Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

Features of Er3+ luminescence in fluorine-doped amorphous silicon dioxide fabricated by low-temperature plasma CVD

Spectra and photoluminescence kinetics of Er³⁺ ions incorporated in an amorphous host of fluorine-doped silicon dioxide synthesized by surface plasma chemical vapor deposition (SPCVD) are investigated at temperatures of 27–300 K. Luminescence is excited with an Ar⁺ laser at a wavelength of 514.5 nm and with a diode laser at a wavelength of 975 nm. Narrow and well-defined components of Stark manifolds with a small contribution from inhomogeneous broadening intrinsic to Er³⁺ ions in crystalline, but not amorphous, hosts are revealed and identified in photoluminescence spectra. The structure of the Stark manifolds is well-resolved at low temperatures. The presence of the well-resolved Stark structure in the spectra is indicative of stable anion complexes formating the Er³⁺ local neighborhood presumably associated with fluorine incorporation. This neighborhood is formed during the low temperature plasma chemical synthesis and is destroyed upon glass fusion.     

Laser induced modification on 40BaO–45B2O3–15TiO2 glass composition

This paper describes a study concerning the permanent modification of 40BaO–45B₂O₃–15TiO₂ (mol%) glass composition when irradiated by a continuous CO₂ laser beam. Three parameters of the permanent modification and crystallization processes induced by irradiation were studied: laser power, irradiation time and the presence of micrograins of β-BaB₂O₄ (β-BBO) crystalline phase on the glass sample surface prior to irradiation. The geometry of permanent deformation induced by irradiation depended mainly on the laser power and sample surface condition. Crystallization of the irradiated region was observed when laser power at around 0.72 W and an irradiation time of around 300 s were used. The structural characterization performed by micro-Raman and X-ray diffraction techniques showed the formation of only the BaTi(BO₃)₂ crystalline phase on the as-polished sample. On the other hand, when β-BBO microcrystals were homogeneously dispersed onto the glass surface, the formation of the β-BBO phase was induced. In this condition, the β-BBO becomes the main crystalline phase and presents a certain degree of texture.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

Spectroscopic ellipsometry study of nickel induced crystallization of a-Si

The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness.     

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(3 3 5) surface with a perfect distribution of monoatomic steps separated with (1 1 1) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4 monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (1 1 1) terraces.     

Synthesis,characterization and local changes induced by a cw argon laser in silica glass xerogel doped with transition metal ions

We report on the synthesis, characterization and local changes induced by a cw argon laser in SiO₂ xerogel doped with low metal ion concentrations, and obtained by the sol–gel process. The V, Mn and Cr metal ions were introduced into the SiO₂ matrix as inorganic salts and as an oxide in the case of Mo. The characterization of these materials showed that an amorphous system with high porosity was obtained; the metals were incorporated as ions with several different oxidation states, and the thermal diffusivity was around 10⁻⁹ m²/s. In these xerogel monoliths it is possible to induce local changes in the refractive index with the incidence of a focused laser beam at 488 nm, about 80 μm in diameter, with power intensities from 1 to 48 mW and an incidence time from 0.5 to 5 min. After this process, a high contrast in the transmitted light between the processed area and the homogenous matrix is obtained, resulting in an optical memory effect.     

Structural change of laser-irradiated Ge2Sb2Te5 films studied by electrical property measurement

Sheet resistance of laser-irradiated Ge₂Sb₂Te₅ thin films prepared by magnetron sputtering was measured by the four-point probe method. With increasing laser power the sheet resistance undergoes an abrupt drop from 10⁷ to 10³ Ω/□ at about 580 mW. The abrupt drop in resistance is due to the structural change from amorphous to crystalline state as revealed by X-ray diffraction (XRD) study of the samples around the abrupt change point. Crystallized dots were also formed in the amorphous Ge₂Sb₂Te₅ films by focused short pulse laser-irradiated, the resistivities at the crystallized dots and the non-crystallized area are 3.375 × 10^?3 and 2.725 Ω m, sheet resistance is 3.37 × 10⁴ and 2.725 × 10⁷ Ω/□ respectively, deduced from the I–V curves that is obtained by conductive atomic force microscope (C-AFM).     

