Porosity as a function of stoichiometry and implantation temperature in Ge/Si1−xGex alloys

Abstract

The development of porosity in single-crystal germanium and silicon-germanium alloys (c-Si1−x Gex) of (100) orientation was studied under bombardment with 140 keV Ge− ions over a wide range of temperatures (−180 to 400 °C) and ion fluences up to 1 × 1018 ions/cm2. The surface swelling and morphology were investigated using multi-characterization techniques including optical profilometry, transmission electron microscopy, and scanning electron microscopy. The initiation of porosity and the evolution of the near-surface microstructure strongly depend on the ion fluence, the irradiation temperature, and the stoichiometry of the substrate. Significant results and new findings include: (i) the fact that, over the entire temperature and stoichiometry range, porosity is only developed once the substrate is rendered amorphous; (ii) with increasing Si content in the alloy, the onset of porosity is pushed to higher fluences; (iii) porosity is observed for Si contents in the alloy up to 23% but not higher under the irradiation conditions used; and (iv) in all cases the initiation of porosity was observed to occur at the surface of the amorphous alloy above a threshold fluence. This last result strongly suggests that the mechanism for initiation of porosity is via preferential vacancy segregation and clustering at the surface of the amorphous alloy. Particularly at elevated temperatures, preferential sputtering of the Si-Ge atomic species in the alloy also plays an important role in developing the surface topography and porosity in alloys. Such effects are discussed along with the implications of our results for mechanisms of porosity in Ge and its alloys.

The influence of capping layers on pore formation in Ge during ion implantation

“…In this case, the top 2/3rd of the a-Ge layer contains large columnar pores that intersect the surface, consistent with typical porosity microstructure in irradiated Ge. 14,15 In contrast, capping the surface with a SiO 2 layer of 20 nm thickness totally eliminated pore formation at LN 2 T at the same Sn fluence as shown in Fig. 1(c).…”

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

“…25. Indeed, it is demonstrated 20 that vacancies preferentially form clusters and voids at the a-Ge surface during ion irradiation leading to pores that intersect the surface and develop with increasing fluence into a porous structure like that in Fig. 4(d).…”

“…It was shown, however, that achieving Sn concentrations in Ge higher than about 6 at:% through ion implantation is severely hindered by a high sputtering effect in Ge and the onset of ionimplantation induced porosity once the Ge is rendered amorphous (a-Ge). 18 It has previously been found that irradiation-induced porosity is favoured in the range of implant temperatures between $ À 80 C and $200 C. [19][20][21] The onset of porosity can be suppressed by undertaking implants outside of this unfavourable temperature window. At elevated temperatures above 200 C, the Ge substrate remains crystalline due to the recombination of mobile vacancies and interstitials.…”

“…Nevertheless, Ge porosity studies so far have focused on the physical mechanism or the implant conditions under which porosity occurs. 20,21 Little is known about possible ways to prevent or delay the onset of porosity, which is the main focus of the current study. Fig.…”

“…At implant temperatures lower than À80 C, porosity can be suppressed or pushed to a higher fluence offset due to inefficient migration and clustering of vacancies in a-Ge. 6,19,20 In the case of liquid nitrogen (LN 2 ) temperature implantation of Ge in Ge, for example, porosity does not occur up to fluences exceeding 1:0 Â 10 17 ion cm À2 . However, for LN 2 implantation of heavier elements such as Sb or Sn, porosity occurs above about 7 Â 10 15 ion cm À2 and 2:0 Â 10 16 ion cm À2 , respectively, thus limiting obtainable impurity concentrations in Ge.…”