**1. Introduction**

In semiconductor technology, doping is a process that introduces delicately controlled amounts of impurities (called dopants) into an intrinsic semiconductor to modify its electrical, optical, and structural properties significantly. The intrinsic semiconductors are pure semiconductors without impurities (typical semiconductors of group IV in the periodic table: Si and Ge), in which the number of excited electrons equals the number of holes. In the doping process, a dopant is added, which could play a role as either a donor to contribute an electron or an acceptor to create a hole with the semiconductor crystal that respectively generates two types of semiconductors: n-type and p-type. The dopants belonging to group III, such as boron (B), aluminum (Al), gallium (Ga), and indium (In), are referred to as acceptors for p-type semiconductors. Moreover, group V elements, including phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and lithium (Li), are donors to contribute free electrons in n-type semiconductors. Boron is a p-type dopant with only three electrons in its valence shell. During the boron incorporation process into the silicon crystal, the one atom of boron can bond with four silicon atoms. Still, since boron only has three free electrons to provide, a hole is created. This hole acts like a positive charge, so boron-doped (B-doped) semiconductors are referred to as p-type semiconductors (**Figure 1a**). In the p-type semiconductors, the holes, like the positive charge, attract

**Figure 1.** *Schematic of (a) boron-doped silicon, (b) an ion implanter, and (c) ion penetration path into a silicon substrate.*

electrons. But when an electron moves into a hole, the electron leaves a new hole in the previous position. Thus, in a boron-doped semiconductor, the holes constantly move around inside the crystal as electrons continuously try to fill them. This appears like the moving of the positive carrier.

The unstoppable development of electronic technology demands the detailed design and effective performance of microelectronics. The formation of shallow and low resistivity junctions is required for contact resistance reduction and leakage current consideration. The precise control of dimension and dopant concentration of source/drain region to achieve a high shallow doping efficiency is crucial for junction fabrication [1–3]. Shallow doping could create doped layers with depths ranging about dozens of nanometers. It required low-energy ions for implantation by considering thermal redistribution [4]. At low energy, the penetrated navigation of ions was mainly directed along crystalline channels rather than moving randomly into semiconductors [5]. Boron is one of the essential dopants for shallow doping in silicon because of its good diffusivity [3].

Boron doping has grabbed attention for several decades. Studies can be classified as ion implantation, solid-phase doping, monolayer doping, and other methods such as sputtering and chemical solution mixing. These techniques are used widely in semiconductor technology. This chapter studied the characteristics of widely used and recently developing methods, such as monolayer doping, by showing the advantages and disadvantages of these doping techniques to give an overall sight of the doping methodology of boron, so it is easier to choose and use suitable doping techniques to meet a specific requirement in further boron-doping application.
