) deposition of materials with technological interest (cobalt ferrite, chalcogenides of Ge-Sb-Te based structures) under various experimental conditions (varied parameters: pressure, target-substrate distance, the length of filing laser radiation characteristics);  

c) correlation between the thin films properties, experimental parameters and plasma characteristics in order to identify a particular deposition protocol for specific application. 

The main research activities done during this project were: 

• The synthesis and characterization of bulk materials of un-doped and rare earth doped cobalt ferrite.
• Optical and spectral analysis of laser-induced plasma plume
• Deposition and characterization of magnetic and chalcogenide thin films

Detailed below are the main activities carried out for this purpose:  

a)       Synthesis and characterization of bulk materials of un-doped and rare earth doped cobalt ferrite.  

• RE doped cobalt ferrite bulk materials with chemical formula CoFe_1.97RE_0.03O₄ (RE=La, Ce, Sm, Gd, Dy, Ho, Er, Yb) were obtained by conventional ceramic technique. Commercially available pure oxides (Fe₂O₃, Co₃O₄ and RE₂O₃) were used as starting materials, mixed in adequate proportions, calcined in air at 950°C for 5 h and then ball milled for 8 h. Finally the milled powders were pressed into disks at 250 MPa and sintered in air at 1250°C for 5 h with a 100°C/h heating rate followed by a natural cooling to room temperature. For the structural characterization of the resulting calcined powders and sintered pallets, different subsequent analyzing methods were used: X-Ray diffraction (XRD Bruker D8 advanced diffractometer with a Cu-Kα radiation, λ=1.5406 Ǻ), Mössbauer spectroscopy (WissEL-ICE Oxford Mössbauer cryomagnetic system), Fourier transform infrared spectroscopy (FT-IR – JASCO 660 Plus spectrophotometer), Scanning Electron Microscopy and Energy-Dispersive X-ray spectroscopy analysis (SEM/EDX, Vega Tescan LMH II microscope and a Bruker AXS Microanalysis GmbH detector). The magnetic behavior was investigated by Vibrating Sample Magnetometry (VSM PRINCETON/Lakeshore M3900) while the magnetostriction coefficient and its field derivative were measured using the strain gauge method with a homemade set-up. For dielectric constants measurement an E4980A Precision LCR Meter was used to analyze the real permittivity and tangent loss values from 20 Hz to 20 MHz frequency range. 
•  Both XRD patterns and Mössbauer transmission spectra of the CoFe_1.97RE_0.03O₄ bulk materials presented peaks corresponding to two types of structures: one of the spinel cobalt ferrite and a second phase which for most of the samples was associated with the RE orthoferrite. Further structural studies revealed an increased porosity and a decreased grain size of the doped sintered samples. The shift in FT-IR vibrational mode of the octahedral sublattice to higher frequencies was consistent with the decrease cell parameter derived from XRD measurements. Small differences in saturation magnetization and coercive field were found for the doped samples as compared to the stoichiometric cobalt ferrite. The coercive filed variation with RE type was ascribed to the frequency shift of vibrational modes and was explained on the basis of the changes in magnetocrystalline anisotropy. The magnetostriction measurements revealed an increased maximum magnetostriction coefficient and strain derivative for most of the doped samples (the highest being observed for the CoFe_1.97Yb_0.03O₄), while comparable values with the stoichiometric cobalt ferrite were found for the Gd, Dy and Er doped cobalt ferrites. The RE addition led to a decrease in real permittivity which was ascribed to higher porosity of the doped samples and a shift of the resonance frequency derived from tangent loss measurements.  

