Direct synthesis of β-FeSi2 by ion beam mixing of Fe/Si bilayers

The iron di-silicide #-FeSi₂ is a promising direct band gap semiconductor but difficult to produce. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 450 to 550 °C is reported. The obtained single-phase #-FeSi₂ layers and their structure are confirmed by Rutherford backscattering spectrometry, X-ray diffraction and conversion electron Mössbauer spectroscopy.     

Terahertz transmission properties of Cr ion implantation glass

<正>Cr ion implantations in glass with the different doses of D=1.493×10~(17) and 4.976×10~(17) ion/cm~2 are obtained by metal vapor vacuum arc(MEVVA).The effects of the different Cr ion implanted doses on terahertz(THz) transmission property are analyzed from THz time-domain spectroscopy.The results show that the more the Cr ion implanted dose in the micro-area implantation glasses,the larger the THz transmission except the larger absorption at 0.24 THz.This is an effect attributed to the coupling of plasmas on both the implantation and the implantation affected zones of the Cr ion implantation glass.     

Photoluminescence of Er-doped Si-SiO2 and Al–Si-SiO2 sputtered thin films

Er-doped Si-SiO₂ and Al–Si-SiO₂ films have been deposited by rf-sputtering being annealed afterwards. Annealing behavior of the Er³⁺: ⁴I_13/2→⁴I_15/2 emission of Er-doped Si-SiO₂ yields a maximum intensity for annealing at 700–800 °C. ⁴I_13/2→⁴I_15/2 peak emission for Er-doped Al–Si-SiO₂ at 1525 nm is shifted from that for Er-doped Si-SiO₂ at 1530 nm and the bandwidth increases from 29 to 42 nm. ⁴I_13/2→⁴I_15/2 emission decays present a fast decaying component related to Er ions coupled to Si nanoparticles, defects, or other ions, and a slow decaying component related to isolated Er ions. Excitation wavelength dependence and excitation power dependence for the ⁴I_13/2→⁴I_15/2 emission correspond with energy transfer from Si nanoparticles. Populating of the ⁴I_11/2 level in Er-doped Si-SiO₂ involves branching and energy transfer upconversion involving two or more Er ions. Addition of Al reduces the populating of this level to an energy transfer upconversion involving two ions.     

Surface structure of β-FeSi2(101) epitaxially grown on Si(111)

The surface morphology and structure of -FeSi₂(101) films epitaxially grown on Si(111) has been studied by means of Scanning Tunneling Microscopy (STM). The films are formed by large crystallites which are single domain. Each crystallite has only one of the three possible azimuthal orientations with respect to the substrate. A large density of planar defects, however, is detected on top of each crystallite. They are assigned to intrinsic stacking faults and their existence seems hard to avoid. This high density of intrinsic defects casts serious doubts on the use of -FeSi₂ as an optoelectronic material.     

Microstructure and photoluminescence of Er-doped SiOx films synthesized by ion beam assisted deposition

Er-doped SiO_x films were synthesized at 500℃ by ion beam assisted deposition technique and annealed at 800 and 1100℃ for 2h in the air atmosphere. The analysis by using energy dispersive x-ray spectroscopy showed that the ratio of Si to O decreased from 3 in the as-deposited films to about 1 in the annealed films. The investigation by using transmission electron microscopy and x-ray diffraction indicated that annealing induces a microstructure change from amorphous to crystalline. The grain sizes in the films were about 10 and 40nm when annealed at 800 and 1100℃, respectively. The films annealed at temperatures of 800 and 1100℃ exhibited a sharp photoluminescence (PL) at 1.533μm from the Er centres when pumped by 980nm laser. The influence of microstructure and grain size on the PL from Er-doped SiO_x films has been studied and discussed.     

