**2.4 Application of monolayer doping**

### *2.4.1 Boron MLD for electronic devices*

As explained above, ion implantation is not feasible to dope boron on the sidewalls of finFET structures due to irreparable crystal damage [99]. Homogeneous and conformal doping is required for small dimension devices. CVD doping can be used for 3D structures, but this method needs to control parameters, including the growth temperature, reactor pressure, and precursor dose. Therefore, CVD doping is challenging in mass production to generate uniform thin layers [100]. Due to the ability to doping a thin uniform layer of boron in 3D structures, MLD promises a practical technique applied in the semiconductor industry to fabricate small electronic devices such as CMOS or finFET with affordable expense. Monolayer doping sulfur on CMOS device designed by Barnett et al. using ammonium sulfide, (NH4)2S as sulfur monolayer source. A uniformly doped ultra-shallow junction with 9 nm of depth and low sheet resistance of 164 Ω/sq. was achieved without damage to the substrate [99]. Ang and coworkers first applied MLD to fabricate ultra-shallow junction in 20 nm finFET with phosphorous MLD. The authors successfully produced a 5 nm- n+/p junction with a sheet resistance of 8.3 × 103 Ω/sq. [101]. The 3-D finFET devices recently require a channel thickness scaled down to sub-10 nm [101] (**Figure 7a**). Boron and phosphorus co-monolayer doping was used to create a conform thin shell doping on polysilicon junctionless finFET devices [102, 103]. The ultra-shallow doping profiles of n-and p-type were obtained with sub-5 nm of depths. The FinFETs showed excellent gate control with Ion/Ioff ~ 106 , lower off-current, and an exceptional subthreshold slope of 67 mV/dec [102]. The recent publication uses conformal monolayer doping to prepare devices with complex-geometry structures, allowing for the formation of multilayer Ge nanosheet gate-all-around field-effect transistors (**Figure 7b**). This can overcome the limitation of the Wrap-Around Contact method normally used for epi source/drain formation [104].

## *2.4.2 MLD for solar cells*

Electrical energy generation in solar cells depends on splitting holes and electrons efficiently at a p-n junction. Therefore, MLD plays a vital role in the manufacture of silicon solar cells. Boron is introduced in silicon to generate p-type semiconductors that allow the transport of electrons from one atomic layer to another. The borondoped silicon is used to increase conduction efficiency and lower the production expense of solar panels by focusing on growing surface-to-volume ratios and p-n junction dimensions. Therefore, the solar cells can absorb the larger light converted into energy to separate more electron-hole pairs. Moreover, the non-planar doping

*Boron Doping in Next-Generation Materials for Semiconductor Device DOI: http://dx.doi.org/10.5772/intechopen.106450*

#### **Figure 7.**

*The graphic diagram of monolayer doping of the five-stacked Ge nanosheets FET printed with permission from Ref. [104]. Copyright 2022 American Chemical Society.*

capability of MLD makes it ideal for this application [105]. In the report of Garozzo et al., MLD was utilized to fabricate a doped layer covering the entire nanohole surface of solar cells. The radial junctions were formed inside the nanoholes with a carrier concentration of around 1019 cm−3 for both n-/p- type doping [106].
