Effect of substrate temperature on the optical parameters of thermally evaporated Ge-Se-Te thin films

Thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10, 20, 30, 40, 50) glassy alloys were deposited at three substrate temperatures (303 K, 363 K and 423 K) using conventional thermal evaporation technique at base pressure of ~ 10^− 4 Pa. X-ray diffraction results show that films deposited at 303 K are of amorphous nature while films deposited at 363 K and 423 K are of polycrystalline nature. The optical parameters, refractive index and optical gap have been derived from the transmission spectra (using UV-Vis-NIR spectrophotometer) of the thin films in the spectral region 400-1500 nm. This has been observed that refractive index values remain almost constant while the optical gap is found to decrease considerably with the increase of substrate temperature. The decrease in optical gap is explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase of substrate temperature. The optical gap has also been observed to decrease with the increase of Te content.     

Physical properties of flash evaporated In2O3 films prepared at different substrate temperatures

Indium oxide (In₂O₃) thin films were prepared by flash evaporated technique under various substrate temperatures in the range of 303-673 K and systematically studied the structural, electrical and optical properties of the deposited films. The films formed at substrate temperatures of < 373 K were amorphous while those deposited at higher substrate temperatures (≥ 373 K) were polycrystalline in nature. The optical band gap of the films decreased from 3.71 eV to 2.86 eV with the increase of substrate temperature from 303 K to 673 K. Figure of merit of the films increased from 2.8 × 10³ Ω^− 1 cm^− 1 to 4.2 × 10³ Ω^− 1 cm^− 1 with increasing substrate temperature from 303 K to 573 K, thereafter decreased to 2.2 × 10³ Ω^− 1 cm^− 1 at higher temperature of 673 K.     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.     

In-situ energy-dispersive X-ray diffraction of metal sulfide assisted crystallization of strongly (001) textured photoactive tungsten disulfide thin films

Highly (001) textured tungsten disulphide (WS₂) thin films were grown by rapid metal (Ni, Pd) sulfide assisted crystallization of amorphous reactively sputtered sulfur-rich tungsten sulfide (WS_3 + x) and by metal sulfide assisted sulfurization of tungsten metal films. The rapid crystallization was monitored by real-time in-situ energy dispersive X-ray diffraction (EDXRD). Provided that a thin nickel or palladium film was deposited prior to the deposition of WS_3 + x or W, the films crystallized very fast (about 20 nm/s) at temperatures above the metal sulfide eutectic temperature. After crystallization, isolated MeS_x crystallites are located on the surface of the WS₂ layer, which was proved by scanning electron microscopy. The metal sulfide assisted crystallized WS₂ layers exhibit a pronounced (001) orientation with large crystallites up to 2 µm. The in-situ EDXRD analysis revealed distinct differences of the two crystallization routes from tungsten and from amorphous, sulfur-rich WS_3 + x precursors, respectively. The crystallized WS₂ films showed photoactivity. Combined with the high absorption coefficient of 10⁵ cm^− 1 and a indirect band gap of 1.8 eV these properties make such films suitable for absorber layers in thin film solar cells.     

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Large tensile stresses (up to 3 GPa) were previously observed in low-mobility metallic Mo_1 − xSi_x films grown on amorphous Si and they were ascribed to the densification strain at the amorphous-crystalline transition occurring at a critical film thickness. Here, we focus on the influence of the nucleation conditions on the subsequent stress build-up in sputter-deposited Mo_0.84Si_0.16 alloy films. For this purpose, growth was initiated on various underlayers, including amorphous layers and crystalline templates with different lattice mismatch, and the stress evolution was measured in situ during growth using the wafer curvature technique. Tensile stress evolutions were observed on amorphous SiO₂ and (111) Ni underlayers, similarly to the stress behaviour found on amorphous Si. For these series, the films were characterized by large in-plane grain size (~ 500 nm). However, on a (110) Mo buffer layer, a different stress behaviour occurred: after an initial tensile rise ascribed to coherence stress, a reversal towards a compressive steady state stress was observed. A change in film microstructure was also noticed, the typical grain size being ~ 30 nm. The origin of the compressive stress source in the metastable Mo_0.84Si_0.16 alloy grown on (110) Mo is discussed based on the stress evolutions measured at varying deposition rates and Ar working pressures, as well as in comparison with stress evolutions in pure Mo films.     

