Annealing Of Mg Research Articles

(Invited) Effect of Oxygen on Activation Annealing of Mg-Doped GaN

The effects of N2 and O2:N2 (1:1) as ambient gases during activation annealing of Mg as p-type doping of GaN grown by MOCVD are investigated. The purpose is to understand the mechanisms involved and especially the impact of O2 on the resulting free hole concentration and hole mobility. The addition of O2 to the ambient gas during annealing is very effective in reducing the H level of the Mg-doped GaN layer, especially at lower activation temperatures, but the maximum achievable hole concentration and mobility, as determined by Hall characterization, is still higher with pure N2.The difference between activation in pure N2 or O2:N2 (1:1) ambient a is explained by an in-diffusion of O to the GaN layer acting as n-dopant and thus giving rise to a compensation effect. It is found that to a large degree only the Mg-H complexes at substitutional (MgGa), i.e., the electrically active acceptor sites that provide free holes, are activated by annealing with N2 only as ambient gas, while annealing with O2:N2 (1:1) also dissociates electrically inactive Mg-H complexes resulting in much less residual H. Thus, the residual H level in relation to the Mg level after activation annealing with N2 only may provide a representative measure of the resulting free hole concentration of the Mg-doped GaN layer.An additional factor to be taken into account for in order to maximize the hole concentration is the growth pressure. At the lower growth pressure of 150 mbar compared to higher growth pressure of 300 mbar, an increasing carbon incorporation leads to a compensation effect drastically reducing the free hole concentration while the mobility is unaffected. Conclusions Addition of O2 during RTP is very effective in reducing the H level however, the maximum hole concentration and mobility are still higher with pure N2.Mg-H complexes at substitutional (MgGa) electrically active acceptor are preferentially dissociated by annealing with N2 as ambient gas.By employing a higher growth pressure, carbon incorporation that leads to a compensation effect drastically reducing the free hole concentration can be minimized.By activation annealing with N2 the residual H level provides a measure of the free hole concentration. Figure 1

Insights into the evolution of texture and mechanical properties during warm rolling and annealing of Mg–2Zn-0.5Ca alloy

In this work, we systematically investigated the microstructure, texture evolution and mechanical properties of Mg–2Zn-0.5Ca alloy during warm rolling and annealing. The results showed that the net-like Mg6Zn3Ca2 secondary phases are evolved into short wire-like and fragmented into smaller pieces with the increasing rolling reduction. Besides, some recrystallization grains can be generated with a higher rolling reduction, resulting in a significant grain refinement, which is prone to suppress the activation of twinning. The formation of a double-peak texture with basal poles splitting from the normal direction (ND) towards the transverse direction (TD) can be mainly ascribed to the prismatic <a> slip during warm rolling. The formation of TD-oriented recrystallized grain leads to a transformation from a broadening of angular distribution towards the rolling direction (RD) to a broadening of angular distribution towards the TD. Moreover, the yield strength is dramatically enhanced by the synergetic strengthening effect of grain refinement, work hardening, and texture evolution. After annealing, the sample with high rolling reduction can achieve a combination of high yield strength and high formability due to the fine grains structure containing massive precipitates, which is also featured by an elliptical shape texture with basal poles inclining towards RD and TD.

Acceptor activation of Mg-doped GaN—Effects of N2/O2 vs N2 as ambient gas during annealing

Here, we investigate the effects of O2:N2 (1:1) as ambient gas as compared with pure N2 during activation annealing of Mg as p-type doping in GaN layers grown by MOCVD. The purpose is to understand the impact of O2 on the resulting free hole concentration and hole mobility using SIMS, XRD, STEM, AFM, and Hall effect measurements. Even though the presence of O2 in the ambient gas during annealing is very effective in reducing the H level of the Mg-doped GaN layers, the maximum achievable hole concentration and mobility is still higher with pure N2. The differences are explained by an in-diffusion of O to the GaN layer acting as n-dopant and, thus, giving rise to a compensation effect. The Mg–H complexes at substitutional (MgGa), i.e., the electrically active acceptor sites that provide free holes, are preferentially activated by annealing with N2 only as ambient gas, while annealing with O2:N2 (1:1) also dissociates electrically inactive Mg–H complexes resulting in much less residual H. At the lower growth pressure of 150 mbar compared to higher growth pressure of 300 mbar, an increasing carbon incorporation leads to a compensation effect drastically reducing the free hole concentration while the mobility is unaffected.

Phase transformations due to isochronal annealing of Mg – rare earth – Sc–Mn squeeze cast alloys

AbstractMagnesium alloys containing rare earths (Gd, Y, Ce, ≈3– 10 wt.%), Sc (&lt; 1 wt.%) and Mn (≈1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20 –580 °C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn2Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg12Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn2Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.

