Optical behaviour of Er doped rutile by ion implantation

Rutile single crystals were implanted at room temperature with fluences of 5 × 10¹⁵ Er⁺/cm² ions with 150 keV energy. Rutherford backscattering/channeling along the 0 0 1 axis reveals complete amorphization of the implanted region. Photoluminescence reveals the presence of an optical centre close to the intra-ionic emission of Er³⁺ in the as-implanted samples. After annealing at 800 °C in air no changes were observed in the aligned RBS spectrum. On the contrary, annealing in reducing atmosphere (vacuum) induces the epitaxy of the damage layer. These results are unexpected, since for implantations of other ions under the same conditions, epitaxial recrystallization of the damage region occurs at this temperature. On the other hand, photoluminescence studies show the presence of new Er-related optical centres with high thermal stability in the samples annealed under oxidizing conditions. Annealing at 1000 °C in vacuum leads to the complete recrystallization of the damaged region. At this temperature a large fraction of Er segregates to the surface.     

Observation of micro-cavities in nickel during He and D irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He⁺ irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D⁺ irradiated nickel was different from the case of He⁺ irradiation. On annealing of the He⁺ irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D⁺ irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.     

Radiation damage and annealing behaviour of Ge-implanted SiC

In recent years, single-crystal SiC has become an important electronic material due to its excellent physical and chemical properties. The present paper reports a study of the defect reduction and recrysallisation during annealing of Ge⁺-implanted 6H-SiC. Implants have been performed at 200 keV with doses of 1 × 10¹⁴ and 1 × 10¹⁵ cm⁻². Furnace annealing has been carried out at temperatures of 500, 950 and 1500°C. Three analytical techniques including Rutherford backscattering spectrometry in conjunction with channelling (RBS/C), positron annihilation spectroscopy (PAS) and cross-sectional transmission electron microscopy (XTEM) have been employed for sample characterisation. It has been shown that damage removal is more complicated than in ion-implanted Si. The recrystallisation of amorphised SiC layers has been found to be unsatisfactory for temperatures up to 1500°C. The use of ion-beam-induced epitaxial crystallisation (IBIEC) has been more successful as lattice regrowth, although still imperfect, has been observed to occur at a temperature as low as 500°C.     

The formation of compound layers in silicon by ion beam synthesis

The technique of ion beam synthesis (IBS) using high doses of energetic ions has been successfully implemented to produce a variety of compounds, the physical properties of which are dependent on the implanted species and range from insulators, e.g. SiO₂, through semiconductors, e.g. SiC, to conductors, e.g. CoSi₂. In this paper we study the evolution of these compounds and compare and contrast their methods of formation. To demonstrate the versatility of the technique we look at three examples of IBS layers: (1) To date most of the interest in IBS has concentrated on the production of buried oxide layers for silicon-on-insulator (SOI) device applications. Recently it has been shown that by using a series of sequential implants and high-temperature anneals the defect density in the silicon overlayer can be dramatically reduced. To study how this process occurs, we followed the redistribution of the implanted species during implantation and annealing using both ¹⁶O⁺ and ¹⁸O⁺. (2) Buried CoSi₂ layers can be fabricated in (100) single-crystal silicon by implanting high doses of energetic cobalt ions at elevated temperatures. For the higher doses (≥ 4 × 10¹⁷ O⁺/cm² at 350 keV), a continuous coherent layer of CoSi₂ grows epitaxially during implantation. For lower doses, precipitates of both A- and B-type CoSi₂ are observed. After annealing at 1000° C for 30 min, single-crystal aligned layers are produced for the higher doses, while for lower doses discrete octahedral A-type precipitates are formed. (3) The microstructures of synthesized SiC layers are more complex than analogous synthesized oxide or silicide layers. Unlike buried oxide layers, the carbon concentration at the peak of the implanted distribution does not saturate at a value equivalent to that in the stoichiometric compound, but continues to rise, reflecting the lower diffusivity of the C in the synthesized compound layer. To achieve chemical segregation of the implanted carbon, very-high-temperature (≥ 1300°C), long-time (typically 20 h) anneals are required. At the interface with the silicon substrate the synthesized layer grows with a degree of epitaxy. This is also found to occur during implantation if the temperature is ≥ 650° C.     

Low temperature epitaxial regrowth of mercury implanted sapphire

Hg ions were implanted into sapphire at room temperature and 80 keV energy to a fluence of 1 × 10¹⁵ Hg⁺ / cm². This fluence was enough to produce an amorphous surface layer. The annealing behaviour was studied combining RBS/channeling and hyperfine interaction techniques. Surprisingly, the RBS/channeling results show there is an epitaxial regrowth of the damaged layer after annealing at 800°C for 20 min. Although some of the implanted Hg segregates to the surface during the epitaxial regrowth, a significant fraction is incorporated into regular sites along the c-axis. The hyperfine interactions results, obtained after implantation of a dose of 5 × 10¹² Hg⁺ / cm², show that a small fraction of Hg is probably bound to oxygen. This result is in agreement with the RBS/channeling measurements which also show that the system formed after annealing is stable even at high temperatures.     

