Optical and structural properties of Ge–Se bulk glasses and Ag–Ge–Se thin films

The optical and structural properties of Ge₂₀Se₈₀, Ge₂₅Se₇₅ and Ge₃₀Se₇₀ bulk glasses and Ag_x(Ge_0.20Se_0.80)_100−x thin films, where x = 0, 6, 11, 16, 20 and 23 at.% were studied. All samples were confirmed as amorphous according to XRD. The Raman spectra showed increase in 260 cm⁻¹ and 237 cm⁻¹ and decrease in 198 cm⁻¹ and 216 cm⁻¹ bands with different Se content in the bulk samples. The optical bandgap energy of bulk samples decreased (2.17–2.08 eV) and refractive index increased (2.389–2.426 at 1550 nm) with increasing Se content in bulk glasses. The Ge₂₀Se₈₀ thin films were prepared by vacuum thermal evaporation from Ge₃₀Se₇₀ bulk glasses. The Raman spectra of the films showed that peaks at 260 cm⁻¹ and 216 cm⁻¹ decreased their intensities with increasing Ag content in the thin films. The significant red shift of bandgap energy occurred upon different Ag content. The optically induced dissolution and diffusion resulted in graded refractive index profile along the film thickness caused by different Ag concentration. The refractive index increased from the substrate side to the top of thin films. The graded profile was getting more uniform with increasing content of silver in the thin film.     

Anomalous anisotropic deformations of As2S3 flakes induced by linearly-polarized bandgap illumination

Semi-free As₂S₃ flakes undergo visible-scale anisotropic deformations when exposed to linearly-polarized bandgap illumination. We investigate the behavior and also those in amorphous Se, GeS₂, AgAsS₂, and crystalline As₂S₃. These results suggest that the deformation occurs through photo-induced birefringence, photo-induced fluidity, and optical force.     

Micron scale and nanoscale structure of mesoporous silica thin films

Ordered mesoporous silica thin films have been prepared on silicon substrates by spin-coating technique using poly(alkaline oxide) triblock copolymers EO₂₀PO₇₀EO₂₀ (P123) as structure-directing agent. The X-ray diffraction and transmission electron microscopy investigations show that the obtained mesoporous silica thin films have an ordered pore array structure in nanoscale. The atomic force microscopy analysis reveals that the obtained mesoporous silica thin films exhibit a tile arrangement structure in micron scale.     

Light structured deposition (1): Material properties

An original light-assisted approach for optical-material deposition is introduced, and preliminary results concerning the properties of the deposits, in the case of As–S binary glass alloys, are reported for the first time. Such approach uses a continuous-wave laser source and on-axis geometry, to grow a semiconducting glass alloy onto a transparent substrate under the concurrent actuation of a given light-intensity distribution. Significant differences are observed when the material is prepared by using this technique, in comparison with those reported for similar alloys prepared by both physical- and chemical-vapor based techniques.     

Morphology and structural studies of Ag photo-diffused GeySe1−y thin films prepared by RF-sputtering

Thin films of Ag photo-doped-Ge_ySe_1−y films were prepared by RF co-sputtering technique. A systematic study of the relation existing between the host layer composition and the saturation rate in silver was carried out. Morphology and composition studies before and after chemical etchings were performed by scanning electron microscopy and electron probe micro-analyses, respectively. Raman spectroscopy studies showed the presence of corner-sharing and edge-sharing Ge(Se_1/2)₄ tetrahedra in all thin films. The vibration mode corresponding to Se_n-chains is observed for the Ge-poor host layer and on the contrary, Ge-rich thin films exhibited some tendency to form homopolar Ge–Ge bonds as a part of ethane-like Ge₂Se₆ units. These investigations revealed the amount of Ag that could be incorporated in the host layer. Such an amount strongly depends on the relative ratio Ge/Se in the Ge_ySe_1−y thin film. For the Ge-poor host layer, an incorporation of Ag (54 at.%) was observed but also a drastic increase in the film heterogeneity. On the other hand, the host layers with higher Ge content showed homogeneous surfaces and a saturation level of 32–41 at.% Ag.     

