Lattice Site Location Research Articles

Photoluminescence and lattice location of Eu and Pr implanted GaN samples

Rare earth (RE) ions implanted GaN films were studied by optical spectroscopy and RBS techniques. Sharp emission lines due to intra-4f n shell transitions can be observed even at room temperature for the Eu 3+ and Pr 3+. The photoluminescence spectra recorded by the above band gap excitation reveal dominant transitions due to the 5D 0→ 7F 1,2,3 lines at 6004, 6211 and 6632 Å for the Eu 3+ and 3P 0,1→ 3F 2,3 at 6450 and 6518 Å, respectively, for the Pr 3+. We report on the temperature dependence of the intra-ionic emissions as well as on the lattice site location of the RE detailed angular scans through the 〈0 0 0 1〉 and 〈1 0 1 ̄ 1〉 axial directions; which indicates that for Pr, complete substitutionality on the Ga sites was achieved while for Eu a Ga displaced site was found.

Fe ion implantation in GaN: Damage, annealing, and lattice site location

In this article we report the damage and annealing behavior as well as lattice site location of Fe atoms in GaN. The Fe ions were homogeneously implanted in GaN films with an energy of 150 keV at room temperature. A two-step annealing (650 °C 15 min and then 1000 °C 2 min) was performed to remove the implantation-induced damage and to drive the dopants into the lattice site. The structure of GaN films before and after the implantation as well as at each stage of the annealing was characterized by Rutherford backscattering/channeling combined with particle induced x-ray emission and high resolution x-ray diffraction. The Fe+ implanted GaN films exhibits an expanded lattice. After the two-step annealing, the lattice distortion does not fully recover. Angular scans along both [0001] and [101̄1] directions show that the Fe atoms occupy the lattice site of Ga atoms in the case of low dose implantation after annealing. However, for the high dose implanted GaN, about 75% of the implanted Fe atoms substitutes Ga atoms, showing that the damage is not annealed out and the dopants are not completely activated.

Coherent precipitate formation and diffusion effects in Ge-implanted TiO 2-single crystals

The group IVA element Ge was implanted into TiO 2-single crystals (rutile) with fluences of 5×10 15 and 3×10 15 ions/ cm 2 16° off and parallel to the 〈0 0 1〉 crystal axis, respectively. The production of damage and its recovery, ion distribution and lattice site location were studied as a function of the annealing temperature, using Rutherford backscattering and Channeling (RBS–C). Coherent precipitate formation was observed after implantation at room temperature (RT). During annealing the precipitates are stable until 800 K and decompose at 900 K. The Ge atoms then started to diffuse and moved to Ti-lattice sites.

Lattice site location of ultra-shallow implanted B in Si using ion beam analysis

ABSTRACTWe have investigated the lattice site location of B in Si using ion channeling in combination with nuclear reaction analysis (NRA). Silicon samples implanted with Boron at an energy of 10 keV and a dose of 5 × 1014 cm−2 (low dose samples) or 5 × 1015 cm−2 (high dose samples) were annealed at 1000 °C for 10 seconds (RTA) or at 800 °C for 10 minutes (FA). The activation efficiencies of these samples were estimated from the B atomic concentration and the hole concentration obtained by secondary ion mass spectrometry (SIMS) and spreading resistance profiling (SRP), respectively. We also studied the ion implantation damage of Si crystals using ion channeling combined with Rutherford backscattering spectrometry (RBS). We found that the activation efficiency is proportional to the substitutionality, meaning that substitutional B is fully activated without any carrier compensation. We also found that B atoms go to the substitutional sites and are activated up to the solubility limit in the high dose samples. However, the ion implantation damage of the crystalline Si in the high dose samples increases somewhat after annealing.

Lattice site determination of Co in low doped congruent LiNbO 3 single crystal using PIXE/channelling

The lattice site location of Co has been determined in congruent LiNbO 3 crystals doped with Co by particle-induced X-ray emission combined with channelling techniques. Co has been found to occupy exclusively the Li site.

