Mg-doping Level Research Articles

Correlation between Infrared Absorption and Lithium Sublattice Disorder in Magnesium-Doped Lithium Niobate

Lithium niobate is a ferro- and piezoelectric material with excellent optical properties and a wide variety of applications. The defect structures of congruent and Mg-doped crystals are still under intense discussion. In this work, undoped lithium niobate and magnesium-doped lithium niobate grown from congruent melt with the addition of 0 to 9 mol% MgO were investigated by infrared absorption, establishing the dependence of the absorbance on the Mg-doping level in two bands related to OH- stretching vibrations. The absorption band at 3485 cm-1 peaks at a MgO concentration in melt of 1 mol% and vanishes for MgO concentrations above the threshold level for optical damage suppression (4.8 mol%). A corresponding peak occurs in the minimum yield of the 7Li(p,α)4He reaction during ion channeling measurements, indicating a maximum of disorder in the Li sublattice. A possible explanation for this correlation is the attribution of this absorption band to ilmenite stacking fault sequences instead of isolated NbLi antisites in undoped and low-doped material. On the other hand, the OH- absorption band at 3535 cm-1 stays weak up to the MgO concentration threshold, and then increases, hinting to a defect related to the increase of vacancies due to the lack of charge compensation.

Effects of Ⅴ/Ⅲ ratio and Cp2Mg flow rate on characteristics of non-polar a-plane Mg-delta-doped p-AlGaN epi-layer

Abstract The non-polar a-plane Mg-delta-doped p-AlGaN epi-layers with excellent electrical conduction were achieved on r-plane sapphire substrates via metal organic chemical vapor deposition technology. The impacts of Ⅴ/Ⅲ ratio and Cp2Mg flow rate on the properties of the non-polar p-AlGaN epi-layers were investigated with scanning electron microscopy, high-resolution X-ray diffraction, Raman spectroscopy, and Hall effect measurement. It was discovered that the crystalline quality and the electrical conductivity were extremely relied upon the Ⅴ/Ⅲ ratio and Mg-doping level. Actually, a hole concentration of 3.7 × 1017 cm−3 and an electrical resistivity of 2.6 Ω ⋅ cm were obtained for non-polar p-Al0.12Ga0.88N epi-layer by carefully optimizing the Ⅴ/Ⅲ ratio and Cp2Mg flow rate during the epitaxial procedure. In addition, the results revealed that improvement on electrical conductivity was ascribed to the evident repression of self-compensation effect generated by decreasing in the densities of the nitrogen vacancy (VN) as well as the VN-related complexes and the Mg-related defects via the employment of the proper Ⅴ/Ⅲ ratio and the Mg-doping concentration.

Influence of Mg Doping Levels on the Sensing Properties of SnO2 Films.

This work presents the effect of magnesium (Mg) doping on the sensing properties of tin dioxide (SnO2) thin films. Mg-doped SnO2 films were prepared via a spray pyrolysis method using three doping concentrations (0.8 at.%, 1.2 at.%, and 1.6 at.%) and the sensing responses were obtained at a comparatively low operating temperature (160 °C) compared to other gas sensitive materials in the literature. The morphological, structural and chemical composition analysis of the doped films show local lattice disorders and a proportional decrease in the average crystallite size as the Mg-doping level increases. These results also indicate an excess of Mg (in the samples prepared with 1.6 at.% of magnesium) which causes the formation of a secondary magnesium oxide phase. The films are tested towards three volatile organic compounds (VOCs), including ethanol, acetone, and toluene. The gas sensing tests show an enhancement of the sensing properties to these vapors as the Mg-doping level rises. This improvement is particularly observed for ethanol and, thus, the gas sensing analysis is focused on this analyte. Results to 80 ppm of ethanol, for instance, show that the response of the 1.6 at.% Mg-doped SnO2 film is four times higher and 90 s faster than that of the 0.8 at.% Mg-doped SnO2 film. This enhancement is attributed to the Mg-incorporation into the SnO2 cell and to the formation of MgO within the film. These two factors maximize the electrical resistance change in the gas adsorption stage, and thus, raise ethanol sensitivity.

