Investigation of size dependent structural and optical properties of thin films of CdSe quantum dots

Cadmium selenide (CdSe) quantum dots were grown on indium tin oxide substrate using wet chemical technique for possible application as light emitting devices. The structural, morphological and luminescence properties of the as deposited thin films of CdSe Q-dot have been investigated, using X-ray diffraction, transmission electron microscopy, atomic force microscopy and optical and luminescence spectroscopy. The quantum dots have been shown to deposit in an organized array on ITO/glass substrate. The as grown Q-dots exhibited size dependent blue shift in the absorption edge. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the nanocrystalline CdSe exhibits intense photoluminescence as compared to the large grained polycrystalline CdSe films.     

Structural morphology and magnetism of electroless-plated NiP films on a surface-modified Si substrate

NiP thin films were deposited on an H₃PO₄-etched Si substrate by means of electroless-plating. By varying the plating time, we were able to deposit NiP films with various thicknesses. Thickness effects upon the structural morphology and magnetic properties of the NiP films were investigated, and they can be comprehended with a model of the deposition mechanism. The results demonstrate that the etched Si surface contained groove-like microstructure which shaped the columnar structure inside the NiP films and favored the film deposition; this was hard to achieve with a smooth-surfaced Si substrate. The well-developed columnar structure resulted in a perpendicular anisotropy due to its vertical nature, which can become superior to the film's longitudinal anisotropy if the applied field is sufficient.     

Titanium dioxide thin films, their structure and its effect on their photoactivity and photocatalytic properties

Atomic Layer Deposition has been used to deposit titanium dioxide thin films on soda-lime glass substrates. A series of films with thicknesses from 2.6 to 260 nm has been created and the film structure has been studied with X-ray diffraction. It has been observed that at a reaction temperature of 350 °C, titanium dioxide thin films initially grow as anatase but after a certain thickness, growth continues as rutile. The photoactivity and photocatalytic activity of the films have been found to reach their maximum at a film thickness of 15 nm. At this thickness, the film structure shows a small fraction of rutile crystallites in a largely anatase matrix indicating that both crystal phases are necessary for the maximum activity.     

Long-range order of cylinders in diblock copolymer thin films using graphoepitaxy

Topographically patterned substrates are known to induce long-range lateral order in spherical diblock copolymers, but it is not clear that similar confinement will also order cylindrical diblock copolymers across the whole surface. The role of graphoepitaxial parameters including trough width and mesa height on the ordering process of cylindrical domains in diblock copolymers thin films is monitored in this study. The quantification of order was achieved by the calculation of an order parameter of the hexagonally packed cylinders. These results demonstrate that graphoepitaxy is an effective method to induce long-range order in cylindrical domain diblock copolymer systems. An increase in order was observed in samples prepared on the mesas and in troughs of widths up to 20 μm, and mesa heights greater than 1.0 but less than 5.0 L₀ The role of molecular weight on the kinetics of the ordering process of cylindrical domains in diblock copolymers thin films is also monitored in this study, where ordering is readily observed for lower molecular weight copolymers (number average molecular weight, M_n = 63,000), but not for larger copolymers (M_n = 230,000). The reduction of the rate of formation of long-range order is attributed to the impeded diffusion of higher molecular weight polymers. These results demonstrate that there will exist upper limits on the molecular weights of diblock copolymers that can be used to create nanoscale templates with long-range order, which also translates to an upper limit in pore size and spacing in these templates.     

