Mg-doped Samples Research Articles

Structural study of MgO and Mg-doped ZnO thin films grown by atomic layer deposition

Pure MgO, 1 and 10 at% Mg-doped ZnO layers were grown by atomic layer deposition technique onto (001)-oriented α-Al2O3 and GaN substrates. The structure and the microstructure of the deposited layers were studied by TEM in detail. The pure MgO layer starts to grow with epitaxy with (100)-type defects that transforms to random orientation. In the 10 at% Mg-doped samples epitaxial cubic MgO buffer layer forms at the interface. This buffer layer helps the ZnO to grow epitaxially in the case of α-Al2O3 substrate forming a ZnO/MgO/c-plane α-Al2O3 hetero-structure showing higher mobility with lower carrier concentration, while in the case on GaN substrate the ZnO is strongly textured. Consequently for higher concentration Mg doping (with the formation of MgO buffer layer) α-Al2O3 is better choice to grow epitaxial ZnO layer. The 1 at% Mg-doping of the epitaxial ZnO layer grown onto GaN substrate was successfully implemented, while in the case of α-Al2O3 substrate a thin cubic MgO buffer layer forms. This shows that the success of low concentration Mg doping in ZnO largely depends on the choice of substrate material as well.

The origin of carbon-related carrier compensation in p-type GaN layers grown by MOVPE

The role of carbon impurities in p-type GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) was investigated. The lightly Mg-doped (∼1017 cm−3) p-type GaN samples with different carbon concentration [C] were prepared by controlling growth temperature and pressure. Temperature-dependent Hall-effect analyses exhibited an increase in donor concentration with increasing [C]. The low-temperature mobility also decreased with increasing [C], as a result of mobility limitation due to ionized impurity scattering. These results show that carbon atoms in MOVPE-grown p-GaN layers act as ionized donors and cause carrier compensation. Deep-level transient spectroscopy (DLTS) using bias pulses detected the existence of Hd traps (EV +0.88 eV) arising from the 0/−1 charge state of carbon on nitrogen sites (CN). The concentrations of Hd traps closely corresponded to [C] values in p-type GaN layers. Employing low-frequency capacitance DLTS to avoid carrier freeze-out at low temperatures, we newly discovered the Ha trap (EV +0.29 eV) whose concentration was directly proportional to the [C] value. These findings suggest that the Ha trap originates from CN identical to the Hd trap. Based on prior theoretical calculations of energy levels, the Ha trap can reasonably be assigned to a +1/0 donor state of CN. These results strongly suggest that a CN having two different charged states can compensate an electron and a hole in n-type and p-type GaN layers, respectively.

Wide range doping control and defect characterization of GaN layers with various Mg concentrations

We demonstrated a wide range of magnesium (Mg) doping control (1016–1020 cm−3) in a GaN layer grown by metalorganic vapor phase epitaxy on a freestanding GaN substrate and investigated the defect states at low and high Mg concentrations ([Mg]). Hydrogen concentrations ([H]) in as-grown GaN samples showed a one-to-one relationship with [Mg] over the wide Mg doping range of 6 × 1016–3 × 1019 cm−3 due to the formation of Mg-H complexes but exhibited gaps between [Mg] and [H] at the low and the high ends of the doping range. At low [Mg], we found that [H] was in good agreement with the sum of [Mg] and carbon (C) concentrations, indicating the formation of C-H complexes. The acceptor concentration (Na) was significantly decreased for heavily Mg-doped samples with lower [H], while Na values close to [Mg] were obtained for samples having [H] close to [Mg]. These suggest that an Mg atom forming an Mg–H bond in the as-grown samples plays as an acceptor after annealing while an Mg atom not forming an Mg-H complex has other states. In the heavily Mg-doped layers, transmission electron microscopy (TEM) and scanning TEM (STEM) analyses showed that nano-scale defects are formed and that these defects are pyramidal inversion domains (IDs) with Mg segregation at the top (0001) boundary. We estimated the amount of segregated Mg atoms on the basis of our TEM-STEM analyses and concluded that most parts of the Mg atoms not forming Mg-H complexes are segregated at the ID boundaries.

