Stoichiometry Of Thin Films Research Articles

A correlation study of layer growth rate, thickness uniformity, stoichiometry, and hydrogen impurity level in HfO2 thin films grown by ALD between [formula omitted] and [formula omitted]

Hafnium oxide was deposited from tetrakis(dimethylamino)hafnium (TDMAHf) and water by atomic layer deposition (ALD) on heated 4″ Si wafers covered with native oxide in the temperature range from 100°C to 350°C. Optimized self-limiting ALD reaction and smallest hydrogen impurity level have been realized for a substrate temperature of 300°C. The stoichiometry of deposited films and hydrogen impurity level were measured by elastic recoil detection analysis. The hafnium to oxygen ratio showed the expected 1:2 value. Besides hydrogen, no other impurities could be detected. Furthermore, a strong correlation between the growth rate per cycle (GPC), film uniformity and level of hydrogen impurities was observed.In addition, the characterization of the crystal structure showed the appearance of some crystallites in an amorphous matrix already for a growth temperature of 250°C and a pure crystalline layer at a growth temperature of 350°C. The increased crystallinity with increasing growth temperature was attributed to a higher seed concentration and a nearly constant crystal size.

Electrical and Optical Properties of Yttrium Titanate Thin Films Synthesized by Sol-Gel Technique

Yttrium titanate (Y2Ti2O7) thin films, having pyrochlore-type structure, were synthesized in single phase by sol-gel synthesis technique. Single-phase formation was confirmed by X-ray diffraction (XRD) study. Surface morphology of thin films was studied by using FESEM and average grain size was found to be in nanometer range. Retention of stoichiometry in thin films was confirmed by EDX analysis. Optical property of Y2Ti2O7 thin films was studied with the help of UV-Vis spectrometer and the energy band gap was found to be ∼3.5 eV. Dielectric properties of thin films are also studied and discussed in details.

Reversible phase switching between antiferromagnetic SrCoO2.5 and ferromagnetic SrCoO3−δ by a flexible solid-state electrolyte gate

Manipulation of oxygen vacancies (VO) in transition metal oxides (TMOs) can largely alter their physical and chemical properties, such as electrical conductivity, magnetic state, optical band-gap, and electrocatalytic reactivity. Many experimental and theoretical works have been conducted to study the formation/annihilation of VO and its corresponding effect on the properties in TMOs. In this paper, a solid-state approach to modulate the oxygen stoichiometry in high quality SrCoOx epitaxial thin films was demonstrated. Dependence of the magnetic and electrical conducting properties on VO was investigated. Room temperature reversible phase switching between brownmillerite antiferromagnetic insulating SrCoO2.5 and perovskite ferromagnetic metallic SrCoO3−δ was achieved by electric-field induced oxygen non-stoichiometry. This room temperature reversible phase switching indicates that SrCoOx thin films are a promising candidate for practical applications in resistive random access memory and spintronic devices.

Fabrication of CZTSSe Thin Film Solar Cell By Single Step Sulfo-Selenization of Stacked Metal Precursors

The Cu2ZnSn(S,Se)4 (CZTSSe) thin films are the most promising photovoltaic absorber to replace CIGS, because it is comprised of abundant and inexpensive elements, and have a very high absorption coefficient of over 104 cm−1 with a direct bandgap energy. The theoretical solar conversion efficiency of CZTS-based thin film solar cells is as high as 32.2%. However, the maximum conversion efficiencies reported for electrical characteristics of devices based on these materials are still far from those accomplished more grown CIGS technologies. In order to achieve the efficiency of 20 % or more required for commercial technology, it is necessary to understand the limiting factors of the CZTSSe thin film. A parameter impacting the performance of increasing photovoltaic efficiencies in CZTSSe solar cells is band gap control of absorber through the control of S/(S+Se) ratio of CZTSSe thin films. Most of reported synthesized CZTSSe with high efficiency, the chalcogen composition of the absorption layer was controlled to optimize the characteristics. Most of them explain only the role of S in controlling the band gap in the CZTSSe thin film. It also only describes the characteristics of CZTSSe thin films and devices with different S ratios. However, S element plays another important role in CZTSSe crystal growth process. In this work, we have discussed the different effects of S in the CZTSSe thin film synthesis process. We fabricated CZTSSe thin films using optimized SLG-Mo/Zn/Cu/Sn (MZCT) as a stack structure and described the volatilization phenomenon of Zn element using MZCT stack structure. In addition, we have introduced H2S gas to effectively control the S/(S+Se) of the film in the sulfo-selenization process and to suppress the volatilization of Zn. The S injection process using H2S gas is more uniform and reproducible than any other processes. Unlike the pure selenization process, it was confirmed that a stable ZnSSe thin film was formed on the surface of the precursor during the sulfo-selenization process. The formation of ZnSSe thin film inhibited the volatilization of Zn, which facilitated the control of thin-film stoichiometry and also played an important role in crystal growth. Finally, the characteristics of thin films and devices fabricated through the optimization process are discussed. Acknowledge: This work was supported by the DGIST R&D Program of the Ministry of Science and ICT (19-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).

