Mg-doped Crystals Research Articles

Micro Defects in Nearly Dislocation Free GaN Doped with Mg during High Pressure Crystallization

GaN crystals grown from solutions of liquid gallium alloyed with Mg, at high pressure of nitrogen, are highly resistive and of very good structural quality, evidenced by X-ray diffraction measurements. The high structural quality of these crystals is also confirmed by selective etching of the N-polar (000—1) surface in molten KOH–NaOH eutectic which results in EPD as low as few tens on cm2. On the other hand, the etching of the Mg-doped crystals in the hot mixture of H3PO4 and H2SO4 beside the etch pits corresponding to dislocations, results in etch hillocks which are not observed for crystals grown without Mg. The hillocks are often arranged into periodic arrays corresponding to growth macrosteps on the crystal surface. In order to identify defects responsible for such etching results the crystals were studied by TEM. The measurements confirmed that the crystals are free of grown-in dislocations, however, bands of microdefects have been observed. Their correlation with the etching results will be shown and discussed in the paper.

Growth and characterisation of L-arginine phosphate family crystals

In order to improve the device characteristics of L-arginine phosphate (LAP) family of crystals, metals doping with oxides of copper and magnesium were examined. Mg-doped crystals enhance the growth along the c-axis; X-ray powder diffraction patterns were used to calculate the lattice parameters of the crystals. Fourier transform infra red (FTIR) spectroscopy and thermal analysis (DTA, TGA) were performed to study the molecular vibrations and thermal behaviour of the crystals, respectively. Mechanical characterisations such as micro-hardness studies on (1 0 0) and (0 1 0) planes were also carried out. The weight percentage of the doped copper and magnesium were calculated by atomic absorption spectroscopy (AAS).

Mg Segregation, Difficulties of P-Doping in GaN

AbstractTransmission 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 1N polarity). These planar defects exhibit the characteristics of stacking faults with a shift vector of a 1/3 [1100] +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.

Near Defect Free GaN Substrates

The current status of GaN crystallization under high nitrogen pressure will be presented. Both conductive and semi-insulating GaN crystals will be characterized.In particular the influence of Mg on the growth mechanisms will be discussed. The influence of Mg doping on morphology of Mg-doped crystals grown under pressure and Mg-doped homoepitaxial layers will be shown. It will be also shown that the addition of about 1 at.% of Mg into the solution improves significantly the structural quality of crystals reducing dislocation density at least by 3 orders of magnitude comparing to the crystals grown without an intentional doping. As it was estimated by selective wet etching and transmission electron microscopy the dislocation densities in the Mg-doped GaN is as low as 10 cm−1. The introduction of Mg also lowers the optical absorption coefficients for energies below fundamental edge by 2 to 3 orders of magnitude what is explained by disappearance of defect related states in the gap.The procedures for preparation of atomically flat epi-ready (000 1 ) surfaces without subsurface damage will be described. It will be shown that high quality homoepitaxial layers growing by monoatomic steps are possible on these substrates.

Near Defect Free GaN Substrates

AbstractThe current status of GaN crystallization under high nitrogen pressure will be presented. Both conductive and semi-insulating GaN crystals will be characterized.In particular the influence of Mg on the growth mechanisms will be discussed. The influence of Mg doping on morphology of Mg-doped crystals grown under pressure and Mg-doped homoepitaxial layers will be shown. It will be also shown that the addition of about I at.% of Mg into the solution improves significantly the structural quality of crystals reducing dislocation density at least by 3 orders of magnitude comparing to the crystals grown without an intentional doping. As it was estimated by selective wet etching and transmission electron microscopy the dislocation densities in the Mg-doped GaN is as low as 10 cm-1. The introduction of Mg also lowers the optical absorption coefficients for energies below fundamental edge by 2 to 3 orders of magnitude what is explained by disappearance of defect related states in the gap.The procedures for preparation of atomically flat epi-ready (000 1 ) surfaces without subsurface damage will be described. It will be shown that high quality homoepitaxial layers growing by monoatomic steps are possible on these substrates.

High Resistivity GaN Single Crystalline Substrates

High resistivity 104 -106 Ω cm (300 K) GaN single crystals were obtained by solution growth under high Ν2 pressure from melted Ga with 0.1-0.5at.% of Mg. Properties of these crystals are compared with properties of conductive crystals grown by a similar method from pure Ga melt. In • particular, it is shown that Mg-doped GaN crystals have better structural quality in terms of FWHM of X-ray rocking curve and low angle boundaries. Temperature dependence of electrical resistivity suggests hopping mechanism of conductivity. It is also shown that strain free GaN homoepitaxial layers can be grown on the Mg-doped GaN substrates. PACS numbers: 71.55.Eq, 72.80.Ey

Enhancement of the magnetoresistance due to structural transition in Mg-doped perovskite Mn oxides

Substitution of magnesium for manganese in mixed perovskite La1−xSrxMnO3was found to reduce the electrical conductivity and the Curie temperature Tc. A remarkable jump in the resistivity ρ was observed in the 6% Mg-doped crystal with x=0.175. This sample remains semiconducting at low temperatures, contrary to the Mg-free crystal. The abrupt change of ρ reflects a first-order structural transition at a temperature Ts just below Tc. An applied magnetic field decreased Ts, leading to an enhancement of the magnetoresistance between Ts (0) and Ts (H). The microscopic properties of Mn3+ and Mn4+ ions due to Sr2+and Mg2+ substitutions were illustrated by 55Mn nuclear magnetic resonance study.

