Tuning the cathodoluminescence of porous silicon films

We have obtained intense cathodoluminescence (CL) emission from electron beam modified porous silicon films by excitation with electrons with kinetic energies below 2 keV. Two types of CL emissions were observed, a stable one and a non-stable one. The first type is obtained in well-oxidized samples and is characterized by a spectral peak that is red shifted with respect to the photoluminescence (PL) peak. The physically interesting and technologically promising CL is however the CL that correlates closely with the PL. Tuning of this CL emission was achieved by controlling the average size of the nanostructure thus showing that the origin of this CL emission is associated with the quantum confinement and the surface chemistry effects that are known to exist in the porous silicon system. We also found that the electron bombardment causes microscale morphological modifications of the films, but the nanoscale features appear to be unchanged. The structural changes are manifested by the increase in the density of the nanoparticles which explains the significant enhancement of the PL that follows the electron irradiation.     

Structural and morphological study of ZnO thin films electrodeposited on n-type silicon

In this work, we report on the electrodeposition of ZnO thin films on n-Si (1 0 0) and glass substrates. The influence of the deposition time on the morphology of ZnO thin films was investigated. The ZnO thin films were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDS) and scanning electron microscopy (SEM). The results show a variation of ZnO texture from main (0 0 2) at 10 min to totally (1 0 1) at 15 min deposition time. The photoluminescence (PL) studies show that both UV (∼382 nm) and blue (∼432 nm) luminescences are the main emissions for the electrodeposited ZnO films. In addition, the film grown at 15 min indicates an evident decrease of the yellow-green (∼520 nm) emission band comparing with that of 10 min. Finally, transmittance spectra show a high transmission value up to 85% in the visible wavelength range. Such results would be very interesting for solar cells applications.     

Structural evolution of single-layer films during deposition of silicon on silver: a first-principles study

In the quest for the construction of silicene, the silicon analogue of graphene, recent experimental studies have identified a number of distinct ultrathin Si over-layer structures on a Ag(111) surface. Here we use first-principles calculations to probe associated atomic-scale mechanisms that can give rise to this rich behavior of Si wetting layers. We find that the interaction between the Si film and the Ag substrate, neither too strong nor too weak, combined with the possibility of buckling, allows for the incorporation of a number of excess Si adatoms in continuous overlayers with a honeycomb network topology. Depending on the Si coverage, we thus obtain a hierarchy of Si mono-atomic films, in agreement with experiments.     

Structural properties of polycrystalline silicon films formed by pulsed rapid thermal processing

A novel pulsed rapid thermal processing (PRTP) method has been used for realizing solid-phase crystallization of amorphous silicon films prepared by plasma-enhanced chemical vapour deposition.The microstructure and surface morphology of the crystallized films were investigated using x-ray diffraction and atomic force microscopy.The results indicate that PRTP is a suitable post-crystallization technique for fabricating large-area polycrystalline silicon films with good structural quality,such as large grain size,small lattice microstrain and smooth surface morphology on low-cost glass substrates.     

On the cathodoluminescence of thin films

An analysis is given of the voltage dependence of the cathodoluminescence of thin luminescent films, taking into consideration the surface recombination and diffusion processes of induced carriers. In the analysis Young's laws were used for the dissipation and range of incident cathode rays. The smooth maximum of experimental voltage response curves can be explained by strong surface recombination on the support.The author wishes to express his sincere thanks to Mr. Gy. Katona for his help in the computations.     

非晶微晶过渡区域硅薄膜的微区喇曼散射研究

通过改变氢气对硅烷气体的稀释程度,并保持其他的沉积参量不变,用等离子体增强化学气相沉积(PECVD)方法成功地制备出处于非晶微晶相变过渡区域的硅薄膜样品.测量了样品的室温光电导和暗电导,样品的光电性能优越,在50mW·cm-2的白光照射下,光电导和暗电导的比值达到106.在室温下用微区喇曼谱研究了薄膜的微结构特性,用高斯函数对喇曼谱进行了拟合分析.结果表明,在我们的样品制备条件下,当H2和SiH4的流量比R较小时,样品表现出典型的非晶硅薄膜的结构特性;随流量比R的增大,薄膜表现出两相结构,其中的微晶成分随 关键词： 非晶硅 薄膜 喇曼散射 微结构     

