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.     

Heteroepitaxial growth of Cu2O thin film on ZnO by metal organic chemical vapor deposition

Cuprous oxide (Cu₂O) thin films were grown epitaxially on c-axis-oriented polycrystalline zinc oxide (ZnO) thin films by low-pressure metal organic chemical vapor deposition (MOCVD) from Copper(II) hexafluoroacetylacetonate [Cu(C₅HF₆O₂)₂] at various substrate temperatures, between 250 and 400 °C, and pressures, between 0.6 and 2.1 Torr. Polycrystalline thin films of Cu₂O grow as single phase with [1 1 0] axis aligned perpendicular to the ZnO surface and with in-plane rotational alignment due to (2 2 0)_Cu2O(0 0 0 2)_ZnO; [0 0 1]_Cu2O[1 2¯ 1 0]_ZnO epitaxy. The resulting interface is rectifying and may be suitable for oxide-based p–n junction solar cells or diodes.     

Effect of silver doping on the structure and phase separation of sulfur-rich As–S glasses: Raman and SEM studies

We report on the structural details and microphase separation of the bulk glasses Ag_x·(As₃₃S₆₇)_100-x for 0x25. Glass–glass phase separation occurs over a wide range of Ag content, i.e. 4x20. An off-resonant polarized Raman spectroscopic study has been carried out to elucidate structural aspects at the short- and medium-range structural order of the glasses. Analysis of Raman spectra revealed quantitative changes of the sulfur-rich microenvironments that reduce upon adding Ag. Scanning electron microscopy combined with X-rays microanalysis have been utilized to examine the type and extent of phase separation, and to provide quantitative details on the atomic concentrations in the Ag-poor and Ag-rich phases. It has been shown that at 7 at.% Ag the Ag-rich phase percolates through the structure; this effect can be associated with an ionic-to-superionic behavior of these glasses in accordance with similar studies on the stoichiometric arsenic sulfide glass; although the phase separation observed in the present glasses is qualitatively different.     

Growth, thermal and spectral properties of Nd-doped NaGd(MoO4)2 crystal

Nd³⁺-doped NaGd(MoO₄)₂ crystal with dimensions were grown by Czochralski method. Nd³⁺:NaGd(MoO₄)₂ crystal melts at 1182 °C. The hardness of Nd³⁺:NaGd(MoO₄)₂ crystal is 334 VDH. The specific heat is 72.6 cal/mol K. The thermal expansion coefficients are for c-axis and for a-axis, respectively. The absorption cross-sections of Nd³⁺:NaGd(MoO₄)₂ crystal are with a FWHM of 9 nm at the 804 nm for π-polarization and with a FWHM of 17 nm at 807 nm for σ-polarization, respectively. The emission cross-section σ_em are at 1063 nm for π-polarization and 1.94×10^-20 at 1070 nm cm² for σ-polarization, respectively. The fluorescence lifetime τ_f is 93.9 μs at room temperature.     

Optical properties of chalcogenide multilayer deposited on Au layer

The chalcogenide multilayers were prepared as dielectric mirrors having the first order stop bands in the near infrared region 1.55 μm. The 7.5 layer pairs of the alternating amorphous Sb–Se and As–S layers were deposited on glass substrates using a conventional thermal evaporation method. To center the stop bands of the 15-layer dielectric mirrors at 1.55 μm, the layer thicknesses 117 nm for Sb–Se and 169 nm for As–S single layers were calculated from the quarter wave stack condition. The optical reflection and transmission spectra of the prepared mirrors were measured using a UV/VIS/NIR and FT-IR spectroscopy at the ambient and elevated temperatures. The optical reflection of the annealed 15-layer chalcogenide mirror was found higher than 99% in the range of 1440–1600 nm. As the 200 nm thick gold layer was added between the substrate and the chalcogenide mirror, the stop band of the annealed Au/multilayer system broadened to 1360–1740 nm simultaneously with an appearance of the 15% transmission peak at 1.55 μm. A preparation of similar metal/multilayer systems is one of the possible ways how to design the dielectric filters for near infrared region exploiting the good optical quality of the chalcogenide films and their simple deposition.     

