Charge states and optical transitions of vanadium in Bi4Ge3O12 identified by MCD and ODMR

The spectroscopic properties of V-doped Bi₄Ge₃O₁₂ have been investigated in detail via several complementary techniques bringing a series of consistent results. The optical absorption spectrum of annealed samples is strongly modified under UV-illumination and the initial state can be restored optically with visible light. Optical absorption and magnetic circular dichroism (MCD) demonstrate that a diamagnetic defect is partly destroyed during UV-illumination while a paramagnetic one is created. The latter shows a very characteristic S-shaped MCD pattern in the near-IR, which is readily assigned to the ²E→²T₂ internal transition of tetragonal V⁴⁺ centers at the Ge sites. This assignment was further confirmed by optically detected magnetic resonance (ODMR), via the change of the MCD under microwaves at 35 GHz. The g tensor of V⁴⁺ was found to be anisotropic with principal values g_||=1.81±0.03 and g=1.94±0.02. Two additional MCD bands in the visible spectral region could be attributed to V⁴⁺ via ODMR measurements. Ionization thresholds for holes (V⁵⁺) and electrons (V⁴⁺) were determined by optical absorption experiments and the V^4+/5+ donor level was positioned 1.9 eV below the conduction band. The presence of the V^4+/3+ acceptor state in the forbidden band is also suggested.     

Enhancement of Er I13/2 population in Y2O3 by energy transfer to Ce

We report measurements of the energy transfer between Er³⁺ and Ce³⁺ in Y₂O₃. The transition between the Er^{3+ 4}I_11/2 and ⁴I_13/2 excited states can be stimulated by energy transfer to Ce³⁺, augmenting the population in the ⁴I_13/2 state at the expense of that in the ⁴I_11/2 state. Experiments were performed on Y₂O₃ planar waveguides doped with 0.2 at.% erbium and 0–0.42 at.% cerium by ion implantation. From measurements of Er³⁺ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10⁻¹⁸ cm³/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y₂O₃ is two times smaller for Er³⁺→Ce³⁺ than that for Er³⁺→Eu³⁺ in the same material.     

Optical properties of rare earth ion (Nd, Er and Tb)-doped alumina films prepared by the sol–gel method

Rare earth ion (Nd³⁺, Er³⁺ and Tb³⁺)-doped alumina films were prepared by the sol–gel method using aqueous alumina sol. The effects of dopant concentration and treatment temperature on the optical properties, absorption and emission were examined for the doped films. Alumina films prepared by this method gave a high dopant concentration (0–15 mol% per alumina). Significant concentration quenching did not occur in this concentration range. The emissions from ⁵D₃ and ⁵D₄ of Tb³⁺-doped film reflected sensitively a matrix environment around Tb³⁺ ions. Er³⁺- and Nd³⁺-doped alumina films resonantly excited by cw Ti–sapphire laser (800 nm) showed upconversion emission at room temperature. The former gave 548 nm (⁴S_3/2→⁴I_15/2) and 640 nm (⁴F_9/2→⁴I_11/2) signals, and the latter 640 nm (⁴G_7/2→⁴I_11/2), which were dependent on alumina.     

紫外光激发Ce3+掺YVO4蓝光发射性能研究

利用提拉法生长了YVO₄和掺2.0at% CeO₂(或Ce₂(CO₃)₃)的YVO₄: Ce³⁺晶体。样品的XRD测试表明Ce³⁺代替Y³⁺进入晶格, Ce³⁺的加入并没有影响YVO₄的晶格结构。XPS测试显示YVO₄: Ce³⁺晶体中Ce离子3d分裂为882.0、885.8、902.9、908.0和915.9 eV等5个峰, 峰位表明样品中铈离子是以Ce³⁺和Ce⁴⁺两种价态形式存在。YVO₄和YVO₄: Ce³⁺激发谱都呈现出260~360 nm宽带激发, 此激发带源于基质中VO₄^3-离子团的配体O到V的电荷迁移吸收。使用325 nm的紫外线激发时, 两种样品均可发出以440 nm 为中心的宽带蓝光,其中YVO₄发射峰应归属于VO₄^3-离子团中³T₂→¹A₁和³T₁→¹A₁跃迁; 而YVO₄: Ce³⁺的蓝光发射则来源于Ce³⁺的5d→4f 的跃迁。分析可知YVO₄: Ce³⁺中VO₄^3-的π轨道和Ce³⁺的电子波函数能有效地重叠, 使得VO₄^3-和Ce³⁺可通过交换作用有效地向Ce³⁺传递能量, 可大幅提高Ce³⁺的蓝光发射强度。实验结果显示YVO₄: Ce³⁺可作为UV-LED管芯激发的白光发光二极管用高亮度蓝色发光材料。     

