**3. Boron chemical vapor deposition (CVD) doping**

#### **3.1 Introduction to CVD doping technique**

Chemical vapor deposition (CVD) or solid-phase doping (SPD) is one technique to grow a thin film layer that involves a chemical reaction of the volatile molecule containing atom precursors. The principle of this method is the interaction between the vapor gas of the precursors with the substrate surface that is heated inside the reaction chamber. Resulting in a condensation layer that grows on the substrate surface and unreacted vapor gas that is later removed. This method is broadly used due to its potential for mass production and flexible controllability of growth parameters (temperature, pressure, precursor concentration, substrates) during the process [107].

The mechanism of this technique is to break the bond between each volatile molecule and leave the targeted atom precursor that is later reassembled as a layer of the atom. A high temperature is needed to break chemical bonding depending on each chemical bonding of the molecules. Then it becomes reasonable why this method needs a quite high temperature.

The formation of the Boron-Si junctions is summarized in **Figure 8**. The H atom on the passive silicon surface (Si−H) is first desorbed to provide a free H-Si dangling bond. H then releases in the form of H2 after borane deposition due to the B−Si bond that formed. Incoming borane develops bonding with Si-B as a boron cross-link over the silicon surface. Thus, a boron layer formed on the silicon surface.

The junctions of boron-doped silicon can be introduced using two methods: *exsitu* and *in-situ* methods. The *ex-situ* steps involve removing oxides and contaminants at the Si surface and effectively passivating the surface [108]. First is depositing boron on the Si surface in the form of B2O3. The oxide is then reduced on the Si surface by

**Figure 8.**

*Chemical interaction scheme of CVD boron deposition printed with permission from ref. [108]. Copyright 2017, Vahid Mohammadi et al.*

oxidation, resulting in a boron-rich SiOx layer to the formation of a shallow p-n junction. The boron is then diffused into the Si and activated (incorporated into a substitutional site) during high-temperature drive-in anneal [109]. The second method uses precursors that containing boron, silicon, and a catalyst in gaseous form to grow both silicon and boron layers simultaneously at high temperatures.

#### **3.2 Current development of boron CVD doping**

Boron sources that are usually used are boron hydrides (diborane), boron halides (boron trichlorides), and organoboron (triethyl boron). Boron sources are chosen depending on which precursor and gas environment that used. Even though diborane seems an upcoming boron source, it only contains hydrogen apart from boron. It is known that diborane (B2H6) has a toxic, flammable, and explosive nature, so it needs a handful of treatments. Boron halides (BCl3) are expected to be a safe boron source because it is nonflammable and less toxic. Otherwise, boron trichlorides (BCl3) will not be suitable as a precursor for BN (boron nitride) since it will produce NH4Cl as the HCl reacts with NH3, which can damage the vacuum pump. At the same time, the hydrogen chloride is corrosive to a metallic substrate. The organoboron (such as B(CH3)3 and B(C2H5)3) seems an excellent precursor to obtaining B4C (boron carbides) because it can act as a boron and carbon source at the same time [110].

Sarubbi et al. demonstrated that diborane has selectively deposited only on Si with ~6 nm thickness at 500°C for 10 min diborane exposure as TEM result does not observe any B deposited on the slope or flat SiO2 surface. The SIMS profile of the B layer formed by CVD after HNO3 treatment has a concentration peak of 6 × 1020 cm−3 and shows a 5.9 × 104 Ω/sq. sheet resistance. They also mention that it has a 2.44 × 10−2 A/μm−2 saturation current density and a 13 nm junction depth [111]. Mok et al. in 2013 demonstrated the pure boron deposition using B2H6 as a boron source and H2 as a carrier at 700°C for 9 min of deposit time. It was found that nanometer-thick pure B layers, upon annealing in the presence of oxygen, function as a catalyst for silicon oxide growth. Based on the HRTEM result, the pure B is successfully doped on the surface (100) with 2.9 nm thickness and 2.1 nm on the surface (111) after TMAH texturing. They also reported the effect of oxygen concentration on the oxide forming. The thickness changes to 24.4 nm (100) and 23.4 nm (111) after furnace anneal at 950°C for 30 min in nitrogen ambient. For dry oxidation at 950°C for 30 min in 14% oxygen concentration, the thickness is changed to 37.6 nm (100) and 43.4 nm

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

(111). They also mention that an ample oxygen supply during annealing results in boron depletion of the boron-doped Si surface due to enhanced oxidization, resulting in a lower surface concentration and higher sheet resistance. The sample that was processed in nitrogen condition and then etched using HNO3 and HF had 61.2 Ω/sq. sheet resistance compared with the sample that was processed using in the presence of oxygen condition, which had 205 Ω/sq. sheet resistance that measured at 105 cm−3 of carrier concentration [112]. Higher oxygen concentrations of O2 lead to increased growth of an intermediate SiO2 layer, which acts as a diffusion barrier and results in an increase in sheet resistance with increasing O2 [113].

