Series Of Isochronal Anneals Research Articles

Nano-analysis of Ta/FeCoB/MgO tunnel magneto resistance structures

The partitioning and segregation of B during the crystallization of amorphous FeCoB in Ta/FeCoB/MgO layered structures is investigated by atom probe tomography to obtain a better understanding of the beneficial impact of Ta capping layers on FeCoB/MgO/FeCoB magnetic tunneling junctions. Boron, initially uniformly dissolved in the amorphous FeCoB layer, is rejected from FeCo grains on crystallization and first segregates at the interface to the Ta layer where it prevents nucleation of crystalline FeCo. Only later, it is fully absorbed by the Ta layer. In the studied thin film structures, B neither segregates at nor dissolves into the MgO barrier even after prolonged heat treatment. Important kinetic parameters are derived from detailed isochronal annealing series. They allow the quantitative modelling of the observed process. The combination of high affinity to B but low diffusivity makes Ta the unique capping material to achieve an optimum performance.

Capacitance spectroscopy study of deep levels in Cl-implanted 4H-SiC

We report on a deep level transient spectroscopy study of Cl-implanted n- and p-type 4H-SiC epilayers. Samples were electrically characterized after each step of an isochronal annealing series, between room temperature and 1800 ∘C, and twelve deep traps were detected. Of these, seven traps were found in n-type material, ranging from 0.15 to 1.6 eV below the conduction band minimum (EC), and five were detected in p-type material, located between 0.5 and 1.7 eV above the valence band maximum (EV). Besides the presence of the well known Z1/2 and EH6/7 levels in n-type 4H-SiC and of the D-center in p-type samples, we found that Cl implantation gives rise to three new traps in n-type material at EC−0.37 eV, EC−1.06 eV, and EC−1.3 eV and one new level in p-type at EV+0.97 eV. These traps are persistent after annealing at 1800 ∘C, and no data were found in the previous experimental studies reported in the literature. The possible involvement of Cl in the microscopic structure of these defects is discussed based on a depth profiling analysis of their concentration.

The effects of displacement threshold irradiation energy on deep levels in p-type6H-SiC

We report on the electrical characterization, by means of deep level transient spectroscopy,of electron-irradiated Al-doped 6H-SiC epilayers. Samples were irradiated with either116 keV, in order to displace only carbon atoms, or 400 keV. Seven deep traps, in the0.1–1.6 eV range above the valence band, were found. The thermal stability of thedetected levels was analyzed by performing an isochronal annealing series in the 100–1800 °C temperature range and the atomic structure of most of the detected traps was found to berelated to C-displacement.

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M 2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M 2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron–positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M 2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.

Thermal stability of deep levels between room temperature and 1500 °C in as-grown 3C-SiC

We report on the thermal stability of deep levels detected in as-grown bulk 3C-SiC. The investigation was performed by Fourier-transform deep level transient spectroscopy and an isochronal annealing series was carried out in the 100–1500 °C temperature range. We found three traps located between 0.14–0.50 eV below the conduction band edge minimum (EC). The shallower trap anneals out at temperatures below 1200 °C while the others display a high thermal stability up to at least 1500 °C. The nature of the former trap is discussed in detail on the basis of its annealing behavior and previous theoretical data found in the literature.

Enhanced oxygen precipitation in neutron-irradiated nitrogen-doped Czochralski silicon crystal

Nitrogen and vacancies were intentionally introduced into Czochralski silicon crystal by doping and neutron irradiation, respectively, to study the effect on oxygen precipitation. A series of isochronal anneals in the temperature range of 700–1150 °C revealed that vacancies showed the prominent enhancement effect on oxygen precipitation at temperatures below 1000 °C, whereas nitrogen exhibited the evident enhancement in oxygen precipitation at temperatures above 900 °C. It is suggested that the vacancies induced by neutron irradiation form VnOm (n, m≥1) complexes acting as nuclei to facilitate oxygen precipitation in silicon. However, the VnOm complexes are not stable at higher temperatures; thus, the enhancement effect of vacancies on oxygen precipitation decreases evidently as the temperature increases above 1000 °C. In addition to the formation of new nuclei based on the VnOm complexes, the growth of grown-in oxygen precipitates is also facilitated by the incorporation of high concentration vacancies in the neutron-irradiated silicon. In nitrogen-doped silicon, the enhanced oxygen precipitation at high temperatures primarily depends on nitrogen concentration rather than vacancy concentration as a result of forming stable N2V2 complexes acting as precipitate nuclei.