Back contacted a-Si:H/c-Si heterostructure solar cells

This paper shows how the amorphous/crystalline silicon technology can be implemented in the interdigitated back contact solar cell design. We have fabricated rear-junction, backside contact cells in which both the emitter and the back contact are formed by amorphous/crystalline silicon heterostructure, and the grid-less textured front surface is passivated by a double layer of amorphous silicon and silicon nitride, which also provides an anti-reflection coating. The entire self-aligned mask and photolithography-free process is performed at temperature below 300 °C with the aid of one metallic mask to create the interdigitated pattern. An open circuit voltage of 687 mV has been measured on a 0.5 Ωcm p-type monocrystalline silicon wafer. On the other hand, several technological aspects that limit the fill factor (50%) and the short circuit current density (32 mA/cm²) still need improvement. We show that the uniformity of the deposited amorphous silicon layers is not influenced by the mask-assisted deposition process and that the alignment is feasible. Moreover, this paper investigates the photocurrent limiting factors by one-dimensional modeling and quantum efficiency measurements.     

Synthesis of CdSe nanoparticles immersed in an organic matrix of amylopectin by means of rf sputtering

CdSe nanoparticles immersed in an organic matrix of amylopectin have been synthesized by means of the rf magnetron-sputtering growth technique. The target was elaborated employing a mixture of CdSe and amylopectin high purity powders. X-ray diffraction (XRD) shows the cubic zinc blende crystalline phase of the nanoparticle and the amorphous structure of the amylopectin film. The average radius of the grains, calculated using the Scherrer formula in XRD peaks, is of the order of 8 nm. Results of resonant Raman scattering show that no confinement effects of optical phonons are observed in these CdSe nanoparticles. The band gap of CdSe nanoparticles measured from optical absorption measurements is 1.9±0.1 eV.     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.     

Silicon nanowire solar cells grown by PECVD

Silicon nanowires offer an opportunity to improve light trapping in low-cost silicon photovoltaic cells. We have grown radial junctions of hydrogenated amorphous silicon over p-doped crystalline silicon nanowires in a single pump-down plasma enhanced chemical vapor deposition process on glass substrates. By using Sn catalysts and boosting p-type doping in the nanowires, the open-circuit voltage of the devices increased from 200 to 800 mV. Light trapping was optimized by extending the length of nanowires in these devices from 1 to 3 μm, producing currents in excess of – 13 mA cm^? 2 and energy conversion efficiencies of 5.6%. The advantages of using thinner window layers to increase blue spectral response were also assessed.     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

Effect of Silane flow rate on microstructure of Silicon films deposited by HWCVD

Hydrogenated silicon films ranging from pure amorphous to those containing small crystallites in large crystalline fraction are prepared using the HWCVD technique without using any hydrogen dilution which is supposed to be necessary for the deposition of nanocrystalline Si films. The only parameter that is varied is Silane flow rate. The deposition rate ranges from 6–27 Å/s. The band gap of the films (1.8–2.0 eV) is high compared to the regular films, which is attributed to the improved short and medium range order as well as the presence of low density amorphous tissues in the grain boundary regions. The films show improved stability under long term light exposure due to more ordered structure and presence of hydrogen mostly as strong Si―H bonds.     

Formation and structure of Na2S + P2S5 amorphous materials prepared by melt-quenching and mechanical milling

xNa₂S + (1 ? x)P₂S₅ amorphous and partially crystalline materials were prepared by melt-quenching and mechanical milling. These products were characterized using x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and Raman spectroscopy. Compared to the narrower x-ray amorphous range for this system obtained through melt-quenching as found in this study, 0.50 ≤ x ≤ 0.67, the x-ray amorphous range for this system could be extended from the low-alkali ultra-thiophosphate composition of x ~ 0.25 to slightly above the high-alkali pyro-thiophosphate composition of x ~ 0.70 using mechanical milling. Mechanically milled samples with Na₂S of composition x = 0.75 yielded a partially crystalline material that had diffuse XRD peaks associated to the α-Na₃PS₄ phase. A similar result was obtained for the x = 0.80 composition except that, as expected, it also showed peaks for unreacted (over stoichiometric) Na₂S. The melt-quenched and mechanically milled samples with the same compositions 0.50 ≤ x ≤ 0.67 showed similar FT-IR and Raman spectra, indicating very similar chemical short-range structures are present in both of these amorphous materials. It was found that the Na₂S + P₂S₅ system exhibited similar behavior to that of the Li₂S + P₂S₅ in that chemical reaction between Na₂S and P₂S₅ could be induced by mechanical milling near room temperature to produce both amorphous and polycrystalline materials.     