Study of microstructure in Agx(As0.33Se0.67)100−x chalcogenide glasses

Glasses with general formula Ag_x(As_0.33Se_0.67)_100−x were studied by means of modulated differential scanning calorimetry (MDSC), conductivity and permittivity measurements and atomic force acoustic microscopy (AFAM). In the whole range of doping (x = 2–12 at.% Ag) a phase separation was supposed. The proportional amount of the ion-conductive Ag rich phase increased with the Ag content. A rapid increase (almost four orders of magnitude) in the conductivity was observed in the Ag concentration range 4–8 at.%, which could be associated with the interconnection of this ion-conductive phase. The transition from a hole conductivity (at low Ag concentration) to a mixed ionic-hole conductivity at higher Ag concentration was studied by permittivity measurements.     

Peculiarities of photoluminescence excited by 157 nm wavelength F2 excimer laser in fused and unfused silicon dioxide

Photoluminescence (PL) spectra and kinetics of high purity amorphous silicon dioxide with ultra low hydroxyl content is studied under the excitation by F₂ excimer laser (157 nm wavelength) pulses. Materials synthesized in the SPCVD plasma chemical process are studied before and after fusion. Two bands are found in the PL spectra: one centered at 2.6–2.9 eV (a blue band) and the other at 4.4 eV (a UV band). Luminescence intensity of unfused material is found to increase significantly with exposure time starting from a very small level, whereas in fused counterpart it does not depend on irradiation time. Both bands show complicated decay kinetics, to which add exponential and hyperbolic functions. The UV band of the unfused material is characterized by decay with exponential time constant τ  4.5 ns and hyperbolic function t⁻ⁿ, where n = 1.5 ± 0.4. For the blue band the hyperbolic decay kinetics with n  1.5 extends to several milliseconds, gradually transforming to the exponential one with τ = 11 ± 0.5 ms. In fused glass relative contribution of the fast component to the UV band is small whereas for the blue one it is great, that allows one to more accurately determine the hyperbolic law factor n = 1.1 ± 0.1 typical for tunneling recombination. Simultaneous intracenter and recombination luminescence, the later occurring with the participation of laser radiation induced defects, add particular features to the decay kinetics. Spectra of the above luminescence processes are different. A less sharp position of bands is associated with the recombination luminescence. The origin of the observed PL features we attribute to the presence of oxygen deficient centers in glass network in the form of twofold coordinated silicon. Such centers being affected by network irregularities can be responsible for the recombination PL component. A great variety of network irregularities is responsible for centers’ structural inequivalence, which causes a non-uniform broadening of PL spectral and kinetic parameters.     

Structure, electrical, optical and thermal properties of Ge4Sb4Tex (x = 8, 9 and 10) thin films

The crystalline samples of Ge₄Sb₄Te₁₀, Ge₄Sb₄Te₉, and Ge₄Sb₄Te₈ were prepared and their amorphous semiconducting thin films obtained by flash evaporation. Their sheet resistance decreased slowly with temperature up to 147–160 °C with activation energy of electrical conductivity ΔE = 0.40–0.44 eV. Above these temperatures, the sheet resistance drops abruptly by several orders due to crystallization. The drop of resistivity proceeds in two steps. Two steps of phase change were also found on curves of DSC and on the temperature dependence of index of refraction. It pays for slow heating rates, crystallization induced by short (≈30 ns) laser pulses proceeds probably in one step only for all studied samples (as it follows from X-ray diffraction), not only for Ge₂Sb₂Te₅ in which a single phase formation was confirmed. The crystallization temperatures are increasing slightly with decreasing Te content in the series Ge₄Sb₄Te₁₀–Ge₄Sb₄Te₉–Ge₄Sb₄Te₈ from 147 to 160 °C. The X-ray diffractograms revealed that in laser crystallized samples can be found only cubic modification of Ge₂Sb₂Te₅ type (a = 0.6 nm), while the samples annealed (230 °C, 2 h) or annealed after the crystallization with laser pulse, contain also small amounts of hexagonal phase.     

Er-doped silica–hafnia films for optical waveguides and spherical resonators

In this paper we present some result on sol–gel derived silica–hafnia systems. In particular we focus on fabrication, morphological and spectroscopic assessment of Er³⁺-activated thin films. Two examples of silica–hafnia-derived waveguiding glass ceramics, prepared by top–down and bottom–up techniques are reported, and the main optical properties are discussed. Finally, some properties of activated microspherical resonators, having a silica core, obtained by melting the end of a telecom fiber, coated with an Er³⁺-doped 70SiO₂–30HfO₂ film, are presented.     