Comparative study of surface morphology of copper phthalocyanine ultra thin films deposited on Si (111) native and RCA-cleaned substrates

Comparative study of substrate doping influence on surface morphology of 16-nm CuPc ultra-thin layers deposited on RCA-cleaned Si (111)/SiO₂ substrates was carried out. The structure and the morphology of thin films were investigated by X-ray photoelectron spectroscopy and atomic force microscopy. The investigations were aimed to provide information whether substrate doping type can be used as one of the parameters for engineering of the sensing layers structure. Atomic force microscopy images and results of photoemission experiments did not reveal any significant differences in morphology and surface chemistry between used substrates. Observed differences in surface morphology of organic overlayer could be caused by different substrate doping. The CuPc film grown on p-type RCA-Si (111) shows a compact network of densely packed crystallites, while the CuPc film deposited on n-type RCA-Si (111) reveals a slightly more open network of larger crystallites. These observations are confirmed by values of roughness, which is 0.97 nm and 1.47 nm for CuPc film on RCA-cleaned p- and n-type substrates, respectively. Results were compared with data obtained for similar 16-nm-thick CuPc layers deposited on n- and p-type Si (111) covered with native oxide. Good agreement between results of both studies was found out.     

Pulsed-laser deposition of textured cerium oxide thin films on glass substrates at room temperature

Cerium oxide thin films are deposited on glass by pulsed laser deposition at room temperature and characterized by X-ray diffraction and atomic force microscopy. The effects of ambient gas, rate of deposition and fluence on growth of films have been studied. The films grown in forming gas and with a high rate of deposition are polycrystalline and show preferential orientation along <011> direction with a roughness of ∼ 2 nm. Films prepared in oxygen have low crystallinity.     

Investigations on luminescence characteristics of Eu and Cr codoped BaAl2O4

Green phosphor BaAl₂O₄:Eu²⁺, Cr³⁺ with varying concentrations of Cr codoping were prepared by solid-state synthesis method. The synthesized compositions were characterized for their phase, morphology and crystallinity by powder X-ray diffraction, SEM and TEM techniques. Photoluminescence properties were investigated measuring PL and decay time. Broad band UV excited luminescence was observed for BaAl₂O₄:Eu²⁺, Cr³⁺ in the green region (λ_max = 500 nm) due to transitions from 4f⁶ 5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. The effect of Cr doping on crystalline quality, morphology and photoluminescence characteristics of prepared compositions was investigated.     

Influence of substrate doping on the surface chemistry and morphology of Copper Phthalocyanine ultra thin films on Si (111) substrates

16 nm thick Copper Phthalocyanine (CuPc) films were deposited at room temperature in Ultra High Vacuum onto “n” and “p” type doped Si(111) substrates covered with a native SiO₂ overlayer. Atomic Force Microscopy indicates that the two substrates are both atomically flat (0.15 nm root mean square roughness). Angle dependent X-ray photoemission spectroscopy shows that the thickness of the native SiO₂ over-layer is 0.8 nm (both for the “n” and “p” type Si substrate). Despite the identity of the substrate roughness, of the SiO₂ thickness, and of the CuPc film growth conditions, the organic films (made out of crystallites in the α-phase, as checked with X-ray Diffraction) grown on the “p” and “n” type substrate show clearly different morphological features (determined with Atomic Force Microscopy and Scanning Electron Microscopy measurements). While the CuPc film on “p” Si(111) shows a compact network of densely packed crystallites with sizes (along the substrate plane) ranging from 50 to 100 nm, the CuPc film on “n” Si(111) shows a slightly more open network of larger crystallites (with 75-150 nm size range). Accordingly, the CuPc film roughness is 0.67 nm and 1.15 nm for the “p” and “n” type substrate respectively. Due to the increased surface to volume effects (lower crystallite size) affecting the CuPc film on “p” Si(111), this film exhibits stronger interaction with oxygen and water vapor of the ambient air, as determined by photoemission spectroscopy experiments performed on samples as grown “in situ” and after prolonged (1 year) exposure to air.     

Damage mode tensile testing of thin gold films on polyimide substrates by X-ray diffraction and atomic force microscopy

In situ tensile testing has been performed on thin gold film, 320 nm thick, deposited on polyimide substrates. During the tensile testing, strain/stress measurements have been carried out by X-ray diffraction using the d-sin²ψ method. The X-ray stress analysis suggests crack formation in the films for stresses greater than 670 MPa. The surface of the deformed specimen observed by atomic force microscopy (AFM) exhibits both cracks and two types of straight-sided buckling patterns lying perpendicular to the tensile axis. These buckling patterns can have a symmetrical or asymmetrical shape. The evolution of these two kinds of buckling structures under tensile stress has been observed in situ by AFM and compared to X-ray stress data. The results indicate that symmetrical straight-sided buckling patterns are induced by the compressive stress during unloading, whereas the asymmetrical result from the delamination of the film during the tensile deformation.     

