Effect of a pulsed Nd:YAG laser irradiation on multi-walled carbon nanotubes film

Multi-walled carbon nanotubes (MWCNTs) film have been analyzed by Raman spectroscopy to clarify the effect of a pulsed Nd:YAG laser heating. The MWCNTs film surface was flashed with the fundamental harmonic (λ = 1064 nm) or the second harmonic (λ = 532 nm) of a single pulse of Nd:YAG laser in the air. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. At the laser fluence of 500 mJ/cm² with Nd:YAG laser (λ = 1064 nm), the surface reached the maximum temperature 1395 °C at 12 ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I_D (defect-mode) and I_G (graphite-mode) was measured by the Raman spectroscopy. The maximum surface temperature was calculated and compared with the I_G/I_D ratio of MWCNTs film. The graphitization occurred on the sample after irradiation.     

Growth of metal-oxide semiconductor nanocomposite thin films by a dual-laser, dual target deposition system

Nanocomposite formed by gold nanoparticles embedded in a titanium dioxide matrix thin films have been synthesized by a synchronized two laser system. An ArF? excimer (λ = 193 nm, τ_FWHM ∼ 12 ns) laser and a frequency tripled Nd:Yttrium Aluminium Garnet (YAG; λ = 355 nm, τ_FWHM ∼ 10 ns) laser were used for the irradiation of titanium dioxide and gold targets. The investigations showed that there exists the possibility for tailoring the optical properties of gold-titanium dioxide nanocomposites by the proper choice of the laser irradiation parameters. The band gap narrowing and additional absorption in the visible spectral region induced by the incorporation of gold in the host TiO₂ matrix allows for the design of nanostructured thin films for new generation of photocatalysts or solar energy converters.     

Effect of NH3 flow rate on growth, structure and luminescence of amorphous silicon nitride films by electron cyclotron resonance plasma

In this paper, hydrogenated amorphous silicon nitride (a-SiN_x:H) films have been deposited using an electron cyclotron resonance chemical vapor deposition system. The effect of NH₃ flow rate R on the deposition rate, structure and luminescence were studied using various techniques such as optical emission spectroscopy, Fourier Transform Infrared absorption (FTIR), X-ray photoelectron spectroscopy (XPS) and fluoro-spectroscopy, respectively. Optical emission behavior of SiH₄ + NH₃ plasma shows that atomic Si radical concentration determines the film deposition rate. Structural transition of a-SiN_x film from Si-rich one to near-stoichiometric/N-rich one with R was revealed by FTIR and the two phase separation of a-Si and a-Si₃N₄ was also convinced in Si-rich SiN_x films by XPS. Either photo- or electroluminescence for all the SiN_x films with R > 3 sccm shows a strong light emission in visible light wavelength range. As R < 6 sccm, recombination of electrons and holes in a-Si quantum dots is the main mechanism of photo/electroluminescence for Si-rich SiN_x films, however, for photoluminescence, gap states' luminescence is also in competition; as R > 6 sccm, light emission of the SiN_x film originates from defect states in its band gap.     

Influence of laser wavelength and beam profile on Nd:YAG laser assisted formation of polycrystalline-Si films

Influence of wavelengths and beam profiles of a pulsed Nd³⁺:YAG laser on the formation of a polycrystalline-silicon (poly-Si) on a-Si thin film is investigated. Two sets of samples of amorphous-Silicon (a-Si) thin films deposited on glass (a-Si/glass) and crystalline Si (a-Si/c-Si) substrates were treated with different laser-fluence values. After the laser treatment, the films were analyzed by a scanning electron microscope, the Raman spectroscopy technique and the resistance-measurement technique. In the case of the third harmonics (355 nm) of the Nd³⁺:YAG laser, poly-Si films were obtained with laser-fluence values ranging from 260 mJ/cm² to 560 mJ/cm², where as in the case of the second harmonics (532 nm), the process window for the formation of poly-Si films, in terms of the laser fluence, was ranging from 300 mJ/cm² to 480 mJ/cm². On the other hand, in the case of samples treated with the fundamental wavelength (1064 nm), a narrow process window with higher laser-fluence values around 1100 mJ/cm² was observed. Further, the substrate was also affected because of the higher laser-fluence value. It has also been observed that the crystallization characteristics of poly-Si films improved with the flat-top intensity distribution as compared to the Gaussian intensity distribution of the Nd³⁺:YAG laser beam. A theoretical simulation based on thermal modeling was performed to understand the mechanism of crystallization.     

