The carbon vacancy (V_C) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the V_C concentration is typically in the range of 10¹²cm⁻³, and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present study, both as-grown layers and a high-temperature processed one have been annealed at 1500 °C and the V_C concentration is demonstrated to be strongly reduced, exhibiting a value of only a few times 10¹¹cm⁻³ as determined by deep-level transient spectroscopy measurements. The value is reached already after annealing times on the order of 1 h and is evidenced to reflect thermodynamic equilibrium under C-rich ambient conditions. The physical processes controlling the kinetics for establishment of the V_C equilibrium are estimated to have an activation energy below ∼3 eV and both in-diffusion of carbon interstitials and out-diffusion of V_C's are discussed as candidates. This concept of V_C elimination is flexible and readily integrated in a materials and device processing sequence.

As-grown and pre-oxidized silicon carbide (SiC) samples of polytype 4H have been annealed at temperatures up to 1950 °C for 10 min duration using inductive heating, or at 2000 °C for 30 s using microwave heating. The samples consisted of a n-type high-purity epitaxial layer grown on 4° off-axis ⟨0001⟩ n⁺-substrate and the evolution of the carbon vacancy (V_C) concentration in the epitaxial layer was monitored by deep level transient spectroscopy via the characteristic Z_1/2 peak. Z_1/2 appears at ∼0.7 eV below the conduction band edge and arises from the doubly negative charge state of V_C. The concentration of V_C increases strongly after treatment at temperatures ≥ 1600 °C and it reaches almost 10¹⁵ cm⁻³ after the inductive heating at 1950 °C. A formation enthalpy of ∼5.0 eV is deduced for V_C, in close agreement with recent theoretical predictions in the literature, and the entropy factor is found to be ∼5 $k$ ($k$ denotes Boltzmann's constant). The latter value indicates substantial lattice relaxation around V_C, consistent with V_C being a negative-U system exhibiting considerable Jahn-Teller distortion. The microwave heated samples show evidence of non-equilibrium conditions due to the short duration used and display a lower content of V_C than the inductively heated ones. Finally, concentration-versus-depth profiles of V_C favour formation in the “bulk” of the epitaxial layer as the prevailing process and not a Schottky type process at the surface.

The carbon vacancy (V_C) is a prevailing point defect in high-purity 4H-SiC epitaxial layers, and it plays a decisive role in controlling the charge carrier lifetime. One concept of reducing the V_C-concentration is based on carbon self-ion implantation in a near surface layer followed by thermal annealing. This leads to injection of carbon interstitials (C_i's) and annihilation of V_C's in the epi-layer “bulk”. Here, we show that the excess of C atoms introduced by the self-ion implantation plays a negligible role in the V_C annihilation. Actually, employing normalized implantation conditions with respect to displaced C atoms, other heavier ions like Al and Si are found to be more efficient in annihilating V_C's. Concentrations of V_C below ∼2 × 10¹¹ cm⁻³ can be reached already after annealing at 1400 °C, as monitored by deep-level transient spectroscopy. This corresponds to a reduction in the V_C-concentration by about a factor of 40 relative to the as-grown state of the epi-layers studied. The negligible role of the implanted species itself can be understood from simulation results showing that the concentration of displaced C atoms exceeds the concentration of implanted species by two to three orders of magnitude. The higher efficiency for Al and Si ions is attributed to the generation of collision cascades with a sufficiently high energy density to promote C_i-clustering and reduce dynamic defect annealing. These C_i-related clusters will subsequently dissolve during the post-implant annealing giving rise to enhanced C_i injection. However, at annealing temperatures above 1500 °C, thermodynamic equilibrium conditions start to apply for the V_C-concentration, which limit the net effect of the C_i injection, and a competition between the two processes occurs.

The carbon vacancy (V_C) is a prominent defect in as-grown 4H-SiC epitaxial layers for high power bipolar devices. V_C is electrically active with several deep levels in the bandgap, and it is an efficient “killer” of the minority carrier lifetime in n-type layers, limiting device performance. In this study, we provide new insight into the equilibration kinetics of the thermodynamic processes governing the V_C concentration and how these processes can be tailored. A slow cooling rate after heat treatment at ∼2000 °C, typically employed to activate dopants in 4H-SiC, is shown to yield a strong reduction of the V_C concentration relative to that for a fast rate. Further, post-growth heat treatment of epitaxial layers has been conducted over a wide temperature range (800–1600 °C) under C-rich surface conditions. It is found that the thermodynamic equilibration of V_C at 1500 °C requires a duration less than 1 h resulting in a V_C concentration of only ∼10¹¹ cm⁻³, which is, indeed, beneficial for high voltage devices. In order to elucidate the physical processes controlling the equilibration of V_C, a defect kinetics model is put forward. The model assumes Frenkel pair generation, injection of carbon interstitials (C_i's) from the C-rich surface (followed by recombination with V_C's), and diffusion of V_C's towards the surface as the major processes during the equilibration, and it exhibits good quantitative agreement with experiment.

A vertical 4H-SiC p-i-n diode with 2×10²⁰cm^-3 Al⁺ implanted emitter and 1950°C/5min post implantation annealing has been characterized by deep level transient spectroscopy (DLTS). Majority (electron) and minority (hole) carrier traps have been found. Electron traps with a homogeneous depth profile, are positioned at 0.16, 0.67 and 1.5 eV below the minimum edge of the conduction band, and have 3×10^-15, 1.7×10¹⁴, and 1.8×10^-14 cm² capture cross section, respectively. A hole trap decreasing in intensity with decreasing pulse voltage occurs at 0.35 eV above the maximum edge of the valence band with 1×10¹³ cm² apparent capture cross section. The highest density is observed for the refractory 0.67 eV electron trap that is due to the double negative acceptor states of the carbon vacancy.