Phase Formation In Thin Films Research Articles

Mg-Ag-Sb thin films produced by solid-state reactive diffusion

α-MgAgSb is a tellurium-free thermoelectric material that exhibits good thermoelectric properties near room temperature. Being made of relatively abundant elements compatible with the complementary metal oxide semiconductor (CMOS) technology, it is considered as a possible solution for the development of high-efficiency thermoelectric devices for heat waste harvesting in microelectronic setups. This study presents a first attempt to investigate the structural properties of MgAgSb thin films prepared by solid-state reactive diffusion. X-ray diffraction (XRD) was used to follow phase formation in thin films, first, in the case of the binary Ag3Sb and Mg3Sb2 compounds, and then, in the case of the ternary system Mg-Ag-Sb. For the later, in situ XRD was used to follow real-time phase formations during the reaction of the bilayerAg3Sb/Mg3Sb2. The results show that the phase α-MgAgSb can be produced by reactive diffusion at the interface of the bilayer. Furthermore, the three phases α, β, and γ are shown to coexist at 360 °C, which can be the result of the thin film geometry (surface and interface effects) or due to a different stoichiometry between these three phases contrasting with usual belief. At temperatures higher than 450 °C, γ-MgAgSb is the only phase stabilized in the film. This study serves as a benchmark for the production of pure α-MgAgSb thermoelectric thin films by reactive diffusion.

Nuclear magnetic resonance study of thinCo2FeAl0.5Si0.5Heusler films with varying thickness

Type, degree and evolution of structural order are important aspects for understanding and controlling the properties of highly spin polarized Heusler compounds, in particular with respect to the optimal film growth procedure. In this work, we compare the structural order and the local magnetic properties revealed by nuclear magnetic resonance (NMR) spectroscopy with the macroscopic properties of thin Co$_2$FeAl$_{0.5}$Si$_{0.5}$ Heusler films with varying thickness. A detailed analysis of the measured NMR spectra presented in this paper enables us to find a very high degree of $L2_1$ type ordering up to 81% concomitantly with excess Fe of 8 to 13% at the expense of Al and Si. We show, that the formation of certain types of order do not only depend on the thermodynamic phase diagrams as in bulk samples, but that the kinetic control may contribute to the phase formation in thin films. It is an exciting finding that Co$_2$FeAl$_{0.5}$Si$_{0.5}$ can form an almost ideal $L2_1$ structure in films though with a considerable amount of Fe-Al/Si off-stoichiometry. Moreover, the very good quality of the films as demonstrated by our NMR study suggests that the novel technique of off-axis sputtering technique used to grow the films sets stage for the optimized performance of Co$_2$FeAl$_{0.5}$Si$_{0.5}$ in spintronic devices.

Comparison of optical and structural properties of nanostructure TiO2 thin film doped by Sn and Nb

The thin films of TiO2 doped by Sn or Nb were prepared by sol–gel method under process control. The effects of Sn and Nb doping on the structural, optical and photo-catalytic properties of applied thin films have been studied by X-ray diffraction (XRD) high resolution transmission electron microscopy and UV–Vis absorption spectroscopy. Surface chemical state of thin films was examined by atomic X-ray photoelectron spectroscopy. XRD results suggest that adding impurities has a great effect on the crystallinity and particle size of TiO2. Titania rutile phase formation in thin film was promoted by Sn4+ addition but was inhibited by Nb5+ doping. The activity of the photocatalyst was evaluated by photocatalytic degradation kinetics of aqueous methylene blue under UV and Visible radiation. The results show that the photocatalytic activity of the Sn-doped TiO2 thin film have a larger degradation efficiency than Nb-doped TiO2 under visible light, but under UV light photocatalytic activity of the Nb-doped TiO2 thin film is better.

Enhanced photo-catalytic activity of TiO2 nanostructured thin films under solar light by Sn and Nb co-doping

In this study, preparation of Sn and Nb co-doped TiO2 dip-coated thin films on glazed porcelain substrates via sol–gel process have been investigated. The effects of co-doping content on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction (XRD), field emission SEM (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and UV–Vis absorption spectroscopy. Surface chemical state of thin films was examined by atomic X-ray photoelectron spectroscopy (XPS). XRD results suggest that adding impurities has a great effect on the crystallinity and particle size of TiO2. Titania Rutile phase formation in thin film was promoted by Sn4+ addition but was inhibited by Nb5+ doping. The prepared co-doped TiO2 photo-catalyst films showed optical absorption edge in the visible light area and exhibited excellent photo-catalytic ability for degradation of methylene blue (MB) solution under solar irradiation. Comparison with undoped and Sn or Nb-doped TiO2, codoped TiO2 shows an obviously higher catalytic activity under solar irradiation.

