Jean-Claude Launay

Growth, spectroscopic and photorefractive investigation of vanadium-doped cadmium telluride

We present new results on the growth of semi-insulating vanadium-doped cadmium telluride crystals and their characterization by different optical techniques such as photoinduced current transient spectroscopy, absorption, photoconductivity spectra, and photorefractive wave mixing. Our joint research program aims at developing optimized crystals for efficient optical processing in the near infrared through the photorefractive effect.

Novel theoretical aspects on photorefractive ultrasonic detection and implementation of a sensor with an optimum sensitivity

We here expose theoretical and experimental results on homodyne detection using near-infrared laser sources, at 1.06, 1.32, and 1.55 μm wavelengths. The used photorefractive crystals are two large size CdZnTe:V samples. With speckled beams such as the ones scattered by diffusive objects, we reach a detection limit which, at 1.55 μm, is only 1.6 times above the one obtained with plane waves in a classical interferometer and only 2 and 2.2 times above at 1.32 and 1.06 μm, respectively. It is then demonstrated that the electron–hole competition, which varies enormously between these three wavelengths and gives a nearly zero two-wave-mixing gain at 1.32 μm, does not influence the sensitivity of the system. Moreover, we show that the frequency cutoff of the system is four times higher in the attenuation regime than in the amplification one.

Structural aspects of the metal-insulator transitions in V0.985Al0.015O2

Abstract The crystal structure of V 0.985 Al 0.015 O 2 has been refined from single-crystal X-ray data at four temperatures. At 373°K it has the tetragonal rutile structure. At 323°K, which is below the first metal-insulator transition, it has the monoclinic M 2 structure, wh...

Growth and electrical properties of pure and Ni-doped Co3O4 single crystals

Abstract Single crystals of pure and Ni-doped Co 3 O 4 spinel-type phases have been grown by C.V.T. Electrical conductivity and thermopower measurements have been performed between 25 and 950 K. Ni-doping gives rise to a significant increase of electrical conductivity at room temperature by three orders of magnitude. The obtained results suggest that the Ni atoms are located in the octahedral sites of the spinel lattice with both oxidation states 2+ and 3+, as illustrated by the formula : Co 2+ 1−y Co 3+ y [Co 3+ 2−x Ni 2+ y Ni 3+ x−y ]O 4 .

Characterization of deep levels in Bi12GeO20 by photoinduced current transient spectroscopy

A systematic investigation of deep levels in undoped and doped (Fe, Fe/V, excess of Bi) single crystals of Bi12 GeO20 (BGO) has been performed by photoinduced current transient spectroscopy. A list of traps in the energy range 0.1–1.0 eV with their thermal characteristics and their concentrations has been drawn up. Ten different levels have been detected, a few of them subdividing probably into substructures. In all probability, these trap species are present simultaneously in all the crystals but their concentrations differ considerably (typically from less than 1012 up to 5×1015 cm−3 ) so that dominant levels emerge. Variations in the number, nature, and concentration of the dominant centers were observed from preparation to preparation even for undoped BGO. This makes it difficult to draw reliable conclusions concerning the effect of doping, though some of the observed changes are probably directly related to Fe or Fe/V doping. This study of BGO also enabled us to test the experimental tool that we hav...

Elaboration et caracterisation de monocristaux de V2O3 par transport en phase vapeur

Abstract Single crystals of V 2 O 3 are obtained by HCl or TeCl 4 chemical transport method in sealed quartz tubes with temperature gradients of 1050–930°C and 990–900°C respectively. The hexagonal prims are 5 mm long and 1 or 2 mm wide. Both kinds of crystals are characterized by sharp insulator-metal transition at 156 and 137 K respectively, they present a large hysteresis. The sharp decrease in resistivity with increasing temperature is of the order of 10 8 at the transition.

Photorefractive response of CdTe: V under ac electric field from 1 to 1.5 μm

Abstract The ac field technique leading to the enhancement of the photorefractive effect is applied to vanadium-doped cadmium telluride. Net photorefractive gain is obtained for the three wavelenghts studied: 1.06 μm and 1.55 μm. A change in the sign of the photorefractive gain between 1.06 μm and 1.55 μm is observed, which represents the first evidence of an electron-hole competition in this material. Theoretical interpretation is performed using an extension of the ac field theoretical model with bipolar conduction.

Optical evidence of a photorefractive effect due to holes in Bi12GeO20 crystals

Abstract We report on the first observation to our knowledge of the photorefractive effect due to hole photoconduction in Bi 12 GeO 20 (BGO) crystals. In cubic crystals such as BGO, the study of energy redistribution in two beam coupling experiments does not allow by itself the determination of the sign of the photocarriers. Additional experiments such as ellipsometric measurements have to be performed to give the sign of the effective electrooptic coefficient. Such experiments points out that either holes or electrons govern the photorefractive effect this behavior is demonstrated in both undoped and iron-doped BGO crystals.

Role of the charge state of deep vanadium impurities and associations of defects in photoelectric and optical properties of semi-insulating CdTe crystals

Photoexcited carrier and space charge field subnanosecond dynamics has been investigated in bulk vanadium-doped and Cl- (or As-) co-doped CdTe crystals by using a degenerate four-wave mixing technique. Time-resolved measurements of picosecond grating decay at various illumination intensities revealed peculiarities of defect transformation with doping or under illumination. Novel features in carrier generation and dynamics were observed for the first time and allowed parameters of the defects to be extracted. A fast electron capture and its saturation with increasing intensity of illumination were observed in CdTe:V and attributed to a Cd divacancy, which is known as a deep double acceptor. Numerical modelling by using the two-deep-trap model allowed us to extract the concentration and recombination activity of Cd divacancies. An enhanced electron generation rate by factor of 3-4 was found in Cl-co-doped CdTe:V crystals in spite of a decreased density of donor vanadium states after the co-doping. We attribute this additional channel of electron generation to photoexcitation of the DX centre, which activation energy of 1-1.2 eV is close to a quantum energy of the YAG:Nd laser used ( eV).

Optical, photoelectrical, deep level and photorefractive characterization of CdTe:V

Abstract Characterization of different optical techniques, such as photo-induced current transient spectroscopy (PICTS), absorption, photoconductivity spectra and photorefractive wave mixing, of semi-insulating V-doped Cdytals prepared by the modified Bridgman method are presented. The aim of this joint research programme is to provide information which can lead to the understanding of the complex processes which determine photorefractive effects and to optimize the key parameters.