LaF3 Thin Films Research Articles

Structural and Electrical Properties of Spin-Coated LaF3 Thin Film on Porous Silicon

Structural and Electrical Properties of Spin-Coated LaF3 Thin Film on Porous Silicon

Epitaxial deposition of LaF3 thin films on Si using deep eutectic solvent based facile and green chemical route

Epitaxial lanthanum fluoride (LaF3) is considered an important and outstanding material for the fabrication of Metal–Insulator–Semiconductor (MIS) capacitive devices. In the quest of finding a green and low-cost scalable technique for the epitaxial deposition of LaF3 thin-film, this article presents an investigation on a home-made deep eutectic solvent (DES)-based chemical route for LaF3 thin film deposition on p-type silicon (p-Si) using the spin coating technique. The x-ray diffraction study confirmed the epitaxial deposition of LaF3 film on the p-Si substrate. An almost pinhole-free homogeneous surface and nearly stoichiometric epitaxial LaF3 were observed on the Si substrate through scanning electron microscopy and energy dispersive x-ray spectroscopy, respectively. From the capacitance–voltage (C–V) characteristics, the capacitance of the Ag/LaF3/p-Si/Ag device with four-layer LaF3 was maximum among the Ag/LaF3/p-Si/Ag devices with two-layer, four-layer, and six-layer LaF3. The flat band potential of the Ag/LaF3/p-Si/Ag structure was determined from the Mott–Schottky plot. The experimental results indicate that the DES-based epitaxial deposition of LaF3 film on the p-Si substrate could be a highly promising technique for the fabrication of LaF3-based MIS capacitive devices.

热蒸发与离子束溅射制备LaF3薄膜的光学特性

研究了钼舟热蒸发工艺和离子束溅射方法制备的单层LaF3薄膜的特性。首先，采用分光光度计测量了LaF3薄膜的透射率和反射率光谱，使用不同模型拟合得出薄膜的折射率和消光系数。然后，采用应力仪测量了加热和降温过程中LaF3薄膜的应力-温度曲线。最后，采用X射线衍射仪测试了薄膜的晶体结构。实验结果表明，热蒸发制备的LaF3(RH LaF3)存在折射率的不均匀性，在193 nm，其折射率和消光系数分别为1.687和5×10-4，而离子束溅射制备的LaF3(IBS LaF3)折射率和消光系数分别为1.714和9×10-4。 两种薄膜表现出相反的应力状态，RH LaF3薄膜具有张应力，而IBS LaF3具有压应力，退火之后其压应力减小。 热蒸发制备的MgF2/LaF3减反膜在193 nm透过率为99.4%，反射率为0.04%，离子束溅射制备的AlF3/LaF3减反膜透过率为99.2%，反射率为0.1%。

Optical Characterization of LaF<sub>3</sub> by Variable Angle Purged UV Spectroscopic Ellipsometry

The optical characterization of LaF3 thin film in DUV spectral range was experimental investigated by using a variable angle purged UV spectroscopic ellipsometer. In order to take into account the inhomogeneity, a theory model that dividing the single thin film into several sublayers was adopted. Two kinds of LaF3 thin films fabricated on fused silicate substrate with different substrates temperature were tested. From the obtained optical index and the physical thickness of different sublayer in the two different kinds of LaF3 thin films, it was found that, the inhomogeneity of the LaF3 thin film deposited with substrate temperature at 300°C was stronger than that of the LaF3 thin film deposited with substrate temperature at 250°C, indicating that the substrate temperature has important influence on the optical index and inhomogeneity of LaF3 thin films. For both of the two kinds LaF3 thin films, the agreement between the measured transmittance and the simulated transmittance using the parameters from regression of SE was nice, indicating that the selection of the material dispersion law and regression procedure were successful.

Influence of ion assistance on LaF3 films deposited by molybdenum boat evaporation

LaF3 thin films at 193 nm were deposited by the molybdenum boat evaporation with ion-assisted deposition (IAD). Various optical characteristics, stress, and microstructures that formed under different ion-beam voltages of IAD deposition were investigated. The relation between these properties is also discussed. LaF3 films deposited with IAD exhibited small rough surfaces and large optical loss at 193 nm. The largest value of optical loss for films at 193 nm, which were prepared at an ion-beam voltage of 400 V, was 1.55% and the extinction coefficient was smaller than 0.0015. Microstructures and crystalline structures of films were influenced and changed by the ion-assisted deposition process. Tensile stress value of films increased as the ion-beam voltage rose. Refractive index, related to the packing density and microstructures, also increased as the ion-beam voltage rose.

Note: Response characteristics of the sensor based on LaF3 thin film to different humidified gases

Electromotive force (EMF) transient curves presented that the sensor showed good repeatable response in the humidity environments using ambient atmosphere as the carrier gas at different temperatures. The 90% response time and recovery time were within 40 s and 50 s, respectively. The sensor also presented stable response characteristics in 75.1% RH and 83.6% RH humidity environments using N(2), 5% O(2), and 50% O(2) as the carrier gases, respectively. The EMF always increased with the partial pressure of oxygen in certain relative humidity. However, the ΔEMF was decreased with the increase of O(2) content in the carrier gas under the condition of the variation of relative humidity from 75.1% to 83.6%. These phenomena revealed that the sensor was sensitive to water vapor without oxygen in the sample gas and too much water vapor had adverse effect on the response to oxygen. Non-Nernst behavior of the sensor was discussed in detail.

