(Invited) High Power Diamond Devices with 2-D Transport Channels
Authors: Shahin David I , Christou Aristos , Butler James E 
Published: February 2020

Diamond transistors with surface 2D conduction channels are projected to be radiation hard with respect to neutrons. However, ionizing radiation hardness may be a problem due to the disruption of the 2D hole concentration. In this paper, the processing of three terminal devices with 2D surface conducting channels is presented as well as analysis of device transfer characteristics. This paper addresses the fabrication, radiation induced defects and device characteristics of microwave power field effect transistors (FET) built on 1) high quality diamond surfaces exploiting the 2 dimensional surface conductivity, and 2) the surface two dimensional “hydrogen terminated” conduction layer in diamond single crystals grown by CVD.

Diamond is an ultra-wide bandgap semiconductor material that exhibits a number of ideal properties for future power and RF electronics applications beyond the current generation of SiC- and GaN-based technology. These properties include a bandgap of 5.5 eV, a high breakdown field (>10 MV/cm), high theoretical room-temperature carrier mobilities (>3800 cm2/V*sec), extremely high thermal conductivity (>22 W/cm*K), and excellent mechanical and thermal stability in vacuum. However, diamond doping is difficult, as the activation energies for typical dopants (B, P) exceed 0.37 eV and lead to very low dopant activation levels.[3] This drawback of dopant activation efficiency has been, in part, mitigated by two-dimensional conductivity found in hydrogen-terminated diamond surfaces. Hydrogen-termination (usually accomplished via exposure to hydrogen plasma) has been found to create a C-H surface dipole layer that reduces the carrier ionization energy by nearly 1.5 eV. Adsorbed species, such as atmospheric H2O, NO2, or intentionally-introduced passivation species contact this dipole layer, and together generate a two-dimensional hole gas (2DHG) with 1013-1014 holes/cm2 carrier density several nanometers below the diamond surface.

Hydrogen-terminated diamond has been used as the basis for metal-semiconductor field effect transistors (MESFETs) and metal-oxide-semiconductor field effect transistors (MOSFETs) hydrogen-terminated diamond. Both unpassivated and passivated surface conduction devices have been fabricated. In the case of unpassivated devices, exposure to air (and thus atmospheric H2O) causes the devices to exhibit sheet charge densities in the 1013 holes/cm2 range.[1] MESFETs typically rely on Au metal for Ohmic contacts and Al metal for Schottky contacts to the hydrogen-terminated diamond. These devices have been normally-on devices, with threshold voltages around between -1V to +1V, depending on device dimensions.[1-4] MOSFETs fabricated using atomic layer deposited (ALD) oxides such as Al2O3 have been fabricated with either normally-off or normally-on behavior, depending on thermal history. Annealing of these devices at 180°C caused the devices to switch from normally-on to normally-off, with threshold voltages again in the range of ±1V range; the change was likely due to the removal of adsorbed species in the unpassivated diamond surfaces between the source, gate, and drain regions in the device, causing a reduction in the 2DHG sheet carrier density.[4]

The diamond materials and surface preparation supplied by Euclid Techlabs using proprietary processes to produce defect free, ultra-smooth (Sa ~ 0.2 nm) diamond surfaces and buried boron ‘delta doped’ CVD layers. Euclid techlabs has demonstrated the delta doped transport channels in Diamond have mobilities in excess of 700 cm2/V sec. Device design and process science development was carried out within the facilities of the UMD NanoCenter and the NanoFab Lab. Radiation testing has been carried out at the radiation facilities of the University of Maryland.

Device electrical performance and characterization has been carried out. We perform baseband noise, as well transconductance/output resistance dispersion characterization of 2D Surface Channel Diamond FETs fabricated under different processes and having different geometry and designs in order to identify device defects and their reliability and their susceptibility to radiation effects. The impact of the various types of noise on device defect and radiation hardness will be reported. Materials characterization techniques including Raman spectroscopy (strain, sp2 vs sp3 bonding), FTIR, and SIMS. Common diamond point defects are characterized by photoluminescence and FTIR.

References:

[1] M. Kubovic, M. Kasu, Y. Yamauchi, K. Ueda, H. Kageshima, “Structural and electrical properties of H-terminated diamond field-effect transistor,” Diamond and Related Materials
18 796-799 (2009).

[2] M. Kasu, K. Ueda, H. Kageshima, Y. Yamauchi, “Gate interfacial layer in hydrogen-terminated diamond field-effect transistors,” Diamond and Related Materials
17 741-744 (2008).

[3] D.A.J. Moran, O.J.L. Fox, H. McLelland, S. Russell, P.W. May, “Scaling of Hydrogen-Terminated Diamond FETs to Sub-100nm Gate Dimensions,” IEEE Electron Device Letters
32 [5] 599-601 (2011).

[4] S.A.O Russell, S. Sharabi, A. Tallaire, D.A.J. Moran, “Hydrogen-Terminated Diamond Field-Effect Transistors With Cutoff Frequency of 53 GHz,” IEEE Electron Device Letters
33 [10] 1471-1473 (2012).