Variable-range hopping in Fe70Pt30 catalyzed multi-walled carbon nanotubes film

Using low-pressure chemical vapour deposition (LPCVD), multi-walled carbon nanotubes (MWNTs) are grown on nanocrystalline Fe₇₀Pt₃₀ film. The Fe₇₀Pt₃₀ nanocrystalline film is deposited by vapour condensation technique. The size of the nanoparticles varies from 5–10 nm, as inferred from SEM micrographs of Fe₇₀Pt₃₀ film. SEM and TEM observations of as-grown CNTs film reveal that these are multi-walled and their diameter varies from 30–80 nm and length is of the order of several micrometers respectively. There is a structural change from ordinary geometry of CNTs to bamboo shaped as suggested by TEM image. Raman spectra shows sharp G and D bands with a higher intensity of G band showing the presence of graphitic nature of the nanotubes. An experimental study of the temperature dependence of electrical conductivity of MWNTs film is done over a wide temperature range from (293–4 K). The measured data gives a good fit to variable-range hopping (VRH) and the results are interpreted using Mott's (VRH) model. The conduction mechanism of the MWNTs film shows a crossover from the exp[ -(T_o/T)^1/4] law in the temperature range (293–110 K) to exp[ -(T_m/T)^1/3] in the low temperature range (110–4 K). This behaviour is attributed to temperature-induced transition from three-dimension (3D) to two-dimension (2D) VRH. Various Mott's parameters like characteristic temperature (T_m), density of states at Fermi level N(E_F), localization length (ξ), hopping distance (R), hopping energy (W) have also been calculated using above-mentioned model.     

Electrical conduction mechanism in Fe<Subscript>70</Subscript>Pd<Subscript>30</Subscript> catalyzed multi-walled carbon nanotubes

Carbon nanotubes (CNTs) are synthesized by the catalytic decomposition of acetylene using low pressure chemical vapour deposition method (LPCVD) at 800 °C and at a chamber pressure of 10 Torr over a supported catalyst film of Fe₇₀Pd₃₀. Morphology of these CNTs is studied using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). From HRTEM image of these multi-walled carbon nanotubes (MWNTs), it is clear that these MWNTs do not possess a co-axial cylindrical structure, but are composed of imperfect and broken graphite cylinders of different sizes. The average diameter and length of the nanotubes varies between 20–70 nm and 5–60 μm respectively. Electrical transport measurements of these MWNTs are studied over a temperature range of 298–4.2 K. The results have been interpreted in terms of variable-range hopping (VRH) over the entire temperature range of 298–4.2 K. Three-dimensional variable-range hopping (VRH) is suggested for the temperature range (298–125 K), while two-dimensional VRH is observed for the temperature range (125–4.2 K).     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.     

Structural and electrical properties of fluorine doped tin oxide films prepared by spray-pyrolysis technique

Fluorine doped SnO₂ films have been successfully prepared at optimized substrate temperature of 723 K by spray pyrolysis technique. The XRD analysis confirmed that films deposited with F/Sn ratio of 0.05 showed a partial amorphous nature whereas films deposited with F/Sn = 0.10 exhibited tetragonal structure (2 0 0) as the preferred orientation and polycrystalline structure. The lattice constants were found to be a = 0.4750 and c = 0.3197 nm. The theoretically constructed XRD pattern for SnO₂ was used to compare with experimental pattern, the difference between them is discussed. By using SEM analysis, the surface morphology of the films was observed as an effect of the variation of F/Sn ratio. At low temperature, the mobility due to lattice, polar, impurity, grain boundary and neutral scattering was estimated for SnO₂ and the possible scattering mechanisms were assigned to SnO₂:F films using experimentally obtained electrical data. The Mott parameters were determined by applying variable range hopping (VRH) conduction mechanism for SnO₂:F films (F/Sn = 0.05) where band conduction mechanism shifted to VRH conduction at below about 250 K.     

Non-adiabatic polaron hopping conduction in CaMn1−xCrxO3 (0?x?0.3)

DC electrical conductivity (σ_dc) of electron-doped antiferromagnetic CaMn_1−xCr_xO₃ (0?x?0.3) has been discussed elaborately in the light of polaron hopping conduction. The increase in Cr doping concentration increases the conductivity and decreases the activation energy. Non-adiabatic polaron hopping conduction is observed in all the manganites at high temperatures. The analysis of σ_dc data shows that small polarons are formed at lower concentrations (?5%) of Cr doping and undoped samples. However, large polarons are materialized at higher doping (?10%) concentrations. This is consistent with the fact that doped Cr³⁺ has larger ionic size compared to that of Mn⁴⁺. Again, strong electron-phonon (e-ph) interaction is perceived in undoped and 5% Cr-doped samples but not in manganites with larger doping concentration. This also confirms the formation of larger polarons with the increase of x. Mott's variable range hopping (VRH) model can elucidate the dc conductivity at very low temperatures. It has been detected that single phonon-assisted hopping is responsible for the dc conduction in the Cr-doped CaMnO₃ manganites.     

