Spectroscopic ellipsometry analysis of GaAs1 − xNx layers grown by molecular beam epitaxy

In this work, we present the effect of nitrogen incorporation on the dielectric function of GaAsN samples, grown by molecular beam epitaxy (MBE) followed by a rapid thermal annealing (for 90 s at 680 °C). The GaAs_1 − xN_x samples with N content up to 1.5% (x = 0.0%, 0.1%, 0.5%, 1.5%), are investigated using room temperature spectroscopic ellipsometry (SE). The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. It was found that the features associated with E₁ and E₁ + Δ₁ transitions are blue-shifted and become less sharp with increasing nitrogen incorporation, in contrast to the case of E₀ transition energy in GaAs_1 − xN_x. An increase of the split-off Δ₁ energy with nitrogen content was also obtained, in agreement to results found with MOVPE GaAs_1 − xN_x grown samples.     

VUV spectroscopy of a new fluoride system NaF–(Er, Y)F3

Emission and excitation spectra as well as luminescence decay kinetics of a new complex fluoride system Na_0.4(Y_1−xEr_x)_0.6F_2.2 (x=0.01, 0.1, 1) have been studied in the vacuum ultraviolet (VUV) spectral range. It has been shown that these crystals have intense VUV luminescence due to the interconfiguration 5d–4f transitions in Er³⁺ ion. The spin-allowed 5d–4f luminescence of Er³⁺ in this system is very weak, i.e., in these crystals there exists an efficient non-radiative relaxation from higher-lying 5d states of Er³⁺ to the lowest 5d level responsible for spin-forbidden luminescence. This makes the studied new fluoride system a promising active medium for the production of VUV solid state laser with optical pumping. Due to rather large bandwidth of Er³⁺ 5d–4f luminescence in this system there is a possibility for the construction of tuneable VUV laser.     

Photoluminescence study of the nitrogen content effect on GaAs/GaAs1 − xNx/GaAs/AlGaAs: (Si) quantum well

We study the effect of nitrogen content in modulation-doped GaAs/GaAs_1 − xN_x/GaAs/GaAlAs:(Si) quantum well using low-temperature photoluminescence spectroscopy. The samples were grown on GaAs (001) substrates by molecular-beam epitaxy with different nitrogen compositions. The variation of the nitrogen composition from 0.04% to 0.32% associated to the bi-dimensional electron gas gives a new interaction mode between the nitrogen localized states and the GaAs_1 − xN_x/GaAs energies levels. The red-shift observed in photoluminescence spectra as function of nitrogen content has been interpreted in the frame of the band anticrossing model.     

Investigation in the VUV range of the excitation efficiency of the Tb ion luminescence in Y3 (Alx, Gay)5O12 host lattices

VUV excitation spectra of the visible luminescence of Tb³⁺ activated Y₃(Al_x, Ga_y)₅O₁₂ compounds and their diffuse reflectance spectra are presented. The crystal field strength, the absorption threshold of the matrix and the external quantum efficiency (Q_ext) associated with the different excitation mechanisms (4f⁸ → 4f⁷ 5d transitions in the activator or interband transitions in the host) decrease when the gallium concentration increases. Such variations are ascribed to an increase of the oxidizing character of the host matrix along with the gallium concentration. The decrease of the external quantum efficiency (Q_ext) is discussed in terms of a trivalent rare-earth ion (RE³⁺) configurational coordinate model and related to the intervention of the photoionized state [RE³⁺ → RE⁴⁺ + e⁻]. The non radiative relaxation rate increases when this state is stabilized.affd     

Optical absorption of Nd, Sm and Dy in bismuth borate glasses with large radiative transition probabilities

