Optical absorption in PECVD deposited thin hydrogenated silicon in light of ordering effects

We report results obtained from measurements of optical transmittance spectra carried out on a series of silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band gap energy, the B factor of the Tauc plots, E ₀₄ (energy at which the absorption coefficient is equal to 10⁴ cm⁻¹), and the Urbach energy as a function of the hydrogen dilution. The results were correlated with microstructure, namely volume fractions of the amorphous and crystalline phase with voids, and with the grain size.      

Switching phenomenon and optical properties of Se85Te10Bi5 films

Se₈₅Te₁₀Bi₅ films of different thicknesses ranging from 126 to 512 nm have been prepared. Energy-dispersive X-ray (EDX) spectroscopy technique showed that films are nearly stoichiometric. X-ray diffraction (XRD) measurements have showed that the Se₈₅Te₁₀Bi₅ films were amorphous. Electrical conduction activation energy (ΔE_σ) for the obtained films is found to be 0.662 eV independent of thickness in the investigated range. Investigation of the current voltage (I-V) characteristics in amorphous Se₈₅Te₁₀Bi₅ films reveals that it is typical for a memory switch. The switching voltage V_th increases with the increase of the thickness and decreases exponentially with temperature in the range from 298 to 383 K. The switching voltage activation energy (ε) calculated from the temperature dependence of V_th is found to be 0.325 eV. The switching phenomenon in amorphous Se₈₅Te₁₀Bi₅ films is explained according to an electrothermal model for the switching process. The optical constants, the refractive index (n) and the absorption index (k) have been determined from transmittance (T) and reflectance (R) of Se₈₅Te₁₀Bi₅ films. Allowed non-direct transitions with an optical energy gap (E_g^opt) of 1.33 eV have been obtained. ΔE_σ is almost half the obtained value of E_g^opt, which suggested band to band conduction as indicated by Davis and Mott.     

Optical properties of nanoscale BiFeO3/BaTiO3/Ni0.5Zn0.5Fe2O4 composite films obtained by the pulsed-laser deposition method

Nanoscale (30–100 nm) films of BiFeO₃/BaTiO₃/Ni_0.5Zn_0.5Fe₂O₄ complex composition have been obtained by the pulsed-laser deposition method. Optical properties of the films were studied in the wavelength range of 250–1000 nm. It is shown that the optical properties of amorphous films deposited at room temperature are explained by the Tauc model for amorphous semiconductors. An increase in the optical gap from 1.7 to 1.95 eV was observed with decreasing film thickness. Allowed direct-band transitions (E _g = 3.1 eV) were observed after annealing of films independent of their thickness.     

Investigation of energy band gap and optical properties of cubic CdS epilayers

High quality cubic CdS epilayers were grown on GaAs (1 0 0) substrates by the hot-wall epitaxy method. The crystal structure of the grown epilayers was confirmed to be the cubic structure by X-ray diffraction patterns. The optical properties of the epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) and were studied in the transmittance spectra at a wavelength range of 400-700 nm at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, 〈?(E)〉 = 〈?₁(E)〉 + i〈?₂(E)〉, such as E₀, E₁, E₂, E^′₀, and E^′₁ structures. In addition, the optical properties related to the pseudodielectric function of CdS, such as the absorption coefficient α(E), were investigated. All the critical point structures were observed, for the first time, at 300 K by ellipsometric measurements for the cubic CdS epilayers. Also, the energy band gap was determined by the transmittance spectra of the free-standing film, and the results were compared with the E₀ structure obtained by SE measurement.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Laser synthesis of nanostructures based on transition metal oxides