Synthesis and the luminescent properties of europium-activated Ca2SnO4 phosphors

The synthesis and photoluminescent (PL) properties of calcium stannate crystals doped with europium grown by mechanically activated in a high energy vibro-mill have been investigated. The characteristics of Ca₂SnO₄:Eu³⁺ phosphors were found to depend on the amounts of europium ions. The XRD profiles revealed that the system, (Ca_1−xEu_x)₂SnO₄, could form stable solid solutions in the composition range of x = 0–7% after being calcined at 1200 °C. The calcined powders emit bright red luminescence centered at 618 nm due to ⁵D₀ → ⁷F₂ electric dipole transition. Both XRD data and the emission ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) reveal that the site symmetry of Eu³⁺ ions decreases with increasing doping concentration. The maximum PL intensity has been obtained for 7 mol% concentration of Eu³⁺ in Ca₂SnO₄.     

Effect of heat treatment on photoluminescence behavior of BaMgAl10O17:Eu phosphors

The blue phosphor of BaMgAl₁₀O₁₇:Eu²⁺ (BAM) powders were prepared by solid-state reaction. The thermal degradation of BAM phosphor significantly reduces the intensity of the blue emission. BAM is reduced by an amount of 50% after heating at around 800 °C for 1 h. Photoluminescence (PL) excitation and emission spectra showed that the blue emission of 450 nm peak decreased with increasing annealing temperature. The ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ were observed at 590 and 615 nm emission lines over 1100 °C. Electron paramagnetic resonance (EPR) spectrum also detected two signals of Eu²⁺, corresponding to g=3.7156(9) for 88 mT, and g=2.9507(9) for 133 mT. X-ray absorption near edge structure (XANES) spectrum decreased the intensity of Eu²⁺ for 6977 eV with increasing annealing temperature, while high-energy peak of Eu³⁺ for 6984 eV was increased. The combined use of X-ray and neutron data by the Rietveld refinement appears to support that the secondary phase of EuMgAl₁₁O₁₉ magnetoplumbite structure in BAM may be formed by heat treatment.     

Highly conducting doped poly-Si deposited by hot wire CVD and its applicability as gate material for CMOS devices

Highly conducting p- and n-type poly-Si:H films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄+H₂+B₂H₆ and SiH₄+H₂+PH₃ gas mixtures, respectively. Conductivity of 1.2×10² (Ω cm)⁻¹ for the p-type films and 2.25×10² (Ω cm)⁻¹ for the n-type films was obtained. These are the highest values obtained so far by this technique. The increase in conductivity with substrate temperature (T_s) is attributed to the increase in grain size as reflected in the atomic force microscopy results. Interestingly conductivity of n-type films is higher than the p-type films deposited at the same T_s. To test the applicability of these films as gate contact Al/poly-Si/SiO₂/Si capacitor structures with oxide thickness of 4 nm were fabricated on n-type c-Si wafers. Sputter etching of the poly-Si was optimized in order to fabricate the devices. The performance of the HWCVD poly-Si as gate material was monitored using C–V measurements on a MOS test device at different frequencies. The results reveal that as deposited poly-Si without annealing shows low series resistance.     

Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic contacts on 4H-SiC

Ohmic contacts to the top p-type layers of 4H-SiC p⁺–n–n⁺ epitaxial structures having an acceptor concentration lower than 1×10¹⁹ cm⁻³ were fabricated by the rapid thermal anneal of multilayer Al/Ti/Pt/Ni metal composition. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to the formation of duplex cermet composition containing Ni₂Si and TiC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. The contacts exhibited a contact resistivity R_c of 9×10⁻⁵ Ω cm⁻² at 21°C, decreasing to 3.1×10⁻⁵ Ω cm⁻² at 186°C. It was found that thermionic emission through the barrier having a height of 0.097 eV is the predominant current transport mechanism in the fabricated contacts.     

