**2. Experiment**

composition in GeSn p-MOSFETs increases the effective hole mobility. However, due to the low surface energy and large covalent radius of Sn, Sn atoms may segregate to the surface during growth of GeSn [20–22]. Hence, the thermal stability may be worse at a higher Sn composition. Li et al. reported that a Sn-rich surface layer would form when Ge0.922Sn0.078 is annealed at 620°C [23]. A similar phenomenon occurs on Ge0.915Sn0.085 surface after annealing at 500°C [24]. For Sn composition as high as 17%, self-assembled Sn wires can form at an annealing temperature as low as 280°C [25]. Severe Sn segregation may reduce carrier mobility and degrade the drive current in MOSFETs. Therefore, in order to achieve high performance GeSn p-MOSFETs with high Sn composition, a fabrication process with low thermal budget may be required to maintain a good quality of the GeSn channel material and the

Various passivation techniques have been demonstrated to be effective in improving the

It has already been reported that Sn can segregate out to the GeSn surface during Si passivation process at a temperature of 370°C and degrades the device performance [17]. Therefore, low temperature passivation technique was investigated in this work for the fabrication of

The adsorption of sulfur atoms is a promising route to chemically and electrically passivate the highly reactive Ge and GeSn surface [15, 29–31]. Compared with other passivation techniques,

surface could be effectively passivated through the formation of covalent S-Ge and S-Sn bond. This will reduce oxide formation which degrades device performance; (2) The formed sulfur passivation layer is very thin with very little increase on the effective oxide thickness (EOT); (3) Sn segregation can be suppressed during the passivation process owning to a lower thermal budget. Sulfur passivation has already been implemented into Ge0.947Sn0.053 p-MOSFETs fabrica-

has not been investigated. In addition, the impact of sulfur passivation on the reduction of *Dit*

In this chapter, sulfur passivation of GeSn surface at room temperature was investigated and implemented in the fabrication of Ge0.83Sn0.17 p-channel MOSFETs. To study the impact of sulfur passivation on the quality of high-*k* dielectric/GeSn interface, extensive X-ray photoelectron spectroscopy (XPS) analysis was carried out. Sulfur passivation is found to be effective in suppressing the formation of Sn oxides and Ge oxides, and Sn surface segregation. In addition, sulfur passivation helps to reduce the high-*k* dielectric/GeSn interface trap density *Dit* as extracted using the conductance method. Material study of nickel stanogermanide [Ni(GeSn)] contact formation at low temperatures was also performed for low resistivity [Ni(GeSn)] S/D contact. The sulfur-passivated Ge0.83Sn0.17 p-MOSFETs exhibit smaller subthreshold swing *S*, higher intrinsic transconductance *Gm,int*, and higher effective hole mobility *μeff* as compared to the non-passivated control. At a high inversion carrier density of 1 × 10<sup>13</sup> cm−2, sulfur passiv-

) 2

passivation require a process temperature higher than 370°C and 400°C, respectively.

H6

H6

S solution has several advantages: (1) GeSn

H6

passivation [27, 28], and sulfur passivation [15, 29, 30]. Among

passivation

passivation and

passivation [15].

interface quality

gate stack quality of both Ge and GeSn channel p-MOSFETs, such as Si<sup>2</sup>

these passivation techniques adopted for GeSn p-MOSFETs fabrication, Si<sup>2</sup>

tion and demonstrated enhanced peak hole mobility as compared with Si<sup>2</sup>

ation enhances *μeff* by 25% as compared with the non-passivated control.

However, the mechanism of the effect of sulfur passivation on the GeSn/HfO<sup>2</sup>

GeSn/high-*k* dielectric interface.

98 Design, Simulation and Construction of Field Effect Transistors

[12, 17], Ge capping [26], GeSnO*<sup>x</sup>*

GeSn p-MOSFETs with Sn composition of 17%.

room temperature sulfur passivation using (NH<sup>4</sup>

GeSnO*<sup>x</sup>*

was not quantified.