Effects of electron blocking layer configuration on the dynamics of laser diodes emitting at 450 nm

In this paper, a theoretical study is presented, analysing the AlxGa(1−x)N electron blocking layer (EBL) at different Al content and Mg-doping levels for the InGaN-based laser diode (LD) emitting at 450 nm. It is observed that the performance of LDs not only depends on the Al content in the EBL but also on the Mg doping levels in the EBL and the In content in the InGaN waveguide. The optimum selection of AlGaN EBL strongly depends upon the In content in the InGaN waveguide. It is found that, for an In0.08Ga0.92N waveguide, the higher Al content in the EBL degrades the performance due to the increase in leakage current, thus an Al-free EBL i.e. a GaN EBL, is optimal for an In0.08Ga0.92N waveguide. However, the increased Mg doping in the p-GaN (Al = 0) EBL shows improvement in the performance of LDs. For the p-GaN EBL with a Mg-doping level of 1 × 1019 cm−3, the output power is 138 mW, for 3 × 1019 cm−3 doping it is 364 mW and for 5 × 1019 cm−3 doping it increased to 556 mW. For the p-GaN EBL, the threshold current at 1 × 1019 cm−3 is 319 mA, at 3 × 1019 cm−3 doping it falls to 231 mA and for 5 × 1019 cm−3 doping it is 227 mA. For the heavily doped p-GaN EBL, the threshold current dependency is negligible at higher Al content in the EBL, but power output shows a strong dependency on the Al content. In the case of an In-free waveguide i.e. a GaN waveguide (In = 0), the performance of the LD is optimal at 15% Al content in the EBL.

Experimental and ab initio study of Mg doping in the intrinsic ferromagnetic semiconductor GdN

We report persistent photoconductivity in Mg-doped GdN thin films grown by molecular beam epitaxy. Temperature-dependent measurements were carried out in the time and frequency domains to probe the nature of Mg impurities in GdN. The results reveal an initial fast decay followed by a slow persistent photoconductivity. The magnitude of the photoconductivity as well as the characteristic fast- and slow-decay times was found to decrease systematically with increasing the Mg-doping level. Our experimental results suggest that Mg impurities in epitaxial GdN thin films act as acceptor-like centres. Interestingly they also show that the incorporation of Mg result in a significant decrease in the concentration of nitrogen vacancies, as is demonstrated also to be in agreement with an ab initio calculation.

Effects of Mg-doping on characteristics of semi-polar [formula omitted] plane p-AlGaN films

The effects of Mg-doping on the characteristics of semi-polar (112¯2) plane Al0.15Ga0.85N films grown on (101¯0) m-plane sapphire substrates with metal-organic chemical vapor deposition (MOCVD) technique were investigated intensively and systematically. The characterization results showed that the crystalline quality of the (112¯2) plane AlGaN films could be improved by optimizing the Mg-delta-doping process. Whereas, the surface morphology for the samples was monotonously degraded with increasing the Mg-doping level. Meanwhile, a hole concentration as high as 5.4×1017cm−3 were achieved by optimizing the Mg-doping level. Furthermore, the luminescence mechanism of the samples was investigated by photoluminescence spectra at room temperature.

Enhancing the Hole-Injection Efficiency of a Light-Emitting Diode by Increasing Mg Doping in the p-AlGaN Electron-Blocking Layer

By increasing the Mg-doping level and hence the hole concentration in the p-AlGaN electron-blocking layer (EBL), particularly around the interface between the EBL and the top quantum barrier (QB), of a light-emitting diode (LED), the polarization field in this layer can be screened for reducing the potential barrier of hole and hence enhancing the hole-tunneling efficiency such that the overall LED emission efficiency is increased. The increase of Mg-doping level is implemented based on an Mg preflow growth technique, in which Mg source is supplied into the metalorganic chemical vapor deposition chamber for several minutes before the growth of p-AlGaN or p-GaN. It is found that by increasing the Mg-doping level by ~20 times near the interface between the EBL and the top QB, LED emission intensity can be enhanced by ~9.4 times. Based on a simulation study, we observe that the energy difference between the valence band edge and the quasi-Fermi level of hole in the EBL is reduced by increasing the Mg-doping level in this layer such that the total hole density in the quantum wells is increased for enhancing the LED emission efficiency.

