Photoluminescence in amorphous (PbLa)TiO3 thin films deposited on different substrates

Pb_1−XLa_XTiO₃ thin films, (X=0.0; 13 and 0.27 mol%) were prepared by the polymeric precursor method. Thin films were deposited on Pt/Ti/SiO₂/Si (1 1 1), Si (1 0 0) and glass substrates by spin coating, and annealed in the 200-300°C range in an O₂ atmosphere. X-ray diffraction, scanning electron microscopy and atomic force microscopy were used for the microstructural characterization of the thin films. Photoluminescence (PL) at room temperature has been observed in thin films of (PbLa)TiO₃. The films deposited on Pt/Ti/SiO₂/Si substrates present PL intensity greater than those deposited on glass and silicon substrates. The intensity of PL in these thin films was found to be dependent on the thermal treatment and lanthanum molar concentration.     

Influence of annealing temperature on structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by sol-gel method

Thin films of Zn_1−xMn_xO (x=0.01) diluted magnetic semiconductor were prepared on Si (1 0 0) substrates by the sol-gel method. The influence of annealing temperature on the structural, optical and magnetic properties was studied by X-ray diffraction (XRD), atom force microscopy (AFM), photoluminescence (PL) and SQUID magnetometer (MPMS, Quantum Design). The XRD spectrum shows that all the films are single crystalline with (0 0 2) preferential orientation along c-axis, indicating there are not any secondary phases. The atomic force microscopy images show the surfaces morphologies change greatly with an increase in annealing temperature. PL spectra reveal that the films marginally shift the near band-edge (NBE) position due to stress. The magnetic measurements of the films using SQUID clearly indicate the room temperature ferromagnetic behavior, and the Curie temperature of the samples is above room temperature. X-ray photoelectron spectroscopy (XPS) patterns suggest that Mn²⁺ ions were successfully incorporated into the lattice position of Zn²⁺ ions in ZnO host. It is also found that the post-annealing treatment can affect the ferromagnetic behavior of the films effectively.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.     

Influence of deposition temperature on the structural and morphological properties of Be3N2 thin films grown by reactive laser ablation

Be₃N₂ thin films have been grown on Si(1 1 1) substrates using the pulsed laser deposition method at different substrate temperatures: room temperature (RT), 200 °C, 400 °C, 600 °C and 700 °C. Additionally, two samples were deposited at RT and were annealed after deposition in situ at 600 °C and 700 °C. In order to obtain the stoichiometry of the samples, they have been characterized in situ by X-ray photoelectron (XPS) and reflection electron energy loss spectroscopy (REELS). The influence of the substrate temperature on the morphological and structural properties of the films was investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The results show that all prepared films presented the Be₃N₂ stoichiometry. Formation of whiskers with diameters of 100-200 nm appears at the surface of the films prepared with a substrate temperature of 600 °C or 700 °C. However, the samples grown at RT and annealed at 600 °C or 700 °C do not show whiskers on the surface. The average root mean square (RMS) roughness and the average grain size of the samples grown with respect the substrate temperature is presented. The films grown with a substrate temperature between the room temperature to 400 °C, and the sample annealed in situ at 600 °C were amorphous; while the αBe₃N₂ phase was presented on the samples with a substrate temperature of 600 °C, 700 °C and that deposited with the substrate at RT and annealed in situ at 700 °C.     

In-plane magnetization with high coercivity in terbium iron garnet thin films deposited on Pt/Si substrate by PLD

We report on the growth of terbium iron garnet (TbIG, Tb₃Fe₅O₁₂) thin films having anomalously large coercivity and in-plane easy axis of magnetization. The TbIG thin films were prepared at room temperature (RT) on Pt/Si(1 0 0) substrates by pulsed laser deposition technique. The films deposited at RT were X-ray amorphous and do not show any magnetic order. Annealing of the RT deposited film at 900 °C resulted into fully textured (532) TbIG film. Atomic force microscopy and cross-sectional scanning electron microscopy studies of the TbIG films showed good surface quality with an average surface roughness of 5.0 nm and thickness of about 300 nm, respectively. The M-H loops measured at 20 K for TbIG films, exhibit about an order of magnitude enhancement in the coercivity value (H_c) than the single crystal. In-plane and out-of-plane M-H loops revealed that the easy axis of the magnetization lies within the film’s plane. In-plane magnetization combining with large H_c value of the TbIG thin film may be of scientific interest for the possible applications.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Fabrication and characterization of Au/SiO2 nanocomposite films

