Interstitial-type Defects Research Articles

Extended Defects Formation in Si Crystals by Clustering of Intrinsic Point Defects Studied by in-situ Electron Irradiation in an HREM

In situ irradiation experiments in a high resolution electron microscope JEOL-4000EX at room temperature resulted in discovery of the isolated and combined clustering of vacancies and self-interstitial atoms on {111}- and {113}-habit planes bothleading to an extended defect formation in Si crystals. The type of the defect is strongly affected by the type of supersaturation of point defects depending on the crystal thickness during electron irradiation. Because of the existence of energy barriers against recombination of interstitials with the extended aggregates of vacancies, a large family of intermediate defect configurations (IDCs) is formed on {113}- and {111}-habit planes at a low temperature under interstitial supersaturation in addition to the well-known {113}-defects of interstitial type. The formation of metastable IDCs inside vacancy aggregates prevents a way of recombination of defects in extended shape.

Defects and Their Annealing Properties in B+-Implanted Hg0.78Cd0.22Te Studied by Positron Annihilation

Defects and their annealing properties in B+-implanted Hg0.78Cd0.22Te (p-type) were studied using the positron annihilation technique. For an unimplanted specimen, the major species of defects was identified to be Hg vacancies, VHg, and its concentration was estimated to be 6×1015 cm-3. For the B+-implanted specimen, a damaged region showed n-type character, and vacancy-type defects were found to be present in this region. Below the n-type layer, about 50% VHg was annihilated by the recombination between VHg and Hg atoms. After 150° C annealing, the mean size of open space of the vacancy-type defects in the n-type layer was found to decrease. At the same annealing temperature, the electron concentration in the n-type layer also decreased. Thus, an origin of donors was attributed to interstitial-type defects. The annealing temperature of the vacancy-type defects introduced by ion implantation was determined to be 300° C

Harnessing reverse annealing phenomenon for shallow p-n junction formation

Monocrystalline silicon was implanted with 60 keV fluorine and 20 keV boron ions and annealed. Carrier profile, fluorine and boron redistribution, and the parameters of p+-n junctions were investigated. In ion implanted Si two specific regions were observed in which peculiarities in carrier concentration, resistivity, and F atoms redistribution occurred. It was reasoned that the “under-surface” specific region is enriched with vacancy-type defects while the “amorphous/crystalline (a/c) interface” region is enriched with interstitial type defects. After annealing at a temperature corresponding to the reverse annealing phenomenon, boron atoms were activated in the “under-surface” and deactivated in the “a/c interface” region. The possibility of PMOS transistor fabrication with ultrashallow p+-n junction (60 nm) and low leakage current by F++B+ implantation and low temperature (600–700) °C annealing by using this phenomenon was demonstrated.

Evolution of interstitial- and vacancy-type defects upon thermal annealing in ion-implanted Si

We have quantitatively analyzed the structure and the annealing behavior of the point defects introduced by ion implantation in Si. We used deep-level transient spectroscopy to monitor and count interstitial-type (e.g., carbon–oxygen complexes) and vacancy-type (e.g., divacancies) defects introduced by MeV Si implants in crystalline Si and to monitor their annealing behavior for temperatures up to 400 °C. A small fraction (∼4%) of the initial interstitial–vacancy pairs generated by the ions escapes recombination and forms equal concentrations of interstitial- and vacancy-type room-temperature stable defect pairs. At T⩽300 °C, vacancy-type defects dissociate, releasing free vacancies, which recombine with interstitial-type defects, producing their dissolution. This defect annihilation occurs preferentially in the bulk. At temperatures above 300 °C, all vacancy-type defects are annealed and the residual damage contains only ∼3 interstitial-type defects per implanted ion. This imbalance between vacancies and interstitials is not observed in electron-irradiated samples, demonstrating that it is the direct consequence of the extra ion introduced by the implantation process.

