GeSn and GeSi are promising channel materials for gate-all-around (GAA) devices to achieve the high I_ON thanks to the high mobility [1, 2]. The undoped channels are used to reduce the impurity scattering and to further enhance the mobility. The GAA structure with high number of stacked channels can further enhance the I_ON for a fixed footprint. The dopant diffusion and segregation play an important role on GAA device structure design. In this work, the dopant diffusion and segregation effects of B and P in highly stacked Ge_0.9Sn_0.1 and Ge_0.95Si_0.05 epilayers are investigated, respectively.

The Ge:B/Ge_0.9Sn_0.1/Ge:B epilayers were grown on the Ge-buffered SOI substrate at 320°C and 100 torr by chemical vapor deposition (CVD) using SnCl₄, Ge₂H₆, and B₂H₆ precursors. The 160nm undoped Ge buffer was grown with an 800℃ annealing to confine the misfit dislocations at Ge/Si interface. For the 8 stacked Ge_0.9Sn_0.1 epilayers, 36 nm Ge_0.9Sn_0.1 layers sandwiched by 24 nm Ge:B layers were grown. Transmission electron microscopy (TEM) image of Ge:B/Ge_0.9Sn_0.1/Ge:B epilayers is shown in Fig. 1(a). The Ge:P/Ge_0.95Si_0.05/Ge:P epilayers were grown on the Ge-buffered SOI substrate at 350°C and 100 torr by SiH₄, GeH₄, and PH₃ precursors using similar Ge buffer layers. For the 8 stacked Ge_0.95Si_0.05 epilayers, 24 nm Ge_0.95Si_0.05 layers sandwiched by 25 nm Ge:P layers were grown. TEM image of Ge:P/Ge_0.95Si_0.05/Ge:P epilayers is shown in Fig. 1(b). Note that both Ge_0.9Sn_0.1 and Ge_0.95Si_0.05 channel layers are unintentionally doped. In the 8 stacked Ge_0.9Sn_0.1 epilayers, the [B] in the Ge:B layers is as high as ~2.0 ×10²¹ cm⁻³ (Fig. 2(a)). In the 8 stacked Ge_0.95Si_0.05 epilayers, the [P] in the Ge:P layers is as high as ~2.2 × 10²⁰ cm⁻³ , and the minimum [P] in the Ge_0.95Si_0.05 channel layers is from ~4.2 ×10¹⁷ cm⁻³ to ~1.5 ×10¹⁸ cm⁻³ (Fig. 2(b)).

The B and P diffusion phenomena in the GeSn and GeSi layers are investigated, respectively. The deep undoped GeSi layers have higher [P] due to higher thermal budget (Fig. 3). Note that the P atoms diffuse from the Ge:P layers into GeSi channels during the epitaxial growth. The similar [B] ~3x10¹⁶cm^-3 in the 2nd to 8th GeSn layers is due to the B detection limit of SIMS analysis (Fig. 3). Note that the decay length (nm/decade) is defined in Fig. 4 and Fig. 5. The decay length of [B] on the top side (4.3 nm/decade) is larger than that on the bottom side (2.9 nm/decade) due to SIMS knock-on effect [3] (Fig. 4). On the other hand, the P segregation [4] increases the decay length on the bottom side of GeSi layers (Fig. 5). Both the decay length of [B] and [P] increase with the increasing depth (Fig. 6) due to the higher thermal budget of the deeper layers in the CVD chamber. In addition, the difference of [B] decay length between top and bottom sides increases with the increasing depth of epilayers (Fig. 6). For the Ge:P/GeSi/Ge:P epilayers, the difference between top and bottom [P] decay length (less than 0.9 nm/decade) is smaller than that of [B] decay length in Ge:B/GeSn/Ge:B epilayers (larger than 1.4 nm/decade) due to P segregation, and the difference decreases for deep layers.

Acknowledgment

This work is supported by the Ministry of Science and Technology, Taiwan (MOST 110-2622-8-002-014-, 110-2218-E-002-030-, and 110-2218-E-002-042-MBK) and the Ministry of Education, Taiwan (NTU-CC-110L892601). The support by Taiwan Semiconductor Research Institute (TSRI), Hsinchu, Taiwan, is also highly appreciated.

References

[1] Y.-S. Huang et al., IEDM, 2020, pp. 23. [2] Y.-C. Liu et al., VLSI, 2021, T15-2. [3] W.-H. Tu et al., IEEE Trans. Electron Devices, vol. 61, no. 7, pp. 2595–2598, Jul. 2014. [4] S. Kobayashi et al., Journal of Applied Physics 86, 5480, 1999.