Humidity response properties of a potentiometric sensor using LaF3 thin film as the solid electrolyte

A thin-film type potentiometric sensor has been prepared by the implementation of electro-beam evaporation, rf magnetron sputtering methods, and micromachining processes. Sn film was deposited on n-Si/SiO(2) (400 nm) substrate. A deposited LaF(3) film was applied as solid electrolyte and sputtered Pt film was used as the sensing electrode. The patterns of the Pt and LaF(3) were realized by the micromachining processes. The LaF(3) film was characterized by scanning electron microscopy and energy dispersive x ray. Saturated aqueous solutions were used to achieve controlled humidity environments. When the sensor was exposed to humidity environments, the electromotive force (EMF) of the sensor was examined. It was found that the sensor varies with the relative humidity (RH). The stable response curve was presented and non-Nernst behavior between the average EMF values and RH may be shown.

Comparison study of microstructure, chemical composition and optical properties between resistive heating and electron beam evaporated LaF3 thin films

LaF3 thin films were deposited by electron beam (EB) and resistive heating (RH) evaporation, respectively. Properties such as microstructure, chemical composition, surface morphology and optical constants of the LaF3 thin films were characterized by measurements of X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and spectrophotometer, then comparison was made between this two deposition methods. It's found that the microstructure properties of the LaF3 films deposited by these two methods were different, and slight content of oxyfluoride films was formed during deposition according to the result of chemical composition analysis. The microstructure of LaF3 bulk materials after interaction with electron beam and resistive heating was also characterized to analyze how the two deposition processes affect the formation of LaF3 thin films and their microstructure properties. When it was for the laser resistance of the films, although the EB evaporated LaF3 thin films occupied lower absorption and optical loss than those of the RH films, they showed slightly smaller laser induced damage thresholds at 355nm, which was thought to be related to their much more rougher surface and higher tensile stress.

Atomic layer deposition of metal fluorides through oxide chemistry

In this paper, we report on the atomic layer deposition of magnesium, calcium and lanthanum fluorides utilising two different approaches with hexafluoroacetylacetonate as a fluorine source. The first approach is based on the reaction of fluorinated metal precursors with a strong oxygen source. A deposition rate of 0.3 A per cycle was obtained when calcium hexafluoroacetylacetonate (Ca(hfac)2) was used as a metal precursor and ozone as a second reactant at 300 °C. From Rutherford backscattering spectroscopy (RBS) measurements, the film stoichiometry was determined to be CaF2.17 with less than 5 atomic% of oxygen. The second and more feasible approach is based on metal oxide formation from traditional nonfluorinated β-diketonate type metal precursors and an oxygen source, followed by fluorination using hexafluoroacetylacetonate (Hhfac) and ozone. The MgF2, CaF2 and LaF3 thin films prepared using this method showed refractive indices of 1.429, 1.472 and 1.687, respectively. The oxygen content of these films was below the detection limit of the RBS, which is 2 at%.

ChemInform Abstract: LaF3 Thin Films as Chemically Sensitive Material for Semiconductor Sensors.

A new kind of semiconductor based fluoride sensor was prepared by growing thin polycrystalline LaF3 films directly on silicon substrates using vacuum vapour deposition technique. The EICS (Electrolyte Ion Conductor Semiconductor) structure was investigated by means of impedance spectroscopy, CV measurements and exchange measurements with labeled ions (18F). Whereas charge and potential conditions at the LaF3/ electrolyte interface are governed by the fast fluoride exchange the LaF3 bulk and the blocked Si/LaF3 interface determine the electrical behavior. Although the Si/LaF3 contact is not reversible the potential stability of the EICS structure is surprisingly high. Additional results at epitaxial LaF3 layers, prepared by MBE, were taken into account for comparision with those at polycrystalline layers.

Electron beam evaporated LaF3 thin films prepared by different temperatures and deposition rates

Abstract LaF 3 thin films were prepared by electron beam evaporation with different temperatures and deposition rates. Microstructure properties including crystalline structure and surface roughness were investigated by X-ray diffraction (XRD) and optical profilograph. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical composition of the films. Optical properties (transmittance and refractive index) and laser induce damage threshold (LIDT) at 355 nm of the films were also characterized. The effects of deposition rate and substrate temperature on microstructure, optical properties and LIDT of LaF 3 thin films were discussed, respectively.