Influence of RF power on the stoichiometry, optical, and electrical properties of chromium oxide coatings prepared by reactive magnetron sputtering

An investigation has been carried out on the stoichiometry, optical and electrical properties of chromium oxide thin films prepared under various deposition powers using RF magnetron sputtering technique. The elemental and chemical composition of the prepared films were characterised by energy dispersive x-ray (EDX) analysis, Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The wettability and surface energies of the thin films were investigated with a goniometer. The four-point electrical probe method was used to determine the electrical resistivity of the films while the optical properties of the films were measured with a spectrophotometer. The coatings were found to be mainly phase based on the dominance of A1g and Eg symmetric modes in the Raman investigations and the Eu vibration mode in the FTIR measurements. The RF powers used during the deposition process were found to have played a vital role in the formation of rich films. It was observed that higher deposition power facilitated both the dislodging of more chromium atoms from the target and dissociation of oxygen used during the deposition process. The wettability results show that the thin films are hydrophilic and interact well with water and this behaviour is linked to the contribution of the polar component to the total surface energy. Optical transmittance values exceeding 70% were obtained for the films prepared at a lower RF power. The resistivity values varied from 0.061 Ω cm to 0.152 Ω cm for the deposited chromium oxide films. The variation in the electrical resistivity and the optical transmittance of the films with RF power indicate that these film properties can be altered or tuned to suit specific applications e.g. as transparent conducting oxide (TCO) for optoelectronic devices and other similar applications.

Tuning the stoichiometry and electrical properties of tantalum oxide thin films

Tantalum oxide has a wide range of applications and has drawn much attention especially for its useful properties in resistive random-access memories, in which the Ta oxide composition plays an important role to control the electrical properties of the TaOx thin films. In this paper, we present a way to tune the composition of TaOx thin films by varying the oxygen partial pressure during growth using pulsed laser deposition. TaOx thin films were deposited at room temperature, under oxygen partial pressures ranging from 10−6 mbar to 2 × 10−2 mbar. Using angle resolved X-ray photoelectron spectroscopy, we show that the composition of the film varies systematically with the oxygen partial pressure during the film growth. We then correlate the oxygen content with the electrical properties of the film and the results show that the composition has a great influence on the resistivity of the TaOx thin films. As the oxygen partial pressure during deposition increases, the percentage of tantalum pentoxide (Ta2O5) as well as the resistivity of the films increases. This experimental approach provides a pathway to control the TaOx thin film stoichiometry and its electrical properties during growth.

Stress and optical constants in thin films of BaF2-PrF3: A potential Th-free infrared low-index evaporation material

Although PrF3 can be regarded as a potential infrared low-index evaporation material used in infrared antireflection to substitute for radioactive ThF4 because its thin films have a good transparency, the lower refractive index n and extinction coefficient k in the spectral range of thermal infrared, the greater tensile stress emerging in its layers will deteriorate the reliability and durability of antireflection coatings. In our investigation, BaF2-PrF3 thin films were deposited using electron beam evaporation from the sintered pellets of PrF3 admixed with BaF2. Stress in thin films was calculated from Stoney’s equation on the basis of measuring the changes of radius of curvature of thin silicon strips. The stoichiometry of thin films was determined using the energy dispersive X-ray analysis (EDX). The optical constants of thin films were determined by fitting the measured spectral transmittance curves using Lorentz oscillators as a dispersion model. It can be observed that stress in BaF2-PrF3 thin films is obviously reduced with the increasing of the concentration of BaF2 in thin films and the sintered pellets. Moreover, it was also demonstrated that incorporation of BaF2 into PrF3 can reduce the refractive index of thin films, although BaF2-PrF3 thin films have a greater dispersion than that of PrF3 thin films. In addition, the lower value of extinction coefficient is also presented in thin films, like those of rare-earth fluorides.

