A. Kling

Optoelectronic and structural properties of amorphous silicon–carbon alloys deposited by low-power electron-cyclotron resonance plasma-enhanced chemical-vapor deposition

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys ~a-SiC:H! are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance ~ECR! plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane ~or ethylene-!-to-silane gas phase ratios and by introducing the methane ~or ethylene! either remotely into the plasma stream or directly through the ECR source, together with the excitation gas ~hydrogen!. Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 ~defined as the energy at which the absorption coefficient is equal to 10 4 cm 21 !. Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.

Solid-phase crystallization of amorphous SiGe films deposited by LPCVD on SiO2 and glass

The crystallization kinetics and film microstructure of poly-SiGe layers obtained by solid-phase crystallization (SPC) of amorphous SiGe with Ge fractions (x) in the 0 to 0.42 range have been studied in detail. Amorphous SiGe layers 100 nm thick were deposited by LPCVD at 450°C on thermally oxidized Si afers and 7059 Corning glasses, using Si 2 H 6 and GeH 4 as gas sources. The samples were crystallized at 550°C and low pressure (below 9 Pa The evolution of the crystallization and the resulting film microstructure were characterized by X-ray diffractometry and transmission electron microscopy. The experimental results on growth kinetics fit the Avramis model. The characteristic crystallization time decreases with x, slowly for x < 0.3 and more abruptly for higher values of x. The transient time depends exponentially on x in all the intervals. The crystallized films have a (111) preferred orientation for low values of x and evolve to a randomly oriented polycrystal as x increases. The grain size in the fully crystallized layers decreases with increasing x. The results are similar for the films deposited on silicon dioxide and glass.

Polarization and local disorder effects on the properties of Er 3+ -doped XBi(YO 4 ) 2 , X=Li or Na and Y=W or Mo, crystalline tunable laser hosts

Pure and Er-doped ([Er]crystal≤2.4 × 1020 cm−3), NaBi(WO4)2 (NBW), NaBi(MoO4)2, and LiBi(MoO4)2 crystals have been grown by the Czochralski method. The three crystal hosts have similar structural and optical properties. The noncentrosymmetric space group I4¯ (No. 82) crystallographic structure has been established through 300 K single-crystal x-ray and neutron (for NBW only) diffraction. Er3+ energy levels were determined experimentally and simulated in the S4 symmetry through a crystal-field analysis. With this background, the large spectroscopic bandwidths observed were ascribed to the presence of two 2b and 2d sites for Er3+ and to different short-range Na+ and Bi3+ distributions around both sites. The radiative properties of Er3+ are described by the Judd-Ofelt theory achieving branching ratios and radiative lifetimes for transitions useful as laser channels. The 4I13/2—>4I15/2 laser channel (λ≈1.5 μm) shows a peak emission cross-section σEMI(λ≈1530 nm)≈0.5×10−20 cm2 and a quantum efficiency η∞0.68 to 0.74. The laser emission is envisaged to be tunable by Δλ≈100 nm.

Comparative study of the luminescence of structures with Ge nanocrystals formed by dry and wet oxidation of SiGe films

The luminescence emission of structures containing Ge nanocrystals embedded in a dielectric matrix obtained by dry and wet oxidation of polycrystalline SiGe layers has been studied as a function of the oxidation time and initial SiGe layer thickness. A clear relationship between the intensity of the luminescence, the structure of the sample, the formation of Ge nanocrystals and the oxidation process parameters that allows us to select the appropriate process conditions to get the most efficient emission has been established. The evolution of the composition and thickness of the growing oxides and the remaining SiGe layer during the oxidation processes has been characterized using Raman spectroscopy, x-ray diffraction, Fourier-transform infrared spectroscopy, Rutherford backscattering spectrometry and transmission electron microscopy. For dry oxidation, the luminescence appears suddenly, regardless of the initial SiGe layer thickness, when all the Si of the SiGe has been oxidized and the remaining layer of the segregated Ge starts to be oxidized forming Ge nanocrystals. Luminescence is observed as long as Ge nanocrystals are present. For wet oxidation, the luminescence appears from the first stages of the oxidation, and is related to the formation of Ge-rich nanoclusters trapped in the mixed (Si and Ge) growing oxide. A sharp increase of the luminescence intensity for long oxidation times is also observed, due to the formation of Ge nanocrystals by the oxidation of the layer of segregated Ge. For both processes the luminescence is quenched when the oxidation time is long enough to cause the full oxidation of the Ge nanocrystals. The intensity of the luminescence in the dry oxidized samples is about ten times higher than in the wet oxidized ones for equal initial thickness of the SiGe layer.