Structure and optical properties of chalcohalide glasses doped with Pr and Yb ions

Glasses of systems 100-y((GeS₂)₈₀(Sb₂S₃)_20−x(PbI₂)_x)yPr₂S₃, x = 0; 2; 5, 8; y = 0; 0.01; 0.1; 0.5 and 99.9-z((GeS₂)₈₀(Sb₂S₃)₁₈(PbI₂)₂)0.1Pr₂S₃zYb₂S₃, z = 0.05; 0.1; 0.15) were synthesized in high purity. Optically well transparent glasses were obtained for x  5 mol.% PbI₂, for y  0.1 mol.% Pr₂S₃ and for z  0.15 mol.% Yb₂S₃. The glasses were stable and homogeneous, as confirmed by X-ray diffraction and electron microscopy, with high optical transmittivity from visible (red) region up to infrared region (900 cm⁻¹). The density of the glasses was 3.26–3.33 gcm⁻³ for PbI₂ containing glasses. The glass transition temperature, T_g, was 320–336 °C. The optical absorption bands in rare-earth doped glasses corresponded to ³H₄–³F₄, ³H₄–³F₃, ³H₄–(³F₂ + ³H₆) f–f electron transitions of Pr³⁺ ions and to ²F_7/2–²F_5/2 f–f electron transitions of Yb³⁺ ions. Strong luminescence band with maximum near 1340 nm (electron transition ¹G₄–³H₅) was found in Pr₂S₃ doped glasses. The intensity of this band was rising with doping by Yb³⁺ ions. The possible mechanism of the luminescence enhancement is suggested.     

Surface morphology of as-deposited and illuminated As–Se chalcogenide thin films

The role of the composition and of the related changes of the structure in the formation of the surface of amorphous As_xSe_1−x (0 < x < 0.5) layers before and after light treatment was investigated by direct measurements of the surface roughness at nanometer-scale and surface deformations at micrometer-scale under influence of illumination. It was established that the surface roughness of the films, deposited by vacuum thermal evaporation, decreased with increasing As content, especially in compositions 0.1  x  0.3, where the maximum light stimulated surface deformations (localized expansion) occurs. Both relate to the rigidity percolation range and the maximum photoplastic effects, which are not directly connected to the known photodarkening effect, since it is minimal for these compositions.     

Optical and spectroscopic characterization of germanium selenide glass films

A previews study of germanium selenide glass films by scanning electron microscopy and atomic force microscopy revealed a heterogeneous surface morphology consisting of granular regions 15–50 nm in size, which cause high optical losses. The present work was performed in order to further characterize such materials using spectroscopic ellipsometry, infrared (IR) and Raman spectroscopies. Chalcogenide glass films with GeSe₂, Ge₂₈Sb₁₂Se₆₀ and GeSe compositions have been deposited on single crystal silicon and silica glass substrates by vacuum thermal evaporation. The film thickness and the optical constants were obtained from spectroscopic ellipsometry using the Tauc-Lorenz dispersion formula. A model was derived for the film structure, which included a roughness layer at the surface. This top layer was found to have a thickness of 5–15 nm, of the order of the size of the granular regions previously reported. The optical bandgap of the samples increased with increasing selenium content, while the refractive index decreased. Despite a previous report of large scale phase separation in bulk Ge₂₆Sb₁₄Se₆₀ glass, the fundamental IR and Raman spectra obtained in the present work did not provide any clear evidence for such phase separation which could be associated with the heterogeneous nanostructure observed at the surface of the films.     