Microstructure- and composition-related characteristics of <inline-formula><math altimg="none" display="inline" overflow="scroll"><mrow><mi mathvariant="normal">La</mi><msub><mi mathvariant="normal">F</mi><mn>3</mn></msub></mrow></math></inline-formula> thin films at <inline-formula><math altimg="none" display="inline" overflow="scroll"><mrow><mn>193</mn><mspace width="0.3em" /><mi>nm</mi></mrow></math></inline-formula>

The characteristics of lanthanum fluoride (LaF3) thin films deposited with a resistive heating (RH) boat and by e-beam gun evaporation at different substrate temperatures have been studied. The characteristics of the deposited films, including optical properties, stress, and laser-induced damage threshold (LIDT), were related to the microstructure as well as the stoichiometry of the film. The films exhibit obvious different characteristics between these two processes at various substrate temperatures. It was found that optical properties, stress, and LIDT were affected by the microstructure and composition of the films. To obtain a high value of the LIDT and good optical properties, LaF3 thin films should be deposited by the RH process at a substrate temperature of 300 °C.

Characterization of oxygen impurities in thermally evaporated LaF3 thin films suitable for oxygen sensor

The lanthanum fluoride thin films has been prepared by means of thermal evaporation method. The XRD analysis shows the formation of polycrystalline hexagonal LaF3. The depth profile X-ray photoelectron spectroscopy (XPS) analysis shows the presence of oxide ions throughout the films. The formation of lanthanum oxyfluoride (LaOF) has been identified. The [O]/[F] ratio has been found to be 0.35 which is higher than the previously reported values of LaF3 film applied for the oxygen sensor.

Ion conduction studies on LaF3 thin film by impedance spectroscopy

Abstract Lanthanum fluoride thin film has been prepared by vacuum thermal evaporation method. The hexagonal-phase LaF 3 film has been detected by using glancing angle XRD analysis. The XPS analysis confirms the composition of lanthanum fluoride. The F − ion conduction through the grain and grain boundary has been analyzed using impedance analysis. The modulus spectra reveal the non-Debye nature and the distribution of relaxation times of the film.

Hetero- and homoepitaxial Nd3+-doped LaF3 thin films grown by molecular beam epitaxy: A spectroscopic study

Hetero- and homoepitaxial Nd3+-doped LaF3 thin films have been grown by molecular beam epitaxy. Two different orientations of CaF2 substrates, (111) and (110), have been used for the heteroepitaxial structures. High-resolution emission and excitation spectra as well as the decay time of the emission have been measured. The spectroscopic measurements demonstrate that one Nd3+ site is present in the LaF3 layers grown on CaF2(111) substrates but two slightly different Nd3+ centers are resolved in the films on CaF2(110) substrates. One Nd3+ site has been found in the homoepitaxial sample. Slight differences are observed between the centers found in the LaF3 layers and the one observed in the Nd3+-doped LaF3 bulk crystal. For the homoepitaxial layer, the linewidths are similar to those of the bulk crystals, whereas for the heteroepitaxial layers, a broadening is observed.

Sol-Gel Processing of LaF<sub>3</sub> Thin Films

Lanthanum fluoride thin films were prepared on silica glass substrates by a sol-gel method using trifluoroacetic acid (TFA) as a fluorine source. A coating solution was produced from lanthanum acetate and trifluoroacetic acid dissolved in 2-propanol. Triuoroacetate gel films were formed by spin coating the sol, which were converted to LaF3 thin films by heat-treatment at 300-500°C for 10min. The formation of LaF3 is considered to result from the reaction between lanthanum ions and the fluorine species generated by the decomposition of trifluoroacetate ions at elevated temperatures. The film fired at 300°C had a smooth surface with a grain size of 30nm. Cracks and voids were only observed in the films fired at 400 and 500°C. The films fired at 300 and 400°C exhibited transmittance of greater than 90% in the visible region.

1.5 μm photoluminescence of Er3+ in YF3, LuF3, and LaF3 thin films

We have studied the photoluminescence at 1.5 μm of the 4I13/2→4I15/2 transition of the Er3+ ion, incorporated as ErF3 into polycrystalline YF3, LuF3, and LaF3 films, deposited onto SiO2 or Si wafers. We conclude that LaF3 is an exceptionally suitable host for Er3+, since it permits high Er3+ concentrations, long lifetimes of the excited level, and a strong Stark splitting. This results in a spectral shape suitable for broadband optical amplifiers with a usable optical bandwidth exceeding 8000 GHz (Δλ≂60 nm).

Electrical conduction of LaF3 thin films

Thin film capacitors of lanthanum fluoride were fabricated by electron beam gun deposition. The current-voltage characteristics of these films were studied at different temperatures (300–470 K). It was observed that at high electric fields (>104 V cm−1 the current increases linearly with the square root of the field. The conduction mechanism appears to be predominantly by Poole-Frenkel emission in these films. The low field activation energy was estimated to be 0.40 eV and the results are discussed.

Transport Properties of LaF3 Thin Films

Pulsed current‐voltage measurements have been carried out on thin films of varying in thickness from 1000 to 10.000Aå. The I–V data fit a relationship of the form normally associated with ionic conduction. If it is assumed that the internal field is equal to the externally applied field, the derived ionic jump distance is of the order of 20Aå. However, potential probe measurements show a strong concentration of positive space charge near the cathode. When the effect of the space charge on the internal electric field is accounted for, a smaller value is obtained for the ionic jump distance, which is more in agreement with the crystal structure of .