Density Reduction: A Mechanism for Amorphization at High Ion Doses
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ABSTRACT At cryogenic temperatures, the accumulation of vacancy-interstitial pairs in A120 3 from atomic displacements associated with ion implantation produces amorphization. At room temperature, these pairs recombine, and amorphization occurs only at high doses. Xray reflectivity measurements show that amorphization of the surface of A120 3 implanted at room temperature with 160 keV Cr+ ions is preceded by a progressive reduction in nearsurface density. Monte Carlo simulations show that this density reduction can be accounted for by high-energy-transfer collisions which knock atoms deep into the target, leaving widely separated vacancies and interstitials, which do not recombine. Electron microscopy shows that at least some of these vacancies condense into voids. We propose that this reduction in near-surface density can lead to amorphization at high doses. We present simple approximations for the density reduction expected for different ions and targets.
INTRODUCTION The mechanisms by which ion implantation causes amorphization can be broadly categorized as structural and chemical. Chemical effects occur in the end-of-range (eor), where implanted ions come to rest, changing the chemical composition of the target. Structural effects are dominant in the midrange: as the i6ns pass through they displace target atoms from lattice sites. At relatively low temperatures, midrange amorphization typically requires a dose producing on the order of one displacement per target atom (dpa), while at higher temperatures structural damage is annealed out, and amorphization does not occur. Room temperature amorphization occurs at much higher ion doses for A120 3, ZrO2 , MgO, and WC. While a dose -1 dpa produces amorphization at cryogenic temperatures, -100 dpa is required at room temperature."'5 8 The mechanism for this high-dose amorphization has been unclear. We address the origin of this anomalously high-dose amorphization by examining the density and microstructure of A120 3 films implanted with ion fluences below the amorphization threshold. Implantation with Cr÷ was chosen to minimize chemical effects: it has been shown9 that elements such as Cr which occupy substitutional sites induce amorphization at a higher dose. Room temperature amorphization of A12 0 3 occurs at a dose between 1 x 1021 and 6 x 1021 Cr+/m 2 [Ref. 6]; we examine samples implanted with 4 x 1020 and 1 x 1021 Cr÷/cm 2. We summarize x-ray reflectivity, electron microscopy, and Monte Carlo simulation results1" which show that amorphization is preceded by a progressive reduction in near399 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
surface density. We derive approximate formulae for the expected density change as a function of the choice of target and incident ion.
X-RAY REFLECTIVITY X rays with wavelength k are totally reflected by a material with density p, average atomic number Z, and average atomic weight M below a critical incident angle 0, = (ApZV/M)"1 , where A = 2.70 x 10"° in." Because x rays penetrate only -5 nm under
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