在不同的退火温度下钴注入二氧化钛结构与磁性

选用能量为180 keV,温度为623 K,注入4×10¹⁶ cm^-2剂量的钴离子束注入TiO₂样品.在不同的退火温度下,用高分辨的扫描电镜、同步辐射X射线衍射、卢瑟福背散射/沟道实验和超导量子干涉仪,分别对样品进行结构与磁性的测试. SR-XRD和HRTEM测试结果表明: 在Co注入TiO₂后,形成了钴的体心立方(hcp)相和面心立方(fcc)相,且在TiO₂中,钴-纳米粒子也已经形成. 随着退     

Ar离子注入YBa2Cu3O7-x超导薄膜中微结构变化的透射电子显微镜研究

Ar离子注入YBa₂Cu₃O_7-x超导薄膜后,不仅会引起样品超导转变温度T_c和临界电流密度J_c的下降,还会使样品的正常态由金属型变为半导体型。透射电子显微镜观察发现在小剂量(<5×10¹²Ar/cm²)注入情况下,样品的晶格结构几乎不受影响。随着注入剂量的增加,晶格损伤越来越严重,最终变成非晶态。对实验结果的分析表明,Ar离子注入引起YBa_{2< 关键词：}     

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar⁺ ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10¹³ ions/cm² to 1 × 10¹⁸ ions/cm². Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N₂⁺ ions and 7.2 × 10¹⁷ ions/cm² are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar⁺ ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.     

First order Raman scattering analysis of transition metal ions implanted GaN

Transition Metal (TM) ions V, Cr, Mn and Co were implanted into GaN/sapphire films at fluences 5×10¹⁴, 5×10¹⁵ and 5×10¹⁶ cm⁻². First order Raman Scattering (RS) measurements were carried out to study the effects of ion implantation on the microstructure of the materials, which revealed the appearance of disorder and new phonon modes in the lattice. The variations in characteristic modes 1GaN i.e. E₂(high) and A₁(LO), observed for different implanted samples is discussed in detail. The intensity of nitrogen vacancy related vibrational modes appearing at 363 and 665 cm⁻¹ was observed for samples having different fluences. A gallium vacancy related mode observed at 277/281 cm⁻¹ for TM ions implanted at 5×10¹⁴ cm⁻² disappeared for all samples implanted with rest of fluences. The fluence dependent production of implantation induced disorder and substitution of TM ions on cationic sites is discussed, which is expected to provide necessary information for the potential use of these materials as diluted magnetic semiconductors in future spintronic devices.     

Thermal stability of F ion-implant isolated AlGaN/GaN heterostructures

The thermal stability of F ion-implanted isolated AlGaN/GaN heterostructures was investigated,with B ion-implanted isolation shown for comparison.The sheet resistance of as-implanted samples with F ions was lower than that with B ions due to enhanced hopping conduction.The leakage current for both implanted samples initially decreased and then increased with increases in post-annealing temperature,indicating a damage-induced isolation mechanism.Leakage reached a minimum value after 400~oC annealing,which was over 10～8times lower than the saturated current(I_(sat))of the as-grown structure,suggesting a successful isolation.After relatively high-temperature annealing,the leakage for F implantation showed a small change,whereas that for B implantation showed significant increase,suggesting that F-implanted isolation exhibited excellent thermal stability.Leakage and F or B ion-implanted samples indicated that the leakage current originated from the region above the high-resistance GaN layer.Variation in E_arevealed that the optimal contributing states for conduction changed with operating temperature,annealing temperature,and ion species.     

Effects of ion implantation on yellow luminescence in unintentional doped n-type GaN

The effects of Si, O, C and N ion implantation with different implantation doses on yellow luminescence (YL) of GaN have been investigated. The as-grown GaN samples used in the work were of unintentional doped n-type, and the photoluminescence (PL) spectra of samples had strong YL. The experimental results showed that YL of ion implanted samples exhibited marked reductions compared to samples with no implantation, while the near band edge (NBE) emissions were reduced to a lesser extent. The deep-level centers associated with YL may be produced in GaN films by O and C ion implantation, and identities of these deep-level centers were analyzed. It was also found that the dose dependence of YL was analogous with the one of the intensity ratios of YL to the near band edge (NBE) emission (I _YL /I _NBE ) for ion implanted samples. The possible reason for this comparability has been proposed.      