Enhancement of red emission assigned to inversion defects in ZnAl2O4:Cr3+ hollow spheres

ZnAl₂O₄:Cr³⁺ hollow spheres composed of secondary nanoparticles with single spinel phase were fabricated using carbon templets. Monitoring the emission of 687 nm, two wide excitation bands attributed to the electrons of Cr³⁺ transiting from ⁴A_2g (⁴F) → ⁴T_1g (⁴F) and ⁴A_2g (⁴F) → ⁴T_2g (⁴F) were observed. The broad excitation band at about 397 nm was asymmetric and consisted of two peaks, indicating that there was a trigonal distortion existing in the lattices. The intensity of all emitting peaks revealed sharp increasing trend with the sintering temperature increase, and the intensity of emission at 698 nm assigned to inversion defects was more intense than that of emission at 687 nm assigned to octahedral Cr³⁺ ions in the undistorted spinel lattice. The samples with higher synthesized temperature revealed longer decay time, and the relative weightage of shorter decay time component decreased with the increase of sintering temperature, indicating that the surface defects decreased.     

Blue, green and 0.8 μm Tm, Ho doped upconversion laser glasses, sensitized by Yb

To obtain efficient upconversion laser glass, the optical properties of Tm³⁺ and Ho³⁺ were investigated in various glasses. Fluoride glass was selected as base glass for upconversion. The efficient upconversion fluorescences corresponding to the ¹G₄→³H₆ and ³H₄→³H₆ transitions of Tm³⁺ and the ⁵S₂→⁵I₈ transition of Ho³⁺ were observed in Yb³⁺-Tm³⁺ and Yb³⁺-Ho³⁺ doped aluminozircofluoride glasses excited at 980 nm. The very stronge blue and green emission light can be observed visually. The upconversion processes observed were two-photon processes for ³F₄→³H₆, ⁵S₂→⁵I₈ transitions and three-photon processes for the ¹G₄→³H₆ transition and can be described by a rate equation model. The energy transfer and energy back-transfer were analyzed in Yb³⁺-Tm³⁺ and Yb³⁺-Ho³⁺ systems. The relationship between emission intensity, pumping intensity and dopant concentrations is described using a rate equation model and shows good agreement with experiments. The dynamics of excited state ( ) is also analyzed with the diffusion-limited model based on Yokota-Tanimoto theory.     

Optical properties, fluorescence mechanisms and energy transfer in Tm, Ho and Tm -Ho doped near-infrared laser glasses, sensitized by Yb

The optical properties of the rare elements Tm³⁺, Ho³⁺ and Yb³⁺ were systematically investigated in various glasses. The Tm³⁺ doped aluminozircofluoride glass shows higher quantum efficiency, longer lifetime and stronger fluorescence intensity than Tm³⁺ doped YSGG crystal and other Tm³⁺ doped glasses for the ³H₄ → ³H₆ transition. Similar quantum efficiency, longer lifetime and stronger fluorescence intensity were also found in Ho³⁺ doped aluminozircofluoride glass for the ⁵I₇ → ⁵I₈ transition. The higher quantum efficiencies of Tm³⁺ and Ho³⁺ in aluminozircofluoride glass are due to the longer lifetime and the lower phonon energy. The fluorescence mechanisms and energy transfer in the Yb³⁺ -Tm³⁺ system, Yb³⁺ -Ho³⁺ system and Yb³⁺ - Tm³⁺ -Ho³⁺ system were studied. The very strong fluorescence intensities in the Yb³⁺ -Tm³⁺ system for Tm³⁺ and the Yb³⁺ -Tm³⁺ -Ho³⁺ system for Ho³⁺ which are 1.68 times that of Tm³⁺ doped YSGG crystal and 2.25 times that of Tm³⁺---Ho³⁺ codoped YSGG crystal are attributed to the efficient Yb³⁺ → Tm³⁺, Yb³⁺ → Ho³⁺ and Tm³⁺ → Ho³⁺ energy transfer processes. The fluorescence processes are described by cross relaxations of ²F_5/2 → ³H₅ → ³H₄ → ³H₆ → ²F_7/2 and²F_5/2 → ³H₅ (or ²F_5/2 → ⁵I₆ → ³H₅) → ³H₄ → ⁵I₇ → ⁵I₈ → ³H₆ → ²F_7/2.     

