Growth of Al2O3 thin film by oxidation of resistively evaporated Al on top of SnO2, and electrical properties of the heterojunction SnO2/Al2O3

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Nanohardness of a Ti thin film and its interface deposited by an electron beam on a 304 SS substrate

The results of nanohardness measurements at a film surface and film-substrate interface are presented and discussed. An electron beam device was used to deposit a Ti film on a 304 stainless steel (304 SS) substrate. The diluted interface was obtained by thermal activated atomic diffusion. The. Ti film and Ti film-304 SS interface were analyzed by energy dispersive spectrometry and were observed using atomic force microscopy. The nanohardness of the Ti film-304 SS system was measured by a nanoindentation technique. The results showed the Ti film-304 SS interface had a higher hardness value than the Ti film and 304 SS substrate. The Ti film surface had a lower hardness due to the presence of a TiO2 thin layer.

Ferroelectric and optical properties of Ba0.8Sr0.2TiO3 thin film

Barium strontium titanate (Ba0.8Sr0.2TiO3) thin films have been prepared on Pt/Ti/SiO2/Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba0.8Sr0.2TiO3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm(-1) peaks. The tetragonal-to-cubic phase transition in the Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed at about 35 degreesC, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba0.8Sr0.2TiO3 thin films showed a remnant polarization (P-r) of 6.5 muC/cm(2), a coercive field (E-c) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (E-g) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27-2.10, respectively. (C) 2002 American Institute of Physics.

High Curie point CaBi2Nb2O9 thin films: A potential candidate for lead-free thin-film piezoelectrics

CaBi2Nb2O9 (CBNO) thin films deposited on platinum coated silicon substrates by the polymeric precursor method exhibited good structural, dielectric, and piezoelectric characteristics. Capacitance-voltage measurements indicated good ferroelectric polarization switching characteristics. Remanent polarization and drive voltage values were 4.2 mu C/cm(2) and 1.7 V for a maximum applied voltage of 10 V. The film has a piezoelectric coefficient d(33) equal to 60 pm/V, current density of 0.7 mu A/cm(2), and Curie temperature of 940 degrees C. The polar-axis-oriented CBNO is a promising candidate for use in lead-free high Curie point in ferroelectric and piezoelectric devices. (c) 2006 American Institute of Physics.

Microstructure and electrical properties of perovskite (Pb, La)TiO3 thin film deposited at low temperature by the polymeric precursor method

High-quality (Pb, La)TiO3 ferroelectric thin films were successfully prepared on a Pt(111)/Ti/SiO2/Si(100) substrate for the first time by spin coating, using the polymeric precursor method. The X-ray diffraction patterns show that the films are polycrystalline in nature. This method allows for low temperature (500 degrees C) synthesis, a high quality microstructure and superior dielectric properties. The effects on the microstructure and electrical properties were studied by changing the La content. The films annealed at 500 degreesC have a single perovskite phase with only a tetragonal or pseudocubic structure. As the La content is increased, the dielectric constant of PLT thin films increases from 570 up to 1138 at room temperature. The C-V and P-E characteristics of perovskite thin films prepared at a low temperature show normal ferroelectric behavior, representing the ferroelectric switching property. The remanent polarization and coercive field of the films deposited decreased due to the transformation from the ferroelectric to the paraelectric phase with an increased La content. (C) 2001 Kluwer Academic Publishers.

Fabrication, Electrical Characterization and Simulation of Thin Film Solar Cells: CdTe and CIGS Materials

CdTe and Cu(In,Ga)Se2 (CIGS) thin film solar cells are fabricated, electrically characterized and modelled in this thesis. We start from the fabrication of CdTe thin film devices where the R.F. magnetron sputtering system is used to deposit the CdS/CdTe based solar cells. The chlorine post-growth treatment is modified in order to uniformly cover the cell surface and reduce the probability of pinholes and shunting pathways creation which, in turn, reduces the series resistance. The deionized water etching is proposed, for the first time, as the simplest solution to optimize the effect of shunt resistance, stability and metal-semiconductor inter-diffusion at the back contact. In continue, oxygen incorporation is proposed while CdTe layer deposition. This technique has been rarely examined through R.F sputtering deposition of such devices. The above experiments are characterized electrically and optically by current-voltage characterization, scanning electron microscopy, x-ray diffraction and optical spectroscopy. Furthermore, for the first time, the degradation rate of CdTe devices over time is numerically simulated through AMPS and SCAPS simulators. It is proposed that the instability of electrical parameters is coupled with the material properties and external stresses (bias, temperature and illumination). Then, CIGS materials are simulated and characterized by several techniques such as surface photovoltage spectroscopy is used (as a novel idea) to extract the band gap of graded band gap CIGS layers, surface or bulk defect states. The surface roughness is scanned by atomic force microscopy on nanometre scale to obtain the surface topography of the film. The modified equivalent circuits are proposed and the band gap graded profiles are simulated by AMPS simulator and several graded profiles are examined in order to optimize their thickness, grading strength and electrical parameters. Furthermore, the transport mechanisms and Auger generation phenomenon are modelled in CIGS devices.