High-performance polycrystalline RuOx cathodes for thin film Li-ion batteries

A process for sputter deposition of RuOx films with high volumetric Li capacities and good cyclability was developed, demonstrating the potential for use of RuOx films as cathodes in high-performance thin-film microbatteries. Using a stoichiometric RuO2 sputtering target, the effects of oxygen content within the Ar/O2 plasma on the stoichiometry of thin films deposited at room temperature was investigated and correlated with electrochemical performance. The oxygen content of the films was found to increase with the O2 to Ar ratio of the sputtering gas. RuOx thin films with x ≥ 1.92 delivering capacities of ∼550 μAh/cm2μm in the 0.75–3.6 V vs. Li/Li+ voltage range, which is much higher than that of under-stoichiometric RuOx (320 μAh/cm2μm). The capacities obtained for RuOx films significantly exceed the capacity of LiCoO2 films (∼60 μAh/cm2μm) typically used in thin-film microbatteries. Given that high capacity RuOx films can be obtained using only room temperature processing, their use in thin-film microbatteries will also allow integration with silicon-based integrated circuits.

Effect of deposition time on the growth mode and stoichiometric of amorphous boron carbide thin films deposited by electron beam evaporation

Electron beam evaporation (EBE) deposition technique was employed to prepare amorphous boron carbide thin films at room temperature. In this work, a new contribution to the knowledge of deposition time dependence of stoichiometric was reported. The results indicated that the thickness of the amorphous boron carbide thin films were approximately 87–830 nm with a deposition time of 20–120 min. The growth mode of the film is layer-island growth. The XPS analysis demonstrated the B/C atomic ratio increased with the increasing of deposition time due to the different evaporation behaviors between B and C atoms in EBE processes. The stoichiometry of amorphous boron carbide thin films was controlled in the range of 1.3 ± 0.1–3.1 ± 0.1 via electron beam evaporation by the deposition time of 20–120 min. This work will be useful to advance the application of amorphous boron carbide thin films as an ICF ablator, and has a certain reference value for electron beams to evaporate other compounds.

Effects of annealing on quality and stoichiometry of HfO2 thin films grown by RF magnetron sputtering

In this paper, the effects of annealing temperature on HfO2 thin films prepared by RF sputtering have been investigated. Thin films of hafnium oxide were deposited using sputtering onto p-type Si substrates and the pristine films were annealed at different temperatures in air atmosphere to obtain crystallinity. The Raman and XRD results of the as-deposited films show amorphous nature, whereas the annealed films at 600 °C results in crystallization. AFM was used to study the surface morphology of the films and to estimate the skewness, kurtosis and the roughness values. The core level orbitals of Hf 4f spectra of as-deposited films not only reveal the formation of hafnium rich hafnium oxide but also nearly stoichiometric composition of HfO2 and exhibit a shift in binding energy with annealing. Electrical measurements of the films suggest that the leakage current is increased for crystalline films as compared to the as-deposited ones. The effects of annealing temperature on quality and stoichiometry of hafnia thin films and their possible applications are reported.

Dynamic x-ray spectrometry method for determining stoichiometry of thin films during growth

A method and technique of dynamic x-ray spectrometry is presented for real-time, in situ composition determination of multi-element films during pulsed laser deposition. The method, named as Low-Angle X-ray Spectrometry (LAXS), is capable of determining the composition with an uncertainty of ±5% and requires neither theoretical calculations nor multiple calibration standards of known composition and thickness. Algorithm of LAXS is validated by comparing film composition results with Rutherford Backscattering Spectroscopy analyses. LAXS provides an immediate visual feedback on film composition and guidance for optimizations, which is critical for applications. As a material example, compositions of several Y-Ba-Cu-O films were analyzed and it is shown that LAXS can rapidly identify an optimum oxygen pressure for stoichiometric film deposition.