Flux growth of Mg- or Rb-doped K x[Ga xGa 8+ xTi 16− xO 56] single crystals

Mg- or Rb-doped single crystals of potassium gallotitanogallate (KGGTO), K x [Ga 8Ga 8+ x Ti 16− x O 56], were grown from a flux melt of the system K 2OMoO 3. The value of x in undoped KGGTO crystals is nearly 1.0. The value of x in the Mg-doped KGGTO crystals had a maximum of 1.6 when Ga was replaced by Mg. This increment for Mg doping was accompanied by an increase in ionic conductivity at high frequency, and by a small increase of lattice parameters. The Rb content in the Rb-doped KGGTO crystals had a maximum of 0.6 wt%, i.e. (K 1.0Rb 0.2) 1.2. The value of x did not change within error. This was accompanied a small increase of ionic conductivity at low frequency. Although both pure and Mg-doped KGGTO crystals melted incongruently at 1530 °C, different exotherms were seen in the DTA curves during cooling.

Phase transition in CsCoCl 3 and CsCo 1− xMg xCl 3 in magnetic fields

Two peaks of the differential magnetization are observed in CsCoCl 3 at 32.9 and 43.5 T and two additional peaks at 16 and 24 T in Mg-doped crystal, CsCo 1− x Mg x Cl 3. The two peaks in pure CsCoCl 3 are attributed to the antiferromagnetic (AF)-ferrimagnetic (FRI) and FRI-ferromagnetic (FRO) boundaries and the two additional peaks in Mg-doped crystal understood by the local spin-flip of Co 2+ ions around the Mg 2+ impurities. The intrachain and the first nearest interchain exchange parameters are consistently obtained as J 0 = −54cm −1 and J 1 = −5.2cm −1, respectively.

Layer intermixing and related long-term instability in heavily carbon-doped AlGaAs/GaAs superlattices

Superlattices (SLs) of Alo.3Gao.7As/GaAs grown by metalorganic chemical vapor deposition and heavily doped with carbon using CClp4 were annealed for 24 h at 825° C under a variety of ambient and surface encapsulation conditions. Photoluminescence atT = 1.7 K has been employed to determine approximate Al-Ga interdiffusion coefficients (D Ai-Ga) for different annealing conditions. For all encapsulants studiedD Al-Ga increases with increasing As4 pressure in the annealing ampoule. This result disagrees with trends reported for Mg-doped crystals, and with predictions of the charged point-defect (Fermilevel) model. The Si3N4 cap provides the most effecitve surface sealing against ambientstimulated layer interdiffusion (D Al-Ga ≈ 1.5-3.9 x 10-19 cm2/sec). The most extensive layer intermixing has occurred for an uncapped SL annealed under As-rich ambient (D Al-Ga ≈ 3.3 x 10-18 cm2/sec). These values are up to ~40 times greater than those previously reported for nominally undoped AIGaAs/GaAs SLs, implying that theC As doping slightly enhances layer intermixing, but significantly less than predicted by the Fermi-level effect. The discrepancies between the experimental data and the model are discussed. Pronounced changes in the optical properties of the annealed SLs with storage time at room temperature are also reported. These changes may indicate a degraded thermal stability of the annealed crystals due to high-temperature-induced lattice defects. A possibly related effect of the systematic failure to fabricate buried heterostructure quantum well lasers via impurity-induced layer disordering in similarly doped AIGaAs/GaAs crystals is discussed.

Non-photorefractive LiNbO3:Mg as the effective material for the nonlinear optics

Abstract The comparative study of optical properties of Mg-doped and Mg-nondoped LiNbO3 and Fe-doped LiNbO3 crystals was carried out. In all the crystals doped with Mg ≥ 1 wt.% the value of photorefraction decreases not less than by 102 in spite of Fe incorporation up to 0.1 wt%. At room temperature the nonlinear optical elements of LiNbO3:2.4 wt% Mg reveal the conversion efficiency of SHG more than 25% with phase matching angle of 90[ddot]. X-ray and UV-induced changes of the optical absorption spectra reveal the alteration of the electron acceptor type in Mg-doped crystals. On the contrary to pure LiNbO3, in LiNbO3 doped by Mg the Fe3+-ion is not more the electron acceptor center, what is followed by the change of the photoelectric and consequently photorefractive properties. Mg incorporation affects the ESR spectrum of Fe3+ in LiNbO3 as well.

