Lattice Site Location Research Articles

Tracing of low-energy protons implanted in different Si crystal orientations by keV recoil detection in transmission geometry

The potential of recoil detection in transmission geometry in examining a possible preferential lattice site location of H atoms implanted into thin single crystalline silicon membranes is explored. Low-energy protons were directed onto the membranes along the 〈001〉 Si crystal axis, as well as in random orientation. Position-sensitive and time-resolved detection of recoiling hydrogen species from a pulsed beam of 280 keV 22Ne+ primary ions was performed in a time-of-flight medium energy ion scattering system. From the primary beam incidence along different crystal axes, a preferential detection of recoiled hydrogen along the 〈011〉 axis can be revealed, as compared to the 〈001〉 axis. The present approach and possible future developments potentially enabling real-space location of interstitial hydrogen are discussed.

Correlations between the structural transformations and concentration quenching effect for RE-implanted ZnO systems

In this paper, we present optical, structural and electrical studies of the phenomenon called concentration quenching effect occurring in ZnO doped with Rare Earth (RE) ions. For this purpose, the epitaxial ZnO layers grown by the Atomic Layer Deposition (ALD) are doped by ion implantation with Yb and Er elements with fluencies ranging from 5 × 1013 to 1 × 1016/cm2. In order to activate optically the implanted RE and to remove defects, the post-implantation thermal annealing was performed at 800 °C for 10 min in the O2atmosphere using aRapid Thermal Annealing (RTA) system. Two-step processed samples, before and after annealing, were evaluated by Rutherford Backscattering Spectrometry (RBS/c) to investigate the damage build-up process in the ZnO lattice after RE ion bombardment and the lattice site location of RE. The annealed samples were examined using the photoluminescence (PL) spectroscopy and Hall effect measurements. Our studies show that the luminescence quenching effect, as well as the electrical resistivity response to the increased RE concentration, are strongly connected with the threshold of the structural transformation due to defects accumulation. It suggests that during structural transformations the RE-ion centers are sufficiently close together to be able to interact and transfer the excitation energy between each other, increasing ipso facto the probability to lose the excitation energy by non-radiative processes. Moreover, in contrast to the popular belief, that the concentration quenching effect in RE-doped ZnO depends strongly on the kind of RE-doped ion, the presented results do not provide any evidence to support such an assumption.

XANES/EXAFS study of the Lu and Y incorporation in the single crystal Tb3Sc2Al3O12 Faraday rotator

Tb3Sc2Al3O12 (TSAG) is a paramagnetic insulator which, in spite of its congruent nature and excellent magneto-optical properties as Faraday rotators for optical isolators (OIs), cannot be industrially produced. The reason is that its lattice structure is so stressed that single crystals easily crack upon cooling after growth or during the cutting process. By the incorporation of a small amount of Lu, it has been demonstrated that the stress in the structure can be effectively released, and, therefore, the crystal cracking can be avoided. This has enabled the substitution of standard Tb3Ga5O12 (TGG), achieving better high-power performance and compacter OIs. In this study, the charge state and the lattice site location of Lu as well as Y dopants in TSAG are investigated by X-ray Absorption Near Edge Structure and Extended X-ray Absorption Fine Structure Analysis. Measurements on single crystal and powdered samples indicate that both dopants are incorporated in the 3+ charge state and substitute predominantly in the dodecahedral Tb-site (>87.5%), while a minor fraction is found in the octahedral Sc-site.

Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density Nb3Sn Superconductor Wires

To meet critical current density, Jc, targets for the Future Circular Collider (FCC), the planned replacement for the Large Hadron Collider (LHC), the high field performance of Nb3Sn must be improved, but champion Jc values have remained static for the last 10 years. Making the A15 phase stoichiometric and enhancing the upper critical field Hc2 by Ti or Ta dopants are the standard strategies for enhancing high field performance but detailed recent studies show that even the best modern wires have broad composition ranges. To assess whether further improvement might be possible, we employed Extended X-ray Absorption Fine Structure (EXAFS) to determine the lattice site location of dopants in modern high-performance Nb3Sn strands with Jc values amongst the best so far achieved. Although Ti and Ta primarily occupy the Nb sites in the A15 structure, we also find significant Ta occupancy on the Sn site. These findings indicate that the best performing Ti-doped stand is strongly sub-stoichiometric in Sn and that antisite disorder likely explains its high average Hc2 behavior. These new results suggest an important role for dopant and antisite disorder in minimizing superconducting property distributions and maximizing high field Jc properties.

