Growth kinetics of CdSe1−x Sx microcrystallites in silicate glass

The absorption spectra of silicate glass CdSe_1−x S_x composite as a function of temperature are studied. At the early stage of thermal treatment, a shift in the absorption edge toward longer waves is associated with an increase in the density of critical nuclei. A further shift of the spectra is due to the growth of semiconductor nanocrystallites.     

Optical Properties of Films of Cu x Ag1−x InSe2 Solid Solutions Obtained by Laser Evaporation

The structure and optical properties of the films of ternary compounds AgInSe₂, CuInSe₂ and Cu _x Ag_1–x InSe₂ solid solutions obtained by pulsed laser evaporation are investigated. It has been established that the films like bulk crystals have the structure of chalcopyrite. By the transmission and reflection spectra near the edge of natural absorption the refractive index and coefficient of optical absorption are calculated and the energies of interband transitions and the crystalline and spinorbit splitting are determined.     

Compositional dependence of the optical properties of amorphous semiconducting glass Ge10AsxSe(90−x) thin films

Optical properties of ternary chalcognide amorphous Ge₁₀As_xSe_(90−x) (with 10?x?25 at%) thin films prepared by thermal evaporation have been measured in visible and near-infrared spectral region. The straightforward analysis proposed by Swanepoel has been successfully employed, and it has allowed us to determine the average thickness , and the refractive index, n, of the films, with high accuracy. The refractive index, n and the average thickness has been determined from the upper and lower envelopes of the transmission spectra measured at normal incidence, in the spectral range 400-2500 nm. The absorption coefficient α, and therefore extinction coefficient k, have been determined from the transmission spectra in the strong-absorption region. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-DiDomenico model, and the optical absorption edge is described using the ‘nondirect transition’ model proposed by Tauc. Likewise, the optical energy gap is derived from Tauc's extrapolation. The relationship between the optical gap and chemical composition in Ge₁₀As_xSe_(90−x) amorphous system is discussed in terms of the average heat of atomization H_s and average coordination number N_c. Finally, the chemical bond approach has been also applied successfully to interpret the decrease of the glass optical gap with increasing As content.     

Molecular Beam Epitaxy of Si–Ge–Sn Heterostructures for Monolithically Integrated Optoelectronic Devices Based on Silicon

Elastically strained metastable Ge_1–xSn _x layers with molar fractions of tin of up to 0.15 are grown on Si (001) substrates. To analyze the optical properties of the samples, photoluminescence (PL) spectra are measured at room temperature and IR transmission spectra are measured at the liquid helium temperature. The room temperature direct intrinsic absorption edge at 0.71–0.72 eV is visible in the spectra of the studied structures with tin contents of 12–13%.     

Structural study of NiXMg1–XFe2O4 ferrites

The ferrite system Ni _x Mg_1-x Fe₂O₄ with 0≤x≤1 was prepared using the usual ceramic technique. The prepared samples were studied by X-ray diffraction and IR spectroscopy. X-ray diffraction analysis proved that all the samples were single-phase with the cubic spinel structure. The lattice constant, radius of the tetrahedral ion, unshared octahedral edge, tetrahedral bond and tetrahedral edge decrease while the bulk and theoretical densities, radius of octahedral ion, octahedral bond and shared octahedral edge increase as nickel ion substitution increases. The positions and intensities of the four bands of IR absorption spectra characterizing ferrites are composition dependent.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals

Oxyfluoride glasses, 30.9SiO₂–15.5AlO_1.5–30.9PbF₂–22.7CdF₂–1.5TmF₃–xHoF₃ ($x=0.1,1,1.5$) (mol %), were prepared by traditional melt-quenching method. By appropriate heat treatment of the precursor glasses as described above, transparent glass ceramics were obtained with $\beta$- PbF₂ nano-crystals embedded in glassy matrix as confirmed by X-ray diffraction analysis. Optical absorption and 2.0 μm emission properties of the precursor glass and glass ceramic samples have been studied. Rare-earth ions were confirmed to enter into nano-crystals, by absorption spectra. The emission intensity in 2.0 μm region increased several times after crystallization, due to shortened distance between codopants and low phonon energy of the $\beta$- PbF₂ nano-crystals. Of the investigated samples, glass ceramic sample containing 1.5 mol% TmF₃ and 0.1 mol% HoF₃ has the broadest and the most efficient emission in 2.0 μm region. The Tm³⁺/Ho³⁺codoped oxyfluoride glass ceramics show potential applications in 2.0 μm laser devices.     