High energy oxygen ion induced modifications in lead based perovskite thin films

The lead based ferroelectric PbZr_0.53Ti_0.47O₃ (PZT), (Pb_0.90La_0.10)TiO₃ (PLT10) and (Pb_0.80La_0.20)TiO₃ (PLT20) thin films, prepared by pulsed laser ablation technique, were studied for their response to the 70 MeV oxygen ion irradiation. The dielectric analysis, capacitance–voltage (C–V) and DC leakage current measurements were performed before and after the irradiation to high-energy oxygen ions. The irradiation produced considerable changes in the dielectric, C–V, leakage characteristics and induced some amount of amorphization. The PZT films showed partial recrystallization after a thermal annealing at 400 °C for 10 min. The phase transition temperature [T_c] of PLT20 increased from 115 °C to 120 °C. The DC conductivity measurements showed a shift in the onset of non-linear conduction region. The current density decreased by two orders of magnitude after irradiation. After annealing the irradiated films at a temperature of 400 °C for 10 min, the films partially regained the dielectric and electrical properties. The results are discussed in terms of the irradiation-induced amorphization, the pinning of the ferroelectric domains by trapped charges and the thermal annealing of the defects generated during the irradiation.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Effect of annealing on the optical absorption of Ni nanoparticles in MgO single crystals

Ni⁺ ion implantation with an energy of 64 keV in MgO single crystals was conducted at room temperature up to a fluence of 1 × 10¹⁷ ion/cm². The as-implanted crystals were annealed isochronally at temperatures up to 900 °C. Optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) have been utilized to characterize the changes of optical properties and the microstructure of the annealed samples. XPS results showed that the charge state of implanted Ni was still mainly in metallic Ni⁰ after annealing at 900 °C. TEM analysis revealed metallic Ni nanoparticles with depth-dependant dimensions of 1–10 nm in the annealed sample. Optical absorption spectroscopy indicated that the Ni nanoparticles exhibited a broad surface plasmon resonance absorption band in annealed samples and the band shifted to a longer wavelength with the increasing annealing temperature.     

Three-dimensional depth profiles of boron ions implanted in SiO2

Silicon dioxide films were implanted at room temperature with boron ions at 7° and 35°. Implantation energies ranged from 20 to 250 keV and the dose was 5×10¹⁴ ions/cm². The depth profiles of ion-implanted boron in SiO₂ were measured using secondary ion mass spectrometry and least-squares fitted to a Pearson distribution. The results demonstrated that the measured depth profiles are well approximated by Pearson distributions, while the experimentally determined range parameters correspond fairly well to the theoretical predictions yielded by the SRIM (stopping and range of ions in matter) Monte Carlo simulation code. The overall differences between the measured and calculated values are 4%, 7%, 15%, 22% and 10%, for projected range, longitudinal range straggling, skewness, kurtosis and transversal range straggling, respectively. Rapid thermal annealing (1050°C for 30 s) of the as-implanted specimens revealed that radiation-enhanced diffusion tends to increase projected range, longitudinal range straggling and transversal range straggling, but decrease (in absolute values) skewness and kurtosis. Notably, the increase in transversal range straggling is smaller than that of longitudinal range straggling.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

RBS studies of the lattice damage caused by 1 Me V Si implantation into Al0.3Ga0.7As/GaAs superlattices at elevated temperature

The lattice damage accumulation in GaAs and Al_0.3Ga_0.7As/GaAs superlattices by 1 MeV Si⁺irradiation at room temperature and 350°C has been studied. For irradiations at 350°C, at lower doses the samples were almost defect-free after irradiation, while a large density of accumulated defects was induced at a higher dose. The critical dose above which the damage accumulation is more efficient is estimated to be 2 × 10¹⁵ + Si/cm² for GaAs, and is 5 × 10¹⁵ Si/cm² for Al_0.8Ga_0.7As/GaAs superlattice for implantation with 1.0 MeV Si ions at 350°C. The damage accumulation rate for 1 MeV Si ion implantation in Al_0.3Ga_0.7As/GaAs superlattice is less than that in GaAs.     

Atom probe characterization of copper solubility in the Midland weld after neutron irradiation and thermal annealing

An atom probe field ion microscopy characterization has been performed to determine the copper matrix concentration in a submerged arc beltline weld of the Midland Unit 1 pressurized water reactor after four conditions: unirradiated, unirradiated and annealed for 168 h at 454°C, neutron-irradiated in a test reactor to a fluence of 1.1×10²³ n m⁻² (E>1 MeV) at a temperature of 288°C, and neutron-irradiated and annealed for 168 h at 454°C. Atom probe analysis of the unirradiated material revealed a substantial depletion of the copper in the matrix to 0.119±0.007 at.% Cu from the bulk value of between 0.18 and 0.28 at.% Cu. Annealing the unirradiated material produced intragranular copper-enriched precipitates and reduced the matrix copper level by 25% to 0.088±0.012 at.% Cu. Neutron irradiation also produced copper-enriched precipitates and reduced the matrix copper level by almost 50% over the stress relieved material to 0.058±0.008 at.% Cu. Annealing the neutron-irradiated material reduced the matrix copper level further to 0.050±0.010 at.% Cu. These results indicate that the annealing treatment coarsens the copper-enriched precipitates produced during neutron irradiation with a slight decrease in the matrix copper content.     