Characterisation of defects in rare earth implanted GaN by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) was used to investigate the electrical properties of GaN implanted with the rare earth (RE) ions erbium and thulium. The GaN layers have been grown by metal-organic chemical vapor deposition (MOCVD) onto (0001) sapphire substrates. We used the channeled implantation geometry to implant a dose of 5×10¹⁴ RE cm⁻² with an energy of 150 keV. For each species, two different annealing procedures were used in a nitrogen atmosphere for 120 s. Indeed, the annealing temperature plays an important role in the lattice recovery, even if RE-related defects remain present. After annealing at 1000 C, the appearance of two new peaks, for both studied RE ions, is associated with the lattice damage induced by the implantation, such as the presence of nitrogen vacancies. After annealing at 1100 C, the recovery of the lattice is observed while a hole trap appears for both implanted RE ions with corresponding energy values E_v+0.61 eV and E_v+1.59 eV, in the case of Er and Tm, respectively.     

X-ray diffraction studies of the structure of nanocrystals in Fe<Subscript>73.5</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> soft magnetic alloys before and after thermomechanical treatment

The structure of the Fe_73.5Si_13.5B₉Nb₃Cu₁ soft magnetic alloy has been investigated using X-ray diffraction in transmission geometry. The initial alloy prepared by rapid quenching from the melt has a short-range order (∼2 nm) in the atomic arrangement, which is characteristic of the Fe-Si structure with a body-centered cubic lattice. The alloy subjected to annealing contains Fe-Si nanocrystals with sizes as large as 10–12 nm. The annealing under a tensile load leads to an extension of the nanocrystal lattice so that, after cooling, a significant residual deformation is retained. This can be judged from the relative shifts of the (hkl) peaks in the X-ray diffraction patterns measured for two orientations of the scattering vector, namely, parallel and perpendicular to the direction of the load applied. The deformation is anisotropic: within the accuracy of the experiment, no distortions in the [111] direction are observed and the distortions in the [100] direction are maximum. It is known that crystals with a composition close to Fe₃Si exhibit a negative magnetostriction; i.e., their magnetization induced under a load (Villari effect) applied along the [100] direction is perpendicular to this direction along one of the easy magnetization ([010] or [001]) axes. In the alloy, the orientation of the nanocrystal axes is isotropic and the majority of the nanocrystals have a composition close to Fe₃Si. The direction of magnetization of these nanocrystals is determined by the residual deformation of their lattice and lies near the plane perpendicular to the direction of the tensile load applied during heat treatment. This is responsible for the appearance of transverse magnetic anisotropy of the easy-plane type in the Fe_73.5Si_13.5B9Nb₃Cu₁ alloy.     

A-site and B-site substitutions and the emission properties of Eu3+ ions in ABO3 –type cubic perovskite: A case study of BaZrO3

The emission properties of the rare earth (RE) elements have been found to be sensitive to the local lattice environment around them, and could be employed for probing the local lattice environment. Because each constituent ion of a material should have its own particular structural environment, the emission profiles in RE elements depend strongly on the doping sites of RE elements inside the host crystals. We investigated the emission properties of the Eu³⁺ ion doped BaZrO₃ (BaZrO₃:Eu) with doping site dependence (A-site vs. B-site), as well as ambient dependence in the post-annealing process. The site-selective doping was identified from the Rietveld refinement analysis on the XRD patterns of our BaZrO₃:Eu samples. Photoluminescence and photoluminescence excitation measurements showed that the emission properties of the samples with the Eu³⁺ ions substituted at Zr sites revealed much greater emission properties than those at the Ba sites. This behavior was found to accord with the change in intensity ratio of ⁵D₀ → ⁷F_0,2 to ⁵D₀ → ⁷F₁ in Eu ions, which should be a measure of the local lattice asymmetry around the Eu ions. We also found that with post-annealing in H₂ atmosphere, the emission intensities of the Eu ions increased significantly, while with post-annealing in O₂ atmosphere, they were suppressed. Our findings indicated that the cation/oxygen vacancies could change the local lattice environment around the Eu ions, as well as the valence states of Eu ions, depending on the doping site in the cubic perovskite.     

Effects of high-energy ion-beam irradiation on structural and optical properties of (Mg0.95Co0.05)TiO3 thin films

We report the structural and optical properties of high-energy ion-beam irradiated Co-doped magnesium titanate thin films. (Mg_0.95Co_0.05)TiO₃ (MCT) thin films were deposited on quartz substrates using radio frequency magnetron sputtering. Subsequently, the films were annealed for crystallinity and were irradiated with 100?MeV Ag ions by varying the ion fluence. The X-ray diffraction patterns of the films before and after the irradiation were refined using the Rietveld refinement and the variations in the lattice parameters were correlated with the ion fluence. Although, annealing of thin films results in an enhancement in refractive index and optical bandgap, the ion fluence induces significant changes in the refractive index and optical bandgap. Atomic force microscopy is employed to study the surface morphology of the films. The impact of ion fluence on structural and optical properties of MCT thin films has been investigated.     