溶胶-凝胶法制备PbS量子点玻璃的研究

采用溶胶-凝胶法合成了半导体PbS量子点掺杂的Na₂O-B₂O₃-SiO₂玻璃,研究了不同热处理工艺对玻璃结构的影响,利用多种表征手段研究了量子点掺杂玻璃中的微晶结构及其光学性能.孔径分析结果表明随着热处理温度的升高玻璃内部孔径不断减小,最终孔结构几乎完全消失;红外光谱分析表明玻璃网络结构在较低温度下己经形成,随温度的升高不断密实化; X射线光电子能谱证明了玻璃中存在PbS,高分辨透射电镜表征了玻璃基质中掺杂的微晶结构是PbS,统计计算表明,玻璃中微晶的平均粒径尺寸为3.5nm;吸收光谱分析发现,微晶掺杂玻璃的吸收边界较PbS的块体材料发生了明显的蓝移,产生了量子尺寸效应;通过Z扫描技术测得其非线性折射率γ为-2.03×10^-14cm²/GW. 关键词： PbS量子点 半导体 非线性光学效应 溶胶-凝胶法     

The determination of the x value in doped Hg1-xCdxTe by transmission spectra

Variable-temperature transmission/absorption spectra are measured on As-doped Hg_1-xCd_xTe grown by molecular beam epitaxy. The nonlinear temperature-dependent shift of the absorption edge is also observed, which is similar to our previous report on V_textrmHg (unintentionally)-doped HgCdTe. By referring to the empirical formulas of E_g(x, T), the x value of the epilayer is calculated and its inconsistency between the extreme temperatures (e.g. 10 and 300 K) is discussed. The results confirm the assumption of the effect of shallow levels on the shift of the absorption edge, and suggest that the x value (or E_g) in intrinsic/extrinsic-doped HgCdTe should be determined by referring to as low a temperature as possible (e.g. 10 K), and not the commonly used temperatures of 77 or 300 K, when the transmission spectrum should be employed. This can give brief guidelines for fabricating HgCdTe-related devices.     

Structural,electrical and optical properties of Cdx Zn1−xSe thin films

X-ray diffraction and electron diffraction techniques indicate that Cd_x Zn_1–xSe thin films on glass substrates have a polycrystalline nature, with sphalerite structure for x0.5 and wurtzite structure for x0.6. The crystalline size in each composition increases with increasing the film thickness. The room temperature dark resistivity varies from one composition to another showing a transition at x=0.55The temperature dependence of of the deposited films revealed two conduction mechanisms, one below 352 K due to shallow levels, surface states, and defects introduced during the film growth, and over 352 K due to deep-level ionization following the ordinary semiconducting behaviour.The thermal activation energy of the free charge carriers decreases linearly with increasing the molar fraction x of the CdSe content up to x=0.55, above which it increases with increasing x.The optical constants of Cd_x Zn_1–xSe thin films of different compositions were determined in the spectral range 400–2000 nm. The analysis of the absorption coefticient at and near the absorption edge indicates the existence of allowed direct transition energy gaps decreasing with increasing x.     

Electronic and optical properties of CdS/CdZnS nanocrystals

Cd1-x ZnxS nanocrystals are prepared by a co-precipitation method with different atomic fractions of Zn.The texture,structural transformation and optical properties with increasing x value in Cd1-x ZnxS are studied with scanning electron microscopy,electron diffraction patterning,and absorption spectra respectively.Quantum confinement in a strained CdS/Cd1-xZnxS related nanodot with various Zn content values is investigated theoretically.Binding energies on exciton bound CdS/CdxZn1-xS quantum dot are computed,with consideration of the internal electric field induced by the spontaneous and piezoelectric polarizations,and thereby the interband emission energy is calculated as a function of the dot radius.The optical band gap from the UV absorption spectrum is compared with the interband emission energy computed theoretically.Our results show that the average diameter of composite nanoparticles ranges from 3 nm to 6 nm.The X-ray diffraction pattern shows that all the peaks shift towards the higher diffracting angles with an increase in Zn content.The lattice constant gradually decreases as the Zn content increases.The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases.The values of the absorption edge are found to shift towards the shorter wave length region and hence the direct band gap energy varies from 2.5 eV for the CdS film and 3.5 eV for the ZnS film.Our numerical results are in good agreement with the experimental results.     

Optical characterization of Cu ion-doped zinc lead borate glasses

We have developed a new series of zinc lead borate (ZLB) glasses by varying ZnO content, to enhance UV transmission, in the chemical composition of xZnO-15PbO-(85−x)B₂O₃, where x=0, 5, 10, 15, 20, 25, 30, 35, 40 and 45 mol% ZnO. From the measurement of UV absorption spectra both the direct and indirect band gaps have been evaluated. Also different physical properties of a reference glass of 45ZnO-15PbO-40B₂O₃ have been studied. From the measurement of refractive indices at six different wavelengths, Cauchy's constants (A=1.578743209; and ) have been computed and a satisfactory correlation has been achieved between the theoretical and the experimental results. Absorption spectra of Cu²⁺(45−x)ZnO-15PbO-40B₂O₃ (where x=0.1, 0.2, 0.5 and 1.0 mol%) have shown two absorption bands at 428 nm (²B_1g→²E_g) and 777 nm (²B_1g→²B_2g). Emission spectra of (1.0 mol%) Cu²⁺:ZLB have revealed two emission transitions at 400 and 493 nm with excitations at 288 and 316 nm.     