Diffusion effects and conductivity of Ge- and Pb-implanted TiO 2 single crystals

Single crystals of TiO 2 (rutile) were implanted with Ge and Pb ions. The damage distribution, the ion distribution and the lattice site location of the ions were determined as a function of the annealing temperature, using Rutherford Backscattering and Channeling (RBS-C). The electrical conductivity was measured as a function of temperature in the region between 5 and 300 K. Upon annealing to 1100 K, a nearly complete recovery of the damage was observed. Ge and Pb atoms moved partly towards the surface, leading to precipitation at the surface in the case of high-dose Pb implants. This was confirmed by EDX-analysis. Ions residing in the implanted region moved from coherent precipitates sites to perfect substitutional Ti lattice sites. The resistivity at room temperature as well as the temperature dependence of the resistivity, strongly depended on the treatment of the sample. For samples implanted at 300 K, the resistivity was thermally activated over the whole temperature region, indicating carrier hopping between localised defect states. For samples implanted at high temperatures (900 or 1000 K) or annealed in vacuum, metallic behaviour was observed at low temperatures between approximately 30 and 100 K. Oxygen vacancies seem to be the main defect causing this behaviour.

Implantation of group IVA elements in TiO 2: lattice site location and diffusion

Ge-, Sn- and Pb-ions with oxidation state +4, similar to that of Ti, were implanted into rutile single crystals. Ion profiles and lattice site location were determined by Rutherford-backscattering and channeling. Previously it was noted that the widths of the angular dip curves for Ge and Sn were wider and deeper than those of Ti, indicating the formation of coherent precipitates, while Pb atoms were displaced along the c-axis. Annealing at high temperatures caused the impurities to move to regular substitutional Ti lattice sites. The influence of diffusion effects on the lattice site occupation and on the formation of precipitates has been studied here in more detail for aligned and randomly oriented implants. For aligned implants Sn-diffusion occurred between 850 and 950 K, about 200 K below the temperature region where diffusion is observed for random implants. The steep decrease of the Sn peak area without broadening indicates that the diffusion is governed by trap release. The local damage density near the impurity at the end of the ion track seems to play a big role: for aligned implants the diffusion occurs in the temperature region where the damage anneals, while for random implants ion diffusion is not connected to damage annealing and occurs due to a substitutional process via cation lattice sites.

Doping of GaN by ion implantation

ABSTRACTIn this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [1011] directions show that 50% of the Er ions implanted at 550 oC occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 μm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing.

Doping of GaN by ion implantation

AbstractIn this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [1011] directions show that 50% of the Er ions implanted at 550 °C occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 µm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing.

Annealing behavior and lattice site location of Hf implanted GaN

The defect recovery and lattice site location of Hf implanted into GaN single crystalline epilayers were studied combining RBS/channelling and hyperfine interactions measurements. The RBS/channelling measurements performed after implantation of 5×10 14 Hf + cm −2 at 100 keV show that nearly all the implanted ions were incorporated into substitutional sites of the GaN lattice. The damage produced by the implantation recovers almost completely after one hour annealing at 900°C and all the Hf ions then occupy substitutional sites. The hyperfine interaction measurements were performed with the radioactive 181Hf/ 181Ta probe, after implantation of 181Hf to a fluence of 5×10 12 Hf + cm −2 with 80 keV. These measurements show that the defect recovery occurs in the 600–800°C annealing temperature range.

Tin implanted in rutile single crystals: disorder, lattice location and the influence of the analyzing He-beam

Sn ions of 300 keV were implanted into 〈0 0 1〉 TiO 2 single crystals (rutile) at room-temperature. The lattice disorder and the lattice site location was determined by Rutherford Backscattering and Channeling (RBS-C). Successive increase of the implanted Sn dose from 5 × 10 14 Sn +/cm 2 to 3 × 10 15 Sn +/cm 2, followed by angular scan measurements at each dose leads to a strong increase of the dechanneling component over the whole depth range. This effect was studied in detail and could be attributed to the analyzing He-beam. Prolonged irradiation with the He-beam causes strain and blistering of the irradiated spot. Sn is fully substitutional on Ti lattice sites and its lattice site location is not affected by the analyzing beam. Annealing studies of a sample with a mixture of He-beam and implantation damage revealed a sharp recovery stage between 900 and 1000 K. Such a stage was not observed for samples partially damaged with a similar Sn dose. Partially damaged rutile reveals complete annealing by columnar regrowth in a temperature region between 800 and 1000 K.