Structural and optical properties of amorphous and crystalline antimony sulfide thin-films

Smooth and compact thin films of amorphous and crystalline antimony sulfide (Sb₂S₃) were prepared by radio frequency sputtering of an Sb₂S₃ target. As-deposited films are amorphous. Polycrystalline antimony sulfide films composed of ∼ 500 nm grains are obtained by annealing the as-deposited films at 400 °C in sulfur vapors. Both amorphous and crystalline antimony sulfide have strong absorption coefficients of 1.8 × 10⁵ cm^− 1 at 450 nm and 7.5 × 10⁴ cm^− 1 at 550 nm, and have direct bandgaps with band energies of 2.24 eV and 1.73 eV, respectively. These results suggest the potential use of both amorphous and crystalline antimony sulfide films in various solid state devices.     

Dielectric properties of sol-gel derived barium strontium titanate thin films

Barium strontium titanate (Ba_xSr_1−xTiO₃) films were deposited by sol-gel technique on platinized silicon substrate for the composition range x = 0.0 to 1.0 in steps of 0.1. The as-deposited films were found to be amorphous. The films crystallize on annealing in air at 700 °C for 1 h. Dielectric constant (ε′) and loss tangent (tanδ) were measured in the temperature range − 180 °C to 150 °C in the frequency range 0.1 to 100 kHz. Both ε′ and tanδ show a small dispersion for all the compositions. This dispersion is more at the peak value than at room temperature. A comparison of the room temperature and peak value of the dielectric constant for various compositions are made with the reported values. Transition temperatures are reported for the entire composition range. All the compositions show a transition from ferroelectric to paraelectric phase except strontium titanate. Transition temperature shows a systematic decrease with increase in strontium content. The variation is at a rate of 3.4 °C/mol% of SrTiO₃. Curie constants are also reported for the entire composition range.     

Preparation of LaNiO3 thin films with very low room-temperature electrical resistivity by room temperature sputtering and high oxygen-pressure processing

LaNiO₃ (LNO) thin films were deposited by radio frequency magnetron sputtering on n-type Si (100) wafers at room temperature (RT). The as-sputtered LNO thin films were amorphous and had very high RT electrical resistivity even after post-annealing at 800 °C. The amorphous as-sputtered LNO films could be transformed to polycrystalline LNO films in rhombohedral phase by heating at 400 °C in an O₂ atmosphere at pressure ranging from 1.5 to 8.0 MPa. Very low RT resistivity of LNO films were obtained by this high oxygen-pressure processing. The lowest value was as low as 1.09 × 10^− 4 Ω cm by processing at oxygen pressure of 8 MPa. Such preparation of LNO thin films is compatible with the Si-based readout integrated circuits. Highly (100)-oriented perovskite structure of Pb(Zr_0.52Ti_0.48)O₃ thin films was formed on this rhombohedral phase LNO, and good ferroelectricity could also be obtained on these HOPP-processed rhombohedral phase LNO films.     

Preparation of La0.8Sr0.2Cr0.97V0.03O3 − δ films for solid oxide fuel cell application

La_0.8Sr_0.2Cr_0.97V_0.03O_3 − δ (LSC) is commonly studied as a ceramic interconnect material as well as a coating material for metallic interconnects for solid oxide fuel cell applications. However, it is difficult to sinter this type of material to high density. In order to overcome this problem and to study the material in form of a thin film we have used Pulsed Laser Deposition to obtain a dense, uniform film with the right stochiometry. Investigation of preparation-parameter dependence of the LSC films deposited on a stainless steel substrate during pulsed-laser deposition was carried out. The LSC films were deposited with KrF excimer laser (248 nm) on a stainless steel substrate at different oxygen pressure and substrate temperatures. The substrate temperature (873-1073 K) and the oxygen background pressure (5-20 Pa) were varied in order to obtain optimal growth conditions. The surface morphology and structural information of the films were obtained using scanning electron microscope (SEM) and X-ray diffraction, respectively. Under the optimal preparation-parameter conditions: substrate temperature of 1023 K and an oxygen pressure of 10 Pa the structure of the film agreed with the target structure and the SEM micrographs show that the surfaces are homogeneous, smooth, crack-free and dense.     

