Mg-doped Crystals Research Articles

Effect of Mg doping on the growth aspects, crystalline perfection, and optical and thermal properties of congruent LiNbO3single crystals

Mg-doped congruent lithium niobate single crystals were grown by the Czochralski technique. High-quality single crystals were grown using a novel seeding technique in a resistive heating furnace. Analysis of crystalline perfection carried out by a multi-crystal X-ray diffractometer revealed that the grown crystals do not contain any structural grain boundaries but do contain point defects. The transmission characteristics showed an enhancement of band gap with an increase in Mg concentration. Conoscopy patterns revealed that the grown crystals are homogeneous and the incorporation of Mg into the lattice does not affect the optical sign (negative uniaxial) of the crystal. The refractive index measurements carried out using a prism coupler showed an increase in the optical birefringence (Δn), while the refractive index was found to decrease with the increase in doping concentration. Further, thermal conductivity was found to decrease with Mg incorporation in the lattice owing to phonon scattering from the Mg ions and, as a consequence, at high concentrations (>4 mol%) crack formation occurred. However, optimization of growth conditions reveals that a slower pulling rate leads to crack-free lithium niobate crystals even at 6 mol% Mg doping.

Optical breakdown threshold in nanosecond high repetition second harmonic generation by periodically poled Mg-doped LiTaO3 crystal

Our study shows that a local temperature increase of ≈1 K in the crystal lattice caused by second harmonic generation (SHG) and two-photon absorption of 532 nm radiation at the rear of periodically poled stoichiometric LiTaO3 crystal by changing spontaneous polarization induces a pyroelectric field ≈10 kV/cm, accelerating free electrons to an energy of ≈10 eV, followed by optical breakdown and crystal damage. Theoretical analysis leads to an explicit expression for the threshold laser fluence of optical breakdown giving ≈1.2 J/cm2 for 1064 nm input radiation in 6.8 kHz pulsed SHG operation, agreeing well with the experimentally found value ≈1.32 J/cm2.

Infiltration growth of Mg-doped Nd–Ba–Cu–O seed crystals for the fabrication of large grain RE–Ba–Cu–O bulk superconductors

By employing the infiltration growth (IG) technique, Mg-doped Nd–Ba–Cu–O (NdBCO) seed crystals were successfully prepared through 1 wt% MgO addition both to a Nd4Ba2Cu2O10 (Nd-422) precursor powder and to a liquid source with the composition Nd2O3:CuO:BaCuO2 = 1:6:10. Differential thermal analysis (DTA) results showed that the decomposition temperature of the processed Mg-doped NdBCO crystals increased by about 20 °C compared with the general, Mg-free NdBCO seeds, thus they can be used as generic seed crystals to induce the growth of any RE–Ba–Cu–O (REBCO) single grain. This result is consistent with that observed in melt growth (MG) processed Mg-doped NdBCO crystals. Finally, highly c-axis oriented Gd–Ba–Cu–O (GdBCO) and Y–Ba–Cu–O (YBCO) large grains were successfully grown using the produced seed crystals.

Extended defects in bulk GaN and III-nitrides grown on this substrate

A short review of the structural perfection of high-pressure grown bulk crystals is given. As-grown undoped and Mg-doped crystals are described. The dependence of defect arrangement and quality of the surface on growth polarity is described. A high perfection of homoepitaxial layers grown on these substrates is shown. However, growth of thick layers by HVPE may lead to the formation of differently arranged dislocations and the formation of low angle grain boundaries associated with cracks. It is shown that the introduction of dopant or growth of mismatched layers on undoped high-pressure substrates may lead to the formation of additional defects.

The effect of Nd and Mg doping on the micro-Raman spectra ofLiNbO3 single-crystals

The LiNbO3 congruent crystals doped with small Nd concentrations,<1 mol% Nd, and co-doped with Mg ions, 0–9 mol% Mg, weresystematically investigated by means of micro-Raman spectroscopy in theY and Z crystal directions. Results obtained from an undoped congruent crystal,an Nd-doped crystal, a Mg-doped crystal and Nd, Mg-co-dopedcrystals are compared. From the analyses of the results obtained in theY direction, the Nd and Mg content dependence of the two lowest-RamanA1(TO1) and A1(TO2) modes, the half-width composition and the area ratio of theA1(TO4) and E(TO8) bands, we reached several conclusions about the incorporation mechanism of the Nd and Mg ions intothe LiNbO3 lattice. Likewise the Raman shift and half-width of theE(TO1) andE(TO7) modes wereinvestigated in the Z direction. Results indicate that Mg and Nd ions are located in the Li site for low dopingconcentrations and for larger concentrations there is a replacement in both Li and Nb ionsites.