Evolution from point to extended defects in ion implanted silicon

We present a quantitative study of the evolution of point defects into clusters and extended defects in ion-implanted Si. Deep level transient spectroscopy (DLTS) measurements are used to identify and count the electrically active defects in the damaged region produced by Si ion implantation at energies of 145 keV–2 MeV, and fluences from 1×108 to 5×1013 Si/cm2. Analyses of silicon annealed in the temperature range 100–680 °C allow us to monitor the transition from simple point defects to defect clusters and extended defects that occur upon increasing the ion fluence and the annealing temperature. At low doses, <1010 Si/cm2, only about 2% of the Frenkel pairs generated by the ion beam escape recombination and are stored into an equal number of interstitial and vacancy-type point defects. Thermal treatments produce a concomitant annealing of interstitial and vacancy-type defects until, at temperatures above 350 °C, only two to three interstitial-type defects per ion are left, and the DLTS spectra contain signatures of second-order point defects. Interstitial clusters at Ev+0.29 and Ev+0.48 eV are found to dominate the residual damage of silicon implanted at higher fluences, 1×1012–7×1013 Si/cm2, and at annealing temperatures, T⩾600 °C. These interstitial clusters have point defect capture kinetics and are not observable in transmission electron microscopy (TEM), suggesting that they are smaller than ≈50 Å. Finally, for silicon implanted at higher Si doses, ⩾5×1013 Si/cm2, thermal treatments at 680 °C result in a strong decrease in the concentration of the interstitial cluster signatures and in the introduction of a different DLTS signal, Ev+0.50 eV, which exhibits logarithmic rather than exponential carrier capture kinetics, a feature typical of an extended defect. Comparison of the formation and dissolution of this extended defect signature with TEM analyses indicates that this level is a signature of the rodlike {311} defects that are known to store the interstitials responsible for transient enhanced diffusion. These results suggest that the small interstitial clusters are either the precursors of the {311} defects or that they compete with {311} defects as sinks for self-interstitials.

The effect of impurity content on point defect evolution in ion implanted and electron irradiated Si

We compare the defect complexes generated in crystalline Si by electron irradiation and ion implantation, using irradiation fluences which deposit the same total energy in nuclear collisions. Deep level transient spectroscopy was used to monitor both vacancy-type (e.g., divacancies) and interstitial-type (e.g., carbon–oxygen complexes) defects produced on p-type Si samples. We show that identical defect structures and annealing behavior, T⩽300 °C, are produced by both Si implantation and electron irradiation. After annealing at higher temperatures, we observe a higher residual damage in ion implanted samples, which is a direct consequence of the extra incorporated ions. We demonstrate that the substrate impurity content rather than the ion cascade dominates defect formation and evolution. In high purity Si, B-related instead of C-related (e.g., the carbon–oxygen complex) defects preferentially store the interstitials which escape direct recombination with vacancies, and the thermal stability of the CiOi complexes is decreased in Si containing low concentration of impurities.

X-ray diffraction studies of silicon implanted with high-energy erbium ions

Silicon crystals after implantation of erbium ions with energies in the range 0.8–2.0 MeV and doses in the range 1×1012–1×1014 cm−2 have been studied by two-and three-crystal x-ray diffraction. Three types of two-crystal reflection curves are observed. They correspond to different structural states of the implanted layers. At moderate doses (1×1012–1×1013 cm−2) a positive strain is observed, due to the formation of secondary radiation defects of interstitial type. An increase of the implantation dose is accompanied by the formation of an amorphous layer separating the bulk layer and a thin monocrystalline surface layer. At an implantation dose of 1×1014 cm−2 the monocrystalline surface layer is completely amorphized. Parameters of the implantation layers are determined. A model of the transformation of structural damage is discussed.

Characteristic features of the accumulation of vacancy-and interstitial-type radiation defects in dislocation-free silicon with different oxygen contents

The formation of the main radiation defects in silicon (A and E centers, C i -C s and C i -O i complexes) in dislocation-free crystals and crystals with a low dislocation density (N D ≃1×104 cm−2) have been investigated as a function of the oxygen density N O. The characteristic features of the accumulation and annealing of radiation defects observed in dislocation-free silicon are interpreted taking into account the presence of interstitial inclusions in the volume of such crystals. It has been determined that the gettering properties of the inclusions depend in a complicated manner on the oxygen concentration and are most obvious when N O⋍3×1016 cm−3.

