Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997     

Electrical conductivity of pulverulent iron-cobalt molybdates Co1−xFexMoO4: evidence for hopping conduction

Different mixed iron-cobalt molybdates Co_1−xFe_xMoO₄ (0 < x ≤ 1) were prepared by means of a ceramic process. The influence of the isostructural substitution of Co²⁺ by Fe²⁺ and Fe³⁺ on the electrical conductivity of CoMoO₄ was studied in the temperature range (50–600°C). The results show that the iron substitution increases the electrical conductivity and changes the conduction mechanism of CoMoO₄. From a band conduction mechanism with an activation energy higher than 0.8 eV the conduction mode transforms into a hopping mechanism between the Fe²⁺ and Fe³⁺ ions in the octahedrally coordinated divalent cation sublattice. The activation energy is lower (0.4 eV) and does not alter around the polymorphic transition temperature. Owing to careful oxidations of the samples into cation deficient phases it was shown that the conductivity is proportional to the [Fe²⁺]/[Fe³⁺] ratio. These mild oxidations confirm the hopping mechanism. The presence of Co²⁺/Co³⁺ pairs has a minor contribution to the overall conductivity process. Paper presented at the 2nd Euroconference, Funchal, Madeira, Portugal, 10 – 16 Sept. 1995     

Gamma-ray induced color centers in Yb:YAG crystals grown by Czochralski method

20 at.% Yb:YAG single crystals have been grown by the CZ method and gamma-ray irradiation induced color centers and valence change of Fe³⁺ and Y b³⁺ ions in Yb:YAG have been studied. One significant 255 nm absorption band was observed in as-grown crystals and was attributed to Fe³⁺ ions. Two additional absorption (AA) bands located at 255 nm and 345 nm, respectively, were produced after gamma irradiation. The changes in the AA spectra after gamma irradiation and air annealing are mainly related to the charge exchange of the Fe³⁺, Fe²⁺, oxygen vacancies and F-type color centers. Analysis shows that the broad AA band is associated with Fe²⁺ ions and F-type color centers. The transition Y b³⁺→Y b²⁺ takes place as an effect of recharging of one of the Y b³⁺ ions from a pair in the process of gamma irradiation.     

Identification of Fe4+ and Fe5+ charge-transfer photochromic absorption bands in SrTiO3

Optical absorption and the enhancement and bleaching of Fe³⁺ and Fe⁵⁺ electron paramagnetic resonance in SrTiO₃: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O^2- valence states to Fe⁴⁺ and Fe⁵⁺. They occur at 2.09 and 2.82 eV for Fe⁴⁺ and at 1.99 and 2.53 eV for Fe⁵⁺.     

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO₃∶Fe and LiNbO₃∶Cu have been studied in combination with optical absorption-, Mössbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO₃∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe²⁺ impurities and the optical absorption band around 2.6 eV in LiNbO₃∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe³⁺ to Fe²⁺ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably. In the case of LiNbO₃∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed qualitatively. However, in contradiction to LiNbO₃∶Fe the Cu²⁺ centers cannot be related to the photorefractive sensitivity of LiNbO₃∶Cu. These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive process in doped LiNbO₃.     

X-ray diffraction and photoelectron spectroscopy study of swift heavy ion irradiated Mn/p-Si structure

The electronic structure and interfacial chemistry of thin manganese films on p-Si (1 0 0) have been studied by photoelectron spectroscopy measurements using synchrotron radiation of 134 eV and from X-ray diffraction data. The Mn/p-Si structures have been irradiated from swift heavy ions (∼100 MeV) of Fe⁷⁺ with a fluence of 1 × 10¹⁴ ions/cm². Evolution of valence band spectrum with a sharp Fermi edge has been obtained. The observed Mn 3d peak has been related to the bonding of Mn 3d-Si 3sp states. Mn 3p (46.4 eV), Mn 3s (81.4 eV) and Si 2p (99.5 eV) core levels have also been observed which show a binding energy shift towards lower side as compared to their corresponding elemental values. From the photoelectron spectroscopic and X-ray diffraction results, Mn₅Si₃ metallic phase of manganese silicide has been found. The silicide phase has been found to grow on the irradiation.     

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe²⁺-doped) lithium niobate crystals (LiNbO₃) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP _s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO₃ and LiNbO₃:Fe (0.1 ppm Fe²⁺) crystals with c axis along the direction of laser propagation.     

