One-Dimensional Models for Diffusion and Segregation of Boron and Ion Implantation of Aluminum in 4H-Silicon Carbide

Abstract

Silicon carbide with a poly-type 4H structure (4H-SiC) is an attractive material for power devices. While bipolar devices mainly utilize 4H-SiC p-n junctions, unipolar devices use p-n junctions both within the active region (to control the electric field distribution) and at the edges of the devices (to reduce electric-field crowding) (Baliga, 2005). In a p-type region, very high doping is necessary since common acceptors have deep energy levels (B: 0.3 eV; Al: 0.2 eV) (Heera et al., 2001). Boron is known to exhibit complex diffusion behaviour (Linnarsson et al., 2003), while aluminum has extremely low diffusivity (Heera et al., 2001). Precise modeling of boron diffusion and aluminum-ion implantation is therefore crucial for developing high-performance 4H-SiC power devices. For carbon-doped silicon, a boron diffusion model has been proposed (Cho et al., 2007). Unfortunately, the results cannot be directly applied to boron diffusion in SiC because of the existence of silicon and carbon sublattices. In addition, knowledge of boron segregation in 4H-SiC is lacking, preventing improvement of such novel devices as boron-doped polycrystalline silicon (poly-Si)/nitrogen-doped 4H-SiC heterojunction diodes (Hoshi et al., 2007). Dopant segregation in elementary-semiconductor/compound-semiconductor heterostructures—in which point defects in an elementary semiconductor undergo a configuration change when they enter a compound semiconductor—has yet to be studied. A framework for such analysis needs to be provided. With regards to aluminum distribution, to precisely design p-n junctions in 4H-SiC power devices, as-implanted profiles have to be accurately determined. For that purpose, Monte Carlo simulation using binary collision approximation (BCA) was investigated (Chakarov and Temkin, 2006). However, according to a multiday BCA simulation using a large number of ion trajectories, values of the simulated aluminum concentration do not monotonically decrease when the aluminum concentration becomes comparable to an n-type drift-layerdoping level (in the order of 1015 cm-3). A continuous-function approximation, just like the dual-Pearson approach established for ion implantation into silicon (Tasch et al., 1989), is thus needed. The historic development and basic concepts of boron diffusion in SiC are reviewed as follows. It took 16 years for the vacancy model (Mokhov et al., 1984) to be refuted by the 2