Depth profiles of vacancy- and interstitial-type defects in MeV implanted Si

We demonstrate that the depth distribution of defects in MeV implanted n-type and p-type crystalline Si is severely affected by the impurity content of the material. Silicon samples with different concentrations of dopants (P or B) and intrinsic contaminants (i.e., C and O) were implanted with 1 or 2 MeV He ions to fluences in the range 2.5×108–1×1013/cm2. Using deep-level transient spectroscopy and spreading resistance measurements, we have identified the defects and determined their concentration and depth distribution. It is found that less than 4% of the defects generated by the beam escape recombination and are stored in electrically active, room temperature stable defect clusters, such as divacancies and carbon–oxygen pairs. When the concentration of these defects is much smaller than the doping level, their profile mirrors the initial defect distribution, as calculated by transport of ions in matter (TRIM), a Monte Carlo code. In particular, the profile presents a maximum at the same depth predicted by TRIM and a width which is strongly dependent on the impurity content of the substrate. Indeed this width can be as large as 2 μm when implants are performed on a lightly doped, pure epitaxial substrate and returns to the value predicted by TRIM (∼0.5 μm) upon increasing the concentration of dopants and intrinsic contaminants which act as traps for the diffusing point defects. The broadening of the concentration profiles is however shown to be unavoidable at high implantation fluences when most of the traps are already full and unable to interrupt the free migration of newly generated defects. Finally, by comparing defect distributions in n-type and p-type samples we have detected the spatial separation between vacancy-type and interstitial-type defects, resulting from the ion momentum transfer. The observed phenomena are explained in terms of a trap limited diffusion of the defects generated by the beam. These effects are only observable for a light ion such as He since direct defect clustering within the diluted collision cascades is expected to be significantly inhibited.

Defect Evolution in Ion Implanted Si: from Point to Extended Defects

ABSTRACTSeveral recent experiments assessing the role of impurities (C, O), dopants (P, B) and clustering on defect evolution in ion implanted Si are reviewed. Deep level transient spectroscopy measurements were used to analyze the defect structure in a wide range of ion implantation fluences (1×108–5×1013 cm−2) and annealing temperatures (100–800 °C). By using substrates with a different impurity content and comparing ion implanted and electron irradiated Si samples, many interesting features of defect evolution in Si have been elucidated. It is found that only a small percentage, 4–16 % depending on ion mass, of the Frenkel pairs generated by the beam escape direct recombination and is stored into an equal number of room temperature stable vacancy- (V-) and interstitial-type (I-) defect complexes. Identical defect structures and annealing behavior have been measured in ion implanted (1.2 MeV Si, 1×108–1×1010/cm2) and electron irradiated (9.2 MeV to fluences between 1 and 3×1015/cm2) samples in spite of the fact that denser collision cascades are produced by the ions. The O and C content of the substrate plays a major role in determining the point defect migration, the room temperature stable defect structures and their annealing behavior. Annealing at temperatures up to 300 °C produces a concomitant reduction of the I- and V-type defect complexes concentration, demonstrating that defect annihilation occurs preferentially in the bulk. At temperatures above 300 °C, when all V-type complexes have been annealed out, ion implanted samples present a residual I-type damage, storing 2–3 I per implanted ion. This unbalance is not observed in electron irradiated samples and it is a direct consequence of the extra implanted ion. The simple point defect structures produced at low ion fluence (1×108–1×1011 /cm2) anneal at ∼ 550 °C. At higher fluences (∼ 1012–1013/cm2) and for annealing temperatures above 500 °C the deep level spectrum is dominated by two signatures at Ev+0.33 eV and at Ev+0.52 eV that we have associated to Si interstitial clusters. Impurities (C, O and B) play a role in determining nucleation kinetics of these defects, but they are not their main constituents. The dissolution temperature of these clusters indicates that they might store the interstitials that drive transient enhanced diffusion phenomena occurring in the absence of extended defects. Finally, at higher implantation fluence, a signature of extended defects is observed and associated to the presence of {311} defects detected by transmission electron microscopy analyses.

Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope

Vacancy accumulation in pure FZ-Si crystals covered with thin Si 3N 4 films leads upon irradiation with a high intensity electron beam in a high voltage electron microscope (HVEM) at temperatures between 20 and 250°C, to the suppression of the formation of 113 -defects of interstitial type in the central part of the irradiated area. Very small secondary clusters of interstitial type with a density of about 10 11 cm −2 appear, however, in this area after prolonged irradiation. Small stacking fault tetrahedra and Frank loops of vacancy type connected with interstitial clusters were analysed on an atomic scale in the thinnest part of the irradiated area in a 400 keV high resolution electron microscope. Vacancy clusters might act as nucleartion centres for the observed secondary defect formation.

