Doping and interface of homoepitaxial diamond for electronic applications
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Introduction Diamond is widely known as a superior material for electronics applications because of its high hardness, high thermal conductivity, and high breakdown voltage. Diamond Schottky junction diodes and field-effect transistors (FETs) are mainly being developed. In addition, diamond has unique properties for electronics applications such as extremely long spin relaxation times of point defects, negative electron affinity (NEA) of hydrogen terminated surfaces, stable exciton states at room temperature, and high conductivity for heavy doping. Using these unique properties, new types of electronic devices can be fabricated: room-temperature operating quantum devices by long spin-lattice relaxation times,1,2 electron emission p-i-n (or p–n) diodes by NEA,3,4 exciton light-emitting diodes by stable excitons,5,6 and high current density diodes by hopping conduction in heavily doping diamond.7,8 These achievements are made possible by success in high-quality diamond growth. In this article, we introduce recent progress in diamond film growth to realize electronics applications, including intrinsic and n-type doping of diamond, p-type doping of diamond, and heterostructures of diamond and other compound semiconductors.
Intrinsic and n-type doped diamond film growth Research on diamond within the field of electronics has accelerated since the end of the 1990s when high-quality intrinsic
and boron-doped p-type diamond was fabricated. High-quality diamond is homoepitaxially grown by the plasma-enhanced chemical vapor deposition (PECVD) technique on high temperature and high pressure diamond substrates. The typical growth conditions are a substrate temperature around 1000°C, pressure of 20–100 Torr, and microwave power of 200–5000 W. The source gas is hydrogen-diluted methane, and the dilution ratio is less than 1 vol%.9–16 For p-type diamond, boron as an impurity is easily incorporated into both natural and synthetic diamond by CVD without any restriction on crystal orientation. On the other hand, n-type diamond is not present in nature, and controlled n-type doping had been considered almost impossible until 1997.17 Among the group V elements as dopants, nitrogen was one of the candidates for n-type doping because of its similar covalent bond length (0.074 nm) to that of diamond (0.077 nm), but it forms a deep donor level ∼1.7 eV below the bottom of the conduction band due to its structural distortion from the substitutional position in the diamond lattice. An important breakthrough was the success of P-doped n-type diamond by Koizumi et al.17 They experimentally demonstrated the growth of P-doped n-type diamond on (111)-oriented diamond substrates by PECVD using a mixture of PH3/CH4 (concentration: 1,000–20,000 ppm) as a dopant source and H2 gas. They carefully controlled the growth conditions to improve crystallinity and avoid undesirable impurities
Satoshi Yamasaki, National Institute of Advanced Industrial Science and Technology, Japan; [email protected] Etienne Gheeraert, University of Grenoble Alpes, Institu
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