High-temperature annealing behavior of deep levels in 1MeV electron irradiated p-type 6H-SiC

We report on the thermal stability of deep levels detected after 1MeV electron irradiated p-type 6H-SiC. The investigation was performed by deep level transient spectroscopy, and an isochronal annealing series was carried out in the 373–2073K temperature range. We found seven traps located between 0.23 and 1.3eV above the valence band edge (EV). Two traps anneal out at temperatures below 1273K, while the others display a high thermal stability up to 2073K. The nature of the detected traps is discussed on the basis of their annealing behavior and previous data found in the literature.

Isochronal Annealing Study of Deep Levels in Hydrogen Implanted p-Type 4H-SiC

Aluminum doped 4H-SiC samples have been implanted with 200 and 100 keV hydrogen at five different temperatures. An isochronal annealing series has been carried out up to 1000 °C and deep level transient spectroscopy (DLTS) was performed after every annealing step in the 150-700 K temperature range. After studying the role of the annealing treatments on the reactivation of aluminum acceptors, we discuss the results of the DLTS experiment. Eight deep levels were found in the 0.15 - 0.7 eV range above the valence band (E v ), and their annealing behavior, as a function of the temperature is presented. We found that deep levels implanted at temperatures below 300 °C are less thermally stable. Furthermore, we compare the experimental results with theoretical data present in the literature, and suggest a possible microscopic structure of the detected levels.

Thermal stability of defects in p-type as-grown 6H-SiC

An isochronal annealing series was performed in order to investigate the thermalbehaviour of as-grown defects in Al-doped 6H-SiC. Five hole traps have been detected,by means of deep level transient spectroscopy (DLTS), above the valence band(EV) in the 0.2–1.3 eV energy range. One trap, labelled MZ2, anneals out after a1800 °C heat treatment, while the others display higher thermal stability. The nature of thesecentres is discussed on the basis of the previous experimental and theoretical data found inthe literature. Particular emphasis has been given to a trap labelled MZ3, which weidentified as the D-centre, and to the effect that the electric field has on its emission timeconstant.

Deep level transient spectroscopy study of defects in hydrogen implanted p-type 4H-SiC

p -type 4H-SiC epitaxial layers grown by chemical vapor deposition have been implanted with 200 and 100keV protons at five different implantation temperatures. An isochronal annealing series was performed from 100to1800°C, and Al-doped epitaxial layers have been characterized by means of deep level transient spectroscopy (DLTS) after each annealing step. DLTS measurements were carried out in the 150–670K temperature range and revealed the presence of eight hole traps located in the 0.18–1.8eV range above the valence band (EV). Heat treatments for temperatures above 700°C showed the progressive reactivation of the Al doping in the implanted region, which is completed after a 1500°C annealing treatment. Two traps located at EV+0.44eV and EV+1.8eV are persistent even after annealing at 1800°C, while the other traps anneal out after heat treatments at ⩽1700°C. An activation energy for dissociation of 6.2eV is estimated for the hole trap at EV+0.79eV, and the nature of this defect is discussed on the basis of previous experimental results and theoretical calculations. Furthermore, the study of the annealing behavior as a function of the implantation temperature shows that the detected traps display an increase of concentration for increasing implantation temperatures.

Annealing behavior between room temperature and 2000 °C of deep level defects in electron-irradiated n-type 4H silicon carbide

The annealing behavior of irradiation-induced defects in 4H-SiC epitaxial layers grown by chemical-vapor deposition has been systematically studied by means of deep level transient spectroscopy (DLTS). The nitrogen-doped epitaxial layers have been irradiated with 15-MeV electrons at room temperature and an isochronal annealing series from 100 to 2000 °C has been performed. The DLTS measurements, which have been carried out in the temperature range from 120 to 630 K after each annealing step, revealed the presence of six electron traps located in the energy range of 0.45–1.6 eV below the conduction-band edge (Ec). The most prominent and stable ones occur at Ec−0.70eV (labeled Z1∕2) and Ec−1.60eV(EH6∕7). After exhibiting a multistage annealing process over a wide temperature range, presumably caused by reactions with migrating defects, a significant fraction of both Z1∕2 and EH6∕7 (25%) still persists at 2000 °C and activation energies for dissociation in excess of 8 and ∼7.5eV are estimated for Z1∕2 and EH6∕7, respectively. On the basis of these results, the identity of Z1∕2 and EH6∕7 is discussed and related to previous assignments in the literature.

