**4. Doping boron in typical semiconductor materials**

In semiconductor manufacture, boron doping is a crucial technique to introduce boron atoms into a semiconductor to modify its physical properties. There are intrinsic semiconductor materials, including silicon (Si),germanium (Ge), and compound semiconductors, which is combinations of elements such as group II–VI (ZnSe, ZnTe, CdS, CdTe), group IV-VI (PbS, PbSe, PbTe) of the periodic table, group III–V (AlN, GaAs, InGaN, InP, InGaAlP), or elements in the same group IV–IV (SiC, SiGe), other advanced materials including carbon nanotube, diamond, 2D materials (graphene, hexagonal boron nitride), etc. In intrinsic semiconductors, their atoms connect by sharing electrons to create stable covalent bonds. Generating conduction in a semiconductor requires energy to break the crystal bond and create conduction electrons moving around in a crystal and leaving holes. For example, silicon requires approximately 1.12 eV of energy to free an electron at room temperature. This energy is called bandgap energy or energy gap (Eg), which is necessary energy to excite an electron trapped in the valence band to the electrical conduction band. Silicon doped by boron is introduced a more significant number of conduction electrons and mobile holes that can lift the valence band close to the conduction band, decreasing the bandgap energy of boron-doped silicon to 0.045 eV [133]. The number of holes (positive charge carriers) rises with the increased amount of active boron concentration. In p-type semiconductors, the conduction is attributable to an enormous number of holes; therefore, holes and electrons are referred majority carriers and minority carriers, respectively.

The electrical conductivity of boron-doped silicon depends on the amount of boron and temperature. According to the calculation modeling of hole mobility on

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

boron concentration of Masetti and coworkers, the hole mobility of boron-doped silicon can be estimated around 424–25 cm2 /Vs, correlating with the range of 1014–1021 cm−3 for boron concentration. The higher the boron concentration, the lower the carrier mobility and resistivity [134]. Moreover, boron doping improves the hardness property of silicon, the hardness increases with increasing boron-doped concentrations. For example, the hardness at boron concentration of 1.3 × 1020 atoms/ cm3 was 30% higher than that at 2.9 × 1017 atoms/cm3 [135]. The thermal conductivity of boron-doped silicon (with a B concentration of 5 × 1020 atom/cm3 ) was lower than undoped silicon at 300 K. Lee et al. found that the mass disorder effect is the main reason for the thermal transport suppression in boron-doped Si [136]. Like silicon, germanium (Ge) is an intrinsic semiconductor as silicon with a bandgap of 0.67 eV [137]. Introducing boron in Ge causes changes in electrical, mechanical, and thermal properties that are approximate to boron-doped silicon. The carrier mobility in boron-doped Ge monocrystals decreases with the increase of boron concentrations. The elasticity limit of Ge enhances after doping with low boron concentration. The mechanical property of boron-doped Ge at high boron concentration [138]. Si and Ge are primary materials for the semiconductor industry. Boron-doped Si and Ge show highly electrical conductivity that more effective for application in the electronic device fabrication including diodes [139], transistors [104], integrated chips/circuits [140], microcontrollers [141] and other applications for sensors [142, 143], light-emitting diodes (LEDs) [144], energy storage such as solar cells [145–147], photovoltaic devices [139, 148], capacitors [149], etc.

Boron doped in carbon nanotubes using CVD doping method that lowered HOMO-LUMO bandgap, featured for chemical reactivity and kinetic stability, of CNTs from 0.56 eV of original CNTs to Eg ~ 0.44 eV of B-CNTs after doping [150]. Introducing boron into CNTs increases the defects that break inertness and improves the reactivity in CNTs. The changes in electrical properties of CNTs varied depending on the boron concentrations. Yi and coworkers investigated that the acceptor state after doping boron was located at 0.16 eV above the Fermi energy for the ratio of B/C ~ 1/80 [151]. Boron doping improves the metallic property of CNTs. Moreover, the mechanical and thermal properties of CNTs were modified after doping with boron. The rupture stress of the B-CNTs was reduced compared with pristine CNTs, but at higher temperatures, B-CNTs showed drawbacks on maximum stress [152]. The thermal conductivity of B-CNTs depends on the temperature. At low temperatures, the thermal conductivity decreases with a rise in boron concentration in zigzag CNTs. However, the thermal transport enhances with increased boron concentrations at higher temperatures [153]. Boron-doped CNTs were applied in various application from hydrogen energy storage [154–156], catalysis [157], electrocatalysis [158], sensors [157, 159, 160].