This paper reports on the optical properties of Nd³⁺, Sm³⁺ and Dy³⁺ in bismuth borate glasses, with Bi₂O₃ content varying from 30 to 60 mol%. The variation of the optical properties with composition plays a dominant role in determining a good laser host material. The variation of the Judd–Ofelt intensity parameters Ω_t (t=2,4,6) and the radiative transition probabilities and the hypersensitive band positions, with composition, have been discussed in detail. The changes in position and intensity parameters of the transitions in the optical absorption spectra are correlated to the structural changes in the host glass matrix. The variation of Ω₂ with Bi₂O₃ content has been attributed to changes in the asymmetry of the ligand field at the rare earth ion site and to the changes in their rare earth–oxygen (R–O) covalency, whereas the variation of Ω₆ strongly depends on nephlauxetic effect. The shift of the hypersensitive band shows that the covalency of the R–O bond increases with increase of Bi₂O₃ content, due to increased interaction between the rare earth ions and the non-bridging oxygens. The radiative transition probabilities of the rare earth ions are large in bismuth borate glasses, suggesting their suitability for laser applications.     

Optical properties of a-(Se70Te30)100−x(Se98 Bi2)x thin films

Measurements of optical constants (absorption coefficient, refractive index, extinction coefficient, real and imaginary part of the dielectric constant) have been made on a-(Se₇₀Te₃₀)_100−x (Se₉₈Bi₂)_x thin films (where x=0, 5, 10, 15 and 20) of thickness 2000 Å in the wavelength range 450–1000 nm. It is found that the optical bandgap decreases with the increase of Se₉₈Bi₂ concentration in the a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system. The value of refractive index (n) decreases, while the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states varying effective Fermi level.     

Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In₂O₃ thin films have been prepared from commercially available pure In₂O₃ powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In₂O_3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In₂O₃ (HVTE) and nearly stoichiometric In₂O_3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO₂ gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO₂ the calculated sensitivities (S=R_g/R_a) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In₂O₃ films to CO and CH₄. Negligible H₂O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl₂ and 10 ppm NO. Only 1000 ppm C₂H₅OH has resulted to have a significant cross to the NO₂ response.     

Effects of oxygen partial pressure on lead content of PLZT thin films produced by excimer laser deposition

Excimer laser ablation has been used to produce thin films of lanthanum-modified lead zirconate titanate (PLZT), or Pb_1−xLa_x(Zr_1−yTi_y)_1−x/4O₃. PLZT is an interesting class of materials since it has a wide range of compositionally dependent electro-optical properties and strong non-linear optical characteristics. PLZT thin films of 7/0/100, 28/0/100 and 0/0/100 compositions have been deposited onto crystalline Si100 and amorphous fused silica substrates. Effects of oxygen backfill pressure on the Pb:(Ti + La) ratios were investigated. The results indicate that controlling the oxygen backfill pressure during laser deposition strongly influences the stoichiometry and crystal structure of PLZT thin films.     

AlxGa−xAs/GaAs heterostructure characterization by wet chemical etching

Selective wet chemical etching of the Al_xGa_1−xAs/GaAs system has been applied to heterostructure characterization. Samples of LPE grown AlGaAs/GaAs laser double-heterostructures and separate confinement heterostructures as well as antiresonant reflecting optical waveguides heterostructures were treated with “I2 solution” (I₂:KI:H₂O) and hydrochloric acid. These compounds selectively etch the ternary Al_xGa_1−xAs layers, but with different “threshold composition” x_th values (the x value is that above which the etching rate of a given compound increases sharply). Selectively etched samples have been examined by SEM. The experimental dependence of etching rate on the x value for I2 solution has been derived. From this dependence, the x composition of any ternary layer can be estimated simply. Observations were made of the “microscopic” properties of the heterostructure, such as the smoothness of the interfaces and the uniformity of layers. All imperfections resulting from the growth process, such as interface perturbations or compositional nonuniformity of layers, are clearly seen. An additional advantage of this etching technique is its simplicity. It allows quick examination of grown heterostructure for the selection of wafers for further processing.     

Mass flow limitations in reactive sputtering

Reactive sputtering may be accomplished by mixing the inert gas argon with some reactive gas during sputtering. To form ZrN, sputtered zirconium atoms must react with nitrogen. At a specific deposition rate, a certain number F_N0 of nitrogen atoms must arrive at the substrate. This requirement is fulfilled at the partial pressure P_N = P_N0. The minimum partial pressure P_N0 to obtain a nitride is often detected by a sharp decrease in sputtering rate at a specific N₂ partial pressure. However, during reactive sputtering of for example ZrN we found that the value of P_N0 was strongly dependent on the total gas throughout in the vacuum system. An increase in total gas throughput causes a decrease in the value of P_N0.