Chemical bonding and optical properties of germanium–carbon alloy films prepared by magnetron co-sputtering as a function of substrate temperature

Amorphous non-hydrogenated germanium carbide (a-Ge_{1 − x}C_x) films have been prepared by magnetron co-sputtering system designed by ourselves. The chemical bonding and microstructure have been analyzed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The optical properties of the films have been investigated by means of spectroscopic ellipsometry. The relationship between the chemical bonding and the optical properties has been explored. It has been found that all films with the constant carbon content are amorphous. The sp² CC and sp³ GeC bonds increase with T_s, and some sp² CC bonds gain infrared activity. The fraction of sp³ GeC bonds rises with T_s, but the fraction of sp³ GeGe bonds gradually drops down. In addition, the refractive index and extinction coefficient increase with T_s. The film optical gap is seen to reach 1.15 eV when T_s is 200 °C. However, the optical properties of a-Ge_{1 − x}C_x films almost remain stable with the substrate temperature.     

The effect of aluminum sources on synthesis of low expansion glass–ceramics in lithia–alumina–silica system by sol–gel route

This paper investigated the effect of different aluminum sources on the crystallization behaviors, thermal expansion and morphology of lithium aluminosilicate glass–ceramics sintered at different temperatures. Specimens were prepared by sol–gel technology. The crystalline phase in the specimen with Al(NO₃)₃9H₂O aluminum source sintered at 1300 °C has shown a major phase of Li₂Al₂Si₃O₁₀ accompanied by minor phase of LiAl₅O₈ and trace amount of SiO₂. The β-eucryptite appeared as the only phase in the specimens with aluminum isopropoxide or aluminum isopropoxide mixed with Al(NO₃)₃9H₂O aluminum sources sintered at 1300 °C. The sintering temperature has significant effect on the thermal expansion behavior of all the specimens. SEM indicated that microstructures were spherulitic crystals and no distinct differences were observed in the specimens with different aluminum sources.     

Growth and characterization of nonlinear optical single crystals of l-arginine diiodate

Growth of good optical quality single crystals of l-arginine diiodate (abbreviated as LADI), a semiorganic nonlinear optical (NLO) material is reported. Crystals of dimension up to were obtained from its aqueous solution by slow solvent evaporation technique. The crystals were characterized by single crystal XRD, FTIR, optical absorption spectrum, microhardness, dielectric and photoconductivity studies. The DTA and TGA traces of LADI confirm the decomposition of the sample at 145 °C.     

Vibrational modes and structure of (AgI)x (GeS1.5)100−x chalcohalide glasses

We report a structural investigation of bulk Ge-rich Ge–S–AgI chalcohalide glasses. A vibrational spectroscopic study of the quaternary system (AgI)_x (GeS_1.5)_100−x (0  x_AgI  20) has been undertaken using infrared spectroscopy and Fourier transform Raman scattering. It was found that the GeS_1.5 Raman spectrum is compatible with a glass structure composed of corner- and edge-sharing mixed GeS_nGe_4−n (n = 0–4) tetrahedra where units with n = 2–4 dominate, whilst the fraction of corner-sharing units are significantly lower than the corresponding fraction in the stoichiometric GeS₂ glass. The addition of AgI has revealed a subtle but systematic effect in the structure of the Ge-rich glass matrix, manifested by mild decrease of the ES units and the concomitant increase of complex GeS_nI_4−n or GeS_nGe_mI_4-n−m tetrahedra whose vibrational modes form a continuum at low frequencies. Although, AgI seems to cause subtle structural changes due to the formation of Ge–I bonds, it is also evident that AgI does not act as a real modifier that would depolymerize appreciably the Ge–S network structure.     

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.     