Similarly, boron was introduced into graphene to modify its physical, chemical, mechanical, and electrical properties. The nature of graphene structure changes from ductile to brittle after being doped with boron. The thermal conductive property of graphene is reported to weaken after boron doping. Thermal conductivity dropped around 60% after introducing 0.75% boron concentration in graphene. Pristine graphene is a zero-gap semiconductor with semi-metallic property [161]. Boron-doped graphene monolayer shows a p-type semiconductor behavior with a high carrier mobility level of approximately 800 cm<sup>2</sup> /Vs at ambient temperature [162]. Wu et al. fabricated B-doped Graphene-based back-gate FETs with mobilities of 450–650 cm<sup>2</sup> /Vs [163]. Graphene doped with boron exhibits excellent electrochemical properties for diverse applications, including electrocatalysis [164], energy storage (batteries, supercapacitors) [165], sensors [166], and photovoltaics [167].

B-doped graphene can obtain a small band gap of 0.05 eV combined with n-type silicon to fabricate a p-n junction for solar cell application. The B-graphene/siliconbased solar cell showed a higher short-circuit current density of 18.8 mA/cm [168].

The sp3 -hybridized diamond is an insulation material with a wide bandgap of 5.47 eV and extremely high resistivity of roundly 1012 Ω/cm. Doping with boron turns an insulative pure diamond into a conductive p-type semiconductor. After introducing boron into the diamond, the acceptor level is quite deep, around 0.37 eV above the valance band. Boron doped diamond shows high-level conductivity and enhanced electron transport compared to undoped diamond. The average boron doping level in diamond ranges from 1018 to 1020 atoms/cm3 [169, 170]. The hole mobility of boron-doped diamond was examined, reaching the maximum of about 2000 cm2 / Vs at ambient temperature [170]. Heavy boron-doped diamond with a higher boron concentration of 1021–1023 atoms/cm3 for superconductivity can obtain at high pressure (105 atmospheres) and temperature (2500–2800 K) [171]. The sheet resistance of B-doped diamond was dropped from 1014 to about 1010 Ω/sq. [30]. Doping boron in diamond also changes its physical and mechanical properties. Similar to B-CNTs and B-doped graphene, B-doped diamond exhibits a comparable tendency in thermal conductivities [172]. The higher the boron-doped concentration, the weaker the thermal transport. The surface area of a diamond is larger after doped with boron. B-doped diamond is electrode material for numerous fields of electroanalysis [173], electrochemical energy storage [174, 175], and sensors [171, 176].

Apart from the above materials, doping boron is applied to improve the mechanical property of semiconductor compounds. Boron doping using ion implantation has been proven to change the roughness, hardness, stress/strain of materials, and other morphological characteristics of materials. The Zinc Selenide (ZnSe) thin films were implanted with boron ions at 75 keV and ranging in doses from 1012 to 1016 ions/ cm2 in the research of Venkatachalam et al. that revealed the increase of film surface roughness and the decrease in the optical band gap value while increasing the dose of boron ions [177]. The hardness and elastic modulus of the hosts were also increased in some substrates of 60NiTi/NiTi after being doped by boron atoms [9, 178]. This is accounted for by replacing boron atoms in lattice matrix to create new nanocrystals, for example, TiB2 in B-doped 60NiTi [9]. In addition, a study by Zhu et al. proved boron ion implantation can enhance a hardening effect in the TiAlN. This hardening resulted from of the increase of excess stresses and the formation of new forms (TiB2 and BN nanocrystals) in the structure after the boron implants [179]. Similarly, boron ion implantation at 150 KeV and a fluence of 1 × 1015 ions/cm2 in hexagonal boron nitride (h-BN) induced the formation of c-BN nanocrystals due to the collisions of ions with the radical atoms and created the displacement of these atoms out from the lattice positions, which generated an atomic vacancy and temporary accumulation of defects in the interstitial site in h-BN. This increases the stress/strain level in h-BN and increases the electron density in the interatomic and interlayer places in the material [27]. Additionally, boron implants modified the structure of two-dimensional carbonfiber-reinforced carbon-carbon (C/C) composites to generate the boron carbide composition, improving resistance during exposure to air at high temperature [180].