Oxidation state and lattice site occupation of ions implanted into rutile

Ions with different oxidation states Ga(+3), Ge(+4) and As(±3,5) were implanted in TiO 2 and their lattice site location was determined using Rutherford Backscattering and Channeling (RBS-C). Angular scan measurements were performed through the 〈0 0 1〉 and 〈1 0 0〉 crystal directions. Ga was substitutional with a fraction of 1 for concentrations up to 0.2 at %. Arsenic was displaced perpendicular to both directions by an average displacement amplitude of about 0.007 nm, indicating an interaction with oxygen point defects. The Ge dip curve was slightly broader and deeper as that of Ti which may indicate that Ge formed coherent precipitations after implantation at 296 K. Ge became substitutional after annealing at 900 K.

Ion implantation in TiO 2: damage production and recovery, lattice site location and electrical conductivity

Ions with different oxidation states were implanted in TiO 2 (rutile). The lattice disorder as well as the lattice site location of the implanted ions were determined using Rutherford Backscattering and Channeling (RBS-C) spectrometry. The production of disorder as a function of dose and temperature, and its recovery was studied in detail. Important results are the observation of dynamic recovery at 293 K and above, and one isothermal at 77 K and two thermal recovery stages between 170 and 210 K and between 260 and 293 K. The recovery at 77 K is proportional to ln t, indicating that the activation energy increases with decreasing disorder density. The results concerning the lattice site location of 14 ion species reveal that 13 ions occupy Ti lattice site. With increasing netcharge, the maximum soluble concentration decreases by the formation of impurity–defect complexes probably enforced by charge compensation. Directed displacements from the substitutional lattice site provide some hints on the structures of these complexes. The electrical conductivity of the implanted samples increased by about 12 orders of magnitude irrespective of the oxidation state of the implanted species. From the temperature and dose dependence of the electrical conductivity as well as from its similar behaviour for noble-gas ions and other species it is concluded, that the carrier transport occurs by single energy states excitation at low doses and by variable range hopping between localized states at high doses.

RBS Lattice Site Location and Damage Recovery Studies in GaN

Erbium was implanted with 160 keV at doses between 5×1014 and 5×1015 at/cm2 into (0001) epitaxial GaN on sapphire and annealed at various temperatures between 600° and 1000° C. The RBS/Channeling technique was used to analyze the damage recovery during different annealing steps and to determine the lattice location of the implanted Er. For a sample implanted with 5×1014 and annealed for 30 min at 600° C a complete overlap of the Er and Ga angular scans across the <0001> axis was observed, indicating that 100% of Er occupies substitutional sites. Measurements along the <10 1> channel show that Er is located on Ga sites. The damage recovery was slightly better for the samples co-implanted with the same dose of Oxygen in an overlapping profile (E=25 keV). However, a complete recovery of the damage caused by the implantation was not achieved. Samples implanted with higher Er and O doses (5×1015 at/cm2 ) and at the same energies as above were annealed at 600° for 30 min and at 900°, 1000° C for 120 s using a proximity cap. The higher dose caused an almost complete amorphisation of the surface layer. After annealing indications of epitaxial regrowth were observed, however, the substitutional fraction remains substantially lower and the damage recovery is less complete.

Ion beam and photoluminescence studies of Er and O implanted GaN

The annealing behavior and the lattice site location of Er and Er+O (only Er was localized) implanted into GaN single crystalline epilayers were studied with the RBS/channeling and photoluminescence techniques. After implantation the results show that for doses of 6 × 10 14 Er +/cm 2 the Er ions occupy substitutional sites in the GaN lattice as revealed by detailed angular scans along the 〈1 0 11〉 and 〈0 0 0 1〉 axes. An increase of the dose to 6 × 10 15 Er +/cm 2 produces a continuous amorphous layer. The damage produced by the implantation starts to anneal at 600°C. For the samples implanted with the lower dose the recovery is significant after 2 min annealing at 900°C. During the annealing process Er remains in substitutional sites. The presence of O seems to slightly increase the substitutional fraction after annealing at 900°C. Further the photoluminescence signal is more intense for the sample co-doped with oxygen.