Controlled release of DNA from zinc and magnesium ion-doped hydroxyapatites

Hydroxyapatite (HA) is a biocompatible and porous material that is useful for gene delivery. In this study, various Zn- and Mg-doped HA samples were produced by adding the ions at different dopant concentrations to control the association and dissociation of DNA. Although the HA crystal structure was retained after doping, it became distorted with increasing doped metal concentration. In 20% Zn-doped HA, a secondary Zn(OH)2 phase was observed. Mg-doped HA did not show significant cytotoxicity against breast tumor cells and osteoblasts. But, highly Zn-doped HAs were toxic, suggesting that doped Zn ions affected the cells. DNA could effectively adsorb to HA, regardless of metal ion doping, but the dissociation behavior of DNA differed. DNA was gradually released from Mg-doped HA for over a week, independent of Mg dope concentration, but not from Zn-doped HA, except for 20% doping. Therefore, the release properties of HA can be controlled by doping with suitable metal ion species at an appropriate dosage.

Theoretical confirmation of the polaron model for the Mg acceptor in β-Ga2O3

β-Ga2O3 has recently been considered for power electronics applications but p-type doping is an issue, especially since small hole polarons were predicted and observed in this material. Recently, by using a gap-optimized, Koopmans-compliant hybrid functional, we were able to reproduce the observed charge transition levels of defects, including the hole polaron in the intrinsic material. In this study, the same hybrid functional is used to confirm the assignment of the electron paramagnetic resonance (EPR) spectrum observed in Mg-doped β-Ga2O3 samples. The Mg impurity introduces a deep acceptor level in β-Ga2O3 corresponding to a small polaron, localized at an oxygen site adjacent to the substitutional Mg. The so-called O1 site is energetically favored and the superhyperfine interactions with the neighboring tetrahedrally coordinated Ga atom are 1.9 times stronger than with the neighboring octahedrally coordinated Ga. The calculated hyperfine tensor agrees well with the EPR measurement. These results confirm that Mg cannot act as a shallow acceptor in β-Ga2O3.

Optical Properties of Mg- and Ni- Doped Ag2S Colloidal Nanoparticles

In this work, we report the optical properties of Mg- and Ni-doped Ag2S colloidal nanoparticles. The Mg- and Ni-doped Ag2S nanoparticles were prepared using a wet chemical method. The influence of doping on the optical properties of the Mg- and Ni-doped Ag2S nanoparticles was investigated. The transmission electron microscopy (TEM) images showed the shapes of particles to be spherical with an average particle size of about 5–18 nm for all pure and doped Ag2S nanoparticles. The absorption spectra of the Mg-doped samples were red shifted from 2.42 to 2.20 eV, however, the UV-vis spectra of the Ni-doped Ag2S showed a blue shift from 2.42 to 2.60 eV. The observed blue shift in the band gap of Ni-doped Ag2S may be due to the substitution of Ni in to the Ag2S lattice. The photoluminescence intensity of the Mg-doped Ag2S nanoparticles increased as the Mg concentration was increased. However, the photoluminescence (PL) intensity of the Ni-doped Ag2S decreased as the concentration of Ni was increased.

Magnesium-doped Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode with high rate capability and improved cyclic stability

Mg-doped Li[Li0.2Mn0.54 − x/3Ni0.13 − x/3Co0.13 − x/3Mgx]O2 (x = 0, 0.005, 0.007, 0.01, and 0.02) cathode materials have been synthesized by mixing Mn0.54Ni0.13Co0.13(CO3)0.8 precursor, Li2CO3, and MgO, followed by high-temperature solid-state method. X-ray diffraction (XRD) results show that Mg-doped samples have enlarged interlayer distance and orderly layered structure. Scanning electron microscope (SEM) results indicate that all the samples have similar morphologies. Electrochemical tests indicate that Mg doping facilitates the activation of Li2MnO3 phase and suppresses the transformation from layered to spinel-like phase. Mg-doped samples possess improved cyclic and rate performance. The Li[Li0.2Mn0.54 − x/3Ni0.13 − x/3Co0.13 − x/3Mgx]O2 (x = 0.01) sample delivers a capacity retention of 92.07% compared with 75.25% of the undoped one after 200 cycles at 250 mA g−1 and exhibits the discharge capacity of 198.56 mAh g−1 at 500 mA g−1. The reasons for the improved electrochemical performance are the enlarged interslab spacing and the enhanced structural stability.

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

Defect-related photoluminescence (PL) is analyzed in detail for $n$-type, $p$-type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive $n$-type and $p$-type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped and Mg-doped GaN samples is attributed to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence $(\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}})$ band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The $\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}}$ band is attributed to electron transitions from an unknown deep donor to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor. Basic properties of the observed PL are explained with a phenomenological model.