Capacitance Spectroscopy Study of High Energy Electron Irradiated and Annealed 4H-SiC

Deep level transient spectroscopy (DLTS) was employed to investigate the annealing behaviour and thermal stability of radiation induced defects in nitrogen doped 4H-SiC epitaxial layers, grown by chemical vapor deposition (CVD). The epilayers have been irradiated with 15 MeV electrons and an isochronal annealing series has been carried out. The measurements have been performed after each annealing step and six electron traps located in the energy band gap range of 0.42-1.6 eV below the conduction band edge (Ec) have been detected.

New experiments on the relationship between light-induced defects and photoconductivity degradation

ABSTRACTWe report experimental results that help settle apparent inconsistencies in earlier work on photoconductivity and light-induced defects in hydrogenated amorphous silicon (a-Si:H) and point toward a new understanding of this subject. After observing that light-induced photoconductivity degradation anneals out at much lower T than the light-induced increase in deep defect density, Han and Fritzsche[1] suggested that two kinds of defects are created during illumination of a-Si:H. In this view, one kind of defect degrades the photoconductivity and the other increases defect sub-bandgap optical absorption. However, the light-induced degradation model of Stutzmann et al.[2] assumes that photoconductivity is inversely proportional to the dangling-bond defect density. We observe two kinds of defects that are distinguished by their annealing activation energies, but because their densities remain in strictlinear proportionduring their creation, the two kinds of defects cannot be completely independent.In our measurements of photoconductivity and defect absorption (constant photocurrent method) during 25°C light soaking and during a series of isochronal anneals between 25 < T < 190°C, we find that the absorption measured with E ≤1.1 eV, first increases during annealing, then exhibits the usual absorption decrease found for deeper defects. The maximum in this absorption at E ≤1.1eV occurs simultaneously with a transition from fast to slow recovery of photoconductivity. The absorption for E ≤1.1eV showstwodistinct annealing activation energies: the signal rises with about 0.87 eV and falls with about 1.15 eV. The 0.87 eV activation energy roughly equals the activation energy for the dominant, fast, recovery of photoconductivity. The 1.15 eV activation energy roughly equals the single activation energy for annealing of the light-induced dangling bond absorption.

Fast neutron radiation on single-crystal thorium dioxide. An optical absorption spectroscopy study of the Fo Colour centre

The effects of fast neutrons have been recorded for the first time on the optical absorption spectra of arc-fused single crystals of thorium dioxide in the region 40 000–5000 cm–1(5.0–0.6 eV). The crystals were subjected to anneals at 600 or 1000 °C in oxygen or at 1000 °C in hydrogen prior to irradiation with neutrons of mean energy 2 MeV. Doses ranged from (1 × 1016) to (5 × 1017) neutrons cm–2 and the crystals became blue owing to three main bands generated in the region 13 000–20 000 cm–1(1.61–2.48 eV) from the formation of colour centres. The spectra also exhibited a well pronounced shoulder, 26 000–38 000 cm–1(3.223–4.711 eV), containing several bands, on the fundamental absorption edge in the UV region. After the induced radioactivity had decayed, the dark decay, at room temperature, of the radiation-induced absorption was first studied. The crystals were then given a series of isochronal anneals in oxygen or hydrogen (to correspond with their pre-irradiation anneal atmosphere) and their spectra recorded before and after a saturating dose of 60Co γ radiation. The induced profiles were resolved into their component Gaussian bands. Our results on the colour-centre bands are compared with the similar spectra arising from proton and 16O ion bombardment studies by Childs et al., but our conclusions are different. The induced profiles arise from defect centres consequent upon displacement of oxygen ions via collision processes. Electron paramagnetic resonance (EPR) studies showed that the colour centres were not paramagnetic and therefore were most likely Fo centres, with two spin-paired electrons occupying the oxygen vacancy. This is the first time that Fo centres have been identified in crystals having the fluorite lattice. Other potential centres were examined but considered less likely. Post-neutron irradiation anneals in hydrogen indicated that, whether or not the defects involved trapped electrons, all were shallow traps since the profiles were optically bleached by light below 17 000 cm–1(2.13 eV) and also showed thermal decay at room temperature. The bands produced in the shoulder region were attributed to transitions involving impurity ions.

