Process-Induced Defects in Germanium

The fabrication of devices in a germanium substrate relies on processing techniques that can be quite aggressive to the material. Chemical etching can introduce metallic and other impurities in the near-surface layer, which diffuse deeper in the bulk during a subsequent heat treatment, thereby affecting the electrical (lifetime and resistivity) properties. Dry etching, on the other hand, results in the creation of radiation damage, as energetic ions are used to sputter/remove locally the Ge atoms from the surface. While in the early days doping profiles were tailored by solid- or gas-source diffusion of Group III and V impurities, the industrial standard is now ion implantation, whereby the depth of the profile (junction) is selected through the energy of the ions, while the dose settles the sheet resistance. However, ion implantation creates radiation damage by displacing Ge atoms from their lattice site. At sufficiently high doses, this results in a complete amorphization of the implanted layer. Therefore, to cure the damage and activate the dopants, that is, to move them on a substitutional lattice site, a thermal anneal is required. For shallow junction formation, the thermal budget of this anneal should be well controlled, to avoid excessive diffusion of the dopant atoms. Generally, there exists a trade-off between damage removal (low leakage current), on the one hand, and maintaining a shallow doping profile on the other. The traditional furnace annealing (FA), is nowadays replaced by the so-called rapid thermal annealing (RTA) and even spike annealing, in the case of silicon CMOS. At the moment, extensive research is being performed on more advanced activation techniques, combining preamorphization of the substrate with solid-phase epitaxial regrowth (SPER) of the implanted layer. The presence of an amorphized layer also opens the door for the implementation of intense laser annealing. At the same time, flash lamp annealing enables to heat the sample on a very short time scale (millisecond) even compared with the spike annealing achievable in standard RTA equipment.

In this chapter, processing-induced defect formation and removal will be discussed, with particular emphasis on ion implantation damage. In the first paragraph, the fundamental ion-implantation damage mechanisms will be discussed in terms of the collision cascade (CC) theory. It will be shown that for high energy density cascades, a deviation occurs which is related to a collective movement of atoms, most likely due to the occurrence of so-called thermal spikes. It will also be shown that MeV self-ion implantation helps to reveal the nature of the damage nucleation — homogeneous (Si) or heterogeneous (Ge). As will be shown for Ge, for high-dose heavy-ion implantations, there exists a damage phase beyond complete amorphization, whereby voids or pores are being formed. In a third paragraph, high-temperature annealing mechanisms will be discussed. Next, ion implantation of the traditional Group III and V dopants is highlighted. As we see in Sect. 5.5, there exists also some interest in implanting oxygen and nitrogen in Ge for certain applications. Currently, also the implantation of low-energy, high-dose hydrogen in Ge is of technological relevance, in the frame of the smart-cut fabrication of Germanium-on-Insulator (GeOI) substrates (Sect. 5.6). Finally, the defect formation during other processing steps will be briefly summarized.