Microstructure evolution and grain size distribution in nanocrystalline FeNbBCu from synchrotron XRD and TEM analysis

The microstructure evolution during nanocrystallization of an Fe₇₇Nb₇B₁₅Cu₁ amorphous alloy is investigated using in situ synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the nanocrystallized alloy consists in dispersion of bcc-Fe nanocrystals of 4–6 nm of diameter embedded in a stabilized amorphous remaining matrix. The grain size distribution of the nanocrystalline Fe₇₇Nb₇B₁₅Cu₁ alloy was obtained using three different methodologies: statistical analysis of TEM images, the Warren–Averbach and Langford methods to analyse the XRD patterns and modelling of the diffraction pattern from the Debye equation. A lognormal distribution function has been assumed in all three methods in order to obtain comparable results. A good agreement is found in the calculated average radius and dispersion although some deviations are found with the Langford approach. The microstructure evolution during crystallization was obtained from the XRD patterns during heating (5.0 · 10^?3 K s^?1) at temperatures between 700 and 900 K. A decrease and prompt saturation of the growth rate is obtained, indicative of the diffusion barrier caused to the overlap between the concentration gradients at the interface of growing grains (soft impingement). A simple model assuming nucleation and initial fast growth of the crystalline grains followed by reduced growth capable of predicting microstructural evolution is presented. The modelling results agree with the experimental observations.     

Electrical properties of bulk amorphous semiconductor (GaSb)38Ge24

Temperature dependences of the dc conductivity and thermopower of a (GaSb)₃₈Ge₂₄ homogeneous bulk amorphous alloy are investigated at 110–425 K and at 180–400 K, respectively. The samples were prepared by spontaneous solid-state amorphization of a quenched crystalline high-pressure phase heated from 77 to 430 K at ambient pressure. In contrast to the parent amorphous GaSb compound exhibiting an unusual combination of electrical properties, amorphous (GaSb)₃₈Ge₂₄ is found to be a typical p-type semiconductor well described by the conventional Mott–Davis model.     

Investigation of hopping transport in n-a-Si:H/c-Si solar cells with pulsed electrically detected magnetic resonance

Hopping transport through heterostructure solar cells based on B-doped crystalline silicon wafers with highly P-doped hydrogenated amorphous silicon emitters with different thicknesses is investigated at T = 10 K with pulsed electrically detected magnetic resonance. The measurements show that transport is dominated by conduction band tail states (g ≈ 2.0046) with a distribution of their mutual coupling strength. The signal intensity correlates to the sample thickness and the g-factors do not exhibit an anisotropy which suggests that transport is still dominated by bulk properties of amorphous silicon. In addition, two broad P-donor hyperfine satellites can be detected. Influences of interface defects such as P_b-like states known from silicon dioxide interfaces are either suppressed by the high Fermi energy at the interface or not present.     

Infrared semiconductor laser irradiation used for crystallization of silicon thin films

We report the rapid thermal crystallization of silicon films using infrared semiconductor laser. Carbon films were used on silicon films to absorb the laser light. Uniform crystalline regions were achieved by a line shape laser beam with a length of 20 μm. Polycrystalline silicon thin film transistors were fabricated in crystallized regions. The effective electron carrier mobility and threshold voltage were achieved to be 130 cm²/Vs and 0.4 V, respectively, when the crystalline volume ratio of the silicon films was 0.95.     

Anatase phase formation kinetics in Ti and TiOx nanoparticles produced by gas-phase condensation

Anatase TiO₂ nanoparticles were successfully synthesized by post-heat treatments of partially crystalline Ti and amorphous TiO_x nanoparticles, respectively produced by inert gas condensation and subsequent oxidation. The nanoparticles condensed on a liquid-nitrogen containing cooling finger (sample LN) were identified to be partially crystalline Ti phase with ~ 10–20 vol.% amorphous TiO_x. On the other hand, those condensed on a room-temperature cooling finger (sample RT) were almost completely amorphous TiO_x phase. Differential scanning calorimetry scan curves of as-oxidized samples were interpreted using Kissinger analysis, the non-isothermal kinetics, and activation energy for the anatase formation was determined as ~ 455 and 865 kJ/mol for samples LN and RT, respectively. As-oxidized samples LN and RT were heat treated at 400 °C for 2 h, respectively (samples LN-H and RT-H). Samples LN-H and RT-H showed the onset of UV–visible light absorption near 400 nm and the optical band gap of 3.12 and 3.21 eV, respectively, corresponding to anatase. The sample LN-H showed faster photocatalytic decomposition of methylene blue and rhodamine B dyes compared to the sample RT-H due to high crystallinity of anatase and rutile phases.