Electron-beam induced amorphization of stishovite: Silicon-coordination change observed using Si K-edge extended electron
energy-loss fine structure
van Aken PA, Sharp TG, Seifert F
25: (2) 83-93 JAN 1998
The analysis of the extended energy-loss fine structure (EXELFS) of the Si K-edge for sixfold-coordinated Si in synthetic stishovite and
fourfold-coordinated Si in natural alpha-quartz is reported by using electron energy-loss spectroscopy (EELS) in combination with trans
mission electron microscopy (TEM). The stishovite Si K-edge EXELFS spectra were measured as a time-dependent series to document
irradiation-induced amorphization. The amorphization was also investigated through the change in Si K- and O K-edge energy-loss near
edge structure (ELNES). For alpha-quartz, in contrast to stishovite, electron irradiation-induced vitrification, verified by selected area electron
diffraction (SAED), produced no detectable changes of the EXELFS. The Si K-edge EXELFS were analysed with the classical extended X-ray
absorption fine structure (EXAFS) treatment and compared to ab initio curve-waved multiple-scattering (MS) calculations of EXAFS spectra
for stishovite and alpha-quartz. Highly accurate information on the local atomic environment of the silicon atoms during the
irradiation-induced amorphization of stishovite is obtained from the EXELFS structure parameters (Si-O bond distances, coordination
numbers and Debye-Waller factors). The mean Si-O bond distance R and mean Si coordination number N changes from R=0.1775nm and
N=6 for stishovite through a disordered intermediate state (R approximate to 0.172nm and N approximate to 5) to R approximate to 0.167 nm
and N approximate to 4.5 for a nearly amorphous state similar to alpha-quartz (R=0.1609 nn and N=4). During the amorphization process,
the Debye-Waller factor (DWF) passes through a maximum value of sigma(N)(2) approximate to 83.8pm(2) as it changes from sigma(st)(2)=
51.8pm(2) for sixfold to sigma(qu)(2) = 18.4pm(2) for fourfold coordination of Si. This Increase in Debye-Waller factor indicates an increase
in mean-square relative displacement (MSRD) between the central silicon atom and its oxygen neighbours that is consistent with the presence
of an intermediate structural state with fivefold coordination of Si. The distribution of coordination states can be estimated by modelling the
amorphization as a decay process. Using the EXELFS data for amorphization, a new method is developed to derive the relative amounts of
Si coordinations in high-pressure minerals with mixed coordination. For the radiation-induced amorphization process of stishovite the
formation of a transitory structure with Si largely in fivefold coordination is deduced.
Defect production and annealing kinetics in elemental metals and semiconductors
de la Rubia TD, Soneda N, Caturla MJ, Alonso EA
251: 13-33 NOV 1997
We present a review of recent results of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations of defect production and
annealing in irradiated metals and semiconductors. The MD simulations describe the primary damage state in elemental metals Fe, V and Au,
and in an elemental semiconductor Si. We describe the production of interstitial and vacancy clusters in the cascades and highlight the
differences among the various materials. In particular, we discuss how covalent bonding in Si affects defect production and amorphization
resulting in a very different primary damage state from the metals. We also use MD simulations to extract prefactors and activation energies
for migration of point defects, as well as to investigate the energetics, geometry and diffusivity of small vacancy and interstitial clusters. We
show that, in the metals, small interstitial clusters are highly mobile and glide in one dimension along the direction of the Burger's vector. In
silicon, we show that, in contrast to the metals, the neutral vacancy diffuses faster than the neutral self-interstitial. The results for the primary
damage state and for the defect energetics and kinetics are then combined and used in a kinetic Monte Carlo simulation to investigate the
escape efficiency of defects from their nascent cascade in metals, and the effect of dose rate on damage accumulation and amorphization in
silicon. We show that in fee metals Au and Pb at or above stage V the escape probability is approximately 40% for 30 keV recoils so that the
freely migrating defect fraction is approximately 10% of the dpa standard. In silicon, we show that damage accumulation at room temperature
during light ion implantation can be significantly reduced by decreasing the dose rate. The results are compared to scanning tunneling
microscopy experiments. (C) 1997 Published by Elsevier Science B.V.
Disorder-induced amorphization
Lam NQ, Okamoto PR, Li M
251: 89-97 NOV 1997
Many crystalline materials undergo a crystalline-to-amorphous (c-a) phase transition when subjected to energetic particle irradiation at low
temperatures. By focusing on the mean-square static atomic displacement as a generic measure of chemical and topological disorder, we are
led quite naturally to a generalized version of the Lindemann melting criterion as a conceptual framework for a unified thermodynamic
approach to solid-state amorphizing transformations. Tn its simplest form, the generalized Lindemann criterion assumes that the sum of the
static and dynamic mean-square atomic displacements is constant along the polymorphous melting curve, so that c-a transformations can be
understood simply as melting of critically-disordered crystals at temperatures below the glass transition temperature where the supercooled
liquid can persist indefinitely in a configurationally-frozen state. Evidence in support of the generalized Lindemann melting criterion for
amorphization is provided by a large variety of experimental observations and by molecular dynamics simulations of heat-induced melting
and defect-induced amorphization of intermetallic compounds. (C) 1997 Elsevier Science B.V.
Behavior and computer simulation of SiC under irradiation with energetic particles