Effects of Recoil-Implanted Oxygen on Depth Profiles of Defects and Annealing Processes in P+-Implanted Si Studied Using Monoenergetic Positron Beams

Effects of oxygen atoms recoiled from SiO2 films on depth profiles of defects and annealing processes in P+-implanted Si were studied using monoenergetic positron beams. For an epitaxial Si specimen, the depth profile of defects was found to be shifted toward the surface by recoil implantation of oxygen atoms. This was attributed to the formation of vacancy-oxygen complexes and a resultant decrease in the diffusion length of vacancy-type defects. The recoiled oxygen atoms stabilized amorphous regions introduced by P+-implantation, and the annealing of these regions was observed after rapid thermal annealing (RTA) at 700° C. For a Czochralski-grown Si specimen fabricated by through-oxide implantation, the recoiled oxygen atoms introduced interstitial-type defects upon RTA below the SiO2/Si interface, and such defects were dissociated by annealing at 1000° C.

Influence of oxygen and nitrogen on point defect aggregation in silicon single crystals

Oxygen- and nitrogen-doped floating zone (FZ) crystals were investigated with regard to radial oxygen precipitation, D-defect formation and gate oxide integrity (GOI) behavior as a function of the nitrogen content. It is shown that the inactivation of the nitrogen-enhanced D-defect suppression and GOI improvement is not a result of the reduction of the N-pair concentration due to nitrogen-oxygen interaction. Instead, it is suggested that the diffusivity of vacancies, Si interstitials and nitrogen is reduced due to interaction with oxygen. This leads to a considerable decrease in the nitrogen- enhanced recombination between vacancies and interstitials, which was previously proposed to explain the simultaneous suppression of vacancy- and Si interstitial-type defects by nitrogen doping. The radial oxygen precipitation behavior is explained by applying the formula of Vanhellemont and Claeys (J. Appl. Phys., 62 (9) (1987) 3960; 71 (2) (1992) 1073) for the critical radius of oxygen precipitates which accounts for the influence of vacancies and self- interstitials. A qualitative picture of the radial variation of the vacancy and interstitial concentrations is derived assuming that D-defect formation occurs before the onset of oxygen precipitation.

Secondary defect formation in self-ion irradiated silicon

A detailed experimental study has been made of the evolution of extended secondary decondary defects which form during rapid thermal anneals of 0.5 MeV energy self-ion irradiated silicon. The implant fluence (2 × 10 15 ions/cm 2), flux and substrate temperature (91°C) were chosen so that primary damage levels were well below saturation. Cross-sectional transmission electron microscopy (X-TEM), Rutherford backscattering-channeling spectroscopy (RBS-C) and variable-energy positron annihilation techniques (VEP) have been used to allow partial discrimination between vacancy- and interstitial-type defects. The growth and development of the defect band and of specific types of extended defects within the band has been followed up to anneal temperatures of 1000°C, where the majority are shown to have dissipated. X-TEM has revealed the formation of a previously unreported tubular defect which is found in a narrow temperature range of 700–765°C. The occurrence of this defect correlates with the positron annihilation analysis which shows that a small concentration of defects with vacancy character is present after annealing in the same temperature range. In addition, positron annihilation analysis has allowed an assessment of the role played by defects lying in regions appearing defect-free by the other techniques (RBS-C and TEM). The implications of these findings to existing models involving secondary defect production are discussed.

A comparison of displacement cascades in copper and iron by molecular dynamics and its application to microstructural evolution

The use of molecular dynamics simulation and improved many-body potentials make it possible to compare displacement cascade evolution in different materials. However, the extreme variability between individual cascades requires multiple simulations at nominally identical conditions of temperature and energy in order to assure that the comparison is statistically valid. We describe such a comparison of copper and iron in this paper. Over 600 cascades have been investigated, with simulation energies in the range 60 eV to 10 keV and temperatures from 100 to 900 K. The evolution of the cascades is similar in both materials, with the development of a highly disordered core and the emission of focusons and replacement collision sequences during the collisional phase of the cascade. The majority of vacancy-type defects are found in the cascade core when in-cascade recombination is complete, while the interstitial-type defects tend to be distributed around the periphery of this region. The final defect structure has been characterized by the total surviving defect fraction, and the number and size of the point defect clusters produced. Since these parameters have significant implications for the nuclear industry in its assessment of radiation damage, we show how they depend on cascade energy and temperature. To illustrate their importance, we provide an example of how the molecular dynamics results can be used in a rate theory model of ferritic steel embrittlement.