Design of graded biomimetic osteochondral composite scaffolds

With the ultimate goal to generate suitable materials for the repair of osteochondral defects, in this work we aimed at developing composite osteochondral scaffolds organized in different integrated layers, with features which are biomimetic for articular cartilage and subchondral bone and can differentially support formation of such tissues. A biologically inspired mineralization process was first developed to nucleate Mg-doped hydroxyapatite crystals on type I collagen fibers during their self-assembling. The resulting mineral phase was non-stoichiometric and amorphous, resembling chemico-physical features of newly deposited, natural bone matrix. A graded material was then generated, consisting of (i) a lower layer of the developed biomineralized collagen, corresponding to the subchondral bone, (ii) an upper layer of hyaluronic acid-charged collagen, mimicking the cartilaginous region, and (iii) an intermediate layer of the same nature as the biomineralized collagen, but with a lower extent of mineral, resembling the tidemark. The layers were stacked and freeze-dried to obtain an integrated monolithic composite. Culture of the material for 2 weeks after loading with articular chondrocytes yielded cartilaginous tissue formation selectively in the upper layer. Conversely, ectopic implantation in nude mice of the material after loading with bone marrow stromal cells resulted in bone formation which remained confined within the lower layer. In conclusion, we developed a composite material with cues which are biomimetic of an osteochondral tissue and with the capacity to differentially support cartilage and bone tissue generation. The results warrant testing of the material as a substitute for the repair of osteochondral lesions in orthotopic animal models.

Effect of gamma irradiation on the optical spectra of undoped, Mg-doped, and Gd-doped LiNbO3 single crystals

We have measured the optical spectra of gamma-irradiated lithium niobate crystals of different compositions: nominally undoped and doped with rare-earth (Gd) and alkaline-earth (Mg) metals in the range 0.003–0.27 wt %. The difference in behavior between gamma-irradiated and vacuum-annealed nominally undoped lithium niobate single crystals has made it possible to elucidate the mechanisms underlying the formation of electronic and point defects in the crystals and to develop a model for the processes involved. The effects of gamma dose, doping level, and the nature of the dopant on the optical absorption of gamma-irradiated doped lithium niobate single crystals are analyzed.

Optical properties of vacancies in thermochemically reduced Mg-doped sapphire single crystals

Optical absorption and emission experiments were used to characterize defects and defect aggregates in Mg-doped Al2O3 crystals due to thermochemical reduction at high temperatures. Oxygen vacancies and higher-order defects are produced much more readily in Mg-doped than in undoped Al2O3 crystals. F+ and F centers (oxygen vacancies with one or two electrons, respectively) were monitored by their optical absorption bands at about 4.8 and 6.0eV, respectively. In contrast with undoped crystals, where the reduction produces primarily F centers and a small amount of F+ centers, in Mg-doped crystals both F and F+ centers are created in comparable concentrations. These thermally generated F and F+ centers are much more stable than those produced in undoped crystals irradiated with neutrons. Clustering of individual oxygen vacancies forming higher-order defects, such as anion divacancy F22+ and F2+ centers, was investigated by low temperature absorption and luminescence experiments, in conjunction with UV irradiation and thermal treatments. The strong absorption bands at 2.87 and 3.69eV were shown to be due to Mg-perturbed F22+ and F2+ centers, respectively. In addition, photoconversion of F22+ and F2+ centers was observed. In crystals containing large concentrations of F22+ centers, electrons excited by 5.0eV light are trapped by F22+ and F2+ centers resulting in the conversion of F22+ centers into F2+. A model of F-type centers was extended to F22+ and F2+ centers. The calculated optical parameters are in very good agreement with those determined experimentally. A simple analysis of the lattice energy suggests that the environments of the F2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals.

Colour centres in [formula omitted]:Fe and [formula omitted]:Cu crystals irradiated by 12C ions

The present work is devoted to the investigation of optical absorption of pure and doped LiNbO 3 (LN) single crystals in the spectral range 5000– 32000 cm - 1 induced under influence of the 12C ions irradiation with energy 4.5 MeV/ u (MeV per nucleon) and a fluence 2 × 10 9 cm - 2 at room temperature. It was found that no remarkable absorption changes occur in pure and Mg-doped LN crystals, but in Fe- and Cu-doped ones. The induced additional absorption in LN:Fe crystals is attributed with Fe ions reducing Fe 3 + → Fe 2 + occurring simultaneously with hole centres formation O - , while only Cu 2 + → Cu + reduction was observed in LN:Cu crystals.