Perlado JM
251: 98-106 NOV 1997
This paper reviews previous works focused on fundamental neutron damage simulations in beta-SiC. The reason ru propose ceramic
composites based on SiC, instead of the monolithic form, is concluded. and the effect of radiation (neutron irradiation) on the macroscopic
responses of the material are shown. Molecular dynamics simulations of threshold displacement energy, defect energetics, melting point. and
3-5 keV cascades are presented. together with a criticism of various interatomic potentials such as Pearson, Tersoff, and Modified Embedded
Atom Model. Most of the presented results have been performed with Tersoff's potential which has demonstrated a better physics description
of the material's parameters. The directional dependence of the threshold energy is mentioned, and the consequences of energetic cascades
such as potential amorphization are compared with results of Si. The effect of the accumulation of damage. at a rate higher than expected in
real Inertial Confinement Fusion applications is also mentioned. (C) 1997 Elsevier Science B.V.
Change of optical properties during ion induced amorphization of AuIn2 films
Heinz B, Ziemann P
132: (4) 589-598 DEC 1997
The attenuated total reflection (ATR) technique has been combined with low temperature ion irradiation allowing to study the changes of
optical properties of AuIn2 films under bombardment. For this purpose, crystalline films were irradiated at 80 K with 350 keV Ar+ ions
resulting in a stepwise amorphization as indicated by accompanying in situ resistance measurements. It is demonstrated that this ion induced
amorphization also leads to significant changes of the real and imaginary part of the index of refraction and its dispersion relation, which can
be traced back to a disappearance of band structure effects in the amorphous phase. Based on the optical information, a procedure can be
developed to extract the amorphous volume fraction, which, as a function of the ion fluence, compares very well with the corresponding
results obtained from the resistance data. (C) 1997 Elsevier Science B.V.
Ion mass and temperature dependence of damage production in ion implanted InP
Wendler E, Opfermann T, Gaiduk PI
82: (12) 5965-5975 DEC 15 1997
Ion beam induced damaging and amorphization of crystalline InP is investigated. 100 keV B+, 300 keV Si+, 200 keV Ar+ and 600 keV Se+
ions are implanted into (100) InP at temperatures ranging from 80 K to 420 K. The implanted layers are analyzed using Rutherford
backscattering spectrometry in channeling configuration, cross section transmission electron microscopy and optical spectroscopy in the
sub-gap frequency region. The temperature dependence of damage production can be represented assuming a thermally stimulated defect
diffusion within the primary collision cascades, resulting in a shrinkage of the remaining defect clusters. At a critical temperature T-infinity
these clusters dissolve completely and only point defect complexes nucleate. Then, amorphization occurs only at very large ion fluences
(approximate to 10(16) cm(-2)) and the process seems to be influenced by the high amount of implanted ions. A defect band forms around the
projected range of the implanted ions, which may act as a diffusion barrier for point defects. The range of T, from approximate to 350 K for
B+ and approximate to 420 K for Se+ ions corresponds to the annealing stage II of defects in InP, which can be related to the mobility of
phosphorous interstitials. This indicates that phosphorous interstitials play an important role during ion irradiation of InP. (C) 1997 American
Institute of Physics.
Lattice parameter changes and point defect reactions in low temperature electron irradiated AlAs
Gaber A, Zillgen H, Ehrhart P, Partyka P, Averback RS
82: (11) 5348-5351 DEC 1 1997
X-ray diffraction was employed to investigate damage accumulation and the subsequent thermally activated annealing reactions in AlAs layers
on GaAs substrates. irradiations were performed at 4.6 K with 2.5 MeV electrons up to a total dose of 2x10(19) electrons/cm(2). The
irradiation-induced increase of the lattice parameter amounts to about half of the changes observed in the GaAs substrates. There is a major
annealing step near room temperature, a rather continuous annealing up to 500 K, and a final recovery stage between 700 and 900 K. The
observations are discussed in relation to the resistance of AlAs against amorphization under ion irradiation. (C) 1997 American Institute of
Kinetics of ion-beam-induced interfacial amorphization in silicon
Henkel T, Heera V, Kogler R, Skorupa W, Seibt M
82: (11) 5360-5373 DEC 1 1997
This article reviews modeling and experimental results of ion-beam-induced interfacial amorphization (IBIIA) in silicon. It is shown that this
process differs from the well-known bulk amorphization with regard to the critical energy density approach and the evolution of the
roughness of the amorphous/crystalline interface during ion irradiation. IBIIA depends on the substrate temperature, ion flux, and nuclear
energy deposition at the amorphous/crystalline interface, which is discussed in detail. Within this scope, new results about the temperature and
ion Aux dependence of IBIIA are presented that cannot be explained by previous models. Therefore, a new model based on ballistic transport
effects that allows one to understand experimental results at low temperatures is proposed. According to this concept IBIIA is controlled by
three processes interacting at the amorphous/crystalline interface: an athermal ion-beam-induced defect generation, a thermally activated
recombination of defects, and an athermal transport of defects towards the amorphous/crystalline interface as a result of ballistic processes. It
is speculated that these defects are mainly interstitials and vacancies involved in those processes. (C) 1997 American Institute of Physics.
  Application of a mechanistic model for radiation-induced amorphization and crystallization of uranium silicide to recrystallization of
Rest J
248: 180-184 SEP 1997