Growth of thin Ag films produced by radio frequency magnetron sputtering

Thin Ag films in the thickness range D = 14–320 nm were deposited by radio frequency magnetron sputtering on glass substrates at room temperature inside a vacuum chamber with base pressure of about 5 × 10^− 6 Pa. The growth of the films was studied via X-ray diffraction and atomic force microscopy experiments. The two techniques are complementary and give us the opportunity to study the surface roughness, the statistical distribution and the average value of the grain size, as well as the texture of the samples. It is shown that the film roughness increases negligibly within the first 60 atomic layers of growth. The thicker films (D 300 nm) develop a nanocrystalline structure with a root mean square roughness of about 2.5 nm. The grain size evolves linearly with the thickness from 9.4 nm at D = 54 nm to 31.6 nm at D = 320 nm.     

Specular beam intensity and surface morphology of BSCO films grown by MBE

Bi-Sr-Cu-O (BSCO) thin films have been epitaxially grown on cleaned SrTiO₃ (001) substrates by a sequentially shutter-controlled molecular beam epitaxy system using an oxygen radical beam. A spot intensity of specular beam in in-situ reflection high-energy electron diffraction (RHEED) was monitored during the atomic layer epitaxy. Atomic force microscopy (AFM) images of the epitaxial thin films were observed in the atmosphere at some oscillation points of the specular beam intensities. The chemical composition ratios of the films (about 100 Å) were determined from intensities of X-ray photoemission spectroscopy. The crystallinity of the films was measured by X-ray diffraction.

The amount of metal deposition corresponding to a half cycle of the intensity oscillation of the specular spot was found to be appropriate to form the flat surface. Characteristic islands were found at the surfaces covered with excess bismuth or excess copper atoms in the AFM image. The intrinsic modulated structure of the BSCO crystal was observed at the surface after the first copper deposition on Sr/Bi/SrTiO₃ in the RHEED pattern.     

Interfacial electron density profile in Nb/Si bilayer films: an X-ray reflectivity study

This article describes a systematic study of the nature of interfaces involved in a Nb layer deposited on Si (Nb-on-Si) and Si layer deposited on Nb (Si-on-Nb) bilayer films by using a UHV electron beam evaporation technique, having individual layer thickness of 35 and 100 Å each. By using Grazing angle X-ray reflectivity and adopting a proper modelling technique the electron density profile (EDP) as a function of depth has been determined in the samples. EDP determined in as-deposited 35 Å Nb and 35 Å Si bilayer films show that the width of Si-on-Nb and Nb-on-Si interfaces are 20 Å and 40 Å, respectively. The difference observed in the width of two interfaces is attributed to the different growth morphology of 35 Å Nb and 35 Å Si single-layer films as revealed by atomic force microscopy (AFM) investigations. EDP determined from measured XRR data for 100 Å Nb and 100 Å Si deposited bilayer film shows that the width of Si-on-Nb interface is 10 Å. This observed width is smaller than the similar interface in the case of samples having an individual layer thickness of 35 Å. The corresponding interface width of Nb-on-Si is found to be 45 Å and marginally more than the similar interface in the case of the 35 Å Nb/35 Å Si bilayer samples. AFM studies carried out on 100 Å Nb and Si layers deposited separately on float glass substrate indicate similar gross as well as subtle morphological features and cannot be attributed to the observed asymmetry in this case. The observed asymmetry in EDP of two interfaces in this case is due to the enhanced diffusion of Si into the formed metal layer relative to the diffusion into the already deposited metal layer.     