Magnesium acceptor in gallium nitride. II. Koopmans-tuned Heyd-Scuseria-Ernzerhof hybrid functional calculations of its dual nature and optical properties

The problem of magnesium acceptor in gallium nitride is that experimental photoluminescence measurements clearly reveal a shallow defect state, while most theoretical predictions favor a localized polaronic defect state. To resolve this contradiction, we calculate properties of magnesium acceptor using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, tuned to fulfill the generalized Koopmans condition. We test Koopmans tuning of HSE for defect calculations in GaN using two contrasting test cases: a deep state of gallium vacancy and a shallow state of magnesium acceptor. The obtained parametrization of HSE allows calculations of optical properties of acceptors using neutral defect-state eigenvalues, without relying on corrections due to charged defects in periodic supercells. Optical transitions and vibrational properties of $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ defect are analyzed to bring the dual (shallow and deep) nature of this defect into accord with experimental photoluminescence measurements of the ultraviolet band in Mg-doped GaN samples.

Performance improvements in AlGaN-based ultraviolet light-emitting diodes due to electrical doping effects

We report a new doping method for the fabrication of wide-bandgap (WB) semiconductors such as p-AlGaN using electric fields and the application of this method to AlGaN-based UV light-emitting diodes (LEDs) to evaluate its effect at the device level. We prepared four LED samples with different work function (WF) energies using Pt, Ni, Ti, or Mg as contact metals and applied electric fields between these metals and the p-AlGaN surface across indium-doped tin oxide (ITO)/AlN thin films to facilitate diffusion of the metal atoms into the p-AlGaN layer. Compared to the samples with reference ITO electrodes (10 or 100 nm), ohmic behavior on the p-AlGaN surface was improved in the samples doped with Pt, Ni (high WF), and Mg (low WF but shallow dopant), but not for the sample doped with Ti (low WF). Furthermore, Mg-doped samples exhibited the lowest contact resistance with reasonably high transmittance among the four samples; accordingly, the lowest forward voltage and highest light-output power were achieved with UV LEDs using ITO/AlN/Mg electrodes. This electrical doping method could be useful for WB semiconductors fabricated with materials such as p-AlGaN and p-ZnO, which are difficult to dope using either thermal or optical doping method.

Radiophotoluminescence and thermoluminescence characteristics of undoped and Mg doped LiF phosphor in the high dose region

Radiophotoluminescence (RPL) and thermoluminescence (TL) from undoped and Mg-doped LiF powder samples were investigated for high dose dosimetric applications. F-aggregate centres in LiF are created by radiolysis. The RPL emission at 530 nm and 650 nm from F3+ and F2 centres respectively (on excitation at 450 nm) in undoped commercial LiF powder enhanced with increasing sintering temperature. Its TL glow curve showed peaks at 145, 290 and 365 °C. The RPL-TL correlation studies carried out for the first time showed that the thermal annihilation of F3+ and F2 centes following Fi0 - Va (interstitial-vacancy) recombination cause 290 °C and 365° TL peaks respectively. This showed that F2 centres are thermally more stable than F3+ centres. Its integrated TL response kept increasing nearly linearly up to 14 kGy, in contrast to the response of the TL peaks in dosimetry grade LiF:Mg, Ti (TLD-100). However, the TL from LiF suffers from fading problems. A 100 °C, 15 min pre-read annealing treatment erased most of the 145 °C TL peak without disturbing the other high temperature peaks. A 750° C, 22 h sintering treatment in carbon atmosphere shifted the major TL glow peak of undoped LiF to 200 °C but reduced its TL sensitivity by more than a factor of 10. Mg-doped LiF samples exhibit a TL glow curve peaking at ∼240 °C with satellite peaks typical of Mg doped LiF whose shape did not change significantly with gamma dose. However it showed lower TL and RPL sensitivities as compared to those of undoped LiF. All LiF except LiF alfa (750 °C, 1 h, CB) showed sublinear increase in TL with dose and therefore will cause inaccuracy in dose measurements especially at high doses due to reduced sensitivity. LiF alfa 750 °C, 22 h and 1000 °C, 6 min sintered powder samples are better than others because their RPL sensitivity increases nearly linearly with dose. However, a low fading observed in the latter sample supports its application. The bright yellow RPL emission seen through a 550 nm cut on filter illuminated with a 450 nm blue LED or He-Cd laser light could quickly confirm an irradiated consignment on which LiF samples are affixed. The increase in green to red emission ratio with dose and post-irradiation storage period might be related to slow migration of anion vacancies created by one secondary electron track to F centres created by another track and consequent conversion of F2 → F3+ centres.