In 2020, Muroi et al. [114] used BCl3, H2, and SiHCl2 as gas precursors on the silicon surface. They observe the deposition and etching behavior at different temperatures. Boron adsorption occurs at a temperature lower than 800°C, the deposition occurs at 900–1000°C, and at a temperature higher than 1000°C, they observe etching behavior due to chlorosilanes that occur in gaseous form. In their further research in 2021 [115], they reported using a similar boron gas source at 800°C. The etching does not occur on the surface based on the HRTEM result that demonstrated the dense film without void. The work that was done by Taniguchi and Inasawa using BCl3 as a boron source in 2020 showed that the presence of boron-doped silicon nanowires could change sheet resistivity from 105 Ωcm to be in the range of 10−3–101 Ωcm [116].

B dopants' diffusion can occur under severe conditions, often simultaneous, such as very large concentration gradients, non-equilibrium point defect density, amorphous-crystalline transition, extrinsic doping level, co-doping, B clusters formation and dissolution, ultra-short high-temperature annealing [117]. The vacancies (V) and self-interstitials (I) are intrinsic point defects significant for dopant diffusion. In germanium, both p-type and n-type are mediated by the vacancies. Boron has a slow diffusion rate compared with other p-type dopants, which helps form ultra-shallow doped regions in Ge. The slow diffusion of B is associated with a high diffusion activation enthalpy that exceeds the activation enthalpy of self-diffusion by more than 1 eV. This indicates that B atoms are not likely associated with vacancies, thus meaning that B diffusions are via self-interstitials [118]. Tu et al. [119] successfully introduced a 5 nm thickness of the boron layer in epitaxial Ge on Silicon with a peak surface of 7 × 1021 cm−3 boron concentration.

### **3.3 Typical applications of boron CVD doping**

In their report, Liu et al. said a pure boron layer deposited using the CVD method could be used as an a-Si mask to protect from TMAH and KOH etching for long hours of exposure [120]. Other literature also shows the potential ability of boron-doped CVD as anti-corrosion on mild steel [121], used to reduce diamond growth rate to achieve a certain thickness of diamond [122, 123], used to develop boron carbide [124], boron nitride [107, 125], and also to fabricate the uniform p-type doping of silicon nanowires [109, 126], it also found that boron can be used to make a superconductor by heavily doped boron on diamond [127].

#### **3.4 Advantages and shortcomings of the CVD technique**

There are many advantages of the boron deposition using the CVD method, namely able to control the growth parameters, it can deposit a single diffusion source only on one side of the wafer, so it can be used to introduce different doping profiles and structures of the diffusion source to achieve dopant concentration profiles next

to each other [128]. It also requires fewer steps than other methods and allows better tunings of dopant profiles. It has a lower thermal budget as in-situ B-doped Ge can be grown at low temperatures (400°C), and B is already activated during growth, so it does not need activation annealing [129]. Unlike ion implantation, B doped using CVD does not destroy the structure due to annealing. Other advantages are that it can perform ultra-shallow junction, it can be used to develop boron sheets (2D structure) or boron carbide or boron nitride (3D structure), and the deposit does not depend on the position or flat surface. It is known that it can perform deposits on silicon wires [116, 130]. Furthermore, high-energy boron ion implantation in diamond enhanced the concentration of active boron up to for CVD method 1021–1022 ion/cm3 to reach superconductor, while normally concentration of boron is around 1019 for boron ion implantation [30]. Therefore, a nanometer-thin boron amorphous layer can be created on the surface of crystalline silicon through a chemical vapor deposition (CVD) process in the temperature range from 700°C to 400°C [108].

Besides its promising advantages, introducing boron using CVD has a few shortcomings. Such as it is lack of a precursor that is highly volatile and, nontoxic and nonpyrophoric, it needs metal boride compounds that can form on the catalytic substrate and the toxicity of boron gas source that used must be concerned and controlled tightly. The boron will continue to diffuse at higher temperatures, so it must be suppressed [131]. The solid solubility of the dopant at operating temperature also becomes a shortcoming because it will be related to dopant concentration [132]. Unproperly removes oxide and boron-rich layer from the surface leading to poor surface passivation [113].
