Structural and Electrical Properties of In and C + In implanted Ge, Si, and Si₁ˍₓGeₓ

Abstract

It is widely accepted that when transistors are scaled beyond the 10-nm technology generation in the near future, Ge, or alternatively Si1-xGex will potentially replace Si as the channel material to maintain CMOS device performance improvement. In is one of the most promising p-type dopant in Ge and Si1-xGex. Hence, this thesis has contributed to the field of semiconductors by performing systematic studies on the structural and electrical properties of In doped and C + In co-doped Ge, Si, and Si1-xGex.

In (and C) atoms were incorporated into Si, Si0.35Ge0.65, Si0.1Ge0.9, and Ge thin films by ion implantation. Electrical properties of the implanted samples were determined by Hall effect measurement, while the identification of the sample structural properties was performed using TEM, Raman spectroscopy, DFT calculation, and X-ray absorption spectroscopy.

The dopant concentration effects on the structural and electrical properties of In-implanted Ge were investigated. For In concentrations lower than 0.2 atomic percent, all In atoms occupy a substitutional lattice site. The formation of metallic In precipitates and In-vacancy complexes are apparent for In concentrations greater than 0.6 atomic percent. Electrical measurement results were correlated with the determined structural properties.

Aiming to enhance the electrical activation of In in Ge, C was introduced as a co-dopant with In. With C + In co-doping, the electrically active fraction was significantly enhanced. This dramatic improvement was found to be the result of C-In pair formation such that In-induced strain in the Ge lattice was reduced while the precipitation of In and the formation of In-vacancy clusters were both suppressed.

Si1-xGex alloys have the potential to combine the positive aspects of Si and Ge as the substrate for In doping. Thus, we performed a systematic study on the dopant concentration and substrate stoichiometry effects on the electrical and structural properties of In-implanted Si1-xGex (including Si and Ge). Correlating the fraction of electrically-active In atoms and the In atomic environment, we observed the transition from electrically-active, substitutional In at low In concentration to electrically-inactive metallic In at high In concentration. The In solid-solubility limit has been quantified, which was found increased as the Ge fraction of the Si1-xGex alloy increased.

Since we found that the Si fraction in Si1-xGex affected its In solid solubility significantly, an above-equilibrium solid solubility threshold is needed to maintain a high In electrical activation. To that end, the co-doping strategy with C was again employed. With C + In co-doping, the solid solubility of In in Si0.35Ge0.65 is at least tripled from that of In doped Si0.35Ge0.65, as a result of C-In pair formation in suppression of In metal precipitation. Dramatic improvement of the sample electrical properties was attained.

This thesis demonstrated a promising future for In doped Ge and Si1-xGex, towards the application to p-type field-effect transistors in future CMOS devices. C co-doping was verified as an effective method for enhancing the In solid solubility and electrical activation in both Ge and Si1-xGex. These results inspire potential future work like In implantation in Si1-x-yGexCy. Show more