Field-lowering carrier excitation in cadmium arsenide thin films

Cadmium arsenide is a II–V semiconductor which exhibits n-type intrinsic conductivity with high mobility up to μ_n=1.0–1.5 m²/V s. Potential applications include magnetoresistors and both thermal and photodetectors, which require electrical characterization over a wide range of deposition and measurement conditions. The films were prepared by vacuum evaporation with deposition rates in the range 0.5–6.0 nm/s and substrate temperatures maintained at constant values of 20–120°C. Sandwich-type samples were deposited with film thicknesses of 0.1–1.1 μm using evaporated electrodes of Ag and occasionally Au or Al. Above a typical electric field F_b of up to 5×10⁷ V/m all samples showed instabilities characteristic of dielectric breakdown or electroforming. Below this field they showed a high-field conduction process with logJ∝V^1/2, where J is the current density and V the applied voltage. This type of dependence is indicative of carrier excitation over a potential barrier whose effective barrier height has been lowered by the high electric field. The field-lowering coefficient β had a value of (1.2–5.3)×10⁻⁵ eV m^1/2/V^1/2 which is reasonably consistent with the theoretical value of β_PF=2.19×10⁻⁵ eV m^1/2/V^1/2 expected when the field-lowering occurs at donor-like centres in the semiconductor (Poole–Frenkel effect). For thinner films Schottky emission was more probable. The effects of the film thickness, electrode materials, deposition rate, and substrate temperature on the conductivity behaviour are discussed.     

P-type InP grown by liquid phase epitaxy with rare earths: not intentional Ge acceptor doping

InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP:Fe substrates with praseodymium added to the melt. Room temperature Hall effect measurements revealed p-type conductivity of the layers with the hole concentration 6×10¹⁴ cm⁻³ and mobility 150 cm² V⁻¹ s⁻¹. By measuring temperature dependence of the hole concentration the binding energy of the dominant acceptor was determined as 223 meV. A photoluminescence line was found at 1.195 eV, close to the previously estimated no-phonon line of Ge acceptor transitions in Ge doped n-type InP. It was concluded that Ge acceptors cause the p-type conductivity of the grown layers.     

混合尖晶石型Zn6Ga8TiO20:Cr3+荧光粉的合成、结构表征与发光性能

采用高温固相法合成新型红色荧光粉Zn₆Ga_8-xTiO₂₀:xCr³⁺。用XRD、XRF和TEM对样品的成分和晶体结构进行表征, 发现所合成的荧光粉为单一的混合尖晶石结构, Cr³⁺能有效地掺杂进入基质Zn₆Ga₈TiO₂₀中, 并占据八面体格位。荧光光谱分析表明, 激发谱由四个峰组成, 峰值分别为281、337、420和555 nm, 其中281 nm对应Cr³⁺离子的⁴A₂→⁴T₁(4P)跃迁, 337 nm来自O^2-的2p轨道电子向Ga³⁺的4s4p轨道迁移跃迁, 420和555 nm分别对应Cr³⁺离子的⁴A₂→⁴T₁和⁴A₂→⁴T_2g的跃迁。发射光谱是由²E→⁴A₂的跃迁辐射零声子线(689 nm, R锐线)、处于畸变的环境中Cr³⁺发射的N线(696 nm)以及由晶格振动导致的声子伴随发射峰组成。这种荧光粉是一种可能应用在白光LED上的红色荧光粉。     

Spectroscopic evidence of inhomogeneous distribution of Nd in YVO4, YPO4 and YAsO4 crystals

Nd³⁺ : YVO₄ is one of the most interesting laser hosts for micro and diode-pumped solid state lasers. We have studied magnetic and optical properties of Nd³⁺ in three zircon type crystals YMO₄ (M=V, As, P). In particular, Nd³⁺ ions exhibit in the three hosts a multisite character observed in the absorption and emission spectra. However, the emission and its dynamics are strongly dependant on the reabsorption mechanisms. In Nd : YVO₄, single crystals containing 7 ± 1 × 10¹⁸ Nd³⁺ ions/cm³, the lifetime is 95 ± 2 μs in good agreement with the calculated radiative lifetime. Electron Paramagnetic Resonance (EPR) measurements are performed to identify the nature of the different substitution sites for Nd³⁺ ions. Nd³⁺ ions are found to be inhomogeneously distributed in tetragonal D_2d symmetry sites, in isolated ions, “shallow clusters” and pairs. Proportions of the different local environments depend on the total neodymium concentration. For instance, 15% of the Nd³⁺ ions are gathered in Nd³⁺–Nd³⁺ pairs for 7.2 ± 0.2 × 10¹⁹ Nd³⁺ ions/cm³.     