Oxygen Self‐Diffusion in Magnesium‐ or Titanium‐Doped Alumina Single Crystals

Oxygen Setf‐diffusion coefficients have been measured in single crystals of N2O3 doped with Mg or Ti under an oxygen partial pressure of 20 kPa in the temperature range 1400° to 1700°C. Diffusion coefficients in Mg‐doped crystals obey the equation Do (cm2/s) = 1 × 1011∼ (1×1013) exp[−915 ± 50(kJ/mol)/RT]. The diffusivity of oxygen in Ti‐doped A12O3 is lower than Mg‐doped A12O3. A vacancy mechanism explains these results.

The OH−absorption spectrum as a probe of defect structure of LiNbO3crystal

Abstract The OH− infrared absorption band in the LiNbO3 crystal with various Li/Nb ratios and contents of MgO have been measured at room temperature. The structure of the absorption band of nearly stoichiometric crystals consisted of three sub-bands with slightly different peak position, and their relative height change with Li/Nb ratios. The OH− absorption band in heavily Mg-doped crystal had a structure consisting of two peaks, and the shift of peak position to shorter wavelength was about 54 cm−1. On the basis of the crystal structure and the model of the defect structure of the LiNbO3 crystal suggested by Abrahams and Smyth,1–2 these observations have been discussed with different structure argument: stoichiometric, congruent and Mg-doped crystals, respectively. The shift of the peak position of the OH− absorption band in heavily Mg-doped crystal to shorter wavelength is suggested to be caused by the Mg2+ ions getting into sites when the Mg-doping level was higher than the threshold level in Mg-doped ...

X-ray and UV influence on the optical absorption spectra of the non-photorefractive lithium niobate

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THE OH ABSORPTION SPECTRUM As A PROBE FOR DEFECT STRUCTURE OF LiNbO3 CRYSTAL

The OH- infrared absorption band in the LiNbO3 crystal with various Li/Nb ratios and MgO contents have been measured at room temperature. The structure of the absorption band of nearly stoichiometric crystals consists of three sub-bands with slightly different peak position, their relative heights change with Li/Nb ratios. The OH- absorption band in heavily Mg-doped crystal has a structure consisting of two peaks, the shift of peak position to shorter wavelength is about 54 cm-1. Basing on the crystal structure and the model of the defect structure of the LiNbO3 crystal suggested by Abrahams and Smyth (1-2), these observations are discussed for three different structures, i.e., stroichiometric, congruent and Mg-doped crystals, respectively. The shift of the peak position of the OH- absorption band in heavily Mg-doped crystal to shorter wavelength is suggested to be caused by the Mg2+ ions getting into Nb5+ sites when the Mg-doping level was higher than the threshold level in Mg-doped LiNbO3 crystals.

Laser-induced grating characteristics in doped lithium niobate crystals

The properties of laser-induced gratings were characterized in doped LiNbO3 crystals using small-angle scattering pattern analysis, the analysis of erasure decay patterns, and two-beam mixing. The results show the usefulness of each of these types of experimental techniques in obtaining different types of information about the photorefractive response of the material. The samples investigated included crystals doped with copper, erbium, magnesium, and hydrogen. The results obtained on a series of Mg-doped crystals showed a strong increase in the charge relocation rate at high concentration levels resulting in a decrease in the grating modulation depth. The results of the two-beam mixing experiments are consistent with increased conductivity in the samples with high Mg concentrations.

Grain-Boundary Migration in LiF: I, Mobility Measurements

Grain-boundary migration rates in LiF were determined by monitoring discontinuous recrystallization rates in deformed single crystals at temperatures between 0.7 Tm and 0.98 Tm, the melting temperature. Significant reductions in migration rates, relative to those expected for pure material, result from solute additions of Mg, Al, or Na, as well as from background impurities. The magnitude of the solute drag effect differs greatly; solute drag coefficients for the dopants studied ranked as αAi/(7 × 104) ≧αMg≧5αNa. Most work was done with Mg-doped crystals; the behavior under some conditions was consistent with that expected for stable solute-drag-limited boundary migration. In contrast, under some conditions of driving force, temperature, and composition, significant variability in mobilities was found both from several measurements for the same experimental conditions and from variability in the amount of displacement along individual growth fronts. This variability is attributed to transitions between different stable mobility extremes.

The variation of electron range in pure and Mg-doped crystals of NaCl due to thermal treatment

Pure and Mg-doped NaCl crystals were subjected to heat treatment at different elevated temperatures and subsequently quenched to room temperature. The specimens were irradiated with 50 keV electrons. F centres were produced uniformly in the crystals. From the depth of coloration, the electron range was estimated by observing the number of F centres/cm 2 at different depths below the surface of the crystal. The electron range is found to diminish with the rise of temperature of the heat treatment up to a certain temperature above which the range indicates an increasing trend. The phenomenon is found to occur both in pure and heat-treated impurity doped samples.

Effect of Plastic Deformation on the Density of Mg-doped LiF Crystals

The thermal gradient technique is used to study the deformation-induced bulk density change of LiF crystals doped with Mg2+ precipitated as a Suzuki phase of MgF2 6 LiF. The results are discussed in terms of the jog-dragging mechanism of production of vacancy pairs. Two extreme cases corresponding to the absence of annihilation and fast annihilation of full jogs on dislocations are considered. [Russian Text Ignored].

Recombination luminescence in single crystal Al2O3

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