For many decades, there has been a desire to employ diamond, a wide bandgap electronic material with superlative materials properties for electronic applications, to build electronic devices whose performance exceeds that of existing technologies. In many optical, mechanical, and thermal applications recent advances in the Chemical Vapor Deposition (CVD) of Diamond has already had an impact. However, for active electronic devices, diamond has only found a few niche applications.
This program will address at least one of the major barriers to using diamond as an active electronic device, that of achieving reasonable carrier mobilities and concentrations at the same time through a technique well known for other semiconductors: “Delta Doping”.
Intrinsic (pure) diamond has a wide bandgap (5.5 eV) and very high charge carrier mobilities (2/Vs for hole and 2/Vs for electrons), but is an excellent insulator (e.g. No thermally activated charge carriers). Impurity doping with Boron can create p-type doping with an activation energy of 0.37 eV and with Phosphorous, n-type doping with an activation barrier of 0.6 eV. Thus far, no shallower dopants have been discovered.
To achieve high carrier concentrations at reasonable temperature, one has to heavily dope diamond, typically to 1019/cc or higher. But this dramatically lowers the mobility with so many scattering centers, and attains only a modest conductivity. A solution to this problem is to create “delta doped” layers in intrinsic diamond. Each layer is heavily doped (> 1019/cc) to create a very thin (ca. 2 nm), semi-metallic layer with a higher degree of ionization where the carrier wavefunctions extend out into the intrinsic region. This creates carriers in a two dimensional layer of high mobility, thus increasing the conductivity. What is critical is the sharpness of the doping change across the interface, changing the doping by 5 orders of magnitude in 1 nm.
The megagrant team is comprised of two teams: IAP-NN and LETI. The first team, IAP-NN, has an established track record in CVD diamond growth and plasma reactor design. This team is primarily responsible for the growth of state of the art single crystal diamond materials (doped and undoped) and designing and developing a novel microwave plasma CVD reactor to produce the Delta Doped layers. The second team, LETI, is to apply their extensive experience with other wide bandgap materials (e.g. SiC and GaN) to the characterization and implementation of novel electronic devices based on the diamond materials produced in the program.
Groups from Japan[1], Germany[2,3], UK[4,5], and France[6,7] have attempted “Delta Doping” of diamond with boron in the past with no reported success in conductivity. The primary problem lies is the sharpness of the interface between the heavily doped and the undoped material compared to the penetration of the carrier wave functions into the intrinsic material. Our approach is to design a diamond CVD reactor specifically to address the issues of the interface sharpness through a detailed understanding of the chemistry and fluid dynamics at the growing interface, along with the morphology of the surface.
Acknowledgements
This work was partly supported by Act 220 of the Russian Government (Agreement No. 14.B25.31.0021 with the host organization IAPRAS).
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