Oxidation of magnetron sputtered La-Si thin films for solide oxide fuel cell electronlytes

La-Si thin films were deposited on stainless steel substrates by magnetron sputtering from pure La and Si targets. The Si/(Si + La) atomic ratio in the films was varied from 43.2 to 59.3% by adjusting the discharge current on the La target. The films had a homogeneous chemical composition down to the substrate and sharp interfaces. Annealing the films in air at 1173 K promotes the formation of apatite-structure La_9.33Si₆O₂₆ and the diffusion of different species from the film to the substrate and vice-versa, resulting in broadening the interfaces. X-Ray diffraction showed that all the as-deposited films had an amorphous structure. The formation of the LaSi₂ phase at intermediate temperatures was observed for the films deposited with higher Si contents while the films deposited with lower Si contents remained amorphous up to the start of the apatite structure crystallization process. The lanthanum silicate apatite-like phase (La_9.33Si₆O₂₆) was obtained only after annealing at 1173 K, excepted for the film with the lower Si content which is already partially crystallized after annealing at 1073 K. Quite pure La_9.33Si₆O₂₆ was obtained only after annealing the film with the highest Si content (Si/(Si + La) = 59.3%) although the theoretical Si/(Si + La) atomic ratio for apatite structure lanthanum silicate is 39%. For the other films, La₂O₃ was always detected when the lanthanum silicate phase was formed. Both phenomena clearly resulted from the strong diffusion of silicon excess towards the stainless steel substrate.     

Thin film preparation and characterization of wide band gap Cu3TaQ4 (Q = S or Se) p-type semiconductors

The structural, optical, and electronic properties of thin films of a family of wide band gap (E_g > 2.3 eV) p-type semiconductors Cu₃TaQ₄ (Q = S or Se) are presented. Thin films prepared by pulsed laser deposition of ceramic Cu₃TaQ₄ targets and ex-situ annealing of the as-deposited films in chalcogenide vapor exhibit mixed polycrystalline/[100]-directed growth on amorphous SiO₂ substrates and strong (100) preferential orientation on single-crystal yttria-stabilized zirconia substrates. Cu₃TaS₄ (E_g = 2.70 eV) thin films are transparent over the entire visible spectrum while Cu₃TaSe₄ (E_g = 2.35 eV) thin films show some absorption in the blue. Thin film solid solutions of Cu₃TaSe_4 − xS_x and Cu₃TaSe_4 − xTe_x can be prepared by annealing Cu₃TaSe₄ films in a mixed chalcogenide vapor. Powders and thin films of Cu₃TaS₄ exhibit visible photoluminescence when illuminated by UV light.     

(0 0 6)-oriented α-Al2O3 films prepared in CO2-H2 atmosphere by laser chemical vapor deposition using a diode laser

(0 0 6)-oriented α-Al₂O₃ films were prepared by laser chemical vapor deposition (LCVD) using aluminum acetylacetonate (Al(acac)₃) in CO₂-H₂ atmosphere. The effects of the CO₂ mole fraction (FCO₂) and laser power (P_L) on the crystal phase, microstructure, and deposition rate (R_dep) were investigated. α- and γ-Al₂O₃ mixture films were prepared at P_L = 90 W (deposition temperature of 818 K), whereas (0 0 6)-oriented single-phase α-Al₂O₃ films were obtained at P_L = 110 W (863 K). The texture coefficient and the grain size of the (0 0 6)-oriented films increased with increasing FCO₂. The orientation of the α-Al₂O₃ films changed from (0 0 6) to (1 0 4) to (0 1 2) with increasing P_L (T_dep). The R_dep of the (0 0 6)-oriented α-Al₂O₃ films increased with increasing FCO₂.     

On the synthesis of ultra smooth ITO thin films by conventional direct current magnetron sputtering

Highly transparent, conductive Sn-doped In₂O₃ (ITO) thin films with a characteristic root mean square surface roughness RMS below 1 nm were obtained from deposition of amorphous ITO and subsequent annealing treatment. ITO thin films with ultra flat surface were produced by (i) controlling crystallization mechanisms (nucleation and growth) of amorphous ITO through optimization of hydrogen content and temperature profile during sputtering and annealing process and (ii) preventing formation of agglomerated atoms/clusters in the gas phase and hence reducing large surface particles through fine tuning the sputtering rate and process pressure. Characterization of the coatings revealed specific resistivities below 2.5 × 10^− 4 Ω cm and transparencies above 90% in the visible range of light.     