Defect formation and optical activation of Tb implanted AlxGa1−xN films using channeled implantation at different temperatures

AlxGa1−xN (x = 0.20, 0.50, 0.63) films grown on (0001) sapphire substrates by metal organic chemical vapor phase deposition were implanted with Tb ions at 150 keV and a fluence of 7 × 1014 Tb cm−2. The implantation was carried out with the beam aligned with the c-axis and the implantation temperature was varied from room temperature (RT) to 550 °C. Additionally, post-implantation rapid thermal annealing was performed at 1200 °C during 120 s. The impact of implantation temperature on the damage accumulation is investigated. Rutherford backscattering spectrometry/channeling (RBS/C) measurements show that two damage regions are forming and the damage is not decreasing monotonically with the increase of implantation temperature. Bulk damage follows the same trend for all AlxGa1−xN compositions, increasing when the temperature is raised up to ~100 °C, and then decreasing for higher temperatures. A good agreement is observed between the damage accumulation obtained from RBS/C and the strain profile derived from symmetric (0002) 2θ-ω X-ray diffraction scans. Transmission electron microscopy results show the suppression of basal stacking faults after implantation at 550 °C. Tb lattice site location studies revealed two preferential sites: the substitutional cation site and a site displaced from that by 0.2 Å along the c-axis. A higher substitutional fraction of Tb ions is obtained for implantation temperature of 550 °C. The optical activity investigated by photoluminescence showed that after thermal annealing, the 5D4 → 7FJ intra-4f8 transition is detected in all the samples.

Studies of O18 impurity trapping at interstitial dislocation loops in ion implanted Fe (1 1 0) by ion channeling and ab initio calculations

Abstract The O18 and self ions are implanted at same depth in Fe (1 1 0) crystal and annealed to study the oxygen trapping under excess self interstitial defects. Rutherford backscattering spectrometry, nuclear reaction analysis and channeling measurements have been performed to determine the lattice site position of O18. The presence of dislocation loops is confirmed by energy-dependent dechanneling parameter measurements. From the tilt angular scans of Fe and O18 signals along 〈1 0 0〉, 〈1 1 0〉 axes, O18 is found to be displaced 0.2 A from tetrahedral towards octahedral interstitial site in O18. Similar lattice site location of oxygen with the displacement of 0.37 A is predicted by density functional theory calculations for the interaction of oxygen with 〈1 0 0〉 interstitial dislocation loop structure. Our results provide strong evidence on oxygen trapping at interstitial dislocation loops in the presence of excess interstitial defects in iron.

The photoluminescence response to structural changes of Yb implanted ZnO crystals subjected to non-equilibrium processing

In this paper, we present the detailed study of optical and structural properties of Yb implanted single ZnO crystals. Hydrothermally grown wurtzite (0001) ZnO crystals were implanted with 150 keV Yb ions to fluencies of 5 × 1014 and 1 × 1015 at/cm2. After ion implantation, two different types of annealing were performed: rapid thermal annealing (RTA) and millisecond range flash lamp annealing (FLA). Crystalline quality, damage recovery, and Yb lattice site location were evaluated by the Channeling Rutherford Backscattering Spectrometry (RBS/c). It is shown that independent of the used annealing technique, defects formed in ZnO during ion implantation can be removed. Upon RTA performed at the temperature higher than 800 °C, strong out-diffusion of implanted Yb atoms and precipitation on the surface takes place. Consequently, the degradation of the photoluminescence (PL) efficiency is observed. The diffusion of implanted Yb during millisecond range FLA does not occur for such experimental conditions. Moreover, FLA treatment for 20 ms leads to the formation of single crystalline ZnO layer with Yb incorporated in the substitutional lattice sites. According to RBS/c and PL data, Yb atoms substituted in the Zn sublattice are predominantly in the 2+ oxidation state. The most intensive PL has been observed after annealing at 800 °C for 20 min which is accompanied with the reduction of Yb substitutional fraction and formation of octahedron Yb-oxygen clusters within ZnO.

