Optical Properties Of Cr Research Articles

CrO codoping effect on electronic and optical properties of GaN

The crystal structure,electronic and optical properties of Cr,O and CrO codoped GaN were calculated by first-principle calculation.The results show that codoping method brings in new intermediate band as well as lowering the formation energy in comparison with single oxygen atom doping.The formation of intermediate band enhances absorption of low energy photons,especially in infrared range of the sun spectrum.Our calculation predicts the feasibility of CrO codoped GaN used as a third-generation solar cell semiconductor material.

Dopant induced changes in structural and optical properties of Cr3+ doped TiO2 nanoparticles

Chromium doped TiO2 nanoparticles were synthesized by sol–gel method and studied by X-ray diffraction, Raman, UV–vis and photoluminescence spectroscopy. The size distribution and composition of the material were investigated by TEM and EDAX respectively. Incorporation of Cr3+ leads to the distortion of lattice structure and creation of oxygen vacancies as revealed by XRD and Raman spectra. Crystal field splitting of Cr3+ in substituted TiO2 results in a spin allowed and spin forbidden absorption peaks arising from Fano antiresonance. Luminescence spectra give emission lines of dopant as well as of oxygen related defect centers.

Cr- and Mo-Doping Effects on Structural and Orbital Order Phase Transition in Spinel-Type MnV2O4

Spinel-type vanadium oxides have been studied from the viewpoints of spin and orbital order in a geometrically frustrated system. We have investigated the doping effects of Cr 3+ (3 d 3 ) and Mo 3+ (4 d 3 ), which have no orbital degrees of freedom, to the V 3+ (3 d 2 ) site of MnV 2 O 4 , in order to clarify the structural, transport and optical properties of Cr- and Mo-doped samples. The orbital order is suppressed by Cr and Mo doping and the electronic structure depends on the doped elements. The results of transport and optics indicate that electrons are localized in the Cr-doped system, while they become nearly itinerant in the Mo-doped one.

Growth and optical properties of Cr3+-doped CdWO4 single crystals

A high-quality Cr3+:CdWO4 single crystal at a size of approximately \Phi 25 \times 80 mm is grown using the Bridgman method with CdO, WO3, and Cr2O3 as raw materials and their molar ratio of 100:100:0.5. The temperature gradient of solid-liquid interface at growth is approximately 50 oC/cm and the growth rate is 0.05 mm/h. The X-ray diffraction (XRD), absorption, excitation, and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated. Two strong broad optical absorption bands of about 472 and 708 nm are observed, and they are associated with the transitions 4A2->4T1 and 4A2->4T2. The weak 4T2->2E transition (the R-line) at 632 nm is also observed. The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4, 776.8, and 3 427.6 cm-1, respectively, according to the absorption spectra and crystal-splitting theory. A broadband fluorescence at about 1 000 nm due to 4T2->4A2 transition is produced by exciting the samples at 675 nm. After being annealed in an O2 atmosphere, the crystals become more transparent, while the effective light absorption of Cr3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.

Cr3+-Doped Fluorides and Oxides: Role of Internal Fields and Limitations of the Tanabe–Sugano Approach