Surface morphology and growth mechanisms for sputtered amorphous silicon nitride thin films

Evolution of surface of sputter-deposited amorphous Si₃N₄ films growth on Si (100) substrates was investigated using atomic force microscopy (AFM). The scaling behaviors of the AFM topographical profiles were analyzed using the one-dimensional power spectral density. The results of root-mean-square surface height variation showed that there is a power law relationship between the surface roughness and deposition time. It is interesting to note that the growth exponent can be divided into one region and two regions, respectively, when Si₃N₄ films are deposited at different working pressures. A very low growth exponent of β = 0.07 ± 0.01 was found when Si₃N₄ films were deposited at a working pressure of 1.6 × 10^− 1 Pa. However, the growth exponent β can be divided into two regions, which is β₁ = 0.09 ± 0.01, β₂ = 0.24 ± 0.03 and β₁ = 0.09 ± 0.01, β₂ = 0.33 ± 0.04, when the films were deposited at a working pressure of 2.1 × 10^− 1 Pa and 2.7 × 10^− 1 Pa, respectively. The mechanisms of anomalous dynamic scaling exponents of Si₃N₄ films deposited at different working pressures were discussed.     

Lipon thin films grown by plasma-enhanced metalorganic chemical vapor deposition in a N2-H2-Ar gas mixture

Lithium phosphorus oxynitride (Lipon) thin films have been deposited by a plasma-enhanced metalorganic chemical vapor deposition method. Lipon thin films were deposited on approximately 0.2 μm thick Au-coated alumina substrates in a N₂-H₂-Ar plasma at 13.56 MHz, a power of 150 W, and at 180 °C using triethyl phosphate [(CH₂CH₃)₃PO₄] and lithium tert-butoxide [(LiOC(CH₃)₃] precursors. Lipon growth rates ranged from 10 to 42 nm/min and thicknesses varied from 1 to 2.5 μm. X-ray powder diffraction showed that the films were amorphous, and X-ray photoelectron spectroscopy (XPS) revealed approximately 4 at.% N in the films. The ionic conductivity of Lipon was measured by electrochemical impedance spectroscopy to be approximately 1.02 μS/cm, which is consistent with the ionic conductivity of Lipon deposited by radio frequency magnetron sputtering of Li₃PO₄ targets in either mixed Ar-N₂ or pure N₂ atmosphere. Attempts to deposit Lipon in a N₂-O₂-Ar plasma resulted in the growth of Li₃PO₄ thin films. The XPS analysis shows no C and N atom peaks. Due to the high impedance of these films, reliable conductivity measurements could not be obtained for films grown in N₂-O₂-Ar plasma.     

Composition, structural and electrical properties of thin films prepared by laser ablation of neodymium-doped potassium gadolinium tungstate

Thin films were grown on (001) SiO₂, SiO₂/(100) Si or (100) MgO substrates by laser ablation of neodymium-doped potassium gadolinium tungstate (Nd:KGW) single crystal target. The films were deposited at temperatures between room temperature and 750 °C and pressures between 1 × 10^− 4 Pa and 50 Pa of oxygen ambient. The influence of the deposition conditions on the composition, structure, morphology and electrical properties of the films was investigated. Special attention was paid to the films deposited in vacuum (1 × 10^− 4 Pa) or at very low oxygen pressures. Under such conditions, the potassium (K), gadolinium (Gd) and oxygen (O) content decreased strongly as the temperature was increased. At room temperature, the films were K and O stoichiometric, in contrast with Gd, which showed a concentration twice higher. The films were polycrystalline, with the exception of those deposited at temperatures below 500 °C, which were amorphous. However, all were smooth and dense. The films grown in vacuum and at temperatures between 500 and 700 °C consist mainly of “â-tungsten” - tungsten oxide (W₃O) phase. The films grown on SiO₂/Si possessed the best surface quality with nano-size relief. The resistivity measurements as a function of the temperature showed that the films produced in vacuum and at temperatures below 500 °C were highly insulating, whereas at 600 °C they exhibited semiconducting behavior or a metallic one at 700 °C. This behavior can be attributed to the existence of various valence states for tungsten below W⁶⁺ in the films and to their crystal structure.     