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E) = ε₁(E) + iε₂(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5-4.5 eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4 nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.     

Rapid thermal-plasma annealing of ZnO:Al films for silicon thin-film solar cells

Rapid thermal annealing of sputter-deposited ZnO and Al-doped ZnO (AZO) films with and without an amorphous silicon (a-Si) capping layer was investigated using a radio-frequency (rf) argon thermal plasma jet at atmospheric pressure. The resistivity of bare ZnO films on glass decreased drastically from 10⁶ to 10³ Ω·cm at maximum surface temperatures T_max above 650 °C, whereas the resistivity increased from 10^− 4 to 10^− 3-10^− 2 Ω·cm for bare AZO films. On the other hand, the resistivity of AZO films with a 30-nm-thick a-Si capping layer remained below 10^− 4 Ω·cm, even after TPJ annealing at a T_max of 825 °C. X-ray diffraction and X-ray photoemission electron studies revealed that the film crystallization of both AZO and a-Si layers was promoted without the formation of an intermixing layer. Additionally, the crystallization of phosphorous- and boron-doped a-Si layers at the sample surface was promoted, compared to that of intrinsic a-Si under identical plasma annealing conditions. The role of the a-Si capping layer on sputter-deposited AZO and ZnO films during TPJ annealing is demonstrated. The effects of the mixing of phosphorous and boron impurities in a-Si:H during TPJ annealing of flat and textured AZOs are also discussed.     

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.     

Crystallization of amorphous Si thin films by the reaction of MoO3/Al nanoengineered thermite

In this work, we investigated a new crystallization method for amorphous silicon (a-Si) using a mixture of nano-energetic materials: molybdenum oxide and aluminum (MoO₃/Al). The purpose of using nano-energetic materials is to improve the performance of a-Si films with a self-propagating exothermic reaction over a period of microseconds without any substrate damage. The mixture of MoO₃/Al nanopowders was used for a thermite reaction for crystallization of a-Si thin films.Characterization results showed that a-Si thin films were successfully crystallized to poly-Si as evidenced by a Raman peak near 519 cm^− 1. The crystalline volume fraction of poly-Si after the nanoengineered thermite reaction was about 94.7% and poly-Si grains was uniformly distributed with an average grain size of around 40-50 nm. These results indicate that high quality poly-Si thin films were successfully prepared on the substrate.     

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi₂ and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi₂ and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.     

Structural and opto-electrical properties of the tin-doped indium oxide thin films fabricated by the wet chemical method with different indium starting materials

Tin-doped indium oxide (ITO) thin films were fabricated by the sol-gel spin-coating method with different indium precursor solutions synthesized from In(NO₃)₃ or InCl₃ (denoted as N-ITO and Cl-ITO, respectively). For both N-ITO and Cl-ITO thin films, the increase of mobility/conductivity and the reduction of carrier concentration with increasing annealing temperatures from 400 to 700 °C are related to the increase of crystallization/densification and the annihilation of oxygen vacancies. The refractive index (1.84 at λ = 550 nm), packing density (0.83), conductivity [(234 (Ω-cm)^− 1], and optical band gap (3.95 eV) of N-ITO thin films are higher than that of Cl-ITO thin films, which can be attributed to the higher densification, lower crystallinity, and more free charge carriers of N-ITO thin films. These properties make the indium nitrate-derived ITO thin films have better potential applications for some commercial products.     

