Determination of the optical constants of HfO2–SiO2 composite thin films through reverse fitting of transmission spectra

Composite thin films of HfO₂:SiO₂ with wide range of relative composition from 100:0 (pure HfO₂) to 10:90 have been deposited on fused silica substrates by co-evaporation technique and the optical properties of the films have been studied by measuring the transmission spectra of the samples by spectrophotometer. Different important optical parameters viz., band gap, refractive index and absorption coefficients of the samples have been obtained by fitting the measured optical spectra with theoretically generated spectra and the variation of the optical constants as a function of SiO₂ content in the films have been obtained. Two different dispersion models viz., the single effective oscillator model and the Tauc–Lorentz model have been used to generate the theoretical spectra in the above fitting procedure. X-ray reflectivity (XRR) measurement technique has been used to find the densities of the films in order to explain the observed variation in optical properties of the films with increase in SiO₂ content.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Synthesis and characterization of HfO2 and ZrO2 thin films deposited by plasma assisted reactive pulsed laser deposition at low temperature

A plasma assisted reactive pulsed laser deposition process was demonstrated for low-temperature deposition of thin hafnia (HfO₂) and zirconia (ZrO₂) films from metallic hafnium or zirconium with assistance of an oxygen plasma generated by electron cyclotron resonance microwave discharge. The structure and the interface of the deposited films on silicon were characterized by means of Fourier transform infrared spectroscopy, which reveals the monoclinic phases of HfO₂ and ZrO₂ in the films with no interfacial SiO_x layer between the oxide film and the Si substrate. The optical properties of the deposited films were investigated by measuring the refractive indexes and extinction coefficients with the aid of spectroscopic ellipsometry technique. The films deposited on fused silica plates show excellent transparency from the ultraviolet to near infrared with sharp ultraviolet absorption edges corresponding to direct band gap.     

Optical properties of the Al2O3/SiO2 and Al2O3/HfO2/SiO2 antireflective coatings

Investigations of bilayer and trilayer Al₂O₃/SiO₂ and Al₂O₃/HfO₂/SiO₂ antireflective coatings are presented in this paper. The oxide films were deposited on a heated quartz glass by e-gun evaporation in a vacuum of 5 × 10^?3 [Pa] in the presence of oxygen. Depositions were performed at three different temperatures of the substrates: 100 °C, 200 °C and 300 °C. The coatings were deposited onto optical quartz glass (Corning HPFS). The thickness and deposition rate were controlled with Inficon XTC/2 thickness measuring system. Deposition rate was equal to 0.6 nm/s for Al₂O₃, 0.6 nm ? 0.8 nm/s for HfO₂ and 0.6 nm/s for SiO₂. Simulations leading to optimization of the thin film thickness and the experimental results of optical measurements, which were carried out during and after the deposition process, have been presented. The optical thickness values, obtained from the measurements performed during the deposition process were as follows: 78 nm/78 nm for Al₂O₃/SiO₂ and 78 nm/156 nm/78 nm for Al₂O₃/HfO₂/SiO₂. The results were then checked by ellipsometric technique. Reflectance of the films depended on the substrate temperature during the deposition process. Starting from 240 nm to the beginning of visible region, the average reflectance of the trilayer system was below 1 % and for the bilayer, minima of the reflectance were equal to 1.6 %, 1.15 % and 0.8 % for deposition temperatures of 100 °C, 200 °C and 300 °C, respectively.     

Effects of annealing temperature on the characteristics of ALD-deposited HfO2 in MIM capacitors

We have investigated the annealing effects of HfO₂ films deposited by an atomic layer deposition (ALD) method on the electrical and physical properties in the Si/SiO₂/Pt/ALD-HfO₂/Pd metal-insulator-metal (MIM) capacitors. If the annealing temperature for HfO₂ films was restricted below 500 °C, an annealing step using a rapid thermal processor (RTP) improves the electrical properties such as the dissipation factor and the dielectric constant. On the other hand, annealing at 700 °C degrades the electrical characteristics in general; the dissipation factor increases over the frequency range of 1∼4 MHz, and the leakage current increases up to 2 orders at the low electric field regions. We found that the degradation of electrical properties is due to the grain growth in the HfO₂ film (i.e., poly-crystallization of the film) by the high temperature annealing processing. We suggested that the annealing temperature must be restricted below 500 °C to obtain the high quality high-k film for the MIM capacitors.     