Field- and current-driven degradation of GaN-based power HEMTs with p-GaN gate: Dependence on Mg-doping level

Within this paper we investigate the degradation of GaN-HEMTs with p-GaN gate submitted to stress at forward gate bias. We studied the effect of both constant-voltage stress and short-pulse stress (induced by TLP, Transmission Line Pulser); devices having three different Mg-doping levels (ranging from 2.1·1019/cm3 to 2.9·1019/cm3) were used for the study.We demonstrated the existence of two different degradation mechanisms, depending on the stress conditions: (i) when submitted to TLP stress (100ns pulses with increasing amplitude), the failure occurs through a field-driven process, i.e. the breakdown of the metal/p-GaN Schottky junction, which is reversely biased when the gate is at positive voltage. Failure voltage decreases with increasing Mg doping, since higher acceptor levels result in a higher electric field. (ii) Conversely, during constant-voltage stress, the long-term stability is undermined by a current-driven process, namely the accumulation of positive charges at the p-GaN/AlGaN interface, which promotes an increase of the leakage current, first gradual and then catastrophic. Increasing Mg-concentration in the p-GaN results in a reduction of the gate leakage at high forward gate bias. As a consequence, devices with higher Mg doping have long TTF (more than two orders of magnitude with respect to the samples with lower Mg doping).

Controllable phase separation and improved grain growth mode of Mg-doped Sb7Te3 films

The effects of Mg on the structure and phase-change kinetics of Sb7Te3 were investigated for phase change memory application. The results revealed that the addition of Mg increased the crystallization temperature, and crystalline activation energy, while hindering grain growth and suppressing phase separation from Sb+Sb2Te, Sb2Te, to Sb phases. Laser-induced phase-change behavior showed four regions in the Mg-Sb7Te3 films: crystallization, transition toward stability, amorphization and ablation. A reversible switching between crystallization and amorphization occurred more rapidly in Mg-doped Sb7Te3 films than in conventional Ge2Sb2Te5. Evaluation of Johnson-Mehl-Avrami plots indicated that the crystallization mode changed from three, two to one-dimensional grain growth as the Mg-doping level increased. This phenomenon contributed to the rapid phase change of Mg-doped Sb7Te3 films.

Enhanced voltage blocking ability of AlGaN/GaN heterojunction FETs-on-Si by eliminating leakage path introduced by low-temperature-AlN interlayers

In this work, we demonstrate an enhanced voltage-blocking ability for GaN-on-Si epitaxial structure with the novel feature of combined Mg-doped-GaN/low-temperature-AlN (LT-AlN) interlayers in the buffer. With an optimized Mg-doping level, such combined interlayers can eliminate the original leakage path at the LT-AlN/GaN interface due to the interfacial roughening effect caused by the Mg doping. As a result, for AlGaN/GaN heterojunction FETs (HFETs) fabricated on such epitaxial structure (template), its off-state leakage current could be reduced by 1–2 orders of magnitude and its breakdown voltage could be remarkably raise to 252%. Although, the Mg-doped-GaN interlayer would cause a little degradation on device on-state performance, such as 6.6% increase of on-resistance, its contribution on improving device’s voltage-blocking ability and reducing its off-state power dissipation outweighs the drawbacks.

Effect of Mg-doping on pyrolysis evolution and phase constitutes of Ba0.7Sr0.3TiO3 (BST) system derived from sol-gel

Gels and powders of dielectric Ba0.7Sr0.3(Ti1-xMgx)O3 (BST) were prepared by sol-gel technique. Effect of Mg-doping of on the pyrolysis evolution and phase constitutes of BSTs was investigated using DSC-TG and XRD. Results show that Mg-doping leads to a remarkable change of crystallizing process of BSTs from a double phase-transformation of non-doping sample into a single phase-transformation and quickens the crystallizing reaction of formation of BST perovskite. The maximum Mg-doping level should be less than x = 0.15 in Ba0.7Sr0.3 (Ti1-xMgx)O3 system. The amount of (Ba,Sr)CO3 phase in the BST powders increases considerably with the Mg-doping level, indicating Mg atoms should occupy the A positions of ABO3 structure of BST, instead of the B position.