Fabrication of high-k dielectric thin films for spintronics

Lo scopo di questa tesi è la fabbricazione di ossidi complessi aventi struttura perovskitica, per mezzo della tecnica Channel Spark Ablation (CSA). Più precisamente sono stati depositati film sottili di manganite (LSMO), SrTiO3 (STO) e NdGaO3 (NGO). Inoltre nel laboratorio ospite è stata effettuata la caratterizzazione elettrica e dielettrica (spettroscopia di impedenza), mentre per l'analisi strutturale e chimica ci si è avvalsi di collaborazioni. Sono stati fabbricati dispositivi LSMO/STO/Co e se ne è studiato il comportamento magnetoresistivo e la bistabilità elettrica a seconda del carattere epitassiale od amorfo dell'STO. I risultati più promettenti sono stati ottenuti con STO amorfo. Sono stati costruiti diversi set di condensatori nella configurazione Metallo/Isolante/Semiconduttore (MIS), con M=Au, I=STO o NGO ed S=Nb:STO, allo scopo di indagare la dipendenza delle proprietà dielettriche ed isolanti dai parametri di crescita. In particolare ci si è concentrati sulla temperatura di deposizione e, nel caso dei film di STO, anche sulla dipendenza della costante dielettrica dallo spessore del film. Come ci si aspettava, la costante dielettrica relativa dei film di STO (65 per un film spesso 40 nm e 175 per uno di 170 nm) si è rivelata maggiore di quella dei film di NGO per i quali abbiamo ottenuto un valore di 20, che coincide con il valore del bulk. Nonostante l'elevata capacità per unità di area ottenibile con l'STO, la costante dielettrica di questo materiale risulta fortemente dipendente dallo spessore del film. Un ulteriore aspetto critico relativo all'STO è dato dal livello di ossidazione del film: le vacanze di ossigeno, infatti, possono ridurre la resistività dell'STO (nominalmente molto elevata), ed aumentarne la corrente di perdita. Al contrario l'NGO è meno sensibile ai processi tecnologici e, allo stesso tempo, ha un valore di costante dielettrica più alto rispetto ad un tipico dielettrico come l'ossido di silicio.

Investigation into the impact of grain boundaries, film interface, and crystallographic orientation on the ionic conductivity of thin film gadolinium-doped ceria

The research reported in this dissertation investigates the impact of grain boundaries, film interface, and crystallographic orientation on the ionic conductivity of thin film Gd-doped CeO2 (GDC). Chapter 2 of this work addresses claims in the literature that submicron grain boundaries have the potential to dramatically increase the ionic conductivity of GDC films. Unambiguous testing of this claim requires directly comparing the ionic conductivity of single-crystal GDC films to films that are identical except for the presence of submicron grain boundaries. In this work techniques have been developed to grow GDC films by RF magnetron sputtering from a GDC target on single crystal r plane sapphire substrates. These techniques allow the growth of films that are single crystals or polycrystalline with 80 nm diameter grains. The ionic conductivities of these films have been measured and the data shows that the ionic conductivity of single crystal GDC is greater than that of the polycrystalline films by more than a factor of 4 over the 400-700°C temperature range. Chapter 3 of this work investigates the ionic conductivity of surface and interface regions of thin film Gd-doped CeO2. In this study, single crystal GDC films have been grown to thicknesses varying from 20 to 500 nm and their conductivities have been measured in the 500-700°C temperature range. Decreasing conductivity with decreasing film thickness was observed. Analysis of the conductivity data is consistent with the presence of an approximately 50 nm layer of less conductive material in every film. This study concludes that the surface and interface regions of thin film GDC are less conductive than the bulk single crystal regions, rather than being highly conductive paths. Chapter 4 of this work investigates the ionic conductivity of thin film Gd-doped CeO2 (GDC) as a function of crystallographic orientation. A theoretical expression has been developed for the ionic conductivity of the [100] and [110] directions in single crystal GDC. This relationship is compared to experimental data collected from a single crystal GDC film. The film was grown to a thickness of _300 nm and its conductivity measured along the [100] and [110] orientations in the 500-700°C temperature range. The experimental data shows no statistically significant difference in the conductivities of the [100] and [110] directions in single crystal GDC. This result agrees with the theoretical model which predicts no difference between the conductivities of the two directions.