This work is aimed at clarifying the changes on optical spectra of Cr(3+) impurities due to either a host lattice variation or a hydrostatic pressure, which can hardly be understood by means of the usual Tanabe-Sugano (TS) approach assuming that the Racah parameter, B, grows when covalency decreases. For achieving this goal, the optical properties of Cr(3+)-doped LiBaF(3) and KMgF(3) model systems have been explored by means of high level ab initio calculations on CrF(6)(3-) units subject to the electric field, E(R)(r), created by the rest of the lattice ions. These calculations, which reproduce available experimental data, indicate that the energy, E((2)E), of the (2)E(t(2g)(3)) → (4)A(2)(t(2g)(3)) emission transition is nearly independent of the host lattice. By contrast, the energy difference corresponding to (4)A(2)(t(2g)(3)) → (4)T(1)(t(2g)(2)e(g)(1)) and (4)A(2)(t(2g)(3)) → (4)T(2)(t(2g)(2)e(g)(1)) excitations, Δ((4)T(1); (4)T(2)), is shown to increase on passing from the normal to the inverted perovskite host lattice despite the increase in covalency, a fact which cannot be accounted for through the usual TS model. Similarly, when the Cr(3+)-F(-) distance, R, is reduced both Δ((4)T(1); (4)T(2)) and the covalency are found to increase. By analyzing the limitations of the usual model, we found surprising results that are shown to arise from the deformation of both 3d(Cr) and ligand orbitals in the antibonding e(g) orbital, which has a σ character and is more extended than the π t(2g) orbital. By contrast, because of the higher stiffness of the t(2g) orbital, the dependence of E((2)E) with R basically follows the corresponding variation of covalency in that level. Bearing in mind the similarities of the optical properties displayed by Cr(3+) impurities in oxides and fluorides, the present results can be useful for understanding experimental data on Cr(3+)-based gemstones where the local symmetry is lower than cubic.

Correlation between electrical and optical properties of Cr:ZnO thin films grown by pulsed laser deposition

Highly transparent and conducting Chromium doped ZnO (Cr:ZnO) thin films with preferential c-axis orientation were grown on (0 0 0 1) sapphire substrates using buffer assisted pulsed laser deposition. The resistivity of Cr:ZnO thin films was found to decrease to a minimum value of ∼1.13×10 −3Ω cm with the increasing Cr concentration up to ∼1.9 at.% and then increase with further increase of Cr concentration. On the contrary, the band gap and carrier concentration of Cr:ZnO thin films increased up to ∼3.37 eV and ∼2×10 20 cm −3, respectively, with the increase of Cr concentration up to ∼1.9 at.%, then decreased with further increase of Cr concentration. The increase of carrier concentration and conductivity with Cr doping at low Cr concentrations (<1.9 at.%) could be attributed to the presence of Cr in +3 valence state in ZnO thus acting as donor while decrease of carrier concentration beyond ∼1.9 at.% of Cr concentration could be attributed to the charge compensating effect due to the presence of acceptor like point defects such as oxygen interstitials. This was experimentally confirmed using x-ray photoelectron spectroscopy. The observed variation in the band gap of Cr:ZnO thin films with increasing Cr doping was attributed to the competing effects of the high free carrier concentration induced Burstein-Moss blue shift and band gap narrowing.

Temperature and pressure dependence of the optical properties ofCr3 + -dopedGd3Ga5O12 nanoparticles

Since the crystal-field strength at theCr3 + siteis very close to the excited-state crossover (ESCO), this work investigates the optical properties ofCr3 + -dopedGd3Ga5O12 (GGG) nanoparticles as a function of temperature and pressure in order to establish theeffect of the ESCO on the optical behaviour of nanocrystalline GGG. Luminescence,time-resolved emission and lifetime measurements have been performed onGGG:0.5% Cr3 + nanoparticles in the 25–300 K temperature range, as well as under hydrostatic pressureup to 20 GPa. We show how low temperature and high pressure progressivelytransforms broadband emission into a ruby-like luminescence. This behaviour together with the lifetime dependence on pressure andtemperature are explained on the basis of the spin–orbit interaction between the 4T2 and 2E statesof Cr3 + .

Electronic structure and optical properties of Cr doped SnO2 superlattice

By the full-potential linearized augmented plane-wave method (FP-LAPW), we investigate the electronic structure, the band structure, the dielectric function, the absorption spectrum, the reflectivity and the refraction of Cr doped SnO2 superlattice. The generalized gradient approximation (GGA) is used for handling correlation energy. Calculation results show that due to the Cr doping, SnO2 superlattice forms new electron occupied state near Fermi energy level and uncentinuous impurity band comes into being, which is contributed by Cr-3d and O-2p, Sn-5s. In dielectric spectrum appear three new dielectric peaks between 0 eV to 5.5 eV. In high-energy area, the position of main peak has a blue-shift and the peak intensity reduces. Absorption spectrum, reflectivity spectrum and refraction spectrum also have peaks corresponding to the dielectric peaks, which are caused by d—d transition of Cr atom.

