Growth and characterization of tris(8-hydroxyquinoline)-aluminum molecular films

Various tris(8-hydroxyquinoline)-aluminum (Alq3) molecular solid films were grown on top of indium-tin-oxide (ITO) glass substrates using physical vapor deposition. The effect of changing the growth conditions on the properties of the films was studied. From scanning electron microscopy, an Alq3 planar layer over an ITO-substrate was observed at the initial period, and an Alq3 tubular structure (which becomes dominant at substrate temperature T_sub ≧ 90 °C) was found to nucleate from this layer. From X-ray diffraction, the Alq3 planar layer possesses an amorphous character while the Alq3 tubular layer has a triclinic α-phase structure. Based on an Arrhenius plot of the growth rate versus 1/T_sub, the growth behaviors in various T_sub-regions were discussed to be dominated by adhesion (for T_sub < 90 °C), steric effect (90 °C < T_sub < 150 °C), and re-evaporation (T_sub > 150 °C). Then, from optical transmission and photoluminescence spectra performed on the high crystalline Alq3 films, two signals associated with the optical-bandgap E_g absorption and the gap-state absorption were determined and discussed in terms of the optical properties of the constituent Alq3 molecules. Finally, from a fit of E_g(T) by an effective electron-phonon interaction model, the physical significance of these fitting parameters for the Alq3 molecular solid was investigated.     

Phase transitions of room temperature RF-sputtered ZnO/Mg0.4Zn0.6O multilayer thin films after thermal annealing

We report the thermal stability of room-temperature RF-sputtered Mg_0.4Zn_0.6O thin films and ZnO/Mg_0.4Zn_0.6O superlattices at 600 °C and 800 °C. The phase of room-temperature as-sputtered Mg_0.4Zn_0.6O is crystalline ZnO embedded in an amorphous or short-range-ordered hexagonal MgZnO matrix. Annealing at either 600 °C or 800 °C for 5 min transforms the matrix into a crystalline hexagonal wurtzite structure, leading to a decrease of the optical bandgap (E_g) of Mg_0.4Zn_0.6O. This also results in a slight change near the absorption edge of the superlattice transmission spectrum. The films precipitate cubic MgZnO after heating Mg_0.4Zn_0.6O at 800 °C for 5 min; by contrast, precipitations take at least 3 h if the samples are heated at 600 °C. Heating at 800 °C for more than 3 h significantly reduces the film thickness and E_g, attributed to the decomposition of superlattices and diffusion of magnesium into the substrate, respectively. On the other hand, annealing the ZnO/Mg_0.4Zn_0.6O superlattice at 600 °C for 12 h also produces an initial slight change in the optical transmission spectra, yet the spectra remain essentially unchanged for the remainder of the annealing process.     

Annealing effect on cadmium in situ doping of chemical bath deposited PbS thin films

This paper describes the effect of annealing on PbS and Cd-doped PbS thin films prepared by chemical bath deposition at different bath temperatures (T_b). The X-ray diffraction (XRD), optical absorption, scanning electron microscopy, and energy dispersive X-ray (EDX) analyses have been performed to explore the properties of PbS and PbCdS films. From the XRD measurements, the particle size (D) of as-deposited PbS and PbCdS films is estimated to be 22 (27) and 12 (9) nm, respectively, for a T_b of 75 (85) °C. A reduction in D was noticed upon annealing the films at 200 °C, irrespective of the T_b and the doping. The optical band gap energy (E_g) of as-deposited PbS films grown at different T_b is found to be in the range of 1.22-1.42 eV. Doping of PbS with Cd and annealing have led to increase in E_g up to 2.61 (2.66) eV. Optical studies revealed prominent blue shifts in the E_g of as-deposited and annealed films due to quantum confinement effect. The addition of Cd into PbS was confirmed by EDX analysis.     