Nanostructures based on iron oxides in the form of thin films were synthesized while laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors (Fe(CO)₅) under the action of Ar⁺ laser radiation (λ_L = 488 nm) on the Si substrate surface with power density about 10² W/cm² and vapor pressure 666 Pa. Analysis of surface morphology and relief of the deposited films was carried out with scanning electron microscopy (SEM) and atomic force microscopy (AFM). This analysis demonstrated their cluster structure with average size no more than 100 nm. It was found out that the thicker the deposited film, the larger sizes of clusters with more oxides of higher oxidized phases were formed. The film thickness (d) was 10 and 28 nm. The deposited films exhibited semiconductor properties in the range 170-340 K which were stipulated by oxide content with different oxidized phases. The width of the band gap E_g depends on oxide content in the deposited film and was varied in the range 0.30-0.64 eV at an electrical field of 1.6 × 10³ V/m. The band gap E_g was varied in the range 0.46-0.58 eV at an electrical field of 45 V/m. The band gap which is stipulated by impurities in iron oxides E_i was varied in the range 0.009-0.026 eV at an electrical field of 1.6 × 10³ V/m and was varied in the range 0-0.16 eV at an electrical field 45 V/m. These narrow band gap semiconductor thin films displayed of the quantum dimensional effect.     

The influence of growth parameters on the optical properties and morphology of UHV deposited Ni thin films

Nickel films of different thickness ranging from 15 nm to 350 nm were deposited on glass substrates, at different substrate temperatures (313-600 K) under UHV condition. The nano-structure of the films was obtained, using X-ray diffraction (XRD) and atomic force microscopy (AFM). The nano-strain in these films was obtained using the Warren-Averbach method. Their optical properties were measured by spectrophotometry in the spectral range of 190-2500 nm. Kramers-Kronig method was used for the analysis of the reflectivity curves. The absorption peaks of Ni thin films at ∼1.4 eV (transition between the bands near W and K symmetry points) and ∼5.0 eV (transition from L_2^′ to L₁ upper) are observed, with an additional bump at about 2 eV. The over-layer thickness was calculated to be less than 3.0 nm, using the Transfer Matrix method. The changes in optical data are related to different phenomena, such as different crystallographic orientations of the grains in these polycrystalline films (film texture), nano-strain, and film surface roughness.     

Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films

Nb₂O₅ films with the thickness (d) ranging from 55 to 2900 nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, d_cri =2010 nm. The structure of the film remains amorphous as d < d_cri. However, it becomes crystallized as d > d_cri. The root mean square of surface roughness increases with increasing thickness as d > 1080 nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000 nm for the crystalline films. Refractive index increases with increasing thickness as d < d_cri, while it decreases with increasing thickness as d > d_cri. In addition, the extinction coefficient increases with increasing thickness as d > d_cri.     

AlF3 film deposited by IAD with end-Hall ion source using SF6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF₆ as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10⁻⁴ at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF₆ working gas in IAD process is a good choice and significantly improves the quality of AlF₃ films.     

Laser synthesis of semiconductor nanostructures with narrow band gap

Semiconductor nanostructures with narrow band gap were synthesized by means of laser chemical vapor deposition (LCVD) of elements from iron carbonyl vapors [Fe(CO)₅] under the action of Ar⁺ laser radiation (λ_L = 488 nm) on the Si substrate surface. The temperature dependence of the specific conductivity of these nanostructures in the form of thin films demonstrated typical semiconductor tendency and gave the possibility to calculate the band gap for intrinsic conductivity (E_g) and the band gap assigned for impurities (E_i), which were depended upon film thickness and applied electrical field. Analysis of deposited films with scanning electron microscopy (SEM) and atomic force microscopy (AFM) demonstrated their cluster structure with average size not more than 100 nm. Semiconductor properties of deposited nanostructures were stipulated with iron oxides in different oxidized phases according to X-ray photoelectron spectroscopy (XPS) analysis.These deposited nanostructures were irradiated with Q-switched YAG laser (λ_L = 1064 nm) at power density about 6 × 10⁷ W/cm². This irradiation resulted in the crystallization process of deposited films on the Si substrate surface. The crystallization process resulted in the synthesis of iron carbide-silicide (FeSi_2−xC_x) layer with semiconductor properties too. The width of the band gap E_g of the synthesized layer of iron carbide-silicide was less than for deposited films based on iron oxides Fe₂O_3−x (0 ≤ x ≤ 1).     