Document type: Article
Language: English
Cited References: 20
Times Cited: 0

An alternative mechanism for the evolution of recrystallization nuclei is described for a model of irradiation-induced recrystallization of UO2
wherein the stored energy in the UO2 is concentrated in a network of sinklike nuclei that heretofore were assumed to diminish with dose due
to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular dislocation structures that evolve relatively early in
the irradiation period. In the alternative approach, a generalized theory of radiation-induced amorphization and crystallization, developed for
uranium silicide, is applied to UO2. The complicated kinetics involved in the formation of a cellular dislocation network are approximated by
the formation and growth of subgrains due to the interaction of shock waves produced by fission-induced damage to the UO2. (C) 1997
Elsevier Science B.V.
  Behavior of neutron-irradiated U3Si
Ugajin M, Akabori M, Itoh A, Ooka N, Nakakura Y
248: 204-208 SEP 1997

Document type: Article
Language: English
Cited References: 14
Times Cited: 0

The behavior of U3Si-based alloys has been studied under neutron irradiation. Maximum burnup reached similar to 62% of the initially
contained 19.6% U-235 and irradiation temperatures were in the approximate range of 190-280 degrees C. Postirradiation examinations
revealed the following. The dimensional instability of the high uranium-density fuels is attributed to the radiation-induced amorphization and
plastic deformation. The occurrence of amorphization is suggested by the liquid-like behavior of U3Si under irradiation at temperatures well
below the melting point of crystalline material. The accelerated swelling of U3Si due to the large fission-gas-bubble growth can be suppressed
by the cladding restraint. The reaction layer is formed at the U3Si-Al interface. The thickness of the reaction layer of surface-oxidized U3Si is
significantly reduced in comparison with that of non-oxidized U3Si. This reduction in thickness is caused by the thin film of UO2 that has been
formed at the surface of the U3Si by oxidation. (C) 1997 Elsevier Science B.V.
  Irradiation-induced amorphization of graphite (vol 52, pg 15785, 1995)
Niwase K
56: (9) 5685-5685 SEP 1 1997
  Crystalline to amorphous transformation during irradiation
Nair KGM, Krishan K
20: (4) 583-589 JUL 1997

Document type: Article
Language: English
Cited References: 6
Times Cited: 0

Irradiation influences the phase stability through increased defect concentration, enhanced diffusion, segregation and ballistic processes. A
variety of phase instabilities are observed during irradiation which include precipitation of thermally unstable phases, dissolution of second
phase particles, irradiation induced coarsening, order-disorder transformation and amorphization. Radiation induced amorphization has
been observed in a number of ordered intermetallic compounds, such as Al6Mn (Nair er al 1993), Zr3Al(Howe and Rainville 1979), Cu4Ti3
(Luzzi 1991) and NiAl (Jaouen et al 1991). A number of empirical correlations have been established in these studies. The most important
amongst them is that compounds with limited compositional range of existence in the equilibrium phase diagram are the ones which are most
susceptible to amorphization. Large negative heat of formation, complicated crystal structure and large difference in the atomic radii of the
constituents are the few other features seen in those compounds which undergo amorphization during irradiation. In addition, it is found
that temperature of irradiation has a strong influence in the amorphization process. The irradiation induced amorphization is inherently a
low temperature phenomenon, since at higher temperatures thermally activated recovery processes become dominant. Various salient features
of the radiation induced amorphization process are discussed in the paper by giving specific examples from investigations carried out in
Al-Mn alloys.
  Ion beam induced amorphization of crystalline solids: Mechanisms and modeling
Trinkaus H
248-: 3-12 1997

Document type: Article
Language: English
Cited References: 37
Times Cited: 0

There are two main mechanisms of irradiation induced amorphization of crystalline solids: (1) continuous accumulation of defects which
destabilize the crystal structure at some critical level, and (2) rapid quenching of irradiation induced liquid thermal spike regions. Which
mechanism occurs or dominates depends on type of material, energy and type of projectile and irradiation temperature. In connection with a
brief review of the main experimental findings and their current understanding the role of thermal spikes in ion beam induced amorphization
(IBIA) is emphasized. An amorphous nucleus is considered to be the result of melting and incomplete recrystallization of a thermal spike
region. The evolution of a liquid/amorphous region during as well as after the thermal spike within the defective crystalline environment is
controlled by crystalline/liquid (amorphous) interface kinetics. On the basis of these ideas, the dependence of IBIA on main parameters such as
type of material, projectile and energy deposition is discussed.
Void formation and surface rippling in Ge induced by high energetic Au irradiation
Huber H, Assmann W, Grotzschel R, Mieskes HD, Mucklich A, Nolte H, Prusseit W
248-: 301-312 1997