Structure and optical properties of Zr1−xSixN thin films on sapphire

A series of zirconium silicon nitride (Zr_1−xSi_xN) thin films were grown on r-plane sapphire substrates using reactive RF magnetron co-sputtering of Zr and Si targets in a N₂/Ar plasma. X-ray diffraction pole figure analysis, X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), optical microscopy, and optical absorption spectroscopy were used to characterize the film stoichiometries and structures after growth at 200 °C and post-deposition annealing up to 1000 °C in ultra-high vacuum. The atomically clean r-plane sapphire substrates induce high quality (100) heteroepitaxy of ZrN films rather than the (111) orientation observed on steel and silicon substrates, but the addition of Si yields amorphous films at the 200 °C growth temperature. After the annealing treatment, films with Si content x < 0.15 have compressive stress and crystallize into a polycrystalline structure with (100) fiber texture. For x > 0.15, the films are amorphous and remain so even after ultra-high vacuum annealing at 1000 °C. XPS spectra indicate that the bonding changes from covalent to more ionic in character as Si―N bonds form instead of Zr―N bonds. X-ray reflectivity, atomic force microscopy (AFM) and optical microscopy data reveal that after post-deposition annealing the 100 nm thick films have an average roughness < 2 nm, except for Si content near x = 0.15 corresponding to where the film becomes amorphous rather than being polycrystalline. At this stoichiometry, evidence was found for regions of film delamination and hillock formation, which is presumably driven by strain at the interface between the film and sapphire substrate. UV-visible absorption spectra also were found to depend on the film stoichiometry. For the amorphous Si-rich films (x > 0.15), the optical band gap increases with Si content, whereas for Zr-rich films (x < 0.15), there is no band gap and the films are highly conductive.     

Optical, electrical and structural properties of Cu2Te thin films deposited by magnetron sputtering

Thin films of Cu₂Te were deposited, at room temperature, on glass substrates by magnetron sputtering from independent Cu and Te sources. This work presents the effect of annealing temperature on the optical, structural, and electrical properties of sputtered Cu₂Te films. Annealing above 300 °C resulted in stoichiometric and near stoichiometric Cu₂Te phases, whereas temperatures above 400 °C yielded films with single Cu₂Te phase. In contrast, annealing at temperatures of 250 °C and below resulted in mixed phases of CuTe, Cu₇Te₅, Cu_1.8Te, and Cu₂Te. Analyses of transmittance and reflectance measurements for Cu₂Te indicate that photon absorption occurs via indirect band transitions for incident photons with energy above the band gap energy and free carrier absorption below the band gap energy. The determined indirect band gap was 0.90 eV and its associated phonon energy was 0.065 eV. Optical phonon scattering was identified as the mechanism through which the momentum is conserved during absorption by free carriers. Electrical measurements show p-type conductivity and highly degenerate semiconducting behavior with a hole carrier concentration p = 5.18 × 10²¹ cm^− 3.     

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.     

Thermal stability of amorphous molybdenum trioxide films prepared at different oxygen partial pressures by reactive DC magnetron sputtering

Thin films of MoO₃ were prepared on quartz and Si (1 0 0) substrates by reactive dc magnetron sputtering of a Mo target in an oxygen and argon atmosphere. The structural and optical changes induced in the films due to post-growth annealing have been systematically studied by Rutherford backscattering (RBS), X-ray diffraction (XRD), X-ray reflectivity (XRR) and by optical methods. RBS studies reveal no change in composition of the films upon annealing at high temperatures. Grazing angle XRD studies show that the as-deposited films are amorphous and crystallize to β-MoO₃ phase with small contribution of α-MoO₃ upon annealing at 300 °C. The film prepared at 0.40 Pa transforms to α-MoO₃ upon annealing at 650 °C, while the film deposited at 0.19 Pa still has some β-MoO₃ phase contribution. XRR measurements reveal that the film thickness decreases upon annealing with simultaneous increase of film density. The surface roughness of the films strongly increases after crystallization. The contraction of the film deposited at 0.40 Pa is much greater than the contraction of the film prepared at 0.19 Pa. The mass variation of the film deposited at 0.19 Pa and that deposited at 0.40 Pa are completely different. The optical properties of MoO₃ films deposited at 0.19 and 0.40 Pa are changed strongly by annealing.     

Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Optical and electrical properties of TiOx thin films deposited by electron beam evaporation

The TiO_x thin films were prepared by electron beam evaporation using TiO as the starting material. The effect of the annealing temperature on the optical and electrical properties was investigated. The spectra of X-ray photoelectron spectroscopy reveal that Ti in the films mainly exist in the forms of Ti²⁺ and Ti³⁺ below 400 °C 24 h annealing. The charge transfer between different titanium ion contribute greatly to the color, absorption, and electrical resistance of the films.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

The influence of annealing temperature on the structural, electrical and optical properties of ion beam sputtered CuInSe2 thin films

CuInSe₂ (CIS) thin films were prepared by ion beam sputtering deposition of copper layer, indium layer and selenium layer on BK7 glass substrates followed by annealing at different temperatures for 1 h in the same vacuum chamber. The influence of annealing temperature (100-400 °C) on the structural, optical and electrical properties of CIS thin films was investigated. X-ray diffraction (XRD) analysis revealed that CIS thin films exhibit chalcopyrite phase and preferential (112) orientation when the annealing temperature is over 300 °C. Both XRD and Raman show that the crystalline quality of CIS thin film and the grain size increase with increasing annealing temperature. The reduction of the stoichiometry deviation during the deposition of CIS thin films is achieved and the elemental composition of Cu, In and Se in the sample annealed at 400 °C is very near to the stoichiometric ratio of 1:1:2. This sample also has an optical energy band gap of about 1.05 eV, a high absorption coefficient of 10⁵ cm⁻¹ and a resistivity of about 0.01 Ω cm.     

Preparation and hydrothermal annealing of pure metastable β-MnS thin films by chemical bath deposition (CBD)

Pure metastable β-MnS thin films have been deposited by chemical bath deposition (CBD) method and subsequently annealed in a Na₂S solution (denoted as hydrothermal annealing). The effects of preparative parameters and hydrothermal annealing on structure, morphology and optical property of the films have been investigated. Experimental results indicate that the crystalline β-MnS thin films can be prepared at a low concentration of Mn and S ions without using any organic chelator. The as-deposited β-MnS can be transformed to γ-phase MnS after the hydrothermal treatment in an autoclave at 200 °C for 1 h. The estimated E_g values are in the range of 3.15-3.18 eV.     

Control of crystallographic orientation of LiNbO3 films grown by electron-cyclotron resonance plasma sputtering on TiN films

We investigated the orientation of domains in LiNbO₃ (LN) thin films grown by electron-cyclotron resonance plasma sputtering on TiN films with various crystalline states. Deposition at 400 °C on an amorphous TiN produced partially crystallized and apparently c-axis-oriented LN. When TiN crystallized at 460 °C to become polycrystalline grains, the roughened surface randomized the orientation of LN. At 600 °C, the reaction of TiN with oxygen atoms supplied from the plasma created a TiO_x layer. Rapid thermal annealing of amorphous LN films at 460 °C was the best solution for removing these disorientation factors, but annealing of amorphous LN on poly-crystalline TiN yielded no c-axis-oriented domains.     

Thin gold films on SnO2:In: Temperature-dependent effects on the optical properties

Gold films with thicknesses of 5 ± 0.5 nm were sputter deposited onto SnO₂:In-coated glass kept at different temperatures up to 140 °C, and similar films, deposited onto substrates at 25 °C, were annealing post treated at the same temperatures. Nanostructures and optical properties were recorded by scanning electron microscopy and spectrophotometry in the 0.3 to 2.5 μm wavelength range, respectively. Annealing had a minor influence on the optical transmittance despite significant changes in the scale of the nanostructure, whereas deposition onto substrates heated to 140 °C yielded granular films with strong plasmon absorption of luminous radiation. These results are of considerable interest for optical devices with gold films prepared at elevated temperature or operating at such temperature.     

