J.C. Launay

Anisotropie des proprietes electriques de l'oxyde de fer Fe2O3α

Abstract The electrical conductivity and the thermoelectric power of trivalent iron oxide α-Fe 2 O 3 , studied in the crystallographic plane (001) and along the [001] axis, have shown typical anisotropic behaviour. In the same material, the existence of two electronic transitions at 2.14 and 2.50 eV respectively has been found after optical absorption measurements. At high temperature (T > 1020 K) the electrical conductivity has an essentially intrinsic origin, and is practically independent of the oxygen pressure, while the extrinsic contribution predominates below 720 K. According to the model developped for the interpretation of the above experimental results, the peculiar behaviour of the electrical conductivity results from variations in the mobility of the carriers, depending on the electronic levels they are likely to occupy. These levels were identified as the results of a split in the t 2g orbitals of iron under the influence of the rhombohedral. distortion of the crystal field. Furthermore, the anisotropy of mobility is strongly dependent on the magnetic order below Neel temperature.

Crystal growth and photoelectrochemical study of Zr-doped α-Fe2O3 single crystal

Abstract Undoped and zirconium doped α-Fe 2 O 3 single crystals have been grown by CVD. Undoped crystals had a very high electrical resistivity (about 10 6 Ω cm at room temperature), so that they could not be used as photoelectrochemical anodes. The resistivity of doped crystals ranged from 4 to 75 Ω cm at 300 K. Their photoresponse has been studied. No anodic dissolution was observed in an alkaline solution. Quantum efficiency increases with decreasing doping level. From differential capacitance measurements, flat band potential and donor concentration of the semiconducting electrodes have been determined. The doping concentration, together with resistivity measurements, led to a free carrier mobility of about 0.1 cm 2 /V · s. The variation of the flat band potential with donor concentration has been discussed.

Determinations of the photorefractive parameters of Bi12GeO20 crystals using transient grating analysis

Abstract A method to characterize the photorefractive parameters of Bi 12 GeO 20 crystal is described. An analysis of the transient photoinduced grating under an external applied electric field allows to determine both the diffusion length and the Debyes screening length which are the important parameters for device applications.

Defects in photorefractive CdTe:V: An electron paramagnetic resonance study

Vanadium‐doped CdTe has recently been shown to have a high sensitivity for optical processing at 1.5 μm but the role of the vanadium dopant has been questioned. We present the results of an electron paramagnetic resonance study of this material, which demonstrates that vanadium is the dominant paramagnetic defect, which pins the Fermi level and gives rise to photoconductivity at 1.5 μm. The vanadium is substitutionally incorporated on a Cd site at concentrations of ≊5×1016 cm−3 and acts as a deep donor with a 2+/3+ level at Ev+0.8 eV. The defect is only observed in the 3+ charge state. The spin Hamiltonian parameters of the V3+ defect are determined as follows: electron spin S=1, Lande g‐factor g=1.962±0.001, central hyperfine interaction constant A=60×10−4 cm−1, Cd Ligand hyperfine interaction constant T=4×10−4 cm−1.

Determination of the factors controlling the optical background absorption in nominally undoped and doped sillenites

Abstract We investigated the background absorption spectrum of undoped and doped crystals within the BMO family (Bi 12 MO 20 , M=Ge, Si, Ti). Dopants were Al, Fe, Cu, Cu/V, and Cr. Many of the observed effects are consistent with the antisite Bi M absorption centre model and the derived predictions. The absorption level primarily depends on the presence of neutral Bi M centres. Their density in undoped material is determined by the nature of M. The effect of added chemical impurities is either to reduce or to enlarge the absorption coefficient. Concerning the influence of dopants, important factors are the valence state, their ability to take different valences, their concentration, their solubility, the symmetry of the occupied site, and magnitude and spectral dependence of their absorption cross section. Doping of BGO with Cu and V highly shifts the optical absorption towards the red and IR region.

Correlations between microscopic properties and the photorefractive response for vanadium-doped CdTe

We present a summary of the experimental results, the attempts of interpretation at the present stage, and the conclusions of coupled investigations on semi-insulating, n-type and p-type vanadium-doped CdTe crystals. Combining electron paramagnetic resonance and magnetic circular dichroism permits the monitoring and the quantitative assessment of [V2+] and [V3+] defect concentrations. Photoabsorption results are presented and are discussed in terms of photoionization processes at various wavelengths. The temperature and the spectral dependence of the steady-state photoconductivity are carefully studied in the near infrared. The results are discussed in terms of electronic transitions for the two vanadium charge states. Finally, the photorefractive performances of the semi-insulating sample are analyzed in a two-beam-coupling experiment and are correlated to the properties of the V2+/V3+ donor states to derive photoionization cross sections.

Magnetic circular dichroism and the optical detection of magnetic resonance for the Bi antisite defect in Bi12GeO20

We report on the optical detection of electron paramagnetic resonance at 36 GHz, 67 GHz and 70 GHz for all the absorption bands of Bi12GeO20 (BGO) in the visible spectral range. This completes our previous studies and demonstrates clearly that the three major magnetic circular dichroism features do belong to the same isotropic centre, i.e., a Bi3+Ge+h defect with the hole being delocalized onto the four surrounding oxygen atoms. We provide a simple explanation for the unusual sign of ODMR and the transient behaviour of the magnetic circular dichroism at resonance.

Experimental and theoretical treatments of the Czochralski growth of Bi12GeO20

Abstract This paper reports on a combined experimental and numerical investigation of BGO Czochralski growth. Different stages of the growth process are investigated. The effect of varying the Grashof number and the rotation Reynolds number on the flow and temperature distribution in the melt is explored over a range of parameters. Some aspects resulting from a non-isothermal crucible and from different sizes of the crystal radius are discussed.

Investigation of nonequilibrium carrier transport in vanadium-doped CdTe and CdZnTe crystals using the time-resolved four-wave mixing technique

We present a novel way to determine the type of dominant carrier photoexcited from deep traps in a photorefractive semiconductor. A numerical analysis of a picosecond free-carrier grating dynamics has revealed an excitation intensity dependent grating diffusive decay time τD as well as effective carrier diffusion coefficient D, when the intensity varied in the range below that required to create a bipolar carrier plasma. According to the numerical analysis, an increase or decrease of effective diffusion coefficient D with excitation can be used as a criterion to distinguish the type of photogenerated carrier. We have verified this method experimentally by measuring dependences of effective D versus excitation density in a number of vanadium-doped and shallow-impurity codoped CdTe and ZnCdTe crystals, using for excitation a picosecond YAG:Nd laser (hν = 1.17 eV). The results were found to be in good agreement with predictions, based on carrier transport peculiarities in photorefractive crystals, and correlated well with the secondary ion mass spectroscopy data for each crystal.

Wavelength dependent effective trap density in CdTe : Evidence for the presence of two photorefractive species

An accurate determination of the effective trap density in CdTe:V is performed using both counterpropagating (small grating spacing) and copropagating (large grating spacing) two wave mixing. This enhanced precision allows to show a variation of the effective trap density with wavelength. We show that such a change is explained when two traps both coupled to the conduction and the valence bands are present. New theoretical expressions for the electron hole competition coefficient and Debye screening length are derived and used