Document type: Article
Language: English
Cited References: 25
Times Cited: 0

Ge wafer have been irradiated at room temperature with 100-266 MeV Au ions at different incidence angles. For an incidence angle of 15
degrees, the Au bombardment leads to a surface roughening when fluences higher than 10(14) ions/cm(2) are applied, while at normal
incidence no surface modification could be seen up to a fluence of 1.2x10(15) ions/cm(2). In the case of flat entrance angles the surfaces of
the bombarded samples start to ripple with wave-lengths between 30 nm and 60 nm after a dose of 3.0x10(14) ions/cm(2). With increasing
fluence these ripples are transformed into a chaotic surface structure. Cross section TEM as well as channeling and in situ blocking
measurements performed on the samples have shown that the modified surface layer has been amorphized during the irradiation, while the
material underneath is damaged but still crystalline. These effects are completely absent after irradiation of Ge with 165 MeV Ni and 185
MeV I ions. Furthermore the heavy ion bombardment leads to the formation of a buried sponge-like layer. The formation depth of this layer
depends on the ion energy as well as on the entrance angle theta of the ions. We discuss the amorphization of a surface layer with the
underlying material being still crystalline and the dependence of the formation depth of the porous layer on the electronic and nuclear energy
loss. Moreover we present a simple model which is capable to describe the wave-length of the surface-rippling observed.
Radiation-induced phase transformations in MgAl2O4 spinel
Yu N, Devanathan R, Sickafus KE, Nastasi M
12: (7) 1766-1770 JUL 1997

Document type: Article
Language: English
Cited References: 25
Times Cited: 0

Ion-irradiation was observed to transform MgAl2O4 spinel first to a metastable crystalline phase and then to an amorphous phase at
cryogenic temperatures. Elastic stiffening of 15% occurred upon formation of the metastable crystalline phase. A second transformation from
the metastable crystalline spinel to an amorphous state was accompanied by elastic softening of 25% relative to unirradiated spinel. This phase
transformation behavior in spinel appears to be different from that in intermetallic compounds where only elastic softening associated with
radiation damage accumulation is observed. A two-stage radiation damage model is proposed to explain the observed phase transformations.
  Ion irradiation and annealing effects in Al2O3 and MgAl2O4
Furuno S, Sasajima N, Hojou K, Izui K, Otsu H, Muromura T, Matsui T
127: 181-185 MAY 1997

Document type: Article
Language: English
Cited References: 9
Times Cited: 0

In situ observation of structural changes of Al2O3 and MgAl2O4 induced by Xe and He ion irradiations and successive annealing were
performed in an electron microscope equipped with ion accelerator. By Xe ion irradiation amorphization occurred in both the materials and
bubbles were formed in MgAl2O4. By He ion irradiation bubbles were formed in both the materials, but amorphization was not observed to
a fluence of 1.8 x 10(21)/m(2) (11 dpa). By annealing after the irradiation, bubbles began to flow large by coalescence at 1000 degrees C in
Al2O3, and at 900 degrees C in MgAl2O4, respectively. Amorphization began to recover in both materials at these temperatures. In the case
of MgAl2O4 amorphous region recovers completely to polycrystalline state at 1000 degrees C, and in the case of Al2O3 to original single
crystal, respectively. Pit formation and exfoliation also occur in both materials at about 1100 degrees C.
  In situ and ex situ investigation of ion-beam-induced amorphization in alpha-SiC
Weber WJ, Yu N
127: 191-194 MAY 1997

Document type: Article
Language: English
Cited References: 26
Times Cited: 0

Single crystals of alpha-SiC with [0001] orientation have been irradiated at 300 K with 360 keV Ar2+ ions at an incident angle of 25 degrees.
The damage accumulation in one sample was followed in situ by Rutherford backscattering spectroscopy in channeling geometry (RBS/C)
along the [<1(1)over bar 02>] direction until the aligned backscattering yield reached the random level throughout the irradiated layer at a
fluence of 8 Ar2+ ions/nm(2). Six other samples were irradiated at fluences ranging from 1 to 6 Ar2+ ions/nm(2), and the damage
accumulation was characterized ex situ by RBS/C along the [0001] direction. The relative disorder, as measured by RBS/C, increased with ion
fluence; however, the observed rate of disordering was much lower when measured ex situ along the [0001] direction.
  Amorphization and solid phase epitaxy of high-energy ion implanted 6H-SiC
Ishimaru M, Harada S, Motooka T, Nakata T, Yoneda T, Inoue M
127: 195-197 MAY 1997

Document type: Article
Language: English
Cited References: 10
Times Cited: 1

We have investigated microstructures of damaged and recrystallized layers in MeV-ion implanted 6H-SiC (0001) wafers by means of
cross-sectional transmission electron microscopy. The substrate surfaces were implanted at 160 degrees C with 1 x 10(17)/cm(2) 8 MeV Si3+
ions using a tandem accelerator. A buried amorphous layer was formed ranging from similar to 1.6 mu m to similar to 3.4 mu m in depth. The
amorphous/crystalline transition regions consisted of many stacking faults perpendicular to the [0001] direction, and their density increased
toward the amorphous region. The amorphous layer regrew epitaxially from the undamaged substrate at an annealing temperature of similar to
1000 degrees C. This epitaxial 6H-SiC layer changed to columnar 6H-SiC with crystal orientations different from the substrate. In addition to
these crystalline 6H-SiC, the existence of polycrystalline 3C-SiC was confirmed in the middle part of the recrystallized layer.
  Ion irradiation damage in ilmenite at 100 K
Mitchell JN, Yu N, Devanathan R, Sickafus KE, Nastasi MA, Nord GL
127: 629-633 MAY 1997