Tuning the functional properties of Silicon Carbide Thin Film and Nanowires

The present thesis has been devoted to the synthesis and investigation of functional properties of silicon carbide thin films and nanowires. The work took profit from the experience of the research group in the synthesis of 3C-SiC from vapour phase. 3C-SiC thin films Thin films heteroepitaxy on silicon substrates was carried out in a vapour phase epitaxy reactor. The initial efforts were committed to the process development in order to enhance the crystal quality of the epi-layer. The carbonization process and a buffer layer procedure were optimized in order to obtain good quality monocrystalline 3C-SiC layers. The films characterization was used not only to improve the entire process, but also to assess the crystalline quality and to identify the defects. Methyltrichlorosilane (MTS) was introduced during the synthesis to increase the growth rate and enhance crystalline quality. The effect of synthesis parameters such as MTS flow and process temperature was studied in order to promote defect density reduction and the release of the strain due to lattice mismatch between 3C-SiC and silicon substrate. In-growth n-type doping was implemented using a nitrogen gas line and the effect of different synthesis parameters on doping level was studied. Raman measurements allowed a contactless characterization and evaluation of electrically active dopant. The effect of MTS on nitrogen incorporation was investigated and a promotion of dopant concentration together with a higher growth rate were demonstrated. This result allows to obtain higher doping concentrations without deteriorating crystal quality in 3C-SiC and, to the best of our knowledge, it has never been demonstrated before. 3C-SiC nanowires Core-shell SiC-SiO2 nanowires were synthesized using a chemical vapour deposition technique in an open tube configuration reactor on silicon substrates. Metal catalyst were used to promote a uniaxial growth and a dense bundle of nanowires 100 µm long and 60 nm thick was obtained. Substrate preparation was found to be fundamental in order to obtain a uniform nanowire density. Morphological characterization was carried out using scanning electron microscopy and the analysis of structural, compositional, optical properties is reported.

Microstructure of MmM(5)/Mg multi-layer hydrogen storage films prepared by magnetron sputtering

Multi-layer hydrogen storage thin films with Mg and MmNi(3.5)(CoAlMn)(1.5) (here Mm denotes La-rich mischmetal) as alternative layers were prepared by direct current magnetron sputtering. Transmission electron microscopy investigation shows that the microstructure of the MmNi(3.5)(CoAlMn)(1.5) and Mg layers are significantly different although their deposition conditions are the same. The MmNi(3.5)(CoAlMn)(1.5) layer is composed of two regions: one is an amorphous region approximately 4 nm thick at the bottom of the layer and the other is a nanocrystalline region on top of the amorphous region. The Mg layer is also composed of two regions: one is a randomly orientated nanocrystalline region 50 nm thick at the bottom of the layer and the other is a columnar crystallite region on top of the nanocrystalline region. These Mg columnar crystallites have their [001] directions parallel to the growth direction and the average lateral size of these columnar crystallites is about 100 nm. A growth mechanism of the multi-layer thin films is discussed based on the experiment results. Wiley-Liss, Inc.

Microstructure of MmM(5)/Mg multi-layer films prepared by magnetron sputtering

Microstructure of MmNi(3.5)(CoAlMn)(1.5)/Mg (here Mm denotes La-rich mischmetal) multi-layer hydrogen storage thin films prepared by direct current magnetron sputtering was investigated by cross-sectional transmission electron microscopy (XTEM). It was shown that the MMM5 layers are composed of two regions: an amorphous region with a thickness of similar to 4nm at the bottom of the layers and a randomly orientated nanocrystallite region on the top of the amorphous region and the Mg layers consist of typical columnar crystallite with their [001] direction nearly parallel to the growth direction. The mechanism for the formation of the above microstructure characteristics in the multi-layer thin films has been proposed. Based on the microstructure feature of the multi-layer films, mechanism for the apparent improvement of hydrogen absorption/desorption kinetics was discussed. (c) 2005 Elsevier B.V. All rights reserved.

