Abstract
This paper benchmarks various epitaxial growth schemes based on n-type group-IV materials as viable source/drain candidates for Ge nMOS devices. Si:P grown at low temperature on Ge, gives an active carrier concentration as high as 3.5 × 1020 cm−3 and a contact resistivity down to 7.5 × 10−9 Ω.cm2. However, Si:P growth is highly defective due to large lattice mismatch between Si and Ge. Within the material stacks assessed, one option for Ge nMOS source/drain stressors would be to stack Si:P, deposited at contact level, on top of a selectively grown n-SiyGe1−x−ySnx at source/drain level, in line with the concept of Si passivation of n-Ge surfaces to achieve low contact resistivities as reported in literature (Martens et al. 2011 Appl. Phys. Lett., 98, 013 504). The saturation in active carrier concentration with increasing P (or As)-doping is the major bottleneck in achieving low contact resistivities for as-grown Ge or SiyGe1−x−ySnx. We focus on understanding various dopant deactivation mechanisms in P-doped Ge and Ge1−xSnx alloys. First principles simulation results suggest that P deactivation in Ge and Ge1−xSnx can be explained both by P-clustering and donor-vacancy complexes. Positron annihilation spectroscopy analysis, suggests that dopant deactivation in P-doped Ge and Ge1−xSnx is primarily due to the formation of Pn-V and SnmPn-V clusters.
Highlights
Source/Drain Materials for Ge nMOS Devices: Phosphorus Activation in Epitaxial Si, Ge, Ge1−x Sn x and Si y Ge1−x−y Sn x
Point-defect engineering approaches supported by density functional theory (DFT) calculations, suggest that the codoping with Sn could help in achieving enhanced phosphorus activation in Ge, as Sn can act as a trap for vacancies.[4,5]
In our previous work (Ref. 25), we have demonstrated that the relaxation effects for the atoms present around the vacancy play an important role in positron trapping
Summary
Source/Drain Materials for Ge nMOS Devices: Phosphorus Activation in Epitaxial Si, Ge, Ge1−x Sn x and Si y Ge1−x−y Sn x. The saturation in active carrier concentration with increasing P (As)-doping for as-grown pure Ge films or SiyGe1−x−ySnx alloys is one of the major hindrances to achieve low contact resistivity, besides the need for a lower Schottky barrier height at the metal/ semiconductor interface. E.g. the degree of dopant deactivation in Ge and Si films with the highest P concentration of 1.1 × 1020 cm−3 and 1.6 × 1021 cm−3 as we achieved in our previous studies is as high as 75% and 78%, respectively.[2,10] The aim of the current manuscript is to get fundamental insights into P deactivation in epitaxial Ge and the impact of Sn-doping on dopant activation in Ge. The manifestation of As-vacancy clusters in As-doped Ge1−xSnx will be discussed in Ref. 15. The structure of the current manuscript is as follows, we will first benchmark the different materials by comparing the maximum achieved active carrier concentration and the lowest contact resistivities as extracted from micro-Hall effect (MHE) and MR-CTLM measurements, respectively. Results from the PAS measurements are corroborated with positron modeling using two-component density functional theory (TCDFT) to identify the type of dominant open-volume defect (i.e. mono- or di-vacancy) and its chemical environment
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