## 3 P-Type B-Doped Diamond Films  
Compared with other wide band gap semiconducting substrates (NiO\({}_{2}\), GaN, SiC, etc. ), diamond has favorable intrinsic optoelectronic performance, for instance, high thermal conductivity (22 W\(\cdot\)cm\({}^{-1}\cdot\)K\({}^{-1}\)), high carrier mobility (2200 and 1800 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)), high electrical breakdown field (10 MV\(\cdot\)cm\({}^{-1}\)) and high saturation velocity (2.7 \(\times\) 10\({}^{-7}\) cm\(\cdot\)s\({}^{-1}\)) [60; 61]. Therefore, diamond is regarded as a suitable material for high power and high temperature to cooperate with metal oxide semiconductors in optoelectronic devices. So far, the main requirements include high-quality epitaxial growth as well as doping. However, because of the contradiction between structural quality and the electrical properties of p-diamond, it is still a challenge to produce high-quality p-type diamond structures. With boron doping, diamond can be transformed from an insulator into a semiconductor or even a superconductor, wherein a boron atom is in the form of a host impurity in the diamond. High-boron doping in the diamond film may enhance its electrical resistivity (up to 10\({}^{-3}\)\(\Omega\cdot\)cm order). In order to improve the efficiency of B-doping pursuing high mobility and high crystal quality, many efforts have been implemented. Experimentally, [PERSON]'s group obtained the BDD film synthesized by hot filament chemical vapor deposition (HFCVD) [62]. B(OCH\({}_{3}\))\({}_{3}\) was utilized as the boron doping source in a methane (CH\({}_{4}\)) and hydrogen (H\({}_{2}\)) reaction atmosphere with a flow rate of 0, 2, 5, 10 and 20 sccm. The resistances decrease for the B-doping diamond films grew with increasing the H\({}_{2}\) flow rate tested by Hall-effect measurement. The undoped diamond film consists of pyramid-shaped grains. With the increase of the B-flow rate up to 2 sccm and 10 sccm, the majority of grains showed lamellar-shaped twin characteristic, on account of the renucleation induced by B-doping. As for 20 sccm, the grains showed a dominating pyramid-shaped morphology, and a twinned crystal appeared (Figure 3). Raman spectroscopy (Figure 4) is an effective technique to investigate the structure of doped diamond. It is worth noting that owing to the high content of BDD, the p-degenerated diamond peak (1332 cm\({}^{-1}\)) shows an asymmetric curve and shifts towards the lower wavelength values in the region-centered phonon bands. Moreover, two wide bands that appear at 500 cm\({}^{-1}\) and 1200 cm\({}^{-1}\) in the low-frequency spectrum portion are consistent with two maximum values of phonon density in the diamond state. Figure 3: SEM images of the boron-doped diamond (BDD) with plane view grown with the boron of (**a**) 2 sccm, (**b**) 2 sccm, (**c**) 10 sccm and (**d**) 20 sccm [62]. In the most recent study [63], a small amount of sulfur was added during the deposition of BDD films by microwave plasma chemical vapor deposition (MPCVD), as reported by [PERSON]'s group. The results show the highest values of at once doping efficiency, growth rate, hole mobility and concentration, crystal mass and surface morphology of boron attained with the addition of sulfur (Figure 5). In the presence of an appropriate amount of sulfur, a high carrier concentration of 1.2 \(\times\) 10\({}^{19}\) at/cm\({}^{3}\) may be obtained during the growth process when the B-C ratio is only 2.5 ppm, which denotes a high efficiency of boron doping. The regulation mechanism of sulfur addition has been considered in terms of sulfur-induced plasma changes and possible boron-sulfur complex formation. [PERSON] et al. [64] reported on the B-doped double-layer diamond films fabricated by MPCVD and discussed the influence of B-doping concentrations on the surface morphology, crystal quality, surface composition, conductivity and secondary electron emission properties. With increasing boron doping amount, the conductivity becomes beneficial to the emission of secondary electrons.