Effect of AlN content on the lattice site location of terbium ions in AlxGa1−xN compounds

Terbium lattice site location and optical emission in Tb implanted AlxGa1−xN (0 ≤ x ≤ 1) samples grown by halide vapour phase epitaxy on (0001) sapphire substrates are investigated as a function of AlN content. The samples were implanted with a fluence of 5 × 1014 cm−2 of terbium ions and an energy of 150 keV. Lattice implantation damage is reduced using channelled ion implantation performed along the 〈0001〉 axis, normal to the sample surface. Afterwards, thermal annealing treatments at 1400 °C for GaN and 1200 °C for samples with x > 0 were performed to reduce the damage and to activate the optical emission of Tb3+ ions. The study of lattice site location is achieved measuring detailed angular ion channelling scans across the 〈0001〉, and axial directions. The precise location of the implanted Tb ions is obtained by combining the information of these angular scans with simulations using the Monte Carlo code FLUX. In addition to a Ga/Al substitutional fraction and a random fraction, a fraction of Tb ions occupying a site displaced by 0.2 Å along c-axis from the Ga/Al substitutional site was considered, giving a good agreement between the experimental results and the simulation. Photoluminescence studies proved the optical activation of Tb3+ after thermal annealing and the enhancement of the 5D4 to 7F6 transition intensity with increasing AlN content.

Precise lattice location of substitutional and interstitial Mg in AlN

The lattice site location of radioactive 27Mg implanted in AlN was determined by means of emission channeling. The majority of the 27Mg was found to substitute for Al, yet significant fractions (up to 33%) were also identified close to the octahedral interstitial site. The activation energy for interstitial Mg diffusion is estimated to be between 1.1 eV and 1.7 eV. Substitutional Mg is shown to occupy ideal Al sites within a 0.1 Å experimental uncertainty. We discuss the absence of significant displacements from ideal Al sites, in the context of the current debate, on Mg doped nitride semiconductors.

Lattice site location and luminescence studies of AlxGa1−xN alloys doped with thulium ions

Rare earth doped AlGaN is being considered for the development of new optoelectronic devices. This wide band gap compound provides lower thermal quenching of rare earth intra-4fn transitions making room temperature operation possible. In this work a detailed study of AlxGa1−xN (x=0.15, 0.77) samples grown by halide vapor phase epitaxy on (0001) sapphire implanted with Tm is presented. The samples were implanted at room temperature with 300keVTm ions to fluences of 1×1014cm−2 and 1×1015cm−2. The damage accumulation and the rare earth lattice site location are investigated by Rutherford Backscattering/Channeling Spectrometry. The structural measurements show a higher resistance of the lattice to irradiation damage with the increase of the AlN content. The analysis of angular scans along different axes using the Monte Carlo code FLUX allows the determination of the rare earth lattice sites. Results for Al0.15Ga0.85N show that Tm ions occupy two preferential sites: the high symmetry substitutional Ga/Al site and a site displaced by ∼0.3Å along the c-axis from this regular site.Rapid thermal annealing treatments at 1200°C under N2 ambient were performed to remove implantation damage and promote the optical activation of rare earth intra-4fn transitions. Photoluminescence measurements were performed to characterize the thulium ions spectroscopic transitions. The effects of thermal annealing and fluence on the fraction of rare earth optically active ions are exploited and discussed.