Optical properties of Cr^2+:ZnSe single crystal and film

To realize an electrically pumped compact mid-IR microlaser, the optical properties of a Cr(2+):ZnSe film deposited by rf magnetron cosputtering were compared with those of a single crystal. The mid-IR room temperature photoluminescence efficiency of the film appears only twice less than the one of the single crystal under direct (1850 nm) and indirect (458 nm) excitations. The smaller Cr(2+) fluorescence lifetime values of the film were attributed to the presence in the film of a large amount of chromium in a valence state different from Cr(2+) and of structural defects in the ZnSe host matrix.

Optical Properties of Cr(III) doped YAG Nanoceramics

The Cr3+ doped Y3Al5O12 ceramics composed of nanosized grains were fabricated by the low temperature high pressure sintering technique. The photo - and cathodoluminescence spectra of Cr3+:Y3Al5O12 nanoceramics were measured. The effect of applied pressure on luminescence properties of Cr3+ doped Y3Al5O12 nanoceramics was investigated.

Optical Properties of Cr(III) Doped YAG Nanoceramics

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Luminescence studies of Cr 3+ doped MgAl 2O 4 nanocrystalline powders

A method of synthesis of Cr 3+:MgAl 2O 4 nanocrystalline powders is presented. Their morphology and structure were characterized by XRD and TEM methods. The sizes of nanocrystals were controlled by temperature annealing process. The size effect of nanocrystals on optical properties of Cr 3+:MgAl 2O 4 was investigated. Following the luminescence, excitation and lifetime measurements, it was found that the optical properties of Cr 3+:MgAl 2O 4 nanocrystals were significantly dependent on their size.

Optical properties of Cr3+ ion in lithium metasilicate Li2O · SiO2 transparent glass–ceramics

The optical properties of Cr3+ ions in lithium metasilicate (Li2O·SiO2) transparent glass–ceramics were investigated. The main crystalline phase precipitated was the lithium metasilicate (Li2O·SiO2) crystal. The percent crystallinity and crystalline size were ranging 65–75% and 20–35nm, respectively. The color changes drastically to deep pink from emerald green upon crystallization. New and strong absorption bands appeared and the absorption intensity increases by about 10 times that in glass. These new absorption bands are found to be derived from Cr3+ ions in octahedral sites in the lithium metasilicate crystal lattice. Cr3+ ions substitute for three Li+ ions and occupy the distorted octahedral site between single [SiO4]n chains of lithium metasilicate crystal. The ligand field parameters can be estimated: 10Dq=13088 cm−1, B=453cm−1, Dq/B=2.89 and C=2036cm−1. The near-infrared luminescence centered at 1250nm was not detected in the deep pink glass–ceramics unlike emerald green glass.