Phase Formation in Thin Films of Cu-In-S Ternary System

In this paper we present the results of studies on the interaction of thin films of Cu-In-S, which is obtained by evaporation of individual components in the plane of condensation, consisting of fresh chips NaCl. Electron-diffraction analysis shows that the length of the regions forming binary compounds and ternary compounds of CuInS2 with simultaneous and sequential deposition of components is the same. Thus we can speak with certainty that the existence of thin film of tetragonal phase of ternary compound is present in CuInS2.

In-situ investigation of the icosahedral Al–Cu–Fe phase formation in thin films

This work is an investigation of the formation by reactive diffusion at high temperatures of the icosahedral phase, i-Al62.5Cu25Fe12.5, in thin films. The samples were prepared by sputtering at room temperature. The elements Al, Cu and Fe were sequentially deposited onto oxidized silicon substrates. The two following stacking sequences, Al/Cu/Fe and Al/Fe/Cu, were investigated. The phase formation was studied using in situ resistivity, in situ X-ray Diffraction and Differential Scanning Calorimetry measurements. Whatever the stacking sequence, the sequences of phase formation evidenced during the heating treatment are similar. However the temperatures of formation for the first phases that are formed are different; they are higher in the case of the Al/Fe/Cu stacking sequence.

Influence of annealing on humidity response of RF sputtered nanocrystalline MgFe 2O 4 thin films

Humidity response of Radio Frequency sputtered MgFe 2O 4 thin films onto alumina substrate, annealed at 400 °C, 600 °C and 800 °C has been investigated. Crystalline phase formation of thin films annealed at different temperature was analyzed by X-ray Diffraction. A particle/grain like microstructure in the grown thin films was observed by Scanning Electron Microscope and Atomic Force Microscope images. Film thickness for different samples was measured in the range 820–830 nm by stylus profiler. Log R (Ω) response measurement was taken for all thin films for 10–90% relative humidity (% RH) change at 25 °C. Resistance of the film increased from 5.9 × 10 10 to 3 × 10 12 at 10% RH with increase in annealing temperature from 400 °C to 800 °C. A three-order magnitude, 10 12 Ω to 10 9 Ω drop in resistance was observed for the change of 10 to 90% RH for 800 °C annealed thin film. A good linear humidity response, negligible humidity hysteresis and minimum response/recovery time of 4 s/6 s have been measured for 800 °C annealed thin film.

An infrared imaging method for high-throughput combinatorial investigation of hydrogenation-dehydrogenation and new phase formation of thin films

We have developed an infrared imaging setup enabling in situ infrared images to be acquired, and expanded on capabilities of an infrared imaging as a high-throughput screening technique, determination of a critical thickness of a Pd capping layer which significantly blocks infrared emission from below, enhancement of sensitivity to hydrogenation and dehydrogenation by normalizing raw infrared intensity of a Mg thin film to an inert reference, rapid and systematic screening of hydrogenation and dehydrogenation properties of a Mg-Ni composition spread covered by a thickness gradient Pd capping layer, and detection of formation of a Mg2Si phase in a Mg thin film on a thermally oxidized Si substrate during annealing.

Organic charge transfer phase formation in thin films of the BEDT-TTF/TCNQ donor–acceptor system

We have performed charge transfer phase formation studies on the donor/acceptor system bis-(ethylendithio)tetrathiafulvalene (BEDT-TTF)/tetracyanoquinodimethane,(TCNQ) by means of physical vapor deposition. We prepared donor/acceptor bilayer structures on glass and Si(100)/SiO 2 substrates held at room temperature and analyzed the layer structures by optical microscopy, X-ray diffraction and focused ion beam cross sectioning before and after annealing. We found clear evidence for the formation of a charge transfer phase during the annealing procedure. For the as-grown samples we could not detect the occurrence of a charge transfer phase. X-ray diffraction indicated that the monoclinic variant of the (BEDT-TTF)-TCNQ was formed. This was further corroborated by single-source evaporation experiments from pre-reacted (BEDT-TTF)-TCNQ obtained from solution growth, and in particular from co-evaporation experiments of (BEDT-TTF)-TCNQ and TCNQ. In the course of these experiments we found that (0 ℓℓ)-oriented BEDT-TTF layers can be prepared on α-Al 2O 3 (112̄0) substrates at about 100 °C using (BEDT-TTF)-TCNQ as source material. We speculate that due to its high vapor pressure the TCNQ component serves as a carrier gas for BEDT-TTF vapor phase transport.