Six complexing agents and their effects on optical, structural, morphological and photoluminescence properties of lead sulphide thin films prepared by chemical route

Lead sulphide (PbS) nanocrystalline thin films were deposited on silica glass substrate by chemical bath deposition method at a bath temperature of 70 °C from six different complexing agents. Different characterization techniques were employed to investigate the effect of applying various complexing agents on the growth mechanism and physical properties of PbS thin films. The XRD results revealed that all the deposited thin films shows a face centered cubic crystal structure however, the preferred orientations of the crystallites varied along the (111) and (200) planes with complexing agents. It was also observed that the complexing agents had a strong influence on the average crystalline size, microstrain and dislocation density of the PbS thin films. The EDX study confirmed that complexing agents had a noticeable effect on the stoichiometry of PbS thin films. The optical absorption spectroscopy study revealed that the optical band gap of the PbS thin films found in the range of 0.77–1.34 eV. The room temperature photoluminescence study verified that irrespective of complexing agents all the deposited thin films experienced emission at 761 nm. Raman studies showed a strong peak around 134 cm−1 irrespective of complexing agents.

Complexing agent-assisted highly dense CuInSe2 thin films prepared by one-step electrochemical deposition

Highly dense CuInSe2 (CISe) thin films were electrochemically deposited on Mo-coated soda-lime glass substrates in aqueous solution. Three compounds, CuCl2, InCl3, and SeO2, were used as precursors with glycine or oxalic acid as complexing agents. The deposition process was carefully selected through preliminary study of the individual electrochemical properties of the three precursors. To achieve ideal stoichiometry and uniform deposition of CISe thin films, the reaction temperature and acidity of precursor solution were investigated. As the reaction temperature increased, Cu deficiency was observed in the CISe thin film. The density and surface morphology of CISe films prepared by one-step electrodeposition were affected strongly by the acidity of deposition solution. The addition of HCl to solution improved the density and surface roughness of as-deposited CISe films. Denser and larger grain size CISe polycrystalline films were prepared in oxalic acid solution. Two-step heat treatments for the prepared CISe samples were applied to fabrication of CISe photovoltaic devices without an additional Se source.

How to control the stoichiometry of cadmium telluride thin film for photovoltaic applications

In this study, Bube's growth model for a cadmium telluride (CdTe) polycrystalline thin film was re-examined with a view of avoiding his assumptions that neglect the vapor pressures of Cd and Te2 near the film. We proposed a new thermodynamic growth model based on the fact that there is an experimentally verified characteristic ratio (α) of equilibrium partial pressures PCd/2PTe2 that depends on the temperature T and CdTe stoichiometry. By writing PCd(0)=2α(0)PTe2(0) and PCd(h)=2α(h)PTe2(h), where α(0) is determined by source stoichiometry, we can solve the equations for α(h) and thereby determine the stoichiometry of the CdTe thin film grown under physical vapor deposition (PVD) conditions. Simulation was performed to predict the stoichiometry of the CdTe thin film as a function of source stoichiometry for various source-film temperature combinations. The results show that for a typical CdTe PVD process with Tsource>Tthin film: (1) stable deposition, without a non-stoichiometric composition shift, can be achieved at congruent-growth stoichiometry; (2) any stoichiometric deviation from the congruent sublimation point becomes more substantial (in the same direction) in the thin film than in the source; and (3) larger ΔT between the source and the thin film results in a more composition shift.

Investigation of phase evolution and control over phase transformation temperature and thermal hysteresis using stoichiometry and co-doping in VO2 thin films

Structural phase transition temperature and its associated hysteresis in VO2 have been controlled by high valent dopant induced local structural modification which acts as phase nucleation site during phase transformation. Monoclinic phase intensity loops plotted against temperature has been observed to exhibit thermal hysteresis. It is shown that the size, shape and central position of hysteresis loop depend on stoichiometry and doping concentration. Highest reduction in phase transformation temperature and thermal hysteresis width has been observed in case of W+6-Mo+6 co-doping and W+6 doping, respectively. Hence energy barrier associated with the structural phase transition has been successfully manipulated to vary Tc and hysteresis width. These findings have implications for designing the phase switching devices and smart window applications.

Sensing and structural properties of Ti-doped tin oxide (SnO2) membrane for bio-sensor applications

Abstract We investigated the effect of rapid thermal annealing (RTA) on the sensing and structural characteristics of Ti-doped SnO2 based electrolyte-insulator-semiconductor (EIS) pH sensors. The Ti-doped Tin oxide (SnO2) EIS sensor annealed at 700 °C post deposition annealing, exhibits the best sensing characteristics in terms of sensitivity, linearity, hysteresis and drift rate. This derives from Ti incorporation that improves the thin-film stoichiometry by reducing dangling bonds on the dielectric surface and improves the temperature stability. We further studied the structural, compositional, and morphological characteristics of the deposited thin-film. RTA treatment increases the sensitivity and linearity by creating larger grain size and higher number of surface sites. Sodium (Na+) and potassium (K+) sensing performance was also measured using the fabricated EIS sensors.

