Showing papers in "Radiation Effects and Defects in Solids in 1971"

Damaged regions in neutron-irradiated and ion-bombarded Ge and Si

Abstract: The formation of amorphous or crystalline damaged regions by ion bombardment or neutron irradiation of Ge and Si is discussed. Several experiments have demonstrated that Ge and Si become amorphous when bombarded to high fluences with ions of 20–200 keV energy, but remain crystalline after low ion fluences (<1014/cm2) or after fission neutron irradiations up to very high fluences. The present results indicate that a fission neutron fluence of 5 × 1035 n/cm2 at ∼50 °C and at a flux of 6 × 1012 n/cm2/sec produces no amorphism in Ge or Si, but that microcrystals appear after annealing at 450 °C. It is concluded that the individual damaged regions produced by high-energy ions or by primary knock-on atoms from fission neutron collisions are crystalline. When these damaged regions are created rapidly enough so that overlap occurs before appreciable annealing of point defects takes place, the damaged regions become amorphous. The critical defect concentration required for spontaneous transformation to am...

On the reflection coefficient of keV heavy-ion beams from solid targets

Abstract: Measurements of the reflected fractions of sodium, potassium, and krypton ions impinging on polycrystalline silver and gold and amorphous germanium targets have been made at medium keV energies. The reflected fraction depends strongly on mass ratio and weakly on energy. Previous calculations of ion reflection (based on random slowing-down by elastic collisions in an infinite medium) have been extended. Corrections to the calculated values due to electronic stopping and surface effects have been estimated. Agreement between measurements and calculations is satisfactory.

The use of channeling-effect techniques to locate interstitial foreign atoms in silicon

Abstract: When the channeling-effect technique is used to determine the lattice location of an impurity which is not completely substitutional, quantitative interpretation of the results requires knowledge of the interaction yield between a channeled beam and an interstitial atom. We have investigated this problem for Yb implanted into silicon. Along the direction, a peak of almost a factor of two is observed in backscattering yield from the Yb atoms, using a 1-MeV He beam. The height and angular width of the peak is satisfactorily interpreted in terms of flux-peaking of the channeled beam in the central region of the channels. The existence of such a large flux-peaking effect seriously complicates quantitative determination of the location of non-substitutional impurities. However, it is still possible to establish rather accurately the lattice position of the impurity, provided the measured minimum yields and angular widths of the and dips are taken into account.

Nucleation of damage centres during ion implantation of silicon

Abstract: Nucleation of damage centres in single crystal silicon held at room temperature and bombarded in a “random” direction with light ions is not at all times heterogeneous, as has sometimes been assumed. Homogeneous nucleation of interstitial clusters occurs at low ion fluences and this is characterized, for an extremely short period, by a linear dependence of the state of disorder on ion fluence, followed by a longer period during which disorder a (fluence)1/2. During these periods of ‘nucleation’ and ‘primary growth’ small interstitial clusters behave as unsaturable traps. For higher fluences there is a smooth transfer to a form of damage increase which is not of displacement cascade origin. In this period of ‘secondary growth’ the state of disorder varies linearly with ion fluence, and interstitial clusters behave as nucleation traps. It is shown how marked effects on the state of disorder due to changes in the mass of the bombarding ion, in the flux of the impinging beam, or in the temperature an...

DISORDER PRODUCED IN SiC BY ION BOMBARDMENT.

Abstract: We have measured the amount of residual lattice disorder produced in α-Sic after implantation of 40-keV Sb+ and 30-keV N+. The implantations were performed at 23°C and, to avoid channeling effects, at a preselected random equivalent orientation of the target with respect to the ion beam. The disorder, defined as the number of lattice atoms displaced greater than ∼0.2A from lattice sites, was determined by backscattered energy analysis of 280-keV He++ and 140-keV H+ incident along the channel and in a random direction. The amount of disorder increased linearly with dose of the heavy ion (Sb+) to a saturation condition, corresponding to the formation of an amorphous layer at a dose of ∼1014 Sb+/cm2. The disorder produced by the light ion (N+) at first increased as the square root of N+ dose but at ∼1014 N+/cm2 increased approximately linearly with dose to a saturation value at ∼1015 N+/cm2. Similar behavior for these two ions has been observed in Si. The number of displaced atoms of Si and C...