Transient and end silicide phase formation in thin film Ni/polycrystalline-Si reactions for fully silicided gate applications

The Ni/polycrystalline-Si thin film reaction was monitored by in situ x-ray diffraction during ramp annealings, obtaining a detailed view of the formation and evolution of silicide phases in stacks of interest for fully silicided gate applications. Samples consisted of Ni (30–170nm)/polycrystalline-Si (100nm)∕SiO2 (10–30nm) stacks deposited on (100) Si. The dominant end phase (after full silicidation) was found to be well controlled by the deposited Ni to polycrystalline-Si thickness ratio (tNi∕tSi), with formation of NiSi2 (∼600°C), NiSi (∼400°C), Ni3Si2 (∼500°C), Ni2Si, Ni31Si12 (∼420°C), and Ni3Si (∼600°C) in stacks with tNi∕tSi of 0.3, 0.6, 0.9, 1.2, 1.4, and 1.7, respectively. NiSi and Ni31Si12 were observed to precede formation of NiSi2 and Ni3Si, respectively, as expected for the phase sequence conventionally reported. Formation of Ni2Si was observed at early stages of the reaction. These studies revealed, in addition, the formation of transient phases that appeared and disappeared in narrow temperature ranges, competing with formation of the phases expected in the conventional phase sequence. These included the transient formation of NiSi and Ni31Si12 in stacks in which these phases are not expected to form (e.g., tNi∕tSi of 1.7 and 0.9, respectively), at temperatures similar to those in which these phases normally grow.

MAGNETOELASTIC NANOCRYSTALLINE Co–Ni ALLOYS

Magnetic susceptibility of the Co – Ni suck-cast plates underwent an abrupt change at 32 at% Ni due to phase transformation. Strain value of Co -32 at% Ni cast plate increased from 54 to 850 με as temperature decreased to 150 K. Phase formation of thin film is dependent upon deposition condition.

Suppression of intermediate phase nucleation in binary couples with metastable solubility

This paper investigates the influence of sharp concentration gradients and/or chemical potentials on the nucleation of intermediate phases during solid state reactions between elements with limited solubility. These factors would not appear to have any significant influence on first intermediate phase nucleation. On the other hand, for the second and subsequent phases, thermodynamic suppression (in addition to the kinetic factor) of nucleation can be quite considerable and may well be the reason for sequential phase formation in thin films. Thermodynamic suppression criteria for intermediate phase nucleation are proposed.

Phase transformations in nitrogen-implanted iron layers

Phase transformations in implanted iron may strongly depend on the sample thickness. The effect essentially relies on lower impurity concentration required for given phase formation in thin films when compared with thick layers or bulk samples. The phase transformation in thin films is also more complete, a full transformation of thin Fe layers into iron nitrides was observed, whereas in thick or bulk samples about 20% of Fe atoms remain in crystalline α-Fe phase.

In situ controlled reactions and phase formation of thin films on GaAs

In situ sequential depositions and reactions are used to control a reaction sequence first to consume GaAs and then to release GaAs from the reaction product. The first reaction involves annealing in situ deposited Ni on molecular beam epitaxially grown GaAs (100) at 300 °C to form a Ni3GaAs/GaAs structure. Exposure of this structure to As4 results in a reaction which consumes the Ni3GaAs by the layer-by-layer formation of NiAs at the surface and epitaxially regrown GaAs at the Ni3GaAs/GaAs interface. The NiAs formation and GaAs regrowth are controlled by the As4 flux. Ni diffusion dominates both the Ni3GaAs formation and decomposition mechanisms. Reflection high-energy electron diffraction, Rutherford backscattering, x-ray diffraction, and transmission electron microscopy data are used to confirm the phase formation and reaction sequences.

High Resolution X-Ray Microanalysis of Nb/Al Multilayer Thin Films

A study of reactive phase formation in thin films requires the understanding of the thermodynamics and kinetics of the reactions. Previous investigations of phase formation in Nb/Al multilayers using calorimetry and x-ray diffraction have observed that despite the presence of rather large driving forces for phase formation, a pronounced nucleation event occurs. This behavior has been reported in other systems including Ti/Al and Ni/Al. The initial nature of the interface can affect the nucleation, therefore careful studies of the structure and composition of the initial interfacial region may explain the reason for a kinetically separated nucleation and lateral growth step preceding normal phase growth. In addition, examination of the interface may also lend insight into the effects of different deposition processes and can provide evidence for the formation of solid solutions or compounds that can lower driving forces available for phase formation. Finally, the cross-sectional multilayer sample provides a means of characterizing spatial resolution and isolating specimen preparation artifacts.