The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10¹³ to 2×10¹⁶ at/cm², using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10¹⁵ at/cm², a highly disordered ‘nanocrystalline layer’ (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I₁, E and I₂ types have been noticed from the lowest fluence, they are I₁ in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I₁ BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs.When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10¹⁶ at/cm². The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.     

Effect of fast neutron irradiation on magnetic properties of fullerene superconductor Rb3C60

Effects of fast neutron irradiation and post-annealing on magnetic properties of Rb₃C₆₀ were studied through the dc magnetization measurement. Rb₃C₆₀ powder samples were prepared in an evacuated quartz glass tube, and the temperature and the magnetic field dependences of dc magnetization were measured before and after irradiation and after post-annealing. The neutron fluences were 1.0, 1.8 and 3.3 × 10¹⁶ n/cm², and the post-annealing was made at a temperature of 473 K for 3 h. Magnetic hysteresis of the samples irradiated at the fluence of 1.8 and 3.3 × 10¹⁶ n/cm² disappeared, and the hysteresis curves hardly changed at the fluence of 1.0 × 10¹⁶ n/cm². As for the post-annealing effect, the hysteresis curves of the sample irradiated at the fluence of 1.8 × 10¹⁶ n/cm² were completely recovered after annealing, while those of the other samples, which had a magnetic background before irradiation, were not recovered. In this study, it was found that the loss of superconductivity in Rb₃C₆₀ powder is observed when the neutron irradiation fluence exceeds 1.0 × 10¹⁶ n/cm², and the lost superconductivity is completely recovered by the post-annealing at 473 K for 3 h.     

Effects of different ions implantation on yellow luminescence from GaN

The influence of C, N, O, Mg, Si and co-implants (Mg+Si) ions implantation with fluences in the wide range 10¹³-10¹⁷ cm⁻² on the yellow luminescence (YL) properties of wurtzite GaN has been studied by photoluminescence (PL) spectroscopy. Two types of n-type GaN samples grown by metal-organic chemical vapor deposition method (MOCVD) and labeled as No-1 and No-2 were studied. In their as-grown states, No-1 samples had strong YL, while No-2 samples had weak YL. Results of the frontside and backside PL measurements in one of the as-grown GaN epifilms are also presented. Comparing the intensity of YL between frontside and backside PL spectra, the backside PL spectrum shows the more intense YL intensity. This implies that most of the intrinsic defects giving rise to YL exist mainly near the interface between the epilayer and buffer layer. Our experimental results show that the intensity ratio of YL to near-band-edge UV emission (I_YL/I_UV) decreases gradually by increasing the C implantation fluence from 10¹³ to 10¹⁶ cm⁻² for No-1 samples after annealing at 900 °C. When the fluence is 10¹⁷ cm⁻², a distinct change of the I_YL/I_UV is observed, which is strongly increased after annealing. For No-2 samples, after annealing the I_YL/I_UV decreases gradually with increase in the C implantation fluence from 10¹³ to 10¹⁵ cm⁻². The I_YL/I_UV is gradually increased with increasing C fluence from 10¹⁶ to 10¹⁷ cm⁻² after annealing, while I_YL/I_UV for other ions-implanted GaN samples decreases monotonically with increase in the ions implantation fluences from 10¹³ to 10¹⁷ cm⁻² for both No-1 samples and No-2 samples. It is noted that for annealed C-implanted No-2 samples I_YL/I_UV is much higher than that of the as-grown one and other ion-implanted ones. In addition, I_YL/I_UV for the Mg, Si, and co-implants (Mg+Si) implanted No-2 samples with a fluence of 10¹³ cm⁻² after being annealed at 900 °C is higher than that of the as-grown one. Based on our experimental data and literature results reported previously, the origins of the YL band have been discussed.     

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Cu₆PS₅Br superionic crystals were implanted by sulphur ions. The effect of ion implantation on the phase transition (PT) temperatures in Cu₆PS₅Br is observed from isoabsorption studies of the optical absorption edge. The dependences of the ferroelastic PT temperature on the ion fluence are analysed.     