The luminescent quantum efficiency of Cr ions in Cs2NaAlF6 single crystals

The luminescent quantum efficiency of Cr³⁺ ions in single fluoride crystal Cs₂NaAlF₆ was determined by using the simultaneous multiple-wavelength photoacoustic and luminescent experiments method, based on the generation of photoacoustic and luminescence signals after pulsed laser excitation. The luminescent quantum yield for the most important transition between the ⁴T₂→⁴A₂ vibronic levels was found to be 68±3%. This value agrees with that obtained from the ratio of the lifetimes of the corresponding transition at different temperatures.     

Optical properties and energy transfer among Nd in Nd:Ca2Al2SiO7 crystals for diode pumped lasers

The energy levels of neodymium in the Nd³⁺:Ca₂Al₂SiO₇ (CAS) laser material with gehlenite structure are reported. As the Nd³⁺:Ca₂Al₂SiO₇ compound presents a broad absorption around 806 nm, it is a good candidate for diode pumped laser. The ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I11/2 emission have been recorded and the fluorescence branching ratios calculated from the Judd-Ofelt analysis are 0.41 and 0.47 respectively. The emission cross section at 1.06 μm (⁴F_3/2→⁴I11/2 transition) is 5 × 10^-20 cm². The decay profiles of the Nd³⁺ emission have been analyzed for several Nd³⁺ concentrations using the kinetic microparameters related to the cross relaxation ( and R₀≈6 Å) and the energy migration probabilities ( ). In the Nd:CAS laser material, the optimal concentration corresponding to the maximum of the fluorescence intensity is determined to be around 2.7 × 10²⁰ Nd³⁺ ions cm^-3. The Nd³⁺-Nd³⁺ interactions are not very strong in this material as the optical concentration value is two times higher than in the Nd:YAG laser material.     

Infrared absorption and Zeeman effect in Nd doped YVO4

Low temperature infrared transmission studies of Nd³⁺ doped YVO₄ were performed, under a magnetic field B c, in the 1800–8000 cm⁻¹ range of the ⁴I_9/2→⁴I_11/2, ⁴I_13/2, and ⁴I_15/2 Nd³⁺ crystal-field transitions. Good agreement is obtained between the experimental and calculated g-factors. Frequencies of the satellites in the ⁴I_9/2→⁴F_3/2 transitions of the Nd³⁺ isolated ion confirm the presence of ferromagnetic interactions between pairs of coupled Nd³⁺ ions that lift the Kramers doublet degeneracies of their ground state and excited multiplets.     

Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V⁴⁺ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V₂O₅ by 1 mol.% Fe₂O₃ which may be due to a decrease in the V⁴⁺/V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe₂O₃ which may be due to possible hopping conduction between Fe²⁺−Fe³⁺, V⁴⁺−Fe³⁺ and Fe²⁺−V⁵⁺. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V⁴⁺/V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V⁴⁺/V ratio and the microstructure.     

Yb to Er energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca2Al2SiO7

Rate equations formalism is used to predict the population ratio of the Er^{3+ 4}I_13/2 levels involved in the 1.55 μm laser transition in the Yb:Er:CAS laser materials. An effective Yb → Er energy transfer, favourable to the Er³⁺ 1.55 μm laser emission, is demonstrated in this laser host. Indeed, the Yb → Er transfer and the Er → Yb back transfer rates are calculated to be 6 x 10⁻¹⁶ and 0.45 x 10⁻¹⁶ cm³ s⁻¹, respectively. Attempts of codoping the system with Nd³⁺, Eu³⁺ and Ce³⁺ have been realised in order to increase the population of the Er^{3+ 4}I_13/2 laser emitting level. Best results are obtained with Ce³⁺ ion since in the sample containing 6 x 10²⁰ Ce³⁺/cm³, the Er^{3+ 4}I_11/2 level lifetime is divided by a factor of 3 while the Er^{3+ 4}I_13/2 fluorescence lifetime remains unaffected. On the contrary, codoping with Nd³⁺ or Eu³⁺ ions simultaneously decreases the Er^{3+ 4}I_11/2 and ⁴I_13/2 kinetics parameters. The role of the other parameters such as Yb/Er concentrations ratios is also discussed.