Laser-diode pumped self-Q-switched microchip lasers

Optical properties of Cr,Yb:YAG, Cr,Nd:YAG crystals, and composite Yb:YAG/Cr:YAG ceramics self-Q-switched solid-state laser materials are presented. The merits of these self-Q-switched laser materials are given and the potentials of such lasers can be chosen by the applications. Cr,Yb:YAG and composite Yb:YAG/Cr:YAG ceramics self-Q-switched laser are conducted. Although several tens of kW peak power can be obtained with a monolithic microchip Cr,Yb:YAG laser, the experimental results show that the performance of this laser is limited by the absorption of Cr4+ ions at a pump wavelength of 940 nm and strong fluorescence quenching at high Cr concentration. Composite Yb:YAG/Cr:YAG ceramics are more suitable to realize high pulse energy and peak power (up to MW level) with optimized lasing and Q-switching parts. In addition, the instabilities induced by the multi-longitudinal mode competition in Cr,Nd:YAG and Cr,Yb:YAG microchip lasers are addressed. The different gain bandwidths of Yb:YAG and Nd:YAG play an important role in the instability of the output laser pulse trains. Stable laser pulses from the Cr,Yb:YAG microchip laser were obtained due to the antiphase dynamics. For the Cr,Nd:YAG microchip laser, the instability caused by the multi-longitudinal mode competition is an intrinsic property. Different transverse patterns were observed in Cr,Nd:YAG microchip lasers when a pump beam with larger diameter was used. Saturated inversion population distribution inside the gain medium plays an important role in the transverse pattern formation. Different transverse patterns were reconstructed by combining different sets of the Hermite-Gaussian modes.

Crystal growth and infrared spectroscopy of Cr:Cd 1−xZn xTe and Cr:Cd 1−xMg xTe

The absorption and emission properties of Cr 2+-doped II–VI semiconductors are of considerable current interest for applications in mid-infrared (MIR) solid-state lasers and for passive optical Q-switches. In this paper, the crystal growth and compositional effects on the optical properties of Cr:Cd 1− x Zn x Te ( x=0.05, 0.1, 0.2) and Cr:Cd 1− x Mg x Te ( x=0.15, 0.35) were investigated. Undoped Cd 1− x Zn x Te and Cd 1− x Mg x Te crystals were grown by Bridgman technique. Cr doping of both materials was achieved either in situ during growth or through a thermal diffusion process. For Cr:Cd 1− x Zn x Te, the Cr 2+ absorption properties were strongly dependent on the host composition and spectral blue shifts were observed with increasing Zn content. The Cr 2+ absorption peak shifted from ∼1910 nm for Cr:CdTe to ∼1815 nm for Cr:Cd 0.8Zn 0.2Te. Only small spectral shifts were observed for the MIR emission from Cr:Cd 1− x Zn x Te. On the contrary, the optical properties of Cr 2+ in Cr:Cd 1− x Mg x Te were nearly independent of the host composition and resembled Cr:CdTe.

Magnetic and optical properties of Cr +-implanted GaN

In this paper, we report the fabrication of ferromagnetic semiconductor on GaN substrates by high-dose Cr ion implantation. Both magnetic and optical properties for Cr +-implanted GaN were studied by photoluminescence and superconducting quantum interference device. The results of photoluminescence reveal that the implanted chromium incorporates substitutionally on Ga site and forms deep acceptor level in GaN. The materials show ferromagnetic-like order up to 300 K as indicated by temperature-dependent magnetization measurement. The M vs. T curve also implies the presence of different components to the magnetization in Cr +-implanted GaN.

Compositional Changes in the Infrared Optical Properties of Cr Doped CdZnTe Crystals

AbstractWe are currently investigating the infrared (IR) optical properties of Cr doped ternary cadmium chalcogenides for potential applications in solid-state lasers and passive optical Qswitches. In this paper, we present compositional changes in the IR optical properties of Cr doped Cd1-xZnxTe single crystals with x=0.05, 0.1, and 0.2. Undoped CdZnTe crystals were grown by vertical Bridgman technique. Cr doping of CdZnTe was achieved through either in-situ doping or through a thermal diffusion process. For comparison, Cr: CdTe and Cr: ZnTe crystals were also prepared. The optical properties of Cr2+ ions were strongly dependent on the host composition and spectral blue shifts were observed with increasing Zn content in Cr: CdZnTe. The IR absorption peak of Cr2+ ions shifted from ∼1910 nm for Cr: CdTe to ∼1815 nm for Cr: Cd0.8Zn0.2Te. Less pronounced blue shifts were observed for the IR emission from Cr: CdZnTe crystals. The spectral shifts can be explained by the decrease in bond-length when going from CdTe to CdZnTe leading to an increased crystal-field effect experienced by Cr2+ ions. A slight broadening of the absorption and emission was also observed in ternary Cr: CdZnTe compounds compared to Cr: CdTe, which suggests that Cr2+ ions were incorporated in multiple lattice sites in CdZnTe. Moreover, the Cr2+ emission dynamics revealed a slightly non-exponential decay behavior for Cr: CdZnTe crystals, whereas the decay time of Cr: CdTe was single-exponential.