Strain modification of AlGaN layers using swift heavy ions

Epitaxial AlGaN/GaN layers grown by molecular beam epitaxy (MBE) on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×10¹² ions/cm². The samples used in this study are 50 nm Al_0.2Ga_0.8N/1 nm AlN/1 μ m GaN/0.1 μ m AlN grown on SI 4H-SiC. Rutherford backscattering spectrometry/channeling strain measurements were carried out on off-normal axis of irradiated and unirradiated samples. In an as-grown sample, AlGaN layer is partially relaxed with a small tensile strain. After irradiation, this strain increases by 0.22% in AlGaN layer. Incident ion energy dependence of dechanneling parameter shows E ^1/2 dependence, which corresponds to the dislocations. Defect densities were calculated from the E ^1/2 graph. As a result of irradiation, the defect density increased on both GaN and AlGaN layers. The effect of irradiation induced-damages are analyzed as a function of material properties. Observed results from different characterization techniques such as RBS/channeling, high-resolution XRD and AFM are compared and complemented with each other to deduce the information. Possible mechanisms responsible for the observations have been discussed in detail.     

Effect of oxygen pressure on microstructure and magnetic properties of strontium hexaferrite (SrFe12O19) film prepared by pulsed laser deposition

The effects of oxygen pressure during deposition on microstructure and magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films grown on Si (100) substrate with Pt (111) underlayer by pulsed laser deposition have been investigated. X-ray diffraction pattern confirms that the films have c-axis perpendicular orientation. The c-axis dispersion (Δθ₅₀) increases and c-axis lattice parameter decreases with increasing oxygen pressure. The films have hexagonal shape grains with diameter of 150-250 nm as determined by atomic force microscopy. The coercivities in perpendicular direction are higher than those in in-plane direction, which shows the films have perpendicular magnetic anisotropy. The saturation magnetization and anisotropy field for the film deposited in oxygen pressure of 0.13 mbar are comparable to those of the bulk strontium hexaferrite. Higher oxygen pressure leads to the films having higher coercivity and squareness. The coercivity in perpendicular and in-plane directions of the film deposited in oxygen pressure of 0.13 mbar are 2520 Oe and 870 Oe, respectively.     

Structural modifications of GaN after cerium implantation

Among the family of rare earth (RE) dopants, the doping of first member Ce into GaN is the least studied system. This article reports structure properties of Ce‐doped GaN realized by technique of ion implantation. Ce ions were implanted into metal organic chemical vapor deposition grown n‐ and p‐GaN/sapphire thin films at doses 3 × 10¹⁴ and 2 × 10¹⁵ cm⁻². X‐ray diffraction scans and Raman scattering measurements exhibited expansion of lattice in the implanted portion of the samples. First order Raman scattering spectra show appearance of several disorder‐activated Raman scattering modes in addition to typical GaN features. A dose‐dependent decrease in intensity of E₂ mode was observed in Raman the spectra of the implanted samples. Ultraviolet Raman spectra of implanted samples show complete quenching of photoluminescence emission and appearance of multiple A₁(LO) phonon scattering modes up to fifth order. Moreover, a decrease in intensity and an increase in line width of LO modes as a function of wavenumber were observed for implanted samples. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrical characterization of rare-earth implanted GaN

Deep-level transient spectroscopy (DLTS) measurements were used to characterize the electrical properties of MOCVD grown, europium- (Eu) and xenon- (Xe) implanted GaN films on sapphire substrates. Implantation energy was 80 keV with a fluence of 1×10¹⁴ cm⁻² along a channeled crystallographic direction. Defect levels were observed at E_C−0.19 eV for both Eu- and Xe-implantation which were predicted to be a rare-earth related donor level by theoretical calculations. Other defect levels are observed with energy levels located at 0.22, 0.68, 0.49, 0.60, 0.77 eV and 0.48, 0.64, 0.45, 0.72 eV below the conduction band for Eu and Xe implantation, respectively. Some of these levels have similar defect signatures and can be related to other implantation related defects introduced in erbium, praseodymium and helium implantations.     

Synthesis and characterization of GaN nanowires by ammoniating Ga2O3/Cr thin films deposited on Si(1 1 1) substrates

GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Cr thin films at 950 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrophotometer, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high-quality crystalline, have the size of 30-80 nm in diameter and several tens of microns in length with good emission properties. The growth direction of GaN nanowires is perpendicular to the fringe of (1 0 1) plane. The growth mechanism of GaN nanowires is also discussed in detail.     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001