Aluminium-induced crystallization of amorphous silicon films deposited by DC magnetron sputtering on glasses

Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by thermal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 °C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuum-annealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evidence that crystallization is initiated at 450 °C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 °C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC).     

Compositional dependence of the electrical conductivity of calcium vanadate glassy semiconductors

Scanning electron microscopy (SEM), X- ray diffraction (XRD), density (d), oxygen molar volume (V_m) and dc conductivity of different compositions of calcium vanadate glasses are reported. SEM exhibits a surface without any presence of a microstructure which is a characteristic of the amorphous phase. The overall features of these XRD curves confirm the amorphous nature of the present glasses. Density was observed to decrease with an increase in V₂O₅ content. The experimental results were analyzed with reference to theoretical models existing in the literature. It has been observed that the high-temperature conductivity data are consistent with Mott's nearest-neighbor hopping model. However, both Mott variable-range hopping (VRH) and Greaves intermediate range hopping models are found to be applicable. The hopping at high temperatures in the calcium vanadate glasses occurs by non-adiabatic process in contrast to the vanadate glasses formed with conventional network formers. The hopping model of Schnakenberg can predict the temperature dependence of the conductivity data. The percolation model of Triberis and Friedman applied to the small polaron hopping (SPH) regime is also consistent with data. The various model parameters such as density of states, hopping energy, etc., obtained from the best fits were found to be consistent with the glass compositions.     

Influence of substrate crystallography on the room temperature synthesis of AlN thin films by reactive sputtering

A pulsed DC reactive ion beam sputtering system has been used to synthesize aluminium nitride (AlN) thin films at room temperature by reactive sputtering. After systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on silicon and Au/Si substrates with an Al target under a N₂/(N₂ + Ar) gas flow ratio of 55%, 2 mTorr processing pressure and keeping the temperature of the substrate holder at room temperature. The crystalline quality of the AlN layer as well as the influence of the substrate crystallography on the AlN orientation has been characterized by high-resolution X-ray diffraction (HR-XRD). Best ω-FWHM (Full Width at Half Maximum) values of the (0 0 0 2) reflection rocking curve in the 1 μm thick AlN layers are 1.3°. Atomic Force Microscopy (AFM) measurements have been used to study the surface morphology of the AlN layer and Transmission Electron Microscopy (TEM) measurements to investigate the AlN/substrate interaction. AlN grew off-axis from the Si substrate but on-axis to the surface normal. When the AlN thin film is deposited on top of an Au layer, it grows along the [0 0 0 1] direction but showing a two-domain structure with two in-plane orientations rotated 30° between them.     

Improvement in electrical and photovoltaic properties of a-Si/c-Si heterojunction with slanted nano-columnar amorphous silicon thin films for photovoltaic applications

The flat a-Si and slanted nanocolumnar (S-nC) a-Si thin films were prepared on c-Si and corning glass substrates by e-beam physical vapor deposition (EB-PVD) technique. The structural properties of all the grown thin films were determined by X-Ray Diffraction (XRD) analysis and Raman spectroscopy. Surface and cross-sectional morphology of a-Si/c-Si and S-nC a-Si/c-Si heterojunctions were investigated by Field Emission Scanning Electron Microscopy (FE-SEM). Sculptured thin films demonstrate potential for significant nanoscale applications in the area of thin film technology. The electrical and photovoltaic properties of these heterojunctions have been investigated by means of dc current–voltage (I–V) measurements at room temperature in dark and light conditions. The S-nC STFs' performance has been found to be improvable on changing the morphology of the thin film. We have found that, the porous morphology of this structure improves the photosensitivity features in photovoltaic devices and solar cell technology. We gained a high open voltage value, such as 900 mV in S-nC a-Si/c-Si thin film, without any doping process.     

AlCl3-induced crystallization of amorphous silicon thin films

It is reported that the direct contact between Al and amorphous silicon (a-Si) enhances the crystallization of a-Si films. But the polycrystalline silicon (poly-Si) films crystallized by the direct contact of Al metal film suffer the problems of rough surface. In our study, we utilized the AlCl₃ vapor during the a-Si films deposition instead of Al metal film to enhance crystallization. X-ray diffraction (XRD) shows that the AlCl₃ vapor so successfully enhanced the crystallization of a-Si films that the crystallization was completed in 5 h at 540 °C. And the orientation of the poly-Si film deposited with AlCl₃ vapor is much more random than that of annealed with Al metal under layer. But the average grain size is much larger than that. Moreover, the surface of the AlCl₃-induced crystallized poly-Si film was much smoother than that of the Al-induced poly-Si film. The Al and Cl incorporation into the poly-Si film was confirmed using X-ray photoelectron spectroscopy (XPS) and found that the quantity of Al and Cl incorporated into the Si film was below the detection limit of XPS.     

