210th ECS Meeting , Abstract #1003, copyright ECS

Understanding Ion Implantation Defects in Germanium

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43 M eV Si ionsΦ = 1x1010 cm -2

Ge0.9992Si0.0008:P U b= -5.0 VU p= 0 Vtp= 1 m s

en = 80 s -1

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ΔC, p

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Tem perature, K

A. R. Peaker, V. P. Markevich, I. Kovačević

University of Manchester, School of Electrical & Electronic Engineering, Sackville Street Building,

Manchester, M60 1QD, UK

Germanium is being seriously considered by many groups as a future channel material for PMOS. Pure germanium has one of the highest hole mobilities of any semiconductor and the superficial similarity to silicon processing expected from another group IV material is an important benefit compared to III-Vs. Techniques for localized deposition of germanium or germanium rich silicon-germanium are advancing rapidly, enabling strained silicon to be used for NMOS and unstrained or strained germanium for PMOS, enabling an ultrahigh mobility balanced CMOS circuit to be fabricated. However a major problem with germanium is the issue of ion implantation, because germanium bipolar transistors were phased out before ion implantation became a commonly used tool in the industry, there is remarkably little fundamental science that has been done on ion implantation in germanium and early attempts by the industry have not surprisingly revealed severe difficulties in terms of maximum achievable doping levels, activation of n type dopants and the removal of defects [1]. In this paper we present some results from which we attempt to gain a fundamental understanding of the defect reactions which occur in germanium during implantation and subsequent thermal processing. Ion implantation into any semiconductor is notoriously difficult to study systematically because of the inhomogeneous nature of the damage that is produced. Even the simplest case of self-implantation produces a region near the surface that is vacancy rich and a region beyond the peak of the implant that is interstitial rich. In silicon fundamental understanding of the implantation process was achieved by careful study of electron and gamma ray damage and we have followed a similar procedure in germanium. Our recent work on this topic combined with that from other groups now gives us a very secure base for moving on to ion implantation. Figure 1, summarizes the electronic levels in the gap produced by electron irradiation in Ge crystals doped with different impurities [2,3]. Although there are quite dramatic differences in the spectra of damage produced according to the oxygen content of the germanium it is evident that all electron damage can be removed easily from germanium with a very low thermal budget.

Fig. 2. DLTS spectra of electron emission from P-doped Ge0.9992Si0.0008 after (1) implantation with 3 MeV Si ions and a subsequent 30-min anneal at temperatures: (2) 100°C, (3) 200°C, (4) 300°C, and (5) 400°C.

However, just as in the silicon case there are quite dramatic differences between electron damage and ion implantation damage. The high generation rate of primary defects in the immediate vicinity of the track of the implanted particle enables defect reactions to occur during implantation that result in the formation of more stable complexes. Phenomenologicaly we see that defect removal in implanted germanium is very much more difficult than in the electron irradiation case. Figure 2 shows thermal annealing of defects resulting from a very low dose silicon implant into germanium rich SiGe. With the maximum thermal budget shown a large concentration of defects remain leading to a significant leakage current, yet with this budget the diffusion of both p and n type dopants has become important for sub-45nm transistors. Studying the kinetics of these ion implantation defects by techniques, which average over a substantial volume, is virtually impossible and so because of the difficulties of doing fundamental work on this extremely inhomogeneous distribution of defects, we have examined neutron irradiated Ge crystals. Neutrons produce cluster damage somewhat similar to ions but the defect population is much more uniformly distributed through the measurement volume of the material and is essentially uniform apart from the near surface region which acts as a powerful sink for primary defects. In this paper we analyze these results in the context that diffusion of dopants in germanium is vacancy mediated and that vacancies play the key role in removing carriers from n type material, hence establishing the n type doping ceiling.

Acknowledgements We would like to thank Umicore for material used in this study. EPSRC and INTAS for their financial support of this work. We also thank the UK Ion Beam Centre at Surrey University for the ion implantations.

References 1. A. Satta, M. Meuris, T. Janssens, T. Clarysse, E. Simoen, C. Demeurisse, B. Brijs, I. Hoflijk, and W. Vandervorst Electrochem Soc Proc 2005-05, 468 (2005) 2. J.Fage-Pedersen, A. Nylandsted Larsen and A. Mesli Phys. Rev. B, 62, 10116 (2000) Fig. 1. Electronic levels in the gap produced by

electron irradiation of germanium. The figures at thebottom indicate the temperature at which the defectanneals out to half the original concentration.

3. V.P. Markevich, I.D Hawkins, A.R. Peaker, K.V. Emtsev, V.V. Emtsev, V.V. Litvinov, L.I. Murin and L. Dobaczewski, Phys Rev B, 70, 235213 (2004)

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