Synthesis of Ag metallic nanoparticles by 120 keV Ag− ion implantation in TiO2 matrix

TiO₂ thin film synthesized by the RF sputtering method has been implanted by 120 keV Ag^? ion with different doses (3?×?10¹⁴, 1?×?10¹⁵, 3?×?10¹⁵, 1?×?10¹⁶ and 3?×?10¹⁶ ions/cm²). Further, these were characterized by Rutherford back Scattering, XRD, X-ray photoelectron spectroscopy (XPS), UV–visible and fluorescence spectroscopy. Here we reported that after implantation, localized surface Plasmon resonance has been observed for the fluence 3?×?10¹⁶ ions/cm², which was due to the formation of silver nanoparticles. Ag is in metallic form in the matrix of TiO₂, which is very interestingly as oxidation of Ag was reported after implantation. Also, we have observed the interaction between nanoparticles of Ag and TiO₂, which results in an increasing intensity in lower charge states (Ti³⁺) of Ti. This interaction is supported by XPS and fluorescence spectroscopy, which can help improve photo catalysis and antibacterial properties.     

Optical and electrical properties of Sb-doped β-Ga2O3 crystals grown by OFZ method

Sb-doped β-Ga₂O₃ crystals were grown using the optical floating zone(OFZ) method.X-ray diffraction data and X-ray rocking curves were obtained,and the results revealed that the Sb-doped single crystals were of high quality.Raman spectra revealed that Sb substituted Ga mainly in the octahedral lattice.The carrier concentration of the Sb-doped single crystals increased from 9.55 × 10¹⁶ to 8.10 × 10¹⁸ cm^-3,the electronic mobility depicted a dec...     

Isolation of fe charge states by ion implantation and their study by mössbauer spectroscopy

Matrix isolation of impurity atoms in a frozen rare-gas lattice is a well-known technique for the study of atoms in quasi-free states. The usual procedure consists of co-evaporating these atoms together with the matrix atoms and depositing them simultaneously on a cold substrate. Subsequently, the frozen samples can be studied by optical spectroscopy or, as has been proved more recently, by Mössbauer spectroscopy.

The technical limitations of the co-evaporation procedure narrow the application range of this method. We have therefore tried to develop the ion implantation technique as a suitable alternative to the co-evaporation. Ion implantation does not put any limitation on the species and the concentration of the isolated atoms; moreover, it is particularly suited for the isolation of radioactive impurities, which may be investigated by Mossbauer spectroscopy.

In a first series of experiments, we have implanted ⁵⁷Co into a frozen Xe matrix, hereby producing isolated Fe⁺ and Fe⁰ states by the electron capture decay of the implanted nuclei.¹ A second experiment was devoted to the study of ⁵⁷Fe implanted in solid O₂.² Due to the high reactivity of oxygen, it would have been impossible to produce this sample by co-evaoporation. Mössbauer data show some evidence that in this case Fe⁶⁺ states have been produced. The results of these experiments will be discussed and further perspectives will be sketched.     

Mössbauer Optimization of the Direct Synthesis of β-FeSi2 by Ion Beam Mixing of Fe/Si Bilayers

At present, there is an increasing interest in the iron di-silicide phase -FeSi₂, which is supposed to be a direct band gap semiconductor and one of the most promising materials for silicon-based optoelectronics, e.g., light-emitting devices, solar cells, and photo detectors. But this phase is very difficult to be produced. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 400 to 600°C is reported. The aim of the experiments was to achieve a complete reaction of the deposited Fe layer with the Si substrate that results in the formation of a pure, single-phased -FeSi₂ surface layer. The obtained silicide layers, their structure and composition are investigated by conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The fraction of the -FeSi₂ formed is determined by CEMS as function of ion species, energy, fluence and temperature. Complete growth and formation of a single-phased -FeSi₂ layer was achieved by 205 keV Xe ion irradiation at a fluence of 2×10¹⁶ ions/cm² at 600°C.     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

X-ray and luminescent analysis of finely dispersed β-FeSi2 films formed in Si by pulsed ion-beam treatment

Finely dispersed β-FeSi₂ films were formed by implanting Fe⁺ ions with an energy of 40 keV and a dose of 1×10¹⁶ cm⁻² in Si single crystals, followed by nanosecond pulsed ion-beam treatment. The results of glancing incidence x-ray diffraction indicate the formation of a highly grain-oriented film consisting of inclusions of the iron disilicide phase (β-FeSi₂) with a grain size of approximately 40 nm surrounded by a polycrystalline Si matrix. The photoluminescence spectroscopy data reveal that the photoluminescence signal with a peak around 1.56 μm, which is observed up to 210 K, is associated with direct interband transitions in β-FeSi₂ and not with the contribution from the dislocation-induced line D1. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 9, 2001, pp. 1569–1572. Original Russian Text Copyright ? 2001 by Bayazitov, Batalov, Terukov, Kudoyarova.     