The decrease in sputtering rate has indirectly been measured by optical emission spectroscopy from the sputtering plasma.     

Fluorination of CNx Nanotubes

Multiwall CN_x nanotubes have been prepared by thermal decomposition of acetonitrile over Co/Ni catalytic particles. The fluorination of nanotubes was performed at room temperature by using a gaseous mixture of BrF₃ and Br₂. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) indicated that only the outer shells of CN_x nanotubes were fluorinated, whereas the inner shells remained intact. X-ray photoelectron spectroscopy (XPS) showed an oxidation of pyridinic-type nitrogen with tube fluorination.     

Optical characterization of MBE-grown GaNAs

We report on low-temperature (T=2 K) optical studies of GaNAs epi-layers with different nitrogen concentrations grown by molecular beam epitaxy (MBE). The content of N in the layers was analyzed by X-ray diffraction (XRD) and secondary-ion mass spectrometry (SIMS). Using absorption data we have determined the fundamental band gap energy and the bowing parameter as a function of nitrogen concentration in the GaN_xAs_1-x alloy up to x=4%.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

Doping and electrical characteristics of in-situ heavily B-doped Si_1−x−yGe_xC_y (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si_1−x−yGe_xC_y with y=0.0054 or below, the carrier concentration was nearly equal to C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ cm⁻³, regardless of the Ge fraction. The B-doped Si_1−x−yGe_xC_y with high Ge and C fractions contained some electrically inactive B even at the lower C_B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.     

Quadrupolar moment calculations and mesomorphic character of model dimeric liquid crystals

Model dimeric compounds CH₃---(CH₂)_nCOO---R---OOC---(CH₂)_p---COO---R---OOC(CH₂)_n---CH₃, where (R=---C₆H₄---C(CH₃)=HC---C₆H₄---), of semiflexible liquid crystal polymers (LCP) have been studied by molecular modeling techniques. First, a conformational analysis of the molecules was made in order to find the lowest energy conformation structure. In a second stage, geometry optimization calculations of these conformers were performed at the quantum mechanics semiempirical PM3 level. Some molecular parameters (electric dipolar and quadrupolar moments) were obtained at the improved ab initio Hartree–Fock HF/6-31G(d)//PM3 level and are related with the mesomorphic character of these systems. It is found that the trace of the quadrupole moment tensor correlates with the spacer (---(CH₂)_p-linkage) length, and this can be an indication of the trend that this magnitude exhibits in aromatic liquid crystal systems.     

Conformation and orientation analysis of modified polyglutamates in thin films by ATR infrared spectroscopy

The molecular conformation and orientation in solution cast films of poly(-methyl l-glutamate)_x-co-(γ-n-octadecyl l-glutamate)_y), a coplymer of the ‘hairy rod’ type, was investigated by ATR FTIR spectroscopy. The influence of a structured interface on the molecular order of the polymer film was studied by introducing mechanical grooves in the surface of the solid support (ATR crystal). The resulting preferential orientation of the ‘hairy rods’ was quantified by recording polarized IR-spectra and yielded an order parameter of S = 0.8. A necessary condition for orientation was a high enough mobility of the molecules, which could be obtained by a swelling of the film in solvent vapor. Temperature dependent measurement revealed the influence of the molecular composition, i.e. the mole fraction y of the long side chain, on the phase state of the ‘hairy rod’ molecules. Our results are in agreement with the model by Tsujita et al. [Y. Tsujita, R. Ojika, K. Tsuzuki, A. Takizawa, T. Kinoshita, J. Polym. Sci., Part A: Polym. Chem. 25 (1987) 1041], proposing a glassy phase state of the long side chains for small mole fractions and a partly crystalline phase for high mole fractions.     