Electronic structure and charge transport properties of amorphous Ta2O5 films

Amorphous Ta₂O₅ films were deposited by sputtering Ta onto silicon substrates with reactive ion beam. Electron energy loss spectroscopy measurements on the film found that the plasma oscillation energy is 23.1 eV. The refractive index and the extinction coefficient were measured with spectroscopic ellipsometry over the spectral range of 1.9–4.9 eV. The optical band gap is found to be 4.2 ± 0.05 eV. The valence band consists of three bands separated by ionic gaps. The values of electron effective masses were estimated with DFT quantum-chemical calculation. Experiments on injection of minority carriers from silicon into oxide were also conducted and we found that the electron component of conduction current governed by the electron current in the amorphous Ta₂O₅.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Absorption and photoluminescence of PbS QDs in glasses

Optical properties of PbS quantum dots (QDs) precipitated inside the oxide glass matrix were investigated. Photoluminescence (PL) from the PbS QDs showed peak wavelengths located at 1170–1680 nm with widths of 150–550 nm. Radii of QDs in glasses were 2.3–4.7 nm depending upon the thermal treatment. Peak wavelengths of PL bands shifted as much as 70 nm as the temperatures and excitation irradiances increased. Calculated effective local temperatures indicated that these shifts of PL spectra were associated with local heating induced by the temperatures and laser beam.     

Structure and growth morphology of Gd2O3-doped CeO2 thin films

Gd₂O₃-doped CeO₂ (Gd_0.1Ce_0.9O_1.95, GDC) thin films were synthesized on (1 0 0) Si single crystal substrates by a reactive radio frequency magnetron sputtering technique. Structures and surface morphologies were characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and one-dimensional power spectral density (1DPSD) analysis. The XRD patterns indicated that, in the temperature range of 200–700 °C, f.c.c. structured GDC thin films were formed with growth orientations varying with temperature—random growth at 200 °C, (2 2 0) textures at 300–600 °C and (1 1 1) texture at 700 °C. GDC film synthesized at 200 °C had the smoothest surface with roughness of R_rms=0.973 nm. Its 1DPSD plot was characterized with a constant part at the low frequencies and a part at the high frequencies that could be fitted by the f^−2.4 power law decay. Such surface feature and scaling behavior were probably caused by the high deposition rate and random growth in the GDC film at this temperature. At higher temperatures (300–700 °C), however, an intermediate frequency slope (−γ₂≈−2) appeared in the 1DPSD plots between the low frequency constant part and the high frequency part fitted by f⁻⁴ power law decay, which indicated a roughing mechanism dominated by crystallographic orientation growth that caused much rougher surfaces in GDC films (R_rms>4 nm).     

Growth and characterization of pendeo-epitaxial GaN on 4H–SiC substrates

Growth on AlN/4H–SiC substrates of coalesced, non-polar GaN films having volumes of material with reduced densities of dislocations and stacking faults has been achieved from etched stripes via the statistical and experimental determination of the effect of temperature and V/III ratio on the lateral and vertical growth rates of the GaN{0 0 0 1} faces combined with pendeo-epitaxy. AFM of the uncoalesced GaN(0 0 0 1) and GaN vertical faces revealed growth steps with some steps terminating at dislocations on the former and a pitted surface without growth steps, indicative of decomposition, on the latter. Coalescence was achieved via (a) a two-step route and the parameters of (1) and V/III=1323 for 40 min and (2) 1020 °C and V/III=660 for 40 min and (b) a one-step route that employed and a V/III ratio=660 for 6 h. The densities of dislocations in the GaN grown vertically over and laterally from the stripes were 4×10¹⁰ cm⁻² and 2×10⁸ cm⁻², respectively; the densities of stacking fault in these volumes were 1×10⁶ cm⁻¹ and 2×10⁴ cm⁻¹, respectively. The defects in the wing material were observed primarily at the bottom of the film where lateral growth of the GaN occurred from the AlN and the SiC. Plan view AFM also revealed different microstructures and a reduction in the RMS roughness values from 1.2 to 0.95 nm in these respective regions.