b)       Optical and spectral characterization of laser induced plasma 

• The experimental set-up consists of a stainless steel vacuum chamber, equipped with a dry pump which can ensure a 10^-2 Torr pressure. The cobalt ferrite target was placed on a micrometric precision 3D translation stage. The second harmonic of an Nd-YAG laser (Quantel Brilliant, 532 nm wavelength, 10 ns pulse duration, 10 Hz repetition rate) was focused at normal incidence on the target by a 25 cm focal length lens. The estimated impact area was 0.5 mm² while the laser energy was set at 50 mJ which led to a fluence of 10 J/cm² . The plasma plume analysis was done using an ICCD PI-MAX camera (PIMAX2- 91003-UNIGEN2, 1024 × 1024 pixels, minimum gating time 2 ns) connected with high resolution monochromator (Acton SP2500i, 500 mm focal length) fitted with one mirror and two diffraction gratings (300 lines/mm, 2400 lines/mm) mounted on the same three-position turret. The investigation of the plasma global dynamics was done by analyzing sequential snapshots taken at different delays with respect to the laser pulse. These images were recorded using the mirror position of the monochromatic turret and a low value of 5ns ICCD gate time for a higher temporal resolution of the expansion process. For spectroscopic analysis the monochromator 2400 lines/mm diffraction grating was used. First, optical emission spectra over 300 nm – 550 nm spectral range were recorded using an ICCD gate time of 5 µs to obtain a global overview of all the species present in the plasma. Once identified, space – and time – resolved optical emission analysis was done on several selected lines, using narrower spectral ranges (~7nm). The spatial distribution of the line intensity was achieved by placing a 1mmx5mm slit on the optical path between the plume and the monochromator, thus analyzing only a 0.1 mm wide plasma slice.   
• For all three types of targets, the ICCD images revealed the presence of two structures with distinct dynamics in the plume. To determine the “center-of-mass” (COM) velocity of these two structures, we plotted the distance from the target of the highest intensity emitting point (in each structure) versus the time at which the snapshot was taken. The first structure displays velocity values five times higher than the second one which is closer to the target surface. Despite the low amount of RE elements (RE:Fe = 1:9), it seems that this is enough to induce a small variation of the COM velocities on both structures. While the second structure presents slightly increased velocities for the RE-doped samples, the first structure (fast component) of the RE containing ferrite plasmas have lower velocity values than the stoichiometric cobalt ferrite. These different dynamics could also be related to the differences in the density of the bulk material (5.19g/cm3 - CoFe₂O₄, 3.81g/cm³ - CoFe_1.8Dy_0.2O₄, 4.05g/cm³ - CoFe_1.8Gd_0.2O₄) and to the physical characteristics of the main elements (atomic mass, ionic radius).  
• The time and space rezolved optical emission spectrosciopy analysis results reveal the presence of two groups with distinct dynamics: one formed by neutrals with velocities in the range of 1 to 3·10³ m/s (“slow” structure) and a second one which consists mainly in ionic species with velocities of almost one order of magnitude higher than the previous one (“fast” structure). The existence of two structures formed by different types of species can be explained on the basis of two distinct ejection mechanisms which are described in our previous papers: one based on electrostatic repulsion and another one linked to thermal mechanisms. 
• The optical emission spectroscopy results revealed a higher excitation temperature for the Dy-doped cobalt ferrite compared to the undoped and Gd doped plasma and lower velocities of the Dy ions and neutrals compared to those of the Gd species. At smaller distances from the target surface, a 600 K electronic temperature difference between the Dy doped and Gd doped cobalt ferrite plasma was observed. Both this dependence and the velocity variations of the RE elements can be tentatively explained on the basis of the difference between the vaporization temperature of Dy and Gd. The lower evaporation heat of Dy compared to Gd means that more Dy atomic systems will pass into an excited state, thus the excitation temperature will be higher. There is also a difference between the ionization potential of the two elements (Dy: 5.93eV; Gd: 6.15eV) but its value is relatively small to explain the higher ionization degree of Dy compared to Gd species. Moreover, the higher kinetic energy of Dy right after the laser-target interaction is lost through excitation mechanisms, leading to a lower velocity for this element. 

