Control of the Interface Charge in Diamond Power Metal Oxide Semiconductor Field Effect Transistors

Abstract

The main aim of this Ph.D. dissertation was to study C-H diamond surface electrical properties and MOSFET performance by controlling the interface charge. Negatively charged adsorbate ions are required for the formation of a C-H conductive accumulation layer, while the positive charge plays an important role in controlling the threshold voltage, needed to achieve normally-off operation. 1. The mechanism regarding the conductivity of the C-H diamond surface, by the adsorption of negative ions on the C-H surface, using corona discharge ions, was investigated. From the results of this study, it was revealed that negatively charged adsorbates on the positive side of the C-H dipole induced accumulation of minority carriers (hole) by band-bending (inversion layer of MOSFET). A large number of negative ions O2− (found in the air) were introduced to the three different kinds of C-H diamond surface, which provided the holes. The three different kinds of C-H diamond substrate; pure C-H surface, partially oxidized C-H surface, and C-H surface after the adsorbates were removed when the diamond inside the chamber was annealed at 600 K. For the pure C-H surface, the value of the conductive surface was increased from the initial value of 6.8×10-5 S to 1.2×10-4 S, due to the negative ions. Regarding the second substrate, directing negative charge ions toward the oxidized C-H surface caused an increase in the carrier density, and the conductance, by three orders of magnitude, from 10−8 S to 10−5 S. The third substrate comprises a C-H diamond material, in which adsorbates have been removed from the surface, and is highly resistive due to the heat treatment in a vacuum. By depositing negatively charged ions to C-H diamond, the electrical conductivity for this third substrate demonstrated a significant increase by 5.5 orders of magnitude, to 3.5 ×10−6 S. Furthermore, the effect of positive corona discharge ions on the C-H conductivity was investigated in this chapter. The decrease in surface conductivity of C-H diamond can be related to a sharp decrease of carrier density, due to the presence of positive adsorbates ions, which are not common. The negatively charged ions must be responsible for the surface conductivity of C-H diamond. 2.The effect of the negative fixed surface charge model on C-H diamond MOSFETs conductivity, depicted using simulation, has been laid out in this Ph.D. dissertation. The p-type conductive channel with a high hole concentration of 7×1019 cm−3 was confirmed in C-H diamond using Atlas TCAD device simulator. The output characteristics showed high performance of MOSFETs, such as normally-on operation, with the negative surface charge 90 model. The maximum drain current density was obtained at IDS= −125 mA/mm, upon the application of a drain voltage of VDS= −50 V, and gate voltages within the range of VG= −26 V~ 26 V. Surface conductivity cannot be produced without negative ions on the C-H surface, although the Ohmic contact is also important to inject holes that flow from source into the channel and toward the material, as FETs. In this simulation, the SBH was 0.2 eV, which is responsible for the formation of Ohmic contacts at room temperature. When the negative interface charge is applied close to the interface, the SBH potential raises. This allows the holes to enter the valence band maximum indicated, forming a hole accumulation layer. In addition, the drain current maximum at IDS= −84 mA/mm with control mobility was obtained in the neutral charge model. In this model, the confirmed conductive channel is based on the limit of hole mobility. The hole mobility may have been limited due to the effect of neutral surface scattering, hence the increases in channel mobility, which may lead to improvement of device performance. The device demonstrated high electric performance in both models; negative interface charge and n