Document type: Article
Language: English
Cited References: 26
Times Cited: 0

A natural single crystal of ilmenite (FeTiO3) was irradiated at 100 K with 200 keV Ar2+. Rutherford backscattering spectroscopy and ion
channeling with 2 MeV He+ ions were used to monitor damage accumulation in the surface region of the implanted crystal, At an irradiation
fluence of 1 x 10(15) Ar2+/cm(2), considerable near-surface He+ ion dechanneling was observed, to the extent that ion yield from a portion of
the aligned crystal spectrum reached the yield level of a random spectrum. This observation suggests that the near-surface region of the crystal
was amorphized by the implantation. Cross-sectional transmission electron microscopy and electron diffraction on this sample confirmed the
presence of a 150 nm thick amorphous layer. These results are compared to similar investigations on geikielite (MgTiO3) and spinel
(MgAl2O4) to explore factors that may influence radiation damage response in oxides.
  Amorphization of intermetallic compounds under irradiation - A review
Motta AT
244: (3) 227-250 APR 1997

Document type: Article
Language: English
Cited References: 189
Times Cited: 1

This is a review of the field of irradiation-induced amorphization of intermetallic compounds. It includes an update of recent experimental
results using in-situ particle irradiation showing the effects of dose rate, temperature, crystal orientation, electron energy and the presence of
stacking faults. The review describes amorphization by ion, electron and neutron irradiation in the context of a kinetic description, where
the rate-limiting step is the accumulation of enough radiation damage in the lattice opposed by thermal annealing. Stability criteria,
thermodynamic or otherwise, are combined with kinetics of radiation damage and annealing to provide an overall description of the
amorphization process, and of the experimentally measured critical dose and critical temperature of amorphization. From the experimental
observations, it is proposed that irradiation-induced amorphization in intermetallic compounds is an entropy-driven transformation, caused
by the need of the material to maintain short-range order while accommodating the random ballistic motions of the atoms caused by
Amorphization of U3Si by ion or neutron irradiation
Birtcher RC, Richardson JW, Mueller MH
244: (3) 251-257 APR 1997

Document type: Article
Language: English
Cited References: 29
Times Cited: 1

Changes in crystal structure of U3Si during irradiation have been monitored by diffraction techniques. Neutron diffraction was used to follow
crystallographic changes produced by uranium fission during neutron irradiation at 30 degrees C. The uranium fission fragments produce
tracks of damage in the form of amorphous zones. Strain from the small volumes of amorphous material drives a transformation of the U3Si
crystal structure from the tetragonal to the cubic phase. Lattice strains develop at an initial rate of (2.20 X 10(22) fissions/m(3))(-1) or (0.076
dpa)(-1), and the total lattice volume change after amorphization is +2.04b in U3Si. At high doses, plastic flow in the amorphous volume
fraction relieves strain in the remaining crystalline volume fraction of U3Si. Complete amorphization of U3Si occurs between 0.85 and 1.11 x
10(23) fissions/m(3) or 0.29 to 0.38 dpa. Electron diffraction during in situ 1.5 MeV Kr ion irradiation was used to determine the doses
required for amorphization of both U3Si at temperatures above 30 degrees C. As with fission fragments, individual Kr ions produce
amorphous volumes that coexist with strained crystalline material. The temperature limit for complete amorphization is 290 degrees C for
U3Si. The same amount of damage is required for amorphization of U3Si or U3Si2 by ion or neutron irradiation.
  Temperature and dose dependence of ion-beam-induced amorphization in alpha-SiC
Weber WJ, Yu N, Wang LM, Hess NJ
244: (3) 258-265 APR 1997

Document type: Article
Language: English
Cited References: 32
Times Cited: 0

Single crystal alpha-SiC with [0001] orientation has been irradiated at 170, 300, and 370 K with 360 keV Ar2+ ions at an incident angle of 25
degrees and the damage accumulation process followed in situ by Rutherford backscattering spectroscopy in channeling geometry (RBS/C)
along [1 (1) over bar 02]. At 170 and 300 K, the increase in relative disorder with ion fluence, as measured by RBS/C, is consistent with a
multiple-cascade overlap process. Then is a significant deviation from the cascade overlap model at 370 K. The RBS/C results indicate that
below a critical damage level the relative disordering rate is nearly temperature independent. Post-irradiation characterization of the fully
disordered samples indicate a significant loss in the intensity of the Raman modes and decreases of 47 and 24% in hardness and elastic
modulus, respectively, Cross sectional transmission electron microscopy has confirmed the amorphous nature of the damaged surface layer
irradiated at 170 K; however, at 370 K, some residual crystallinity was observed over the depth range from 10 to 160 nm. The decrease in
density associated with amorphization at 170 K is estimated to be 22 +/- 3%.
  Nucleation-limited amorphization of GaAs at elevated temperatures
Brown RA, Williams JS
55: (19) 12852-12855 MAY 15 1997