R.f.-sputtered SiO2 films for optical applications

We investigated the refractive index variation of r.f.-sputtered SiO₂ films in the direction normal to the substrate surface as a function of some important deposition conditions. As confirmed by Rutherford backscattering measurements the impurity content and its distribution over the film thickness profoundly affect the optical homogeneity and the UV absorption of SiO₂ films. Homogeneous SiO₂ films with refractive indices and transmission properties close to those of fused silica can be prepared by judicious selection of the substrate temperatures. This is possible even under such rough mutual contamination conditions as occur in the fabrication of multilayer stacks.     

Composition and optical properties of silicon nitride films grown on SiO2-glass and R-Al2O3 substrates by reactive RF magnetron sputtering

We have grown silicon nitride (Si₃N₄) films on SiO₂-glass and R-Al₂O₃ substrates by using reactive RF magnetron sputtering deposition methods with N₂ pure gas and N₂ + Ar mixture gas. The film composition, thickness and impurities have been examined by ion beam analysis. It is shown that the films have stoichiometric composition and are free from Ar contamination, when N₂ gas was used for the film deposition. Effects of impurities on the film properties, e.g., optical properties will be discussed.     

Characteristics of atomic layer deposition grown HfO2 films after exposure to plasma treatments

Ultra thin HfO₂ films were grown by the atomic layer deposition (ALD) technique using tetrakismethylethylaminohafnium (Hf[N(CH)₃(C₂H₅)]₄) and ozone (O₃) as the precursors and subsequently exposed to various plasma conditions, i.e., CCP (capacitively coupled plasma) and MMT (modified magnetron typed plasma) in N₂ or N₂/O₂ ambient. The conventional CCP treatment was not effective in removing the carbon impurities, which were incorporated during the ALD process, from the HfO₂ films. However, according to the X-ray photoelectron spectroscopy measurements, the MMT treated films exhibited a significant reduction in their carbon contents and the efficient incorporation of nitrogen atoms. Although the incorporated nitrogen was easily released during the post-thermal annealing of the MMT treated samples, it was more effective than the CCP treatment in removing the film impurities. Consequently, the MMT treated samples exhibited excellent electrical properties as compared to the as-deposited HfO₂ films, including negligible hysteresis (flatband voltage shift), a low leakage current, and the reduced equivalent oxide thickness of the gate stack. In conclusion, MMT post treatment is more effective than conventional CCP treatment in improving the electrical properties of high-k films by reducing the carbon contamination and densifying the as-deposited defective films.     

Kinetics of interfacial layer formation during deposition of HfO2 on silicon

Very thin HfO₂ films were deposited directly on Si substrates by the pulsed laser deposition technique in a wide range of substrate temperatures and oxygen pressures to investigate the kinetics of the interfacial layer formation. Angle-resolved X-ray photoelectron spectroscopy (XPS) investigations showed that the interfacial layer formed between the Si substrate and the deposited oxide contains a mixture of HfO₂ and SiO₂ without any strong evidence to support the formation of a silicate-type compound. X-Ray reflectivity measurements showed that the mass density of the interfacial layer is higher than that of pure SiO₂, while spectroscopic ellipsometry measurements showed that the refractive index is higher than that of pure SiO₂, therefore corroborating the XPS results.     

Natural and persistent superhydrophilicity of SiO2/TiO2 and TiO2/SiO2 bi-layer films

Sol-gel SiO₂/TiO₂ and TiO₂/SiO₂ bi-layer films have been deposited from a polymeric SiO₂ solution and either a polymeric TiO₂ mother solution (MS) or a derived TiO₂ crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at 500 °C have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscospy. Water contact angle measurements show that MS SiO₂/TiO₂ and CS TiO₂/SiO₂ bi-layer films exhibit a natural superhydrophilicity, but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO₂/TiO₂ bi-layer films exhibit a natural, persistent, and regenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the nature of the TiO₂ single-layer component and arrangement of the bi-layer structure, i.e. TiO₂ underlayer or overlayer.     