含氮石墨烯量子点的制备及其光学性质研究

采用水热法一步合成了含氮石墨烯量子点(NGQDs), 通过原子力显微镜(AFM)、透射电镜(TEM)、X射线光电子能谱(XPS)等对NGQDs的形貌和组成进行表征, 并进一步通过紫外-可见光谱(UV-Vis)、荧光光谱(PL)等手段研究了NGQDs的光学性质。AFM和TEM分析结果表明, NGQDs尺寸约为8.9 nm、厚度为0.6 ~2.0 nm (即1~3个碳原子层)。XPS分析结果表明NGQDs中氮含量约为17%, 且氮元素主要以“吡咯N”形式存在。光谱学实验表明, NGQDs的激发光谱与吸收光谱基本一致, 且其发射光谱与激发波长之间不存在依赖关系。此外, NGQDs的量子产率为~18%, 并随着含氮量的增加而增加, 且其荧光寿命衰变曲线可以被拟合成很好的双指数衰变曲线(τ₁=2.93 ns, τ₂=9.00 ns), 表明NGQDs有两种发色源, 即边缘富有含氧官能团的sp²碳簇和含氮五元环-吡咯环。     

Relation between the hue and composition of TiCxNy films prepared using multiple hollow cathode discharge ion plating

There is a distinct colour difference between electroplated gold layers and gold-like TiN films deposited by ion plating. In order to improve the hue of TiN coatings we have added carbon atoms to the Ti---N matrix and studied the influence of the carbon content on the hue of Ti---C---N complex films.

The films were prepared by means of the reactive ion-plating method using an in-line system composed of three chambers. The film composition was controlled by regulating the throughput of each reactant gas, i.e. nitrogen, acetylene, methane and hydrogen.

The colour of materials is dependent on the optical reflectivity and its dependence on the wavelength and on the lightness. It was found that a colour close to that of electroplated gold could be obtained with an appropriate TiC_xN_y composition, although the lightness of the film surface decreased as the reactant gas throughput increased.     

Structural characterization of CxByNz (x=0.1 to x=0.2) layers obtained by laser-driven synthesis

C_xB_yN_z layers have been prepared by laser-assisted chemical vapour deposition (CVD) in a gas atmosphere containing C₂H₄, B₂H₆ and NH₃ where the starting composition ratio could vary in a large range. The characterization of these C–B–N materials was made by XRD, EPMA, XPS, optical and scanning electron microscopy. The chemical composition of C–B–N layers tended to a composition C₁B₆N₃ on the line BN–‘B₃C’. The turbostratic structure of C–B–N layers could be influenced and modified as a function of composition. XPS investigations confirmed the single-phase nature and the existence of bonding between all the elements. Some planar structures, containing especially CB₂N groups, were suggested for the ‘unit cell’ of these C–B–N solid solutions, in agreement with EPMA and XPS analysis.     

Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content

Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers.     

Optical properties and laser performance of neodymium doped fluoroapatites SrxCa5−x(PO4)3F (x = 0, 1, 2, 3, 4, and 5)

Nd doped fluoroapatites Sr_xCa_5−x(PO₄)₃F(S_xC_5−xPF, X = 0, 1, 2, 3, 4, and 5) single crystals have been grown by the Czochralski technique. Their polarized absorption and emission spectra have been recorded at room temperature and used to calculate the absorption and stimulated emission cross sections. Broadening of the absorption and emission bands is observed for Nd³⁺ in the solid solutions SPF-CPF compared to Nd³⁺ in CPF or SPF. 1% Nd:S_xC_5−xPF, X = 0, 2, 3, 4, and 5, laser rods have been tested in a cavity longitudinally pumped by a 1 W AlGaAs laser diode and compared to Nd:YAG and Nd:YVO₄ rods. All fluoroapatites exhibit very good laser performance with low thresholds and high slope efficiencies, higher than in the case of YAG and equal to the YVO₄ samples. The dependance of the laser output power versus the diode temperature has also been measured for all materials. The laser output was found to be as sensitive to the diode temperature fluctuations for fluoroapatites as for YAG.