Effect of sol concentration on the structural, morphological, optical and photoluminescence properties of zirconia thin films

ZrO₂ thin films were deposited on quartz substrates from 10 wt.%, 20 wt.% and 40 wt.% solutions of Zirconium-n-butoxide in isopropanol by sol-gel dip-coating technique. Higher concentrated sols of 20 wt.% and 40 wt.% exhibited faster gelation, where as 10 wt.% sol remained stable for two months and films synthesized from this sol remained transparent and continuous even for 12 coatings. Ellipsometric study revealed that refractive index of the films increased with increase in sol concentration which is ascribed to the decrease in porosity. X-ray diffraction study showed that a tailoring of grain size from 7.9 to 39.2 nm is possible with increase in sol concentration. Atomic force microscopy studies showed a change in growth mode from vertical to lateral mode with increase in sol concentration. The film surface revealed positive skewness and high kurtosis values which make them favorable for tribological applications. The average optical transmittance in the visible region is highest (greater than 90%) for the film deposited from 10 wt.% sol. The optical band gap decreased from 5.74 to 5.62 eV with increase in the sol concentration. Photoluminescence (PL) spectra of the films exhibit an increase in the emission intensity with increase in sol concentration which substantiates better crystalline quality of the film deposited from 40 wt.% sol and increase in oxygen vacancies. The “Red shift” of the PL spectra with increase in sol concentration originates from the increase in the grain size with sol concentration which makes it suitable for generation of solid state lighting in light emitting diode.     

Structural characteristics and mechanical properties of Ti(Cr) films produced on Si substrate

A series of nanogranular Ti₉₀Cr₁₀ thin films have been fabricated by pulsed-laser deposition on Si substrates at different temperatures. The crystal structure and mechanical properties of these films were investigated. The X-ray diffraction and transmission electron microscope images with selected area diffraction showed that the structure of as-prepared films is dependent on film thickness and deposition temperature. It was found that the Ti₉₀Cr₁₀ films consisted of fine hexagonal close packed microstructure with columnar grains, while body close-packed cubic structure of Cr films are composed of irregular grains, meanwhile, a chromium disilicide (CrSi₂) layer formed in the interface between the substrate and Cr films which deposited at temperature of greater than 600 °C. The crystalline and columnar grains improved with an increase of the thickness of the films and an optimum microstructure is obtained under the present experimental condition of about 50 nm thickness and deposited temperature of 500 °C for Ti₉₀Cr₁₀ films. Deposited at 300 °C, the Ti₉₀Cr₁₀ films have hardness of 12.7 GPa and elastic modulus of 174.6 GPa. Improved to 600 °C the sample shows higher hardness of 13.1 GPa and higher elastic modulus of 183.2 GPa. Using Benjamin-Weaver model, adhesion shearing force can be calculated as 34.9 MPa for 300 °C Ti₉₀Cr₁₀ film while higher value of 44.4 MPa for higher temperature of 600 °C.     

Influence of deposition temperature on structure and morphology of nanostructured SnO2 films synthesized by pulsed laser deposition

Nanostructured tin oxide thin films were deposited on the Si (100) substrate using the pulsed laser deposition technique at different substrate temperatures (300, 450 and 600 °C) in an oxygen atmosphere. The structure and morphology of the as-deposited films indicate that the film crystallinity and surface topography are influenced by the deposition temperature by changing from an almost amorphous to crystalline microstructure and smoother topography at a higher substrate temperature. The photoluminescence measurement of the SnO₂ films shows three stable emission peaks centered at respective wavelengths of 591, 554 and 560 nm with increasing deposition temperature, contributed by the oxygen vacancies.     

Phase stabilization and microstructural studies of lead lanthanum titanate thin films

Thin ferroelectric films of PLTx (Pb_1−xLa_xTi_1−x/4O₃) have been prepared by a sol-gel spin coating process. As deposited films were thermally treated for crystallization and formation of perovskite structure. Characterization of these films by X-ray diffraction (XRD) have been carried out for various concentrations of La (x = 0.04, 0.08 and 0.12) on ITO coated corning glass substrates. For a better understanding of the crystallization mechanism, the investigations were carried out on films annealed at temperatures (350, 450, 550 and 650 °C). Characterization of these films by X-ray diffraction shows that the films annealed at 650 °C exhibit tetragonal phase with perovskite structure. Atomic force microscope (AFM) images are characterized by slight surface roughness with a uniform crack free, densely packed structure. Fourier transform infrared spectra (FTIR) studies of PLTx thin films (x = 0.08) deposited on Si substrates have been carried out to get more information about the phase stabilization.     

