Effective Channel Mobility in Epitaxial and Implanted 4H-SiC Lateral MOSFETs
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1069-D07-18
Effective Channel Mobility in Epitaxial and Implanted 4H-SiC Lateral MOSFETs Sarah Kay Haney1,2, Sei-Hyung Ryu1, Sarit Dhar1, Anant Agarwal1, and Mark Johnson2 1 Power R&D, Cree, Inc, 4600 Silicon Drive, Durham, NC, 27703 2 Department of Materials Science, North Carolina State University, Raleigh, NC, 27695 Abstract In this paper, we investigate the effective inversion layer mobility of lateral 4H-SiC MOSFETs. Initially, lateral n-channel MOSFETs were fabricated to determine the effect of ptype epi-regrowth on a highly doped p-well surface. The negative effects of the high p-well doping are still seen with 1500 Å p-type regrowth, while growing 0.5 µm or more appears to be sufficient to grow out of the damaged area. A second experiment was performed to examine the effects of doping during epitaxial regrowth versus using ion implantation after regrowth. Comparable mobilities and threshold voltages were observed for equivalent epitaxial and implanted doping concentrations. Introduction The successful, commercial production of a SiC MOSFET has been a long-term goal of the silicon carbide community for the past decade. While much progress has been made, there are still many problems that must be solved before this goal can be truly achieved. The reduction of interface states in SiC MOS system has been a primary focus of research during the last 15 years. In the past decade, much work has been done through the use of NO anneals to passivate the MOS interface.1,2 NO annealing is effective in reducing the density of interface traps, Dit, near the conduction band edge by an order of magnitude. However this increase in effective mobility also yields a negative shift in the threshold voltage, Vth, which can lead to MOSFETs with negative Vth. Controlling the threshold voltage is one of the most critical issues faced in current MOSFET fabrication. This difficulty stems from the fact that the threshold voltage is determined by a difference of two large numbers, namely, a large positive fixed charge and a large negative charge in the interface traps and possibly in-bulk traps.3 So in order to effectively increase the mobility by reducing the Dit, one must also work to decrease the large amount of fixed charge in the oxide in order to maintain a reasonable threshold voltage. These problems have proved to be crucial in the production of DMOSFETs, where highly doped p-wells are needed. This high surface p-well doping reduces the inversion layer mobility by means of impurity scattering. In this series of initial experiments, the regrowth of lightly doped p-type layer was examined as a way to reduce impurity scattering. Experimental Experiment I For experiment I, lateral n-channel MOSFETs with a W/L equal to 400 µm/ 400 µm were fabricated on 1e1016 cm-3 Al-doped p-type epilayers grown on p-type 4H-SiC substrates off-cut 4o from the (0001) Si-face vicinal surface. This initial experiment was designed to determine the effect of p-type epitaxial regrowth on the highly doped p-well surface. In this experiment, a p-well was fir
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