Formation of Excess Donors During High-Dose 74 Ge + Ion Implantation
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74
Ge+ ION
Z. Xia*, E. Ristolainen**, R. Elliman***, H. Ronkainen*, S. Erdnen*, P. Kuivalainen*, M. Sopanen**, T. Tuomi**, and P. Holloway**** *VTT Electronics, P.O. Box 11012, FIN-02044 VTT, Finland **Helsinki University of Technology, FIN-02150 Espoo, Finland *Australian National University, Canberra, ACT 0200, Australia ****University of Florida, Gainesville, FL 32601, USA
ABSTRACT Recently observations that high-dose Ge implantations into Si substrates caused the n-type carrier concentration to increase were attributed to residual structural defects after activation annealing [7,12]. However, co-implantation of an n-type impurity is another possibility. The origin of this excess donor concentration has been studied in this work. The possibilities of residual defects versus implantation of impurities have been investigated using two different implanters and materials analysis. Comparison of data from different implanters showed that the concentration of excess donors was sensitive to the implanter configuration. Furthermore, transmission electron microscopy (TEM), Rutherford backscattering channeling (RBS-C), and spreading resistance profiling (SRP) data showed that the excess donor effect was related to impurities rather than residual defects. Secondar-ion mass spectroscopy (SIMS) and SI measurements confirmed that impurities such as 7 5 As ions were present after implants. This impurity easily explains the excess donor concentration when 7 4 Ge implants are performed into silicon wafers doped with phosphorous. INTRODUCTION In recent years, strained epitaxial SilxGex/Si heterojunction bipolar transistors (HBTs) have attracted much interest due to their excellent high frequency and high current capabilities. In most cases, high performance SilxGex/Si HBTs have been fabricated using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) [1,2]. Alternatively, SilixGex alloy can be formed by using Ge ion implantation into Si and subsequent solid phase epitaxial regrowth [3-9]. The main advantage of this implantation technique is that it is fully compatible with standard silicon VLSI processing, and convenient for selective area growth. However, for implanters used for ion implantations of B+, As+, and P+ into Si, one must consider the possibility of impurities when extracting a high intensity Ge+ ion current from a conventional hot-filament type ion sources. Since germanium has five isotopes, 70, 72, 73, 74, and 76, with natural abundances of 20.5%, 27.4%, 7.7%, 36.7%, and 7.7%, respectively, the most abundant isotope 74 may be preferred to achieve a larger beam current. At present, germanium hydrides and halides (GeH 4 , GeCI4 , and GeF 4 ), are commonly used as implanter source materials, as well as alternative solid compounds such as GeS for microwave ion sources [10]. Of these sources, the gaseous GeH 4 is not recommended because of two major drawbacks: (i) monogermane reacts with oxygen to form solid Ge0 2 deposits capable of blocking the ion source gas entrance and reducing the beam intensity, an
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