On the spatial distribution of channelled ions

Abstract: The spatial distribution of channelled ions in the transverse plane is studied theoretically using models based on the continuum approximation and confirmed by computer simulation. The variation of the flux across the crystal channels is found to be quite pronounced. In particular the averaged flux at the centre of the channel can be as high as 4 or 5 times normal under experimentally attainable conditions or, due to the oscillatory nature of channelled trajectories, even greater at certain fixed depths. The dependence of the maximum flux on the multiple scattering and the channel geometry is examined in detail. A general expression for the flux at any point in the transverse plane is derived, and various simplifications are introduced which take advantage of the symmetry about either the atomic rows or the mid-channel axes. Both the ideal situation as well as perturbations due to misalignment and multiple scattering are treated. The results have a direct application to any channelling experiment...

Spatial distribution of ions incident on a solid target as a function of instantaneous energy.

Abstract: Integro-differential equations are derived which govern the spatial distribution of ions of energy E' moving in a solid target after the ions were incident on the target surface with energy E. Thes...

Energy deposition and penetration depth of heavy ions in the electronic stopping region

Abstract: Previous calculations of spatial moments over the distribution of ion ranges and bombardment damage have been extended into the range of ion energies where electronic stopping is important. Numerical solutions are given of well-known integral equations, under the assumption of Thomas-Fermi scattering and velocity-proportional electronic stopping, for equaI masses of ion and target and five values of the electronic stopping constant, over a range of four decades of ion energy. The results are compared with experimental damage distributions, with good success. Implications on sputtering are mentioned briefly.

Observations of a regular void array in high purity molybdenum and T.Z.M. irradiated at high temperatures with 2MeV nitrogen ions

Abstract: Electron microscope observations are presented to demonstrate the existence of a three-dimensional defect array in high purity molybdenum, and in T.Z.M., irradiated at high temperatures with 2 MeV nitrogen ions to doses of approximately 1018 ions/cm2. It is shown that the defect array has a body centred cubic structure with a ‘lattice parameter’ of 220 A and with crystallographic axes coincident with those of the molybdenum matrix. The identification of the defects as voids is discussed together with possible mechanisms for the array formation.

Structure of multiple-vacancy (oxygen) centers in irradiated silicon.

Abstract: Analysis of a new paramagnetic spin 1 spectrum, labeled Si-S1, indicates that it corresponds to the neutral charge state of the Si-B1 center in an excited spin-triplet state. An extension of this analysis to the Si-P1, -P2, -P3, -P4, and -P5 centers suggest that these centers comprise a family of defects which consist of a string of neighboring vacancies in a single (110) plane. In particular, it appears that the Si-P1 center is an odd ( 3)-vacancy defect; the Si-P2 center is a 2-vacancy, oxygen defect; the Si-P3 center is a 4-vacancy defect; the Si-P4 center is a 3-vacancy, oxygen defect of symmetry lower than C2v ; and the Si-P5 center is a 3-vacancy, oxygen defect having C2v symmetry.

Irradiation damage in carbon-doped silicon irradiated at low temperatures by 2 MeV electrons

Abstract: Irradiation of silicon by 2 MeV electrons at 130°K leads to the removal of carbon from substitutional sites and the formation of centres with axial symmetry having vibrational modes at 921 and 930 cm−1 for 12C; large isotope shifts are found in crystals containing 13C and 14C. This centre is considered to involve interstitial carbon atoms but not oxygen impurities. On annealing such irradiated crystals to room temperature the concentration of these centres is reduced and a new transient centre involving carbon has been detected. Further annealing leads to the formation of the well known 11.56μm absorption band in pulled crystals and it is shown that this may be correlated with another band at 1115.5 cm−1. Again large isotope effects are found in crystals containing 13C and 14C and this centre is ascribed to a [O-C] complex involving an interstitial carbon atom.