Nucleation and growth of platelet bubble structures in He implanted silicon

He a ions were implanted into (1 0 0) Si at energies from 30 to 120 keV and fluences from 5 · 10 15 to 1 · 10 16 cm ˇ2 . After implantation, pieces of these samples were subjected to rapid thermal annealing for 600 s at temperatures ranging from 300∞C to 700∞C. The samples were analyzed by Transmission Electron Microscopy (TEM) and by Rutherford Backscattering and channeling spectrometry (RBS/C). The TEM observations were related to the RBS/C measurements and the results discussed in terms of a nucleation model to explain the formation of overpressurized bubbles in He implanted and annealed silicon. ” 1998 Published by Elsevier Science B.V.

Optical and structural properties of chromium implanted lithium niobate – cluster formation and substitutional incorporation

Abstract In order to study the possibility of Cr doping of lithium niobate (LiNbO 3 ) by ion implantation Y -cut congruent lithium niobate samples were implanted with 100 keV Cr + ions at room temperature to fluences of 1×10 16 and 5×10 16 cm −2 . Annealings were performed in a conventional furnace at 600°C, 800°C and 1000°C for 15 min to 240 min under a flowing wet oxygen atmosphere. The optical absorption of the samples was studied in the spectral range from the ultraviolet (300 nm) to the infrared (3.5 μm). Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-ray Emission (PIXE) studies were performed in order to investigate the lattice recovery and the incorporation of the Cr dopant. Annealings at 600°C and 1000°C yielded a virtually complete lattice recovery and the Cr dopant was incorporated substitutionally in the same way as in melt-doped crystals. On the other hand at 800°C the annealing was hampered by the formation of clusters.

Lattice site location of Hf in LiNbO3: Influence of dopant concentration and crystal stoichiometry

Abstract The influence of dopant concentration and crystal stoichiometry on the lattice site of Hf in LiNbO3 single crystals was studied combining ion beam and hyperfine interaction methods. Two cases were studied: A near-stoichiometric LiNbO3 crystal doped with 1 mol% HfO2 and a congruent crystal doped with 6 mol% HfO2. In both cases it is shown that Hf occupies both Li and Nb sites, in contrast with the case of congruent crystals doped with a small amount of Hf, where it occupies only Li sites. The roles played by the stoichiometry of the crystal and the dopant concentration are discussed in the framework of the current models for the defect structure of lithium niobate.

Study of structural differences between stoichiometric and congruent lithium niobate

Abstract The structural differences between stoichiometric and congruent (lithium deficient) lithium niobate single crystals were studied by RBS- and NRA-channeling as well as perturbed angular correlation (PAC) measurements. The 111 Cd-PAC investigations point out that a second Li site can be detected in congruent material, while only one is present in stoichiometric. Channeling studies of different axes and the comparison of the results with computer simulations corroborated former indications that this additional lattice site can be attributed to the formation of ilmenite type stacking faults. A comparative study of the energy dependence of the dechanneling showed that a remarkable disorder is also present in the Nb sublattice of the congruent crystals and that these defects have a point-like character.

Strain and defects depth distributions in undoped and boron-doped Si1−xGex layers grown by solid phase epitaxy

A detailed characterization of undoped and heavily boron-doped Si1−xGex layers with x=0.21, 0.26, and 0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The starting material for solid phase epitaxial growth was prepared by amorphization of epitaxial SiGe-Si heterostructures by ion implantation. In order to obtain doped layers, boron was also implanted into some of the amorphous samples. After regrowth, the strain depth distributions of the SiGe layers were measured using axial channeling angular scans and the defect distributions were observed by high-resolution electron microscopy. A defect-free region ranging from 0 nm (undoped layer of x=0.34) to 30 nm (doped layer of x=0.21) in thickness was observed next to the layer-substrate interface. In the upper region of the layers, strain-relieving defects, identified as planar faults, were observed. Some isolated defects were also present at the layer-substrate interfaces of most of the samples. The measured strain depth profiles show that (i...

Simulation of channeling in crystals with defects using the CASSIS code

Abstract The concepts for the introduction of defects into the Monte Carlo simulation code CASSIS are discussed. The feasibility of the code to describe correctly effects on the channeling of light ions in cubic crystals containing point defects and dislocations is demonstrated for several examples. Calculations for intrinsic defects in the complex LiNbO 3 structure indicate that channeling measurements combined with Monte Carlos simulations yield a valuable contribution to the solution of the problem of stoichiometry related defects in this material