Polarity inversion in high Mg-doped In-polar InN epitaxial layers

To investigate the Mg-dopability in In-polar InN epilayers grown by molecular beam epitaxy, polarity inversion dependence on Mg-doping level is studied. A multiple-InN layer-structure sample with different Mg-doping levels is grown and analyzed by transmission electron microscopy. Formation of high density V-shaped inversion domains is observed for the Mg-doped InN with Mg concentration ([Mg]) of 2.9×1019cm−3. These domains lead to polarity inversion from In to N polarity. Further study for Mg-doped InN epilayers shows that polarity inversion takes place when [Mg] increases above 1.6×1019cm−3. It is also shown that the Mg-sticking coefficient is almost independent of the polarity.

Mg doping in (1 1¯ 0 1)GaN grown on a 7° off-axis (0 0 1)Si substrate by selective MOVPE

Mg doping was attempted in (1 1¯ 0 1)GaN grown on a patterned (0 0 1)Si by selective metal-organic vapor phase epitaxy (MOVPE). The source material for the Mg doping was EtCp 2Mg and the electrical properties were studied on samples with different doping levels. All samples showed p-type conduction. However, the hole concentration was decreased by the Mg doping at low doping levels followed by an increase at high doping levels. The activation energy was around 106–130 meV depending on the doping level, which was larger than that found in undoped or carbon-doped samples. The AFM images showed gradual change in the surface structure in accordance with the Mg-doping level. The variation of the optical spectra was discussed in relation to the Mg-doping levels.

Critical Mg doping on the blue-light emission in p-type GaN thin films grown by metal–organic chemical-vapor deposition

The photoluminescence and photocurrent from p-type GaN films were investigated at temperatures of 30 and 297 K for various Mg-doping concentrations. At a low Mg-doping level, there exists a photoluminescence center of the donor and acceptor pair transition at the 3.28 eV band. This center correlates with the defects for a shallow donor of the VGa and for an acceptor of MgGa. The acceptor level shows a binding energy of 0.2–0.25 eV, which was observed by measuring the photocurrent signal at a photon energy of 3.02–3.31 eV. At a high Mg-doping level, we found a photoluminescence center of a deep donor and acceptor pair transition of the 2.76 eV blue band. This center is attributed to the defect structures of MgGa –VN for the deep donor and MgGa for the acceptor. For low-doped samples, thermal annealing provides an additional photocurrent signal for unoccupied deep acceptor levels of 0.87–1.35 eV above the valence band, indicating p-type activation.

Detailed deep trap analysis in Mg-doped p-type GaN layers grown by MOVPE

p-Type Mg-doped GaN layers grown by metal organic vapor phase epitaxy on undoped GaN buffer layers on (0001) sapphire substrates were investigated by thermal admittance spectroscopy, temperature dependence conductivity and thermally stimulated currents. A detailed trap characterization in the p-type GaN:Mg as well as in the undoped buffer layers shows that the TAS peaks observed from the p-type samples are caused by Mg-defects. Furthermore, the undoped buffer layer mainly dominates the optical induced deep defect-to-band transitions as measured by photoelectrical optical admittance spectroscopy. An increasing Mg-doping level introduces an enhancement of a deep defect signal at about E G-(280–360) meV. These results show the complex nature of the Mg defect and the introduction of further deep defects by the doping process.

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 ...

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.

The preparation and selected properties of Mg-doped p-type iron oxide as a photocathode for the photoelectrolysis of water using visible light

Stable Mg-doped iron oxides that exhibit p-type character have been synthesized. These have been utilized in the form of sintered polycrystalline disks for the photodissociation of water. We report the influence of sintering temperature, surface oxygen deficiency, Mg-doping level, and solution chemistry on the photoactivity of the Mg-doped iron oxide photocathodes.