Observation of ε′-Ag17 Mg54 phase induced by plasma immersion ion implantation

This paper provides a confirmation of the effectiveness of the recently suggested ab initio approach to the theoretical prediction of phase transformations which may be induced in metallic alloys by metal plasma immersion and ion implantation processing. The approach is based on an assumption that at certain concentrations of the implanted species, the relaxation of the exited electronic state of the implanted structure should be accompanied by the rearrangement of atoms leading to the formation of a new phase. Recently, on the basis of density functional theory calculations of the energetic characteristics of the electronic subsystems of the implanted Mg–Ag system, it was predicted that concentrations of the implanted Ag ions within the range from ～18 to 23 at% Ag, favor transition to the phase ε′-Ag₁₇Mg₅₄. Our transmission electron microscopy observations and electron diffraction analysis of the Mg-based alloy subjected to the implantation of Ag ions at dose of ～5×10¹⁵ ion/cm² confirmed that the formation of the ε′-Ag₁₇Mg₅₄ phase indeed takes place.     

The feasibility of Al-based oxide precipitation in Fe–10%Cr alloy by ion implantation

This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al⁺ and O⁺ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30?nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM.     

Electrical contact properties of Cu2S nanowires grown vertically on Cu foil by gas–solid reaction

We grew Cu₂S nanowires vertically on Cu foil by gas–solid reaction with a gas mixture of O₂ and H₂S. The electrical contact properties between the Cu₂S nanowires and Cu foil were investigated using a modified current–voltage–temperature plot. The Cu/Cu₂S layer exhibited the characteristics of a Schottky barrier with a barrier height of ∼0.72 eV, which was closer to the value for Cu/Cu₂O than to Cu/Cu₂S. Energy dispersive spectroscopy results showed the presence of Cu-oxide between the Cu₂S nanowires and Cu foil. The overall structure was Cu/Cu-oxide/Cu₂S and the electrical properties were controlled by the Cu/Cu-oxide.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors, Ca_0.736?ySi_9.6Al_2.4O_0.8N_15.2:0.064 Eu²⁺, yMn²⁺, were firstly synthesized by the high temperature solid state reaction method. The effects of doped Eu²⁺ and Eu²⁺–Mn²⁺ concentrations on the photoluminescence properties of the as-prepared phosphors were investigated systematically. Powder X-ray diffraction shows that pure Ca-α-SiAlON phase is synthesized after sintering at 1700 °C for 2 h under 0.5 MPa N₂ atmosphere. The excitation spectra of Eu²⁺-doped Ca-α-SiAlON phosphors are characterized by two dominant bands centered at 286 nm and 395 nm, respectively. The photoluminescent spectrum of Eu²⁺-doped Ca-α-SiAlON phosphor exhibits an intense emission band centered at 580 nm due to the allowed 4f ⁶5d→4f ⁷ transition of Eu²⁺, showing that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. The intensities of both excitation and emission spectra monotonously decrease with the increment of codoped Mn²⁺ content (i.e. y value), indicating that energy transfer between Eu²⁺ and Mn²⁺ is inefficient in the case of Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors.     

Effect of void formation during MEVVA tungsten ion implantation on the microstructure and surface properties of H13 die steel

H13 die steel was implanted with tungsten using a metal vapour vacuum arc (MEVVA) ion source. When the pulsed beam current density of tungsten ions increased to 6mA?cm^-2, some voids appeared in the high voltage electron microscope (HVEM) micrograph, which would disappear at an annealing temperature of 600℃. HVEM and x-ray diffraction were used for observing the phase structure of the annealed and un-annealed H13 steel after the steel was implanted. Results of wear and hardness tests indicated that whether the voids appear significantly influences the hardness and wear of H13 steel. Reasons for the formation of voids and the relation between the surface mechanical property and voids are discussed in terms of collision theory.     

Optical properties of bulk gallium nitride single crystals grown by chloride–hydride vapor-phase epitaxy

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.