Cd ion implantation in AlN

AlN thin films were implanted with cadmium, to fluences of 1×1013 and 8×1014at/cm2. The implanted samples were annealed at 950°C under flowing nitrogen. Although implantation damage in AlN is known to be extremely stable the crystal could be fully recovered at low fluences. At high fluences the implantation damage was only partially removed. Implantation defects cause an expansion of the c-lattice parameter. For the high fluence sample the lattice site location of the ions was studied by Rutherford Backscattering/Channelling Spectrometry. Cd ions are found to be incorporated in substitutional Al sites in the crystal and no significant diffusion is seen upon thermal annealing. The observed high solubility limit and site stability are prerequisite for using Cd as p-type dopant in AlN.

Effects of concentration and thermal annealing on the optical activation of Er implanted into GaN layers

The wide band gap semiconductor, GaN, has emerged as an important host for rare earth-electroluminescence. The annealing behaviour and lattice site location of Er implanted into GaN were studied with the Rutherford Backscattering Spectrometry (RBS)/channelling and photoluminescence (PL) techniques. Also Er site dependence on the annealing temperature and implantation dose has been studied in detail. The optical properties of the Er-doped GaN system, evidencing their dependence on the parameters adopted during the synthesis procedure (Er implantation dose, annealing temperature) have been discussed. RBS/channelling measurements suggested that mostly Er occupy substitutional site and depends on the Er concentration. The main result is the activation of a typical Er giving rise to PL emission in the 1450–1650 nm range, related to radiative 4 I 13/2→4 I 15/2 transitions. Depending on the Er dose, we observe a specific behaviour linked to variation of the annealing temperature that strongly determines PL emission band. We observed a PL spectral shape with the main peak located at 1542 nm and shoulder peak at 1558 nm (and full width at half maximum (FWHM) of 33 nm) with a series of weaker PL structures at 1519, 1572 and 1591 nm, due to the Stark sub-level splitting.

ChemInform Abstract: Electronic Structure and Lattice Site Location of Mn in III‐Mn‐V Ferromagnetic Semiconductors

AbstractReview: 73 refs.

Ion implantation of Cd and Ag into AlN and GaN

AbstractGaN and AlN thin films were implanted with cadmium (Cd) or silver (Ag), to fluences ranging from 1×1013 to 1.7 × 1015 at/cm2. The implanted samples were annealed at 950 °C under flowing nitrogen. While implantation damage could be fully removed for the lowest fluences, for higher fluences the crystal quality was only partially recovered. For the high fluence samples the lattice site location of the ions was studied by Rutherford Backscattering/channelling (RBS/C). Cd ions are found to be incorporated in substitutional cation sites (Al or Ga) while Ag is slightly displaced from this position. To further investigate the incorporation sites, Perturbed Angular Correlation (PAC) measurements were performed and the electric field gradients at the site of the probe nuclei were determined (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single phase a-plane MgZnO epilayers for UV optoelectronics: substitutional behaviour of Mg at large contents

High quality 1 μm thick a-plane MgxZn1−xO layers were produced by molecular beam epitaxy with Mg contents higher than 50%. Resonant Rutherford backscattering spectrometry combined with ion channeling revealed a uniform growth in both composition and atomic order. The lattice-site location of Mg, Zn and O elements was determined independently, proving the substitutional behaviour of Mg in Zn-sites of the wurtzite lattice. X-Ray diffraction pole figure analysis also confirms the absence of phase separation. Optical properties at such high Mg contents were studied in Schottky photodiodes.

Damage formation and recovery in Fe implanted 6H–SiC

Silicon carbide doped with magnetic ions such as Fe, Mn, Ni or Co could make this wide band gap semiconductor part of the diluted magnetic semiconductor family. In this study, we report the implantation of 6H–SiC single crystals with magnetic 56Fe+ ions with an energy of 150keV. The samples were implanted with 5×1014Fe+/cm2 and 1×1016Fe+/cm2 at different temperatures to study the damage formation and lattice site location. The samples were subsequently annealed up to 1500°C in vacuum in order to remove the implantation damage. The effect of the annealing was followed by Rutherford Backscattering/Channeling (RBS/C) measurements. The results show that samples implanted above the critical amorphization temperature reveal a high fraction of Fe incorporated into regular sites along the 〈0001〉 axis. After the annealing at 1000°C, a maximum fraction of 75%, corresponding to a total of 3.8×1014Fe+/cm2, was measured in regular sites along the 〈0001〉 axis.A comparison is made between the observed annealing behavior and the one measured in low fluence (2×1013cm−2) β emission channeling experiments using the radioactive isotope 59Fe (t1/2=45d), where the sample was implanted at room temperature and the β− emission channeling yield was measured by means of a position-sensitive detector.