Excitonic photoluminescence characteristics of amorphous silicon nanoparticles embedded in silicon nitride film

We have investigated the photoluminescence (PL) properties of amorphous silicon nanoparticles (a-Si NPs) embedded in silicon nitride film (Si-in-SiN_x) grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The PL spectrum of the film exhibits a broad band constituted of two Gaussian components. From photoluminescence excitation (PLE) measurements, it is elucidated that the two PL bands are associated with the a-Si NPs and the silicon nitride matrix surrounding a-Si NPs, respectively. The existence of Stokes shift between PL and absorption edge indicates that radiative recombination of carriers occurs in the states at the surface of the Si NPs, whereas their generation takes place in the a-Si NPs cores and the silicon nitride matrix, respectively. The visible PL of the film originates from the radiative recombination of excitons trapped in the surface states. At decreasing excitation energy (E_ex), the PL peak energy was found to be redshifted, accompanied by a narrowing of the bandwidth. These results are explained by surface exciton recombination model taking into account there existing a size distribution of a-Si NPs in the silicon nitride matrix.     

Structural and optical properties of amorphous hydrogenated silicon carbonitride films produced by PECVD

Amorphous hydrogenated silicon carbonitride thin films (a-Si:C:N:H), deposited by plasma enhanced chemical vapour deposition (PECVD) using hexamethyldisilazane (HMDSN) as monomer and Ar as feed gas, have been investigated for their structural and optical properties as a function of the deposition RF plasma power, in the range of 100-300 W. The films have been analysed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-vis-NIR spectrophotometry and atomic force microscopy (AFM). From the analysis of the FT-IR spectra it results that the films become more amorphous and inorganic as RF plasma power increases. The incorporation of oxygen in the deposited layers, mainly due to the atmospheric attack, has been evaluated by XPS and FT-IR spectroscopy. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow to descrive the film absorption edge for interband transitions. A relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition RF plasma power has been investigated. The reduction of the optical energy gap from 3.85 to 3.69 eV and the broadening of the optical absorption tail with RF plasma power increasing from 100 to 300 W are ascribed to the growth of structural disorder, while the increase of the refractive index, evaluated at 630 nm, is attributed to a slight densification of the film. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become rougher when RF plasma power increases. The wettability of the film has been studied and related to the chemical composition and to the morphology of the deposited layers.     

Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromagnetic insulating (FMI) Ln_0.85Ca_0.15MnO₃ (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θ_D/2, θ_D being the Debye temperature) dc conductivity (σ_dc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σ_dc data at low temperature (T<θ_D/2).     

The effect of etching time on the CdZnTe surface

The surface quality of CdZnTe plays an important role in the performance of sensors based on this material. In this paper the effect of chemical etching on Cd_0.9Zn_0.1Te sensor performance was examined. Sample surfaces were treated with the same concentration 2% Br-MeOH for different etching times (30 s, 2, 4, 6, 8 min). The surfaces were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and I-V Measurement. These results demonstrate that the best surface quality can be obtained by chemical etching for 30 s. Under these experimental conditions, the surface composition Te/Cd + Zn approaches 1, the roughness is lower than 3 nm, and the leakage current shows a value lower than 10 nA.     

Electronic transport mechanisms in tetraphenyleprophyrin thin films

Thermally-evaporated thin films of tetraphenylporphyrin, TPP, with thickness range from (175 to 735) nm had been prepared. Annealing temperatures ranging from (273–473) K do not influence the amorphous structure of these films. The influence of environmental conditions: film thickness, temperature and frequency on the electrical properties of TPP thin films had been reported. It was found that dc conductivity increases with increasing temperature and film thickness. The extrinsic conduction mechanism is operating in temperature range of (293–380) K with activation energy of 0.13 eV. The intrinsic one is in temperatures >380 K via phonon assisted hopping of small polaron with activation energy of 0.855 eV. The ac electrical conductivity and dielectric relaxation in the temperature range (293–473) K and in frequency range (0.1–100) kHz had been also studied. It had been shown that theoretical curves generated from correlated barrier hopping (CBH) model gives the best fitting with experimental results. Analysis of these results proved that conduction occurs at low temperatures (300–370) K by phonon assisted hopping between localized states and it is performed by single polaron hopping process at higher temperatures. The temperature and frequency dependence of both the real and imaginary parts of dielectric constant had been reported.     