Insight into excimer laser crystallization exploiting ellipsometry: Effect of silicon film precursor

The optical diagnostic of spectroscopic ellipsometry is shown to be an effective tool to investigate the mechanism of excimer laser crystallization (ELC) of silicon thin films. A detailed spectroscopic ellipsometric investigation of the microstructures of polycrystalline Si films obtained on SiO₂/Si wafers by ELC of a-Si:H and nc-Si films deposited, respectively, by SiH₄ plasma enhanced chemical vapor deposition (PECVD) and SiF₄-PECVD is presented. It is shown that ellipsometric spectra of the pseudodielectric function of polysilicon thin films allows to discern the three different ELC regimes of partial melting, super lateral growth and complete melting. Exploiting ellipsometry and atomic force microscopy, it is shown that ELC of nc-Si has very low energy density threshold of 95 mJ/cm² for complete melting, and that re-crystallization to large grains of ∼ 2 μm can be achieved by multi-shot irradiation at an energy density as low as 260 mJ/cm² when using nc-Si when compared to 340 mJ/cm² for the ELC of a-Si films.     

Band alignment of Cd(1 − x)ZnxS produced by spray pyrolysis method

Cd_(1 − x)Zn_xS thin films have been grown on glass substrates by the spray pyrolysis method using CdCl₂ (0.05 M), ZnCl₂ (0.05 M) and H₂NCSNH₂ (0.05 M) solutions and a substrate temperature of 260 °C. The energy band gap, which depends on the mole fraction × in the spray solution used for preparing the Cd_(1 − x)Zn_xS thin films, was determined. The energy band gaps of CdS and ZnS were determined from absorbance measurements in the visible range as 2.445 eV and 3.75 eV, respectively, using Tauc theory. On the other hand, the values calculated using Elliott-Toyozawa theory were 2.486 eV and 3.87 eV, respectively. The exciton binding energies of Cd_0.8Zn_0.2S and ZnS determined using Elliott-Toyozawa theory were 38 meV and 40 meV, respectively. X-ray diffraction results showed that the Cd_(1 − x)Zn_xS thin films formed were polycrystalline with hexagonal grain structure. Atomic force microscopy studies showed that the surface roughness of the Cd_(1 − x)Zn_xS thin films was about 50 nm. Grain sizes of the Cd_(1 − x)Zn_xS thin films varied between 100 and 760 nm.     

Laser-induced amorphization and crystallization on Se80Te20−xPbx thin films

Thin films of glassy alloys of a-Se₈₀Te_20−xPb_x (x=2, 6 and 10) was crystallized in a specially designed sample holder under a vacuum of 10⁻² Pa. The amorphous and crystallized films were induced by pulse laser (wavelength: 337.1 nm, frequency: 10 Hz, pulse duration: 4 ns and pulse energy: 0.963 mJ). After laser irradiation on amorphous and crystalline films: optical band gaps were measured. Crystallization and amorphization of chalcogenide films is accompanied by the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. This change in the optical band gap may be due to the increase in the grain size and the reduction in the disorder of the system.     

Laser-induced damage threshold at different wavelengths of Ta2O5 films annealed over a wide temperature range

The laser-induced damage of Ta₂O₅ films annealed at a wide range of temperature (473-1273 K) at the laser wavelengths of 1064 and 355 nm was investigated. The relations between microstructure, optical properties, chemical composition, absorption and laser-induced damage threshold (LIDT) were studied. The dependence of a damage mechanism on laser wavelengths was discussed. It was found that the LIDT either at 1064 or 355 nm first increased and then decreased with increase of annealing temperature. The LIDT at 1064 nm was influenced by the substoichiometric defects, structural defects and thermal diffusion, whereas at 355 nm it was affected mainly by the intrinsic absorption and structural defects. Both the maximum LIDT at the two wavelengths were obtained at the annealing temperature of 873 K, which could be attributed to the lowest defect density in films.     

Microstructure and optical properties of MgxZn1−xO thin films grown by means of pulsed laser deposition

The single-phase epitaxial Mg_xZn_1−xO (0.4 < x < 0.9) alloy films with wide band gap have been deposited on cubic LaAlO₃ (LAO) (100) substrates by pulsed laser deposition (PLD). X-ray diffraction measurement and TEM photograph indicate that the cubic phase could be stabilized up to Zn content about 0.6 without any phase separation. Films and substrates have a good heteroepitaxial relationship of (100) _MgxZn1−xO||(100)_LAO (out-of-plane) and (011)_MgxZn1−xO||(010)_LAO (in-plane)_. The lattice parameters a of Mg_xZn_1−xO films increase almost linearly with increasing ZnO composition, while the band gap energy of the materials increases from 5.17 to 5.27 eV by alloying with more MgO. The cross-section morphology reveals layer thickness of about 250-300 nm and AFM scan over a 30 μm × 30 μm area reveals a surface roughness R_a of about 100 nm.     