Glass formation in the GexTe100−x binary system: Synthesis by twin roller quenching and co-thermal evaporation techniques

The Ge–Te system exhibits one main composition domain where glasses can be easily prepared by melt quenching technique; this domain is centered on the eutectic composition Ge₁₅Te₈₅. In this work, bulk flakes and films of composition Ge_xTe_100−x with x  50 at.% were prepared by two different quenching techniques: (i) the twin roller quenching for bulk flakes and, (ii) the co-thermal evaporation for films (with thickness comprised between 2 and 14 μm). Electron Probe Micro-Analysis was used to check the composition of the materials while X-ray diffraction allowed identifying the amorphous state and/or the crystalline phases present in the Ge_xTe_100−x samples. Thermal properties for both types of materials were investigated by differential scanning calorimetry. The glass-forming regions were: 11.7–22.0 at.% Ge for bulk flakes and 10.2–35.9 at.% Ge for films. A similar thermal behavior of bulk flakes and thick films was highlighted by Differential Scanning Calorimetry.     

Stimulated interdiffusion and optical recording in Sb/As2S3 nanomultilayers

The role of the compositional modulation at nano-scale dimensions (2–10 nm) in the enhancement of optical recording parameters in nanomultilayers, which contain Sb as active, optical absorbing and diffusing layers and As₂S₃ as barrier (matrix) layers was investigated. Comparison was made with single homogeneous layers made of ternary (As₂S3)_xSb_1−x glasses and co-deposited from Sb and As₂S₃. It was shown that essential increase of the recording efficiency, sensitivity of the bleaching process, broadening of its spectral range occurs due to the stimulated interdiffusion of adjacent components in Sb/As₂S₃ nanomultilayers with optimized Sb layer thickness.     

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.     

Characterization of deposits produced by TEA CO2 pulsed laser ablation of silicon mono- and dioxide

Silicon based deposits were prepared by TEA CO₂ pulsed laser ablation (PLA) of SiO and SiO₂ targets in the atmosphere of selected gases (N₂, He, Ne, Ar, Kr). These deposits possess high specific area of several hundreds m² per gram. Owing to the high specific area, some chemical groups and hydrogen related radical were detected by means of FTIR and EPR analyses and theoretical calculations: silyl (E′ center) Si, silylen Si:, silanon SiO, POL (peroxy linkage) SiOOSi and/or NBOHC (non-bridging oxygen hole center) SiO, POR (peroxy radical) SiOO and dioxysilirane Si(O)₂. In SiO₂ deposits the concentration of silyl Si resp. POR SiOO was determined to be 5.8 × 10¹⁸/g resp. 6.2 × 10¹⁹/g. In SiO deposits the ratio [Si:]:[Si] = (3.1-5.7) × 10¹⁹/g: (5.3-9.8) × 10¹⁹/g was measured. Estimated concentration of [Si] in deposits was increased nearly five times in comparison with SiO target. After exposure of the SiO deposits to H₂ EPR doublet with hyperfine splitting of 7.7 mT was observed. The best agreement between calculated theoretical and experimental values was found for the model [(HO)₃SiO]₂HSi. FTIR measurements and calculations of the silanol theoretical model clusters enabled us to discuss the chemical surroundings of the silanol and to determine the defects in the deposits.     

Hg/Sn amalgam degradation of ancient glass mirrors

Tin amalgam, which is obtained by pouring mercury onto a sheet of tin, has been used in the production of reflective coatings for mirrors. The corrosion processes of the amalgam layer were investigated in several mirrors from historical buildings located in southern Spain using SEM/EDS, XPS, and GID. Mercury and Sn⁴⁺ are present as spheres on the amalgam surface due to the evaporation process (5 nm). The profile shows a mixture of Sn²⁺ and Sn⁴⁺. The original amalgam was composed of a binary alloy of tin and mercury (Hg0.1Sn0.9) and metallic tin. In this paper the tin oxidation mechanism of the amalgam is described. Liquid mercury is volatile and evaporates slowly, leaving fine tin particles that oxidize easily, forming tin monoxide (SnO) and tin dioxide (SnO₂). The mercury-rich phase accelerates the corrosion of the tin-rich phase.