Impact of heterogeneous boundary nucleation on transformation kinetics and microstructure

We examine quantitatively the impact of heterogenous nucleation on the temporal evolution of a phase transformation with particular emphasis on the correlation of nucleation site distribution and product phase microstructure. This is accomplished by investigating the spatial correlations in the transforming system via the calculation of several non-equilibrium, n-point correlation functions and thereby defining associated transformation fractions and correlation lengths. Computer simulations of transformations are also employed in order to validate the theoretical description and to relate the microstructural features of the evolving phase to relevant length and time scales in the problem. Finally, our findings are related to the results of experimental calorimetric studies of phase formation in thin films.

Interfacial Reactions in Thin Film and Bulk Iron/Silicon Couples

ABSTRACTInitial phase formation in thin film and bulk Fe/Si couples has been investigated using transmission and scanning electron microscopy (TEM and SEM). For the thin film couples, ≈165 nm of Fe was deposited by electron beam evaporation onto <111> oriented Si substrates. SiO2 capping layers (≈100nm thick) were used to protect the Fe from oxidation during subsequent annealing. Bulk diffusion couples were fabricated by clamping together polycrystalline Fe pieces and <111> oriented single crystal Si pieces and sealed in evacuated (≈10-4 torr) quartz capsules. Annealing of thin film couples was done at temperatures ranging from 300°C to 500°C for up to several hours. Bulk couples were annealed at 700°C for up to ≈1000hrs.Interfacial reactions were detected in as deposited thin film couples. A layer =5nm thick was identified, through electron diffraction, as poorly crystalline off-stoichiometric Fe3Si. Iron was the major diffuser during the formation of Fe3Si. During annnealing off-stoichiometric Fe3Si transformed to stoichiometric Fe3Si. FeSi was the next phase to form - initially detected after annealing at 300°C for 3 hrs. Similar results were obtained for bulk couples. The first phase to form was ordered stoichiometric Fe3Si (initially detected after 7 hrs), followed by FeSi (≈23 hrs) and then FeSi2 (>200 hrs). The formation behavior of these phase is discussed.

Reactive Phase Formation in Thin Films: Evolution of Grain Structure

AbstractIt is a well known fact that the properties and performance of polycrystalline materials, including polycrystalline thin films, are strongly affected by the grain structure. Therefore, in treating reactive phase formation in these films, it is (or it will inevitably be) necessary to quantify the grain structure of reactant and product phases and its evolution during the course of the reaction. Theoretical models and the conventional view of thin film reactions, however, have been largely extensions, to small and finite dimensions, of theories and descriptions developed for bulk diffusion couples. These models and descriptions primarily focus on the growth stage and to a much lesser extent on the nucleation stage. Consequently, these models and descriptions are not able to treat the development of product phase grain structure. Recent calorimetric investigations of several thin film systems demonstrate the importance of nucleation kinetics (and hence nucleation barriers) in product phase formation and provide quantitative measures of the thermodynamics and kinetics of formation of the product phases, thereby allowing some degree of comparison with reaction models. Furthermore, microstructural investigations of thin-film reactions demonstrate the non-planarity of the growth front and highlight the role of reactant-phase grain boundaries. In this paper, a summary of these experimental studies and recent theoretical treatments, which combine nucleation and growth in an integrated manner, is presented, with particular emphasis on the Nb/Al system. These experiments and models lead to a new view of reactive phase formation and grain structure evolution as one in which the latter is an integral part of the former. Based on this view, directions for future research are discussed.

Quasicrystalline phase formation in thin films

The phenomena of reactive diffusion developing the icosahedral quasicrystalline structure (i-QC) has been analysed in the Al-Mn thin film system prepared by sequential deposition of Al and Mn at the reaction temperatures (HTSD). The development of Kirkendall voids in the Al film indicates that Al is the dominant diffusion component and that growth takes place on the top surface of the QC layer were Mn deposited. Al has a high diffusion rate across the QC layer and depends on the local structure of the QC phase. The reaction rate of Al and Mn is large if Mn is present in vapour phase while it is extremely small at the interface of QC-Mn layers, during the heat treatment of Al-QC-Mn layer system. A concentration profile of Al has been proposed for the reactive diffusion taking place in the case of HTSD.

A15 structure formation in TiPd

Vapor phase formation of thin films in the Ti-Pd alloy system is investigated using magnetron sputter deposition. Crystalline as-deposited structures are produced with nominal compositions of 20, 25 and 30 at.%Pd. The high temperature b.c.c. phase is produced on room temperature substrates via quenching from the vapor phase. An A15 structure of Ti 3Pd is rigorously identified for the first time, in agreement with predictions from electronic structure calculations.