Investigation of damage threshold to TiO2 coatings at different laser wavelength and pulse duration

Laser-induced damages to TiO₂ single layers and TiO₂/SiO₂ high reflectors at laser wavelength of 1064 nm, 800 nm, 532 nm, and pulse width of 12 ns, 220 ps, 50 fs, 8 ns are investigated. All films are prepared by electron beam evaporation. The relations among microstructure, chemical composition, optical properties and laser-induced damage threshold (LIDT), have been researched. The dependence of damage mechanism on laser wavelength and pulse width is discussed. It is found that from 1064 nm to 532 nm, LIDT is mainly absorption related, which is determined by film's extinction coefficient and stoichiometric defects. The rapid decrease of LIDT at 800 nm is due to the pulse width factor. TiO₂ coatings are mainly thermally by damaged at long pulse (τ ≥ 220 ps). The damage shows ablation feature at 50 fs.     

Plasma deposition of n-SiOx nanocrystalline thin film for enhancing the performance of silicon thin film solar cells

In this paper, we firstly optimized the properties of n-SiO_x nanocrystalline thin film through tuning deposition parameters by plasma enhanced chemical vapor deposition, so that we can actively control the properties of materials obtained. Secondly, we proposed using n-SiO_x/Al as back reflector for amorphous silicon (a-Si:H) solar cells. Compared to Al single-layer as back reflector, adding an n-SiO_x layer into the back reflector could improve the solar cell performance, which not only enhances the short circuit current density by an improvement of spectral response in the wavelength range of 550-750 nm, but also improves the open circuit voltage. With an optimized n-SiO_x/Al back reflector, a-Si:H solar cells with an intrinsic layer thickness of 270 nm show 13.1% enhancement in efficiency. In addition, a-Si:H/μc-Si:H tandem solar cells with n-SiO_x as intermediate reflector were also researched. As a result, it evidently balanced the current matching between top and bottom cell.     

Excellent passivation effect of Cat-CVD SiNx/i-a-Si stack films on Si substrates

We have demonstrated that the surface recombination velocity can be lowered to as low as 1.3 cm/s for n-type c-Si wafers and to 9.0 cm/s for p-type wafers by using amorphous Si (a-Si) and Si nitride (SiN_x) stacked films prepared by catalytic chemical vapor deposition (Cat-CVD). These values are much lower than those of c-Si wafers passivated by same stacked structures formed by low-damage remote plasma-enhanced CVD (PECVD). It is revealed that Cat-CVD a-Si insertion layers play an important role to improve interface quality, and also SiN_x films are also essential for reducing the surface recombination velocity down to such low levels.     

Study of non-isothermal kinetics, electrical and optical properties of (GaSeTe) films

Ternary Ga_xSe_86−xTe₁₄ amorphous films (x=15 and 36) were prepared by thermal evaporation. The results of differential scanning calorimetry (DSC) at different heating rates are reported and discussed. The glass transition activation energy, E_t, and the crystallization activation energy, E_c, were evaluated by measuring the heating rate dependence of the glass transition, crystallization onset and peak crystallization temperatures. The average calculated values of E_t and E_c are 140.29 and 97.89 kJ/mol, respectively. The electrical conductivity of amorphous Ga_xSe_86−xTe₁₄ thin films with different thickness has been measured in the temperature range (263.2-333.3 K) and this allows the effect of introducing a metallic impurity to be observed. It was observed that conductivity increases with increasing activation energy and with a lowering of the pre-exponential factor, which suggests the results can be explained in terms of hopping conduction. The optical constants of these films were determined by transmission and reflection measurements at normal incidence in the spectral range of 500-800 nm. The refractive index has anomalous behavior in the spectral range 400-500 nm. The refractive index dispersion can be fitted to a single oscillator model.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.