Novel chemical synthetic route and characterization of zinc selenide thin films

Zinc selenide (ZnSe) thin film have been deposited using chemical bath method on non-conducting glass substrate in a tartarate bath containing zinc sulfate, ammonia, hydrazine hydrate, sodium selenosulfate in an aqueous alkaline medium at 333 K. The deposition parameter of the ZnSe thin film is interpreted in the present investigation. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption, electrical measurements, atomic absorption spectroscopy (AAS). The ZnSe thin layers grown with polycrystalline zinc blende system along with some amorphous phase present in ZnSe film. The direct optical band gap ‘E_g’ for the film was found to be 2.81 eV and electrical conductivity in the order of 10⁻⁸(Ω cm)⁻¹ with n-type conduction mechanism.     

Photoluminescence properties of SiNx/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH₂Cl₂/H₂/NH₃ plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV-VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N-H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN_x interfaces.     

Influence of thickness on the optical properties of amorphous GeSe2 thin films: analysis using Raman spectra,Urbach energy and Tauc parameter

An analysis is reported of thickness-induced defects in amorphous GeSe₂ thin films deposited by the vacuum evaporation technique. X-ray diffraction studies confirmed the amorphous nature of the thin films. Optical absorption measurements revealed an indirect transition with an energy gap that increases with film thickness. A blue shift in optical transmittance edges was observed in annealed GeSe₂ thin films. The obtained lower values of Urbach energy (E _U) indicate that as thickness increases more ordered films can be produced. Raman spectra suggest that annealing promotes corner-sharing GeSe_4/2 tetrahedra and edge-sharing Ge₂Se_8/2 bi-tetrahedra bonding and leads to the reduction in disorder in bonding network, which is amply supported by the way of increase in band gap, increase in Tauc parameter (B ^1/2) and reduction in E _U from the analysis of transmittance spectra. Increasing the thickness promotes tetrahedral and bi-tetrahedral bonding through the reduction in bonding defects.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Effect of gamma doses on the optical parameters of Se76Te15Sb9 thin films

The effect of γ-radiation dose on the optical spectra and optical energy gap (E_opt.) of Se₇₆Te₁₅Sb₉ thin films was studied. The dependence of the absorption coefficient (α) on the photon energy (hν) was determined as a function of radiation dose. The films show indirect allowed interband transition that is influenced by the radiation dose. Both the optical energy gap and the absorption coefficient were found to be dose dependent. The indirect optical energy gap was found to decrease from 1.257 to 0.664 eV with increasing the radiation dose from 10 to 250 krad, respectively. The results can be discussed on the basis of γ-irradiation-induced defects in the film. The width of the tail of localized states in the band gap (E_e) was evaluated using the Urbach edge method. The refractive index (n) was determined from the analysis of the transmittance and reflectance data. Analysis of the refractive index yields the values of high frequency dielectric constant (ε_∞) and the carrier concentration (N/m^*). The dependence of refractive index on the radiation dose has also been discussed. Other optical parameters such as real and imaginary parts of the dielectric constant (ε₁, ε₂) and the extinction coefficient (k) have been evaluated. It was found that the spectral absorption coefficient is expected to a suitable control parameter of γ-irradiation-sensitive elements of dosimetric systems for high energy ionizing radiation (0.06-1.33 MeV).     

Optical properties of selenium-tellurium nanostructured thin film grown by thermal evaporation

Se-Te nanostructured thin films were deposited on glass substrates in the presence of oxygen and argon by thermal evaporation. The properties of Se-Te thin films strongly depend on the deposition method. During the process used, the substrate is cooled to a temperature of 77 K employing liquid nitrogen. The nanostructured thin films of Se_100−xTe_x (where x=4, 8 and 16) are deposited on glass substrate. The surface morphology of the deposited films was investigated through Scanning Electron Microscopy (SEM). The typical size of these nanostructures is in the range 40-100 nm and the length is of the order of several micrometers. The optical parameters i.e. optical gap (E_g), absorption coefficient (α), and extinction coefficient (k) are calculated in the wavelength range 190-1100 nm. It was found that the optical band gap decreased from 3.4 to 2.9 eV when Te concentration was increased in the Se_100−xTe_x nanostructured thin films. The large bandgap may be attributed to the decrease in particle size which clearly exhibits a quantum size effect. XRD analysis was performed to confirm glassy nature of the nanostructured thin films.     