Effect of annealing temperature on the tribological behavior of ZnO films prepared by sol-gel method

The tribological behavior of zinc oxide (ZnO) films grown on glass and silicon (100) substrates by sol-gel method was investigated. Particularly, the as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature. Crystal structural and surface morphology of the films were measured by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). XRD patterns and AFM images indicated that the crystallinity and grain size of the films were enhanced and increased, respectively, with temperature. The tribological behavior of films was evaluated by sliding the ZnO films against a Si₃N₄ ball under 0.5 gf normal load using a reciprocating pin-on-plate tribo-tester. The wear tracks of the films were measured by AFM to quantify the wear resistance of the films. The results showed that the wear resistance of the films could be improved by the annealing process. The wear resistance of the films generally increased with annealing temperature. Specifically, the wear resistance of the films was significantly improved when the annealing temperature was higher than 550° C. The increase in the wear resistance is attributed to the increase in hardness and modulus of the film with annealing temperature.     

Effects of a ZnTe buffer layer on structural quality and morphology of CdTe epilayer grown on (001)GaAs by molecular beam epitaxy

The structural properties, crystalline quality and surface morphology of CdTe thin films without and with a ZnTe buffer layer grown on (001)GaAs by molecular beam epitaxy (MBE) have been studied. CdTe thin film directly prepared on GaAs substrate displays (111) orientation with an island growth mode, whereas the CdTe epilayers with a ZnTe buffer are (001)-oriented single-crystalline film with a two-dimensional (2D) growth mode. The morphology and surface root-mean-square (RMS) roughness of CdTe epilayers are also dramatically improved by using a ZnTe buffer. Furthermore, it is suggested that the high-temperature (HT) ZnTe buffer grown at 360 °C is more efficient for enhancing CdTe structural quality than the low-temperature (LT) one at 320 °C. The CdTe epilayer on the HT-ZnTe buffer shows a narrower full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (004) reflection and a smaller RMS roughness.     

InAs0.45P0.55/InP strained multiple quantum wells intermixed by inductively coupled plasma etching

The intermixing effect on InAs_0.45P_0.55/InP strained multiple quantum wells (SMQWs) by inductively coupled plasma (ICP) etching and rapid thermal annealing (RTA) is investigated. Experiments show that the process of ICP etching followed RTA induces the blue shift of low temperature photoluminescence (PL) peaks of QWs. With increasing etching depth, the PL intensities are firstly enhanced and then diminished. This phenomenon is attributed to the variation of surface roughness and microstructure transformation inside the QW structure during ICP processing.     

Nitrogen-doped p-type ZnO thin films and ZnO/ZnSe p-n heterojunctions grown on ZnSe substrate by radical beam gettering epitaxy

We investigate the p-type doping in ZnO prepared by the method of radical beam gettering epitaxy using NO gas as the oxygen source and nitrogen dopant. Secondary ion mass spectroscopy measurements demonstrate that N is incorporated into ZnO film in concentration of about 8 × 10¹⁸ cm^− 3. The hole concentration of the N-doped p-type ZnO films was between 1.4 × 10¹⁷ and 7.2 × 10¹⁷ cm^− 3, and the hole mobility was 0.9-1.2 cm²/Vs as demonstrated by Hall effect measurements. The emission peak of 3.312 eV is observed in the photoluminescence spectra at 4.2  of N-doped p-type ZnO films, probably neutral acceptor bound. The activation energy of the nitrogen acceptor was obtained by temperature-dependent Hall-effect measurement and equals about 145 meV. The p-n heterojunctions ZnO/ZnSe were grown on n-type ZnSe substrate and have a turn-on voltage of about 3.5 V.