Photochromism of vacancy-related defects in thermochemically reduced α-Al2O3:Mg single crystals

Oxygen vacancies and their aggregates are produced much more readily in Mg-doped α-Al2O3 than in undoped α-Al2O3 single crystals during thermochemical reduction at high temperatures. A reversible photochromic effect was discovered in Mg-doped Al2O3 crystals containing large concentrations of oxygen divacancies. Alternate excitation with 5.0 and 3.69 eV light results in brown or yellowish–green coloration, respectively. The yellowish–green coloration can also be restored by thermal anneals at temperatures of about 750 K.

Relationship between photorefractive activity and Raman scattering in lithium niobate crystals

We performed Raman scattering experiments in pure, Mg-doped and Fe-doped single crystals of lithium niobate. The measurements were carried out in 90° geometry at several laser beam intensities, and in several polarization arrangements. The possible occurrence of light-induced polarization-anisotropic scattering, and its effects on Raman spectra, were checked by simultaneously measuring the time dependence of the laser beam intensity transmitted by the sample with unchanged polarization, and the time dependence of the intensity of Raman lines from vibrational modes of known symmetry. The results give no indication of any effect of photorefractive activity on Raman observations in our pure and Mg-doped samples. On the contrary, in Fe-doped lithium niobate the build-up of light-induced o–e scattering strongly affected the relative intensities of Raman lines in specific experimental arrangements. We discuss the possible application of these effects to the characterization of lithium-niobate-based materials with strong photovoltaic and photorefractive activity.

X-ray damage characterization in BaLiF3,KMgF3 and LiCaAlF6 complex fluorides

Two-inch sized KMgF3,BaLiF3 and LiCaAlF6 (LiCAF) single crystals were grown by the Czochralski method under a CF4 atmosphere. X-ray irradiation was used to carry out a comparative study of induced optical absorption phenomena and colour centre creation in the ultra-violet and visible spectral regions. The integral of the induced absorption spectra is significantly lower in LiCAF with respect to the other studied materials. It is found that the amplitude of the F-absorption band is suppressed more than a factor of 3 by Mg-doping. For Mg-doped crystals, the optimum doping concentration is about 0.2mol% of Mg2+.

Refractive index changes in Mg-doped LiNbO3caused by photorefraction and thermal effects

The origin of light-induced refraction changes in Mg-doped congruent and stoichiometric LiNbO3 (LN) has been investigated using the Z-scan method. The limitations of this method are discussed. We showed that in 5 mol% Mg-doped congruent LN photorefraction (PR) dominates among the nonlinear effects at the MW cm−2 light intensity level. For 2–5 mol% Mg-doped stoichiometric samples at the same intensity level PR are absent as shown by CCD recordings of the beam cross section, the sign of the changes and the excellent fitting of the Z-scan curves. From the magnitude and the intensity dependence of the measured nonlinear refraction of the Mg-doped stoichiometric crystals we concluded that a thermal effect is responsible for the changes observed. The thermally induced refractive index changes are associated with nonlinear absorption.

69,71GaNMR spectra and relaxation in wurtzite GaN

Magnetic properties of wurtzite GaN are studied by Ga nuclear magnetic resonance (NMR) in a GaN bulk crystal containing a high carrier concentration, as well as in highly resistive Mg-doped crystals. The quadrupole coupling constant is derived from satellite lines and from the shift of the central line in the quadrupolar perturbed 6 9 , 7 1 Ga NMR spectra. The electric-field gradient is in good agreement with the value calculated by the ah initio full-potential linear-muffin-tin-orbital method, using the local-density approximation to describe exchange and correlation effects. The 6 9 , 7 1 Ga spin-lattice relaxations in the Mg-compensated sample in the range 80-400 K are due to the quadrupolar interaction mechanism, similar to what is observed in GaAs, and the proper scaling factor, given by the square of the ratio of the nuclear quadrupole moments, ( 6 9 Q/ 7 1 Q) 2 , is observed for the two isotopes. In contrast, in the conductive sample, the relaxation mechanism is caused by magnetic interaction with the conduction electrons and one finds a relaxation rate, WT. The isotope ratio 6 9 W/ 7 1 W is close to ( 6 9 γ/ 7 1 γ) 2 , where y is the gyromagnetic factor. Only above 200 K, there is evidence of a relaxation process due to interactions with phonons also in the degenerate sample, with a temperature dependence approximately of the form WT 3 , consistent with a Debye temperature around 600 K.