Structural study of Mn-doped ZnO films by TEM

The structural study of diluted magnetic semiconductors is important for interpreting the ferromagnetic behavior associated with the materials. In the present work, a series of low concentration Mn-doped ZnO thin films synthesized by pulsed laser deposition was studied by electron microscopy. All films show the wurtzite structure with (001) preferred growth orientation on the Si substrate. Electron diffraction experiments indicate the deterioration of the growth orientation in some areas of the films with increasing Mn concentration, and the existence of a secondary phase, of Mn₂O₃-type, in the films with larger Mn concentrations. High-resolution electron microscopy images confirm the existence of the secondary phase in the grain boundary of the Mn-doped ZnO phase. The magnetic properties of Mn-doped ZnO are discussed in relation to the structures of the films.     

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.     

Photo- and cathodoluminescence investigation of ZnO films

Photo- and cathodoluminescence spectra of ZnO films obtained by magnetron sputtering and annealed at different temperatures have been studied. The effect of irradiation in a scanning electron microscope on the cathodoluminescence spectra of these films is investigated. It is shown that even relatively small irradiation durations result in a noticeable change in signal intensity. Both signal enhancement and reduction can occur depending on the annealing temperature. Data obtained are explained by defect formation and rearrangement stimulated by electron irradiation.     

Structural study of supercooled liquid silicon

For many years, theoretical studies using model and ab initio potentials have predicated the existence of a liquid/liquid phase transition in silicon, based on a continuous change of the liquid A5 structure to A4. In contrast, we report here a quantitative analysis of data from high-energy X-ray diffraction measurements of containerlessly-processed supercooled liquid silicon that demonstrates that the fractions of regions with A5 and A4 order instead remain essentially constant with supercooling, but that the coherence length of the A5 order increases.     

Identification of various luminescence centers in CuI films by cathodoluminescence technique

CuI films are prepared by different techniques at room temperature. An expansion of band gap energy was observed for the thin films prepared by pulse laser deposition technique. Various luminescence centers are identified in CuI films and different mechanisms are proposed for cathodoluminescence at different centers.     

Enhanced ultraviolet-visible cathodoluminescence from Ar beam-induced nano-patterned silicon

Cathodoluminescence (CL) of 60 keV Ar ion beam-induced ripple patterned Si in a high resolution scanning electron microscope (HRSEM) shows strong room temperature (RT) luminescence bands compared to a nonpatterned or patterned recrystallized Si. Site-specific CL spectroscopy and imaging data indicate while the top and front surface of ripples contribute predominantly to the red and near infra-red (IR) emission at ∼650 and 750 nm respectively, the back surface contributes mostly to ultraviolet (UV) emission at . When the patterned sample is recrystallized after high temperature annealing, one observes a blue shift of the red peak to a yellow peak at . Nanostructured Si of varying sizes () located around amorphous/crystalline (a/c)-interface and beyond it appears to be probable origin of luminescence observed in the present study.     

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5-10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon-oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25-30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO₂) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.     

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO₂ thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.     

XRR and GISAXS study of silicon oxynitride films

Thin films of silicon oxynitride have largely replaced pure silicon oxide films as gate and tunnel oxide films in modern technology due to their superior properties in terms of efficiency as boron barrier, resistance to electrical stress and high dielectric strength. A single chamber system for plasma enhanced chemical vapor deposition was employed to deposit different films of SiO_xN_yH_z with 0.85 < x < 1.91. All films were previously characterized by Rutherford back-scattering and infrared spectroscopy to determine the stoichiometry and the presence of various bonding configurations of constituent atoms. We used X-ray reflectivity to determine the electron density profile across the depth, and we showed that the top layer is densified. Moreover, grazing incidence small-angle X-ray scattering was used to study inhomogeneities (clustering) in the films, and it is shown that plate-like inhomogeneities exist in the top and sphere-like particles at the bottom part of the film. Their shape and size depend on the stoichiometry of the films.     

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K₂S₂O₈ solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K₂S₂O₈ solution has been proposed.