The isothermal annealing of boron implanted silicon

Abstract: Isothermal annealing studies of boron implanted silicon have been made at temperatures above 600°C for boron doses in the range of 1014 to 1015 ions/cm2. A 5 eV activation energy is obtained in the temperature range 600°C to 900°C. This value is the same as that associated with the generation and subsequent migration of vacancies in silicon. This indicates that the mechanism responsible for the increase in electrical activity of the implanted boron is due to the thermal generation of vacancies and the diffusion of these vacancies or vacancy complexes to interstitial boron atoms, which then become substitutional. The characteristic annealing times are dose dependent. No simple integral-order kinetic process will explain all the data. Good agreement between the data and a calculation is obtained if it is assumed that the characteristic annealing times are position dependent. Detailed data are presented which provide the time and temperature necessary to obtain complete electrical activity of boron ...

On the annealing of damage produced by boron ion implantation of silicon single crystals

Abstract: Using the techniques of Rutherford scattering, electron microscopy and infra-red absorption, the damage produced by implantation of 100 to 400 keV boron (11B) ions in silicon single crystals, held at room and liquid nitrogen temperatures, has been examined, and the annealing behaviour investigated. The rapid annealing of disorder in the range between room temperature and 325°C, as observed by the Rutherford scattering of channelled analysing particle beams, correlates with a shrinkage of the 1. 8μ divacancy infra-red absorption band, and with a coarsening of some of the defects observed in the electron microscope. Amorphous zones observed in samples implanted with 1015 boron ions/sq. cm. at liquid nitrogen temperature do not anneal until temperatures in the vicinity of 450 °C are attained. The consequences of aggregation of point defects, first as small crystallographic clusters, for temperatures in the vicinity of 300 °C, and then as large circular dislocation loops following isochronal anneals ...

Ionization, thermal, and flux dependences of implantation disorder in silicon

Abstract: Proton channeling effect measurements were used to study the implantation lattice disorder in silicon as a function of ion flux (dose-rate), fluence, implant temperature, ion mass, and the presence of ionizing radiation. For both O+ and Sb+ implantation at 87°K the lattice disorder production is the same for equal total energy into atomic processes/cm3 at a fixed rate of energy into atomic processes/cm3-sec. The disorder production for both light (O+)and heavy (Sb+) ions exhibits a flux dependence at low temperatures and at room temperature. At low temperatures an increase in disorder is observed for increasing flux at constant fluences; however, for low fluence Sb implantation at 300°K, lower disorder is observed with increasing flux. The lattice disorder increases for all ions with decreasing implant temperature from 300°K down to temperatures as low as 38°K, but the variation with temperature is greater for lighter ions. The stimulation of lattice disorder annealing (typically ≍ 10 per cent) b...

Microstructural analysis of neutron irradiated titanium and rhenium

Abstract: The defect structure in α-titanium and rhenium irradiated with neutrons at 04T m (T m = absolute melting temperature) has been analyzed using transmission electron microscopy In rhenium, the vacancies agglomerate into voids whereas in titanium, vacancy dislocation loops lying on the prism planes are the only vacancy type defects observed In both metals, dislocation segments and network fragments are the main evidence of interstitial type defects The presence of dislocation loops rather than voids in titanium irradiated at this temperature is an anamalous result when compared to results on other similarly irradiated pure metals Possible explanations for the preferential formation of loops rather than voids in the titanium are discussed

Void formation in irradiated Nickel 270

Abstract: Nickel 270 (99.98 per cent nominal purity) was irradiated in EBR-II to fluences ranging from 1 × 1018 to 1.5 × 1022 neutrons/cm2 at temperatures between 375 and 525°C. Voids were observed in all specimens in concentrations of 1 to 3 × 1014/cm3, independent of temperature and fluence. At low fluences the voids were non-homogeneously distributed. These observations are interpreted in terms of void nucleation on sites existing in the material prior to irradiation. The results are compared with other observations on nickel of comparable and higher purity. Large differences exist not only in the magnitude of void concentrations but also in temperature and fluence dependencies. These differences indicate that a single, void nucleation mechanism is not operative and that impurities play an important role in determining the nature and amount of damage produced by neutron irradiation at elevated temperatures.