Half Ca-doped LaMnO3 films on (0 0 1) SLAO and STO substrate studied by magnetization and transport measurements

In this study, La_0.5Ca_0.5MnO₃ (LCMO) films, at the boundary between ferromagnetic metallic and charge-ordered antiferromagnetic insulator according to the bulk phase diagram, were epitaxially grown on (0 0 1) SrTiO₃ (STO) and SrLaAlO₄ (SLAO) substrates by pulsed laser deposition technique. The films were analyzed by X-ray diffraction, magnetization and magnetoresistance measurements. A considerably higher magnetization was measured for 290-nm-thick film on SLAO substrate compared to the film on STO substrate, although both films have the same chemical composition, thickness and epitaxial orientation. The film on SLAO shows a metal-insulator (MI) transition, which occurs at higher temperatures with increasing applied magnetic field, whereas only insulating behavior was observed for the 290-nm-thick film on STO except for the highest applied magnetic field (7 T). In addition, transport measurements were performed and analyzed by Mott's variable range hopping (VRH) model to correlate the resistivity of the films with the Jahn-Teller strain (ε_J−T) in the structure.     

Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films

Ge thin films with a thickness of about 110 nm have been deposited by electron beam evaporation of 99.999% pure Ge powder and annealed in air at 100-500 °C for 2 h. Their optical, electrical and structural properties were studied as a function of annealing temperature. The films are amorphous below an annealing temperature of 400 °C as confirmed by XRD, FESEM and AFM. The films annealed at 400 and 450 °C exhibit X-ray diffraction pattern of Ge with cubic-F structure. The Raman spectrum of the as-deposited film exhibits peak at 298 cm⁻¹, which is left-shifted as compared to that for bulk Ge (i.e. 302 cm⁻¹), indicating nanostructure and quantum confinement in the as-deposited film. The Raman peak shifts further towards lower wavenumbers with annealing temperature. Optical band gap energy of amorphous Ge films changes from 1.1 eV with a substantial increase to ∼1.35 eV on crystallization at 400 and 450 °C and with an abrupt rise to 4.14 eV due to oxidation. The oxidation of Ge has been confirmed by FTIR analysis. The quantum confinement effects cause tailoring of optical band gap energy of Ge thin films making them better absorber of photons for their applications in photo-detectors and solar cells. XRD, FESEM and AFM suggest that the deposited Ge films are composed of nanoparticles in the range of 8-20 nm. The initial surface RMS roughness measured with AFM is 9.56 nm which rises to 12.25 nm with the increase of annealing temperature in the amorphous phase, but reduces to 6.57 nm due to orderedness of the atoms at the surface when crystallization takes place. Electrical resistivity measured as a function of annealing temperature is found to reduce from 460 to 240 Ω-cm in the amorphous phase but drops suddenly to 250 Ω-cm with crystallization at 450 °C. The film shows a steep rise in resistivity to about 22.7 KΩ-cm at 500 °C due to oxidation. RMS roughness and resistivity show almost opposite trends with annealing in the amorphous phase.     

Electrical properties of nanocrystalline magnetite with large non-stoichiometry, near Verwey transition

DC electrical measurements were carried out on compacted powders of magnetite with an average particle diameter of 50 nm over the temperature range 10-300 K. The non-stoichiometry was estimated from Mossbauer spectroscopy analysis. High-resolution X-ray diffraction studies in the temperature range 93-300 K did not show any phase transition. There was a drastic change in resistivity around 80 K but no discontinuity thereof. Electrical resistivity vs. temperature data were analysed on the basis of Mott's small polaron and variable-range hopping models, respectively. The Verwey temperature as estimated from this analysis was 93 K. From voltage-current characteristics it was concluded that there was a small intrinsic gap at the Fermi level above the transition temperature and the same increased drastically below the transition temperature. This was ascribed to a transition from short-range order to long-range order as the temperature was lowered.     

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

Nanostructured sapphire vicinal surfaces as templates for the growth of self-organized oxide nanostructures

Vicinal substrates of sapphire with miscut angle of 10° from the (0 0 1) planes towards the [1 1 0] direction have been annealed in air in the range from 1000 to 1500 °C. The behaviour of these surfaces has been characterized as a function of the temperature and the thermal treatment time by Atomic Force Microscopy observations. A thermal treatment at 1250 °C allows to stabilize a surface made of periodically spaced nanosized step-bunches. Such stepped surfaces were used as template to grow self-patterned epitaxial oxide nanoparticles by thermal annealing of yttria-stabilized zirconia thin films produced by sol-gel dip-coating. Grazing Incidence Small Angle X-ray Scattering and High-Resolution Transmission Electron Microscopy were used to study the morphology of the nanoparticles and their epitaxial relationships with the substrate.