Structure and magnetic properties of sputtered (Fe1 − xCrx)0.09Cu0.91 solid solution films

The surface morphology of the (Fe_1 − xCr_x)_0.09Cu_0.91 films does not change significantly as x increases. No Fe or Cr granules form in the films because of the low deposition temperature and the non-equilibrium deposition procedure, suggesting that Fe and Cr atoms disperse in the Cu matrix. With increasing x, the lattice constant c and single cell volume decrease, but the lattice constants (a, b) first increase and latterly decrease. The films show spin-glass-like manner that looks like superparamagnetism at high temperatures and are ferromagnetic at low temperatures. The peak temperature of the zero-field-cooling curves decreases from 37 to 23 K as x increases from 0 to 0.25. Below the peak temperatures, the field-cooled magnetization decreases with decreasing temperature due to the antiferromagnetically coupling of the disordered spin-glass-like moments. The coercivity increases greatly below 50 K because of the pinning effect of the frozen spin-glass-like moments at low temperatures.     

Influence of Se on the electron mobility in thermal evaporated Bi2(Te1−xSex)3 thin films

Thermoelectric solid solutions of Bi₂ (Te_1−xSe_x)₃ with x = 0, 0.2, 0.4, 0.6, 0.8 and 1 were grown using the Bridgman technique. Thin films of these materials of different compositions were prepared by conventional thermal evaporation of the prepared bulk materials. The temperature dependence of the electrical conductivity σ, free carriers concentration n, mobility μ_H, and seebeck coefficient S, of the as-deposited and films annealed at different temperatures, have been studied at temperature ranging from 300 to 500 K. The temperature dependence of σ revealed an intrinsic conduction mechanism above 400 K, while for temperatures less than 400 K an extrinsic conduction is dominant.The activation energy, ΔE, and the energy gap, E_g, were found to increase with increasing Se content. The variation of S with temperature revealed that the samples with different compositions x are degenerate semiconductors with n-type conduction. Both, the annealing and composition effects on Hall constant, R_H, density of electron carriers, n, Hall mobility, μ_H, and the effective mass, m^∗/m₀ are studied in the above temperature range.     

Nickel diffusion in polycrystalline CuInSe2 thin films with a <112> fiber texture

Nickel diffusion in CuInSe₂ thin films was studied in the temperature range 430-520 °C. Thin films of copper indium diselenide (CuInSe₂) were prepared by selenization of CuInSe₂-Cu-In multilayered structure on glass substrate. A thin film of Nickel was deposited and annealed at different temperatures. Surface morphologies of the Ni diffused and undiffused CuInSe2 films were investigated using scanning electron microscope. The alteration of Nickel concentration in the CuInSe₂ thin film was measured by Energy Dispersive X-Ray Fluorescence (EDXRF) technique. These measurements were fitted to a complementary error function solution and the diffusion coefficients at four different temperatures were evaluated. The diffusion coefficients of Ni in CuInSe₂ films were estimated from concentration profiles at temperatures 430-520 °C as D = 1.86 × 10^− 7(cm²s^− 1)exp[− 0.68(eV)/kT].     

Optical properties of tungsten oxide thin films by non-reactive sputtering

Tungsten oxide thin films were grown on glass substrates by RF sputtering at room temperature using a tungsten trioxide target for several values of the argon pressure (P_Ar). The structural and morphological properties of these films were studied using X-ray diffraction and atomic force microscopy. The as-deposited films were amorphous irrespective of the argon pressure, and crystallized in a mixture of hexagonal and monoclinic phases after annealing at a temperature of 350 °C in air. Surface-roughness increased by an order of magnitude (from 1 nm to 20 nm) after thermal treatment. The argon pressure, however, had a strong influence on the optical properties of the films. Three different regions are clearly identified: deep blue films for P_Ar ≤ 2.67 Pa with low transmittance values, light blue films for 2.67 Pa < P_Ar < 6 Pa with intermediate transmittance values and transparent films for P_Ar ≥ 6 Pa with high transmittance values. We suggest that the observed changes in optical properties are due to an increasing number of oxygen vacancies as the growth argon pressure decreases.