Effect of Ca and Nd on the microstructural development during dynamic and static recrystallization of indirectly extruded Mg–Zn based alloys

The present study examines the influence of Ca or Nd on the microstructure and texture modification during indirect extrusion and after subsequent annealing of Mg–Zn based alloys. The addition of such elements influence the recrystallization processes, i.e. dynamic (DRX) and static recrystallization (SRX), and leads to distinctive texture changes of samples extruded at different extrusion speeds. In the binary Mg–Zn alloys, the resulting texture after extrusion is the classical basal type texture, where there is an alignment of basal planes along the arc between the <10–10〉> and <11–20> poles. This development is independent of the extrusion speed, as the resulting microstructures are almost completely recrystallized. In the case of the Mg–Zn–Ca, there is a distinctive <10–11> component together with a <11–20> pole, while in the Mg–Zn-Nd alloy the so-called <11–21> rare-earth texture component and the <20–23> pole intensity are observed as DRX dominates the microstructural development. It is found that this distinctive behavior can be changed if partially recrystallized microstructures are subsequently annealed. This leads to an increase of the importance of SRX and a different resulting texture while maintaining a similar grain structure. Both recrystallization processes lead to different mechanical properties, yielding behavior and toughness as a result of the different texture development.

Thermal stability of nanocrystalline Mg-based alloys prepared via mechanical alloying

The thermal stability and the kinetics of grain growth of nanocrystalline Mg–6Al–1Zn and Mg–6Al–1Zn–1Si alloys prepared via mechanical alloying were investigated. It started with elemental powders, using a variety of analytical techniques including differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry. The kinetics of grain growth in isothermal annealing was investigated. The XRD results show that, although the grain sizes of both material systems increase as the annealing temperature rises, the Si-containing system displays a relatively smaller grain size, i.e., 60 nm compared with 72 nm in Mg–6Al–1Zn system, after being exposed to 350 °C for 1 h. The second-phase intermetallic particle Mg2Si formed during the isothermal annealing of Mg–6Al–1Zn–1Si system could influence not only the activation energy but also the exponent of kinetic equation. Higher hardness values obtained in the Si-containing system would be due to the formation of Mg2Si intermetallic phase.

Grain growth and texture evolution during annealing in an indirect-extruded Mg–1Gd alloy

Abstract RE (rare earth)-texture component has been reported to be responsible for improved formability and ductility in RE-containing Mg alloys. Grain growth and texture evolution during annealing of Mg–1Gd alloy was investigated after indirect extrusion and quenching to reveal the formation mechanisms of the RE texture component. In the as-extruded Mg–1Gd sample, both the recrystallized grains and large elongated grainsare oriented mainly with 〈 1 0 1 ¯ 0 〉 orientation parallel to the extrusion direction (ED). The preferred growth of the 〈 2 1 ¯ 1 ¯ 1 〉 grain with its 〈 2 1 ¯ 1 ¯ 1 〉 orientation parallel to ED has been observed during recrystallization annealing, leading to the gradual strengthening of 〈 2 1 ¯ 1 ¯ 1 〉 RE-texture component at the expense of the 〈 1 0 1 ¯ 0 〉 component with increasing annealing temperature. The driving force for the preferred grain growth is the difference in stored energy between the 〈 2 1 ¯ 1 ¯ 1 〉 grains and the 〈 1 0 1 ¯ 0 〉 grains. Abnormal growth of the 〈 1 0 1 ¯ 0 〉 grain takes place during long-time annealing, resulting in the strengthening of 〈 1 0 1 ¯ 0 〉 component texture, which can be explained by a texture effect.

Microstructure and texture evolution during hot rolling and subsequent annealing of Mg–1Gd alloy

The microstructure, texture and tensile ductility of Mg–1Gd alloy were investigated and compared to pure Mg following multipass rolling at 300°C and isothermal annealing at 400°C. The addition of Gd weakens the basal texture in both the as-rolled and annealed conditions, which is related to the enhanced activity of pyramidal 〈c+a〉-slip during hot rolling and the suppressed grain boundaries migration due to Gd segregation at grain boundary during annealing. A large number of secondary twins and shear bands formed during hot rolling of Mg–1Gd sheet may serve as favorable nucleation sites for static recrystallization during annealing. The recrystallized grains at bands/twins in Mg–1Gd alloy display a wide spread of orientations, which is similar to that in conventional Mg alloys. Pure Mg sheet shows a strong {0002} 〈11−20〉 texture component due to the preferred growth of grains with the 〈11−20〉 component during annealing. However, Gd solute segregation at grain boundary could inhibit the preferred growth of 〈11−20〉 grains, leading to a weak basal texture and a single 〈10−10〉 texture component in Mg–1Gd alloy sheet after annealing. The room-temperature ductility is significantly improved by the addition of Gd, which is mainly attributed to the texture weakening and grain refinement.

Oxygen vacancy enhanced room temperature magnetism in Al-doped MgO nanoparticles

We have measured the room temperature magnetization in Al-substituted magnesium oxide, Mg(Al)O nanoparticles with Al fractions of up to 5 at. %. All samples, including undoped MgO nanoparticles, exhibit weak room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at. % of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization for 2 at. % Al-doped MgO. Our results suggest that the oxygen vacancies are largely responsible for the weak room temperature ferromagnetism in MgO.