Dependence of observed cascade defects on neutron spectrum and dose in Au and Ag irradiated with fission and fusion neutrons at low temperature

Cryotransfer transmission electron microscopy (TEM) experiments have been performed at about 110 K for Ag and Au irradiated with fission neutrons at 6 K and fusion neutrons at 20 K. More than 50% of the observed defect clusters were of interstitial type. The average size of defect clusters increased as neutron dose became higher. This result was explained by the effect of freely migrating interstitial atoms produced by newly developed displacement cascades. The effect of the difference in neutron spectrum was manifested in the number of defect clusters in a group of closely existing clusters (‘subcascade’ structure). There was a possibility that interstitial-type defect clusters were nucleated in the close vicinity of subcascades.

Radiation effects in FCC metals and intermetallic compounds: A molecular dynamics computer simulation study

Abstract We review recent results on the mechanisms of atomic mixing, radiation-induced disordering, and defect production and clustering induced by displacement cascades in Cu, Ag, Cu3Au and Ni3Al. We employ molecular dynamics computer simulation (MD) methods with isotropic many body potentials and recoil energies near the subcascade formation regime (up to 30 keV). Atomic mixing is shown to be dominated by diffusion in the locally molten cascade core in both the pure metals and the intermetallic compounds. Disordering of the intermetallics was found to take place in the core of the cascades. However, because of the short lifetime of the displacement cascade, chemical short range order is preserved in the molten zone. Our results reveal the appearance of very large vacancy and interstitial type defect clusters at high recoil energy and cascade energy density. Vacancies agglomerate and collapse into Frank dislocation loops as the result of the quenching of the cascade molten core. Large interstitial clusters are directly produced in cascades and form prismatic dislocation loops. The fraction of defects in clusters for low temperature cascades increases with recoil energy and approaches ∼70% and 60% for interstitials and vacancies, respectively, at recoil energies near the threshold for subcascade formation. In the case of intermetallic (A3B) compounds, the large energy required to produce and transport a superdislocation appears to inhibit the interstitial prismatic loop punching process and interstitials appear as isolated (100) dumbbells formed by pairs of majority-type atoms.

Impact of in situ photoexcitation on the defectivity of silicon layer implanted with different dose rates of nitrogen ions

It was shown using x-ray diffractometry method that dose rate does not influence the defectivity degree of N+-implanted Si if the implantation is carried out with in situ photoexcitation. It was also shown that photoexcitation does not appreciably reduce the concentration of interstitial-type defects.

Exposure time and recoil energy dependence of defect accumulation

Experimental information frorn irradiations with a wide variety of energetic particles, electrons, ions and fission and fusion neutrons, is combined and analyzed to elucidate the mechanism of microstructure evolution during irradiation, with particular interest placed on the case of cascade damage. A brief survey of the case of homogeneous introduction of point defects, electron irradiation with high voltage electron microscopes, is made. Stochastic fluctuation of point defect reaction and positional instability of shall point defect clusters are among new subjects. A kinetics analysis is made for each of the categorized cases of time evolution of defect structures during irradiation with cascade damage. The cases analyzed include vacancy cluster accumulation from cascades by the impact effect from cascades (proportional to the square of irradiation dose), and the extent of impact influence from cascade collisions is estimated. Vacancy cluster accumulation from cascades with efficient permanent sinks (proportional to irradiation dose) and with poor permanent sinks (proportional to the square-root of irradiation dose). The case with interstitial cluster formation (proportional but with smaller accumulation rate) is treated separately from the case without. For the nucleation of interstitial clusters from cascades, the important role of the fluctuation of point defect reactions to suppress the nucleation is emphasized. The influence of the positional instability of small interstitial clusters plays a role in interstitial type defect structures. Finally, a method is proposed to improve the currently poor situation of fission-reactor neutron irradiation into a systematic and reliable experiment.

Aggregations of silicon interstitials and interstitial defect species in Czochralski silicon degenerately doped with boron during low temperature processing

The generation mechanism of crystallographic defects in Czochralski silicon degenerately doped with boron during low temperature processing was studied. After an extended annealing at 900 °C, the primary defects were identified as the a/2〈110〉-type elongated hexagonal- and a/6〈211〉-type rhombus-shaped dislocation loops using the transmission electron microscope. A low density of bulk stacking faults of the type a/3〈111〉 was also observed. No oxide precipitates were detected. After subsequent high temperature annealing, no significant change in the morphology or density of the interstitial-type defects was observed. Based upon the present results, it is hypothesized that aggregations of silicon interstitials and interstitial defect species during low temperature annealing could be one of the generation mechanisms of crystallographic defects in degenerately doped silicon.