c)       Thin film deposition 

• CoFe₂O₄, CoFe_1.8RE_0.2O₄ ( RE = Dy , Gd , La) and CoFe_1.97RE_0.03O₄ ( RE = Dy, La, Gd, Ho, Sm, Yb, Er, Ce) thin films were deposited by PLD in various experimental conditions. The experiments were done at a pressure of ~ 10^-6 Torr achieved by a preliminary vacuum pump and a turbomolecular pump. The monocrystalline silicon substrate (100) was heated during deposition at a temperature of 400^oC. The experimental parameters which were varied were: deposition time, distance between the target and substrate and laser energy. Structural and chemical properties of the deposited layers were analyzed by Raman spectroscopy, profilometry, Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy. 
• We observed a dependency between the thickness of the cobalt ferrite thin films and the varied experimental parameters. The thickness increases when increasing the deposition time and laser fluence and decreases as the target substrate is changed from 40 mm to 70 mm. Raman spectra show peaks corresponding to the vibration modes of cobalt ferrite. Lack of displacement of these peaks toward higher or lower frequencies indicates the absence of internal stress which can be induced by the lattice parameter mismatch between the substrate and the thin film of by the differences in thermal expansion coefficients. 
• Magnetic properties of the samples were analyzed using a vibrating sample magnetometer. Hysteresis curves were recorded in two configurations: 1) the magnetic field is applied parallel to the thin film plane and 2) the magnetic field is applied perpendicular to the layer to analyze a possible preference in magnetic orientation of the grains that form the thin film. The recorded maximum magnetizations were close to 300 emu/cm³, with small variations from one sample to another. The out-of-plane hysteresis curves presented a very weak signal compared to the in plane ones. A decrease in maximum magnetization was observed as the rare earth ions were added. This result can be explained by the structural properties of thin layers. RE doped cobalt ferrite films were found to be amorphous. The rare earth addition restrains the crystallization process and can also account for the decrease in maximum magnetization and coercive field. The use of more elevated laser fluence or a higher substrate temperature can lead to the crystallization of the sample in a spinel type structure and can increase the magnetic properties. These aspects are considered for further studies.   

ISI indexed papers: 

1. Effect of rare earth substitution in cobalt ferrite bulk materials, G. Bulai, L. Diamandescu, I. Dumitru, S. Gurlui, M. Feder, O.F. Caltun, Journal of Magnetism and Magnetic Materials, Volum 390, 2015, Pag. 123–131 
2. Advanced metallic materials response at laser excitation for medical applications; I. Stirbu, P. Vizureanu, R. Cimpoesu, G. Dascălu (BULAI), S. O. Gurlui, M. Bernevig, M. Benchea, N. Cimpoeşu, P. Postolache, Journal of Optoelectronics and Advanced Materials, Vol. 17, No. 7-8, July – August 2015, p. 1179 – 1185; 
3. Pure and rare earth doped cobalt ferrite laser ablation: space and time resolved optical emission spectroscopy, G. Bulai, S. Gurlui, O. F. Caltun, C. Focsa, Digest Journal of Nanomaterials and Biostructures Vol. 10, No. 3, July - September 2015, p. 1043 – 1053. 

Conference participations: 

1. Synthesis and characterization of RE (RE=Dy, Gd, La, Sm, Yb, Er, Ho) doped cobalt ferrite bulk materials, G. Bulai, L. Diamandescu, I. Dumitru, S. Gurlui, M. Feder, O. F. Caltun, ELECTROCERAMICS 2014, 16-20 June 2014, Bucuresti, Romania.  
2. Influence of ablation conditions on the structural and optical properties of Ge-Sb-Te based thin films deposited by PLD, G. Bulai, O. Pompilian, V. Nazabal, P. Nemec, B. Chazallon, S. Gurlui, C. Focsa, ELECTROCERAMICS 2014, 16-20 June 2014, Bucuresti, Romania.  
3. Study of cobalt ferrite bulk materials doped with rare earth ions, G. Dascalu, L. Diamandescu, M. Feder, A. Pui, I. Dumitru, S. Gurlui, O.F. Caltun, IEEE ROMSC 2014, Iasi, Romania; 
4. Magnetic and magnetostrictive behavior of rare earth doped cobalt ferrite bulk materials, Georgiana Bulai, Lucian Diamandescu, Silviu Gurlui, Marcel Feder, Ovidiu Florin altun, Ioan Dumitru, European Conference on Magnetic Sensors and Actuators, 6-9 July 2014, Vienna, Austria. 
5. Optical and spectral investigations of CoFe2O4 and CoFe_1.8RE_0.2O₄ (RE=Dy, Gd) plasma plume generated by laser ablation; G. Bulai, M. M. Cazacu, S. Gurlui, C. Focsa; International Physics Conference TIM14, 20-22 November, Timisoara, Romania. 
6. Influence of rare earth addition on structural and magnetic properties of cobalt ferrite thin films, G. Bulai, V. Nica, B. Chazallon, S. Gurlui, C. Focsa, EMRS Spring Meeting, 11-15 May 2015, Lille, France.