Infrared studies of low temperature electron irradiated silicon containing germanium oxygen and carbon

Abstract: Single crystal silicon containing various concentrations of germanium, oxygen, and carbon has been irradiated at 90 °K with 2.8 MeV electrons and the ensuing defects studied by infrared absorption measurements. We find that, in n-type silicon containing 3 × 1020 Ge atoms/cm3, there is no 12μ absorption line (A-center) after a 90 °K electron irradiation. The A-centers appear only after annealing between 200 °K and 280 °K. It is therefore inferred that Ge in silicon acts as an efficient trap for vacancies at low temperatures, the trapped vacancies being released from the Ge-V centers by thermal activation in the 200 °K-280 °K range. These vacancies then migrate through the lattice and are trapped on oxygen atoms. On the other hand we find that the interaction between silicon interstitials and Ge atoms is very low. The 935 cm−1 absorption line is unusually intense in our experiments. Results suggest that the defects (‘B-centers’) which produce this absorption line are the result of the interaction o...

Low temperature channeling measurements of ion implantation lattice disorder in GaAs

Abstract: Lattice disorder resulting from 140 keV Zn and 151 keV Xe implantations below 100 °K was studied by channeling effect analysis using 400 keV protons. Implantations and analyses were performed in the same system without warmup. Isochronal anneal curves show significant annealing of disorder below room temperature for low temperature, low fluence implants. The annealing is similar for both Zn and Xe implants and also is similar to previous measurements of the annealing of thermal conductivity following low temperature electron irradiation. For 298 °K implants, the disorder production is strongly dose rate dependent and increases significantly with increasing dose rate. For dose rates used in these measurements, the disorder production at 298 °K was greatly reduced from that for the low temperature implantations.

Flux and fluence dependence of disorder produced during implantation of 11B in silicon

Abstract: The flux and fluence dependence of disorder produced in silicon during the implantation of 11B has been investigated at room temperature, -50°C. and -120°C. Implantations were carried out with 200 keV 11B ions using current densities in the range from 0.06μA/cm2 to 15μA/cm2, and the disorder monitored by measuring the energy spectra of backscattered protons which were incident on the sample at 450 keV parallel to a (110) axis. Significant differences in the dependence of the disorder on 11B flux and fluence were observed between the implantations performed at room temperature and those carried out at the two lower temperatures.

Relation between surface structures and sputtering ratios of copper single crystals

Abstract: Polished (100) Cu crystals have been bombarded at target temperatures of 204 K, 294 K and 456 K by 10 and 20 keV Ne+ ions up to a total dose of 1.7 × 1019 ions/cm2. The plane of incidence was chosen to be a {100} plane perpendicular to the surface. Measurements have been performed for incident angles between 36° and 44° with respect to the surface normal. In this angular interval the sputtering ratio and the surface structure have been studied by weightloss and replica electron microscope techniques respectively. At target temperatures of 204 K and 294 K an anomaly was observed in the curve of the sputtering ratio versus angle of incidence. A small peak appears where the curve slopes towards the 〈110〉 minimum. The position and height of the peak is a function of target temperature and ion energy. This sputtering submaximum is accompanied by the formation of {100} orientated furrows perpendicular to the ion beam. The nucleation of this relief is tentatively discussed in terms of local deviations f...