Low-cost growth of magnesium doped gallium nitride thin films by sol-gel spin coating method

Low-cost sol-gel spin coating growth of magnesium (Mg) doped gallium nitride (GaN) thin films with different concentrations of Mg was reported. The effects of the Mg concentration on the structural, surface morphology, elemental compositions, lattice vibrational, and electrical properties of the deposited films were investigated. X-ray diffraction results show that the Mg-doped samples have wurtzite structure with preferred orientation of GaN(002). The crystallite size decreases and the surface of the films with pits/pores were formed, while the crystalline quality of the films degraded as the Mg concentration increases from 2% to 6. %. All the Raman active phonon modes of the wurtzite GaN were observed while a broad peak attributed to the Mg-related lattice vibrational mode was detected at 669 cm-1. Hall effect results show that the resistivity of the thin films decreases while the hole concentration and hall mobility of thin films increases as the concentration of the Mg increases.

Laser-induced local activation of Mg-doped GaN with a high lateral resolution for high power vertical devices

A method for laser-induced local p-type activation of an as-grown Mg-doped GaN sample with a high lateral resolution is developed for realizing high power vertical devices for the first time. As-grown Mg-doped GaN is converted to p-type GaN in a confined local area. The transition from an insulating to a p-type area is realized to take place within about 1–2 μm fine resolution. The results show that the technique can be applied in fabricating the devices such as vertical field effect transistors, vertical bipolar transistors and vertical Schottkey diode so on with a current confinement region using a p-type carrier-blocking layer formed by this technique.

Mg-doped Li1.2Mn0.54Ni0.13Co0.13O2 nano flakes with improved electrochemical performance for lithium-ion battery application

Mg-doped Li1.2-xMgxMn0.54Ni0.13Co0.13O2 cathode materials were prepared by a molten salt method. Our results showed that the introduction of NaCl is rather important and will provide a fusion environment, which not only leads to an evenly distribution of the transition metals within the cathode materials but is also helpful for the formation of a unique two dimensional (2D) nano structures. Mg doping results in a structural variation of the cathode materials, leading to a continuous increase of the capacity at the first dozen cycles. Although Mg doping can significantly improve the rate capability, the cycling stability, and the capacity retention of the cathode materials, a large Mg doping amount (>6%) will cause the formation of impurity phases, which in turn reduce the electrochemical performance of the materials. Electrochemical impedance spectroscopy (EIS) measurement showed that the charge transfer resistance was reduced significantly due to Mg doping, and 4% Mg-doped sample exhibits promising lithium diffusion kinetics and rate capability.

Comprehensive study of the structural, optical and electrical properties of InAlAs: Mg films lattice matched to InP grown by MOVPE

An extensive photoreflectance (PR) and photoluminescence (PL) study is done on an undoped n-type and p-type InAlAs metalorganic vapor phase epitaxy (MOVPE) grown layers on Fe-doped semi-insulating (SI)-InP substrates. A p-type doping with doping concentration of (1.57 × 1018 cm−3) at a growth temperature of 560 °C and V/III ratio of 50 is achieved. Photoluminescence (PL) spectra show a significant improvement in peak intensity of the type II transition for the Mg-doped InAlAs sample proving a better quality of the interface. Moreover, the abundant narrow satellite peaks of X-ray diffraction and the reciprocal space mapping recorded in the vicinity of (004) using high resolution X-ray diffraction (HRXRD) demonstrate a good quality of interfaces. Besides, photoreflectance spectroscopy (PR) at room temperature is useful to experimentally establish the valence-band splitting, a band-gap energy of InP substrate and InAlAs layers. A good correlation between experimental results was obtained.

<inline-formula> <tex-math notation="LaTeX">$\beta$</tex-math> </inline-formula>-Ga2O3Solid-State Devices for Fast Neutron Detection

Examination of Ga2O3 as solid-state nuclear detector was carried out. Ga2O3 is a wide bandgap material with beneficial physical properties that enable its application in harsh environmental conditions, such as elevated temperature or strong electromagnetic field; therefore, Ga2O3 could become a competitor of diamond and 4H silicon-carbide nuclear detectors. Furthermore, because of its high oxygen content the new detector material can play an important role in the field of reactor research due to the 16O(n, $\alpha )^{13}\text{C}$ reaction. Monocrystalline $\beta $ -Ga2O3 samples were investigated under 14 MeV fast neutron irradiation. On unintentionally doped semiconducting and Mg-doped insulating crystals metallic films were deposited in order to form the contact electrodes for biasing and to collect the electron-hole pairs generated by secondary particles after nuclear interactions between neutrons and the nuclei of the Ga2O3 crystal. The Mg-doped sample could be operated from zero up to more than 1000 V biasing level. The recorded electric signal and energy histograms were investigated.