Au/SiO₂ nanocomposite films were prepared by radio frequency sputtering technique and annealing. The above nanocomposite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The surface of the nanocomposite films was uniform with the particle diameter of 100-300 nm. The size of Au crystallites increased on increasing annealing time. The luminescent behavior of the nanocomposite films was characterized by photoluminescence (PL) with different excitation wavelengths. Two emission peaks at around 525 nm and 560 nm were observed with the excitation wavelength at 325 nm. An intensive emission peak at around 325 nm was observed with the excitation wavelength at 250 nm, which is related to the defective structure of the amorphous SiO₂ layer because of oxygen deficiency, and could be applied to many fields, such as ultraviolet laser and ultraviolet detector.     

Structural and photoluminescence properties of Si nanoclusters obtained by ion implantation into Si3N4 films

In the present paper we report structural and photoluminescence (PL) results from samples obtained by Si implantation into stoichiometric silicon nitride (Si₃N₄) films. The Si excess was introduced in the matrix by 170 keV Si implantation performed at different temperatures with a fluence of Φ=1×10¹⁷ Si/cm². The annealing temperature was varied between 350 and 900 °C in order to form the Si precipitates. PL measurements, with a 488 nm Ar laser as an excitation source, show two superimposed broad PL bands centered around 760 and 900 nm. The maximum PL yield is achieved for the samples annealed at 475 °C. Transmission electron microscopy (TEM) measurements show the formation of amorphous nanoclusters and their evolution with the annealing temperature.     

A combinatorial approach to the discovery and optimization of YCa4O(BO3)3-based luminescent materials

Thin films of YCa₄O(BO₃)₃ (YCOB)-based new luminescent materials were explored by the combinatorial pulsed laser deposition (PLD) method which enabled us to fabricate continuous composition spread film libraries. Strong red and green luminescence were found in the Y_1−xEu_xCOB (0 ≤ x ≤ 1), (YEuCOB) and Y_1−yTb_yCOB (0 ≤ y ≤ 1) (YTbCOB) films, respectively. The film libraries were characterized by photoluminescence (PL), PL decay, an electron-probe microanalyzer and an electron diffraction analysis. The luminescent intensities in the amorphous film libraries strongly depended on the chemical composition of each rare-earth (RE) ion. The optimum concentration of rare-earth ions in YEuCOB and YTbCOB were experimentally determined to be Eu = 7.5% and Tb = 20-30%, respectively.     

Room temperature photoluminescence mechanism of SiOx film after annealing at different temperatures

SiO_x films were deposited on Si(1 0 0) substrates by evaporation of SiO powder. The samples were annealed from room-temperature (RT) to 1100 °C. After the samples were cooled down to RT, photoluminescence (PL) spectra from these samples were measured. It was found that when the annealing temperature Ta is not higher than 1000 °C, there is a PL centered at 620 nm, and with Ta increasing the intensity increases at first and then decreases when Ta is higher than 500 °C. When Ta is no less than 1000 °C another PL peak located at 720 nm appears. Combined with Raman and XRD spectra, we confirm that the latter PL is from Si nanocrystals that start to form when Ta is higher than 1000 °C. PL spectra for Ta<900 °C were discussed in detail and was attributed to defects in the matrix rather than from Si clusters.     

Pure UV photoluminescence from ZnO thin film by thermal retardation and using an amorphous SiO2 buffer layer

ZnO/SiO₂ thin films were fabricated on Si substrates by E-beam evaporation with thermal retardation. The as-prepared films were annealed for 2 h every 100 °C in the temperature range 400-800 °C under ambient air. The structural and optical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). The XRD analysis indicated that all ZnO thin films had a highly preferred orientation with the c-axis perpendicular to the substrate. From AFM images (AFM scan size is 1 μm×1 μm), the RMS roughnesses of the films were 3.82, 5.18, 3.65, 3.40 and 13.2 nm, respectively. PL measurements indicated that UV luminescence at only 374 nm was observed for all samples. The optical quality of the ZnO film was increased by thermal retardation and by using an amorphous SiO₂ buffer layer.     