Optical,photo-acoustic and electrical switching studies of amorphous GeS2 thin films

This paper reports optical, photo-acoustic and electrical switching investigations of GeS₂ amorphous thin films of different thicknesses, deposited on glass substrates in vacuum. The Tauc parameter (B ^1/2) and Urbach energy (E _U) have been determined from the transmittance spectra, to understand the changes in structural disorder; it is found that B ^1/2 increases whereas E _U decreases as the thickness of the films increases. Based on the results, it is suggested that bond re-arrangement, i.e. transformation from homopolar bonds to heteropolar bonds, takes place with increase in thickness. The thermal diffusivity values of GeS₂ thin films also show the presence of a chemically ordered network in the GeS₂ thin films. Further, it is found that these films exhibit memory-type electrical switching. The observed variation in the switching voltages has been understood on the basis of increase in chemical order.     

Third-order optical nonlinearity of (Ba0.7Sr0.3)TiO3 ferroelectric thin films fabricated by soft solution processing

(Ba_0.7Sr_0.3)TiO₃ (BST) ferroelectric thin films with perovskite crystal structure were fabricated by soft solution processing on a quartz substrate. The third-order nonlinear optical properties were investigated by using Z-scan technique. Positive nonlinear refractive index and nonlinear absorption coefficient were determined to be 4×10⁻⁷ esu and 1.2×10⁻⁶ m/w, respectively. The real part and imaginary part of third-order optical nonlinear susceptibility were calculated and the values were 6.43×10⁻⁸ and 5.14×10⁻⁸ esu, respectively. All of these results show ferroelectric BST thin film is promising for applications in nonlinear optical devices.     

Amorphous film thickness dependence for epitaxy of perovskite oxide films under excimer laser irradiation

We have studied the epitaxial growth of perovskite manganite LaMnO₃ (LMO) on SrTiO₃(1 0 0) in the excimer laser assisted metal organic deposition process. The LMO was preferentially grown from the substrate surface by the KrF laser irradiation. The study of amorphous LMO film thickness dependence on epitaxial growth under the excimer laser irradiation revealed that the photo-thermal heating effect strongly depended on the amorphous film thickness due to a low thermal conductivity of amorphous LMO: the ion-migration for chemical bond-forming at the reaction interface would be strongly enhanced in the amorphous LMO film with the large film thickness about 210 nm. On the other hand, the photo-chemical effect occurred efficiently for the amorphous film thickness in the range of 35-210 nm. These results indicate that the epitaxial growing rate was dominated by the photo-thermal heating after the photo-chemical activation at the growth interface.     

Characteristics of filtered vacuum arc deposited ZnO-SnO2 thin films on room temperature substrates

ZnO, SnO₂ and zinc stannate thin films were deposited using filtered vacuum arc deposition (FVAD) system on commercial microscope glass and UV fused silica substrates (UVFS) at room temperature (RT). The structural and morphological analyses were performed using X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), respectively. XRD patterns of ZnO films deposited at RT had strongly c-axis orientation, whereas SnO₂ and zinc stannate films had amorphous structure as they did not have any defined patterns. Average crystalline size and surface grain size of ZnO films were ∼16 nm, as determined from diffraction line broadening and AFM images, respectively. Optical constants in the 250-1100 nm wavelength range were determined by variable angle spectroscopic ellipsometry and transmission measurements. The transmission of the deposited films in the VIS was 80-90%, affected by interference. The refractive indices and the extinction coefficients of deposited ZnO, SnO₂ and zinc stannate films were in the range 1.87-2.15 and 0.02-0.04, depending on wavelengths and deposition parameters. The optical band gap (E_g) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values for ZnO, SnO₂ and zinc stannate were in the range 3.25-3.30 eV, 3.60-3.98 eV and 3.43-3.52 eV, respectively, depending on the deposition pressure.