7.1 SCOPE OF THE CHAPTER

Electronic devices based on Si/Ge alloys are already important for high frequency applications [1] and infrared detectors [2]. Heterojunction bipolar transistors (HBTs) which were first reported in 1988 [3], have demonstrated operating frequencies significantly greater than 100 GHz. Moreover, the electronic properties of Si/Ge with respect to and in combination with silicon offer significant advantages for integrated circuit technology [4, 5]. Some examples of the potential of Si/Ge in CMOS technology are displayed in FIGURE 7.1. In fact, the combination of HBT and CMOS will provide the potential for monolithic silicon-based RF systems [1].

Figure 7.1: Schematic MOSFETs with the grey regions showing where the Si/Ge alloys can be present to enhance the device performance: (a) compressively strained channel, (b) relaxed virtual substrate for tensile-strained Si atop, (c) tuneable workfunction gate, and (d) low-resistivity source/drain extension. Reprinted with permission from Reference 5, S.-L. Zhang, Microelectron. Engng. (UK) vol.70 (2003) p. 174 85.

The development of ohmic or rectifying contacts to these semiconducting Si/Ge layers is a critical step in the development of the device technologies. Understanding the formation and electronic states of contacts to Si/Ge semiconducting layers begins with the clean surfaces. The formation and stability of the contacts will depend on the interface chemistry, and for Si/Ge, the system shows significant complexity in comparison to silicide contacts to silicon surfaces. Finally, the device operation will depend on the electrical properties of the interface. This chapter presents summaries...