Synthesis and materials properties of transparent conducting In2O3 films prepared by sol-gel-spin coating technique

Transparent conducting indium oxide (In₂O₃) thin films have been prepared on glass substrates by the simple sol-gel-spin coating technique. These films have been characterized by X-ray diffraction, resistivity and Hall effect measurements, optical transmission, scanning electron microscopy and atomic force microscopy for their structural, electrical, optical and morphological properties. The influence of spin parameters, number of coating, process temperature on the quality of In₂O₃ films are studied. In the operating range of deposition, 400-475 °C, all the films showed predominant (2 2 2) orientation. Films deposited at optimum process conditions exhibited a resistivity of 2×10⁻² Ω cm along with the average transmittance of about 80% in the visible spectral range (400-700 nm).     

Photoluminescence properties of powder and pulsed laser-deposited PbS nanoparticles in SiO2

Thin films of lead sulfide (PbS) nanoparticles embedded in an amorphous silica (SiO₂) host were grown on Si(1 0 0) substrates at different temperatures by the pulsed laser deposition (PLD) technique. Surface morphology and photoluminescence (PL) properties of samples were analyzed with scanning electron microscopy (SEM) and a 458 nm Ar⁺ laser, respectively. The PL data show a blue-shift from the normal emission at ∼3200 nm in PbS bulk to ∼560-700 nm in nanoparticulate PbS powders and thin films. Furthermore, the PL emission of the films was red-shifted from that of the powders at ∼560 to ∼660 nm. The blue-shifting of the emission wavelengths from 3200 to ∼560-700 nm is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles, while the red-shift between powders and thin-film PbS nanoparticles is speculated to be due to an increase in the defect concentration. The red-shift increased slightly with an increase in deposition temperature, which suggests that there has been a relative growth in particle sizes during the PLD of the films at higher temperatures. Generally, the PL emission of the powders was more intense than that of the films, although the intensity of some of the films was improved marginally by post-deposition annealing at 400 °C. This paper compares the PL properties of powder and pulsed laser-deposited thin films of PbS nanoparticles and the effects of deposition temperatures.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Highly conductive and transparent laser ablated nanostructured Al: ZnO thin films

Al doped ZnO thin films are prepared by pulsed laser deposition on quartz substrate at substrate temperature 873 K under a background oxygen pressure of 0.02 mbar. The films are systematically analyzed using X-ray diffraction, atomic force microscopy, micro-Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, z-scan and temperature-dependent electrical resistivity measurements in the temperature range 70-300 K. XRD patterns show that all the films are well crystallized with hexagonal wurtzite structure with preferred orientation along (0 0 2) plane. Particle size calculations based on XRD analysis show that all the films are nanocrystalline in nature with the size of the quantum dots ranging from 8 to 17 nm. The presence of high frequency E₂ mode and longitudinal optical A₁ (LO) modes in the Raman spectra suggest a hexagonal wurtzite structure for the films. AFM analysis reveals the agglomerated growth mode in the doped films and it reduces the nucleation barrier of ZnO by Al doping. The 1% Al doped ZnO film presents high transmittance of ∼75% in the visible and near infrared region and low dc electrical resistivity of 5.94 × 10⁻⁶ Ω m. PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. Nonlinear optical measurements using the z-scan technique shows optical limiting behavior for the 5% Al doped ZnO film.     

Influence of Al concentration on electrical, structural and optical properties of Al-As codoped p-ZnO thin films

We report the influence of Al concentration on electrical, structural, optical and morphological properties of Al-As codoped p-ZnO thin films using RF magnetron sputtering. Al-As codoped p-ZnO films with different Al concentrations were fabricated using As back diffusion from the GaAs substrate and sputtering Al₂O₃ mixed ZnO targets (1, 2 and 4 at%). The grown films were investigated by Hall effect measurement, X-ray diffraction (XRD), electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and atomic force microscopy (AFM) to study the electrical, structural, optical and morphological properties of the films. From the XRD, it was observed that both full-width at half-maximum (FWHM) and c-axis lattice constant have similar trends with respect to Al concentration. Hall measurements showed that the hole concentration increases as the Al concentration increases from 10¹⁵ to 10²⁰ cm⁻³. The increase in hole concentration upon codoping was supported by the red shift in the near-band-edge (NBE) emission observed from room temperature PL spectra. The proposed p-type mechanism due to As_Zn-2V_Zn complex was confirmed by low temperature PL and XPS analysis. The low FWHM, resistivity and peak-to-valley roughness observed by XRD, Hall measurement and AFM, respectively, suggest that 1 at% Al-doped ZnO:As film is the best codoped film.     

