Erik Johnson

Structure, morphology and melting hysteresis of ion-implanted nanocrystals

Abstract Investigations of nanosized metal and semimetal inclusions produced by ion implantation in aluminium are reviewed. The inclusions are from 1 nm to 15 nm in size and contain from 80 to 100 000 atoms. Embedded crystallites, which are topotactically aligned with the surrounding matrix, may not be produced in this size range by any other method. The inclusions offer unique possibilities for study of the influence of interfaces on the crystal structure of the inclusions as well as on their melting and solidification behaviour. Studies are made with transmission electron microscopy (TEM), electron- and X-ray diffraction and in situ RBS-channeling measurements. Bi, Cd, In, Pb and Tl inclusions all show a substantial melting/solidification temperature hysteresis, which, in all cases except for Bi, is placed around the bulk melting temperature, while bismuth melts below that temperature.

NANOSIZED LEAD INCLUSIONS IN SILICON PRODUCED BY ION IMPLANTATION

Abstract Silicon single crystals have been implanted with 80 keV lead ions to a concentration of about 3 at.%. TEM and RBS/channeling analysis showed that implantations at 300 and 625 K induced amorphization of the implanted layer with nanoscale lead inclusions embedded in the amorphous matrix. At 300 K the lead inclusions had sizes around 1–2 nm and showed no clear signs of crystallinity. At 625 K the inclusions with size around 5 nm had a distinct fcc structure and were oriented randomly in the amorphous matrix. Implantations at 925 K retained the crystalline nature of the matrix and the implanted layer contained faceted fcc lead inclusions about 10 nm in size growing in parallel-cube alignment with the matrix. Annealing of the samples implanted at 300 and 625 K, respectively, for one hour at 1175 K led to recrystallization of the amorphous matrix with simultaneous loss of nearly all the implanted lead, while about 80% of the implanted lead was retained after annealing of samples implanted at 925 K.

TEM and RBS/channelling of nanosized bicrystalline (Pb,Cd) inclusions in Al made by sequential ion implantation

Abstract Sequential ion implantation of Pb and Cd in Al at 425 K and 475 K respectively has been used to produce dense distributions of nanosized (Pb,Cd) inclusions with equiatomic composition. Pb and Cd form a simple eutectic system, but both elements are insoluble in solid Al, and the inclusions are Cd-rich in comparison with the eutectic composition. Inclusions in the size range from 1 to 20 nm were observed in as-implanted samples. Their overall shape was nearly cuboctahedral. Most of the inclusions were bicrystalline with an fcc Pb part forming a segment of a cuboctahedron and an hcp Cd slab attached to one of the {111}Pb facets. The orientation relationship had close-packed planes and directions parallel in the three structures. In situ melting/solidification experiments combining TEM and RBS/channelling showed that melting of the inclusion ensemble occurs within a narrow temperature interval of 10–15 K around the eutectic temperature of 521 K. Solidification takes place with undercoolings of about 50–65 K below the liquidus line in a two-stage process where Cd solidifies 15 to 20 K before Pb.

Nanosized f.c.c. thallium inclusions in aluminium

Abstract Ion implantation of pure aluminium with thallium induces the formation of nanosized crystalline inclusions of thallium with a f.c.c. structure. The size of the inclusions depends on the implantation conditions and subsequent annealing treatments, and is typically in the range from 1 to 10 nm. The inclusions are aligned topotactically with the aluminium matrix with a cube-cube orientation relationship, and they have a truncated octahedral shape bounded by {111} and (001) planes. The lattice parameter of the f.c.c. thallium inclusions is 0.484±0.002 nm, which is slightly but significantly larger than in the high-pressure f.c.c. thallium phase known to be stable above 3.8 GPa.

Lattice location of erbium in high-fluence implanted silicon–germanium: Backscattering/channeling study

High-quality crystalline Si0.75Ge0.25 alloy crystals were implanted with 70 keV Er+ ions at 550u200a°C to a fluence of 1019u200am−2. In situ Rutherford backscattering/channeling spectrometry with a 500 keV He2+ beam revealed Er atoms located on regular lattice sites of the host matrix. Angular scans taken around the 〈100〉, 〈110〉, and 〈111〉 crystallographic axes showed that a considerable fraction of Er atoms occupy tetrahedral interstitial sites.