An investigation of thin film magnetic recording media

Mechanical, physical and chemical changes in the surface of commercial thin film metal evaporated magnetic recording media have been correlated to recording error and signal degradation measurements. Modified and adapted commercial Hi-8 video recorders have been used for sample generation whilst analytical techniques such as SXPS,IMS and SEM have been employed in the surface characterisation. The durability of the media was assessed through stop motion (still frame) and cycling tests, where error growth and signal degradation were measured as a function of running time. The tests were performed under ambient (22°C, 40% RH) and high humidity (22°C, 80% RH) conditions. Characterisation of the lubricant layer on each tape was performed through models based on XPS and angle resolved XPS. The lubricant thickness can significantly affect the durability and signal output level of a thin film tape and thus it is important that reliable quantification can be achieved. Various models were considered for determining the lubricant thickness although ultimately, the most suitable technique was deemed to be a model that assumed a uniform layer structure. In addition to thin film metal evaporated media, equivalent durability tests and surface analysis experiments were performed using a commercial metal particle tape in order that comparisons could be made between the two types of recording media. The signal performance of the thin film metal evaporated media was found to be quite different from that for the metal particle tape since dropout errors and signal degradation increased at a much earlier stage. Extensive surface analyses enabled the mechanisms responsible for media failure and error growth to be identified in the ME and MP tapes and these were found to result from cyclic stressing and fatigue on the immediate substrate of the media.

Synthesis, thermal processing, and thin film morphology of poly(3-hexylthiophene)-poly(styrenesulfonate) block copolymers

A series of novel block copolymers, processable from single organic solvents and subsequently rendered amphiphilic by thermolysis, have been synthesized using Grignard metathesis (GRIM) and reversible addition-fragmentation chain transfer (RAFT) polymerizations and azide-alkyne click chemistry. This chemistry is simple and allows the fabrication of well-defined block copolymers with controllable block lengths. The block copolymers, designed for use as interfacial adhesive layers in organic photovoltaics to enhance contact between the photoactive and hole transport layers, comprise printable poly(3-hexylthiophene)-block-poly(neopentyl p-styrenesulfonate), P3HT-b-PNSS. Subsequently, they are converted to P3HT-b-poly(p-styrenesulfonate), P3HT-b-PSS, following deposition and thermal treatment at 150 °C. Grazing incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) revealed that thin films of the amphiphilic block copolymers comprise lamellar nanodomains of P3HT crystallites that can be pushed further apart by increasing the PSS block lengths. The approach of using a thermally modifiable block allows deposition of this copolymer from a single organic solvent and subsequent conversion to an amphiphilic layer by nonchemical means, particularly attractive to large scale roll-to-roll industrial printing processes.

Direct laser writing of a channel waveguide in Al2O3:Nd thin film

The femtosecond laser modification of refractive index in amorphous Al2O3:Nd thin film prepared by rf magnetron sputtering is investigated. Modifications of the refractive index in a sample with a single Al2O3:Nd layer and in a sample composed of the Al2O3:Nd layer and SiO2 layer on the top were compared. Advantages arising from addition of the SiO2 layer are shown. The film was patterned in order to form an active waveguide. Waveguide loss and mode composition were investigated experimentally and theoretically. Spectrum and kinetics of luminescence in the region of 1.06 μm were measured.

Elastic modulus study of gold thin film for use as an actuated membrane in a superconductive radio frequency (RF) microelectromechanical (MEM) switch

The objective of this research was to find Young's elastic modulus for thin gold films at room and cryogenic temperatures based on the flexional model which has not been previously attempted. Electrical Sonnet simulations and numerical methods using Abacus for the mechanical responses were employed for this purpose. A RF MEM shunt switch was designed and a fabrication process developed in house. The switch is composed of a superconducting YBa2 Cu3O7 coplanar waveguide structure with an Au bridge membrane suspended above an area of the center conductor covered with BaTiO3 dielectric. The Au membrane is actuated by the electrostatic attractive force acting between the transmission line and the membrane when voltage is applied. The value of the actuation force will greatly depend on the switch pull-down voltage and on the geometry and mechanical properties of the bridge material. Results show that the elastic modulus for Au thin film can be 484 times higher at cryogenic temperature than it is at room temperature. ^