Document type: Article
Language: English
Cited References: 15
Times Cited: 0

By a detailed correlation of damage profiles from Rutherford backscattering and channeling with cross-sectional transmission electron
microscopy images, we have identified an intriguing nucleation-limited amorphization regime in GaAs irradiated with ions at elevated
temperatures. When the rate of dynamic annealing during irradiation exceeds the damage production rate, amorphization can take place at
depths significantly different from the maximum in the energy deposition density. This process results from the incomplete annihilation of
mobile irradiation-induced defects and occurs either at the surface or at a dislocation hand, formed hy the agglomeration of interstitials. Once
formed such amorphous layers grow by a layer-by-layer process.
  A thermal spike model for nanophase formation in yttrium iron garnet under swift heavy ion beams
Costantini JM, Brisard F, Toulemonde M, Studer F
122: (3) 514-521 FEB 1997

Document type: Article
Language: English
Cited References: 40
Times Cited: 0

Amorphous latent tracks can be produced in the magnetic insulators, like yttrium iron garnet (Y3Fe5O12 or YIG), by irradiations with swift
heavy ions in the electronic slowing down regime. For this, the electronic stopping power must be higher than a threshold, around 4.0 keV
nm(-1) in the case of YIG, to obtain the amorphization of the material, when the irradiation proceeds with heavy ions in the 0.8-6 MeV
amu(-1) range. In the same energy range, we find that a nanophase can be formed in YIG by track overlapping. This occurs at fluences
depending on the electronic stopping power, and corresponding to the random impacts of the heavy ions on the amorphous tracks. Yet, no
subsequent amorphization of the nanophase is observed even at high fluences. These results are interpreted with a model based on the
thermal spike induced by the high density of ionizations and electronic excitations in the material.
  Laser-induced luminescence in glassy SiO2 and neutron-irradiated alpha-quartz: Three types of non-bridging oxygen hole centers
Skuja L, Naber A
239-: 25-28 1997

Document type: Article
Language: English
Cited References: 8
Times Cited: 0

Site-selective photoluminescence (PL) spectra of alpha-quartz samples irradiated by 10(18) and 10(19) fast neutrons/cm(2) were studied at
temperatures between 6 and 300 K under dye-laser excitation in the spectral region between 1.9 and 2.1 eV. At the room temperature an
irradiation-induced PL band with the peak at 1.9 eV and decay time 17 mu s was found. It is similar to the pi, band of the non-bridging
oxygen hole centers (NBOHC's) characteristic of amorphous forms of SiO2. The low temperature (T <80 K) site-selective pi, spectra show a
number of sharp lines. Two of them, at 1.933 and 1.883 eV are identified as zero-phonon lines (R(1) and R(2) lines) due to two distinct
variants of NBOHC's, embedded in an ordered crystalline environment and undergoing relatively small inhomogeneous broadening. They are
superimposed by a broad continuum of significantly broadened zero-phonon lines, which are similar to the ones observed for NBOHC's in
irradiated glassy SiO2. Vibronic sidebands at 897 cm(-1) in emission spectra and at 849 cm(-1) in emission spectra are due to the
silicon-(nonbridging oxygen) stretching mode of NBOHC.
Transmission electron microscopy studies of crystal-to-amorphous transition in ion implanted silicon
Ishimaru M, Harada S, Motooka T
81: (3) 1126-1130 FEB 1 1997

Document type: Article
Language: English
Cited References: 20
Times Cited: 0

The microstructure of 5 MeV ion implanted silicon at room temperature has been investigated in detail by means of cross-sectional transmission electron microscopy. Buried amorphous layers were observed in the specimens obtained by ion doses of 1X10(17) and 2X10(17)/cm(2) with abrupt amorphous-to-crystal interfaces. Damaged layers adjacent to the amorphous layers included many dislocation loops and the concentration increased toward the amorphous region. Microdiffraction patterns and high-resolution images showed that this damaged region is defective crystalline silicon, suggesting that homogeneous amorphization occurs due to an accumulation of defects. The atomistic structure of the damaged regions was analyzed by comparing high-resolution electron microscopy images with those calculated on the basis of a model for amorphization processes proposed previously [T. Motooka, Phys. Rev. B 49, 16 367 (1994)]. (C) 1997 American Institute of Physics.
  A generalized model for radiation-induced amorphization and crystallization of U3Si and U3Si2 and recrystallization of UO2
Rest J
240: (3) 205-214 FEB 1997

Document type: Article
Language: English
Cited References: 29
Times Cited: 1

A rate-theory model of radiation-induced amorphization and crystallization of U3Si during ion irradiation has been generalized to include
U3Si2 and UO2. The generalized model has been applied to ion-irradiation and in-reactor experiments on U3Si and U3Si2 and provides an
interpretation for the amorphization curve (dose required to amorphize the material as a function of temperature), for the
ion-radiation-induced nanoscale polycrystallization of these materials at temperatures above the critical temperature for amorphization, as
well as for the role of the small crystallites in retarding amorphization. An alternative mechanism for the evolution of recrystallization nuclei is
described for a model of irradiation-induced recrystallization of UO2 wherein the stored energy in the UO2 is concentrated in a network of
sinklike nuclei that diminish with dose due to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular
dislocation structures that evolve relatively early in the irradiation period. The complicated kinetics involved in the formation of a cellular
dislocation network are approximated by the formation and growth of subgrains due to the interaction of shock waves produced by
fission-induced damage to the UO2.
Radiation effects in silicon carbide: High energy cascades and damage accumulation at high temperature
delaRubia TD, Perlado JM, Tobin M
237: 1096-1101, Part B OCT 1996