Solute-defect interactions in a metastable Pb–Ni alloy formed by high-fluence ion implantation

Implantation of 240 keV Pb+ ions into a Ni (110) single crystal to a fluence of 1016u2009cm−2 at room temperature and 470 K, respectively, resulted in the formation of a metastable supersaturated Pb–Ni solid solution with a maximum lead concentration of 2.4 at.u200a%. Rutherford backscattering/channeling analysis and transmission electron microscopy have shown that in the as-implanted state most of the Pb atoms were distributed on substitutional lattice sites in the host matrix while a small fraction of Pb was confined within nanoscale precipitates. Most of the precipitates, with sizes ranging from 2 to 15 nm, were single crystalline although bi-, tri-, and tetracrystals were occasionally observed. Upon heating, decomposition of the metastable alloy was observed, with strong outdiffusion of a large fraction of Pb to the surface. By means of angular scan channeling analysis, the lattice location of the implanted Pb atoms was followed directly during in situ isochronal annealing at different temperatures up to 860 ...

High-fluence ion implantation of In into Al crystals: Formation and evolution of buried layers

Abstract Al(110) single-crystal samples were implanted at T = 200°C with In+ ions of 250 keV energy with fluences ranging from 4×1020 to 3×1021 m−2. The implantation resulted in the formation of In precipitates growing with fluence in topotactical alignment with the host matrix. High-fluence implantation was used in an attempt to produce single-crystal buried layers of In embedded into Al. Rutherford back-scattering (RBS)-channelling analysis of the implanted samples and transmission electron microscopy studies of the incorporated In layer morphology were carried out. With increasing fluence the peak In concentration was observed to increase gradually until a maximum value of about 38 at.% was reached at an implantation dose of 1·5×1021 m−2. The buried layer thus formed was found to have fragmentary morphology consisting of large In precipitates. During further implantation the peak concentration decreased drastically to reach a steady state. In order to study a possible effect of sputtering on the In pro...

Channeling study of melting and solidification of lead nanocrystals in aluminium

Abstract Nanometer-sized Pb crystallites were obtained in Al single crystals by 423 K implantation of 150 keV Pb+ ions with a fluence of 2.3 × 1016 cm−2. The crystallites grow in perfect topotactical alignment with the matrix with a cube/cube orientation relationship. Lead depth profiles were obtained using the Rutherford backscattering (RBS) technique. With the RBS and channeling analysis two major distributions of nanocrystals were observed. The first one with the average size of crystallites of about 13 nm is located within depth region 30–65 nm, and the second distribution (70–105 nm) has the average size of crystallites of 9.9 nm. Measurements of melting/solidification of Pb nanocrystals were performed with the channeling technique. A thermal hysteresis for crystallites as well as for channeling in Al matrix was observed. The nanocrystals show large superheating (∼ 75 K above the bulk melting point of Pb) as well as supercooling (∼ 35 K) during the heating cycle. The size dependence of melting of the crystallites is deduced from the measurements using Monte Carlo channeling simulations. These results are compared with those obtained by transmission electron microscopy (TEM). They are discussed in a phenomenological context considering the lack of free surfaces and a reduction of thermal vibrations for surface atoms in topotactical nanocrystals.

Channeling and TEM investigation of nanosized thallium inclusions in aluminium

Abstract Metastable and stable nanosized crystalline inclusions of thallium and lead have been produced in an aluminium matrix by implantation of the respective ions into pure aluminium single crystals. The implanted samples were analyzed by in-situ Rutherford backscattering/channeling spectrometry (RBS) and transmission electron microscopy (TEM). Samples implanted with lead have microstructures consisting of dense distributions of nanosized lead inclusions with fcc structure growing in topotactical alignment with the matrix in a cube/cube orientation relationship. Single crystals implanted with lead therefore show channeling in the lead inclusions in step with channeling in the aluminium matrix both in the 〈110〉 and the 〈111〉 directions. Channeling in the lead inclusions is most significant for samples implanted at higher temperatures where the inclusions are the largest. Conversely, channeling in thallium inclusions from samples implanted above 525 K is only moderate in the 〈110〉 direction and nearly absent in the 〈111〉 direction. The thallium inclusions formed in samples implanted at temperatures up to around 450 K are a few nm in size and have a metastable fcc structure determined by the aluminium matrix. Their channeling properties are comparable to that of the lead inclusions. However, at higher implantation temperatures the size of the inclusions grow markedly, and it is no longer possible to sustain the metastable fcc structure. Instead the inclusions have bcc structure with a variety of orientational variants responsible for the changes in the channeling properties.

Finite size melting of spherical solid–liquid aluminium interfaces

We have investigated the melting of nano-sized cone shaped aluminium needles coated with amorphous carbon using transmission electron microscopy. The interface between solid and liquid aluminium was found to have spherical topology. For needles with fixed apex angle, the depressed melting temperature of this spherical interface, with radius R, was found to scale linearly with the inverse radius 1/R. However, by varying the apex angle of the needles we show that the proportionality constant between the depressed melting temperature and the inverse radius changes significantly. This led us to the conclusion that the depressed melting temperature is not controlled solely by the inverse radius 1/R. Instead, we found a direct relation between the depressed melting temperature and the ratio between the solid–liquid interface area and the molten volume.