C. Hernández-Rodríguez

Anisotropic textured silicon obtained by stain-etching at low etching rates

The structure, luminescence and etching kinetics for porous silicon stain-etched at different temperatures are studied. The results reveal that for temperatures below 10 °C and for short etching times, a novel anisotropic structure based on surface roughness preferentially oriented in the 100 direction is observed. At temperatures higher than 10 °C or large etching times, typical macropores and mesopores with non-preferential pore wall orientation are detected. The luminescence spectra of the samples with preferential surface roughness orientation are red-shifted with respect to the samples with typical isotropic orientation. The results are interpreted in terms of average etching rates and pore growth.

Optical and compositional characterisation of stain-etched porous silicon subjected to anodic oxidation and thermal treatments

Abstract In this work, we study optical and compositional properties of stain-etched porous silicon (PS) passivated by means of anodic oxidation or thermal treatments in a N2 atmosphere. We search a passivation method that allows us to stabilise the surface properties of this material and to make it attractive to its implementation in a low-cost silicon-based solar cell fabrication process. We observe that the thermal treatment maintains the reflectance coefficient at very low values, and the anodic oxidation generates surfaces with a more homogeneous oxide composition than the thermal treatment. These results are appropriate with the requirements for future solar cells that can use these passivated porous layers as antireflection coatings.

Optical anisotropy of quartz in the presence of temperature-dependent multiple reflections using a high-accuracy universal polarimeter

Optical activity and birefringence in the presence of multiple reflections for (100), (010) and (101) planes of quartz were studied by using a high-accuracy universal polarimeter (HAUP) in a temperature range between 20 and 300 °C. Likewise, optical activity in the presence of multiple reflections for the (001) plane of quartz was studied by using a conventional polarimeter in the same temperature range at 632.8 nm wavelength. In all cases a modulation of the HAUP parameters determined by linear least-square fit is observed when the temperature is changed. This effect can be explained by the multiple reflections within the slab of a high degree of plane parallelism. The Levenberg-Marquardt method of nonlinear least-square routines was used to compare the experimental values of the HAUP coefficients with the results obtained using the Jones formalism. The method gives directly the optimum temperature dependence of birefringence and gyration tensor components of quartz.

Systematic errors in the high-accuracy universal polarimeter: application to determining temperature-dependent optical anisotropy of KDC and KDP crystals

The problem of removing the characteristic parasitic errors of the high-accuracy universal polarimeter method in a linearly birefringent and optically active crystal section is examined. The use of constant parasitic errors typical of each particular polarimetric system is shown to be inappropriate. The parasitics should preferably be determined in each measurement process, since the instrumental parasitics depend not only on the optical and mechanical elements of the experimental system (polarizers, rotators, detection unit etc.), but on sample quality, alignment of the system, and even in many cases on exactly where the light beam passes through the sample. Thus, measurements with different samples give different values of parasitic errors. Such instrumental parasitics can be held within the same order of magnitude for different samples (∼10−4) if they are of good optical quality. However, the parasitics are increased by an order of magnitude (∼10−3) when the samples are of moderate or bad optical quality. Optical anisotropy properties as coefficients of thermal variation of the birefringence of KDC and KDP single crystals and the optical activity of KDP at 632.8 nm wavelength are obtained, in the ranges from room temperature to 353 K and to 373 K, respectively.

Absolute optical rotation of CsLiB6O10

The optical rotation (OR) of CsLiB6O10 (CLBO, space group I\bar{4}2d) along the a axis has been determined by the HAUP method [Kobayashi & Uesu (1983). J Appl. Cryst. 16, 204–211] at a wavelength of 632.8 nm and by the TILTER method [Kaminsky & Glazer (1996). Ferroelectrics, 183, 133–141] at 532 nm and 650 nm. The respective rotatory powers were found to be 17 (1), 24 (2) and 19 (2)° mm−1. The absolute chirality has been established by comparing Bijvoet differences, {hkl} and {k\bar{h}l}, on the same crystal on which OR was measured. Atomic positions and electron density Fourier peak heights were exploited as input for semi-empirical calculations of refractive indices and OR, using WinOPTACT [Glazer (2002). J. Appl. Cryst. 35, 652] with only one free parameter fitted to match the average refractive index.

