*3.1. Diamond Substrates*

Homoepitaxially deposited diamond is carried out over commercially available diamond substrates. Diamond substrates can be synthetized by high-pressure hightemperature (HPHT) or chemical vapor deposition (CVD) techniques. Synthetic and natural diamonds are classified on the basis of their impurity concentrations. All natural and lab-grown diamonds contain some nitrogen (N) impurities [6]. When the N content is high (hundreds or thousands of ppm), so that it can be characterized by infrared (IR) spectroscopy, diamond is classified as Type I. These N atoms can be placed replacing carbon atoms in the lattice forming aggregated (Type Ia) or isolated from each other N atoms (Type Ib). Type Ia is also subdivided into IaA, with nitrogen atoms forming pairs, and IaB, with four nitrogen atoms symmetrically surrounding a vacancy in the diamond structure. On the other hand, when nitrogen content is low enough to not be detected by IR (<10<sup>17</sup> cm−<sup>3</sup> ), diamond is classified as Type II. Usually, the nitrogen reduction is a consequence of the incorporation of Boron impurities. When both impurities contents are very low, the substrate is named Type IIa, which corresponds to the purest diamond crystals. When the boron concentration increases so it is higher than nitrogen, the substrate is called Type IIb, and it corresponds to a p-type semiconductive diamond crystal.

Above the impurities content, there are two main restrictions related to the available substrate: the crystalline defects density and the substrate size. The HPHT process provides high-purity and low-defect-density substrates (<10<sup>3</sup> cm−<sup>2</sup> in Type IIa). However, the size is restricted to <sup>≤</sup>1 cm<sup>2</sup> by the technological limitations of the method. Moreover, the prize increases drastically when a high-purity diamond crystal is required. For this reason, Type Ib is the most usual diamond substrate for electronic applications, which means low cost and an acceptable dislocation density of about 10<sup>5</sup> cm−<sup>2</sup> . Larger substrate sizes can be obtained by CVD growth [7]. The largest single-crystal diamond substrate has been reported to have a diameter of ~3.5-inches based on Ir/YSZ/Si [8]. ® Other groups have reported 2-inch-scale substrates labs also using Ir hetereoepitaxy [9,10]. However, the crystalline quality is still worse than that of HPHT (commonly dislocation density of 10<sup>7</sup> to 10<sup>9</sup> cm−<sup>2</sup> in heteroepitaxy growth) [11–13]. An alternative method to obtain large diamond substrates is growing on a mosaic configuration by CVD. This technique results in substrates sizes above 5 cm<sup>2</sup> but the bonding boundaries of the mosaic are very strained and defective [14]. Thus, up to now, it has not been possible to obtain larger than 1 cm<sup>2</sup> good-quality diamond substrates.