Cr 4+-doped Li 2CaSiO 4 crystal: growth and spectroscopic properties

The growth and optical properties of Cr 4+-doped Li 2CaSiO 4 single crystals are described. Absorption spectroscopy together with calculations of energy levels shows distortion of tetrahedral sites with Cr 4+ doping. The symmetry of the local environment was assigned to D 2d; ligand field parameters were determined for both undistorted and distorted tetrahedral sites. Emission spectra show broadband fluorescence from 1000 to 1500 nm. The fluorescence spectrum is structureless at room temperature and exhibits sharp lines at low temperature. The temperature dependence of nonradiative relaxation shows a quantum efficiency ∼46%. From the experimental data and the best fit with the Struck and Fonger model, the following spectroscopic parameters were obtained: effective energies of odd- and even-parity phonons are hω odd=158 and hω even=564 cm −1, respectively, Huang–Rhys parameter S=2.1, nonradiative constant K nr=10 14 s −1, and the number of phonons bridging the energy gap is ∼19. Long lifetime (41 μs at 300 K) and high quantum efficiency make this crystal a new promising tunable laser medium for the near-infrared region.

Indofluorides in the AMF 3–InF 3 systems (A=K, Rb, Cs and M=Mg, Zn) and optical properties of Cr 3+:K 0.38Mg 0.38In 0.62F 3

Among the fluorides disclosed in the AMF 3–InF 3 systems (A=K, Rb, Cs/M=Mg, Zn), the most favorable material for optical applications in relation with the optical quality of its crystals is K 0.38Mg 0.38In 0.62F 3 (KMI), a distorted monoclinic Magnéli-bronze, which crystals are grown from the melt. Cr 3+ introduced in this host is optically characterized by optical spectroscopy. The chromium ions in the single crystals occupy simultaneously centrosymmetric and non-centrosymmetric sites. The broad emission band ( 4T 2 g → 4A 2 g transition) obtained in the near infrared range is characteristic of a small crystal field effect (Dq=1476 cm −1). Crystal field analysis leads to a very good agreement between calculated and experimentally observed energy levels. Furthermore, dips observed in the absorption at about 15370 cm −1 are related to the calculated mixing between the wave functions, connected with the 4T 2 g ( 4F) and 2E g ( 2G) and 2T 1 g ( 2G) electronic levels.

Electrical and optical properties of Cr and Fe implanted n-GaN

Deep levels introduced into n-GaN films by Fe and Cr implantation have been studied by means of optical absorption and microcathodoluminescence spectroscopy measurements and by deep level transient spectroscopy, admittance spectroscopy, and capacitance-voltage profiling. The results are compared with previous measurements on Mn and Co implanted GaN. It is shown that the acceptor levels of substitutional Mn, Co, Fe, and Cr in n-GaN are located, respectively, near Ev+1.6 eV, Ev+1.7 eV, Ev+1.8 eV, and Ev+2 eV, the trend being similar to that observed in GaAs, GaP, and InP. The Fermi level in the implanted region is pinned near deep electron traps at Ec−0.5 eV that are tentatively attributed to complexes between substitutional transition metal ions and native defects such as nitrogen vacancies. It is shown that for all implanted species after 700 °C annealing a damaged region with relatively high resistivity is formed down to the depth of about 1 μm much, exceeding the projected range of implanted ions. This region is enriched with radiation-damage-related defects and is most likely formed by outdiffusion of these defects from the implanted region during annealing. The thickness of this damaged region is shown to be the lowest for Cr implantation.