Simulation of high fluence fast neutron damage with heavy ion bombardment

Abstract: 316 stainless steel has been irradiated with 5 MeV Cu ions to a fluence of 2 × 1016 ions/cm2 at 500°C. Transmission electron microscopy of this sample reveals that 6 × 1015 voids/cm2 of average diameter equal to 180 A were produced. A method for correlating the fluence of ions with equivalent neutron fluences is described. This method predicts that the Cu bombardment in this study should produce a microstructure similar to that found in steel irradiated with 2–5 × 1122 neutrons/cm2. A comparison of the ion produced voids with those found after previous neutron irradiation experiments confirms this prediction.

The effects of irradiation temperature and preinjected gases on voids in aluminum

Abstract: Voids in high purity aluminum irradiated to a fast (E>1 MeV) fluence of 4 × 1020 n/cm2 at 125 (0.43T m) and 150°C (0.45T m) are fewer in number but very much larger in size than those in material irradiated at 55°C (0.35T m). Additionally, at 125 and 150°C, the voids adopt a variety of shapes including plates, ribbons, cylinders and more equiaxed polyhedra, and are frequently associated with particles of transmutation-produced silicon. At the higher temperatures voids are larger near grain boundaries than in grain interiors. Injection of hydrogen or helium prior to irradiation causes an increase in the number of voids and a corresponding decrease in size in specimens irradiated at 150°C; 3 at. ppm He is more effective than either 3 or 9 at. ppm H. The gases do not appear to influence swelling. A commercial purity (99 per cent) aluminum subjected to the same irradiation treatments did not develop voids whether preinjected with gases or not; the visible radiation damage consisted solely of small lo...

Production and mutual annihilation of frenkel pairs in low temperature irradiated zirconium

Abstract: 12.5 μ thick zirconium foils were irradiated by monoenergetic electrons in the range 0.5–1.7 MeV at liquid helium temperature. To the measured defect production rate, a theoretical curve was fitted, assuming an atomic displacement probability function P d, (T) and estimating the multiple displacement contribution. The best fit was obtained with: Isochronal and isothermal annealing studies were performed in the recovery stage I (up to 150 °K) and compared with results obtained earlier by Neely. An effect of radiation doping on the behaviour of the different substages has been observed. and a Frenkel pair resistivity of 4 × 10−3 ω cm. The implications of such a multiple displacement function are considered.

Thermal Neutron damage in cadmium

Abstract: The isochronal annealing of the damage produced by thermal neutron irradiation of cadmium at 3.6 °K has been studied for several initial doses which vary by a factor of 1000. The recovery results show a strong dependence upon initial dose. This effect, which is not seen to this extent in the fcc metals, cannot be accounted for by an irradiation annealing mechanism. In contrast to the observation of two processes involving long range defect migration for several fcc metals only one process, at high temperatures, is discernable from isochronal annealing of Cd. The presence of another process at low temperatures is clearly established by other means. Irradiation annealing effects observed during the production of damage at high defect concentrations indicate that the spontaneous annihilation volume between the defects of a new capture event and the defects from an earlier event is 80 atomic volumes. Other results suggest that damage production and recovery mechanisms may be associated with the aniso...

Spatial distribution of defects in ion bombarded silicon and germanium.

Abstract: The damage produced in Si and Ge by 10–400 keV heavy-ion bombardment has been measured by using the channeling technique. By optimizing the experimental conditions, the depth distribution of the damage can be measured with a depth resolution of ∼ 10% in the range of 50–10000 A. The present paper gives a short description of the method and the apparatus applied in these measurements. The results of the measurements may be summarized as follows: (i) The measured depth distributions demonstrate the necessity of taking the energy transported by recoils away from their point of origin into account. (ii) The average damage depth and the depth straggling agree with recent theoretical calculations of the distribution of energy deposited by ions in solids. (iii) The damage production exhibits a dose rate dependence indicative of room temperature annealing. This is supported by the observation of the annealing effect of a 0.5 MeV He bombardment. (iv) Channeling of a heavy ion has a marked influence on the ...