Synthesis and optical properties of Cu2O/SiO2 composite films via gamma-irradiation route

The Cu₂O particles or clusters dispersed mesoporous SiO₂ composite films were prepared by a new method: First the matrix SiO₂ films were prepared by sol-gel process combined with the dip-coating technique, and then they were soaked in Cu(NO₃)₂ solutions followed by γ-ray irradiation at room temperature and in ambient pressure. Thus prepared Cu₂O/SiO₂ composite films were examined by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and optical absorption spectroscopy. Compared with the pure Cu₂O nanoparticles, an obvious blue shift existed in the obtained Cu₂O/SiO₂ composite films.     

Al2O3/SiO2 and HfO2/SiO2 dichroic mirrors for UV solid-state lasers

HfO₂/SiO₂ and Al₂O₃/SiO₂ multilayers to be employed as high reflectance end mirrors in Cerium-doped fluoride solid-state lasers were produced by radio frequency sputtering. The components were designed to have high transmittance at the pumping wavelength and high reflectance in a wavelength band corresponding to the active medium emission. A photoacoustic beam deflection technique and inspection of the irradiated area under a microscope were used to measure the laser induced damage threshold of the mirrors at the pumping wavelength. These coatings were tested in a laser cavity.     

Gamma-irradiation-induced Ag/SiO2 composite films and their optical absorption properties

Small Ag particles or clusters dispersed mesoporous SiO₂ composite films were prepared by a new method: First the matrix SiO₂ films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO₃ solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO₂ films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements.     

Structural and morphological characterization of Pr and Er-containing SiO2-P2O5 sol-gel thin films

Present work is focused on obtaining and characterization of sol-gel thin films belonging to SiO₂-P₂O₅-Er₂O₃ (I) and SiO₂-P₂O₅-Pr₂O₃ (II) systems. The films have been obtained by spin coating technique for three rotation speeds: 2000, 3500 and 5000 rpm. The deposition of the films was performed at different periods of time, i.e. 24 h, 96 h, 120 h, 144 h and 168 h after instant preparation of the precursor sols. FTIR (Fourier transform infrared spectroscopy) and Raman characterization aimed at investigating the structural changes that occurred in silicophosphate network in dependence on the spin rate of the substrate as well as on the time period elapsed since the sol preparation till the deposition day. FTIR spectra recorded in the 400-4000 cm⁻¹ range revealed Si-O-Si, P-O-P and Si-O-P vibration modes and optical phonons specific for OH units. Raman spectra collected in the 100-4000 cm⁻¹ range put in evidence stretching, bending and mixed vibration modes specific for silicophosphate network as well as rare-earth ion peaks specific to certain electronic transitions. Morphological investigation made by AFM (atomic force microscopy) on Er and Pr-doped silicophosphate sol-gel films evidenced specific features depending on the parameters mentioned above and SEM (scanning electron microscopy) analysis revealed micron sphere structural units, exfoliation of the films and micro cracks.     

The effects of O2/Ar ratio on the structure and properties of hafnium dioxide (HfO2) films

HfO₂ films at various O₂/Ar flow ratios were prepared by reactive dc magnetron sputtering. The effects of O₂/Ar ratio on the structure and properties of HfO₂ films were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and UV-Visible spectroscopy. The results showed that the HfO₂ films were amorphous at different O₂/Ar ratios, and the atomic ratio of O/Hf in the HfO₂ films at high O₂/Ar ratio was nearly to 2:1. The peaks of Hf4f and O1s shifted to higher binding energy with increasing the oxygen flow proportion. The HfO₂ films at high O₂/Ar ratio had high transmissivity at the range of 400-1100 nm.     

Atomic layer deposition of HfO2 on self-assembled monolayer-passivated Ge surfaces

HfO₂ films are not easily deposited on hydrophobic self-assembled monolayer (SAM)-passivated surfaces. However, in this study, we deposited HfO₂ films on a tetradecyl-modified SAM with a Ge surface using atomic layer deposition at 350 °C. A slightly thinner HfO₂ film thickness was obtained on the tetradecyl-modified SAM passivated samples than that typically obtained on GeO_x-passivated samples. The resulting electrical properties are explained by the physical thickness and stoichiometry of the interfacial layer.     