Document type: Article
Language: English
Cited References: 22
Times Cited: 0

We discuss results of molecular dynamics computer simulation studies of 3 keV and 5 keV displacement cascades in beta-SiC, as well as
damage accumulation under conditions similar to those in Inertial Fusion environments. The simulations are performed with the Tersoff
potential. The cascade lifetime in SiC is found to be extremely short. This, combined with the high melting temperature of SIC, precludes
direct lattice amorphization during the cascade. Although large disordered regions result, these retain their basic crystalline structure. The SiC
results also show anisotropy in the number of Si and C recoils as well as in the number of replacements in each sublattice. Details of the
damage configurations and of the accumulation dynamics obtained will be discussed.
Amorphization of alpha-quartz under irradiation
Douillard L, Duraud JP
6: (12) 1677-1687 DEC 1996

Document type: Article
Language: English
Cited References: 35
Times Cited: 0

The course of radiation induced damage produced in alpha-quartz by neutrons, ions, electrons or photons - commonly known as
metamictization - has been re-analyzed by careful comparison of available experimental data. Specific interest was devoted to confront
experimental metamict state features with current structural models. It comes out that the metamict state of irradiated quartz should exhibit
some structural characteristics of the modulated structure proposed for vitreous silica. The metamictization process is consistent with a
structural relaxation process of a highly defective quartz matrix. According to this new point defect analysis, structural relaxation should be
triggered by a critical concentration of oxygen vacancy point defects likely to significantly lower the connectivity of the SiO2 network. Various
experimental results are interpreted by incorporating the influence of the SiO2 crystalline polymorph and the influence of the nature of the
irradiating particle to the point defect model.
  Defect accumulation during room temperature N+ irradiation of silicon
Titov AI, Carter G
119: (4) 491-500 DEC 1996

Document type: Article
Language: English
Cited References: 58
Times Cited: 1

The accumulation of disorder in Si crystals implanted with 40 keV N+ ions at room temperature to fluences between 1X10(14) ions cm(-2)
and 2X10(15) ions cm(-2) and with ion fluxes between 5X10(11) ions cm(-2)s(-1) and 5X10(13) ions cm(-2)s(-1), was measured using low
angle Rutherford Backscattering Channelling techniques. It was found that fractional disorder-fluence functions were sigmoidal and four
disordering regimes were identified. At very low disorder fractions the disorder is ion flux independent but at slightly higher disorder fractions
the disordering rate is a function of ion flux. For still higher disorder fractions the flux density dependence of disordering rate is again absent
and disorder finally saturates at complete layer amorphisation. A set of models involving intra-cascade or bombardment induced defect - native
defect interaction processes, converting to intercascade process and finally enhanced direct impact amorphisation of defected crystal is
developed and shown to explain the observed results.
  Modeling fast heavy ion induced amorphization in pure metals
Ossi PM, Pastorelli R
101: (3) 415-423 NOV 1996

Document type: Article
Language: English
Cited References: 29
Times Cited: 0

Bombardment of metallic targets with heavy, GeV energy ions results in electronic excitation beyond a threshold value of stopping power. Due
to the spatial homogeneity of ion tracks, the projectile homogeneously deposits its energy onto all atoms within a cylinder. The radius of such
an ionization cylinder is calculated, thereafter the amount of energy transferred to each single atom in the cylinder is evaluated: n ionization
events per atoms occur, which change the atomic configuration from (Z) to (Z-n). Ionized atoms are ejected out of the ionization cylinder, and
they interact with matrix atoms inside a damage cylinder. Locally a starting compound [(Z) (Z-n)] is formed. Segregation at the matrix-damage
cylinder interface of one component of the starting compound gives rise to a non-equilibrium compositional profile. Relaxation to metastable
equilibrium of the associated non-equilibrium electronic density is simulated via charge transfer reactions, each of which involves a matrix atom
and an ionized atom. The reaction product is a dimer, considered a nucleus of an effective compound. The energy cost to introduce in the
matrix an effective compound dimer is calculated, together with the surface properties of starting and effective compounds. Qualitative
differences are found between compounds which form in metals amorphized or, respectively crystallized under fast heavy ion irradiation.
Structural characteristic of irradiated and unirradiated ices
Leto G, Palumbo ME, Strazzulla G
116: (1-4) 49-52 AUG 1996

Document type: Article
Language: English
Cited References: 25
Times Cited: 2

We have studied experimentally the structure of H2O and H2O:CO ices, as well as structural changes induced by ion irradiation, using
infrared (IR) spectroscopy. In fact, IR band profiles (shape, width and peak position) are sensitive to the structure of ices. Results show that
the structure of water ice, formed by vapor deposition onto a given substratum, depends both on the temperature (in the range 10-160 K) at
which ice films are formed and on thermal and/or irradiation history. CO molecules can occupy two different sites in a water matrix and give
rise to an asymmetric IR absorption band profile. A detailed study of this latter, as a function of thermal and/or irradiation history, gives
relevant information on che structure of the ice. Ice mixtures have been prepared both by codeposition of H2O and CO and by diffusion of
CO into a preformed water-ice film. These studies are important to understand physical and chemical characteristics of ices even in
astrophysical environments. In this latter case information on the history of molecular ices are obtained comparing observed spectra with those
taken in the laboratory.
  Ion damage buildup and amorphization processes in GaAs-AlxGa1-xAs multilayers
Tan HH, Jagadish C, Williams JS, Zou J, Cockayne DJH
80: (5) 2691-2701 SEP 1 1996