Calibrated U-TH glasses for neutron dosimetry and determination of uranium and thorium concentration by the fission track method

Abstract: We have prepared a series of standard uranium glasses. A primary standard containing 41 ppm of added U had its concentration and isotope ratio verified by mass spectrometry and by fission track techniques. Secondary standards were compared to the primary standard using the fission track technique. These glasses are useful for measuring both Th and U in solids. The U concentration in an unknown is determined by comparing the fission track density induced by thermal neutron irradiation with the corresponding density in the standard. Th is measured by a cyclotron irradiation using the same U standards. The glasses are also useful for neutron dosimetry. Investigators may obtain small samples of most of these standards from R. H. Brill of the Corning Museum of Glass. Although most of the glasses were fabricated from depleted uranium, one standard containing natural uranium is available. A standard Th glass with concentration of 41 ppm was also made.

Selective X-ray generation by heavy ions (Part 2) measurement of the concentration distribution of ion-implanted antimony in silicon by the use of selective heavy ion X-ray excitation

Abstract: This investigation demonstrates how the total, concentration distribution of antimony, previously implanted into silicon at 100 keV, may be elucidated without recourse to the usual radioactive isotope techniques It uses the fact that 100 keV Kr+ ions can preferentially excite characteristic X-rays from antimony, even in the presence of a huge excess of silicon The resultant high sensitivity for the detection of antimony in silicon is accompanied by the fact that the X-rays arise predominantly from less than one hundred Angstroms below the surface of the specimen Thus bombardment by 100 keV Kr+ ions is used in conjunction with an anodic stripping technique (which removes 169±20 A at a time) to obtain the antimony distribution profile in silicon Consideration is also given to the possibility of obtaining the implanted antimony range distribution by using 100 keV Kr+ ions to detect the antimony and simultaneously remove silicon by sputtering

Investigation of point defects in silicon and germanium by non-irradiation techniques

Abstract: The paper reviews the information on vacancies and interstitials from such techniques as diffusion studies, precipitation from supersaturated solid solutions, quenching from high temperatures, and plastic deformation. Self- and impurity diffusion are considered in some detail and the evidence is presented according to which the high-temperature self-diffusion occurs via extended interstitials in silicon and via extended vacancies in germanium. From the diffusion coefficients of group-V impurities and from the precipitation of substitutional nickel impurities the selfdiffusion coefficient for a vacancy self-diffusion mechanism is derived and found to be lower than that from high-temperature self-diffusion experiments. This constitutes evidence for a change-over in the self-diffusion mechanism in silicon at about 900 °C to a low-temperature vacancy mechanism with lower activation energy and lower preexponential factor than the interstitial mechanism. It is proposed that self-interstitials in silico...

Critical approach distances and critical angles for channelling

Abstract: The critical angles and critical approach distances, which determine the limits for the channelling of energetic ions, down open crystal directions are established by the methods of computer simulation and compared with simple analytical models. For non-vibrating rows or planes a good agreement between the two is obtained. The complicating effects of thermal vibrations can be accounted for by empirical formulae.

The dependence of sputtering efficiency on ion energy and angle of incidence

Abstract: Earlier measurements of sputtering efficiency of polycrystalline targets (fraction of impinging ion energy leaving the target through sputtering and backscattering) have been extended to higher energies. Lead and copper targets were bombarded with several different projectiles with energies between 80 and 1200 keV. The sputtering efficiency decreases with increasing energy. This decrease is ascribed to the combined influence of changes in the scattering cross section with energy, and to electronic stopping. The results may be described as a function of the mass ratio M 2/M 1 and the reduced energy ∊ only. The sputtering efficiency was measured as a function of angle of incidence of the bombarding ions. To ensure complete collection of sputtered and backscattered particles, it was possible to cover only the region of incidence angle from 0° to 45°. Targets of copper, silver, and lead were investigated with 17 different ion-target combinations. The sputtering efficiency increased with angle of inci...