Tailoring Bi-Te based nanomaterials by electrodeposition: Morphology and crystalline structure

Bi2Te3 is the most commonly used thermoelectric material in modern solid-state refrigerators and power generators based on this basic principle. Due to predictions of significant improvements in their efficiency by using nanostructured materials, a thorough study on thin films and nanowires deposited by the electrodeposition method are here presented. The study of the deposition applied potential effect on the morphology, stoichiometry and crystallinity of both thin films and nanowires has been conducted. The morphology and stoichiometry was found to highly depend on the deposition potential, where by increasing it one was able to accurately control the Te% content of the deposits. X-ray diffraction measurements have shown the presence of a strong relation between the material's crystallinity and the deposition potential, where samples ranged from monocrystalline, at very low potentials, to almost completely amorphous, at high potentials. Finally, nanowire diameters were seen to diminish with the applied potential, in conjunction with the general array.

Atomic layer deposition of HfO2 using HfCp(NMe2)3 and O2 plasma

HfO2 thin films were prepared by plasma-enhanced atomic layer deposition using a cyclopentadienyl-alkylamido precursor [HfCp(NMe2)3, HyALD™] and an O2 plasma over a temperature range of 150–400 °C at a growth per cycle around 1.1 Å/cycle. The high purity of the films was demonstrated by x-ray photoelectron spectroscopy and elastic recoil detection analyses which revealed that by increasing the deposition temperature from 200 to 400 °C, the atomic concentrations of residual carbon and hydrogen reduced from 1.0 to <0.5 at. % and 3.4 to 0.8 at. %, respectively. Moreover, Rutherford backscattering spectroscopy studies showed an improvement in stoichiometry of HfO2 thin films with the increase in deposition temperature, resulting in Hf/O ratio close to ∼0.5 at 400 °C. Furthermore, grazing incidence x-ray diffraction measurements detected a transition from amorphous at the deposition temperature of 300 °C to fully polycrystalline films at 400 °C, consisting of a mixture of monoclinic, tetragonal, and cubic phases. Finally, the surface morphology and conformality of HfO2 thin films studied by atomic force microscopy and transmission electron microscopy are also reported.

Tin monosulfide (SnS) thin films grown by liquid-phase deposition

Tin monosulfide (SnS) is an earth-abundant semiconductor material and shows potential as an absorber for solar cells. However, it is difficult to obtain the pure SnS phase because other binary phases of Sn-S (SnS2, Sn2S3 and Sn3S4) are more favorable in air. In this report, solution and liquid precursors were used to fabricate pure SnS thin films on Mo-coated soda lime glass using a direct solution coating method and subsequent annealing at different reaction temperatures. Direct solution coating is an easy and rapid process for large-scale fabrication. Additionally, it allows for precise control of the thin film stoichiometry. The phase and morphology of the fabricated SnS thin films were confirmed by XRD, XPS, Raman and SEM. The films prepared using the solution precursor showed additional phases along with SnS. However, for the films prepared with the liquid precursor, only the SnS orthorhombic structure was observed.

Sequential pulsed laser deposition of homoepitaxial SrTiO3 thin films

The control of thin film stoichiometry is of primary relevance to achieve desired functionality. Pulsed laser deposition ablating from binary-oxide targets (sequential deposition) can be applied to precisely control the film composition, offsetting the importance of growth conditions on the film stoichiometry. In this work, we demonstrate that the cation stoichiometry of SrTiO3 thin films can be finely tuned by sequential deposition from SrO and TiO2 targets. Homoepitaxial SrTiO3 films were deposited at different substrate temperatures and Ti/Sr pulse ratios, allowing the establishment of a growth window for stoichiometric SrTiO3. The growth kinetics and nucleation processes were studied by reflection high-energy electron diffraction and atomic force microscopy, providing information about the growth mode and the degree of off-stoichiometry. At the optimal (stoichiometric) growth conditions, films exhibit atomically flat surfaces, whereas off-stoichiometry is accommodated by crystal defects, 3D islands, and/or surface precipitates depending on the substrate temperature and the excess cation. This technique opens the way to precisely control stoichiometry and doping of oxide thin films.