Transient annealing of Sn implanted GaAs

High temperature ( 900°C) transient annealing of Sn⁺ implants into GaAs have been studied by secondary ion mass spectroscopy, electrical measurements and transmission electron microscopy. A two-layer encapsulant (Si₃N₄ + AlN) has been used prior to annealing using an incoherent light furnace. Secondary ion mass spectroscopy measurements show that outdiffusion of tin has occurred which depends both on the dose and annealing conditions. The as implanted atomic profiles are wider than the theoretical profiles and up to 17% further broadening occurs during annealing. Electron concentrations approaching 10¹⁹ cm⁻³ have been measured reproducibly. Transmission electron microscopy results show both faulted and unfaulted dislocation loops and dislocation lines, all defects being decorated with precipitates which contain metallic tin. A large concentration of stacking fault tetrahedra is also produced.     

Effect of forming gas anneals on the photoluminescence from nanocrystalline silicon formed by Si implantation into SiO2 matrix

The blue region of the room temperature photoluminescence spectrum from Si nanocrystallites formed in SiO₂ by Si⁺ ion implantation has been observed for the first time after annealing in a forming gas (10% H₂ + 90% N₂) ambient. Thermally grown SiO₂ on Si substrates were implanted with a dose of 2 × 10¹⁷ Si⁺ cm⁻² at energies of 200 keV and 400 keV. For reference purposes, quartz silica was implanted also with the same dose of 200 keV Si⁺ ions. The implanted samples were annealed in nitrogen and forming gas at 900°C for 3 to 180 min. Both the SiO₂ and quartz samples exhibited luminescence at about 380 nm which was weak, but detectable, before annealing. During extended anneals in forming gas, the intensity increased by a factor of about 2 above that recorded after a nitrogen anneal but the peak position was unchanged. The intensity was greater in samples annealed in forming gas which is due to the additional hydrogen. It would seem that this blue luminescence originates from new luminescent centres in the matrix caused by the Si⁺ ion implantation.     

Ion beam synthesis and recrystallization of amorphous SiGe/SiC structures

Si_1−xGe_x amorphous layers implanted with different doses of carbon (between 5 × 10¹⁵ and 2 × 10¹⁷ cm⁻² and annealed at 700°C and 900°C have been analyzed by Raman and Infrared spectroscopies, electron microscopy and Auger electron spectroscopy. The obtained data show the synthesis of amorphous SiC by implanting at the highest doses. In these cases, recrystallization only occurs at the highest annealing temperature (900°C). The structure of the synthesized SiC strongly depends on the implantation dose, in addition to the anneal temperature. For the highest dose (2 × 10¹⁷ cm⁻²), crystalline β-SiC is formed. Finally, a strong migration of Ge towards the Si substrate is observed from the region where SiC precipitation occurs.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

Er site in Er + Au-implanted SiO2: Effect of annealing in reducing atmosphere

The Er site in Er + Au-implanted silica has been investigated by x-ray absorption spectroscopy, in particular after annealing in reducing atmosphere (H₂(4%):N₂(95%)) at temperature ranging from 100 to 800 °C. The EXAFS analysis shows that Er ions are surrounded by a first shell of O atoms, while the absence of signal from further coordination shell indicates a disordered site. The Er-O distance is lower than that of the Er₂O₃; it is suggested that the annealing in reducing atmosphere leads to a significant reduction of the first shell coordination number. Correspondingly, in the XANES region of the spectrum, it is observed a decrease in the white line intensity for annealing temperature higher than 400 °C; similar annealing treatments in inert atmosphere did not result in significant changes of the near-edge region of the X-ray absorption spectrum. These results enlighten that the annealing procedure, normally used to tailor the size distribution of the metal clusters present in the matrix and/or to modify the matrix structure, can also have an effect on the site of the Er ions, and possibly on the rare-earth optical properties.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.     

Ion beam induced nucleation in amorphous GaAs layers during MeV implantation

To investigate the nonlinear dose dependence of the thickness of the recrystallized layer during ion beam induced epitaxial recrystallization at amorphous/crystalline interfaces GaAs samples were irradiated with 1.0 MeV Ar⁺, 1.6 MeV Ar⁺ or 2.5 MeV Kr⁺ ions using a dose rate of 1.4 × 10¹² cm⁻² s⁻¹ at temperatures between 50°C and 180°C. It has been found that the thickness of the recrystallized layer reaches a maximum value at T_max = 90°C and 135°C for the Ar⁺ and Kr⁺ implantations, respectively. This means that the crystallization rate deviates from an Arrhenius dependence due to ion beam induced nucleation and growth within the remaining amorphous layer. The size of the crystallites depends on the implantation dose. This nucleation and growth of the crystallites disturbes and at least blocks the interface movement because the remaining surface layer becomes polycrystalline. Choosing temperatures sufficiently below T_max the thickness of the recrystallized layer increases linearly with the implantation dose indicating that the irradiation temperature is too low for ion induced nucleation.