Interstitial iron and iron-acceptor pairs in silicon

Deep levels in iron-dopedp-type silicon are investigated by means of Deep Level Transient Spectroscopy (DLTS) and the Hall effect. Pairs of Fe with the acceptors B, Al, and Ga are observed at 0.1, 0.19, and 0.24 eV above the valence band edgeE _v. For interstitial iron, (Fe _i ), a level energy ofE _v+0.39+-0.02 eV is obtained with DLTS after correction with a measured temperature dependence of the capture cross section. The Hall effect yieldsE _v+0.37 eV for Fe_i. The annealing behavior of levels related to Fe is investigated up to 160 °C. Iron participates in at least four different types of impurity states: Fe _i , Fe-acceptor pairs, precipitations (formed above 120 °C) and an additional electrically inactive state, which is formed at room temperature.     

Electron paramagnetic resonance (EPR) and photo-EPR studies of aggregate centers with two iron atoms in silicon

We have investigated iron by EPR (electron paramagnetic resonance) and photo-EPR in initially boron doped silicon samples in which the iron concentration exceeded the boron content. A new EPR spectrum, showing orthorhombic-I symmetry was observed and could be fitted by an effective spin Hamiltonian with the parameters S=3/2, g=2.07, and E/D=0.68. We identify this spectrum as a new modification of a Fe₂B center which has the same symmetry but different configuration of the constituent atoms.Furthermore, we were able to determine the donor level of the old Fe₂B center to E=E_V+0.57±0.02 eV above the valence band.We have also investigated the Fe₂ donor. According to our straightforward interpretation the energy levels of the transitions from Fe₂⁺ to Fe₂⁰ and from Fe₂⁰ to Fe₂⁺ were determined as E=E_V+0.61±0.02 eV and E=E_C-0.67±0.02 eV, respectively, suggesting a lattice relaxation on electron capture, which is unusual for transition metal centers in silicon. PACS 71.55.Cn; 76.30.Fc     

Light-induced charge transport in BaTiO3 via three charge states of rhodium

By combined studies of electron spin resonance and optical absorption at low temperatures, the charge-transfer bands of Rh⁵⁺ and Rh⁴⁺ are identified to be peaked near 1.6 and 1.9 eV, respectively. On this basis, the light-induced charge-transfer processes in BaTiO₃:Rh are unraveled at room temperature. It is shown that three charge states of Rh are involved, leading to two levels: the shallow Rh^4+/5+ and the deeper Rh^3+/4+ level. The optical behaviour of these two levels corresponds to those expected from a two-center model. The present paper represents the first atomic-scale identification of three charge states of one element leading to optical two-level response.     

Electrical conduction mechanism and carrier trapping in β-metal free phthalocyanine single crystals

The dark electrical conductivity of β-metal free phthalocyanine single crystals has been investigated over the temperature range 273–600°K, at a reduced pressure of 10^?7 torr. The results obtained are in accordance with the model proposed by Barbe and Westgate[5] for this material, in which the energy gap between the top of the valence band and the bottom of the conduction band is determined to be 2·00 eV. At temperatures below about 410°K, the conduction process is consistent with the presence of an electron trapping level located 0·32 eV below the conduction band edge, with a density of 7×10¹⁶ cm^?3, and a donor level of density 2×10⁷ cm^?3 at the same energy. Above about 410°K, there is evidence to suggest that the conduction process is intrinsic.     

I.R.-Absorption peculiarities of GaP:Fe crystals irradiated by electrons

Using photo-ionization transitions theory by Allen and Langer the existence of two kinds of transitions was demonstrated for GaP:Fe crystals, namely, transition from valence band to local level (E_{2 = 1.27eV}). The effect of that form local level to conduction band (E_{2 = 1.27eV}). The effect of electron irradiation (E_{el = 3 MeV}) on both intracenter absorption and absorption in the 0.7?1.6 eV spectral region was investigated and the conclusion was made that absorption E₁ is probably due to Fe²⁺+e → Fe³⁺ photoionization transition while E₂-absorption is due to Fe³⁺ → Fe⁴⁺ +e transition.     

X-Ray photoelectron and mössbauer spectroscopy studies on molybdenum-bearing ferrite

The X-ray photoelectron spectra of Mo 3d electrons (232.4–232.9 and 229.4–229.6 eV) for Mo-bearing ferrites have suggested that molybdenum ions are in the 4+ valence state on the lattice points in the spinel structure. The XPS data for Mo 3d and Fe 2p electrons combined with the Mössbauer data at room temperature suggest that Fe₂MoO₄ takes a valence state (Fe²⁺)_tet[Fe²⁺Mo⁴⁺]_octO₄.     