Transmission spectra of films of Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se solid solutions

Films of Mn_1−x Fe _x Se (x = 0–0.45) solid solutions were flash-sputtered. We measured the transmission spectra of the films in the wavelength range 200–1000 nm at room temperature. From these spectra, we calculated the absorption coefficients and determined the fundamental absorption edge, the position of which is shifted from 2.65 eV in MnSe to 2.30 eV in solid solutions of compositions x = 0.20–0.45. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 136–138, January–February, 2007.     

Optical band gap studies and estimation of two photon absorption coefficient in alkali bismuth borate glasses

The optical absorption spectra of the glasses with composition xBi₂O₃·(30???x)R₂O·70B₂O₃ (R?Li, Na, K) and xBi₂O₃·(70???x)B₂O₃·30Li₂O (0?≤?x?≤?20) have been recorded in the wavelength range 350–650?nm. The glass samples were prepared by the normal melt–quench technique. The fundamental absorption edge for all the series of glasses is analyzed using the theory of Davis and Mott. The position of absorption edges and the values of optical band gap are dependent on the mol% of Bi₂O₃. The absorption in these glasses is associated with indirect transitions. The values of Urbach's energy and band tailing parameters are reported. The two photon absorption coefficient, β, in these glasses has also been estimated from the optical band gap and its value ranges from 1.3 to 11.6?cm/GW. The relationship between β and glass composition has also been discussed in terms of the electronic structure of the glass system.     

Charge correlations in cobaltates La2–xSrx CoO4

We report on an elastic neutron scattering study of the charge correlations in La_2–xSr_x CoO₄ with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La_2–xSr_x CoO₄.

Nano‐phase separation in La_2–xSr_x CoO₄ (schematically). Red areas: undoped La₂CoO₄ islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co³⁺ ions, magnetic Co²⁺ ions and oxygen ions, respectively; green arrows indicate Co²⁺ spins [1, 2].     

Evolution of ferroelectric SrBi2Nb2O9 phase embedded in tellurite glass

Glasses with the composition, [(100-x)TeO₂- x(SrO–Bi₂O₃–Nb₂O₅)] with x = 20, 30 and 40 (in mol %) were prepared. The X-ray diffraction (XRD) pattern and differential thermal analysis (DTA) for the as-prepared samples confirmed the amorphous and glassy characteristics, respectively. The SrBi₂Nb₂O₉ phase in tellurite glass for HT773 sample at x = 40 mol % is formed and confirmed by the Rietveld refinement. DTA curves for all glass samples exhibit two endothermic dips while the two broad exothermic peaks at lower x reduced to one at higher x. Infrared (IR) results revealed that the glassy matrix are composed of TeO₃, TeO₃₊₁, TeO_4, BiO₆ and NbO₆ structural units. The changes in the density (ρ), molar volume (V_m), oxygen molar volume (V₀) and oxygen packing fraction (OPD) have correlated with structural changes in the glass network. The optical studies show an absorption bands below the absorption edge in the glass samples.     

Study of glasses and glass ceramics in Bi2O3-GeO2-Fe2O3 and Bi2O3-GeO2-Cr2O3 systems by optical spectroscopy

The ferroelectric Bi₂GeO₅ crystalline phase is synthesized by heat treatment of 1Bi₂O₃-1GeO₂-xFe₂O₃ and 1Bi₂O₃-1GeO₂-yCr₂O₃ glasses. The obtained glass ceramics and initial glasses are studied using X-ray diffraction analysis and optical spectroscopy. The dielectric characteristics are measured, and the Curie temperature is determined. The effect of chromium and iron ions on the absorption spectra and dielectric properties of glasses and glass ceramics is determined.     

Effect of thallium on magnetic susceptibility of arsenic sulphides

Magnetic susceptibility was studied of samples of Tl_2xAs₂S_3+x and Tl_2xAs₂S₄ series where 0x1 differing in abundance of bonds in structural units. Absolute value of specific magnetic susceptibility decreases with increasing thallium content. Separation of susceptibility in dia- and paramagnetic parts made it possible to refer to the correlation of absorption edge position and paramagnetic susceptibility component.     

Cluster glass-like behavior of the diluted antiferromagnet Fe x Mg1−x TiO3 (x=0.2,x=0.3)

A diluted antiferromagnet Fe _x Mg_1–x TiO₃ has been shown to behave as a spin glass (x=0.2) and a reentrant spin glass (x=0.3) near the Fe percolation concentrationx 0.25. In order to obtain microscopic information on these samples, we performed Mössbauer measurements. At considerably higher temperatures than the transition temperatures, magnetically broadened spectra appear superimposed upon the paramagnetic doublets. A remarkable feature is that the intensity of the magnetic spectra increases accompanying the decrease of their linewidth. This behavior can be ascribed to the gradual slow-down of fluctuations of the antiferromagnetic clusters formed at high temperatures. To investigate the temperature variations of the relaxation time of the clusters, we analyzed the Mössbauer spectra using the method formulated by Blume. It has been shown that becomes long with decreasing temperature and the rate of the slow-down of is hastened aroundT _SG andT _N.