SiGe is a very interesting material because of its technological applications. Heterojunctions formed with SiGe have many benefits over conventional homojunction approaches. One of the advantages is that the bandgap can be altered by changing the proportion of the Ge content and strain caused by the lattice mismatch [67]. SiGe is also faster than Si in terms of carrier mobility [2]. Development in epitaxial growth methods, such as ultra-high vacuum chemical vapour deposition (UHV/CVD),
Figure 10: A top view of one cascade event. Initial and final positions of atoms that have moved more than 0.5 ˚A are joined. Strain relief seems to favour the 110 direction and this results in a four-leaf clover pattern on the surface in case of symmetrical amorphous zone.
has enabled rapid production of highly uniform SiGe structures [68]. It has been found that ion implantation of strained SiGe can cause strain relaxation [69]. Equally, as the ion bombardment can change the strain in the sample, the strain can affect the outcome of a collision cascade.
We decided to approach this problem with a case study of 5 keV Xe bombardment of strained Ge [VI]. Thick strained Ge layers cannot be formed experimentally. However, sometimes a non-reality based case can be a more efficient way to start, than a study that directly reflects reality, as the effects of interest can be more distinct in them. This time the (modified) Stilliger-Weber potential [9] for Ge–Ge interactions was used, as the liquid material had a large role in this problem.
We found that large adatom islands are produced on top of amorphous zones. We also found that lattice atoms around the molten zone move radially inwards and thus cause strain relief in the sample (see Fig. 10). The strain was increased perpendicular to the surface, which has also been found experimentally [69]. The mechanism for adatom island creation is as follows: During the collision cascade, a liquid volume is first formed. As the atomic volume of the liquid atoms is somewhat smaller than the atomic volumes in the surrounding lattice, the lattice atoms relax radially inwards to the soft liquid core, causing the pressure in the liquid to rise. As the liquid cools, the transition to the amorphous phase further increases the pressure owing to its lower density, causing formation of an
adatom island on the surface. At the same time atoms below the amorphous zone are pushed deeper into the bulk and thus create strain in the z -direction.
Clearly none of the previously mentioned four surface damage mechanisms can explain the observed phenomenon. The formation of adatom islands by nucleation of single adatoms and small adatom clusters has been known for a long time, but in this case a large single adatom cluster is formed by collective movement of the atoms to the surface. Thus, this phenomenon can be classified as the fifth surface damage creation mechanism.
Preliminary tests show that the effect is also present in SiGe, although it is a lot weaker than in Ge.
7 CONCLUSIONS
It has been shown that the MD method is a very versatile tool for examining ion implantation caused effects in covalently bonded materials. The method has been used for determining energy depen-dences in defect production, defect migration in conjunction with experiments, mixing, chemical reactions and strain effect on collision cascades.
The results from MD simulations may not be considered, mainly because of the limitations of the potential models, to be quite correct at the quantitative level. However, the results demonstrate that the MD method can be used powerfully to look for, for example, energy dependences or damage creation mechanisms.
We have found that the vacancy production at low energies in Si is a superlinear function of energy.
It has also been found that the threshold temperature for interstitial migration is Si is between 130 and 180 K. We have shown that there are definite differences in damage creation in GaAs and Ge.
The decrease of carbon erosion at high hydrogen bombardment fluxes was found to originate from hydrogen buildup at the carbon surface. We have also found a new damage creation mechanism in strained Ge which cannot be attribute to ballistic effects.
ACKNOWLEDGEMENTS
This study was carried out at the Accelerator Laboratory between the years 1996 and 2000. I wish to thank the former and current heads of the laboratory, Doc. Armas Fontell, Doc. Jyrki R ¨ais¨anen and Doc. Eero Rauhala for placing the facilities of the laboratory at my disposal.
I am grateful for my supervisor, Prof. Juhani Keinonen, the head of the Department of Physics, for the guidance and advice during this work.
My special thanks are due to Prof. Kai Nordlund, who has been the driving force for me to get this work done. I thank both Kai and Doc. Antti Kuronen for demystifying the wonders of molecular dynamics for me. I also wish to thank all my co-authors and colleagues at the Accelerator Laboratory for the informative discussions and refreshing moments we have had.
My warmest thanks are due to my wife, Tiina, who has given all the love and support during these years.
Financial support from the Academy of Finland and Magnus Ehrnrooth foundation is gratefully ac-knowledged.
Helsinki, May 2000 Jura Tarus
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