Investigations on Fe doped polyvinyl alcohol films with and without gamma (γ)-irradiation

This paper deals with the preparation of pure and ferric chloride (FeCl₃) doped polyvinyl alcohol (PVA) films by solution casting method. Optical and electrical properties were systematically investigated. We have found the decrease in optical band gap energy of PVA films on doping FeCl₃. The optical band gap energy values in the present work are found to be 3.10 eV for pure PVA, 2 eV for PVA:Fe³⁺ (5 mol%), 1.91 eV for PVA:Fe³⁺(15 mol%) and 1.8 eV for PVA:Fe³⁺(25 mol%). Direct current electrical conductivity (σ) of pure, FeCl₃ doped PVA films in the temperature range 70-127 °C has been studied. At 387 K dc electrical conductivity of pure PVA film is 5.5795 μ Ω⁻¹ cm⁻¹, PVA:Fe³⁺ (5 mol%) film is 10.0936 μ Ω⁻¹ cm⁻¹ and γ-Irradiated PVA:Fe³⁺ (5 mol%) film for 900 CGY/min is 22.1950 μ Ω⁻¹ cm⁻¹. The result reveals the enhancement of the electrical conductivity with γ-irradiation. FT-IR study signifies the intermolecular hydrogen bonding between Fe³⁺ ions of FeCl₃ with OH group of PVA.     

Thermoelectric power and ac conductivity of A-type Nd2O3

Measurement of thermoelectric power Θ of pressed pellets of A-type Nd₂O₃ from 550 to 1180K and electrical conductivity (σ) at dc, 50 Hz, 1.542 kHz and 3 kHz at different temperatures is reported. It is concluded that electrical conduction at high temperature (T>600K) in this solid is due to positive large polarons in O²⁻ : 2p (valence) band and negative intermediate polarons in Nd³⁺ : 5d (conduction band). The energy band gap of the solid has been found to be 2.44 eV. At low temperatures, conduction by hopping of charge carriers from one impurity centre to another has been predicted.     

γ辐照室温短波HgCdTe光伏器件的导纳谱研究

利用变频导纳谱研究了γ辐照前后Hg_1－xCd_xTe(x=0.6)n⁺-on-p结中的深能级缺陷．辐照前其缺陷能级位置在价带上0.15 eV,俘获截面σ_p＝2.9×10^-18cm²,缺陷密度N_t＝6.5×10¹⁵cm^-3,初步认为是Hg空位或与其相关的复合缺陷;经过10⁴Gy的γ辐照后其能级变得更深,在价带上0.19 eV,同时其俘获截面增加了近一个数量级,而缺陷密度基本上没有变化．γ辐照引入的这种能级变化最终使器件的性能(探测率)下降了1/2以上． 关键词：     

Electronic structure of thin film iron-tetracyanoethylene: Fe(TCNE)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Thin film iron-tetracyanoethylene Fe(TCNE) _x , x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron spectroscopy results show no spurious trace elements in the films, and the iron is of Fe²⁺ valency. The highest occupied molecular orbital of Fe(TCNE) _x is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE⁻ singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results. The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)₂ where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe²⁺ (3d⁶) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE) _x system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)₂ (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T _C ) seen for Fe(TCNE) _x -type magnets.     

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi₂O₅ glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.     

Transfert électronique d'intervalence entre sites inéquivalents: Étude de CaFe3O5 par spectrométrie Mössbauer

As in magnetite Fe₃O₄, calcium ferrite CaFe₃O₅ is an oxide in which electron transfer occurs between the iron ions ($\text{Fe}{}^{\mathrm{3+}}\text{Fe}{}^{\mathrm{2+}}\text{= 2}$). This intervalence exchange process has been studied by ⁵⁷Fe Mössbauer spectroscopy and by electrical conductivity measurements. In CaFe₃O₅, the Fe³⁺ and Fe²⁺ ions occupy different crystallographic sites and have a deformed octahedral coordination. Each Fe²⁺O₆ octahedron shares an edge with two Fe³⁺O₆ octahedra. In the antiferromagnetic region (T_N = 282 ± 2 K), the Fe³⁺ and Fe²⁺ ions are well differentiated. Thermally-activated electron transfer is observed above t_n, in the paramagnetic region and is well characterized by the Mössbauer spectra. These are analyzed using the hypothesis of an electron jump limited to a trimer Fe³⁺Fe²⁺Fe³⁺ which leads to a relaxation time of 180 ns at 298 K and 80 ns at 400 K. Within this temperature interval, the process follows the Arrhenius law with an activation energy of 0.10 eV. Electrical conductivity measurements lead to similar results with an activation energy of 0.09 ± 0.02 eV.