Analysis of compressive fracture in rock using statistical techniques: Part II. Effect of microscale heterogeneity on macroscopic deformation

Abstract We have performed a parameter-sensitivity analysis to evaluate the relative importance of different types of grain-scale heterogeneity on fracture processes and compressive strength in simulated compression tests of brittle, heterogeneous materials such as rock. This was done using a non-linear, rule-based model described in a companion paper. Results presented here indicate that heterogeneity in local stress field (due to grain shape and loading) has a first-order effect on macroscopic properties and is much more important than heterogeneity in site strength or location. In particular, increasing local stress heterogeneity lowers the mean ultimate strength following an inverse power law. Increasing heterogeneity in the lattice-site locations (i.e. irregular lattice spacing) decreases crack localization and decreases normalized crack strain energy. This result is consistent with the postulate that systems with increasing disorder require more energy to break. Heterogeneity in site-strength distribution had a relatively minor effect on macroscopic behavior. Peak strength is dependent on the mean site strength, not on the width of the site-strength distributions. This study also revealed that percolation thresholds are much lower than those predicted from stochastic fracture models. Consequently, statistical models for rock fracture must consider alternative percolation algorithms such as directed-bond percolation, because the standard percolation models may not be appropriate for analyzing systems where crack interaction dominates behavior at a low fraction of sites broken.

Lattice site location studies of ion implanted Li8 in GaN

The lattice sites of ion implanted Li atoms in GaN were studied as a function of implantation temperature between room temperature and 770 K. We measured the channeling and blocking patterns of α-particles emitted in the radioactive decay of implanted Li8 ions to determine the Li lattice sites. Below 700 K Li atoms occupy mainly interstitial sites in the center of the c-axis hexagons at positions c/4 and 3c/4, where c is the lattice constant in c-axis direction. Around 700 K Li starts to diffuse and presumably interacts with vacancies created in the implantation process. This leads to the formation of substitutional Li above 700 K. An activation energy of about 1.7 eV for interstitial Li diffusion was determined.

Ion implantation in TiO 2: Effect of the charge state on the lattice site occupation

Ions with different charge states Hg(+2), Sn(+4), Sb(+5), and Pb(+4) were implanted in TiO 2 (rutile) and their lattice site location was determined using Rutherford Backscattering and Channeling (RBS-C). The maximum solubility concentrations on Ti sites (+4) were about 5 × 10 13cm −2 for Hg, 1 × 10 15cm −2 for Sb and greater than 3 × 10 15cm −2 for Sn. For Pb a value of 5 × 10 13cm −2 was observed, although Pb should be insoluble as estimated from the solid solubility rules. The results cannot be explained by the size mismatch energy, as in the case of metals but seem to depend on the charge state. The non-substitutional fractions were larger for Hg and Sb, having a net charge on Ti sites. This behavior is attributed to the formation of impurity-defect complexes, where it is electrostatically favorable for different defects to associate for charge compensation. Supersaturation is reached for Pb, where charge compensation is not required.

Lattice site location and annealing behavior of W implanted TiO 2

The implantation damage and lattice site location of W in TiO 2 (rutile) was studied using the Rutherford backscattering technique in the channeling mode (RBS-C). The W ions were implanted at room temperature with fluences in the range of 10 15 to 10 17/cm 2 into both 〈1 0 0〉 and 〈0 0 1〉 oriented single crystals. The implanted region becomes completely disordered for W doses higher than 10 16/cm 2. After annealing experiments at temperatures up to 1100 K the results suggest that lattice recovery depends on the type of TiO 2 single crystal used. While in 〈0 0 1〉 oriented single crystals partial epitaxial solid phase regrowth of the damaged region is seen, for 〈1 0 0〉 oriented single crystals a recrystallization process occurs almost to completion. During damage recovery the high dose samples lose more than 80% of the W from the implanted region. Detailed angular scans for the main axial directions show that in the case of full recovery about 82% of the W remaining in the implanted region are incorporated on Ti lattice sites. These observations suggest that there is a strong anisotropy of the lattice recovery and diffusion properties in W implanted TiO 2.

Lattice site location of Hf in LiNbO 3: Influence of dopant concentration and crystal stoichiometry

The influence of dopant concentration and crystal stoichiometry on the lattice site of Hf in LiNbO 3 single crystals was studied combining ion beam and hyperfine interaction methods. Two cases were studied: A near-stoichiometric LiNbO 3 crystal doped with 1 mol% HfO 2 and a congruent crystal doped with 6 mol% HfO 2. In both cases it is shown that Hf occupies both Li and Nb sites, in contrast with the case of congruent crystals doped with a small amount of Hf, where it occupies only Li sites. The roles played by the stoichiometry of the crystal and the dopant concentration are discussed in the framework of the current models for the defect structure of lithium niobate.