Optical birefringence imaging of the phase transition of K2Mn2(SO4)3

The present paper reports on the observation of the optical birefringence as a function of temperature of the langbeinite-type crystal K 2 Mn 2 (SO 4 ) 3 by using an imaging version of the so-called rotating-polarizer method. This method provides separate moving images of the orientation of the optical indicatrix and the magnitude of the optical anisotropy, showing the domain-wall behaviour during temperature changes. Some new results on the domain behaviour of the phase transition of K 2 Mn 2 (SO 4 ) 3 have been obtained. It is concluded that the sample single crystal consists of local regions, indicating the coexistence of two phases in this langbeinite-type crystal.

Temperature-dependent gyration tensor of LiIO3 single crystal using the high-accuracy universal polarimeter

The gyration tensor of LiIO3 has been measured in the temperature range between 293 and 493 K at a wavelength of 632.8 nm. Optical activity and birefringence for the (010) plane were determined by using a high-accuracy universal polarimeter (HAUP). Likewise, optical activity for the (001) plane was studied by using a conventional polarimeter in the same temperature range at 632.8 nm wavelength. In the latter case, a modulation of the optical activity was observed. This effect can be explained by multiple reflections within the slab with a high degree of plane parallelism.

Photoluminescence of monocrystalline and stain-etched porous silicon doped with high temperature annealed europium

In this work, for the first time, the photoluminescent emission and excitation spectra of non-textured layers and stain-etched porous silicon layers (PSLs) doped with high temperature annealed europium (Eu) are evaluated. The PSLs are evaluated as a host for rare earth ions and as an antireflection coating. The applied doping process, which consists in a simple impregnation method followed by a high-temperature annealing step, is compatible with the standard processes in the fabrication of solar cells. The results show down-shifting processes with a maximum photoluminescent intensity at 615 nm, related to the transition 5D0 → 7F2. Different initial concentrations of Eu(NO3)3 are evaluated to study the influence of the rare earth concentration on the photoluminescent intensity. The chemical composition and the morphology of Eu-doped PSLs are examined by means of x-ray dispersion spectroscopy, Fourier-transform infrared spectroscopy and scanning electron microscopy. These Eu-doped layers are considered to be applied as energy converters in silicon-based third generation solar cells.

Optical Activity of Deuterated Analog of L-Arginine Phosphate Single Crystals

The complete optical gyration tensor of monoclinic pure L-arginine phosphate monohydrate (LAP) and deuterated LAP (d-LAP) crystals has been determined. Variation of the optical activity in the presence of birefringence of d-LAP crystal with pure LAP is found from the high-accuracy universal polarimeter (HAUP) method at a wavelength of 632.8 nm. The gyration tensor components in terms of rotatory power were found to be r11 = -6.6(3) º/mm, r22 = 19(2) º/mm; r33 = 49.7(7) º/mm; and r13 = -43.7(8) º/mm for the LAP, and r11 = -1.2(1) º/mm, r22 = 5.0(4) º/mm; r33 = 41.4(3) º/mm; and r13 = -38.0(4) º/mm for d-LAP at room temperature.

Temperature-dependent optical rotatory power in the presence of birefringence of KTA and KTP crystals by the high-accuracy universal polarimeter method at 632.8 nm wavelength

Temperature-dependent simultaneous measurement of birefringence and optical rotatory power (ORP) for orthorhombic crystals of potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) has been achieved by using the high-accuracy universal polarimeter method at 632.8 nm wavelength. The birefringence and ORP changes along the (110) planes for KTA and KTP crystals were found to have a nearly parabolic form for the temperature range 297–493 K. The thermal variation coefficients were found to be 0.9 (5) × 10−5 K−1 and 5.6 (3) × 10−8 K−2 for KTA and 0.9 (5) × 10−5 K−1 and 5.7 (3) × 10−8 K−2 for KTP. The ORPs at 297 K were found to be 20.0 (20) and 20.5 (15)° mm−1 for KTA and KTP, respectively. The thermal variation coefficients of the ORP were found to be 1.8 (2) × 10−3° mm−1 K−1 and 1.1 (1) × 10–5° mm−1 K−2 for KTA and 1.9 (2) × 10−3° mm−1 K−1 and 1.2 (1) × 10−5° mm−1 K−2 for KTP.