Structural and electrical properties of BiFeO3 thin films by solution deposition and microwave annealing

Lead-free polycrystalline BiFeO₃ (BFO) thin films were developed using a chemical solution deposition method to deposit the films and the multi-mode 2.45 GHz microwave furnace to optimize the annealing condition of the films. Phase-pure BFO films were obtained at 500 °C-600 °C for 1-5 min with a heating rate of 10 °C/min. The film by microwave annealing (MW) at 550 °C for 5 min exhibited a (012)-preferred orientation with a dense morphology of grain size ~ 294 nm. Its dielectric constant of 96.2, low leakage current density of 2.466 × 10^− 6 A/cm², polarization (2P_r) and coercive field (2E_c) of 0.931 μC/cm² and 57.37 kV/cm, respectively, were improved compared to those by conventional annealing (CA) at the same annealing conditions.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Influence of the substrate temperature on material properties

Successful p-type doping of μc-SiC:H with Al introduced from trimethylaluminum has been already demonstrated. In this work we focus on the influence of substrate temperature (T_S = 300-390 °C) on the Al-doping. As T_S is reduced from 390 °C to 300 °C, the crystallinity decreases from 75% to 55% and the dark conductivity σ_D decreases first by about three orders of magnitude before increasing again at T_S = 300 °C. Both microstructure, as determined from Raman spectroscopy, and optical absorption are little affected by the change in T_S. Upon annealing at 450 °C in vacuum, σ_D increases typically by two orders of magnitude up to 10⁻⁴ S/cm, which is explained by dopant activation as a result of hydrogen desorption. It is concluded that a process temperature > 350 °C is needed to obtain effective Al-doping for p-type μc-SiC:H thin films.     

Optical properties of sputter deposited transparent and conducting TiO2:Nb films

Transparent and conducting thin films of TiO₂:Nb were prepared on glass by reactive dc magnetron sputtering in Ar + O₂. Post-deposition annealing in vacuum at 450 °C led to good electrical conductivity and optical transparency. The optical properties in the sub-bandgap region were in good agreement with Drude free electron theory, which accounts for intraband absorption. The band gap of the films was found to be in the range of 3.3 to 3.5 eV and signifies the onset of interband absorption. Electrical conductivities in the 10^− 3 Ω cm range were obtained both from dc electrical measurements and from analysis of the optical measurements.     

Effects of post-deposition surface treatment on the optical, structural and hydrogen sensing properties of TiO2 thin films

TiO₂ thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO₂ thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO₂ thin films were tested as sensors of hydrogen at concentrations 10,000-1000 ppm and operating temperatures within the 180-200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.     

Influence of magnetic annealing on high-frequency magnetic properties of FeCoNd films

FeCoNd thin film with thickness of 166 nm has been fabricated on silicon (1 1 1) substrates by magnetron co-sputtering and annealed for one hour under magnetic field at different temperatures (T_a) from 200 °C to 700 °C. The As-deposited and annealed FeCoNd film samples at T_a ≤ 500 °C were amorphous while the ones obtained at T_a ≥ 600 °C were crystallized. We found that the perpendicular anisotropy field gradually decreases as the annealing temperature increases from room temperature to 300 °C. A well induced in-plane uniaxial anisotropy is achieved at the annealing temperature between 400 and 600 °C. The variation of the dynamic magnetic properties of annealed FeCoNd films can be well explained by the Landau-Lifshitz equation with the variation of the anisotropy field re-distribution and the damping constant upon magnetic annealing. The magnetic annealing might be a powerful post treatment method for high frequency application of magnetic thin films.     

Structural, optical, and electrical properties of tin sulfide thin films grown by spray pyrolysis

Tin sulfide (SnS) thin films have been prepared by spray pyrolysis (SP) technique using tin chloride and N, N-dimethylthiourea as precursor compounds. Thin films prepared at different temperatures have been characterized using several techniques. X-ray diffraction studies have shown that substrate temperature (T_s) affects the crystalline structure of the deposited material as well as the optoelectronic properties. The calculated optical band gap (E_g) value for films deposited at T_s = 320-396 °C was 1.70 eV (SnS). Additional phases of SnS₂ at 455 °C and SnO₂ at 488 °C were formed. The measured electrical resistivity value for SnS films was ∼ 1 × 10⁴ Ω-cm.