Growth and characterization of aliovalent ion-doped LiCaAlF 6 single crystals

Two-inch size LiCaAlF 6 (LiCAF) single crystals doped with Mg 2+ and Ba 2+ were successfully grown by the Czochralski technique. Optical absorption measurements in the UV/VIS spectral regions following X-ray irradiation (radiation-induced absorption) were performed in order to investigate the radiation damage of the crystals. It is found that the amplitude of the F-absorption band is suppressed more than a factor of 3 by Mg doping, while in the case of Ba-doped LiCAF, no significant variation is observed. For Mg-doped crystals, the optimum doping concentration is about 0.2 mol% of Mg 2+. The crystallinity was studied using X-ray rocking curve analysis. The precise measurement of the refractive index showed that refractive index of LiCAF dose not change by the Mg 2+ doping, within the doping range of Mg 2+<1.0 mol%.

Color centers in LiCaAlF6 single crystals and their suppression by doping

LiCaAlF 6 (LiCAF) single crystals pure and doped with MgF2 and BaF2 were successfully grown by the Czochralski technique. Optical absorption measurements in the UV/Visible spectral regions following x-ray irradiation (radiation induced absorption) were performed in order to investigate the radiation damage of the crystals. The F-center absorption band at 262 nm is the dominating induced absorption feature. The amplitude of the F-absorption band is suppressed by more than a factor of 3 in Mg-doped LiCAF compared to undoped, while for Ba-doped LiCAF, almost no variation is obtained. In Mg-doped crystals the optimum doping concentration is of about 0.2 mol % of MgF2 in the melt.

Characterization of Mg-Doped GaN Micro-Crystals Grown by Direct Reaction of Gallium and Ammonia

Mg-doped GaN micro-crystals were prepared by the direct reaction of metal gallium and ammonia using magnesium chloride (MgCl2) as Mg doping source. The growth of microcrystalline hexagonal GaN was clearly found from scanning electron microscopy and TEM measurements. The grain size of Mg-doped GaN crystals was larger than that of undoped GaN. Room temperature PL and CL spectra for Mg-doped GaN micro-crystal showed the blue emission at 2.75 and 2.85 eV, respectively.

High-nitrogen-pressure growth of GaN single crystals: doping and physicalproperties

Growth of GaN under high-pressure high-temperature conditions allows one to obtainlarge-size high-quality GaN single crystals. These crystals have highconcentration of free electrons, most likely due to a high concentration of Oimpurity replacing nitrogen in the N sublattice. The incorporation of oxygen impurityduring high-pressure growth of GaN single crystals was investigated using quantum mechanical density functional theorycalculations. It was shown that the adsorption of oxygen in liquid groupIII metals (Al, Ga and In) leads to dissociation of the O2 molecule. Thedissociation process proceeds without energy barrier.The transition of oxygen from the adsorbed position into the interior of the Al has beenalso investigated. The results of calculations indicate that the directtransition energy barrier is about 3 eV. This indicates that the dissolution ofoxygen into liquid group III metals proceeds via Brownian motion of O-containing clusters.This also explains the difference between the solid and liquidsurfaces: the solid surfaces undergo passivation by oxygen, whereas in the liquid metalthe oxygen is dissolved.The doping of Mg during growth leads to a change of the electric properties of GaNcrystals - they become highly resistive. Mg doping changes the morphology of theplate-like GaN crystals. The physical properties of GaN:Mg crystals will bereviewed and compared with undoped GaN crystals.Beryllium doping is considered as an alternative route to obtaining p-type GaN. Thedoping with Be during growth increases the resistivity of the Be-doped GaN.However,the optical properties of Be-doped GaN crystals are different. These propertieswill be compared with Mg-doped and undoped GaN crystals.

Mg Segregation, Difficulties of P-Doping in GaN

Transmission electron microscopy has been used to study defects formed in Mg-doped GaN crystals. Three types of crystals have been studied: bulk crystals grown by a high pressure and high temperature process with Mg added to the Ga solution and two types of crystals grown by metal-organic chemical vapor deposition (MOCVD) where Mg was either delta-doped or continuously doped. Spontaneous ordering was observed in bulk crystals. The ordering consists of Mg rich planar defects on basal planes separated by 10.4 nm and occurs only for growth in the N to Ga polar direction (000 N polarity). These planar defects exhibit the characteristics of stacking faults with a shift vector of a 1/3 [1 00] +c/2 but some other features identify these defects as inversion domains. Different type of defects were formed on the opposite site of the crystal (Ga to N polar direction), where the growth rate is also an order of magnitude faster compared to the growth with N-polarity. These defects are three-dimensional: pyramidal and rectangular, empty inside with Mg segregation on internal surfaces. The same types of defects seen for the two growth polarities in the bulk crystals were also observed in the MOCVD grown GaN samples with Mg delta doping, but were not observed in the crystals where Mg was added continuously.

Mechanism of radiative recombination in acceptor-doped bulk GaN crystals

Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN:Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.