Size reduction-induced chain breaking in Haldane-chain compounds SrNi2−xMgxV2O8 (x = 0 and 0.1)

We report size reduction-induced chain breaking in the spin-1 Haldane-chain SrNi2−xMgxV2O8 (x = 0 and 0.1) by magnetization and electron spin resonance measurements. For x = 0.0, the magnetic susceptibility of all samples can be well described by a temperature-independent term, a Curie–Weiss term and a Haldane-gap term. This implies that a reduced sample grain size breaks the long chain and creates a considerable number of S = 1/2 edge spins, resulting in the enhancement of magnetization and the decrease of Haldane gap in the samples. These edge spins as well as the other paramagnetic ions at grain boundary and surface might be weakly coupled with each other. For the Mg-doped sample with x = 0.1, there are more S = 1/2 spins creased in relative to x = 0.0 because of a combined effect of lattice defects, Mg-doping and reduced size. In addition, the antiferromagnetic resonance of x = 0.1 is also presented.

Thermoelectric properties of LiCo1−xMxO2 (M = Cu, Mg, Ni, Zn): Comparison with LiyCoO2 and NayCoO2 systems

LiyCoO2 has a similar layered structure to NayCoO2, which is a typical p-type oxide thermoelectric material, and the average Co valence of 3 + y is controlled by the Li content y. We investigated the thermoelectric properties of LiCo1−xMxO2 samples (M = Cu, Mg, Ni, Zn) for the first time at high temperatures, in which Co3+ was substituted by the divalent M2+ ions, and the average Co valence of 3 + x can be controlled similarly to the Li content y in LiyCoO2. The substitution of the M2+ ions for the Co site was found to show thermoelectric properties similar to those of LiyCoO2 with the same average Co valence. The Mg-doped sample showed the highest thermoelectric performance at high temperatures in this study; the thermoelectric power factor P is 2.38 × 10−4 W m−1 K−2 at 1173 K and the dimensionless figure of merit ZT is 0.024 at 876 K. The thermoelectric potential of LiCo1−xMxO2 is discussed and compared with those of LiyCoO2 and NayCO2 systems.

Role of Mg doping in the structural, optical, and electrical characteristics of ZnO-based DSSCs

ZnO- and Mg-doped ZnO samples are prepared by spray pyrolysis on conducting glass substrates to fabricate ZnO-based dye-sensitized solar cells (DSSCs). Influences of Mg-doping content on the power conversion efficiencies of ZnO-based DSSCs are investigated. X-ray diffraction results show that all the samples exhibit a hexagonal wurtzite structure. Scanning electron microscopy data indicate that the ZnO sample has uniform rods with 1 $\mu $m diameter. With respect to ZnO, the band gap value of 4 at.% Mg-doped ZnO samples improves to the value of 3.27 eV and a further increase in Mg level up to 6 at.% gives rise to a decline in the band gap value of 3.22 eV. Photoluminescence measurements illustrate that intensities of the ultraviolet peak and a red luminescence peak take their maximum values for 4 at.% Mg doping. From solar cell performance measurements, the best power conversion efficiency of 0.08% is obtained for the doping amount of 4 at.% Mg.

Highly Efficient Epoxidation of Cyclopentene Catalyzed by Magnetically Recoverable Mg-doped Cobalt Ferrites with Greatly Improved Performances

The alkaline earth metal Mg-doped CoFe2O4 composite ferrite samples MgxCo1-xFe2O4 (x=0, 0.1, 0.3, 0.5, 0.7 and 1) were prepared using a sol-gel auto-combustion route. The as-prepared samples were characterized by Raman spectrometry, X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), ICP–atomic emission spectroscopy and N2-physisorption. The catalytic activities of the samples were tested in the cyclopentene oxidation using hydrogen peroxide (30 %) as the oxidant. Compared with the pristine CoFe2O4 and MgFe2O4, the Mg−Co composite ferrites showed much enhanced catalytic performances in the oxidation of cyclopentene with cyclopentene oxide as the main product. In particular, 95.2 % of cyclopentene conversion and 96.3 % of selectivity for cyclopentene oxide were achieved when Mg0.5Co0.5Fe2O4 was used at 60 oC for 8 h in the oxidation reaction. The catalyst can be easily separated by applying magnetic field and exhibits great reusability.