Defect production by X- and gamma-rays in gamma lithium aluminate

Using electron paramagnetic resonance measurements (EPR), we detected paramagnetic defects in γ-LiAlO 2 single crystals irradiated by X- or gamma-rays at room temperature. We studied (i) their production as a function of dose and (ii) their recovery during an isochronal annealing series from room temperature up to 300°C. At low doses (⩽10 6 Gy) an eleven-line signal is observed; in addition, two distinct six-line signals appear at higher doses (⩾ 10 7 Gy). The colour centres originating these EPR signals involve an electron located near two equivalent aluminium nuclei (eleven-line signal) or one aluminium nucleus (six-line signals). Furthermore, as observed by both EPR and optical absorption measurements, there is no trace, in the X-ray- and gamma-ray-irradiated samples, of the major defect that we detected previously in electron-irradiated samples [M.H. Auvray-Gely, A. Perez and A. Dunlop, to appear in Philos. Mag.]. This defect—that could be an F +-centre—is therefore not created through a radiolysis mechanism.

Light-induced defect creation in hydrogenated amorphous silicon: A detailed examination using junction-capacitance methods.

From junction-capacitance measurements we have determined the density, energy distribution, and electronic occupation of light-induced metastable defects near the midgap for a set of nearly intrinsic hydrogenated-amorphous-silicon samples with well-characterized impurity levels. In particular, we have inferred the changes in both neutral and negative dangling bonds for a series of isochronal anneals between the light-soaked state and full-dark-annealed state of each sample. Our results indicate that the dominant defect-creation mechanism is not Si--Si bond breaking, and also indicate the existence of at least two quasi-independent metastable defect-creation processes.

Structural relaxation of transition-metal-metalloid metallic glasses

The structural relaxation of ferromagnetic glasses containing about 20at.% metalloid has been studied by using differential scanning calorimetry to measure the Curie temperature (T c). The kinetics of the T c changes on annealing are shown to depend not only on the annealing temperature, but also on the direction of the change: the change in T c is slower in a structure with a lower fictive temperature. A series of isochronal anneals on FeB, FeNiB, FePB and FeNiPB glasses suggests that, whilst the addition of a second transition metal roughly doubles the rise of T c on annealing at a given T/T x, the metalloids broaden and shift to lower T/T x the temperature range over which relaxation occurs. Care has to be exercised in interpreting the T c changes, since they may be due, not only to structural relaxation, but also to primary crystallisation and phase separation: anomalous results are obtained from FeMoB and FePB glasses. In a FeB glass cold-rolling appears to have no effect on the T c changes induced by subsequent annealing. Interpretations of the T c changes are considered both in terms of specific mechanisms and in terms of glass relaxation in general.

Sheet resistance variation on color-banded silicon following high dose implantations at high dose rates

With some scanning systems color bands are produced on silicon wafers after high dose implantation. The color bands arise from the amorphous zones within the silicon and the thickness of these zones is determined by the temperature attained by the wafer during implantation. The sheet resistance of ’’color-banded’’ silicon wafers has been measured following a series of isochronal anneals and found to vary considerably across a wafer, in a manner correlated with the color bands. The sheet resistance variation persists through a series of isochronal anneals to 1100°C, although it is reduced in magnitude. The implications for high dose implantation of silicon wafers are briefly discussed.

Neutron-induced disorder in V 3Ga as revealed by isochronal annealing

Fast neutron irradiation has been found to have a deleterious effect on the superconducting properties of V 3Ga similar to that observed in other A15 materials. The recovery of the transition temperature, T 0, of a V 3Ga sample irradiated with fast neutrons (6×10 18neutrons/cm, E>1 MEV) was measured after each of a series of isochronal anneals. These results were compared with a similar annealing study of V 3Ga by other workers, where the disorder was that introduced by arc melting and quenching. We have found that T 0 of the irradiated sample recovered at annealing temperatures considerably lower than required for the unirradiated sample. We attribite this lower recovery temperature to the enhancement of diffusion provided by those defects produced during irradiation.