AIN content influence on the properties of Al xGa 1−xN doped with Pr ions

The purpose of this work is the study of the structural and optical properties of Al x Ga 1− x N films grown on (0 0 0 1) sapphire substrates with different AlN molar fraction implanted with 150 keV of praseodymium ions with a fluence of 2.5 × 10 14 cm −2. The main goal is to achieve the optical doping of Al x Ga 1− x N with the rare earth Pr. Structural properties, damage accumulation and Pr lattice site location were studied combining Rutherford backscattering/channeling spectrometry and high resolution X-ray diffraction. The channeling data clearly indicate a higher resistance of the lattice to irradiation damage with the increase of the AlN content. Detailed angular scans reveal a fraction above 90% of Pr incorporated in near substitutional sites. A pronounced narrowing at the bottom of the Pr angular curve along the c-axis suggests the presence of a minor fraction with a higher displacement towards the center of the channel. The displacement is less pronounced with the increase of Ga content in the samples. Ionoluminescence with a 1H + microbeam reveal an enhancement of the optical activity when the AlN content is in the intermediate range of concentrations.

Defect studies and optical activation of Yb doped GaN

Wide band-gap semiconductors, particularly III-nitrides, became one of the most studied materials during the last decades. These compounds are the base of a new generation of optoelectronic devices operating in the UV-Blue region of the electromagnetic spectrum. Incorporation of rare-earth (RE) ions into nitrides creates new routes to build all-nitride electroluminescent devices, using the sharp intra-4fn transitions of these elements. The introduction of the RE ions in the nitride lattice during the growth or by ion implantation creates defects which influence the optical behaviour of the doped region. In this work we report the results on Yb implanted GaN. A combination of techniques (Rutherford backscattering/Channeling and Photoluminescence) was used to assess the mechanisms responsible for the optical and structural behaviour of the doped materials. Lattice site location experiments showed that Yb is incorporated into positions slightly displaced from the Ga-site. Clearly the optical activity of the RE could be enhanced by orders of magnitude reducing the number of non-radiative recombination paths related with defects.

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 °C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 °C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (∼0.2 Å) in the sample grown at 900 °C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu.

Boron lattice site location in (BGa)As and (BGa)P thin films studied using RBS and NRA with a channeled He+ ion beam

Abstract In this study the boron lattice site location in ternary B x Ga 1− x As and B x Ga 1− x P thin films grown on (0 0 1) GaAs and (0 0 1) GaP, respectively, using low pressure metal-organic vapour-phase epitaxy (MOVPE) with boron concentrations between x = 0.8% and x = 3.2% was investigated with RBS and the 10 B(α,p) 13 C nuclear reaction using a 2.3 MeV He + ion beam. For this purpose, the ion beam was aligned with the [0 0 1], [0 1 1] and [1 1 1] axis and the RBS and proton yield from the nuclear reaction compared with random ion incidence. For comparison, theoretical proton yields which assume boron to be located on substitutional lattice sites only were calculated for each sample/axis combination and compared with the experimental yields. The RBS/channeling measurements show a very good crystal quality of the films with χ min being in the range of 3–5% for the [0 1 1] axis. The best crystal qualities, i.e. the lowest χ min values and dechanneling rates, are achieved for low boron concentrations. From NRA/channeling it can be deduced that in the B x Ga 1− x As films the fraction of interstitial boron is approximately 5% for low boron concentrations of x = 1% and 6–10% for concentrations up to x = 3.2%, whereas the fraction of interstitial boron is less than 3% in the B x Ga 1− x P film studied despite a concentration of x = 2.0%. This indicates that antisite effects of the boron incorporation are more likely in GaAs compared to GaP.