MgB2 films prepared by rapid annealing method

The experiment for preparing MgB2 thin films by electron beam (e-beam) annealing of Mg–B multilayer is reported. The annealing process lasts less than 1s, which effectively suppresses the volatility of Mg as well as the reaction of Mg and oxygen. The influence of annealing parameters on the film quality including exposure time, accelerating voltage and beam current is investigated. It is found that e-beam with lower accelerating voltage can acquire MgB2 film with smaller annealing energy density. Also, double e-beam scan annealing process helps to obtain more uniform MgB2 thin films than the single scan. This rapid annealing method synthesizes MgB2 thin films with critical temperature Tc of 35K and critical current density Jc at 15K of 7.6×106Acm−2.

Phase transformations due to isochronal annealing of Mg – rare earth – Sc–Mn squeeze cast alloys

Abstract Magnesium alloys containing rare earths (Gd, Y, Ce, ≈ 3–10 wt.%), Sc (< 1 wt.%) and Mn (≈ 1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20–580°C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn2Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg12Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn2Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.

Ion implantation of Si, Mg and C into Al 0.12Ga 0.88N

Si, Mg and C were implanted in undoped semi-insulating layers of Al x Ga 1− x N with x=0.12. Activation of implanted Si occurred after annealing at 1140°C in ammonia with subsequent anneal at 800°C in nitrogen. For a dose of 5 × 10 14cm −2 an average electron concentration of 1.2 × 10 18cm −3 was achieved. Implantation and annealing of Mg and C resulted in very highly resistive AlGaN layers for all annealing conditions, with no evidence of shallow acceptor activation. Proton implantation into initially conducting AlGaN produced highly resistive material suitable for electrical device isolation.

Deep levels in p+-n junctions fabricated by rapid thermal annealing of Mg or Mg/P implanted InP

In this work, we investigate the deep levels present in ion implanted and rapid thermal annealed (RTA) InP p+-n junctions. The samples were implanted with magnesium or coimplanted with magnesium and phosphorus. These levels were characterized using deep level transient spectroscopy (DLTS) and capacitance–voltage transient technique (CVTT). Seven majority deep levels located in the upper half of the band gap were detected in the junctions by using DLTS measurements, four of which (at 0.6, 0.45, 0.425, and 0.2 eV below the conduction band) result from RTA, while the origin of the other three levels (at 0.46, 0.25, and 0.27 eV below the conduction band) can be ascribed to implantation damage. An RTA-induced origin was assigned to a minority deep level at 1.33 eV above the valence band. From CVTT measurements, several characteristics of each trap were derived. Tentative assignments have been proposed for the physical nature of all deep levels.

Rapid thermal annealing of Mg + and P + co-implanted GaAs

The electrical activation of Mg and carrier profile distribution after rapid thermal annealing (RTA) in GaAs samples dually implanted with Mg + (1.0 × 10 15cm −2 at 100 keV) and P + to different doses and energies were studied. The main results include the enhancement of electrical activation yield (EAY) of Mg and the reduction of profile redistribution with the increasing of the P + dose. A maximum of the EAY of about 68% and carrier peak concentration of ⋍3 × 10 19 cm −3 were measured after RTA at 950°C for 5 s. The coexistence of high damage concentration level and a P concentration similar or higher than that of Mg, are the requirements for obtaining higher sheet carrier concentrations and shallower carrier profiles. The results are discussed considering possible effect of the P + implantation on the concentration of Ga sublattice point defects.

Rapid thermal annealing of Mg + and P + dually implanted InP

125 keV M + ions and 160 keV P + ions were implanted at RT, 100° C and 200° C with doses ranging from 1 × 10 14 to 1 × 10 15 cm −2 to study the effect of the dual implantations on the electrical properties of Fe doped InP. The samples were characterized by electrochemical C- V measurement and RBS-channelling techniques. It has been found that Mg + + P + dual implantations into InP can result in an enhanced activation of implanted dopants. The highest peak hole concentration and dopant activation obtained in this work were 1.1 × 10 19 cm −3 and 68% in the case of Mg + + P + dual implantations at a dose of 1 × 10 15 cm −2 while that for Mg + single implantation at the same dose were 5.4 ×10 18 cm −3 and 40%. A thin n-type layer was found in the carrier concentration profile of Mg + singly implanted samples whereas there was not such a layer in that of Mg + + P + dually implanted ones. RBS spectra showed that the implantation at 100° C leads to damage similar to that in the sample implanted at 200° C. A deep penetration of Mg into InP during annealing, which is supposed to be related to the (Mg,Fe) complex, can be suppressed to some extent by P + co-implantation.

Ion implantation and rapid thermal annealing of Mg, Cd, and Si in AlxGa1−xAs grown by molecular-beam epitaxy

Ion implantation and rapid thermal annealing of Mg, Cd, and Si in AlxGa1−xAs are reported. The dependence of activation on various Al compositions (x=0.15–0.35) and annealing temperatures was investigated. Activations of over 60% were achieved for both Mg and Cd. However, for the same annealing cycle, the activation efficiency of the Mg implants was found to be generally higher than that of Cd implants. No particular dependence of the activation efficiency on Al composition was observed for either ion. For Si29 implants, poor activation was observed in samples with x above 0.2, while for x=0.2 a maximum activation of 80% was achieved with a 1000 °C, 5 s annealing cycle.