Spin-deposited nanocrystalline lithium ferrite thin films: Fabrication and characterization

Thin films of lithium ferrite (with general composition Li_0.5Fe_2.5O₄) were fabricated at low temperatures (up to 650 °C) by citrate-route using spin-deposition technique. Deposited films consisted of nanometer-sized grains as evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD patterns for annealed films showed broad peaks exhibiting a spinel phase. Size of nanocrystallites is estimated to be 3-7 nm using Scherrer's equation. Average grain size ∼8.5 nm is observed from TEM images of films annealed at 650 °C. Scanning electron micrographs show the formation of spherical aggregates of around 130 nm in diameter. The AFM analysis clearly evidenced the development of nanograins even at low (∼500 °C) annealing temperatures. Significant decrease in complex dielectric permittivity (∈′ − j∈″) with frequency is observed in the low frequency (100 Hz-1 MHz) as well as in X-band microwave frequency (8-12 GHz) region. ∈′ is found to be in the range of 15.7-33.9 in low frequency region, whereas in X-band microwave frequency region, it is found to lie between 3.9 and 4.9. Similarly, ∈″ is found to be 0.16-5.9 in the low frequency region, and 0.002-0.024 in the X-band microwave frequency region. Room temperature dc resistivity of these films is estimated to lie in the range of 10⁶-10⁸ Ω cm. These results strongly suggest that citrate-route processed nanocrystalline lithium ferrite thin films are promising candidates for monolithic microwave integrated circuits (MMICs).     

Epitaxial growth and optical characterization of compound semiconductor β-FeSi2 prepared by pulsed laser deposition

β-FeSi₂ thin films were prepared on FZ n-Si (1 1 1) substrates by pulsed laser deposition (PLD). The structural properties and crystallographic orientation of the films were investigated by X-ray diffraction (XRD) analysis. This indicates that β-FeSi₂/Si (2 0 2/2 2 0) and the single-crystalline β-FeSi₂ can be prepared using PLD. In photoluminescence (PL) measurements at 8 K detected by Ge detector, the PL spectra of the samples annealed at 900 °C for 1, 5, 8 and 20 h showed that the PL intensity of the A-band peak increased depending on annealing time in comparison with those of as-deposited samples. The intrinsic PL intensity of the A-band peak at 0.808 eV of the β-FeSi₂ from the 20-h-annealed sample was investigated for the first time by the PLD method detected by an InGaAs detector. This result has been confirmed by temperature dependence and excitation power density of the 20-h-annealed sample with the comparison of other defect-related band peaks of the sample. Cross-sectional scanning electron microscopy (SEM) observation was also performed and the thickness of the thin films was found to be at 75 nm for 20-h-annealed. The thermal diffusion for the epitaxial growth of β−FeSi₂/Si was observed when the compositional ratio of Fe to Si was around Fe:Si=1:2 for 20-h-annealed carried out by energy dispersive X-ray spectroscopy (EDX). We discussed high crystal quality of the epitaxial growth and optical characterization of β-FeSi₂ achieved after annealing at 900 °C for 20 h.     

The effect of different gas atmospheres on luminescent properties of pulsed laser ablated SrAl2O4:Eu,Dy thinfilms

SrAl₂O₄:Eu²⁺,Dy³⁺ thin films were grown on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique to investigate the effect of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological, photoluminescence (PL) and cathodoluminescence (CL) properties of the films. The films were ablated using a 248 nm KrF excimer laser. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and fluorescence spectrophotometry were used to characterize the thin films. Auger electron spectroscopy (AES) combined with CL spectroscopy were employed for the surface characterization and electron-beam induced degradation of the films. Better PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres with respect to those prepared in vacuum. A stable green emission peak at 515 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions were obtained with less intense peaks at 619 nm, which were attributed to transitions in Eu³⁺. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The CL intensity increased under prolonged electron bombardment during the removal of C due to electron stimulated surface chemical reactions (ESSCRs) on the surface of the SrAl₂O₄:Eu²⁺, Dy³⁺ thin films. The CL stabilized and stayed constant thereafter.     

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0 0 0 1) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800 °C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (0 0 2) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800 °C in N₂. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (V_O) and interstitial oxygen (O_i). The biggest ratio of the PL intensity of UV emission to that of visible emission (I_NBE/I_DLE) is observed from ZnO thin films annealed at 800 °C in N₂. Therefore, we suggest that annealing temperature of 800 °C and annealing atmosphere of N₂ are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.