Reactive sputtering of SiO2-TiO2 thin film from composite Six/TiO2 targets

Coatings of SiO₂-TiO₂ films are frequently used in a number of optical thin film applications. In this work we present results from depositing films with variable Si/Ti ratios prepared by reactive sputtering. The different Si/Ti ratios were obtained by varying the target composition of composite single targets. Compared to co-sputtering this facilitates process control and composition uniformity of the films. Varying the oxygen supply during sputter deposition can result in films ranging from metallic/substoichiometric to stoichiometric oxides. Transmittance spectra of the different films are presented and the optical constants are determined from these spectra. Furthermore, the deposition process, films structure and composition of the films are discussed. The study shows that by choosing the right composition and working in the proper oxygen flow range, it is possible to tune the refractive index.     

Spectroscopic ellipsometry and positron annihilation investigation of sputtered HfO2 films

Hafnium oxide (HfO₂) films were prepared using a pulsed sputtering method and different O₂/(O₂ + Ar) ratios, deposition pressures, and sputtering powers. Spectroscopic ellipsometry (SE) and positron annihilation spectroscopy (PAS) were used to investigate the influence of the deposition parameters on the number of open volume defects (OVDs) in the HfO₂ films. The results reveal that a low O₂/(O₂ + Ar) ratio is critical for obtaining films with a dense structure and low OVDs. The film density increased and OVDs decreased when the deposition pressure was increased. The film deposited at high sputtering power showed a denser structure and lower OVDs. Our results suggest that SE and PAS are effective techniques for studying the optical properties of and defects in HfO₂ and provide an insight into the fabrication of high-quality HfO₂ thin films for optical applications.     

Characterization of mesoporous ZnO:SiO2 films obtained by the sol-gel method

ZnO:SiO₂ films are intensively investigated for optical and electronic applications. Additionally, porous ZnO:SiO₂ films are of great interest as catalyst and gas-sensing materials. The sol-gel method is an efficient and low-cost process for the deposition of meso- and microporous silica-based films. The present paper studies the effect of the withdrawal speed on the microstructure and optical properties of mesoporous ZnO:SiO₂ films obtained by the sol-gel method. The morphology of the films was investigated by atomic force microscopy and the overall structure was studied by X-ray diffraction. The structure and size of the zinc oxide nanoparticles embedded in the silica matrix were investigated in more detail by transmission electron microscopy. These techniques showed ZnO:SiO₂ films with crack-free mesoporous morphology and highly efficient embedding of ZnO nanoparticles with (100) preferred orientation. Furthermore, the optical transmittance (in the visible and near infrared regions) and the optical band gap value were observed to vary with withdrawal speed. It is shown that ZnO:SiO₂ nanocomposites films which possess ZnO particles exhibiting a (100) orientation, with possible special applications in non-linear optics, could be prepared by the low-temperature crystallization sol-gel method.     

Spectroellipsometric assessment of HfO2 thin films

Present paper looks into the possibilities and limitations of near ultraviolet-visible range spectroscopic ellipsometry in investigating HfO₂ thin films (thickness < 7 nm). The “high k” dielectric films were produced by Atomic Layer Deposition—ALD, sputtering, and Metalo-Organic Chemical Vapour Deposition—MOCVD, on silicon and on silicon/silicon dioxide structures. Using a simple optical model (Cauchy dispersion, with an Urbach absorption tail), suitable for the optical range investigated, we extract the thickness of the layers and their optical constants. Results related to the optical properties show the important impact made by the initial surface and the growth/deposition procedure. It is also shown that for the case of ALD HfO₂ films grown on RTO oxides a significant increase in the absorption coefficient is recorded in the 4.7-5.15 eV range; this can be linked with the formation of defects related to oxygen vacancies. Subsequent anneal cycles performed in oxygen reveal that changes do occur both at the transition layer level, and in the structure of the HfO₂ film, for which an increase in the absorption is recorded.