Document type: Article
Language: English
Cited References: 36
Times Cited: 1

The nature of ion damage buildup and amorphization in GaAs-AlxGa1-xAs multilayers at liquid-nitrogen temperature is investigated for a
variety of compositions and structures using Rutherford backscattering-channeling and cross-sectional transmission electron microscopy
techniques. In this multilayer system, damage accumulates preferentially in the GaAs layers; however, the presence of AlGaAs enhances the
dynamic annealing process in adjacent GaAs regions and thus amorphization is retarded close to the GaAs-AlGaAs interfaces even when
such regions suffer maximum collisional displacements. This dynamic annealing in AlGaAs and at GaAs-AlGaAs interfaces is more efficient
with increasing Al content; however, the dynamic annealing process is not perfect and an amorphous phase may be formed at the interface
above a critical defect level or ion dose. Once an amorphous phase is nucleated, amorphization proceeds rapidly into the adjacent AlGaAs.
This is explained in terms of the interplay between defect migration and defect trapping at an amorphous-crystalline or GaAs-AlGaAs interface.
In addition, enhanced recrystallization of the amorphous GaAs at the interface may occur during heating if an amorphous phase is not formed
in the adjacent AlGaAs layer. This is most likely the result of mobile defects injected from the AlGaAs layer during heating. (C) 1996
American Institute of Physics.
  Temperature dependent microstructural modification in ion-irradiated Tl-type high temperature superconductors
Newcomer PP, Barbour JC, Wang LM, Venturini EL, Kwak JF, Ewing RC, Miller ML, Morosin B
267: (3-4) 243-253 AUG 20 1996

Document type: Article
Language: English
Cited References: 24
Times Cited: 2

Ion irradiation damage creation and recovery were examined in TI-based high temperature superconductors, HTSC, using TEM, resistivity,
and magnetic measurements for irradiation temperatures of 20 to 650 K. During 1.5 MeV Kr+ and Xe+ ion irradiations of single-crystal
Tl-1212 and Tl-2212 Tl-Ba-Ca-Cu-O HTSC, microstructural modification was observed in situ by electron diffraction and shows a remarkable
temperature dependence. At selected sample temperatures, irradiations continued until a critical fluence, D-c, was reached where the original
structure disappeared, The temperature dependence of D-c shows a minimum near the superconducting transition temperature, T-c, and is
correlated with the temperature dependence of the thermal conductivity, which has a maximum near T-c. At an irradiation temperature near
this maximum in thermal conductivity, a minimum amount of damage recovery occurs because heat can be dissipated away from the
displacement cascade, Ion irradiation suppresses the T-c. The rate of decrease in the T-c as a function of damage (measured in
displacements per atom, dpa) was found to be the same for various incident ions (He+, O2+, Au5+ which shows that the damage
accumulation is a result of atomic collisions. Further, the rate of decrease in T-c was found to be the same for both transport and
magnetization measurements, indicating that the displacements effect the bulk of the samples through point defect creation, An activation
energy of 0.4 eV for ion irradiation damage recovery over the temperature range from 100 to 650 K was determined from normal. state
resistance versus time immediately after irradiation.
  Local structural modification in ion damaged InGaAs
Yu KM, Hsu L
69: (6) 824-826 AUG 5 1996

Document type: Article
Language: English
Cited References: 26
Times Cited: 1

The indium nearest-neighbor environment in InGaAs thin films damaged by Ar ion implantation has been studied by extended x-ray absorption
fine structure spectroscopy. We find that before the material turns amorphous by Ar irradiation, the In-As nearest-neighbor distance remains
close to its crystalline value even when the layer is heavily damaged but not entirely amorphous. Once the Ar dose exceeds the threshold for
amorphizing the InGaAs layer, the In-As bond distance relaxes to that of pure crystalline InAs. This sudden change in local structure as the
material transforms from crystalline to amorphous suggests that the transition is due to simultaneous amorphous nucleation rather than the
accumulation and overlapping of isolated amorphous regions. Moreover, this change in the local structure in a ternary alloy can be used as a
criterion for determining the crystalline-to-amorphous transition of the alloy. (C) 1996 American Institute of Physics.
  The effects of flux, fluence and temperature on amorphization in ion implanted semiconductors
Carter G
79: (11) 8285-8289 JUN 1 1996

Document type: Article
Language: English
Cited References: 25
Times Cited: 2

The Avrami-Johnson-Mehl [P. C. Shewmon, Transformation in Metals (McGraw Hill, New York (1969)] approach to phase transformations
is extended to include finite area amorphous zone nucleation or generation by ion impact onto semiconductors together with bombardment
induced zone expansion and thermal annealing zone contraction (recrystallization) in order to predict the variation of amorphized material
fraction with ion fluence. The ion flux and substrate temperature are found to play decisive roles in determining this variation. The model
predictions are shown to agree qualitatively with currently available experimental data with respect to the system variables of ion flux, fluence,
and substrate temperature. (C) 1996 American Institute of Physics.